Resistance to state transitions in responsiveness is differentially modulated by different volatile anaesthetics in male mice.

British Journal of Anaesthesia, 125 (3): 308e320 (2020) doi: 10.1016/j.bja.2020.05.031 Advance Access Publication Date: 11 July 2020 Neuroscience and Neuroanaesthesia Resistance to state transitions in responsiveness is differentially modulated by different volatile anaesthetics in male mice Andrzej Z. Wasilczuk1,2, Benjamin A. Harrison1, Paula Kwasniewska1, Bo Ku1, Max B. Kelz1,2,3, Andrew R. McKinstry-Wu1,* and Alex Proekt1,3 1 Department of Anaesthesiology and Critical Care, Perelman School of Medicine, Philadelphia, PA, USA, 2Department of Bioengineering, School of Engineering and Applied Science, Philadelphia, PA, USA and 3Chronobiology and Sleep Institute, University of Pennsylvania, Philadelphia, PA, USA *Corresponding author. E-mail: This article is accompanied by an editorial: Advances in precision anaesthesia may be found by testing our resistance to change by Eagleman & MacIver Br J Anaesth 2020:125:235e237, doi: 10.1016/j.bja.2020.06.007 Abstract Background: Recent studies point to a fundamental distinction between population-based and individual-based anaesthetic pharmacology. At the population level, anaesthetic potency is defined as the relationship between drug concentration and the likelihood of response to a stimulus. At the individual level, even when the anaesthetic concentration is held constant, fluctuations between the responsive and unresponsive states are observed. Notably, these spontaneous fluctuations exhibit resistance to state transitions Rst. Therefore, the response probability in each individual depends not just upon the drug concentration, but also upon responses to previous stimuli. Here, we hypothesise that Rst is distinct from drug potency and is differentially modulated by different anaesthetics. Methods: Adult (14e24 weeks old) C57BL/6J male mice (n¼60) were subjected to repeated righting reflex (RR) assays at equipotent steady-state concentrations of isoflurane (0.6 vol%), sevoflurane (1.0 vol%), and halothane (0.4 vol%). Results: Fluctuations in RR were observed for all tested anaesthetics. Analysis of these fluctuations revealed that Rst was differentially modulated by different anaesthetics (F[2, 56.01]¼49.59; P<0.0001). Fluctuations in RR were modelled using a stochastic dynamical system. This analysis confirmed that the amount of noise that drives behavioural state transitions depends on the anaesthetic agent (F[2, 42.86]¼16.72; P<0.0001). Conclusions: Whilst equipotent doses of distinct anaesthetics produce comparable population response probabilities, they engage dramatically different dynamics in each individual animal. This manifests as a differential aggregate propensity to exhibit state transitions. Thus, resistance to state transitions is a fundamentally distinct, novel measure of individualised anaesthetic pharmacology. Keywords: general anaesthesia; individual-based pharmacology; inhalational anaesthetics; population-based pharmacology; responsiveness; state transitions Received: January 31, 2020; Accepted: 3 May 2020 © 2020 British Journal of Anaesthesia. Published by Elsevier Ltd. All rights reserved. For Permissions, please email: 308 Resistance to state transitions under anaesthesia Editor’s key points  Fluctuations in the state of responsiveness at fixed anaesthetic concentrations exhibit resistance to state transitions, such that knowing just the drug concentration is insufficient to predict the probability of response.  Resistance to state transitions could depend solely on the effective concentration of an anaesthetic, or be independent of drug potency and differentially modulated by different anaesthetics.  To distinguish between these possibilities, the authors exposed male mice to equipotent concentrations of three volatile anaesthetics and quantified the degree of resistance to state transitions.  At equipotent concentrations, resistance to state transitions depended on the anaesthetic agent, and is thus a dissociable feature of anaesthetic pharmacology that has been obscured by conventional population-based measures of drug efficacy.  Even when the population-based measures, such as effective concentration, are the same, the state of anaesthesia induced with different volatile anaesthetics is associated with distinct dynamics at the level of individual mice, which has implications for defining depth of anaesthesia. Anaesthesiologists aim to deliver an appropriate anaesthetic dose, such that each individual patient remains unconscious throughout the procedure and recovers consciousness swiftly and uneventfully once the procedure concludes. To accomplish this goal, clinicians currently rely on fundamental principles of pharmacology. The relationship between anaesthetic concentration and its effect in a target population is quantified by constructing a concentrationeresponse curve.1 A classic measure of inhaled anaesthetic pharmacology, the minimal alveolar concentration (MAC), is defined as the anaesthetic concentration at which the likelihood of a response to a painful stimulus is 50%.2e4 Other anaesthetic endpoints, such as MAC-awake, have been defined as the concentration at which 50% of subjects respond to a verbal command.2 In - 309 rodent research, the ability of an animal placed on its back to turn over onto its paws, the righting reflex (RR), has been used as a proxy for the state of wakefulness.5,6 Across many mechanistically distinct anaesthetics, the effective concentrations for 50% of subjects (EC50) for loss of RR in rodents and MAC-awake in humans are closely correlated.7 Current measures of anaesthetic efficacy, including MACawake, define the response at the level of the population rather than of the individual. To apply population-based measures to an individual, it is typically assumed that the individual’s probability of wakefulness mirrors that of the population for every anaesthetic concentration. At the EC50, for instance, each individual is expected to be responsive on 50% of trials. However, this line of reasoning reveals an essential shortcoming of population-based measures of anaesthetic potency. It is not clear which 50% of stimuli will trigger a response in each individual. To illustrate this limitation, consider two hypothetical examples of individuals at EC50 (Fig. 1). With respect to the across-trial probability of wakefulness, these individuals are the same and each responds to 50% of stimuli. Yet, the overall righting probability does not directly inform the underlying dynamics of the responses in the two individuals. The first individual randomly fluctuates between responsive and unresponsive states (Fig. 1a). Thus, on average, the state switches approximately every other trial. In contrast, the fluctuations in the second individual are more predictable. The second individual tends to dwell in its previously observed state (Fig. 1b). We define this inertial tendency ‘resistance to state transitions’ Rst. One manifestation of this inertia is that the pro (...truncated)