The mRNA level of the 17-kDa protein neurogranin (NG), a postsynaptic substrate of the protein kinase C, has previously been found to be decreased in rat forebrain after 24-h sleep deprivation (SD). To investigate the functional significance of this finding in various forebrain regions, the effect of 24-h SD on the mRNA level and the protein level of NG was determined in the cerebral cortex, hippocampus, and the total of the remaining subcortical forebrain plus midbrain areas (SFMA) of rats. In these areas, high levels of both NG mRNA and NG protein were detected by in situ hybridization and immunohistochemistry, respectively. NG protein was recognized in brain tissue by newly developed polyclonal antibodies. As determined by RNase protection assays, the level of NG mRNA was decreased in SFMA by 34 +/- 7% (P < 0.05) after 24-h SD, and was not significantly affected in the cerebral cortex and hippocampus. In contrast, on Western blots, the protein concentration of NG was reduced in the cerebral cortex by 37 +/- 7% (P < 0.05) whereas no significant changes were present in other brain areas tested. The results indicate that the mRNA and protein levels of NG are differentially modulated in rat brain by the prolongation of the waking period.