GABA INTERNEURONS IN THE RAT MEDIAL FRONTAL-CORTEX - CHARACTERIZATIONBY QUANTITATIVE INSITU HYBRIDIZATION OF THE GLUTAMIC-ACID DECARBOXYLASE (GAD67) MESSENGER-RNA

In situ hybridization of mRNA encoding one isoform of glutamic acid de carboxylase (GAD67) was performed in the rat medial frontal cortex (MF C) to characterize GABA interneurons. Qualitatively, the labelling obt ained with a [S-35]cDNA probe was in register with neurons and was nev er associated with glial cells. No obvious differences in the density of labelled cells were observed between the different areas of the MFC examined (infralimbic, prelimbic, anterior cingulate and precentral m edial) and between the various cortical layers. Grain counting was per formed on single cells in the various layers of the prelimbic and the anterior cingulate area, two main areas of the MFC. According to their grain density, neurons were arbitrarily classified as low, high and v ery high GAD67 mRNA content. The neurons with the high GAD67 mRNA cont ent corresponded to around 50% of the labelled cells in all the layers and in both areas. In the prelimbic area, the neuronal population wit h a low GAD67 mRNA content varied from 50% in layers I and II-III to 4 0% in layers V-VI whereas the very high GAD67 mRNA content neurons cor responded to around 5% of the labelled neurons in all layers. In the a nterior cingulate area the neuronal population showing low GAD67 mRNA content varied from 35% in layers I and II-III to 20% in layers V-VI. In this area, neurons with a very high GAD67 mRNA content were more nu merous than in the prelimbic area: they varied from 15% in layers I an d II-III to 30% in layers V-VI. Parallel to the presence of very highl y labelled cells, GAD enzymatic activity measured both in the presence and in the absence of pyridoxal 5'-phosphate was higher in the anteri or cingulate area than in the prelimbic area. The heterogeneity of GAD 67 mRNA content at the cellular level might underlie the existence of subpopulations of GABA interneurons in the MFC and suggests a higher G ABAergic inhibitory control in the anterior cingulate area than in the prelimbic area.