PINEALECTOMY AND CONSTANT ILLUMINATION INCREASE THE DENSITY OF MELATONIN BINDING-SITES IN THE PARS-TUBERALIS OF RODENTS

We report here the effects of pinealectomy and light exposure on the melatonin receptor density in the pars tuberalis of the rat and the European hamster using quantitative autoradiography. Scatchard analysis revealed that 24 and 72 h of constant light exposure (LL) before sacrifice did not modify the K(d) value of melatonin for its receptors in rats and European hamsters (about 70 pM). In contrast, the B(max) value was significantly increased in both species when the animals were kept in constant illumination for 72 h before sacrifice (50%-70% compared with the controls). A similar increase was also observed in rats pinealectomized 3 days before sacrifice and then kept in either constant illumination or in 12L/12D. Pinealectomy or constant light exposure are known to result in a clear decrease in the concentration of circulating melatonin. We demonstrate here that they also result in an increase in the density of melatonin receptors. This could suggest a direct effect of melatonin on its own receptors.