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

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Abstract
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) (≥0.05 mg/kg) and by Levo-5-hydroxytryptophan (L-5-HTP) (≥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.