Induction of a non-rhythmic motor pattern by nitric oxide in hatchling Rana temporaria embryos

Nitric oxide (NO) is a ubiquitous neuromodulator with a diverse array of fu nctions in a variety of brain regions, but a role for NO in the generation of locomotor activity has yet to be demonstrated. The possibility that NO i s involved in the generation of motor activity in embryos of the frog Rana temporaria was investigated using the NO donors S-nitroso-n-acetylpenicilla mine (SNAP; 100-500 mu mol l(-1)) and diethylamine nitric oxide complex sod ium (DEANO; 25-100 mu mol l(-1)). Immobilised Rana temporaria embryos gener ate a non-rhythmic 'lashing' motor pattern either spontaneously or in respo nse to dimming of the experimental bath illumination, Path-applied NO donor s triggered a qualitatively similar motor pattern in which non-rhythmic mot or bursts were generated contra- and ipsilaterally down the length of the b ody. The inactive precursor of SNAP, n-acetyl-penicillamine (NAP), at equiv alent concentrations did not trigger motor activity. NO donors failed to in itiate swimming and had no measurable effects on the parameters of swimming induced by electrical stimulation. Intracellular recordings with potassium -acetate-filled electrodes revealed that the bursts of ventral root dischar ge induced by NO donors were accompanied by phasic depolarisations in motor neurons, During the inter-burst intervals, periods of substantial membrane hyperpolarisation below the normal resting potential were observed, presum ably coincident with contralateral ventral root activity, With KCl-filled e lectrodes, inhibitory potentials were strongly depolarising, suggesting tha t inhibition was Cl--dependent, The synaptic drive seen in motor neurons af ter dimming of the illumination was very similar to that induced by the NO donors. NADPH-diaphorase histochemistry identified putative endogenous sour ces of NO in the central nervous system and the skin. Three populations of bilaterally symmetrical neurons mere identified within the brainstem, Some of these neurons had contralateral projections and many had axonal processe s that projected to and entered the marginal zones of the spinal cord, sugg esting that they were reticulospinal.