Copulation induces Arc expression in sex-relevant brain regions
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The study of copulation has contributed to knowledge of hormonal effects on behavior and natural reward mechanisms in the brain. In male rats, olfactory cues are particularly important for sexual behavior. Several brain areas are key for the processing of sexually-relevant olfactory stimuli, in particular the medial amygdala (MeA), bed nucleus of the stria terminalis (BNST), and the medial preoptic area (mPOA). These areas also play crucial roles in generating copulatory behavior. Sexual experience is another important factor that improves subsequent sexual behavior and renders males more resistant to the detrimental effects of damage to the aforementioned brain areas. In an effort to identify the brain areas in which changes occur as a result of sexual experience, immunohistochemistry was used to visualize the presence of the immediate early gene (IEG) Arc, which is indicative of activity-dependent synaptic plasticity. Sexually naïve and experienced male rats were either placed in the mating arena alone, with an inaccessible estrous female, or with a receptive female with which they could copulate on the test day. Patterns of Arc and c-Fos expression in their brains were then examined. Sexual experience reduced latencies to mount, intromit, and ejaculate, and also increased the frequency of intromissions during copulation. As expected, copulation induced c-Fos expression in the posterior dorsal MeA, posteromedial BNST, and central mPOA regardless of prior experience. Arc expression was induced by copulation much more widely throughout the anterior BNST, posterior BNST, and MeA, as well as in the posterior mPOA, but not in the central mPOA. Surprisingly, Arc induction did not vary based on prior sexual experience, indicating that neural plasticity induced by copulation is important for both sexually naïve and experienced males. Correlations between measures of sexual behavior and IEG induction revealed that increased Arc in the BNST of naïve males was associated with higher mount latencies and numbers of mounts, while increased Arc in the MeA and mPOA of naïve males was associated with higher intromission latencies and numbers of intromissions. This suggests that Arc induction may be particularly important for improving behavior in naïve males that perform poorest.