Temporal dynamics of sensory input to the olfactory bulb imaged in the awake rat.

CAREY, R.M.; WESSON, D.W.; VERHAGEN, J.V.; PÍREZ, N.; WACHOWIAK, M.

HHMI Janelia Farm, Virginia, Estados Unidos

Conferencia; Using In vivo Physiology to Understand Neural Circuits in Genetic Systems; 2008

HHMI Janelia Farm Research Campus

The temporal dynamics of activity in the olfactory bulb are strongly coupled to respiration and are thought to play an important role in odor coding. In awake animals, respiration is heavily modulated according to the behavioral state and task demands, but the effect of this modulation on sensory input to the bulb and subsequent postsynaptic activity is unknown. Here, we use calcium imaging from the axon terminals of the olfactory receptor neurons to monitor sensory input to the olfactory bulb in awake, head-fixed rat as it performs odor discriminations. In this paradigm, rats typically sample odors (i.e. sniff) at low frequencies (1-3 Hz); under these conditions, sensory inputs are phasic and tightly coupled to respiration, but nonetheless show diverse temporal patterns of activation that are both glomerulus- and odorant-specific. By comparing these dynamics with odor discrimination times within the same trials, we have found that odor identification typically occurs even before the initial volley of sensory inputs to glomeruli has ended. These results place constraints on map- and rate-based coding schemes for encoding odor identity. We also found that the temporal structure of input to the bulb changes dramatically during high-frequency (< 4 Hz) sniffing typical of active exploration. Under these conditions, inputs are attenuated and principally tonic, with little synchronization to  the sniff cycle, suggesting that models of odor coding that rely on timing of bulbar activity relative to the respiratory cycle may not apply at high sniff frequencies. Thus, odor coding and processing strategies may differ depending on sampling frequency, possibly in a way that is adaptive according to task demands.