Nitric oxide (NO) is a ubiquitous neuromodulator with a diverse array of functions in a variety of brain regions, but a role for NO in the generation of locomotor activity

behaviour patterns according to prevailing behavioural and developmental requirements. Even among closely related species, the expression and emergence of different patterns of motor behaviour during development can follow different temporal sequences, presumably to meet the demands imposed by their distinct ecological niches. However, the way in which motor networks are modified during ontogeny to enable species-specific temporal expression of different motor patterns is not fully understood. Neuronal inputs originating in the brainstem and descending to the spinal cord both modulate ongoing motor programs and prime, activate and suppress motor behaviour. As such, developmentally regulated ingrowth to the spinal cord by the axonal processes of brainstem neurons may be important for the temporal expression of different behaviour patterns. For example, at the time of hatching, the tadpole of Rana temporaria (stage 20: Gosner, 1960) is capable of generating a well-coordinated swimming motor pattern (Soffe, 1991; Soffe and Sillar, 1991). However, fictive swimming in Rana temporaria is relatively more mature than in hatchling tadpoles of the related anuran Xenopus laevis in that motor bursts are longer and more variable on a cycle-bycycle basis (Sillar and Soffe, 1989). It has been speculated that this interspecific difference at equivalent stages of development is due to the greater development of descending serotonergic projections in Rana temporaria because serotonin (5-HT) is known to facilitate motor burst generation in both species (Woolston et al., 1994; Sillar et al., 1993). The free radical gas nitric oxide (NO) is recognised as a ubiquitous signalling molecule that is intimately involved with 1307 The Journal of Experimental Biology 204, 1307–1317 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB3072