title: Tryptamine-induced myoclonus in guinea-pigs pretreated with a monoamine oxidase inhibitor indicates pre- and post-synaptic actions of tryptamine upon central indoleamine systems.
abstract: Tryptamine (1-320 mg/kg) evoked only slight muscle jerking in naive guinea-pigs but, in animals pretreated with pargyline (75 mg/kg; 1 hr previously), tryptamine induced a dose-dependent (6-160 mg/kg) myoclonus. The myoclonus induced by tryptamine (40 mg/kg) plus pargyline (75 mg/kg) was differentially inhibited by the indoleamine receptor antagonists, methergoline (5 mg/kg) which was more potent than methysergide (10 mg/kg), mianserin (10 mg/kg) which was more potent that cyproheptadine (10 mg/kg) and propranolol (20 mg/kg) which was more potent than cinanserin (10 mg/kg). This rank order of potency differed from that observed for the order of potency of these drugs in inhibiting the myoclonus induced by L-5-hydroxytryptophan (5HTP) plus carbidopa in guinea-pigs (Luscombe, Jenner and Marsden, Neuropharmacology, 1981), perhaps indicating involvement of pharmacologically distinct indoleamine receptors. Manipulation of presynaptic function of 5-hydroxytryptamine (5HT) by tryptophan hydroxylase inhibition with p-chlorophenylalanine to produce depletion of cerebral 5HT, or by an L-tryptophan load to elevate 5HT in brain, suggested that the functional integrity of serotonergic neurones is required for the expression of myoclonus induced by tryptamine plus pargyline. A range of blockers of 5HT re-uptake did not alter the jerking produced by tryptamine (40 mg/kg) in guinea pigs pretreated with pargyline (75 mg/kg; 1 hr previously), or the threshold myoclonus induced by a smaller dose of tryptamine (10 mg/kg; plus pargyline 75 mg/kg). It is suggested that myoclonus induced by tryptamine in guinea pigs pretreated with pargyline involves activation of post-synaptic indoleamine receptors by tryptamine by a mechanism which requires intact presynaptic function of 5HT.
mesh_terms: D006916:5-Hydroxytryptophan / Q000494:pharmacology; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D005260:Female; D006168:Guinea Pigs; D008996:Monoamine Oxidase Inhibitors / Q000494:pharmacology*; D009207:Myoclonus / Q000139:chemically induced*; D018377:Neurotransmitter Agents / Q000378:metabolism; D010293:Pargyline / Q000494:pharmacology; D012701:Serotonin / Q000494:pharmacology; D013569:Synapses / Q000187:drug effects; D014363:Tryptamines / Q000378:metabolism / Q000494:pharmacology*; D014365:Tryptophan Hydroxylase / Q000378:metabolism
chemical_list: D008996:Monoamine Oxidase Inhibitors; D018377:Neurotransmitter Agents; D014363:Tryptamines; D012701:Serotonin; D010293:Pargyline; D006916:5-Hydroxytryptophan; D014365:Tryptophan Hydroxylase