MULTIPLE 2ND MESSENGER SYSTEMS ACT SEQUENTIALLY TO MEDIATE ROLIPRAM-INDUCED PROLONGATION OF PROSTAGLANDIN-E(2)-INDUCED MECHANICAL HYPERALGESIA IN THE RAT

In this study we have evaluated the mechanisms mediating the prolonged hyperalgesia induced by administration of prostaglandin E(2) plus rolipram, an inhibitor of type IV phosphodiesterase. The Randall-Selitto paw pressure device was employed to measure the effect of intradermal injection of test agents on the time course of the decrease in mechanical nociceptive threshold produced by prostaglandin E(2) plus rolipram in the hairy skin of the hindpaw of the rat. The intradermal injection of prostaglandin E(2) produced a dose-dependent decrease in the nociceptive threshold which lasted approximately 2 h. While rolipram alone had no significant effect on nociceptive threshold, it enhanced and prolonged (> 72 h) prostaglandin E(2)-induced hyperalgesia. WIPTIDE, a protein kinase A inhibitor, when administered 30 min after prostaglandin E(2), or with prostaglandin E(2) plus rolipram, a time when prostaglandin E(2)-induced hyperalgesia was at its peak, produced a significant reduction in hyperalgesia. However, at 90 or at 180 min after injection of prostaglandin E(2) plus rolipram, WIPTIDE was found to be without effect. H-8, a protein kinase G inhibitor, and okadaic acid, a protein phosphatase inhibitor, when administered 30 min after prostaglandin E(2), or 180 min after prostaglandin E(2) plus rolipram, produced no significant effect. However, when administered 90 min after prostaglandin E(2) plus rolipram, each produced a significant reduction in the hyperalgesia induced by prostaglandin E(2) plus rolipram. When 8-bromo cyclic GMP (1 mu g) was injected 30 min after prostaglandin E(2) it enhanced and prolonged for another 30 min the peak effect of prostaglandin E(2)-induced hyperalgesia, whereas the injection of 8-bromo cyclic AMP (1 mu g) only enhanced the prostaglandin E(2)-induced hyperalgesia. When 8-bromo cyclic GMP or 8-bromo cyclic AMP were injected alone, both reduced the mechanical threshold with time courses similar to each other. TMB-8 and Quin-2, inhibitors of intracellular calcium mobilization were found to be without effect 30 min after prostaglandin E(2), or 90 min after prostaglandin E(2) plus rolipram. However, at 180 min and 24 h after prostaglandin E(2) plus rolipram, both inhibited hyperalgesia. Rolipram-induced prolongation of prostaglandin E(2) hyperalgesia was also inhibited by surgical sympathectomy. These observations suggest that the prolonged hyperalgesia induced by prostaglandin E(2) plus rolipram is mediated sequentially, over time, by different signaling systems. Although the adenylyl cyclase/protein kinase A pathway mechanism contributes initially to hyperalgesia induced by prostaglandin E(2) plus rolipram, after a period of time corresponding to the duration of prostaglandin E(2)-induced hyperalgesia, a second signaling system plays a crucial role in the prolonged hyperalgesia induced by prostaglandin E(2) plus rolipram. The selective action of H-8 to inhibit hyperalgesia 90 min after prostaglandin E(2) plus rolipram is compatible with a contribution of protein kinase G. A third phase in the prolonged hyperalgesia induced by prostaglandin E(2) plus rolipram, lasting approximately 72 h, can be detected by its sensitivity to TMB-8 and Quin-2, suggesting the involvement of calcium/phospholipase C signaling in this late phase of hyperalgesia. Furthermore, the prevention of rolipram-induced prolongation of prostaglandin E(2) hyperalgesia by sympathectomy suggests the involvement of sympathetic postganglionic neurons in this prolongation process.