The effect of etomidate on sensory transmission in the dorsal column pathway in the urethane-anaesthetized rat

Closing the gap between the molecular and systemic actions of anesthetic agents

Genetic approaches have been successfully used to relate the diverse molecular actions of anesthetic agents to their amnestic, sedative, hypnotic, and immobilizing properties. The hypnotic effect of etomidate, quantified as the duration of the loss of righting reflex in mice, is equally mediated by GABAA receptors containing β2- and β3-protein subunits. However, only β3-containing receptors are involved in producing electroencephalogram (EEG)-patterns typical of general anesthesia. The sedative action of diazepam is produced by α1-subunit-containing receptors, but these receptors do not contribute to the drug's characteristic EEG-"fingerprint." Thus, GABAA receptors with α1- and β2-subunits take a central role in causing benzodiazepine-induced sedation and etomidate-induced hypnosis, but the corresponding EEG-signature is difficult to resolve. Contrastingly, actions of etomidate and benzodiazepines mediated via α2- and β3-subunits modify rhythmic brain activity in vitro and in vivo at least in part by enhancing neuronal synchrony. The immobilizing action of GABAergic anesthetics predominantly involves β3-subunit-containing GABAA receptors in the spinal cord. Interestingly, this action is self-limiting as GABA-release is attenuated via the same receptors. Anesthetic-induced amnesia is in part mediated by GABAA receptors harboring α5-subunits that are highly enriched in the hippocampus and, in addition, by α1-containing receptors in the forebrain. Because there is accumulating evidence that in patients the expression pattern of GABAA receptor subtypes varies with age, is altered by the long-term use of drugs, and is affected by pathological conditions like inflammation and sepsis, further research is recommended to adapt the use of anesthetic agents to the specific requirements of individual patients.

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.

Sensory and motor effects of etomidate anesthesia

The effects of anesthesia with etomidate on the cellular mechanisms of sensory processing and sensorimotor coordination have been studied in the active electric sense of the mormyrid fish Gnathonemus petersii. Like many anesthetics, etomidate is known to potentiate GABA(A) receptors, but little is known about the effects on sensory processing at the systems level. A better understanding is necessary for experimental studies of sensory processing, in particular regarding possible effects on the dynamic structure of excitatory and inhibitory receptive fields and to improve the knowledge of the mechanisms of anesthesia in general. Etomidate slowed the electromotor discharge rhythm, probably because of feedback inhibition at the premotor level, but did not alter the structure of the electromotor command. Sensory translation through primary afferents projecting to the cerebellum-like electrosensory lobe (ELL) was not changed. However, central interneurons and projection neurons were hyperpolarized under etomidate, and their spiking activity was reduced. Although the spatial extent and the center/surround organization of sensory receptive fields were not changed, initial excitatory responses were followed by prolonged inhibition. Corollary discharge input to ELL was maintained, and the temporal sequence of excitatory and inhibitory components of this descending signal remained intact. Later inhibitory corollary discharge responses were prolonged by several hundred milliseconds. The result was that excitatory reafferent sensory input was conserved with enhanced precision of timing, whereas background activity was greatly reduced. Anti-Hebbian synaptic plasticity evoked by association of sensory and corollary discharge input was still present under anesthesia, and differences compared with the nonanesthetized condition are discussed.

Chloramphenicol decreases brain glucose utilization and modifies the sleep-wake cycle architecture in rats

We studied the effects of chloramphenicol on brain glucose utilization and sleep-wake cycles in rat. After slightly anaesthetized animals were injected with [18F]fluoro-2-deoxy-D-glucose, we acquired time-concentration curves from three radiosensitive beta microprobes inserted into the right and left frontal cortices and the cerebellum, and applied a three-compartment model to calculate the cerebral metabolic rates for glucose. The sleep-wake cycle architecture was analysed in anaesthetic-free rats by recording electroencephalographic and electromyographic signals. Although chloramphenicol is a well-established inhibitor of oxidative phosphorylation, no compensatory increase in glucose utilization was detected in frontal cortex. Instead, chloramphenicol induced a significant 23% decrease in the regional cerebral metabolic rate for glucose. Such a metabolic response indicates a potential mismatch between energy supply and neuronal activity induced by chloramphenicol administration. Regarding sleep-wake states, chloramphenicol treatment was followed by a 64% increase in waking, a 20% decrease in slow-wave sleep, and a marked 59% loss in paradoxical sleep. Spectral analysis of the electroencephalogram indicates that chloramphenicol induces long-lasting modifications of delta-band power during slow-wave sleep.

Molecular and systemic mechanisms of general anaesthesia: the ‘multi-site and multiple mechanisms’ concept

PURPOSE OF REVIEW

Amnesia, hypnosis and immobility are essential components of general anaesthesia. This review highlights recent advances in our understanding of how these components are achieved at a molecular level.

RECENT FINDINGS

Commonly used volatile anaesthetic agents such as isoflurane or sevoflurane cause immobility by modulating multiple molecular targets predominantly in the spinal cord, including gamma-aminobutyric acidA receptors, glycine receptors, glutamate receptors and TREK-1 potassium channels. In contrast, intravenously applied drugs such as propofol or etomidate depress spinal motor reflexes almost exclusively via enhancing gamma-aminobutyric acidA receptor function. Studies on knock-in animals showed that etomidate and propofol act via gamma-aminobutyric acidA receptors containing beta3 subunits, whereas gamma-aminobutyric acidA receptors including alpha2 and gamma subunits mediate the myorelaxant properties of diazepam. These findings suggest that a large fraction of gamma-aminobutyric acidA receptors in the spinal cord assemble from alpha2, beta3 and most probably gamma2 subunits. The hypnotic actions of etomidate are mediated by beta3-containing gamma-aminobutyric acidA receptors expressed in the brain. In contrast, gamma-aminobutyric acidA receptors harbouring beta2 subunits produce sedation, but not hypnosis. Furthermore, there is growing evidence that extrasynaptic gamma-aminobutyric acidA receptors in the hippocampus containing alpha5 subunits contribute to amnesia.

SUMMARY

Clinical anaesthesia is based on drug actions at multiple anatomical sites in the brain. The finding that amnesia, hypnosis and immobility involve distinct molecular targets opens new avenues for developing improved therapeutic strategies in anaesthesia.

Migrating from target-controlled infusion to closed-loop control in general anaesthesia

The target-controlled infusion (TCI) technique has been successfully and commercially used in clinical general anaesthesia with the intravenous anaesthetic agent propofol. The technique is based on a population pharmacokinetic model and is an open-loop control system. Closed-loop control requires a reliable and consistent signal for feedback utilisation. With all anaesthetic agents the somatosensory evoked potentials (SEP) have been shown to give increased latency as anaesthetic depth is increased. Using infusion rate and SEP response data from rats anaesthetised with propofol a mathematical model was derived to describe the anaesthetic process. This model was used as a design reference to develop a proportional integral (PI) closed-loop control system using SEP as the feedback measure. A serials of 10 trials were conducted to investigate the difference between continuous bolus injection and infusion, all under closed-loop control. The trials showed that the use of SEPs in closed-loop control of anaesthesia is feasible.

Somatosensory cortical neuronal population activity across states of anaesthesia

Experiments were carried out to learn about changes in sensory cortical processing associated with different levels of anaesthesia. Traditionally this question has been addressed by studying single neurons. Because state changes are likely to influence the relationships between neurons, the present experiments were undertaken to investigate the spatial and temporal firing patterns distributed across cortex. Using 5 x 5 or 10 x 10 microelectrode arrays, spontaneous and stimulus-evoked activity of multineuron clusters was recorded from rat somatosensory 'barrel' cortex (the whisker representation) during a light surgical stage of urethane anaesthesia, and after two supplemental doses of urethane which led to intermediate and deep levels of anaesthesia. At all depths of anaesthesia, spontaneously occurring action potentials at a single electrode tended to be clustered into 'bursts.' With increasing anaesthetic depth, bursts became more prominent and rhythmic, and increasingly synchronized between cortical barrel-columns. Burst frequency decreased and fewer spikes occurred outside bursts, leading to a decrease in the overall spontaneous firing rate. The cortical territory engaged by individual whiskers contracted with increasing depth of anaesthesia, leading to the spatial segregation of whisker representations. At all stages of anaesthesia, whisker stimulation produced the maximal cortical response when delivered close to burst onset. These observations show that ongoing spontaneous activity modulates sensory response properties and makes peripheral tactile information accessible to a cortical territory whose size is determined by the phase of burst cycle. The possible significance of the cyclic cortical responsiveness encountered during urethane anaesthesia to cortical processing in awake rats is considered.