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

In the present study, we applied a combination of pre-embedding peroxidase-based immunocytochemistry and post-embedding immunogold staining to examine the synaptic interactions of substance P (SP) and gamma-aminobutyric acid (GABA) in the superficial laminae of the dorsal horn of cat and rat spinal cord. We demonstrate for the first time the co-existence 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 dendrites. These synapses were exclusively symmetric. Quantitative analysis showed that the percentage of SP/GABA double labelled bouton profiles was 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. However, in agreement with previous studies, the co-localization of SP and GABA immunoreactivities was never detected in the rat dorsal horn. In both species, SP + GABA-IR or GABA-IR axonal bouton profiles were never seen presynaptic to SP-IR boutons. These findings provide a morphological basis for the interaction of excitatory and inhibitory agents in the nociceptive circuits in the dorsal horn of the cat and rat spinal cord.