The development of neuromodulatory systems and the maturation of motor patterns in amphibian tadpoles

The relative simplicity of the amphibian tadpole nervous system has been utilised as a model for the mechanisms underlying the generation and development of vertebrate locomotion. In this paper, we review evidence on the role of descending brainstem projections in the maturation and intrinsic modulation of tadpole spinal motor networks. Three transmitter systems that have been investigated utilise the biogenic amines serotonin (5HT) and noradrenaline (NA) and the inhibitory amino acid γ-aminobutyric acid (GABA). The distribution, development and spinal targets of these systems will be reviewed. More recent data on the role of nitric oxide (NO) will also be discussed. This ubiquitous gaseous signalling molecule is known to play a crucial role in the developing nervous system, but until recently, had not been directly implicated in the brain regions involved in motor control. NO appears to be produced by three homologous brainstem clusters in the developing motor networks of two closely related amphibian species, Xenopus laevis and Rana temporaria but, surprisingly, it plays contrasting roles in these species. Given the presumed co-localisation and interaction of nitric oxide with conventional neurotransmitters, we discuss the potential relationship of nitrergic neurons with 5HT, NA and GABA in these amphibian models.