Although intravenous anesthetic agents are typically considered as safe to be administered during pediatric surgery, preclinical and clinical evidence has recently emerged regarding their potential neurotoxicity. Several studies have demonstrated that anesthetic exposure in early age may lead to long-term cognitive impairment as well as learning deficits. The United States Food and Drug Administration has raised the concern of pediatric anesthetic neurotoxicity as a major public health issue, and toward that direction, the Smart-Tots initiative has been carried out. Moreover, a number of clinical studies have been performed in recent years, and symposia are now assessing both the preclinical and clinical data on the potential correlation between anesthetic exposure and developmental neurocognitive impairment. A central role in the ongoing debate on the potential developmental neurotoxicity of anesthetic agents is played by ketamine [Figure 1a], a N-methyl-D-aspartate (NMDA) receptor antagonist that is widely used in the pediatric anesthesia practice and in sub-anesthetic doses for sedation during diagnostic procedures.
Most of the currently available clinical evidence is derived from retrospective and observational clinical studies, and thus, very little can be concluded from them with regard to the mechanisms involved. We, herein, attempt a brief review of the cellular and molecular mechanisms suggested to mediate ketamine-induced developmental neurotoxicity, utilizing a selected number of recent in vivo experimental evidence [Table 1]. Nevertheless, the translational value of the preclinical data discussed in this review is interpreted with caution.
Main pathways of ketamine-induced developmental neurotoxicity
Neuroapoptosis, a major consequence of ketamine's developmental toxicity and one of the first to be reported, is now known to be not only dose-dependent but also time-evolving and dependent on the exposure time-window. Caspase-3 protein level increase as well as the induction of neuroapoptosis seems to be “hallmarks” of ketamine-induced developmental neurotoxicity in both rodents and non-human primates. Recent in vivo experimental evidence suggests that neuroapoptosis is only an aspect of a more complex pathophysiological cascade involved in ketamine-induced developmental neurotoxicity [Figure 1b]. Specifically, the deregulation of the NMDA receptors' expression (overexpression) and the induction of oxidative stress as a result of increased cellular susceptibility to glutamate (Glu) and calcium (Ca2+) mobilization are evident and/or implied by a number of studies in rodents.
Deregulation of NMDA receptors' expression
As the antagonistic action on the NMDA receptor is one of the main mechanisms of the anesthetic and analgesic effect of ketamine, it comes as no surprise that a study performed on Sprague-Dawley rats has revealed a major and fulminant ketamine-induced upregulation of NMDA receptor subunit NR1 in the PND7 frontal cortex. An earlier study investigating gene expression profiling in frontal cortical areas of age-matched (PND7) Sprague-Dawley rats that received ketamine, identified perturbations and confirmed an upregulation of NMDA receptors.
The deregulation of the expression of the NMDA receptors contributes to the neuronal susceptibility to the excitotoxic effects of Glu after the clearance of ketamine, leading to a major deregulation of the neuronal Ca2+-signaling, and to the generation of oxidative stress. Moreover, due to the fact that Glu is an established regulator of neural progenitor cell (NPC) differentiation, and as NMDA receptors are considered to promote neuronal differentiation (through the overexpression of NeuroD as a result of neuronal excitation), premature neuronal differentiation becomes an additional consequence of the exposure to ketamine during neurodevelopment [Figure 1b].
Mitochondrial dysfunction and oxidative stress/mitochondrial apoptotic pathway
The deregulation of the neuronal Ca2+ signaling as a result of the increased susceptibility to the excitotoxic effects of Glu has been reported to provoke mitochondrial dysfunction and the generation of oxidative stress in the hippocampi of rats exposed to ketamine during neurodevelopment. Mitochondrial dysfunction in ketamine-exposed rat brains has been associated with a downregulation of critical components of the extracellular signal regulated kinase (ERK) signaling cascade, implying a decreased capacity to perform critical gene transcription and translation. In the hippocampus, the latter could result to an impairment of synaptic consolidation and to a difficulty in the maintenance of long-term potentiation. Also, triggering of the mitochondrial apoptotic pathway has been reported to evolve autophagy and caspase-1-dependent pyroptosis in rodents.
Deregulation of neurogenesis through premature neuronal differentiation
An experimental study on transgenic zebrafish embryos has put forward a new candidate pathway of ketamine-induced developmental neurotoxicity through the manipulation of differentiating and differentiated neurons [Figure 1c]. More specifically, zebrafish embryos were exposed to 0.5 and 2 mM of ketamine for 2 or 20 h; when administered for 20 h, ketamine at 2 mM was found not only to decrease cranial and motor neuron populations, and the axon length of the latter, but also to: (i) suppress the expression of the Notch 1 α gene, (ii) downregulate the expression of the motor neuron-inducing NeuroD and Gli2b, and (iii) upregulate the expression of Ngn1.
The reported ketamine-induced downregulation of Notch 1 α is expected to affect negatively the ligand-dependent Notch signaling in the proneural domain. The latter inhibition would upregulate Ngn1 in the NPCs and decrease the possibility of neuronal survival in differentiated neurons. In the first case, the ketamine-induced upregulation of the Ngn1 expression could lead to an upregulation of NeuroD expression, leading to premature neuronal differentiation. A downregulation of the NeuroD expression has been reported and it was suggested to be a result of fewer surviving differentiated neurons as a result of the exposure to ketamine [Figure 1c].
This—yet to be confirmed in mammals—mode of ketamine-induced developmental neurotoxicity could explain the findings of Aligny et al. on FVB-Tg(GadGFP) 45704Swn transgenic mice, where both the migration and the cytomorphology of GABAergic interneurons in the cortical layers II-IV were significantly affected by maternal exposure to ketamine from GD15 to GD20. It could also account for the dose-dependent inhibition of cell proliferation in critical rat neurogenic regions such as the ventricular and the subventricular zones by a single intraperitoneal injection of ketamine on GD 17.
Other findings of interest
Interestingly, the reduction of parvalbumin-expressing interneurons in the adult murine medial prefrontal cortex as a result of exposure to ketamine during the PND7 to PND11 time-window seems to be compatible with the expression of a phenotype that could act as a model for the experimental simulation of the “cognitive and negative symptoms of schizophrenia”. Moreover, a critical and noteworthy study for the understanding of the role of the hippocampus in the ketamine-induced developmental neurotoxicity, with important leads regarding neuronal migration and glial growth, has been performed by Huang et al. In that well-designed study on Sprague-Dawley rats, ketamine-exposed rats on PND7 demonstrated a transient disruption of their neural stem cell proliferation and differentiation, and an inhibition of neuronal migration and in the granule cell layer of the hippocampal dentate gyrus upon reaching PND37 and PND44, which were accompanied by reduced growth of astrocytes in the hippocampal dentate gyrus.
The “bigger picture” and translational perspectives
Despite the progress recorded regarding the understanding of the neurodevelopmental toxicity of ketamine, the clinical translation of the aforementioned experimental findings should be done with caution and only after considering that: (i) in clinical pediatric or obstetric practice, ketamine is rarely used as a stand-alone anesthetic agent, (ii) sex-dependent differences with regards to the developmental neurotoxicity of ketamine seem to exist, and (iii) in vivo experimental studies involving a maternal exposure to ketamine rarely provide details of the maternal hemodynamic and respiratory stability; the latter being a critical interfering factor for the reliability of that type of study. The pathways presented in this review seem to form a bigger picture in which the extent and the nature of the neuronal susceptibility to ketamine during neurodevelopment is strongly dependent on the experimental conditions employed.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
1. Flick RP, Katusic SK, Colligan RC, Wilder RT, Voigt RG, Olson MD, et al Cognitive and behavioral outcomes after early exposure to anesthesia and surgery Pediatrics. 2011;128:e1053–61
2. Ikonomidou C, Bosch F, Miksa M, Bittigau P, Vöckler J, Dikranian K, et al Blockade of NMDA receptors and apoptotic neurodegeneration in the developing brain Science. 1999;283:70–4
3. Vutskits L, Davidson A. Update on developmental anesthesia neurotoxicity Curr Opin Anaesthesiol. 2017;30:337–42
4. Walters JL, Paule MG. Review of preclinical studies on pediatric general anesthesia-induced developmental neurotoxicity Neurotoxicol Teratol. 2017;60:2–23
5. Kuehn BM. FDA considers data on potential risks of anesthesia use in infants, children JAMA. 2011;305:1749–50, 1753
6. Ramsay JG, Rappaport BA. SmartTots: A multidisciplinary effort to determine anesthetic safety in young children Anesth Analg. 2011;113:963–4
7. Ramsay JG, Roizen M. SmartTots: A public-private partnership between the United States Food and Drug Administration (FDA) and the International Anesthesia Research Society (IARS) Paediatr Anaesth. 2012;22:969–72
8. Davidson AJ, Disma N, de Graaff JC, Withington DE, Dorris L, Bell G, et al Neurodevelopmental outcome at 2 years of age after general anaesthesia and awake-regional anaesthesia in infancy (GAS): An international multicentre, randomised controlled trial Lancet. 2016;387:239–50
9. Gleich SJ, Flick R, Hu D, Zaccariello MJ, Colligan RC, Katusic SK, et al Neurodevelopment of children exposed to anesthesia: Design of the Mayo Anesthesia Safety in Kids (MASK) study Contemp Clin Trials. 2015;41:45–54
10. Lee JJ, Sun LS, Levy RJ. Report on the Sixth Pediatric Anesthesia Neurodevelopmental Assessment (PANDA) Symposium, “Anesthesia and Neurodevelopment in Children” J Neurosurg Anesthesiol. 2019;31:103–7
11. Aligny C, Roux C, Dourmap N, Ramdani Y, Do-Rego JC, Jégou S, et al Ketamine alters cortical integration of GABAergic interneurons and induces long-term sex-dependent impairments in transgenic Gad67-GFP mice Cell Death Dis. 2014;5:e1311
12. Brambrink AM, Evers AS, Avidan MS, Farber NB, Smith DJ, Martin LD, et al Ketamine-induced neuroapoptosis in the fetal and neonatal rhesus macaque brain Anesthesiology. 2012;116:372–84
13. Dong C, Rovnaghi CR, Anand KJ. Ketamine exposure during embryogenesis inhibits cellular proliferation in rat fetal cortical neurogenic regions Acta Anaesthesiol Scand. 2016;60:579–87
14. Huang H, Liu CM, Sun J, Hao T, Xu CM, Wang D, et al Ketamine affects the neurogenesis of the hippocampal dentate gyrus in 7-day-old rats Neurotox Res. 2016;30:185–98
15. Huang L, Liu Y, Jin W, Ji X, Dong Z. Ketamine potentiates hippocampal neurodegeneration and persistent learning and memory impairment through the PKCγ-ERK signaling pathway in the developing brain Brain Res. 2012;1476:164–71
16. Jeevakumar V, Driskill C, Paine A, Sobhanian M, Vakil H, Morris B, et al Ketamine administration during the second postnatal week induces enduring schizophrenia-like behavioral symptoms and reduces parvalbumin expression in the medial prefrontal cortex of adult mice Behav Brain Res. 2015;282:165–75
17. Kanungo J, Cuevas E, Ali SF, Paule MG. Ketamine induces motor neuron toxicity and alters neurogenic and proneural gene expression in zebrafish J Appl Toxicol. 2013;33:410–7
18. Li X, Guo C, Li Y, Li L, Wang Y, Zhang Y, et al Ketamine administered pregnant rats impair learning and memory in offspring via the CREB pathway Oncotarget. 2017;8:32433–49
19. Li Y, Li X, Zhao J, Li L, Wang Y, Zhang Y, et al Midazolam attenuates autophagy and apoptosis caused by ketamine by decreasing reactive oxygen species in the hippocampus of fetal rats Neuroscience. 2018;388:460–71
20. Liu F, Paule MG, Ali S, Wang C. Ketamine-induced neurotoxicity and changes in gene expression in the developing rat brain Curr Neuropharmacol. 2011;9:256–61
21. Shi Q, Guo L, Patterson TA, Dial S, Li Q, Sadovova N, et al Gene expression profiling in the developing rat brain exposed to ketamine Neuroscience. 2010;166:852–63
22. Yan J, Huang Y, Lu Y, Chen J, Jiang H. Repeated administration of ketamine can induce hippocampal neurodegeneration and long-term cognitive impairment via the ROS/HIF-1α pathway in developing rats Cell Physiol Biochem. 2014;33:1715–32
23. Ye Z, Li Q, Guo Q, Xiong Y, Guo D, Yang H, et al Ketamine induces hippocampal apoptosis through a mechanism associated with the caspase-1 dependent pyroptosis Neuropharmacology. 2018;128:63–75
24. Zhao T, Li C, Wei W, Zhang H, Ma D, Song X, et al Prenatal ketamine exposure causes abnormal development of prefrontal cortex in rat Sci Rep. 2016;6:26865
25. Disma N, Hansen TG. Pediatric anesthesia and neurotoxicity: Can findings be translated from animals to humans? Minerva Anestesiol. 2016;82:791–6
26. Dong C, Anand KJ. Developmental neurotoxicity of ketamine in pediatric clinical use Toxicol Lett. 2013;220:53–60
27. Semple BD, Blomgren K, Gimlin K, Ferriero DM, Noble-Haeusslein LJ. Brain development in rodents and humans: Identifying benchmarks of maturation and vulnerability to injury across species Prog Neurobiol. 2013;106-107:1–16
28. Zanghi CN, Jevtovic-Todorovic V. A holistic approach to anesthesia-induced neurotoxicity and its implications for future mechanistic studies Neurotoxicol Teratol. 2017;60:24–32
29. Cheung HM, Yew DTW. Effects of perinatal exposure to ketamine on the developing brain Front Neurosci. 2019;13:138
30. Zarros A, Byrne AM, Boomkamp SD, Tsakiris S, Baillie GS. Lanthanum-induced neurotoxicity: Solving the riddle of its involvement in cognitive impairment? Arch Toxicol. 2013;87:2031–5