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

Developmental elevation of NO/cGMP signalling decreases NMJ numbers.

<p><b>A</b>. Left: schematic illustration of a zebrafish embryo at 2 days post fertilisation (dpf). Dashed box indicates region used for puncta analysis. Right: schematic illustration of a motoneuron at 2 dpf. Motoneuron somata (black circle) are located within the spinal cord (sc) and their axons extend along fascicles (solid black line) into the somitic region. Axon branches (solid grey lines) extend from the main fascicle. Dashed lines indicate somitic boundaries <b>B.</b> Left hand panels: lateral trunk views of anti-SV2 (green)/Rh-α-BTX (red) co-staining in control (top) and DETA-NO treated (bottom) zebrafish at 2 dpf. Right hand panels: expanded regions showing staining localised to a single somitic region. Insets show SV2 (upper) and Rh-α-BTX (lower) z-projections from which merged images were derived. <b>C</b>. Bar chart depicting the mean (± SEM) number of synapses located within each somite (som), along motor fascicles (fas) and along branch-associated regions (branch) of control (black) and DETA-NO (grey) treated fish. <b>D</b>. Mean density of branch-associated puncta in control (black) and DETA-NO treated (grey) fish. <b>E.</b> Left hand panels: lateral trunk views of anti-SV2 (green)/Rh-α-BTX (red) co-staining in control (top) and 8-pCPT-cGMP treated (cGMP, bottom) zebrafish at 2 dpf. Right hand panels: expanded regions showing staining localised to a single somitic region. <b>F</b>. Bar chart depicting the mean (± SEM) number of synapses located within each somite (som), along motor fascicles (fas) and along branches of control (black) and 8-pCPT-cGMP (cGMP, grey) treated fish. <b>G.</b> Mean density of branch-associated puncta in control (black) and 8-pCPT-cGMP treated (cGMP, grey) fish. Scale bars = 30 µm. ***p≤0.001.</p

Effects of nitric oxide on neuromuscular properties of developing zebrafish embryos

Nitric oxide is a bioactive signalling molecule that is known to affect a wide range of neurodevelopmental processes. However, its functional relevance to neuromuscular development is not fully understood. Here we have examined developmental roles of nitric oxide during formation and maturation of neuromuscular contacts in zebrafish. Using histochemical approaches we show that elevating nitric oxide levels reduces the number of neuromuscular synapses within the axial swimming muscles whilst inhibition of nitric oxide biosynthesis has the opposite effect. We further show that nitric oxide signalling does not change synapse density, suggesting that the observed effects are a consequence of previously reported changes in motor axon branch formation. Moreover, we have used in vivo patch clamp electrophysiology to examine the effects of nitric oxide on physiological maturation of zebrafish neuromuscular junctions. We show that developmental exposure to nitric oxide affects the kinetics of spontaneous miniature end plate currents and impacts the neuromuscular drive for locomotion. Taken together, our findings implicate nitrergic signalling in the regulation of zebrafish neuromuscular development and locomotor maturation

Developmental effects of NO on the frequency and duration on the neuromuscular drive for locomotion.

<p><b>A</b>. Line graph of mean (± SEM) locomotor-related end plate potential (EPP) frequency at the beginning, middle and end of evoked fictive swim episodes in control fish and fish exposed to DETA-NO or L-NAME during development. <b>B</b>. Bar chart showing mean duration of fictive motor episodes in control, DETA-NO and L-NAME treated fish. *** p<0.001.</p

Effects of 18βGA on electrical coupling in EF and ES fibres.

<p><b>A, B.</b> Images of embryonic fast (EF, <b>A</b>) and embryonic slow (ES, <b>B</b>) muscle fibres that were dialysed with sulforhodamine during whole cell recording. Note that in control saline, dye spreads to neighbouring muscle fibres whilst 18βGA pre-treatment abolishes this effect. <b>C,D.</b> Representative traces of miniature end plate currents (mEPCs) occurring in EF (<b>C</b>) and ES (<b>D</b>) fibres exposed to 18βGA. Right: example events captured on an expanded time scale. <b>E,F.</b> Amplitude versus rise time plots for mEPCs recorded from EF (<b>E</b>) and ES (<b>F</b>) fibres exposed to 18βGA. Insets show histograms of mEPC rise time in control (black) and 18βGA-treated (grey) fibres. <b>G.</b> Paired recording between ES fibres reveal that coincident events, presumed to be carried by electrical synapses, are occasionally observed in the presence of 18βGA; asterisked events with extended time scale on right. Scale bars = 50 µM.</p

Developmental inhibition of NO/cGMP synthesis increases NMJ numbers.

<p><b>A</b>. Left hand panels: lateral trunk views of control (top) and L-NAME (bottom) treated zebrafish at 2 days post fertilisation (dpf) processed with anti-SV2 (green) and Rh-α-BTX (red) staining. Right hand panels: expanded regions showing staining localised to a single somitic region. <b>B</b>. Bar chart depicting the mean (± SEM) number of synapses located within each somite (som), along motor fascicles (fas) and along branch-associated domains (branch) of control (black) and L-NAME (grey) treated fish. <b>C.</b> Mean density of branch-associated puncta in control (black) and L-NAME treated (grey) fish. <b>D</b>. Left hand panels: lateral trunk views of control (top) and ODQ (bottom) treated zebrafish at 2 dpf. Right hand panels: expanded regions showing staining localised to a single somitic region. <b>E</b>. Bar chart depicting the mean (± SEM) number of synapses located within each somite (som), along motor fascicles (fas) and along branch-associated domains (branch) of control (black) and ODQ (grey) treated fish. <b>F.</b> Mean density of branch-associated puncta in control (black) and ODQ treated (grey) fish. Scale bars = 30 µm. **p≤0.01, ***p≤0.001.</p

Firing Dynamics and Modulatory Actions of Supraspinal Dopaminergic Neurons During Zebrafish Locomotor Behaviour

Dopamine (DA) is a known modulator of motor circuits. Here, Jay et al. use the zebrafish to study in vivo activity patterns and functional roles of identified DAergic diencephalospinal neurons. Their findings provide important insights into the behavioral relevance of this evolutionarily conserved cell population

Homogenized stool subsamples have less variance compared to non-homogenized stool subsamples.

<p>Mean, standard deviation and variance values for each of the bacterial taxa detected via qPCR collected from four different subjects`stool subsamples that had been homogenized on liquid nitrogen or not homogenized. Levene's p-values are reported where a significant p-value denotes whether variance is significantly different between groups; the p value is italicized if the non-homogenized subsamples (underlined) have significantly higher variance compared to the homogenized subsamples. The averaged mean, standard deviation and variance between the four subjects is included in the right column (bolded text; large variance seen in non-homogenized subsamples).</p

The mean variances of bacterial taxa are lower in homogenized subsamples compared to non-homogenized stool subsamples.

<p>The variance values were calculated for each of the bacterial taxa tested using qPCR from five subsamples where the stool was homogenized by crushing on liquid nitrogen into a fine powder and compared to stool not homogenized. The mean variance was calculated by taking the average of the variances determined for each bacteria taxa from the four subjects that were examined.</p

Stool storage at room temperature alters the abundance of bacterial taxa.

<p>Ten subsamples from the same stool were either stored at room temperature for 15 minutes or for 30 minutes, followed by DNA extraction and used to compare bacterial taxa via qPCR. Bacteroidetes detection decreased after 30 minutes at room temperature, whereas Firmicutes increased after 30 minutes. *, p > 0.05.</p