Plateau properties in mammalian spinal interneurons during transmitter-induced locomotor activity

We examined the organization of spinal networks controlling locomotion in the isolated spinal cord of the neonatal rat, and in this study we provide the first demonstration of plateau and bursting mechanisms in mammalian interneurons that show locomotor-related activity. Using tight-seal whole-cell recordings, we characterized the activity of interneurons from spinal regions previously suggested to be involved in locomotor rhythm generation. Most (63%) interneurons showed rhythmic, oscillating membrane potentials in phase with rhythmic ventral root activity induced by the glutamate receptor agonist, N-methyl-D-aspartate and 5-hydroxytryptamine or activation of muscarinic acetylcholine receptors. We focused our attention on these cells because they appeared most likely to be participating in locomotor networks. The rhythmic oscillations of most of these interneurons (88%) appeared to be driven mainly by excitatory and inhibitory synaptic inputs. A smaller number of interneurons, however, also displayed intrinsic plateau properties or bursting capabilities which amplified their response to excitatory input, and which were correlated with the presence of negative slope regions in the steady-state I-V curve, and with the ability to burst in the absence of synaptic drive. Although the bursting properties of these neurons may contribute to the generation of the locomotor rhythm, as suggested previously in studies of lower vertebrates, we suggest that a prime role of intrinsic plateau properties in mammalian locomotor networks is to facilitate or shape and time the propagation of information in the network.