Evolutionary divergence in developmental strategies and neuromodulatory control systems of two amphibian locomotor networks

Attempts to understand the neural mechanisms which produce behaviourmust consider both prevailing sensory cues and the central cellularand synaptic changes they direct. At each level, neuromodulationcan additionally shape the final output. We have investigatedneuromodulation in the developing spinal motor networks in hatchlingtadpoles of two closely related amphibians, Xenopus laevis andRana temporaria to examine the subtle differences in their behavioursthat could be attributed to their evolutionary divergence. At the point of hatching, both species can swim in responseto a mechanosensory stimulus, however Rana embryos often displaya more forceful, non-locomotory coiling behaviour. Whilst thesynaptic drive that underlies these behaviours appears similar,subtle inter-specific differences in neuronal properties shapemotor outputs in different ways. For example, Rana neurons expressN-methyl-D-aspartate (NMDA)/serotonin (5-HT)-dependent oscillations,not present in hatchling Xenopus and many also exhibit a prominentslow spike after-hyperpolarisation. Such properties may endowthe spinal circuitry of Rana with the ability to produce a moreflexible range of outputs. Finally, we compare the roles of the neuromodulators 5-HT, noradrenaline(NA) and nitric oxide (NO) in shaping motor outputs. 5-HT increasesburst durations during swimming in both Xenopus and Rana, but5-HT dramatically slows the cycle period in Rana with littleeffect in Xenopus. Three distinct, but presumably homologousNO-containing brainstem clusters of neurons have been described,yet the effects of NO differ between species. In Xenopus, NOslows and shortens swimming in a manner similar to NA, yet inRana NO and NA elicit the non-rhythmic coiling pattern.