Dopaminergic signaling in the spinal cord suppresses locomotion in larval zebrafish development

Abstract

The significance of dopamine (DA) and its multifaceted role as a neurotransmitter in the central nervous system has undergone extensive investigation. The research focus of my project centers on dopamine’s role in modulating spinal locomotor circuits in larvae zebrafish. Previous research from our lab showed that larval zebrafish swimming patterns change during development from long episodes durations at 3 days post fertilization (dpf) to short episode durations at 4 dpf and coincides with gross to fine motor control. Dopamine receptor D4 signaling in the spinal cord is necessary in facilitating this switch, likely by modulating dopamine signaling and regulating the activity of motor neurons involved in generating locomotor patterns. We demonstrated that antagonism of D4R signaling starting at 3 dpf prevents the switch from long to short episode durations, while D4R antagonism at 4 dpf reverses the switch from short to long episode durations. We hypothesized that 3 dpf larvae possess sufficient dopaminergic receptors in the spinal cord to bind to DA, enabling the advancement of the developmental switch from immature, long swim patterns to a mature state resembling 4 dpf larvae by exposing larvae at 3 dpf to exogenous DA. To test this, we used transgenic zebrafish that expressed Channelrhodopsin (ChR) in glutamatergic neurons within the spinal cord, allowing for the activation of these neurons using blue-light stimulation. Fictive swimming was measured using peripheral nerve recordings in different conditions, of a baseline (t0), treatment of dopamine (t1), and washout (saline) (t2). Control (untreated) preparations exhibited no significant changes between conditions, indicating that repeated optogenetic stimulation by itself did not induce notable changes in locomotor activity. Dopamine application significantly decreased the number of bursts and episode duration during optogenetic stimulation locomotor activity without affecting number of episodes, burst duration, or inter-burst intervals. These results suggest that exogenous DA affected swim patterns in 3 dpf larvae to resemble their 5 dpf counterparts, indicating a sufficient expression level of dopamine receptors in spinal locomotor networks of 3 dpf larvae to prematurely advance the developmental switch. These results could elucidate how neurodegenerative and motor disorders develop and progress, and shed light on the mechanisms underlying spinal cord injury. These findings could potentially inform translational medical approaches creating novel therapeutic interventions for treating neurodegenerative diseases.