Nitric oxide synthase and the contribution of nitric oxide to vertebrate motor control

Abstract

I have explored the role of nitric oxide (NO) and the distribution of its synthetic enzyme in neurons, nNOS, in vertebrate motor control. The CNS of Xenopus laevis tadpoles was used primarily in this investigation as a developmental model of neural plasticity. During metamorphosis, spinal locomotor networks for axial‐based swimming in the tadpole undergo dynamic reconfiguration in order to generate mature rhythmic motor patterns for limb‐based propulsion in the frog.

Thyroid hormones (THs) orchestrate this change, possibly involving NO signalling in the CNS. Anatomical data were obtained from organotypic brainstem‐spinal cord cultures using histochemical techniques. THs induced NOS expression in the spinal cord of stage 47, premetamorphic CNSs, a developmental stage when NOS is normally virtually absent in this region, after 3 days of organotypic culture. The pattern of NOS‐expressing cells was found to be similar to that of more mature tadpoles, providing evidence that one developmental effect of THs in Xenopus CNS is to induce the expression of the NOS enzyme in a regionally specific manner.

The role of NO on the maturation of motor systems in the spinal cord was subsequently explored further, again using organotypic culture methods. Xenopus CNSs at stage 47 were cultured in the presence of the NO donor SNAP, or the NOS inhibitor L‐NAME. Primary motor neurons in the spinal cord serve embryonic swimming; secondary motor neuron pools provide motor innervation to the limbs, but also to mature larval and adult axial musculature. During development, secondary motor systems eventually supersede the primary system. In my organotypic cultures at stage 47, L‐NAME had no significant effect, but exogenous NO was found to cause an increase in the number of presumptive secondary motor neurons in the brachial region of the spinal cord, as indicated using choline acetyltransferase (ChAT) immunohistochemistry. This indicates that NO has a differentiating effect on secondary motor neurons in this region and provides evidence suggesting a maturational role of NO in the reconfiguration of spinal locomotor circuitry.

Finally, the presence of NOS in the spinal cord of the lamprey, a primitive vertebrate, was explored using histochemical techniques. The lamprey spinal cord is a well‐studied model of rhythm generation by CPGs. NOS expression was found in motor and sensory cells involved in the generation and modulation of locomotion: motor neurons, dorsal cells, edge cells, and giant interneurons labelled for both NADPH‐diaphorase and nNOS. This suggests that NO might exert a modulatory influence on rhythmic locomotion in the lamprey. Furthermore, my data indicate NO’s role in motor control appeared early in vertebrate evolution, and that NO signalling in the CNS has been evolutionarily conserved. Taken together, data from Xenopus and lamprey suggest an important role for NO in the development and execution of vertebrate motor control.