Two isoforms of glutamic acid decarboxylase (GAD67 and GAD65) and their mRNAs were localized in the rat brain by immunohistochemistry and nonradioactive in situ hybridization methods with digoxigenin-labeled cRNA probes. In most brain regions, both GAD isoforms were present in neuronal cell bodies as well as axon terminals. A few populations of neurons, such as those in the reticular nucleus of the thalamus, exhibited similar cell body labeling for both GADs. However, in many brain regions, the cell bodies that were immunoreactive for GAD67 were often more numerous than those that were immunoreactive for GAD65. In contrast, the density (quantity) of GAD65-immunoreactive axon terminals was higher than that of GAD67-immunoreactive terminals. Strong parallels were observed between the intensity of immunohistochemical labeling of cell bodies and the levels of mRNA labeling for both GAD isoforms. Many groups of GAD-containing cell bodies were distinctly labeled for GAD67, and these same groups of neurons were heavily labeled for GAD67 mRNA. Such neurons included Purkinje cells of the cerebellar cortex, nonpyramidal cells in the cerebral cortex, and neurons of the reticular nucleus of the thalamus. Similar parallels in labeling were observed for GAD65 and its mRNA. Distinct cell body labeling for the protein and associated high levels of GAD65 mRNA were found in neurons of the reticular nucleus of the thalamus and periglomerular cells in the olfactory bulb. However, many cell bodies were not readily labeled for GAD65 with immunohistochemical methods. Such absence or weakness of cell body labeling for the protein was associated with low or moderate levels of GAD65 mRNA. Even though light cell body staining was frequently observed for GAD65 and its mRNA, strong axon terminal labeling for GAD65 was present. Thus, in the deep cerebellar nuclei to which the Purkinje cells of the cerebellar cortex project, strong terminal labeling was observed for both GAD isoforms even though only light cell body labeling of the Purkinje cells was obtained for GAD65 and its mRNA. The findings suggest that the two isoforms of GAD are present in most classes of GABA neurons but that they are not similarly distributed within the neurons. GAD67 is present in readily detectable amounts in many GAD-containing cell bodies whereas GAD65 is particularly prominent in many axon terminals. In addition, neurons that express either form of GAD mRNA also express the corresponding protein. Levels of labeling for the GAD mRNAs suggest that, under normal conditions, the synthesis of GAD65 is frequently lower than that of GAD67.(ABSTRACT TRUNCATED AT 400 WORDS)

Impaired inhibition is thought to be important in temporal lobe epilepsy (TLE), the most common form of epilepsy in adult patients. We report that, in experimental TLE, spontaneous GABAergic inhibition was increased in the soma but reduced in the dendrites of pyramidal neurons. The former resulted from the hyperactivity of somatic projecting interneurons, whereas the latter was probably due to the degeneration of a subpopulation of dendritic projecting interneurons. A deficit in dendritic inhibition could reduce seizure threshold, whereas enhanced somatic inhibition would prevent the continuous occurrence of epileptiform activity.

We studied the modulation of GABAergic inhibition by glutamate and kainate acting on GluR5-containing kainate receptors in the CA1 hippocampal region. Glutamate, kainate or ATPA, a selective agonist of GluR5-containing receptors, generates an inward current in inhibitory interneurons and cause repetitive action potential firing. This results in a massive increase of tonic GABAergic inhibition in the somata and apical dendrites of pyramidal neurons. These effects are prevented by the GluR5 antagonist LY 293558. Electrical stimulation of excitatory afferents generates kainate receptor-mediated excitatory postsynaptic currents (EPSCs) and action potentials in identified interneurons that project to the dendrites and somata of pyramidal neurons. Therefore glutamate acting on kainate receptors containing the GluR5 subunit may provide a protective mechanism against hyperexcitability.