ABSTRACT
RATIONALE: There is suggestive evidence that the neural mechanisms mediating one-trial and multi-trial behavioral sensitization differ, especially when the effects of various classes of dopamine (DA) agonists are examined. OBJECTIVE: The purpose of the present study was to determine the role of the D2 receptor for the induction of one-trial and multi-trial methamphetamine sensitization in preweanling rats. METHODS: In a series of experiments, rats were injected with saline or raclopride (a selective D2 receptor antagonist), either alone or in combination with SCH23390 (a selective D1 receptor antagonist), 15 min prior to treatment with the indirect DA agonist methamphetamine. Acute control groups were given two injections of saline. This pretreatment regimen occurred on either postnatal days (PD) 13-16 (multi-trial) or PD 16 (one-trial). On PD 17, rats were challenged with methamphetamine and locomotor sensitization was determined. RESULTS: Blockade of D2 or D1/D2 receptors reduced or prevented, respectively, the induction of multi-trial methamphetamine sensitization in young rats, while the same manipulations had minimal effects on one-trial behavioral sensitization. CONCLUSIONS: DA antagonist treatment differentially affected the methamphetamine-induced sensitized responding of preweanling rats depending on whether a one-trial or multi-trial procedure was used. The basis for this effect is uncertain, but there was some evidence that repeated DA antagonist treatment caused nonspecific changes that produced a weakened sensitized response. Importantly, DA antagonist treatment did not prevent the one-trial behavioral sensitization of preweanling rats. The latter result brings into question whether DA receptor stimulation is necessary for the induction of psychostimulant-induced behavioral sensitization during early ontogeny.