Trigeminal antihyperalgesic effect of intranasal carbon dioxide

Abstract

Aims: Clinical studies demonstrate attenuation of trigeminal-related pain states such as migraine by intranasal CO(2) application. This study investigated the underlying mechanisms of this observation and its potential use to reverse trigeminal pain and hypersensitivity.

Main methods: We used a behavioral rat model of capsaicin-induced trigeminal thermal hyperalgesia, intranasal CO2 application and several pharmacologic agents such as carbonic anhydrase, acid-sensing ion channels (ASICs), and TRPV1 blocker as well as acidic buffer solutions to investigate and mimic the underlying mechanism.

Key findings: Intranasal CO(2) application produced a robust dose-dependent antihyperalgesic effect in rats that lasted at least one hour. Blockade of nasal carbonic anhydrase with a dorzolamide solution (Trusopt ophthalmic solution) showed only a non-significant decrease of the antihyperalgesic effect of intranasal CO(2) application. Pharmacologic blockade of ASICs or TRPV(1) receptor significantly attenuated the antihyperalgesic effect of CO(2) application. The effect of intranasal CO(2) application could be mimicked by application of pH 4, but not pH 5, buffer solution to the nasal mucosa. As with CO(2) application, the antihyperalgesic effect of intranasal pH 4 buffer was blocked by nasal application of antagonists to ASICs and TRPV(1) receptors.

Significance: Our results indicate that intranasal CO(2) application results in a subsequent attenuation of trigeminal nociception, mediated by protonic activation of TRPV(1) and ASIC channels. A potential central mechanism for this attenuation is discussed. The antihyperalgesic effects of intranasal CO(2) application might be useful for the treatment of trigeminal pain states.

Fig. 1

Effect of intranasal CO₂ on capsaicin-induced heat sensitization: After topical capsaicin application, there was a robust decrease in withdrawal latencies to a heat stimulus applied to the cheek (circles). Air applied with a flowrate of 0.8 l/min for 40 s (squares) and CO₂ with a flowrate of 0.4 l/min for 40 s (dark grey triangles), respectively, did not show significant antihyperalgesic effects. CO₂ with a flowrate of 0.8 l/min for 20 s (light grey triangles) exhibited a significant (p < 0.05, ANOVA) antihyperalgesic effect immediately after application, which decreased with time, whereas CO₂ with a flowrate of 0.8 l/min for 40 s (rhombs) demonstrated a significant (p < 0.05, ANOVA) antihyperalgesic effect for every observed time point after application. * indicates statistical significance as compared to “capsaicin only” (two way-ANOVA, p<0.05).

Fig. 2

Effect of intranasal CO₂ insufflation (0.8l/min for 40 s) on heat withdrawal latencies on capsaicin vehicle treated (non sensitized) cheeks. Heat withdrawal latency one minute after CO₂ insufflation was significantly different from baseline indicating a short-termed antinociceptive effect of CO₂ in non-sensitized skin (*: p<0.05, t-test)

Fig. 3

Effect of intranasal CO₂ insufflation (0.8l/min for 40 s) on right hind paw withdrawal latencies after capsaicin sensitization of the left cheek. * indicates statistical significance as compared to “capsaicin only” (two way-ANOVA, p<0.05)

Fig. 4

Effects of drugs on CO₂ induced antihyperalgesic effect: A) Application of the carbonic anhydrase blocker dorzolamide before intranasal administration of CO₂ with a flowrate of 0.8 l/min for 40 s non-significantly (p > 0.05, ANOVA) blocked the CO₂ induced antihyperalgesia as compared to CO₂ application without pretreatment. B) Application of an the acid-sensing ion channel blocker amiloride before intranasal administration of CO₂ with a flow rate of 0.8 l/min for 40 s significantly (p < 0.05, ANOVA) blocked the CO₂ induced antihyperalgesic effect immediately after application. C) Application of the TRPV₁ blocker capsazepine before intranasal administration of CO₂ with a flowrate of 0.8 l/min for 40 s also significantly (p < 0.05, ANOVA) blocked the CO₂ induced antihyperalgesic effect 30 and 60 minutes after application of CO₂. ^# indicates statistical significance as compared to “CO₂ 0.8 l/min 40s” (two way-ANOVA, p<0.05).

Fig. 5

Effects of pH buffer: A) Intranasal administration of pH5 buffer did not change the decrease in withdrawal latency to a nociceptive heat stimuli after capsaicin sensitization. B) Intranasal administration of pH4 buffer produced a significant (p<0.05, ANOVA) antihyperalgesic effect immediately after application (+1min). This effect was significantly (p < 0.05, ANOVA) blocked by pretreatment with the acid-sensing ion channel blocker amiloride (black triangles), as well as by the TRPV₁ blocker capsazepine (rhombs). * indicates statistical significance as compared to “capsaicin only” (two way-ANOVA, p<0.05). ^# indicates statistical significance as compared to “pH4” (two way-ANOVA, p<0.05).