Anesthetic action on extra-synaptic receptors: effects in neural population models of EEG activity.
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ABSTRACT:
PROVIDER: S-EPMC4261904 | BioStudies | 2014-01-01T00:00:00Z
REPOSITORIES: biostudies
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Project description:The activity of histaminergic neurons in the tuberomammillary nucleus (TMN) of the hypothalamus correlates with an animal's behavioral state and maintains arousal. We examined how GABAergic inputs onto histaminergic neurons regulate this behavior. A prominent hypothesis, the "flip-flop" model, predicts that increased and sustained GABAergic drive onto these cells promotes sleep. Similarly, because of the histaminergic neurons' key hub-like place in the arousal circuitry, it has also been suggested that anesthetics such as propofol induce loss of consciousness by acting primarily at histaminergic neurons. We tested both these hypotheses in mice by genetically removing ionotropic GABA(A) or metabotropic GABA(B) receptors from histidine decarboxylase-expressing neurons. At the cellular level, histaminergic neurons deficient in synaptic GABA(A) receptors were significantly more excitable and were insensitive to the anesthetic propofol. At the behavioral level, EEG profiles were recorded in nontethered mice over 24 h. Surprisingly, GABAergic transmission onto histaminergic neurons had no effect in regulating the natural sleep-wake cycle and, in the case of GABA(A) receptors, for propofol-induced loss of righting reflex. The latter finding makes it unlikely that the histaminergic TMN has a central role in anesthesia. GABA(B) receptors on histaminergic neurons were dispensable for all behaviors examined. Synaptic inhibition of histaminergic cells by GABA(A) receptors, however, was essential for habituation to a novel environment.
| S-EPMC3466043 | BioStudies
Project description:Anesthetic agents can induce a paradox activation and sensitization of nociceptive sensory neurons and, thus, potentially facilitate pain processing. Here we identify distinct molecular mechanisms that mediate an activation of sensory neurons by 2,6-diisopropylphenol (propofol), a commonly used intravenous anesthetic known to elicit intense pain upon injection. Clinically relevant concentrations of propofol activated the recombinant transient receptor potential (TRP) receptors TRPA1 and TRPV1 heterologously expressed in HEK293t cells. In dorsal root ganglion (DRG) neurons, propofol-induced activation correlated better to expression of TRPA1 than of TRPV1. However, pretreatment with the protein kinase C activator 4β-phorbol 12-myristate 13-acetate (PMA) resulted in a significantly sensitized propofol-induced activation of TRPV1 in DRG neurons as well as in HEK293t cells. Pharmacological and genetic silencing of both TRPA1 and TRPV1 only partially abrogated propofol-induced responses in DRG neurons. The remaining propofol-induced activation was abolished by the selective γ-aminobutyric acid, type A (GABA(A)) receptor antagonist picrotoxin. Propofol but not GABA evokes a release of calcitonin gene-related peptide, a key component of neurogenic inflammation, from isolated peripheral nerves of wild-type but not TRPV1 and TRPA1-deficient mice. Moreover, propofol but not GABA induced an intense pain upon intracutaneous injection. As both the release of calcitonin gene-related peptide and injection pain by propofol seem to be independent of GABA(A) receptors, our data identify TRPV1 and TRPA1 as key molecules for propofol-induced excitation of sensory neurons. This study warrants further investigations into the role of anesthetics to induce nociceptor sensitization and to foster postoperative pain.
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Project description:High-affinity extrasynaptic GABA(A) receptors (GABA(A)Rs) are a prominent feature of cerebellar granule neurons and thalamic relay neurons. In both cell types, the presence of synaptic glomeruli would be expected to promote activation of these GABA(A)Rs, contributing to phasic spillover-mediated currents and tonic inhibition. However, the precise role of different receptor subtypes in these two phenomena is unclear. To address this question, we made recordings from neurons in acute brain slices from mice, and from tsA201 cells expressing recombinant GABA(A)Rs. We found that ? subunit-containing GABA(A)Rs of both cerebellar granule neurons and thalamic relay neurons of the lateral geniculate nucleus contributed to tonic conductance caused by ambient GABA but not to spillover-mediated currents. In the presence of a low "ambient" GABA concentration, recombinant "extrasynaptic" ? subunit-containing GABA(A)Rs exhibited profound desensitization, rendering them insensitive to brief synaptic- or spillover-like GABA transients. Together, our results demonstrate that phasic spillover and tonic inhibition reflect the activation of distinct receptor populations.
2011-01-01 | S-EPMC3059572 | BioStudies
Project description:Many patients who undergo general anesthesia and surgery experience cognitive dysfunction, particularly memory deficits that can persist for days to months. The mechanisms underlying this postoperative cognitive dysfunction in the adult brain remain poorly understood. Depression of brain function during anesthesia is attributed primarily to increased activity of γ-aminobutyric acid type A receptors (GABA(A)Rs), and it is assumed that once the anesthetic drug is eliminated, the activity of GABA(A)Rs rapidly returns to baseline and these receptors no longer impair memory. Here, using a murine model, we found that a single in vivo treatment with the injectable anesthetic etomidate increased a tonic inhibitory current generated by α5 subunit-containing GABA(A)Rs (α5GABA(A)Rs) and cell-surface expression of α5GABA(A)Rs for at least 1 week. The sustained increase in α5GABA(A)R activity impaired memory performance and synaptic plasticity in the hippocampus. Inhibition of α5GABA(A)Rs completely reversed the memory deficits after anesthesia. Similarly, the inhaled anesthetic isoflurane triggered a persistent increase in tonic current and cell-surface expression of α5GABA(A)Rs. Thus, α5GABA(A)R function does not return to baseline after the anesthetic is eliminated, suggesting a mechanism to account for persistent memory deficits after general anesthesia.
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Project description:The medial habenula (MHb) plays an important role in nicotine-related behaviors such as nicotine aversion and withdrawal. The MHb receives GABAergic input from the medial septum/diagonal band of Broca (MS/DB), yet the synaptic mechanism that regulates MHb activity is unclear. GABA (? -aminobutyric acid) is a major inhibitory neurotransmitter activating both GABA<sub>A</sub> receptors and GABA<sub>B</sub> receptors. Depending on intracellular chloride concentration, however, GABA<sub>A</sub> receptors also function in an excitatory manner. In the absence of various synaptic inputs, we found that MHb neurons displayed spontaneous tonic firing at a rate of about ~4.4?Hz. Optogenetic stimulation of MS/DB inputs to the MHb evoked GABA<sub>A</sub> receptor-mediated synaptic currents, which produced stimulus-locked neuronal firing. Subsequent delayed yet lasting activation of GABA<sub>B</sub> receptors attenuated the intrinsic tonic firing. Consequently, septal GABAergic input alone orchestrates both excitatory GABA<sub>A</sub> and inhibitory GABA<sub>B</sub> receptors, thereby entraining the firing of MHb neurons.
| S-EPMC5050514 | BioStudies
Project description:GABA (gamma-aminobutyric acid) is predominantly released by local interneurons in the cerebral cortex to particular subcellular domains of the target cells. This suggests that compartmentalized, synapse-specific action of GABA is required in cortical networks for phasic inhibition. However, GABA released at the synaptic cleft diffuses to receptors outside the postsynaptic density and thus tonically activates extrasynaptic GABA(A) and GABA(B) receptors, which include subtypes of both receptor families especially sensitive to low concentrations of GABA. The synaptic and extrasynaptic action of GABA corroborates the idea that neurons of the brain use synaptic (or wiring) transmission and non-synaptic (or volume) transmission for communication. However, re-uptake mechanisms restrict the spatial extent of extrasynaptic GABA-mediated effects, and it has been proposed that the concerted action of several presynaptic interneurons, the sustained firing of individual cells or an increase in release-site density is required to reach ambient GABA levels sufficient to activate extrasynaptic receptors. Here we show that individual neurogliaform cells release enough GABA for volume transmission within the axonal cloud and, thus, that neurogliaform cells do not require synapses to produce inhibitory responses in the overwhelming majority of nearby neurons. Neurogliaform cells suppress connections between other neurons acting on presynaptic terminals that do not receive synapses at all in the cerebral cortex. They also reach extrasynaptic, delta-subunit-containing GABA(A) (GABA(Adelta)) receptors responsible for tonic inhibition. We show that GABA(Adelta) receptors are localized to neurogliaform cells preferentially among cortical interneurons. Neurosteroids, which are modulators of GABA(Adelta) receptors, alter unitary GABA-mediated effects between neurogliaform cells. In contrast to the specifically placed synapses formed by other interneurons, the output of neurosteroid-sensitive neurogliaform cells represents the ultimate form of the lack of spatial specificity in GABA-mediated systems, leading to long-lasting network hyperpolarization combined with widespread suppression of communication in the local circuit.
| S-EPMC2771344 | BioStudies
Project description:Gamma-frequency oscillations depend on phasic synaptic GABA(A) receptor (GABA(A)R)-mediated inhibition to synchronize spike timing. The spillover of synaptically released GABA can also activate extrasynaptic GABA(A)Rs, and such tonic inhibition may also contribute to modulating network dynamics. In many neuronal cell types, tonic inhibition is mediated by delta subunit-containing GABA(A)Rs. We found that the frequency of in vitro cholinergically induced gamma oscillations in the mouse hippocampal CA3 region was increased by the activation of NMDA receptors (NMDARs) on interneurons. The NMDAR-dependent increase of gamma oscillation frequency was counteracted by the tonic inhibition of the interneurons mediated by delta subunit-containing GABA(A)Rs. Recordings of synaptic currents during gamma activity revealed that NMDAR-mediated increases in oscillation frequency correlated with a progressive synchronization of phasic excitation and inhibition in the network. Thus, the balance between tonic excitation and tonic inhibition of interneurons may modulate gamma frequency by shaping interneuronal synchronization.
| S-EPMC2843436 | BioStudies
Project description:<h4>Background</h4>The most currently used general anaesthetics are potent potentiators of γ-aminobutyric acid A (GABA<sub>A</sub>) receptors and are invariably neurotoxic during the early stages of brain development in preclinical animal models. As causality between GABA<sub>A</sub> potentiation and anaesthetic-induced developmental neurotoxicity has not been established, the question remains whether GABAergic activity is crucial for promoting/enhancing neurotoxicity. Using the neurosteroid analogue, (3α,5α)-3-hydroxy-13,24-cyclo-18,21-dinorchol-22-en-24-ol (CDNC24), which potentiates recombinant GABA<sub>A</sub> receptors, we examined whether this potentiation is the driving force in inducing neurotoxicity during development.<h4>Methods</h4>The neurotoxic potential of CDNC24 was examined vis-à-vis propofol (2,6-diisopropylphenol) and alphaxalone (5α-pregnan-3α-ol-11,20-dione) at the peak of rat synaptogenesis. In addition to the morphological neurotoxicity studies of the subiculum and medial prefrontal cortex (mPFC), we assessed the extra-, pre-, and postsynaptic effects of these agents on GABAergic neurotransmission in acute subicular slices from rat pups.<h4>Results</h4>CDNC24, like alphaxalone and propofol, caused dose-dependent hypnosis in vivo, with a higher therapeutic index. CDNC24 and alphaxalone, unlike propofol, did not cause developmental neuroapoptosis in the subiculum and mPFC. Propofol potentiated post- and extrasynaptic GABA<sub>A</sub> currents as evidenced by increased spontaneous inhibitory postsynaptic current (sIPSC) decay time and prominent tonic currents, respectively. CDNC24 and alphaxalone had a similar postsynaptic effect, but also displayed a strong presynaptic effect as evidenced by decreased frequency of sIPSCs and induced moderate tonic currents.<h4>Conclusions</h4>The lack of neurotoxicity of CDNC24 and alphaxalone may be at least partly related to suppression of presynaptic GABA release in the developing brain.
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Project description:Anesthetic contact residues in ?-aminobutyric acid type A (GABAA) receptors have been identified using photolabels, including two propofol derivatives. O-propofol diazirine labels H267 in ?3 and ?1?3 receptors, whereas m-azi-propofol labels other residues in intersubunit clefts of ?1?3. Neither label has been studied in ??? receptors, the most common isoform in mammalian brain. In ??? receptors, other anesthetic derivatives photolabel m-azi-propofol-labeled residues, but not ?H267. The authors' structural homology model of ?1?3?2L receptors suggests that ?3H267 may abut some of these sites.Substituted cysteine modification-protection was used to test ?3H267C interactions with four potent anesthetics: propofol, etomidate, alphaxalone, and R-5-allyl-1-methyl-5-(m-trifluoromethyl-diazirinylphenyl) barbituric acid (mTFD-MPAB). The authors expressed ?1?3?2L or ?1?3H267C?2L GABAA receptors in Xenopus oocytes. The authors used voltage clamp electrophysiology to assess receptor sensitivity to ?-aminobutyric acid (GABA) and anesthetics and to compare p-chloromercuribenzenesulfonate modification rates with GABA versus GABA plus anesthetics.Enhancement of low GABA (eliciting 5% of maximum) responses by equihypnotic concentrations of all four anesthetics was similar in ?1?3?2L and ?1?3H267C?2L receptors (n > 3). Direct activation of ?1?3H267C?2L receptors, but not ?1?3?2L, by mTFD-MPAB and propofol was significantly greater than the other anesthetics. Modification of ?3H267C by p-chloromercuribenzenesulfonate (n > 4) was rapid and accelerated by GABA. Only mTFD-MPAB slowed ?3H267C modification (approximately twofold; P = 0.011).?3H267 in ?1?3?2L GABAA receptors contacts mTFD-MPAB, but not propofol. The study results suggest that ?3H267 is near the periphery of one or both transmembrane intersubunit (?+/?- and ?+/?-) pockets where both mTFD-MPAB and propofol bind.
| S-EPMC4681607 | BioStudies
Project description:GABA(A) receptors play a crucial role in the actions of general anesthetics. The recently published crystal structure of the general anesthetic propofol bound to Gloeobacter violaceus ligand-gated ion channel (GLIC), a bacterial homolog of GABA(A) receptors, provided an opportunity to explore structure-based ligand discovery for pentameric ligand-gated ion channels (pLGICs). We used molecular docking of 153,000 commercially available compounds to identify molecules that interact with the propofol binding site in GLIC. In total, 29 compounds were selected for functional testing on recombinant GLIC, and 16 of these compounds modulated GLIC function. Active compounds were also tested on recombinant GABA(A) receptors, and point mutations around the presumed binding pocket were introduced into GLIC and GABA(A) receptors to test for binding specificity. The potency of active compounds was only weakly correlated with properties such as lipophilicity or molecular weight. One compound was found to mimic the actions of propofol on GLIC and GABA(A), and to be sensitive to mutations that reduce the action of propofol in both receptors. Mutant receptors also provided insight about the position of the binding sites and the relevance of the receptor's conformation for anesthetic actions. Overall, the findings support the feasibility of the use of virtual screening to discover allosteric modulators of pLGICs, and suggest that GLIC is a valid model system to identify novel GABA(A) receptor ligands.
| S-EPMC3807073 | BioStudies