脑神经活动光学显微成像技术

In vivo optical imaging of brain activity

大脑包含数亿至数千亿的神经元以及更为复杂的神经突触连接网络,是生物体中最复杂的器官.脑科学是21世纪以来最重要的前沿新兴学科之一,它的兴起标志着人类在认识自我、探索智慧和意识的本质中进入了一个新时代.在活体中对大脑神经活动进行长时间、大视野、高时空分辨率的观测,是解析大脑功能的关键.光学显微成像技术以其时空分辨率高,光学探针的特异性和多样性等优势,成为了脑神经活动研究的重要工具.针对大脑的高度散射、高速神经信号传递、超大神经元规模、精细突触连接结构等特性以及自由活动动物的脑神经活动观测需求,本文将从超深、超快、大视场、超分辨、微型化5个发展方向,概述包括多光子、红外二区、光声、光片、结构光以及自适应光学在内的多种光学显微成像技术在脑神经活动显微观测领域的发展进展及前沿动态,并展望脑神经活动光学显微成像技术的未来发展方向与前景.

In the new millennium,the brain and neuroscience have taken center-stage in international collaborative efforts.The brain comprises billions of neurons interconnected at trillions of synapses.To decipher its structure and function is one of the boldest projects ever pursued by the scientific community.The development of better imaging technology once again affords powerful tools to meet this grand challenge.Different imaging modalities have been widely used in revealing the complex structural organization and functional dynamics of the brain,including X-ray computed tomography,magnetic resonance imaging,positron emission computed tomography,ultrasound imaging,electron microscopy,and fluorescence microscopy.Among these,only fluorescence microscopy provides high contrast,high specificity,and high spatiotemporal resolution imaging in vivo. Thanks to the recent progress in photonics,laser physics,computer and information science,and nanomaterial science, the century-old optical imaging field is now being revitalizing and is booming.Recent developments in three-photon microscopy enable the optical resolution of single-neuron activity as deep as 2mm beneath the surface of the cortex,and non-invasively visualization of single-neuron activity through the intact opaque skull.The emergence of photon-efficient super-resolution Hessian structured illumination microscopy allows live cells to be imaged with a spatial resolution <90 nm and an acquisition rate of 564 frames per second,and enables time-lapse super-resolution imaging for over an hour with minimal photo-bleaching.Light-sheet microscopy,on the other hand,is capable of imaging -100000neurons in the entire zebrafish brain,at a volumetric imaging rate >10Hz.Other innovations such as near-infrared II imaging,photoa- coustic tomography,and adaptive optics are also extending the spatial and temporal resolution,imaging depth,and trans-scale volumetric imaging capacity. Another paradigm-shift is to record brain activity in freely-moving and behaving animals,which involves technological innovation in miniaturized microscopy with high spatiotemporal resolution.In this regard,we recently developed a fast,high-resolution,miniaturized two-photon microscope (FHIRM-TPM),with a headpiece weighing only 2.2g and occupying less than 1 cm^3,equipped with a GRIN lens of NA 0.8.Because the fluorescent Ca^2+indicators GFP and GCaMP6 are commonly used in biomedical science,we designed and custom-manufactured a hollow-core photonic crystal fiber to deliver 920-nm femtosecond laser pulses with little dispersion and attenuation.FHIRM-TPM is capable of long-term recording neuronal activity in freely-behaving mice at single-spine and sub-millisecond spatiotemporal resolution (0.64μm laterally and 3.35μm axially,40Hz at 256pixel×256pixel for raster scanning and 10000Hz for free-line scanning).Future applications of this technology in many behavioral paradigms will help to address many fundamental questions such as spatial and temporal information-processing,learning and memory,decision-making,and social interactions. In summary,with the emphasis on developing more advanced imaging technologies,scientists can directly visualize neuronal activity deeper in the brain,markedly faster with super-resolution,and over many orders of spatiotemporal scales.With technical advances unfolding in multiple fronts (e.g.,probes,detectors,modality fusion,and deep-learning assisted imaging),systematic breakthroughs will provide brain scientists and neuroscientists with the ability to gain a holistic view of multi-layered brain activity at the levels of neuron clusters,nuclei,and the circuitry of long-range connectivity.Finally,we envision that high-resolution imaging methods enabling in tort recording of brain activity at single-neuron resolution in small mammals may become a reality within the next decade.