The Isolation and Identification of Spinal Neurons That Control Movement in the Xenopus Embryo.

A procedure for isolating spinal neurons of the Xenopus embryo has been devised. Using a variety of methods four of the eight previously described categories of neuron can be distinguished in vitro. Firstly, since many anatomical features of the neurons survived dissociation, sensory neurons and glycinergic inhibitory interneurons could be identified after isolation using anatomical criteria. Secondly, by selective labelling of the neurons in the intact spinal cord with fluorescein-conjugated dextran amines prior to dissociation, three classes of isolated neuron could be identified: motoneurons, putative excitatory interneurons and, once again, inhibitory interneurons. The identity of the inhibitory interneurons has been confirmed using glycine immunocytochemistry. The physiological properties of the isolated neurons were similar to those of their in vivo counterparts. The dissociated neurons exhibited strong membrane accommodation and outward rectification, both of which could be blocked by the injection of Cs+ ions. The neurons also retained their receptors for the agonists N-methyl-d-aspartate, kainate, quisqualate, glycine and GABA. Both the membrane properties and pharmacological sensitivity of the isolated neurons therefore appeared to be unaltered by the dissociation procedure. Thus the four classes of neuron from the Xenopus embryonic nervous system that can be identified after isolation constitute the basis for a valid model for detailed study of how the properties of individual neurons contribute to the functioning of the circuitry that underlies locomotor pattern generation in vertebrates.