INVESTIGATIONS OF THE REGULATION OF SPECIFIC 2-[I-125]IODOMELATONIN BINDING-SITES IN SIBERIAN HAMSTERS BY ENDOGENOUS AND EXOGENOUS MELATONIN

These studies used quantitative in vitro autoradiography to investigat e whether endogenous or exogenous melatonin modulate specific 2-[I-125 ]iodomelatonin binding sites in the pars tuberalis or suprachiasmatic nuclei of Siberian hamsters. Saturation analyses were conducted on tis sue sections from hamsters that were pinealectomized, exposed to const ant illumination (72 h), or injected with melatonin, and from hamsters that were treated as controls. High affinity (K-d approximately 20-75 pM) specific 2-[I-125]iodomelatonin binding sites were detected in th e suprachiasmatic nuclei and pars tuberalis of all animals. Neither pi nealectomy nor constant illumination significantly affected either the affinity or the density of the specific 2-[I-125]iodomelatonin bindin g sites in either region. Melatonin injections led to a decrease in sp ecific 2-[I-125]iodomelatonin binding to the pars tuberalis at 3-4 h a fter the last injection. However, washing the sections before incubati on with 2-[I-125]iodomelatonin eliminated this effect, suggesting that melatonin was occupying the binding sites rather than decreasing thei r actual number. Furthermore, when hamsters were sacrificed 18 h after the last melatonin injection, no effect of melatonin on either the af finity or density of specific 2-[I-125]iodomelatonin sites was observe d. These data suggest that 2-[I-125]iodomelatonin binding sites in Sib erian hamsters are not regulated by changes in circulating melatonin l evels.