Multiple sevoflurane exposures in infant monkeys do not impact the mother-infant bond

Neonatal exposures to sevoflurane in rhesus monkeys alter synaptic ultrastructure in later life

Repeated or prolonged early life exposure to anesthesia is neurotoxic in animals and associated with neurocognitive impairment in later life in humans. We used electron microscopy with unbiased stereological sampling to assess synaptic ultrastructure in dorsolateral prefrontal cortex (dlPFC) and hippocampal CA1 of female and male rhesus monkeys, four years after three 4-h exposures to sevoflurane during the first five postnatal weeks. This allowed us to ascertain long-term consequences of anesthesia exposure without confounding effects of surgery or illness. Synapse areas were reduced in the largest synapses in CA1 and dlPFC, predominantly in perforated spinous synapses in CA1 and nonperforated spinous synapses in dlPFC. Mitochondrial morphology and localization changed subtly in both areas. Synapse areas in CA1 correlated with response to a mild social stressor. Thus, exposure to anesthesia in infancy can cause long-term ultrastructural changes in primates, which may be substrates for long-term alterations in synaptic transmission and behavioral deficits.

Application of Nonhuman Primate Models in the Studies of Pediatric Anesthesia Neurotoxicity

Numerous animal models have been used to study developmental neurotoxicity associated with short-term or prolonged exposure of common general anesthetics at clinically relevant concentrations. Pediatric anesthesia models using the nonhuman primate (NHP) may more accurately reflect the human condition because of their phylogenetic similarity to humans with regard to reproduction, development, neuroanatomy, and cognition. Although they are not as widely used as other animal models, the contribution of NHP models in the study of anesthetic-induced developmental neurotoxicity has been essential. In this review, we discuss how neonatal NHP animals have been used for modeling pediatric anesthetic exposure; how NHPs have addressed key data gaps and application of the NHP model for the studies of general anesthetic-induced developmental neurotoxicity. The appropriate application and evaluation of the NHP model in the study of general anesthetic-induced developmental neurotoxicity have played a key role in enhancing the understanding and awareness of the potential neurotoxicity associated with pediatric general anesthetics.

General anesthetic neurotoxicity in the young: Mechanism and prevention

General anesthesia (GA) is usually considered to safely induce a reversible unconscious state allowing surgery to be performed without pain. A growing number of studies, in particular pre-clinical studies, however, demonstrate that general anesthetics can cause neuronal death and even long-term neurological deficits. Herein, we report our literature review and meta-analysis data of the neurological outcomes after anesthesia in the young. We also review available mechanistic and epigenetic data of GA exposure related to cognitive impairment per se and the potential preventive strategies including natural herbal compounds to attenuate those side effects. In summary, anesthetic-induced neurotoxicity may be treatable and natural herbal compounds and other medications may have great potential for such use but warrants further study before clinical applications can be initiated.

Repeated neonatal isoflurane exposures in the mouse induce apoptotic degenerative changes in the brain and relatively mild long-term behavioral deficits

Epidemiological studies suggest exposures to anesthetic agents and/or sedative drugs (AASDs) in children under three years old, or pregnant women during the third trimester, may adversely affect brain development. Evidence suggests lengthy or repeated AASD exposures are associated with increased risk of neurobehavioral deficits. Animal models have been valuable in determining the type of acute damage in the developing brain induced by AASD exposures, as well as in elucidating long-term functional consequences. Few studies examining very early exposure to AASDs suggest this may be a critical period for inducing long-term functional consequences, but the impact of repeated exposures at these ages has not yet been assessed. To address this, we exposed mouse pups to a prototypical general anesthetic, isoflurane (ISO, 1.5% for 3 hr), at three early postnatal ages (P3, P5 and P7). We quantified the acute neuroapoptotic response to a single versus repeated exposure, and found age- and brain region-specific effects. We also found that repeated early exposures to ISO induced subtle, sex-specific disruptions to activity levels, motor coordination, anxiety-related behavior and social preference. Our findings provide evidence that repeated ISO exposures may induce behavioral disturbances that are subtle in nature following early repeated exposures to a single AASD.

Visual recognition memory is impaired in rhesus monkeys repeatedly exposed to sevoflurane in infancy

Background

Experimental studies in animals have shown that exposure to general anaesthesia in infancy can cause loss of cells in the central nervous system and long-term impairments in neurocognitive function. Some human epidemiological studies have shown increased risk of learning disability after repeated anaesthesia exposure in early childhood. Thus, we investigated in a highly translational rhesus monkey model, whether repeated exposure in infancy to the inhalation anaesthetic sevoflurane is associated with impaired visual recognition memory during the first two yr of life.

Methods

Rhesus monkeys of both sexes were exposed to sevoflurane inhalation anaesthesia on approximately postnatal day 7 and then again 14 and 28 days later, for four h each time. Visual recognition memory was tested using the visual paired comparison task, which measures memory by assessing preference for looking at a new image over a previously-viewed image. Monkeys were tested at 6-10 months of age, again at 12-18 months of age, and again at 24-30 months of age.

Results

No memory impairment was detected at 6-10 months old, but significant impairment (reduced time looking at the novel image) was observed at 12-18 and 24-30 months old.

Conclusions

Repeated exposure of infant rhesus monkeys to sevoflurane results in visual recognition memory impairment that emerges after the first yr of life. This is consistent with epidemiological studies that show increased risk of learning disability after repeated exposure to anaesthesia in infancy/early childhood. Moreover, these deficits may emerge at later developmental stages, even when memory performance is unaffected earlier in development.

Neurotoxicity of anesthetic drugs: an update

PURPOSE OF REVIEW

This article reviews the most recently published evidence that investigated anesthesia-induced neurotoxicity in both animals and humans, especially as it pertains to the perinatal period.

RECENT FINDINGS

Several recent studies have focused on better understanding the complex mechanisms that underlie intravenous and volatile anesthesia-induced neurotoxicity in animals. Adjuvant agents that target these pathways have been investigated for their effectiveness in attenuating the neuroapoptosis and neurocognitive deficits that result from anesthesia exposure, including dexmedetomidine, rutin, vitamin C, tumor necrosis factor α, lithium, apocynin, carreic acid phenethyl ester. Five clinical studies, including one randomized control trial, provided inconsistent evidence on anesthesia-induced neurotoxicity in humans.

SUMMARY

Despite a growing body of preclinical studies that have demonstrated anesthesia-induced neurotoxic effects in the developing and aging brain, their effects on the human brain remains to be determined. The performance of large-scale human studies is limited by several important factors, and noninvasive biomarkers and neuroimaging modalities should be employed to define the injury phenotypes that reflect anesthesia-induced neurotoxicity. Ultimately, the use of these modalities may provide new insights into whether the concerns of anesthetics are justified in humans.

Persistent alteration in behavioural reactivity to a mild social stressor in rhesus monkeys repeatedly exposed to sevoflurane in infancy

BACKGROUND

Socio-emotional development is the expression and management of emotions, which in non-human primates can be examined using responses toward increasing levels of threat. Damage to the limbic system alters socio-emotional development in primates. Thus, neuronal and glial cell loss caused by exposure to general anaesthesia early in infancy might also impact socio-emotional development. We recently reported that repeated sevoflurane exposure in the first month of life alters emotional behaviours at 6 months of age and impairs visual recognition memory after the first year of life in rhesus monkeys. The present study evaluated socio-emotional behaviour at 1 and 2 yr of age in those same monkeys to determine the persistence of altered emotional behaviour.

METHODS

Rhesus monkeys of both sexes were exposed to sevoflurane anaesthesia three times for 4 h each time in the first 6 weeks of life. At 1 and 2 yr of age, they were tested on the human intruder task, a well-established mild acute social stressor.

RESULTS

Monkeys exposed to sevoflurane as infants exhibited normal fear and hostile responses, but exaggerated self-directed (displacement) behaviours, a general indicator of stress and anxiety in non-human primates.

CONCLUSIONS

Early repeated sevoflurane exposure in infant non-human primates results in an anxious phenotype that was first detected at 6 months, and persists for at least 2 yr of age. This is the first demonstration of such a prolonged impact of early anaesthesia exposure on emotional reactivity.

Postoperative Delirium, Learning, and Anesthetic Neurotoxicity: Some Perspectives and Directions

Evidence of anesthetic neurotoxicity is unequivocal when studied in animal models. These findings have translated poorly to the clinical domain when equated to postoperative delirium (POD) in adults and postoperative cognitive dysfunction (POCD) in either children or the elderly. In this perspective, we examine various reasons for the differences between animal modeling of neurotoxicity and the clinical situation of POD and POCD and make suggestions as to potential directions for ongoing research. We hypothesize that the animal anesthetic neurotoxicity models are limited, in part, due to failed scaling correction of physiological time. We posit that important insights into POCD in children and adults may be gleaned from studies in adults examining alterations in perioperative management designed to limit POD. In this way, POD may be more useful as the proxy for POCD rather than neuronal dropout or behavioral abnormalities that have been used in animal models but which may not be proxies for the human condition. We argue that it is time to move beyond animal models of neurotoxicity to directly examine these problems in well-conducted clinical trials with comprehensive preoperative neuropsychometric and psychiatric testing, high fidelity intraoperative monitoring of physiological parameters during the anesthetic course and postoperative assessment of subthreshold and full classification of POD. In this manner, we can “model ourselves” to better understand these important and poorly understood conditions.

Using animal models to evaluate the functional consequences of anesthesia during early neurodevelopment

Fifteen years ago Olney and colleagues began using animal models to evaluate the effects of anesthetic and sedative agents (ASAs) on neurodevelopment. The results from ongoing studies indicate that, under certain conditions, exposure to these drugs during development induces an acute elevated apoptotic neurodegenerative response in the brain and long-term functional impairments. These animal models have played a significant role in bringing attention to the possible adverse effects of exposing the developing brain to ASAs when few concerns had been raised previously in the medical community. The apoptotic degenerative response resulting from neonatal exposure to ASAs has been replicated in many studies in both rodents and non-human primates, suggesting that a similar effect may occur in humans. In both rodents and non-human primates, significantly increased levels of apoptotic degeneration are often associated with functional impairments later in life. However, behavioral deficits following developmental ASA exposure have not been consistently reported even when significantly elevated levels of apoptotic degeneration have been documented in animal models. In the present work, we review this literature and propose a rodent model for assessing potential functional deficits following neonatal ASA exposure with special reference to experimental design and procedural issues. Our intent is to improve test sensitivity and replicability for detecting subtle behavioral effects, and thus enhance the translational significance of ASA models.

Update on developmental anesthesia neurotoxicity

PURPOSE OF REVIEW

Adverse long-term impact of general anesthesia on the developing brain is a widely discussed and controversial issue with potential public health relevance. The goal of this article is to give insights into the most recent experimental and clinical observations aimed to advance our understanding in this field.

RECENT FINDINGS

Recent investigations demonstrate long-term behavioral consequences of early-life anesthesia exposure in nonhuman primates under experimental conditions that are translationally relevant to human clinical practice. Converging evidence from rodent experiments strongly suggest that anesthetics exert developmental stage-dependent and context-dependent impact on developing neuronal circuitry and, therefore, may induce lasting changes in neuronal plasticity. Although three recent population-based human studies found a strong evidence for small increase in risk, the two most robust studies (General Anaesthesia compared to Spinal anaesthesia trial and Pediatric Anesthesia Neurodevelopment Assessment) did not find an association between brief anesthesia exposure and poor neurodevelopmental outcome.

SUMMARY

Experimental data with reasonable translational relevance suggest that early-life exposure to general anesthetics can induce lasting behavioral and cognitive deficits. In contrast, human studies provide, at best, mixed evidence about developmental anesthesia neurotoxicity. Future research, both experimental and human, is needed to clarify this important issue.

Lasting impact of general anaesthesia on the brain: mechanisms and relevance

General anaesthesia is usually considered to safely induce a reversible brain state allowing the performance of surgery under optimal conditions. An increasing number of clinical and experimental observations, however, suggest that anaesthetic drugs, especially when they are administered at the extremes of age, can trigger long-term morphological and functional alterations in the brain. Here, we review available mechanistic data linking general-anaesthesia exposure to impaired cognitive performance in both young and mature nervous systems. We also provide a critical appraisal of the translational value of animal models and highlight the important challenges that need to be addressed to strengthen the link between laboratory work and clinical investigations in the field of anaesthesia-neurotoxicity research.

From the Cover: Volatile Anesthetics Transiently Disrupt Neuronal Development in Neonatal Rats

Volatile anesthetics can cause neuronal and glial toxicity in the developing mammalian brain, as well as long-term defects in learning and memory. The goals of this study were to compare anesthetics using a clinically relevant exposure paradigm, and to assess the anesthetic effects on hippocampal development and behavior. Our hypothesis was that volatile anesthetics disrupt hippocampal development, causing neurobehavioral defects later in life. Bromodeoxyuridine (BrdU) was administered to rats on postnatal day (P)1, and the rats were exposed to volatile anesthetics (isoflurane, sevoflurane, or desflurane) for 2 h on P2. On days P7 and P14, the BrdU-labeled cells were quantified in the hippocampal dentate gyrus using immunohistochemical assays and fluorescent microscopy. Caspase-3 positive cells were quantified on P2 to evaluate apoptosis. The remaining animals underwent behavioral testing at ages 6 weeks and 6 months, using the Morris Water Maze. Significantly fewer BrdU-positive cells were detected in the hippocampal dentate gyrus in both isoflurane and desflurane-treated animals compared with controls at P7, but there were no changes in cell numbers after sevoflurane exposure. Cell counts for all three anesthetics compared with controls were equivalent at P14. Isoflurane or desflurane exposure yielded slight differences in the behavioral tests at 6 weeks, but no differences at 6 months post-exposure. We conclude that a single 2-h exposure at P2 to either isoflurane or desflurane causes a transient disruption of hippocampal neuronal development with no significant detectable long-term effects on learning and memory, whereas the same exposure to sevoflurane has no effects.