Rises in [Ca2+]i mediate capsaicin- and proton-induced heat sensitization of rat primary nociceptive neurons.

Capsaicin (CAPS) as well as acidic pH induces Ca2+ influx in a subset of rat dorsal root ganglion neurons. Here we show that CAPS as well as three different approaches to induce experimental tissue acidification (phosphate buffered solution pH 5.4, CO2-gassed solution pH 6.1 and NPE-caged protons) yielded a transient heat sensitization of peripheral nociceptive terminals in rat skin in vitro. The heat sensitization induced by CAPS (1 microM) could be prevented by preloading the neurons with the neuroprotective calcium chelator BAPTA-AM (1 mM). However, this pretreatment had no effect on the sensitization following exposure to acidic solutions (pH 5.4 and pH 6.1). Therefore, the membrane-permeant proton buffer SNARF-AM (200 microM) was used together with BAPTA-AM in order to prevent changes in intracellular pH. Under these conditions heat sensitization by low pH did not occur. To investigate the underlying membrane mechanisms, current recordings together with simultaneous calcium measurements using FURA-2 were performed in neurons isolated from rat dorsal root ganglia. In a subset of these neurons, an increase in [Ca2+]i and concomitant facilitation of heat-activated ionic currents was observed after application of CAPS as well as pH 5.6. Rises in [Ca2+]i thus appear to play an essential role in plastic changes not only of central neurons but also of peripheral nociceptive terminals which may account for heat hyperalgesia.