Inhibition of the high-affinity uptake of D-[3H]aspartate in rate by L-alpha-aminoadipate and arachidonic acid.

The mechanism of inhibition of the high-affinity sodium-dependent transport of D-[3H]aspartate by the gliotoxin, L-alpha-aminoadipate, and also by the endogenous fatty acid, arachidonic acid (cis-5,8,11,14 eicosatetraenoic acid), into rat brain synaptosomes has been investigated. L-alpha-Aminoadipate competitively inhibited the transport of D-[3H]aspartate with a K1 value of 192 microM. Superfusion of coronal slices of rat brain for 40 min with 1 mM L-alpha-aminoadipate reduced the glutathione concentration of the tissue by 20%. Neither glutamate nor kainate depleted the glutathione level of the slices. Pre-incubation of synaptosomes with arachidonic acid (10 microM) for 10-60 min produced a marked potentiation of the inhibition of D-[3H]aspartate transport, compared to experiments in which the acid was added concurrently with the D-[3H]aspartate ('co-incubation' experiments). Inhibition of D-[3H]aspartate transport by arachidonic acid was not blocked by addition of nordihydroguaretic acid to the pre-incubation medium. Staurosporine (50 nM) reduced the inhibition of transport occurring during pre-incubation with 10 microM arachidonic acid, and there was no longer any significant difference from the level of inhibition obtained in co-incubation experiments. Phorbol, 12-myristate, 13-acetate (1 microM) reduced the transport of D-[3H]aspartate to 73% of control after 20 min pre-incubation of the synaptosomes. This study highlights the fact that inhibition of glutamate transport may affect brain function in a number of different ways. Competitive inhibition by a structural analogue of glutamate, such as L-alpha-aminoadipate, leads to a reduction in the glutathione level, which may be an important factor in L-alpha-aminoadipate-mediated toxicity. On the other hand, the more long-term effects of non-competitive inhibition of glutamate transport by arachidonic acid, in a mechanism involving protein kinase C, may represent a physiological means for regulation of transporter activity in the brain.

Pre-incubation of synaptosomes with arachidonic acid potentiates inhibition of [3H]D-aspartate transport.

The ability of low micromolar concentrations of the polyunsaturated fatty acid, arachidonic acid (cis-5,8,11,14-eicosatetraenoic acid) to inhibit the high-affinity, sodium-dependent transport of [3H]D-aspartate into purified synaptosomes of rat brain has been examined. Pre-incubation of the synaptosomes with arachidonic acid for 10-60 min produced a marked potentiation of the response to 10 microM arachidonic acid compared to co-incubation, and the threshold for inhibition of [3H]D-aspartate transport occurred at a concentration of 1 microM. Minimal inhibition of transport was seen with the unsaturated fatty acids, cis-oleic (cis-9-octadecenoic acid) and cis-linolenic (cis-9,12,15-octadecatrienoic acid), nor with the 20-carbon saturated fatty acid, arachidic acid (n-eicosanoic acid). Inclusion of the cyclo-oxygenase inhibitor, nor-dihydroguaretic acid (NDGA), in the presence of 5 microM arachidonic acid did not alter the inhibition of [3H]D-aspartate transport between 0-10 min, but did enhance the response at longer pre-incubation times. Inhibition of [3H]D-aspartate transport by arachidonic acid persisted during addition of the calcium ionophore, A23187, whereas removal of calcium ions from the incubation medium potentiated the response to arachidonic acid. The results are discussed in terms of the physiological relevance of the inhibition of glutamate transport by arachidonic acid, and suggest that regulation of inhibition of the glutamate transporter by arachidonic acid may be achieved by changes in the extracellular, as well as the intracellular, concentration of calcium ions.