Sex difference in the effect of mating on c-fos expression in luteinizing hormone-releasing hormone neurons of the ferret forebrain.

The pulsatile secretion of LH was previously found to rise in female ferrets after receipt of an intromission, whereas in males that achieved an intromission, both LH and testosterone secretion were either reduced or unchanged. We sought to determine whether this sexually dimorphic pattern of LH secretion reflects a sex difference in the effect of mating on the activity of forebrain neurons that secrete LHRH. Immunocytochemical methods were used to localize the nuclear protein product of the immediate early gene, c-fos, as an index of increased neuronal activity after mating. Nuclear FOS immunoreactivity (FOS-IR) was monitored in LHRH-IR neurons as well as other non-LHRH forebrain neurons. In confirmation of previous reports, LHRH-IR perikarya in ferrets of both sexes were located medially along the base of the brain at rostral, medial, and caudal levels of the preoptic-hypothalamic continuum. In each of these regions a significantly higher percentage of LHRH-IR neurons was colabeled with nuclear FOS-IR in mated than in unpaired females. By contrast, an equivalent low percentage of LHRH-IR neurons was colabeled with FOS-IR in mated and unpaired male ferrets. Significantly more FOS-IR neurons (not colabeled with LHRH) were detected in the bed nucleus of the stria terminalis, the medial preoptic area, the dorsal-medial hypothalamus, and the medial amygdala (MA) of mated vs. unpaired females. By contrast, mating significantly augmented FOS-IR only in the MA of male ferrets. The results suggest that the sexually dimorphic pattern of LH secretion that occurs in ferrets after mating reflects a selective activation of LHRH neurons in the female forebrain. This sex-specific increase in the responsiveness of LHRH neurons to mating may depend on input from a limbic circuit which includes the medial amygdala, bed nucleus of the stria terminalis, and medial preoptic area.