Ultrastructural organization of transmitters in the cat lateralis medialis-suprageniculate nucleus of the thalamus: An immunohistochemical study

The lateralis medialis-suprageniculate nuclear (LM-Sg) complex of the cat's posterior thalamus receives a rather wide variety of inputs from diverse c ortical and subcortical areas. Previous ultrastructural studies of this nuc leus demonstrated the presence of four types of vesicle-containing profiles and characterized some of these as gamma-aminobutyric acid (GABA)-containi ng terminals (Norita and Katoh [1987] J. Comp. Neurol. 263:54-67; Norita an d Katoh [1988] Frog. Brain Res. 75:109-118). The present study has extended these observations by examining the immunoreactivity (ir) of LM-Sg, with a ntibodies raised against aspartate (Asp), glutamate (Glu), GABA, the acetyl choline (ACh) marker, choline acetyltransferase (ChAT), and substance P (SP ), by using light and electron microscopy. Neuronal somata immunopositive f or the excitatory amino acids (EAAs) Asp and Glu, were of medium size. EAA- ir terminals also were of medium size and contained round synaptic vesicles ; they made asymmetrical synaptic contacts with dendritic profiles. Neurona l somata immunopositive for GABA were small. GABA-positive terminals also w ere small and contained pleomorphic synaptic vesicles; they formed symmetri cal synaptic contacts with dendritic profiles. No neurons immunolabeled for ChAT were found. Terminals immunopositive for ChAT were small and containe d round synaptic vesicles; these made symmetrical synaptic contacts, asymme trical synaptic contacts, or both, of the en passant type with dendritic pr ofiles. SP-immunolabeled neuronal somata were not found. Immunolabeled term inals were small, contained round synaptic vesicles, and made asymmetrical synaptic contacts with dendritic profiles. ChAT-ir and SP-ir axon terminals were not expressed evenly within LM-Sg. This difference in distribution su ggests that within the LM-Sg, there may be a difference in specific sensory processing functions which correlate with transmitter type. (C) 2000 Wiley -Liss, Inc.