Isoflurane – the most common preclinical anesthetic

Isoflurane is one of the most commonly utilized general anesthetics in preclinical work. It has high potency and stability and is easy to use for anaesthesia maintenance. However, isoflurane can evoke side-effects such as respiration depression, reductions in blood pressure, vasodilation, and elevated airway irritation (Wren-Dail et al., 2017). In the clinic, isoflurane has nowadays been largely replaced by sevoflurane or desflurane, mainly because of the good safety record and less irritative nature of these agents.

Pharmacokinetics

Isoflurane, as an inhalation anesthetic, is absorbed into the bloodstream by diffusion. The minimum alveolar concentration which is enough to cause a loss of reactivity to a painful stimulus in adult rats is 1.22-1.35% (Orliaguet et al., 2001).

The effect time of isoflurane is dependent on the ratio of the alveolar concentration to the inspired concentration over time (Stock et al., 2013). In comparison to other volatile anesthetics, isoflurane has a relatively high solubility in blood, thus increasing blood equilibrium time. Nonetheless, the relative alveolar uptake of isoflurane is rather rapid as 50% of the relative alveolar concentration is reached within ~2 mins. Inhalation drugs in general are delivered rapidly to the vessel-rich compartments, including the brain. Since isoflurane has a high brain-blood partition coefficient, it is quickly distributed to the brain tissue where its anesthetic effects take place.

Mechanism of action

al., 2009), Ca²⁺ and K⁺ channel currents (Buljubasic et al., 1992) and thalamocortical neurons in the ventrobasal thalamus (Ying et al., 2009).

Burst suppression

In the presence of isoflurane concentrations of 1.25-2.0%, brain activity shifts to a burst suppression state with quasi-periodical peaks and silent states (Derbyshire et al., 1936; Hudetz and Imas, 2007; Xiao Liu et al., 2013b). The BS phenomenon has been thought to be initiated by a depletion of extracellular calcium stores (Amzica, 2009), diminished cortical inhibition (Ferron et al., 2009) and an overall decrease in metabolic and neuronal activity (Ching et al., 2012). Even though BS activity is detected in the unconscious brain, the brain is thought to attempt to recover normal neuronal dynamics and exchange of information during the burst phase (Ching et al., 2012; Japaridze et al., 2015). Accordingly, it has been reported that cortical bursts are initiated by rhythmic thalamocortical oscillations (Steriade et al., 1994;

Zhang et al., 2019) or can be triggered by subthreshold sensory stimuli (Kroeger et al., 2013).

2.2.4.1 Isoflurane – long-term effects

In addition to the initial isoflurane evoked changes in brain dynamics or FC, isoflurane has been found to exert long-term effects in brain activity, behavior, memory and gene-expression (Colon et al., 2017). Recently, several preclinical experiments have been conducted to reveal the effects of isoflurane on gene expression or behavior (Colon et al., 2017). Despite the extensive research on this subject, there are still contradictory findings in the literature about the possible long-term consequences; these are possibly attributable to the large variability in anesthesia concentrations, repetitiveness of anesthesia, combination of multiple anesthetics, or the age of the subjects being anesthetized. However, it has been found that neurodegeneration and behavioral deficits might be initiated by anesthetic agents and the effects can be pronounced with agents that act through a combination of both NMDA and GABA-A receptors (Fredriksson et al., 2007), which is also considered as a mechanism of action of isoflurane.

Gene and protein expression

Protein expression is a highly dynamic and complex process. In response to

apoptosis have been some of the most common findings in preclinical gene expression studies (Cao et al., 2012; Ge et al., 2015; Kong et al., 2013; Zhang et al., 2015). The developing brain may be more susceptible to long-term changes, but changes have been also detected in the adult or aged brain (Colon et al., 2017).

Behavior, memory and brain function

Usually the brain is able to recover fully from anesthesia, and normal brain function is stabilized within weeks to months (Ii et al., 2016; Rammes et al., 2009; Uchimoto et al., 2014). However, if the stimulus is repeated or sufficiently strong, behavioral changes and altered brain function may be evident in the long-term or, potentially, even for the lifespan (Figure 1). As an analog example, multiple bursts of protein synthesis in response to environmental cues are needed for memory formation, to support neuronal growth and synaptic plasticity (Alberini, 2009; Costa-Mattioli et al., 2009). Thus, with regard to isoflurane anesthesia, repetitive or even a single anesthetic treatment can lead to prolonged functional, behavioral or memory impairments, especially in hippocampal dependent memory tasks (Kodama et al., 2011; Zhong et al., 2015) (However see Walters et al., 2016). Notably, as the isoflurane evoked BS pattern is highly distinct from typical neuronal oscillations, it is not surprising that this type of neural activity can be involved in brain plasticity (Broad et al., 2016). In addition to apoptotic or inflammatory activation, neural plasticity may contribute to the detected changes in behavior, learning and memory. Indeed, a correlation has been found between changes in learning and the upregulation of NMDA-Rs (Rammes et al., 2009) or altered regulation of GluA1-containing AMPA-Rs- trafficking (Uchimoto et al., 2014), supporting the link between neural plasticity and brain function. Furthermore, the possibility has been raised that isoflurane may be involved in the development of a condition called post-operative cognitive dysfunction (POCD). Even though the main reason for generation of POCD has been hypothesized to be the inflammatory responses to surgery (Safavynia and Goldstein, 2019), data also point at a potential role of anesthesia evoked POCD related symptoms (Geng et al., 2017). In addition to direct modulation of neuronal plasticity, isoflurane can also affect the brain through indirect mechanisms. Interestingly, at high doses, isoflurane is known to open the blood brain barrier (Tétrault et al., 2008), thus leaving the brain tissue exposed to peripheral inflammatory responses (Safavynia and Goldstein, 2019). Thus, even if not having a direct causal role, isoflurane can potentially contribute to POCD together with the surgical operation.

In contrast, also positive or no responses between isoflurane anesthesia and behavioral outcome have been reported in several animal studies (Alkire et al.,

potential cerebral protection effects of BS during surgery through decreased cerebral metabolism (Ching et al., 2012) (However see. Roach et al., 1999).

Figure 1. Anesthesia can have both acute and chronic effects on brain circuits.

Figure adapted from (Colon et al., 2017).