Effects of Early Exposure of Isoflurane on Chronic Pain via the Mammalian Target of Rapamycin Signal Pathway

Early Postnatal Exposure to Midazolam Causes Lasting Histological and Neurobehavioral Deficits via Activation of the mTOR Pathway

Exposure to general anesthetics can adversely affect brain development, but there is little study of sedative agents used in intensive care that act via similar pharmacologic mechanisms. Using quantitative immunohistochemistry and neurobehavioral testing and an established protocol for murine sedation, we tested the hypothesis that lengthy, repetitive exposure to midazolam, a commonly used sedative in pediatric intensive care, interferes with neuronal development and subsequent cognitive function via actions on the mechanistic target of rapamycin (mTOR) pathway. We found that mice in the midazolam sedation group exhibited a chronic, significant increase in the expression of mTOR activity pathway markers in comparison to controls. Furthermore, both neurobehavioral outcomes, deficits in Y-maze and fear-conditioning performance, and neuropathologic effects of midazolam sedation exposure, including disrupted dendritic arborization and synaptogenesis, were ameliorated via treatment with rapamycin, a pharmacologic mTOR pathway inhibitor. We conclude that prolonged, repetitive exposure to midazolam sedation interferes with the development of neural circuitry via a pathologic increase in mTOR pathway signaling during brain development that has lasting consequences for both brain structure and function.

Effects of Early Exposure to Isoflurane on Susceptibility to Chronic Pain Are Mediated by Increased Neural Activity Due to Actions of the Mammalian Target of the Rapamycin Pathway

Patients who have undergone surgery in early life may be at elevated risk for suffering neuropathic pain in later life. The risk factors for this susceptibility are not fully understood. Here, we used a mouse chronic pain model to test the hypothesis that early exposure to the general anesthetic (GA) Isoflurane causes cellular and molecular alterations in dorsal spinal cord (DSC) and dorsal root ganglion (DRG) that produces a predisposition to neuropathic pain via an upregulation of the mammalian target of the rapamycin (mTOR) signaling pathway. Mice were exposed to isoflurane at postnatal day 7 (P7) and underwent spared nerve injury at P28 which causes chronic pain. Selected groups were treated with rapamycin, an mTOR inhibitor, for eight weeks. Behavioral tests showed that early isoflurane exposure enhanced susceptibility to chronic pain, and rapamycin treatment improved outcomes. Immunohistochemistry, Western blotting, and q-PCR indicated that isoflurane upregulated mTOR expression and neural activity in DSC and DRG. Accompanying upregulation of mTOR and rapamycin-reversible changes in chronic pain-associated markers, including N-cadherin, cAMP response element-binding protein (CREB), purinergic P2Y12 receptor, glial fibrillary acidic protein (GFAP) in DSC; and connexin 43, phospho-extracellular signal-regulated kinase (p-ERK), GFAP, Iba1 in DRG, were observed. We concluded that early GA exposure, at least with isoflurane, alters the development of pain circuits such that mice are subsequently more vulnerable to chronic neuropathic pain states.

mTOR-mediated autophagy in the hippocampus is involved in perioperative neurocognitive disorders in diabetic rats

Introduction

Perioperative neurocognitive disorders (PND) are common neurological complications after surgery. Diabetes mellitus (DM) has been reported to be an independent risk factor for PND, but little is known about its mechanism of action. Mammalian target of rapamycin (mTOR) signaling is crucial for neuronal growth, development, apoptosis, and autophagy, but the dysregulation of mTOR signaling leads to neurological disorders. The present study investigated whether rapamycin can attenuate PND by inhibiting mTOR and activating autophagy in diabetic rats.

Methods

Male diabetic Sprague‐Dawley rats underwent tibial fracture surgery under isoflurane anesthesia to establish a PND model. Cognitive functions were examined using the Morris water maze test. The levels of phosphorylated mTOR (p‐mTOR), phosphorylated tau (p‐tau), autophagy‐related proteins (Beclin‐1, LC3), and apoptosis‐related proteins (Bax, Bcl‐2, cleaved caspase‐3) in the hippocampus were examined on postoperative days 3, 7, and 14 by Western blot. Hippocampal amyloid β (Aβ) levels were examined by immunohistochemistry.

Results

The data showed that surgical trauma and/or DM impaired cognitive function, induced mTOR activation, and decreased Beclin‐1 levels and the LC3‐II/I ratio. The levels of Aβ and p‐tau and the hippocampal apoptotic responses were significantly higher in diabetic or surgery‐treated rats than in control rats and were further increased in diabetic rats subjected to surgery. Pretreatment of rats with rapamycin inhibited mTOR hyperactivation and restored autophagic function, effectively decreasing tau hyperphosphorylation, Aβ deposition, and apoptosis in the hippocampus. Furthermore, surgical trauma‐induced neurocognitive disorders were also reversed by pretreatment of diabetic rats with rapamycin.

Conclusion

The results demonstrate that mTOR hyperactivation regulates autophagy, playing a critical role in the mechanism underlying PND, and reveal that the modulation of mTOR signaling could be a promising therapeutic strategy for PND in patients with diabetes.

A novel social fear conditioning procedure alters social behavior and mTOR signaling in differentially housed adolescent rats

Vulnerabilities to fear-related disorders can be enhanced following early life adversity. This study sought to determine whether post-weaning social isolation (PSI), an animal model of early life adversity, alters the development of social fear in an innovative model of conditioned social fear. Male and female Sprague-Dawley rats underwent either social rearing (SR) or PSI for 4 weeks following weaning. Rats were then assigned to groups consisting of either Footshock only, Social conditioned stimulus (CS) only, or Paired footshock with a social CS. Social behavior was assessed the next day. We observed a novel behavioral response in PSI rats, running in circles, that was rarely observed in SR rats; moreover, this behavior was augmented after Paired treatment in PSI rats. Other social behaviors were altered by both PSI and Paired footshock and social CS. The mammalian target of rapamycin (mTOR) pathway was assessed using immunohistochemistry for phosphorylated ribosomal protein S6 (pS6) in subregions of the prefrontal cortex (PFC) and amygdala. Paired treatment produced opposite effects in the PFC and amygdala in males, but no differences were observed in females. Conditioned social fear produced alterations in social behavior and the mTOR pathway that are dependent upon rearing condition and sex.