Early olfactory-induced rhythmic limb activity in the newborn rat

Locomotor-like rhythmic movements without postural constraints were elicited in newborn rats aged from a few hours to five days, using an olfactory stimulus provided by bedding materials. The rats were held in a sling with the front and the hind legs hanging on each side. The step frequency increased between postnatal days 0 and 4 (P0-P4); the step period was around 1 s at P0 and decreased during the following days. This decrease was larger in the forelimbs (650 ms at P4) than in the hindlimbs (750 ms at P4) and was mainly due to a decrease in stance duration. Both ipsilateral and contralateral legs moved in an alternating pattern. Analysis of the regulation of this pattern when a 1:1 or a 1:2 inter-leg coordination (with double steps) occurred showed that both anterior and posterior locomotion pattern generators were coupled very early. Results are discussed in relation to the locomotor activities studied at this early stage of life in other behavioral situations (swimming and air stepping), and in relation to fictive locomotion induced in vitro.