Restricted feeding does not modify discriminative stimulus effects of morphine in the rat

An experiment was performed to determine if the discriminative stimulus effects of morphine are modified by mild food deprivation, a condition that increases drug-reinforced behavior. Rats were trained to discriminate between SC injections of saline and morphine (3.0 mg/kg) in a discrete-trial shock-avoidance procedure. Stimulus generalization curves for morphine were then determined on three occasions by a cumulative-dosing procedure: before, during and after the body weight of the rats had been reduced to 86-87% of normal by restricted feeding. Food deprivation had little or no effect on the morphine generalization curve. Despite a close relationship between the reinforcing and discriminative stimulus properties of opioid drugs, stimulus control of behavior by morphine was not modified by a condition that enhances opioid-reinforced behavior.