Glucose is Necessary to Maintain Neurotransmitter Homeostasis during Synaptic Activity in Cultured Glutamatergic Neurons

Abstract

Glucose is the primary energy substrate for the adult mammalian brain. However, lactate produced within the brain might be able to serve this purpose in neurons. In the present study, the relative significance of glucose and lactate as substrates to maintain neurotransmitter homeostasis was investigated. Cultured cerebellar (primarily glutamatergic) neurons were superfused in medium containing [U-13 C]glucose (2.5 mmol/L) and lactate (1 or 5 mmol/L) or glucose (2.5 mmol/L) and [U-13 C]lactate (1 mmol/L), and exposed to pulses of N-methyl-D-aspartate (300 lmol/L), leading to synaptic activity including vesicular release. The incorporation of 13 C label into intracellular lactate, alanine, succinate, glutamate, and aspartate was determined by mass spectrometry. The metabolism of [U-13 C]lactate under non-depolarizing conditions was high compared with that of [U-13 C]glucose; however, it decreased significantly during induced depolarization. In contrast, at both concentrations of extracellular lactate, the metabolism of [U-13 C]glucose was increased during neuronal depolarization. The role of glucose and lactate as energy substrates during vesicular release as well as transporter-mediated influx and efflux of glutamate was examined using preloaded D-[ 3 H]aspartate as a glutamate tracer and DL-threo-b-benzyloxyaspartate to inhibit glutamate transporters. The results suggest that glucose is essential to prevent depolarization-induced reversal of the transporter (efflux), whereas vesicular release was unaffected by the choice of substrate. In conclusion, the present study shows that glucose is a necessary substrate to maintain neurotransmitter homeostasis during synaptic activity and that synaptic activity does not induce an upregulation of lactate metabolism in glutamatergic neurons.

Physiological glucose is critical for optimized neuronal viability and AMPK responsiveness in vitro

“…However, it is common practice in many studies to maintain cultures in high glucose and then transfer them into media containing lower glucose concentrations prior to experimentation (Ioudina, et al, 2004;Abe, et al, 2006;Bak, et al, 2006;Morgenthaler, et al, 2006). As mentioned in the results, the osmolarity of our NB-A 3 and NB-A 25 medias differ by approximately 20 mOsm and to be consistent with the majority of published "switching" paradigms we did not adjust the osmolarity in this study.…”

“…For these studies, a previously established primary dissociated cortical neuronal culture (Dawson, et al, 1993;Landree, et al, 2004) was maintained in a glucose-free media supplemented with glutamine and B27 and either a physiological concentration of 3 mM glucose (NB-A 3 ), or a non-physiological high concentration of 25 mM glucose (NB-A 25 ). The present study was designed to further develop previous studies that considered the role of media glucose concentrations present during short-term culture (used by DIV4) and/or experimental medias (Wang, et al, 2004;Song, et al, 2005;Morgenthaler, et al, 2006;Abe, et al, 2006;Bak, et al, 2006;Kang, et al, 2006;Canabal, et al, 2007a;Canabal, et al, 2007b).…”

“…In these situations, brain glucose can rise to 4.5 mM or drop as low as 0.16 mM (Silver, et al, 1994). Select studies acknowledge these relationships, and emphasize the importance of in vitro culture conditions for CNS research (Wang, et al, 2004;Song, et al, 2005;Morgenthaler, et al, 2006;Abe, et al, 2006;Bak, et al, 2006;Kang, et al, 2006;Canabal, et al, 2007a;Canabal, et al, 2007b); however, most studies do not. It is not unusual for culture media to contain 25 mM glucose, a level perhaps seen in the plasma of obese ob/ob mice (Schwartz, et al, 1996), but never seen in the brain.…”

A Combined Metabonomic and Proteomic Approach Identifies Frontal Cortex Changes in a Chronic Phencyclidine Rat Model in Relation to Human Schizophrenia Brain Pathology

“…These alterations might be caused by regulatory effects of the Ca 2 þ signaling on the mitochondrial system (Cali et al, 2012). Calcium stimulates the activity of two Ca 2 þ -sensitive tricarboxylic acid cycle (TCA) dehydrogenases, the pyruvate dehydrogenase (Hansford, 1994;McCormack et al, 1990), and enhances electron flow and ATP production through the electron transport chain (Bak et al, 2006;Hajnoczky et al, 1995;Jouaville et al, 1999). The identified mitochondrial abnormalities might lead to abnormal neuronal metabolism, function, plasticity, and brain circuitry (Albensi et al, 2000;Calabresi et al, 2001;Weeber et al, 2002), which are likely to result in the behavioral and cognitive dysfunctions observed in cPCP animal models.…”

“…Alterations in these proteins is thought to lead to cytoskeletal remodeling (ARP2/3) and changes in long-term potentiation, synaptic transmission, and endocytosis (Berridge, 2012). The disturbed intracellular Ca 2 þ levels (regulated by taurine via voltagedependent Ca 2 þ channels and ionotropic glutamate receptors (Bulley and Shen, 2010) affect the energy metabolism via modulating glycolytic and tricarboxylic acid (TCA) kinase activities and changes in amino-acid metabolism (Bak et al, 2006;Hajnoczky et al, 1995;Hansford, 1994;Jouaville et al, 1999;McCormack et al, 1990). Morphological and energy balance changes might ultimately lead to an abnormal neural activity.…”