In vivo beta and gamma subthreshold oscillations in rat mitral cells: origin and gating by respiratory dynamics

In mammals, olfactory bulb (OB) dynamics are paced by slow and fast oscillatory rhythms at multiple levels: local field potential, spike discharge, and/or membrane potential oscillations. Interactions between these levels have been well studied for the slow rhythm linked to animal respiration. However, less is known regarding rhythms in the fast beta (10-35Hz) and gamma (35-100Hz) frequency ranges, particularly at the membrane potential level. Using a combination of intracellular and extracellular recordings in the OB of freely breathing rats, we show that beta and gamma subthreshold oscillations (STO) coexist intracellularly and are related to extracellular LFP oscillations in the same frequency range. However, they are differentially affected by changes in cell excitability and by odor stimulation. This leads us to suggest that beta and gamma STOs may rely on distinct mechanisms: gamma STOs would mainly depend on mitral cell intrinsic resonance while beta STOs could be mainly driven by synaptic activity. In a second part, we found that STO occurrence and timing are constrained by the influence of the slow respiratory rhythm on mitral/tufted cells. First, respiratory-driven excitation seems to favor gamma STOs while respiratory-driven inhibition favors beta STOs. Second, the respiratory rhythm is needed at the subthreshold level in order to lock gamma and beta STOs in similar phases as their LFP counterparts and to favor the correlation between STO frequency and spike discharge. Overall, this study helps to understand how the interaction between slow and fast rhythms at all levels of OB dynamics shapes its functional output.

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