Phosphorylation of extracellular signal-related protein kinase is required for rapid facilitation of heat-induced currents in rat dorsal root ganglion neurons

A subgroup of dorsal root ganglion (DRG) neurons responds to noxious heat with an influx of cations carried by specific ion channels such as the transient receptor potential channel of the vanilloid receptor type, subtype 1 (TRPV1). Application of capsaicin induces a reversible facilitation of these currents. This facilitation could be an interaction of two agonists at their common receptor or be caused by an influx of calcium ions into the cell. Calcium influx into the cell can activate protein kinases such as the extracellular signal-related protein kinase (ERK) pathway. This study explored the kinetics, calcium-dependency and intracellular signals following application of capsaicin and leading to facilitation of heat-induced currents (Iheat) in rat DRG neurons. Application of 0.5 microM capsaicin caused a 2.65-fold increase of Iheat within 2 s, which was significantly correlated to a small capsaicin-induced current. Intracellular application of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), a fast calcium chelator, did not change capsaicin-induced currents or Iheat itself, but inhibited facilitation of Iheat by capsaicin. ERK is activated by calcium influx and membrane depolarization via the mitogen-activated protein kinase/extracellular signal-related protein kinase kinase (MEK). Application of the MEK inhibitor U0126 also inhibited facilitation of Iheat by capsaicin. We conclude that the MEK/ERK cascade is an intracellular signaling pathway playing a vital role in the regulation of nociceptive neurons' sensitivity. The very fast kinetics (less than two seconds) are only explainable with a membrane-attached or at least membrane-near localization of these kinases.