Relative roles of protein kinase A and protein kinase C in modulation of transient receptor potential vanilloid type 1 receptor responsiveness in rat sensory neurons in vitro and peripheral nociceptors in vivo

The function of the transient receptor potential vanilloid type 1 capsaicin receptor is subject to modulation by phosphorylation catalyzed by various enzymes including protein kinase C and cAMP-dependent protein kinase. The aim of this study was to compare the significance of the basal and stimulated activity of protein kinase C and cAMP-dependent protein kinase in transient receptor potential vanilloid type 1 receptor responsiveness in the rat in vitro by measurement of the intracellular calcium concentration in cultured trigeminal ganglion neurons and in vivo by determination of the behavioral noxious heat threshold. KT5720, a selective inhibitor of cAMP-dependent protein kinase, reduced the calcium transients induced by capsaicin or the other, much more potent transient receptor potential vanilloid type 1 receptor agonist resiniferatoxin in trigeminal sensory neurons and diminished the drop of the noxious heat threshold (heat allodynia) evoked by intraplantar resiniferatoxin injection. Chelerythrine chloride, a selective inhibitor of protein kinase C, failed to alter either of these responses, although it inhibited the effect of phorbol 12-myristate 13-acetate in the in vitro assay. Staurosporine, a rather nonselective protein kinase inhibitor, failed to reduce the capsaicin- and resiniferatoxin-induced calcium transients but inhibited the resiniferatoxin-evoked heat allodynia. Dibutyryl-cAMP and phorbol 12-myristate 13-acetate, activator(s) of cAMP-dependent protein kinase and protein kinase C, respectively, enhanced the effect of capsaicin in the calcium uptake assay while forskolin, an activator of adenylyl cyclase, augmented that of resiniferatoxin in the heat allodynia model. None of the protein kinase inhibitors or activators altered the calcium transients evoked by high potassium, a nonspecific depolarizing stimulus. It is concluded that basal activity of cAMP-dependent protein kinase, unlike protein kinase C, is involved in the maintenance of transient receptor potential vanilloid type 1 receptor function in somata of trigeminal sensory neurons but stimulation of either cAMP-dependent protein kinase or protein kinase C above the resting level can lead to an enhanced transient receptor potential vanilloid type 1 receptor responsiveness. Similar mechanisms are likely to operate in vivo in peripheral terminals of nociceptive dorsal root ganglion neurons.