Light and electron microscopic immunocytochemical localization of glutamic acid decarboxylase in monkey geniculate complex: Evidence for gabaergic neurons and synapses

Abstract

The morphology and distribution of neuronal cell bodies and synapses that utilize γ-aminobutyric acid (GABA) as a transmitter have been studied in Macaca dorsal lateral geniculate nucleus (dLGN), ventral lateral geniculate nucleus (vLGN), and reticular nucleus of the thalamus (RNT). These cells are identified by immunocytochemical staining for the antiserum to the enzyme that synthesizes GABA, glutamic acid decarboxylase (GAD) and visualized by the peroxidase-antiperoxidase method for light and electron microscopy. In the dLGN, GAD-positive (GAD+) cell bodies are found in all layers and in the interlaminar zones. The neuropil is more heavily labeled in the laminar than the interlaminar zones, with the magnocellular layers being most densely labeled. In the vLGN no stained cell bodies are seen, but the neuropil is intensely GAD+, especially in the retinal input layer. The RNT is just the opposite, with many GAD+ neurons but sparsely labeled neuropil. The GAD+ neurons in the dLGN have small cell bodies with a large infolded, unlabeled nucleus. GAD reactivity is found in the thin rim of cytoplasm and in the two to four thick, straight dendrites, and on the cytoplasmic surfaces of mitochondria, Golgi and membraneous cisternae, and vesicles. GAD+ synaptic profiles are found in both pre- and postsynaptic relationships in the dLGN. Some GAD+ profiles are both pre- and postsynaptic in the same section, suggesting that these are presynaptic dendrites. In the laminae, GAD+ presynaptic profiles are very commonly found within synaptic glomeruli where they lie postsynaptic to a retinal axon terminal and presynaptic to an unlabeled dendrite, which also receives input from the retinal terminal. Outside of glomeruli, individual GAD+ profiles are presynaptic to either labeled or unlabeled cell bodies and dendrites. Other GAD+ profiles are postsynaptic to unlabeled F or RSD-type terminals. Individual GAD+ profiles ending on unlabeled or labeled dendrites are found in the interlaminar zones. These data, when combined with previously published work on dLGN circuitry, suggest that GAD+ glomerular profiles are the presynaptic dendrites of the dLGN interneuron, whereas the individual GAD+ contacts arise from axons of RNT neurons.