The discriminative stimulus properties of morphine in female rats chronically depleted of dopamine.

Female rats, 14 days of age, were chronically depleted of brain dopamine (DA) by the intracisternal administration of 6-OHDA 30 minutes following (i.p.) administration of desmethylimipramine (DMI). Dopamine depleted (DA) rats and their respective controls were trained to discriminate morphine from saline in a double bar discrimination task. While dopamine depletion did not hinder learning of the task, it markedly reduced morphine's suppressive effects as measured by response rates. Indications of stimulus generalization to methadone and haloperidol were observed in both groups via dose response tests. In addition, chronic morphine injections appeared to cause a repletion of DA in the DA group. Acute morphine-doses or chronic saline treated DA rats exhibited a 64-85% depletion of DA, while DA rats administered morphine chronically displayed a lesser depletion (42-46%).