Sniff rhythm-paced fast and slow gamma-oscillations in the olfactory bulb: relation to tufted and mitral cells and behavioral states.

Odor signals are conveyed from the olfactory bulb (OB) to the olfactory cortex by two types of projection neurons, tufted cells and mitral cells, which differ in signal timing and firing frequency in response to odor inhalation. Whereas tufted cells respond with early-onset high-frequency burst discharges starting at the middle of the inhalation phase of sniff, mitral cells show odor responses with later-onset lower-frequency burst discharges. Since odor inhalation induces prominent gamma-oscillations of local field potentials (LFPs) in the OB during the transition period from inhalation to exhalation that accompany synchronized spike discharges of tufted cells and mitral cells, we addressed the question of whether the odor-induced gamma-oscillations encompass two distinct gamma-oscillatory sources, tufted cell and mitral cell subsystems, by simultaneously recording the sniff rhythms and LFPs in the OB of freely behaving rats. We observed that individual sniffs induced nested gamma-oscillations with two distinct parts during the inhalation-exhalation transition period: early-onset fast gamma-oscillations followed by later-onset slow gamma-oscillations. These results suggest that tufted cells carry odor signals with early-onset fast gamma-synchronization at the early phase of sniff, whereas mitral cells send them with later-onset slow gamma-synchronization. We also observed that each sniff typically induced both fast and slow gamma-oscillations during awake, whereas respiration during slow-wave sleep and rapid-eye-movement sleep failed to induce these oscillations. These results suggest that behavioral states regulate the generation of sniff rhythm-paced fast and slow gamma-oscillations in the OB.