Background: Systemic alcohol exposure lowers plasma testosterone (T) levels in adult males, but the relative role of impaired luteinizing hormone (LH)-releasing hormone synthesis and decreased pituitary LH release versus that of a direct ability of circulating alcohol to inhibit testicular steroidogenesis remains poorly understood. We had reported preliminary evidence that alcohol might stimulate a pituitary-independent, neural pathway between the hypothalamus and the testes whose activation blunts T secretion in response to human chorionic gonadotropin (hCG). The present work was done to further investigate the influence of alcohol on this pathway by comparing the effect of the intragastric (i.g.) and intracerebroventricular (i.c.v.) injection of alcohol on the T response to hCG, to probe the role of LH and corticotropin-releasing factor (CRF) in both models, and to examine potential changes in levels of the cholesterol transfer protein steroidogenic acute regulatory (StAR) protein.
Methods: Male Sprague Dawley rats were implanted with chronic i.c.v., i.g., and/or intravenous cannulae that allowed drug delivery and blood sampling in nonanesthetized, undisturbed animals. T blood levels were measured by radioimmunoassay. The role of LH and of hormones of the hypothalamic-pituitary-adrenal axis such as adrenocorticotropic hormone and corticosterone was investigated in rats pretreated with an LH-releasing hormone antagonist or CRF antibodies. The potential presence of neuronal damage was assessed by Fluoro-Jade methodology. StAR protein levels were measured by Western blot in Leydig cells isolated from rats injected with the vehicle or alcohol.
Results: Although it was not accompanied by measurable blood alcohol levels, i.c.v. administered alcohol, at a dose (5 microl of 200 proof, 86 microM) that did not cause neuronal damage and did not lead to detectable levels of the drug in the cerebrospinal fluid of the fourth ventricle of the brain, significantly blunted hCG-induced T release. The ig injection of alcohol, which in contrast induced significant increases in blood alcohol levels, also significantly interfered with the ability of hCG to induce T release. This effect was comparable in 40- and 65-day-old rats. Neither prior blockade of LH-releasing hormone receptors with a potent LH-releasing hormone antagonist nor immunoneutralization of endogenous CRF altered the inhibitory effect of alcohol injected i.c.v. or i.g. on T secretion. Preliminary results suggested that testicular levels of StAR protein may be slightly decreased by both alcohol regimens.
Conclusions: Collectively, our results indicate that alcohol can act within the brain to influence testicular activity independently of LH, independently of hormones of the hypothalamic-pituitary-adrenal axis, and/or independently of the presence of the drug in the circulation. Our present working hypothesis is that the i.c.v. injection of alcohol stimulates an inhibitory neural pathway that connects the hypothalamus to the testes.