Introduction to the Autonomic Nervous System

1. Introduction to the Autonomic Nervous System Ed Bilsky, Ph.D. Department of Pharmacology University of New England Phone 283-0170, x2707 E-mail: ebilsky@une.edu

2. Autonomic Nervous System • A largely autonomous system that monitors and controls internal body functions to maintain homeostasis and meet the organisms demands • cardiac output • blood volume and pressure • digestive processes • Contains both afferent and efferent components, along with integrating centers • Drugs which modify the function of the autonomic nervous system can be used therapeutically for many disease states

3. Autonomic Nervous System • There are two efferent divisions that act antagonistically to each other • allows for a greater degree of control over various processes than one system would allow • Sympathetic branch (fight or flight) • increased cardiac output • redirection of blood flow from GI system and skin to skeletal muscle • Parasympathetic branch (rest and maintenance) • decreased cardiac output • increased GI motility and secretions

4. Autonomic Nervous System Divisions of the ANS use a two neuron system: Preganglionic neuron: • cell bodies in the spinal cord • nerves terminate in ganglion Postganglionic neuron: • cell bodies in the ganglion • nerves terminate on effector organs including smooth muscle and cardiac muscle Ganglion: aggregation of nerve cells in the peripheral nervous system

5. Autonomic Nervous System Preganglionic Cell Locations Sympathetic: • thoracic spinal cord • lumbar spinal cord Parasympathetic: • cranial nerves (CN III, VII, IX, X) • sacral spinal cord

6. Neurotransmitters of the ANS Two primary neurotransmitters in the ANS: Acetylcholine: • preganglionic cells of the parasympathetic and sympathetic branches • postganglionic cells of the parasympathetic branch • some postganglionic cells of the sympathetic branch Norepinephrine: • most postganglionic cells of the sympathetic branch

7. Neurotransmitters of the ANS cranial parasympathetic nerves visceral effectors Ach Ach visceral effectors NE Ach sympathetic (thoracolumbar) nerves visceral effectors NE Ach visceral effector organs sacral parasympathetic nerves Ach Ach

8. Neuromodulators of the ANS • There are numerous other substances found in cholinergic and noradrenergic neurons, as well as other neurons of the ANS • These substances may modulate the actions of the primary neurotransmitters or have functions of their own Examples: • Substance P • CGRP • serotonin • VIP • CCK

9. Primary Receptors of the ANS Adrenergic Cholinergic 1 2 3 1 2 Muscarinic M1 M2 M3 Nicotinic NN NM

10. Cholinergic Receptors Receptor Primary Locations Main Biochemical Effects M1 sympathetic post-ganglionic neurons, formation of IP3 and DAG --> CNS neurons increased intracellular Ca2+ M2 myocardium, smooth muscle inhibition of adenylyl cyclase open K+ channels M3 vessels (smooth muscle/endothelial), formation of IP3 and DAG --> exocrine glands increased intracellular Ca2+ NN postganglionic neurons increased Na+ conductance --> depolarization of neuron NM neuromuscular junction increased Na+ conductance --> initiation of muscle contraction

11. Adrenergic Receptors Receptor Primary Locations Main Biochemical Effects 1 smooth muscle formation of IP3 and DAG --> increased intracellular Ca2+ 2 presynaptic nerve terminals inhibition of adenylyl cyclase --> platelets, lipocytes, smooth muscle decreased cAMP 1 cardiac muscle, lipocytes, CNS stimulation of adenylyl cyclase --> presynaptic ANS nerve terminals increased cAMP 2 smooth muscle, cardiac muscle stimulation of adenylyl cyclase --> increased cAMP 3 lipocytes stimulation of adenylyl cyclase --> increased cAMP

12. Neurotransmission Four Major Steps: 1. Synthesis and Storage of the neurotransmitter in the presynaptic neuron 2. Release of the neurotransmitter into the synaptic cleft 3. Interaction of the neurotransmitter with receptors on the post-synaptic cell 4. Termination of the synaptic actions of the neurotransmitter

13. Synthesis and Storage • Acetylcholine example: • The precursor choline is transported into cholinergic nerve terminals • hemicholinums can block the transporter --> decreased synthesis of ACh • Once synthesized, acetylcholine is transported into vesicles for storage • vesamicol can block the vesicular transporter, decreasing stores of releasable ACh • Because of the ubiquitous nature of acetylcholine, these drugs are not used in clinical pharmacology

14. Release of Neurotransmitter

15. Release • Acetycholine example: • Botulinum toxins are among the most potent pharmacological agents known • The various botulinum toxins are produced by distinct strains of Clostridium botulinum • The light chain of the protein exerts a metalloprotease effect that cleaves proteins involved in exocytosis • SNAP-25 • syntaxin • VAMP-1 and 2

16. Clinical Correlate • Intramuscular injections of botulinum toxin type A are the most effective treatment for focal dystonia and may be used in a limited form in patients with segmental or generalized dystonia • Treatment is necessary every 3 to 5 months in most patients, and this therapy has been used safely in some patients for more than 15 years • some patients develop resistance to the clinical response, and antibodies to the A toxin may develop • if the dose is limited to less than 300 U per procedure and the treatment is given no more frequently than every 3 months, the risk of immunoresistance is minimized

17.  Effector  Interaction of Neurotransmitters with Receptors Na+ ACh Ligand-gated channel Agonist G-protein regulated Opioid receptor G protein complex

18. Termination of Neurotransmitter Effect Enzymatic breakdown of neurotransmitter:

19. Acetylcholinesterase • Acetylcholinesterase (AChE) is one of only a few enzymes that have obtained near catalytic perfection • the rate of hydrolysis is close to the rate of diffusion to the active site • a single enzyme can hydrolyze 14,000 ACh molecules/second • Blockade of acetylcholinesterase will rapidly increase synaptic levels of acetylcholine • neostigmine-reversible inhibitor • sarin, malathion-irreversible inhibitors

20. Termination of Neurotransmitter Effect Reuptake of neurotransmitter:

21. Reuptake of Catecholamines • Dopamine and norepinephrine are inactivated primarily via reuptake • specific transporters that transport the catecholamines back into the presynaptic terminal • The effects of cocaine and amphetamine are mediated in part through the dopamine transporter

22. Cholinomimetic Drugs Ed Bilsky, Ph.D. Department of Pharmacology University of New England Phone 283-0170, x2707 E-mail: ebilsky@une.edu

23. Drugs that Increase Cholinergic Activity Cholinergic agonists • muscarinic agonists (pilocarpine) • nicotinic agonists (nicotine) Inhibitors of acetylcholinesterase • reversible inhibitors (neostigmine) • irreversible inhibitors (nerve gas, insecticides)

24. Direct Acting Cholinomimetics Structure: • Major differences exist between drugs in this class • The choline esters have quaternary structures that possess positive charges (e.g., bethanechol) • water soluble • Other agents do not have have a charge (e.g., pilocarpine) • There is a strong stereoselective binding requirement for the muscarinic receptor • (S)-bethanechol >> (R)-bethanechol

25. Direct Acting Cholinomimetics Pharmacokinetics: • The quaternary amines are poorly absorbed and poorly distributed into the CNS compared to the tertiary amines • bethanechol versus pilocarpine • Some of these compounds are more resistant to cholinesterases than others • bethanechol >> acetylcholine • Modification of the structure can influence the affinity of the drug for muscarinic and nicotinic receptors • bethanechol versus acetylcholine

26. Direct Acting Cholinomimetics Pharmacodynamics: • Muscarinic receptors are coupled to G-proteins that activate phospholipase C (M1 and M3) or inhibit adenylyl cylase (M2) • increased production of IP3 and DAG, decreased levels of cAMP • These second messengers produce a number of intracellular effects • increase intracellular Ca2+ levels and activation of protein kinase C • opening of K+ channels --> hyperpolarization of the cell • Activation of nicotinic receptors produces an influx of Na+ ions and depolarization of the cell --> action potential

27. Organ System Effects Cardiovascular system: • Primary effects of muscarinic agonists are a decrease in peripheral resistance and changes in heart rate • Direct effects of the heart include: • increased K+ current in atrial muscle, SA and AV nodes • decreased Ca2+ current in cardiac cells • a reduction in hyperpolarization-activated current that underlies diastolic depolarization • net effect is to slow the pace maker cells and decrease atrial contractility • the ventricles are less densely innervated than the atrial tissue

28. Organ System Effects Cardiovascular system (continued): • The direct effects of muscarinic agonists on the heart are usually opposed by reflex sympathetic discharge • elicited by the fall in blood pressure • Muscarinic agonists can produce marked vasodilation • generation of EDRF from endothelial cells (NO main contributor) Respiratory system: • Muscarinic agonists produce smooth muscle contraction and stimulate secretion in the bronchial tree • can aggravate symptoms associated with asthma

29. Organ System Effects Genitourinary tract: • Stimulation of muscarinic receptors increases tone of the detrusor muscle and relaxes the trigone and sphincter muscles of the bladder • promotes voiding of urine • No major effects on uterine contractility

30. Organ System Effects Eye: • muscarinic stimulation leads to contraction of the smooth muscle of the iris sphincter and of the cilliary muscle • responsible for miosis and accomodation, respectively • Both effects promote the outflow of aqueous humor • decreases intraoccular pressure Miscellaneous secretory glands: • muscarinic agonists stimulate the secretory activity of sweat, lacrimal and nasopharyngeal glands

31. Organ System Effects CNS effects: • The CNS contains both muscarinic and nicotinic receptors • Nicotine has important effects on the brainstem and cortex • stimulant type effects, addiction liability • high doses can cause tremor and convulsions • Muscarinic receptors play a role in movement, cognition, learning and memory, and vestibular function • potential therapeutic applications to CNS diseases, though side-effects limit the clinical use of these agents

32. Organ System Effects PNS effects: • Activation of nicotinic receptors produces action potentials in post-ganglionic nerves of the ANS • The activation of both branches of the ANS results in complex effects on the organism • cardiovascular effects are primarily sympathomimetic • GI and genitourinary effects primarily parasympathomimetic Neuromuscular junction: • nicotine receptors initiate muscle action potentials • fasciculations to strong contractions of an entire muscle possible • can produce depolarization blockade

33. Indirect Acting Cholinomimetics Structure: • Three major classes of compounds • simple alcohols bearing quaternary ammonium group • carbamic acid esters of alcohols bearing quaternary or tertiary ammonium groups • organic derivatives of phosphoric acid (organophosphates) Pharmacokinetics: • The quaternary derivatives are poorly absorbed and poorly distributed into the CNS compared to the tertiary amines • physostigmine > neostigmine • Differences in insecticide absorption and metabolism can affect the safety of these products • malathion metabolized quickly in mammals and birds, not insects

34. Indirect Acting Cholinomimetics Pharmacodynamics: • The affinity of the drug to acetycholinesterase determines the duration of action • edrophonium and related quaternary alcohols interact weakly (electrostatic and hydrogen bonds) --> 2-10 min interaction • carbamate esters (e.g., neostigmine) form covalent bonds --> 30 min to 6 hr interactions • organophospahtes can form very strong covalent bonds that are basically irreversible --> hundreds of hours • An aging process can strengthen the organophosphate bonds making treatment of nerve gas poisoning very difficult to manage

35. Organ System Effects Cardiovascular system: • These drugs exert negative chronotropic, inotropic and dromotropic effects on the heart --> decreased CO • Limited effects on the vasculature • Net effect of moderate doses is modest bradycardia and a fall in CO, with only minimal effects on blood pressure • higher doses produce marked bradycardia and hypotension Respiratory, GI and GU systems: • Similar to effects produced by direct acting agents

36. Organ System Effects Neuromuscular junction: • Low (therapeutic) doses prolong and intensify the effects of physiologically released acetylcholine • Higher doses can lead to muscle fibrillation and fasiculations of an entire motor unit

37. Therapeutic Applications: Myasthenia Gravis • Myasthenia gravis is an autoimmune disorder that attacks the nicotinic ACh receptors at the neuromuscular junction • leads to profound muscle weakness • Acetylcholinesterase inhibitors increase the amount of acetylcholine in the neuromuscular junction • neostigmine is frequently used for this disorder • If muscarinic side-effects are prominent, anticholinergics can be administered (e.g., atropine) • tolerance usually occurs to the muscarinic side-effects Why are the direct acting cholinomimetics not used for myasthenia gravis?

38. Therapeutic Applications: Reversal of NMB • By increasing levels of acetylcholine in the NMJ, the compounds are able to facilitate recovery from competitive neuromuscular blockade • restores neuromuscular transmission • Edrophonium has a more rapid onset of action than neostigmine, and shorter duration of action • Neostigmine is preferable to other agents when >90% twitch depression is to be antagonized

39. Therapeutic Applications: Glaucoma • Constriction of the ciliary body promotes aqueous humor outflow --> decreased intraoccular pressure • Direct and indirect cholinomimetics can be used to treat glaucoma • pilocarpine is the most commonly used agent • typically formulated as eye drops

40. Therapeutic Applications: Atonic GI/GU • The smooth muscle of the GI and GU systems can show depressed activity in certain states • post-operative ileus • congenital megacolon • Bethanechol and neostigmine are the most widely used agents • increases secretion and motility in the G.I. tract • can be given orally or by injection These agents can not be used if there is a mechanical obstruction of the GI or urinary tract

41. Therapeutic Applications: Other Uses • Physostigmine is rarely used for reversing the effects of anticholinergic poisoning • has many side-effects of its own that are difficult to control • The use of edrophonium for treating supraventricular tachyarrhythmias has been discontinued • newer agents that act at adenosine receptors and calcium channels have replaced its use in this condition

42. Anticholinergics Neuromuscular receptor antagonists • Tubocurarine (nicotinic antagonist) Ganglionic receptor antagonists • hexamethonium Muscarinic receptor antagonists • atropine and scopolamine (belladonna alkaloids) • pirenzepine

43. Anticholinergics Structure: • Atropine is the prototypic drug in this class • found in Atropa belladonna (deadly nightshade) and Datura stramonium (Jimson Weed) • tertiary amine structure allows passage across the BBB • Other drug classes possess anticholinergic activity by virtue of their similar chemical structures • many antihistamines, antipsychotics and antidepressants • Anticholinergics that are quaternary amines have been developed for limiting CNS effects • ipratropium for asthma • propantheline for GI use

44. Anticholinergics Pharmacokinetics: • The quaternary amines are poorly absorbed from the GI tract and poorly distributed into the CNS compared to the tertiary amines • atropine >> propantheline • Metabolism is drug specific • atropine has a relatively short half-life, with the majority of the drug being eliminated in the urine unchanged, some metabolism in the urine (hydrolysis and conjugation)

45. Anticholinergics Pharmacodynamics: • Atropine produces reversible blockade of muscarinic receptors • very selective for muscarinic receptors • does not differentiate between M1, M2 and M3 receptors • Other anticholinergics possess subtype selective profiles • pirenzepine M1 > M2 > M3

46. Organ System Effects CNS: • Clinical doses of atropine typically produce minimal CNS effects • scopolamine has greater CNS effects (sedation, amnesia) • higher doses of these agents can produce hallucinations • Blockade of muscarinic receptors has been used to treat tremors associated with Parkinson’s disease • newer agents have replaced anticholinergics as a primary treatment, sometimes used as an adjunct • Vestibular disturbances, especially motion sickness, appears to be mediated by CNS muscarinic receptors • scopolamine can be given orally or by transdermal patch

47. Organ System Effects Eye: • Tertiary anticholinergics produced marked mydriasis due to unopposed sympathetic activity • Decreased contraction of the ciliary muscle produces cycloplegia and a loss of accommodation • These effects are useful for certain ophthalmology procedures • contraindicated in patients with glaucoma Cardiovascular effects: • Moderate doses have pronounced effects on the SA node to increase heart rate • low doses can cause bradycardia due to presynaptic muscarinic receptor blockade

48. Organ System Effects Respiratory system: • Blockade of muscarinic receptors in the bronchial tree produces bronchodilation and decreased secretions • Older class of agents used for treating asthma • largely replaced in the treatment of asthma by beta-2 agonists • Ipratropium is sometimes used in asthma and COPD as an inhalational drug • decreased systemic distribution compared to atropine • Anticholinergics can decrease secretions during intubation procedure and during the delivery of volatile anesthetics

49. Organ System Effects GI effects: • Decreases secretions and motility in the GI system • dry mouth and constipation are frequent side-effects • infrequently used for treating peptic ulcer and diarrhea • better agents available that have produce less side-effects • selective M1 blockers are being developed (pirenzepine) • Decreases spasms of the bladder and ureters is useful in treating some inflammatory conditions where incontinence is a problem (M3 antagonists) Sweat glands: • thermoregulatory sweating is inhibited by atropine • sympathetic nervous system effect • Large doses of atropine may increase body temperature in adults • infants and children are much more sensitive to this effect

50. Cholinergic poisoning • A number of insecticides and nerve gasses can produce cholinergic toxicity • Many of the signs and symptoms can be reversed by administering atropine • There are several compounds that can hydrolyze the phosphoryalted acetylcholinesterase and reverse organic phosphate poisoning • need to be administered soon after the exposure • pralidoxine (PAM) “regenerates” the acetycholinesterase • Atropine can be used to treat certain types of mushroom poisoning (Inocybe genus and others)