Altered Calcitonin Gene-related Peptide, Substance P and Enkephalin Immunoreactivities and Receptor Binding Sites in the Dorsal Spinal Cord of the Polyarthritic Rat

The dorsal horn of the spinal cord, which forms the locus of first synapses in pain pathways, is an important site of interaction between calcitonin gene‐related peptide (CGRP), substance P and enkephalin—the neuropeptides considered to be especially involved in the regulation of pain perception. Since adjuvant‐induced arthritic rats provide a suitable model for peripheral inflammation and hyperalgesia, the possible alterations of immunoreactive CGRP, substance P and enkephalin as well as the binding sites for [125I]hCGRPα, [125I]substance Plneurokinin‐1, (NK1) and [125I]FK‐33‐824/μ‐opioid receptors were studied in the dorsal horn of the spinal cord receiving projections from the inflamed limbs. In arthritic rats compared to control animals, a bilateral increase in CGRP‐ and substance P‐immunoreactive fibres and the presence of enkephalin‐immunoreactive cell bodies were noted in the dorsal horn of the spinal cord. As for receptors, while a significant decrease in [125I]hCGRPα and [125I]substance P/NK1 binding sites was observed in selective layers, no measurable alteration in [125I]FK‐33‐824/μ‐opioid binding sites was noted in any regions of the arthritic rat dorsal horn compared to the unaffected control rats. Following unilateral section of the peripheral nerve prior to induction of arthritis, CGRP‐ and substance P‐immunoreactive fibres were markedly depleted and no enkephalin‐positive neurons were observed in the ipsilateral dorsal horn. Analysis of receptor binding sites in denervated arthritic rats, however, exhibited differential responses, i.e. a significant increase in [125I]hCGRPα, a marked decrease in [125I]FK‐33‐824/μ‐opioid and apparently no alteration in [125I]substance P/NK1 receptor binding sites were observed in the ipsilateral dorsal horn compared to the intact contralateral side. These results taken together provide anatomical evidence for a concerted role of these peptides in the regulation of adjuvant‐induced hyperalgesia accompanying peripheral inflammation.