Electron Microscopic Reconstruction of Functionally Identified Cells in a Neural Integrator

• Published in: Current biology 2017-07, Vol.27 (14), p.2137-2147.e3
• Main Authors: Vishwanathan, Ashwin, Daie, Kayvon, Ramirez, Alexandro D., Lichtman, Jeff W., Aksay, Emre R.F., Seung, H. Sebastian
• Format: Article
• Language: English
• Subjects: Animals; Axons; connectomics; Eye Movements - physiology; eye-position; Microscopy, Electron, Scanning; neural integrator; Neurons - physiology; Neurons - ultrastructure; positive feedback; Rhombencephalon - physiology; serial section electron microscopy; two-photon microscopy; zebrafish; Zebrafish - physiology
• ISSN: 0960-9822; 1879-0445; 1879-0445
• DOI: 10.1016/j.cub.2017.06.028

Neural integrators are involved in a variety of sensorimotor and cognitive behaviors. The oculomotor system contains a simple example, a hindbrain neural circuit that takes velocity signals as inputs and temporally integrates them to control eye position. Here we investigated the structural underpinnings of temporal integration in the larval zebrafish by first identifying integrator neurons using two-photon calcium imaging and then reconstructing the same neurons through serial electron microscopic analysis. Integrator neurons were identified as those neurons with activities highly correlated with eye position during spontaneous eye movements. Three morphological classes of neurons were observed: ipsilaterally projecting neurons located medially, contralaterally projecting neurons located more laterally, and a population at the extreme lateral edge of the hindbrain for which we were not able to identify axons. Based on their somatic locations, we inferred that neurons with only ipsilaterally projecting axons are glutamatergic, whereas neurons with only contralaterally projecting axons are largely GABAergic. Dendritic and synaptic organization of the ipsilaterally projecting neurons suggests a broad sampling from inputs on the ipsilateral side. We also observed the first conclusive evidence of synapses between integrator neurons, which have long been hypothesized by recurrent network models of integration via positive feedback. [Display omitted] •Correlated light and electron microscopy from larval zebrafish•Electron microscopic reconstruction of neurons encoding for eye-movement behavior•Imaged neurons differed in axonal projections, dendritic arbors, and synaptic densities•Synaptic connections between integrator cells Vishwanathan et al. combined two-photon calcium imaging and serial-section electron microscopy in the larval zebrafish to reconstruct neurons in the oculomotor integrator. They observed three classes of neurons and evidence of synaptic connections between these classes.