Efferent modulation in the mechanosensory lateral line of the larval zebrafish

Summary

Date: 
November 29, 2017 - 1:00pm
Location: 
Northwest Building, Room 243
About the Speaker
Name: 
Iris Odstrcil
Speaker Title: 
PhD Candidate, Engert Lab

Animals need to identify the source of sensory stimuli to guide appropriate responses. However, sensory systems can be activated both by events occurring in an animal’s environment, as well as self-stimulation generated by an animal’s own movement. This poses a fundamental challenge given that sensory receptors are intrinsically unable to discriminate between external and self-generated inputs; they can merely report their presence. In addition to informational ambiguity, movement also generates sensitivity challenges. By strongly driving sensory activity, movement may mask external inputs that occur concurrently or desensitize primary afferents leaving the animal unresponsive to stimuli following motion.

Aquatic organisms are particularly confronted with this challenge. Fish and amphibians guide multiple behaviors by using the lateral line, a collection of mechanosensory organs distributed along their body that sense water currents or displacements caused by other animals or objects. However, fluid drag during locomotion also strongly activates this sensory modality. Nevertheless, fish are able to filter out this confounding contribution and respond appropriately to external stimuli.

To understand the neuronal circuits that underlie such stimulus discernment, we study the lateral line system of larval zebrafish as a model circuit. Using standard labeling techniques, we have found two distinct nuclei providing direct descending inputs to mechanosensory organs: a dopaminergic hypothalamic nucleus and a cholinergic hindbrain nucleus. Using calcium imaging in a head-embedded preparation, we have observed that both descending nuclei are activated during locomotive events such as swims and escapes, while the primary sensory stream gets inhibited. Interestingly, pharmacological manipulations, laser ablations, and targeted mutations have shown that inhibition is largely mediated by descending cholinergic neurons. This is suggestive of an efference copy generation system, whereby motor areas inform sensory processing areas about impending movements such that the expected self-generated stimulation can be nullified or compensated for via subtraction.