MEDICAL PHARMACOLOGY – PCOL 425

FALL 2009 – CHOLINERGIC AGONISM AND ANTAGONISM MODULE

MARCELO G. BONINIAssistant Professor College of Medicine

mbonini@uic.edu

312-996-4741

Overview

- Cholinergic actionThe junction, molecular mechanism, sites of action- Muscarinic ReceptorStructure, types, coupling with G-Proteins- Cholinomimetic DrugsStructure, general features, organ effects- Cholinomimetic DrugsTherapeutic uses, toxicology- Muscarinic AntagonismDrugs, therapeutic uses, toxicity

MOVIE

http://www.youtube.com/watch?v=DF04XPBj5uc

Neurons and neurotransmitters – general overview

Generalized Cholinergic Junction

Acetylcholine binding and receptor structural changes

Cholinergic action (molecular mechanism)

Intracellular signaling triggered by acetylcholine in the smooth muscle

Main molecular players: M3, Heterotrimeric protein Gq, PLC/IP3, Ca(2+), MLCK

Cholinergic action (molecular mechanism)

Intracellular signaling triggered by acetylcholine in the Heart

Main molecular players: M2, heterotrimeric G Protein Gi, Adenylyl cyclase

Intracellular signaling triggered by acetylcholine in the endothelium

eNOS

●NO

L-Arg

L-Citruline

Major molecular players: M3, heterotrimeric G Protein Gq, Ca(2+)-CaM, eNOS, NO

Cholinergic action (molecular mechanism)

eNOS Nitric oxide synthase

Cholinergic action (molecular mechanism)acetylcholine mediated endothelium-dependent vasodilation

Start Here

+AChNO probe

- AChNO probe

Cholinergic action (molecular mechanism)

(endothelium present) (endothelium absent)

-

+

Endothelium dependence of Ach-induced vasodilation ofpre-constricted arterial rings

Sites of Cholinergic Activity

-Preganglionic synapses of both sympathetic and parasympathetic ganglia

- Parasympathetic postganglionic neuroeffector junctions

- All somatic motor end plates on skeletal muscles

M2 M4 M5M3M1

Gi Go

Adenylyl cyclasecAMP

Hyperpolarization (heart)Cardiac inhibitionAntagonism of smooth muscle relaxation

RECEPTOR

INTRACELLULARTRANSDUCER

ELECTRICALMECHANICALPHYSIOLOGICALRESPONSES

Gq

Phospholipase CDiacyl-glycerol IP3

DepolarizationSmooth muscle contractionGlandular secretion

Muscarinic receptor types experiments that led to their discovery

M1 - Neurotransmission in Cortex and Ganglia (-/-) mice - abrogation of pilocarpine – induced seizures

M2 - Agonist-mediated bradycardia, tremor, autoinhibition of release in severalbrain regions (-/-) mice - loss of oxytremorine-induced tremors; loss ofagonist-induced bradycardia; diminished hypothermia

M3 - Smooth muscle contraction, gland secreation, pupil dilation, food intakeand possibly weight gain(-/-) mice - loss of agonist-induced bronchoconstriction, higher basal pupil dilation, reduction of agonist-induced salivation

M4 and M5 – Central Nervous System (CNS) roles.

Classes of cholinergic stimulants

Direct-acting

Receptor agonists

Choline estersACETYLCHOLINEBETHANECOL

AlkaloidsPILOCARPINE

Cholinesterase inhibitors

CarbamatesPHYSOSTIGMINENEOSTIGMINEPYRIDOSTIGMINEEDROPHONIUM

PhosphatesISOFLUROPHATEAntidotePRALIDOXIMINE

Indirect-acting

Chemical Structure of Cholinergic agonists

Tertiary amine

Quaternary ammonium

Pilocarpine – source/history

• Chewing pilocarpus caused salivation

Amazon

Experiments performed in Brazil in 1874, isolated in 1875, methacholine and carbachol studies in 1911

Absorption, metabolism, distribution

- Absorption: polarity dependent (poor for ACh, quaternary ammonium), intravenous, subcutaneous and intramuscular for local effects (Ach)

- Metabolism: Highly dependent on the susceptibility to acetylcholinesterase (AChE)

Compound Susceptibility (AChE) Muscarinic Effect

Acetylcholine chloride High (++++) High (limited by AChE) Methacholine chloride Low (+) Highest (++++) Carbachol chloride Negligible Medium (++) Bethanechol chloride Negligible Medium (++)

Organ effects – Eye/Cardiovascular

- Eyes: contraction of ciliary muscle and smooth muscle of the iris sphincter (miosis) – aqueous humor outflow, drainage of the anterior chamber

- Cardiovascular: Bradycardia (possibly preceded by tachycardia), vasodilation (all vascular beds including pulmonary and coronary – M3) and hypotension, reduction of the contraction strength (atrial and ventricular cells, IK+ , ICa2+ diastolic depolarization , NO-inhibitable ATP?), negative chronotropic effect (inhibition of adrenergic activation).

- GI - increases in tone, amplitude of contractions, and peristaltic activity of the stomach and intestines, enhances secretory activity of the gastrointestinal tract.

- Urinary bladder - increase ureteral peristalsis, contract the detrusor muscle of the urinary bladder, increase the maximal voluntary voiding pressure, and decrease the capacity of the bladder.

- Other effects – Increased secretion from all glands that receive parasymphatetic enervation (salivary, lacrimal, tracheobronchial, digestive and exocrine sweat glands)

- IMPORTANT - BROCHOCONSTRICTION

Organ effects – GI/urinary bladder

Therapeutic uses (BETHANECHOL)

• Bethanechol chloride (carbamylmethylcholine chloride; URECHOLINE)

• Stimulant of the smooth muscle of the GI tract and the urinary bladder.

• Postoperative abdominal distension and gastric retention or gastroparesis.

Urinary retention and inadequate emptying of the bladder when organic obstruction is absent:

- postoperative - postpartum urinary retention- certain cases of chronic hypotonic or neurogenic bladder.

-alternative to pilocarpine to promote salivation Xerostomia (dryness of the mouth).

-Sjogren syndrome (immunologic disorder with destruction of theexocrine glands) leading to mucosal dryness

Administration/Precaution/Toxicity• Bethanechol should be administered only by the oral or subcutaneous route for systemic effects;

they also are used locally in the eye.

•Antidote - atropine.

•Epinephrine may be used to overcome severe cardiovascular or bronchoconstrictor responses.

•Among the major contra-indications to the use of the choline esters are asthma, hyperthyroidism, coronary insufficiency, and acid-peptic disease.

• Bronchoconstrictor action could precipitate an asthmatic attack

• Hyperthyroid patients may develop atrial fibrillation.

• Hypotension induced by these agents can severely reduce coronary blood flow, especially if it is already compromised.

• The gastric acid secretion produced by the choline esters can aggravate the symptoms of acid-peptic disease.

POSSIBLE SIDE EFFECTS : sweating (very common), abdominal cramps, a sensation of tightness in the urinary bladder, difficulty in visual accommodation for far vision, headache, and salivation.

Therapeutic use/toxicity (carbachol/methacholine)

• Carbachol usually is not employed for these purposesbecause of its relatively larger component of nicotinicaction at autonomic ganglia.

The unpredictability of the intensity of response hasvirtually eliminated the use of methacholine or othercholinergic agonists as vasodilators and cardiacvagomimetic agents.

• Methacholine chloride (acetyl-b-methylcholine chloride;PROVOCHOLINE) may be administered for diagnosisof bronchial hyperreactivity and asthmatic conditions.

Toxicity/Mycetism

• Exageration of all symptoms of muscarinic agonism• Significance: Higher consumption of wild mushrooms

(culinary)

30-60 minutes, salivation, lacrimation, excessive sweating, nausea, vomitingdiarrhea, bronchospasm, headache, visual disturbances, abdominal colic, bradychardia, hypotension, shock

ALL SYMPTOMS REVERTED BY ATROPINE1 - 2 mg intramuscular

A. muscaria

Mycetism/non muscarinic

• Amanita phalloides – deadly nightcap

• Inhibits mRNA synthesis – 24 h symptom free period followed by liver and

kidney malfunction, death within 4-7 days

A. phalloides A. muscaria

Muscarinic antagonism

Attropa belladona

History/sources

• Atropa belladona - used in the renaissance• Deadly nightshade - used in the middle ages to produce polonged

poisoning

Jimson plant leaves burned in India to treat Asthma (1800) purification of atropine (1831)

Muscarinic Antagonists

ATROPINE

SCOPOLAMINE

Muscarinic Antagonists

ATROPINE

SCOPOLAMINE Attropa belladona

- Atropine and Scopolamine are belladona alkaloids (competitive inhibitors)

-Drugs differ in their CNS effects, scopolamine permeates the blood-brain barrier

-At therapeutic doses atropine has negligible effect upon the CNS, scopolamine even at low doses has prominent CNS effects.

Mechanism of drug action

- Competitively block muscarinic receptors

- Salivary, bronchial, and sweat glands are

most sensitive to atropine

- Smooth muscle and heart are intermediate

in responsiveness- In the eye, causes pupil dilation and difficulty for far

vision accomodation- Relaxation of the GI, slows peristalsis

Effect of muscarinic inhibitor in the eyePupil dilation vs accomodation

Effect of muscarinic inhibition in the heart and salivary glands

- Increases the heart rate after a transient bradychardia at the low dose- Diminishes gland excretory function

Graphic summary of atropine effects

Organ effect – drug reviewAntidotes

• ORGAN DRUG APPLICATIONCNS Benztropine Treat Parkinson’s disease

Scopolamine Prevent/Reduce motion sickness

Eye Atropine Pupil dilation

Bronchi Ipatropium Bronchodilate in Asthma, COPD

GI Methscopolamine Reduce motility/cramps

GU Oxybutinin Treat transient cystitisPostoperative bladder spasms

Toxicity of muscarinic antagonists

• “DRY AS BONE, RED AS A BEET, MAD AS HATTER.”

• Dry is a consequence of decreased sweating, salivation and lacrimation

• Red is a result of reflex peripheral (cutaneous) vasodilation to dissipate heat (hyperthermia)

• Mad is a result of the CNS effects of muscarinic inhibition which can lead to sedation, amnesia (hypersensitivity), or hallucination

Preview

- Indirect cholinergic agonism (Inhibition of AChE)

- Nicotine-acetylcholine agonism / antagonism

- Therapeutic use and toxicology