The general anaesthetic etomidate inhibits the excitability of mouse thalamocortical relay neurons by modulating multiple modes of GABAA receptor-mediated inhibition

Opposing actions of CRF-R1 and CB1 receptor on facial stimulation-induced MLI-PC plasticity in mouse cerebellar cortex

BACKGROUND

Corticotropin-releasing factor (CRF) is the major neuromodulator orchestrating the stress response, and is secreted by neurons in various regions of the brain. Cerebellar CRF is released by afferents from inferior olivary neurons and other brainstem nuclei in response to stressful challenges, and contributes to modulation of synaptic plasticity and motor learning behavior via its receptors. We recently found that CRF modulates facial stimulation-evoked molecular layer interneuron-Purkinje cell (MLI-PC) synaptic transmission via CRF type 1 receptor (CRF-R1) in vivo in mice, suggesting that CRF modulates sensory stimulation-evoked MLI-PC synaptic plasticity. However, the mechanism of how CRF modulates MLI-PC synaptic plasticity is unclear. We investigated the effect of CRF on facial stimulation-evoked MLI-PC long-term depression (LTD) in urethane-anesthetized mice by cell-attached recording technique and pharmacological methods.

RESULTS

Facial stimulation at 1 Hz induced LTD of MLI-PC synaptic transmission under control conditions, but not in the presence of CRF (100 nM). The CRF-abolished MLI-PC LTD was restored by application of a selective CRF-R1 antagonist, BMS-763,534 (200 nM), but it was not restored by application of a selective CRF-R2 antagonist, antisauvagine-30 (200 nM). Blocking cannabinoid type 1 (CB1) receptor abolished the facial stimulation-induced MLI-PC LTD, and revealed a CRF-triggered MLI-PC long-term potentiation (LTP) via CRF-R1. Notably, either inhibition of protein kinase C (PKC) with chelerythrine (5 µM) or depletion of intracellular Ca²⁺ with cyclopiazonic acid (100 µM), completely prevented CRF-triggered MLI-PC LTP in mouse cerebellar cortex in vivo.

CONCLUSIONS

The present results indicated that CRF blocked sensory stimulation-induced opioid-dependent MLI-PC LTD by triggering MLI-PC LTP through CRF-R1/PKC and intracellular Ca²⁺ signaling pathway in mouse cerebellar cortex. These results suggest that activation of CRF-R1 opposes opioid-mediated cerebellar MLI-PC plasticity in vivo in mice.

Discovery of the EL-0052 as a potential anesthetic drug

As a γ-aminobutyric acid A receptor (GABA_AR) inhibitor, etomidate fulfills several characteristics of an ideal anesthetic agent, such as rapid onset with rapid clearance and high potency, along with cardiovascular stability. Unfortunately, etomidate has been reported to inhibit CYP11B1 at hypnotic doses, which is associated with a marked increase in patient deaths due to this unexpected off-target effect. In this study, molecular docking was used to simulate the binding mode of etomidate with GABA_AR and CYP11B1. Based on the in-depth analysis of the binding mode, strong electron-withdrawing group on the C4 position of the imidazole ring was introduced to reduce the charge density of the nitrogen, which is beneficial in reducing the coordination bond between the imidazole nitrogen and heme iron in CYP11B1, as well as in reducing the adrenocortical suppression. Based on the results of ADMET property prediction, MEP analysis, and molecular docking simulation, 4-fluoroetomidate (EL-0052) was designed and synthesized. In vivo studies in rats and mice confirmed that EL-0052 had the efficacy similar to etomidate, but without adrenocortical suppression. These findings suggested that EL-0052 was superior to etomidate and support the continued development of EL-0052 as a preclinical candidate as an anesthetic.

Realising the therapeutic potential of neuroactive steroid modulators of the GABAA receptor

In the 1980s particular endogenous metabolites of progesterone and of deoxycorticosterone were revealed to be potent, efficacious, positive allosteric modulators (PAMs) of the GABA_A receptor (GABA_AR). These reports were followed by the discovery that such steroids may be synthesised not only in peripheral endocrine glands, but locally in the central nervous system (CNS), to potentially act as paracrine, or autocrine "neurosteroid" messengers, thereby fine tuning neuronal inhibition. These discoveries triggered enthusiasm to elucidate the physiological role of such neurosteroids and explore whether their levels may be perturbed in particular psychiatric and neurological disorders. In preclinical studies the GABA_AR-active steroids were shown to exhibit anxiolytic, anticonvulsant, analgesic and sedative properties and at relatively high doses to induce a state of general anaesthesia. Collectively, these findings encouraged efforts to investigate the therapeutic potential of neurosteroids and related synthetic analogues. However, following over 30 years of investigation, realising their possible medical potential has proved challenging. The recent FDA approval for the natural neurosteroid allopregnanolone (brexanolone) to treat postpartum depression (PPD) should trigger renewed enthusiasm for neurosteroid research. Here we focus on the influence of neuroactive steroids on GABA-ergic signalling and on the challenges faced in developing such steroids as anaesthetics, sedatives, analgesics, anticonvulsants, antidepressants and as treatments for neurodegenerative disorders.

Sleep Spindles: Mechanisms and Functions

Sleep spindles are burstlike signals in the electroencephalogram (EEG) of the sleeping mammalian brain and electrical surface correlates of neuronal oscillations in thalamus. As one of the most inheritable sleep EEG signatures, sleep spindles probably reflect the strength and malleability of thalamocortical circuits that underlie individual cognitive profiles. We review the characteristics, organization, regulation, and origins of sleep spindles and their implication in non-rapid-eye-movement sleep (NREMS) and its functions, focusing on human and rodent. Spatially, sleep spindle-related neuronal activity appears on scales ranging from small thalamic circuits to functional cortical areas, and generates a cortical state favoring intracortical plasticity while limiting cortical output. Temporally, sleep spindles are discrete events, part of a continuous power band, and elements grouped on an infraslow time scale over which NREMS alternates between continuity and fragility. We synthesize diverse and seemingly unlinked functions of sleep spindles for sleep architecture, sensory processing, synaptic plasticity, memory formation, and cognitive abilities into a unifying sleep spindle concept, according to which sleep spindles 1) generate neural conditions of large-scale functional connectivity and plasticity that outlast their appearance as discrete EEG events, 2) appear preferentially in thalamic circuits engaged in learning and attention-based experience during wakefulness, and 3) enable a selective reactivation and routing of wake-instated neuronal traces between brain areas such as hippocampus and cortex. Their fine spatiotemporal organization reflects NREMS as a physiological state coordinated over brain and body and may indicate, if not anticipate and ultimately differentiate, pathologies in sleep and neurodevelopmental, -degenerative, and -psychiatric conditions.

Role of GABAA receptor subtypes in the behavioural effects of intravenous general anaesthetics

Since the introduction of general anaesthetics into clinical practice, researchers have been mystified as to how these chemically disparate drugs act to produce their dramatic effects on central nervous system function and behaviour. Scientific advances, particularly during the last 25 years, have now begun to reveal the molecular mechanisms underpinning their behavioural effects. For certain i.v. general anaesthetics, such as etomidate and propofol, a persuasive case can now be made that the GABAA receptor, a major inhibitory receptor in the mammalian central nervous system, is an important target. Advances in molecular pharmacology and in genetic manipulation of rodent genes reveal that different subtypes of the GABAA receptor are responsible for mediating particular aspects of the anaesthetic behavioural repertoire. Such studies provide a better understanding of the neuronal circuitry involved in the various anaesthetic-induced behaviours and, in the future, may result in the development of novel therapeutics with a reduced propensity for side-effects.

Etomidate Effects on Desensitization and Deactivation of α4β3δ GABAA Receptors Inducibly Expressed in HEK293 TetR Cells

Central α4βδ receptors are the most abundant isoform of δ subunit-containing extrasynaptic GABAA receptors that mediate tonic inhibition. Although the amplitude of GABA-activated currents through α4βδ receptors is modulated by multiple general anesthetics, the effects of general anesthetics on desensitization and deactivation of α4βδ receptors remain unknown. In the current study, we investigated the effect of etomidate, a potent general anesthetic, on the kinetics and the pseudo steady-state current amplitude of α4β3δ receptors inducibly expressed in human embryonic kidney 293 TetR cells. Etomidate directly activates α4β3δ receptors in a concentration-dependent manner. Etomidate at a clinically relevant concentration (3.2 μM) enhances maximal response without altering the EC50 of GABA concentration response. Etomidate also increases the extent of desensitization and prolongs the deactivation of α4β3δ receptors in the presence of maximally activating concentrations of GABA (1 mM). To mimic the modulatory effect of etomidate on tonic currents, long pulses (30-60 seconds) of a low GABA concentration (1 μM) were applied to activate α4β3δ receptors in the absence and presence of etomidate. Although etomidate increases the desensitization of α4β3δ receptors, the pseudo steady-state current amplitude at 1 μM GABA is augmented by etomidate. Our data demonstrate that etomidate enhances the pseudo steady-state current of α4β3δ receptors evoked by a GABA concentration comparable to an ambient GABA level, suggesting that α4β3δ receptors may mediate etomidate's anesthetic effect in the brain.

Comparison of αβδ and αβγ GABAA receptors: Allosteric modulation and identification of subunit arrangement by site-selective general anesthetics

GABA_A receptors play a dominant role in mediating inhibition in the mature mammalian brain, and defects of GABAergic neurotransmission contribute to the pathogenesis of a variety of neurological and psychiatric disorders. Two types of GABAergic inhibition have been described: αβγ receptors mediate phasic inhibition in response to transient high-concentrations of synaptic GABA release, and αβδ receptors produce tonic inhibitory currents activated by low-concentration extrasynaptic GABA. Both αβδ and αβγ receptors are important targets for general anesthetics, which induce apparently different changes both in GABA-dependent receptor activation and in desensitization in currents mediated by αβγ vs. αβδ receptors. Many of these differences are explained by correcting for the high agonist efficacy of GABA at most αβγ receptors vs. much lower efficacy at αβδ receptors. The stoichiometry and subunit arrangement of recombinant αβγ receptors are well established as β-α-γ-β-α, while those of αβδ receptors remain controversial. Importantly, some potent general anesthetics selectively bind in transmembrane inter-subunit pockets of αβγ receptors: etomidate acts at β⁺/α^- interfaces, and the barbiturate R-5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid (R-mTFD-MPAB) acts at α⁺/β^- and γ⁺/β^- interfaces. Thus, these drugs are useful as structural probes in αβδ receptors formed from free subunits or concatenated subunit assemblies designed to constrain subunit arrangement. Although a definite conclusion cannot be drawn, studies using etomidate and R-mTFD-MPAB support the idea that recombinant α1β3δ receptors may share stoichiometry and subunit arrangement with α1β3γ2 receptors.

Enhanced Thalamic Spillover Inhibition during Non-rapid-eye-movement Sleep Triggers an Electrocortical Signature of Anesthetic Hypnosis

BACKGROUND

Alterations in thalamic γ-aminobutyric acid-mediated signaling are thought to underlie the increased frontal α-β frequency electrocortical activity that signals anesthetic-induced loss of consciousness with γ-aminobutyric acid receptor type A (GABAAR)-targeting general anesthetics. The general anesthetic etomidate elicits phasic extrasynaptic GABAAR activation ("spillover" inhibition) at thalamocortical neurons in vitro. We hypothesize that this action of etomidate at the thalamus is sufficient to trigger an increase in frontal α-β frequency electrocortical activity and that this effect of etomidate is fully recapitulated by enhanced thalamic spillover inhibition in vivo.

METHODS

We recorded electrocortical activity and sleep-wake behavior in freely behaving wild-type (n = 33) and extrasynaptic δ-subunit-containing GABAAR knockout mice (n = 9) during bilateral microperfusion of the thalamus with etomidate and/or other pharmacologic agents that influence GABAAR or T-type Ca channel activity.

RESULTS

Microperfusion of etomidate into the thalamus elicited an increase in α-β frequency electrocortical activity that occurred only during non-rapid-eye-movement (REM) sleep (11.0 ± 11.8% and 16.0 ± 14.2% greater 8 to 12- and 12 to 30-Hz power, respectively; mean ± SD; both P < 0.031) and was not affected by blockade of thalamic T-type Ca channels. Etomidate at the thalamus also increased spindle-like oscillations during non-REM sleep (4.5 ± 2.4 spindle per minute with etomidate vs. 3.2 ± 1.7 at baseline; P = 0.002). These effects of etomidate were fully recapitulated by enhanced thalamic extrasynaptic GABAAR-mediated spillover inhibition.

CONCLUSIONS

These findings identify how a prototypic GABAAR-targeting general anesthetic agent can elicit the characteristic brain wave pattern associated with anesthetic hypnosis when acting at the thalamus by promoting spillover inhibition and the necessity of a preexisting non-REM mode of activity in the thalamus to generate this effect.

Auditory thalamic circuits and GABAA receptor function: Putative mechanisms in tinnitus pathology

Tinnitus is defined as a phantom sound (ringing in the ears), and can significantly reduce the quality of life for those who suffer its effects. Ten to fifteen percent of the general adult population report symptoms of tinnitus with 1-2% reporting that tinnitus negatively impacts their quality of life. Noise exposure is the most common cause of tinnitus and the military environment presents many challenging high-noise situations. Military noise levels can be so intense that standard hearing protection is not adequate. Recent studies suggest a role for inhibitory neurotransmitter dysfunction in response to noise-induced peripheral deafferentation as a key element in the pathology of tinnitus. The auditory thalamus, or medial geniculate body (MGB), is an obligate auditory brain center in a unique position to gate the percept of sound as it projects to auditory cortex and to limbic structures. Both areas are thought to be involved in those individuals most impacted by tinnitus. For MGB, opposing hypotheses have posited either a tinnitus-related pathologic decrease or pathologic increase in GABAergic inhibition. In sensory thalamus, GABA mediates fast synaptic inhibition via synaptic GABAA receptors (GABAARs) as well as a persistent tonic inhibition via high-affinity extrasynaptic GABAARs and slow synaptic inhibition via GABABRs. Down-regulation of inhibitory neurotransmission, related to partial peripheral deafferentation, is consistently presented as partially underpinning neuronal hyperactivity seen in animal models of tinnitus. This maladaptive plasticity/Gain Control Theory of tinnitus pathology (see Auerbach et al., 2014; Richardson et al., 2012) is characterized by reduced inhibition associated with increased spontaneous and abnormal neuronal activity, including bursting and increased synchrony throughout much of the central auditory pathway. A competing hypothesis suggests that maladaptive oscillations between the MGB and auditory cortex, thalamocortical dysrhythmia, predict tinnitus pathology (De Ridder et al., 2015). These unusual oscillations/rhythms reflect net increased tonic inhibition in a subset of thalamocortical projection neurons resulting in abnormal bursting. Hyperpolarizing de-inactivation of T-type Ca2+ channels switches thalamocortical projection neurons into burst mode. Thalamocortical dysrhythmia originating in sensory thalamus has been postulated to underpin neuropathies including tinnitus and chronic pain. Here we review the relationship between noise-induced tinnitus and altered inhibition in the MGB.

Propofol Anesthesia Is Reduced in Phospholipase C-Related Inactive Protein Type-1 Knockout Mice

The GABA type A receptor (GABA_A-R) is a major target of intravenous anesthetics. Phospholipase C-related inactive protein type-1 (PRIP-1) is important in GABA_A-R phosphorylation and membrane trafficking. In this study, we investigated the role of PRIP-1 in general anesthetic action. The anesthetic effects of propofol, etomidate, and pentobarbital were evaluated in wild-type and PRIP-1 knockout (PRIP-1 KO) mice by measuring the latency and duration of loss of righting reflex (LORR) and loss of tail-pinch withdrawal response (LTWR). The effect of pretreatment with okadaic acid (OA), a protein phosphatase 1/2A inhibitor, on propofol- and etomidate-induced LORR was also examined. PRIP-1 deficiency provided the reduction of LORR and LTWR induced by propofol but not by etomidate or pentobarbital, indicating that PRIP-1 could determine the potency of the anesthetic action of propofol. Pretreatment with OA recovered the anesthetic potency induced by propofol in PRIP-1 KO mice. OA injection enhanced phosphorylation of cortical the GABA_A-R β3 subunit in PRIP-1 KO mice. These results suggest that PRIP-1-mediated GABA_A-R β3 subunit phosphorylation might be involved in the general anesthetic action induced by propofol but not by etomidate or pentobarbital.

Thalamic δ-subunit containing GABAA receptors promote electrocortical signatures of deep non-REM sleep but do not mediate the effects of etomidate at the thalamus in vivo

Extrasynaptic δ-subunits containing GABAA receptors (δGABAARs) are sensitive targets for several commonly used hypnotic agents and mediate tonic neuronal inhibition. δGABAARs are highly expressed within the thalamus and their activation promotes a switch from tonic to burst firing in vitro. Here we test two hypotheses in vivo. (1) Activation of thalamic δGABAARs will elicit electrocortical signatures consistent with widespread thalamocortical burst firing such as increased delta oscillations (1-4 Hz) and reciprocal changes in spindle-like oscillations (7-14 Hz). (2) These signatures will be recapitulated by the general anesthetic etomidate, if the electrocortical effects of etomidate at the thalamus are mediated by δGABAARs. Microperfusion of the δGABAAR-preferring agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP; 10 and 50 μM) into the ventrobasal complex produced significant effects on electrocortical activity in wild-type mice, but not in mice lacking δGABAARs (Gabrd(-/-)), i.e., the effects with THIP were dependent on δGABAARs. THIP (1) increased 1-4 Hz power in wakefulness and nonrapid-eye movement (NREM) sleep; (2) reduced spindle-like oscillations in NREM sleep; and (3) increased the speed of stable transitions into NREM sleep, indicating effects on state-space dynamics. In contrast, microperfusion of etomidate (10 and 30 μM) into the ventrobasal complex produced effects on electrocortical activity that were independent of δGABAARs, i.e., effects occurred in wild-type and Gabrd(-/-) mice. Etomidate (1) decreased 1-4 Hz power, increased 8-12 Hz, and/or 12-30 Hz power in all sleep-wake states; (2) increased spindle-like oscillations; and (3) increased REM sleep expression. These results indicate that thalamic δGABAARs promote electrocortical signatures of deep NREM sleep, but do not mediate the effects of etomidate at the thalamus in vivo.

Developmentally regulated neurosteroid synthesis enhances GABAergic neurotransmission in mouse thalamocortical neurones

KEY POINTS

During neuronal development synaptic events mediated by GABAA receptors are progressively reduced in their duration, allowing for rapid and precise network function. Here we focused on ventrobasal thalamocortical neurones, which contribute to behaviourally relevant oscillations between thalamus and cortex. We demonstrate that the developmental decrease in the duration of inhibitory phasic events results predominantly from a precisely timed loss of locally produced neurosteroids, which act as positive allosteric modulators of the GABAA receptor. The mature thalamus retains the ability to synthesise neurosteroids, thus preserving the capacity to enhance both phasic and tonic inhibition, mediated by synaptic and extrasynaptic GABAA receptors, respectively, in physiological and pathophysiological scenarios associated with perturbed neurosteroid levels. Our data establish a potent, endogenous mechanism to locally regulate the GABAA receptor function and thereby influence thalamocortical activity. During brain development the duration of miniature inhibitory postsynaptic currents (mIPSCs) mediated by GABAA receptors (GABAA Rs) progressively reduces, to accommodate the temporal demands required for precise network activity. Conventionally, this synaptic plasticity results from GABAA R subunit reorganisation. In particular, in certain developing neurones synaptic α2-GABAA Rs are replaced by α1-GABAA Rs. However, in thalamocortical neurones of the mouse ventrobasal (VB) thalamus, the major alteration to mIPSC kinetics occurs on postnatal (P) day 10, some days prior to the GABAA R isoform change. Here, whole-cell voltage-clamp recordings from VB neurones of mouse thalamic slices revealed that early in postnatal development (P7-P8), the mIPSC duration is prolonged by local neurosteroids acting in a paracrine or autocrine manner to enhance GABAA R function. However, by P10, this neurosteroid 'tone' rapidly dissipates, thereby producing brief mIPSCs. This plasticity results from a lack of steroid substrate as pre-treatment of mature thalamic slices (P20-24) with the GABAA R-inactive precursor 5α-dihydroprogesterone (5α-DHP) resulted in markedly prolonged mIPSCs and a greatly enhanced tonic conductance, mediated by synaptic and extrasynaptic GABAA Rs, respectively. In summary, endogenous neurosteroids profoundly influence GABAergic neurotransmission in developing VB neurones and govern a transition from slow to fast phasic synaptic events. Furthermore, the retained capacity for steroidogenesis in the mature thalamus raises the prospect that certain physiological or pathophysiological conditions may trigger neurosteroid neosynthesis, thereby providing a local mechanism for fine-tuning neuronal excitability.