Quantitative analysis of immunogold labeling indicates low levels and non-vesicular localization of L-aspartate in rat primary afferent terminals

The role of L-aspartate as an excitatory neurotransmitter in primary affere nt synapses in the spinal cord dorsal horn is disputed. To further investig ate this issue, we examined the presence of aspartate-like immunoreactivity in primary afferent nerve terminals and other tissue components of the dor sal horn. We also examined the relationship between aspartate and glutamate immunogold labeling density and the density of synaptic vesicles in primar y afferent terminals and presumed inhibitory terminals forming symmetric sy napses. Weak aspartate immunosignals, similar to or lower than those displa yed by presumed inhibitory terminals, were detected in both C-fiber primary afferent terminals in lamina II (dense sinusoid axon terminals, identified by morphological criteria) and in A-fiber primary afferent terminals in la minae III-TV (identified with anterograde transport of choleragenoid-horser adish peroxidase conjugate). The aspartate immunogold signal in primary aff erent terminals was only about one-fourth of that in deep dorsal horn neuro nal eel bodies. Further, whereas significant positive correlations were evi dent between synaptic vesicle density and glutamate immunogold labeling den sity in both A- and C-fiber primary afferent terminals, none of the examine d terminal populations displayed a significant correlation between synaptic vesicle density and aspartate immunogold labeling density. Thus, our resul ts indicate relatively low levels and a non-vesicular localization of aspar tate in primary afferent terminals. It is therefore suggested that aspartat e, rather than being a primary afferent neurotransmitter, serves a role in the intermediary metabolism in primary afferent terminals. J. Comp. Neurol. 430:147-159, 2001. (C) 2001 Wiley-Liss, Inc.