Enhancement by progesterone of 5-hydroxytryptophan inhibition of the copulatory response in the female rat.

A study was made of the influence of different hormonal treatments used to induce copulatory behavior in ovariectomized female rats (lordosis behavior), on the effects of an endogenous increase of 5-HT or catecholamines achieved by DL-5-HTP and L-Dopa. The lordosis response (LR) has been shown to be inhibited by increased serotonergic and catecholaminergic neuronal activity. The 5-HT agonist lysergic acid diethylamide (LSD) has been found to inhibit the LR. This effect was recently shown to be enhanced by increasing doses of progesterone. In the present study it was demonstrated that the inhibitory effect of the 5-HT precursor 5-hydroxytryptophan was also potentiated by increased doses of progesterone. However, the effect of DL-5-HTP on spontaneous behaviors in an exploratory situation was not influenced by progesterone treatment. In contrast to the results with DL-5-HTP, progesterone had no modulating effects on the lordosis-inhibiting action of the catecholamine precursor levo-dopa. This elective effect on the influence of a progesterone-dependent response suggests a direct relationship between 5-HT mechanisms and the progesterone action involved in the LR. Possible mechanisms underlying the observed interactive effect of progesterone and serotonin on lordosis behavior, such as a progesterone-induced alteration of serotonergic transmission, are discussed.