Involvement of 5-HT2 receptors in the LSD- and L-5-HTP-induced suppression of lordotic behavior in the female rat.

Copulatory behavior in the ovariectomized rat, the lordotic response (L.R.), was induced by estrogen followed by progesterone. L.R. is inhibited by lysergic acid diethylamide (LSD) (greater than or equal to 0.05 mg/kg) and by Levo-5-hydroxytryptophan (L-5-HTP) (greater than or equal to 2.5 mg/kg). The effects of the putative 5-HT antagonists lisuride, metergoline, methysergide, mianserin, cinanserin, cyproheptadine, pirenperone and altanserin on the LSD-induced inhibition of L.R. were tested. Lisuride, metergoline, methysergide and mianserin were found to have no LSD-blocking effect. In contrast, cinanserin, cyproheptadine and pirenperone acted antagonistically to LSD, within a critical dose range. The selective 5-hydroxytryptamine2 (5-HT2) receptor antagonist altanserin effectively prevented the LSD-induced inhibition of L.R., and the doses required (0.05-0.20 mg/kg) indicated a comparatively high antagonistic potency. In addition altanserin (0.2 mg/kg) effectively prevented the lordosis inhibitory effect induced by L-5-HTP (2.5 mg/kg), after pretreatment with pargyline and RO4-4602. It is suggested that the suppression of copulatory behavior caused by LSD and L-5-HTP is mediated by 5-HT2 receptors.