Subcellular Dissection of a Simple Neural Circuit: Functional Domains of the Mauthner-Cell During Habituation

Abstract

Sensorimotor integration is a pivotal feature of the nervous system for ensuring a coordinated motor response to external stimuli. In essence, such neural circuits can optimize behavioral performance based on the saliency of environmental cues. In zebrafish, habituation of the acoustic startle response (ASR) is a simple behavior integrated into the startle command neurons, called the Mauthner cells. Whereas the essential neuronal components that regulate the startle response have been identified, the principles of how this regulation is integrated at the subcellular regions of the Mauthner cell, which in turn modulate the performance of the behavior, is still not well understood. Here, we reveal mechanistically distinct dynamics of excitatory inputs converging onto the lateral dendrite (LD) and axon initial segment (AIS) of the Mauthner cell by in vivo imaging glutamate release using iGluSnFR, an ultrafast glutamate sensing fluorescent reporter. We find that modulation of glutamate release is dependent on NMDA receptor activity exclusively at the AIS, which is responsible for setting the sensitivity of the startle reflex and inducing a depression of synaptic activity during habituation. In contrast, glutamate-release at the LD is not regulated by NMDA receptors and serves as a baseline component of Mauthner cell activation. Finally, using in vivo calcium imaging at the feed-forward interneuron population component of the startle circuit, we reveal that these cells indeed play pivotal roles in both setting the startle threshold and habituation by modulating the AIS of the Mauthner cell. These results indicate that a command neuron may have several functionally distinct regions to regulate complex aspects of behavior.

The Adaptor Protein 2 (AP2) complex modulates habituation and behavioral selection across multiple pathways and time windows

“…SLC latency shortens after ablating feedforward glycinergic inputs to the Mauthner neuron, pharmacological inhibition of glycine receptors, and cacna2d3 mutation, which all also reduce SLC habituation [75][76][77][78] . In contrast, the NMDA antagonist l-7,01,324 disrupts habituation through the feed-forward excitatory spiral fiber pathway with no impact on SLC latency 79 , consistent with our transgenic dissociation of SLC habituation and latency. Together these suggest the AP2 complex could both enhance the activity of the feed-forward inhibitory glycinergic pathway to regulate acoustically-evoked SLC latency and regulate the feed-forward excitatory pathway to modulate habituation learning.…”

Silicon-Rhodamine Functionalized Evocalcet Probes Potently and Selectively Label Calcium Sensing Receptors In Vitro, In Vivo, and Ex Vivo

“…We found no significant difference in either the percentage of elicited startle responses or the ratio of short-latency escapes (Figure S6). To assess target engagement of EvoSiR4 in the brain, we imaged the transgenic Tol056 zebrafish strain, which express eGFP in the Mauthner-neurons in the hindbrain responsible for the initiation of the short-latency escape response using confocal microscopy . In agreement with the behavioral data, we found no significant EvoSiR label, indicating that the bath-applied probe does not enter the zebrafish CNS (Figure S6).…”

Integration of cooperative and opposing molecular programs drives learning-associated behavioral plasticity

“…We previously showed that Hip14 can palmitoylate Kv1.1 and that it regulates its localization to the spiral fiber terminals [ 28 ]. Moreover spiral fibers are known to undergo plasticity during habituation learning [ 37 ]. Interestingly, these neurons were not reliably identified as showing differential activity in MK-801 or Butaclamol-treated animals (Module 1).…”