1Autonomic Receptor FunctionsPart IISummary of ANS overview• Pharmacological classification of ANS is based on neurotransmitters: cholinergic, adrenergic, and dopaminergic.• Major sites for pharmacological interventions are neurotransmitter synthesis, storage, release, action and metabolism.• Acetylcholine synthesis is a one-step enzyme conversion. Its metabolism involves one major enzyme: cholinesterase.• Synthesis of catecholamines (dopamine, norepinephrine, and epinephrine) requires multiple enzymes. Their metabolism requires two enzymes.• The actions of neurotransmitters are mediated through receptors. An action on a receptor is a direct action. Indirect-acting drugs can affect receptor function through acting on a different molecule. When the net effect mimics that of a neurotransmitter, it is an agonistic effect. When the net effect negates or is opposite of the effect of a neurotransmitter, it is antagonistic.2Physostigmine’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. An example of indirect-acting pharmacological agents:Autonomic Receptor Classification:Cholinergic RAdrenergic RDopamine RMuscarinic RNicotinic RM1, M3, M5 (Gq coupled)M2, M4 (Gi coupled)NM(neuromuscular, or muscle type)NN(neuronal, or ganglion type)β1,α2α1,β2, β3D1, D2, D3, D4, D5Other receptors in ANS (or receptors for NANC transmitters,e.g. nitric oxide, vasoactive intestinal peptide, neuropeptide Y)(mAChR)(nAChR)3The “Muscarinic Actions” -- reproduced by injection of muscarine, fromAmanita muscaria. Similar to those ofparasympathetic stimulation• Neural (M1): CNS, PNS, gastric parietal cells (excitatory; Gq)• Cardiac (M2): atria & conducting tissue; presynaptic (inhibitory; Gi)• Glandular (M3): exocrine glands; smooth muscle (excitatory; Gq)The “Nicotinic Actions” -- similar to those induced by nicotine• stimulation of all autonomic ganglia (NN)• stimulation of voluntary muscle (NM)• secretion of epinephrine from the adrenal medulla (NN)Nicotinic acetylcholine receptor: Structure• All nAchRs have a pentameric structure, consisting of subunits in different combinations:A total of 9 different α subunits and 4 different β subunits have been identified• In skeletal muscle: α1(2), β, γ, δ (γ is replaced by ε in adult muscle)• In autonomic ganglion: α3, α5, α7, β2, β4• Other combinations of α and β subunits are found among the nAchRs in CNS• nAchRs with different composition have different affinities for some ligands. For example,α-bungarotoxin binds to nAchR in motor end plate with high affinity4Nicotinic acetylcholine receptor: Function• Ligand-gated ion (Na+) channel• Acetylcholine binds to the α subunits at the boundary of αand γ, and α and δ subunits.• Channel opening requires binding of 2 acetylcholine molecules, with positive cooperativity. • Structurally and functionally similar to the sodium channel, which can be blocked by local anesthetics• Acetylcholine binds to the α-subunits of the receptor makingthe 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.• In addition to α-bungarotoxin, there are other blocking agents for autonomic ganglions that include hexamethonium, tetraethylammonium, mecamylamine, and trimethaphan. Blocking at this level stops all autonomic outflow and produces a broad effect.5Two 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.Sir Henry Hallett Dale(Nobel laureate, 1936)(Arterial pressure of ananesthetized cat wasmeasured)ThoracolumbarCranialSacralCNS Pre-ganglionic Ganglion Post-ganglionicParasympatheticAchNicotinicAchNicotinicAchNicotinicAchNicotinicAchNicotinicEpiSympatheticSympatheticSympatheticSympathetic (adrenal medulla)Motor (somatic)AchAchMuscarinicMuscarinicNEAdrenergic(α, β)DDopaminergic(D1)AchNicotinicCardiac & smoothmuscles, gland cells,nerve terminalsCardiac & smoothmuscles, gland cells,nerve terminalsSweat glandsRenal vascularsmooth muscleReleased intobloodSkeletal muscleAch = acetylcholine NE = norepinephrineEpi = epinephrineD = dopamineEffectors6Gα•GDPGAPGEFGα•GTPGG--proteins are activated by proteins are activated by muscarinicmuscarinicreceptorsreceptorsβγβγPiDownstreameffectorsPLC-βIon channelsAdenylyl cyclaseKinasesOthersAgonistGs and Gs and GiGiproteins have different functionsproteins have different functionsAgonistβγαsAgonistβγαiACACαsβγαiβγPLCβPI3KγKir (channel)Other effectorsGs = stimulatory G proteinGi = inhibitory G proteinAC = adenylyl cyclase (convert ATP to cAMP)7AgonistMuscarinic acetylcholine receptors (mAChR)AgonistAgonistM1“neural”M2“cardiac”M3“glandular”Gq Gi Gq↑ Inositol phosphates(IP3)↑ Diacyl glycerol (DAG)(IP3)↑ Inositol phosphates(IP3)↑ Diacyl glycerol (DAG)(IP3)↑ Intracellular calcium↓ cAMP↓ Calcium channels↑ K+ conductance↓ K+ conductanceMostly excitatory(decrease of M1 activity inCNS may be a cause ofdementia)Mostly inhibitory(responsible for thevagal inhibition of theheart)Mostly exitatory(stimulation of glandularsecretion, contraction ofvisceral smooth muscle)DepolarizationMuscarinic agonistsDrugReceptor specificityHydrolysis by AchEmAChR nAchRAcetylcholineCarbacholBethanecholMuscarinePilocarpineMethacholine+++ ++++++++ +++( − )+++ + ++++++++++( − ) ( − )( − ) ( − )( − ) ( − )Muscarinic antagonistsAtropine, scopolamine, and pirenzepine (relatively selective for M1 mAChR)8Classification of adrenergic receptors by agonist potencyα -- NE > Epi > Isoβ -- Iso > Epi > NENE = norepinephrineEpi = epinephrineIso = isoproterenolHOHOCH2NHCH3OHCHEpiHOHOCH2NH2OHCHNEHOHOCH2NHOHCHIsoCH(CH3)2AgonistSignaling properties of adrenergic