JOLJRNALOFNEUROPHYSIOLOCY Vol. 69, No. 4. April 1993. Printed irz l,‘..‘;., 1. Timing of Odor Stimulation Does Not Alter Patterning of Olfactory Bulb Unit Activity in Freely Breathing Rats ERIK C. SOBEL AND DAVID W. TANK Biological Computation Research Department, AT&T Bell Laboratories, Murray Hill, New Jersey 07974 SUMMARY AND CONCLUSIONS I. The effect of the timing between nasal airflow and ongoing tracheal respiration on single-unit activity in the olfactory bulb (OB) of the rat was examined. Nasal and tracheal breathing were dissociated with the use of a double tracheal cannulation tech- nique that allowed independent control of nasal airflow and control of the synchronization of nasal airflow and tracheal breathing. 2. When amyl acetate-saturated air was presented to the nose, OB units showed a distinct reorganization of activity known as respiratory patterning. Of 43 cells examined, 29 fired maximally after inspiration, and 14 fired maximally after expiration. In all 43 cells the patterning of OB activity was synchronized with the time course of the nasal stimulation. This synchronization was indepen- dent of the point in the ongoing respiratory cycle (tracheal breath- ing) at which the odor stimulation was applied. 3. Patterning of OB single-unit activity was also observed when odor was applied as a series of “inspirations” without intervening expirations. Patterning was observed to follow the time course of the odor stimulation even when this was considerably longer and slower than normal breathing. No patterning of activity was ob- served during continuous odor stimulation or in the absence of odor stimulation. 4. It is concluded that respiratory patterning of OB single-unit activity in the rat is not directly dependent on centrifugal inputs synchronized to respiration. Rather, the observed pattern of neural activity reflects the phasic stimulation of the olfactory re- ceptors with each inspiration and the dynamics of the circuitry intrinsic to the bulb itself. INTRODUCTION A general question in sensory processing concerns whether neural responses are strictly stimulus driven or de- termined in part by the internal state of the brain. This question is particularly relevant in the olfactory bulb (OB), where neural activity of freely breathing mammals is stimu- lus driven in a striking manner. In the presence of odor, neural activity increases and becomes strongly synchro- nized with respiration. In the absence of odor, spontaneous activity is not correlated with respiration. At first blush, this phenomenon might seem to be the obvious consequence of the periodic arrival of odorant at the olfactory receptors with each inspiration and the ensuing washout of odor mol- ecules with expiration. However, the modulation of activity does not appear to be a simple superposition of stimulus- driven activity on spontaneous activity. Units firing with inspiration are strongly inhibited during expiration, and units firing with expiration are strongly inhibited during inspiration (Chaput and Holley 1980, 1985). The origin of this partitioning of activity is not under- stood. There are two possible mechanisms (which are not mutually exclusive) : 1) the partitioning is driven primarily by the phasic nature of the input and reflects the dynamics of the circuitry intrinsic to the OB (i.e., inhibitory feedback loops); and 2) partitioning involves a phasic modulation of OB activity by centrifugal inputs to the bulb that are syn- chronized to respiration. Chaput and Holley ( 1980) noted that the suppression of activity between inspirations is not likely to be due to mitral cell self-inhibition via granule cells, because the observed interruption of activity is longer than the inhibition from mitral-granule-mitral interaction. They suggested an inhibitory process with a longer time constant, perhaps acting at the glomerular level, as a more likely mechanism. Previous results do not clearly resolve whether centrifu- gal inputs to the bulb are essential to respiratory synchroni- zation. Some studies have attempted to distinguish between intrinsic and extrinsic sources for respiratory patterning by sectioning centrifugal pathways to the OB (Chaput 1983; Onoda and Mori 1980; Pager 1980; Potter and Chorover 1976) and by examining OB activity in the absence of odors (Ravel et al. 1987; Ravel and Pager 1990). Ravel et al. ( 1987) found that respiratory patterning of OB activity per- sisted when nasal airflow was interrupted. This result is in contrast to those of Macrides and Chorover ( 1972) and Onoda and Mori ( 1980), who reported that respiratory pat- terning disappeared immediately when nasal airflow was interrupted. Macrides and Chorover ( 1972) also observed respiratory patterning of OB activity when “clean air” was used as a stimulus and attributed this effect to the mechani- cal stimulation of olfactory receptors that had been sug- gested by Walsh ( 1956). Chaput ( 1983) explored the effect of olfactory peduncle sectioning on single-unit responses in OB and found instances where respiratory patterning was diminished by this manipulation (Fig. 5 of Chaput 1983) and instances where patterning was enhanced (Fig. 6 of Chaput 1983 ) . Pager ( 1980) reported respiratory pattern- ing of OB activity that persisted in tracheotomized animals and disappeared after peduncle sectioning. Experiments in- volving sectioning raise the concern that all centrifugal in- puts might not have been eliminated and that such proce- dures might have affected the general ph.ysiology of the OB in a global manner (i.e., by removal of tonic inputs or non- specific effects of surgery such as damage to blood supply). In the experiments described here, we have examined the role of centrifugal inputs to the respiratory patterning of OB activity by uncoupling periodic stimulation of the nasal epi- thelium from respiration while disturbing the circuitry itself as little as possible. This was achieved by controlling the relative timing between odor stimulation and ongoing tra- cheal breathing. This method seeks to avoid some of the 0022-3077/93 $2.00 Copyright 0 1993 The American Physiological Society 1331