Exposure to methylphenidate during peri-adolescence affects endocrine functioning and sexual behavior in female Long-Evans rats.

The present study was designed to test the effects of methylphenidate (MPH) exposure on the maturation of endocrine functioning and sexual behavior. Female rat pups received either MPH (2.0mg/kg, i.p.) or saline twice daily between postnatal days 20-35. This period of exposure represents the time just prior to puberty as well as puberty onset. Approximately five weeks after the last injection of MPH or saline, female subjects were hormone-primed and tested during their first sexual experience. Subjects were given the choice to interact with a sexually active male or a sexually receptive female rat (i.e., the partner-preference test). The partner-preference paradigm allows us to assess multiple aspects of female sexual behavior. MPH exposure during peri-adolescence delayed puberty and, when mated for the first time, affected sexual behavior (e.g., increased time spent with the male stimulus and decreased the likelihood of leaving after mounts) during the test of partner preference. When monitoring estrous cyclicity, female subjects treated with MPH during peri-adolescence frequently experienced irregular estrous cycles. The results of the present study suggest that chronic exposure to a therapeutic dose of MPH around the onset of puberty alters long-term endocrine functioning, but with hormone priming, increases sensitivity to sexual stimuli.

Endocannabinoid influence on partner preference in female rats.

The present study investigated the role of the endocannabinoid system on sexual motivation in the female rat. In Experiment 1, gonadally intact female rats were first tested for partner preference after a vehicle injection. Approximately 2 weeks later, all rats were tested again after an injection of the endocannabinoid antagonist, SR141716 (SR; also known as Rimonabant; 1.0mg/kg). During the first 10 min of each partner preference test, subjects could spend time near either a male or female stimulus animal that was placed behind a wire mesh (No-Contact). During the second 10 min of each partner preference test, subjects had unrestricted access to both stimulus animals (Contact). When the female subjects were treated with SR, they made fewer visits to either stimulus animal during the no-contact phase of the partner preference test compared to when they were treated with vehicle. In Experiment 2, ovariectomized (OVX) subjects primed with estrogen were administered SR or vehicle and tested for partner preference (Experiment 2A). Approximately 2 weeks later, the subjects from the control group were tested again after an injection of SR (Experiment 2B). In contrast to Experiment 1, treatment with SR reduced the number of visits specifically to the male stimulus during the contact phase of the test in Experiment 2. Experiment 3 tested the effects of SR on general locomotion and found no effect of SR on line crossings in an open field. Finally, in Experiment 4, OVX estrogen- and progesterone-primed subjects were administered the endocannabinoid agonist anandamide (AEA: 1.0mg/kg) or vehicle and tested for partner preference. AEA-treated subjects made more visits to the male stimulus than vehicle-treated subjects during the contact phase of the test. The results of the present study suggest that the endocannabinoid system may contribute to sexual motivation in female rats by specifically altering approach behavior.