Glutamate and γ-aminobutyric acid content and release of synaptosomes from temporal lobe epilepsy patients

Abstract

During surgical intervention in medically refractory temporal lobe epilepsy (TLE) patients, diagnosed with either mesial temporal lobe sclerosis (MTS)- or tumor (T)-associated TLE, biopsies were taken from the anterior temporal neocortex and the hippocampal region. Synaptosomes, isolated from these biopsies were used to study intrasynaptosomal Ca2+ levels ([Ca2+]i), and glutamate and -aminobutyric acid (GABA) contents and release. All synaptosomal preparations demonstrated a basal [Ca2+]i of about 200 nM, except neocortical synaptosomes from MTS-associated TLE patients (420 nM). K+-induced depolarization resulted in a robust increase of the basal [Ca2+]i in all preparations. Neocortical synaptosomes from TLE patients contained 22.9 ± 3.0 nmol glutamate and 4.6 ± 0.5 nmol GABA per milligram synaptosomal protein, whereas rat cortical synaptosomes contained twice as much glutamate and four times as much GABA. Hippocampal synaptosomes from MTS-associated TLE patients, unlike those from T-associated TLE patients, contained about 70% less glutamate and 55% less GABA than neocortical synaptosomes. Expressed as percentage of total synaptosomal content, synaptosomes from MTS-associated TLE patients exhibited an increased basal and a reduced K+-induced glutamate and GABA release compared to rat cortical synaptosomes. In MTS-associated TLE patients, only GABA release from neocortical synaptosomes was partially Ca2+-dependent. Control experiments in rat synaptosomes demonstrated that at least part of the reduction in K+-induced release can be ascribed to resection-induced hypoxia in biopsies. Thus, synaptosomes from MTS-associated TLE patients exhibit a significant K+-induced increase in [Ca2+]i, but the consequent release of glutamate and GABA is severely impaired. Our data show that at least part of the differences in glutamate and GABA content and release between human biopsy material and fresh rat tissue is due to the resection time.