Mitral/tufted cells (MC) are the only output neurons of the olfactory bulb. They play an important role in transmitting sensory information processed by the bulb to the higher brain structures. However, the in vivo properties of these cells, especially the spatiotemporal pattern of their activation remain unclear. We addressed this issue using in vivo 2-photon Ca²⁺-imaging in transgenic mice expressing the FRET-based Ca²⁺-indicator TN-XXL. This mouse model enables imaging of odor-evoked responses in dozens of MCs simultaneously. Application of an odorant to the snout of freely-breathing TN-XXL mice caused clear odor-evoked Ca²⁺-transients in MCs with an amplitude of 5 - 35%DR/R at 1% of saturated vapor. The vast majority (98%) of odor-evoked Ca²⁺- transients in MCs occurred at the onset of odorant application and were therefore ON responses. The amplitude of the Ca²⁺-transients varied between the cells in the same field of view, with neighboring cells often responding to different odorants. The amplitude of the odor-evoked Ca²⁺- transients in a given cell increased gradually with an increasing odorant concentration. We observed striking similarity of the dose-response curves recorded from MCs and their parent glomeruli.

Taken together, our data validate the TN-XXL expressing transgenic mouse line as a useful model for in vivo studies of MC function. In contrast to what was expected based on the in situ data (Carlson et al., 2000; Gire & Schoppa 2009), MCs in vivo did not show a bimodal (on/off) response behaviour.