Progesterone Enhancement of Lysergic Acid Diethylamide and Levo-5-Hydroxytryptophan Stimulation of the Copulatory Response in the Female Rat

In female rats whose ovaries have been removed, sexual receptivity (the lordosis response) can be triggered by estrogen alone or estrogen plus progesterone. Large doses of LSD (≥50 μg/kg) or L-5-HTP (≥2.5 mg/kg) inhibit this behavior, with progesterone enhancing the inhibition. Conversely, small doses of LSD (5–30 μg/kg) increase lordosis when only estrogen is given. This study tested how different hormone treatments affect this stimulatory action. When lordosis was activated by estradiol benzoate alone, a 10 μg/kg dose of LSD increased the response within 10 minutes, but 1 μg/kg had no effect. However, when progesterone was also given, 1 μg/kg of LSD did increase lordosis. Similar results occurred with very small doses of L-5-HTP after certain pretreatments. Progesterone appears to influence serotonin-related mechanisms in this behavior.

Study at a glance

Abstract

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 (≥50 μg/kg) and by Levo-5-hydroxytryptophan (L-5-HTP) (≥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 μg/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 × 2 μg/kg), LR increased 10 min after injection of LSD in a dose of 10 μg/kg. There were no detectable effects in animals treated with estrogen alone when the dose of LSD was lowered to 1 μg/kg. LSD in the latter dose gave an increased response, however, when LR was activated by EB (5 μg/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.