Anesthesiology 2009; 110:709–11 Copyright © 2009, the American Society of Anesthesiologists, Inc. Lippincott Williams & Wilkins, Inc. Between Clotho and Lachesis How Isoflurane Seals Neuronal Fate “Alle Ding sind Gift, und nichts ohn Gift; allein die Dosis macht, BDNF. This hypothesis connects the physiologic mech- daß ein Ding kein Gift ist.” anisms that regulate neuronal survival with the neuro- “All things are poison and nothing is without poison, only the dose permits something not to be poisonous.” pharmacology of anesthetics. — Theophrastus Phillippus Aureolus Bombastus von Hohenheim, The developing brain is a veritable battleground be- known as Paracelsus (1493–1541) tween factors promoting neuronal survival and death. FOR more than 150 yr, general anesthetics have repre- Many more neurons and synapses are formed through- sented the quintessential triumph of medical pharmacol- out development than survive into adulthood to form Downloaded from http://pubs.asahq.org/anesthesiology/article-pdf/110/4/709/368150/0000542-200904000-00008.pdf by guest on 25 September 2021 11 14 ogy, providing painless oblivion for patients and a quiet the 10 neurons and 10 synapses in the adult human field for surgeons. But now, concerns over long-term brain. Programmed cell death (apoptosis) and synaptic sequelae in the vulnerable brain have begun to tarnish pruning are critical to plasticity and stabilization of cir- the heretofore almost spotless image of general anesthet- cuits in the developing nervous system. These are active ics. The extremely low therapeutic ratios of these pow- processes tightly controlled by complex neurotrophin erful agents (which would likely preclude regulatory signaling mechanisms to ensure normal development, approval today if not for their extreme utility) are well facilitate synaptic plasticity, and prevent neoplasia. The known; indeed their expert use and careful dosing forms four mature neurotrophins, which include BDNF, nerve the essence of the practice of anesthesiology. However, growth factor (NGF), neurotrophin-3, and neurotro- it is the reports of delayed and long-term damage to the phin-4, activate one or more of the three Trk (tropomy- developing and the aged brain that have attracted atten- osin-related kinase) transmembrane receptor tyrosine ki- tion and concern from both scientists and the public.‡ nases. Dimerization of these receptors induced by The sophisticated and timely report by Head et al. in this neurotrophin binding activates an intrinsic receptor ty- 1 issue of ANESTHESIOLOGY illuminates some the complexity rosine kinase and subsequently downstream intracellular and context-specificity of anesthetic interactions with signaling pathways that ultimately control neuronal the mammalian brain. survival, differentiation, and connectivity.5 Neuronal Head et al. investigated the interactions of isoflurane activity regulates secretion of BDNF, which influences with neuronal growth factor brain-derived neurotrophic synaptic efficacy and morphology via activation of factor (BDNF)2 and tissue plasminogen activator (tPA), a presynaptic and postsynaptic TrkB receptors. Although serine protease better known for its hematologic role in BDNF is clearly critical to normal neuronal development fibrinolysis and its therapeutic application as a thrombo- and survival, it also has a dark side. BDNF can be se- lytic agent.3 BDNF-dependent signaling had been impli- creted in its precursor form as pro-BDNF, which, far cated previously in the activation of neuroapoptosis by a from being an inactive precursor, binds to and activates triple anesthetic cocktail (nitrous oxide, midazolam, p75NTR, an alternative pan-neurotrophin receptor that isoflurane) in neonatal rats in vivo.4 Head et al. extend binds all neurotrophins with similar affinity.6 This recep- these findings by providing evidence that exposure to tor is structurally unrelated to Trk receptors, but it is a isoflurane alone reduces release of tPA from neurons and member of the tumor necrosis factor family of receptors thereby upsets the delicate balance between survival- and contains a type II intracellular death domain. Acti- promoting and death-promoting aspects of BDNF signal- vation of p75NTR by pro-BDNF initiates signaling path- ing. This effect is proposed to be critical to isoflurane- ways that can lead to trophic effects at low levels and to mediated neurotoxicity in the developing rodent brain apoptosis with more intense stimulation.7,8 Under nor- by reducing the processing of pro-BDNF to mature mal conditions, BDNF and pro-BDNF maintain a fine balance between neuronal life and death that might be 9 This Editorial View accompanies the following article: Head upset by anesthetics. ᭜ BP, Patel HH, Niesman IR, Drummond JC, Roth DM, Patel PM: Head et al. hypothesized that tPA might play the de- Inhibition of p75 neurotrophin receptor attenuates isoflurane- ciding role of the Fate Atropos of Greek mythology. In mediated neuronal apoptosis in the neonatal central nervous contrast to the shears of the most feared and powerful of system. ANESTHESIOLOGY 2009; 110:813–25. the Moirae, however, the proteolytic shears of tPA en- able survival rather than signal death. In addition to its well known role in intravascular fibrinolysis (conversion Accepted for publication December 24, 2008. The authors are not supported by, nor maintain any financial interest in, any commercial activity that may be of plasminogen to plasmin), tPA can cleave pro-neuro- associated with the topic of this editorial. trophins (including pro-BDNF) to their mature forms in ‡ Shekar C: Anesthesia: A medical mainstay re-examined. Available at: http:// the brain. By studying the effects of isoflurane exposure articles.latimes.com/2007/may/14/health/he-anesthesia14. Accessed December 30, 2008. on markers for neuronal apoptosis and dendritic spines Anesthesiology, V 110, No 4, Apr 2009 709 710 EDITORIAL VIEWS in cultures of rat hippocampal and cortical neurons, these and lithium,14 these findings provide leads for targeted investigators provide evidence that isoflurane reduces den- therapeutic approaches for the protection of the vulner- dritic spine number and increases apoptotic neuronal able rodent brain. Nevertheless, a number of crucial death after 5 days, but not after 2 weeks, in culture hurdles remain to establish the clinical significance of (roughly equivalent to 4 and 6 months gestation in humans, experimental observations relating to the neurotoxicity respectively). This is supported by ultrastructural evidence of anesthetics. First, available evidence indicates that in vivo that isoflurane exposure of 5-day-old rat pups re- isoflurane exposure (concentration times duration) must duces the number of synapses. These effects are attributed be substantial to be detrimental, and must occur within to reduced vesicular secretion of tPA, which the authors a narrow developmental window. In the current exam- speculate is the result of depressed neuronal activity. This ple, toxicity was observed in neurons from rats equiva- reduction in tPA release diminishes proteolytic processing lent to postnatal day 5 but was absent by postnatal day Downloaded from http://pubs.asahq.org/anesthesiology/article-pdf/110/4/709/368150/0000542-200904000-00008.pdf by guest on 25 September 2021 of pro-BDNF to BDNF, shifting the balance of power from 14. This narrow window of vulnerability (the exact the survival-promoting spinner of life Clotho (BDNF/TrkB timing is region-specific) correlates with lower levels signaling) to the proapoptotic life-terminating Lachesis of BDNF secretion early in development that might (pro-BDNF/p75NTR signaling). contribute to this sensitivity.15 This narrow window of This intriguing proposal is supported by complemen- susceptibility is consistent with studies of alcohol tox- tary neurobiological evidence that implicates specific icity in rats16 and ketamine toxicity in monkeys.17 elements of critical signaling pathways in neuronal sur- Translating such developmental windows between spe- vival and spinogenesis. Overall, Head et al. provide a cies is not trivial,18 so the relevance of effects observed case against isoflurane that has interesting testable impli- in altricial rodents for precocial humans is difficult to cations. Several controversial aspects of the case will extrapolate. require confirmation. For example, previous evidence Even more intriguing is the notion that the end-result of that pro-BDNF is secreted and then processed extracel- isoflurane-induced reduction of tPA release (and hence lularly was obtained from cultured cells engineered to plasmin levels) might be determined by exposure time and 10 overexpress the growth factor and so might not apply context. Increased tPA/plasmin levels have been impli- to physiologic conditions where pro-BDNF processing cated in stroke, seizure, and other ischemia/neurotoxicity takes place intracellularly and therefore would not re- models,19 and tPA contributes to increased permeability of 11 quire activity-dependent release, a key aspect of the the blood-brain barrier and hence to cerebral edema in the current model. Demonstration of isoflurane-induced ischemic brain20 such that under these pathologic inhibition of activity-dependent release of BDNF/ conditions inhibition of tPA secretion might be bene- proBDNF and/or tPA is critical to solidify support for the ficial. On the other hand, tPA and plasmin might also proposed model. Previous work has shown that triple contribute to ␤-amyloid clearance in the Alzheimer cocktail anesthesia can reduce or