DAILY AND PHOTOPERIODIC 2-I-125-MELATONIN BINDING CHANGES IN THE PARSTUBERALIS OF THE SYRIAN-HAMSTER (MESOCRICETUS-AURATUS) - EFFECT OF CONSTANT LIGHT EXPOSURE AND PINEALECTOMY

Using quantitative autoradiography, 2(125)I-melatonin binding was inve stigated throughout the light/dark cycle in the pars tuberalis (PT) of the pituitary of adult Syrian hamsters kept for 8 weeks either in lon g or short photoperiod (LP or SP, respectively). Melatonin receptor de nsity in the PT displayed photoperiod dependent daily variations (maxi mal values in LP). Indeed, in LP, melatonin receptor density underwent strong daily variations with maximal values during the first half of the light period and minimal values at the end of the night. These var iations depended on changes in the maximal binding (B-max) without dif ferences in the dissociation constant (K-d). In contrast, PT melatonin receptor density was constant and at a very low level throughout the light:dark cycle in SP exposed animals. Daily PT melatonin receptor de nsity variations of LP exposed animals were abolished by pinealectomy or continuous light exposure, These results show clearly that both at the daily and at the seasonal level the regulation of PT melatonin rec eptors is strongly dependent on circulating melatonin concentrations i n the Syrian hamster, but that other regulatory factors, yet unclarifi ed, might also play a role.