AMINO-ACID IMMUNOCYTOCHEMISTRY OF PRIMARY AFFERENT TERMINALS IN THE RAT DORSAL HORN

We combined transganglionic tracing methods with postembedding electro n microscopic immunocytochemistry to determine whether identified prim ary afferent fibers terminating in spinal laminae I-IV may use glutama te and aspartate as neurotransmitters. Sciatic injections of wheat-ger m agglutinin conjugated to horseradish peroxidase labeled fine afferen t fibers with terminals in laminae I-II of the lumbar spinal cord, whe reas injections of the B subunit of cholera toxin conjugated to horser adish peroxidase labeled primary afferent terminals in deeper laminae. Many labeled primary afferent terminals in superficial laminae were i nvolved in glomerular synaptic arrangements; others established nonglo merular contacts. Most glomerular arrangements were clearly immunoposi tive for glutamate, compared with dendrites, astrocytes, or terminals immunopositive for gamma-aminobutyic acid (GABA). The degree of enrich ment varied in labeled terminals of different morphological types. Asp artate was enriched, though to a lesser degree than glutamate, in labe led central terminals of glomeruli in superficial laminae. Labeled pri mary afferent terminals in laminae III-TV were immunopositive for glut amate, though at lower levels than glomerular terminals in superficial laminae. Aspartate was not enriched in these terminals compared with dendrites, glia, and C:ABA-positive terminals. These results support a neurotransmitter role for glutamate in primary afferents to the dorsa l horn. Quantitative differences in the content of glutamate in identi fied primary afferent terminals may be related to functional differenc es. Enrichment of aspartate in terminals in superficial but not deep l aminae is compatible with a role for this amino acid in sustained, NMD A-mediated phenomena characteristic of activity in fine caliber affere nts. (C) 1994 Wiley-Liss, Inc.