P.1.017 Isoflurane anesthesia rapidly activates TrkB receptor signaling and produces antidepressant-like behavioral effects

Abstract

Purpose of the study: Emerging evidence suggests that induction of TrkB signaling and the subsequent changes in synaptic plasticity are involved in the therapeutic effects of antidepressant drugs, including rapid-acting antidepressant ketamine [1]. Intriguingly, isoflurane, a commonly used volatile anesthetic, has been shown to produce rapid antidepressant effects in treatment resistant depressive patients [2]. The mechanism of isoflurane’s antidepressant action, however, is completely unknown. We have here investigated the effects of isoflurane anesthesia on TrkB activation and on its downstream signaling pathways related to synaptogenesis and neuronal plasticity. Furthermore, we have studied the effects of single isoflurane treatment on cortical spine formation and morphology, hippocampal long-term potentiation and antidepressant-like behavior in mice. Methods: Adult mice were anesthetized with isoflurane (maximum 30min). For biochemical experiments the animals were killed during the anesthesia or after described recovery period. Hippocampus and prefrontal cortex were dissected out and protein and mRNA levels were analysed using western blotting/ELISA and qPCR, respectively. For electrophysiological recordings mice were decapitated by guillotine 24 h after isoflurane treatment. Parasagittal 400mm hippocampal slices were cut using vibroslicer. Field excitatory potentials (fEPSP) were evoked in Schaffer collateral pathway of the hippocampal slices and LTP was induced by 100 Hz tetanic stimulation. For spine density analysis animals expressing YFP in a subset of cortical layer V pyramidal neurons were perfused with PFA 24 hours after isoflurane treatment. Brains were extracted and 70mm sections were cut using a vibratome. Immunohistochemistry was performed to enhance the fluorescence and sections were analyzed using confocal microscope. Isoflurane-induced antidepressantlike behavior in the forced swim test was monitored 15 minutes and 2 weeks after anesthesia. Results: Isoflurane anesthesia (30min) increased TrkB phosphorylation and its downstream signaling related to neuronal plasticity (pCREB, pAkt) and to synaptogenesis (pmTOR, pP70S6k) in the mouse hippocampus and prefrontal cortex but BDNF protein and mRNA levels remained unaltered. Increased pTrkB, pCREB and pmTOR levels were detected already within 2 minutes from isoflurane treatment onset. Isoflurane’s effects on TrkB signaling could not be blocked with pre-treatment of AMPA receptor blocker NBQX (10mg/kg, ip). LTP was facilitated in hippocampal slices obtained from mice treated with isoflurane 24 h before but any change in cortical spine density and morphological types could not be observed. A single isoflurane anesthesia led to rapid antidepressant-like behavioral effect in the forced swim test that persisted for at least 2 weeks. Conclusions: We show that isoflurane anesthesia rapidly activates TrkB neurotrophin signaling in the mouse brain. This phenomenon is followed by antidepressantlike behavioral responses and facilitation of LTP but not by changes in spine formation or morphology. The biochemical, functional and behavioral effects of isoflurane partially resemble those of rapid-acting antidepressant ketamine. Our findings may provide the neurobiological basis for the previously observed rapid antidepressant effect of isoflurane narcotherapy and encourage investigating further the therapeutic potential of isoflurane.