Depolarization‐induced release of [3H]d‐aspartate from GABAergic neurons caused by reversal of glutamate transporters

Publisher Website
Google Scholar
Abstract
Cultured neocortical neurons, which predominantly consist of GABAergic neurons exhibit a pronounced stimulus‐coupled GABA release. Since the cultures may contain a small population of glutamatergic neurons and the GABAergic neurons have a high content of glutamate it was of interest to examine if glutamate in addition to γ‐aminobutyric acid (GABA) could be released from these cultures. The neurons were preloaded with [3H]d‐aspartate and subsequently its release was followed during depolarization induced by a high potassium concentration or the α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) receptor agonists, AMPA and kainate. Depolarization of the neurons with 55 mM potassium increased the release of [3H]d‐aspartate by more than 10‐fold. When the non‐specific calcium‐channel blockers cobalt or lanthanum were included in the stimulation buffer with potassium, the release of [3H]d‐aspartate was decreased by about 40%. These results indicated that some of the released [3H]d‐aspartate might originate from a vesicular pool. When AMPA was applied to the neurons, the release of [3H]d‐aspartate was increased 2‐fold and could not be prevented or decreased by addition of cobalt. Since AMPA has a rapid desensitizing effect on AMPA receptors, it was examined whether AMPA under non‐desensitizing conditions was able to induce an increased release of [3H]d‐aspartate as compared to the conditions of applying AMPA alone. The desensitization of AMPA receptors was blocked by 6‐chloro‐3,4‐dihydro‐3‐(2‐norbornen‐5‐yl)‐2H‐1,2,4‐benzothiadiazine‐7‐sulphonamide‐1,1‐dioxide (cyclothiazide). Under the non‐desensitizing conditions, the AMPA‐induced release of [3H]d‐aspartate was highly enhanced showing about a 10‐fold increase over basal release. Addition of cobalt or lanthanum did not decrease the amount of [3H]d‐aspartate released, indicating that the release originated from a cytoplasmic pool. Kainate, which induces an almost non‐desensitizing effect on AMPA receptors, showed similar results as observed for AMPA under non‐desensitizing conditions. The NMDA receptor antagonist (5R,10 S)‐(+)‐5‐methyl‐10,11‐dihydro‐5H‐dibenzo[a,d]cyclohepten‐5,10‐imine (MK‐801) had only minor effects on the [3H]d‐aspartate release induced by AMPA and kainate. Thus, the depolarization‐induced release of [3H]d‐aspartate from cultured GABAergic neurons appears to be caused mainly by reversal of the glutamate transporters.
Funding Information
  • Sundhed og Sygdom, Det Frie Forskningsråd (9700761)
  • Lundbeckfonden

This publication has 35 references indexed in Scilit: