Plasticity of Dopaminergic Phenotype and Locomotion in Larval Zebrafish Induced by Brain Excitability Changes during the Embryonic Period

Abstract

Significance Statement Spontaneous calcium (Ca21) transients, used here as a proxy for embryonic neural excitability (ENE), are detected in the forebrain of embryonic zebrafish. Using short-term pharmacological treatments by bath application that increase or decrease ENE we detected changes in the postmitotic differentiation of the dopaminergic phenotype, occuring within a reserve pool of vMAT2-positive cells. We also report changes in locomotion, with transient increase of ENE leading to hyperlocomotion, a phenotype associated with attention deficit hyperactivity disorder (ADHD) in this model. Our results provide a convenient framework to study the molecular mechanisms linking ENE to neurotransmitter specification. During the embryonic period, neuronal communication starts before the establishment of the synapses with alternative forms of neuronal excitability, called here embryonic neural excitability (ENE). ENE has been shown to modulate the unfolding of development transcriptional programs, but the global consequences for developing organisms are not all understood. Here, we monitored calcium (Ca21) transients in the telencephalon of ze-brafish embryos as a proxy for ENE to assess the efficacy of transient pharmacological treatments to either increase or decrease ENE. Increasing or decreasing ENE at the end of the embryonic period promoted an increase or a decrease in the numbers of dopamine (DA) neurons, respectively. This plasticity of dopaminergic specification occurs in the subpallium (SP) of zebrafish larvae at 6 d postfertilization (dpf), within a relatively stable population of vMAT2-positive cells. Nondopaminergic vMAT2-positive cells hence constitute an unanti-cipated biological marker for a reserve pool of DA neurons that can be recruited by ENE. Modulating ENE also affected larval locomotion several days after the end of the treatments. In particular, the increase of ENE from 2 to 3 dpf promoted hyperlocomotion of larvae at 6 dpf, reminiscent of zebrafish endophenotypes reported for attention deficit hyperactivity disorders (ADHDs). These results provide a convenient framework for identifying environmental factors that could disturb ENE as well as to study the molecular mechanisms linking ENE to neurotransmitter specification.