Fig. 1. Effects of intraplantar administration of the full opioid agonist fentanyl (A, B ) or the partial opioid agonist buprenorphine (C, D ) on paw pressure thresholds (PPTs) after Freund's complete adjuvant (FCA), FCA plus anti–nerve growth factor (NGF), or vehicle treatment. PPT values are given as mean ± SD of the percentage of maximum possible effect (% MPE) according to the following formula: (PPTpostinjection− PPTbasal)/(250cut-off− PPTbasal). (A ) Intraplantar injection of fentanyl produced a significant and dose-dependent increase in PPT of vehicle-treated rats. In FCA-treated animals, significantly lower doses of fentanyl produced a dose-dependent elevation of PPT, indicating a leftward shift in potency. (B ) Similar to intraplantar FCA, intraplantar NGF treatment resulted in a leftward shift in the dose-dependent antinociceptive effects of intraplantar fentanyl compared with vehicle treatment. (C, D ) Intraplantar buprenorphine produced a significant and dose-dependent increase in PPT of FCA- and NGF-treated, but not vehicle-treated, rats. Immunoneutralization of endogenous NGF in FCA-treated animals with intraplantar anti-NGF antiserum reversed the leftward shift in fentanyl-induced antinociception (A ) or abolished the buprenorphine evoked antinociceptive efficacy (B ). *P < 0.05 versus vehicle (two-way ANOVA, Newman-Keuls test). †P < 0.05 versus FCA (two-way ANOVA, Newman-Keuls test).