SODIUM-NITROPRUSSIDE EVOKES THE RELEASE OF IMMUNOREACTIVE CALCITONIN-GENE-RELATED PEPTIDE AND SUBSTANCE-P FROM DORSAL HORN SLICES VIA NITRIC OXIDE-DEPENDENT AND NITRIC OXIDE-INDEPENDENT MECHANISMS

The results of behavioral studies suggest that nitric oxide (NO) parti cipates in certain spinal mechanisms that contribute to hyperalgesia. Additionally, previous studies indicate that the release of immunoreac tive calcitonin gene-related peptide (iCGRP) and substance P (iSP) is increased in the dorsal horn of the spinal cord during hyperalgesia. T herefore, the aim of this study was to determine whether NO acts to en hance peptide release in the dorsal horn of rats using an in vitro sup erfusion technique. Sodium nitroprusside (SNP) was used as an NO donor . The results of this study indicate that SNP caused a dose-related, c alcium-dependent increase in the release of iCGRP and iSP from dorsal horn slices of the rat spinal cord. Furthermore, pretreatment with SNP reduced the ability of capsaicin to evoke the release of either pepti de, suggesting that a target for SNP exists on certain capsaicin-sensi tive primary afferent terminals. In addition to increasing peptide rel ease, SNP also caused a significant five to sixfold increase in the le vels of immunoreactive guanosine 3',5'-monophosphate (i-cGMP) in the d orsal horn. This SNP-evoked increase was significantly decreased by th e guanylate cyclase inhibitor methylene blue in a dose-dependent manne r. In addition, the release of iCGRP was also significantly reduced in the presence of methylene blue, although the relationship between pep tide release and i-cGMP production remains unclear. Sodium nitroprussi de-evoked peptide release was significantly reduced in the presence of hemoglobin (an oxide radical scavenger), suggesting that the drug eff ect was due to the generation of NO. However, the release of iCGRP and iSP was also evoked by sodium ferricyanide (the coproduct of SNP) and by 7-d-old, photoinactivated SNP. Taken together, these data indicate that SNP acts to evoke the release of iCGRP and iSP from primary affe rent neurons in the dorsal horn. In addition, these data suggest that the SNP-evoked increase in iCGRP release may be associated with increa sing tissue levels of i-cGMP. Finally, these data indicate that SNP ha s NO-dependent and NO-independent mechanisms of action in the dorsal h orn.