Investigations of the regulation of specific 2-[¹²⁵I]iodomelatonin binding sites in Siberian hamsters by endogenous and exogenous melatonin

These studies used quantitative in vitro autoradiography to investigate whether endogenous or exogenous melatonin modulate specific 2-[¹²⁵I]iodomelatonin binding sites in the pars tuberalis or suprachiasmatic nuclei of Siberian hamsters. Saturation analyses were conducted on tissue sections from hamsters that were pinealectomized, exposed to constant 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-[¹²⁵I]iodomelatonin binding sites were detected in the suprachiasmatic nuclei and pars tuberalis of all animals. Neither pinealectomy nor constant illumination significantly affected either the affinity or the density of the specific 2-[¹²⁵I]iodomelatonin binding sites in either region. Melatonin injections led to a decrease in specific 2-[¹²⁵I]iodomelatonin binding to the pars tuberalis at 3-4 h after the last injection. However, washing the sections before incubation with 2-[¹²⁵I]iodomelatonin eliminated this effect, suggesting that melatonin was occupying the binding sites rather than decreasing their actual number. Furthermore, when hamsters were sacrificed 18 h after the last melatonin injection, no effect of melatonin on either the affinity or density of specific 2-[¹²⁵I]iodomelatonin sites was observed. These data suggest that 2-[¹²⁵I]iodomelatonin binding sites in Siberian hamsters are not regulated by changes in circulating melatonin levels.