Activation of CaMKII and ERK1/2 contributes to the time-dependent potentiation of Ca2+ response elicited by repeated application of capsaicin in rat DRG neurons

When capsaicin is applied repeatedly to dorsal root ganglion (DRG) neurons for brief periods (10-15 s) at short intervals (5-10 min), the evoked responses rapidly decline, a phenomenon termed tachyphylaxis. In addition to this phenomenon, the present study using Ca2+ imaging revealed that repeated application of capsaicin to rat dissociated DRG neurons at longer intervals (20-40 min) or during multiple applications at short intervals elicited an enhancement of the responses, termed potentiation. The potentiation occurred in 50-60% of the capsaicin-responsive cells, on average representing a 20- to 30% increase in the peak amplitude of the Ca2+ signal, and was maximal at a 40-min application interval. An analysis of the mechanisms underlying potentiation revealed that it was suppressed by block of Ca2+/calmodulin-dependent protein kinase II (CaMKII) with 5 mu M KN-93 or block of the activation of extracellular signal-regulated kinase (ERK) 1/2 with 2 mu M U-0126. Lowering the extracellular Ca2+ concentration from 2 to 1 mM or pretreatment with deltamethrin (1 mu M), which blocks calcineurin and tachyphylaxis, enhanced potentiation. Potentiation was not affected by: 1) inhibition of protein kinase C or protein kinase A, 2) block of the three subtypes of neurokinin receptors, or 3) block of the trafficking of transient receptor potential V1 channel to the membrane. These results indicate that the potentiation is a slowly developing Ca2+-modulated process that is mediated by a complex intracellular signaling pathway involving activation of CaMKII and ERK1/2. Potentiation may be an important peripheral autosensitization mechanism that occurs independently of the pronociceptive effects of inflammatory mediators and neurotrophic factors.