Inactivation and tachyphylaxis of heat-evoked inward currents in nociceptive primary sensory neurones of rats

1. Membrane currents evoked by repeated noxious heat stimuli (43-47 degreesC) of 3 s duration were investigated in acutely dissociated dorsal root ganglion (DRG) neurones of adult rats. The heat stimuli generated by a fast solution exchanger had a rise time of 114 +/- 6 ms and a fall time of 146 +/- 13 ms. 2. When heat stimuli were applied to heat-sensitive small (less than or equal to 32.5 mum) DRG neurones, an inward membrane current (I-heat) with a mean peak of 2430 +/- 550 pA was observed (n = 19). This current started to activate and deactivate with no significant latency with respect to the heat stimulus. The peak of I-heat was reached with a rise time of 625 +/- 115 ms. When the heat stimulus tvas switched off I-heat deactivated with a fall time of 263 +/- 17 ms. 3. During constant heat stimulation I-heat decreased with time constants of 4-5 s (inactivation). At the end of a 3 a heat stimulus the peak current was reduced by 44 +/- 5% (n = 19). 4. Current-voltage curves revealed outward rectifying properties of I-heat and a reversal potential of -6.3 +/- 2.2 mV (n = 6). Inactivation was observed at all membrane potentials investigated (-80 to 60 mV); however, inactivation was more pronounced for inward currents (37 +/- 5%) than for outward currents (23 +/- 6%, P<0.05). 5. When neurones were investigated with repeated heat stimuli (3 to 5 times) of the same temperature, the peak current relative to the first I-heat declined by 48 +/- 6% at the 3rd stimulus (n = 19) and by 54 +/- 18% at the 5th stimulus (n = 4; tachyphylaxis). 6. In the absence of extracellular Ca2+ (buffered with 10 mM EGTA) inactivation (by 53 +/- 6%) and tachyphylaxis (by 42 +/- 7% across three stimuli) were still observed (n = 8). The same was true when intracellular Ca2+ was buffered by 10 mw BAPTA (inactivation by 49 +/- 4%, tachyphylaxis by 52 +/- 7% across three stimuli; n=13). Thus, inactivation and tachyphylaxis were mainly independent of intra- and extracellular Ca2+. 7. These results indicate that inactivation and tachyphylaxis of heat-evoked inward currents can be observed in vitro, similar to adaptation and suppression of action potential discharges elicited by comparably fast heat stimuli in vivo. Whereas the voltage dependence of I-heat resembles that of capsaicin-induced membrane currents (I-Caps), the independence of inactivation and tachyphylaxis of I-heat from calcium is in clear contrast to I-heat. A similar difference in calcium dependence of inactivation has been reported between heat-evoked and capsaicin-induced currents through the cloned capsaicin receptor channel VR1. Thus, the properties of I-heat and of VR1 largely account for the adaptation and suppression of heat-evoked nociceptor discharges.