HYPEROSMOLALITY-INDUCED GABA RELEASE FROM RAT BRAIN SLICES: STUDIES OF CALCIUM DEPENDENCY AND SOURCES OF RELEASE

Abstract— The effects of hyperosmolal superfusion upon the release of preloaded, radio-labeled GABA has been studied, using both first cortical and first pontine brain slices. GABA release was stimulated with either hyperosmolal Na⁺ or sucrose superfusion in cortical slices. This stimulated release of radio-labeled GABA was partially Ca²⁺-dependent in cortical slices. When barium ions replaced Ca²⁺ in hyperosmolal medium, a similar effect was seen. High concentration of magnesium in Ca²⁺ -free hyperosmolal medium did not induce stimulation. The increased release of α-aminoisobutyric acid (AIBA), a non-metabolized amino acid induced by hyperosmolality, was not Ca²⁺-dependent.
GABA release was also stimulated with hyperosmolal sucrose superfusion in pontine slices. The effect of pre-treatment of cortical and pontine slices with β-alanine or L-2,4-diaminobutyric acid (DABA) was used to study the source of exogenous GABA release induced by hyperosmolality. In cortical slices, β-alanine blocked the hyperosmolal release of GABA and also slightly inhibited GABA uptake. DABA did not change hyperosmolal GABA release, although it inhibited GABA uptake. In pontine slices, both DABA and β-alanine inhibited GABA uptake, but were unable to inhibit the hyperosmolal release of GABA.
The data suggest that hyperosmolality causes increased release of GABA from neurons, analogous to that seen with K⁺-depolarization. AIBA, unlike GABA, is released from brain cells as a non-Ca²⁺ -dependent response to osmotic equilibration. The observation that pre-treatment with β-alanine inhibits the hyperosmolal release of GABA suggests that hyperosmolality alters glial cell function.