SUBSTANCE-P-LIKE AND GABA-LIKE IMMUNOREACTIVITIES ARE CO-LOCALIZED INAXONAL VARICOSITIES IN THE SUPERFICIAL LAMINAE OF CAT BUT NOT RAT SPINAL-CORD

In the present study, we applied a combination of pre-embedding peroxi dase-based immunocytochemistry and post-embedding immunogold staining to examine the synaptic interactions of substance P (SP) and gamma-ami nobutyric acid (GABA) in the superficial laminae of the dorsal horn of cat and rat spinal cord. We demonstrate for the first time the co-exi stence of SP and GABA immunoreactivities in axonal boutons in laminae I-III of cat spinal dorsal horn. In cat, most SP + GABA immunoreactive (IR) axonal boutons established synapses with SP-IR or non-IR dendrit es. These synapses were exclusively symmetric. Quantitative analysis s howed that the percentage of SP/GABA double labelled bouton profiles w as higher (7%) in lamina I but was considerably lower in laminae IIo, IIi and III. Similarly, the density (number of bouton profiles per 100 mu m(2)) of SP + GABA-IR bouton profiles was highest in lamina I. How ever, in agreement with previous studies, the co-localization of SP an d GABA immunoreactivities was never detected in the rat dorsal horn. I n both species, SP + GABA-IR or GABA-IR axonal bouton profiles were ne ver seen presynaptic to SP-IR boutons. These findings provide a morpho logical basis for the interaction of excitatory and inhibitory agents in the nociceptive circuits in the dorsal horn of the cat and rat spin al cord.