Anesthetic drug & AMPK activator Propofol shown for the first time to excite neurons before loss & return of consciousness: Hypothesis substantiated

Source: By Lt.Cmdr. Jesse Ehrenfeld - https://www.army.mil/article/141145/anesthesiologists_keep_soldiers_safe_in_afghanistan, Public Domain, https://commons.wikimedia.org/w/index.php?curid=71554756

 A recently published paper in December of 2018 in the journal Frontiers in Molecular Neuroscience demonstrated for the first time that the popular anesthetic drug and AMPK activator propofol (the anti-diabetic drug Metformin also acts via AMPK) paradoxically activated neurons in the brain of rats just before loss of consciousness and return of consciousness [3]. These results provide direct support and substantiate my previous hypotheses in which I proposed for the first time that low doses of anesthetic drugs including propofol promote paradoxical excitation in neurons before loss of consciousness and may potentially facilitate restoration of consciousness via cellular stress induction and AMPK activation (i.e. increases in calcium [Ca2+], reactive oxygen species [ROS]), etc.) [1,2].

As seen in the figure below, the authors showed that propofol induced an abrupt increase in Ca2+ levels and activated parabrachial nucleus (PBN) neurons in vivo just before loss of righting reflex (LORR, analogous to loss of consciousness in humans) and recovery of righting reflex (RORR, analogous to return of consciousness in humans) in rats [3]. Such evidence indicates that propofol is excitatory at low doses and promotes paradoxical excitation and possible facilitation of return of consciousness via cellular stress induction (i.e. Ca2+ and/or ROS), after the concentration of propofol decreases in the brain to a low, stimulatory level upon anesthetic removal [1,2].

Source: See Reference #3

During the initial period of general anesthesia, known as induction, a bolus dose of an anesthetic drug is administered that leads to loss of consciousness as evidenced by a lack of response to an oral command [1]. However, an intriguing phenomenon known as paradoxical excitation may also occur after initial administration of an anesthetic drug [1]. When administered at a low dose, nearly every anesthetic induces behavioral signs of neuronal activation such as eccentric body movements and a transient increase in beta activity (13–25 Hz) on the electroencephalogram (EEG) [1]. Consequently, many anesthetics appear to paradoxically excite the brain before inducing unconsciousness [1]. Anesthesiologists and neuroscientists are currently unable to explain how anesthetics are able to induce paradoxical excitation.

However, as noted in the figure above, the increase in Ca2+ levels in PBN neurons just before LORR in rats is analogous to paradoxical excitation in humans demonstrated by several anesthetics, including propofol [4]. PBN neurons have also been shown to play critical role in maintaining consciousness [9].  In addition to behavioral signs of neuronal activation, an increase in EEG beta power has also been observed during paradoxical excitation and just before return of consciousness in humans [4,5]. Because propofol increased Ca2+ levels and activated PBN neurons just before both LORR and RORR, propofol is likely excitatory at low doses and promotes paradoxical excitation and possible facilitation of return of consciousness via cellular stress induction. This notion is in line with the recent findings that lower concentrations of anesthetics are present in the brain during emergence from general anesthesia compared to the initial induction phase [6,7]. Additionally, activation of glutamatergic neurons in the PBN accelerates emergence from sevoflurane anesthesia in mice and lesions in the PBN leads to a coma-like state in rats, indicating that low doses of propofol and other anesthetics may also paradoxically facilitate return of consciousness in disorders of consciousness (e.g. coma) [8,9].

Preconditioning refers to the exposure of a cell or an organism to a mild or sublethal stressor that leads to an adaptive response and protection against a subsequent and potentially lethal application of the same or a similar stressor [1]. Interestingly, propofol and many other anesthetics used clinically act as preconditioning agents at low doses and transient increases in intracellular ROS, Ca2+, and AMPK activation exert preconditioning effects in various cell types (e.g. neurons), suggesting that paradoxical excitation is analogous to both the increase in Ca2+ in PBN neurons just before RORR and preconditioning [10-15].

Anesthetic-induced paradoxical excitation has also been demonstrated in non-mammalian organisms, with exposure of the nematode C. elegans to volatile anesthetics initially resulting in a paradoxical increase in movement, later followed by a progressive lack of coordination, immobility, and ultimately unresponsiveness [16,17]. Loss of neural AMPK (aak-2 in C. elegans) inhibits movement whereas isoflurane acts as a preconditioning agent in C. elegans [18,19]. Additionally, the anesthetic drug diethyl ether was recently shown to induce a “sedation-like” effect in plants, epitomized by a lack of response to a stimulus that normally induces movement in the Venus flytrap [20]. Preliminary data however demonstrated that the production of ROS by cold (i.e. room-temperature) plasma induced activation and trap closing of the Venus flytrap [21], suggesting that a common mechanism of cellular stress-induced AMPK activation crosses species boundaries and underlies the phenomenon of anesthetic-induced paradoxical excitation.

Anesthetics and increases in ROS have also been shown to promote seed germination (analogous to paradoxical excitation) and AMPK (SnRK1 in plants), ROS, and Ca2+ promotes pollen germination and fertilization in Arabidopsis thaliana [22-25]. Also, although they do not have a nervous system, plants produce nearly all neurotransmitters (i.e. glutamate, acetylcholine, histamine, dopamine, serotonin and norepinephrine) that are critical for maintaining consciousness in humans and biotic and abiotic stressors have been well-described to increase the production and activity of these neurotransmitters in plants [26-28]. As several neurotransmitters that play key roles in human consciousness also act as preconditioning agents (i.e. glutamate, acetylcholine, histamine, dopamine, and norepinephrine), a common mechanism of cellular stress-induced AMPK activation by neurotransmitters may have been evolutionarily conserved to promote neuronal activation in the human brain [29-33].

AMPK, known as the master regulator of cellular metabolism, increases lifespan and healthspan in several model organisms, is present throughout the mammalian brain (e.g. neurons of the thalamus, hypothalamus, striatum, hippocampus, and cortex), and is activated by cellular stress (i.e. increases in ROS, Ca2+, AMP/ATP ratio, etc.) [34-37]. AMPK is also activated by nearly all neurotransmitters that play a critical role in maintaining consciousness (glutamate, acetylcholine, histamine, orexin-A, dopamine, and norepinephrine) as well as by several anesthetic drugs that are commonly used to induce and/or maintain loss of consciousness in humans (e.g. propofol, sevoflurane, isoflurane, ketamine, dexmedetomidine, and midazolam) [38-50]. Indeed, propofol has been shown to inhibit mitochondrial electron transport chain function and increase ROS levels in human neuroblastoma SH-SY5Y cells, effects that were enhanced via the addition of the AMPK activator metformin [51]. Metformin also promotes neurogenesis in both the subventricular zone and the dentate gyrus in vitro and in vivo, potentially enhancing brain repair and recovery from disorders of consciousness (e.g. coma) [52-55].

Interestingly, as long-term potentiation (LTP) is considered the cellular correlate for learning and memory, ROS inhibition prevents LTP in vitro and AMPK knockdown severely impairs hippocampal CA1 LTP and blocks long-term memory formation in mice (as I first hypothesized), indicating that cellular stress-induced AMPK activation is not only required for learning and memory, but may also play an indispensable role in the neural correlates of consciousness [56-58]. Indeed, AMPK has recently been shown to improve postoperative cognitive dysfunction in sevoflurane-anesthetized rats [59]. Overexpression of AMPKα1 upregulated the expression of phosphorylated/activated AMPK in the hippocampus of rats anesthetized with sevoflurane and also decreased escape latencies, increased target quadrant swimming times, swimming distances, and platform crossing times during Morris water maze tests. The AMPK inhibitor compound C however abolished AMPKα1-mediated improvement of postoperative cognitive dysfunction [59].

Lastly, as noted in my most recent publication, cellular stress and AMPK activation may link human consciousness with seemingly disparate physiological and pathophysiological phenomena, including aging (metformin and AMPK alleviate accelerated aging), human reproduction (stress and AMPK are critical for oocyte maturation and sperm acrosome reaction), gene regulation (e.g. transposable elements, stress beneficially activates “jumping genes” in human cells), plasma medicine (cold plasma induces beneficial effects in cells by increasing ROS), meditation (meditation increases genes in the AMPK signaling pathway in humans), parabiosis (i.e. young blood, young plasma activates AMPK), planarian regeneration (stress and AMPK play crucial roles in regeneration of worm body parts), and stress-induced CRISPR-Cas activation in bacteria (e.g. gene editing technology, various stressors including nutrient starvation and temperature stress activate CRISPR-Cas systems in bacteria) [60-70].

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