In order to get more detailed information on the neural circuit of the lateralis medialis‐suprageniculate nuclear (LM‐Sg) complex of the cat, the GABAergic innervation of this complex was studied by GABA immunohistochemical techniques. Small immunoreactive cells were found throughout the LM‐Sg complex. On the basis of their ultrastructural features, these GABAergic cells were identified as Golgi type II interneurons. The neuropil of this nucleus displayed a conspicuous granular immunoreactivity. Ultra‐structurally, the immunoreactive neural profiles in the neuropil were identified as the presynaptic dendrites of interneurons, myelinated axons, or axon terminals. The GABAergic‐dendritic profiles, containing pleomorphic synaptic vesicles, were involved in synaptic glomeruli. Additionally, GABAergic axon terminals containing pleomorphic synaptic vesicles formed symmetric axodendritic synaptic contacts mainly in the extraglomerular neuropil. They appeared to correspond to either axon terminals from the thalamic reticular nucleus (TRN) or the axon terminals of interneurons. The projections from the TRN to the LM‐Sg complex were studied by using wheat germ agglutinin‐conjugated horseradish peroxidase (WGA‐HRP). Following injection of WGA‐HRP into the LM‐Sg complex, a number of retrogradely labeled cells were observed in the TRN. The connections between the TEN and the LM‐Sg complex appeared to be topographically organized, the dorsal TRN being connected mainly with the dorsomedial portion of the LM‐Sg complex, and the ventral TRN being connected chiefly with the ventrolateral portion of the LM‐Sg complex. Following injection of the tracer into the TRN. ultrastructural examination of anterograde labeling in the LM‐Sg complex revealed that labeled terminals contain pleomorphic vesicles and make symmetric synaptic contacts mainly with small to medium‐sized dendrites. The labeled terminals were not involved in synaptic glomeruli. The present results provide anatomic support for the contention that the projection cells of the LM‐Sg complex may be inhibited by both the TRN axons and interneurons, probably through the mediation of GABA.
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