Spontaneous firing of olfactory receptor neurons (ORNs) was recently shown to be required for the survival of ORNs and the maintenance of their appropriate synaptic connections with mitral cells in the olfactory bulb. ORN spontaneous activity has never been described or characterized quantitatively in mammals. To do so we have made extracellular single unit recordings from ORNs of freely breathing (FB) and tracheotomized (TT) rats. We show that the firing behavior of TT neurons was relatively simple: they tended to fire spikes at the same average frequency according to purely random (Poisson) or simple (Gamma or Weibull) statistical laws. A minority of them were bursting with relatively infrequent and short bursts. The activity of FB neurons was less simple: their firing rates were more diverse, some of them showed trends or were driven by breathing. Although more of them were regular, only a minority could be described by simple laws; the majority displayed random bursts with more spikes than the bursts of TT neurons. In both categories bursts and isolated spikes (outside bursts) occurred completely at random. The spontaneous activity of ORNs in rats resembles that of frogs, but is higher, which may be due to a difference in body temperature. These results suggest that, in addition to the intrinsic thermal noise, spontaneous activity is provoked in part by mechanical, thermal, or chemical (odorant molecules) effects of air movements due to respiration, this extrinsic part being naturally larger in FB neurons. It is suggested that spontaneous activity may be modulated by respiration. Because natural sampling of odors is synchronized with breathing, such modulation may prepare and keep olfactory bulb circuits tuned to process odor stimuli.
(c) 2005 Wiley Periodicals, Inc.