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  • Title: Reinforcer-dependent enhancement of operant responding in opioid-withdrawn rats.
    Author: Cooper ZD, Shi YG, Woods JH.
    Journal: Psychopharmacology (Berl); 2010 Oct; 212(3):369-78. PubMed ID: 20686752.
    Abstract:
    RATIONALE AND OBJECTIVE: Opioid withdrawal increases the reinforcing effectiveness of the μ-opioid agonist remifentanil in rodents. The current study explored the selectivity of this effect by assessing operant behavior maintained by drug and non-drug reinforcers, remifentanil, cocaine, a palatable liquid food, and standard food pellets, as a function of opioid dependence and withdrawal. MATERIALS AND METHODS: Operant responding exhibited by nondependent, morphine-naïve groups was compared with responding exhibited by morphine-dependent and withdrawn groups. Dependence was established using a noncontingent morphine dosing procedure that has been previously verified to maintain dependence while allowing for daily behavioral observation during a withdrawn state. Behavior maintained by remifentanil (0.10-10.0 μg/kg/infusion), cocaine (0.032-1.0 mg/kg/infusion), a palatable liquid food reinforcer (3.2-100.0% Vanilla Ensure® and water), or food pellets was assessed in dependent and nondependent groups. RESULTS: Morphine withdrawal enhanced remifentanil self-administration, resulting in an upward and rightward shift of the descending limb of the dose-response curve, and increased operant responding for both food reinforcers. However, opioid withdrawal did not affect cocaine self-administration, nor did it affect responding for water. CONCLUSIONS: Enhanced operant responding observed under opioid-dependent and withdrawn conditions, while selective, is generalized to some nonopioid reinforcers.
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