Ultrastructural immunocytochemical localization of excitatory amino acids in the somatosensory system.

We studied the ultrastructure and the synaptic arrangement of glutamate-immunoreactive terminals in rats, in the superficial laminae of the spinal cord, the brainstem cuneate nucleus, and the thalamic ventroposterolateral nucleus, where a role for glutamate as neurotransmitter has been suggested by biochemical, physiological and pharmacological approaches. The antiserum employed was raised against glutaramate conjugated to keyhole limpet hemocyanin with glutaraldehyde, and was used for pre-embedding staining with an avidin-biotin-peroxidase method and for post-embedding staining with an immunogold procedure. Both methods yielded similar results, consisting of labeling of selected terminals in all the areas examined. Double immunogold labeling on the same thin section using antisera against gamma-amino-butyric acid (GABA) or substance P (SP), in combination with the anti-glutamate serum, showed that staining for glutamate and GABA was present in different terminals in all the regions examined; glutamate and SP were co-localized in a few terminals only in the superficial laminae of the spinal cord. By performing immunogold staining in combination with anterograde tracing, glutamate immunoreactivity could be localized in identified primary afferents to the dorsal spinal cord and cuneate nucleus, and in lemniscal afferents to the thalamus.