Purpose: We investigated the causative role of testosterone in copulatory disorder and the expression of c-fos messenger (m)RNA in the medial preoptic area in rats after social stress.

Materials and methods: To generate copulatory disorder rats in the experimental defeated group were attacked by residents for 10 minutes daily for 7 consecutive days (social stress). We then investigated the effect of repeat defeat on the frequency of mounting behavior and plasma testosterone levels. The effects of testosterone replacement and/or apomorphine (100 microg./kg. subcutaneously), a dopamine receptor agonist, on the frequency of mounting behavior were also studied. After experiencing social stress the brain area within the medial preoptic area was removed for analysis of c-fos and androgen receptor mRNA expression. Real-time reverse transcription-polymerase chain reaction was done to analyze gene expression.

Results: Rats in the defeated group showed a reduced frequency of mounting behavior and a decrease in plasma testosterone levels compared with values in control rats (p <0.01). After testosterone replacement the frequency of mounting behavior became significantly higher than that of socially stressed rat (p <0.05) but did not achieve control levels. The frequency of mounting behavior by socially stressed rats after apomorphine treatment was significantly higher than that of vehicle treated rats (p <0.05) but the frequency produced by the combination of testosterone replacement and apomorphine injection did not achieve control levels. After the social stress experience c-fos mRNA expression was significantly increased compared with that in control rats (p <0.05). The expression of androgen receptor mRNA was not affected by social stress. Testosterone replacement significantly reduced the expression of c-fos mRNA in the medial preoptic area (p <0.05).

Conclusions: Our results indicate that a reduction in plasma testosterone may have a causative role in copulatory disorder induced by social stress. Changes in c-fos mRNA expression in the medial preoptic area correlated with copulatory disorder and, thus, they are suitable for monitoring that disorder.