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Abstract
Different aspects of spinal locomotor organization have been studied in the mouse
during embryonic and neonatal development using in vitro preparations of isolated
lumbosacral cords. The first consideration was the embryonic development of an alternating
bilateral pattern. From embryonic day (E) 12, perfusion of serotonin could induce
relatively synchronous lumbar bursts across the cord. Bilateral activity became progressively
alternate at E15 due to the appearance of glycinergic inhibitory interactions (revealed
by strychnine application). Strictly alternating patterns were expressed at E18 and
were maintained after birth. In a second step, we investigated cellular properties
involved in lumbar rhythmogenesis in postnatal day 0-2 preparations which displayed
spontaneous locomotor-like activity. Perfusion of receptor antagonists showed the
co-operative involvement of N-methyl-D-aspartate (NMDA)- and non-NMDA-receptors for
excitatory amino acids-mediated operation of locomotor networks. In a final step we
investigated the localization of locomotor networks within the lumbar cord. Data obtained
from preparations exhibiting spontaneous or Mg2+-free induced bursts revealed that
the networks are present throughout the lumbar cord and that rhythmogenesis is distributed
throughout all segmental levels.