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 (PT) 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 2-125I-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.(ABSTRACT TRUNCATED AT 250 WORDS)
Gauer
F
F
Neurobiologie des Fonctions Rythmiques et Saisonnières, URA-CNRS 1332, Université Louis Pasteur, Strasbourg, France.
Masson-Pévet
M
M
Skene
D J
DJ
Vivien-Roels
B
B
Pévet
P
P
D000818:Animals; D001345:Autoradiography; D002940:Circadian Rhythm / Q000502:physiology*; D008027:Light; D008297:Male; D008550:Melatonin / Q000378:metabolism / Q000502:physiology*; D010903:Pituitary Gland, Anterior / Q000378:metabolism*; D011869:Radioligand Assay; D051381:Rats; D017208:Rats, Wistar; D044094:Receptors, Melatonin; D017981:Receptors, Neurotransmitter / Q000378:metabolism*; D013493:Suprachiasmatic Nucleus / Q000378:metabolism*
D044094:Receptors, Melatonin; D017981:Receptors, Neurotransmitter; D008550:Melatonin