Metabolic coupling between glial cells and neurones was first recognised in the honey bee retina, where glial cells released alanine, which is taken up and oxidatively metabolised by photoreceptors (Tsacopoulos et al. 1994). Analogous studies have since demonstrated similar coupling in a variety of central mammalian preparations where glucose is taken up by astrocytes and metabolised to lactate, which is then shuttled to neural elements across the narrow extracellular space. In this study we used electrophysiological and pharmacological procedures to investigate the ability of D-lactate to act as a metabolically inert competitive inhibitor of astrocyte-axon intercellular L-lactate transport in the adult mouse optic nerve (MON), a typical central white matter tract.
Adult male Swiss Webster mice (20-25 g) were killed by decapitation under deep anaesthesia with CO2. The optic nerves were placed in an interface perfusion chamber with oxygenated aCSF at 37 °C. All experiments were carried out in accordance with the guidelines for Animal Care of the University of Washington. Data are presented as means ± S.E.M. and Student’s unpaired t test was employed to determine significance.
A 50 µs, supramaximal stimulus evoked a compound action potential (CAP) that characteristically displayed three peaks (Brown et al. 2003). MONs superfused with control aCSF containing 10 mM glucose had stable robust CAPs for several hours. Replacement of glucose with 20 mM L-lactate sustained the CAP for 2 h indicating that L-lactate can substitute for glucose as a metabolic substrate. During a 60 min period of aglycaemia the CAP was maintained for 22.8 ± 2.2 min (n = 8) before failing. The CAP could be partially rescued by subsequent superfusion with control aCSF; CAP recovered to 38.2 ± 4.2 % (n = 8) of control. Superfusion with 20 mM D-lactate following aglycaemia did not rescue the CAP, indicating that D-lactate is metabolically inert. Aglycaemia in the presence of 20 mM D-lactate resulted in accelerated CAP decline compared to aglycaemia alone (14.4 ± 0.6 min; n = 4; P < 0.05). The CAP could be maintained in the relatively hypoglycaemic glucose concentration of 2 mM for up to 2 h, but prior depletion of glycogen rendered 2 mM glucose incapable of supporting function, suggesting that astrocytic glycogen was supplementing glucose to support function. Addition of 150 µM cinnemate or 20 mM D-lactate in the presence of 2 mM glucose resulted in loss of the CAP after 7.9 ± 2.2 min (n = 3) or 18.3 ± 0.4 min (n = 3), respectively.
These results indicate that in the MON L-lactate is transferred between astrocytes and axons. Glycogen-derived L-lactate presumably sustains axon function either in the absence of glucose, or when ambient glucose is insufficient to meet axonal energy requirements. D-Lactate is a valuable new pharmacological tool to probe the metabolic coupling between astrocytes and neural elements as it has no apparent secondary effects (cf. cinnemate) and is metabolically inert.
This work was supported by NIH Grant 15589 (BRR) and the EPVA (AMB & BRR).