Effects of photoperiod, beta-endorphin, and naloxone on in vitro secretion of testosterone in white-footed mouse (Peromyscus leucopus) testes.

Abstract

The effects of the pro-opiomelanocortin-derived beta-endorphin (B-EP) and the opioid antagonist naloxone on in vitro secretion (accumulation of testosterone (T) in the medium) of T by testicular cells were assessed in adult white-footed mice (Peromyscus leucopus). Animals were housed under long days (16L:8D) to maintain testicular function or under short days (8L:16D) to induce gonadal regression. In vitro treatment with B-EP or naloxone did not affect basal secretion of T in dispersed cells from active or regressed testes. However, B-EP caused a dose-dependent reduction in secretion of T from cells stimulated maximally with human chorionic gonadotropin (hCG) or dibutyryl cyclic adenosine 3', 5'-monophosphate (dbcAMP). Conversely, naloxone enhanced maximal hCG- and dbcAMP-stimulated secretion of T in testicular incubates from both long- (1.5-fold) and short-day (3.5-fold)-exposed mice. The finding that the addition of naloxone to maximally stimulated cells increased further the secretion of T is evidence that B-EP may act to inhibit gonadotropin-stimulated secretion of T. Also, the stimulatory effect of naloxone on cells from regressed testes indicates that B-EP may be involved in suppressing production of T during the gonadally regressed state. Testicular B-EP-like immunostaining is present within the cytoplasm of interstitial cells and is not apparent in the seminiferous tubules. Together, these results support the idea that in P. leucopus endogenous opioid peptides in the testes may aid in the regulation of testicular function throughout the yearly breeding cycle.