Rationale: Aripiprazole is an atypical antipsychotic drug with high in vitro affinity for 5-HT(1A), 5-HT(2A) and dopamine (DA) D2 receptors. However, its in vivo actions in the brain are still poorly characterized.
Objective: The aim was to study the in vivo actions of aripiprazole in the rat and mouse brain.
Methods: Brain microdialysis and single-unit extracellular recordings were performed.
Results: The systemic administration of aripiprazole reduced 5-HT output in the medial prefrontal cortex (mPFC) and dorsal raphe nucleus of the rat. Aripiprazole also reduced extracellular 5-HT in the mPFC of wild-type (WT) but not of 5-HT(1A) (-/-) knockout (KO) mice. Aripiprazole reversed the elevation in extracellular 5-HT output produced by the local application of the 5-HT(2A/2C) receptor agonist DOI in mPFC. Aripiprazole also increased the DA output in mPFC of WT but not of 5-HT(1A) KO mice, as observed for atypical antipsychotic drugs, in contrast to haloperidol. Contrary to haloperidol, which increases the firing rate of DA neurons in the ventral tegmental area (VTA), aripiprazole induced a very moderate reduction in dopaminergic activity. Haloperidol fully reversed the inhibition in dopaminergic firing rate induced by apomorphine, whereas aripiprazole evoked a partial reversal that was significantly different from that evoked by haloperidol and from the spontaneous reversal of dopaminergic activity in rats treated with apomorphine.
Conclusions: These results indicate that aripiprazole modulates the in vivo 5-HT and DA release in mPFC through the activation of 5-HT(1A) receptors. Moreover, aripiprazole behaves as a partial agonist at DA D2 autoreceptors in vivo, an action which clearly distinguishes it from haloperidol.