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Decreases in Mediobasal Hypothalamic and Preoptic Area Opioid ([3H]Naloxone) Binding Are Associated with the Progesterone-Induced Luteinizing Hormone Surge^*

Department of Obstetrics and Gynecobgy, University of Florida College of Medicine Gainesville, Florida 32610

Address all correspondence and requests for reprints to: Satya P. Kalra, Ph.D., Department of Obstetrics and Gynecology, University of Florida College of Medicine, P.O. Box J-294 JHMHC, Gainesville, Florida 32610.

Abstract

In view of evidence implicating hypothalamic opioid systems in the control of LH release, we have examined the binding of [³H]naloxone (NAL) to slices of mediobasal hypothalamus (MBH) and preoptic area (POA) during the induction of an afternoon LH surge (1630–1700 h) in estradiol benzoate (EB)-primed ovariectomized (OVX; day 0) rats by treatment with progesterone (P; day 2). Such a surge was invariably accompanied by a decrease from early morning (1000 h) values in the number of NAL-binding sites detectable in the MBH, while the affinity of the binding site was not affected over the course of the day. Time-course studies indicated that P injection at 1000 h on day 2 was followed by a transient midday elevation in the amount of NAL bound to slices of MBH; the binding decreased significantly before the onset of and during the LH surge. A similar diurnal change was not observed in MBH slices of either oil-treated OVX rats (controls) or EBtreated OVX rats, which displayed only a 2-fold increase in LH release in the afternoon. Further studies indicated a similar change in NAL binding to slices of the POA of EBP-treated rats. Since hypothalamic opioid systems inhibit LH release, the decrease in opioid binding to MBH as well as POA slices suggests that P may curtail the existing opioid inhibitory influence in these areas before and during the course of the afternoon LH surge. (Endocrinology 124: 199–206, 1989)

Footnotes

^* Presented at the 70th Annual Meeting of The Endocrine Society, New Orleans, LA, June 8-11, 1988, abstract p. 319. This work was supported by NIH Grant HD-08634.

Received August 1, 1988.

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