COMPARATIVE LOCALIZATION OF 2 FORMS OF GLUTAMIC-ACID DECARBOXYLASE AND THEIR MESSENGER-RNAS IN RAT-BRAIN SUPPORTS THE CONCEPT OF FUNCTIONALDIFFERENCES BETWEEN THE FORMS

two isoforms of glutamic acid decarboxylase (GAD67 and GAD65) and thei r mRNAs were localized in the rat brain by immunohistochemistry and no nradioactive in situ hybridization methods with digoxigenin-labeled cR NA probes. In most brain regions, both GAD isoforms were present in ne uronal cell bodies as well as axon terminals. A few populations of neu rons, such as those in the reticular nucleus of the thalamus, exhibite d similar cell body labeling for both GADs. However, in many brain reg ions, the cell bodies that were immunoreactive for GAD67 were often mo re numerous than those that were immunoreactive for GAD65. In contrast , the density (quantity) of GAD65-immunoreactive axon terminals was hi gher than that of GAD67-immunoreactive terminals. Strong parallels wer e observed between the intensity of immunohistochemical labeling of ce ll 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, nonpy ramidal cells in the cerebral cortex, and neurons of the reticular nuc leus of the thalamus. Similar parallels in labeling were observed for GAD65 and its mRNA. Distinct cell body labeling for the protein and as sociated high levels of GAD65 mRNA were found in neurons of the reticu lar nucleus of the thalamus and periglomerular cells in the olfactory bulb. However, many cell bodies were not readily labeled for GAD65 wit h immunohistochemical methods. Such absence or weakness of cell body l abeling for the protein was associated with low or moderate levels of GAD65 mRNA. Even though light cell body staining was frequently observ ed for GAD65 and its mRNA, strong axon terminal labeling for GAD65 was present. Thus, in the deep cerebellar nuclei to which the Purkinje ce lls of the cerebellar cortex project, strong terminal labeling was obs erved for both GAD isoforms even though only light cell body labeling of the Purkinje cells was obtained for GAD65 and its mRNA. The finding s suggest that the two isoforms of GAD are present in most classes of GABA neurons but that they are not similarly distributed within the ne urons. GAD67 is present in readily detectable amounts in many CAD-cont aining cell bodies whereas GAD65 is particularly prominent in many axo n terminals. In addition, neurons that express either form of GAD mRNA also express the corresponding protein. Levels of labeling for the GA D mRNAs suggest that, under normal conditions, the synthesis of GAD65 is frequently lower than that of GAD67. However, strong terminal label ing for GAD65 suggests that this protein accumulates in the axon termi nals. Different levels of GAD67 and GAD65 as well as differential intr aneuronal localizations may be related to different functions and asso ciated rates of synthesis of the two proteins.