The Journal of Neurophysiology Vol. 86 No. 6 December 2001, pp. 3030-3042
Copyright ©2001 by the American Physiological Society

NMDA Receptor-Dependent Periodic Oscillations in Cultured Spinal Cord Networks

 ¹The Neurosciences Institute, San Diego, California 92121;  ²Department of Biological Sciences and Center for Network Neuroscience, University of North Texas, Denton, Texas 76203; and  ³Institut für Zellbiologie und Biosystemtechnik, University of Rostock, 18051 Rostock, Germany

Keefer, Edward W., Alexandra Gramowski, and Guenter W. Gross. NMDA Receptor-Dependent Periodic Oscillations in Cultured Spinal Cord Networks. J. Neurophysiol. 86: 3030-3042, 2001. Cultured spinal cord networks grown on microelectrode arrays display complex patterns of spontaneous burst and spike activity. During disinhibition with bicuculline and strychnine, synchronized burst patterns routinely emerge. However, the variability of both intra- and interculture burst periods and durations are typically large under these conditions. As a further step in simplification of synaptic interactions, we blocked excitatory AMPA synapses with 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzoquinoxaline-7-sulphonamide (NBQX), resulting in network activity mediated through the N-methyl-D-aspartate (NMDA) receptor (NMDA_ONLY). This activity was APV sensitive. The oscillation under NMDA_ONLY conditions at 37°C was characterized by a period of 2.9 ± 0.3 s (16 separate cultures). More than 98% of all neurons recorded participated in this highly rhythmic activity. The temporal coefficients of variation, reflecting the rhythmic nature of the oscillation, were 3.7, 4.7, and 4.9% for burst rate, burst duration, and interburst interval, respectively [mean coefficients of variation (CVs) for 16 cultures]. The oscillation persisted for at least 12 h without change (maximum observation time). Once established, it was not perturbed by agents that block mGlu receptors, GABA_B receptors, cholinergic receptors, purinergic receptors, tachykinin receptors, serotonin (5-HT) receptors, dopamine receptors, electrical synapses, burst afterhyperpolarization, NMDA receptor desensitization, or the hyperpolarization-activated current. However, the oscillation was destroyed by bath application of NMDA (20-50 µM). These results suggest a presynaptic mechanism underlying this periodic rhythm that is solely dependent on the NMDA synapse. When the AMPA/kainate synapse was the sole driving force (n = 6), the resulting burst patterns showed much higher variability and did not develop the highly periodic, synchronized nature of the NMDA_ONLY activity. Network size or age did not appear to influence the reliability of expression of the NMDA_ONLY activity pattern. For this reason, we suggest that the NMDA_ONLY condition unmasks a fundamental rhythmogenic mechanism of possible functional importance during periods of NMDA receptor-dominated activity, such as embryonic and early postnatal development.