Circadian rhythm of mt₁ melatonin receptor expression in the suprachiasmatic nucleus of the C3H/HeN mouse

This report studied the diurnal and circadian rhythms of mt₁ melatonin receptor expression in the SCN of C3H/HeN mice maintained in either a light:dark (LD) cycle or in constant dark for a minimum of 6 wk. Diurnal times (ZT) were assessed with reference to the onset of the light period (ZT0) and circadian times (CT) were established by determining the phase of wheel running activity of each mouse before sacrifice. 2-[¹²⁵I]-Iodomelatonin binding in the SCN revealed low amplitude diurnal and circadian rhythms with highest levels of binding 2 hr after lights on (41.3 ± 1.7 fmol/mg protein, n = 5, at ZT2) or at the beginning of the subjective day (48.6 ± 2.1 fmol/mg protein, n = 6, CT2), respectively. The expression of mt₁ mRNA, determined by in situ hybridization with a ³⁵S-labeled mouse mt₁ riboprobe, showed robust diurnal and circadian rhythms. In animals housed under a LD cycle, low levels of expression were observed during the day, with a rapid rise in mt₁ melatonin receptor expression at the beginning of the dark period (ZT14), coincident with an abrupt increase in levels of circulating melatonin measured by radioimmunoassay. In animals housed under constant dark conditions, a robust peak of mt₁ mRNA expression occurred in the middle of the subjective night (CT18), 8 hr before the peak of protein expression, while the lowest levels of mt₁ mRNA expression were observed during the day (CT10). Results suggest that mt₁ melatonin receptor rhythm in the C3H/HeN mouse SCN is regulated both by light and by the biological clock as distinct rhythms of both mRNA and protein are differentially expressed under a LD cycle and constant dark conditions.