PLASTICITY IN THE SYNTHESIS AND STORAGE OF SUBSTANCE-P AND CALCITONIN-GENE-RELATED PEPTIDE IN PRIMARY AFFERENT NEURONS DURING PERIPHERAL INFLAMMATION

Several indices of peptidergic, primary afferent neural transmission i n rat at the level of the lumbar spinal cord exhibited differential ch anges over time in response to adjuvant-induced inflammation of the hi ndpaw. The indices were measurements of the production of messenger RN A encoding the precursors for substance P and calcitonin gene-related peptide in dorsal root ganglia, the storage of substance P and calcito nin gene-related peptide in the dorsal spinal cord and the release of the peptides evoked by application of capsaicin to the dorsal spinal c ord. A 47% decrease in the content of immunoreactive substance P in th e dorsal half of the lumbar spinal cord, as determined by radioimmunoa ssay, was measured at 6 h following the injection of complete Freund's adjuvant into the hindpaw. Decreased content of immunoreactive SP per sisted for four days, but was no longer present at eight days after th e adjuvant injection. The content of immunoreactive calcitonin gene-re lated peptide in the dorsal spinal cord was decreased by 29% at one da y following the injection of adjuvant into the rat hindpaw and 43% at two days; the content then increased to a level greater than that of c ontrol animals at eight days. The amount of messenger RNA encoding pre protachykinin and prepro-calcitonin gene-related peptide in L4-L6 dors al root ganglia was determined from northern blot analysis of the tota l messenger RNA extracted from the dorsal root ganglia. Each species o f messenger RNA had increased compared to the control animals at two d ays following the injection of adjuvant into the rat hindpaws and rema ined elevated after eight days. Thus, an increase in the messenger RNA s encoding substance P and calcitonin gene-related peptide in the dors al root ganglia preceeded the recovery of the content of the peptides in the spinal cord. Morphometric studies of calcitonin gene-related pe ptide-immunoreactive perikarya in the L4 dorsal root ganglia indicated that the increase in messenger RNA occurred in neurons of the size th at normally express calcitonin gene-related protein. Radioimmunoassay of the superfusate of the dorsal half of the lumbar spinal cord was us ed to measure the release of immunoreactive substance P and immunoreac tive calcitonin gene-related protein in vitro. Although the basal rele ase of immunoreactive substance P and immunoreactive calcitonin-gene r elated protein from the dorsal spinal cord was constant throughout the time points examined, changes occurred in the release of peptide evok ed by 10 mu M capsaicin. The capsaicin-evoked release of immunoreactiv e substance P was decreased at 6 h and eight days post-injection of ad juvant. In contrast, at four days after the injection of adjuvant into the rat hindpaw, the capsaicin-evoked release of immunoreactive calci tonin gene-related protein from the dorsal half of the spinal cord was increased. Thus, the basal release and capsaicin-releasable pool of i mmunoreactive substance P and immunoreactive calcitonin gene-related p rotein were largely maintained in spite of the persistent, decreased c ontent of the peptides in the spinal cord. In total, these data illust rate the time course of the plasticity that occurs presynaptically in response to adjuvant-induced inflammation in primary afferent neurons containing substance P and calcitonin gene-related protein. The change s support the hypothesis that substance P and calcitonin gene-related protein neurotransmission have a role. in generating and maintaining t he hyperalgesia and edema that accompany peripheral inflammation.