Escape behavior elicited by single, channelrhodopsin-2-evoked spikes in zebrafish somatosensory neurons

Abstract

Somatosensory neurons in teleosts and amphibians are sensitive to thermal, mechanical, or nociceptive stimuli [1, 2]. The two main types of such cells in zebrafish--Rohon-Beard and trigeminal neurons--have served as models for neural development [3-6], but little is known about how they encode tactile stimuli. The hindbrain networks that transduce somatosensory stimuli into a motor output encode information by using very few spikes in a small number of cells [7], but it is unclear whether activity in the primary receptor neurons is similarly efficient. To address this question, we manipulated the activity of zebrafish neurons with the light-activated cation channel, Channelrhodopsin-2 (ChR2) [8, 9]. We found that photoactivation of ChR2 in genetically defined populations of somatosensory neurons triggered escape behaviors in 24-hr-old zebrafish. Electrophysiological recordings from ChR2-positive trigeminal neurons in intact fish revealed that these cells have extremely low rates of spontaneous activity and can be induced to fire by brief pulses of blue light. Using this technique, we find that even a single action potential in a single sensory neuron was at times sufficient to evoke an escape behavior. These results establish ChR2 as a powerful tool for the manipulation of neural activity in zebrafish and reveal a degree of efficiency in coding that has not been found in primary sensory neurons.

Figure 1. Photoactivation of ChR2 in Zebrafish Somatosensory Neurons Triggers Escape Behaviors

(A) Lateral view of a 24 hpf embryo expressing ChR2-YFP and EGFP in touch-sensitive Rohon-Beard and trigeminal neurons. Anterior is at left, and dorsal is at top. The scale bar represents 100 µm. (B) Maximum-intensity Z projections of two-photon stacks showing ChR2-YFP and EGFP expression in trigeminal (top panel) and Rohon-Beard (bottom panel) neurons at 24 hpf. The scale bar represents 10 µm. (C) Time-series projection of a ChR2-YFP-expressing embryo performing an escape in response to illumination at 488 nm. Images were acquired at 500 fps. The scale bar represents 100 µm. (D) Percentage of experimental (Isl1::Gal4-VP16::UAS-E1b::ChR2-YFP, UAS::GFP) and control (Isl1::Gal4-VP16, UAS::GFP) embryos showing light-evoked escape behaviors. Data are mean ± SEM across three (ChR2) and two (control) clutches of 50–100 injected embryos. Experimental: 79% ± 4%, n = 149 embryos; control: 1% ± 1%, n = 66 embryos.

Figure 2. Extracellular Recording of ChR2-Evoked Spiking Activity in Trigeminal Neurons

(A) Loose-patch recordings were made by targeting GFP-positive cells (asterisk) with a glass microelectrode (dashed lines) and forming a loose seal of ~50–100 MΩ. The scale bar represents 20 µm. (B) Current traces from ChR2-expressing neurons showed light-evoked spikes occurring after stimulus onset (arrow). (C) Prolonged stimuli of 50 ms trigger repetitive firing. Sixteen stimuli of 50 ms (blue bar) were delivered to the target cell at 0.2 Hz. (D) Failures occur more often with high-frequency stimulation and after many stimulus repetitions. Light pulses of 50 ms (blue bar) were delivered at 1 Hz for 100 s; every tenth trial in the series is shown. (E) The dependence of failure rate on stimulation frequency varies between cells. Two neurons in two fish were exposed to 20× 50 ms pulses at varying frequencies, and the probability of showing one or more spikes per stimulus was plotted as a function of frequency. (F) Response probability increases with pulse duration (n = 9). Data are mean ± SEM.

Figure 3. Single Spikes in Single Somatosensory Neurons Can Trigger Escape Behaviors

(A) By closing the microscope’s epifluorescence-field aperture, illumination could be restricted to single cells in sparsely labeled embryos. Red channel: illumination light; green channel: ChR2-expressing Rohon-Beard neuron. The scale bar represents 20 µm. (B) Stimulation of a single cell reliably evokes a behavioral response. One hundred millisecond stimuli were delivered at different positions along the A–P axis of the fish, and behavioral responses were monitored by watching the local contraction of the muscle. A–P displacements are given relative to the field in which the cell was located in the center of the illumination spot (0 µm). The number of successful trials (out of five) at each field is given at the bottom of each panel, above the displacement value. At −80 and +80 µm, the cell has moved completely out of the illumination spot. The scale bar represents 10 µm. (C) Data pooled for five cells in five different fish that showed single-cell responsiveness. Mean ± SEM response probabilities are shown at several A–P displacements. (D) Dependence of behavior probability on pulse duration. Cells identified as being sufficient to elicit a behavior were stimulated five times at each of several different pulse durations, and the behavioral output was monitored. Differently colored traces represent different cells (n = 5 cells in four fish; includes three trigeminal and two Rohon-Beard neurons). (E) Number of spikes evoked by light pulses of different durations. Data show the probability of a given number of spikes occurring at a given pulse length and are pooled from six neurons. Pulses of 10 ms or less never evoked more than a single spike. (F) Single spikes are reliably evoked by 5 ms light pulses (blue bar) in a trigeminal neuron. Stimuli were delivered at 0.2 Hz.