DAILY AND PHOTOPERIODIC MELATONIN BINDING CHANGES IN THE SUPRACHIASMATIC NUCLEI, PARAVENTRICULAR THALAMIC NUCLEI, AND PARS TUBERALIS OF THEFEMALE SIBERIAN HAMSTER (PHODOPUS-SUNGORUS)

Using quantitative autoradiography, 2-I-125-melatonin binding was inve stigated throughout the Light:dark cycle in the suprachiasmatic nuclei (SCN), paraventricular nuclei (PVT), and pars tuberalis (PT) of adult female Siberian hamsters kept for 10 weeks in either long or short ph otoperiods (LP or SP, respectively). Plasma melatonin concentrations w ere measured by radioimmunoassay, and the sexual status of the animals was established by visual inspection of vaginal smears and by weighin g uteri after sacrifice. The SCN displayed neither daily nor photoperi od-dependent variations in specific binding. Melatonin receptors in th ese nuclei would be regulated neither by plasma melatonin nor by the l ight:dark cycle or sexual steroids. By contrast, melatonin receptor de nsity in the PT displayed both strong daily (maximal values during the first half of the light period and minimal values during the night) a nd photoperiod-dependent (maximal values in LP) variations. These vari ations depended on changes in the maximal binding (B-max) without diff erences in the dissociation constant (K-d). Daily melatonin receptor d ensities in the PT of LP- and SP-exposed animals might be regulated by the dark:light transition but not by melatonin. Daily profiles of 2-I -125-melatonin-specific binding in the PT were independent of photoper iod. Factors underlying the photoperiod-related variations presently a re unknown. Concerning the PVT, weak variations in specific binding we re detected in SP only when time points were grouped according to the Light or dark periods. It is not yet possible to conclude whether they have any physiological relevance. These results show clearly that the regulation of melatonin receptors varies among structures (SCN, PVT, and PT) in the Siberian hamster and is also totally different from tha t found in the rat.