The Plasticity Of Evoked Responses Of Barrel Cortex By Natural Stimulus To Whisker And Its Mechanisms

Background and Objective:Barrel cortex(BC) was firstly defined by Woolsey and Van der Loos in 1970.BC,as a major part of primary sensory cortex(SI), contains 6 layers.LayerⅣhas an exquisite somatotopic map where each individual whisker in rodent face is represented in a discrete anatomical unit,the "barrel".Each neuron in the barrel mainly receives information input from its corresponding whisker which is so called principal whisker(PW) and generates evoked responses.Current studies are mainly focus on the analysis of evoked response in barrel cortex by all kinds of ways of stimulating whiskers in vivo or the experience-dependent plasticity of the evoked responses of BC by electrical stimulation of thalamus in vitro.But there is no research of that natural stimulation to whiskers by different frequencies can induce plasticity of the evoked response in BC and its mechanisms.Because the frequency is a very important parameter for rodents exploratory behavior,and methods for evoking plasticity is usually electrical stimulation presynaptically,so in our studies,we want to illuminate whether different frequencies' natural stimulation to whiskers could induce synaptic plasticity of the evoked response in BC.And the possible mechanisms of this plasticity evoked by natural stimuli to whisker are also explored.Methods:Sprague-Dawley(SD) rats weighing 220-280g were used in this study, ignoring sex distinction.The rats were anaesthetized by intraperitoneal injection of urethane(20%,0.8ml/100g).The rat was fixed in a stereotaxic instrument.A small craniotomy was made according to the atlas and a tungsten electrode(FHC) was inserted into the layerⅣof the barrel cortex(centered 1.8～2.5mm to bregma and 5.5mm lateral to the midline,and a depth of about 0.8-0.9mm from surface of cortex).A whisker was stimulated by an air puff from glass microelectrode which was generated from a picospritzer controlled by an electrical stimulator.The direction of air puff was vertical to the stimulated whisker.The amplitude of vibration of the whisker caused by airflow was about 1mm.After PW was identified,we recorded evoked responses of BC after stimulating the PW by different frequencies(100Hzls/50Hz10s/50Hz2s/20Hz30s /20Hz5s/5Hz10min) and recorded the changes of the evoked responses of BC. Intracerebral microinjection was achieved by micro-syringes whose capacity was 1μl, and our injection volume to BC's layerⅣwas also 1μl.The injection was finished in 3-5 min at average rate.In whisker deprivation experiment,we trimmed all the whiskers of the right face during the first postnatal week in SD rats or ICR mice.In adult,the brains were taken out after doing heart perfusion.Then we did cytochrome oxidase histochemistry reaction for the brains.Results:1.Changes of the evoked responses of BC caused by different frequencies of conditioning stimuli were different.Long-term potentiation(LTP) of the evoked response could be seen after high-frequency,such as 100Hzls,50Hz10s and 50Hz2s stimulation.The higher the frequencies,the more obvious the LTP.Long-term depression(LTD) of the evoked response of barrel cortex was shown after low frequency such as 5Hz10min stimulation.The duration of all the long-term changes lasted for more than 1h.But the plastic change of the evoked response of BC after 20Hz stimulation was not obvious.2.After injecting APV(200mM,1μl) into layerⅣof BC, the evoked responses of BC decreased significantly.After APV injection,the long term change of the evoked responses of BC couldn't be generated by stimulation of 100Hz1s or 5Hz10min.3.After injecting KN-62(5mM,1μl) into layerⅣof BC,the evoked responses of BC decreased significantly.After KN-62 injection,the long term change of the evoked responses of BC couldn't be generated by stimulation of 100Hz1s again.4. Compared to the normal SD rats' plasticity,those SD rats whose whiskers had been deprived in their first week after birth took on a different change.The enhancement of the evoked responses of BC after 100Hz1s stimulation was significantly reduced compared to the control group.The decrease of the evoked responses of BC after 5Hz10min stimulation was also obviously reduced by the trimming at the first week.5. In this experiment,the effect of the KYN could last for 40-90min.Thirty minutes after injecting KYN(10mM,1μl) into layerⅣof BC,the amplitude of the evoked responses was about half lower than that before KYN,then 100Hz1s stimuli could not increase the evoked response more than that before KYN injection.Conclusion:The natural stimulus to whisker can induce the plasticity of evoked response of BC in vivo,and this plasticity has a certain relevance to the frequencies of the natural stimulus.High-frequency stimulus can induce LTP,low-frequency stimulus can induce LTD;The first postnatal week plays an extremely important role in the development and plasticity of somatosensory cortex;These synaptic plasticity changes may occur in the level of cortex,and may be related to the activation of the NMDA receptor and CaMKⅡin the barrel cortex.