The Circadian Clock, Light/Dark Cycle and Melatonin Are Differentially Involved in the Expression of Daily and Photoperiodic Variations in mt₁ Melatonin Receptors in the Siberian and Syrian Hamsters

Abstract

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.

International Union of Basic and Clinical Pharmacology. LXXV. Nomenclature, Classification, and Pharmacology of G Protein-Coupled Melatonin Receptors

“…In addition, 24-h variations in MT 1 receptor mRNA expression, as well as repressed MT 1 receptor transcription, under short photoperiod conditions have been reported (Masana et al, 2000;Schuster et al, 2001;Poirel et al, 2002). Little is known, however, about the mechanisms that drive these changes in melatonin receptor gene expression.…”

The regulation of neuroendocrine function: Timing is everything

“…In addition, exogenous melatonin alters the phase of SCN electrical activity measured in a slice preparation in vitro in a manner predicted by the phase response curve (McArthur et al, 1991). The sensitivity of the SCN to melatonin may be a function of daily variation in the density of melatonin binding sites within the SCN (Schuster et al, 2001). In humans, melatonin administration causes phase delays during late night or early morning and phase advances in late morning to early afternoon (Lewy and Sack, 1997).…”

The mt1 Melatonin Receptor and RORβ Receptor Are Co-localized in Specific TSH-immunoreactive Cells in the Pars Tuberalis of the Rat Pituitary

“…Therefore, the mt1 receptor subtype appears sufficient to drive the melatonin-induced photoperiodic response. Although many studies have been performed on the circadian and seasonal transcriptional regulation of mt1 receptors in the PT (Gauer et al 1993;Guerrero et al 2000;Schuster et al 2001), the exact cell type expressing this receptor has not yet been identified. Therefore, one aim of this study was the identification of this cell type in the rat by combining the nonradioactive in situ hybridization (ISH) for the mt1 mRNA with immunocytochemistry (ICC) for the different pituitary hormones.…”