The circadian clock, light/dark cycle and melatonin are differentially involved in the expression of daily and photoperiodic variations in mt(1) melatonin receptors in the Siberian and Syrian hamsters

Mechanisms underlying the daily and photoperiodic variations in mt(1) melat onin receptors were investigated in the pars tuberalis (PT) and suprachiasm atic nuclei (SCN) of Siberian and Syrian hamsters, Whatever its daily profi le, melatonin receptor density was strongly increased in both structures an d species after constant light exposure or pinealectomy, and decreased afte r a single melatonin injection, indicating melatonin involvement in the dai ly regulation of the receptor protein. This was confirmed by a strong inver se correlation between melatonin binding capacity and plasma melatonin conc entration. In contrast, regulation of mt(1) mRNA appeared more complex. The circadian clock, the light/dark cycle and melatonin are all implicated in mt(1) gene daily fluctuations, but the extent of their involvement depends upon the structure and the species studied. The photoperiodic decrease in m elatonin receptor density observed in short photoperiod (PT of the two hams ter species and Syrian hamster SCN) seems to be the consequence of a long-t erm mt(1) gene repression induced by the lengthening of the melatonin peak. Altogether, these results show that during daily variations, mt(1) melaton in receptor mRNA and protein are differentially regulated, while at the pho toperiodic level, the mt(1) protein status depends on mRNA transcription. C opyright (C) 2001 S. Karger AG, Basel.