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

Neuroleptics block high- but not low-dose heroin place preferences: Further evidence for a two-system model of motivation.

“…One difficulty in designing a self-administration protocol that would be sensitive enough to assay for the TPP-nondeprived motivational system is that once rats are in a deprived motivational state, the motivational effects of various classes of stimuli are mediated by the dopaminergic, deprived motivational system (Nader & van der Kooy, 1994). This rules out the possibility of food depriving rats to facilitate self-administration of drugs because once rats are in a food-deprived state the motivational effects of drugs can substitute for the motivational effects of food via the dopaminergic-deprived motivational system (Nader & van der Kooy, 1994). However, preliminary findings have demonstrated that bilateral lesions of the TPP block the initial acquisition of heroin self-administration in previously drug-naive rats (Olmstead, Munn, & Wise, 1993).…”

Tegmental pedunculopontine lesions in rats decrease saccharin's rewarding effects but not its memory-improving effect.

“…The TPP-lesion effect in water-deprived rats is somewhat inconsistent with the findings that deprivation states (food deprivation and morphine deprivation in opiate dependent rats) overshadow the effects of motivational stimuli that act through the TPP system (Bechara et al, 1992; Bechara & van der Kooy, 1992b). Indeed, food deprivation appears to overshadow the TPP-mediated motivational effects of morphine (Nader & van der Kooy, 1993). The present findings demonstrating that sham-lesioned rats show saccharin place conditioning under water-deprivation conditions and that TPP lesions block this conditioning suggest that the water-deprivation conditions used here are not sufficient to overshadow saccharin's TPP-mediated motivational effects.…”

Clonidine antagonizes the aversive effects of opiate withdrawal and the rewarding effects of morphine only in opiate withdrawn rats.

“…Although the motivational effects of morphine in drug-naive rats are TPP dependent, morphine place preferences in food-deprived (but drug-naive) rats were blocked by dopamine antagonist pretreatment. TPP lesions had no effect on these place preferences (Nader & van der Kooy, 1994). Thus, the acute rewarding properties of morphine were mediated by a dopamine-sensitive motivational substrate that was activated by food deprivation (Nader & van der Kooy, 1994).…”

“…TPP lesions had no effect on these place preferences (Nader & van der Kooy, 1994). Thus, the acute rewarding properties of morphine were mediated by a dopamine-sensitive motivational substrate that was activated by food deprivation (Nader & van der Kooy, 1994). It is possible, then, that water deprivation during the conditioned taste aversion training may have activated a dopaminergic motivational substrate.…”

“…Once rats are in a state of deprivation, the motivational effects of certain classes of stimuli act by way of a dopamine-dependent motivational substrate (Nader & van der Kooy, 1994). Although the motivational effects of morphine in drug-naive rats are TPP dependent, morphine place preferences in food-deprived (but drug-naive) rats were blocked by dopamine antagonist pretreatment.…”