Neuromodulation of spinal networks in embryonic and larval zebrafish

Spinal networks, once considered an inflexible ensemble of excitatory and inhibitory components organised into fixed circuits, are in fact modulated by a range of neuromodulators which impart levels of flexibility that permit adaptation to changing environments. In this thesis the roles of two known neuromodulators, nitric oxide (NO) and dopamine (DA), have been examined within the developing zebrafish nervous system. In the first results chapter, the anatomical and functional effects of perturbing NO signalling during neuromuscular junction (NMJ) development have been investigated. This revealed that prolonged exposure to NO decreased NMJ number. Additionally, miniature end plate current (mEPC) frequency was reduced, kinetics slowed, and locomotor drive affected, suggesting NO is a potent modulator of NMJ maturation and function. In the second and third chapters, the physiological maturation and functional roles of a population of DAergic neurons which project to spinal networks have been studied. To understand when and how cellular activity patterns develop, targeted in vivo electrophysiological recordings were made from dopaminergic diencephalospinal neurons (DDNs) at embryonic and larval stages, where locomotor network development and output undergo profound changes. These investigations demonstrated that DDNs functionally mature during development, engaging in low frequency tonic spiking at embryonic stages which is accompanied by high frequency bursting at larval stages. Paired recordings of DDNs with spinal neurons revealed that at free swimming (larval) stages, tonic spiking is associated with periods of locomotor inactivity, whereas bursts are associated with periods of swimming. Ablation of DDNs was sufficient to suppress locomotor output suggesting that these cells modulate spinal network excitability. In sum, these investigations provide important insights into the roles of NO and DA during locomotor network ontogeny: NO modulates NMJ maturation while DA contributes to locomotor output.