Indoleamine antagonists: relative potencies as inhibitors of tryptamine- and 5-hydroxytryptophan-evoked responses

Four indoleamine antagonists were evaluated for relative potencies as inhibitors of tryptamine-induced forepaw clonus and 5-hydroxytryptophan-evoked head twitches. Methergoline was approximately three times more potent against the forepaw clonus than the head twitch response, whereas methysergide exhibited nearly equal activity in both tests. Cyproheptadine and cinanserin showed a profile opposite to methergoline and a greater degree of selectivity, being 25 to 40 times more potent as inhibitors of the 5-hydroxytryptophan- than of the tryptamine-induced response. These findings clearly demonstrate that the rank order of potency of indoleamine antagonists varies greatly depending upon the test procedure employed.