CAPSAICIN-INDUCED CURRENTS WITH DISTINCT DESENSITIZATION AND CA2-CELLS( DEPENDENCE IN RAT TRIGEMINAL GANGLION)

1. Whole cell patch-clamp records from cultured rat trigeminal ganglio n cells having soma diameters ranging from 20 to 50 mu m revealed that capsaicin activated two inward currents and an outward current. At -6 0 mV, the inward currents could be distinguished by their different pe ak times, which were 4.2 +/- 3.1 and 41.4 +/- 16.4 (SD) s. 2. Cells wi th the smallest soma diameters had the largest current densities. 3. T he more rapidly activating current had a linear current-voltage relati on and a reversal potential near 0 mV. 4. The more slowly activating c urrent is not a Ca2+-activated Cl- current. 5. The peak of the rapid c urrent (I-p)-capsaicin concentration (C) relationship was characterize d by I-p/I-pmax = [1 + (C/K-d)(n)](-1), where n = 1.2 and the dissocia tion constant (K-d) = 0.68 mu M. 6. The rapidly activating current was heterogeneous in regards to both its rate of activation and extent of desensitization. In cells bathed in buffer containing calcium and hel d at -60 mV, most of the capsaicin-activated currents desensitized. Re moval of extracellular Ca2+ could reduce, eliminate, or have no effect on desensitization. 7. At positive holding potentials the currents ve ry slowly desensitized, even in the presence of Ca2+. 8. Repeated 30-s applications of 1 mu M capsaicin separated by 0.5, 2.5, and 5.5 min a ll induced tachyphylaxis. Tachyphylaxis decreased exponentially until the current remained approximately constant. Decreasing the time betwe en capsaicin applications increased the extent of tachyphylaxis, where as elimination of extracellular Ca2+ markedly reduced tachyphylaxis.