Nociception Coding In Laser-evoked Brain Responses Of Primary Somatosensory Cortex And Ventral Posterior Lateral Thalamus In Rats

Pain is an extremely complex subjective experience,which is closely related to human survival and daily life.Acute pain warns individuals to protect their bodies.Chronic pain damages the physical and mental health of patients,and brings serious economic and social burden.At present,using laser stimulation to evoke acute pain,researchers have revealed the key brain regions involved in pain processing at the macro level,and preliminarily depicted the brain response characteristics related to pain processing in the brain.Specifically,primary somatosensory cortex and thalamus are extremely important for the perception of acute pain under normal physiological conditions and the somatic symptoms of chronic pain under pathological conditions,N1 signal and GBOs signal in evoked brain response are closely related to the early pain information processing,and the primary somatosensory cortex may be the main brain source of the two signals.However,pain information processing requires the transmission of neural pathways and the participation of multiple brain regions.Thalamus,as the hub of pain information from peripheral to somatosensory cortex,its characteristics of laser-evoked brain response are still unknown,and the contribution of somatosensory cortex and thalamus to pain signal is still unclear.Therefore,based on the cross species consistency of human and rodents in brain structure and brain function,we use the electrocorticography(ECo G)and multichannel local field potential recording technique to record the laser-evoked brain responses of rats.The response of brain related pain was extracted and the neural activity and coding characteristics of the primary somatosensory cortex and the ventral posterolateral nucleus of thalamus were revealed.In the ECo G study,the laser-evoked brain responses of twelve Sprague-Dawley(SD)rats were recorded by applying the fixed intensity of 3J laser on the left and right forepaws of rats,and the auditory evoked brain responses of six rats were recorded as the control.The results showed that 3J laser stimulation can evoke the clearly laser-evoked brain responses in rats.In accordance with previous studies,the spatial topographic map of N1 signal in the early time domain has obvious maximal response to stimulus in the lateral hemisphere,and the amplitude of the contralateral electrode on the stimulation position is significantly higher than that of the ipsilateral electrode.Therefore,N1 signal may be related to the early processing of somatic projection of nociception information.In time frequency domain,the time and frequency characteristics of gamma band oscillations(GBOs)signal are consistent with those of the previous studies,but the topographic map features lack obvious lateralization effect,and there is no significant difference between the magnitude of GBOs signal in the contralateral electrode and the ipsilateral electrode.In study 2,in order to further understand the neural origin of laser-evoked brain responses and the contribution of thalamus and primary somatosensory cortex to the nociception information coding,we use the multichannel local field potential recording technique has recorded the laser-evoked brain responses in the bilateral somatosensory cortex forepaw representative area(S1FL),and the ventral posterolateral nucleus(VPL)in thalamus of eight rats.Distinct from study 1,we used 2.5J,3J and 3.5J three intensities of laser stimulation,and the pain behaviors of rats were recorded simultaneously.The results showed that the pain behavior score of rats was significantly modulated by the intensity of stimulation,and increased with the stimulation intensity.In the time domain,the amplitude of N1 signal in S1 FL and VPL brain area were significantly modulated by the intensity of stimulation.In addition,the amplitude of N1 in S1 FL was significantly more negative than that in VPL,and the latency was significantly shorter than that in the VPL.The N1 in the contralateral S1 FL and VPL was significantly stronger than that in the ipsilateral side.In time frequency domain,the magnitude of the high frequency GBOs in S1 FL and VPL was also significantly modulated by stimulation intensity.Correlation analysis showed that the amplitude of N1 signal in time domain of S1 FL and VPL brain area was significantly negatively correlated with pain behavior score of rats,and the magnitude of high frequency GBOs signal in the contralateral S1 FL was positively correlated with pain behavior score of rats.Consequently,it can be observed that N1 and GBOs signals related to pain intensity information processing in the laser-evoked responses of rats,and the animal model of acute pain based on laser-evoked brain responses is an effective model to study the physiological mechanism of pain.The two signals mainly come from the neural activity in the primary somatosensory cortex,but the contribution of VPL in thalamus is not excluded.The contralateral brain area plays an important role in the early processing of pain information,especially the contralateral somatosensory cortex.In conclusion,the study verified and supported the existing research findings of pain related signals in the laser-evoked brain responses of rat at macro and mesoscopic level.Additionally,we have preliminarily described the neuro-electric activity characteristics of somatosensory cortex and ventral posterolateral nucleus of thalamus in nociception information processing.We hope that these findings can promote people’s understanding of pain related neurophysiological mechanisms to a certain extent,and provide support for the development of objective pain assessment indicators.