Local control of Leydig cell arginine vasopressin receptor by naloxone

Arginine vasopressin (AVP) and beta-endorphin are present within the testis where they could act as paracrine effectors of steroidogenesis. In this study we investigated the effect of naloxone, an opioid receptor antagonist on Leydig cell AVP receptor. Intratesticular injection of increasing doses of naloxone (0.1-100 micrograms) resulted 24 h later in a dose-dependent increase in Leydig cell AVP binding capacity. This effect occurred locally since s.c. injection of similar doses of naloxone did not alter the testicular AVP receptor content and intratesticular injection enhanced AVP receptor density only in the naloxone-treated testis but not in the contralateral vehicle-treated testis. Scatchard plot analysis of the data revealed that naloxone locally injected altered AVP binding capacity without change in affinity. These results suggest that in addition to their known paracrine effects in the testis, endogenous opioid peptides may locally control the testicular AVP system by modulating AVP receptor capacity.