RELEASE OF ENDOGENOUS GLUTAMATE FROM RAT CORTICAL SLICES IN PRESENCE OF THE GLUTAMATE UPTAKE INHIBITOR L-TRANS-PYRROLIDINE-2,4-DICARBOXYLICACID

The effect of the new glutamate uptake inhibitor, L-trans-pyrrolidine- 2,4-dicarboxylic acid (L-trans-PDC), on the electrically evoked releas e or, rather, overflow of endogenous glutamate in superfusates from ra t cortical slices was compared with that of dihydrokainate. In the abs ence of these presumed uptake inhibitors, electrical stimulation for 4 min at 1 Hz did not elicit a measurable glutamate overflow over basel ine at all. Basal overflow increased concentration-dependently in the presence of 10 - 100 muM L-trans-PDC, about 5-fold at 100 muM. Also, e lectrical stimulation caused increases of glutamate overflow over basa l levels progressive with increasing concentrations of trans-PDC; a st imulated overflow corresponding to about 50% of basal overflow was obt ained at 100 muM. Basal as well as evoked release in the presence of d ihydrokainate did not exceed ca. 60% of that obtained with 100 muM L-t rans-PDC. In synaptosomes, L-trans-PDC much more than dihydrokainate c aused a transient increase of spontaneous glutamate release which was diminished in the absence of Na+, indicating that it is transported in to the cytoplasm by the glutamate carrier and induces some efflux of t he amino acid from this compartment. Moreover, trans-PDC caused a weak to moderate inhibition of K+-evoked glutamate release from synaptosom es at 10-300 muM, without obvious concentration-dependence. Glutamate overflow elicited from rat cortical slices by electrical field stimula tion at 1 Hz was Ca2+-dependent to about 80%. Tetrodotoxin (0.3 muM) r educed it by about 90%. Lowering the temperature from 37-degrees-C to 22-degrees-C increased the ratio between evoked and basal overflow. As an application for L-trans-PDC as a glutamate uptake inhibitor in rel ease studies, the regulation of glutamate release by GABA(B) receptors was investigated. At 1 Hz, (-)-baclofen reduced evoked glutamate over flow at and above 3 muM by maximally 40% at 30 muM. This maximal effec t was not increased when higher or lower stimulation frequencies were used nor when the Ca2+ concentration in the medium was increased or lo wered, nor when the slices were prepared from other brain areas (hippo campus or striatum). The GABA uptake inhibitor, SK&F 89976, had no sig nificant effect on evoked glutamate overflow, and the potent GABA(B) a ntagonist, CGP 55845, induced only a small increase, indicating that t onic inhibition of glutamate by GABA via GABA(B) receptors was not mar ked. On the other hand, the GABA(B) antagonist was able to prevent the inhibitory effect of (-)baclofen when applied before it and to abolis h it when applied afterwards. The conclusion is that L-trans-PDC is a useful tool in glutamate release studies in brain slices for many purp oses, with the reservation that its inhibitory effect on evoked glutam ate release in synaptosomes is not yet understood.