GABA-Receptor–Independent Dorsal Root Afferents Depolarization in the Neonatal Rat Spinal Cord

Abstract

Dorsal root afferent depolarization and antidromic firing were studied in isolated spinal cords of neonatal rats. Spontaneous firing accompanied by occasional bursts could be recorded from most dorsal roots in the majority of the cords. The afferent bursts were enhanced after elevation of the extracellular potassium concentration ([K+]e) by 1-2 mM. More substantial afferent bursts were produced when the cords were isolated with intact brain stems. Rhythmic afferent bursts could be recorded from dorsal roots in some of the cords during motor rhythm induced by bath-applied serotonin and N-methyl--aspartate (NMDA). Bilaterally synchronous afferent bursts were produced in pairs of dorsal roots after replacing the NaCl in the perfusate with sodium-2-hydroxyethansulfonate or after application of the gamma-aminobutyric acid-A (GABAA) receptor antagonist bicuculline with or without serotonin (5-HT) and NMDA. Antidromic afferent bursts also could be elicited under these conditions by stimulation of adjacent dorsal roots, ventrolateral funiculus axons, or ventral white commissural (VWC) fibers. The antidromic bursts were superimposed on prolonged dorsal root potentials (DRPs) and accompanied by a prolonged increase in intraspinal afferent excitability. Surgical manipulations of the cord revealed that afferent firing in the presence of bicuculline persisted in the hemicords after hemisection and still was observed after removal of their ventral horns. Cutting the VWC throughout its length did not perturb the bilateral synchronicity of the discharge. These findings suggest that the activity of dorsal horn neurons is sufficient to produce the discharge and that the bilateral synchronicity can be maintained by cross connectivity that is relayed from side to side dorsal to the VWC. Antagonists of GABAB, 5-HT2/5-HT1C, or glutamate metabotropic group II and III receptors could not abolish afferent depolarization in the presence of bicuculline. Depolarization comparable in amplitude to DRPs, could be produced in tetrodotoxin-treated cords by elevation of [K+]e to the levels reported to develop in the neonatal rat spinal cord in response to dorsal root stimulation. A mechanism involving potassium transients produced by neuronal activity therefore is suggested to be the major cause of the GABA-independent afferent depolarization reported in our study. Possible implications of potassium transients in the developing and the adult mammalian spinal cord are discussed.

Sensory feedback synchronizes motor and sensory neuronal networks in the neonatal rat spinal cord

“…Spontaneous activity also has been described in vitro , using the neonatal rat spinal cord preparation or neonatal rat spinal cord slices, in which correlated sensorimotor activity may be supported by intrinsic connections and emphatic interactions33343543. Therefore, we further investigated interactions between sensory and motor zones in the in vitro spinal cord (from P5-7 rats) preparation, through silicone probe recordings, as performed in vivo (Supplementary Fig.…”

“…Gating of sensory feedback could occur at the level of spinal cord, for example via local GABAergic interneuron mediated inhibitory corollary discharge on dorsal sensory neurons and primary afferents32, but this remains hypothetical. Even more complexity is added by observations made in the in vitro spinal cord preparation, which displays spontaneous motor bursts33, and in which other forms of communication between motor and sensory neurons have been suggested, including excitatory efferent copy, mediated either by interneurons activating primary afferents via depolarizing GABA actions or through ephaptic interactions3435. However, it remains unknown whether sensorimotor network activity and the local interactions described in the in vitro spinal cord preparation correspond to the spinal cord network dynamics in behaving animals.…”

Primary Afferent Terminals Acting as Excitatory Interneurons Contribute to Spontaneous Motor Activities in the Immature Spinal Cord

“…A robust bursting antidromic activity is recorded from lumbar DRs in the in vitro spinal cord preparation isolated from neonatal rats (see Results) (see also Kremer and Lev-Tov, 1998;FellippaMarques et al, 2000). The sensory fibers exhibiting antidromic discharges were not identified in the present study because, in contrast to adults, primary afferents cannot be characterized by their conduction velocities in neonates since they are nonmyelinated.…”

Modeling the mammalian locomotor CPG: insights from mistakes and perturbations

“…1B and 2A). As shown experimentally, antagonists of glycinergic and GABAergic inhibition can produce rhythmic activity characterized by synchronized bursting in flexor and extensor motoneurons (e.g., Noga et al, 1993;Cowley and Schmidt, 1995;Kremer and Lev-Tov, 1998;Beato and Nistri, 1999) instead of an alternating, locomotor pattern. To simulate oscillations evoked by blocking inhibitory transmission, the weights of all inhibitory connections in the model were set to zero.…”

“…The finding that synchronized oscillations are evoked when synaptic inhibition is blocked in mammalian spinal cord preparations (Noga et al, 1993;Cowley and Schmidt, 1995;Kremer and Lev-Tov, 1998;Beato and Nistri, 1999) has led to the suggestion that the spinal locomotor network has endogenous rhythmogenic properties, which do not require inhibition (Kiehn, 2006;Rossignol et al, 2006). While we agree that a rhythm can be produced in the absence of network inhibition and our model can indeed produce such a rhythm, we do not accept that these oscillations represent the locomotor rhythm.…”