The cerebellum, a brain region with a high degree of plasticity, is pivotal in motor control, learning, and cognition. The cerebellar reserve is the capacity of the cerebellum to respond and adapt to various disorders via resilience and reversibility. Although structural and functional recovery has been reported in mammals and has attracted attention regarding treatments for cerebellar dysfunction, such as spinocerebellar degeneration, the regulatory mechanisms of the cerebellar reserve are largely unidentified, particularly at the circuit level. Herein, we established an optical approach using zebrafish, an ideal vertebrate model in optical techniques, neuroscience, and developmental biology. By combing two-photon laser ablation of the inferior olive (IO) and long-term non-invasive imaging of whole-brain imaging at a single-cell resolution, we succeeded in visualization of the morphological changes occurring in the IO neuron population and showed at a single-cell level that structural remodeling of the olivocerebellar circuit occurred in a relatively short period. This system, in combination with various functional analyses, represents a novel and powerful approach for uncovering the mechanisms of the cerebellar reserve, and highlights the potential of the zebrafish model to elucidate the organizing principles of neuronal circuits and their homeostasis in health and disease.

Unraveling the function of brain circuitry is fundamental to understanding how the brain works during various behavioral conditions. A challenge is to define how populations of neurons within brain circuits communicate with each other to yield a network function. Recent advances in optical techniques have provided powerful tools to analyze organizing principles and the development of neuronal circuits. Prominent examples include optical measurements and control of neuronal activity known as optogenetic (Boyden et al., 2005;Lin & Schnitzer, 2016;Mancuso et al., 2011;Yizhar et al., 2011). These approaches have expanded the functional analysis of neuronal populations, which were previously analyzed mainly using electrophysiological recordings. For example, compared with electrophysiological detection, which records a limited number of neurons, optical approaches help analyze spatiotemporal dynamics from a large number of neurons.

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In vivo deuterium MR imaging (2H MR) was investigated in rats after intraperitoneal administration of deuterated saline, and a dynamic study of the water movement in rat eyes was performed. Deuterium MR imaging was carried out by means of a gradient-echo (GRE) and a spin-echo (SE) pulse sequence. The rat eye was imaged in 2H MR more selectively by SE than by GRE, but a lower signal-to-noise ratio was obtained in *H MR imaging using the SE sequence. The MR signal intensity of the rat eye was followed by a 3compartment model, which enabled determination of the flow rate constant of the water in the eye (0.359/min). Deuterium MR imaging is useful to visualize the dynamic change of water in rat eyes using 2H MR at the same magnetic field (2 T) that can also be used for conventional MR imaging in

A central question of brain development lies in how individual neurons emerge and organize communities to acquire various functions. Voltage imaging provides unique approaches to address this by enabling simultaneous, non-invasive, in vivo recording of voltage dynamics from a population of cells. Recently, genetically encoded voltage indicators (GEVIs) facilitate cell-type specific imaging of voltage dynamics. However, it has not been applied to brain development. Here, we applied ArcLight, a GEVI utilizing voltage-sensitive domain, to zebrafish and established experimental approaches for analyzing voltage and morphology during development, focusing on the spinal cord and cerebellum. We initially demonstrated that Arclight was widely distributed in the neural tissues. By voltage imaging, we successfully visualized the coordinated, spontaneous activity of spinal cord neurons in their early stage of development at a high spatiotemporal resolution, at subcellular and population levels. Hyperpolarization and subthreshold signals were also detected. Finally, long-term voltage imaging during development revealed the process of changes in voltage dynamics in neuron populations, which were accompanied by axonal outgrowth. Voltage imaging could greatly contribute to our understanding of the functional organization of the nervous system during development.