Increased expression of c-fos in the medial preoptic area after mating in male rats: role of afferent inputs from the medial amygdala and midbrain central tegmental field.

Immunocytochemical methods were used to localize the protein product of the immediate-early gene, c-fos, in male rats after exposure to, or direct physical interaction with, oestrous females. Increasing amounts of physical contact with a female, with resultant olfactory-vomeronasal and/or genital-somatosensory inputs, caused corresponding increments in c-fos expression in the medial preoptic area, the caudal part of the bed nucleus of the stria terminalis, the medial amygdala, and the midbrain central tegmental field. Males bearing unilateral electrothermal lesions of the olfactory peduncle showed a significant reduction in c-fos expression in the ipsilateral medial amygdala, but not in other structures, provided their coital interaction with oestrous females was restricted to mount-thrust and occasional intromissive patterns due to repeated application of lidocaine anaesthetic to the penis. No such lateralization of c-fos expression occurred in other males with unilateral olfactory lesions which were allowed to intromit and ejaculate with a female. These results suggest that olfactory inputs, possibly of vomeronasal origin, contribute to the activation of c-fos in the medial amygdala. However, lesion-induced deficits in this type of afferent input to the nervous system appear to be readily compensated for by the genital somatosensory input derived from repeated intromissions. Unilateral excitotoxic lesions of the medial preoptic area, made by infusing quinolinic acid, failed to reduce c-fos expression in the ipsilateral or contralateral medial amygdala or central tegmental field following ejaculation. By contrast, combined, unilateral excitotoxic lesions of the medial amygdala and the central tegmental field significantly reduced c-fos expression in the ipsilateral bed nucleus of the stria terminalis and medial preoptic area after mating; no such asymmetry in c-fos expression occurred when lesions were restricted to either the medial amygdala or central tegmental field. This suggests that afferent inputs from the central tegmental field (probably of genital-somatosensory origin) and from the medial amygdala (probably of olfactory-vomeronasal origin) interact to promote cellular activity, and the resultant induction of c-fos, in the ipsilateral bed nucleus of the stria terminalis and medial preoptic area. The monitoring of neuronal c-fos expression provides an effective means of studying the role of sensory factors in governing the activity of integrated neural structures which control the expression of a complex social behaviour.