In guinea pigs,
myoclonus can be induced by
5-hydroxytryptamine (5-HT, serotonin) precursors and synthetic
5-HT receptor agonists, yet the receptor subtype specificity of this behavior is not fully delineated. Guinea pigs were pre-treated with
carbidopa (50 mg) followed by one of eight
5-HT antagonists: (-)-N-tert-butyl-3-[4-(2-methoxyphenyl) piperazin-1-yl]-2-phenyl
propionamide ((-)-WAY 100135) (5-HT1A), N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]-N-(2-pyridyl)-cy clohexancarboxamide (
WAY 100635) (5-HT1A),
methiothepin mesylate (5-HT1/2),
mesulergine hydrochloride (5-HT2A/2C), N[4-methoxy-3-(4-methyl-L-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2 ,4-oxadizol-3-yl) (
GR 127935) (5-HT1D), trans-4-[(3Z)3-(2-dimethylaminoethyl)oxyimino-3(2-fluorop hen yl) propen-1-yl]
phenol, hemifumarate (
SR 46349) (5-HT2),
ondansetron hydrochloride (5-HT3), and [1-[2-[methylsulphonyl)amino]ethyl]-4-piperidinyl]methyl-5-fluoro-2-meth oxy-1H-
indole-3-carboxylate (
GR 125487) (5-HT4). Thirty minutes later, they received
5-hydroxytryptophan (5-HTP) (75 mg/kg, sc) and myoclonic jumping rates were assessed every 10 min for 200 min by a blinded observer. Repeated measures analysis of variance of
drug-induced antagonism of 5-HTP-induced
myoclonus revealed a significant effect for the
5-HT receptor antagonists
methiothepin mesylate, GR127935, and
mesulergine hydrochloride compared to placebo, and each of these drugs inhibited 5-HTP-induced
myoclonus in a dose-dependent fashion. Based on the receptor profiles of the three effective antagonists, 5-HTP-induced
myoclonus is influenced by the 5-HT1/2 receptor systems. The absence of a significant change with any other receptor subtype antagonist suggests that
myoclonus is not related to diffuse activation of central serotonergic mechanisms.