ON THE MECHANISM BY WHICH VERATRIDINE CAUSES A CALCIUM‐INDEPENDENT RELEASE OF γ‐AMINOBUTYRIC ACID FROM BRAIN SLICES

Abstract

1 The mechanisms by which veratridine increases the release of γ-aminobutyric acid (GABA) from brain slices have been studied. 2 Exposure of superfused cerebro-cortical, nigral or cerebellar slices to veratridine (5 μm) or KC1 (50 mM) caused large increases in the efflux of [³H]-GABA. 3 Reduction of the external Ca concentration [Ca]₀ to zero had strikingly different effects on the veratridine and K-evoked release of [³H]-GABA. The K-evoked release from all three areas was greatly reduced in Ca-free medium, but the veratridine-evoked release from cerebeller slices was not affected, and the release of [³H]-GABA from cortical and nigral slices was increased three fold. The potentiation of the veratridine evoked release of GABA which occurred in Ca-free medium was not due to the reduction in divalent ions, because it still occurred in medium in which the Ca was replaced by an equivalent amount of Mg. 4 The veratridine-evoked release of [¹⁴C]-glycine from slices of spinal cord was also significantly increased in Ca-free medium. In contrast, the release of cortical [³H]-noradrenaline and [¹⁴C]-acetylcholine caused by the alkaloid was greatly diminished in Ca-free medium. 5 The veratridine but not the K-evoked release of [³H]-GABA was abolished when the external Na concentration [Na]₀ was reduced to zero and by tetrodotoxin (TTX) (0.2 μm). Cl-free medium did not affect the veratridine-evoked release of [³H]-GABA or its potentiation by Ca-free medium. 6 Exposure of the tissue to depolarizing concentrations of external K ([K]₀ = 120 mM) did not abolish the veratridine evoked release of [³H]-GABA or its potentiation by Ca-free medium. 7 Pre-incubation of cortical slices with L-2,4, diaminobutyric acid (DABA), or substitution of Na in the superfusion medium with Li, did not affect the veratridine-evoked release of [³H]-GABA, indicating that the alkaloid does not stimulate GABA efflux by a carrier-mediated transport process. 8 Exposure of the tissue to ruthenium red (10 μm) increased the veratridine evoked release of [³H]-GABA in both normal and in Ca-free medium but almost abolished the K-evoked release. 9 It is suggested that veratridine causes GABA release by increasing the permeability of the nerve terminals to Na. In normal medium, the resulting influx of Ca²⁺ ions through voltage-dependent Ca²⁺ channels may be involved in triggering the release of GABA. However, a major part of the GABA efflux appears to be triggered by the release of Ca²⁺ ions from intraterminal mitochondria, which results from the increase in[Na]i. Since Ca²⁺ ions antagonize the action of veratridine, the potentiation of the drug-evoked release of GABA that occurs in Ca-free medium, might be due to the absence of the antagonistic Ca²⁺ ions. The resulting greater increase in Na entry and [Ca]_i caused by Ca release from intracellular stores, must presumably more than balance the contribution normally made by any influx of extracellular Ca²⁺.