Initiation of Mauthner- or Non-Mauthner-Mediated Fast Escape Evoked by Different Modes of Sensory Input

Abstract

Brainstem reticulospinal neurons (RSNs) serve as the major descending system in vertebrate sensorimotor integration. One of the paired RSNs in zebrafish, the Mauthner (M) cell, is thought to initiate fast escape from sudden noxious stimuli. Two other paired RSNs, morphologically homologous to the M-cell, are also suggested to play key roles in controlling fast escape. However, the relationship among activities of the M-cell and its homologs during fast escape and the sensory inputs that elicit escape via their activation are unclear. We have monitored hindbrain RSN activity simultaneously with tail flip movement during fast escape in zebrafish. Confocal calcium imaging of RSNs was performed on larvae rostrally embedded in agar but with their tails allowed to move freely. Application of a pulsed waterjet to the otic vesicle (OV) to activate acousticovestibular input elicited contralateral fast tail flips with short latency and an apparent Ca 2ϩ increase, reflecting a single action potential, in the ipsilateral M-cell (M-escape). Application of waterjet to head skin for tactile stimulation elicited fast escapes, but onset was delayed and the M-cell did not fire (non-M-escape). After eliminating either the M-cell or OV, only non-M-escape was initiated. Simultaneous high-speed confocal imaging of the M-cell and one of its homologs, MiD3cm, revealed complementary activation during fast escape: MiD3cm activity was low during M-escape but high during non-M-escape. These results suggest that M-cell firing is necessary for fast escape with short latency elicited by acousticovestibular input and that MiD3cm is more involved in non-M-escape driven by head-tactile input.

Functional Role of a Specialized Class of Spinal Commissural Inhibitory Neurons during Fast Escapes in Zebrafish

“…Calcium imaging of M-cells was performed as described previously (O'Malley et al, 1996;Takahashi et al, 2002;Kohashi and Oda, 2008). Briefly, M-cells were labeled with a fluorescent Ca 2ϩ indicator, Calcium Green dextran (3000 or 10,000 molecular weight; Invitrogen) by injecting the dye into the spinal cord of 4 or 5 dpf wildtype larvae.…”

“…After Ca 2ϩ imaging with the sound/vibration stimuli, D-tubocurarine was added to the solution (final concentration, 0.1 mg/ml) to immobilize the larvae. Then, fluorescence responses associated with antidromic (AD) spikes were examined as described previously (Kohashi and Oda, 2008). For illustration purposes, the images were smoothed to reduce pixel-to-pixel noise.…”

“…We expected that these stimulations would activate the auditory nerves and that this in turn would activate the M-cell. In addition to the M-cell, other reticulospinal neurons that are important for initiating escape behavior could also be activated (O'Malley et al, 1996;Kohashi and Oda, 2008). Stimulations were applied to the contralateral side of the recorded CoLo neuron so that that side of the recorded CoLo neuron would initiate escape (Fig.…”

“…Therefore, we focused our analyses on sound/vibration-elicited escapes (Fig. 7A), because M-cells have been shown to play a major role in escapes evoked by acoustic-vestibular stimulations in larval zebrafish (Burgess and Granato, 2007;Kohashi and Oda, 2008).…”

“…To further obtain parallel evidence that both M-cells were coactivated by the sound/vibration stimulus, we performed calcium imaging (O'Malley et al, 1996;Takahashi et al, 2002;Kohashi and Oda, 2008). The M-cells were retrogradely labeled with a Ca 2ϩ indicator, Calcium Green dextran, and imaged with confocal microscopy.…”