Desflurane and the Nonimmobilizer 1,2-Dichlorohexafluorocyclobutane Suppress Learning by a Mechanism Independent of the Level of Unconditioned Stimulation

Assessing anesthetic activity through modulation of the membrane dipole potential

There is great individual variation in response to general anesthetics (GAs) leading to difficulties in optimal dosing and sometimes even accidental awareness during general anesthesia (AAGA). AAGA is a rare, but potentially devastating, complication affecting between 0.1% and 2% of patients undergoing surgery. The development of novel personalized screening techniques to accurately predict a patient’s response to GAs and the risk of AAGA remains an unmet clinical need. In the present study, we demonstrate the principle of using a fluorescent reporter of the membrane dipole potential, di-8-ANEPPs, as a novel method to monitor anesthetic activity using a well-described inducer/noninducer pair. The membrane dipole potential has previously been suggested to contribute a novel mechanism of anesthetic action. We show that the fluorescence ratio of di-8-ANEPPs changed in response to physiological concentrations of the anesthetic, 1-chloro-1,2,2-trifluorocyclobutane (F₃), but not the structurally similar noninducer, 1,2-dichlorohexafluorocyclobutane (F₆), to artificial membranes and in vitro retinal cell systems. Modulation of the membrane dipole provides an explanation to overcome the limitations associated with the alternative membrane-mediated mechanisms of GA action. Furthermore, by combining this technique with noninvasive retinal imaging technologies, we propose that this technique could provide a novel and noninvasive technique to monitor GA susceptibility and identify patients at risk of AAGA.

Sevoflurane impairs memory consolidation in rats, possibly through inhibiting phosphorylation of glycogen synthase kinase-3β in the hippocampus

Sevoflurane administration impairs memory processes in both humans and animals. Increasing evidence suggests that enhancement of the phosphorylation state of glycogen synthase kinase-3β (GSK-3β), as a result of acute administration of lithium chloride (LiCl), may enhance memory consolidation. The current experiments examined whether GSK-3β phosphorylation was involved in mediating the memory impairing effects of posttraining sevoflurane on inhibitory avoidance (IA) retention. In experiment 1, adult male Sprague-Dawley rats were exposed to sevoflurane (0.5%, 1%, or 2%) for 2h immediately after training in a continuous multiple-trail IA paradigm. Sevoflurane (2% inspired) induced significant impairment of retention performance on a 24-h test and inhibited phosphorylation of GSK-3β in the hippocampus 2h after training. In experiment 2, administration of LiCl (100mg/kg, intraperitoneally) 30 min before IA training not only blocked the sevoflurane-induced impairment of consolidation, but also reversed the inhibitory effect of sevoflurane on GSK-3β phosphorylation in the hippocampus. Collectively, these findings support the hypothesis that sevoflurane exposure can impair consolidation of IA memory in rats. Sevoflurane-induced amnesia may be due, at least in part, to suppression of GSK-3β phosphorylation in the hippocampus.

Anaesthetic mechanisms: update on the challenge of unravelling the mystery of anaesthesia

General anaesthesia is administered each day to thousands of patients worldwide. Although more than 160 years have passed since the first successful public demonstration of anaesthesia, a detailed understanding of the anaesthetic mechanism of action of these drugs is still lacking. An important early observation was the Meyer-Overton correlation, which associated the potency of an anaesthetic with its lipid solubility. This work focuses attention on the lipid membrane as a likely location for anaesthetic action. With the advent of cellular electrophysiology and molecular biology techniques, tools to dissect the components of the lipid membrane have led, in recent years, to the widespread acceptance of proteins, namely receptors and ion channels, as more likely targets for the anaesthetic effect. Yet these accumulated data have not produced a comprehensive explanation for how these drugs produce central nervous system depression. In this review, we follow the story of anaesthesia mechanisms research from its historical roots to the intensely neurophysiological research regarding it today. We will also describe recent findings that identify specific neuroanatomical locations mediating the actions of some anaesthetic agents.

Behavioural effects of chronic exposure to sub-anesthetic concentrations of halothane, sevoflurane and desflurane in rats

BACKGROUND

A double-blind, randomized trial was conducted to determine the behavioural effects of chronic exposure to subanesthetic concentrations of halothane, sevoflurane and desflurane in rats.

METHODS

Halothane, sevoflurane and desflurane group rats received 0.1%, 0.3%, and 0.6% concentrations in a flow rate of 3 L.min(-1) O(2) respectively. Control animals also received 3 L.min(-1) O(2) in another investigation room, which had the same properties as the study group rooms. Rats breathed inhaled agents or oxygen between 09:00-13:00 hr every day for 30 days. After 30 days of inhalation of subanesthetic doses of inhaled agents or oxygen, behavioural tests were applied.

RESULTS

Tests of exploratory activity and curiosity (hole-board test), anxiety (elevated plus maze test) and learning and memory functions (multiple T maze test), demonstrated that chronic exposure to subanesthetic concentrations of all three anesthetics alters behavioural functions in rats. However, impairment of learning (P<0.05) and memory function (P<0.05) were greater in association with desflurane, in comparison to halothane and sevoflurane-treated rats.

CONCLUSION

Chronic exposure to subanesthetic concentrations of halothane, sevoflurane and desflurane is associated with behavioural change in rats. Of the three drugs, desflurane was associated with the lowest learning and memory function test scores.

Short-term memory resists the depressant effect of the nonimmobilizer 1-2-dichlorohexafluorocyclobutane (2N) more than long-term memory

The nonimmobilizer 1,2-dichlorohexafluorocyclobutane (2N, also termed F6) does not suppress movement to noxious stimuli but does suppress learning of fear-potentiated startle. The mechanism whereby 2N suppresses this learning is unknown. Herein, we report the effect of 2N on suppression of two other forms of learning, fear conditioning to context and to tone. Because 2N does not cause sedation, we could study the effect of 2N on short-term memory (memory for fear conditioning measured during or immediately after training) as well as on long-term memory (measured 24 h after training). The EC(50) for suppression of long-term memory (the concentration decreasing memory by 50%) of fear conditioning to context was 2.00% plus/minus 0.01% (mean plus/minus SEM), and for fear conditioning to tone was 3.45% plus/minus 0.26%, (P < 0.05). The EC(50) for suppression of short-term memory of fear conditioning to context was 2.59% plus/minus 0.21% (P < 0.05, compared with long-term memory of context conditioning), whereas short-term memory of fear conditioning to tone was not suppressed by 3.5%, the largest concentration studied. Thus, short-term memory resists the depressant effect of 2N more than long-term memory, fear conditioning to tone is less vulnerable to the effect of 2N than fear conditioning to context, and 3.5% 2N does not preclude transmission of tone and shock signals to the site where tone-shock associations are formed.

IMPLICATIONS

The nonimmobilizer 1,2-dichlorohexafluorocyclobutane has a greater depressant effect on long-term memory than short-term memory, suggesting that it impairs the processes responsible for the retention of memory more than for the formation of memory itself.