Ultrastructural analysis of GABA-immunoreactive elements in the monkey thalamic ventrobasal complex.

This study describes the ventrobasal complex of the primate by using GABA immunocytochemistry at the electron microscopic level. The primate ventrobasal complex has a similar synaptic organization to sensory thalamic nuclei in other species. Two synaptic profiles within the ventrobasal complex contain flattened or pleomorphic synaptic vesicles and are GABA-immunoreactive. F-boutons (= F1 type, Guillery's classification; Guillery: Z. Zellforsch. 96:1-38, '69) are located principally in the extraglomerular neuropil and contain densely packed flattened synaptic vesicles and several elongate mitochondria and establish symmetric (Gray's type II) synaptic contacts. These boutons are not found postsynaptic to any other element and are presynaptic principally to nonimmunoreactive elements that are thought to be thalamocortical relay cell dendrites. PSD-boutons (= F2 type, Guillery's classification) contain a moderate number of flattened or pleomorphic synaptic vesicles and fewer mitochondria than F-boutons. PSD-boutons are found in glomerular and extraglomerular areas of neuropil and establish symmetric synaptic contacts. These boutons are considered to be appendages of interneuron dendrites and are postsynaptic to RL-, RS (Guillery's classification)-, F-, and other PSD-boutons. PSD-boutons are presynaptic to thalamocortical relay neurons and interneuron dendrites including PSD-boutons. Problems in distinguishing F- from PSD-boutons are addressed by comparing immunostained and nonimmunostained material and by the use of serial sections. The majority of synaptic contacts between pleomorphic vesicle-containing profiles appear to be between PSD-boutons and other components of interneurons. Few contacts between F-boutons and local circuit neurons are seen. These data suggest the principal GABAergic input to interneurons in the primate ventrobasal complex is derived from other interneurons.