The effects of protein phosphatase inhibitors on nociceptive behavioral responses of rats following intradermal injection of capsaicin.

The functions of crucial proteins in the nervous system are modulated by kinases and phosphatases which catalyze opposing reactions of phosphorylation and dephosphorylation. During spinal cord central sensitization, serine/threonine protein phosphatase 2A (PP2A) may play an important role in determining the excitability of nociceptive neurons in the spinal cord by modulating the phosphorylation state of some critical proteins. The effects of a general inhibitor of PP2A, okadaic acid (OA), and a specific inhibitor, fostriecin, on the behavioral responses of rats following capsaicin injection were investigated in this study. Hyperalgesia was initiated by injection of capsaicin into the plantar surface of the hindpaw of rats. An intrathecal catheter was previously implanted into the subarachnoid space of the spinal cord for the administration of a variety of drugs. Rats were tested for responses to mechanical stimuli using von Frey filaments of different bending forces applied at a site outside the area of injection. Responses to heat stimuli were detected from a site near the injection area. The responses were recorded before and after injection of capsaicin with the perfusion of ACSF, OA negative control, OA or fostriecin at different time points. The results demonstrated that secondary mechanical hyperalgesia and allodynia can be induced by the intradermal injection of capsaicin. Compared to administration of ACSF or the OA negative control, infusion of the phosphatase inhibitor OA or of fostriecin into the subarachnoid space enhanced the secondary mechanical hyperalgesia and allodynia by making the intradermal capsaicin-induced hyperalgesia and allodynia last longer.