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 ut ilised as a model for the mechanisms underlying the generation and developm ent of vertebrate locomotion. In this paper, we review evidence on the role of descending brainstem projections in the maturation and intrinsic modula tion of tadpole spinal motor networks. Three transmitter systems that have been investigated utilise the biogenic amines serotonin (5HT) and noradrena line (NA) and the inhibitory amino acid gamma -aminobutyric acid (GABA), Th e distribution, development and spinal targets of these systems will be rev iewed. More recent data on the role of nitric oxide (NO) will also be discu ssed. This ubiquitous gaseous signalling molecule is known to play a crucia l role in the developing nervous system, but until recently, had not been d irectly implicated in the brain regions involved in motor control. NO appea rs 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 spe cies. Given the presumed colocalisation and interaction of nitric oxide wit h conventional neurotransmitters, we discuss the potential relationship of nitrergic neurons with 5HT, NA and GABA in these amphibian models. (C) 2001 Elsevier Science Inc.