Opioid Inhibition of Luteinizing Hormone Release Declines with Age and Acyclicity in Female Rats*

Abstract

The current study assesses changes in opioid inhibition of LH secretion with age in female rats. We administered naloxone (NAL; 2 mg/kg, iv) to regularly cycling estrous rats of three age groups and measured serum LH in serial samples drawn from intraatrial catheters before and after treatment. The serum LH rise 10 min after NAL treatment in 4- to 6-month-old rats was significantly reduced (P less than 0.01) compared with that in 1.5- to 3-month-old animals, and no LH response was observed in 8- to 11-month-old rats. On early proestrus, LH secretion was also reduced 10 min after NAL treatment in older vs. younger rats, but all groups demonstrated belated LH rises 1 h after treatment during proestrus. Persistent estrous (PE) rats released less LH after NAL treatment than age-matched estrous rats (P less than 0.025). Higher dose NAL treatment did not increase LH release in estrous or PE rats. These results support the hypothesis that opioid inhibition of LH secretion diminishes with age in cycling rats. Furthermore, opioid tone is a function of estrous state as well as age. PE rats have lower opioid tone than cycling animals of the same age. Our findings suggest a possible role of diminished opiate tone in reproductive senescence.