The Impact Of Excitability Properties And Postsynaptic Subthreshold Depolarization Of Pyramidal Neurons On Synaptic Modification In Rat Visual Cortex

ObjectivesTo investigate the excitability electrophysiological properties of layer IV pyramidal neurons and the impact on synaptic modification which evoked by paired stimulation during the critical period of visual development in rats, and discuss the role of postsynaptic membrane subthreshold depolarization on the synaptic modification.MethodsChosen healthy Wistar rats during postnatal13～24days, divided into two groups(group A:P13-P17d and group B:P20-P24d). By combining infrared differential interference contrast (IR-DIC) technique and CCD-Camera system,and performed whole-cell recordings with Axon700B and give electric stimulus with Master-8-vp in slices of rat visual cortex.①Give the step stimulus on postsynaptic pyramidal neurons, and to compare the AP latency and intensity between the two groups.②Chosen the stimulus intensity which close to the rheobase of pyramidal neurons as postsynaptic stimulus, combining with presynaptic stimulus to condition the cell, and to record postsynaptic voltage which evoked by the postsynaptic subthreshold depolarization current. Stimulation electrode was put on the white matter which correspond of monocular region, and gave postsynaptic excitatory input to the neurons of layerⅣof primary visual cortex.③A biref subthreshold current (30ms) was injected into the cell with different time interval (positive or negtive) with the presynaptic white matter stimulus. Characteristics and relative factors of synaptic plasticity of rat visual cortex were observed by two different stimulation modes in two groups.Results1. The electrophysiological properties in layer IV pyramidal neurons of two groups were observed.①Group A had a weaker capacity for evoked action potentials in layer IVpyramidal neurons than group B, and the delay to first spike of AP of two groups were0.0361±0.10s and0.0282±0.01s.There were significant difference between two groups(t=5.81,P<0.05).①The mean number of AP between the two were Significant difference (P=0.016, P<0.05) when inject the same current intensity. 2. The cells showed bidirectional long term changes in EPSC amplitude after paired stimulation in different intervals(At=±110ms) in rats during criticol period of visual development. And5ms-20ms were classified as short range time interval,30ms were classified as intermediate range time interval,40ms-110ms were classified as long range time interval. The results showed that cells recorded in group A were more likely to induce LTD in most range. In pre-post mode it is easier to induce LTP in short range time intervals in group B, and LTD in long range time intervals.There was no significant difference in LTD/LTP occurrence rate in post-pre mode. LTD/LTP occurrence rate were21.3%and60.2%in group A,while35.2%and43.5%in group B, there were significant difference between two groups (χ2=6.791, P<0.05).The results showed that group A, B had different plasticity characteristics, which cells recorded in group A were more likely to induce LTD compared with group B.3. The current intensity which induced postsynaptic subthreshold depolarization was positive correlated with the EPSC amplitude changes in group A (r=0.698, P<0.01) and groupB (r=0.639, P<0.01.4. The current intensity of postsynaptic subthreshold depolarization was positive correlated with postsynaptic voltage change in group A (r=0.752, P<0.01) and group B(r=0.837, P<0.01). The results suggested that postsynaptic depolarization voltage as a parameters which reflected directly the level of postsynaptic subthreshold depolarization, and closely related synaptic modification.Conclusions1.During an early stage for visual development in rats visual cortex, layer IV pyramidal neurons had a lower capacity for exciting, thus it was difficult to produce postsynaptic suprathreshold excitatory. In this period of time the postsynaptic subthreshold depolarization was involved in synaptic modification,and played an important role in rats visual cortex plasticity.2. In our study, the changes in synaptic weight depends on the postsynaptic membrane voltage combined electrical activity of presynaptic,but not just depend on the timing.