Patterns of Spontaneous Activity in Single Rat Olfactory Receptor Neurons Are Different in Normally Breathing and Tracheotomized Animals Patricia Duchamp-Viret, 1 Lubomir Kostal, 2 Michel Chaput, 1 Petr La ´ nsky, 2 Jean-Pierre Rospars 3 1 Neurosciences & Syste ` mes Sensoriels, Universite ´ Claude Bernard Lyon 1, CNRS UMR 5020, 50 avenue Tony Garnier, 69366 Lyon Cedex 07, France 2 Institute of Physiology, Academy of Sciences, Videnska 1083, 14220 Prague 4, Czech Republic 3 Physiologie de l’Insecte: Signalisation et Communication, UMR 1272 UPMC – INRA – INA-PG, INRA 78026 Versailles Cedex, France Received 18 March 2005; accepted 18 March 2005 ABSTRACT: Spontaneous firing of olfactory recep- tor 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 record- ings from ORNs of freely breathing (FB) and tracheo- tomized (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) statisti- cal 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 pro- voked in part by mechanical, thermal, or chemical (odor- ant molecules) effects of air movements due to respira- tion, this extrinsic part being naturally larger in FB neu- rons. 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. ' 2005 Wiley Periodicals, Inc. J Neurobiol 65: 97–114, 2005 Keywords: olfactory receptor unit; spontaneous firing; rat; single unit; free breathing; statistical analysis INTRODUCTION Since the pioneering studies of Hubel and Wiesel in the 1970s (Hubel and Wiesel, 1970), the spontaneous activity present in several areas of the central nervous system has been shown to play an essential role in the refinement of neural connections, especially during development (Shatz and Stryker, 1988; Erzurumlu and Kind, 2001; McLaughin et al., 2003; Grubb et al., Correspondence to: J.-P. Rospars (rospars@versailles.inra.fr). Contract grant sponsor: Academy of Sciences of the Czech Republic; contract grant number: 1ET400110401. Contract grant sponsor: Barrande; contract grant number: 9146QL. ' 2005 Wiley Periodicals, Inc. Published online 19 August 2005 in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/neu.20177 97