Abstract

Systemic administration of N-methyl-D-aspartate (NMDA) antagonists prevents the development of behavioral sensitization to amphetamine-like psychostimulants. Pretreatment with the noncompetitive NMDA antagonist, MK-801, resulted in a dose-dependent blockade of behavioral sensitization to cocaine. However, pretreatment with the highest dose of MK-801 (0.25 mg/kg i.p.) alone inhibited the behavioral response to a subsequent cocaine challenge 24 hr later. The induction of behavioral sensitization is known to result, at least partly, from an action by psychostimulants in the ventral tegmental area (VTA). To determine whether the dose-dependent inhibition of behavioral sensitization to cocaine by NMDA antagonists resulted from receptor blockade in the VTA, rats were pretreated in the VTA with the MK-801 or the competitive NMDA antagonist, 3-(2-carboxypiperazine-4-yl) propyl-1-phosphonic acid, before systemically administered cocaine (30 mg/kg i.p.). Two to 3 days later rats were challenged with cocaine alone (15 mg/kg i.p.). Pretreatment with either NMDA antagonist into the VTA prevented the manifestation of behavioral sensitization. Intracranial pretreatment with MK-801 was also made into the nucleus accumbens and amygdala which have been implicated in psychostimulant-induced sensitization. Whereas MK-801 was without effect in the nucleus accumbens, when microinjected into the ventral amygdala it prevented the manifestation of behavioral sensitization to a cocaine challenge. The blockade of sensitization by MK-801 in the VTA was produced with a minimum effective dose of 0.01 nmol, whereas the minimum effective dose in the amygdala was 1.0 nmol. These data demonstrate that stimulation of NMDA receptors in the VTA and amygdala is necessary in the development of behavioral sensitization to cocaine.