Activation and recovery of the PGE2-mediated sensitization of the capsaicin response in rat sensory neurons

Pro-inflammatory prostaglandins are known to enhance the sensitivity of sensory neurons to various modalities of stimulation, including the excitatory chemical agent, capsaicin. In this report, we examined the capacity of prostaglandin E-2 (PGE(2)) to enhance the capsaicin response recorded from sensory neurons isolated from embryonic rats and grown in culture. Previous work demonstrated that the cyclic adenosine 3',5'-monophosphate pathway mediates initiation of the PGE(2)-induced sensitization, however, little is known about the pathways regulating the recovery from sensitization. Therefore, we examined the neuronal transduction cascades that control the duration of sensitization. Treatment with PGE(2) enhanced the capsaicin-evoked current by two-to threefold, however, this sensitization was transient even in the continued presence of prostaglandin. The duration of sensitization produced by PGE, was related inversely to the extracellular Ca2+ concentration with the shortest recovery times observed in cells exposed to 2 mM Ca2+-Ringer. Inclusion of the Ca2+ chelator, bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid, in the recording pipette greatly lengthened the period of sensitization. Pretreatment with either the nitric oxide synthase inhibitor, nitro-l-arginine methyl ester (L-NAME), or the inhibitor of the cyclic guanosine 3',5'-monophosphate (GMP)-dependent protein kinase, KT-5823, before the application of PGE(2) increased the duration of sensitization even in the presence of 2 mM Ca2+. In contrast, after attaining maximal sensitization in 2 mM Ca2+-Ringer containing L-NAME, the addition of either nitric oxide donors (3-morpholinosydnonimine or s-nitroso-n-acetylpenicillamine) or 8-Br-cyclic GMP led to a rapid decrease in the level of sensitization. In the absence of sensitization, nitric oxide-cyclic GMP modulating agents had no effect on the capsaicin-evoked current. Therefore, these results suggest that capsaicin-induced elevations in intracellular Ca2+ levels lead to an enhanced production of cyclic GMP, via the nitric oxide pathway, that ultimately activates cyclic GMP-dependent protein kinase. This protein kinase inactivates or terminates the sensitization produced by PGE(2) by an as yet unidentified mechanism.