Effects of fasting and diabetes on some enzymes and transport of glutamate in cortex slices or synaptosomes from rat brain

Phosphate-activated glutaminase (PAG) and glutamic acid decarboxylase (GAD) were assayed in homogenates and synaptosomes obtained from starved (48 hr or 120 hr) and diabetic (streptozotocin) rat brain cortex. Glutamine synthetase (GS) was assayed in homogenates, microsomal and soluble fractions, from brain cortex of similarly treated rats.l-Glutamate uptake and exit rates were determined in cortex slices and synaptosomes under the same conditions. The specific activity (s.a.) of PAG, a glutamate producing enzyme, decreased (50%) in the homogenate after 120-hr starvation. In synaptosomes it decreased (25%) only after 48-hr starvation. The s.a of GAD and GS, which are glutamate-consuming enzymes, were progressively increased with time of starvation, reaching 39% and 55% respectively after 120 hr. GS in the microsomes or the soluble fraction and GAD in the synaptosomes showed no change in s.a. under these conditions. Diabetes increased (40%) microsomal GS s.a. and decreased GAD s.a. (18%) in the homogenate. Thel-glutamate uptake rate was decreased (48%) by diabetes in slices but not in synaptosomes. It is suggested that a) enzymes of the glutamate system respond differently in different subcellular fractions towards diabetes or deprivation of food and b) diabetes may affect the uptake system in glial cells but not in neurons.Abbreviations used AET 2-aminoethylisourethonium bromide - GAD glutamic acid decarboxylase - GS glutamine synthetase - GSH glutathione - PAG phosphate-activated glutaminase - PLP pyridoxal phosphate - r.c.f. relative centrifugal force - s.a. specific activity