Melatonin Receptors in the Lamb Pars tuberalis/Median Eminence throughout the Day

Abstract

After validation of the methodology, melatonin receptors have been measured by binding of (¹²⁵I)-melatonin to membranes of individual pars tuberalis/median eminence of lambs at different times of the day in the course of three experiments. Plasma melatonin was assessed by radioimmunoassay 60 min, 30 min and just before slaughter. Kd was found in the range of 10–25 pM and did not vary with the time of slaughter. Particularly, the mean Kd values were identical and equal to 17 pM in animals slaughtered either during the light or the dark phase of the day. The pattern of Bmax changes was similar in the three experiments and varied significantly (p < 0.001) with the time of the day. The apparent numbers of receptors were found to be the highest at the end of the day and at the onset of the night and to be the lowest at the end of the night: 63.4 ± 6.3, 61.4 ± 6.5 and 34.4 ± 2.8 fmol/mg protein (mean ± SEM), respectively. Values were found to be intermediary either at the middle of the day (51.1 ± 5.3 fmol/mg protein) or at the middle of the night (40.4 ± 6.3 fmol/mg protein). Furthermore, plasma melatonin was positively correlated to the melatonin receptor number (p < 0.02) but only at the onset of the night. In conclusion, the results are strongly suggestive of the existence of a circadian rhythm in the apparent number of melatonin receptors in the lamb pars tuberalis/median eminence with a possible downregulation occurring during the night.

Prolactin Cycles in Sheep under Constant Photoperiod: Evidence That Photorefractoriness Develops Within the Pituitary Gland Independently of the Prolactin Output Signal

“…The development of photorefractoriness does not appear to result from a loss of melatonin receptors in the PT, a change in receptor-G protein coupling, or a change in the inhibitory cAMP response to melatonin, at least based on the results of early studies in Siberian hamsters rendered refractory to short days [31]. More detailed work has revealed that melatonin inhibits MT1-receptor expression in the PT and that a 24-h cycle exists in receptor expression inverse to the blood melatonin rhythm, with increased amplitude under long days compared with short days [32,33], which also occurs in sheep [34]. Still, no evidence suggests that photorefractoriness results from melatonin receptor-related down-regulation or desensitization in the target cells of the PT.…”

Regulation of the ovine MT1 melatonin receptor promoter: Interaction between multiple pituitary transcription factors at different phases of development

“…Finally we examined the possible role of putative E-box elements observed within the 261 oMT 1 promoter (data not shown) in the reported daily rhythms of PT melatonin 262 receptor expression (Piketty and Pelletier, 1993). The mRNA of circadian clock 263 transcription factors Clock and Bmal1 were both expressed in the ovine PT (Fig.…”

Functional melatonin receptors and metabolic coupling in cultured chick astrocytes

“…To determine specific IMEL binding in cultured cells, a modified version of the radioreceptor assay used by Piketty and Pelletier (1993) was employed. Briefly, 100l of homogenate buffer containing 1 of 9 concentrations (20,50,150,200,400,600,800, and 1,000 pM) of IMEL (DuPont; 2,000 Ci/mmol) was added to the duplicate samples described above to form 200 l samples with final IMEL concentration of 10, 25, 75, 100, 200, 300, 400, and 500 pM.…”