Suppression of network activity in dorsal horn by gabapentin permeation of TRPV1 channels: Implications for drug access to cytoplasmic targets

Abstract

The effectiveness of gabapentin (GBP) in the treatment of neuropathic pain depends on access to the α2δ-1 accessory subunit of voltage-gated Ca2+ channels. Access may be limited by its rate of entry via the neuronal system L-neutral amino acid transporter. The open pore of capsaicin-activated TRPV1 channel admits organic molecules such as local anesthetics and we calculated that GBP entry via this route would be 500× more rapid than via the transporter. Capsaicin should therefore increase GBP effectiveness. We used a quaternary GBP derivative (Q-GBP) as sole charge carrier in whole-cell recording experiments on rat dorsal root ganglion (DRG) neurons. Under these conditions, capsaicin produced a capsazepine-sensitive inward current thereby confirming Q-GBP permeation of TRPV1 channels. We have previously established that 5–6days exposure to 100μM GBP decreases excitability of dorsal horn neurons whereas 10μM is ineffective. Excitability was monitored using confocal Ca2+ imaging of rat spinal cord slices in organotypic culture. GBP effectiveness was augmented by transient exposures of cultures to capsaicin and robust suppression of excitability was seen with 10μM GBP. Experiments with an inhibitor of the neutral amino acid transporter, 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH 300μM), showed the actions of GBP seen in the presence of capsaicin were independent of its entry by this route. Capsaicin potentiation of GBP depression of dorsal horn activity may therefore reflect drug permeation of TRPV1 channels. Agonist activation of TRP channels may provide a means for improving drug access to cytoplasmic targets in selective neuronal populations defined on the basis of type of TRP channel expressed.