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(1)Graded potentials on the post-synaptic membrane: depolarization and hyperpolarization; ligand-gated mechanisms (2) What happens at a synapse? A. Transmitter release Recognition by receptors on post-synaptic membrane Transmitter inactivation B. Graded potential changes across the post- synaptic membrane: EPSPs & IPSPs (3) Summing of EPSPs & IPSPs in TIME & SPACE (4) How neurotransmitters (ligands) produce voltage changes across the post-synaptic membrane: ionotropic and metabotropic (5) Types of neurotransmitters/neuromodulators (6) Drug actions: concepts SYNAPTIC POTENTIALS, TRANSMITTERS, & DRUG ACTIONS Class # 4: Synapses., p. 1
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Graded membrane potentials Depolarization: voltage across the post-synaptic membrane becomes more positive than the resting membrane potential. EPSP: excitatory post- synaptic potential – a temporary depolarization Hyperpolarization: voltage across the post- synaptic membrane become more negative than the resting membrane potential. IPSP: inhibitory post- synaptic potential – a temporary hyperpolarization Class # 4: Synapses., p. 2 These graded deploarizations (EPSPs) and hyperpolarizations (IPSPs) are brought about by the movement of ions across channels of the membrane. The type of channel involved is the ligand-gated channel, which is activated by a neurotransmitter at a synapse.
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What happens at a synapse? Class # 4: Synapses., p. 3
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Class # 4: Synapses., p. 4 EPSPs (excitatory post-synaptic potentials) IPSPs (inhibitory post-synaptic potentials) EPSPs bring the membrane potential closer to its threshold for an action potential (i.e., depolarizes). IPSPs bring membrane further from threshold (i.e., hyperpolarizes).
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SUMMATION OF EPSPs and IPSPs in time and space Class # 4: Synapses., p. 5
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Class # 4: Synapses., p. 6 AN EXERCISE ON SUMMATION (to be done in class)
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LIGAND-GATED ACTIONS on the post-synaptic membrane SOME MAY BE VOLTAGE- GATED AS WELL AS LIGAND- GATED; i.e., transmitter action will not occur unless the membrane voltage has also changed in the required way. Class # 4: Synapses., p. 7
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EXAMPLES of different types of synaptic effects, and of the fact that neurotransmitters do not always have the same effect on the post-synaptic membrane. What happens depends on the receptor-ligand combination, the type of ion that crosses the membrane, and it’s direction of movement: IONOTROPIC: direct action on ion channels Acetylcholine (ACh): acts on several types of nicotinic ionotropic receptors. At receptors in CNS and skeletal muscle, ACh opens Na + channels EPSP; at nictotinic receptors in cardiac muscle, ACh opens K + receptors IPSP Glutamate: acts on at least 4 types of receptors (3 of which are ionotropic). For one major type (the “AMPA” receptor), glutamate opens Na + channels EPSP GABA: acts on many types of receptors. For the GABA A type, it opens CL - channels IPSP METABOTROPIC: indirect action on ion channels via G-protein and sometimes also a second messenger. Norepinephrine (NE): acts on at least 4 types of receptors ( 1, 2, 1, 2 ), all metabotropic. For both beta types, it closes K + channel, extending the duration of EPSPs. (1) The binding of NE to the receptor activates a G-protein in the membrane. (2) The G- protein activates the enzyme adenylyl cyclase. (3) Adenylyl cyclase converts ATP into the second messenger cAMP. (4) cAMP activates a protein kinase. (5) the protein kinase causes a K + channel to close by attaching a phosphate group to it. Class # 4: Synapses., p. 8
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TYPES OF NEUROTRANSMITTERS and NEUROMODULATORS Class # 4: Synapses., p. 9
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SOME FACTORS THAT DETERMINE SYNAPTIC ACTION (drugs can influence all these factors except the first) Location of synapse relative to axon hillock. Nature/quantity of channels and receptors. What transmitter is released, (and how much). Which receptors are located on post-synaptic membrane (and how many). How the transmitter is stored and delivered to synaptic cleft. Transmitter recognition by the receptors (and the effect of recognition – direct vs indirect) If if indirect, which G-proteins and second messengers are involved. Transmitter inactivation: reuptake, enzyme degra- dation, diffusion, uptake into glia, uptake into post-synaptic cell. Synthesis of transmitters and inactivators. Class # 4: Synapses., p. 10
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DRUGS and their ACTIONS (definitions) Ligand: A molecule that binds with the binding site of a receptor. Agonist: A ligand that binds with and activates a receptor (or that mimics or increases the effects of the typical ligand for that the receptor) Affinity: the strength by which the agonist binds to (attaches to) the receptor Efficacy: the intensity of the agonist- produced receptor activation. Antagonist: A drug that opposes or blocks the effects of a particular ligand on a receptor. Pharmacokinetics: How the body acts on the drug (processes by which it is absorbed, distributed, metabolized, and excreted). Pharmacodynamics: How the drug acts on the body (processes by which it exerts its actions). Class # 4: Synapses., p. 11
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Addiction: a primary, chronic, neurobiologic disease, with genetic psychosocial, and enviornmental factors influenceing its development and manifestations. It is characterized by behaviors that include one or more of the following: imparied control over drug use, compulsive use, continued use despite harm, and craving. Physical Dependence: a state of adaptation that is manifested by a drug class-specific withdrawal syndrome that can be produced by abrupt cessation, rapid dose reduction, decreasing blood level of the drug, and/or administration of an antagonist. Tolerance: a state of adaptation in which exposure to a drug induces changes that result in a diminution of one or more of the drug’s effects over time. DRUGS and their ACTIONS (definitions: cont’d) CONSENSUS STATEMENT 2001 by the American Academy of Pain Medicine, American Pain Society, American Society of Addiction Medicine. Class # 4: Synapses., p. 12
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