Identifying excitatory drive for rhythmic locomotor activity during development

Supervisors: HongYan Zhang, Keith Sillar

Project description:

Understanding how rhythmic locomotor movements (e.g. walking and swimming) are controlled by spinal neurons is an important goal of neuroscience research. Spinal neurons controlling movements comprise motor circuits called central pattern generators (CPGs). To generate rhythmic activity, a pacemaker-like excitatory drive is expected to exist in spinal CPGs. Identifying such a special neuron type is still essential. In hatchling Xenopus embryos (2-day old tadpoles), a group of excitatory descending interneurons, dINs, has been found to be the pacemaker-like neurons (Roberts et al., 2010). Such neurons have also been observed at later larval and metamorphic stages (unpublished data), which indicates that this type of excitatory interneuron is preserved during development. In this project, we aim to investigate the role of dINs in rhythm generation over a long developmental period.

dINs in Xenopus embryos display a unique set of electrophysiological properties compared to other CPG neurons (Roberts et al., 2010). It seems dIN unique properties persist at later stages, despite the motor output of the CPG being more mature (Currie et al., 2016). Therefore, we assume that the fundamental mechanisms underlying rhythm generation might persist during development.

To test this hypothesis, firstly, dIN from different stages will be recorded using whole-cell patch-clamp techniques in in vivo, semi in vivo and in vitro preparations. dIN electrical properties and postsynaptic activities can be investigated. The student will be trained in two laboratories located in Edinburgh and St Andrews with electrophysiological techniques, such skills are highly desired in neuroscience research field. The student will be trained with other related skills, e.g. microdissection. Both research experience and academic networking can be broadened by studying in different laboratories/universities.

Secondly, dIN morphology will be observed following patch recordings using different labelling methods. Bright field microscopy and confocal imaging will be utilized to observe dIN morphology on a cellular level.  Electron microscopy will be used to identify subcellular ultrastructure of dINs and their intercellular contacts to other cells, e.g. gap junctions. Our centre has many experts with these skills, and abundant communal equipment for experiments. These imaging techniques are widely utilised in biology and neuroscience. With this training the student will gain an excellent understanding of neuronal circuitry in addition to electrophysiological knowledge. 

Thirdly, unlike other CPG neurons, dINs do not display a Na/K pump mediated ultra-slow after-hyperpolarisation (usAHP) following a swim episode (Zhang & Sillar, 2012). Such an absence of a usAHP seems important for animals to reliably initiate rhythm activity without being suppressed by preceding activity. Our recent data (unpublished) indicate that the usAHP in dINs is masked by a depolarising Ih current that appears during development. Revealing the nature of Ih current and usAHP in dINs is vital for understanding the role of dINs in rhythm generation. Pharmacological approaches will be employed to characterise these components and to reveal modulations by various factors.

This project is the first to investigate the excitatory drive in spinal motor circuit across a long developmental period, from embryonic to metamorphic stages, providing fundamental knowledge of motor circuit organisation and development.

References:

Currie SP, Doherty GH, Sillar KT. (2016) Deep-brain photoreception links luminance detection to motor output in Xenopus frog tadpoles. PNAS, 113: 6053-6058. 

Roberts A., Li W.-C. & Soffe S. R. (2010). How neurons generate behavior in a hatchling amphibian tadpole: an outline. Frontiers in Behavioral Neuroscience, 4, 16.

Zhang H. Y. & Sillar K. T. (2012). Short-term memory of motor network performance via activity-dependent potentiation of Na+/K+ pump function. Current Biology, 22, 526–531.

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