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

Abstract

Early-life exposure to anesthesia in infant humans and monkeys increases the risk for cognitive and socioemotional impairments. However, the long-term effects of neonatal anesthesia on synaptic ultrastructure have not been thoroughly investigated in primates. We used electron microscopy with unbiased stereological sampling to assess synaptic ultrastructure in the CA1 of the hippocampus and the dorsolateral prefrontal cortex (dlPFC) of female and male rhesus macaques four years after three 4-hour exposures to sevoflurane during the first five postnatal weeks. We counted synapses and measured synapse areas for all synapses and those classified as perforated or nonperforated with spine or dendritic shaft targets. We measured numbers and shapes of mitochondria within presynaptic boutons and calculated vesicle docking rates. In monkeys exposed to anesthesia as infants, synapse areas were reduced in the largest 20% of synapses in CA1 and the largest 5% of synapses in dlPFC, with differential sex effects for the largest 10% of synapses in CA1. Synapse areas were reduced by 7.6% for perforated spinous synapses in CA1, and by 10.4% for nonperforated spinous synapses in dlPFC. Perforated and nonperforated dendritic synapse numbers in CA1 increased by 180% and 63% respectively. Curved mitochondria decreased 25% in CA1 after anesthesia exposure, and dlPFC boutons with 0 mitochondria showed an interaction of anesthesia and sex. These results demonstrate that exposure to anesthesia in infancy can cause long-term structural deficits in primates. These structural changes may be substrates for long-term alterations in the strength and efficiency of synaptic transmission in hippocampus and prefrontal cortex.Key pointsExposure to anesthesia in early life causes lasting cognitive and socioemotional impairments in human and nonhuman primates, but the extent to which early-life exposure to anesthesia alters synaptic ultrastructure in later life has not been known.Four years after rhesus monkeys were given multiple exposures to anesthesia in infancy, the area of spinous synapses was reduced in CA1 and dlPFC, dendritic synapse numbers were elevated in CA1, there were fewer curved presynaptic mitochondria in CA1, and numbers of presynaptic boutons without mitochondria were altered in dlPFC.The long-term ultrastructural changes to synapses and presynaptic mitochondria of rhesus monkeys that were exposed to anesthesia as infants could help explain their behavioral deficits later in life.

Violation of the ultrastructural size principle in the dorsolateral prefrontal cortex underlies working memory impairment in the aged common marmoset (Callithrix jacchus)

“…The image stacks were imported into Volume Annotation and Segmentation Tool (VAST Lite) (Berger et al, 2018) for three-dimensional reconstruction of presynaptic boutons and 1 https://ieeexplore.ieee.org/document/8010595 their mitochondria, and the associated synapse surface area. We employed similar sampling methods to prior work in macaque monkeys (Hara et al, 2011(Hara et al, , 2014(Hara et al, , 2016Fehr et al, 2022). The experimenter performing the segmentations was blind to the subject-identifying information of the images.…”

Violation of the ultrastructural size principle in the dorsolateral prefrontal cortex underlies working memory impairment in the aged common marmoset (Callithrix jacchus)

“…The image stacks were imported into Volume Annotation and Segmentation Tool (VAST Lite) (Berger et al, 2018) for three-dimensional reconstruction of presynaptic boutons and their mitochondria, and the associated synapse surface area. We employed similar sampling methods to prior work in macaque monkeys (Hara et al, 2011(Hara et al, , 2014(Hara et al, , 2016Fehr et al, 2022). The experimenter performing the segmentations was blind to the subject-identifying information of the images.…”

An Update on Preclinical Research in Anesthetic-Induced Developmental Neurotoxicity in Nonhuman Primate and Rodent Models

“…Importantly, the amygdala is a major brain area for processing socioemotional behaviors, which are functions that have repeatedly been found to be affected by neonatal anesthesia exposure 9–12. Electron microscopic analyses of NHP brain tissue after neonatal exposure to sevoflurane suggest that early-in-life exposure causes alterations in the synaptic ultrastructure later in life 13. Four years after multiple 4-hour exposures to sevoflurane during infancy, the brains of exposed animals showed a significant reduction in synapse areas in the CA1 area of the hippocampus, while the synapse density was unaffected.…”