Elsevier

Neuroscience

Volume 141, Issue 3, 2006, Pages 1489-1501
Neuroscience

Pain mechanism
An N-methyl-d-aspartate receptor mediated large, low-frequency, spontaneous excitatory postsynaptic current in neonatal rat spinal dorsal horn neurons

https://doi.org/10.1016/j.neuroscience.2006.04.049Get rights and content

Abstract

Examples of spontaneous oscillating neural activity contributing to both pathological and physiological states are abundant throughout the CNS. Here we report a spontaneous oscillating intermittent synaptic current located in lamina I of the neonatal rat spinal cord dorsal horn. The spontaneous oscillating intermittent synaptic current is characterized by its large amplitude, slow decay time, and low-frequency. We demonstrate that post-synaptic N-methyl-d-aspartate receptors (NMDARs) mediate the spontaneous oscillating intermittent synaptic current, as it is inhibited by magnesium, bath-applied d-2-amino-5-phosphonovalerate (APV), or intracellular MK-801. The NR2B subunit of the NMDAR appears important to this phenomenon, as the NR2B subunit selective NMDAR antagonist, alpha-(4-hydroxphenyl)-beta-methyl-4-benzyl-1-piperidineethanol tartrate (ifenprodil), also partially inhibited the spontaneous oscillating intermittent synaptic current. Inhibition of spontaneous glutamate release by the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or the mu-opioid receptor agonist [d-Ala2, N-Me-Phe4, Gly5] enkephalin-ol (DAMGO) inhibited the spontaneous oscillating intermittent synaptic current frequency. Marked inhibition of spontaneous oscillating intermittent synaptic current frequency by tetrodotoxin (TTX), but not post-synaptic N-(2,6-dimethylphenylcarbamoylmethyl)triethylammonium bromide (QX-314), suggests that the glutamate release important to the spontaneous oscillating intermittent synaptic current is dependent on active neural processes. Conversely, increasing dorsal horn synaptic glutamate release by GABAA or glycine inhibition increased spontaneous oscillating intermittent synaptic current frequency. Moreover, inhibiting glutamate transporters with threo-beta-benzyloxyaspartic acid (dl-TBOA) increased spontaneous oscillating intermittent synaptic current frequency and decay time. A possible functional role of this spontaneous NMDAR–mediated excitatory postsynaptic current in modulating nociceptive transmission within the spinal cord is discussed.

Section snippets

Experimental procedures

All experimental methods were approved by the University of Washington animal care and use committee and adhere to the Committee for Research Ethical Issues guidelines published by the International Association for the Study of Pain. All efforts were made to minimize both the number of rats used in the experiments and the distress involved in the experimental procedures.

Characterization of the sEPSCosc and its sensitivity to Mg2+

An unusually large and slow sEPSC was observed to occur intermittently in lamina I of the spinal dorsal horn of the neonatal rat when the bath Mg2+ concentration was decreased from 2 to 1 mM. In comparison to the sEPSCs present in the aCSF containing 2 mM Mg2+, the sEPSC that occurred in aCSF containing 1 mM Mg2+ was larger (mean maximal amplitude=462±169 pA vs. 50±2 pA, n=7 cells), had a longer rise time (mean rise time=110±6 ms vs. 1±0. 2 ms, n=7 cells), was slower to decay (mean decay

Discussion

In this report we have characterized a novel large spontaneous low frequency oscillating EPSC (sEPSCosc) in lamina I of the neonatal rat spinal cord dorsal horn. This sEPSCosc is: blocked by 2 mM Mg2+ and APV; inhibited by ifenprodil, 1 mM Mg2+, TTX, CNQX, and DAMGO; and potentiated by strychnine, bicuculline, and TBOA. Taken together, these data suggest that this sEPSCosc is generated by glutamate released from pre-synaptic neuronal sites that act on post-synaptic NMDARs, at least some of

Acknowledgments

This work was supported by NIH grant DA14588 (GT). L.T. was supported by T32 DA07278, an institutional fellowship from the National Institute on Drug Abuse. DAMGO and naloxone used in these studies were gifts from the National Institute of Drug Abuse (NIDA) drug supply system.

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    Both L. M. Thomson and J. Zeng contributed equally to this work.

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