The degree of GABA immunoreactivity is related to neuronal size in the thalamic reticular nucleus of the Japanese monkey, Macaca fuscata.

The thalamic reticular nucleus (RT) of the Japanese monkey, Macaca fuscata, was examined by light microscopic Golgi impregnation and by light and electron microscopic immunocytochemistry with anti- gamma-aminobutyric acid (GABA) antibody. With the Golgi study, RT neurons were classified into three groups predominantly according to their size. Small neurons were mainly fusiform or ovoid in shape of somata and characterized by beaded structures along dendrites. With GABA-immunocytochemical study, RT neurons were also divided into three classes: large GABA-negative, medium-sized weakly GABA-positive, and small strongly GABA-positive neurons. GABA-positive small neurons were mainly fusiform or ovoid. By electron microscopy, small neurons showed a markedly invaginated nuclear envelope and thin cytoplasm. GABA-positive axon terminals revealed symmetrical synaptic contact with GABA-negative dendrites. The present results suggest that three kinds of neurons are present in monkey RT, and that at least small ones operate as GABAergic interneurons.