Effects of inhibition of fatty acid amide hydrolase vs. the anandamide membrane transporter on TRPV1-mediated calcium responses in adult DRG neurons;: the role of CB1 receptors

The aim of the present study was to investigate the relationship between TRPV1 stimulation and endocannabinoid-driven CB1 receptor-mediated inhibition of activity in adult rat dorsal root ganglion (DRG) neurons, a model of primary afferent nociceptors. Calcium-imaging studies were performed to compare the effects of the fatty acid amide hydrolase (FAAH) inhibitor URB597 (1 mu M) vs. the anandamide (AEA) uptake inhibitor UCM707 (1 mu M) on capsaicin (100 nM) and N-arachidonoyl dopamine (NADA; 1 mu M)-evoked changes in intracellular calcium [Ca2+](i) in DRG neurons. The ability of the CB1 receptor antagonist AM251 (1 mu M) to modulate the effects of URB597 and UCM707 was also determined. Suprafusion of NADA and capsaicin evoked robust increases in [Ca2+](i) in DRG neurons (89 +/- 4% and 132 +/- 6% of the depolarizing KCl response, respectively). Co-incubation with URB597 significantly attenuated both NADA and capsaicin-evoked increases in [Ca2+](i) (39 +/- 3% and 79 +/- 4% of KCl response, respectively). Similarly, co-incubation with UCM707 significantly attenuated both NADA and capsaicin-evoked increases in [Ca2+](i) (59 +/- 7% and 72 +/- 4% of KCl response, respectively). The CB1 receptor antagonist AM251 significantly attenuated the effects of URB597 on NADA-evoked increases in [Ca2+](i) but not the effects of URB597 on capsaicin-evoked increases in [Ca2+](i). By contrast, AM251 significantly attenuated the inhibitory effects of UCM707 on both NADA and capsaicin-evoked increases in [Ca2+](i). These data suggest that transport of both NADA and capsaicin into DRG neurons and the subsequent activation of TRPV1 is partly governed by FAAH-dependent mechanisms as well as via the putative AEA membrane transporter.