We have examined the distribution of neurons and terminals that are immunoreactive for glutamic acid decarboxylase (GAD), the synthesizing enzyme for the inhibitory neurotransmitter gamma-aminobutyric acid within the lateral geniculate nucleus of the cat. We estimate that GAD-positive neurons constitute approximately one-fourth of the neurons in all layers of the lateral geniculate nucleus and in the medial interlaminar nucleus (MIN). In addition, almost all of the neurons within the perigeniculate nucleus are GAD-positive. The mean size of GAD-positive cell bodies is significantly smaller than the mean size of unlabeled neurons in all subdivisions of the lateral geniculate nucleus. GAD-positive neurons have thick primary dendrites which are associated with thin lightly immunoreactive processes that give rise to clusters of GAD-positive terminals. Clusters of GAD-positive terminals are prominent in lamina A, A1, magnocellular C, and MIN but are rare in the parvocellular C laminae. Within the A laminae, GAD immunoreactivity is found within vesicle-containing profiles of the synaptic glomerulus lying postsynaptic to optic axon terminals and presynaptic to unlabeled dendritic profiles. GAD-positive neurons in the A laminae are distinguished from other small to medium-sized neurons by their failure to label following injections of HRP into visual cortex and by their lack of cytoplasmic laminated body. These results support the idea that GAD-positive neurons constitute a distinct population of neurons in the lateral geniculate nucleus of the cat; a population which has a number of features in common with previous descriptions of presumed local circuit neurons based on Golgi staining.

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Intracellular injection of HRP combined with immunocytochemistry for [Leu]enkephalin was used to demonstrate striatal spiny neuron dendritic and local axonal arborizations in the same section as enkephalin-rich patches (striosomes). Cobalt intensification of the first DAB reaction prior to the immunoperoxidase steps resulted in good contrast between the black reaction product in the intracellularly labeled cells and the brown staining for [Leu]enkephalin. Serial reconstructions of the labeled cells and nearby boundaries between the enkephalin-rich striosomes and enkephalin-poor matrix allowed the relationship between the arborizations of the labeled cells and these boundaries to be established. It was also possible to examine the relationship to compartmental boundaries of a second neuronal class consisting of large, pallidallike neurons whose somatodendritic morphology was outlined by immunoperoxidase-labeled terminals. We found that spiny projection neurons in both compartments have dendritic arbors and local axonal collaterals that are confined by compartmental boundaries. The termination or recurvature of dendrites at such boundaries suggests that the cellular basis of striatal compartmental organization is provided by this class of striatal neuron. On the other hand, large pallidumlike striatal neurons were found to have dendrites that extend across compartmental boundaries. These results support previous reports that striatal spiny projection neurons preserve the compartmental segregation of parallel striatal input-output systems, whereas other classes of striatal neurons may serve to provide limited integration between compartments.

We have examined the distribution of neurons and terminals immunoreactive for glutamic acid decarboxylase (GAD) in the thalamus and adjacent structures of the opossum (Didelphis virginiana) and the rabbit and have compared this distribution with the distributions we described previously for the cat and bushbaby (Galago senegalensis). The significance of these experiments depends, first, on the fact that GAD is the synthetic enzyme for GABA, and therefore that GAD immunoreactivity is a marker for GABAergic inhibitory neurons, and second, on previous findings that suggest that GABAergic neurons in the dorsal thalamus are local circuit neurons. In both cat and Galago, GAD-immunoreactive neurons are distributed essentially throughout the entire thalamus. In the opossum, GAD neurons are chiefly confined to the dorsal lateral geniculate nucleus and the lateral extremity of the lateral posterior nucleus. The distribution of GAD neurons in the rabbit is intermediate between that found in the opossum on the one hand and cat and Galago on the other. Like opossum, about 25% of the neurons in the lateral geniculate nucleus of rabbit are GAD immunoreactive. Unlike opossum, however, as many as 18% of the cells in the ventral posterior nucleus of the rabbit are GAD immunoreactive, and scattered cells are also labeled in other thalamic areas, such as the medial geniculate and the lateral group. Aside from the findings in the dorsal thalamus, the chief observation is that GAD-immunoreactive neurons and/or terminals densely fill all principal targets of the optic tract, including the ventral lateral geniculate nucleus; the superficial gray layer of the superior colliculus; the anterior, posterior, and olivary pretectal nuclei; the nucleus of the optic tract; and the medial and lateral terminal nuclei of the accessory optic tract. These results support the idea first put forward by Cajal that local circuit neurons increase in number during the course of the evolution of complex mammalian brains. If we can assume that the conservative opossum retains characteristics reflecting an early stage of mammalian evolution, the results suggest that thalamic local circuit neurons arose first in the visual system and only later in evolution spread throughout the thalamus.

Immunocytochemical methods were used to identify neurons in the ventral posterior nucleus of the cat and Galago senegalensis that contain glutamic acid decarboxylase (GAD), the synthetic enzyme for the inhibitory neurotransmitter, GABA. In both species GAD-immunoreactive neurons make up about 30% of the total neurons in the ventral posterior nucleus and form a distinct class of small cells. After cortical injections of horseradish peroxidase (HRP), GAD-immunoreactive cells are not labeled with HRP and may, therefore, be GABAergic local circuit neurons. Comparison of the dendritic morphology of GAD-immunoreactive neurons with that of HRP-filled projection neurons reveals that the morphology of the GAD-containing neurons is distinct and, in particular, that the GAD-immunoreactive neurons display fewer primary dendrites. The relay neurons, in turn, can be divided into classes based on dendritic morphology and cell body size.The idea that thalamic neurons could be divided into classes based on morphological criteria has a long history. According to Rambn y Cajal (1909, the most fundamental distinction was between the neurons with long axons that project to the cerebral cortex and neurons with short axons that were thought to be local circuit neurons. Cajal found the neurons with short axons or "cellules 6 cylindre-axe court" to be distributed widely in the nervous system and to be especially numerous in the thalamus and neocortex of higher mammals. The neurons with long axons (or projection neurons) could be further subdivided into classes based on their appearance in Golgi material. T6mb61 recently (1967) identified at least two classes in the case of the somatosensory ventral posterior nucleus of the cat.

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