A NON-PUNGENT RESINIFERATOXIN ANALOG, PHORBOL-12 PHENYLACETATE-13 ACETATE 20-HOMOVANILLATE, REVEALS VANILLOID RECEPTOR SUBTYPES ON RAT TRIGEMINAL GANGLION NEURONS

Capsaicin, the vanilloid responsible for the pungent taste of hot pepp ers, binds to receptors found primarily in polymodal nociceptors. Caps aicin initially stimulates polymodal nociceptors and subsequently inhi bits them from responding to a variety of stimuli. This property makes it useful clinically as an analgesic and anti-inflammatory compound. There is mounting, albeit indirect, evidence for the existence of seve ral subtypes of vanilloid receptors. One such piece of evidence comes from studying analogues of capsaicin, such as phorbol 12-phenylacetate 13 acetate 20-homovanillate. This compound binds to (capsaicin) vanil loid receptors on sensory neurons, but unlike capsaicin it is non-pung ent and does not produce hypothermia. To determine how sensory neurons respond to phorbol 12-phenylacetate 13 acetate 20-homovanillate, and to compare these responses with those evoked by capsaicin, whole-cell patch-clamp measurements were performed on cultured rat trigeminal gan glion neurons. It was found that 63% of the neurons held at -60 mV wer e activated by 3 mu M, phorbol 12-phenylacetate 13 acetate 20-homovani llate, and 87% of these were also activated by 1 mu M capsaicin. In a given neuron, phorbol 12-phenylacetate 13 acetate 20-homovanillate, li ke capsaicin, could activate kinetically distinct inward currents. The current-voltage curves characterizing phorbol 12-phenylacetate 13 ace tate 20-homovanillate responses were asymmetric and had reversal poten tials at -5.8 +/- 6.0 mV and 10.4 +/- 4 mV. The averaged dose-response curves for phorbol 12-phenylacetate 13 acetate 20-homovanillate were fit to the Hill equation and had binding constants (K(1/2)s) of 2.73 m u M and 0.96 mu M and Hill coefficients (ns) of approximate to 1 for a rapidly-and slowly-activating current, respectively. These parameters are consistent with those obtained from binding experiments and calci um-influx experiments on sensory nerves. Repeated applications of phor bol 12-phenylacetate 13 acetate 20-homovanillate every 3 min caused a complete reduction in the rapidly-activating currents leaving only a r educed slowly-activating current. This provides strong evidence for th e independence of these currents and the existence of subtypes of vani lloid receptors. Additional evidence for the existence of receptor sub types is that 10 mu M capsazepine, a specific and competitive inhibito r of capsaicin-evoked responses, did not inhibit the phorbol 12-phenyl acetate 13 acetate 20-homovanillate-induced currents in some neurons a nd partially inhibited them in other neurons. Thus, there are capsazep ine-sensitive and capsazepine-insensitive subtypes of vanilloid recept ors. In summary, we have obtained electrophysiological and pharmacolog ical evidence for distinct subtypes of vanilloid receptors. (C) 1998 I BRO. Published by Elsevier Science Ltd.