Modulation of inhibitory neurotransmission by the vanilloid receptor type 1 (TRPV1) in organotypically cultured mouse substantia gelatinosa neurons

The vanilloid receptor type 1 (TRPV1) plays a pivotal role in modulating thermal, chemical, and inflammatory pain. TRPV1s are expressed in some dorsal horn (DH) neurons, but their contribution, if any, to central pain processing still remains unclear. We studied the effects of 2 lM capsaicin-induced TRPV1 activation in organotypically cultured substantia gelatinosa neurons from post-natal (8–12) mice. Capsaicin affected sIPSC frequency (272 ± 60% of control, n = 14, P < 0.02), but not amplitude (131 ± 12% of control, n = 14, P > 0.05) in patch clamp recordings, also in the presence of 50 lM AP-5 (frequency: 265 ± 69% of control; n = 8, P < 0.05; amplitude: 156 ± 28% of control; n = 8, P > 0.05). The frequency increase was reduced by TTX (181 ± 21% of control; n = 12, P < 0.05). Pre-administration of I-RTX (1 lM), a TRPV1 antagonist, prevented the capsaicin effect (frequency: 149 ± 28% of control, P > 0.05, n = 12; amplitude: 97 ± 4% of control, P > 0.05, n = 12). NADA (1 lM), an endovanilloid/endocannabinoid agonist of TRPV1, induced a significant increase of sISPC frequency (191 ± 40% of control; n = 8, P < 0.05) without affecting the amplitude (102 ± 6% of control; n = 8, P > 0.05), and the co-application of two naturally occurring N-acyldopamines, PALDA (5 lM) and STEARDA (5 lM) that facilitate the effect of TRPV1 agonists, also induced a significant increase of sIPSC frequency (278 ± 67% of control, n = 6, P < 0.05). The presence of TRPV1 protein and mRNA in DH neurons was confirmed by histological (immunocytochemistry, in situ PCR) and biochemical (Western blotting, PCR) procedures. These data show that TRPV1 modulates inhibitory neurotransmission in cultured substantia gelatinosa neurons, and suggest that endogenous agonists can activate the spinal receptors in vivo.