Evidence for an involvement of 5-HT1B receptors in the inhibition of male rat ejaculatory behavior produced by 5-HTP

The administration of the 5-hydroxytryptamine (5-HT) precursor 5-hydroxytryptophan (5-HTP) (25 mg/kg IP), in combination with an inhibitor of peripheral 5-HTP decarboxylase, produced a dose-dependent increase in the ejaculation latency of male rats, and this effect was enhanced by additional treatment with the 5-HT1 receptor antagonist (-)-pindolol (2 mg/kg SC). The 5-HT2A/C receptor agonist (+/-)1-(2, 5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.125-0.5 mg/kg SC) did not by itself affect male ejaculatory behavior, but additional treatment with (-)-pindolol (2 mg/kg SC) produced a dose-dependent decrease in number of ejaculating animals. The increased ejaculation latency produced by 5-HTP was fully antagonized by treatment with the 5-HT1B receptor antagonist isamoltane (4 mg/kg SC), but not by ritanserin (2 mg/kg SC) treatment. The selective 5-HT1A receptor antagonist WAY-100635 (0.15 mg/kg SC) enhanced the inhibitory actions of 5-HTP on the male rat ejaculatory behavior, and this dose of WAY-100635 fully antagonized 8-OH-DPAT-induced facilitation (0.25 mg/kg SC) of the ejaculatory behavior. WAY-100635 (0.04-0.60 mg/kg SC) did not, by itself, significantly affect male rat sexual behavior. Taken together, the results suggest an inhibitory role for postsynaptic 5-HT1B receptors in the effects produced by 5-HTP on male rat ejaculatory behavior. Furthermore, 5-HTP-induced inhibition of male rat ejaculatory behavior is partially controlled by stimulation of inhibitory 5-HT1A autoreceptors, since the effects of 5-HTP were accentuated by treatment with (-)-pindolol, as well as by the more selective 5-HT1A receptor antagonist WAY-100635.