CHOLINOCEPTOR-ACTIVATING & CHOLINESTERASE-INHIBITING DRUGS CHOLINOMIMETIC AGENTS: CHOLINOCEPTOR-ACTIVATING & CHOLINESTERASE-INHIBITING DRUGS Two types of classification: By their spectrum of action depending on the type of receptor (muscarinic or nicotinic) By their mechanism of action (direct acting or indirectly acting)

CHOLINOMIMETIC AGENTS:

SPECTRUM OF ACTION OF CHOLINOMIMETIC DRUGS Cholinoceptors are either: 1. muscarinic (G protein-linked) or 2. nicotinic (ion channel) Cholinoceptors are members of either G protein-linked (muscarinic) or ion channel (nicotinic) families; Muscarinic receptors: - Contain 7 transmembrane domains whose third cytoplasmic loop is coupled to G proteins - In general, regulate the production of intracellular second messengers

In general, Muscarinic receptors regulate the production of intracellular 2nd messengers

Muscarinic receptors are located on: plasma membranes of cells in the CNS, in organs innervated by parasympathetic nerves (e.g. heart, smooth muscles, glands) on some tissues that are not innervated by these nerves, eg, endothelial cells on those tissues innervated by cholinergic postganglionic sympathetic nerves, e.g. sweat glands, blood vessels in skeletal muscles. Selectivity of muscarinic receptors depends on receptor subtypes and the type of G proteins

Nicotinic receptors are located on: on plasma membranes of parasympathetic and sympathetic postganglionic cells in all autonomic ganglia, on membranes of muscles innervated by somatic motor fibers, and in the central nervous system Nicotinic receptors = part of a transmembrane polypeptide whose subunits form cation-selective ion channels

Postreceptor Mechanism Subtypes and characteristics of cholinoceptors. Receptor Type Other Names Location Structural Features Postreceptor Mechanism M1 M1a Nerves 7 transmembrane segments, G protein-linked IP3, DAG cascade M2 M2a, cardiac M2 Heart, nerves, smooth m. ↓cAMP,activation of K+ channels M3 M2b, glandular M2 Glands, smooth m., endothelium m41   ?CNS ↓ cAMP m51

Subtypes and characteristics of cholinoceptors (cont’d) NM Muscle type, end plate receptor Skeletal muscle neuromuscular junction Pentamer (α2βδγ) Na+, K+ depolarizing ion channel NN Neuronal type, ganglion receptor Postganglionic cell body, dendrites α and β subunits only as α2β2 or α3β3

Selectivity of drug action: Selectivity of action based on several factors: muscarinic receptors Vs. nicotinic receptors; Some drugs stimulate nicotinic receptors at neuromuscular junctions preferentially/ less effect on nicotinic receptors in ganglia; Pharmacokinetic selectivity: using appropriate routes of administration (e.g topical muscarinic stimulants to modulate ocular function) Unselective cholinoceptor stimulants in sufficient dosage can produce very diffuse and marked alterations in organ system function because acetycholine has multiple sites of action where it initiates both excitatory and inhibitory effects. Fortunately, drugs available have a degree of selectivity, so that desired effects can be achieved while avoiding or minimizing adverse effects. Selectivity of action based on several factors, some drugs stimulate muscarinic receptors or nicotinic receptors; Some drugs stimulate nicotinic receptors at neuromuscular junctions preferentially and have less effect on nicotinic receptors in ganglia; Organ selectivity can also be achieved by using appropriate routes of administration (e.g topical muscarinic stimulants to modulate ocular function)

MODE OF ACTION OF CHOLINOMIMETIC DRUGS Direct-acting agents directly bind to and activate cholinoceptors. Indirect-acting agents: inhibit the action of AChE → the concentration of endogenous ACh ↑ in the synaptic clefts (amplifiers) Neostigmine (1) activates neuromuscular nicotinic cholinoceptors directly in addition to (2) blocking cholinesterase

BASIC PHARMACOLOGY OF THE DIRECT-ACTING CHOLINOCEPTOR STIMULANTS

The direct-acting cholinomimetic drugs can be divided into: - esters of choline (including ACh) - alkaloids ( muscarine and nicotine). A few of these drugs are highly selective for the muscarinic or for the nicotinic receptor. Many have effects on both receptors (Ach)

Chemistry: esters of choline are permanently charged and relatively insoluble in lipids - contain the quaternary ammonium group: 1) acetylcholine 2) methacholine 3) carbachol 4) bethanecol

Longer duration of action

Pharmacokinetics 1.The choline esters: Poorly absorbed & poorly distributed into the CNS (hydrophilic); All hydrolyzed in the GIT (and less active by the oral route); Differ markedly in susceptibility to hydrolysis by cholinesteras, Ach: (5-20 sec), Presence of the β-methyl group  ↓the potency of these drugs at the nicotinic receptor Ach: very rapidly hydrolyzed; a large I.V.bolus injection has a brief effect, (5-20 sec), whereas I.M. & S.C. injections produce only local effects Methacholine: 3 times more resistant to hydrolysis & produces systemic effects even when given SC. The carbamic acid esters, carbachol and bethanechol: extremely resistant to hydrolysis by cholinesterase & have longer durations of action. Presence of the β-methyl group (methacholine, bethanechol) ↓the potency of these drugs at the nicotinic receptor

Susceptibility to Cholinesterase Properties of choline esters. Choline Ester Susceptibility to Cholinesterase Muscarinic Action Nicotinic Action Acetylcholine chloride ++++ +++ Methacholine chloride + None Carbachol chloride Negligible ++ Bethanechol chloride  

Cholinomimetic alkaloids & synthetic analogs: 1) muscarine 2) nicotine 3) pilocarpine 4) lobeline

Structures of cholinomimetic alkaloids TOXIC Tertiary amines: better absorption and distribution well absorbed from most sites of administration; nicotine, a liquid, sufficiently lipid-soluble to be absorbed across the skin; muscarine, a quaternary amine, less completely absorbed from the GIT than the tertiary amines but is nevertheless toxic when ingested, e.g. in certain mushrooms; excretion of these amines is chiefly by the kidneys oxotremorine , an extremely potent synthetic muscarinic agonist, has been used as a research tool. It is well distributed into the CNS. lobeline, a plant derivative with less potency than nicotine but with a similar spectrum of action Structures of cholinomimetic alkaloids

Mechanism of Action: Activation of the parasympathetic nervous system modifies organ function by two major mechanisms: ACh released can activate muscarinic receptors on effector organs; ACh released can interact with muscarinic receptors on nerve terminals to inhibit the release of their neurotransmitter 2. By this mechanism, acetylcholine release and circulating muscarinic agonists indirectly alter organ function by modulating the effects of the parasympathetic and sympathetic nervous systems and perhaps nonadrenergic, noncholinergic (NANC) systems.

Result of activation of muscarinic receptors: activation of the IP3, DAG cascade. DAG → opening of smooth muscle Ca channels; IP3 → release of Ca from endoplasmic and sarcoplasmic reticulum. ↑ cGMP. ↑ K+ flux across cardiac cell membranes and ↓ it in ganglion and smooth muscle cells (by binding of an activated G protein directly to the channel). in some tissues (eg, heart, intestine) ↓adenylyl cyclase activity.

The mechanism of nicotinic receptor activation: Nicotinic receptors NN and NM are ion channels binding an agonist → channel opening → Na+ and K+ ions diffuse rapidly down their concentration gradients → depolarization of the postsynaptic membrane. In case of prolonged agonist occupancy of the receptor, the effector response is abolished; ie, the postganglionic neuron stops firing-ganglionic effect-), and the skeletal muscle cell relaxes-neuromuscular endplate effect- “depolarizing blockade” Furthermore, continued presence of the nicotinic agonist prevents electrical recovery of the postjunctional membrane; Thus, a state of “depolarizing blockade” is induced that is refractory to reversal by other agonists This effect can be exploited for producing muscle paralysis.

Effects of direct-acting cholinoceptor stimulants Effects of direct-acting cholinoceptor stimulants. Only the direct effects are indicated; homeostatic responses to these direct actions may be important. Organ Response Eye   Sphincter muscle of iris Contraction (miosis) Ciliary muscle Contraction for near vision (accomodation) Heart Sinoatrial node Decrease in rate (negative chronotropy) Atria Decrease in contractile strength (negative inotropy). Decrease in refractory period Atrioventricular node Decrease in conduction velocity (negative dromotropy). Increase in refractory period Ventricles Small decrease in contractile strength Blood vessels Arteries Dilation (via EDRF). Constriction (high-dose direct effect) Veins

Effects of direct-acting cholinoceptor stimulants (cont’d)   Lung Bronchial muscle Contraction (bronchoconstriction) Bronchial glands Stimulation Gastrointestinal tract Motility Increase Sphincters Relaxation Secretion Urinary bladder Detrusor Contraction Trigone and sphincter Glands Sweat, salivary, lacrimal, nasopharyngeal The trigone (a.k.a. vesical trigone) is a smooth triangular region of the internal urinary bladder formed by the two ureteral orifices and the internal urethral orifice. The area is very sensitive to expansion and once stretched to a certain degree, the urinary bladder signals the brain of its need to empty.

Organ System Effects 1. Eye- contraction of iris sphincter (miosis) contraction of the ciliary muscle (accommodation). the iris is pulled away from the angle of the anterior chamber→the trabecular meshwork at the base of the ciliary muscle is opened up → ↑outflow of aqueous humor into the canal of Schlemm (drains the anterior chamber)→ ↓ intraocular pressure (IOP)

Organ System Effects (cont’d) The net effect on the heart rate depends on local concentrations of the agonist in the heart and in the vessels and on the level of reflex responsiveness 2. Cardiovascular system (heart) Primary effects of muscarinic agonists: (1) ↓in peripheral vascular resistance & (2) changes in HR (all mediated by M2) direct effects on heart rate are modified by homeostatic reflexes. - I.V. infusions of of min effective dose of Ach (20-50 g/min) → vasodilation → ↓BP → reflex ↑in HR - Larger doses of ACh → bradycardia and ↓ conduction velocity through the AV node in addition to ↓BP

Organ System Effects 2. Cardiovascular system (the vessels) In the intact organism, muscarinic agonists produce marked vasodilation (EDRF (largely NO)  ↑ c-GMP) Isolated blood vessels Ach contraction In the intact organism, muscarinic agonists produce marked vasodilation. (needs intact endothelium for release of NO (endothelium-derived relaxing factor, or EDRF) → smooth muscle relaxation via activation of guanylyl cyclase and ↑ cGMP in smooth muscle;

Organ System Effects Exception: Pilocarpine if given I.V. may produce hypertension after a brief initial hypotensive response. The longer-lasting ↑ BP is due to sympathetic ganglionic discharge caused by activation of M1 receptors in the postganglionic cell membrane, which close K+ channels and elicit slow excitatory (depolarizing) postsynaptic potentials smooth muscle contraction ↑ BP This effect can be blocked by atropine (antimuscarinic agent) The cardiovascular effects of all the choline esters are similar to those of acetylcholine—the main difference being in their potency and duration of action. Because of the resistance of methacholine, carbachol, and bethanechol to acetylcholinesterase, lower doses given intravenously are sufficient to produce effects similar to those of acetylcholine, and the duration of action of these synthetic choline esters is longer. The cardiovascular effects of most of the cholinomimetic natural alkaloids and the synthetic analogs are also generally similar to those of acetylcholine.

Organ System Effects 5. Genitourinary tract stimulation of detrusor muscle and relaxation of trigone and sphincter muscles of the bladder→↑voiding. human uterus is not notably sensitive to muscarinic agonists. 6. Miscellaneous secretory glands: muscarinic agonists ↑secretion of thermoregulatory sweat, lacrimal, and nasopharyngeal glands

Organ System Effects 7. CNS contains both muscarinic and nicotinic receptors, Brain: mainly muscarinic sites Spinal cord: mainly nicotinic sites. However, nicotine has an effect on brain stem and cortex (what is it?) in spite of that, nicotine and lobeline have important effects on the brain stem and cortex.

Nicotine effects on CNS: The mild alerting action of nicotine absorbed from inhaled tobacco is best known of nicotine effects on brainstem and cortex In higher concentrations → nicotine induces tremor, emesis, and stimulation of the respiratory center At still higher concentrations→ convulsions and fatal coma

Organ System Effects (cont’d) 8. Peripheral nervous system The autonomic ganglia are important sites of nicotinic synaptic action Nicotinic agonists cause marked activation of these nicotinic receptors and initiate AP in postganglionic neurons Nicotine itself has greater affinity for NN than for NM The action is the same on both parasympathetic and sympathetic ganglia

Organ System Effects (cont’d) 8. Peripheral nervous system In cardiovascular system, the effects of nicotine are chiefly sympathomimetic: IV injection of nicotine →↑BP & sympathetic tachycardia may alternate with a vagally mediated bradycardia. In the GI and urinary tracts, the effects are largely parasympathomimetic: nausea, vomiting, diarrhea, voiding of urine Prolonged exposure: depolarizing blockade of ganglia

Organ System Effects 9. Neuromuscular nicotinic agonist applied directly → immediate depolarization of the neuromuscular endplate (  permeability Na+ & K+)→ contraction of muscle; depolarizing nicotinic agents that are not rapidly hydrolyzed (eg, nicotine): rapid development of depolarization blockade; transmission blockade persists even when the membrane has repolarized (flaccid paralysis) - flaccid: soft, lacking rigidity