DIFFERENTIAL SEASONAL REGULATION OF MELATONIN RECEPTOR DENSITY IN THE PARS TUBERALIS AND THE SUPRACHIASMATIC NUCLEI - A STUDY IN THE HEDGEHOG (ERINACEUS-EUROPAEUS, L)

Using quantitative autoradiography, we have studied the seasonal changes of high affinity melatonin receptor density in both the SCN and PT of the hedgehog, a seasonal breeder and an hibernator. Animals in 3 different physiological states were studied: sexually active animals, and sexually inactive animals during the hibernation period, being then either euthermic or hypothermic. In sexually active animals, B(max) were 75.8 +/- 7.1 fmol/mg protein in PT and 9.1 +/- 0.5 fmol/mg protein in SCN; and Kd values were: 94 +/- 22 pM in the PT and 101 +/- 15 pM in the SCN. This specific binding was strongly decreased in the PT of sexually inactive animals. Moreover, this decrease was significantly stronger in hypothermic than in euthermic hedgehogs. Saturation studies and Scatchard analysis revealed that the observed decrease in the PT resulted from change in the B(max) but not in the Kd, B(max) values being respectively 56.4 +/- 5.9 and 29.5 +/- 1.9 fmol/mg protein in euthermic and hypothermic sexually at rest animals. In none of the different physiological states, did the density of melatonin receptors of the SCN show any changes, B(max) values being respectively 9.8 +/- 0.5 and 9.8 +/- 0.4 fmol/mg protein in euthermic and hypothermic sexually at rest animals. This shows for the first time a tissue-specific regulation of melatonin receptor density occurring in the PT but not in the SCN. Furthermore, this decrease of binding in the PT is correlated with both sexual inactivity and hibernation period. This strongly suggests that the mediation of the photoperiodic effect on seasonal functions like seasonal hypothermia and reproduction involves an effect of melatonin on the PT rather than on the SCN.