Effects of apomorphine on sexual behavior in young and middle-aged rats

Abstract

Apomorphine (APO), the prototypical dopamine agonist, produced different effects on the copulatory patterns of sexually vigorous male rats depending on the dose, the time between treatment and behavioral observation and the age of the animals. Prior to any drug treatment, middle-aged males (13 months of age) exhibited prolonged latencies to ejaculation and intervals between intromissions when compared to young male rats (3 months of age). In mating tests to young male rats initiated six minutes after treatment, administration of low doses of APO was followed by reductions in the ejaculatory threshold (0.05–0.8 mg/kg). The greatest reduction in the ejaculatory threshold was observed after administration of 0.4 mg/kg APO. No such facilitation of ejaculatory behavior was evident in the middle-aged males. Age-related differences in APO-induced changes in ejaculation latency (0.05–0.8 mg/kg), intromission frequency (0.05–0.4 mg/kg), copulatory efficacy (0.2 and 0.8 mg/kg) and intercopulatory interval (0.8 mg/kg) were evident. Doses of APO above 0.4 mg/kg in young males and above 0.2 mg/kg in middle-aged males elicited dose-related decreases in the number of rats engaging in copulatory activity, with 3 mg/kg effecting a virtual elimination in both age groups. In mating tests initiated 60 minutes after treatment, no decrease in the number of rats engaging in copulatory activity was seen in the young animals, and 3 mg/kg caused only a minor reduction in the number of middle-aged males achieving ejaculation. Further, in those males that mated to ejaculation in tests six minutes posttreatment, APO treatment was followed by age-related differences in ejaculation latency (0.4 and 0.8 mg/kg), intromission frequency (0.4 and 0.8 mg/kg), intercopulatory interval (0.2 and 0.8 mg/kg) and postejaculatory interval (0.8 mg/kg). In general, the alterations seen in tests 60 minutes after treatment were in the opposite direction than those seen in the six minute tests. These data provide support for the recent proposal that the activation of presynaptic dopamine receptors leads to a reduction in the amount of stimulation required to elicit ejaculatory behavior in copula and, further, suggest that stimulation of postsynaptic dopamine receptors results in a suppression of copulatory behavior, evidenced by a decreased number of rats mating. The data also lead to the suggestion that middle-aged male rats are more sensitive to postsynaptic, and less sensitive to presynaptic dopamine receptor stimulation.