Presentation on theme: "Autonomic Nervous System:"— Presentation transcript:
1 Autonomic Nervous System: Introduction to neurotransmitter and receptor specificityThomas GuenthnerProfessor of PharmacologyCollege of MedicineTelRoom E418, CMWThanks to Dr. Richard Ye for Powerpoint concepts and slides
2 Knowledge objectives introduced by these two lectures: Identify the key conceptual similarities and differences between autonomic cholinergic and adrenergic pathways including receptor subtypes, neurotransmitters, transmitter synthesis, storage, and release, and relative specificities of drugs that stimulate or inhibit each branch or activity.List the major systems or organs innervated by the autonomic cholinergic and adrenergic systems.Describe the organ system effects of cholinergic and adrenergic stimulation or antagonism.Relate the tissue expression profiles of cholinergic and adrenergic receptors to their specific functions.
6 Pharmacological division of cholinergic vs. adrenergic neurotransmissionAll preganglionic and parasympathetic postganglionic neurons use acetylcholine as neurotransmitter. Ach is the neurotransmitter at ganglia, nmj, and muscarinic tissue synapses.Most postganglionic sympathetic neurons use norepinephrine which is an adrenergic neurotransmitter.There are exceptions: Cholinergic transmission in sympathetic system – all ganglia, adrenal medulla, sweat glads use Ach (nicotinic or muscarinic). Dopaminergic innervation in sympathetic system – renal blood vessels.
7 Synapse – site most amenable to pharmacologic manipulation: Precursors(choline/tyrosine)SynapticcleftPrecursorNeurotransmitterPre-synapticnerve cellStorageReleaseCa2+Recognitionby receptorsMetabolicdispositionPost-synapticnerve cellManipulation possible at pre-synaptic neuron, where neurotransmitter is synthesized, stored and released upon cell activation, or at post-synaptic neuron or effector cell, where neurotransmitter is detected and its action is translated into cellular activities.
8 Key Steps in Neurotransmission: Synthesis & StorageMetabolismActionpotentialReleaseRecognition(action)ReuptakeStrategies for Pharmacological Intervention:Block synthesis and storage: Usually rate-limiting steps; produce long-term effectsBlock release: Rapid action and effectiveBlock reuptake increases synaptic neurotransmitter concentrationsCan be selective or non-selectiveInterfere with metabolism: Can be reversible or irreversible; blocking metabolismincreases effective neurotransmitter concentrationsInterfere with recognition: Receptor antagonists & agonists; high specificity
9 Definition of Agonist and Antagonist: Agonist: (1) A natural ligand that activates a receptor. (2) A drug that has properties similar to a natural ligand in activating the same receptor.Antagonist: (1) A receptor-specific blocker. (2) A molecule, such as a drug (e.g., enzyme inhibitor) or a physiologic agent (e.g., hormone), that diminishes or prevents the action of another molecule.Mode of Action:Direct-acting: Molecule that physically binds to the target for its effect.Example: carbachol activates cholinergic receptors.Indirect-acting: Molecule that exerts effect on the target by interacting withanother non-target site.Example:neostigmine blocks AchE, causing Ach accumulation.Mode of action and agonism are different concepts. For example, a direct- acting molecule can be either agonistic or antagonistic.
10 Otto Loewi (Nobel Laureate, 1936) Discovered that stimulation of the vagus of a frog heart causes release of a substance that, when applied to a second heart, could slow heart rate. He called this “Vagusstoff”, demonstrating the chemical basis of neurotransmission.Also found that atropine can prevent the inhibitory action, but not the release, of “Vagusstoff”.Exposure of “Vagusstoff” to frog heart homogenate inactivates it.Physostigmine enhances the effect of vagus stimulation on the heart, and prevents the destruction of “Vagusstoff”.
11 + + Synthesis of acetylcholine: Choline Acetylcholine Choline –CH2–CH2–OHCH3N+–CH2–CH2–O–C–CH3OCholineacetyltransferase++CoA–S–C–CH3OCoA-SHAcetyl-CoACoA
12 Synthesis, storage and release of acetylcholine: Na+Choline(10 mM)SynapticcleftCholineAchAc-CoAChATAntiporterAchNerveimpulseAchAchcholine+ acetic acidAchPre-synapticcellNNCa2+AchAchECa2+Recognitionby receptorsNMCAT = choline acetyltransferaseAchE = acetylcholinesterasePost-synapticcellAchE
13 Degradation of acetylcholine: Acetic acidAchE(CH3)3 N+–CH2–CH2–O–C–CH3(CH3)3 N+–CH2–CH2–OH+CH3COOH(-)OHAchEGlu202Tyr337Ser203Glu334His447600,000 Ach molecules / AchE / min= turnover time of 150 microsecondsSteps involved in the action of acetylcholinesterase:1. Binding of substrate (Ach)2. Formation of a transient intermediate (involving -OH on Serine 203, etc.)3. Loss of choline and formation of acetylated enzyme4. Deacylation of AchE (regeneration of enzyme)
15 An example of indirect agonism: Physostigmine’s effect on acetylcholine receptor is indirect. This effect is mediated through the inhibition of cholinesterase, which causes an increase in the local concentration of acetylcholine. The net effect is agonistic on acetylcholine receptor.
16 Julius Axelrod (Nobel Laureate, 1970) His discoveries concern the mechanisms which regulate the formation of norepinephrinein the nerve cells and the mechanisms which are involved in the inactivation of thisimportant neurotransmitter.Synthesis of CatecholaminesTyrosine hydroxylasePhenylethanolamine-N-methyl transferase31HOHCNH2CH2TyrosineCOOHHOHCNH2CH2DOPACOOHTHDopa decarboxylase(L-amino aciddecarboxylase)DD(L-AAD)HOCH2NHCH3OHCHEpinephrineHOCH2NH2OHCHNorepinephrineHOCH2NH2DopaminePNMTDBHDopamine b-hydroxylaseAdrenal medulla
17 Regulation of Norepinephrine Synthesis and Metabolism: Na+TyrosineTyrosineDopaTHDDDopamine(DA)a2RSignalDBHUptake-1NEATPNE(-)NEDBHATPbRPost-synapticPre-synapticCa2+Ca2+Cellular messengersand effectsaRCOMTDiffusion,metabolismNormetanephrine (NMN)
25 Gs and Gi proteins have different functions AgonistAgonistAlpha2 receptorBeta1 receptorACbgbgasaiasbgaibgGs = stimulatory G proteinGi = inhibitory G proteinAC = adenylyl cyclase (convert ATP to cAMP)
26 Distribution and functions of adrenergic receptors: 10/31/12Distribution and functions of adrenergic receptors:a1: postsynaptic effector cells, especially smooth muscleVasoconstriction, hepatic glycogenolysisa2 presynaptic adrenergic nerve terminals (autoreceptor), platelets, lipocytes,0 smooth muscleInhibition of transmitter release, platelet aggregation, relaxation ofgi smooth muscleb1 postsynaptic effector cells: heart, lipocytes, brain, presynaptic adrenergic / cholinergic terminalsIncreased cardiac rate & force, relaxation of gastrointestinal smooth muscleb2 postsynaptic effector cells: smooth muscle, cardiac muscleBronchodilation, vasodilation, relaxation of visceral smooth muscle, hepaticglycogenolysisb3 postsynaptic effector cells: lipocytesLipolysis
32 Dopaminergic receptors in the periphery Dopamine receptors play important roles in CNS. Notably, dopamine neurotransmission is involved in several diseases including Parkinson’s disease, schizophenia, and attention deficiency disorder.There are 5 types of dopamine receptors (D1 – D5). In periphery, D1 dopamine receptor mediates renal vasodilation, and increased myocardial contractility.AgonistAgonistD1,5D2,3,4GsGi cAMP cAMP
34 Cholinergic receptors: Nicotinic Nicotiana tabacum(cultivated tobacco)“Nicotinic actions” -- similar to those induced by nicotine; action mediated by nicotinic cholinergic receptors:stimulation of all autonomic ganglia (NN)stimulation of voluntary muscle (NM)secretion of epinephrine from the adrenal medulla (NN)
35 Nicotinic acetylcholine receptor: Function Ligand-gated ion (Na+) channel- an “Ionotropic Receptor”Acetylcholine binds to the a-subunits of the receptor making the membrane more permeable to cations (sodium) and causing a local depolarization. The local depolarization spreads to an action potential, or leads to muscle contraction when summed with the action of other receptors. The ion channel is open during the active state.Nicotine in small doses stimulates autonomic ganglia and adrenal medulla. When large doses are applied, the stimulatory effect is quickly followed by a blockade of transmission.
36 Nicotinic receptor antagonists Competitive vs. depolarizingDepolarizingBinds and locks the receptoropenCompetitivePhysically blocksAch bindingINHIBITOR
37 Cholinergic receptors: Muscarinic “Muscarinic actions” -- reproduced by injection of muscarine, from Amanita muscaria (fly agaric). Similar to those of parasympathetic stimulationMultiple muscarinic cholinergic receptors distributed in different tissues. Therefore, the “muscarinic actions” are dependent on the receptors in different tissues and cells.Neural/enteric (M1): CNS, ENS, gastric parietal cells (excitatory; Gq)Cardiac (M2): atria & conducting tissue; presynaptic (inhibitory; Gi)Glandular/endothelial (M3): exocrine glands, vessels (excitatory; Gq)Neural (M4): CNS (inhibitory; Gi)Neural (M5): CNS (excitatory; Gq)(Sites of primary expression are listed; all are found in CNS)
41 Effects of muscarinic antagonists “DRY AS A BONE, RED AS A BEET, MAD AS A HATTER.”Decreased sweating, salivation and lacrimationReflex peripheral (cutaneous) vasodilation to dissipate heat (hyperthermia)CNS effects of muscarinic inhibition -- restlessness, delerium, hallucinationALSO:BronchodilationTachycardiaMydriasis (pupil dilation) and Cycloplegia (loss of focus)GI and Bladder atony
42 Physiological Effects of ANS Stimulation and Inhibition
49 Cardiovascular effects of intravenous infusion of epinephrine, norepinephrine, and isoproterenol in man. Norepinephrine (predominantly a-agonist) causes vasoconstriction and increased systolicand diastolic BP, with a reflex bradycardia. Isoproterenol (b-agonist) is a vasodilator, but stronglyincreases cardiac force and rate. Mean arterial pressure falls. Epinephrine combines both actions.
50 Two kinds of effects produced by Ach. Sir Henry Hallett Dale(Nobel laureate, 1936)(Arterial pressure of ananesthetized cat wasmeasured)Two kinds of effects produced by Ach.A. Ach causes a fall in BP due to vasodilation.B. A larger dose of Ach also produces bradycardia, further reducing BP.C. Atropine blocks the effect of Ach in lowering BP.D. Still under the influence of atropine, a much larger dose of Ach causes a rise in BP and tachycardia.A, B: Muscarinic effects of Ach (M3, M2)C: Muscarinic antagonistic effect (M)D. Stimulation of sympathetic ganglia (NN)
51 Receptor distribution and effects in the autonomic nervous system: OrganSympatheticReceptorParasympatheticReceptorHeartSA nodeAtrial muscleAV nodeVentricular muscleRateForce Automaticity 1b1Rate Force Conduction velocity AV blockM2Blood vesselsArteriolesCoronarySkeletal muscleVisceraSkinBrainErectile tissueSalivary glandContractionRelaxationa1b2M3RelaxationM3Vein(Continued, next page)
52 Intracellular signaling triggered by acetylcholine in the endothelium eNOS●NOL-ArgL-CitrulineeNOSNitric oxide synthaseMajor molecular players: M3, heterotrimeric G Protein Gq, Ca(2+)-CaM, eNOS, NO
53 Nitric oxide (NO) signaling pathway for SMC relaxation 10/31/12Nitric oxide (NO) signaling pathway for SMC relaxationSecondmessengerInhibition of PDE causes sustained levelof cGMP that maintains SMC relaxation.Sildenafil (Viagra)is an inhibitor forPDE 5.
59 Contraction of pupillary constrictor muscle -- miosis 10/31/12Cholinergic effects:Adrenergic effects:Contraction of pupillary constrictor muscle-- miosisContraction of ciliary muscle - bulge of lens-- near vision, outflow of aqueous humorContraction of pupillary dilator muscle-- mydriasisStimulation of ciliary epithelium-- production of aqueous humorPupillary dilator muscle (a1)Pupillary constrictor muscle (M3)Trabecular meshwork(opened by pilocarpine)Lens(M3)Secretion of aqueous humor (b)