Potentiation, activation and blockade of GABAA receptors by etomidate in the rat sacral dorsal commissural neurons

Potentiation of the GABAAR reveals variable energetic contributions by etiocholanolone and propofol

The properties of a potentiator are typically evaluated by measuring its ability to enhance the magnitude of the control response. Analysis of the ability of drugs to potentiate responses from receptor channels takes place in the context of particular models to extract parameters for functional effects. In the often-used coagonist model, the agonist generating control activity and the potentiator enhancing the control activity make additive energetic contributions to stabilize the active state of the receptor. The energetic contributions are fixed and, once known, enable calculation of predicted receptor behavior at any concentration combination of agonist and potentiator. Here, we have examined the applicability of the coagonist model by measuring the relationship between the magnitude of receptor potentiation and the level of background activity. Ternary αβγ GABAA receptors were activated by GABA or the allosteric agonist propofol, or by a gain-of-function mutation, and etiocholanolone- or propofol-mediated potentiation of peak responses was measured. We show that the free energy change contributed by the modulators etiocholanolone or propofol is reduced at higher levels of control activity, thereby being in disagreement with basic principles of the coagonist model. Possible mechanisms underlying this discrepancy are discussed.

Crosstalk between GABAA receptors in astrocytes and neurons triggered by general anesthetic drugs

General anesthetic drugs cause cognitive deficits that persist after the drugs have been eliminated. Astrocytes may contribute to such cognition-impairing effects through the release of one or more paracrine factors that increase a tonic inhibitory conductance generated by extrasynaptic γ-aminobutyric acid type A (GABAA) receptors in hippocampal neurons. The mechanisms underlying this astrocyte-to-neuron crosstalk remain unknown. Interestingly, astrocytes express anesthetic-sensitive GABAA receptors. Here, we tested the hypothesis that anesthetic drugs activate astrocytic GABAA receptors to initiate crosstalk leading to a persistent increase in extrasynaptic GABAA receptor function in neurons. We also investigated the signaling pathways in neurons and aimed to identify the paracrine factors released from astrocytes. Astrocytes and neurons from mice were grown in primary cell cultures and studied using in vitro electrophysiological and biochemical assays. We discovered that the commonly used anesthetics etomidate (injectable) and sevoflurane (inhaled) stimulated astrocytic GABAA receptors, which in turn promoted the release paracrine factors, that increased the tonic current in neurons via a p38 MAPK-dependent signaling pathway. The increase in tonic current was mimicked by exogenous IL-1β and abolished by blocking IL-1 receptors; however, unexpectedly, IL-1β and other cytokines were not detected in astrocyte-conditioned media. In summary, we have identified a novel form of crosstalk between GABAA receptors in astrocytes and neurons that engages a p38 MAPK-dependent pathway. Brief commentary BACKGROUND: Many older patients experience cognitive deficits after surgery. Anesthetic drugs may be a contributing factor as they cause a sustained increase in the function of "memory blocking" extrasynaptic GABAA receptors in neurons. Interestingly, astrocytes are required for this increase; however, the mechanisms underlying the astrocyte-to-neuron crosstalk remain unknown. TRANSLATIONAL SIGNIFICANCE: We discovered that commonly used general anesthetic drugs stimulate GABAA receptors in astrocytes, which in turn release paracrine factors that trigger a persistent increase in extrasynaptic GABAA receptor function in neurons via p38 MAPK. This novel form of crosstalk may contribute to persistent cognitive deficits after general anesthesia and surgery.

[Etomidate reduces excitability of the neurons and suppresses the function of nAChR ventral horn in the spinal cord of neonatal rats]

OBJECTIVE

To investigate the effects of etomidate on electrophysiological properties and nicotinic acetylcholine receptors (nAChRs) of ventral horn neurons in the spinal cord.

METHODS

The spinal cord containing lumbosacral enlargement was isolated from 19 neonatal SD rats aged 7-12 days. The spinal cord were sliced and digested with papain (0.18 g/30 mL artificial cerebrospinal fluid) and incubated for 40 min. At the ventral horn, acute mechanical separation of neurons was performed with fire-polished Pasteur pipettes, and perforated patch-clamp recordings combined with pharmacological methods were employed on the adherent healthy neurons. In current-clamp mode, the spontaneous action potential (AP) of the ventral horn neurons in the spinal cord was recorded. The effects of pretreatment with different concentrations of etomidate on AP recorded in the ventral horn neurons were examined. In the voltage-clamp mode, nicotine was applied to induce inward currents in the ventral horn neurons, and the effect of pretreatment with etomidate on the inward currents induced by nicotine were examined with different etomidate concentrations, different holding potentials and different use time.

RESULTS

The isolated ventral horn neurons were in good condition with large diverse somata and intact processes. The isolated spinal ventral horn neurons (n=21) had spontaneous action potentials, and were continuously perfused for 2 min with 0.3, 3.0 and 30.0 μmol/L etomidate. Compared with those before administration, the AP amplitude, spike potential amplitude and overshoot were concentration-dependently suppressed (P < 0.01), and spontaneous discharge frequency was obviously reduced (P < 0.01, n=12). The APs of the other 9 neurons were completely abolished by etomidate at 3.0 or 30 μmol/L. At the same holding potential (VH=-70 mV), pretreatment with 0.3, 3.0 or 30.0 μmol/L etomidate for 2 min concentration-dependently suppressed the current amplitude induced by 0.4 mmol/L nicotine (P < 0.01, n=7). At the holding potentials of - 30, - 50, and - 70 mV, pretreatment with 30.0 μmol/L etomidate for 2 min voltage-dependently suppressed the current amplitude induced by 0.4 mmol/L nicotine (P < 0.01, n=6 for each holding potential). During the 6 min of 30.0 μmol/L etomidate pretreatment, the clamped cells were exposed to 0.4 mmol/L nicotine for 4 times at 0, 2, 4, and 6 min (each exposure time was 2 s), and the nicotinic current amplitude decreased gradually as the number of exposures increased. But at the same concentration, two nicotine exposures (one at the beginning and the other at the end of the 6 min pretreatment) resulted in a significantly lower inhibition rate compared with 4 nicotine exposures (P < 0.01, n=6).

CONCLUSIONS

etomidate reduces the excitability of the spinal ventral neurons in a concentration-dependent manner and suppresses the function of nAChR in a concentration-, voltage-, and use-dependent manner.

Ketamine Increases the Function of γ-Aminobutyric Acid Type A Receptors in Hippocampal and Cortical Neurons

BACKGROUND

The "dissociative " general anesthetic ketamine is a well-known N-methyl-D-aspartate receptor antagonist. However, whether ketamine, at clinically relevant concentrations, increases the activity of inhibitory γ-aminobutyric acid (GABA) receptor type A (GABAA) receptors in different brain regions remains controversial. Here, the authors studied the effects of ketamine on synaptic and extrasynaptic GABAA receptors in hippocampal neurons. Ketamine modulation of extrasynaptic GABAA receptors in cortical neurons was also examined.

METHODS

Whole cell currents were recorded from cultured murine neurons. Current evoked by exogenous GABA, miniature inhibitory postsynaptic currents, and currents directly activated by ketamine were studied.

RESULTS

Ketamine did not alter the amplitude, frequency, or kinetics of postsynaptic currents but increased a tonic inhibitory current generated by extrasynaptic GABAA receptors in hippocampal neurons. For example, ketamine (100 µM) increased the tonic current by 33.6 ± 6.5% (mean ± SEM; 95% CI, 18.2 to 48.9; n = 8, P < 0.001). Ketamine shifted the GABA concentration-response curve to the left, but only when GABAA receptors were activated by low concentrations of GABA (n = 6). The selective increase in tonic current was attributed to ketamine increasing the apparent potency of GABA at high-affinity extrasynaptic GABAA receptors. Ketamine also increased a tonic current in cortical neurons (n = 11). Ketamine directly gated the opening of GABAA receptors, but only at high concentrations that are unlikely to occur during clinical use.

CONCLUSIONS

Clinically relevant concentrations of ketamine increased the activity of high-affinity extrasynaptic GABAA receptors in the hippocampus and cortex, an effect that likely contributes to ketamine's neurodepressive properties.

Potentiating action of propofol at GABAA receptors of retinal bipolar cells

PURPOSE

Propofol (2,6-diisopropyl phenol), a widely used systemic anesthetic, is known to potentiate GABA(A) receptor activity in a number of CNS neurons and to produce changes in electroretinographically recorded responses of the retina. However, little is known about propofol's effects on specific retinal neurons. The authors investigated the action of propofol on GABA-elicited membrane current responses of retinal bipolar cells, which have both GABA(A) and GABA(C) receptors.

METHODS

Single, enzymatically dissociated bipolar cells obtained from rat retina were treated with propofol delivered by brief application in combination with GABA or other pharmacologic agents or as a component of the superfusing medium.

RESULTS

When applied with GABA at subsaturating concentrations and with TPMPA (a known GABA(C) antagonist), propofol markedly increased the peak amplitude and altered the kinetics of the response. Propofol increased the response elicited by THIP (a GABA(A)-selective agonist), and the response was reduced by bicuculline (a GABA(A) antagonist). The response to 5-methyl I4AA, a GABA(C)-selective agonist, was not enhanced by propofol. Serial brief applications of (GABA + TPMPA + propofol) led to a progressive increase in peak response amplitude and, at higher propofol concentrations, additional changes that included a prolonged time course of response recovery. Pre-exposure of the cell to perfusing propofol typically enhanced the rate of development of potentiation produced by (GABA + TPMPA + propofol) applications.

CONCLUSIONS

Propofol exerts a marked and selective potentiation on GABA(A) receptors of retinal bipolar cells. The data encourage the use of propofol in future studies of bipolar cell function.

Valerenic acid potentiates and inhibits GABA(A) receptors: molecular mechanism and subunit specificity

Valerian is a commonly used herbal medicinal product for the treatment of anxiety and insomnia. Here we report the stimulation of chloride currents through GABA(A) receptors (I(GABA)) by valerenic acid (VA), a constituent of Valerian. To analyse the molecular basis of VA action, we expressed GABA(A) receptors with 13 different subunit compositions in Xenopus oocytes and measured I(GABA) using the two-microelectrode voltage-clamp technique. We report a subtype-dependent stimulation of I(GABA) by VA. Only channels incorporating beta(2) or beta(3) subunits were stimulated by VA. Replacing beta(2/3) by beta(1) drastically reduced the sensitivity of the resulting GABA(A) channels. The stimulatory effect of VA on alpha(1)beta(2) receptors was substantially reduced by the point mutation beta(2N265S) (known to inhibit loreclezole action). Mutating the corresponding residue of beta(1) (beta(1S290N)) induced VA sensitivity in alpha(1)beta(1S290N) comparable to alpha(1)beta(2) receptors. Modulation of I(GABA) was not significantly dependent on incorporation of alpha(1), alpha(2), alpha(3) or alpha(5) subunits. VA displayed a significantly lower efficiency on channels incorporating alpha(4) subunits. I(GABA) modulation by VA was not gamma subunit dependent and not inhibited by flumazenil (1 microM). VA shifted the GABA concentration-effect curve towards lower GABA concentrations and elicited substantial currents through GABA(A) channels at > or = 30 microM. At higher concentrations (> or = 100 microM), VA and acetoxy-VA inhibit I(GABA). A possible open channel block mechanism is discussed. In summary, VA was identified as a subunit specific allosteric modulator of GABA(A) receptors that is likely to interact with the loreclezole binding pocket.

Etomidate reduces initiation of backpropagating dendritic action potentials: implications for sensory processing and synaptic plasticity during anesthesia

Anesthetics may induce specific changes that alter the balance of activity within neural networks. Here we describe the effects of the GABA(A) receptor potentiating anesthetic etomidate on sensory processing, studied in a cerebellum-like structure, the electrosensory lateral line lobe (ELL) of mormyrid fish, in vitro. Previous studies have shown that the ELL integrates sensory input and removes predictable features by comparing reafferent sensory signals with a descending electromotor command-driven corollary signal that arrives in part through parallel fiber synapses with the apical dendrites of GABAergic interneurons. These synapses show spike timing-dependent depression when presynaptic activation is associated with postsynaptic backpropagating dendritic action potentials. Under etomidate, almost all neurons become tonically hyperpolarized. The threshold for action potential initiation increased for both synaptic activation and direct intracellular depolarization. Synaptically evoked inhibitory postsynaptic potentials (IPSPs) were also strongly potentiated and prolonged. Current source density analysis showed that backpropagation of action potentials through the apical dendritic arborization in the molecular layer was reduced but could be restored by increasing stimulus strength. These effects of etomidate were blocked by bicuculline or picrotoxin. It is concluded that etomidate affects both tonic and phasic inhibitory conductances at GABA(A) receptors and that increased shunting inhibition at the level of the proximal dendrites also contributes to increasing the threshold for action potential backpropagation. When stimulus strength is sufficient to evoke backpropagation, repetitive association of synaptic excitation with postsynaptic action potential initiation still results in synaptic depression, showing that etomidate does not interfere with the molecular mechanism underlying plastic modulation.

Potentiation of inhibitory amino acid receptors-mediated responses by lanthanum in rat sacral dorsal commissural neurons

Lanthanum is one of rare earth cations with extremely active chemical property and has been reported to influence neuronal transmitter systems. To date, little attention has been directed towards the sacral dorsal commissural nucleus (SDCN), which serves as a relay of sensory information from the pelvic viscera in the spinal cord. Therefore, the effect of lanthanum on the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) and glycine (Gly) responses in neurons acutely dissociated from the rat SDCN was investigated using the nystatin-perforated patch-recording configuration under voltage-clamp conditions. At a holding potential of -40 mV, La(3+) reversibly potentiated GABA (3 microM)-activated currents (I(GABA)) in a concentration-dependent manner over the concentration range of 10 microM to 30 mM, with the EC(50) value of 67.3+/-16.4 microM. Similarly, La(3+) reversibly potentiated glycine (10 microM)-activated currents (I(Gly)) in a concentration-dependent manner over the concentration range of 1 microM to 1 mM, with the EC(50) value of 52.3+/-10.9 microM. The effects of La(3+) on I(GABA) and I(Gly) were voltage-independent. Moreover, both of the potentiations were not use-dependent and were overcome by increasing the concentration of agonist. Our results indicate that La(3+) potentiates the inhibitory amino acid receptors-mediated responses in SDCN, which may reduce the transmission of the pelvic visceral information. The information provided by this work may help to elucidate the mechanisms and effects of lanthanum on brain functions.

Identification of structures within GABAA receptor alpha subunits that regulate the agonist action of pentobarbital

Barbiturates act on GABA(A) receptors (GABARs) through three distinct mechanisms, resulting in positive allosteric modulation, direct activation, and inhibition. These effects are observed at different concentrations and are differentially affected by some mutations and by the receptor's subunit composition. Mammalian GABARs can be formed from a combination of 16 different subunit subtypes. Although the effect of barbiturates depends largely on the beta subunit, their agonist activity is substantially influenced by the alpha subunit subtype. Pentobarbital is a more effective agonist than GABA only when receptors contain an alpha6 subunit. Results from chimeric alpha1/alpha6 subunits suggested that structural differences within the extracellular N-terminal domain were responsible for this characteristic. Within this domain, we examined 15 amino acid residues unique to the alpha6 subtype. Each of these sites was individually mutated in the alpha6 subunit to the corresponding residue of the alpha1 subunit. The effect of the mutation on direct activation by pentobarbital was determined with whole-cell electrophysiological recordings. Our results indicate that only one of these mutations, alpha6(T69K), altered pentobarbital efficacy. This single mutation reduced the response to pentobarbital to a level intermediate to the wild-type alpha1beta1gamma2L and alpha6beta1gamma2L isoforms. The mutation did not affect the sensitivity of the receptor to GABA but did reduce the efficacy of etomidate, another i.v. anesthetic with activity similar to pentobarbital. The reverse mutation in the alpha1 subunit (K70T) did not alter the response to pentobarbital. This is the first identification of a structural difference in GABAR alpha subtypes that regulates direct activation by barbiturates.