The Neuroprotective Effect of Lactate Is Not Due to Improved Glutamate Uptake after Controlled Cortical Impact in Rats

Abstract

For many years lactate was considered to be a waste product of glycolysis. Data are accumulating that suggest that lactate is an important energy substrate for neurons during activation. In severe traumatic brain injury (TBI) glutamate release and ischemic cerebral blood flow (CBF) are major factors for a mismatch between energy demand and supply and for neuronal cell death. Although ATP and behavior could be improved by lactate treatment after TBI, no histological correlate nor any linkage to better astrocytic glutamate uptake or CBF as possible mechanisms have been described. We subjected male rats to a controlled cortical impact (CCI; 5 m/sec, 2.5 mm). To study the effects of lactate treatment on lesion volume, glutamate release, and CBF, animals were infused with either NaCl or 100 mM lactate for up to 3 h. The role of endogenous lactate was investigated by inhibiting transport with α-cyano-4-hydroxy-cinnamic acid (4-CIN; 90 mg/kg). Lactate treatment 15 min post-CCI reduced lesion volume from 21.1±2.8 mm³ to 12.1±1.9 mm³ at day 2 after CCI. Contusion produced a significant three- to fourfold increase of glutamate in microdialysates, but there was no significant difference between treatments that began 30 min before CCI. In this experiment lesion volume was significantly reduced by lactate at day 7 post-CCI (23.7±4 to 9.3±1-2 mm³). CBF increased immediately after CCI and dropped thereafter below baseline in all animals. Lactate infusion 15 min post-CCI elevated CBF for 20 min in 7 of 10 animals, whereas 7 of 8 NaCl-treated animals showed a further CBF decline. Neuroprotection was achieved by lactate treatment following contusion injury, whereas blocking of endogenous lactate transport exerted no adverse effects. Neuroprotection was not achieved by improved glutamate uptake into astrocytes, but was supported by augmented CBF following CCI. Due to its neuroprotective property, lactate might be a beneficial pharmacological treatment for TBI patients.