The mammalian olfactory bulb is characterized by prominent oscillatory activity of its local field potentials. Breathing imposes the most important rhythm. Other rhythms have been described in the beta- and gamma-frequency ranges. We recorded unitary activities in different bulbar layers simultaneously with local field potentials in order to examine the different relationships existing between (i) breathing and field potential oscillations, and (ii) breathing and spiking activity of different cell types. We show that, whatever the layer, odour-induced gamma oscillations always occur around the transition point between inhalation and exhalation while beta oscillations appear during early exhalation and may extend up to the end of inhalation. By contrast, unitary activities exhibit different characteristics according to the layer. They vary in (i) their temporal relationship with respect to the respiratory cycle; (ii) their spike rates; (iii) their temporal patterns defined according to the respiratory cycle. The time window of a respiratory cycle might thus be split into three main epochs based on the deceleration of field potential rhythms (from gamma to beta oscillations) and a simultaneous gradient of spike discharge frequencies ranging from 180 to 30 Hz. We discuss the possibility that each rhythm could serve different functions as priming, gating or tuning for the bulbar network.

In this review we report data from freely breathing animals in an attempt to show how respiratory dynamics can influence bulbar and cortical activity. Relying on in vivo data as well as in vitro observations, we try to emphasize the multiple mechanisms that underlie this modulation, its multiple origins, and its possible functional role.

The role of the piriform cortex (PC) in olfactory information processing remains mainly unknown. Indeed, until recently, only a few studies have investigated the response of PC neurons to odours and these studies did not take into account the functional heterogeneity of the PC previously described using an electrical stimulation paradigm. In this experiment, extracellular activity in response to odour was recorded in urethane anaesthetized rats in the different parts of the cortex ranging from anterior to posterior. A large percentage of cortical cells were silent at rest, and this percentage increased from anterior to posterior. Analysis of odour evoked activity revealed a large percentage of nonresponsive cells that increased from anterior to posterior. Cell activity was largely synchronized with breathing and different temporal patterns were observed. The anterior PC was characterized by odour-evoked responses phase-locked with the inhalation-exhalation transition period. By contrast, activity in the posterior PC was mainly phase-locked with inhalation or exhalation. These data confirm the spatial functional heterogeneity previously reported in the PC. Functional anatomy of the PC suggests that activity in the anterior PC can be mainly driven by afferent activity coming from the OB whereas posterior cells were certainly entrained by more complex mechanisms.