Repeating triplets of spikes and oscillations in the mitral cell discharges of freely breathing rats Re Âmy Lestienne, Henry C. Tuckwell, Monique Chalansonnet 1 and Michel Chaput 1 Institut des Neurosciences (CNRS UMR 7624), Universite  Paris VI, 9 quai St-Bernard, F-75005 Paris, France 1 Lab. Neurosciences et Syste Á mes Sensoriels (CNRS ERS 5643), Universite  Claude Bernard-Lyon I, 43 Bld du 11 Novembre 1918, F-69622 Villeurbanne Cedex, France Keywords: olfactory bulb, temporal coding Abstract The olfactory bulb responses to odours display evident temporal organization, both in the form of high-frequency oscillations and precisely replicating triplets of spikes. In this study, the frequency of replicating triplets in a sample of 118 individual responses from 45 cells was compared with that in simulations of non-homogeneous Poisson processes, constructed from the experimental post- stimulus time histograms (PSTHs). In a large majority of the records, replicating triplets (to a precision of 0.5 ms) are found to be more numerous in the physiological records; in some of them, they are ~ 10 times more abundant. An excess of precisely replicating triplets is also found in records where no oscillations are apparent in the autocorrelograms. Triplet replication thus seems a more robust phenomenon than transient oscillation. Not unlike fast oscillations observed in other preparations, replicating triplets produced by a given mitral cell are generally observed only during a restricted period of time of the respiratory cycle (at least in the case of the responses under olfactory stimulation). No relation was found, however, between the nature and strength of the olfactory stimulus and the frequency of replicating patterns. In the absence of olfactory stimulation, some mitral cell discharges also contain more replicating triplets than the non-homogeneous Poisson simulations. Thus, replicating triplets in single-cell discharges seem to play only an indirect role in the coding of olfactory information at the mitral cell output level. Introduction Over the last 10 years, much attention has been paid to the possible role that the temporal structure of neuronal discharges may play in the processing of sensory information. Oscillations and synchronization are perhaps the two paradigms on which the greatest attention has been concentrated (for a review, see Singer, 1993). The olfactory bulb seems to be the ®rst structure for which evidence has been presented that neuronal responses may display high-frequency oscillations in the g range (30±70 Hz, Adrian, 1950; Freeman, 1985). Mitral cells and their glomeruli constitute the ®rst synaptic relay for incoming olfactory information. Detailed studies of mitral cell responses have shown that their discharges usually display characteristic time courses as a function of the phases of the respiratory cycle (Chaput et al., 1992). More recent studies suggest that in fact odours are probably coded by using a complex spatiotemporal code, whereby both the place of individual mitral cells discharges and the temporal order of their ®ring may carry information about the nature of the stimulant (Laurent & Davidowitz, 1994; Wehr & Laurent, 1996; MacLeod et al., 1998). The neuronal code used by most neurons in general, and by mitral cells in particular is, however, not precisely known. Several studies of single-unit spike trains in the visual system have revealed a high proportion of temporal patterns (doublets of spikes or higher order patterns) that replicate themselves in the same spike train with submillisecond precision, in proportions incompatible with various null hypotheses, e.g. locally steady Poisson process, shuf¯ing of actual intervals and renewal models based upon the overall time interval distributions (Strehler & Lestienne, 1986; Lestienne & Strehler, 1987; 1988; Lestienne, 1996). Recently, Lestienne & Tuckwell (1998) have compared the rate of production of precisely replicating patterns in various central nervous system centres in a variety of mammals. They observed that mitral cells of the olfactory bulb, and cells of the lateral geniculate nucleus in the visual system (both centres being located at the level of the ®rst synaptic relay of the incoming sensory information) were particularly inclined to display precisely replicating patterns in their spike trains (Lestienne & Tuckwell, 1998). Replicating triplets suggest a more complex temporal organization than simple oscillations. Although a functional use of replicating triplets in terms of repeated synchronous discharges at sets of synapses of the downstream neurons (Strehler & Lestienne, 1986; Lestienne & Strehler, 1987) might be found in the future, such a role has not yet been demonstrated. We attempted to assess the propensity of individual mitral cells to produce precisely replicating triplets, by using a new and probably better null hypothesis, in the form of non- homogeneous Poisson process (NHPP)-simulated spike trains (H. C. Tuckwell & R. Lestienne, unpublished results), and studied the possible relationship between these replications and the occurrence of characterized oscillations in their discharges. Materials and methods Physiological recordings Single-unit mitral cells were recorded in the olfactory bulbs of anaesthetized adult Wistar rats, as determined by a careful examination of spike waveforms during recordings and after Correspondence: Dr R. Lestienne, as above. E-mail: Remy.Lestienne@snv.jussieu.fr Received 22 July 1998, revised 21 April 1999, accepted 28 April 1999 European Journal of Neuroscience, Vol. 11, pp. 3185±3193, 1999 ã European Neuroscience Association