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The visual development is completed during the critical period in human and mammals.However, the critical period is not the initial of receiving visual experience.It is known that before the onset of critical period in mammals, such as mouse, there is an earlier stage for visual development, the pre-critical period.The research of response characteristics of the visual cortical neurons and the synaptic plasticity in the pre-critical period is still in the exploratory stage.
The study aimed to preliminarily investigate the response properties of neurons and synaptic plasticity in mouse visual cortex during the pre-critical period.
Forty-eight postnatal day 13-17 C57BL/6J mice were used for in vivo whole-cell recordings and in vitrobrain slice whole-cell recordings.In vivo whole-cell recordings were done in anesthetized mice.Moving bars in different directions were produced and controlled by a Matlab program.Cell recordings were obtained at the depth of layer Ⅳ of visual cortex.Step current stimuli under current clamp were given to measure the membrane response properties of neurons.Optimal visual stimuli were given to measure the in vivo largest responses of membrane potentials.In vitroexperiments were performed after in vivo experiments.All cells were given current step stimuli to measure the membrane response properties of neurons.Different intensities of white-matter-to- layer-Ⅳpathway stimulation were given to measure the evoked response properties.All cells from 48 mice were randomized into 4 groups according to different stimulus training modes, including low frequency stimulation (LFS), high frequency theta-burst stimulation (TBS), pre-post synaptic timing stimulation (pre-post TS) and post-pre synaptic timing stimulation (post-pre TS). Under the voltage clamp of -70 mV, excitatory postsynaptic currents (EPSCs) before and after training were recorded to measure the plastic changes of excitatory synaptic connections.pClamp 10 was used for the pre-analysis of data and Matlab 2008a was used for statistical analysis.The use and care of the animals followed the Statement for the Use of Animals in Ophthalmic and Vision Research.
Thirty-nine cells and 48 cells were successfully recorded in the in vivo and in vitro experiments, respectively.The steady-state average number of action potentials (APs) were (1.01±0.03)/sweep and (1.01±0.05)/sweep, the AP thresholds were (-40.2±3.2)mV and (-39.6±2.0)mV, and the threshold step current levels were (126.7±17.4)pA and (129.6±17.5)pA in the in vivo and in vitro recordings, respectively, with no significant differences between them (APs: t=0.512, P=0.610; AP thresholds: t=-1.074, P=0.286; current levels: t=-0.776, P=0.440). Under the optimal visual or pathway stimulation, the average peak response of membrane potentials was (7.3±4.3)mV and (6.4±2.8)mV with rarely evoked APs in the in vivo and in vitro experiments, respectively, with no significant difference between them (t=1.234, P=0.221). Under the in vitro recording, the EPSCs before LFS were [(138.1±51.9)pA], which was significantly higher than that after LFS [(76.1±34.8)pA] (t=4.437, P=0.001), but no significant differences were seen in EPSCs before and after TBS (t=-0.756, P=0.466). The EPSCs before and after pre-post TS were (122.4±62.2)pA and (78.5±46.7)pA, and those before and after post-pre TS were (131.9±48.0)pA and (74.3±30.7)pA, showing significant differences between them (pre-post TS: t=3.558, P=0.004; post-pre TS: t=4.283, P=0.001).
The construction of fundamental neural circuits in layer Ⅳ of mouse visual cortex is completed during pre-critical period.However, the membrane responsive capability of neurons and the synaptic connections are in an immature state, and the evoked responses to visual pathway inputs are basically subthreshold.The strength of synaptic connections is depressed with low frequency stimulation or pre-post/post-pre synaptic timing stimulation, and kept unchanged with high frequency stimulation.The development of visual neural system of PSP in mouse presents different characteristics from CP.