ULTRASTRUCTURAL FEATURES OF THE COLOCALIZATION OF CALCITONIN-GENE-RELATED PEPTIDE WITH SUBSTANCE-P OR SOMATOSTATIN IN THE DORSAL HORN OF THE SPINAL-CORD

Calcitonin gene related peptide (CGRP), substance P (SP), and somatost atin (SOM) are probably the three most important sensory neuropeptides . However, in contrast to CGRP immunoreactivity (IR), SP- and SOM-IR o ccur also in neurons intrinsic to the dorsal horn. Colocalization of e ither SP- or SOM-IR with CGRP-IR is, therefore, strongly suggestive of a primary sensory origin. In this study, high resolution double-label ling immunocytochemistry was applied to detect CGRP/SP and CGRP/SOM co localizations in axonal boutons of the rat superficial dorsal hem. Mos t boutons colocalizing CGRP-IR and SP-IR were not part of synaptic glo meruli. However, 15% of such boutons represented the central varicosit ies of type I synaptic glomeruli. CGRP-IR was always present in the sm all proportion of type I glomeruli central boutons that displayed SOM- IR. In a study in the cat combining double-labelling immunocytochemist ry with intracellular injection of electrophysiologically characterize d neurons, we found that nociceptive neurons received numerous synapse s from varicosities colocalizing SP-IR and CGRP-IR. In contrast, non-n ociceptive neurons almost completely lacked synaptic input from bouton s colocalizing both immunoreactivities. This study confirms that fibre s colocalizing SP-IR and CGRP-IR probably play a major role in spinal nociceptive mechanisms.