Research reportRegional and subtype-specific loss of GnRH neurons is associated with diminished mating behavior in middle-aged male rats

•There was no difference in the total number of both s-GnRH and i-GnRH neurons between young controls and MIE.•Both s-GnRH and i-GnRH neuron numbers in MIE were higher than those in MI and NC.•Both in the areas of MS and POA, s-GnRH neurons were greatest in Group MIE, lowest in Group NC.•Both in the areas of OVLT and POA, i-GnRH neurons were greatest in Group MIE, lowest in Group NC.•In the area of MS, MIE rats had less s-GnRH neurons than young controls.

The current study was to examine the relationship between the number of gonadotropin-releasing hormone (GnRH) neurons and male sexual behavior in middle-aged rats. Based on their sexual performance, middle-aged male rats (18-19 months) were assigned to three groups: (i) Group MIE (showing mounts, intromissions, and ejaculation), (ii) Group MI (displaying mounts and intromissions, but no ejaculation), and (iii) Group NC (showing no copulatory behavior). The brains of these middle-aged animals and of sexually active, young controls were collected and then examined for immunohistochemical localization of GnRH neurons. The numbers of two subtypes of GnRH neurons, smooth (s-GnRH) and irregular (i-GnRH), in the medial septum (MS), organum vasculosum of the lamina terminalis (OVLT), preoptic area (POA), and anterior hypothalamus (AH), were determined under a light microscope. As compared to young controls, an age-related decrease in the number of s-GnRH neurons was found in the MS of MIE rats. Among three groups of middle-aged rats, Groups MIE and MI had more s-GnRH neurons in the POA and i-GnRH neurons in the OVLT and POA than Group NC. In addition, loss of s-GnRH and i-GnRH neurons in the MS was observed in Groups MI and NC and Group NC, respectively. Our results suggest that a decrease in GnRH neuron subtypes occurring in different brain regions might be critical for the loss of specific components of male rat sexual behavior during aging.