Protein kinase C inhibitors decrease hyperalgesia and C-fiber hyperexcitability in the streptozotocin-diabetic rat.

1. We have previously demonstrated that although rats with streptozotocin-induced diabetes (STZ-D) have decreased behavioral mechanical nociceptive thresholds (hyperalgesia), their C-fiber primary afferent mechanical (von Frey hair) thresholds are not altered. Instead, when stimulated with a standardized sustained suprathreshold mechanical stimulus, C-fibers from STZ-D rats were found to have an increased number of spikes (hyperexcitability). We suggested that this C-fiber hyperexcitability contributes to the behavioral hyperalgesia and that agents that reverse the hyperalgesia may act by decreasing this hyperexcitability. Because protein kinase C activity contributes to C-fiber afferent excitability, we examined the effect of agents that inhibit protein kinases on behavioral mechanical nociceptive thresholds and on the response of C-fiber afferents to sustained mechanical stimulation. 2. The effects of intradermal injection of two protein kinase inhibitors, staurosporine and protein kinase C pseudosubstrate inhibitor peptide [PKC(19-36)], on behavioral mechanical nociceptive thresholds were determined using the Randall-Selitto paw-withdrawal device. These agents increased the mechanical nociceptive threshold of STZ-D rats in a dose-dependent manner but did not alter nociceptive threshold in control rats. 3. The same agents were tested for their effects on single C-fiber mechanical thresholds and excitability in response to suprathreshold (445 g) mechanical stimulation. Intradermal injection of staurosporine or PKC(19-36) significantly reduced the response of C-fibers from STZ-D rats to sustained suprathreshold mechanical stimulation but did not alter the response of C-fibers from control rats to the same stimulation. Neither agent altered mechanical threshold in C-fibers from either STZ-D or control rats. 4. In this study we found that both the mechanical behavioral hyperalgesia and the C-fiber hyperexcitability to mechanical stimuli seen in STZ-D rats are reduced by agents that inhibit protein kinase C. This evidence supports our hypothesis that C-fiber hyperexcitability, in part mediated by PKC activity, contributes to hyperalgesia in this model of diabetic neuropathy.