Neural activation following sexual behavior in the male and female rat brain

Abstract

Neural activation following sexual behavior was studied in the male and female rat brain, using Fos-immunoreactivity (Fos-IR) as a measure. In accordance with the available literature, we observed increased expression of c-fos in the medial preoptic nucleus (MPN), in the posteromedial subdivision of the bed nucleus of the stria terminalis, in the posterodorsal part of the medial amygdala, and in the caudal thalamus, in the parvicellular part of the subparafascicular nucleus. After performance of different behavioral elements (anogenital investigation, mounting, intromission or ejaculation) not only the numbers of Fos-IR neurons varied considerably, but also their distribution. Especially after ejaculation, but in females already after intromissions, dense groups of Fos-IR neurons appeared in specific subdivisions of the areas mentioned above. That these groups of dense Fos-IR appeared as a result of the ejaculation per se, was assessed by administrating the 5-HT1A agonist 8-OH-DPAT to the males, whereupon they ejaculated within a few seconds, without the usual amount of preceding behavioral elements. Since the pattern of Fos-IR was similar to the normal ejaculation pattern, we have described the dense activation areas as 'ejaculation-related clusters'. Our review discusses the stimuli and pathways probably involved in the observed pattern of Fos-IR and we conclude that the 'deep viscero-genital' activation, occurring at the moment of ejaculation, running along the pelvic nerve and ascending from the spinal cord, is most probably responsible. We show that the location of the Fos-IR neurons in the medial subparafascicular nucleus perfectly coincides with the location of Galanin-IR fibers, ascending from the spinal cord. The application of anterograde and retrograde neuroanatomical tracers into the MPN, in combination with Fos-IR showed that the medial preoptic nucleus has very specific relationships with the Fos-IR sub-areas, involved in ejaculation. We conclude that within the larger brain structures involved in sexual and other social activities, a specific ejaculation-related subcircuit exists, which may, under normal conditions in the rat, serve a 'sexual-satiety function'.