The distribution of nadph-diaphorase-labelled interneurons and the role ofnitric oxide in the swimming system of Xenopus laevis larvae

The possible involvement of the free radical gas nitric oxide (NO) in the m odulation of spinal rhythm-generating networks has been studied using Xenop us laevis larvae. Using NADPH-diaphorase histochemistry, three putative pop ulations of nitric oxide synthase (NOS)-containing cells were identified in the brainstem. The position and morphology of the largest and most caudal population suggested that a proportion of these neurons is reticulospinal. The possible contribution of nitrergic neurons to the control of swimming a ctivity was examined by manipulating exogenous and endogenous NO concentrat ions in vivo with an NO donor (SNAP, 100-500 mu mol l(-1)) and NOS inhibito rs (L-NAME and L-NNA, 0.5-5 mmol l(-1)), respectively. In the presence of S NAP, swim episode duration decreased and cycle period increased, whereas th e NOS inhibitors had the opposite effects. We conclude from these data that the endogenous release of NO from brainstem neurons extrinsic to the spina l cord of Xenopus laevis larvae exerts a continuous modulatory influence on swimming activity, functioning like a 'brake'. Although the exact level at which NO impinges upon the swimming rhythm generator has yet to be determi ned, the predominantly inhibitory effect of NO suggests that the underlying mechanisms of NO action could involve modulation of synaptic transmission and/or direct effects on neuronal membrane properties.