Repeated experience with naloxone facilitates acute morphine withdrawal: potential role for conditioning processes in acute opioid dependence.

Single injections with morphine can induce a state of acute opioid dependence in humans and animals, typically measured as precipitated withdrawal when an antagonist such as naloxone is administered 4-24 h after morphine. Repeated treatment with morphine at 24-h intervals can result in a progressive shift in potency of naloxone to produce such acute withdrawal signs, including suppression of operant responding for food reward. The current study characterized fully both morphine and naloxone dose-effect functions in an effort to establish the relative contributions of repeated morphine vs. repeated naloxone (Nal) experience to these potency shifts. Rats trained on an FR15 schedule for food received four vehicle or morphine injections (0.56-5.6 mg/kg sc), spaced 24 h apart. Four hours after each morphine pretreatment (Repeat Nal), or 4 h after the fourth and final morphine pretreatment only (Single Nal), a cumulative dose-effect function for naloxone-induced suppression of responding was determined. Vehicle-pretreated (Morphine Naive) rats showed little change in the naloxone dose effect function even after four cumulative dose-effect determinations. By contrast, a progressive increase in naloxone potency was observed following successive pretreatments with morphine under Repeat Nal conditions, and the magnitude of naloxone potency shift was morphine dose dependent. At a morphine dose of 5.6 mg/kg, repeated naloxone experience in the presence of morphine was not an absolute requirement to produce an increase in naloxone potency across days, but repeated naloxone could potentiate the magnitude of the observed shift, indicating both experience-independent and experience-dependent processes at work. At lower doses of morphine (1.0 and 3.3 mg/kg) no shift in naloxone potency was observed across days of morphine treatment in the absence of repeated naloxone experience (Single Nal conditions), indicating an increasing contribution of naloxone experience-dependent processes as dose of morphine was decreased. It is argued that these experience-dependent processes in the progressive shift of naloxone potency observed in the current study may reflect an important role of conditioning in the early development of opioid dependence.