Reproductive aging in Japanese quail, Coturnix japonica is associated with changes in central opioid receptors

Quantitative in vitro autoradiography was used to measure specific μ and δ opioid receptor densities in regions of the Japanese quail, Coturnix japonica, brain that regulates reproductive endocrine and behavioral responses to determine the possible involvement of the opioid system in reproductive decline seen during aging. Densities were measured in selected brain regions of young sexually active (YAM), young photoregressed (YPM), old reproductively senescent (OIM) male, young active (YF), and old senescent female (OF) Japanese quail. Medial and lateral septum (SM, SL), medial preoptic area (POM), and n. intercollicularis (ICo) were of particular interest for reproductive responses. Similar to previous observations, μ and δ opioid receptors showed differential distributions in the areas measured. Some age-related changes were observed, with lower SM μ receptor densities in aged males (OIM) than females or young males (YAM). Densities of μ receptors in the POM and in other areas examined did not vary with sex or age. Similarly, OIM males had lower densities of δ receptors in the SM than young males (YAM and YPM); POM δ receptor densities were also low in OIM males compared to the YPM males, and YAM males were intermediate. Interestingly, photoregressed males (YPM) had higher SL δ receptor densities than any other group. Thus there were age-related differences detected in μ receptor densities among groups in the SM of OIM relative to other groups; and the μ and δ receptor densities did not differ in females with brain region. Additionally for δ receptors specifically, YF and OF did not differ from OIM for any brain region and similarly had lower densities of δ receptors compared to YAM males. These data provide support for regional differences in opioid receptor distribution and for age- and sex-related differences in δ opioid receptor densities. The direction of change presents an interesting dichotomy in that, compared to young active males, δ opioid receptor densities increased with loss of reproductive function in the YPM, whereas receptor densities decreased in the OIM. Plasma androgen levels were relatively low in both these groups compared to the young active males. This observation suggests that there is an age-related loss in the ability of this receptor system to respond to circulating and centrally produced steroid hormones in the POM and in some septal regions, compared to young animals that are responding to environmental cues. Furthermore, these data support an active role of the opioid peptide system in the inhibition of the reproductive axis in photoregression.