NEUROLEPTICS BLOCK HIGH-DOSE BUT NOT LOW-DOSE HEROIN PLACE PREFERENCES - FURTHER EVIDENCE FOR A 2-SYSTEM MODEL OF MOTIVATION

The researchers studied whether 2 separate motivational systems in the brain underlie the rewarding effects of morphine. The brainstem tegme ntal pedunculopontine nucleus (TPP) is involved in mediating the motiv ational effects of opiates in nondeprived (drug-naive) rats, whereas d opamine transmission is necessary in mediating the motivational effect s of opiates in deprived rats (opiate withdrawal). The results show th at heroin's motivational properties obey the same boundary between a n ondeprived and a deprived motivational state. Bilateral ibotenic acid lesions of the TPP blocked the acquisition of a place preference for a n environment paired with 0.05 mg/kg heroin (a dose that induces no wi thdrawal aversion) but had no effect on place preference for an enviro nment paired with 0.5 mg/kg heroin (a dose that does induce withdrawal aversion). Dopamine antagonist pretreatment produced the opposite pat tern of results.