Synergistic Interactions between Endogenous Neurosteroids and Zolpidem in Cultured Neocortical Tissue Slices

Abstract

Despite their long-standing and widespread use in modern medicine, the mechanisms of action of many anaesthetics still remain unknown. To date, it is mostly unclear which receptors exactly are targeted by these substances and how they interact with each other as well as with endogenous substances produced by the human body itself. This study aims to examine this issue, specifically the effects and interactions of GABAergic drugs both on a synaptic as well as on a network level. It focuses on the action profiles and the interactions of zolpidem, a sedative drug that mainly targets alpha1 subunit-containing GABA(A) receptors, and XBD173, a TSPO ligand which supposedly targets GABA(A) receptors indirectly via neurosteroidogenesis. For this study, electrophysiological recordings of spontaneous neuronal activity in organotypic slice cultures from the neocortex of neonatal mice were done. Neuronal activity on a synaptic level was assessed via patch clamp experiments, while network activity was assessed using extracellular multi-unit recordings. The patch clamp experiments demonstrated that both XBD173 and zolpidem are able to affect synaptic GABAergic transmission in organotypic neocortical slice cultures, although they modify mIPSC kinetics in different ways. While zolpidem alone produced a strong increase in mIPSC decay time, XBD173 altered mIPSC kinetics in a much more complex way and, when co-applied with zolpidem, significantly reduced its effect. In the extracellular experiments, XBD173 and zolpidem both significantly reduced action potential activity, while again displaying different patterns of effect. Zolpidem strongly decreased high frequency action potential activity, while XBD173 only affected phases of moderate sustained action potential activity. During those phases, a synergistic effect could be observed after the combined application of zolpidem and XBD173. In conclusion, this study was able to confirm that zolpidem and XBD173 have different profiles of action in organotypic neocortical slice cultures, offering a plausible explanation for their distinct clinical effects. It also demonstrated a synergistic effect between both substances on a neuronal network level. Considering the combined results of the patch clamp and extracellular experiments, however, this effect cannot be mediated by synaptic GABA(A) receptors alone, indicating that there have to be other receptors involved in the process. These findings may offer novel options for clinical anaesthesia – making use of synergistic effects, which may come in the form of synergistic interactions between classical anaesthetics and neurosteroids, could quite possibly offer a new way to reduce anaesthetic dose requirements and thus their unwanted side-effects, thereby improving the patient-friendly use of anaesthetics overall.