Muscarinic and Nicotinic Receptors - MBBS/BDS PDF

Muscarinic and Nicotinic Cholinergic receptors Ishola IO (Ph.D) Department of Pharmacology, Therapeutics and Toxicology CMUL Cholinergic receptors  These receptors subdivide into nicotinic...

Muscarinic and Nicotinic Cholinergic receptors Ishola IO (Ph.D) Department of Pharmacology, Therapeutics and Toxicology CMUL Cholinergic receptors  These receptors subdivide into nicotinic and muscarinic receptors, which are named secondary to separate activating ligands that contributed to their study.  Nicotinic receptors are responsive to the Activates cholinergic agonist nicotine, while muscarinic receptors at nerve synapses receptors are responsive to muscarine.  and on skeletal muscle The two receptors differ in function as ionotropic ligand-gated and G-protein coupled receptors, respectively.  Nicotinic receptors function within the central nervous system and at the neuromuscular junction.  muscarinic receptors function in both the peripheral and central nervous systems, mediating innervation to visceral organs. Nerve Transmission Peripheral nervous system Skeletal muscle CNS (Somatic) Ach (N) CNS (Autonomic) Synapse Ach (N) NA Sympathetic Adrenaline Ach Adrenal (N) medulla AUTONOMIC Parasympathetic Synapse Ach Ach (M) (N) Smooth muscle Cardiac muscle Neurotransmitter Acetylcholine (Ach) O + C NMe 3 H 3C O Acetyl Choline 3. Transmission process Signal in nerve 1... Nerve 2 Nerve 1...... Signal. Acetylcholine Acetylcholinesterase enzyme Cholinergic receptor Vesicle 3. Transmission process Vesicles fuse with membrane and release Ach Nerve 2 Nerve 1 Signal 3. Transmission process Nerve 2 3. Transmission process Receptor binds Ach 2o Message Induced fit triggers 2o message Triggers firing of nerve 2 Ach undergoes no reaction Nerve 2 3. Transmission process Ach departs receptor Receptor reverts to resting state Ach binds to acetylcholinesterase Nerve 2 3. Transmission process Ach hydrolysed by acetylcholinesterase O O C C HO H 3C O NMe3 Nerve 2 H 3C OH + NMe3 Acetylcholine Acetic acid Choline 3. Transmission process Choline binds to carrier protein Choline Nerve 2 Nerve 1 Carrier protein for choline 3. Transmission process Choline transported into nerve Nerve 2 Nerve 1 3. Transmission process Ach resynthesised Nerve 2 Nerve 1 E 1 = Choline acetyltransferase O O C C E1 NMe3 H 3C SCoA + HO CH2 CH2 NMe3 H 3C O Choline Acetylcholine 3. Transmission process Ach repackaged in vesicles Nerve 2 Nerve 1 Peripheral nervous system Skeletal Muscle CNS (Somatic) SOMATIC Ach (N) CNS (Autonomic) Synapse Ach (N) NA Sympathetic Adrenaline Ach Adrenal (N) medulla AUTONOMIC Parasympathetic Synapse Ach Ach (M) (N) Smooth Muscle Cardiac Muscle Issue of concern???  Due to the diffuse presence of cholinergic receptors, dysfunction can yield various outcomes.  Abnormal muscarinic receptor function has shown correlations in diseases such as Alzheimer disease, Parkinson disease, schizophrenia, and epilepsy.  research shows both nicotinic and muscarinic receptors play a role in the dopamine reward system pathway  At the neuromuscular junction, nicotinic receptors function in signal transduction of voluntary movement.  In myasthenia gravis, competitive receptor inhibition secondary to autoimmune dysfunction can lead to the life-threatening loss of function at the neuromuscular junction. Cont’d  This life-sustaining function works to the clinicians’ advantage during surgical cases where a patient requires paralysis with pharmacologic interference at the neuromuscular junction (great care must be taken in patients with disorders such as lower or upper motor neuron denervation, major trauma, severe infection, or burn injuries, as these may result in the upregulation of nicotinic receptors at the neuromuscular junction, which contributes to an overabundance of receptors)  Use of the neuromuscular-blocking drug succinylcholine to induce paralysis may potentially contribute to life-threatening electrolyte abnormalities.  This condition is secondary to both the function of the nicotinic receptor as an ionotropic channel and succinylcholine activation of the receptor. Nicotinic cholinergic receptors  The nicotinic receptor subdivides into two subtypes, N1 and N2.  N1 may also be referred to as the peripheral or muscle receptor type,  N2 is known as the central or neuronal receptor subtype  N2 receptors are on the cell bodies of postganglionic neurons within the parasympathetic and sympathetic nervous systems.  They are also on the adrenal medulla as a component of the sympathetic nervous system  Expression of various nicotinic receptor subtypes in the brain influences cell migration, neuronal outgrowth, and signaling pathways  nicotinic receptors influence the release of multiple neurotransmitters such as dopamine, noradrenaline, acetylcholine, glutamate, and GABA. Muscarinic cholinergic receptors  Muscarinic receptors are divided into five main subtypes M1, M2, M3, M4, and M5.  each subtype exists within the CNS, they are encoded by separate genes and localized to different tissue types.  The M1 receptor is primarily found in the cerebral cortex, gastric, and salivary glands.  M2 receptors are diffusely located in smooth muscle and cardiac tissue.  M3 receptors are also present on smooth muscle, gastric, and salivary glands.  M4 and M5 receptors are not as well characterized but appear within the hippocampus and substantia nigra Nicotinic receptor  The N2 receptors are responsive to acetylcholine and transmit signals from the preganglionic to the postganglionic cell.  The ionic flux generated at the postganglionic cell is responsible for excitatory signal transduction to effector organs of the autonomic nervous system.  the N1 or muscle nicotinic receptor is at the neuromuscular junction on muscle cells generating voluntary muscle movement.  An excitatory signal may become generated through N1 receptor activation.  Depending on the strength of the signal, receptor activation may result in membrane depolarization with subsequent muscle contraction. Activation of nicotinic receptors  Nicotinic receptors are ionotropic; this means activation of the receptor leads to the formation of an ion channel within the cell membrane, known as a ligand-gated ion channel.  The channel consists of five homologous subunits that form a central pore in the membrane upon activation, which cations may pass through.  Depending on the strength of signals, the influx of cations into the cell can cause depolarization, generating an excitatory action potential. Nicotinic receptor Control of Cationic Ion Channel: Binding Receptor site Messenger Induced Cell fit Cell membrane membrane ‘Gating’ (ion channel opens) Five glycoprotein subunits traversing cell membrane 4.1 Nicotinic receptor The binding sites Binding sites Ion channel b a a g Cell g membrane d a b a d Two ligand binding sites 2xa, b, g, d subunits mainly on a-subunits Muscarinic receptors  G-protein coupled receptors consists of five members designated M1-M5.  The gene family as a whole shows 26.3% overall amino acid identity, with the variation between the receptor subtypes being seen largely within the intracellular loops.  The third intracellular loop is particularly variable, showing only 2.7% identity between receptor, compared with an average of 66% identity found in the conserved transmembrane domains.  Classically these receptors are sub-divided into two broad groups based on their primary coupling efficiency to G-proteins.  M2 and M4-muscarinic receptors are able to couple to the pertusiss-toxin sensitive Gi/o-proteins, and M1, M3 and M5-muscarinic receptors couple to Gq/11-proteins.  muscarinic receptor family can couple to a wide range of diverse signalling pathways, some of which are mediated by G-proteins and others that are G-protein- independent. Muscarinic receptor pharmacology  The orthosteric binding pocket of the muscarinic receptor family is highly conserved, making the development of subtype-specific agonists and antagonists very difficult.  Pirenzepine - an antagonist, 44-fold selectivity for the M1-muscarinic receptor over the M2 , but only a 17.9 and 3.5 fold selectivity for the M3 and M4-muscarinic receptor subtypes, respectively.  darifenacin, which is widely considered as M3 selective and is approved for clinical use in the treatment of over-active bladder has ≤10-fold selectivity over M1/M4/M5 receptors. Muscarinic receptor physiological role  The role of muscarinic receptors in the contraction of smooth muscle, particularly of airway, ileum, iris and bladder, are considered a classical muscarinic response mediated primarily by M3-muscarinic receptors expressed on the smooth muscle cells.  Co-expressed with the M3-muscarinic receptors in smooth muscle is an often larger population of M2-muscarinic receptors which appear to play a much smaller role in the smooth muscle contractile response.  M2-muscarinic receptors expressed in the heart have a profound role in the control of cardiac myocyte contraction.  the release of acetylcholine from vagal parasympathetic neurones reduces HR almost exclusively by acting at M2-muscarinic receptors.  Exocrine secretion, particularly of saliva, insulin is primarily mediated by M3-muscarinic receptors with a smaller role played by M1-receptors particularly in salvation.  Activation of muscarinic receptors  muscarinic receptors (are G-protein coupled receptors, (GPCRs)).  GPCRs constitute a family of receptors that generate a second messenger system when activated via ligand binding.  A second messenger system utilizes the activation of intracellular signaling molecules to produce the excitatory or inhibitory response.  Muscarinic receptors generate separate second messenger systems.  The M1 and M3 receptors are characterized as excitatory GPCRs known as Gq GPCRs = Upon their activation, they generate phospholipase C, producing the second messenger's inositol triphosphate (IP3) and diacylglycerol (DAG) and leading to an increase in intracellular calcium and protein kinase.  The generation of calcium and protein kinase C is responsible for further activation of downstream events, which produce the overall effect of receptor activation.  Alternatively, the M2 receptor is an inhibitory g-protein (Gi) GPCR, which upon ligand binding, leads to the inhibition of adenylate cyclase, resulting in a decrease in the second messenger molecule cAMP  Muscarinic receptors as drug targets  stimulation of the M1-muscarinic receptor subtype - work on the M1- receptor knockout mice has suggested that this receptor subtype play no significant role in cognition as previously thought  novel anti-Alzheimer drugs that target other muscarinic receptor subtypes, such as the M2-, M4- and M5-muscarinic receptors, are now under more intense consideration.  muscarinic receptor modulation of dopaminergic transmission has provided the impetus for the development of muscarinic receptor ligands in the treatment of schizophrenia and PD.  in chronic obstructive pulmonary disease (COPD) and asthma, that are currently treated with the non-selective muscarinic antagonists ipratropium and tiotropium  in overactive bladder, improved selectivity of muscarinic receptor ligands would help to reduce unwanted side effects  PARASYMPATHOMIMETICS (CHOLINOMIMETICS): Drugs that facilitate or mimic some or all of the actions of the parasympathetic nervous system. Direct Acting Indirect Acting Muscarinic Nicotinic Anticholinesterases receptor receptor agonists agonists Reversible Irreversible 32 Direct Acting Cholinergic Agonists  Acetylcholine not used therapeutically – N/M *  Carbachol * (Miostat) – N/M  Bethanechol * (Urecholine) – M  Pilocarpine (Pilocar, Ocusert) – M  Indications – Urinary retention after surgery or postpartum, Glaucoma  Adverse effects –  Muscarinic (M): salivation, flushing, bronchospasm, sweating, nausea, abdominal pain – acid indigestion and GI cramping, diarrhea, and possibly, decreased blood pressure.  Nicotinic (N)- Fasciculations, respiratory arrest 33  At the neuromuscular junction,  Nicotinic receptor agonists are used to induce a state of paralysis.  Nicotinic agonists do this by binding to the receptor, occupying the acetylcholine binding domain.  Two popular classes of drugs are succinylcholine and tubocurarine.  Both of these drugs exhibit specificity for nicotinic receptors at the neuromuscular junction but differ in their mechanism for receptor inactivation.  Succinylcholine binds and activates the nicotinic receptor but remains bound to the active site of the recept.  This binding prevents subsequent activation of the receptor while succinylcholine is bound;  it is commonly referred to as a “depolarizing neuromuscular blocker” due to initial receptor activation and subsequent membrane depolarization.  the tubocurarine class of drugs such as rocuronium, vecuronium, and atracurium falls in the category of “non-depolarizing agents.” These agents act via competitive inhibition, occupying the active receptor site and preventing acetylcholine binding and activation Direct Acting Cholinergic Nicotinic Agonists  Nicotine  MOA**- Low doses – ganglionic stimulation causing euphoria and arousal. CNS effects cause relaxation and improves attention (Acute)  Indications – None  Adverse effects – Vomiting, convulsions, hypertension, cardiac arrhythmias, Respiratory arrest – (depolarizing blockade), Muscarinic effects - PNS ganglia stimulation.  Succinylcholine *:  MOA - Overstimulation results in depolarizing blockade  Indications – muscle relaxation/paralysis associated with intubation, other procedures  Adverse effects – Fasciculations, respiratory arrest, malignant hyperthermia 35 Indirect Acting Cholinergic Agonists - Reversible  Edrophonium *(Tensilon)  Neostigmine *  Pyridostigmine* (Mestinon,)  Physostigmine  MOA - Prolongs duration of acetylcholine by binding with and blocking acetylcholinesterase.  Indications – Myasthenia Gravis, Glaucoma, Atropine Poisoning  –salivation, flushing, bradycardia, Adverse effects bronchospasm, sweating, nausea, abdominal pain, diarrhea, decreased blood pressure, muscle fasciculations (N), and respiratory arrest (N). 36 Indirect Acting Cholinergic Agonists – Reversible - CNS  Used in Alzheimer’s :  Donepezil (Aricept, Aricept ODT)  Galantamine (Razadyne, Razadyne ER)  Rivastigmine (Exelon) – tertiary amine  Mechanism of action – Increase cerebral concentrations of acetylcholine by inhibiting acetylcholinesterase  Adverse effects –Same as other reversible ACHase Inhibitors 37 Indirect Acting Cholinergic Agonists - Irreversible  Echothiophate* (Phospholine)  MOA- Prolongs duration of acetylcholine by permanently inactivating acetylcholinesterase.  Therefore both Nicotinic(N) and Muscarinic(M) effects!  Indications – Glaucoma  Adverse effects –salivation, flushing, bradycardia, bronchospasm, sweating, nausea, abdominal pain, diarrhea, decreased blood pressure, muscle fasciculations (N), and respiratory arrest (N). 38 “SLUDWARMF”  Sweating  Lacrimation  Urination  Diarrhea  Wheezing  Accommodation Muscarin  Rhinorrhea ic  Miosis  Fasciculations Nicotini 39 c  PARASYMPATHOLYTICS (ANTICHOLINERGICS): Drugs that reduce or inhibit some or all of the actions of the parasympathetic nervous system. Muscarinic Nicotinic receptor receptor antagonists antagonists Ganglionic Neuromuscular blocking drugs blocking drugs (Nn) (Nm) 40 Parasympatholytics Muscarinic receptor antagonists  MOA- Block muscarinic receptors on the effector organs of the parasympathetic nervous system and on the sweat glands  Indications – Varied - specificity for muscarinic receptors is a key reason behind their usefulness.  Adverse effects –  Autonomic  PNS - dry mouth, blurred vision, tachycardia, urinary retention, and constipation  SAS - Inhibition of sweating  CNS - restlessness, confusion, and hallucinations  Common mnemonic : “hot as a hare, blind as a bat, dry as a bone, red as a beet, and mad as a hatter" 41  Parasympatholytics Muscarinic receptor antagonists  Atropine Sulfate – Cholinesterase poisoning,  ACLS: Bradycardia, Pulseless Electrical Activity and Asystole  Benztropine (Cogentin) – Parkinsonism  Dicyclomine (Bentyl) – Irritable Bowel Syndrome  Ipratropium *(Atrovent) – COPD, Rhinorrhea  Tiotropium * (Spiriva) – COPD, Rhinorrhea  Oxybutynin (Ditropan) – Overactive bladder  Tolterodine (Detrol) – Overactive bladder  Tropicamide (Mydriacyl) - Mydriasis (short duration)  Scopolamine - Motion Sickness, Amnesia 42  Parasympatholytics Nicotinic receptor antagonists- Ganglionic Blockers (NN)  Mecamylamine – Non-depolarizing, competitive blocker  Nicotine – Depolarizing blocker  High doses – ganglionic blockade causing respiratory paralysis and hypotension  No selectivity - block receptors on both the parasympathetic and sympathetic ganglia  Adverse Effects – Intolerable  Hypotension, Orthostatic Hypotension, atony of bladder and GI Tract, cycloplegia, xerostomia, sexual dysfunction, hyperthermia  Rarely used therapeutically 43  Parasympatholytics Nicotinic receptor antagonists- Neuromuscular Blockers (NM)  Tubocurarine* - Prototype  Atracurium*  Pancuronium *  Rocuronium* Vecuronium* Indications – muscle relaxation/paralysis associated with intubation, other procedures  MOA - Competitive blocker - action can be reversed by increasing concentration of Ach**  Adverse Effects – Respiratory arrest. 44 Thanks for Listening