Differences of somatostatin mRNA in the rat suprachiasmatic nucleus under light-dark and constant dark conditions: an analysis by in situ hybridization.

Daily profiles of somatostatin mRNA expression were investigated in the rat suprachiasmatic nucleus (SCN) by semiquantitative in situ hybridization histochemistry. Under 12 h light/12 h dark conditions, somatostatin mRNA signals were higher during the day time (Zeitgeber time (ZT) 1) than during the night time (ZT 16). This day-night difference was still maintained in constant darkness where the somatostatin mRNA was higher in the subjective day (circadian time (CT) 1) than in the subjective night (CT 16). Together with previous Northern blot hybridization studies, the present observation suggests that the level of somatostatin mRNA in SCN neurons is controlled by the circadian clock, independent of photic environment.