The immobilization stress induces an acute inhibition of testicular steroidogenesis that is mediated by the nitric oxide (NO) signaling pathway. Here we compared the effects of 2-h immobilization stress on in vivo and in vitro rat steroidogenesis at two time points, 0 h and 6 h after the end of the stress session. As expected, serum androgens and human chorionic gonadotropin (hCG)-stimulated progesterone and testosterone production by testicular tissue were inhibited at 0 h, and also at the 6-h time point. Both the acute and sustained inhibitions of in vitro steroidogenesis were accompanied by a significant increase in nitrite, a stable oxidation product of NO. To clarify which subtype of NO synthase (NOS) (constitutive (cNOS) or inducible (iNOS)) participates in down-regulation of testicular steroidogenesis, aminoguanidine hydrochloride (AG), a selective iNOS inhibitor, was employed. Intratesticular injection of AG prevented the sustained, but not the acute, stress-induced decrease in serum testosterone. When added in vitro, it also prevented the sustained decrease in steroid production and increase in nitrite production by testicular tissue, both in a dose-dependent manner and with EC microM. Furthermore, AG added in vivo and in vitro effectively blocked the sustained decrease in 3beta-hydroxysteroid dehydrogenase (3betaHSD) and 17alpha-hydroxylase/C17-20 lyase (P450c17) activities. In all concentrations employed, AG did not affect serum androgens and in vitro steroid and nitrite production in unstressed animals. These results indicate that the NO signaling pathway participates in acute and sustained stress-induced down-regulation of testicular steroidogenesis, presumably through its direct action on 3betaHSD and P450c17. The acute NO production is controlled by cNOS and the sustained production of this messenger is controlled by iNOS.

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