| It was shown in previous studies that endurance training could hyperpolarized the resting membrane potential and lowered the voltage threshold for action potential generation in-motoneurons of rat spinal cord.In contrary,however,increase in both current and voltage thresholds for action potential generation was reported in limb-unweighting rats.These results suggest that electrophysiological properties of spinal motoneurons can be changed by exercise intervention.While endurance training increases the excitability of spinal motoneurons,the limb-unweighting reduces their excitability.It is well known that locomotion in vertebrates is produced through the control of motoneurons by the networks of spinal cord.In addition to the motoneurons,spinal interneurons are required to participate in generating and coordinating locomotion.However,it remains unknown if exercise intervention could modulate the intrinsic membrane properties of the spinal interneurons.Furthermore,previous studies of the exercise intervention were all carried on in adult rats and a few of adolescent mice were used as the experimental models.For the above reasons we used adolescent mice of three weeks in this study and induced three-week treadmill exercise in these mice.And then the electrophysiological properties of the spinal interneurons were measured from the spinal cord slices of T12-L6 with whole-cell patch clamp technique.Results from this study show that the excitability of spinal interneurons can be increased by the exercise training.Objective:In the current study our purpose was to determine if the adolescent mouse allowed access to treadmill training for 3 weeks would possess interneurons with different electrophysiological properties than their control counterparts.And we use the whole cell patch recording in the slice of the spinal cord to test the electrophysiological properties of the interneurons.Method:Experiments were conducted on the B6 mouse obtained from Jackson Lab,The experiment was carried out in strict accordance with the ethical requirements of the animal center of East China Normal University,Ethical code:20141003.The 3 weeks postnatal animals were assigned to either a control group or a treadmill-training group randomly.The training is last for 3 weeks,we test the electrophysiological properties of the interneurons from both the two groups when they were 6 weeks old.(1)Treadmill training:Following the adjustment period,exercise animals were acclimated to treadmill running(13m/min,30min).The acute exercise training protocol:20m/min(85%VO2),lh/day,6 days/week on a motor-driven treadmill for 3weeks.(2)Electrophysiology:Membrane properties observed in the current study include rest membrane potential(Em),input resistance(Rin),voltage threshold(Vth),rheobase,membrane time constant(Tm),whole cell capacitance(Cm),amplitude and width of the action potential,amplitude and half decay time after hyperpolarization(AHP)in the whole cell patch recording.Results:Membrane properties of the 6 weeks old mouse.1.Electrophysiology:1.1 Control group(n=27):Em(-64.2±5.7 mV)?Ith(11.9±6.9pA)?Rin(1483.3±682.3M?2)?Tm(41.0±16.6ms)?Cm(27.6±24.3pF)?Vth(-36.9±5.1mV)?AP height(49.9±10.9mV)?AP width(1.3±0.4ms)?AHP depth(21.2±4.5mV)?AHP1/2 decay time(122.0±95.1ms);1.2 Training group(n=47):Em(-62.2±4.9mV)?Ith(11.7±9.6pA)?Rin(1608.2±586.4 M?)?Tm(41.1±23.3ms)?Cm(28.1 ±24.9pF)?Vth(-40.4±3.2mV)?AP height(52.7±11.9mV)?AP width(1.8±0.6ms)?AHP depth(18.7±5.2mV)?AHP1/2 decay time(117.4±83.0ms).2.Membrane properties in the different lamina.2.1 In the control group,significant differences were found between the dorsal(lamina ?-?)and ventral horn(lamina ?,? and ?)interneurons in the rheobase(16.3 ±4.8pA and 9.1 ±6.4 pA for the dorsal and ventral respectively).2.2 In the training group,there was no significant difference between the neurons distributed in the dorsal and ventral horn.3.Three types of interneurons.All the cells recorded in both groups in this study were classified into three types basing on their response to the injected depolarizing current:type I(single-spike firing),type II(phasic firing)and type III(tonic firing).3.1 For the control group,type ? has significant smaller AHP half decay time and depolarized voltage threshold.3.2 For the training group,type ? has significant hyperpolarized voltage threshold.Membrane currents and special firing pattern.1.Hyperpolarization-activated inward current(Ih)was measured in this study.Ih was observed in 57%of the total neurons in the control group and 47.6%in the training group.2.In current study,we found 3 burst neurons.Two of them were from training group and they were distributed in lamia ? and lamina ?,and the rest one was from the control and was distributed in the lamia VII.Effects of activity.The treadmill training did change the properties of the spinal cord interneurons significantly,and the treatment effect depended on the interneuron type and distribution.1.The ventral horn interneuron changed their properties after 3 week treadmill training.The interneuron from the training group had significant hyperpolarized voltage threshold(-40.2±3.2 mV and-34.9±4.4 mV for training and control group respectively),increased AP width(2.1 ±0.6ms and 1.3±0.4ms for training and control group respectively)and reduced the AHP depth(16.6±4.3mV and 24.1±3.9mV for training and control group respectively)compare with control group.2.After 3 weeks treadmill training type ? neuron had significant increased input resistance(1834.0±580.6M? and 1332.1±419.6MQ for training and control group respectively);and type ? neuron has significant larger AP width(1.9±0.4ms and 1.5± 0.5ms for training and control group respectively).Conclusion:1.The treadmill training do change the properties of the spinal cord interneuron significantly,and the treatment effect depends on the interneuron type and distribution.2.After exercise training the ventral horn interneuron has significant hyperpolarized voltage threshold,larger AP width and smaller AHP depth;These results suggest that the exercise training can improve the excitability of the ventral horn interneurons.3.No significant difference was found in the sensory interneurons located in the dorsal horn between the control and the training group;This result indicates that the interneurons distributed in the ventral horn are more responsible to the exercise intervention than those in dorsal horn.4.Exercise training echanced the excitability of the type ? and type ? neuron,and suggesting that these two types are more responsible to the exercise intervention than type ?. |
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