Proenkephalin and opioid μ-receptor mRNA expression in ovine hypothalamus across the estrous cycle

John P. Walsh, Alexandra Rao, Robert C. Thompson, Iain J. Clarke

Research output: Contribution to journalArticleResearchpeer-review

11 Citations (Scopus)

Abstract

The neural mechanism underlying the preovulatory surge of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) is thought to include reduced opioid inhibition of GnRH secretion (disinhibition). Possible mechanisms for disinhibition include reduced endogenous opioid peptide or receptor mRNA expression. Proenkephalin and opioid μ-receptor mRNA expression were measured by in situ hybridization using 35S-labeled cRNA probes and computer-assisted grain counting in hypothalamic nuclei of ovary-intact ewes (n = 4) killed on day 10 of the luteal phase or 24 or 48 h into the follicular phase. In a second experiment, proenkephalin and μ-receptor mRNA expression were compared in ewes killed on day 10 of the luteal phase or during the preovulatory LH surge. Cells expressing proenkephalin mRNA were more widely distributed in ovine hypothalamus than previously described. In the periventricular nucleus, there was a significant reduction in the grain count per cell and the number of labeled cells during the follicular phase and during the LH surge, as compared to the luteal phase. In the ventromedial hypothalamus, there was a significant reduction in the grain count per cell during the follicular phase and LH surge as compared to the luteal phase, but no change in the number of labeled cells. No differences in proenkephalin mRNA expression were detected in the medial septum, diagonal band of Broca, preoptic area, anterior hypothalamic area or paraventricular nucleus across the estrous cycle. Cells expressing opioid μ-receptor mRNA were also widely distributed. No difference in μ-receptor mRNA expression was detected in the medial septum, diagonal band, medial preoptic area, anterior hypothalamus or bed nucleus of the stria terminalis across the cycle. We conclude that in sheep, proenkephalin gene expression in the periventricular nucleus and ventromedial hypothalamus is reduced during the follicular phase and at the time of the LH surge. This may be part of the neural mechanism underlying the GnRH/LH surge in this species.

Original languageEnglish
Pages (from-to)26-36
Number of pages11
JournalNeuroendocrinology
Volume73
Issue number1
DOIs
Publication statusPublished - 6 Mar 2001
Externally publishedYes

Keywords

  • Estrous cycle
  • Gonadotropin-releasing hormone
  • Gonadotropins
  • In situ hybridization
  • Opioid peptide receptors
  • Ovine hypothalamus
  • Proenkephalin

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