Neurobiology of motivation: double dissociation of two motivational mechanisms mediating opiate reward in drug-naive versus drug-dependent animals.

Separate brain manipulations double dissociate two motivational mechanisms underlying the rewarding effects of opiates. Lesions of the brain stem tegmental pedunculopontine nucleus block the rewarding properties of morphine in drug-naive, but not in drug-dependent, rats. Neuroleptics (which block the action of the neurotransmitter dopamine) abolished opiate motivational effects in drug-dependent, but not in drug-naive, rats in place conditioning paradigms. This second dopaminergic opiate reward mechanism mediates morphine's alleviation of the withdrawal distress associated with abstinence in opiate-dependent animals. Furthermore, neuroleptic-induced blockade of food-related motivational effects in food-deprived, but not in food-sated (non-food-deprived), animals suggests that the neural substrates of motivational events do not dissociate along the line between different rewarding stimuli but along the line between deprivation and nondeprivation.