Selectivity Of Anesthesia-induced Neuroapoptosis

Background:General anaesthesia facilitates surgical operations and painful interventions in millions of patients every year. Recent observations found that exposure to anaesthetics dramatically increases the rate of neuroapoptosis in developing animals and subsequent neurocognitive impairment. Also epidemiological human studies have associated early childhood anesthesia with long-term neurobehavioral abnormalities, raising substantial concerns that anesthetics may cause similar cell death in young children.However, it remained unresolved why some brain regions are more affected than others, why certain neurons are eliminated while neighbouring cells are seemingly unaffected, and what renders the developing brain exquisitely vulnerable, while the adult brain remained apparently resistant to the phenomenon.Importantly, although the phenomenon’s mechanism is unknown, but understanding the selectivity of cellular loss following anesthetic exposure may be important in elucidating the phenomenon’s mechanism.Methods:Part Ⅰ:Neonatal (P7), juvenile (P21), and young adult mice (P49) were anaesthetised with 1.5% isoflurane arid activated, cleaved caspase-3 (AC3), a marker of apoptotic cell death, was quantified in neocortex (RSA), caudoputamen (CPu), hippocampal CA1 and dentate gyrus (DG), cerebellum (Cb), and olfactory bulb (GrO) at the conclusion of anaesthesia and compared with unanaesthetised littermates.Part II:Juvenile (P21) mice were anaesthetised with 1.5% isoflurane for 6 hours. During neuronal maturation, DGCs transiently express (1) Sox2 as radial glia like progenitors, (2) NeuroDl as late progenitors,(3) calretinin as immature granule cells, and (4) calbindin as mature granule cells. We use birth-dating (BrdU Injections) and immunohistochemical techniques to characterize a particularly vulnerable neuronal age to anesthetic toxicity (neuroapoptosis) in dentate granule cells (DGCs) of P21 mice.Results:Part Ⅰ:Following anaesthetic exposure, increased AC3 was detected in neonatal mice in RSA (11-fold), CPu (10-fold), CA1 (3-fold), Cb (4-fold), and GrO (4-fold). Surprisingly, AC3 continued to be elevated in DG and GrO of juvenile (15- and 12-fold, respectively) and young adult mice (2-and 4-fold, respectively).Part II:BrdU birth-dating of affected neurons confirmed their immature state, indicating that cells 13-15 days old were most vulnerable.Apoptotic neurons in dentate gyrus of P21 mice were most frequently identified as late progenitors (NeuroD1+) and immature granule cells (calretinin+). However, at earlier stages, such as Sox2-positive, radial glialike progenitors (type 1) or more mature calbindin-expressing granule cells, were not substantially affected.Conclusions:The present study confirms previous results of peak vulnerability to anaesthesia-induced neuronal cell death in the newborn forebrain,but discovers sustained susceptibility into adulthood in areas of continued neurogenesis, substantially expanding the previously observed age of vulnerability.The differential windows of vulnerability among brain regions, which closely follow 1-2 weeks after regional peaks in neurogenesis, may explain the heightened vulnerability of the developing brain due to its increased number of immature neurons.Even further, we identify a critical period of cellular developmental during which neurons are susceptible to anesthesia induced apoptosis. These results suggest that vulnerability to anesthetic neurotoxicity is determined by neuronal age and maturational stage, not simply by the age of the organism.