The use of ketamine for anesthesia during neurodevelopment caused long-lasting changes in reward behavior in male mice

 https://doi.org/10.1038/s42003-022-03667-4 

The possibility of anesthesia-induced neurotoxicity has received significant research interest in recent decades,¹ as millions of children receive anesthesia for surgical or diagnostic procedures every year. Preclinical studies have shown that anesthetics commonly used in clinical settings may act as neurotoxins during neurodevelopment, inducing widespread neuronal cell death or excitatory/inhibitory (E/I) imbalance. The disruption of E/I balance during the critical neurodevelopmental period is of considerable concern since E/I imbalance is an important mechanism for neurodevelopmental disorders. Based on preclinical studies, clinical studies have attempted to identify long-term negative effects of anesthesia in young children. While recent prospective clinical studies suggested that a short anesthetic exposure does not affect general intelligence, several studies also reported long-term behavioral changes in specific behaviors.^{2, 3} However, it is important to note that anesthesia-induced neurotoxicity was first identified in animal models without direct clinical evidence, making it difficult to speculate how this might manifest in children. Although recent studies have acknowledged this fact and performed a wide range of tests to evaluate diverse cognitive and behavioral aspects, many types of behavior remain unstudied.

Previous studies have shown that changes in E/I synaptic transmission are deeply involved with the development of addiction.^{4, 5} Thus, it is possible that the synaptic changes due to early anesthesia exposures may also affect addiction behavior. Ketamine, an intravenous anesthetic agent, is often used for sedation or anesthesia during short procedures in pediatric patients due to its’ lack of respiratory depression and excellent analgesic effects. However, unlike other anesthetic agents, ketamine is also classified as a ‘dissociative anesthetic’. By disconnecting the thalamo-neocortical and limbic systems, ketamine can induce hallucinations and altered sensory states. As a result of these attributes, ketamine has come to be used as a recreational drug at sub-anesthetic doses, leading to drug abuse. Since ketamine exposures during neurodevelopment induce E/I imbalance, and ketamine itself has addictive properties, it is possible that repeated ketamine anesthesia during neurodevelopment might affect addiction behavior later in life.

To test our hypothesis, postnatal day 16 (PND16) received multiple anesthetic treatments with ketamine (35 mg·kg^-1, i.p.) for five consecutive days. This specific age was used as it may be comparable to a 6 to 9 months-old human infant.^{6, 7} Long-term changes in addiction-related behaviors were evaluated by measuring behavioral sensitization and conditioned place-preference (CPP) to low-dose ketamine and nicotine in adolescent (PND28) and adult (8 weeks old) mice. Although there was no change in behavioral sensitization, we discovered significant increases in place preference for both ketamine and nicotine in male mice who had received early ketamine anesthesia. As the CPP test is an animal model of drug reward, our results suggest that exposures to ketamine during neurodevelopment can cause long-lasting changes in addiction-associated behaviors. To further investigate whether long-term changes in E/I balance were involved with such behavioral changes, we also performed transcriptome analysis and whole-cell recordings in the hippocampus, a brain region involved with CPP. We discovered increased expression of genes encoding axon-localized proteins that regulate neuronal excitability. Such changes were associated with increased intrinsic excitability, spontaneous synaptic transmission, and number of spontaneous action potentials in hippocampal pyramidal neurons. Our results are in line with our hypothesis, as multiple ketamine exposures induced E/I imbalance and increased drug-reward behavior later in life. Most interestingly, we found that the long-term effects of ketamine anesthesia only occurred in male mice, and that female mice were resistant to the changes.

Our results provide several important insights regarding anesthesia-induced neurotoxicity. Not only does our data suggest that multiple ketamine anesthesia may affect abusive drug use later in life, but also implies that future studies must recognize that early anesthesia may affect a wide range of behaviors. Secondly, our results confirm that sex must be considered an important biological variable when studying the changes induced by early anesthesia.

1. Andropoulos DB, Greene MF. Anesthesia and Developing Brains - Implications of the FDA Warning. The New England journal of medicine 376, 905-907 (2017).
2. Walkden GJ, Gill H, Davies NM, Peters AE, Wright I, Pickering AE. Early Childhood General Anesthesia and Neurodevelopmental Outcomes in the Avon Longitudinal Study of Parents and Children Birth Cohort. Anesthesiology 133, 1007-1020 (2020).
3. Ing C, et al. Prospectively assessed neurodevelopmental outcomes in studies of anaesthetic neurotoxicity in children: a systematic review and meta-analysis. British journal of anaesthesia 126, 433-444 (2021).
4. Lüscher C, Malenka RC. Drug-evoked synaptic plasticity in addiction: from molecular changes to circuit remodeling. Neuron 69, 650-663 (2011).
5. van Huijstee AN, Mansvelder HD. Glutamatergic synaptic plasticity in the mesocorticolimbic system in addiction. Frontiers in cellular neuroscience 8, 466 (2014).
6. Workman AD, Charvet CJ, Clancy B, Darlington RB, Finlay BL. Modeling transformations of neurodevelopmental sequences across mammalian species. The Journal of neuroscience : the official journal of the Society for Neuroscience 33, 7368-7383 (2013).
7. Charvet CJ. Closing the gap from transcription to the structural connectome enhances the study of connections in the human brain. Dev Dyn 249, 1047-1061 (2020).