Rhythmic Expression of Functional MT1 Melatonin Receptors in the Rat Adrenal Gland

We previously demonstrated that melatonin is involved in the regulation of adrenal glucocorticoid production in diurnal primates through activation of MT1 membrane-bound melatonin receptors. However, whether melatonin has a similar role in nocturnal rodents remains unclear. Using an integrative approach, here we show that the adult rat adrenal gland expresses a functional MT1 melatonin receptor in a rhythmic fashion. We found that: 1) expression of the cognate mRNA encoding for the MT1 membrane-bound melatonin receptor, displaying higher levels in the day/night transition (1800–2200 h); 2) expression of the predicted 37-kDa MT1 polypeptide in immunoblots from adrenals collected at 2200 h but not 1000 h; 3) no expression of the MT2 melatonin receptor mRNA and protein; 4) specific high-affinity 2-[125I]iodomelatonin binding in membrane fractions and frozen sections from adrenals collected at 2200 h but not 0800 h (dissociation constant = 14.22 ± 1.23 pm; maximal binding capacity = 0.88 ± 0.02 fmol/mg protei...