Release of Neurotransmitter Amino Acids from Synaptosomes: Enhancement of Calcium‐Independent Efflux by Oleic and Arachidonic Acids

Abstract

The release of preloaded [14C]neuroactive amino acids (glutamic acid, proline, gamma-aminobutyric acid) from rat brain synaptosomes can occur via a time-dependent, Ca2+-independent process. This Ca2+-independent efflux is increased by compounds that activate Na+ channels (veratridine, scorpion venoms), by the ionophore gramicidin D, and by low concentrations of unsaturated fatty acids (oleic acid and arachidonic acid). Saturated fatty acids have no effect on the efflux process. Neither saturated nor unsaturated fatty acids have an effect on the release of [14C]leucine, an amino acid not known to possess neurotransmitter properties. The increase in the efflux of neuroactive amino acids by oleic and arachidonic acids can also be demonstrated using synaptosomal membrane vesicles. Under conditions in which unsaturated free fatty acids enhance amino acid efflux, no effect on 22Na+ permeability is observed. Since Na+ permeability is not altered by fatty acids, the synaptosomes are not depolarized in their presence and, thus, the Na+ gradient can be assumed to be undisturbed. We conclude that unsaturated fatty acids represent a potentially important class of endogenous modulators of neuroactive amino acid transport in nerve endings and further postulate that their action is the result of an uncoupling of amino acid transport from the synaptosomal Na+ gradient.

External Ca2+‐Independent Release of Neurotransmitters

“…The consequent depolarization of the nerve terminals is accompanied by an influx of Ca2+ (Blaustein, 1975;Akerman and Nicholls, 1981;Adam-Vizi and Ligeti, 1986), paralleled by an increase in transmitter release (Blaustein, 1975;Adam-Vizi and Ashley, 1987). However, in the absence of external Ca2+, veratridine is also effective in enhancing the release of GABA (Benjamin and Quastel, 1972;Haycock et al, 1978;Sandoval, 1980;Nelson and Blaustein, 1982;Rhoads et al, 1983;Carvalho et al, 1986), noradrenaline (Schoffelmeer and Mulder, 1983), ACh (Meyer and Cooper, 1981;Vyas and Marchbanks, 198 1 ;Adam-Vizi and Ligeti, 1984;Carroll, 1984;Carroll and Benishin, 1984), dopamine (Raiteri et al, 1979), glutamate (Levi et al, 1980;Rhoads et al, 1983;McMahon et al, 1990), glycine and B-alanine (Aragon et al, 1988). These effects of veratndine, in both the presence and the absence of Ca2+, were specific with respect to being sensitive to the sodium channel blocker tetrodotoxin (TTX).…”

Metabotropic glutamate receptors, transmitter output and fatty acids: studies in rat brain slices

“…In striatal slices, this increase was 3.5 + 0.7 and 10 + 2 fold when the challenging solutions contained 30 or 50 mM KCl respectively. Similarly, in cortical slices, the increase of radioactivity output was 2.8 + 0.2 and 8.9 + 1.1 fold (Figures 1 and 2 (Spector, 1975;Rhoads et al, 1983) and in order to test the role of these fatty acids in the mGluR agonist-induced modulation of transmitter release, low concentrations of a few of them were added to BSA containing superfusion solution. As shown in Figure 5, arachidonic acid (1-10,IM) or oleic acid (1-10,UM), added together with lS,3R-ACPD to BSA containing buffer solutions, restored the potentiating effect the mGluR agonist had in control experiments.…”

“…While the inhibition of transmitter output caused by the mGluR agonist in the striatum does not require the availability of fatty acids, the potentiation of the depolarization-induced transmitter release in cortical slices requires the simultaneous presence of 1S,3R-ACPD and of fatty acids. In fact, BSA, which is able to bind fatty acids (Spector, 1975;Rhoads et al, 1983) or inhibitors of fatty acid formation such as bromophenacyl bromide, an inhibitor of phospholipase A2 (Hofmann et al, 1982), and RG-80267, an inhibitor of diacylglycerol lipase (Schimmel, 1988), completely prevented the mGluR-induced potentiation of transmitter output. Similar results have been obtained in studies on 4-aminopyridine-induced exocytosis of glutamate from cortical synaptosomes (Herrero et al, 1992;Vazquez et al, 1994;Coffey et al, 1994).…”

“…However, in this preparation, fatty acids may be endogenously formed, as a consequence of spontaneous or transmitter-induced activation of membrane fatty acids are considered particularly important as retrograde messengers during events leading to synaptic plasticity (Williams et al, 1989;Nicholls, 1992), we thought it interesting to test if the presence of a fatty acid is required both for the facilitatory and the inhibitory events induced by mGluR agonists on transmitter output. One of the approaches used to significantly reduce the availability of fatty acids in perfused tissues is the addition to the perfusion fluid of micromolar concentrations of bovine serum albumin (BSA), which possesses active sites able to bind fatty acids (Reed et al, 1975;Rhoads et al, 1983). Moreover, since fatty acid formation in in vitro tissues is mainly due to the action of the enzymes phospholipase A2 and/or diacylglycerol lipase (see: Axelrod et al, 1988 for a review), we also used inhibitors of these enzymes for a better understanding of the role fatty acids play in the mGluR-modulation of transmitter output.…”

Lithium ion does not protect brain against transient ischemia in gerbils.

“…22 This finding is consistent with the view that glutamate released during ischemia stimulates postsynaptic receptors, which in turn activate phospholipase C in neurons. Furthermore, free arachidonic acid is known to increase the efflux of glutamate from synaptosomes 23 and to inhibit its uptake by glia and neurons. 24 Deacylation of arachidonic acid would, therefore, reinforce the excitatory effect of this neurotransmitter.…”