Basic Principles of Neuropharmacology PDF

[Basic principles of neuropharmacology] [Lecture 1:] [Learning outcomes:] 1. Define ligands and the concept of binding, introduce and explain different types of agonists and antagonists, and their interaction. 2. Understand and explain concentration response relationships; define affin...

[Basic principles of neuropharmacology] [Lecture 1:] [Learning outcomes:] 1. Define ligands and the concept of binding, introduce and explain different types of agonists and antagonists, and their interaction. 2. Understand and explain concentration response relationships; define affinity and efficacy, and potency. 3. Discuss spare receptors and their role for ligand efficacy, discuss selectivity. [Binding of ligands to receptors:] - Ligand is any chemical that binds to (or combines with) a receptor. - Receptor is a cellular macromolecule - or assembly of macromolecules - concerned directly and specifically in chemical signalling between and within cells. - The binding occurs in 3D - Binding of ligands to receptors is an active process, which happens due to the alignment of 3D shape and biophysical properties (forces) between the ligand and the binding site on the receptor: - multiple points of interaction may be needed within a binding site for binding to occur - hydrophilic and hydrophobic, charge sites - van der waal, electrostatic, covalent - Binding happens with a force. Binding of one part of the molecule can facilitate or prevent the binding of another part (e.g. through shape changes). - When a part of molecule binds to a receptor -- there is a change in receptor - High molecules that bind very potently to the receptor would be molecules that bind to the molecules to the shape of the receptor site but creates new access points. Ligands and their binding to receptors: - **Ligands** that are **produced naturally by the body** (e.g. neurotransmitters and other molecules that can bind to receptors) are referred to as **endogenous ligands**. - Endogenous ligands can also be synthesised in the lab or modified to change their properties. Endogenous ligands that are modified or designed by the chemists are termed **exogenous ligands**. - The concept of "binding" of ligands to their receptors in tissues was developed in 1960s using **radioligand** binding assays to quantify the amount of ligand bound to receptors. - A ligand (e.g neurotransmitter) radiolabelled and incubated with a tissue preparation, which is washed extensively to remove loosely bound drug. A radioactive atom, typically 3H, 14C, or 125I, must be added to the ligand without altering its binding properties. [The radioligand binding assay:]  - Many chemicals/molecules have specific binding sites and unspecific binding sites. - Specific binding sites = binding with receptors. - Non-specific binding sites = Binding in the membrane - Provides unspecific labelling + signals - Washing the preparation would remove some of the molecules but molecules that bind with a certain force would still remain. [Specific and non-specific binding:] A diagram of a line Description automatically generated - Total binding is the total amount of binding observed -- contains specific + non-specific binding sites - Nonspecific binding represents the non-saturable portion of binding that is presumably not associated with the specific binding site under investigation. - High potency at the start due to binding occurring at the correct region/receptor. - Non-saturated - Specific binding is calculated as the difference between total and nonspecific binding and reflects the amount of radioligand bound to the specific binding site. - Saturated [Quantification of radioligand binding:]  - To produce a sigmoidal curve -- plot values on logarithmic axis. - Easier to estimate the IC50. - The specific binding of a ligand to a tissue preparation, when the ligand is at equilibrium with the receptors, is quantified according to two properties: the affinity of the binding, which is expressed as a **dissociation constant (Kd),** and the **total amount of binding (Bmax).** - Kd = value at which 50% of the receptors is occupied by a certain concentration of the drug on the compound. - Increase of compound = more non-specific binding not specfic A diagram of a graph Description automatically generated with medium confidence [Autoradiography + protein distribution:] - Tissue is incubated with a radiolabelled ligand to allow it to bind to its targets. - The tissue is then exposed to photo film. - The radiation emitted by the radioactive ligand exposes the film and creates a picture highlighting the specific pattern of binding in the brain - Scientists calibrate the amount of compound they added so they can limit non-specific + maximise specific binding.  Binding studies describe the physical relationship between a drug and its target but do not directly assess the physiological or functional consequences of this association. [Functional classification of ligands: agonists and antagonists:] Ligands, in simple terms, can be defined as agonists or antagonists, described in terms of effects and their ability to bind to their targets. - **Agonists** evoke (produce) effects in biological tissue; they can be full, partial or inverse. - All neurotransmitters are agonists. - Inverse agonist has an opposite effect. - **Antagonists** do not have effects of their own on biological tissue but can block effects evoked by the agonists. Thus, their effect is to antagonise the action of an agonist. Antagonists can be competitive and non-competitive A diagram of a diagram Description automatically generated [Quantification of an agonist effect:] - It is usually a biological response *(in vitro or in vivo)*, which is elicited by an agonist, that we measure, and this is often plotted as a concentration-response curve (*in vitro*) or dose-response curve (*in vivo*). - As we increase the concentration of the agonist, we have an increase in biological response until the maximum (Emax)  - Concentration-response curves allow us to estimate the maximal response that the drug can produce (*Emax*), and the effective concentration (*EC50*) or dose (*ED50*) needed to produce a 50% maximal response. A graph of a plot Description automatically generated with medium confidence - The logarithmic plot is the preferred method for visualising concentration-response relationships because it becomes easier to accurately determine the potency of he ligand: the EC50 value (the concentration which produces 50% of the maximum response) by placing it on the linear portion of the curve. [Comparing potency of different agonists:]  Concentration-response relationships for 3 agonists that vary in potency. Agonists A, B and C have the same efficacy, but differ in terms of their potency. The most potent agonist - A, has the lowest *EC50* value (\~5mM), and is approximately four times more potent than C (\~20mM) - The same effect is produced by different types of agonists at different concentration due to different binding to these receptors but affect the same biological function. [Comparing potency of different agents:] A diagram of a graph Description automatically generated Concentration-response relationships for 3 agonists that vary in efficacy. Each agonist has essentially the same *EC50* value (*equipotent*), but they differ in terms of the maximum response they produce. For example, agonist A has a relative efficacy that is two times greater than agonist B. - Some agonists don't produce maximal effects - C = partial agonist -- can't produce the full effect [Partial vs full agonist: ] When a partial agonist binds to a receptor it elicits only a **small response**. This is because it lacks a portion of the molecule required for the full physiological effect or it binds to a slightly different site on the receptor. Nonetheless a partial agonist, whilst being less efficacious **can be more potent than a full agonist**! - In the **presence of a full agonist, a partial agonist will act as a functional antagonist**, competing with the full agonist for the same receptor. - This **reduces the ability of the full agonist** - Would not express a response to the extent a full agonist would. - Some partial agonists can be very potent compared to a full agonist -- competition between the two agonists binding to the same receptor means that they are co-competing for the same target but one is much more potent -- higher affinity to the receptor and this one outcompetes the others. - **Functional antagonist** = partial agonists that are present with a full agonist -- outcompete the full agonist + minimise the receptor function therefore minimising the biological effect. - Can be used clinically for treatment. [Maximal drug responses and spare receptors:] Repeated application or use of agonists can lead to receptor 'desensitisation' or tolerance that may be overcome by increasing the dose. Why? [Linear relationship:] Maximal response when all receptors occupied [Hyperbolic relationship:] - In most mammalian systems the relationship between receptor occupancy & drug response is hyperbolic - Maximal responses at less than maximal receptor occupancy - Some receptors are "spare" - At 50% occupancy, the maximum effect has occurred -- no matter how much more stimulus is applied, it will not increase further. - Activation + binding of other receptors will not contribute to the biological effect once maximum is reached -- those receptors become 'spare' - Spare receptors relate to amplification factors. [Amplification of efficacy:]  - Image = visualisation of the effects that occur in the biological tissue, in the GPCR environment - Agonist 3 produces more of a response compared to the others -- more binding to the receptor. - Once it goes through G-proteins + second messengers -- all drugs become good at activation the second messenger = agonist 3 is slighter better but 2 is more potent. - Once reaching the effector organs (i.e. muscles) -- all agonists produce the same response but 2 is the most potent + 3 is the least. - All these steps create the 'spare' receptor effect which means we don't need to activate all of the receptors to produce a certain function. [Other types of concentration response curves:] An inverted U-shaped curve indicates that the biological response elicited by an agonist progressively increases as the agonist concentration increases and subsequently peaks at a moderate concentration; **higher concentrations elicit progressively smaller responses.** A graph showing a normal distribution of efficiency Description automatically generated with medium confidence - Cholinesterase inhibitors are used in treatment for Alzheimer's disease -- minimises the memory related symptoms + activates the cholinergic synapses in the brain - Many drugs, inhibitors + activators follow this U-shape as dose is increased + produces many unwanted effects.  [Functional classification of ligands: agonists and antagonists:] Ligands, in simple terms, can be defined as agonists or antagonists, described in terms of effects and their ability to bind to their targets. - **Agonists** evoke (produce) effects in biological tissue; they can be full, partial or inverse. - **Antagonists** do not have effects of their own on biological tissue but can block effects evoked by the agonists. Thus their effect is to antagonise the action of an agonist. - Antagonists can be competitive and non-competitive. [Competitive antagonists:] - A competitive antagonist competes with an agonist (or endogenous ligand) for the same binding site on the receptor. The antagonist does not alter the efficacy of the agonist because the same number of receptors are available to both drugs. - Competitive antagonists = molecules that bind to the same site as agonists -- doesn't activate the receptor = no effect on the receptor but prevents the biological function. - Who wins the competition? Well, it depends on: - Affinity - Strength of binding - Dissociate constant - Ability to dissociate - Amount present A graph of a log concentration Description automatically generated - Grey -- agonist alone has a higher potency - Red -- due to competition between the agonist + antagonist = less potency of agonist - Competitive antagonist -- shifts the curve to the right + creates a situation where more agonist is needed to activate receptors.  - i.e. curare poisoning shifts the acetylcholine effect to the right. Neostigmine increases levels of acetylcholine + produce a beneficial effect. - Overcome the effect of the antagonist by increasing concentrations of the agonist. - That is primarily why, if you increase the concentration of the agonist, it will overcome the effects of a competitive antagonist. [Non-competitive antagonists]: - A noncompetitive antagonist works at a completely different binding site (different to the agonist binding site) and alters the configuration of the receptor for the agonist. It reduces the number of receptors available for the agonist to bind to. - i.e. picrotoxin -- doesn't bind at the GABA binding site on the GABA receptor. A graph of a log concentration Description automatically generated - The potency remains the same, but the efficacy (in this case maximum level) is greatly reduced. - Increasing agonist concentration will not overcome this issue as there aren't as many receptors available. - More 'spare' receptors = less effects of a non-competitive receptors [How do inert antagonists produce behavioural responses?] By preventing the agonist action and therefore preventing the biological effect. - Competitive antagonists compete with agonists (or endogenous ligands) for the same binding site on the receptor. - Noncompetitive antagonists bind to an allosteric (non-agonist) site on the receptor to prevent activation of the receptor. - Can be functional at other binding sites. - Allosteric binding sites = module receptors either positively or negatively - Negative allosteric modulators = classical non-competitive antagonists.  - In the presence of a constant concentration of an agonist (endogenous or exogenous), and by systematically changing the antagonist concentration we can quantify the inhibitory effect of the antagonist action on the agonist evoked response A diagram of a graph Description automatically generated with medium confidence - The IC50 value (half maximal inhibitory concentration) indicates how much antagonist is needed to inhibit a biological process by half. - IC50 values can be used to compare potencies of antagonists in different tissues - TTX is much more potent [Summary: agonists, antagonists and modulators:]  - Inhibitors = act like antagonists - Potentiators = don't become an agonist but they modulate the receptor so there is better binding of agonists to the receptor. - Act as "antagonist" as they have no effect but sometimes modulations increase the function. - A = agonist -- binds to receptor which provokes a response - A + D = agonist + non-competitive antagonist (negative allosteric modulator) - Decrease in maximum number of receptors + maximum available effect to be produced. - Fewer receptors available for the agonist to produce an action. - A + C = agonist + non-competitive antagonist (positive allosteric modulator) - Allows for the production of a greater biological response. - In situations where there cannot be anymore activation, these modulators are unlikely to have an effect. - A + B = competitive antagonist + agonist - Shifts concentration curve/response to the right. - Need more agonist to outcompete the antagonist. - B = antagonist - Produces no effect in the biological system - Full agonist = maximal response in the system where no larger response can be provoked by any other known agonist. - Partial agonist/functional antagonist = might provoke a response which is less than a full agonist but substantially less. - Can be more potent but produce a lesser effect. - Used in treatment for overdoses + poisoning - Prevents overactivation of a system or certain receptors A diagram of a drug action effect Description automatically generated with medium confidence An antagonist may bind to the receptor in a way that prevents the access for the agonist that is required for the response. [Summary: affinity, potency and efficacy]  - Target substrate = receptor - Affinity is determined during radioligand - Potency = measured in ml/kg + molar + used to define EC50 + IC50 values for agonist + antagonists Real life examples: - Isoprenaline + Propanolol A graph of a line graph Description automatically generated with medium confidence - The potency of the agonist decreases with a [competitive antagonist] because the agonist and antagonist compete for the same binding site on the same receptors. - Curve shifts to the right - Isoprenaline (analogue of adrenaline) is a β-adrenergic receptor agonist. - Propanalol is a "β-blocker" - These drugs are used for regulation of blood pressure - 5-HT + Methysergide  - Some antagonists bind covalently to the receptor and cannot be displaced by either competing ligands or washing (effects are irreversible), changing the efficacy of the agonist. - 5-HT is an agonist at serotonin receptors. - Methysergide is a 5-HT antagonist with actions at adrenergic and dopaminergic receptors. - Non-competitive antagonist [Most ligands have affinity to many receptors: the concept of selectivity] A graph of a graph of a graph Description automatically generated with medium confidence - Serotonin = mood neurotransmitter + important in the gut. - Antagonist can be used to treat constipation - As concentration increases, binding occurs to other receptors + transporters become highly unselective. [Lecture 2:] [Learning outcomes:] 1\. Basic concepts: define drugs & medicines, in contrast to ligands. 2\. Review actions of drugs (and ligands) in biological systems 3\. Define the concepts of pharmacodynamics and pharmacokinetics. 4\. Discuss bioavailability: administration, release, absorption, distribution, metabolism and excretion of drugs. 4. Discuss volume of distribution and clearance. [Basic terms:] - **Ligand** is any chemical that binds to a receptor, they can be agonists or antagonists. - Neurochemicals which bind to neuronal, astrocytic or supporting receptors. - Neurotransmitters -- drugs that modulate neurotransmitter function. - Can be through presynaptic or postsynaptic effects. - **Drug** is any substance (other than food) that is used to **prevent, diagnose, treat, or relieve symptoms of a disease or abnormal condition**. Drugs can also affect how the brain and the rest of the body work and cause changes in mood, awareness, thoughts, feelings, or behaviour. Some types of drugs, such as opioids, may be abused or lead to addiction. *(National Cancer Institute, NCI USA)* - **Medicine refers to the practices and procedures** used for the prevention, treatment, or relief of symptoms of diseases or abnormal conditions. This term may also refer to a **legal drug** used for the same purpose. *(National Cancer Institute, NCI USA)* - Difference between drug + medicine is that medicines are legally defined ways to treat/prevent a disease. - Medicines don't always involve chemicals. Medicines can be i.e. behavioural psychotherapy - Drugs that influence behaviour + work on the brain are known as **psychotropic agents** or **psychoactive agents**. But there are many other terms: [chemical, compound, agent,] etc... - [Small molecule] (in pharmacology) = used on different types of ligands which are different developmental stages -- to become a drug or approved as a medicines [Common use of drugs:] - Antidepressant, antianxiety, anticonvulsant and antipsychotic agents are among the most widely prescribed medications. - Some of these act on other organ systems and are associated with unpleasant/unwanted side effects. - Common side effect = stomach issues. - Many people use common substances, such as caffeine, alcohol and nicotine, that also act on the central nervous system. - In some people, drugs are used compulsively, in a manner that constitutes an addiction. [The action of drugs on neural targets:] - Binding to the receptor is the initiation of the process -- many drug related effects are dependent on the subsequent cascades where second messenger cascades that active certain local expression pathways can either amplify or downregulate those effects. - The initial target of a drug determines the cells and circuits on which the drug acts, and at the same time the potential efficacy and side effects. - The initial binding of a drug to its target is only the beginning of a signalling cascade that affects the behaviour of cells, neural circuits and animals [Pharmacodynamics vs Pharmacokinetics:] [Pharmacodynamic]s *(grk, medicine & power)* - The time course of the effect of a drug, and the intensity (power) of the effects. - "What a drug does to the body?" - The ability of a drug to produce an effect on an organism is dependent on the underlying mechanisms of drug action. - Affinity, efficacy, potency, concentration-response relationships, spare receptors & amplification  [Pharmacokinetics] *(grk, medicine & movement)* - The time course of a drug in the body - "What the body does to a drug" - Eventually the drug will be cleared from the system A diagram of a medical condition Description automatically generated with medium confidence Pharmacokinetics + bioavailability:  - The ability of a drug to produce an effect on an organism is dependent on many of its properties (in addition to its mechanism of action), from its absorption to its stability to its elimination - its bioavailability Stages of pharmacokinetics: 1. Route of administration 2. Release / liberation 3. Absorption (dosing regiments) 4. Distribution (compartments) 5. Metabolism (metabolite kinetics, clearance) 6. Excretion (clearance) [Administration -- getting drugs into the brain:] - Route of administration must be considered which can determine whether or how rapidly a drug reaches its target organ and which organs it affects - Enteral (through intestine): - Oral administration typically results in a relatively slow onset of action. - Sublingual / buccal / rectal - Parenteral describes all other routes of administration, including: - Intravenous (into the venous system) - Intramuscular (into a muscle) - Subcutaneous (under the skin) - Inhalation / nasal - Intraperitoneal (into the peritoneal--abdominal cavity) - [Intracerebroventricular (into the cerebral ventricular system)] - [Intracerebral (into the brain parenchyma) delivery]. - Oral = through the mouth - Intravenous - Subcutaneous = injections - [These two are method used on experimental animals to surgically modify behaviour -- no other way to deliver the drug to ensure it reaches it's target.] [Pros and cons -- administration:] A close-up of a table Description automatically generated [Oral bioavailability -- tablet distribution (release/liberation):]  - When the drug enters the stomach, it is broken down into multiple fragments -- fragments increase surface area of the drug. - These fragments dissolve into the solution -- reaches the intestine for absorption - Issue with the stomach = pH is very low (very acidic) in the stomach -- many drugs are broken down due to the high acidity therefore drugs have to be able to withstand the stomach acid. - Can be avoided using an alternative administration - Pepsin can break down the protein structures within the drugs + convert them to amino acids before they reach the intestine. [Oral bioavailabity -- absorption] A diagram of a body Description automatically generated - IMPORTANT: drugs do not get absorbed in the stomach but in the small intestine - The small intestine (specifically the duodenum + jejunum) contains: - Microvilli that increase the surface area which contain transport mechanisms - Drugs are absorbed by these microvilli + enter circulation. - Drug absorption is competed by other aspects: - Food intake - Exercise - Disease states - Drug itself - Time of day - Absorption is the disappearance of the drug from its site of administration and not the appearance of the drug in the general circulation. [Oral bioavailability -- first pass metabolism:]  - Drugs may not be absorbed by the microvilli + excreted - Cytochrome P450 enzymes = metabolise drugs by oxidation -- can leave many drugs inactive - Find even more of these enzymes once the drug enters the portal vein + into the liver. - The liver is one of the main organs that tries to break down drugs + get rid of them. - Therefore, the dose of the drug has to be able to withstand the liver trying to break it down + removal. - Some drugs reach the liver in an inactive state + convert into an active within the liver - Some drugs we do not want to pass the intestine -- intestine related medicines are made for the intestine so they can't be transported into the system. [Bioavailability is affected by binding to plasma proteins:] A diagram of a drug Description automatically generated - Do not need to learn list. - Many drugs bind to circulating plasma drugs i.e. albumin - When bound to proteins, drugs are not active - To be active, drugs need to bind to a receptor - In emergencies -- use intravenous administration [Bioavailability is also affected by barrier:]  - Bioavailability is regulated by the intestinal transport, blood brain barrier (regulates access of molecules to the brain). - In the blood brain barrier, two types of drugs are most likely to work -- either water soluble or lipid soluble - Water soluble = dissolve in water + go through the tight junction to reach the brain - Lipid soluble = the brain is a fatty organ -- the fatty membranes take up fatty drugs through passive transport. - Glucose/amino acids are transported through a transport system -- allow drugs to be taken up + pass the brain barrier - Insulin binds to the receptor before transportation - Charged molecules can be transported via vesicles - many drugs are made to not pass the brain barrier. [Getting drugs into the brain -- bioavailability ] - with every step, we are decreasing the concentration of the drug which is available to produce an effect. The bioavailability of a drug determines how much of the drug that is\ administered actually reaches its target. - Influenced by absorption of the drug (from the gut if administered orally). - Affected by metabolism and excretion. - Affected by binding of the drug to plasma proteins, which makes the drug unavailable to bind to its target. - Influenced by a drug's ability to penetrate the blood-brain barrier, or its ability to permeate cell membranes [Drug pharmacokinetics decides the dosing regiment:] A diagram of a function Description automatically generated - When the drug is first taken up, it reaches a certain level -- maximal availability - Talking about a drug that is not bound to a protein -- has free access - Drug starts to decay exponentially as it reaches its target - Area under the curve -- defines amount of drug which is available - With repeat dosing we can get a steady state which ideal for treatements. [Pharmacokinetics + dosing terms]  [Fundamentals of pharmacokinetics:] - Volume of distribution -- how well a drug is distributed - Clearance -- how well a drug is cleared - Elimination half-life [Volume of distribution:] A diagram of a syringe and a tank Description automatically generated - Volume of an average human = 70L [Clearance: ] - By constantly taking a sample, we can measure the disappearance of the drug  [Case study -- fate of ethanol in body] A diagram of a drug use Description automatically generated with medium confidence [Case study -- ethanol distribution volume:]  - Females have less body water than males due a higher percentage of fat in the body. - A lot of ethanol would be stored in the fat + will not be able to cleared at the same rate. [Variability in drug response:] - appearance and philosophy - physiology and biochemistry - occupations and habits - genetics -- genotype vs phenotype