GABAergic regulation of penile reflexes and copulation in rats

Abstract

In three experiments we investigated the role of the GABAergic system in regulating penile reflexes and copulation in rats. All males had the suspensory ligament of the penis removed 3 weeks before the experiments, thereby permitting the novel observation of penile body erections in addition to penile glans responses. In Experiments 1 and 2, rats were assigned to one of the following treatment groups: control (saline-injected); baclofen, a GABA-B receptor agonist (1 or 2 mg/kg); THIP, a GABA-A receptor agonist (5 mg/kg); or (+)bicuculline, a GABA-A receptor antagonist (0.25 mg/kg). Penile reflex tests began 20 min after SC injection. Baclofen treatment decreased the number of males responding with glans erections within the 30 min latency limit, and significantly reduced the number of glans erections displayed, in a dose-dependent manner. These results indicate that activation of GABA-B receptors inhibits penile reflexes outside the context of copulation (ex copula). Rats treated with baclofen also had a dose-dependent reduction in the ratio of clusters of glans erections to penile body erections, and a significant increase in the number of penile body erections observed prior to the first glans erection. These findings suggest that baclofen inhibited the mechanisms mediating penile glans responses more than those mediating penile body erections. Bicuculline was without effect in both experiments; THIP significantly increased erections in Experiment 2 but had no reliable effect in Experiment 1. Because baclofen inhibited penile responses in ex copula tests, Experiment 3 was designed to assess the effects of the same dosages of this agent on male copulatory behavior. Baclofen treatment did not appear to inhibit penile responses during mating: There were no significant differences between groups in intromission latency, ejaculation latency, or in the postejaculatory interval. Furthermore, all males attained intravaginal intromissions and ejaculations, and there was no systematic effect of baclofen on the proportion of mounts that were accompanied by intromissive patterns. These data indicate that inhibition of penile reflexes mediated by GABA-B receptors is reduced within the context of mating. Our results support the hypothesis that GABA-B receptors play an inhibitory role in the complex synaptic mechanisms which regulate penile reflexes in the rat; no clear role for GABA-A receptors emerged from these studies.