Increased GABA release in the human brain cortex as a potential pathogenetic basis of hyperosmolar diabetic coma

Human cerebral cortical slices preincubated with [3H]GABA, [3H]noradrenaline, or 5-[3H]hydroxytryptamine and superfused with Krebs solution or Mg(2+)-free Krebs solution were used to investigate the influence of increased D-glucose concentrations on the release of these [3H]-neurotransmitters evoked by high K+ content or NMDA receptor activation, respectively. An increase in level of D-glucose (normal content, 11.1 mM) by 32, 60, and/or 100 mM (a range characteristic for hyperosmolar diabetic coma) increased the [3H]GABA release and inhibited the [3H]noradrenaline release evoked by both methods of stimulation. The K(+)-induced 5-[3H]hydroxytryptamine release was also inhibited by high D-glucose content. Blockade of GABAB receptors by p-(3-aminopropyl)-p-diethoxymethylphosphinic acid (CGP 35348) attenuated the inhibitory effect of high D-glucose content on the K(+)-evoked release of [3H]noradrenaline and 5-[3H]hydroxytryptamine, suggesting that the effect on monoamine release is, at least to a major part, the result of the increased GABA release and, as a consequence, of an increased GABA concentration at inhibitory GABAB receptors. The membrane-impermeable sorbitol mimicked the increasing effect of D-glucose on [3H]GABA release and its inhibitory effect on 5-[3H]hydroxytryptamine release. However, dimethyl sulfoxide, which is known to permeate rapidly through biological membranes, had no effect at concentrations equiosmolar to D-glucose. It is concluded that a reduction in brain cell volume caused by increased extracellular, compared with cytoplasmic, osmolarity is crucial for the changes in neuronal function observed at high D-glucose and sorbitol content.(ABSTRACT TRUNCATED AT 250 WORDS)