Single-channel properties of native and cloned rat vanilloid receptors

摘要

The responses of single-channel currents to capsaicin were recorded using the giga-seal patch-clamp technique in cell-attached and excised (inside-out/outside-out) patches from embryonic rat dorsal root ganglion (DRG) neurones in culture and in Xenopus oocytes heterologously expressing the rat vanilloid receptor (rVR1). Native and cloned vanilloid receptor (VR)-mediated currents exhibited outward rectification. In both the DRG neurones and oocytes expressing VR1, the chord conductances at −60 and +60 mV were ≈50 and ≈100 pS, respectively. At positive potentials, the channel exhibited a single conductance state. In contrast, at negative potentials, brief sojourns to subconductance states were apparent. The probability of the channel being open (Po) was dependent on the transmembrane voltage and the patch configuration (i.e. cell-attached vs. excised). In both DRG neurones and oocytes, the Po was greater at positive (+60 mV) than at negative (-60 mV) potentials. In cell-attached patches, the Po was approximately twofold higher, regardless of the applied potential. Most likely, the outward rectification observed in whole-cell currents is due to the voltage dependence of single-channel conductance and Po. The open-time distributions of single-channel currents recorded from native and cloned VRs in the presence of low agonist concentrations (0.01-0.03 μm) were best fitted with three exponential components. The closed-time distributions were best fitted by five exponential components. At higher concentrations (0.5-1 μm), an additional component was required to fit the open-time distribution, and the number of exponential components needed to fit the closed-time distributions was reduced to two. The overall mean open time at +60 mV was ≈4 ms, compared to ≈1.2 ms at −60 mV. However, the overall mean closed time was not voltage dependent. There were no significant differences between the native and cloned receptors. A comparison of single-channel properties of native and heterologously expressed VR channels indicates that expression of the rVR1 subunit alone can account for the single-channel behaviour of the majority of the native VRs. These results suggest that either native VRs are made up of VR1 subunits, or the incorporation of subunits other than VR1 does not influence the functional properties.