Fractional Ca²⁺ Currents through Capsaicin‐ and Proton‐Activated Ion Channels in Rat Dorsal Root Ganglion Neurones

Abstract

Capsaicin and protons cause excitation and sensitization of primary nociceptive afferents. In a subset of dorsal root ganglion (DRG) neurones, which probably represent nociceptive neurones, both capsaicin and protons induce slowly inactivating non‐selective cation currents. Whole‐cell as well as single channel currents activated by these two stimuli share many biophysical and physiological properties in these neurones. This has lead to the suggestion that protons and capsaicin might activate the same ion channels. In this study we simultaneously measured fluorescence signals and whole‐cell currents activated by capsaicin or protons in acutely isolated DRG neurones filled with a high concentration (1 mM) of the Ca2+ indicator dye fura‐2. From these measurements the fractional contribution of Ca2+ (Pf; the portion of the whole‐cell current carried by Ca2+) to capsaicin‐ and two types of proton‐induced (fast and slowly inactivating) membrane currents was determined. Capsaicin‐ and slowly inactivating proton‐induced currents were accompanied by a change in fluorescence that was dependent on the presence of extracellular Ca2+. With 1.6 mM extracellular Ca2+ and at a holding potential of −80 mV Pf of capsaicin‐induced currents (at pH 7.3) was 4.30 ± 0.17% (mean ±s.e.m.; no. of experiments, n= 16) and of slowly inactivating proton‐induced currents (at pH 5.1) was 1.65 ± 0.11% (n= 17). Pf of fast inactivating proton‐induced currents was negligible. P f of capsaicin‐ and slowly inactivating proton‐induced currents increased with increasing extracellular Ca2+ concentration (0.5–4.8 mM). P f of both current types decreased linearly with decreasing extracellular pH by about 0.7% per pH unit over the pH range investigated. When determined at the same extracellular pH Pf values were significantly different for the two current types at all pH values tested. In summary, our results provide evidence that capsaicin and protons activate ion channels which are markedly permeable to Ca2+. The fractional contribution of Ca2+, however, was significantly different for capsaicin‐ and slowly inactivating proton‐induced currents. This strongly suggests that the two stimuli activate different populations of ion channels and supports the possibility that Ca2+ influx through these channels may be important for Ca2+‐dependent sensitization of primary nociceptive neurones.

Capsaicin Binds to the Intracellular Domain of the Capsaicin-Activated Ion Channel

“…5). The average reversal potential with 100 mM Ca 2ϩ in eight experiments was 26.6 Ϯ 2.2 mV, indicating the high C a 2ϩ permeability over Na ϩ (P C a /P Na ϭ 2.9) as reported previously (Wood et al, 1988;C aterina et al, 1997;Z eilhofer et al, 1997). These results further suggest that the channel activated by DA is nonselectively permeable to cations, as observed in the CAP-activated channel.…”

TRPV1 Recapitulates Native Capsaicin Receptor in Sensory Neurons in Association with Fas-Associated Factor 1

“…As seen by others, TRPV1 activity increases as acidity of the solution increases up to pH 4.4 (Tominaga et al, 1998). However, unlike progressive activation of TRPV1, extreme acid reduces neural activity in sensory neurons (Steen et al, 1992;Zeilhofer et al, 1997). Thus, the apparent discrepancy between the response profile of TRPV1 and that of sensory neurons to acid suggests an obligatory regulation of TRPV1 by associated proteins.…”

“…Acid, one of the major causes of the hyperalgesia induced by inflammation or ischemic injury, is known to excite sensory neurons (Steen et al, 1992;Reeh and Steen, 1996;Zeilhofer et al, 1997). Of the ion channels in sensory neurons that respond to acidic solution (Chen et al, 1998;Reeh and Kress, 2001), TRPV1 is a candidate transducer of acid-induced hyperalgesia (Caterina et al, 1997;Tominaga et al, 1998;Kress and Zeilhofer, 1999;Jung et al, 2002).…”

A sensory neuron-specific, proton-gated ion channel

“…Krishtal and Pidoplichko (5) demonstrated that low pH evoked inward currents in rat trigeminal ganglion neurons, and similar observations have been made in rat dorsal root ganglion (DRG) neurons (6)(7)(8) Low pH responses from DRG are characteristically multiphasic in nature, suggesting the existence of distinct types of channel (see, e.g., ref. 7).…”