Neuronal activity of mitral-tufted cells in awake rats during passive and active odorant stimulation Abbreviated Title: Neuronal activity in the OB of awake rats

Odorants induce specific modulation of Mitral/Tufted (MT) cells’ firing rate in the mammalian olfactory bulb (OB), inducing temporal patterns of neuronal discharge embedded in an oscillatory local field potential (LFP). While most studies have examined anesthetized animals, little is known about the firing rate and temporal patterns of OB single units and population activity in awake behaving mammals. We examined the firing rate and oscillatory activity of MT cells, and local field potentials (LFP) signals in behaving rats during two olfactory tasks: passive exposure (PE), and two-alternative choice discrimination task (TA). MT inhibitory responses are predominant in the TA task (76.5%) while MT excitatory responses predominate in the PE task (59.2%). Rhythmic discharge in the 12-100 Hz range was found in 79.0% and 68.9% of MT cells during PE and TA tasks, respectively. Most odorants presented in PE task increase rhythmic discharges at frequencies above 50 Hz, while in TA, 1 of 4 odorants produced a modest increment below 40 Hz. LFP oscillations were clearly modulated by odorants during the TA task, increasing their oscillatory power at frequencies centered at 20 Hz, and decreasing power at frequencies greater than 50 Hz. Our results indicate that firing rate responses of MT cells in awake animals are behaviorally modulated, with inhibition being a prominent feature of this modulation. The occurrence of oscillatory patterns in singleand multi-unitary discharge is also related to stimulation and behavioral context, while the oscillatory patterns of the neuronal population showed a strong dependence on odorant stimulation. Page 2 of 23