Unchanged glutamine synthetase activity and increased NMDA receptor density in epileptic human neocortex: Implications for the pathophysiology of epilepsy

We investigated whether alterations in glutamate metabolising glutamine synthetase activity occur in human epileptic neocortex, as shown previously for human epileptic hippocampus [Eid, T., Thomas, M.J., Spencer, D.D., Rundén-Pran, E., Lai, J.C.K., Malthankar, G.V., Kim, J.H., Danbolt, N.C., Ottersen, O.P., de Lanerolle, N.C., 2004. Loss of glutamine synthetase in the human epileptic hippocampus: possible mechanism for raised extracellular glutamate in mesial temporal lobe epilepsy. Lancet 363, 28-37]. Glutamine synthetase activity was equivalent in both non-epileptic and epileptic human neocortex. Epileptic tissue, however, was characterised by a 37% increase in the density of synaptosomal NMDA receptor sites compared to non-epileptic tissue, as revealed by a radioligand binding assay (Bmax(non-epileptic) 1.45 pmol/mg protein and Bmax(epileptic) 1.99 pmol/mg protein, P < 0.05). Our findings shed some doubts on a role of glutamine synthetase in the pathophysiology of epilepsy in the neocortex. However, the detection of a significantly reduced enzymatic activity in the epileptic amygdala supports the assumption that the enzyme defect is localized to the epileptic mesial temporal lobe of corresponding patients.