Effectiveness of extracellular lactate/pyruvate for sustaining synaptic vesicle proton gradient generation and vesicular accumulation of GABA

The effects of extracellular monocarboxylates pyruvate and lactate on membrane potentials, acidification and neurotransmitter filling of synaptic vesicles were investigated in experiments with rat brain synaptosomes using (3)H]GABA and fluorescent dyes, potential-sensitive rhodamine 6G and pH-sensitive acridine orange. In experiments investigating accumulation of acridine orange in synaptic vesicles within the synaptosomes, monocarboxylates, similarly to glucose, ensured generation of the vesicle proton gradient by available and recycled vesicles, and pyruvate demonstrated the highest efficacy. An increase in the level of proton gradient correlated with enhanced accumulation of (3)H]GABA in synaptic vesicles and resulted in enlarged exocytosis and attenuated the transporter-mediated (3)H]GABA release. Pyruvate added to glucose-contained medium caused more active binding of rhodamine 6G by synaptosomes that reflected mitochondrial membrane hyperpolarization, and this intensification of nerve terminal energy metabolism resulted in an increase in total ATP content by approximately 25%. Pyruvate also prolonged the state of metabolic competence of nerve terminal preparations, keeping the mitochondrial potential and synaptic vesicle proton gradient at steady levels over a long period of time. Thus, besides glucose, the extracellular monocarboxylates pyruvate and lactate can provide sufficient support of energy-dependent processes in isolated nerve terminals, allowing effective functioning of neurotransmitter release and reuptake systems.