The effects of sympathetic outflow on upregulation of vanilloid receptors TRPV1 in primary afferent neurons evoked by intradermal capsaicin

The vanilloid receptor TRPV1 is a key nociceptive molecule located in primary afferent nociceptive neurons in dorsal root ganglia (DRG) for initiating neurogenic inflammation and pain. Our recent study demonstrates that up-regulation of TRPV1 receptors by intradermal injection of capsaicin is modulated by activation of the protein kinase C (PKC) cascade. Neurogenic inflammation and pain resulting from capsaicin injection are sympathetically dependent, responding to norepinephrine, adenosine 5′-triphosphate (ATP) and/or neuropeptide Y released from sympathetic efferents. In a rat model of acute neurogenic inflammatory pain produced by capsaicin injection, we used immunofluorescence and Western blots combined with pharmacology and surgical sympathectomies to analyze whether the capsaicin-evoked up-regulation of TRPV1 in DRG neurons is affected by sympathetic outflow by way of activating the PKC cascade. Sympathetic denervation reduced significantly the capsaicin-evoked expressions of TRPV1, calcitonin gene-related peptide and/or phosphorylated PKC and their co-expression. These reductions could be restored by exogenous pretreatment with an analog of ATP, α,β-methylene ATP. Inhibition of PKC with chelerythrine chloride prevented the ATP effect. Consistent results were obtained from experiments in which capsaicin-evoked changes in cutaneous inflammation (vasodilation and edema) were examined after sympathetic denervation, and the effects of the above pharmacological manipulations were evaluated. Our findings suggest that the capsaicin-evoked up-regulation of TRPV1 receptors in DRG neurons is modulated sympathetically by the action of ATP released from sympathetic efferents to activate the PKC cascade. Thus, this study proposes a potential new mechanism of sympathetic modulation of neurogenic inflammation.