Autonomic Pharmacology: Cholinergic Drugs :Autonomic Pharmacology: Cholinergic Drugs Presenter: Marc Imhotep Cray, M.D. Professor Pharmacology Recommended Reading: Cholinergic Drugs Tutorial Worth Visiting: Cholinergic ANS Clinical: E-Medicine Articles Myasthenia Gravis


Online Reference Resource :10/17/2009 2 Online Reference Resource IVMS Online Textbook Series BRS Pharmacology, 4th Edition Gary C. Rosenfeld PhD;David S. Loose PhD Full Text Online Student Resources Password Protected, for Enrolled Students Only


From IAU Online Textbook SeriesPharm. Book :10/17/2009 3 From IAU Online Textbook SeriesPharm. Book


Cholinergic Biosynthesis :10/17/2009 4 Cholinergic Biosynthesis Acetylcoline is formed from two precursors: choline: which is derived from dietary and intraneuronal sources acetyl coenzyme: which is made from glucose in the mitochondria of neurons Acetylcholine is synthesized from choline and acetyl-CoA by the enzyme choline acetyl transferase (ChAT) to form acetylcholine, which is immediately stored in small vesicular compartments closely attached to the cytoplasmic side of presynaptic membranes. ChAT is a selective marker for cholinergic neurons


Cholinergic Biosynthesis :10/17/2009 5 Cholinergic Biosynthesis 1) Synthesis of acetylcholine (ACh) from acetyl CoA and choline 2) Storage of ACh in synaptic vesicles 3) Release of ACh ( fusion of synaptic vesicle with presysnaptic membrane and release of ACh into the synapse) 4) Action of ACh by binding to and activating receptors (nicotinic in autonomic ganglia and neuromuscular junction and, muscarinic in many sites) 5) Inactivation by enzymatic breakdown of ACh by acetylcholinesterase (AChE) located in the synapse. ACh is degraded in the synaptic cleft by acetylcholinesterase to choline and acetate


Cholinergic Agents-Direct Acting and Indirect Acting :10/17/2009 6 Cholinergic Agents-Direct Acting and Indirect Acting Choline Esters Acetylcholine Bethanechol (Urecholine) Carbachol Methacholine (Provocholine) Alkaloids Muscarine Pilocarpine (Pilocar) There are three main types of cholinesterase: Short-acting: edrophonium medium-acting: neostigmine (2-4h), pyridostigmine (3-6h) physostigmine irreversible: organophosphates, dyflos, ecothiopate Agents-Direct Acting Indirect Acting


Spectrum of Action of Choline Esters :10/17/2009 7 Spectrum of Action of Choline Esters Location of cholinergic synapses mainly determine the spectrum of action of acetycholine and choline esters Cholinergic Synaptic Sites autonomic effector sites: innervated by post-ganglionic parasympathetic fibers some CNS synapses autonomic ganglia and the adrenal medulla skeletal muscle motor endplates (motor nerves)


Spectrum of Action of Choline Esters(2) :10/17/2009 8 Spectrum of Action of Choline Esters(2) Cholinergic influences are prominent in many organ systems:


Spectrum of Action of Choline Esters(3) :10/17/2009 9 Spectrum of Action of Choline Esters(3) Cholinergic Receptors: Cholinergic refers to responses in various systems to the natural transmitter molecule Acetycholine (ACh)  If one looks at a set of responses where ACh is the normal transmitter, observation has shown that thosesame responses are differently sensitive to the extrinisic molecules Nicotine and Muscarine  Nicotine comes from tobacco, Muscarine comes from certain mushrooms See: NS The Reception and Transmission of Extracellular Information Receptors-A Brief Note


Spectrum of Action of Choline Esters(4) :10/17/2009 10 Spectrum of Action of Choline Esters(4) Based on the different sensitivities shown above, Cholinergic receptors are subclassified into two categories,Nicotinic and Muscarinic, named for the extrinsic compounds that stimulate only that category.


Spectrum of Action of Choline Esters(5) :10/17/2009 11 Spectrum of Action of Choline Esters(5) Nicotinic Receptors Stimulated by ACh and nicotine, not stimulated by muscarine. Found at all ganglionic synapses. Also found at neuromuscular junctions Blocked by hexamethonium.


Spectrum of Action of Choline Esters(6) Nicotinic Receptors :10/17/2009 12 Spectrum of Action of Choline Esters(6) Nicotinic Receptors The physiological responses to stimulation and block are complex since both sympathetic and parasympathetic systems are affected The final response of any one organ system depends on which system has a stronger tonic influence EXAMPLE: Under normal circumstances, the heart receives more parasympathetic influence than sympathetic Ganglionic blockade would lower parasympathetic influence more than sympathetic, and thus heart rate would increase


Spectrum of Action of Choline Esters(6) Muscarinic Receptors :10/17/2009 13 Spectrum of Action of Choline Esters(6) Muscarinic Receptors Stimulated by ACh and muscarine, not stimulated by nicotine Found at target organs when ACh is released by post-ganglionic neurons (all of parasympathetic, and some sympathetic) Stimulated selectively by Muscarine and Bethanechol etc. Blocked by Atropine


Spectrum of Action of Choline Esters(7) Muscarinic Receptors :10/17/2009 14 Spectrum of Action of Choline Esters(7) Muscarinic Receptors Stimulation causes: Increased sweating Decreased heart rate Decreased blood pressure due to decreased cardiac output Bronchoconstriction and increased bronchosecretion. Contraction of the pupils, and contraction of ciliary body for near vision Tearing and salivation Increased motility and secretions of the GI system. Urination and defecation Engorgement of genitalia


Cholinergic Receptors: Subtypes, Tissues, Responses and Molecular Mechanisms :10/17/2009 15 Cholinergic Receptors: Subtypes, Tissues, Responses and Molecular Mechanisms Muscarinic Receptor Coupling Mechanisms Five types of cholinergic receptors have been identified by molecular cloning methods. The five muscarinic receptor subtypes, M1 - M5, are associated with specific anatomical sites For example: M1 -ganglia; secretory glands M2 - myocardium, smooth muscle M3 , M4 :smooth muscle, secretory glands


Cholinergic Receptors: Subtypes, Tissues, Responses and Molecular Mechanisms :10/17/2009 16 Cholinergic Receptors: Subtypes, Tissues, Responses and Molecular Mechanisms Nicotinic Muscle Receptor


Cholinergic Receptors: Subtypes, Tissues, Responses and Molecular Mechanisms(2) :10/17/2009 17 Cholinergic Receptors: Subtypes, Tissues, Responses and Molecular Mechanisms(2) Nicotinic Neuronal Receptor


Cholinergic Receptors: Subtypes, Tissues, Responses and Molecular Mechanisms(3) :10/17/2009 18 Cholinergic Receptors: Subtypes, Tissues, Responses and Molecular Mechanisms(3) Muscarinic Type M1


Cholinergic Receptors: Subtypes, Tissues, Responses and Molecular Mechanisms(4) :10/17/2009 19 Cholinergic Receptors: Subtypes, Tissues, Responses and Molecular Mechanisms(4) Muscarinic Type M2


Signal Transduction: Comparison of Muscarinic and Nicotinc Receptors :10/17/2009 20 Signal Transduction: Comparison of Muscarinic and Nicotinc Receptors Nicotinic Receptors  Ligand-gated ion channels  Agonist effects blocked by tubocurarine  Receptor activation results in: rapid increases of Na+ and Ca2+ conductance deplorization excitation  Subtypes based on differing subunit composition: Muscle and Neuronal Classification Discussed Above


Signal Transduction: Comparison of Muscarinic and Nicotinc Receptors :10/17/2009 21 Signal Transduction: Comparison of Muscarinic and Nicotinc Receptors Muscarinic Receptors  G-protein coupled receptor system  Slower responses  Agonist effects blocked by atropine  At least five receptor subtypes have been described by molecular cloning


Muscarinic Receptors: Second Messenger Systems :10/17/2009 22 Muscarinic Receptors: Second Messenger Systems Activation of IP3, DAG cascade DAG may activate smooth muscle Ca2+ channels IP3 releases Ca2+ from endoplasmic and sarcoplasmic reticulum Increase in cGMP Increase in intracellular K+ by cGMP-K+ channel binding inhibition of adenylyl cyclase activity (heart)


Muscarinic Receptors: Second Messenger Systems(2) :10/17/2009 23 Muscarinic Receptors: Second Messenger Systems(2)


Direct vs. Indirect-Acting Cholinomimetics :10/17/2009 24 Direct vs. Indirect-Acting Cholinomimetics A direct-acting cholinomimetic drug produces its pharmacological effect by receptor activation An indirect-acting drug inhibits acetylcholinesterase, thereby increasing endogenous acetylcholine levels, resulting in increased cholinergic response.


Pharmacological Effects of Cholinomimetics :10/17/2009 25 Pharmacological Effects of Cholinomimetics 1)Vasodilation   This effect is mediated by muscarinic receptor activation and is especially prominent in the salivary gland and intestines


Pharmacological Effects of Cholinomimetics(2) :10/17/2009 26 Pharmacological Effects of Cholinomimetics(2) Vasodilation cont.   The vascular response is due to endothelial cell nitric oxide (NO) release following agonist interactions with endothelial muscarinic receptor Increased NO activates guanylate cyclase which increases cyclic GMP concentrations


Pharmacological Effects of Cholinomimetics(3) :10/17/2009 27 Pharmacological Effects of Cholinomimetics(3) Vasodilation cont.  Subsequent activation of a Ca2+ ion pump reduces intracellular Ca2+  Reduction in intracellular Ca2+ causes vascular smooth muscle relaxation  Ca2+ complexes with calmodulin activating light-chain myosin kinase  Increased cGMP promotes dephosphorylation of myosin light-chains.  Smooth-muscle myosin must be phosphorylated in order to interact with actin and cause muscle contraction.


Nitric Oxide (NO) and Vasodilitation :10/17/2009 28 Nitric Oxide (NO) and Vasodilitation Schematic below from: http://www.nature.com/nature/journal/v396/n6708/fig_tab/396213a0_F1.html


Pharmacological Effects of Cholinomimetics(4) :10/17/2009 29 Pharmacological Effects of Cholinomimetics(4) 2)Negative chronotropic effect (Decrease in heart rate)  Decreases phase 4 (diastolic depolarization) As a result, it takes longer for the membrane potential to reach threshold.   Mediated by M2 muscarinic receptors


Pharmacological Effects of Cholinomimetics(5) :10/17/2009 30 Pharmacological Effects of Cholinomimetics(5) 3) Decreased SA nodal and AV nodal conduction velocity Excessive vagal tone may induce bradyarrhythmias including partial or total heart block (impulses cannot pass through the AV node to drive the ventricular rate; in this case, the idioventricular or intrinsic ventricular rate must maintain adequate cardiac output) Transmission through the AV node is especially dependent on Ca2+ currents.  ACh decreases calcium currents in the atrioventricular node


Pharmacological Effects of Cholinomimetics(6) :10/17/2009 31 Pharmacological Effects of Cholinomimetics(6) 4) Negative inotropism (decreased myocardial contractility) more prominent in atrial than ventricular tissue. due to a decrease in Ca2+ inward current in the ventricle, adrenergic tone dominates; at higher levels of sympathetic tone, a reduction in contractility due to muscarinic stimulation is noted. Muscarinic stimulation reduces the response to norepinephrine by opposing increases in cAMP in addition to reducing norepinephrine release from adrenergic terminals


Clinical Uses :10/17/2009 32 Clinical Uses Gastrointestinal & Genitourinary Bethanechol (Urecholine)   GI smooth muscle stimulant postoperative abdominal distention paralytic ileus esophageal reflux; promotes increased esophageal motility (other drugs are more effective, e.g. dopamine antagonist (metoclopramide) or serotonin agonists (cisapride)


Clinical Uses(2) :10/17/2009 33 Clinical Uses(2) Urinary bladder stimulant post-operative; post-partum urinary retention alternative to pilocarpine to treat diminished salivation secondary e.g. to radiation Carbachol not used due to more prominent nicotinic receptor activation Methacholine used for diagnostic purposes. testing for bronchial hyperreactivity and asthma


Clinical Uses(3) :10/17/2009 34 Clinical Uses(3) Opthalmological Uses  Acetylcholine and Carbachol may be used for intraocular use as a miotic in surgery Carbachol may be used also in treatment of glaucoma.  Pilocarpine is used in management of glaucoma and has become the standard initial drug for treating the open-angle form. Sequential adminstration of atropine (mydriatic) and pilocarpine (miotic) is used to break iris-lens adhesions.


Adverse Effects: Muscarinic Agonists :10/17/2009 35 Adverse Effects: Muscarinic Agonists Adverse Effects: Muscarinic Agonists  salivation  diaphoresis  colic  GI hyperactivity  headache  loss of accommodation


Major contraindication to the use of muscarinic agonists :10/17/2009 36 Major contraindication to the use of muscarinic agonists Asthma: Choline esters (muscarinic agonists) can produce bronchoconstriction. In the predisposed patient, an asthmatic attack may be induced. Hyperthyroidism: Choline esters (muscarinic agonists) can induce atrial fibrillation in hyperthyroid patients. Peptic ulcer: Choline esters (muscarinic agonists), by increasing gastric acid secretion, may exacerbate ulcer symptoms. Coronary vascular disease:  Choline esters (muscarinic agonists), as a result of their hypotensive effects, can further compromise coronary blood flow.


Indirect-acting Cholinomimetic Drugs :10/17/2009 37 Indirect-acting Cholinomimetic Drugs Acetylcholinesterase Inhibitors There are three classes of anticholinesterase agents Reversible, Short-Acting Anticholinesterases Carbamylating Agents: Intermediate-Duration Acetylcholinesterase Inhibitors Phosphorylating Agents: Long-Duration Acetylcholinesterase Inhibitors


Reversible, Short-Acting Anticholinesterases :10/17/2009 38 Reversible, Short-Acting Anticholinesterases 1) edrophonium (Tensilon) and 2) tacrine (Cognex) , associate with the choline binding domain The short duration of edrophonium (Tensilon) action is due to its binding reversibility and rapid renal clearance. Tacrine (Cognex), being more lipophillic, has a longer duration.


Carbamylating Agents: Intermediate-Duration Acetylcholinesterase Inhibitors :10/17/2009 39 Carbamylating Agents: Intermediate-Duration Acetylcholinesterase Inhibitors Physostigmine Neostigmine are acetylcholinesterase inhibitors that form a moderately stable carbamyl-enzyme derivative The carbamyl-ester linkage is hydrolyzed by the esterase, but much more slowly compared to acetylcholine. As a result, enzyme inhibition by these drugs last about 3 - 4 h (t ½ = 15 - 30 min). Neostigmine possesses a quaternary nitrogen and thus has a permanent positive charge By contrast, physostigmine is a tertiary amine


Phosphorylating Agents: Long-Duration Acetylcholinesterase Inhibitors :10/17/2009 40 Phosphorylating Agents: Long-Duration Acetylcholinesterase Inhibitors Organophosphate acetylcholinesterase inhibitors, such as diisopropyl fluorophosphate (DFP) form stable phosphorylated serine derivatives. For DFP the enzyme effectively does not regenerate following inhibition.


Phosphorylating Agents: Long-Duration Acetylcholinesterase Inhibitors(2) :10/17/2009 41 Phosphorylating Agents: Long-Duration Acetylcholinesterase Inhibitors(2) Furthermore, in the case of DFP, the loss, termed "aging", of an isopropyl group, further stabilizes the phosphylated enzyme The application of the terms "reversible" and "irreversible" depends on the duration of enzyme inhibition rather than strictly based on mechanism


Organophosphate poisoning :10/17/2009 42 Organophosphate poisoning Parathion Parathion, a low volatility and aqueous-stable, organophosphate is used as an agriculural insecticide. Parathion is converted to paraoxon by mixed function oxidases. Both the parent compound and its metabolite are effective acetylcholinesterase inhibitors (P=S to P=O). Parathion probably is the most common cause of accidental organophosphate poisoning and death The phosphothioate structure is present in other common insecticides: dimpylate, fenthion, and chlorpyrifos.


Tx of Organophosphate poisoning-Pralidoxine :10/17/2009 43 Tx of Organophosphate poisoning-Pralidoxine Pralidoxine is a cholinesterase activator It is used as an antidote to organophosphates poisoning Unfortunately, pralidoxine does not cross the blood brain barrier to treat the central effects of organophosphate poisoning. It has to be given very early after poisoning as within a few hours the phosphorylated enzyme undergoes a change (aging) that renders it no longer susceptible to reactivation


Clinical applications of anticholinesterases :10/17/2009 44 Clinical applications of anticholinesterases They are also used in cases of overdose with either the muscarinic antagonist, atropine, or muscle relaxants (nicotinic antagonists) Pralidoxine is a cholinesterase activator. organophosphates poisoning


Opthalmological Uses of Anticholinesterase Drugs :10/17/2009 45 Opthalmological Uses of Anticholinesterase Drugs When applied to the conjunctiva, acetylcholinesterase inhibitors produce: constriction of the pupillary sphincter muscle (miosis) contraction of the ciliary muscle (paralysis of accommodation or loss of far vision). Loss of accommodation disappears first, while the miotic effect is longer lasting. During miosis, elevated intraocular pressure (glaucoma) declines due to enhanced flow of aqueous humor.  In glaucoma, elevation of intraocular pressure can cause damage to the optic disc and blindness.


Gastrointestinal and Urinary Bladder :10/17/2009 46 Gastrointestinal and Urinary Bladder Neostigmine is the anticholinesterase agent of choice for treatment of paralytic ileus or urinary bladder atony. Direct acting cholinomimetic drugs are also useful.


Myasthenia Gravis See Clinical: E-Medicine ArticleMyasthenia Gravis :10/17/2009 47 Myasthenia Gravis See Clinical: E-Medicine ArticleMyasthenia Gravis Myasthenia Gravis appears to be caused by the binding of anti-nicotinic receptor antibodies to the nicotinic cholinergic receptor. Binding studies using snake alpha-neurotoxins determined a 70% to 90% reduction of nicotinic receptors per motor endplate in myasthenic patients


Myasthenia Gravis(2) :10/17/2009 48 Myasthenia Gravis(2) Receptor number is reduced by: increased receptor turnover (rapid endocytosis) blockade of the receptor binding domain antibody damage of postsynaptic muscle membrane


Myasthenia Gravis(3) :10/17/2009 49 Myasthenia Gravis(3) Anticholinesterase, edrophonium (Tensilon), is useful in differential diagnosis for myasthenia gravis. In this use, edrophonium (Tensilon) with its rapid onset (30 s) and short duration (5 min) may cause an increase in muscle strength.


Myasthenia Gravis(4) :10/17/2009 50 Myasthenia Gravis(4) This change is due to the transient increase in acetylcholine concentration at the end plate. Edrophonium (Tensilon) may also be used to differentiate between muscle weakness due to excessive acetylcholine (cholinergic crisis) and inadequate drug dosing. Anticholinesterase drugs provide


Antimuscarinic Effects on Organ Systems :10/17/2009 51 Antimuscarinic Effects on Organ Systems Central Nervous System Effects of Antimuscarinic Agents In normal doses, atropine produces little CNS effect. In toxic doses, CNS excitation results in restlessness, hallucinations, and disorientation. At very high doses, atropine can lead to CNS depression which causes circulatory and respiratory collapse. By contrast, scopolamine at normal therapeutic doses causes CNS depression, including drowsiness, fatigue and amnesia.


Antimuscarinic Effects on Organ Systems :10/17/2009 52 Antimuscarinic Effects on Organ Systems Central Nervous System Effects of Antimuscarinic Agents cont. Scopolamine also may produce euphoria, a basis for some abuse potential. Scopolamine may exhibit more CNS activity than atropine because scopolamine crosses the blood brain barrier more readily. Scopolamine (transdermal) is effective in preventing motion sickness. Antimuscarinics are used clinically as preanesthetic medication to reduce vagal effects secondary to visceral manipulation during surgery. Antimuscarinics with L-DOPA are used in Parkinson's disease. Extrapyramidal effects induced by some antipsychotic drugs may be treated with antimuscarinic agents.


Antimuscarinic Effects on Organ Systems :10/17/2009 53 Antimuscarinic Effects on Organ Systems Autonomic Ganglia and Autonomic Nerve Terminals The primary cholinergic receptor class at autonomic ganglia is nicotinic; however, muscarinic M1-cholinergic receptors are also present. Muscarinic M1-ganglionic cholinergic receptor activation produce a slow EPSP that may have a modulatory role. Muscarinic receptors are also located at adrenergic and cholinergic presynaptic sites where their activation reduces transmitter release.  Blockade of these presynaptic receptors increase transmitter release.


Antimuscarinic Effects on Organ Systems :10/17/2009 54 Opthalmological Muscarinic receptor antagonists block parasympathetic responses of the ciliary muscle and iris sphincter muscle, resulting in paralysis of accommodation (cycloplegia) and mydriasis (pupillary dilation). Mydriasis results in photophobia, whereas cycloplegia fixes the lens for far vision only (near objects appear blurred). Antimuscarinic Effects on Organ Systems


Antimuscarinic Effects on Organ Systems :10/17/2009 55 Opthalmological cont. Systemic atropine at usual doses does not produce significant ophthalmic effect. By contrast, systemic scopolamine results in both mydriasis and cycloplegia. Note that sympathomimetic-induced mydriasis occurs without loss of accommodation.  Atropine-like drugs can increase intraocular pressure, sometimes dangerously, in patients with narrow-angle glaucoma. Increases in intraocular pressure is not typical in wide-angle glaucoma. Antimuscarinic Effects on Organ Systems


Antimuscarinic Effects on Organ Systems :10/17/2009 56 Antimuscarinic Effects on Organ Systems Muscarinic Type M2