Coexpression of heat-evoked and capsaicin-evoked inward currents in acutely dissociated rat dorsal root ganglion neurons.

Noxious heat is able to activate heat-sensitive nociceptors in the skin very rapidly, but little is known about the mechanisms by which heat is transduced. We used the whole-cell patch-clamp technique to study the effects of noxious heat and capsaicin on freshly dissociated rat dorsal root ganglion neurons in vitro. Using temperatures between 41 degrees C and 53 degrees C, 8 of 19 small neurons (phi < or = 30 microm) exhibited a heat-evoked inward current. All heat-sensitive neurons tested were also capsaicin-sensitive. Moreover, the heat response tended to be enhanced after capsaicin (360 +/- 150 pA versus 125 +/- 45 pA, P < 0.1, n = 7). Two of five heat-insensitive neurons were excited by capsaicin; both neurons developed a heat response after capsaicin. Large neurons (phi > 30 microm) did not respond to heat (0/7), and were not sensitive to capsaicin either. These findings indicate that heat stimuli may directly activate capsaicin-sensitive primary nociceptive afferents.