Elsevier

Neuroscience

Volume 94, Issue 4, November 1999, Pages 1199-1206
Neuroscience

Regulation by glycine, Mg2+ and polyamines of the N-methyl-d-aspartate-induced locomotion in the neonatal rat spinal cord in vitro

https://doi.org/10.1016/S0306-4522(99)00301-2Get rights and content

Abstract

Excitatory amino acids are known to activate the spinal neural network that organize locomotor activity in various species. In this study, the role of various compounds which alter the functioning of the N-methyl-d-aspartate receptor (glycine, Mg2+ ions and spermine) was investigated during fictive locomotion, using an in vitro isolated spinal cord preparation from neonatal rats. Locomotor-like activity induced by excitatory amino acids was recorded both extra- and intracellularly. 7-chlorokynurenic acid, an antagonist of the glycine site at the N-methyl-d-aspartate receptor, depressed the N-methyl-d-aspartate component of the synaptic inputs received by the motoneurons. Glycine at low concentrations had no effect on locomotor activity, while 7-chlorokynurenic acid increased the locomotor period and decreased the burst amplitude in a dose-dependent manner. Removal of Mg2+ ions from the saline facilitated the N-methyl-d-aspartate-mediated response, and triggered spontaneous bursting activity, abolished by 2-amino-5-phosphonovaleric acid, an antagonist of the N-methyl-d-aspartate receptor. The polyamine, spermine, did not change the locomotor parameters. On the contrary, arcaine, a putative antagonist of the polyamine site on the N-methyl-d-aspartate receptor, increased locomotor activity. The effects of arcaine were counteracted by spermine.

These results suggest that glycine and spermine are present at saturating concentrations on the N-methyl-d-aspartate receptor during ongoing locomotion. Together with Mg2+ ions, these endogenous regulators contribute to control the level of activity of the N-methyl-d-aspartate receptor in the spinal cord of the neonatal rat.

Section snippets

Dissection

Experiments were performed on 34 Wistar rats aged one to four days. The animals were deeply anesthetized with ether before decapitation. A laminectomy was done to expose the spinal cord and to remove it. The spinal cord, cut at the thoracic level, was then placed in a recording chamber. The preparation was superfused (3 ml/min) with an oxygenated (95% O2/5% CO2) saline (composition in mM: NaCl 130; KCl 3; CaCl2 3.75; MgSO4 1.3; NaH2PO4 0.58; NaHCO3 25; glucose 10). The solution was adjusted to

Effects of 7-chlorokynurenic acid on the locomotor synaptic inputs on motoneurons

The superfusion of NMA/5-HT or of NMA alone on the lumbar spinal cord (Fig. 2A1) initiated locomotor-like activity which was recorded extracellularly from the L2 ventral roots and intracellularly from a motoneuron (Fig. 2A2). This stereotypical locomotor pattern consisted of bursts of action potentials alternating between the right and left side of each lumbar segment and between the ipsilateral L2 and L5 ventral roots.7., 20., 26.

In order to test whether glycine regulates the excitatory part

Discussion

The results of the present study show that the various allosteric mechanisms controlling the NMDA receptor are already at work at birth. First, the NMDA part of the excitatory synaptic inputs conveyed by the L1–L2 network to the motoneurons is under the control of endogenous glycine. Secondly, the NMA-induced locomotion is positively controlled by glycine and negatively controlled by both Mg2+ and spermine in the neonatal rat spinal cord.

Most of the studies done on the regulation of the NMDA

Conclusion

This study shows that, at birth, the complex allosteric regulatory mechanisms are already at work regulating NMDA receptor activity. As for glycine, it appears to act at multiple levels of locomotor processes. As NMDA receptors are transiently overexpressed in the developing spinal cord,18 glycine, Mg2+ and spermine may play an important role in developmental regulation by determining the level of activity of the NMDA currents in the neonatal rat.

Acknowledgements

The authors wish to thank Dr H. Burnet for his help in statistical analysis, G. Riviere for her technical assistance and Dr F. Clarac and K. Duff for reading and correcting the manuscript. Many thanks to R. Navarro for nursing the pups.

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