Testing the sorption hypothesis in olfaction: a limited role for sniff strength in shaping primary odor representations during behavior

Wireless monitoring of respiration with EEG reveals relationships between respiration, behavior, and brain activity in freely moving mice

Active sampling in the olfactory domain is a fundamental aspect of mouse behavior, and there is increasing evidence that respiration-entrained neural activity outside of the olfactory system sets an important global brain rhythm. It is therefore crucial to accurately measure breathing during natural behaviors. We develop a new approach to do this in freely moving animals, by implanting a telemetry-based pressure sensor into the right jugular vein, which allows for wireless monitoring of thoracic pressure. After verifying this technique against standard head-fixed respiration measurements, we combined it with EEG and EMG recording and used evolving partial coherence analysis to investigate the relationship between respiration and brain activity across a range of experiments in which the mice could move freely. During voluntary exploration of odors and objects, we found that the association between respiration and cortical activity in the delta and theta frequency range decreased, whereas the association between respiration and cortical activity in the alpha range increased. During sleep, however, the presentation of an odor was able to cause a transient increase in sniffing without changing dominant sleep rhythms (delta and theta) in the cortex. Our data align with the emerging idea that the respiration rhythm could act as a synchronizing scaffold for specific brain rhythms during wakefulness and exploration, but suggest that respiratory changes are less able to impact brain activity during sleep. Combining wireless respiration monitoring with different types of brain recording across a variety of behaviors will further increase our understanding of the important links between active sampling, passive respiration, and neural activity.NEW & NOTEWORTHY Animals can alter their respiration rate to actively sample their environment, and increasing evidence suggests that neurons across the brain align their firing to this changing rhythm. We developed a new approach to measure sniffing in freely moving mice while simultaneously recording brain activity, and uncovered how specific cortical rhythms changed their coherence with respiration rhythm during natural behaviors and across arousal states.

Organization and engagement of a prefrontal-olfactory network during olfactory selective attention

BACKGROUND

Sensory perception is profoundly shaped by attention. Attending to an odor strongly regulates if and how it is perceived - yet the brain systems involved in this process are unknown. Here we report integration of the medial prefrontal cortex (mPFC), a collection of brain regions integral to attention, with the olfactory system in the context of selective attention to odors.

METHODS

First, we used tracing methods to establish the tubular striatum (TuS, also known as the olfactory tubercle) as the primary olfactory region to receive direct mPFC input in rats. Next, we recorded (i) local field potentials from the olfactory bulb (OB), mPFC, and TuS, or (ii) sniffing, while rats completed an olfactory selective attention task.

RESULTS

Gamma power and coupling of gamma oscillations with theta phase were consistently high as rats flexibly switched their attention to odors. Beta and theta synchrony between mPFC and olfactory regions were elevated as rats switched their attention to odors. Finally, we found that sniffing was consistent despite shifting attentional demands, suggesting that the mPFC-OB theta coherence is independent of changes in active sampling.

CONCLUSIONS

Together, these findings begin to define an olfactory attention network wherein mPFC activity, as well as that within olfactory regions, are coordinated based upon attentional states.

The sorption/chromatography hypothesis of olfactory discrimination: The rise, fall, and rebirth of a Phoenix

Herein, I discuss the enduring mystery of the receptor layout in the vertebrate olfactory system. Since the awarding of the 2004 Nobel Prize to Axel and Buck for their discovery of the gene family that encodes olfactory receptors, our field has enjoyed a golden era. Despite this Renaissance, an answer to one of the most fundamental questions for any sensory system-what is the anatomical logic of its receptor array?-eludes us, still, for olfaction! Indeed, the only widely debated hypothesis, finding its origins in the musing of another Nobel laureate Sir Edgar Adrian, has it that the vertebrate nose organizes its receptors according to the "sorptive" properties of their ligands. This idea, known as the "sorption" or "chromatography" hypothesis, enjoys considerable support despite being controversial. Here, I review the history of the hypothesis-its rises and falls-and discuss the latest data and future prospects for this perennial idea whose history I liken to the mythical Phoenix.

Similarity and Strength of Glomerular Odor Representations Define a Neural Metric of Sniff-Invariant Discrimination Time

The olfactory environment is first represented by glomerular activity patterns in the olfactory bulb. It remains unclear how these representations intersect with sampling behavior to account for the time required to discriminate odors. Using different chemical classes, we investigate glomerular representations and sniffing behavior during olfactory decision-making. Mice rapidly discriminate odorants and learn to increase sniffing frequency at a fixed latency after trial initiation, independent of odor identity. Relative to the increase in sniffing frequency, monomolecular odorants are discriminated within 10-40 ms, while binary mixtures require an additional 60-70 ms. Intrinsic imaging of glomerular activity in anesthetized and awake mice reveals that Euclidean distance between activity patterns and the time needed for discriminations are anti-correlated. Therefore, the similarity of glomerular patterns and their activation strengths, rather than sampling behavior, define the extent of neuronal processing required for odor discrimination, establishing a neural metric to predict olfactory discrimination time.

Tests of the chromatographic theory of olfaction with highly soluble odors: a combined electro-olfactogram and computational fluid dynamics study in the mouse

The idea that the vertebrate nasal cavity operates like a gas chromatograph to separate and discriminate odors, referred to herein as the ‘chromatographic theory’ (CT), has a long and interesting history. Though the last decade has seen renewed interest in the notion, its validity remains in question. Here we examine a necessary condition of the theory: a correlation between nasal odor deposition patterns based on mucus solubility and the distribution of olfactory sensory neuron odotypes. Our recent work in the mouse failed to find such a relationship even across large sorption gradients within the olfactory epithelium (OE). However, these studies did not test extremely soluble odorants or low odor concentrations, factors that could explain our inability to find supporting evidence for the CT. The current study combined computational fluid dynamics (CFD) simulations of odor sorption patterns and electro-olfactogram (EOG) measurements of olfactory sensory neuron responses. The odorants tested were at the extremes of mucus solubility and at a range of concentrations. Results showed no relationship between local odor sorption patterns and EOG response maps. Together, results again failed to support a necessary condition of the CT casting further doubt on the viability of this classical odor coding mechanism.

Summary: This paper casts doubt on the classical chromatographic theory of olfaction, showing there is no correlation between olfactory receptor spatial layout and odor solubility patterns, a necessary condition of the theory.

Methods in Rodent Chemosensory Cognition

Olfactory information processing and learning are highly developed computational abilities of rodents. These attributes can be exploited to ask questions at several levels of complexity, from aspects of odorant binding by olfactory receptors to higher order learning about the predictive consequences of odorant stimulus presentation. Quantitative understanding of rodent odorant sampling patterns, both baseline nasal breathing and odorant-stimulated sniffing, is critical to elucidating mechanisms of olfactory information processing, from primary olfactory receptors to cortical centers that synthesize olfactory percepts from preprocessed multimodal inputs. This chapter outlines an innovative new method for measuring breathing and sniffing rates in unrestrained mice while the mice are performing odor-guided tasks in a computer controlled olfactometer.The method described here involves implantation of a wireless pressure sensor in the mouse that reports on thoracic pressure transients caused by breathing and sniffing. Recordings of pressure sensor outputs are made simultaneously with optically-sensed nose pokes by the mouse into an odor delivery port or a water delivery port. Odorant delivery timing and water reward delivery are also recorded simultaneously. This method allows for breathing and sniffing dependent thoracic pressure transients to be recorded with high temporal precision before, during, and after the mouse approaches an odor delivery port, samples the delivered odor, and obtains a water reward contingent on the identity of the odor that was presented and sampled.

Sniffing Fast: Paradoxical Effects on Odor Concentration Discrimination at the Levels of Olfactory Bulb Output and Behavior

In awake mice, sniffing behavior is subject to complex contextual modulation. It has been hypothesized that variance in inhalation dynamics alters odor concentration profiles in the naris despite a constant environmental concentration. Using whole-cell recordings in the olfactory bulb of awake mice, we directly demonstrate that rapid sniffing mimics the effect of odor concentration increase at the level of both mitral and tufted cell (MTC) firing rate responses and temporal responses. Paradoxically, we find that mice are capable of discriminating fine concentration differences within short timescales despite highly variable sniffing behavior. One way that the olfactory system could differentiate between a change in sniffing and a change in concentration would be to receive information about the inhalation parameters in parallel with information about the odor. We find that the sniff-driven activity of MTCs without odor input is informative of the kind of inhalation that just occurred, allowing rapid detection of a change in inhalation. Thus, a possible reason for sniff modulation of the early olfactory system may be to directly inform downstream centers of nasal flow dynamics, so that an inference can be made about environmental concentration independent of sniff variance.

Active Sampling State Dynamically Enhances Olfactory Bulb Odor Representation

The olfactory bulb (OB) is the first site of synaptic odor information processing, yet a wealth of contextual and learned information has been described in its activity. To investigate the mechanistic basis of contextual modulation, we use whole-cell recordings to measure odor responses across rapid learning episodes in identified mitral/tufted cells (MTCs). Across these learning episodes, diverse response changes occur already during the first sniff cycle. Motivated mice develop active sniffing strategies across learning that robustly correspond to the odor response changes, resulting in enhanced odor representation. Evoking fast sniffing in different behavioral states demonstrates that response changes during active sampling exceed those predicted from feedforward input alone. Finally, response changes are highly correlated in tufted cells, but not mitral cells, indicating there are cell-type-specific effects on odor representation during active sampling. Altogether, we show that active sampling is strongly associated with enhanced OB responsiveness on rapid timescales.

Tests of the sorption and olfactory "fovea" hypotheses in the mouse

The spatial distribution of receptors within sensory epithelia (e.g., retina and skin) is often markedly nonuniform to gain efficiency in information capture and neural processing. By contrast, odors, unlike visual and tactile stimuli, have no obvious spatial dimension. What need then could there be for either nearest-neighbor relationships or nonuniform distributions of receptor cells in the olfactory epithelium (OE)? Adrian (Adrian ED. J Physiol 100: 459-473, 1942; Adrian ED. Br Med Bull 6: 330-332, 1950) provided the only widely debated answer to this question when he posited that the physical properties of odors, such as volatility and water solubility, determine a spatial pattern of stimulation across the OE that could aid odor discrimination. Unfortunately, despite its longevity, few critical tests of the "sorption hypothesis" exist. Here we test the predictions of this hypothesis by mapping mouse OE responses using the electroolfactogram (EOG) and comparing these response "maps" to computational fluid dynamics (CFD) simulations of airflow and odorant sorption patterns in the nasal cavity. CFD simulations were performed for airflow rates corresponding to quiet breathing and sniffing. Consistent with predictions of the sorption hypothesis, water-soluble odorants tended to evoke larger EOG responses in the central portion of the OE than the peripheral portion. However, sorption simulation patterns along individual nasal turbinates for particular odorants did not correlate with their EOG response gradients. Indeed, the most consistent finding was a rostral-greater to caudal-lesser response gradient for all the odorants tested that is unexplained by sorption patterns. The viability of the sorption and related olfactory "fovea" hypotheses are discussed in light of these findings.NEW & NOTEWORTHY Two classical ideas concerning olfaction's receptor-surface two-dimensional organization-the sorption and olfactory fovea hypotheses-were found wanting in this study that afforded unprecedented comparisons between electrophysiological recordings in the mouse olfactory epithelium and computational fluid dynamic simulations of nasal airflow. Alternatively, it is proposed that the olfactory receptor layouts in macrosmatic mammals may be an evolutionary contingent state devoid of the functional significance found in other sensory epithelia like the cochlea and retina.

Patterned Arrangements of Olfactory Receptor Gene Expression in Zebrafish are Established by Radial Movement of Specified Olfactory Sensory Neurons

Spatial restriction of olfactory receptor (OR) gene expression in peripheral sense organs is a common phenomenon across species, suggesting that zonal OR expression somehow contributes to olfactory function. In zebrafish OR expression patterns reminiscent of zones occur as concentric domains with preferred diameters for different ORs. However, the function and the developmental origin of the pattern are unknown. Here we investigate olfactory sensory neuron (OSN) neurogenesis in the adult zebrafish olfactory epithelium (OE) to understand how the zonally organized OR pattern is established and maintained during the lifetime of the animal. We find that OSNs are generated from two discontinuous proliferation zones located at the central and peripheral edge of the sensory OE. OSNs turn on OR expression soon after they exit mitosis and invade the sensory tissue, approaching each other from both ends of the OE. Biased generation of OSN subpopulations at both neurogenic sites and elimination of OSNs along their route across the OE generates the impression of OR-specific expression domains. We formulated a simple mathematical model based on exact parameters derived from our analysis of OSN neurogenesis, which accurately generates OR-like distributions without the need to invoke molecular signals to pattern the OE.

Type 3 adenylyl cyclase: a key enzyme mediating the cAMP signaling in neuronal cilia

Cilia are rigid, centriole-derived, microtubule-based organelles present in a majority of vertebrate cells including neurons. They are considered the cellular "antennae" attuned for detecting a range of extracellular signals including photons, odorants, morphogens, hormones and mechanical forces. The ciliary microenvironment is distinct from most actin-based subcellular structures such as microvilli or synapses. In the nervous system, there is no evidence that neuronal cilia process any synaptic structure. Apparently, the structural features of neuronal cilia do not allow them to harbor any synaptic connections. Nevertheless, a large number of G protein-coupled receptors (GPCRs) including odorant receptors, rhodopsin, Smoothened, and type 6 serotonin receptor are found in cilia, suggesting that these tiny processes largely depend on metabotropic receptors and their tuned signals to impact neuronal functions. The type 3 adenylyl cyclase (AC3), widely known as a cilia marker, is highly and predominantly expressed in olfactory sensory cilia and primary cilia throughout the brain. We discovered that ablation of AC3 in mice leads to pleiotropic phenotypes including anosmia, failure to detect mechanical stimulation of airflow, cognitive deficit, obesity, and depression-like behaviors. Multiple lines of human genetic evidence also demonstrate that AC3 is associated with obesity, major depressive disorder (MDD), sarcoidosis, and infertility, underscoring its functional importance. Here we review recent progress on AC3, a key enzyme mediating the cAMP signaling in neuronal cilia.

Respiratory modulation of spontaneous subthreshold synaptic activity in olfactory bulb granule cells recorded in awake, head-fixed mice

Although the firing patterns of principal neurons in the olfactory bulb are known to be modulated strongly by respiration even under basal conditions, less is known about whether inhibitory local circuit activity in the olfactory bulb (OB) is modulated phasically. The diverse phase preferences of principal neurons in the OB and olfactory cortex that innervate granule cells (GCs) may interfere and prevent robust respiratory coupling, as suggested by recent findings. Using whole-cell recording, we examined the spontaneous, subthreshold membrane potential of GCs in the OBs of awake head-fixed mice. We found that, during periods of basal respiration, the synaptic input to GCs was strongly phase modulated, leading to a phase preference in the average, cycle-normalized membrane potential. Subthreshold phase tuning was heterogeneous in both mitral and tufted cells (MTCs) and GCs but relatively constant within each GC during periods of increased respiratory frequency. The timing of individual EPSPs in GC recordings also was phase modulated with the phase preference imparted by large-amplitude EPSPs, with fast kinetics often matching the phase tuning of the average membrane potential. These results suggest that activity in a subset of excitatory afferents to GCs, presumably including cortical feedback projections and other sources of large-amplitude unitary EPSPs, function to provide a timing signal linked to respiration. The phase preference we find in the membrane potential may provide a mechanism to dynamically modulate recurrent and lateral dendrodendritic inhibition of MTCs and to selective engage a subpopulation of interneurons based on the alignment of their phase tuning relative to sensory-driven MTC discharges.

Precise detection of direct glomerular input duration by the olfactory bulb

Sensory neuron input to the olfactory bulb (OB) was activated precisely for different durations with blue light in mice expressing channelrhodopsin-2 in olfactory sensory neurons. Behaviorally the mice discriminated differences of 10 ms in duration of direct glomerular activation. In addition, a subset of mitral/tufted cells in the OB of awake mice responded tonically therefore conveying information on stimulus duration. Our study provides evidence that duration of the input to glomeruli not synchronized to sniffing is detected. This potent cue may be used to obtain information on puffs in odor plumes.

Sensory-specific appetite is affected by actively smelled food odors and remains stable over time in normal-weight women

Understanding overconsumption starts with knowledge of how separate factors influence our eating behavior. Food cues such as food odors are known for their effect on general appetite and sensory-specific appetite (SSA). Active sniffing rather than passive exposure may induce satiation over time. The objective of this study was to investigate how actively sniffing banana odors affects general appetite, SSA, and subsequent food intake. In a crossover study, 61 women actively smelled cups containing natural banana, artificial banana odor, or water (no odor) for 10 min. Treatment order was randomly assigned as much as possible. General appetite and SSA were monitored by using 100-mm visual analog scales during the 10 min of active sniffing, followed by ad libitum intake of banana milkshake. Results showed that SSA was consistently high (+12 mm) during actively sniffing natural or artificial banana odors, with no decrease in SSA over time. Sniffing both banana odors increased the appetite for banana (+11 mm) and other sweet products (+4 mm), whereas the appetite for savory products decreased by 7 mm (all P < 0.01) compared with no odor. Actively sniffing banana odor did not significantly influence food intake (P = 0.68) or general appetite scores (P = 0.06). In conclusion, SSA scores during active sniffing were identical to the SSA found in a similar study that used passive smelling, suggesting that SSA is independent of the manner of sniffing and exposure time. Moreover, sweet/savory categorization may suggest that food odors communicate information about the nutrient composition of their associated foods. These data clearly show the appetizing effects of food odors.

Sniff adjustment in an odor discrimination task in the rat: analytical or synthetic strategy?

A growing body of evidence suggests that sniffing is not only the mode of delivery for odorant molecules but also contributes to olfactory perception. However, the precise role of sniffing variations remains unknown. The zonation hypothesis suggests that animals use sniffing variations to optimize the deposition of odorant molecules on the most receptive areas of the olfactory epithelium (OE). Sniffing would thus depend on the physicochemical properties of odorants, particularly their sorption. Rojas-Líbano and Kay (2012) tested this hypothesis and showed that rats used different sniff strategies when they had to target a high-sorption (HS) molecule or a low-sorption (LS) molecule in a binary mixture. Which sniffing strategy is used by rats when they are confronted to discrimination between two similarly sorbent odorants remains unanswered. Particularly, is sniffing adjusted independently for each odorant according to its sorption properties (analytical processing), or is sniffing adjusted based on the pairing context (synthetic processing)? We tested these hypotheses on rats performing a two-alternative choice discrimination of odorants with similar sorption properties. We recorded sniffing in a non-invasive manner using whole-body plethysmography during the behavioral task. We found that sniffing variations were not only a matter of odorant sorption properties and that the same odorant was sniffed differently depending on the odor pair in which it was presented. These results suggest that rather than being adjusted analytically, sniffing is instead adjusted synthetically and depends on the pair of odorants presented during the discrimination task. Our results show that sniffing is a specific sensorimotor act that depends on complex synthetic processes.

Tuning to odor solubility and sorption pattern in olfactory epithelial responses

Odor information is first represented as a spatial activation pattern across the olfactory epithelium, when odor is drawn into the nose through breathing. This epithelial pattern likely results from both the intrinsic olfactory sensory neuron (OSN) sensitivity and the sorptive patterns imposed by the interaction of nasal aerodynamics with physiochemical properties of odorants, although the precise contributions of each are ill defined. Here, we used a computational fluid dynamics (CFD) model of rat nasal cavity to simulate the nasal aerodynamics and sorption patterns for a large number of odorants, and compared the results with the spatial neural activities measured by electro-olfactogram (EOG) under same flow conditions. The computational and experimental results both indicate greater sorption and response to a narrow range odorants as a function of their mucosal solubility, and this range can be further modulated by changes of intranasal flow rates and direction (orthonasal vs retronasal flow). A striking finding is that the profile of intrinsic EOG response measured in surgically opened nose without airflow constraints is similar to the shape of the sorption profile imposed by nasal airflow, strongly indicating a tuning process. As validation, combining the intrinsic response with the mucosal concentration estimated by CFD in nonlinear regression successfully accounts for the measured retronasal and orthonasal EOG response at all flow rates and positions. These observations redefine the role of sorption properties in olfaction and suggest that the peripheral olfactory system, especially the central zone, may be strategically arranged spatially to optimize its functionality, depending on the incoming stimuli.

Comparing thoracic and intra-nasal pressure transients to monitor active odor sampling during odor-guided decision making in the mouse

BACKGROUND

Recording of physiological parameters in behaving mice has seen an immense increase over recent years driven by, for example, increased miniaturization of recording devices. One parameter particularly important for odorant-driven behaviors is the breathing frequency, since the latter dictates the rate of odorant delivery to the nasal cavity and the olfactory receptor neurons located therein.

NEW METHOD

Typically, breathing patterns are monitored by either measuring the breathing-induced temperature or pressure changes in the nasal cavity. Both require the implantation of a nasal cannula and tethering of the mouse to either a cable or tubing. To avoid these limitations we used an implanted pressure sensor which reads the thoracic pressure and transmits the data telemetrically, thus making it suitable for experiments which require a freely moving animal.

RESULTS

Mice performed a Go/NoGo odorant-driven behavioral task with the implanted pressure sensor, which proved to work reliably to allow recording of breathing signals over several weeks from a given animal.

COMPARISON TO EXISTING METHOD(S)

We simultaneously recorded the thoracic and nasal pressure changes and found that measuring the thoracic pressure change yielded similar results compared to measurements of nasal pressure changes.

CONCLUSION

Telemetrically recorded breathing signals are a feasible method to monitor odorant-guided behavioral changes in breathing rates. Its advantages are most significant when recording from a freely moving animal over several weeks. The advantages and disadvantages of different methods to record breathing patterns are discussed.

Odor representations in the olfactory bulb evolve after the first breath and persist as an odor afterimage

Rodents can discriminate odors in one breath, and mammalian olfaction research has thus focused on the first breath. However, sensory representations dynamically change during and after stimuli. To investigate these dynamics, we recorded spike trains from the olfactory bulb of awake, head-fixed mice and found that some mitral cells' odor representations changed following the first breath and others continued after odor cessation. Population analysis revealed that these postodor responses contained odor- and concentration-specific information--an odor afterimage. Using calcium imaging, we found that most olfactory glomerular activity was restricted to the odor presentation, implying that the afterimage is not primarily peripheral. The odor afterimage was not dependent on odorant physicochemical properties. To artificially induce aftereffects, we photostimulated mitral cells using channelrhodopsin and recorded centrally maintained persistent activity. The strength and persistence of the afterimage was dependent on the duration of both artificial and natural stimulation. In summary, we show that the odor representation evolves after the first breath and that there is a centrally maintained odor afterimage, similar to other sensory systems. These dynamics may help identify novel odorants in complex environments.

Sniffing behavior communicates social hierarchy

Sniffing is a specialized respiratory behavior that is essential for the acquisition of odors [1-4]. Perhaps not independent of this, sniffing is commonly displayed during motivated [5-7] and social behaviors [8, 9]. No measures of sniffing among interacting animals are available, however, calling into question the utility of this behavior in the social context. From radiotelemetry recordings of nasal respiration, I found that investigation by one rat toward the facial region of a conspecific often elicits a decrease in sniffing frequency in the conspecific. This reciprocal display of sniffing was found to be dependent upon the rat's social status in two separate paradigms, with subordinates reliably decreasing their sniffing frequency upon being investigated in the face by dominant rats. Failure of subordinates to decrease their sniffing frequency shortened the latency for agonistic behavior by dominant rats, reflecting that decreases in sniffing serve as appeasement signals during social interactions. Rats rendered unable to smell persisted in displaying reciprocal sniffing behavior, demonstrating the independence of this behavior from olfaction. Oxytocin treatment in rats with established social hierarchies abolished agonistic behaviors and reciprocal sniffing displays. Together, these findings demonstrate that rodents utilize sniffing behaviors communicatively, not only to collect [6, 10-14] but also to convey information.