Behavioral and electrophysiological studies of the role of the mechanosensory lateral line in the goldfish escape response
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In goldfish, one of the paired Mauthner (M-) cells, initiates the auditory evoked escape behavior. The behavior is appropriately directional; in the open field a fish reliably turns away from the sound source whereas near the wall it rather turns towards the sound. The mechanism whereby these bilateral neurons extract directional information from their afferent inputs has remained largely obscure. Through complimentary behavioral and electrophysiological experiments this thesis examines the hypothesis that the lateral line (LL) mechanosensory system contributes to escape directionality.;High-speed video was used to analyze escapes triggered in swimming fish by abrupt sound stimuli after complete LL elimination with cobalt chloride or gentamicin, or partial elimination through transection of the posterior LL nerve (pLLn). Cobalt significantly increased escape onset latency and reduced open field directionality to chance. Gentamicin had similar effects, however pLLn transection had a lesser effect on directionality, indicating the anterior LL nerve (aLL) is more likely to transmit directional information to the M-cell.;Electrical stimulation of the aLLn evoked short latency monosynaptic PSPs that contained both a chemical and electrical component. The afferents project bilaterally to both M-cells, with the contralateral side receiving smaller inputs with a longer latency. Ipsilateral inputs were localized proximal to the site of input from the inner ear. Furthermore superimposed on the evoked PSP is a shunting inhibition with a delay of ∼1 ms, that was symmetrical in both cells, a characteristic of other sensory inputs to the M-Cell. Consistent with the behavioral observations, these findings confirm that fast synaptic potentials from the aLL can play a significant modulatory role in shaping the behavior.;Finally using underwater air puffs, threshold responses were evoked in-vivo that often fired the M-cells in a directional manner. Dissection of thresholds PSP revealed two components, an early non-directional bilateral input and a later one that was directional and preferentially activated the ipsilateral M-cell.;This study is the first to demonstrate a source of directional information in teleost escapes. These results pave the way for a long-term hypothesis that directional information is derived from interactions, at the dendritic level, of multisensory stato-acoustic inputs.
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