D-Ala2,D-Leu5-enkephalin generalizes to a discriminative stimulus produced by fentanyl but not ethylketocyclazocine.

Male Sprague-Dawley rats were trained to discriminate between the effects of saline and either fentanyl (0.04 mg/kg) or ethylketocyclazocine (0.32 mg/kg) by responding on a FR 10 schedule of food reinforcement with a lever on one side of a food cup following a subcutaneous saline injection and responding with a lever on the alternate side following the subcutaneous injection of one of those drugs. The effects of an intracerebroventricular injection of either fentanyl (2.89-14.45 nmol) or D-Ala2,D-Leu5-enkephalin (0.172-1.72 nmol) dose-dependently generalized to the discriminative stimulus produced by fentanyl injected subcutaneously. Intracerebroventricular injection of ethylketocyclazocine (15.7-125.8 nmol) but not D-Ala2,D-Leu5-enkephalin (1.72-13.76 nmol) dose-dependently generalized to the discriminative stimulus produced by ethylketocyclazocine injected subcutaneously. These data demonstrate similarities in the discriminative stimulus properties of proposed micron and delta but not kappa and delta opiate receptor agonists.