Differential expression, activity and regulation of the sodium/myo-inositol cotransporter in astrocyte cultures from different regions of the rat brain

The high-affinity sodium/myo-inositol cotransporter (SMIT) is involved in o smoregulation in several cells and tissues. In the CNS the activity of SMIT also determines the individual susceptibility of neural cells to the inosi tol depleting effect of lithium, which is considered to be important in lit hium's therapeutic effects in manic-depressive illness. Among neural cells SMIT is particularly active in astrocytes. In the present work we have clon ed the cDNA of SMIT of the rat and assessed its activity, expression and re gulation in primary astroglia cultures derived from five different rat brai n regions: cerebellum, cortex, diencephalon, hippocampus and tegmentum. Aft er an incubation period of 24 h in medium containing (3)[H]labeled myo-inos itol different steady-state concentrations were detected which were depende nt on the brain region from which the astrocytes were cultured. In addition , myo-inositol uptake in astrocytes from different areas was characterized by two different K-m, values (27 mu M for cerebellum and diencephalon, 50 m u M for cortex, hippocampus and tegmentum) and by three different v(max),,, values (approx. 200 pmol/mg protein/min for astrocytes from cerebellum and tegmentum, 298 for hippocampus and 465 for cortex), indicating that the ac tive myo-inositol uptake into astroglial cells is distinct in the various b rain regions. The efficacy of uptake as determined by v(max),, values of 3[ K]myoinositol uptake correlated with the level of mRNA of SMIT in the astro cyte cultures from the various brain regions as determined by semiquantitat ive reverse transcription-polymerase chain reaction (RT-PCR). Both 3[H]myo- inositol uptake and SMIT mRNA content was upregulated by incubation of astr ocytes in medium of increased osmolarity. In astrocytes from cerebellum, co rtex, hippocampus and tegmentum 3[H]myo-inositol uptake was downregulated b y chronic incubation with 400 mu M inositol. This effect was not observed i n astrocytes from diencephalon. Furthermore, in astrocytes from cortex and hippocampus but not from cerebellum, diencephalon and tegmentum incubation with corticosterone for three days upregulated 3[H]myo-inositol uptake. It is concluded that SMIT is differentially expressed and regulated in astrocy tes from distinct brain regions. These regional differences suggest particu lar consideration of localized effects in investigations of the role of myo -inositol in the mechanism of action of antibipolar drugs. (C) 2000 Elsevie r Science Ltd. All rights reserved.