Inhibitory activity of the novel CB2 receptor agonist, GW833972A, on guinea-pig and human sensory nerve function in the airways

Abstract

Background and purpose: Sensory nerves regulate central and local reflexes such as airway plasma protein leakage, bronchoconstriction and cough. Sensory nerve activity may be enhanced during inflammation such that these protective effects become exacerbated and deleterious. Cannabinoids are known to inhibit airway sensory nerve function. However, there is still controversy surrounding which receptor is involved in eliciting these effects.

Experimental approach: We have adopted a pharmacological approach, including using a novel, more selective CB(2) receptor agonist, GW 833972A (1000-fold selective CB(2)/CB(1)), and receptor selective antagonists to investigate the inhibitory activity of cannabinoids on sensory nerve activity in vitro and in vivo in guinea-pig models of cough and plasma extravasation.

Key results: GW 833972A inhibited capsaicin-induced depolarization of the human and guinea-pig and prostaglandin E(2) (PGE(2)) and hypertonic saline-induced depolarization of the guinea-pig isolated vagus nerve in vitro. GW 833972A also inhibited citric acid-induced cough but not plasma extravasation in the guinea-pig and this effect was blocked by a CB(2) receptor antagonist.

Conclusions and implications: This confirms and extends previous studies highlighting the role of CB(2) receptors in the modulation of sensory nerve activity elicited both by the exogenous ligands capsaicin and hypertonic saline but also by endogenous modulators such as PGE(2) and low pH stimuli. These data establish the CB(2) receptor as an interesting target for the treatment of chronic cough.

Figure 1

Characterization of the depolarization responses elicited by human vagus nerve preparations in response to tussive agents: comparison with responses obtained in guinea-pig tissue. Values are presented as mean±s.e.mean of the percentage change in depolarization responses before and after drug superfusion of n=2–4.

Figure 2

Chemical structure of GW 833972A.

Figure 3

The effect of the selective CB₂ receptor agonist (GW 833972A) on capsaicin-, hypertonic saline- and PGE₂-evoked depolarization of the guinea-pig vagus nerve. GW 833972A (0.03–30 μM), inhibited (a) capsaicin (1 μM)-, (b) hypertonic saline (2%)- and (c) PGE₂ (10 μM)-induced depolarization of guinea-pig vagus nerve in a concentration-dependent manner. Values are presented as mean±s.e.mean of the percentage change in depolarization responses before and after drug superfusion of n=4. V refers to the vehicle control group. PGE_2, prostaglandin E_2.

Figure 4

The effect of CB₁ and CB₂ receptor-selective antagonists on the inhibition of capsaicin-, hypertonic saline- and PGE₂-induced depolarization of the guinea-pig vagus nerve evoked by submaximal concentrations of the selective CB₂ receptor agonist (GW 833972A). The inhibitory action of a submaximal concentration of (a) GW 833972A (30 μM) on capsaicin (1 μM)-induced depolarizations, (b) GW 833972A (10 μM) on hypertonic saline (2%)-induced depolarizations and (c) GW 833972A (10 μM) on PGE₂ (10 μM)-induced depolarizations of the guinea-pig vagus was not affected by prior superfusion (10 min) with the CB₁ receptor antagonist rimonabant, (SR 141716A, 0.01 μM), but was completely abolished in the presence of the CB₂ receptor antagonist SR 144528 (0.01 μM). Values are presented as mean±s.e.mean of the percentage change in responses before and after drug superfusion of n=4. ^*Significant difference (P<0.05) compared with control responses to tussive stimuli before drug superfusion in the same preparation using a paired t-test. PGE_2, prostaglandin E_2.

Figure 5

Histogram and representative trace describing the effect of GW 833972A on capsaicin-evoked depolarization of human vagus nerve. GW 833972A (1 and 10 μM) inhibited capsaicin (10 μM)-evoked depolarization of the human vagus nerve: (a) representative trace from data obtained from a single human vagus nerve (male, 54 years old) and (b) a histogram illustrating the depolarization pre- and post-administration of the compound (prior superfusion for 10 min) and then following washout. Values are presented as mean of the depolarization responses before and after drug superfusion of n=1–2.

Figure 6

The effect of GW 833972A on citric acid (0.3 M)-induced cough in conscious guinea-pigs. GW 833972A (30 mg kg⁻¹, i.p., administered 30 min before citric acid challenge) inhibited the citric acid-induced cough in the guinea-pig. Values are presented as mean±s.e.mean of the number of coughs per min over the 10-min citric acid exposure period, n=20 in each group. ^*Significant difference (P<0.05) between vehicle-treated and compound-treated groups. The Mann–Whitney U-test was used when comparing vehicle- versus drug-treated animals in the cough studies.

Figure 7

The effect of CB₁ and CB₂ receptor antagonists on the inhibitory effect of GW 833972A on citric acid (0.3 M)-induced cough in conscious guinea-pigs. The CB₂ (SR 144528) but not the CB₁ (rimonabant, SR 141716A) receptor antagonist blocked the antitussive activity of GW 833972A (30 mg kg⁻¹, i.p., administered 30 min before citric acid challenge) in the guinea-pig. Values are presented as mean±s.e.mean of the number of coughs per min over the 10-min citric acid exposure period, n=10 in each group. ^*Significant difference (P<0.05) between vehicle-treated and compound-treated groups. The Mann–Whitney U-test was used when comparing vehicle- versus drug-treated animals in the cough studies.