Binding of neuroleptic drugs (trifluoperazine and rimcazole) to vanilloid receptors in porcine dorsal horn

Abstract Neuroleptic drugs were reported to modulate [ 3 H]resiniferatoxin binding to vanilloid receptors in the spinal cord, with marked differences between rat and man. In the present study, we have used a [ 3 H]resiniferatoxin binding assay using porcine dorsal horn membranes to explore further species differences in the interaction of neuroleptic drugs at spinal vanilloid receptors. Specific binding of 13 pM [ 3 H]resiniferatoxin to porcine dorsal horn membranes (corresponding to a 7% fractional receptor occupancy) was affected by trifluoperazine in a bi-phasic fashion, with an initial 90% enhancement of binding preceding inhibition: a fit to the modified Hill equation yielded a cooperativity index of 1.8 and a K i of 5 μM. Under similar conditions, rimcazole, by contrast, had a monophasic effect: it enhanced but, up to 100 μM, did not inhibit [ 3 H]resiniferatoxin binding. These results are in accord with previous findings in human spinal cord but contrast with those in the rat. In experiments in which the concentration of [ 3 H]resiniferatoxin was varied, 20 μM trifluoperazine reduced the B max by 33% (from 181 ± 9 fmol/mg protein to 121 ± 5 fmol/mg protein) without a measurable change in affinity or cooperativity. In parallel experiments, by contrast, neither capsaicin nor capsazepine (both at a concentration of 10 μM) affected the B max or cooperativity but, as expected, reduced the affinity from 61 ± 8 pM to 120 ± 11 pM or to 101 ± 7 pM, respectively. Whereas vanilloid receptor agonists (resiniferatoxin and capsaicin) affected [ 3 H]resiniferatoxin binding at low (approximately 7%) fractional receptor occupancies by the radioligand in a bi-phasic fashion, the competitive vanilloid receptor antagonist capsazepine failed to induce the initial binding enhancement. Thus, capsazepine appears to bind to vanilloid receptors in a non-cooperative fashion, or at least with much reduced positive cooperativity in this system. The mechanism by which neuroleptic drugs modulate resiniferatoxin binding is yet to be clarified and is clearly complicated as well as species-dependent; nonetheless, the reduced B max at higher concentrations suggests that it may at least in part be non-competitive.