Glutamate decarboxylase‐immunoreactive structures in the rat neostriatum: A correlated light and electron microscopic study including a combination of Golgi impregnation with immunocytochemistry

Abstract

An antibody to glutamate decarboxylase has been used in a light and electron microscopic study of the neostriatum of rats that had received intracerebral injections of colchicine. In the light microscope, neuronal perikarya and small punctate structures that displayed immunoreactivity were found. The perikarya could be divided into two classes based on their sizes: small‐to‐medium‐sized and large. Proximal dendrites, axon initial segments, and axon collaterals were occasionally stained. When the nuclei of the neurons were visible, they possessed indentations. The immunoreactive punctate structures were spread evenly throughout the neostriatum but occasionally were associated with immunoreactive and nonimmunoreactive perikarya. When the same sections were examined in the electron microscope, the small‐to‐medium‐sized immunoreactive perikarya were found to be similar in morphology and synaptic input to a class of Golgi‐impregnated neuron that has been previously shown to accumulate locally administered, radiolabelled γ‐aminobutyric acid. Neurons with the ultrastructural characteristics of typical striatonigral neurons did not display immunoreactivity. As neurons in this pathway probably contain γ‐aminobutyric acid, it is possible that our procedure or our antibody does not stain all γ‐aminobutyric‐acid‐containing structures in the neostriatum. A total of 404 immunoreactive punctate structures were examined by correlated light and electron microscopy or by electron microscopy alone. They were identified as immunoreactive axonal boutons and each of them, when examined in serial sections, displayed typical synaptic specialisations. Membrane specialisations were always of the symmetrical type. At least five distinct targets of the immunoreactive terminals were identified: (1) neurons that were themselves immunoreactive for glutamate decarboxylase; the immunoreactive terminals made synaptic contact with all parts of the neurons examined, i.e., perikarya, proximal dendrites, and axon initial segments. (2) Neurons identified by Golgi impregnation of the same sections as medium‐sized and densely spiny; the immunoreactive terminals made contact predominantly with the perikarya and dendritic shafts. (3) Large neurons found only in the ventral caudate‐putamen, whose somata and dendrites were ensheathed in immunoreactive terminals. (4) Medium‐sized nonimmunoreactive perikarya that possessed nuclear indentations. (5) Large nonimmunoreactive perikarya that had the typical structural features of striatal cholinergic neurons. It is concluded that there is a diverse innervation of many types of striatal neuron by synaptic terminals that possess the ability to synthesize γ‐aminobutyric acid and that these terminals probably arise from neurons within the neostriatum.