Regulation of GABA Level in Rat Brain Synaptosomes: Fluxes Through Enzymes of the GABA Shunt and Effects of Glutamate, Calcium, and Ketone Bodies
Article first published online: 23 NOV 2002
Journal of Neurochemistry
Volume 67, Issue 6, pages 2325–2334, December 1996
How to Cite
Erecińska, M., Nelson, D., Daikhin, Y. and Yudkoff, M. (1996), Regulation of GABA Level in Rat Brain Synaptosomes: Fluxes Through Enzymes of the GABA Shunt and Effects of Glutamate, Calcium, and Ketone Bodies. Journal of Neurochemistry, 67: 2325–2334. doi: 10.1046/j.1471-4159.1996.67062325.x
- Issue published online: 23 NOV 2002
- Article first published online: 23 NOV 2002
- Received April 11, 1996; final revised manuscript received July 25, 1996; accepted July 30, 1996.
- Glutamic acid decarboxylase activity;
- GABA-transaminase activity;
- Ketone bodies;
Abstract: Stable isotopes were used to measure both the rate of GABA formation by glutamic acid decarboxylase (GAD) and the rate of utilization by GABA-transaminase (GABA-T). The initial rate of GABA accumulation, determined with either [2-15N]glutamine or [2H5]glutamine as precursor, was 0.3–0.4 nmol/min/mg of protein. Addition of the calcium ionophore A23187 enhanced GAD activity, whereas changes in levels of inorganic phosphate and H+ were without influence. Flux through GABA-T (GABA glutamate), measured with [15N]GABA as precursor, was 0.82 nmol/min/mg of protein, whereas the reamination of succinic acid semialdehyde (reverse flux through GABA-T) was almost sixfold faster, 4.8 nmol/min/mg of protein. The rate of GABA metabolism via the tricarboxylic acid cycle was very slow, with the upper limit on flux being 0.03 nmol/min/mg of protein. Addition of either acetoacetate or β-hydroxybutyrate raised the internal content of glutamate and reduced that of aspartate; the GABA concentration and the rate of its formation increased. It is concluded that in synaptosomes (a) GABA-T is a primary factor in regulating the turnover of GABA, (b) a major regulator of GAD activity is the concentration of internal calcium, (c) GAD in nerve endings may not be saturated with its substrate, glutamate, and the concentration of the latter is a determinant of flux through this pathway, and (d) levels of ketone bodies increase, and maintain at a higher value, the synaptosomal content of GABA, a phenomenon that may contribute to the beneficial effect of a ketogenic diet in the treatment of epilepsy.