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 cortical and subcortical areas. Previous ultrastructural studies of this nucleus demonstrated the presence of four types of vesicle‐containing profiles and characterized some of these as γ‐aminobutyric acid (GABA)–containing terminals (Norita and Katoh [1987] J. Comp. Neurol. 263:54–67; Norita and Katoh [1988] Prog. Brain Res. 75:109–118). The present study has extended these observations by examining the immunoreactivity (ir) of LM‐Sg, with antibodies raised against aspartate (Asp), glutamate (Glu), GABA, the acetylcholine (ACh) marker, choline acetyltransferase (ChAT), and substance P (SP), by using light and electron microscopy. Neuronal somata immunopositive for 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. Neuronal somata immunopositive for GABA were small. GABA‐positive terminals also were small and contained pleomorphic synaptic vesicles; they formed symmetrical synaptic contacts with dendritic profiles. No neurons immunolabeled for ChAT were found. Terminals immunopositive for ChAT were small and contained round synaptic vesicles; these made symmetrical synaptic contacts, asymmetrical synaptic contacts, or both, of the en passant type with dendritic profiles. SP‐immunolabeled neuronal somata were not found. Immunolabeled terminals 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 suggests that within the LM‐Sg, there may be a difference in specific sensory processing functions which correlate with transmitter type. J. Comp. Neurol. 419:257–270, 2000. © 2000 Wiley‐Liss, Inc.