LOCAL-REGULATION OF TESTICULAR IMMUNOREACTIVE-ARGININE VASOPRESSIN AND STEROIDOGENESIS BY NALOXONE

The effects of intratesticular injection of naloxone, a universal opio id antagonist, on testicular immunoreactive (IR)-arginine vasopressin (AVP) content and on in vitro testosterone production by Leydig cells were investigated in the mouse. Bilateral intratesticular injection of increasing doses of naloxone (0.1-10 mug/testis) resulted 24 h later in a dose-dependent increase in testosterone production by Leydig cell s incubated for 3 h in the presence or absence of hCG (100 ng/ml). Uni lateral intratesticular injection of naloxone (10 mug) similarly enhan ced basal and hCG-stimulated testosterone production by Leydig cells, but production was not modified in Leydig cells from the contralateral vehicle-injected testis, nor was it changed when the same dose was in jected subcutaneously. Unilateral intratesticular injection of 10 mug naloxone led to a dose-dependent increase in the hCG-responsiveness wi thout altering the slope of the hCG dose-response curve. In vitro expo sure of Leydig cells to increasing doses of naloxone (10(-9) to 10(-7) M) did not alter either basal or hCG-stimulated testosterone producti on. Testicular IR-AVP content declined in a dose-dependent manner in n aloxone-injected testis, but remained unchanged in the contralateral v ehicle-injected testis and in testis from animals that received simila r doses of naloxone subcutaneously. Since AVP has been shown to locall y exert a negative control on testosterone production within the testi s, it might be hypothesized that the increased Levdig cell activity af ter local naloxone administration results from reduced intratesticular IR-AVP levels. The possibility that endogenous opioid peptides may lo cally control testosterone biosynthesis via testicular AVP system regu lation is discussed.