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
J. Recio et al., 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, Journal of pineal research, 24(3), 1998, pp. 162-167
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.