Cholinergic mechanisms and cataplexy in dogs

Narcolepsy is a disabling neurological disease characterized by excessive daytime somnolence and sudden attacks of partial or complete flaccid paralysis called cataplexy. The disease is known to affect humans as well as dogs. Nineteen dogs diagnosed as narcoleptic were used in this study, which utilized the food-elicited cataplexy test. This test is based on the cataplexy-eliciting effect of food. The results of this study showed that the anticholinesterase physostigmine salicylate (0.05 mg/kg i.v.) and the muscarinic cholinomimetic arecoline hydrochloride (0.15 mg/kg s.c.) significantly increased the amount of cataplexy. Two muscarinic blockers, atropine sulfate (0.1 mg/kg i.v.) and scopolamine hydrobromide (20 μg/kg i.v.), were both effective in significantly reducing the amount of cataplexy. Neostigmine (0.05 mg/kg i.v.), atropine methylnitrate (0.1 mg/kg i.v.), and scopolamine methylnitrate (20 μg/kg i.v.), which do not penetrate the bloodbrain barrier, were ineffective. Nicotine (0.03 mg/kg i.v.) and the nicotinic blocker mecamylamine (0.3 and 1 mg/kg i.v.) were also ineffective. The results of this study suggest that central muscarinic cholinergic receptors are critically involved in the mechanism which produces the motor inhibition of cataplexy.