NMDA-dependent, network-driven oscillatory activity induced by bicuculline or removal of Mg²⁺ in rat olfactory bulb neurons

Spontaneous, low-frequency voltage oscillations (LFOs) were observed in the neurons of rat olfactory bulb upon disinhibition with GABA_A antagonists and/or removal of Mg²⁺ from external saline. Ordinarily, LFOs presented a highly organized temporal structure with bursts recurring regularly at about 0.05 Hz. Slow depolarizing shifts with similar frequencies were observed in all types of bulbar neurons. Simultaneous recordings from mutually independent neurons showed that LFOs were highly synchronized in distinct cells. The occurrence of LFOs was prevented by NMDA, but not AMPA/kainate, receptor antagonists. The oscillations were also halted by Ca²⁺ antagonists and tetrodotoxin. The pace of the oscillations was reset by stimulation of the olfactory nerve but not by direct injection of depolarizing current into the oscillating cell. Removal of the outer portion of the slice with a cut along the external plexiform layer provided crucial evidence that the bursting activity first initiated in the glomerular region and propagated synaptically downstream towards the inner layers, suggesting an organizing role for olfactory glomeruli.