Metabolic properties of nerve endings isolated from rat brain

Energy metabolism was studied in nerve endings isolated from 3-week-old rat brain. Concentrations of glycogen, glucose, ATP, phosphocreatine and lactate were lower in synaptosomes than in the intact brain. The consumption of these endogenous substrates, the ability to generate high-energy phosphate, and the production of ammonia were determined in aerobic and anaerobic conditions. Unlike nerve tissue in general, synaptosomes preferentially utilized endogenous ATP and phosphocreatine stores which, on incubation in the absence of exogenous substrates, were emptied long before glycogen stores were exhausted. The optimal medium for respiratory studies was found to have electrolyte concentrations equal to the extracellular fluid. The synaptosomes had an endogenous respiration rate of 6.3 nmol O2 mg prot. min, measured with an oxygen electrode, and it probably reflects consumption of their glycogen stores. Glucose usually had no effect on the respiration rate of synaptosomes, but sometimes increased it slightly. However, after incubation in the presence of arsenate synaptosomes showed an increase in respiration when glucose was added. ADP, when added with glucose, also stimulated respiration. Pyruvate and succinate always increased respiration, succinate usually having the stronger effect. The present results show that isolated nerve endings are metabolically intact, which justifies their use in research on neurotransmission. In addition, opposite to the present consensus, synaptic transmission does not seem primarily to depend on the availability of glucose but rather on local stores of high-energy phosphate compounds.