MT1 melatonin receptor mRNA expression exhibits a circadian variation in the rat suprachiasmatic nuclei

The aim of the present study was to investigate the daily regulation of both MT1 and MT2 melatonin receptor subtype mRNA expression in the rat SCN in order to clarify their role in the daily variation of SCN melatonin receptors. Existing MT1 and MT2 partial clones were extended by PCR to 982 and 522 bp, respectively. However, while the MT1 clone allowed us to set up a highly sensitive in situ hybridization (ISH) method, we could not detect MT2 expression within the SCN. Therefore, our results suggest that only MT1 mRNA can be correlated with 2-iodo-melatonin binding sites in the rat SCN. Investigation of MT1 mRNA expression throughout the 24 h light/dark cycle or in constant darkness clearly showed that in the two conditions, mRNA expression showed a robust rhythm with two peaks, one after the day/night and one after the night/day transitions in LD, and at the beginning of the subjective night and day in DD, respectively. Furthermore, these variations were not linked to the daily changes in melatonin receptor density. Thus, the transcriptional regulation of MT1 receptors does not appear to play a role in the daily regulation of melatonin binding sites availability.