CIRCADIAN CHANGES OF GLUTAMATE-DECARBOXYLASE-65 AND GLUTAMATE-DECARBOXYLASE-67 MESSENGER-RNA IN THE RAT SUPRACHIASMATIC NUCLEI

GABA has been implicated in the regulation of the circadian clock with in the suprachiasmatic nuclei (SCN). In the present study in situ hybr idization histochemistry was used to assess expression of mRNA for the two isoforms of the enzyme governing GABA synthesis, glutamic acid de carboxylase (GAD) 65 and 67, within the rat SCN during a 24 h cycle in constant darkness. GAD 65 mRNA exhibited a monophasic rhythm, with a peak at the beginning of the subjective day and a nadir early in the s ubjective night. In contrast, there was a biphasic variation in GAD 67 expression, with peaks at the beginning of the subjective day and ear ly in the subjective night. No significant variation in the mRNAs for GAD 65 or GAD 67 was observed in the cingulate cortex. These results s uggest that the clock intrinsic to the SCN controls the expression of the mRNAs encoding GAD 65 and GAD 67 in both a coordinated and a diffe rential manner.