Journul zyxwvutsrqponmlk of zyxwvutsrqponmlkj Neurochemistry zyxwvutsrqpon Raven Press, New zyxwvutsrqponm York zyxwvutsrqpo 0 1986 International Society for Neurochemistry Differences in Anionic Dependence of the Synaptic Efflux of D-Aspartic Acid and y-Aminobutyric Acid Liv Unni Naalsund and Frode Fonnum Norwegian Defence Research Establishment, Division for Environmental Toxicology, Kjeller, Norway Abstract: The synaptosomal effluxes of D-aspartate and y-aminobutyric acid (GABA) induced by a substitution of the C1- ions with propionate in the incubation medium were measured in preparations of hippocampal tissue ho- mogenates. The efflux of aspartate was significantly higher than spontaneous efflux at 125 mM C1- (normal zyxwvut = 144 mM) and was increased with decreasing C1- concen- tration. GABA efflux was much less sensitive to a reduc- tion in C1- concentration than D-aspartate. The efflux was Ca2+-dependent in both cases. Key Words: ?-Amino- butyric acid-D-Aspartate-Transmitters- Chloride-EMux. Naalsund L. U. and Fonnum F. Differ- ences in anionic dependence of the synaptic efflux of D- aspartic acid and y-aminobutyric acid. J. Neurochem. zy 47, 687-690 (1986). The synaptic effluxes of endogenous transmitters or exogenously labelled transmitters and analogues have been studied by several different depolariza- tion methods (for reviews see Szerb, 1983; Fonnum, 1984). The transmitter amino acids are also secreted by depolarization of glial cell prepara- tions, but this efflux can be separated from the syn- aptic efflux by its lack of Cat+ dependence (Sell- strom and Hamberger, 1977). It is well established that a sudden change in the external [K+] or [Cl-] will depolarize the mem- brane of a muscle fiber (Hodgkin and Horowicz, 1960). Stimulation of synaptosomal release by high Kf concentration is a frequently used tool in neu- rochemical research. Recently it has been shown that a severe reduction in the C1- concentration in the perfusion medium was accompanied by an ef- flux of the transmitter amino acids glutamate and aspartate (Hardy et al., 1984). We are interested in studying the differences in efflux mechanisms for different transmitters. The neurotoxic agent trimethyltin has been shown to induce the efflux of glutamate, but has no effect on the efflux of y-aminobutyric acid (GABA) (Naalsund and Fonnum, 1986). Trimethyltin has the capacity to induce Cl-/OH- exchanges across cel- lular membranes (Motais et al., 1977). It has there- fore been suggested that changes in C1- equilibrium could explain the observed effects on eMux. If tri- methyltin has the capacity to produce a change in the C1- equilibrium in the nerve terminal this could probably result in transmitter efflux. GABA is known to act as a C1- ionophore (McBurney and Barker, 1978). We have therefore suggested that the reaction of the GABAergic nerve terminal to changes in extracellular C1- concentra- tion might be different from that of the glutamergic terminals. If this is the case, it would give an expla- nation for the selective effect of trimethyltin on glu- tamergic neurons. This could also represent a basic difference between the excitatory glutamergic and inhibitory GABAergic neurons which could be helpful during discussions on different vulnerabili- ties to neurobiological tools. In this article we present the results of a comparative study of the efflux of D-aspartate and GABA during pulses of low C1- concentration. MATERIALS AND METHODS Tissue homogenates were prepared from hippocampi dissected from brains of young adult male Wistar rats (150-200 g). The hippocampi were homogenized in cold sucrose, 0.32 M, to give a protein concentration of 6 mg/ml. Efflux of ~-[2,3-’H]aspartic acid and [2,3-’H(N)]GABA was measured by a modification of the method described by Levi et al. (1982). In brief, 2 ~1 ho- Received November 22, 1985; revised February 28, 1986; ac- cepted March 1, 1986. Norway. Address correspondence and reprint requests to Dr. F. Fonnum at Norwegian Defence Research Establishment, Divi- sion for Environmental Toxicology, P.O. Box 25, N-2007 Kjeller. Abbreviation used: GABA, y-aminobutyric acid. 68 7