Early Exposure to General Anesthesia Disrupts Spatial Organization of Presynaptic Vesicles in Nerve Terminals of the Developing Rat Subiculum

Abstract

Exposure to general anesthesia (GA) during critical stages of brain development induces widespread neuronal apoptosis and causes long-lasting behavioral deficits in numerous animal species. Although several studies have focused on the morphological fate of neurons dying acutely by GA-induced developmental neuroapoptosis, the effects of an early exposure to GA on the surviving synapses remain unclear. The aim of this study is to study whether exposure to GA disrupts the fine regulation of the dynamic spatial organization and trafficking of synaptic vesicles in presynaptic terminals. We exposed postnatal day 7 (PND7) rat pups to a clinically relevant anesthetic combination of midazolam, nitrous oxide, and isoflurane and performed a detailed ultrastructural analysis of the synaptic vesicle architecture at presynaptic terminals in the subiculum of rats at PND 12. In addition to a significant decrease in the density of presynaptic vesicles, we observed a reduction of docked vesicles, as well as a reduction of vesicles located within 100 nm from the active zone, in animals 5 days after an initial exposure to GA. We also found that the synaptic vesicles of animals exposed to GA are located more distally with respect to the plasma membrane than those of sham control animals and that the distance between presynaptic vesicles is increased in GA-exposed animals compared to sham controls. We report that exposure of immature rats to GA during critical stages of brain development causes significant disruption of the strategic topography of presynaptic vesicles within the nerve terminals of the subiculum.

Neonatal anesthesia impairs synapsin 1 and synaptotagmin 1, two key regulators of synaptic vesicle docking and fusion

“…Our study showed that exposure to anesthesia during a phase of intense synaptic development impairs mRNA and protein expression of Syn1 and Syt1, two presynaptic proteins that are critical for vesicle trafficking, docking and fusion. The results of this study are consistent with our previous ultrastructural observations of a shift in synaptic vesicle localization away from the presynaptic membrane, a reduction in the number of docked vesicles and increased inter-vesicular distance in the nerve terminals of young rats after neonatal anesthesia [8].…”

“…A previous study from our laboratory demonstrated a reduction in the number of vesicles docked at and within 100 nm from the presynaptic plasma membrane in the CA1-subiculum of anesthesia-treated rats five days after exposure [8]. Hence, the aims of this study were to investigate the effects of neonatal anesthesia on Syn 1 and Syt 1, two key regulators of synaptic vesicle trafficking, docking and fusion, and to test the link between changes in Syn 1 and Syt 1 and the learning and memory deficiencies observed after neonatal anesthesia.…”

“…P8 was chosen to evaluate the acute effects on Syn 1 and Syt 1. P12 was selected in keeping with prior work from our laboratory that documented a reduction in docked vesicles five days after neonatal anesthesia [8]. P24 was chosen to test if altered Syn 1 and Syt 1 expression persists after synaptogenesis, which is complete at three weeks of age in rats [17].…”

“…As described previously [8,9], Sprague-Dawley rats received a single intraperitoneal injection of midazolam (9 mg/kg), followed by 6 hours of nitrous oxide 70%, isoflurane 0.75% and oxygen 30% at the peak of synaptogenesis (P7) [2,8,9]. Controls received an equal volume of intraperitoneal vehicle (0.1% dimethyl sulfoxide), and were kept in a separate chamber at room air for 6 hours.…”

Neuroprotection of the Developing Brain by Dexmedetomidine Is Mediated by Attenuating Single Propofol-induced Hippocampal Apoptosis and Synaptic Plasticity Deficits

“…The discrepant reason may be due to their different drug regimen, which somehow decreased number of neurocytes, self-renewal and differentiation capacity of neural stem cells during the period of mature brains. On the one hand, it is acknowledged that the anesthetic mechanism of propofol in the developing CNS relates to multiple biological changes, including inhibition of network connections, inhibition of synaptic transmission efficiency and depression of neuronal activity [12]. On the other hand, the Schaffer collateral CA1 pathway is not the only pathway involved in LTP, thus there may be additional connections that could represent an alternative mechanism for long-term cognition alleviation.…”