Effect of naloxone on serum testosterone in adult male rabbits

Considerable evidence suggests that endogenous opioids may play an important role in the hypothalamic LH-releasing hormone. Administration of high doses of naloxone, an opiate antagonist, produces an increase in plasma concentration of LH. Naltrexone administration to healthy males produces an increase in both immunoactive and bioactive LH. The objective of the present work was to assess the effect of low doses of naloxone during 10 consecutive days on testosterone serum levels in rabbit. Three groups of five rabbits were injected with naloxone or saline. Naloxone was tested at 0.1 and 0.01 mg/kg day-1. Blood samples were taken at 90 min and 1, 2, 4, 7, 10, and 14 days after starting naloxone administration. Plasma testosterone (T) levels were measured by RIA. T levels increased progressively through the study in the experimental groups. The differences were significant after days 4 and 7 for 0.01-mg/kg and 0.1-mg/kg doses, respectively. T levels in both groups peaked at day 10 and decreased at day 14 (4 days after treatment).