Effects Of Chronic Intermittent Hypoxia On The High Voltage-Activated Calcium Currents In Hippocampal CA1 Neurons

Objective:In this study,using a mouse model of chronic intermittent hypoxia?CIH?,we aimed to investigate the role of high voltage-activated?HVA?calcium channel in CIH-mediated calcium overload,and examine the neuroprotective effect of nimodipine,a L-type calcium channel receptor antagonist,as well as memantine,a non-competitive NMDA receptor antagonist,in mediating these changes.Methods:55 ICR male mice,20²5g weight,were randomly divided into the room air group?RA+VEH group?,the chronic intermittent hypoxia group?CIH+VEH group?and the pretreatment nimodipine group?CIH+NIM group?and memantine group?CIH+MEM group?.CIH+VEH group,CIH+NIM group and CIH+MEM group were exposed to intermittent hypoxia while RA+VEH group to room air for 8h per day.Mice in the CIH+NIM group and CIH+MEM group were pretreated with nimodipine?10mg/kg?memantine?5mg/kg?respectively by intraperitoneal injection in approximately 15 minutes prior to start of daily CIH exposure,and the other groups were treated with the same volume of vehicle?alcohol or physiological saline?.The current amplitude and membrane capacitance of hippocampal CA1 neurons were recorded by whole-cell patch-clamp technique.Activation and inactivation curves were constructed by using Boltzmann equation.The expression of Cav1.3 in hippocampus were detected by Western blot.Results:Exposure to 2-week CIH induced a marked increase in HVA calcium current density in hippocampus CA1 neurons compared to the RA+VEH group?RA+VEH:-71.99±9.21pA/pF,n=19 vs CIH+VEH:-135.42±15.41pA/pF,n=25,**p<0.01?.Pretreatment with nimodipine decreased HVA calcium current density compared to the CIH+VEH group?CIH+VEH:-135.42±15.4pA/pF,n=25 vs CIH+NIM:-88.12±9.33pA/pF,n=16,*p<0.05?.Similarly,pretreatment with memantine also decreased HVA calcium current density compared to the CIH+VEH group(CIH+VEH:-135.42±15.4pA/pF,n=25vs CIH+MEM:-46.36±7.21pA/pF,n=11,^##p<0.01).There was no significant difference between RA+VEH group and CIH+NIM group,as well as RA+VEH group and CIH+MEM group.CIH exposure led to a³mV negative shift in the steady-state activation curve?RA+VEH:-22.29±0.88mV,n=19 vs CIH+VEH:-25.39±1.03mV,n=25,*p<0.05?,whereas pretreatment nimodipine led to a⁴mV positive shift in the steady-state activation curve?CIH+VEH:-25.39±1.03mV,n=25 vs CIH+NIM:-21.68±1.24mV,n=16,*p<0.05?.Similarly,pretreatment memantine also led to a⁶mV positive shift in the steady-state activation curve?CIH+VEH:-25.39±1.03mV,n=25 vs CIH+MEM:-20.80±1.89mV,n=11,*p<0.05?.There was no significant difference in the slope factor among the above groups.The shift of the activation curve in a depolarizing direction permits the calcium channel to open in response to weaker depolarization under chronic intermittent hypoxia,which can be reversed by pretreatment nimodipine and memantine.On the other hand,CIH exposure led to a⁷mV positive shift in the steady-state inactivation curve?RA+VEH:-23.29±1.55mV,n=10 vs CIH+VEH:-16.38±1.52mV,n=14,**p<0.01?,whereas pretreatment nimodipine led to a⁴mV negative shift in the steady-state inactivation curve?CIH+VEH:-16.38±1.52mV,n=14 vs CIH+NIM:-20.41±1.12mV,n=12,*p<0.05?.Memantine pretreatment did not significantly change the V_h of the inactivation curve.There were no significant differences in the slopes?K?of the inactivation curves among the above groups.The shift of the inactivation curve in a depolarizing direction permits the calcium channel to open continuously in response to weaker hyperpolarization under chronic intermittent hypoxia,which can be reversed by pretreatment nimodipine.Exposed to 2-week CIH up-regulated the expression of Cav1.3protein?P<0.05?,which can be abolished in CIH group pretreated with nimodipine compared to the RA+VEH group?P<0.05?.Conclusion:The present study using a mouse CIH model provides the experimental evidence that HVA Ca²⁺channels,as well as NMDAR,may serve as an important source of Ca²⁺-mediating hippocampal CA1 neuron excitotoxicity and synaptic plasticity associated with cognitive impairment.L-VDCC-dependent Ca²⁺influx is thought to play a key role in NMDAR-dependent hippocampal excitability and plasticity,which memantine could abolish the increased HVA calcium current density in the CIH mice.These results offer new mechanistic insights for both basic researchers and clinicians in our combat against the neurocognitive deficits in OSAHS patients.