Acid-induced pain and its modulation in humans

Abstract

Despite the discovery of ion channels that are activated by protons, we still know relatively little about the signaling of acid pain. We used a novel technique, iontophoresis of protons, to investigate acid-induced pain in human volunteers. We found that transdermal iontophoresis of protons consistently caused moderate pain that was dose-dependent. A marked desensitization occurred with persistent stimulation, with a time constant of approximately 3 min. Recovery from desensitization occurred slowly, over many hours. Acid-induced pain was significantly augmented in skin sensitized by acute topical application of capsaicin. However, skin desensitized by repeated capsaicin application showed no significant reduction in acid-induced pain, suggesting that both capsaicin-sensitive and insensitive sensory neurons contribute to acid pain. Furthermore, topical application of non-steroidal anti-inflammatory drugs (NSAIDs) significantly attenuated acid-evoked pain but did not affect the heat pain threshold, suggesting a specific interaction between NSAIDs and peripheral acid sensors. Subcutaneous injection of amiloride (1 mm) also significantly inhibited the pain induced by iontophoresis of acid, suggesting an involvement of acid-sensing ion channel (ASIC) receptors. Conversely, iontophoresis of acid over a wide range of skin temperatures from 4 to 40 degrees C produced only minor changes in the induced pain. Together these data suggest a prominent role for ASIC channels and only a minor role for transient receptor potential vanilloid receptor-1 as mediators of cutaneous acid-induced pain.

Figure 1.

Mean pain rating and time course of perceived pain during iontophoresis of acid and saline. A, The mean VAS scores increase with the increase in current applied to the cathode, indicating that the perceived pain increases in a dose-dependent manner. Significantly higher VAS scores were obtained when acid was iontophoresed when compared with iontophoresis of saline. B, The time course of the mean VAS scores obtained when acid is iontophoresed at different currents (0.1-0.4 mA). C, The time course of the mean VAS scores obtained when saline was iontophoresed at different currents (0.1-0.4 mA) (n = 12). Asterisk denotes a significant difference (p < 0.05).

Figure 2.

Time course of resensitization after desensitization with acid. Iontophoresis of acid (0.3 mA, 4 min) significantly desensitizes the skin to acid. The perceived acid pain is significantly reduced for 30 min after the desensitizing stimuli. Recovery from desensitization takes much longer than the initial desensitizing stimuli and does not fully recover until 24 hr later. Asterisk denotes a significant difference (p < 0.05).

Figure 3.

Mean VAS scores experienced when acid and saline were iontophoresed into skin acutely sensitized and desensitized with 1% capsaicin. A, Sensitizing the experimental skin with capsaicin significantly increases the perceived acid-mediated pain when compared with nonsensitized skin. Sensitizing the skin with capsaicin does not increase the pain experienced when saline is iontophoresed, indicating the sensitizing phenomena is specific for acid pain (n = 12). B, Desensitizing the experimental skin with capsaicin does not significantly alter the perceived acid-mediated pain when desensitized and nondesensitized skin are compared (n = 5). Asterisk denotes a significant difference (p < 0.05).

Figure 4.

Time course of perceived pain during iontophoresis of acid on diclofenac-treated skin and placebo-treated skin. Except for the initial time point, topical application of diclofenac decreased the mean acid VAS score throughout the 4 min iontophoretic trial when compared with placebo-treated skin. Diclofenac does not affect the discomfort experienced during iontophoresis of saline (n = 15).

Figure 5.

Time course of perceived pain during iontophoresis of acid and saline on amiloride-injected (1 mm) and saline-injected skin. A, During the 4 min acid iontophoretic trial (0.3 mA), amiloride decreased the mean acid VAS scores at all time points except for the initial time point. B, During the iontophoresis of saline, amiloride marginally decreases the experienced pain associated with saline iontophoresis (n = 10).

Figure 6.

Mean VAS scores experienced during simultaneous iontophoresis of acid (0.3 mA) and heating of the experimental skin. Heating the skin to subthreshold heat pain temperatures does not significantly potentiate the experienced acid pain (n = 8). Asterisk denotes a significant difference (p < 0.05).