Neuroanatomical distribution of mating-induced c-fos expression in the male Syrian hamster brain: Role of pheromones, experience and ejaculations.

Abstract

Mating behavior in the male Syrian hamster (Mesocricetus auratus) is dependent on chemosensory cues and gonadal steroids, and the limbic system pathways important for processing these two signals have been identified. In the experiments described in this dissertation, I have utilized immunocytochemical localization of Fos protein as a marker of neuronal activation in an attempt to identify specific populations of neurons along these pathways that are activated by male sexual behavior. The results of several experiments demonstrated that mating produced a significant increase in the number of neurons expressing Fos protein within specific subdivisions of the medial amygdala, bed nucleus of the stria terminalis, medial preoptic area and septum as well as within several nuclei of the hypothalamus. This mating-induced pattern of neuronal activation was shown to be dependent upon the investigation of female vaginal odors in some brain areas and in other areas on the number of ejaculations achieved. In sexually-experienced males, the number of Fos-immunoreactive neurons also increased in several limbic regions simply after placing the animal in a clean cage in which the male had previously experienced mating tests. This latter observation suggested that neurons within the brain of a sexually-experienced male may be activated in anticipation of mating. In addition, several of the areas that were activated by mating, including subdivisions of the medial amygdala, bed nucleus of the stria terminalis, septum and ventral premammillary nucleus of the hypothalamus, were also activated during agonistic behavior. These similar patterns of Fos expression observed after mating and agonistic behavior were interpreted to reflect an increase in the animal's state of arousal during conspecific interactions. The combined results of these studies suggest that the mating-induced pattern of neuronal activation observed in the male Syrian hamster may be associated with four distinct physiological processes: increased arousal, anticipation of sexual activity, investigation of female vaginal odors and mating to ejaculation.