Glutamate‐immunoreactive terminals synapse on primate spinothalamic tract cells

Glutamate has been shown to excite spinothalamic tract (STT) neurons and has been localized to primary afferent neurons, spinal cord projection neurons, and interneurons in the spinal cord dorsal horn. The likelihood that glutamate‐immunoreactive (GLU‐IR) terminals directly innervate STT neurons was investigated. For these studies three lamina IV or V STT cells in the lumbar spinal cords of three monkeys (Macaca fascicularis) were identified electrophysiologically and characterized. Two were identified as high threshold neurons and one as a wide dynamic range neuron. Following intracellular injection of the cells with HRP and reaction to give the cells a Golgi‐like appearance, the tissues were processed for electron microscopy. Postembedding immunogold methods with antibodies specific for glutamate were used to identify GLU‐IR terminals apposing the somata and dendrites of the STT neurons, including dendrites that extended into laminae IV and III. The GLU‐IR terminals were numerous and constituted a mean of 46% of the population counted that appose the STT soma and 50% of the profiles apposing the dendrites. Fifty‐four percent of the somatic and 50% of the dendritic surface length was contacted by GLU‐IR terminals. Most terminals contained round clear vesicles and some contained a variable number of large dense core vesicles. For one of the three cells examined it was determined that 45% of the terminals apposing the soma were GLU‐IR and 30% of the terminals were gamma aminobutyric acid‐immunoreactive (GABA‐IR). In an additional monkey, a lamina I cell retrogradely labeled from the ventral posterolateral nucleus of the thalamus was found to be ensheathed in glial processes. Of the 9 terminals observed apposing the retrogradely filled STT neuron in one plane of section, 2 were GLU‐IR and 3 were GABA‐IR. The data suggest that a large percentage of the profiles adjacent to STT neurons in intermediate laminae of the spinal cord contain glutamate and many synapse directly with STT neurons.