| As brain neuroscience continues to grow,developing neuromodulation techniques to understand brain function and disease has become an urgent task.Electrical,thermal,chemical,optical,magnetic,and mechanical methods have been used as neuromodulation techniques – each presenting both advantages and disadvantages towards minimally invasive methods of neural modulation.Infrared Neural Modulation(INM)is a relatively new technique and defined as the direct induction or suppression of a neural action potential with near-infrared light – typically by a nonspecific photothermal mechanism.INM has been demonstrated as a contact-free and high spatial resolution tool in the peripheral neural system.However,the differences in anatomy and physiology of the central nervous system(CNS)present a new set of challenges in applying INM to regulate neural function.Under the support of National Natural Science Foundation(NSFC31271060),this thesis performed the theoretical calculations,in-vivo and ex-vivo experiments to explore the effects of the INM on rat primary motor cortex.The detailed work can be summarized as following:(1)To investigate the effect of photothermal-effect on neural acitivity,infrared pulse train with varied radiant exposures and wavelengths were applied to stimulate the rat's primary motor cortex.Acivity(spike)of neurons located in the depth of 300-500?m were recorded,and the relationship between stimulation parameters and neural response were analyzed.Meanwhile,the temperature field simulation model of the laser-irradiated cortex was established in COMSOL Multiphysics,and the temperature distribution under varied stimulation parameters were calculated.Experimental and simulation results suggest that 1860 nm and 1875 nm infrared light can effectively inhibit the neural response in layer 2/3 of the primary motor cortex,and the temperature increase induced by photothemal effect is the critical factor to decide the inhibitory rate: the higher radiant exposure or absorption coeffectient of light is,the velocity of temperature increase would be,and when the temperature change reachs 2.6?,neural activity would be inhibited.What's more,the response durations and inhibition rates increased with temperature change,and inhibitory effect of INM lasted even after irradiation then returned to the initial state as the temperatures stabilized.The results suggest the neural acitivity presents a dynamic time course along the temperature change of tissue.(2)Considering the effect of absorption coefficient on the temperature distribution,980 nm infrared light with higher optical penetration depth was applied to characterize the effect of INM on neurons located in the deep layer of cortex.Under the guidance of the simulation results,animal experiments were carried out to further evaluate the effect of INM on spike firing rate and firing pattern(Fano factor)of neurons located in the depth of 800-1100 ?m.The results showed that the 980 nm infrared light can effectively modulate the neural activity of layer 5,and the post-stimulation effects showed both excitatory neural response and inhibitory neural response.The effect of INM(the number of responsive neurons,firing rate change,response duration,and Fano factor)increased with radiant exposure.Particularly,under high intensity stimulation,neural response showed inhibition followed by excitation,firing pattern showed more irregularity,and the response lasted about 4s.Simulation and experimental results confirmed that after infrared irradiation,the temperature presents an exponential decay,while the neural activity showed more complex pattern.This study suggests that the temperature was not the only factor which alter the post-stimulation neural response,both neural properties and network connection could influence neural acitivy.The prolonged post-stimulaiton response also suggests that the stimulus interval should be considered as a key parameter in the construction of optimal stimulation strategy.(3)To explore the feasibility of INM on the neural disorders,we firstly assessed the effect of INM on the epileptic neural activity in vivo.Animal experiments were performed in 4-AP epileptic rat model,and the 1875 nm infrared light was applied to suppress the abnormal hyper-excited neural activity.The results showed that INM can reversibly suppress epileptic neural activity in 4-AP model.The efficiency decreased when potassium channel was blocked,indicating potassium ions were involved in the inhibition effect of INM.We also explored the post-stimulation effect of INM on the electrical-evoked neural activity.The results showed that INM can modulate the electrical-evoked inhibitory neural response,an enhancement of inhibitory response was observed within 200 ms after stimulation,and then followed by a reduction in the neuron firing inhibition rate and inhibition duration.The results preliminarily suggest the physical/chemical factors-induced non-spontaneous neuronal activity could be modulated by the near-infrared light.This study provides new method for the experimental research and clinical therapy of brain disorders.(4)As an indispensable part of CNS,glial cells play an important role in regulating neural activity and involve in some brain functional diseases,while there are fewer studies reported the direct modulation of infrared light on glial cells.To further explore the mechanism of infrared modulation on central neural system,this thesis applied 1875 nm infrared light pulse in ex-vivo astrocytes.The results showed that infrared light could directly induce calcium responses in astrocytes.Analyzing the effect of stimulation parameters and the spacial distribution of cell's reponse showed that the response intensity increased with the increase of stimulation energy,and decreased as an exponent function of distance between the cell and fiber,which proved that a photothermal effect was a fundamental mechanism of INM in astrocytes.This study preliminary comfirms the efficience of INM in astrocytes,provides a new means for astrocyte regulation,and also opens up a new thought for exploring the mechanism INM in CNS.Besides,In the pharmacological studies,results showed that the efficiency of cell activation and the activity of calcium activity were significantly decreased both in the DPBS solution without calcium ions and in the DMSO solution with RN-1734.The results suggested that the mechanisms of INM in astrocytes were associated with both extracellular calcium influx and heat-sensitive ion channel TRPV-4 activation.In summary,this thesis performed the theoretical calculations and animal experiments to explore the temporal-spatial heat distribution induced by photo-thermal effect of INM and its effect on the neural activity modulation,and also preliminary comfirmed the efficience of INM in abnormal neural activities and astrocytes in cortex.This thesis provides theoretical and experimental basis to optimize the INM strategy in CNS,understand the mechanisms of near-infrared modulation,and establish new methods for functional brain research and brain disorder treatments. |
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