Selective inhibition of mesolimbic behavioral supersensitivity by naloxone

Rats trained on ventral tegmental intracranial self-stimulation (ICSS) were unaffected by acute injection of naloxone (2 mg/kg), while acute morphine (5 mg/kg) increased ICSS rate by 46%. Chronic haloperidol (1 mg/kg/day for 21 days) produced mesolimbic dopaminergic supersensitivity, as seen by 42% increase in ICSS. Acute naloxone partially blocked this supersensitivity (to 13% over basal), while acute morphine augmented it (66% over basal). These findings suggest that enkephalinergic systems may be involved in neuroleptic-induced dopaminergic behavioral supersensitivity.