Neuroendocrinology

MTRN1A Melatonin Receptors in the Ovine Pre-Mammillary Hypothalamus: Day-Night Variation in the Expression of the Transcripts

Abstract
Melatonin regulation of reproductive functions in sheep is mediated by action in the pre-mammillary hypothalamus (PMH). The aim of this study was to identify the high affinity melatonin receptor subtypes expressed in this structure. In order to achieve this, we used RT-PCR and developed in situ hybridization techniques (ISH). By using RT-PCR, we detected a band corresponding to the MT1 melatonin receptor cDNA in the PMH as well as in the pars tuberalis (PT). On the opposite, MT2 melatonin receptor transcripts were not detected using degenerate primers in any of the structures considered, confirming the lack of expression of this receptor subtype in sheep. The expression of MT1 mRNA was further confirmed in the PMH by ISH with a ³⁵S-labelled ovine MT1 riboprobe. We next investigated the variation in the expression of MT1 mRNA between the end of the day and the end of the night (absence and presence of melatonin, respectively). MT1 transcript expression was greater at the end of the night than at the end of the day in the PMH. In contrast, MT1 mRNA expression was lower at the end of the night than at the end of the day in the PT. No significant variation in the MT1 mRNA expression was observed in a more dorsal hypothalamic area. Overall, these results show that MT1 transcripts are expressed in the ovine PMH and that their expression follows a diurnal rhythm which is different from the pattern of expression observed in the PT.

Key words: Neuroendocrinology • Hypothalamus • Melatonin • Seasonal reproduction

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