Photic regulation of mt(1) melatonin receptors in the Siberian hamster pars tuberalis and suprachiasmatic nuclei: Involvement of the circadian clock and intergeniculate leaflet

In the Siberian hamster suprachiasmatic nuclei and pars tuberalis of the pi tuitary, high affinity mt, melatonin receptors are present. We have previou sly shown that night applied light pulse induced an increase in mt(1) mRNA expression in the suprachiasmatic nuclei of this species, independently of the endogenous melatonin, Here, we report the photic regulation of melatoni n receptor density and mRNA expression in the suprachiasmatic nuclei and pa rs tuberalis of pinealectomized Siberian hamsters and the implication in th is control of either the circadian clock or the intergeniculate leaflet. Th e results show that: (1) A 1-h light pulse, delivered during the night, ind uces a transitory increase in mt(1) mRNA expression in the suprachiasmatic nuclei and pars tuberalis. After 3 h this increase has totally disappeared (suprachiasmatic nuclei) or is greatly reduced (pars tuberalis), (2) The me latonin receptor density, in the suprachiasmatic nuclei, is not affected by 1 or 3 h of light, while it is strongly increased in the pars tuberalis. ( 3) In hamsters kept in constant darkness, the mt, mRNA rise is gated to the subjective night in the suprachiasmatic nuclei and pars tuberalis, In cont rast, the light-induced increase in melatonin binding is also observed in t he subjective day in the pars tuberalis, (4) intergeniculate leaflet lesion totally inhibits the mt, mRNA expression rise in the suprachiasmatic nucle i, while it has no effect on the light-induced increase in mt(1) mRNA in th e pars tuberalis, However, the light-induced increase in melatonin receptor density is totally prevented by the intergeniculate leaflet lesion in the pars tuberalis. These results show that: (1) the photic regulations of mt(1 ) mRNA expression and receptor density are independent of each other in bot h the suprachiasmatic nuclei and pars tuberalis; and (2) the circadian cloc k and the intergeniculate leaflet are implicated in the photic regulation o f melatonin receptors but their level of action differs totally between the suprachiasmatic nuclei and pars tuberalis.