The expression of glutamate transporter GLT-1 in the rat cerebral cortex is down-regulated by the antipsychotic drug clozapine

We show here that clozapine, a beneficial antipsychotic, down-regulates the expression of the glutamate transporter GLT-1 in the rat cerebral cortex, thereby reducing glutamate transport and raising extracellular glutamate levels. Clozapine treatment (25-35 mg kg^-1 day^-1 orally) reduced GLT-1 immunoreactivity in several brain regions after 3 weeks; this effect was most prominent after 9 weeks and most evident in the frontal cortex. GLT-1 protein levels were reduced in the cerebral cortex of treated rats compared with controls and were more severely affected in the anterior (71.9 ± 4.5%) than in the posterior (53.2 ± 15.4%) cortex. L-[³H]-glutamate uptake in Xenopus laevis oocytes injected with mRNA extracted from the anterior cerebral cortex of rats treated for 9 weeks was remarkably reduced (to 30.6 ± 8.6%) as compared to controls. In addition, electrophysiological recordings from oocytes following application of glutamate revealed a strong reduction in glutamate uptake currents (46.3 ± 10.2%) as compared to controls. Finally, clozapine treatment led to increases in both the mean basal (8.1 ± 0.7 μM) and the KCl-evoked (28.7 ± 7.7 μM) output of glutamate that were 3.1 and 3.5, respectively, higher than in control rats. These findings indicate that clozapine may potentiate glutamatergic synaptic transmission by regulating glutamate transport.