Fig. 1.
Graphic outline of the experimental protocols to evaluate the effects of preadministration of varenicline or ABT 594 versus vehicle (saline) on fentanyl-induced respiratory depression (plethysmography), analgesia (Formalin test), and sedation (righting reflex test). Varenicline (1 mg/kg, neck subcutaneously, subcutaneously) or ABT 594 (30 µg/kg, subcutaneously) or saline was administrated approximately 7 min before fentanyl (36 µg/kg over 10 min, intravenous infusion). The righting reflex testing started 14 min after fentanyl. The animal was then removed from the chamber for formalin injection (50 μl, 1.5% formalin diluted in saline) into the intraplantar region of the right hind paw at 35 min after fentanyl, followed by assessment of nocifensive behaviors (licking and lifting of the injected paw) in phase I (0 to 5 min after formalin injection, reflecting acute nociceptive response), and phase II (20 to 40 min after formalin injection, reflecting inflammation) of the assay.