Binding of [3H]A-778317 to native transient receptor potential vanilloid-1 (TRPV1) channels in rat dorsal root ganglia and spinal cord

A-778317 (1-((R)-5-tert-butyl-indan-1-yl)-3-isoquinolin-5-yl-urea) is a potent antagonist of human and rat transient receptor potential vanilloid-1 (TRPV1) receptors. We have previously reported that [3H]A-778317 is an excellent radioligand to study the recombinant human TRPV1 receptor in a heterologous expression system. These studies were extended to determine the feasibility of using [3H]A-778317 to label native TRPV1 channels in rat tissues. Saturable high-affinity binding of [3H]A-778317 was detected in membrane preparations of rat dorsal root ganglia (DRG) and spinal cord that was inhibited by TRPV1 receptor agonists and antagonists. [3H]A-778317 labeled a single class of high-affinity binding sites in both rat DRG and spinal cord membranes (KD = 10 and 8.4 nM, respectively). The number of binding sites was 10-fold higher in rat DRG membranes than spinal cord membranes (Bmax = 3.3 and 0.35 pmol/mg protein, respectively). The pharmacology of the high-affinity binding sites was similar in rat DRG and spinal cord, but differed from the recombinant rat TRPV1 (rTRPV1) receptor expressed in transiently transfected HEK293-F cells. In particular, a large disparity in potency (> 300-fold) was observed for the TRPV1 receptor agonist resiniferatoxin between native and recombinant rTRPV1 receptors. Our data indicate that the binding of [3H]A-778317 to native rTRPV1 channels is pharmacologically distinct, and perhaps more complex, than its binding to the recombinant rTRPV1 receptor.