Clinical significance of altered kynurenine metabolism and enhanced glutamatergic activity in MTLE-HS
As KYNA modulates the hippocampal extracellular glutamate release through α7 nAChR (Carpenedo et al., 2001), it is possible that exogenously applied KYNA prevented presynaptic action potential-dependent glutamate release causing reduction in frequency of spontaneous EPSCs recorded from pyramidal neurons in resected hippocampal samples (Fig. 5E). As KYNA competitively inhibits postsynaptic glycine binding site of the NMDA receptors and NMDA receptor levels are high in patients with MTLE-HS, exogenous application of KYNA may increase its binding to postsynaptic NMDA receptors causing reduction in amplitude of spontaneous EPSCs (Figure 5e). In addition, we also observed that the magnitude of reduction of frequency of spontaneous EPSCs by KYNA in MTLE-HS was significantly less than that caused by specific NMDA receptor antagonist, APV (Figure 5i). This might suggests that, besides NMDA receptors, KYNA may have some other targets like GPR35 (Alkondon et al., 2015), aryl hydrocarbon receptors (DiNatale et al., 2010) or some other unknown sites; but their role in MTLE-HS is not known and needs further investigations. Moreover, future experiments involving comparison between KYNA and APV co-perfusion and KYNA or APV alone can further help to understand the presence of a cumulative or occlusive effect of these antagonists on NMDA receptor activity. Increase in endogenous QUIN level in hippocampal samples also contributes to glutamate receptor mediated hyperexcitability. QUIN binds with postsynaptic NMDA receptors which are upregulated and render those functionally hyperactive. It is tempting to speculate that decreased KYNA levels as well as increased QUIN levels in the hippocampus and hyperglutamatergic tone may contribute to seizure generation in patients with MTLE-HS.
The primary limitation for this human study is use of tumour periphery samples obtained from patients with low-grade gliomas as non-seizure control, as ethically it was not possible to get healthy hippocampal samples. The cell type variation, cell-specific expression pattern and subtype differences of glutamate receptors between the MTLE-HS samples and non-seizure control samples could also contribute to the changes observed in the frequency and amplitude of spontaneous EPSCs. Although previous studies have used samples obtained from non-MTLE patients for comparison with MTLE-HS samples, we believe that epileptic samples obtained from patients with non-MTLE (but DRE) could also have their own limitations as “control” as those tissues will also possess, the epilepsy related abnormalities which may significantly affect the comparative results of synaptic transmission between the two groups. In addition, it was not possible to obtain age and gender matched non-seizure control specimens for this study. All the patients recruited in this study were on a combination of anti-epileptic drugs (supplementary table 1), so the effect of these drugs on glutamatergic/GABAergic activity recorded from the pyramidal neurons of resected hippocampal samples cannot be ruled out.