ii
Abstract
Neuronal circuits are capable of generating diverse forms of rhythmic activity.
Mechanisms underlying rhythmogenesis can be separated into two main groups. First,
pacemaker central pattern generators (CPGs) are composed of neurons that have intrinsic
oscillatory properties, such as the lobster stomatogastric ganglion. Second, CPGs driven by
network-based dynamics rely on synapse-mediated cell properties, such as locomotion in
aquatic vertebrates. Despite an existing wealth of knowledge obtained through studying
frog and lamprey swimming CPGs, the means by which a locomotor CPG develops
remains elusive. Here, we propose to address this question using the zebrafish embryo, for
its rapid development, optical transparency and stereotyped behaviour.
Motor activity in zebrafish embryos begins with spontaneous activity around 17
hours post-fertilization (hpf). Studies have shown that this activity is not sensitive to
antagonists of chemical neurotransmission, and does not require the hindbrain. By 28 hpf,
they become able to swim, and generate low-amplitude alternating contractions at a rate of
30 Hz. This study explores the developmental window between the onset of motility and
the onset of a mature locomotor output, such as swimming, with the objective of
uncovering key steps in motor network maturation.
By measuring the grouping of contractions rather than overall frequency of
spontaneous activity, we uncovered a novel form of spontaneous activity, starting around
22 hpf. This activity consists of two alternating contractions in rapid succession. In contrast
to early spontaneous activity, this motor activity requires glutamatergic neurotransmission
and input from the hindbrain, as previously shown for swimming at 28 hpf. This
intermediate behavior may reveal an important step in the maturation of the motor network.
Keywords : rhythmic activity, CPG, motor network, developmental neurobiology