Progesterone enhancement of lysergic acid diethylamide and levo-5-hydroxytryptophan stimulation of the copulatory response in the female rat.

Copulatory behavior in the ovariectomized rat, i.e. the lordosis response (LR) on being mounted by a male, can be induced by administration of either estrogen alone or estrogen followed by progesterone. LR has been shown to be inhibited by lysergic acid diethylamide (LSD) in certain doses (greater than or equal to 50 micrograms/kg) and by Levo-5-hydroxytryptophan (L-5-HTP) (greater than or equal to 2.5 mg/kg). This effect was recently found to be enhanced by increasing doses of progesterone. In contrast, small doses of LSD (5-30 micrograms/kg) have been shown to increase LR activated by estrogen alone. The effects of various hormone treatments on the stimulatory action of LSD were tested in the present study. When the lordosis behavior was activated by estradiol benzoate (EB) alone (25 or 7 X 2 micrograms/kg), LR increased 10 min after injection of LSD in a dose of 10 micrograms/kg. There were no detectable effects in animals treated with estrogen alone when the dose of LSD was lowered to 1 microgram/kg. LSD in the latter dose gave an increased response, however, when LR was activated by EB (5 micrograms/kg) in combination with progesterone (4.0 mg/rat). An analogous study was conducted with L-5-HTP, after pretreatment with pargyline and R04-4602. Small doses of L-5-HTP (0.25 and 0.05 mg/kg) stimulated the LR and the influence of progesterone was the same as for small doses of LSD. Possible mechanisms underlying the observed influence of progesterone on serotonergic mechanisms involved in the lordosis behavior are discussed.