Altering swimming speed by spinal motor circuit of Xenopus larvae

Centre for Neuroregeneration, University of Edinburgh, Chancellor's Building, 49 Little France Crescent, Edinburgh, EH16 4SB

The ability to change speed, direction and strength of rhythmic locomotor activity, like walking and swimming, is vital for animals in everyday life. These movements are controlled by neural circuits in the spinal cord called central pattern generators (CPGs). How CPG networks generate flexible locomotor output during development remains largely unknown. To tackle this question, a simple model, the spinal cord of Xenopus larva, is used to study the control of locomotor speed. Rhythmic motor activity, i.e. swimming, generated by CPG network of Xenopus embryos (2 days old) starts at ~25Hz and then gradually slow down until it stops. Whereas one day later Xenopus larvae (3 days old) can produce a flexible motor pattern with varying frequencies during fictive swimming (motor circuit functions without muscle contraction). Following a hindbrain lesion, 87% of larvae (26 out of 30) display the stereotyped embryonic motor pattern, and only 13% still exhibit the flexible pattern, which indicates that spinal CPG network itself can alter motor speed, but descending inputs from higher brain centres play an important role. Applying D-serine to lesioned larvae restores flexible motor pattern indicating that excitatory synaptic activity is essential for changing motor frequency. Indeed, our whole-cell patch-clamp recordings show that more neurons are recruited or fire more action potentials during the period of increasing motor frequency. Both excitatory and inhibitory post-synaptic activities are enhanced during that period as well. Investigating mechanisms underlying swimming speed control in a simple vertebrate will help us to understand how more complex and flexible movements are generated and modulated in mammalian systems.

Funded by: Biotechnology and Biological Sciences Research Council (BBSRC)