Improvement of sexual behavior in aged rats by p-chlorophenylalanine and methysergide

The effects of methysergide, a serotonin antagonist, and p-chlorophenylalanine (PCPA), a serotonin synthesis blocker on the sexual behavior of intact old male rats at 19-23 months of age and female rats at 17-18 months of age were studied. Female rats with prolonged vaginal cornification and a lordosis quotient (LQ) of less than 50%, and male rats displaying no ejaculation were selected as experimental animals. The receptivity of aged female rats, but not the approach behavior to males nor proceptivity was improved by PCPA or methysergide. The LQ of intact aged female rats in non-exit tests was improved by the i.p. administration of 50 mg/kg B.W. of PCPA 2 to 4 hours prior to the sexual behavior test or 3 mg/kg B.W. of methysergide 4 hours prior to the test. Total mount frequency and mount latency of intact aged male rats were improved by the administration of methysergide (3 mg/kg B.W.), while total mount frequency and intromission latency were improved by the administration of PCPA (20 mg/kg B.W. for 4 days). The present results together with the finding from our previous study suggest that the increase of serotonin (5-HT) activity per se plays an important role in the decline of receptivity of aged female rats and the copulatory activity of aged male rats.