Figure 1.

(A) Schematic representation of the behavioral paradigm used. Boxes represent exposure to the operant training apparatus wherein rats had access to palatable reward contingent upon contacting their muzzles on a heated thermode stimulus. Variations of PBS vehicle, morphine (MOR, 1mg /kg), or naloxone (NLX, 5 mg/kg) were administered 30 mins before the final three sessions. (B) Summary of major groups in the current study.