Peripheral group II metabotropic glutamate receptors (mGluR2/3) regulate prostaglandin E2-mediated sensitization of capsaicin responses and thermal nociception

Abstract

Previous studies have shown that group II metabotropic glutamate receptors (mGluRs) are present on the peripheral terminals of primary sensory neurons, suggesting that they might be involved in nociception. In this study, we investigated the modulation of nociception by peripheral group II mGluRs and the molecular basis of this modulation. Subcutaneous injection of a group II mGluR agonist, 2R,4R 4-aminopyrrolidine-2,4-dicarboxylate (APDC), did not alter thermal sensitivity but blocked prostaglandin E2 (PGE2)-induced thermal hyperalgesia. This effect was blocked by (2s)-2-amino-2-[(1s,2s)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid, a selective group II mGluR antagonist. In cultured primary sensory neurons, APDC blocked PGE2-induced potentiation of capsaicin responses, which was abolished when neurons were pretreated with pertussis toxin. Similar potentiating effects induced by forskolin but not 8-bromo-cAMP were also blocked by the activation of group II mGluRs. These results indicate that peripheral group II mGluRs act via inhibition of adenylyl cyclase to reverse the sensitization of capsaicin receptors and the thermal hyperalgesia induced by PGE2, and suggest that peripheral group II mGluRs might be targeted for therapeutic intervention in inflammatory pain states.

Fig. 1.

Activation of peripheral group II mGluRs blocks PGE₂-induced thermal hyperalgesia. a, PGE₂ (100 ng in 10 μl) injected subcutaneously into the plantar surface of a mouse hindpaw decreased withdrawal latency to radiant heat applied to the hindpaw. Asterisks indicate time points at which PGE₂ (n = 6) was significantly different from vehicle (n = 7).b, Normalized thermal withdrawal latencies at 0.5, 0.75, and 1 hr were averaged and compared with baseline values. Injection of APDC (20 ng in 10 μl; n = 6) had no effect alone, but coinjection of APDC (n = 6) blocked PGE₂-induced hyperalgesia. LY341495 (0.2 ng in 10 μl;n = 7) blocked the APDC effect. c, At 3 hr, all groups were statistically indistinguishable from baseline. In all panels, dashed lines indicate the basal response.

Fig. 2.

Group II mGluRs block PGE₂ enhancement of capsaicin receptor function. a, Representative traces of capsaicin-induced calcium responses recorded from cultured DRG neurons; 20 nm capsaicin was applied twice (bars below the traces; 21 sec each), inducing pronounced desensitization of the calcium response. PGE₂(200 nm, 7 min) decreased this desensitization of capsaicin responses (note the arrows compared with control). APDC (10 μm) blocked the PGE₂ effect, and LY341495 (100 nm) blocked the APDC effect. Dashed linesshow the peaks of the responses. b, Means ± SEM for the data shown in a;n = 28–60; *p < 0.05.

Fig. 3.

Group II mGluRs block PGE₂ enhancement of capsaicin receptor function via a PTX-sensitive G-protein.a, Representative traces of the effect of PGE₂ (200 nm, 7 min) in the presence and absence of APDC (10 μm) in cells treated with PTX (500 ng/ml, overnight). In PTX-treated cells, PGE₂ enhanced capsaicin responses, but APDC failed to block the PGE₂effects. Bars below traces represent time of capsaicin application. b, Means ± SEM;n = 51–73; *p < 0.05.

Fig. 4.

Group II mGluRs block sensitization induced by the activation of AC but not sensitization induced by the direct activation of PKA. a, Representative traces of the effect of forskolin (50 μm) plus IBMX (100 μm) (7 min) in the absence and presence of APDC (10 μm). Forskolin produced an enhancement of capsaicin responses similar to that of PGE₂, which was blocked by APDC. b, Representative traces of the effect of 8-bromo-cAMP (100 μm) (7 min) in the absence and presence of APDC (10 μm). 8-bromo-cAMP produced an enhancement of capsaicin responses similar to that of PGE₂, and this enhancement was not reduced by APDC. Bars below traces represent time of capsaicin application. c, Means ± SEM; n = 30–47 for each group; *p < 0.05. Fsk, Forskolin.