Peptide coexistence in axon terminals within the superficial dorsal horn of the rat spinal cord.

The somata of primary sensory neurons have been shown to contain up to four (and possibly more) neuroactive peptides. Although each of these peptides has been separately located in axon terminals within the superficial dorsal horn of the spinal cord, it is not clear whether multiple peptide coexistence is also a feature of terminal varicosities. The aim of this study was to determine whether the peptides substance P (SP) and calcitonin gene-related peptide (CGRP), which are colocalized in the somata of a large number of primary sensory neurons, coexist in the central terminals of these neurons in the spinal cord. The protein A-gold technique of antigen localization was used to screen single boutons in laminae I and II of the rats spinal cord for SP- and CGRP-like immunoreactivity at the ultrastructural level. The results show that SP and CGRP are colocalized within a large number of synaptic boutons in the superficial dorsal horn. Furthermore, evidence was obtained to suggest that both SP and CGRP may be found in the same synaptic vesicle within these boutons. These findings indicate that both SP and CGRP may be coreleased from single terminals in the superficial dorsal horn. This is of considerable interest in view of the reported interaction between SP and CGRP in nociceptive behavioral responses in the rat.