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Reinforcing properties of morphine chronically used in aversive life conditions: place-preference paradigm, long-term
1Laboratory of Pathophysiology, All-Union Research Center on Medico-Biological Problems of Narcology, USSR Ministry of Health, Moscow.
Abstract:
To substantiate significance of the organism's functional state in the mediation of an opiate's reinforcing activity, the rats' attitude to an initially nonpreferred environment associated with chronic (10) administration of morphine (MOR 0.67 and 2.0 mg/kg) was regularly estimated after drug administrations in conditions of two-hour restraint stress. The absence of any significant changes in the quantity of entries and the time in restraint box in rats received MOR at a low dose and only a slight increase in the quantity of entries found in animals administered MOR at a high dose allowed us to propose a blockade or prominent inhibition of the drug's rewarding efficacy to be typical under these conditions. Inverted changes in movement reactivity (its increase) as response to apomorphine at low "presynaptic" doses found in animals treated by MOR may indicate the involvement of modified presynaptic dopamine receptors in the mediation of revealed changes in an opiate's reinforcing activity.