DIFFERENCES OF SOMATOSTATIN MESSENGER-RNA IN THE RAT SUPRACHIASMATIC NUCLEUS UNDER LIGHT-DARK AND CONSTANT DARK CONDITIONS - AN ANALYSIS BYIN-SITU HYBRIDIZATION

Daily profiles of somatostatin mRNA expression were investigated in th e rat suprachiasmatic nucleus (SCN) by semiquantitative in situ hybrid ization histochemistry, Under 12 h light/12 h dark conditions, somatos tatin 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 subj ective night (CT 16). Together with previous Northern blot hybridizati on studies, the present observation suggests that the level of somatos tatin mRNA in SCN neurons is controlled by the circadian clock, indepe ndent of photic environment.