Differential Expression of Glutamate Dehydrogenase in Cultured Neurons and Astrocytes From Mouse Cerebellum and Cerebral Cortex Ioannis Zaganas, 1 Helle S. Waagepetersen, 2 Panagiotis Georgopoulos, 1 Ursula Sonnewald, 3 Andreas Plaitakis, 1 and Arne Schousboe 2 * 1 Department of Neurology, University of Crete, School of Health Sciences, Crete, Greece 2 Department of Pharmacology, Neuroscience PharmaBiotec Res. Center, Royal Danish School of Pharmacy, Copenhagen, Denmark 3 Department of Clinical Neuroscience, Norwegian University of Science and Technology (NTNU), Trondheim, Norway Glutamate dehydrogenase (GDH) specific activities, ki- netic properties and allosteric regulation were studied in extracts from cultured neurons and astrocytes prepared from mouse cerebral cortex and cerebellum. Consider- able differences were observed in the specific activity of the enzyme among the different cell types with astro- cytes expressing the highest GDH activity. This may reflect the functional importance of these cells in gluta- mate uptake and metabolism. Among the neurons, the glutamatergic cerebellar granule cells showed a GDH specific activity that was 60% higher (P  0.01) than that of the GABAergic cerebral cortical neurons. Also, the K m for ammonia was 1.7-fold higher in the cortical neurons than in the other cell types. These findings may reflect a particular need for the glutamatergic granule cells to synthesize glutamate via the GDH pathway. No differ- ences were observed among the different cell types with regard to the allosteric properties of GDH expressed by these cells. J. Neurosci. Res. 66:909 –913, 2001. © 2001 Wiley-Liss, Inc. Key words: -ketoglutarate; ammonia; GABAergic neu- rons; glutamatergic neurons Glutamate dehydrogenase (GDH, EC 1.4.1.3.) is a mitochondrial enzyme that catalyzes the oxidative deami- nation of glutamate to -ketoglutarate using NAD + or NADP + as cofactors (Smith et al., 1975). There is evi- dence that GDH activity in vivo is controlled allosterically by the need of the cell for ATP as an energy source (Smith et al., 1975). Consistent with this concept are observations on pancreatic beta cells showing that stimulation of GDH activity is associated with insulin release due to increased energy charge (Sener et al., 1981). GDH is present in almost all mammalian tissues with high enzyme activities occurring in the liver, kidney, pancreas, brain, intestine, spleen, lymph nodes and heart (Smith et al., 1975). In mammalian brain, GDH may exist in different isoforms (Colon et al., 1986; Hussain et al., 1989; Cho et al., 1995). Two distinct genes, designated GLUD1 and GLUD2, encoding human GDH have been cloned and characterized (Shashidharan et al., 1994; Plaitakis et al., 2000). In the nerve tissue, glutamate serves as the major excitatory neurotransmitter, besides its func- tion in metabolism. GDH may thus have a dual function in brain that involves glutamate neurotransmitter availabil- ity and energy metabolism. To this date, however, the exact role of GDH in glutamate homeostasis has not been fully elucidated. The cellular and subcellular localization of GDH has previously been studied in rat brain using mainly immunocytochemical methods. Results of those studies showed that GDH immunoreactivity is essentially lim- ited to astrocytes (Aoki et al., 1987; Wenthold et al., 1987; Rothe et al., 1990, 1994). Moreover, a consid- erable regional variation in glial GDH immunoreactiv- ity was observed, with the enzyme being particularly enriched in regions that receive dense glutamatergic innervation (Aoki et al, 1987). Abbreviations used: DMEM, Dulbecco’s minimum essential medium; GDH, glutamate dehydrogenase; SA, specific activity. Contract grant sponsor: Danish Medical Research Council; Contract grant number: 9700761, 22-00-1011, 1747; Contract grant sponsor: Norwegian Research Council; Contract grant sponsor: Association for the Advance- ment of Research and Treatment of Neurologic Disorders of Crete; Con- tract grant sponsor: Lundbeck Foundation; Contract grant sponsor: NOVO Nordisk Foundation; Contract grant sponsor: Blix Foundation; Contract grant sponsor: SINTEF Foundation. *Correspondence to: Prof. A. Schousboe, DSc, Dept. of Pharmacology, Royal Danish School of Pharmacy, 2 Universitetsparken, DK-2100 Copenhagen, Denmark. E-mail: as@dfh.dk Received 11 July 2001; Revised 17 August 2001; Accepted 20 August 2001 Journal of Neuroscience Research 66:909 –913 (2001) © 2001 Wiley-Liss, Inc.