The naturally occurring diterpene resiniferatoxin (RTX) is an ultrapotent analogue of capsaicin. Acting on polymodal afferent neurons, RTX induces a generally similar pattern of responses as does capsaicin. However, the two compounds, as well as other vanilloid derivatives, display different relative potencies for different responses. In the present study, we examined the vanilloid-like activities of two new derivatives, the amide analogue of RTX and phorbol 12,13-dibenzoate 20-homovanillylamide. Structurally, RTX-amide resembles capsaicin more closely than does RTX, and after cleavage of the amide bond the resulting amine would be predicted to not bind to protein kinase C in contrast to resiniferonol 9,13,14-orthophenylacetate, the parent diterpene of RTX. In contrast to our expectations the binding potency of the RTX-amide for the vanilloid receptor present in rat spinal cord was 450-fold lower than that of RTX (Ki values for the RTX-amide and RTX were 10.4 +/- 0.7 nM and 23.1 +/- 3.2 pM, respectively). In the case of phorbol 12,13-dibenzoate 20-homovanillylamide, there was a further loss of affinity for the vanilloid receptor compared with RTX; nonetheless, the Ki (8.56 +/- 0.61 microM) was comparable with that of capsaicin (5.31 +/- 0.37 microM). Computer fitting of the binding data yielded Hill coefficient values of 2.25 +/- 0.03, 2.33 +/- 0.03, and 1.84 +/- 0.05 for RTX, RTX-amide, and phorbol 12,13-dibenzoate 20-homovanillylamide, respectively, indicating that both new compounds induced apparent positive cooperativity among vanilloid binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)