Daily Rhythms of Melatonin Binding Sites in the Rat Pars tuberalis and Suprachiasmatic Nuclei; Evidence for a Regulation of Melatonin Receptors by Melatonin Itself

Using quantitative autoradiography, the density of melatonin binding sites has been measured in the rat pars tuberalis (FT) and suprachiasmatic nuclei (SCN) every 4 h throughout a 24-hour period in animals kept in a light regime of 12L/12D (with lights on at 07.00 h). Slices of PT and SCN were incubated in the presence of 180 and 172 pM, respectively, of 2-l25I-melatonin. In both structures investigated, specific 2-125I-melatonin binding sites showed similar rhythms throughout the 24-hour period with a maximum at 16.00 h (PT: 46.9 ± 2.8 fmol/mg protein, n = 5 and SCN: 5.12 ± 0.30 fmol/mg protein, n = 5) and a minimum at 4.00 h (PT: 28.5 ± 4.5 fmol/mg protein, n = 5 and SCN: 3.07 ± 0.39 fmol/mg protein, n = 5). Similar experiments performed on PT of animals kept in constant light (LL) for 3 days revealed a lack of variations of melatonin binding site density, all the values being significantly higher than those of the respective 12L/12D group (concentration of 2-125I-melatonin used: 180 pM). All these preliminary results were confirmed by saturation studies performed at 16.00 and 4.00 h using quantitative autoradiography and in 12L/12D animals, using radioreceptor binding assays on isolated PT membranes. In 12L/12D animals, the maximum number of melatonin binding sites (Bmax) of both SCN and PT was significantly higher at 16.00 h than at 4.00 h. In all these cases, however, the dissociation constant (Kd) failed to show any significant daily variation. Plasma melatonin concentrations measured by radioimmunoassay at the same time points showed a clear rhythm with a nocturnal peak present in 12L/12D animals (24.00 and 4.00 h) and a lack of variation in LL-exposed animals. These findings (1) show that there is no tissue-specific regulation of melatonin receptors between PT and SCN and (2) suggest that the daily variations in plasma melatonin concentrations could be implicated in the regulation of the density of melatonin binding sites both in the rat PT and SCN, probably by a mechanism of desensitization of the melatonin binding sites by melatonin itself.