Diurnal Influences on Serum Luteinizing Hormone Responses to Opiate Receptor Blockade with Naloxone or to Luteinizing Hormone-Releasing Hormone in the Immature Female Rat

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Abstract
The existence of a temporal pattern was investigated in the gonadotropin response of immature rats to LHRH or the opiate antagonist, naloxone. Thirty-day-old female rats were injected at 3-h intervals over a 24-h period with either naloxone (2.5 mg/kg body wt) or LHRH (8 ng/100 g body wt). Animals were decapitated 15 min later and serum samples were assayed for luteinizing hormone (LH) by radioimmunoassay. The serum LH response to naloxone and LHRH varied significantly with the time of day. Naloxone administration had no statistically significant (P > 0.05) effect on levels of serum LH at 1500 and 1800 h compared to levels in saline-injected controls, but induced a significant rise in serum LH at all other times. Naloxone had its greatest effect during the late evening and early morning hours (2100-0900 h). A similar, but not identical, pattern of LH responsiveness to LHRH was observed, with the 2 rhythms being truly divergent only during the late afternoon when LH sensitivity to LHRH was high but low to naloxone. There is a diurnal pattern of pituitary sensitivity to both naloxone and LHRH in the immature rat. Temporal variations in the LH response to opiate antagonists may result from altered pituitary sensitivity to endogenous LHRH. However, the enhanced response of the pituitary to LHRH during the late afternoon, when opioid inhibition of hypothalamic LHRH secretion appears to be at a nadir, could provide a mechanism in the immature rat whereby adult-like LH surges can be stimulated. The early afternoon LH response to various doses of naloxone was examined in intact and ovariectomized 30-day-old rats. Intacts displayed a lower absolute but higher percentage increase above basal values of LH than did ovariectomized animals. These latter findings contrast with those previously found in adult female rats.

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