Photic Regulation of mt1 Melatonin Receptors in the Siberian Hamster Pars Tuberalis and Suprachiasmatic Nuclei: Involvement of the Circadian Clock and Intergeniculate Leaflet

Abstract

In the Siberian hamster suprachiasmatic nuclei and pars tuberalis of the pituitary, high affinity mt₁ melatonin receptors are present. We have previously shown that night applied light pulse induced an increase in mt₁ mRNA expression in the suprachiasmatic nuclei of this species, independently of the endogenous melatonin. Here, we report the photic regulation of melatonin receptor density and mRNA expression in the suprachiasmatic nuclei and pars tuberalis of pinealectomized Siberian hamsters and the implication in this control of either the circadian clock or the intergeniculate leaflet. The results show that: (1) A 1‐h light pulse, delivered during the night, induces a transitory increase in mt₁ 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 melatonin 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 mt₁ mRNA rise is gated to the subjective night in the suprachiasmatic nuclei and pars tuberalis. In contrast, the light‐induced increase in melatonin binding is also observed in the subjective day in the pars tuberalis. (4) intergeniculate leaflet lesion totally inhibits the mt₁ mRNA expression rise in the suprachiasmatic nuclei, while it has no effect on the light‐induced increase in mt₁ mRNA in the 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 mt₁ mRNA expression and receptor density are independent of each other in both the suprachiasmatic nuclei and pars tuberalis; and (2) the circadian clock and the intergeniculate leaflet are implicated in the photic regulation of melatonin receptors but their level of action differs totally between the suprachiasmatic nuclei and pars tuberalis.