Abstract

The systemic administration of noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists has been considered as a pharmacological model of schizophrenia. In the present work, we used in vivo microdialysis to examine: first, the effects of MK-801, on the efflux of glutamate and serotonin (5-HT) in the medial prefrontal cortex (mPFC) of the rat; second, whether the MK-801-induced changes in the cortical efflux of both transmitters could be blocked by atypical (clozapine and olanzapine) and classical (haloperidol and chlorpromazine) antipsychotic drugs given intra-mPFC; and third, the role of local blockade of dopamine D2/D3/D4, serotonin 5-HT2A and α1-adrenergic receptors as well as agonism at dopamine D1/D5 and 5-HT1A receptors in the mPFC on the increased efflux of glutamate and 5-HT elicited by MK-801. The four antipsychotic drugs blocked the MK-801-induced increase in glutamate, whereas only clozapine and olanzapine were able to block the increased efflux of 5-HT. Furthermore, M100907 (5-HT2A antagonist), BAY x 3702 (5-HT1A agonist) and prazosin (α1-adrenergic antagonist) blocked the MK-801-induced increase of 5-HT and glutamate in the mPFC. In contrast, raclopride (D2/D3 antagonist) and L-745,870 (D4 antagonist) were able to prevent the increased efflux of glutamate (but not that of 5-HT) elicited by MK-801. SKF-38393 (dopamine D1/D5 agonist) also prevented the MK-801-induced increase of glutamate in the mPFC, but the same effect on cortical 5-HT was reached only at the highest concentration tested. We suggest that the blockade of an exacerbated 5- HT release in the mPFC induced by NMDA antagonists can be a characteristic of atypical antipsychotic drugs. Moreover, we propose that D2/D3/D4 receptor antagonists would act predominantly on a subpopulation of GABAergic interneurons of the mPFC, thus enhancing cortical inhibition, which would prevent an excessive glutamatergic transmission. Dopamine D1/D5 agonists would further stimulate GABA release from other subpopulation of interneurons controlling cortical output to the dorsal raphe nucleus. Atypical antipsychotic drugs might further act upon 5-HT2A, 5-HT1A and α1-adrenoceptors present in pyramidal cells (including those projecting to the dorsal raphe nucleus), which would directly inhibit an excessive excitability of these cells.

Keywords: Serotonin, glutamate, prefrontal cortex, MK-801, dopamine D1/D2/D3/D4 receptors, 5-HT2A receptor, 5-HT1A receptor, α1- adrenoceptor