Vanilloid receptor-1 (TRPV1)-dependent activation of inhibitory neurotransmission in spinal substantia gelatinosa neurons of mouse

Abstract

Inhibitory neurotransmission in spinal cord dorsal horn is mainly mediated by γ-amino butyric acid (GABA) and glycine. By patch clamp recordings and correlative immunocytochemistry, we studied here the effect of 2 μM capsaicin-induced vanilloid receptor-1 (TRPV1) activation on IPSCs in spinal lamina II neurons from post-natal mice. Specificity was confirmed after pre-incubation with the competitive antagonist SB366791 (10 μM). After a single capsaicin pulse, an intense increase of spontaneous IPSC (sIPSC) frequency was observed in the presence of NBQX 10 μM (62/81 neurons; ∼76%) or NBQX 10 μM + AP-5 20–100 μM (27/42 neurons; ∼64%). Only a subpopulation (∼40%) of responsive neurons showed a significant amplitude increase. Seventy-two percent of the neurons displayed pure GABA A receptor-mediated sIPSCs, whereas the remaining ones showed mixed GABAergic/glycinergic events. After two consecutive capsaicin pulses, frequency rises were very similar, and both significantly higher than controls. When the second pulse was given in the presence of 4 μM L732,138, a selective antagonist of the substance P (SP) preferred receptor NK1, we observed a significant loss in frequency increase (63.90% with NBQX and 52.35% with NBQX + AP-5). TTX (1 μM) largely (∼81.5%) blocked the effect of capsaicin. These results show that TRPV1 activation on primary afferent fibers releases SP. The peptide then excites inhibitory neurons in laminae I, III and IV, leading to an increased release of GABA/glycine in lamina II via a parallel alternative pathway to glutamate.