The motivation produced by morphine and food is isomorphic: Approaches to specific motivational stimuli are learned

Using an unbiased place preference paradigm, opiate-naive rats were conditioned as follows: one side was paired with 18-h food deprivation, and the other environment was paired with 18-h food deprivation and morphine (20 mg/kg i.p.). On the test day, animals showed a large prefer· ence for the environment paired with hunger and morphine. The acquisition of this preference was unaffected by bilateral ibotenic lesions of the brainstem tegmental pedunculopontine (TPP) nucleus, but was completely blocked by pretreatment with alpha-flupentixol (0.8 mg/kg i.p.). The failure of TPP lesions, which have previously been shown to abolish the motivational effects of morphine in drug-naive animals, to affect the present preferences in food-deprived rats suggests that morphine is not producing its normal (drug-naive) motivational effects. We suggest that mor· phine can substitute for the dopamine-mediated, motivational effects of food in a food-deprived animal.