Melatonin regulates the mRNA expression of the mt(1) melatonin receptor inthe rat pars tuberalis

The pars tuberalis (PT) of the pituitary is a major neuroendocrine target s ite for melatonin as it contains a large number of high-affinity melatonin receptors, We have previously shown that melatonin autoregulates the densit y of its own receptors in the PT. However, whether melatonin regulation inc ludes mRNA expression in vivo is unclear. In the present study we have used quantitative in situ hybridization to (1) follow the daily profile of mt(1 ) mRNA expression in the rat PT and (2) investigate whether mt(1) mRNA expr ession could be regulated in vivo by melatonin, We found clear diurnal vari ations of mt(1) mRNA expression that persist in constant darkness. We also showed, on pinealectomized animals, that the rhythmic pineal melatonin secr etion is necessary for the expression of these daily variations. In a secon d step, we studied the effect of an acute suppression of endogenous melaton in synthesis on mt(1) melatonin receptors by applying a 1-hour light pulse during the night. We found that light induced a dramatic increase in mt(1) mRNA which was totally prevented by a melatonin injection showing that the acute effect of melatonin on the receptor mRNA is strongly inhibitory. A li ght pulse applied to animals with a chronic absence of melatonin was ineffe ctive showing that light only affects melatonin receptors via the light-ind uced plasma melatonin suppression. Altogether our results show that melaton in regulates mt(1) melatonin receptor mRNA expression. However, this regula tion seems to be complex: acute changes in plasma melatonin concentration r egulate negatively the gene transcription, even if the daily endogenous noc turnal melatonin peak seems a prerequisite for variations in its receptor e xpression. Copyright (C) 2000 S. Karger AG, Basel.