Mechanisms underlying the daily and photoperiodic variations in mt₁ melatonin receptors were investigated in the pars tuberalis (PT) and suprachiasmatic nuclei (SCN) of Siberian and Syrian hamsters. Whatever its daily profile, melatonin receptor density was strongly increased in both structures and species after constant light exposure or pinealectomy, and decreased after a single melatonin injection, indicating melatonin involvement in the daily regulation of the receptor protein. This was confirmed by a strong inverse correlation between melatonin binding capacity and plasma melatonin concentration. In contrast, regulation of mt₁ mRNA appeared more complex. The circadian clock, the light/dark cycle and melatonin are all implicated in mt₁ gene daily fluctuations, but the extent of their involvement depends upon the structure and the species studied. The photoperiodic decrease in melatonin receptor density observed in short photoperiod (PT of the two hamster species and Syrian hamster SCN) seems to be the consequence of a long-term mt₁ gene repression induced by the lengthening of the melatonin peak. Altogether, these results show that during daily variations, mt₁ melatonin receptor mRNA and protein are differentially regulated, while at the photoperiodic level, the mt₁ protein status depends on mRNA transcription.

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