COMPARATIVE DISTRIBUTION OF MESSENGER-RNAS ENCODING GLUTAMIC-ACID DECARBOXYLASES (MR 65,000 AND MR 67,000) IN THE BASAL GANGLIA OF THE RAT

Glutamic acid decarboxylase, the enzyme required for GABA synthesis, e xists as distinct isoforms, which have recently been found to be encod ed by different genes. The relative expression of messenger RNAs encod ing two isoforms of glutamic acid decarboxylase (M(r) 67,000 and M(r) 65,000) was measured at the single-cell level in neurons of the rat ba sal ganglia with in situ hybridization histochemistry. Both messenger RNAs were expressed in neurons of the striatum, pallidum, and substant ia nigra pars reticulata, but marked differences in the relative level of labelling were observed with the two probes. In striatum, efferent neurons were more densely labelled for the messenger RNA encoding glu tamic acid decarboxylase (M(r) 65,000) than for the messenger RNA enco ding glutamic acid decarboxylase (M(r) 67,000), whereas the reverse wa s observed for GABA-ergic interneurons. Neurons of the entopeduncular nucleus were much more densely labelled for messenger RNA encoding glu tamic acid decarboxylase (M(r) 65,000) than for messenger RNA encoding glutamic acid decarboxylase (M(r) 67,000). In addition, labelling for messenger RNA encoding glutamic acid decarboxylase (M(r) 65,000) was higher in the entopeduncular nucleus (internal pallidum) than in the g lobus pallidus (external pallidum), a structure which expressed simila r levels of both mRNAs. In contrast to neurons of the internal pallidu m, efferent neurons of the substantia nigra pars reticulata expressed slightly more messenger RNA encoding glutamic acid decarboxylase (M(r) 67,000) than that encoding the other isoform of the enzyme. The resul ts suggest a differential expression of the messenger RNAs encoding th e two isoforms of glutamic acid decarboxylase in subpopulations of bas al ganglia neurons in rats.