The Long-Lasting Effect of Neonatal Repeated Sevoflurane Anesthesia on the Synaptic Plasticity in the Rat Hippocampal CA1 Field

Objective:In the present study,from the functional perspective,the effect of neonatal repeated sevofl urane anesthesia on the synaptic plasticity at juvenile and adult age was observed in the SD rat model. Also,the related mechanism was probed in depth.Methods:1.Grouping and establishing models for repeated inhaled anesthesia:32 healthy P7 Sprague-Dawley(SD) rats,weight 15~19 g, female and male either,were assigned into experimental group(group Sev) and control group(group C) (n=16). Each group was divided into 2 subgroups:juvenile group(P37) and adult group(P97). During juvenile stage,group Sev inhaled 2.6% sevofl urane and carrier gas(1 L·min-1 Air+1 L·min-1 O2) for 2 h at day 7,14 and 21 repeatedly,while group C inhaled carrier gas merely in the same time points. 2. Electrophysiological experiment:On postnatal day 37(P37) and P97,8 rats were extracted from each groups respectively for electrophysiological recordingin vivo.①The tip of recording electrode located in the pyramidal cell body layer of CA1,the bipolar stimulating electrode was inserted into the ipsilateral Schaffer collaterals.②A test stimulation with duration of 0.2 ms and intensity of 3~6 mA was given every 10 s to elicit the adequate wave shape. 50% of intensity to elicit maximum amplitude of population spike was regarded as intensity of basic stimulation. The response to basic stimulation was recorded for 30 min stably.③5 min power spectrum analysis was implemented before basal stimulation, before HFS and after the whole recording to maintain the similar anesthetic level of each sample in the experiment.④After 30 min steady basal stimulus,paired stimulation with different interstimulus interval of 25,50,75,100,150,200 and 250 ms were implemented. High frequency stimulation(HFS) was performed and thereafter shifted into basal stimulation for subsequent 90 min recording.⑤The population spike amplitude(PSA),slope of field excitatory postsynaptic potential(fEPSP) and the response to paired stimulation with different interstimulus interval were recorded at several time points before and after HFS on both P37 and P97. The discrepancy of above-mentioned index between P37 and P97 were compared as well.Results:1. During neonatal anesthesia,rats in both groups breathed steadly,with ruddy lips,tail and skin and without cyanosis. All rats waked up spontaneously after anesthesia without death induced by anesthetic factors 2.5 min power spectrum analysis before basal stimulation,before HFS and after the whole recording indicated:there is no signifi cant difference in the power spectrum of slow wave(0~1Hz),δ(1~4Hz),θ(4~8Hz),α(8~12Hz) andβ(12~25Hz) frequency band between group Sev and group C(P>0.05). 3.The effects of neonatal repeated sevoflurane anesthesia on LTP in the hippocampal CA1 fields of rats:(1)The variation of PSA before and after HFS①During juvenile stage(P37),PSA was increased to(247.58±25.45) %,(206.97±16.73)%, (195.78±11.86) % and(179.66±13.83) % at 5 min,30 min,60 min and 90 min after HFS in group C. The PSA increment of group Sev at each time point reduced remarkably as compared to group C (P<0.05),which were(215.64±28.31) %,(153.15±23.59) %,(146.71±22.87) % and(141.03±16.05) %.②During adulthood(P97), PSA was increased to(267.04±23.89)%,(209.42±21.55)%,(196.47±26.74)% and(180.60±21.48)% at 5 min,30 min,60 min and 90 min after HFS in group C, The PSA increment of group Sev at each time point reduced as well comparing with group C(P<0.05), which were(217.53±28.31)%,(167.38±16.88) %,(161.03±14.63)% and(156.50±12.75)%.③The discrepancy of PSA between group Sev and group C at P97 shrunk comparing with P37,but the variation had no statistical significance(P>0.05).(2)The changes of fEPSP slope①During juvenile stage(P37),fEPSP slope was increased to(238.18±23.11) %,(193.04±19.34) %,(165.56±16.82)% and(151.76±17.85) % at 5 min,30 min,60 min and 90 min after HFS in group C. The slope of group Sev at the same time points were(210.22±22.74) %,(169.27±26.65) %,(146.75±21.18) % and (139.51±14.03) %. Comparing at all time points,fEPSP slope of group Sev was lower than group C at 5 min,10 min,15 min,and 55 min after HFS(P<0.05),whereas there was no discrepancy at other time points.②During juvenile stage(P97),fEPSP slope was increased to(240.44±33.1) %(,194.76±24.04) %, (169.39±27.3) % and(160.65±19.30)%. The slope of group Sev at the same time points were (235.93±27.84) %,(195.52±25.89) %,(174.46±15.03) % and(149.59±15.38)%,there was no signifi cant discrepancy at all time points between two groups(P>0.05). 4.The effects of neonatal repeated sevofl urane anesthesia on PPF in the rat hippocampal CA1 neurons.(1) In P37 rats of group Sev,PPF ratio induced by paired stimulation with interstimulus interval of 100 ms,150 ms,200 ms and 250 ms in group Sev was signifi cantly increased as well(P<0.001). At P97 PPF ratio induced by paired stimulation with interstimulus interval of 150 ms,200 ms and 250 ms in group Sev was also signifi cantly higher than group C(P<0.05).(2)As compared with P37,the discrepancy of PPF ratio in response to stimulation with interstimulus interval of 150 ms and 200 ms between two groups at 97 was signifi cantly decreased (P<0.05).Conclusion:1. Neonatal repeated sevoflurane anesthesia can induce lesion to both long-term and short-term synaptic plasticity in the neurons of hippocampal CA1 region at juvenile and adult age. This alteration may be the functional mechanism underlying dysfunction of learning and memory induced by neonatal repeated sevofl urane anesthesia. 2. The mechanism underlying the lesion of synaptic plasticity in the rat hippocampal neurons induced by neonatal repeated sevofl urane anesthesia may be related to the effects on presynaptic neurotransmitters release and postsynaptic neuronal population response. 3. The long-term lesion on synaptic plasticity induced by neonatal repeated sevoflurane anesthesia was most obvious at juvenile stage with trend of alleviation during adulthood.