| Stimulation: Stimulation of cholinergic nerves
is achieved either directly or indirectly. Direct acting agents
(agonists) activate the receptor site by mimicking the effects of
acetylcholine. Cholinesterase inhibitors act indirectly by preventing
the enzyme from hydrolyzing (inactivating) acetylcholine at the receptor
site. This inhibition permits the buildup of acetylcholine and results
in more intensive and prolonged activation of the receptor site. The
effects of cholinergic stimulation include: vasodilation of blood
vessels; slower heart rate; constriction of bronchioles and increased
secretion of mucus in the respiratory tract; intestinal cramps;
secretion of salvia; sweat and tears; and constriction of eye pupils.
Direct Acting Cholinergic Agents - Agonists:
Direct acting cholinergic agents act as agonists and initiate
stimulant type responses at the receptor site. Direct stimulation of
acetylcholine receptors is achieved by: Arecholine, Pilocarpine,
Urecholine(Betanechol), Carbachol, Choline, Metacholine, Mushrooms
(Boletus sp., Clitocybe sp. , Inocybe sp.)
Drugs: Urecholine and philocarpine are direct acting drugs.
Urecholine is used to restore parasympathetic tone to smooth muscles of
the intestinal tract and bladder following abdominal surgery.
Pilocarpine is used to constrict pupils and reduce pressure caused by
glaucoma. Pilocarpine contracts the ciliary muscle with causes the iris
to be withdrawn. This action permits drainage of the aqueous humor and
thus relieves the pressure due to a glaucoma condition.
Cholinergic Poison agents which mimic the structure of
acetylcholine include two poisons: muscarine - an alkaloid
present in poisonous mushrooms and nicotine from cigarettes.
Muscarinic effects are those of parasympathetic overactivity and include
bradycardia, pinpoint pupils, sweating, blurred vision, excessive
lacrimation, excessive bronchial secretions, wheezing, dyspnoea,
coughing, vomiting, abdominal cramping, diarrhea, and urinary and fecal
incontinence.
Nicotine: Nicotinic effects are those of sympathetic
overactivity and neuromuscular dysfunction and include tachycardia,
hypertension, dilated pupils, muscle fasciculation and muscle weakness.
Accidental ingestion of these poisons may produce death from heart
failure unless treated with a suitable antidote. Atropine blocks the
receptor site to decrease the stimulant effects produced by the
muscarine type poisons, but has no effect on nicotine receptors. |
