ANS Pharmacology Notes PDF

ANS PHARMACOLOGY PHRM202 Section B I. ANATOMY & GENERAL FUNCTIONS OF THE AUTONOMIC & SOMATIC MOTOR NERVOUS SYSTEMS Learning Objectives: 1. List the neurotransmitters of the autonom...

ANS PHARMACOLOGY PHRM202 Section B I. ANATOMY & GENERAL FUNCTIONS OF THE AUTONOMIC & SOMATIC MOTOR NERVOUS SYSTEMS Learning Objectives: 1. List the neurotransmitters of the autonomic sympathetic nervous system and describe their anatomical location 2. List the receptors and receptor-subtypes of the autonomic sympathetic nervous system 3. Predict the responses to activation and inhibition of autonomic sympathetic nervous system receptors Reading material: The nervous system is divided into two anatomical divisions: (i) Central nervous system (CNS) which consists of:  Brain  Spinal cord 1 ANS PHARMACOLOGY PHRM202 (ii) Peripheral nervous system (PNS; neurons outside the CNS) which consists of:  Afferent division – neurons which bring signals from the periphery to the CNS  Efferent (motor) division - neurons carrying signals away from the brain and spinal cord to the peripheral tissues The efferent division is further divided into two major subdivisions: Somatic: The somatic nervous system differs from the autonomic system in that it consist of a single myelinated motor neuron, originating in the CNS; the neuron travels directly to skeletal muscle without the mediation of ganglia. The somatic nervous system is under voluntary control, whereas the autonomic nervous system is an involuntary system. Autonomic: This is also called the visceral, vegetative, or involuntary nervous system. It is largely autonomous (independent) in that its activities are not under direct conscious control and it is concerned primarily with visceral (organs in the abdominal cavity) functions that are necessary for life such as cardiac output, blood flow to various organs, digestion, etc ANS is further subdivided into three systems, i.e:  Sympathetic nervous system  Parasympathetic nervous system  Enteric nervous system a) Anatomy of the Autonomic Nervous System There are two types of efferent ANS neurons responsible for transmitting nerve impulses from the CNS to the effector organs: 1) Preganglionic neurons: They emerge from the brainstem or spinal cord and make a synaptic connection in ganglia (an aggregation of nerve cell bodies located in the peripheral nervous system). Their cell bodies are located within the CNS. 2 ANS PHARMACOLOGY PHRM202 2) Postganglionic neurons The latter neurons have cell bodies originating in the ganglia. They are generally non-myelinated and terminate on effector organs, such as smooth muscles of the viscera, cardiac muscle, and the exocrine glands. b) Functions of the sympathetic nervous system It prepares the body in response to stressful situations, such as trauma, fear, hypoglycemia, cold, or exercise. Stimulation of the sympathetic division increases the heart rate and blood pressure thus mobilising energy stores of the body. Also increase the blood flow to skeletal muscles and the heart, while diverting the blood flow from the skin and internal organs. “Fight or flight” response (in emergency) triggered both by direct sympathetic activation of the effector organs, and by stimulation of the adrenal medulla to release adrenaline (A) and lesser amounts of noradrenaline (NA). NA & A enter the bloodstream and promote responses in effector organs that contain adrenergic receptors throughout the body. 3 ANS PHARMACOLOGY PHRM202 c) Functions of the parasympathetic nervous system The parasympathetic division maintains essential bodily functions required for life such as the conservation of energy & maintenance of organ function during periods of minimal activity. It usually acts to oppose or balance the actions of the sympathetic division. It is generally dominant over the sympathetic system in "rest and digest” situations. The parasympathetic division slows the heart rate, lowers the BP, stimulates the GIT movements & excretion, aids absorption of nutrients, protects the retina from excessive light, & empties the urinary bladder & rectum. d) Innervation by the ANS 4 ANS PHARMACOLOGY PHRM202 Most organs in the body are innervated by both divisions of the autonomic nervous system (sympathetic & parasympathetic). Despite this dual innervation, one system usually predominates in controlling the activity of a given organ e.g. in the heart, the parasympathetic is the predominant factor for controlling the heart rate. Organs receiving only sympathetic innervation:  some effector organs, such as the adrenal medulla, blood vessels, and sweat glands, receive innervation only from the sympathetic system e) Chemical signalling between cells 5 ANS PHARMACOLOGY PHRM202 Chemical communication between cells (including those of the ANS) occurs through the use of chemical mediators which are classified as follows:  Local hormones: Secreted by most cells in the body. They act locally and are rapidly destroyed; they do not enter the bloodstream e.g. histamine secreted by the mast cells during inflammation.  Hormones: Secreted by the endocrine glands into the bloodstream distributed throughout the body.  Neurotransmitters: Released from the nerve terminals - triggered by the arrival of the action potential at the nerve ending. They are responsible for the communication between:  the nerve cells (ganglion), and nerve cell and the effector organ (neuroeffector)  They rapidly diffuse across the synaptic cleft or gap (synapse) and combine with (bind to) specific receptors on the postsynaptic (target) cell. Like local hormones, they are rapidly destroyed Six most common NT are noradrenaline (NA), acetylcholine (Ach), dopamine (DA), serotonin (5-HT), histamine (H), & gama-aminobutyric acid (GABA). Each of these NT binds to a specific family of receptors. Ach and NA are the primary chemical signals in the ANS; others function mostly in the CNS. f) Neurotransmitter Chemistry of the ANS Autonomic nerves are classified on the basis of their primary transmitter molecules: o Acetylcholine (Ach) o Noradrenaline (NA) Nerves that synthesise and release Ach are referred to as cholinergic neurons (fibers), while those synthesising and secreting NA are referred to as noradrenergic (or simply “adrenergic”) fibers. 6 ANS PHARMACOLOGY PHRM202 7 ANS PHARMACOLOGY PHRM202 1. Cholinergic transmission Ach is a NT at autonomic ganglia in both the sympathetic and parasympathetic nervous systems. Also a NT at the adrenal medulla where it modulates the sympathetic nervous system. Also act at autonomic neuroeffector junction (between postganglionic nerves and the effector organs) in the parasympathetic nervous system. In the somatic nervous system, transmission at the neuromuscular junction (that is, between nerve fibers and voluntary muscles) is also cholinergic. 2. Adrenergic fibers constitute both NA and A NA is a transmitter at autonomic neuroeffector junction (between postganglionic nerves and the effector organs) in the sympathetic system, with the exception of the sweat glands where the neurotransmitter is Ach. Both NA and A are also synthesised and released by the adrenal medulla following cholinergic stimulation to control (increase) BP. g) Autonomic Receptors All neurotransmitters are too hydrophilic to penetrate the cell membranes lipid bilayer of target-cell. Instead, their signal is mediated by binding to specific receptors on the cell surface of target organs There are two main types of receptors in the ANS: o Cholinergic receptors o Adrenergic receptors 1. Cholinergic receptors The primary ACh (cholinergic) receptor subtypes were named after the alkaloids originally used in their identification: 8 ANS PHARMACOLOGY PHRM202 o Muscarine - Muscarinic receptors (M) Found mostly as postsynaptic membranes of effector organs in the parasympathetic nervous system. Further subdivided into M1-4 o Nicotine – Nicotinic receptors (N). Found autonomic ganglia of, Sympathetic, Parasympathetic nervous system, and Adrenal medulla. Also found postsynaptically in neuroeffector membrane in the somatic nervous system. Further subdivided into NN & NM 2. Adrenergic receptors Adrenoceptor or simply adrenergic receptors respond to catecholamines such as NA, A, & DA. The adrenoceptors can be subdivided into two main divisions on the basis of their agonist and antagonist selectivity: o Alpha-adrenergic receptors (α-ARs)  Further subdivided into α1-& α2-ARs  The rank order of potency of agonists for α-ARs is A > NA ≥ isoproterenol o Beta-adrenergic receptors (β-ARs)  Further subdivided into β1-, β2- & β3-ARs  The rank order of potency of agonists for β-ARs is isoproterenol > A ≥ NA 9 ANS PHARMACOLOGY PHRM202 10 ANS PHARMACOLOGY PHRM202 II. CHOLINERGIC AGONISTS Learning objectives:  Be able to list the receptors of the parasympathetic nervous system  Contrast the actions and effects of direct and indirect stimulation of muscarinic cholinoreceptors  List the therapeutic uses of parasympathomimetic agents  List the adverse effect of parasympathomimetic agents Overview Cholinergic agonists (cholinomimetic agents) are a large group of drugs that mimic the actions of ACh which includes:  Ach-R stimulants  Cholinesterase inhibitors Ach-R stimulants are further classified according to the type of receptors they selectively stimulate:  Muscarinic cholinomimetics  Nicotinic cholinomimetics They are sometimes classified according to their mechanism of actions, i.e.:  Direct acting (binding directly to the ACh-R) cholinergic agonists  Indirect acting (inhibit the hydrolysis of endogenous ACh) cholinergic agonists a) The Cholinergic Neuron In the ANS, ACh is a neurotransmitter at  the autonomic ganglia (both parasympathetic and sympathetic) 11 ANS PHARMACOLOGY PHRM202  preganglionic fibers terminating in the adrenal medulla  postganglionic fibers of the parasympathetic division In addition, cholinergic neurons innervate the muscles of the somatic system. *Patients with Alzheimer’s disease have a significant loss of cholinergic neurons in the temporal lobe andl cortex. It is a progressive neurodegenerative disorder in the form of dementia occurring in the middle age & later. Most of the drugs available to treat the disease at this stage are acetylcholinesterase (AChE) inhibitors. b) Neurotransmission at cholinergic neurons Neurotransmission in cholinergic neurons involves six steps including: (1) synthesis, (2) storage, (3) release, (4) binding of Ach to a receptor, (5) degradation of the NT in the synaptic gap, and (6) the recycling of choline (1) Synthesis of acetylcholine: Choline is transported from the plasma into the neuron by a specialised carrier system referred to as the high-affinity choline transporter (Na+ dependent). Choline is more polar (i.e. has a quaternary nitrogen -NCH3+) & cannot diffuse through the membrane. The uptake of choline is the rate-limiting step in acetylcholine synthesis. *This carrier system can be inhibited by the drug hemicholinium. 12 ANS PHARMACOLOGY PHRM202 Choline acetyltransferase catalyses the reaction of choline with acetyl CoA to form acetylcholine in the cytosol. (2) Storage of ACh in vesicles: ACh is packaged into vesicles called vesiculin by an active transport process. The mature vesicle also contains adenosine triphosphate (ATP), ions(Ca2+ & Mg2+) and proteoglycan. (3) Release of acetylcholine: An action potential stimulates voltage-sensitive Ca2+ channels in the presynaptic membrane. Elevated Ca2+ levels promote the fusion of synaptic vesicles with the cell membrane and release of their contents into the synaptic cleft. *This release can be blocked by botulinum toxin (a nerve toxin produced by the bacterium Clostridium botulinum). In contrary black widow spider venom causes all the ACh stores in synaptic vesicles to empty into the synaptic gap. (4) Binding to the receptor: Released ACh diffuses across the synaptic space and binds to either  two types of postsynaptic receptors (M & N) on the target cell  Presynaptic (auto-) receptors in the membrane of the neuron that released ACh Binding to a receptor leads to a biological response within the cell (5) Degradation of Ach: The signal at the postsynaptic effector site is rapidly terminated, because acetylcholinesterase (AchE) cleaves ACh to choline and acetate in the synaptic cleft. (6) Recycling of choline: Choline may be recaptured by a Na +-coupled, high-affinity choline uptake system and transported back into the neuron for further ACh synthesis. 13 ANS PHARMACOLOGY PHRM202 c) Cholinergic Receptors (Cholinoreceptors) Muscarinic cholinergic receptors M receptors are 7 transmembrane protein that are coupled to a G-protein (GPCRs). G-protein functions as a transducer (a device capable of converting one form of signal to another). These receptors bind to both ACh and muscarine – but only a weak affinity for nicotine. 5 subclasses of M receptors have been distinguished: M1-5:  M1, M3, and M5 lead to cellular excitation,  whereas M2 and M4 inhibit cellular excitability 14 ANS PHARMACOLOGY PHRM202 Locations of muscarinic receptors All five subtypes have been found on neurons; M1 receptors are also found on gastric parietal cells (where they control gastric acid secretion), M2 receptors on cardiac cells and smooth muscle (slow the heart rate & force of contraction), and M 3 receptors on the bladder, exocrine glands, and smooth muscle. ACh signal transduction mechanisms: M1 & M3 receptor transduction mechanisms When the M1 or M3 receptors are activated, they undergo a conformational change and interacts with a G protein, designated Gq, which in turn activates phospholipase C. This leads to the hydrolysis of phoshatidylinositol-(4,5)-bisphosphate (PIP2) to yield diacylglycerol (DAG) and inositol (1,4,5)-trisphosphate (IP3). IP3 cause an increase in intracellular Ca 2+ from intracellular stores, sarcoplasmic reticulum). Ca2+ can then interact to stimulate or inhibit enzymes, or cause hyperpolarization, secretion, or contraction 15 ANS PHARMACOLOGY PHRM202 M2 & M4 cholinergic receptor transduction mechanisms in cardiac cells In contrast, activation of the M2 subtype on the myocardium muscle stimulates a G protein, designated Gi,. This activity then inhibits adenylyl cyclase (AC) activity – which normally catalises the conversion of ATP to cAMP (second messenger). The resultant effect of such an inhibition is an increased K+ conductance to which the heart responds with a decrease in rate and force of contraction. Nicotinic cholinergic receptors (N) These receptors bind to both ACh & nicotine – and also show only a weak affinity for muscarine. The N cholinergic receptor is composed of five polypeptides transmembrane subunits, and functions as a ligand-gated ion channel Binding elicits a conformational change that allows the entry of Na+, resulting in the depolarization of the effector cell (excitatory post-synaptic potential; EPSP). Nicotine (or ACh) initially stimulates and then blocks the receptor (due to receptor desensitisation) following persistent stimulation. 16 ANS PHARMACOLOGY PHRM202 Nicotinic receptors are located in the  CNS  adrenal medulla  all autonomic ganglia  neuromuscular junction of sympathetic and somatic nervous system Those at the neuromuscular junction are designated NM, while those in the postganglionic cell body & dendrites are designated NN. *NN receptors are selectively blocked by hexamethonium, whereas NM receptors are specifically blocked by tubocurarine. 17 ANS PHARMACOLOGY PHRM202 d) Direct-acting Muscarinic Cholinergic Agonists This group of drugs mimic the effects of ACh by binding directly to muscarinic cholinergic receptors. These agents are classified as either:  synthetic esters of choline, such as carbachol , methacholine, carbamic acid and bethanechol  naturally occurring alkaloids, such as pilocarpine, muscarine, & nicotine All have longer durations of action than Ach. They show little specificity or selectivity for the various subtypes of M receptors in their actions, which limits their clinical usefulness. Some of these agents stimulate both the N & M cholinergic receptors. Pharmacokinetics (absorption, distribution, & metabolism) 18 ANS PHARMACOLOGY PHRM202 Synthetic esters of choline are poorly absorbed (from the GIT) & poorly distributed into the CNS because they are hydrophilic. As a result they are less active by oral route – they are inactivated in the GIT. They are mostly reserved for ophthalmic application – due to their potential toxic effects. They differ in their susceptibility to hydrolysis by AChE. Metacholine is more resistant to AChE – has a longer duration of action. The b-methyl group (methacholine & bethanecol) reduced the potency of these drugs for N cholinergic receptors The natural cholinomimetic alkaloids (pilocarpine, nicotine) are well absorbed from most sites of administration. Nicotine – lipid soluble liquid can also be absorbed across the skin. Muscarine – incompletely absorbed from the GIT, but may be too toxic when ingested. Pilocarpine – moderately lipophilic and well absorbed from the GIT. Also enters the CNS (crosses the BBB). They are excreted mainly by the kidneys. Acidification of the urine enhances the clearance of these tertiary amines (because at acidic pH, they are ionised and more water soluble) i) Acetylcholine (ACh) ACh is a quaternary ammonium compound that cannot penetrate membranes. It is the NT of parasympathetic and somatic nerves as well as ganglia. *It is therapeutically of no importance because of its multiplicity of actions, and its rapid inactivation by both the membrane AChE & plasma butyrylcholinesterase (BuChE; pseudocholinesterases) ACh has both M and N cholinergic activity 19 ANS PHARMACOLOGY PHRM202 Pharmacological properties of Ach ACh is rarely given systemically because of its short t1/2 and its diffuse actions ACh has several primary effects on the cardiovascular system, i.e. 1. Decreased heart rate and cardiac output: ACh, if injected IV, produces a brief decrease in cardiac rate and stroke volume resulting from interaction with the M2 receptors at the sinoatrial (SA) node of the heart. ACh slows the heart rate by: o decreasing the rate of spontaneous diastolic (time between ventricular contractions (systole), at which ventricular filling occurs) depolarisation (the pacemaker current) and o by increasing the repolarising K+ current at the SA node The resultant effect is the delayed cardiac cycle due to the elevated threshold potential. 2. Decrease in blood pressure: Injection of ACh causes vasodilatation and lowers of BP. The parasympathetic division does not innervate the vasculature but there are cholinergic receptors on the blood vessels (especially the M3 cholinergic receptors). The M receptors responsible for relaxation are located on the endothelial cells of the vasculature. Stimulation of these receptors activates the G q-PLC-IP3 of the endothelial cells leading to Ca2+-calmodulin-dependent activation of endothelial NO synthase (eNOS) and production of NO (endothelium-derived relaxing factor; EDRF). 20 ANS PHARMACOLOGY PHRM202 NO also diffuses to adjacent smooth muscle cells and cause them to relax by catalising the conversion of GTP to cGMP (mediator responsible for the relaxation). *Atropine blocks these muscarinic receptors and prevents ACh from producing vasodilation. 3. Decreased strength of contraction (negative inotropic effect) In the artrial muscles, ACh decreases the force of contraction both directly & indirectly. Indirectly – as a result of decreasing cAMP and Ca2+ channel activity at lower concentrations. Directly – at higher concentrations of ACh, there is inhibition of the M 2 cholinergic receptors resulting in receptor-mediated activation of G protein regulated K+ channels. The cells are then hyperpolarised by the K+ influx – the ultimate response is reduced impulse generation and conduction. This decrease in conduction is responsible for the complete heart block observed when large quantities of cholinergic agonists are administered. ii) Bethanechol (carbonyl-b-methylcholine) It is structurally related to ACh but the acetate is replaced by carbamate and the choline methylated. These structural changes renders it resistant to hydrolysis by AChE but not to all other esterases like BuChE. 21 ANS PHARMACOLOGY PHRM202 The b-methyl group reduces the potency of bethanechol to N cholinergic receptors (selective for M cholinergic receptors). It has a duration of action of about 1 hour Bethanechol (carbonyl-b-methylcholine) Actions: Bethanechol directly stimulates muscarinic receptors (M3) causing increased intestinal motility and tone. It also stimulates the detrusor muscles of the bladder while the trigone and sphincter are relaxed, causing expulsion of urine (M3). Therapeutic applications: In urologic treatment, bethanechol is used to stimulate the atonic (nonemptying) bladder, particularly in postpartum or postoperative, nonobstructive urinary retention. Oral bethanechol is useful in certain cases of postoperative abdominal distention (expansion) & gastric atony (lack of tone) or gasreparesis (delayed stomach emptying). Adverse effects: Bethanechol causes the effects of generalized cholinergic stimulation including sweating, salivation, flushing, hypotension, nausea, abdominal pain, diarrhea, and bronchospasm. iii). Carbachol (carbamoycholine) 22 ANS PHARMACOLOGY PHRM202 Carbachol has both M as well as N actions (not selective). Like bethanechol, carbachol is an ester of carbamic acid and a poor substrate for AChE. It is biotransformed by other esterases, but at a much slower rate. A single administration can last as long as 1 hour. Carbachol (carbamoycholine) Actions: Carbachol profoundly affects both the cardiovascular system and the GIT because of its ganglion- stimulating activity (N receptor effect). It may 1st stimulate & then depress these systems. It can cause release of A & NA from the adrenal medulla by its nicotinic action. When locally instilled into the eye, it mimics the effects of ACh, causing miosis (contraction of the pupils) and a spasm of accommodation. Therapeutic uses: Rarely used therapeutically except in the eye as a miotic agent to treat glaucoma (by causing pupillary contraction and a decrease in intraocular pressure). Use is limited by its high potency, lack of selectivity & relatively long duration of action. Adverse effects: At doses used ophthalmologically, little to no side effects occur. iv) Methacholine (acetyl-b-methylcholine) 23 ANS PHARMACOLOGY PHRM202 Methacholine is a synthetic choline ester. It is highly active at all of the M cholinergic receptors, but has little or no effect on the N cholinergic receptors (possible due to the presence of a b-methyl group). It is also resistance to AChE - broken down at a relatively slow rate within the body. Methacholine (acetyl-b-methyl choline) Clinical use Primarily used to diagnose bronchial hyper-reactivity, which occurs in asthma. Asthmatic patients respond to cholinergic agonists with intense bronchoconstriction, secretions, and reduction in vital capacity. Side effects Cardiovascular effects, such as bradycardia and hypotension limit the use of this drug. Use of methacholine, as well as all other M receptor agonists, is contraindicated in patients with coronary insufficiency, gastroduodenal ulcers, and incontinence (inability to control excretory functions) v) Pilocarpine The alkaloid pilocarpine is a tertiary amine, and is stable to hydrolysis by AChE. It is far less potent than ACh and its derivatives. Pilocarpine exhibits M activity and is used primarily in ophthalmology (selective for M receptors). 24 ANS PHARMACOLOGY PHRM202 Actions: When applied topically to the cornea, pilocarpine produces a rapid miosis (M3 stimulation) and contraction of the ciliary muscle (M3 stimilation) making the lens more convex. At this point it is impossible to focus – vision is fixed at to a particular distance. Atropine (M receptor blocker) opposes these effects on the eye. Pilocarpine potently stimulates secretions such as sweat, tears, and saliva – but its use is limited due to its lack of selectivity (amongst M receptor subtypes) e.g. its use in Sjogren syndrome or xerostomia (dryness of the mouth) that follows head & neck radiation treatments is limited by its lack of selectivity. Therapeutic use in glaucoma: Pilocarpine is the drug of choice in the emergency lowering of intraocular pressure of both narrow- angle (also called closed-angle; acute congestive) and wide-angle (also called open-angle; chronic simple) glaucoma. It achieves these because it stimulates the contraction of the smooth muscles of the iris sphincter (resulting in miosis). The iris is then pulled away from the anterior chamber opening the trabecular meshwork around Schlemm canal at the base of ciliary muscles causing an immediate drop in intraocular pressure as a result of the increased drainage of aqueous humor. This action lasts up to one day and can be repeated. 25 ANS PHARMACOLOGY PHRM202 Adverse effects: Pilocarpine can enter the brain and cause CNS disturbances. It stimulates profuse sweating and salivation. iv) CNS application of cholinergic agonists Selective M1 cholinergic receptor agonists have been targets for use in treating cognitive imparement in Alzheimer’s disease (where the is reduced CNS cholinergic neurotransmission). Such a potential agonist should selectively stimulate the postsynaptic M 1 cholinergic receptors without stimulating the presynaptic M2 that inhibit the release of endogenous ACh e) Indirect-Acting Cholinergic Agonists; Anticholinesterases AChE is an enzyme that specifically cleaves ACh to acetate and choline and terminates its actions. It is membrane bound, located both pre- and postsynaptically in the nerve terminal. It is highly concentrated at the postsynaptic end plate of the neuromuscular junction. 26 ANS PHARMACOLOGY PHRM202 Drugs that inhibit AChE are called anticholinesterase (anti-ChE) agents. Anti-ChE indirectly provides cholinergic neurotransmission by prolonging the lifespan of ACh at the synaptic space. This results in the accumulation of ACh in the synaptic space provoking a response at all cholinoceptors in the body (both N & M receptors). Due to the widespread distribution of cholinergic neurons in the body, anti-ChE has found application in as toxic agents in the form of agricultural pesticides and potential chemical warfare “nerve gases” (sarin, soman, & tabun). *Therapeutically anti-ChE are used in conditions such as Alzheimer’s disease. These group of agents are classified as either reversible anti-ChE (physostigmine, neostigmine); or irreversible anti-ChE or organophosphates (sarine; isoflurophate) f) Therapeutic applications of anti-ChE 1. Non-obstructive paralytic ileus (paralysis or inactivity of the intestine) - neostigmine is preferred 2. Atony of the bladder – Neostigmine is also preferred 3. Glaucoma and other ophthalmologic indications Glaucoma is characterised by an increase in intraocular pressure, that if sufficiently high & persistent lead to damage of the optic disc at the juncture of the optic nerve and retina: irreversible blindness can occur. Narrow angle (acute congestive; closed-angle) glaucoma is a medical emergency while wide angle (chronic simple) is controlled by continuous drug therapy. Anti-ChE are also used locally in the treatment of accommodative esotropia & mysthenia gravis of the intraocular & eyelid muscles. Also used in Adie (tonic pupil syndrome) resulting from the dysfunction of the ciliary body – physostigmine decreases the blurred vision & pain associated with the condition 27 ANS PHARMACOLOGY PHRM202 4. Myasthenia gravis: Neuromuscular disease characterised by weakness & marked fatigability of the skeletal muscles caused by autoimmune response primarily to the N ACh receptors at the postjuctional endplate. Symptoms are similar to those seen with curare (muscle relaxant) intoxication. Both respond to anti-ChE therapy. Diagnosis – edrophonium chloride (positive response consists of brief improvement in muscle strength. Treatment – pyridostigmine, neostigmine, & ambenonium are the standard anti-ChE used in the symptomatic treatment of Myasthenia gravis. 5. Prophylaxis in anti-ChE inhibitor poisoning Pretreatment with pyridostimine the incapacitation and mortality associated with nerve gas poisoning – especially for agents like soman with rapid aging. Given to the military in anticipation of nerve-agent attack. 6. Intoxication by anticholinergic drugs Cholinergic intoxication is caused by anticholinergic agents like atropine, and many other drugs with either central or peripheral anticholinergic activity like phenothiazines, antihistamines, and tricyclic antidepressants. Physostigmine is useful in reversing the central anticholinergic effects of these agents 7. Alzheimer’s disease A deficiency of intact cholinergic neurons, especially those extending from the subcortical areas has been observed in patients with progressive dementia of the Alzheimer’s disease. Tacrine is approved for use in mild to moderate Alzheimer’s disease (limited by a high incident of hepatotoxicity – typical of all the anti-ChE). Other anti-ChE approved for the treatment of Alzheimer’s disease include, donepezil, rivastigmine, & galantamine. g) Anticholinesterases (Reversible) 28 ANS PHARMACOLOGY PHRM202 i) Physostigmine Physostigmine is a tertiary amine alkaloid. It is a carbamic acid ester and a substrate of AchE. It forms a relatively stable complex with the enzyme, which then becomes reversibly inactivated. The result is potentiation of cholinergic activity throughout the body. Actions: Physostigmine has a wide range of effects – it lacks selectivity in its actions (because it potentates the effects of ACh). Its duration of action is about 2 to 4 hours. Physostigmine can enter and stimulate the cholinergic sites in the CNS Therapeutic uses: It is used therapeutically stimulate the atonic (nonemptying) bladder. Placed topically in the eye to treat glaucoma, but pilocarpine is more effective. Physostigmine is also used in the treatment of 29 ANS PHARMACOLOGY PHRM202 overdoses of drugs with anticholinergic actions, such as atropine, phenothiazines, and tricyclic antidepressants Adverse effects: CNS related convulsions when high doses are used. Bradycardia & a fall in cardiac output may also occur. At higher dose may paralyze skeletal muscles. ii) Neostigmine Neostigmine is a synthetic compound that is also a carbamic acid ester. Like physostigmine, it also reversibly inhibits AChE. Unlike physostigmine, neostigmine does not enter the CNS (has a quarternary nitrogen - it is more polar). Its effect on skeletal muscle is greater than that of physostigmine, and it can stimulate contractility before it paralyzes. Its duration of action is 30 min to 2 hours. Therapeutic uses: It is used to stimulate the bladder and GI tract, also used as an antidote for tubocurarine and other competitive neuromuscular blocking agents. Also used in the symptomatic treatment of myasthenia gravis Adverse effects of neostigmine include: Those of generalized cholinergic stimulation, such as salivation, flushing, decreased blood pressure, nausea, abdominal pain, diarrhea, and bronchospasms. iii) Pyridostigmine and ambenomium Pyridostigmine & ambemonium are other AChE inhibitors that are used in the chronic management of myasthenia gravis. Their durations of action is 3 to 6 hours and 4 to 8 hours, respectively iv) Edrophonium The actions of edrophonium are similar to those of neostigmine, except that it is more rapidly absorbed and has a short duration of action (10 to 20 minutes). Edrophonium is a quarternary amine and is used in the diagnosis of myasthenia gravis. 30 ANS PHARMACOLOGY PHRM202 IV injection of edrophonium leads to a rapid increase in muscle strength. Care must be taken, because excess drug may provoke a cholinergic crisis. Atropine is the antidote. v) Tacrine, donepezil, rivastigmine, & galantamine Patients with Alzheimer’s disease have a deficiency of cholinergic neurons in the CNS. This observation led to the development of anticholinesterases as possible remedies for the loss of cognitive function. Tacrine was 1st available, but it has been replaced by the others because of its hepatotoxicity. Despite their ability to delay the progression of the disease – they do not stop its progression of the disease. GIT distress is their primary adverse effect. h) Anticholinesterases (Irreversible) Most organic derivatives of phosphoric acid (organophosphate) have the capacity to bind covalently to AChE. The result is a long-lasting increase in ACh at all sites where it is released. However many of these drugs are extremely toxic and were developed by the military as nerve agents e.g. Sarine in the Tokyo subway (20 March 1995). Related compounds, such as parathion, are employed as insecticides. Recently the use of organophosphate insecticides has been replaced by less toxic reversible carbamates such as carbaryl, propoxur (Baygon®), and aldicarb. Isoflurophate (diisopropyl fluorophosphate; DFP) is the prototype agent. i) Isoflurophate Mechanism of action: Isoflurophate is an organophosphate that covalently binds to the active site of AChE (phosphorylating the enzyme). If the alkyl groups of the phosphorylated enzyme are ethyl or methyl, spontaneous regeneration occurs within several hours. 31 ANS PHARMACOLOGY PHRM202 Secondary & tertiary alkyl groups (like isopropyl of isoflurophate) further strengthens the stability of the covalent bond – significant regeneration of the active enzyme is no longer observed. The enzyme is then permanently inactivated – restored by the synthesis of new enzyme molecules. These process by which the phosphorylated enzyme slowly releases one of its alkyl groups is called “aging”. “Aging” makes it impossible for chemical reactivators, such as pralidoxime (PAM) , to break the bond between the remaining drug and the enzyme because “aging” strengthens the phosphorylation of the enzyme. DFP ages in 6 to 8 hours, whereas newer nerve agents, available to the military, age in minutes or seconds. Actions: Cause generalized cholinergic stimulation, paralysis of motor function, intense miosis, and convulsions. Atropine in high dosage can reverse many of the muscarinic and central effects of isoflurophate Therapeutic uses: An ophthalmic ointment of the drug is used topically in the eye for the chronic treatment of open- angle glaucoma. The effects may last for up to one week after a single administration. Echothiophate also covalently binds to acetylcholinesterase, and has largely replaced isoflurophate as a therapy for open-angle glaucoma i) Cholinesterase Reactivators Oximes like pralidoxime (pyridine-2-aldoxime methyl chloride; PAM) and obidoxime (1,1'- (Oxydimethylene)bis(4-formylpyridinium) dioxime chloride; HI-6) are capable of reactivating inhibited AChE enzyme more rapidly than does spontaneous hydrolysis. The presence of a charged group allows them to approach an anionic site on the enzyme, where it essentially displaces the organophosphate and regenerates the enzyme. 32 ANS PHARMACOLOGY PHRM202 If given before aging of the enzyme occurs, they can reverse the effects of isoflurophate, except for those in the CNS (because they do not cross the BBB). PAM is less effective against newer nerve agents - because aging occurs more rapidly. Pharmacology, toxicology, & disposition The reactivating effects of oximes are more evident at the neuromuscular junction – IV administration restores response to stimulation of the motor nerve within few minutes. The antidotal effects of oximes are minimal at the autonomic effector sites & their entry into the CNS is restricted by their quaternary ammonium group. High doses of oximes inhibit the AChE enzyme 33 ANS PHARMACOLOGY PHRM202 and causes neuromuscular blockade – care should be exercised when these agents are administered Current antidotal therapy for organophosphate exposure resulting from warfare or terrorism includes: o Atropine o An oxime o A benzodiazipine as an anticonvulsant 34 ANS PHARMACOLOGY PHRM202 35 ANS PHARMACOLOGY PHRM202 Case study: A 61-year old man is noted to have increased intraocular pressure on a routine eye examination. The visual acuity is normal in both eyes. The dilated eye examination reveals no evidence of optic nerve damage. Visual field testing shows mild loss of peripheral vision. He is diagnosed with primary open-angle glaucoma and is started on pilocarpine ophtalmic drops. 1. What is the action of pilocarpine on the muscles of the iris and cilia? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 2. What receptor mediates this action? ______________________________________________________________________________ 3. Is pilocarpine the appropriate first-line drug for treatment of primary open-angle glaucoma? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ To remember:  Cholinoreceptors are classified as either nicotinic or muscarinic  Muscarinic cholinoreceptors are localized at organs such as the heart, causing a negative chronotropic effect  Stimulation of muscarinic receptors in the smooth muscle, exocrine glands, and vascular endothelium cause bronchoconstriction, increased acid secretion and vasodilation  Methacholine and bethanechol are highly selective for muscarinic cholinoreceptors  Cholinomimetic agents, including anticholinesterase inhibitors, are precluded for treatment of GI or urinary tract disease because of mechanical obstruction, where therapy can result in increased pressure and possible perforation. They are also not indicated for patients with asthma 36 ANS PHARMACOLOGY PHRM202 III) CHOLINERGIC ANTAGONISTS Learning objectives: 1. Describe the mechanism of action of muscarinic cholinoreceptor antagonists 2. Describe the physiologic effects of muscarinic cholinoreceptor antagonists 3. List important therapeutic uses of muscarinic cholinoreceptor antagonists 4. List the adverse effects and contraindications of muscarinic cholinoreceptor antagonists Reading material The cholinergic antagonists (cholinergic blockers or anticholinergic drugs) bind to cholinergic receptors, but they do not trigger the usual receptor-mediated intracellular effects. They are divided into 2 major subgroups on the basis of their specific receptor affinities: o Antimuscarinic (M receptor blockers), and o Antinicotinic (Nicotinic receptor blockers) The antinicotinic drugs consist of o Ganglionic stimulants & blockers o Neuromuscular junction blockers (muscle relaxants) The most useful of these agents are the selective antimuscarinic agents of the parasympathetic nerves followed by the neuromuscular blocking agents. The ganglionic blockers have little clinical usefulness – due to their diffuse or broad actions & lack of selectivity 37 ANS PHARMACOLOGY PHRM202 a) Antimuscarinic Agents Muscarinic antagonists are often called parasympatholytic because they block the effects of parasympathetic autonomic discharge; but the term "antimuscarinic" is preferable. These agents competitively bind to the M cholinergic receptors – blocking ACh’s binding to these receptors, inhibiting its stimulatory effects. In addition they block the few exceptional sympathetic neurons that are cholinergic, such as those innervating salivary & sweat glands. Unlike the cholinergic agonists (with limited therapeutic usefulness), the cholinergic blockers are beneficial in a variety of clinical situations. Because they do not block N cholinergic receptors, they are devoid (or little) of both the skeletal neuromuscular junctions or autonomic ganglia effects 38 ANS PHARMACOLOGY PHRM202 i) Atropine (hyoscyamine) Atropine and its naturally occurring congeners are tertiary amine alkaloid esters of tropic acid. Atropine (hyoscyamine) is found in the plant Atropa belladonna, and in Datura stramonium, also known as thorn apple. At therapeutic doses atropine acts mainly peripherally, it has very low central effects (it does not cross the cross the BBB significantly). Tissues most sensitive to atropine are salivary, bronchial, & sweat glands. Secretion of acid by the gastric parietal cells is the least sensitive In general atropine has a duration of action of about 4 hours, but topically in the eye actions may last for days. Mechanism of action Atropine and related compounds have a higher affinity for M receptors. They bind competitively to the common site on the cholinergic receptors blocking ACh and other cholinergic agonists from stimulating these receptors. Since the binding of atropine to the M cholinergic receptors is competitive, it can be overcome if the concentration of ACh at the receptor sites is increased sufficiently. Thus, blockade by atropine can be overcome by a larger concentration of: o ACh (anti-ChE, like neostigmine) o equivalent muscarinic agonist (bethanechol) Atropine is highly selective for M receptors; potency at N receptors is much lower. Unfortunately it does not discriminate between the M1, M2, and M3 subgroups of muscarinic receptors Actions of atropine i) Eye: Atropine blocks all cholinergic activity of the pupillary sphincter muscles of the iris and the ciliary muscles of the eye controlling lens curvature (M3 effects) resulting in persistent mydriasis (dilated pupil), unresponsiveness to light, cycloplegia (paralysis of the ciliary muscles of the eye), & inability to focus for near vision. The lens is fixed for far vision, near objects are blurred, and objects may appear smaller than they really are. 39 ANS PHARMACOLOGY PHRM202 In patients with close-angle glaucoma, intraocular pressure may rise dangerously. As a result short-acting agents, are generally favored. ii) GIT: It has antispasmodic actions – reducing the activity of the GIT (M3 effects). Atropine & scopolamine are probably the most potent drugs available that produce this effect. It does not significantly reduce HCl production - not effective in promoting healing of peptic ulcer. Pirenzepine & its potent analog telenzepine are selective M1-receptor antagonists – reduces gastric acid secretion at doses that do not antagonize other systems Histamine type-2 H2 receptor antagonists (cimetidine), and proton pump inhibitors (omeprazole) have replaced the nonselective muscarinic antagonists as inhibitors of acid secretion iii) Urinary system: The antimuscarinic action of atropine relaxes smooth muscle of the ureter and bladder wall (M3 effects) and slows voiding. It is employed clinically to reduce hypermotility states of the urinary bladder. It is still occasionally used in enuresis (involuntary voiding of urine) among children, but α-adrenergic agonists with fewer side effects may be more effective iv) Secretions: Atropine blocks the salivary glands, producing a drying effect on the oral mucus membranes (xerostomia) – since salivary glands are exquisitely sensitive to atropine (M3 effects). Sympathetically innervated sweat glands are also affected by small doses of atropine & scopolamine– suppressing thermoregulatory sweating, the skin becomes hot and dry. Sweating may be depressed enough to raise the body temperature. In adults, body temperature is elevated only at high doses of atropine - but in infants & children even ordinary doses may cause "atropine fever" v) Respiratory system Both smooth muscle & secretory glands of the respiratory system are parasympathetically innervated and contain M receptors but the anticholinergic effect is more significant in patients with airway diseases – but not as useful as the β2-AR stimulants in the treatment of asthma 40 ANS PHARMACOLOGY PHRM202 Atropine has limited effectiveness in treating chronic obstructive pulmonary disease (COPD) because it blocks auto-inhibitory M2 receptors on postganglionic presynaptic parasympathetic nerves endings. This opposes the bronchodilation caused by blockade of M3 receptors on the effector smooth muscle (airway smooth muscle). vi) Cardiovascular: Atropine produces varying effects on the cardiovascular system, depending on the dose At low doses - decreased cardiac rate (bradycardia) – this results from blockade of the autoregulatory M1 receptors on the inhibitory prejunctional neurons – permitting increased ACh release At higher doses - modestly increased cardiac rate (tachycardia) due to blockade of M2 receptors on the sinoatrial node. Arterial blood pressure is unaffected, because most vascular beds lack significant cholinergic innervation (peripheral vasodilation does occur – but there is no significant BP changes). At toxic levels – atropine will dilate the cutaneous vasculature. Therapeutic or clinical uses of atropine a. Ophthalmic: Effect limited to the eye are obtained by administration of antimuscarinic agents locally in the Eye. The mydriatic & cycloplegic effects of atropine permits the measurement of refractive errors without interference by the accommodative capacity of the eye in conditions such as iridocyclitis (Inflammation of the iris and ciliary body) & keratitis (inflammation of the cornea) Mydriasis is often necessary for thorough examination of the retina & optic disc Homatropine (semisynthetic derivative of atropine), cyclopentolate, & tropicamide are other agents used in ophthalmology. They are preferred to topical atropine because of their shorter duration of action. They are also better tolerated. b. GIT: Atropine is used as an antispasmodic agent to relax the GIT & bladder (rarely used for this purpose to date). Antimuscarinic agents are no longer used for the management of peptic ulcers. They can reduce the secretion of gastric acid. The anti-secretary dose produces 41 ANS PHARMACOLOGY PHRM202 pronounced adverse effects such as dry mouth, loss of visual accommodation, photophobia, & urinary retention. Pirenzepine & its analog telenzepine have selectivity for M1 over M2 and M3 receptors but their affinity for M1 & M4 receptors are comparable, as a result it does not possesses total M1 selectivity. Both drugs are used in the treatment of peptic ulcer disease in some European countries, Japan, and Canada. Pirenzepine produces about the same rate of healing of both duodenal and gastric ulcers as the H2 receptor antagonists cimetidine & ranitidine (Zantac®). c. Antidote for cholinergic agonists: Atropine (at high doses) is used for the treatment of overdoses of AChE inhibitors including those found in commercial insecticides & some types of mushroom poisoning. The antimuscarinic therapy does not affect the skeletal neuromuscular junction d. Antisecretory agent: Used to block secretions in the upper and lower respiratory tracts prior to surgery e. Anaesthesia Atropine or glycopyrrolate is used with neostigmine (to block the parasympathomimetic effects) when used to reverse the skeletal muscle relaxation after surgery. Pharmacokinetics of atropine Atropine is readily absorbed from the GIT & the conjuctival membrane and well distributed throughout the body including the CNS (limiting its tolerability when intended for peripheral effects) Partially metabolized by the liver with a plasma half-life (t1/2) of 2-4 hours. The drug’s effects on the parasympathetic function declines rapidly in all organs – except in the eye. About 60% of the dose is eliminated unchanged in the urine Adverse effects of atropine 42 ANS PHARMACOLOGY PHRM202 Poisoned individuals manifest as dry mouth, blurred vision, "sandy eyes," tachycardia, hot & flashed skin, urinary retention & constipation for as long as a week. Body temperature is frequently elevated. Effects on the CNS include restlessness, confusion, hallucinations, and delirium, which may progress to depression, collapse of the circulatory & respiratory systems, and death. In older individuals, the use of atropine to induce mydriasis & cycloplegia is too risky, because it may exacerbate an attack of glaucoma ii) Scopolamine (hyoscine) Scopolamine is another belladonna alkaloid with peripheral effects similar to those of atropine. However, scopolamine has greater action on the CNS & a longer duration of action in comparison to those of atropine. Actions of scopolamine Marked central effects – at therapeutic dosages produces drowsiness & amnesia (loss of short- term memory) in sensitive individuals. In toxic doses, scopolamine (& to a lesser degree atropine) can cause excitement, agitation, hallucinations, and coma. The tremor of Parkinson's disease is reduced by centrally acting antimuscarinic drugs. Parkinsonian tremor & rigidity result from a relatively excessive cholinergic activity because of a deficiency of dopaminergic activity in the basal ganglia-striatum system. The combination of an antimuscarinic agent with a dopamine precursor drug (Ievodopa) may provide more effective therapy than either drug alone Motion sickness appear to involve vestibular disturbances of the muscarinic cholinergic transmission. Scopolamine is often effective in preventing or reversing these disturbances Therapeutic uses of scopolamine Although similar to atropine, therapeutic use of scopolamine is limited to prevention of motion sickness (for which is particularly effective) and to block short-term memory. As with all such drugs used for motion sickness, it is much more effective prophylactically than for treating motion sickness once it occurs. The amnesic action of scopolamine makes it an important adjunct drug in anesthetic procedures. 43 ANS PHARMACOLOGY PHRM202 Pharmacokinetics and Adverse effects Mostly similar to those seen with atropine, except scopolamine can be absorbed across the skin (transdermal route). iii) Ipratropium (Atrovent®) Ipratropium is a quaternary derivative of atropine & an antimuscarinic drug that is poorly absorbed orally.nnIt is also poorly distributed into the CNS – free of CNS effects. Inhaled ipratropium and tiotropium (structural analog of ipratropium) is useful in treating asthma in patients who are unable to take b2-AR agonists or, in combination with β2-AR agonist to take advantage of the synergistic effect of the combination Inhaled ipratropium is also beneficial in the management of chronic obstructive pulmonary disease (COPD). COPD, also known as chronic obstructive airway disease (COAD), is a group of diseases characterised by the pathological limitation of airflow in the airway that is not fully reversible. COPD is the general term for chronic bronchitis (inflammation of the bronchioles), emphysema (fluid in lung tissues) and a range of other lung disorders. It is most often due to tobacco smoking but can be due to other airborne irritants such as coal dust, asbestos or solvents Because of ipratropium’s quaternary positive charge, it does not enter the systemic circulation nor the CNS. iv) Hyoscine-N-butylbromide (Scopex®; Buscopan®) Hyoscine-N-butylbromide is a structural analog of hyoscine. Its use is limited for the treatment of GI spasms b) Other anti-muscarinic drugs i) Methscopolamine: A quaternary ammonium derivative of scopolamine devoid of the CNS effects of scopolamine. Its use is limited to mainly to GIT diseases (peptic ulcers & intestinal spasms) ii) Homatropine methylbromide: A quaternary derivative of homatropine which is less potent than atropine in its anti-muscarinic activities (but rather ganglionic blocking properties). Used in 44 ANS PHARMACOLOGY PHRM202 combination with hydrocodone as an antitussive combination. It has also been used to relief GIT spasms & as an adjunct in peptic ulcer disease. iii) Dicyclomine, flavoxate, trospium (quaternary ammonium compound), oxybutynin, & tolterodine are tertiary amines used for their antispasmodic properties. At therapeutic doses, they decrease spasms of the GIT, bilary tract, & uterus. c) Ganglionic Blockers Ganglionic blockers specifically act on the N receptors of the autonomic ganglia. Show NO selectivity for the parasympathetic or sympathetic ganglia. Thus, they block the entire output of the ANS at the N receptor. Therefore, ganglionic blockade is rarely used therapeutically – often serve as tools in experimental pharmacology. Ganglionic blockers are subdivided into 2 groups, i.e. o Depolarising blockers – e.g. natural alkaloids nicotine & lobeline, carbamoylcholine & ACh (if amplified by AChE inhibitor), tetramethylammonium (TMA), dimethylphenylpiperazinium iodide (DMPP) o Non-depolarising blockers – competitive neutral antagonists including trimethaphan, mecamylamine, hexamethonium, & tetraethylammonium (TEA) N cholinergic receptors of the ganglion (N N) & the skeletal muscle neuromuscular junction (N M) are subject to both depolarising & non-depolarising blockade. Mechanism of action Depolarising blockade: Depending on the dose, these agents bind to the N N cholinergic receptors first stimulating and then paralising them. Initial stimulation of the receptors gives rise to an initial excitatory postsynaptic potential (EPSP) & depolarization of the ganglia due to the influx Na + and sometimes Ca2+ through the neuronal type N-receptor channels. Then an action potential is generated in the postganglionic 45 ANS PHARMACOLOGY PHRM202 neuron following the attainment of the threshold potential (critical amplitude) from the initial EPSP. The resultant effect is first stimulation Secondary paralysis occurs because – the depolarising agent remains attached to the receptor for a relatively longer time resulting in persistent depolarization and desensitization of the cholinergic receptors site and continued blockade. Membrane repolarises but the continued binding desensitises the receptor – making it incapable of transmitting further impulses Non-depolarising blockade: The blockade of the autonomic ganglia produced by these agents does not involve prior ganglionic stimulation. They impair transmission either by:  competing with ACh for the ganglionic N receptor (e.g. trimethaphan), or  blocking the ion channels (e.g. hexamethonium). This action shortens the duration of current flow because the open channels becomes closed (clogged) In both the above mechanisms, the initial EPSP is blocked, and ganglionic transmission is inhibited Non-depolarizing competitive antagonists are the only once presently used as ganglion blockers clinically. Hexamethonium produces most of its blockade by occupying sites in/ on the ion channel not by occupying the N receptor itself. In contrast, trimethaphan & mecamylamine block the N receptor, not the channel. Blockade can be at least partially overcome by increasing the concentration of the nicotinic cholinergic agonist like ACh i) Depolarizing blockers 1. Nicotine Nicotine was first isolated from the leafs of tobacco, Nicotina tabacum by Poselt & Reiman in 1828 It is of considerable medical significance because of its toxicity, presence in tobacco, and ability to confer dependence to its users Pharmacological action of nicotine Peripheral nervous system: 46 ANS PHARMACOLOGY PHRM202 Small doses of nicotine stimulate the ganglion cells directly and may facilitate impulse transmission. When large doses of the drug are administered, initial stimulation is followed very quickly by a blockade of transmission Nicotine possesses a biphasic action on the adrenal medulla;  Small doses stimulates the discharge of catecholamines (NA, A)  Large doses prevent their release in response to nerve stimulation Like ACh, Nicotine also stimulates sensory receptors of the tongue, lungs, stomach, thermal receptors of the skin and tongue, and pain receptors. Pre-treatment with hexamithonium prevents the stimulation of these sensory receptors by nicotine CNS (it markedly stimulates the CNS): Low doses produce week analgesia (pain relief) and excitation of respiration. Higher doses produce respiratory depression, and death results from failure of respiration due to both central paralysis (medulla oblongata) and peripheral blockade of the muscles of respiration Nicotine induces vomiting by both central (chemoreceptor trigger zone in the area postrema of the medulla oblongata) and peripheral (vagal and spinal nerves involved in vomiting) actions Toxic levels of nicotine causes tremors and convulsion Chronic exposure to nicotine causes marked increase in density and number of N-cholinergic receptors – probably due to the compensatory response to the desensitization of receptor function by nicotine Cardiovascular system IV administration of nicotine to laboratory animals produces an increase in heart rate and BP. This response is due to stimulation of the sympathetic ganglia and adrenal medulla. Also contributing to the sympathomimetic response to nicotine is the activation of the chemoreceptors of the aortic and carotid bodies – resulting in vasoconstriction, tachycardia, and elevated BP GIT 47 ANS PHARMACOLOGY PHRM202 The combined activation of the parasympathetic ganglia and cholinergic nerve endings by nicotine results in increased tone and motor activity of the bowel resulting in nausea, vomiting, and occasional diarrheoa in nicotine naïve patients Exocrine glands Nicotine produces an initial stimulation of the salivary and bronchial secretions followed by inhibition. Pharmacokinetics of nicotine (absorption, fate, and excretion) Absorption: Nicotine is readily absorbed from the respiratory tract, buccal (directed toward the cheek) membranes, and skin. Severe poisoning from nicotine has resulted from percutaneous (across the skin) absorption. Nicotine is a strong base (pKa = 8.5) – its absorption from the stomach is limited, but intestinal absorption is far more efficient. Nicotine in chewing tobacco has a longer duration of effect because of its slow absorption than the inhaled form. The average cigarette contains 8-11 mg nicotine delivering 1-3 mg systemic nicotine to the smoker – bioavailability increase up to threefold with the intensity of puffing & technique of the smoker. To obtain abstinence from tobacco use and help in preventing a withdrawal or a abstinence syndrome– nicotine is available in several dosage forms; i.e. orally as a gum (Nicorette®), transdermal patch (Nicoderm®), nasal spray (Nicotrol NS®), and vapour inhaler (Nicotrol inhaler®). The efficacy of these dosage forms in producing abstinence from smoking is enhanced by concurrent counselling and motivation therapy. Fate and excretion Approximately 80-90% of absorbed nicotine is metabolised and transformed in the body mainly the liver, but also the kidney and lung. Cotinine is the major metabolite, with nicotine-1’-N-oxide and 3- hydroxycotinine being found in lesser quantities. The t1/2 = is 2 hours following inhalation or IV administration of nicotine. Nicotine and its metabolites are rapidly eliminated by the kidney – the rate of urinary excretion is reduced when the urine in alkaline 48 ANS PHARMACOLOGY PHRM202 Acute nicotine poisoning The acutely fatal dose of nicotine for an adult in approximately 60 mg of the base. The absorption of oral nicotine is delayed because of slow gastric emptying – as a result vomiting caused by the initial absorbed nicotine on the CNS may remove much of the nicotine remaining in the GIT. The rapid onset of acute nicotine poisoning may be in the form of nausea, salivation, abdominal pain, vomiting, diarrhea, cold sweat, headache, dizziness, disturbed hearing and vision, mental confusion, and marked weakness. BP drops; breathing is difficult; the pulse is weak, rapid, irregular; and collapse may be followed by terminal convulsions. Death may results after few minutes due to respiratory failure Therapy Vomiting may be induced, or gastric lavage (a procedure used to empty the stomach of its contents. Performed using a flexible rubber tube that is passed through the mouth and advanced to the stomach) should be performed. Alkaline solutions should be avoided. Activated charcoal may be orally administered & respiratory assistance may be necessary ii) Non-depolarizing blockers e.g. trimethaphan, mecamylamine, hexamethonium, tetraethylammonium Organ systems effects i). Cardiovascular system The blood vessels receive mainly vasoconstrictor fibers from the sympathetic division; therefore, ganglionic blockade causes decrease in arteriolar & venous tone. The BP may drop gradually due to decreased peripheral vascular resistance. Since the sinoatrial node of the heart is usually dominated by the parasympathetic nervous system, a moderate tachycardia may occur following the gangionic blockade ii). Other organ systems 49 ANS PHARMACOLOGY PHRM202 GIT - secretions are reduced (not enough to effectively treat peptic disease) & motility is profoundly inhibited & constipation may be serious. Genitourinary smooth muscles - partially dependent on autonomic innervation for normal function. Ganglionic blockade therefore may precipitate urinary retention in men with prostatic hyperplasia (enlarged prostate). Sexual functions - are impaired in that both erection & ejaculation. Thermoregulatory sweating is blocked by the ganglion-blocking drugs Clinical Applications Because of the increasing availability of selective autonomic blocking agents, ganglionic blockers are rarely used to date. Mecamylamine; – studied for possible use in reducing nicotine craving – Used for the treatment of moderately to severe hypertension – Also used as an alternative therapy for hypertensive crises Trimethaphan; – occasionally used in the treatment of hypertensive crises d) Neuromuscular Blocking Drugs These drugs block cholinergic transmission between motor nerve endings of the somatic division & the N receptors on the neuromuscular end plate of skeletal muscle (muscle relaxants). These neuromuscular blockers are structural analogs of ACh, and are either: o Nondepolarising (competitive) blockers, or o Depolarizing blockers They are useful clinically during surgery to produce complete muscle relaxation. A 2nd group of muscle relaxants are centrally acting and useful in controlling spastic muscle tone - Diazepam & baclofen, which binds GABA receptors. Thirdly dantrolene and botolinum toxin act peripheraly - dantrolene interferes with the release of Ca2+ from the sarcoplasmic reticulum directly on muscle 50 ANS PHARMACOLOGY PHRM202 i) Nondepolarising (competitive) blockers The 1st drug that was found to be capable of blocking the skeletal neuromuscular junction was curare (from the Strychnos spp). Curare is a generic term for various South American arrow poisons which the native hunters of the Amazon in South America used to paralyze their prey. Overview The modern clinical application of curare dates from 1932 when the highly purified fraction was used in patients with tetanus and spastic disorders. The drug tubocurarine was ultimately purified from curare, synthesised, & introduced into clinical practice in the early 1940s. Then structural modifications of tubocurarine where made, and this lead to the development of other non- depolarising neuromuscular blocker Other members of this group include; atracurium, cisatracurium, doxacurium, metocurine, mivacurium, pancuronium, rocuronium, pipecurium & vecuronium The neuromuscular blocking agents have significantly increased the safety of anesthesia because less anesthetic is required to produce muscle relaxation. Mechanism of action of non-depolarizing blockers At low doses: Competitive blockers bind to the N cholinergic receptor (especially the N M) at the end plate, thereby competitively blocking the binding of ACh preventing depolarisation of the muscle cell membrane & inhibit muscular contraction. Their action can be overcome by increasing the concentration of ACh in the synaptic gap At high doses: Can block the ion channels of the end plate. This leads to further weakening of neuromuscular transmission & reduced ability of AChE inhibitors to reverse the actions of non-depolarising muscle relaxants Pharmacological actions of non-depolarizing blockers 51 ANS PHARMACOLOGY PHRM202 Sequence and characteristics of paralysis: Not all muscles are equally sensitive to blockade by competitive blockers. Muscles of the face and eye are most susceptible & are paralyzed 1 st, followed by the jaw and the larynx. Then muscles of the limbs, neck, trunk, and the intercostal muscles are affected. Lastly, the diaphragm muscles are paralyzed and respiration then ceases Recovery of muscle usually occurs in the reverse order to that of the paralysis – thus the diaphragm ordinarily is the 1st muscle to regain function Histamine release: Tubocurarine produces typical histamine-like wheals following intracutaneous or intra-arterial injection in human. It also produces other histamine-like effects such as bronchospasms, hypotension, excessive bronchial and salivary secretions. All the above effects appear to be caused by the release of histamine triggered by tubocurarine Mivacurium, atracurium, succinylcholine, and doxacurium also trigger the release histamine – but to a lesser extent unless administered rapidly The histamine release is a direct action of the muscle relaxant on the mast cells – not the IGE- mediated anaphylaxis. Autonomic ganglia and muscarinic sites: Neuromuscular blocking agents show variable potencies in producing ganglionic blockade. At therapeutic doses, tubocurarine is the least selective in this group of agents followed by pancuronium. The others are more selective for the motor end plate. Pancuronium has a vagolytic action, presumably from blockade of muscarinic cholinergic receptors – that leads to tachycardia Therapeutic uses These blockers are mainly used therapeutically as adjuvant drugs in anesthesia during surgery to relax skeletal muscle. Muscle relaxation is also of value in various orthopedic procedures – such as the correction of dislocations and alignment of fractures Short acting neuromuscular blockers are used in combination with generalised anaesthetics to facilitate intubation of an endotracheal tube in laryngoscopy, bronchoscopy, and esophagoscopy. 52 ANS PHARMACOLOGY PHRM202 These agents are also used in psychiatry to prevent trauma during electroconvulsive therapy (ECT) – seizures induced may cause dislocation of fractures Control of muscle spasms – botulinum toxins are preferred Pharmacokinetics All neuromuscular blocking agents are injected IV – due to their minimal uptake via oral route They penetrate membranes very poorly & do not enter cells or cross the BBB Tubocurarine, pancuronium, mivacurium, metocurine, & doxacurium are excreted in the urine unchanged Atracurium is degraded spontaneously in the plasma and by ester hydrolysis Atracurium has been replaced by its isomer, cisatracurium since atracurium releases histamine & is metabolized to laudanosine, which can provoke seizures. vecuronium and rocuronium are deacetylated in the liver, and their clearance may be prolonged in patients with hepatic disease Drug interactions Cholinesterase Inhibitors: o E.g. neostigmine & edrophonium can overcome the action of nondepolarizing neuromuscular blockers o but with increased dosage, AChE inhibitors can cause a depolarising block as a result of elevated ACh levels at the end-plate membrane Halogenated hydrocarbon anesthetics: o Drugs such as halothane act to enhance neuromuscular blockade by exerting a stabilizing action at the neuromuscular junction Aminoglycosides antibiotics: o E.g. gentamicin or tobramycin inhibit ACh release from cholinergic nerves by interfering with Ca2+ ions conduction o enhancing the blockade Ca2+-channel blockers: 53 ANS PHARMACOLOGY PHRM202 o These agents may increase the neuromuscular block of tubocurarine & other competitive blockers as well as depolarizing blockers Therapeutic advantages and disadvantage of competitive blockers Therapeutic advantages and disadvantage of competitive blockers 54 ANS PHARMACOLOGY PHRM202 b) Depolarising blockers Succinylcholine is the sole member in this group that is available for systemic use in humans Decamethonium also possesses the actions of a depolarising blocker Mechanism of action 55 ANS PHARMACOLOGY PHRM202 Succinylcholine binds to the N cholinergic receptor of the motor end plate & mimic the actions of ACh depolarising the junction Unlike ACh, the depolarising agent persists at high levels in the synaptic cleft for a relatively longer time providing a constant stimulation The depolarizing agent 1st causes the opening of the Na+ channel associated with the N receptors resulting in depolarisation of the receptor- an initial excitatory postsynaptic potential (EPSP) (Phase I) This leads to a transient twitching of the muscle due to the release of Ca 2+ from the sarcoplasmic reticulum The continued binding of the depolarising agent desensitises the receptors (rendering them incapable of transmitting further impulses) Even though the receptor is gradually repolarized as the Na+ channel closes – the resultant effect is resistance to further depolarization (Phase II) & a flaccid (lacking firmness) paralysis 56 ANS PHARMACOLOGY PHRM202 Pharmacological actions of depolarizing blockers The sequence of paralysis may be slightly different - but like competitive blockers, the respiratory muscles are paralyzed last Succinylcholine initially produces short-lasting muscle contractions, followed within a few minutes by paralysis The drug does not produce a ganglionic block except in high doses – but it also triggers histamine release Normally, the duration of action of succinylcholine is extremely short, because this drug is rapidly broken down by both plasma and hepatic cholinesterase (BuChE) to almost undetectable levels Malignant hyperthermia (heatstroke) Malignant hyperthermia can manifest by elevated body temperature, lack of sweating, hot dry skin, and neurologic symptoms (such as unconsciousness, paralysis, headache, vertigo, confusion) It is a potentially life-threatening event triggered by the administration of certain anaesthetics and neuromuscular blocking agents The initiating event is the uncontrolled release of Ca2+ from the sarcoplasmic reticulum of the skeletal muscles Other clinical features include contracture, rigidity, and heat production from the skeletal muscles resulting in severe hyperthemia, accelerated muscle metabolism, metabolic acidosis, and tachycardia Halogenated hydrocarbon anaesthetics (like halothane, isoflurane, & sevoflurane) and succinylcholine OR the combination of these agents has been associated with incidents of malignant hyperthermia Treatment entails the administration of dantrolene, which blocks release of Ca 2+ from the sarcoplasmic reticulum of muscle cells thus reducing heat production & relaxing muscle tone Rapidly cooling of the patient, inhalation of 100% oxygen, and control of acidosis should be considered adjunct therapy in malignant hyperthermia 57 ANS PHARMACOLOGY PHRM202 Therapeutic uses Because of its rapid onset and short duration of action, o succinylcholine is useful when rapid endotracheal intubation is required during the induction of anesthesia o a rapid action is essential if aspiration of gastric contents is to be avoided during intubation It is also employed during electroconvulsive shock therapy Pharmacokinetics Succinylcholine is injected intravenously Its brief duration of action (several minutes) results from redistribution & rapid hydrolysis by plasma cholinesterase It therefore is usually given by continuous infusion Adverse effects Hyperthermia: o Already explained Apnoea (temporary cessation of breathing): o Apnoea due to paralysis of the diaphragm o Especially in patient who is genetically deficient in plasma cholinesterase iv) Cholinergic Poisoning Poisoning occurs especially in rural communities where; o the use of cholinesterase inhibitor (organophosphate) insecticides is common, & o cultures where wild mushrooms are commonly eaten Mushroom poisoning is divided into rapid-onset and delayed-onset types 1. The rapid onset type appears within 15-30 minutes of ingestion of the mushrooms 58 ANS PHARMACOLOGY PHRM202 characterised entirely by signs of muscarinic excess: N, V, & D, vasodilation, reflex tachycardia, sweating, salivation, & sometimes bronchoconstriction Amanita muscaria contains muscarine & numerous other alkaloids, including antimuscarinic agents. In fact, ingestion of A muscaria may produce signs of atropine poisoning, not muscarine. 2. Delayed-onset types Other forms of mushroom poisoning manifests its 1st symptoms 6-12 hours after ingestion E.g. Amanita phalloitks, A. virosa, Galnina autumnalis, or G. marginata,. Although the initial symptoms usually include nausea and vomiting, the major toxicity involves hepatic and renal cellular injury by amatoxins that inhibit RNA polymerase Atropine is of no value in this form of mushroom poisoning Application of organophosphates as chemical warfare "nerve gases" also requires an awareness of the methods for treating acute poisoning Treatment is either by: Antimuscarinic agents Cholinesterase regenerator compounds reversible inhibitors of the AChE to prevent binding to the irreversible organophosphate – via competition Symptomatic treatment 3. Antimuscarinic therapy Both the N and the M effects of the AChE inhibitors can be life-threatening There is no effective method for directly blocking the nicotinic effects of AChE inhibition - because N agonists & blockers cause blockade of the entire autonomic transmission tertiary amine drug preferably atropine must be used to treat both the CNS & PNS effects of the organophosphate inhibitors Large doses of atropine may be needed to combat the muscarinic effects of extremely potent agents like parathion & chemical warfare nerve gases: 59 ANS PHARMACOLOGY PHRM202 1-2 mg of atropine sulfate may be given IV every 5-15 minutes until signs of effect (dry mouth, reversal of miosis) appear Treatment may be as long as 1 month for full control of muscarinic actions 4. Cholinesterase regenerator compounds Members of these group of agents include oxime agents such as pralidoxime (PAM), diacetylmonoxime (DAM) Mechanism of action The oxime group (-NOH) has a very high affinity for the phosphorus atom, & these drugs are able to hydrolyse the phosphorylated enzyme if the complex has not "aged" Pralidoxime (clinically applicable) is most effective in regenerating the cholinesterase associated with skeletal muscle neuromuscular junctions Because of its positive charge, it does not enter the CNS & is ineffective in reversing the central effects of organophosphate poisoning. Diacetylmonoxime (clinical trials), on the other hand, does cross the BBB & can regenerate some of the central nervous system cholinesterase 60 ANS PHARMACOLOGY PHRM202 Pharmacokinetics Pralidoxime is administered by intravenous infusion 1,-2 g given over 15-30 minutes. Even after aging of the phosphate-enzyme complex, multiple doses of pralidoxime over several days may still be useful in severe poisoning Pralidoxime is not recommended for the reversal of inhibition of AChE by carbamate inhibitors 3. Reversible inhibitors of the enzyme Pretreatment with reversible inhibitors of AChE to competitively prevent binding of the irreversible organophosphate inhibitor This prophylaxis can be achieved with pyridostigmine or physostigmine Reserved for situations in which possible lethal poisoning is anticipated, eg, chemical warfare Amanita muscaria 61 ANS PHARMACOLOGY PHRM202 Case study A 53-year old woman comes to see you for a consultation. She is scheduled to take a cruise in about 2 weeks but is concerned about sea sickness. She has been on boats before and is very sensitive to motion sickness. A friend mentioned to her that there is a patch that is effective for this problem. She is in good health and takes no medication regularly. Her examination is normal. You provide her with a scopolamine transdermal patch. 1. What is the mechanism of action of scopolamine? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 2. What are the common side effects of this medication? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 3. What are some relative contraindications to its use? ______________________________________________________________________________ ______________________________________________________________________________ 62 ANS PHARMACOLOGY PHRM202 ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ IV. ADRENERGIC AGONISTS Learning objectives 1. Outline the effects of sympathomimetic agents on peripheral organ systems 2. List the major sympathomimetic agonists and their routes of administration 3. Describe the therapeutic and adverse effects of the major sympathomimetic drugs Reading material Drugs that mimic the actions of A or NA are referred to as sympathomimetic drugs. They have a wide range of effects, such as regulating activities of the heart & peripheral vasculature, especially in response to stress. Sympathetic stimulations are mediated by release of NA, A or DA from the nerve terminals that serve to activate the adrenoceptors on postsynaptic sites. Adrenaline, noradrenaline and dopamine are called catecholamines. a) The Adrenergic Neuron Adrenergic neurons release NA as the neurotransmitter. Adrenergic neurons are mostly found in the CNS and also in the sympathetic nervous system. They form the link between ganglia and the effector organs. NA is released from the nerve terminals of the postganglionic neuron. The adrenergic neurons and receptors are located either presynaptically on the neuron (autoreceptors), or postsynaptically on the effector organ. b) Neurotransmission at adrenergic neurons 63 ANS PHARMACOLOGY PHRM202 It closely resembles that of the cholinergic neurons, except that NA is the neurotransmitter instead of Ach. The process involves 5 steps: (1) the synthesis, (2) storage, (3) release, (4) receptor binding of the NA, (5) followed by removal of the neurotransmitter from the synaptic gap. 1) Synthesis of NA: Tyrosine is transported by a Na+-linked carrier into the axoplasm of the adrenergic neuron where it is 3-hydroxylated to dihydroxyphenylalanine (L-DOPA) by tyrosine hydroxylase. This is the rate- limiting step in the formation of NA. L-DOPA is then decarboxylated to form dopamine (DA). 2) Storage of NA in vesicles: DA is transported into synaptic vesicles by an amine transporter system that is also involved in the re-uptake of preformed NA. This carrier system is blocked by reserpine (anti-hypertensive drug) DA is hydroxylated to form NA by the enzyme, DA β-hydroxylase. 64 ANS PHARMACOLOGY PHRM202 3) Release of NA: An action potential arriving at the nerve junction triggers an influx of Ca2+ ions into the axoplasm. The increase in Ca2+ ions causes vesicles inside the neuron to fuse with the cell membrane & release their contents into the synapse. This release is blocked by drugs such as guanethidine. Tyramine can enter the nerve terminal and displace stored NA. 4) Binding to a receptor: NA released from the synaptic vesicles diffuses across the synaptic space and binds to either postsynaptic receptors, or to presynaptic receptors. The recognition of NA by the membrane receptors triggers a cellular response via formation of intracellular second messengers. Adrenergic receptors use both the cAMP & IP3 2nd messenger system. 5) Removal of norepinephrine: NA may 65 ANS PHARMACOLOGY PHRM202  (i) diffuse out of the synaptic space & enter the general circulation  (ii) be metabolized by postsynaptic cell membrane-associated catechol O-methyltransferase (COMT) in the synaptic space, or  (iii) be recaptured by an uptake system that pumps the NA back into the neuron The uptake by the neuronal membrane involves a Na+/K+-activated ATPase and can be inhibited by tricyclic antidepressants, such as imipramine, or by cocaine. 6) Potential fate of recaptured NA: Once NA re-enters the cytoplasm of the adrenergic neuron, it may be taken up into adrenergic vesicles via the amine transporter system & be ready for release by another action potential, or alternatively, NA can be oxidized by monoamine oxidase (MAO), present in neuronal mitochondria. The inactive products of NA metabolism are excreted in the urine as vanillylmandelic acid, metanephrine, and normetanephrine. 66 ANS PHARMACOLOGY PHRM202 c) Adrenergic receptors (adrenoceptors; ARs) Several classes of ARs have been distinguished pharmacologically. Two families of receptors, designated o α-ARs, and o β-ARs Currently there is a number of AR-subtypes identified. i) Alpha adrenoceptors 67 ANS PHARMACOLOGY PHRM202 The α-ARs are subdivided into two subgroups, α1 & α2 receptors based on their affinities for agonists and blocking drugs. The α1-ARs are postsynaptic and “excitatory”, while the α2-ARs are presynaptic and has “feed-back inhibitory effect”. 1) α1-ARs are found on the postsynaptic membrane of the effector organs. There are three pharmacologically defined α1-ARs (α1A-; α1B-; α1D-ARs) with distinct sequences and tissue distribution. Activation of the α1-ARs initiates a series of reactions through a G q protein activation of phospholipase C, resulting in the generation of inositol-1 ,4,5-trisphosphate (IP 3) causing the release of Ca2+ from the endoplasmic reticulum into the cytosol. Ca2+ can then interact to stimulate or inhibit enzymes, or cause hyperpolarization, secretion, or contraction. 2) α2-ARs are located primarily on presynaptic nerve possessing feedback-inhibitory effect. There are also three subtypes of α2-ARs i.e α2A-; α2B-; & α2D-ARs. They act by inhibiting the ongoing release of NA from the stimulated adrenergic neuron. They are also found on other cells, such as the b cell of the pancreas regulating insulin output. They also play a vital role in the CNS, where its inhibitors are useful as anti-depressants (mianserine, mirtazepine). The effects of binding at α2-ARs are mediated by inhibition of Gi-adenylyl cyclase & a fall in the levels of intracellular cAMP. ii) Beta adrenoceptors The bβ-ARs are subdivided into three subgroups, β1-; β2-; & β3-ARs – but β1-; & β2-ARs are the major receptors. Their division is based on their affinities for adrenergic agonists and antagonists. β1-ARs have approximately equal affinities for A & NA, whereas β2-ARs have a higher affinity for A than for NA. The β3-ARs are about tenfold more sensitive to NA than A Thus, tissues with a predominance of β2-ARs (bronchial smooth muscles; blood vessels of skeletal muscle) are particularly responsive to the hormonal effects of circulating A released by the adrenal medulla. 68 ANS PHARMACOLOGY PHRM202 Activation of all the β-ARs results in activation of Gs-adenylyl cyclase complex &, therefore, increased concentrations of cAMP within the cell. iii) Dopamine receptors Endogenous catecholamine dopamine (DA) produces a variety of biologic effects that are mediated by interactions with specific DA receptors. These receptors are distinct from α and β receptors and are of particularly importance in the brain. They are also located in the splanchnic (innervating the visceral organs) & renal vasculature. There