title: Primary afferent terminals acting as excitatory interneurons contribute to spontaneous motor activities in the immature spinal cord.
abstract: Patterned, spontaneous activity plays a critical role in the development of neuronal networks. A robust spontaneous activity is observed in vitro in spinal cord preparations isolated from immature rats. The rhythmic ventral root discharges rely mainly on the depolarizing/excitatory action of GABA and glycine early during development, whereas at later stages glutamate drive is primarily responsible for the rhythmic activity and GABA/glycine are thought to play an inhibitory role. However, rhythmic discharges mediated by the activation of GABA(A) receptors are recorded from dorsal roots (DRs). In the present study, we used the in vitro spinal cord preparation of neonatal rats to identify the relationship between discharges that are conducted antidromically along DRs and the spontaneous activity recorded from lumbar motoneurons. We show that discharges in DRs precede those in ventral roots and that primary afferent depolarizations (PADs) start earlier than EPSPs in motoneurons. EPSP-triggered averaging revealed that the action potentials propagate not only antidromically in the DR but also centrally and trigger EPSPs in motoneurons. Potentiating GABAergic antidromic discharges by diazepam increased the EPSPs recorded from motoneurons; conversely, blocking DR bursts markedly reduced these EPSPs. High intracellular concentrations of chloride are maintained in primary afferent terminals by the sodium-potassium-chloride cotransporter NKCC1. Blocking these cotransporters by bumetanide decreased both dorsal and ventral root discharges. We conclude that primary afferent fibers act as excitatory interneurons and that GABA, through PADs reaching firing threshold, is still playing a key role in promoting spontaneous activity in neonates.
authors.affiliation: Laboratoire Plasticité et Physio-Pathologie de la Motricité, UMR 6196, Centre National de la Recherche Scientifique, and Aix-Marseille Université, F-13402 Marseille cedex 20, France.
authors.forename: Rémi
authors.initials: R
authors.lastname: Bos
authors.forename: Frédéric
authors.initials: F
authors.lastname: Brocard
authors.forename: Laurent
authors.initials: L
authors.lastname: Vinay
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000818:Animals; D000831:Animals, Newborn; D019706:Excitatory Postsynaptic Potentials / Q000502:physiology*; D005260:Female; D007395:Interneurons / Q000502:physiology*; D008297:Male; D009046:Motor Neurons / Q000502:physiology*; D009475:Neurons, Afferent / Q000502:physiology; D051381:Rats; D017208:Rats, Wistar; D011963:Receptors, GABA-A / Q000502:physiology; D013116:Spinal Cord / Q000502:physiology*; D013126:Spinal Nerve Roots / Q000502:physiology; D013569:Synapses / Q000502:physiology; D005680:gamma-Aminobutyric Acid / Q000502:physiology
chemical_list: D011963:Receptors, GABA-A; D005680:gamma-Aminobutyric Acid