Inhibition of guinea-pig and human sensory nerve activity and the cough reflex in guinea-pigs by cannabinoid (CB2) receptor activation

Abstract

1. There is considerable interest in novel therapies for cough, since currently used agents such as codeine have limited beneficial value due to the associated side effects. Sensory nerves in the airways mediate the cough reflex via activation of C-fibres and RARs. Evidence suggests that cannabinoids may inhibit sensory nerve-mediated responses. 2. We have investigated the inhibitory actions of cannabinoids on sensory nerve depolarisation mediated by capsaicin, hypertonic saline and PGE2 on isolated guinea-pig and human vagus nerve preparations, and the cough reflex in conscious guinea-pigs. 3. The non-selective cannabinoid (CB) receptor agonist, CP 55940, and the selective CB2 agonist, JWH 133 inhibited sensory nerve depolarisations of the guinea-pig vagus nerve induced by hypertonic saline, capsaicin and PGE2. These responses were abolished by the CB2 receptor antagonist SR144528, and unaffected by the CB1 antagonist SR141716A. Similarly, JWH 133 inhibited capsaicin-evoked nerve depolarisations in the human vagus nerve, and was prevented by SR144528. 4. Using a guinea-pig in vivo model of cough, JWH 133 (10 mg kg-1, i.p., 20 min) significantly reduced citric acid-induced cough in conscious guinea pigs compared to those treated with the vehicle control. 5. These data show that activation of the CB2 receptor subtype inhibits sensory nerve activation of guinea-pig and human vagus nerve, and the cough reflex in guinea-pigs, suggesting that the development of CB2 agonists, devoid of CB1-mediated central effects, will provide a new and safe antitussive treatment for chronic cough.

Figure 1

Inhibition of nerve depolarisation by cannabinoids. The nonselective cannabinoid agonist CP 55940 inhibits (a) capsaicin (1 μM), (b) PGE₂ (1 μM) and (c) hypertonic saline (2%)-induced depolarisation of the guinea-pig vagus nerve. Similarly, the CB₂-selective receptor agonist JWH 133 inhibits (d) capsaicin (1 μM), (e) PGE₂ (1 μM) and (f) hypertonic saline (2%)-induced depolarisation of the guinea-pig vagus nerve. Nerve depolarisation responses were expressed as absolute values in mV depolarisation before and after drug additions, and then expressed a percentage change. The data were subjected to a paired two-tailed t-test, since the response to a stimulant was measured before and after drug treatment within the same nerve. ^*P<0.05, ^**P<0.01 and ^***P<0.001 denote statistical significance. Values are presented as the mean±s.e.m. percentage change of n=4 determinations. pD₂ values (−log of the EC₅₀ defined as the concentration of drug required to elicit 50% of the maximum response) and statistical significance were calculated using ‘GraphPad Instat™' (© GraphPad Software).

Figure 2

JWH 133-mediated inhibitory responses on nerve depolarisations are SR144528 sensitive. The inhibitory action of JWH 133 on guinea-pig vagus nerve depolarisations induced by (a) hypertonic saline (2%), (b) capsaicin (1 μM) and (c) PGE₂ (1 μM) is abolished in the presence of the CB₂ receptor antagonist SR144528, and is unaffected by the CB₁ receptor antagonist SR141716A. Nerve depolarisation responses were expressed as absolute values in mV depolarisation before and after drug additions, and then expressed a percentage change. The data were subjected to a paired two-tailed t-test, since the response to a stimulant was measured before and after drug treatment within the same nerve. ^**P<0.01 denotes the statistical significance compared to control responses in the same tissue prior to drug treatment and ^###P<0.001 denotes the statistical significance between two different treatment groups using an unpaired t-test. Values are presented as the mean±s.e.m. percentage change of n=4 determinations.

Figure 3

Effect of cannabinoid ligands on nerve depolarisations of isolated human vagus. Traces showing (a) the inhibitory effect of JWH 133 on nerve depolarisations induced by capsaicin, and (b) the blockade of this response by the CB₂ receptor antagonist SR144528 from human vagus nerve.

Figure 4

JWH 133 inhibits citric acid-induced cough in guinea-pigs. Histogram describing the mean data (a) and a representative trace (b) showing the effect of the CB₂-selective agonist JWH 133 (10 mg kg⁻¹, i.p., 20 min, n=8) on citric acid (0.3 M, 10 min)-induced cough in conscious guinea-pigs compared to that of vehicle control-treated animals (0.5% methyl cellulose with 0.2% Tween 80 in saline, i.p., 20 min, n=8). The number of cough sounds detected by a microphone and recorded on a chart recorder (indicated by spikes) during the 10 min exposure period to citric acid (0.3 M) were compared between treated and control groups using an unpaired t-test (^**P<0.01).