Serotoninergic control of the activity and expression of glial GABA transporters in the rat cerebellum

Gamma-Aminobutyric acid (GABA) transporters (GAT-1, GAT-2, and GAT-3) play a key role in the termination of GABA transmission and the regulation of extracellular GABA concentrations. In the present study, pharmacological, cellular, and molecular analyses provide evidence for a modulatory effect of serotoninergic neurons on the activity and expression of glial GABA transporters in the rat cerebellum. Degeneration of serotoninergic neurons after in vivo 5,7-dihydroxytryptamine (5,7-DHT) treatment resulted in a significant decrease (-27%) in [3H]-GABA uptake into cerebellar punches. This decrease probably occurred via inhibition of GAT-2 or GAT-3 activity since their inhibitor, beta-alanine, induced a decrease in [3H]-GABA uptake in punches of sham-operated rats (-28%), but not in punches of 5,7-DHT-treated rats, demonstrating that serotonin terminal degeneration had already impaired the beta-alanine-sensitive component of GABA uptake. In contrast, nipecotic acid, a preferential inhibitor of GAT-1, induced comparable decreases in [3H]-GABA uptake comparable in punches of 5,7-DHT (-38%) versus sham-operated rats (-37%). The decreases in GAT-1 (-16%), GAT-2 (-34%), and GAT-3 (-32%) mRNA levels after 5,7-DHT treatment (detected by quantitative RT-PCR) are consistent with a serotoninergic control of GABA transporter expression at the transcriptional level. The cellular distribution of GAT-2 and GAT-3 mRNA, shown by in situ hybridization, suggests a glial localization of these transporters in the cerebellum and demonstrated a preferential anatomical localization of GAT-2 mRNA in the granular layer and of GAT-3 mRNA in the deep cerebellar nuclei. A direct serotoninergic control of glial GABA uptake was further demonstrated in vitro since serotonin stimulated the activity and mRNA expression of the GABA transporters in cerebellar astrocyte cultures.