Vanilloid receptors have recently become a subject of intensive research, after the isolation of VR1, an ion channel activated by noxious stimuli: heat (temperatures > 42 °C), low pH (~6), characteristic for inflamed tissue, as well as by several exogenous compounds, generically called vanilloids, of which capsaicin (the active ingredient of hot chilli peppers) is best known. Controlling temperature with a Peltier-based system, we have studied channel activity induced by capsaicin (0.5 µM) or protons (pH 5.4) in cultured dorsal root ganglion (DRG) neurones from adult Wistar rats. Animals were killed by CO₂ inhalation, then DRGs were dissociated with collagenase and dispase; cells were plated on glass coverslips and cultured for 1-3 days.

We have previously shown that the temperature dependence of currents elicited by capsaicin and protons in small (15-30 µm) DRG neurones is greater than the known temperature effect on permeation, which led us to suggest that temperature affects the gating of the channel (Babes et al. 2001). To test this, we performed single-channel recordings in outside-out patches at -80 mV, in Ca²⁺-free Ringer solutions containing 0.5 µM capsaicin and with 140 mM CsCl in the pipette solution. The Q₁₀ of single-channel currents was in the range 1.1-1.5. P_open was deduced by dividing the mean current by the unitary current multiplied by the number of active channels in the patch. It increased from near zero at 10 °C (no channel activity for a period of 20 s, n = 3) to 0.056 ± 0.009 at 20-22 °C (n = 4) and 0.148 ± 0.049 at 30-32 °C (n = 6) (mean ± S.D.). Preliminary experiments using dwell-time distribution analysis revealed a single open lifetime and two closed lifetimes, one brief and one longer. The longer-lived closed state showed the most pronounced temperature sensitivity.

These findings are similar to those obtained by Cesare et al. (1999) with noise analysis of heat-induced currents. We conclude that temperature sensitivity of capsaicin-induced currents is due to effects on channel gating, and not simply to effects on permeation. We were surprised to note that protons induced channel activity in only two out of seven patches where we recorded capsaicin-activated channels; this may indicate that native VR1-like channels in DRG neurones are more heterogeneous than their cloned counterparts.This work was supported by the World Bank, NATO and The Physioloical Society.

Babes, A., Amuzescu, B., Krause, U., Scholz, A., Flonta, M.-L. & Reid, G. (2001). J. Physiol. 533.P, 57-58P.

Cesare, P., Moriondo, A., Vellani, V. & McNaughton, P.A. (1999). Proc. Natl Acad. Sci. USA. 96, 7658-7663.