Modulation von GABAA Rezeptor Untereinheiten durch Neurosteroide und Anästhetika: Identifizierung der molekularen Bindungsstellen entscheidend für Neuroprotektion
Zusammenfassung der Projektergebnisse
Midazolam is a commonly used benzodiazepine (BZD) in perioperative anesthesia, causing amnesia, sedation, hypnosis, and anxiolysis. The molecular targets contributing to these actions are still unknown. GABAA receptors containing an α1-, α2-, α3- or α5-subunit and a g-subunit are sensitive to BZDs whereas receptors with a d-subunit are BZD-resistant. The α1-subtype mediates the drugs’ sedative effect, the α2/3-subtypes the anxiolytic effect and α5-receptors play a key role in memory and learning. BZDs are known to induce prominent side effects, e.g., postoperative cognitive deficits (POCD) causing a slow recovery of patients on the intensive care unit. Neurosteroids (NSs) such as allopregnanolone and 3α,21-dihydroxy-5α-pregnan-20-one (THDOC) potentiate GABAA receptor activation with high affinity to dcontaining receptors. The high-affinity translocator protein (18kDa; TSPO) ligand XBD173 induces neurosteroidogenesis and exerts rapid anxiolytic effects in animal models and humans in the absence of sedation, tolerance development, and withdrawal symptoms. Furthermore, NSs and TSPO ligands have been shown to exert neuroprotection in models of ischaemia. In our recent DFG-project we studied the effects of midazolam, NSs and XBD173 on hippocampal longterm potentiation (LTP), a cellular surrogate of memory and learning, and their ability in providing neuroprotection in a model of hippocampal hypoxic-hypoglycemic excitotoxity (HHE-model). Midazolam (10nM) depresses LTP predominantly via α1-GABAA receptors and reduces the excitability of cortical neurons. On the network level, BZD-induced inhibition immediately kicks in during the onset of highfrequency firing and, at clinically relevant concentrations (about 100nM), is equally effective in phases of high and low neuronal activity. BZDs are also highly effective in modulating GABAergic negative feedback circuits that exhibit very fast response times and approximately linear transfer functions. BZDs are affecting neuronal rate coding mostly via α1-GABAA receptors and temporal coding in the hippocampus and neocortex via α2-GABAA receptors. Taken together, these findings can explain why BZDs, even at very low concentrations, impair memory efficiency and cognitive performance. In our HHE-model, midazolam induced neuroprotection via selective modulation of α5-GABAA, whereas THDOC, allopregnanolone and XBD173 provided neuroprotection exclusively mediated by d-GABAA receptors. XBD173, THDOC and allopregnanolone all failed to depress hippocampal LTP at concentrations that provided neuroprotection. Furthermore, BZDs and NSs modified the activity patterns of cortical neurons in different ways. Prominent inhibition by NSs was only apparent during longer episodes of ongoing activity. However, during brief periods of high-frequency network activity (lasting about 25-50ms) these substances did not alter neuronal activity patterns. This mode of action explains why neurosteroids do not inhibit LTP. Synergistic interactions between BZDs and NSs were absent at GABAergic synapses, suggesting that these classes of agents predominantly act via different subtypes of GABAA receptors. Furthermore, our results are first providing evidence that α1-GABAA receptors, known to mediate BZD-induced POCD, are not major molecular targets of NSs. Taken together, our findings suggest that an already sedative concentration of midazolam blocks LTP via α1-GABAA receptors. Even though midazolam is a very potent neuroprotective agent, it interferes with LTP and this may provide an explanation for its detrimental effect on cognition. XBD173 may represent a promising alternative in perioperative anesthesia with a less severe side effect profile than BZDs.
Projektbezogene Publikationen (Auswahl)
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GABA(A) receptor-targeted drug development -New perspectives in perioperative anesthesia. Expert Opinion on Drug Discovery, 14(7), 683-699.
Antkowiak, Bernd & Rammes, Gerhard
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Midazolam at Low Nanomolar Concentrations Affects Long-term Potentiation and Synaptic Transmission Predominantly via the α1–γ-Aminobutyric Acid Type A Receptor Subunit in Mice. Anesthesiology, 136(6), 954-969.
Puig-Bosch, Xènia; Bieletzki, Stefan; Zeilhofer, Hanns Ulrich; Rudolph, Uwe; Antkowiak, Bernd & Rammes, Gerhard
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γ-Aminobutyric Acid Type A Receptor Subtypes and Circuit Connections in Midazolam-induced Amnesia, Sedation, and Hypnosis. Anesthesiology, 136(6), 880-882.
Forman, Stuart A.
