Regional distribution of rat electroolfactogram

A method for detecting spatiotemporal patterns of cancer biomarkers-evoked activity using radial basis function network extracted time-domain features from calcium imaging data

BACKGROUND

Two-photon calcium imaging is widely used to study the odor-evoked glomerular activity in the dorsal olfactory bulb of macrosmatic animals. The nonstationary character of activated patterns sets a limit on the use of a traditional image processing approaches.

NEW METHOD

The developed method makes it possible to automatically map cancer biomarkers-activated glomeruli in the rat dorsal olfactory bulb. We interpolated fluorescence intensity of calcium dynamics based on the Gaussian RBF network and synthesized the physiological fluorescence model of the receptive glomerular field.

RESULTS

The experiments on 5 rats confirmed the correctness of the developed approach. Patterns evoked by the 6-methyl-5-hepten-2-one (stomach cancer biomarker) and benzene (lung cancer biomarker) were correctly identified.

COMPARISON WITH EXISTING METHODS

The proposed method was compared with the nonnegative matrix factorization method and with the method based on computer vision algorithms. The developed approach showed better accuracy in experiments and provided the mathematical models of the odor-evoked patterns synthesis. These models can be used to generate synthetic images of odor-evoked glomerular activity and thus to overcome the problem of small experimental data collected in calcium imaging.

CONCLUSIONS

The proposed method should be considered part of the toolkit for fully automatic analysis of calcium imaging-based studies. Currently available methodology is not able to use breath biomarkers to reliably discriminate between cancer patients and healthy controls. Nevertheless, the effective identification of the spatial patterns of cancer biomarkers-evoked glomerular activity can serve as the foundation for highly sensitive biohybrid systems for cancer screening.

Computational exploration of molecular receptive fields in the olfactory bulb reveals a glomerulus-centric chemical map

Progress in olfactory research is currently hampered by incomplete knowledge about chemical receptive ranges of primary receptors. Moreover, the chemical logic underlying the arrangement of computational units in the olfactory bulb has still not been resolved. We undertook a large-scale approach at characterising molecular receptive ranges (MRRs) of glomeruli in the dorsal olfactory bulb (dOB) innervated by the MOR18-2 olfactory receptor, also known as Olfr78, with human ortholog OR51E2. Guided by an iterative approach that combined biological screening and machine learning, we selected 214 odorants to characterise the response of MOR18-2 and its neighbouring glomeruli. We found that a combination of conventional physico-chemical and vibrational molecular descriptors performed best in predicting glomerular responses using nonlinear Support-Vector Regression. We also discovered several previously unknown odorants activating MOR18-2 glomeruli, and obtained detailed MRRs of MOR18-2 glomeruli and their neighbours. Our results confirm earlier findings that demonstrated tunotopy, that is, glomeruli with similar tuning curves tend to be located in spatial proximity in the dOB. In addition, our results indicate chemotopy, that is, a preference for glomeruli with similar physico-chemical MRR descriptions being located in spatial proximity. Together, these findings suggest the existence of a partial chemical map underlying glomerular arrangement in the dOB. Our methodology that combines machine learning and physiological measurements lights the way towards future high-throughput studies to deorphanise and characterise structure-activity relationships in olfaction.

Olfactory coding in the turbulent realm

Long-distance olfactory search behaviors depend on odor detection dynamics. Due to turbulence, olfactory signals travel as bursts of variable concentration and spacing and are characterized by long-tail distributions of odor/no-odor events, challenging the computing capacities of olfactory systems. How animals encode complex olfactory scenes to track the plume far from the source remains unclear. Here we focus on the coding of the plume temporal dynamics in moths. We compare responses of olfactory receptor neurons (ORNs) and antennal lobe projection neurons (PNs) to sequences of pheromone stimuli either with white-noise patterns or with realistic turbulent temporal structures simulating a large range of distances (8 to 64 m) from the odor source. For the first time, we analyze what information is extracted by the olfactory system at large distances from the source. Neuronal responses are analyzed using linear-nonlinear models fitted with white-noise stimuli and used for predicting responses to turbulent stimuli. We found that neuronal firing rate is less correlated with the dynamic odor time course when distance to the source increases because of improper coding during long odor and no-odor events that characterize large distances. Rapid adaptation during long puffs does not preclude however the detection of puff transitions in PNs. Individual PNs but not individual ORNs encode the onset and offset of odor puffs for any temporal structure of stimuli. A higher spontaneous firing rate coupled to an inhibition phase at the end of PN responses contributes to this coding property. This allows PNs to decode the temporal structure of the odor plume at any distance to the source, an essential piece of information moths can use in their tracking behavior.

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.

Coding odorant concentration through activation timing between the medial and lateral olfactory bulb

In mammals, each olfactory bulb (OB) contains a pair of mirror-symmetric glomerular maps organized to reflect odorant receptor identity. The functional implication of maintaining these symmetric medial-lateral maps within each OB remains unclear. Here, using in vivo multielectrode recordings to simultaneously detect odorant-induced activity across the entire OB, we reveal a timing difference in the odorant-evoked onset latencies between the medial and lateral halves. Interestingly, the latencies in the medial and lateral OB decreased at different rates as odorant concentration increased, causing the timing difference between them to also diminish. As a result, output neurons in the medial and lateral OB fired with greater synchrony at higher odorant concentrations. Thus, we propose that temporal differences in activity between the medial and lateral OB can dynamically code odorant concentration, which is subsequently decoded in the olfactory cortex through the integration of synchronous action potentials.

Recording odor-evoked response potentials at the human olfactory epithelium

Electro-olfactogram (EOG) represents the sum of generator potentials of olfactory receptor neurons in response to an olfactory stimulus. Although this measurement technique has been used extensively in animal research, its use in human olfaction research has been relatively limited. To understand the promises and limitations of this technique, this review provides an overview of the olfactory epithelium structure and function, and summarizes EOG characteristics and conventions. It describes methodological pitfalls and their possible solutions, artifacts, and questions of debate in the field. In summary, EOG measurements provide a rare opportunity of recording neuronal input from the peripheral olfactory system, while simultaneously obtaining psychophysical responses in awake humans.

Neural activity at the human olfactory epithelium reflects olfactory perception

Organization of receptive surfaces reflects primary axes of perception. In vision, retinal coordinates reflect spatial coordinates. In audition, cochlear coordinates reflect tonal coordinates. However, the rules underlying the organization of the olfactory receptive surface are unknown. To test the hypothesis that organization of the olfactory epithelium reflects olfactory perception, we inserted an electrode into the human olfactory epithelium to directly measure odorant-induced evoked responses. We found that pairwise differences in odorant pleasantness predicted pairwise differences in response magnitude; that is, a location that responded maximally to a pleasant odorant was likely to respond strongly to other pleasant odorants, and a location that responded maximally to an unpleasant odorant was likely to respond strongly to other unpleasant odorants. Moreover, the extent of an individual's perceptual span predicted their span in evoked response. This suggests that, similarly to receptor surfaces for vision and audition, organization of the olfactory receptor surface reflects key axes of perception.

Individual and synergistic effects of sniffing frequency and flow rate on olfactory bulb activity

Is faster or stronger sniffing important for the olfactory system? Odorant molecules are captured by sniffing. The features of sniffing constrain both the temporality and intensity of the input to the olfactory structures. In this context, it is clear that variations in both the sniff frequency and flow rate have a major impact on the activation of olfactory structures. However, the question of how frequency and flow rate individually or synergistically impact bulbar output has not been answered. We have addressed this question using multiple experimental approaches. In double-tracheotomized, anesthetized rats, we recorded both the bulbar local field potential (LFP) and mitral/tufted cells' activities when the sampling flow rate and frequency were controlled independently. We found that a tradeoff between the sampling frequency and the flow rate could maintain olfactory bulb sampling-related rhythmicity and that only an increase in flow rate could induce a faster, odor-evoked response. LFP and sniffing were recorded in awake rats. We found that sampling-related rhythmicity was maintained during high-frequency sniffing. Furthermore, we observed that the covariation between the frequency and flow rate, which was necessary for the tradeoff seen in the anesthetized preparations, also occurred in awake animals. Our study shows that the sampling frequency and flow rate can act either independently or synergistically on bulbar output to shape the neuronal message. The system likely takes advantage of this flexibility to adapt sniffing strategies to animal behavior. Our study provides additional support for the idea that sniffing and olfaction function in an integrated manner.

Analyzing responses of mouse olfactory sensory neurons using the air-phase electroolfactogram recording

Animals depend on olfaction for many critical behaviors, such as finding food sources, avoiding predators, and identifying conspecifics for mating and other social interactions. The electroolfactogram (EOG) recording is an informative, easy to conduct, and reliable method to assay olfactory function at the level of the olfactory epithelium. Since the 1956 description of the EOG by Ottoson in frogs¹, the EOG recording has been applied in many vertebrates including salamanders, rabbits, rats, mice, and humans (reviewed by Scott and Scott-Johnson, 2002, ref. 2). The recent advances in genetic modification in mice have rekindled interest in recording the EOG for physiological characterization of olfactory function in knock-out and knock-in mice. EOG recordings have been successfully applied to demonstrate the central role of olfactory signal transduction components^3-8, and more recently to characterize the contribution of certain regulatory mechanisms to OSN responses^9-12.

Odorant detection occurs at the surface of the olfactory epithelium on the cilia of OSNs, where a signal transduction cascade leads to opening of ion channels, generating a current that flows into the cilia and depolarizes the membrane¹³. The EOG is the negative potential recorded extracellularly at the surface of the olfactory epithelium upon odorant stimulation, resulting from a summation of the potential changes caused by individual responsive OSNs in the recording field². Comparison of the amplitude and kinetics of the EOG thus provide valuable information about how genetic modification and other experimental manipulations influence the molecular signaling underlying the OSN response to odor.

Here we describe an air-phase EOG recording on a preparation of mouse olfactory turbinates. Briefly, after sacrificing the mouse, the olfactory turbinates are exposed by bisecting the head along the midline and removing the septum. The turbinate preparation is then placed in the recording setup, and a recording electrode is placed at the surface of the olfactory epithelium on one of the medial turbinates. A reference electrode is electrically connected to the tissue through a buffer solution. A continuous stream of humidified air is blown over the surface of the epithelium to keep it moist. The vapor of odorant solutions is puffed into the stream of humidified air to stimulate the epithelium. Responses are recorded and digitized for further analysis.

Odorant concentration dependence in electroolfactograms recorded from the human olfactory epithelium

Electroolfactograms (EOGs) are the summated generator potentials of olfactory receptor neurons measured directly from the olfactory epithelium. To validate the sensory origin of the human EOG, we set out to ask whether EOGs measured in humans were odorant concentration dependent. Each of 22 subjects (12 women, mean age = 23.3 yr) was tested with two odorants, either valeric acid and linalool (n = 12) or isovaleric acid and l-carvone (n = 10), each delivered at four concentrations diluted with warm (37 degrees C) and humidified (80%) odorless air. In behavior, increased odorant concentration was associated with increased perceived intensity (all F > 5, all P < 0.001). In EOG, increased odorant concentration was associated with increased area under the EOG curve (all F > 8, all P < 0.001). These findings substantiate EOG as a tool for probing olfactory coding directly at the level of olfactory receptor neurons in humans.

Effects of odor stimulation on antidromic spikes in olfactory sensory neurons

Spikes were evoked in rat olfactory sensory neuron (OSN) populations by electrical stimulation of the olfactory bulb nerve layer in pentobarbital anesthetized rats. The latencies and recording positions for these compound spikes showed that they originated in olfactory epithelium. Dual simultaneous recordings indicated conduction velocities in the C-fiber range, around 0.5 m/s. These spikes are concluded to arise from antidromically activated olfactory sensory neurons. Electrical stimulation at 5 Hz was used to track changes in the size and latency of the antidromic compound population spike during the odor response. Strong odorant stimuli suppressed the spike size and prolonged its latency. The latency was prolonged throughout long odor stimuli, indicating continued activation of olfactory receptor neuron axons. The amounts of spike suppression and latency change were strongly correlated with the electroolfactogram (EOG) peak size evoked at the same site across odorants and across stimulus intensities. We conclude that the curve of antidromic spike suppression gives a reasonable representation of spiking activity in olfactory sensory neurons driven by odorants and that the correlation of peak spike suppression with the peak EOG shows the accuracy of the EOG as an estimate of intracellular potential in the population of olfactory sensory neurons. In addition, these results have important implications about traffic in olfactory nerve bundles. We did not observe multiple peaks corresponding to stimulated and unstimulated receptor neurons. This suggests synchronization of spikes in olfactory nerve, perhaps by ephaptic interactions. The long-lasting effect on spike latency shows that action potentials continue in the nerve throughout the duration of an odor stimulus in spite of many reports of depolarization block in olfactory receptor neuron cell bodies. Finally, strong odor stimulation caused almost complete block of antidromic spikes. This indicates that a very large proportion of olfactory axons was activated by single strong odor stimuli.

Numerical modeling of odorant uptake in the rat nasal cavity

An anatomically accurate 3-dimensional numerical model of the right rat nasal cavity was developed and used to compute low, medium, and high flow rate inspiratory and expiratory mucosal odorant uptake (imposed patterning) for 3 odorants with different mucus solubilities. The computed surface mass flux distributions were compared with anatomic receptor gene expression zones identified in the literature. In general, simulations predicted that odorants that were highly soluble in mucus were absorbed dorsally and medially, corresponding roughly to receptors from one of the gene expression zones. Insoluble odorants tended to be absorbed more peripherally in the rat olfactory region corresponding to the other 2 zones. These findings also agreed in general with the electroolfactogram measurements and the voltage-sensitive dye measurements reported in the literature. This numerical approach is the first to predict detailed odorant flux information across the olfactory mucosa in the rat nasal cavity during inspiratory and expiratory flow and to relate it to anatomic olfactory receptor location, physiological function, and biochemical experiment. This numerical technique can allow us to separate the contributions of imposed and inherent patterning mechanisms on the rat olfactory mucosa.

Long hydrocarbon chains serve as unique molecular features recognized by ventral glomeruli of the rat olfactory bulb

In an effort to understand mammalian olfactory processing, we have been describing the responses to systematically different odorants in the glomerular layer of the main olfactory bulb of rats. To understand the processing of pure hydrocarbon structures in this system, we used the [(14)C]2-deoxyglucose method to determine glomerular responses to a homologous series of alkanes (from six to 16 carbons) that are straight-chained hydrocarbons without functional groups. We found two rostral regions of activity evoked by these odorants, one lateral and one medial, that were observed to shift ventrally with increasing alkane carbon chain length. Furthermore, we successfully predicted that the longest alkanes with carbon chain length greater than our previous odorant selections would stimulate extremely ventral glomerular regions where no activation had been observed with the hundreds of odorants that we had previously studied. Overlaps in response profiles were observed in the patterns evoked by alkanes and by other aliphatic odorants of corresponding carbon chain length despite possessing different oxygen-containing functional groups, which demonstrated that hydrocarbon chains could serve as molecular features in the combinatorial coding of odorant information. We found a close and predictable relationship among the molecular properties of odorants, their induced neural activity, and their perceptual similarities.

Effects of concentration and sniff flow rate on the rat electroolfactogram

Previous reports using the electroolfactogram (EOG) to study the spatial and temporal aspects of response in the rodent olfactory epithelium had focused on high odorant concentrations that gave large responses. This investigation has used lower concentrations to test the difference between responses in the rat dorsomedial and lateral recesses with a range of nasal flow rates and a range of chemical properties. The responses to a highly polar, more hydrophilic odorant changed more steeply with flow rate than responses to a very nonpolar, hydrophobic odorant. With low flow rates there was a response delay in the lateral recess, which is consistent with the models indicating lower flow rates in that region. We observed significant volume conduction effects in which large responses in the dorsomedial region obscured smaller initial portions of the lateral responses. These effects could be removed by destroying the dorsomedial response with a high concentration of a low molecular weight ester. We caution that investigators of EOG recordings from the intact epithelium must attend to the possible presence of volume conduction, which can be assessed by attention to the selectivity of odorant response, response waveform, and response latency.

Temporal dynamics and latency patterns of receptor neuron input to the olfactory bulb

Odorants are first represented in the brain by distributed patterns of activity in the olfactory bulb (OB). Although neurons downstream of sensory inputs respond to odorants with temporally structured activity, sensory inputs to glomeruli are typically described as static maps. Here, we imaged the temporal dynamics of receptor neuron input to the OB with a calcium-sensitive dye in the olfactory receptor nerve terminals in anesthetized mice. We found that diverse, glomerulus- and odorant-dependent temporal dynamics are present even at this initial input stage. Instantaneous spatial patterns of receptor input to glomeruli changed both within and between respiration cycles. Glomerular odorant responses differed in amplitude, latency, rise time, and degree of modulation by sniffing in an odorant-specific manner. Pattern dynamics within the first respiration cycle recurred in a similar manner during consecutive cycles. When sniff rate was increased artificially, pattern dynamics were preserved in the first sniff but were attenuated during subsequent sniffs. Temporal response properties were consistent across individuals on a coarse regional scale and on a fine scale of individual glomeruli. Latency and magnitude of glomerular inputs were only weakly correlated and might therefore convey independent odorant information. These data demonstrate that glomerular maps of primary sensory input to the OB are temporally dynamic. These dynamics may contribute to the representation of odorant information and affect information processing in the central olfactory system of rodents.

A new method for measuring reaction times for odour detection at iso-intensity: Comparison between an unpleasant and pleasant odour

A psychophysical detection test was developed to measure the reaction time of human subjects to a pleasant and an unpleasant odour. The response latencies to stimulation with a malodour (valeric acid) and pleasant odorant (amyl acetate) were compared over a range of different stimulus strengths. By expressing reaction time as a function of detection rate, the responses to the two odours can be compared at iso-intensity across the concentration range. This is the first study that allows odorants to be compared at the same intensity over a range of concentrations. The malodour valeric acid was detected more rapidly than amyl acetate; at the 50% detection level the reaction time for the detection of amyl acetate was 1.74 s compared 1.36 s for valeric acid (380 ms or 22% faster). Women were significantly faster than men at detecting both the unpleasant (by 18%) and pleasant (by 26%) odour at the 50% detection level and this disparity increased with decreasing stimulus strength. In conclusion, we demonstrate the ability of a new method for the measurement of reaction times to odour detection to discriminate between two different odours - a malodour and a non-malodour.

Sniffing and spatiotemporal coding in olfaction

The act of sniffing increases the air velocity and changes the duration of airflow in the nose. It is not yet clear how these changes interact with the intrinsic timing within the olfactory bulb, but this is a matter of current research activity. An action of sniffing in generating a high velocity that alters the sorption of odorants onto the lining of the nasal cavity is expected from the established work on odorant properties and sorption in the frog nose. Recent work indicates that the receptor properties in the olfactory epithelium and olfactory bulb are correlated with the receptor gene expression zones. The responses in both the epithelium and the olfactory bulb are predictable to a considerable extent by the hydrophobicity of odorants. Furthermore, receptor expression in both rodent and salamander nose interacts with the shapes of the nasal cavity to place the receptor sensitivity to odorants in optimal places according to the aerodynamic properties of the nose.

Evidence for the disproportionate mapping of olfactory airspace onto the main olfactory bulb of the hamster

Olfactory receptor neurons (ORNs) project to the rodent main olfactory bulb (MOB) from spatially distinct air channels in the olfactory recesses of the nose. The relatively smooth central channels of the dorsal meatus map onto the dorsal MOB, whereas the highly convoluted peripheral channels of the ethmoid turbinates project to the ventral MOB. Medial and lateral components of each projection stream innervate the medial and lateral MOB, respectively. To ascertain whether such topography entails the disproportionate representation seen in other sensory maps, we used disector-based stereological techniques in hamsters to estimate the number of ORNs associated with each channel in the nose and the number of their targets (glomeruli and mitral and tufted cells) in corresponding divisions of the MOB. Each circumferential half of the MOB (dorsal/ventral, medial/lateral) contained about 50% of the 3,100 glomeruli and about 50% of the 160,000 mitral and tufted cells per bulb. We found equivalent numbers of ORNs with dendritic knobs in the medial and lateral channels (4.5 million each). However, the central channels had only 2 million knobbed ORNs, whereas the peripheral channels had 7 million. Thus, there is a disproportionate mapping of the central-peripheral axis of olfactory airspace onto the dorsal-ventral axis of the MOB, encompassing a greater than threefold variation in the average convergence of ORNs onto MOB secondary neurons. We hypothesize that the disproportionate projections help to optimize chemospecific processing by compensating, with differing sensitivity, for significant variation in the distribution and concentration of odorant molecules along the olfactory air channels during sniffing.

Functional organization of sensory input to the olfactory bulb glomerulus analyzed by two-photon calcium imaging

Glomeruli in the olfactory bulb are anatomically discrete modules receiving input from idiotypic olfactory sensory neurons. To examine the functional organization of sensory inputs to individual glomeruli, we loaded olfactory sensory neurons with a Ca(2+) indicator and measured odorant-evoked presynaptic Ca(2+) signals within single glomeruli by using two-photon microscopy in anaesthetized mice. Odorants evoked patterns of discrete Ca(2+) signals throughout the neuropil of a glomerulus. Across glomeruli, Ca(2+) signals occurred with equal probability in all glomerular regions. Within single glomeruli, the pattern of intraglomerular Ca(2+) signals was indistinguishable for stimuli of different duration, identity, and concentration. Moreover, the response time course of the signals was similar throughout the glomerulus. Hence, sensory inputs to individual glomeruli are spatially heterogeneous but seem to be functionally indiscriminate. These results support the view of olfactory glomeruli as functional units in representing sensory information.

Mucosal activity patterns as a basis for olfactory discrimination: comparing behavior and optical recordings

In over half a century numerous studies have demonstrated that different odorants produce individually different spatial patterns of neural receptor activity on the olfactory mucosa. However, the thought that these differential activity patterns could be the neural code underlying olfactory perception has not been tested directly. In the present study using operant techniques, rats were trained to differentially identify five odors from a homologous series of iso-intensive straight-chain aldehydes that differed serially by only one carbon atom, viz. hexaldehyde to decaldehyde. The rats identified each of the five odorants with greater than 90% correct identification. The degree of perceptual similarity between any pair of the five odorants was determined. Using multidimensional scaling techniques (MDS) the similarity measures yielded a two-dimensional perceptual odorant space. Optical techniques were used to record the olfactory mucosal activity patterns in response to these same five iso-intensive aldehydes. The mucosal activity elicited by each odorant revealed individually distinct band-like patterns that varied both within and across these bands. More importantly, the relative differential responsivity of the bands was related to chain length. An MDS analysis of the dissimilarity measure between all possible pairs of odorant induced activity patterns yielded a two-dimensional neurophysiologic odorant space. Further analysis indicated that the neurophysiologic and psychophysically determined odorant spaces were highly correlated (F(1,39)=23.9, P=nil). These results give additional credence to the concept that the odorant-induced mucosal activity patterns may serve as the substrate for the perception of odorant quality.

Spatio-temporal dynamics of odor representations in the mammalian olfactory bulb

We explored the spatio-temporal dynamics of odor-evoked activity in the rat and mouse main olfactory bulb (MOB) using voltage-sensitive dye imaging (VSDI) with a new probe. The high temporal resolution of VSDI revealed odor-specific sequences of glomerular activation. Increasing odor concentrations reduced response latencies, increased response amplitudes, and recruited new glomerular units. However, the sequence of glomerular activation was maintained. Furthermore, we found distributed MOB activity locked to the nasal respiration cycle. The spatial distribution of its amplitude and phase was heterogeneous and changed by sensory input in an odor-specific manner. Our data show that in the mammalian olfactory bulb, odor identity and concentration are represented by spatio-temporal patterns, rather than spatial patterns alone.

Zonal organization of the mammalian main and accessory olfactory systems

Zonal organization is one of the characteristic features observed in both main and accessory olfactory systems. In the main olfactory system, most of the odorant receptors are classified into four groups according to their zonal expression patterns in the olfactory epithelium. Each group of odorant receptors is expressed by sensory neurons distributed within one of four circumscribed zones. Olfactory sensory neurons in a given zone of the epithelium project their axons to the glomeruli in a corresponding zone of the main olfactory bulb. Glomeruli in the same zone tend to represent similar odorant receptors having similar tuning specificity to odorants. Vomeronasal receptors (or pheromone receptors) are classified into two groups in the accessory olfactory system. Each group of receptors is expressed by vomeronasal sensory neurons in either the apical or basal zone of the vomeronasal epithelium. Sensory neurons in the apical zone project their axons to the rostral zone of the accessory olfactory bulb and form synaptic connections with mitral tufted cells belonging to the rostral zone. Signals originated from basal zone sensory neurons are sent to mitral tufted cells in the caudal zone of the accessory olfactory bulb. We discuss functional implications of the zonal organization in both main and accessory olfactory systems.

What is the significance of gamma wave activity in the pyriform cortex?

The relations between behavior, olfactory input and bursts of gamma wave activity (roughly 30-90 Hz) in the pyriform cortex were studied in freely moving rats. Olfactory input was measured by recording the negative (depolarizing) potentials which occur in the olfactory mucosa during inspiration. Low levels of pyriform gamma activity were associated with spontaneous apnea and slightly higher levels were associated with vigorous sniffing. Intermediate levels of gamma activity occurred during rhythmical breathing during alert immobility, walking, eating, and grooming. High amplitude bursts of gamma activity occurred during very deep breathing during sighing and vocalization. Closing one nostril resulted in the ipsilateral disappearance of pyriform gamma wave bursts, even during vocalization, but contralateral gamma wave activity persisted. It is concluded that pyriform gamma wave activity is entirely dependent on olfactory input and that previously reported relations to conditioning, arousal, motivation, etc., are indirect effects of variations in the pattern of air flow in the upper airway.

3-Methylindole alters both olfactory and trigeminal nasal mucosal potentials in rats

Data from human studies imply that vanillin is an olfactory stimulant, whereas CO2 activates intranasal trigeminal nociceptors. We examined the effects of the olfactotoxin 3-methylindole (3-MI) on nasal mucosal potentials evoked by vanillin and CO2 in rats. A single i.p. administration of 300 mg/kg 3-MI altered both olfactory and trigeminal mucosal responses. Relative to amplitude values determined in non-3-MI-injected rats, the response to vanillin was reduced to 6%, 7%, and 43%, and the response to CO2, recorded in the same rats, decreased to 25%, 38%, and 51% at 4, 8 and 16 days post-3-MI, respectively. The results suggest that 3-MI affects both olfactory and trigeminal elements within the nasal mucosa.

Chemical determinants of the rat electro-olfactogram

The chemical properties that determine the distribution of the electro-olfactogram were studied after exposure of a large area of the rat olfactory epithelium. Multiple electrodes were placed along the rostral border of endoturbinate IV on the midline of the nasal cavity. This array of electrodes spanned a region containing the four receptor gene expression zones described for the rat. The response to a series of odorants containing only carbon, hydrogen, and oxygen was strongly related to electrode position. For most hydrocarbons, the responses were progressively larger toward the ventral epithelium. The only exceptions were aromatic hydrocarbons, which evoked nearly equal response sizes across the epithelium. Ketones and aldehydes evoked relatively larger dorsal responses than did hydrocarbons with similar structures. Aromatic ketones and aldehydes evoked systematically larger responses from the dorsal part of the epithelium. The response profiles for most odorants were well described by a linear fit to the electrode position along the dorsal-ventral position on the epithelium. However, a few bicyclic odorants and carboxylic acids evoked significantly nonlinear profiles. It is concluded that there is a systematic distribution of odorant sensitivity across this part of the epithelium and that this sensitivity is related to general chemical properties. Other evidence suggests that these properties extend to other parts of the epithelium. This spatial sensitivity of the epithelium to odorants probably contributes to olfactory coding in parallel with the convergence of axons from olfactory sensory neurons expressing the same receptor type.

OCAM: A new member of the neural cell adhesion molecule family related to zone-to-zone projection of olfactory and vomeronasal axons

Zone-to-zone projection of olfactory and vomeronasal sensory axons underlies the topographic and functional mapping of chemoreceptor expression zones of the sensory epithelia onto zonally arranged glomeruli in the main and accessory olfactory bulbs. Here we identified OCAM (R4B12 antigen), an axonal surface glycoprotein expressed by subsets of both olfactory and vomeronasal axons in a zone-specific manner. OCAM is a novel homophilic adhesion molecule belonging to the immunoglobulin superfamily with striking structural homology to neural cell adhesion molecule. In both the main and accessory olfactory systems, OCAM mRNA is expressed by sensory neurons in restricted chemoreceptor expression zones, and OCAM protein-expressing axons project to the glomeruli in the corresponding zones of the main and accessory bulbs. OCAM protein is expressed on subsets of growing sensory axons in explant cultures even in the absence of the target bulb. These results demonstrate a precisely coordinated zonal expression of chemoreceptors and OCAM and suggest that OCAM may play important roles in selective fasciculation and zone-to-zone projection of the primary olfactory axons.

Recording the slow potentials evoked by odors in the olfactory mucosa of awake animals

The aim of this report is to expose several improvements which are essential for obtaining good recordings from the basal side of the olfactory mucosa of awake rats with Ag-AgCl electrodes implanted through holes drilled in the roof of the nasal bone. In a first step, we present how this minimally invasive method was developed and validated in anesthetized rats. We insist particularly on several important points such as the size and form of the electrode tip, the careful deposit of silver chloride on this tip or the location of the implanting site. Then we demonstrate that the recorded signals have the characteristics of an electro-olfactogram (EOG), i.e that they have a local origin, that they change with odors and concentrations, and that they do not appear during pure air delivery, nor after ipsilateral nostril closure. Lastly we show that this method was successfully utilized in awake rats. In provided data demonstrating the rhythmicity of EOGs in freely-breathing animals and allowed us to study their relationships with respiration.

Spatially organized response zones in rat olfactory epithelium

Electroolfactogram recordings were made with a four-electrode assembly from the olfactory epithelium overlying the endoturbinate bones facing the nasal septum. In this study we tested whether odors of different chemical structures produce maximal responses along longitudinally oriented regions following the olfactory receptor gene expression zones described in the literature. The distribution of responses along the dorsal-to-ventral direction of this epithelium (i.e., across the expression zones) was tested in two types of experiments. In one, four electrodes were fixed along the dorsal-to-ventral axis of one turbinate bone. In the other, four electrodes were placed in corresponding positions on four turbinate bones and moved together up toward the top of the bone. These experiments compared the odorants limonene and alpha-terpinene, which are simple hydrocarbons, with carvone and menthone, which differ from the hydrocarbons by the presence of ketone groups. All responses were standardized to an amyl acetate or ethyl butyrate standard. The responses to limonene and alpha-terpinene were often larger for the ventral electrodes. The responses to carvone and menthone were largest for the dorsal electrodes. Intermediate electrodes gave responses that were intermediate in amplitude for these odors. The possibility that direction of air flow caused the observed response distributions was directly tested in experiments with odor nozzles placed in two positions. The relatively larger dorsal responses to carvone and relatively larger ventral responses to limonene were present despite odor nozzle position. We conclude that the responses to this set of odors vary systematically in a fashion parallel to the four gene expression zones. The odorant property that governs this response distribution may be related to the presence of oxygen-containing functional groups. Certain odors evoked larger responses at the intermediate electrode sites than at other sites. Cineole was the best example of this effect. This observation shows that not all oxygen-containing functional groups produce the same effect. Although we cannot exclude other possible mechanisms, these three response gradients may be produced by the four receptor expression zones described for many of the putative olfactory receptor genes. Therefore many of the receptors in each zone may share common properties. It remains to be determined whether this zonal input is significant in central odor processing. However, the correlation of odor chemical properties with the structure of receptor molecules in each zone may provide significant leads to structure-function relationships in vertebrate olfaction.

General anosmia caused by a targeted disruption of the mouse olfactory cyclic nucleotide-gated cation channel

Olfactory neurons transduce the binding of odorants into membrane depolarization. Two intracellular messengers, cyclic AMP (cAMP) and inositol trisphosphate (IP3), are thought to mediate this process, with cAMP generating responses to some odorants and IP3 mediating responses to others. cAMP causes membrane depolarization by activating a cation-selective cyclic nucleotide-gated (CNG) channel. We created a mutant "knockout" mouse lacking functional olfactory CNG channels to assess the roles of different second messenger pathways in olfactory transduction. Using an electrophysiological assay, we find that excitatory responses to both cAMP- and IP3-producing odorants are undetectable in knockout mice. Our results provide direct evidence that the CNG channel subserves excitatory olfactory signal transduction, and further suggest that cAMP is the sole second messenger mediating this process.

Olfactory marker protein (OMP) gene deletion causes altered physiological activity of olfactory sensory neurons

Olfactory marker protein (OMP) is an abundant, phylogentically conserved, cytoplasmic protein of unknown function expressed almost exclusively in mature olfactory sensory neurons. To address its function, we generated OMP-deficient mice by gene targeting in embryonic stem cells. We report that these OMP-null mice are compromised in their ability to respond to odor stimull, providing insight to OMP function. The maximal electroolfactogram response of the olfactory neuroepithelium to several odorants was 20-40% smaller in the mutants compared with controls. In addition, the onset and recovery kinetics following isoamyl acetate stimulation are prolonged in the null mice. Furthermore, the ability of the mutants to respond to the second odor pulse of a pair is impaired, over a range of concentrations, compared with controls. These results imply that neural activity directed toward the olfactory bulb is also reduced. The bulbar phenotype observed in the OMP-null mouse is consistent with this hypothesis. Bulbar activity of tyrosine hydroxylase, the rate limiting enzyme of catecholamine biosynthesis, and content of the neuropeptide cholecystokinin are reduced by 65% and 50%, respectively. This similarity to postsynaptic changes in gene expression induced by peripheral olfactory deafferentation or naris blockade confirms that functional neural activity is reduced in both the olfactory neuroepithelium and the olfactory nerve projection to the bulb in the OMP-null mouse. These observations provide strong support for the conclusion that OMP is a novel modulatory component of the odor detection/signal transduction cascade.