Rhythms of glutamic acid decarboxylase mRNA in the suprachiasmatic nucleus

The purpose of the present study was to determine whether there is a rhythm in glutamic acid decarboxylase (GAD) message in the suprachiasmatic nucleus (SCN) of rats housed in a Light:dark cycle. The mRNAs encoding two isoforms of GAD (i.e., GAD(65) and GAD(67)) were examined using in situ hybridization histochemistry. Computerized image analysis of film autoradiographs revealed that GAD(65) mRNA was significantly higher in the light than it was in the dark. GAD(67) mRNA levels were lower overall and did not decrease significantly in the dark. Following emulsion autoradiography, silver grain counts over individual SCN cells indicated that GAD(65) mRNA was highest in the dorsomedial hypothalamus during the light. These data suggest that GAD mRNA varies rhythmically in the SCN and that mRNA levels are regulated differently within SCN subdivisions during the light:dark cycle.