Specificity of spatial patterns of glomerular activation in the mouse olfactory bulb: computer-assisted image analysis of 2-deoxyglucose autoradiograms

Visualizing olfactory learning functional imaging of experience-induced olfactory bulb changes

The anatomical organization of sensory neuron input allows odor information to be transformed into odorant-specific spatial maps of mitral/tufted cell glomerular activity. In other sensory systems, neuronal representations of sensory stimuli can be reorganized or enhanced following learning or experience. Similarly, several studies have demonstrated both structural and physiological experience-induced changes throughout the olfactory system. As experience-induced changes within this circuit likely serve as an initial site for odor memory formation, the olfactory bulb is an ideal site for optical imaging studies of olfactory learning, as they allow for the visualization of experience-induced changes in the glomerular circuit following learning and how these changes impact of odor representations with the bulb. Presently, optical imaging techniques have been used to visualize experience-induced changes in glomerular odor representations in a variety of paradigms in short-term habituation, chronic odor exposure, and olfactory associative conditioning.

Spatial representations of odorants in olfactory bulbs of rats and mice: similarities and differences in chemotopic organization

In previous studies, we mapped glomerular layer 2-deoxyglucose uptake evoked by hundreds of both systematically related and chemically distinct odorants in rat olfactory bulbs. To determine which principles of chemotopic organization revealed in these studies may be more fundamental and which may be more species typical, we now have characterized patterns of responses to 30 of these odorants in mice. We found that only a few odorants evoked their multiple foci of peak activity in exactly the same locations in the two species. In mice, as in rats, odorants that shared molecular features evoked overlapping patterns, but the locations of the feature-responsive domains often differed in rats and mice. In rats, increasing carbon number within a homologous series of aliphatic odorants is generally associated with rostral and ventral progressions of activity within domains responding to odorant functional group and/or hydrocarbon backbone. Such chemotopic progressions were not obvious in mice, which instead showed more abrupt differences in activated glomeruli within the domains for odorants differing by a single methylene group. Despite the differences, quantitative relationships between overall uptake patterns exhibited a similar organization with respect to odorant chemistry for the two species, probably as a result of partial overlaps of peak domains and more extensive overlaps in large, low-activity areas for rats and mice. We conclude that clustering responses to shared odorant features may be a general strategy for odor coding but that the specific locations of high-activity domains may be unique to a species.

Chemotopic odorant coding in a mammalian olfactory system

Systematic mapping studies involving 365 odorant chemicals have shown that glomerular responses in the rat olfactory bulb are organized spatially in patterns that are related to the chemistry of the odorant stimuli. This organization involves the spatial clustering of principal responses to numerous odorants that share key aspects of chemistry such as functional groups, hydrocarbon structural elements, and/or overall molecular properties related to water solubility. In several of the clusters, responses shift progressively in position according to odorant carbon chain length. These response domains appear to be constructed from orderly projections of sensory neurons in the olfactory epithelium and may also involve chromatography across the nasal mucosa. The spatial clustering of glomerular responses may serve to "tune" the principal responses of bulbar projection neurons by way of inhibitory interneuronal networks, allowing the projection neurons to respond to a narrower range of stimuli than their associated sensory neurons. When glomerular activity patterns are viewed relative to the overall level of glomerular activation, the patterns accurately predict the perception of odor quality, thereby supporting the notion that spatial patterns of activity are the key factors underlying that aspect of the olfactory code. A critical analysis suggests that alternative coding mechanisms for odor quality, such as those based on temporal patterns of responses, enjoy little experimental support.

Odorants with multiple oxygen-containing functional groups and other odorants with high water solubility preferentially activate posterior olfactory bulb glomeruli

In past studies in which we mapped 2-deoxyglucose uptake evoked by systematically different odorant chemicals across the entire rat olfactory bulb, glomerular responses could be related to each odorant's particular oxygen-containing functional group. In the present study we tested whether aliphatic odorants containing two such functional groups (esters, ketones, acids, alcohols, and ethers) would stimulate the combination of glomerular regions that are associated with each of the functional groups separately, or whether they would evoke unique responses in different regions of the bulb. We found that these very highly water-soluble molecules rarely evoked activity in the regions responding to the individual functional groups; instead, they activated posterior glomeruli located about halfway between the dorsal and ventral extremes in both the lateral and the medial aspects of the bulb. Additional highly water-soluble odorants, including very small molecules with single oxygenic groups, also strongly stimulated these posterior regions, resulting in a statistically significant correlation between posterior 2-deoxyglucose uptake and molecular properties associated with water solubility. By showing that highly water-soluble odorants stimulate a part of the bulb associated with peripheral and ventral regions of the epithelium, our results challenge a prevalent notion that such odorants would activate class I odorant receptors located in zone 1 of the olfactory epithelium, which projects to the dorsal aspect of the bulb.

Effects of double and triple bonds on the spatial representations of odorants in the rat olfactory bulb

Many naturally occurring volatile chemicals that are detected through the sense of smell contain unsaturated (double or triple) carbon-carbon bonds. These bonds can affect odors perceived by humans, yet in a prior study of unsaturated hydrocarbons we found only very minor effects of unsaturated bonds. In the present study, we tested the possibility that unsaturated bonds affect the recognition of oxygen-containing functional groups, because humans perceive odor differences between such molecules. We therefore compared spatial activity patterns across the entire glomerular layer of the rat olfactory bulb evoked by oxygen-containing odorants differing systematically in the presence, position, number, and stereochemistry of unsaturated bonds. We quantified activity patterns by mapping [(14)C]2-deoxyglucose uptake into anatomically standardized data matrices, which we compared statistically. We found that the presence and number of unsaturated bonds consistently affected activity patterns, with the largest effect related to the presence of a triple bond. Effects of bond saturation included a loss of activity in glomeruli strongly activated by the corresponding saturated odorants and/or the presence of activity in areas not stimulated by the corresponding saturated compounds. The position of double bonds also affected patterns of activity, but cis vs. trans configuration had no measurable impact in all five sets of stereoisomers that we studied. These results simultaneously indicate the importance of interactions between carbon-carbon bond types and functional groups in the neural coding of odorant chemical information and highlight the emerging concept that the rat olfactory system is more sensitive to certain types of chemical differences than others.

Differential responses to branched and unsaturated aliphatic hydrocarbons in the rat olfactory system

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. Previously, we demonstrated chemotopically organized and distinct olfactory responses to a homologous series of straight-chained alkanes that consisted of purely hydrocarbon structures, indicating that hydrocarbon chains could serve as molecular features in the combinatorial coding of odorant information. To better understand the processing of hydrocarbon odorants, we now have examined responses to other types of chemical changes in these kinds of molecules, namely, branching and carbon-carbon bond saturation. To this end, we used the [14C]2-deoxyglucose method to determine glomerular responses to a group of eight-carbon branched alkane isomers, unsaturated octenes (double-bonded), and octynes (triple-bonded). In contrast to the differential responses we observed previously for straight-chained alkanes of differing carbon number, the rat olfactory system was not particularly sensitive to these variations in branching and bond saturation. This result was unexpected, given the distinct molecular conformations and property profiles of the odorants. The similarity in activity patterns was paralleled by a similarity in spontaneous perceptual responses measured using a habituation assay. These results demonstrate again the functional relationship between bulbar activity patterns and odor perception. The results further suggest that the olfactory system does not respond equally to all aspects of odorant chemistry, functioning as a specific, rather than a general, chemical analysis system.

Analysis of training-induced changes in ethyl acetate odor maps using a new computational tool to map the glomerular layer of the olfactory bulb

Odor quality is thought to be encoded by the activation of partially overlapping subsets of glomeruli in the olfactory bulb (odor maps). Mouse genetic studies have demonstrated that olfactory sensory neurons (OSNs) expressing a particular olfactory receptor target their axons to a few individual glomeruli in the bulb. While the specific targeting of OSN axons provides a molecular underpinning for the odor maps, much remains to be understood about the relationship between the functional and molecular maps. In this article, we ask the question whether intensive training of mice in a go/no-go operant conditioning odor discrimination task affects odor maps measured by determining c-fos up-regulation in periglomerular cells. Data analysis is performed using a newly developed suite of computational tools designed to systematically map functional and molecular features of glomeruli in the adult mouse olfactory bulb. This suite provides the necessary tools to process high-resolution digital images, map labeled glomeruli, visualize odor maps, and facilitate statistical analysis of patterns of identified glomeruli in the olfactory bulb. The software generates odor maps (density plots) based on glomerular activity, density, or area. We find that training up-regulates the number of glomeruli that become c-fos positive after stimulation with ethyl acetate.

Effects of functional group position on spatial representations of aliphatic odorants in the rat olfactory bulb

Principles of olfactory coding can be clarified by studying the olfactory bulb activity patterns that are evoked by odorants differing systematically in chemical structure. In the present study, we used series of aliphatic esters, ketones, and alcohols (27 odorants total) to determine the effects of functional group position on glomerular-layer activity patterns. These patterns were measured as uptake of [(14)C]2-deoxyglucose and were mapped into standardized data matrices for statistical comparison across different rats. The magnitude of the effect of position differed greatly for the different functional groups. For ketones, there was little or no effect of position on evoked patterns. For esters, uptake in individual glomerular modules increased, whereas uptake in others decreased with changing group position, yet the overall patterns remained similar. For alcohols, group position had a profound effect on evoked activity patterns. For example, moving the hydroxyl group in either heptanol or nonanol from the first to the fourth carbon changed the activity patterns so greatly that the major areas of response did not overlap. Within every functional group series, however, responses were globally chemotopic, such that pairs of odorants with the smallest difference in functional group position evoked the most similar patterns. These results help to define further the specificities of glomeruli within previously described glomerular modules, and they show that functional group position can be an important feature in encoding an odorant molecule.

Information processing in the mammalian olfactory system

Recently, modern neuroscience has made considerable progress in understanding how the brain perceives, discriminates, and recognizes odorant molecules. This growing knowledge took over when the sense of smell was no longer considered only as a matter for poetry or the perfume industry. Over the last decades, chemical senses captured the attention of scientists who started to investigate the different stages of olfactory pathways. Distinct fields such as genetic, biochemistry, cellular biology, neurophysiology, and behavior have contributed to provide a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. So far, the combination of these approaches has been most effective at the cellular level, but there are already signs, and even greater hope, that the same is gradually happening at the systems level. This review summarizes the current ideas concerning the cellular mechanisms and organizational strategies used by the olfactory system to process olfactory information. We present findings that exemplified the high degree of olfactory plasticity, with special emphasis on the first central relay of the olfactory system. Recent observations supporting the necessity of such plasticity for adult brain functions are also discussed. Due to space constraints, this review focuses mainly on the olfactory systems of vertebrates, and primarily those of mammals.

Local and global chemotopic organization: General features of the glomerular representations of aliphatic odorants differing in carbon number

To determine whether there is a general strategy used by the olfactory system to represent odorants differing in carbon chain length, rats were exposed to homologous series of straight-chained, saturated aliphatic aldehydes, ethyl esters, acetates, ketones, primary alcohols, and secondary alcohols (32 odorants total). Neural activity across the entire glomerular layer of the olfactory bulb was mapped quantitatively by measuring uptake of [14C]2-deoxyglucose evoked by each odorant. Uptake was observed both in dorsal glomerular modules previously associated with the particular odorant functional groups and in more ventral and posterior modules. Aldehyde-evoked activity patterns were dominated by ventral modules that included the area receiving projections from octanal-responsive sensory neurons expressing the I7 odorant receptor. The dorsal area that has been the focus of optical imaging studies of aldehyde responses contained only minor activity. For all functional groups except for ketones, uptake within functional group-sensitive modules displayed local chemotopy, with longer odorants stimulating more ventral and rostral glomeruli. In more posterior regions, chemotopy was observed for all functional groups, again with uptake shifting ventrally and rostrally with increasing chain length. In addition to these local shifts in activity, correlations analysis of entire activity patterns revealed a global chemotopic organization for all odorant series, with each odorant evoking a pattern most similar to odorants possessing the same functional group but differing by only one carbon in length. Thus, global chemotopy and local modular chemotopy appear to be fundamental principles underlying the representation of odorants differing in carbon chain length.

Informatics approaches to functional MRI odor mapping of the rodent olfactory bulb: OdorMapBuilder and OdorMapDB

The present study applies informatics tools to aid and extend fMRI analysis of the coding mechanism of neural signals in the rodent olfactory system. Odor stimulation evokes unique spatial patterns of activity in the glomerular layer of the mammalian olfactory bulb (OB). An open-source software program, OdorMap-Builder, has been developed to process the high resolution anatomical and functional MRI images of the OB and to generate single two-dimensional flat maps, called odor maps, that describe the spatial activity patterns in the entire glomerular layer. Odor maps help identify the spatial activity patterns from the tremendous amount of fMRI data and they serve as ideal representation of space coding for the olfactory signals in the OB in response to a given odor stimulation. Based on the fMRI technology, OdorMapBuilder provides comparable odor maps on the intra-subject basis, a significant step towards the detailed analyses of the effects of odor types and/or concentrations. In addition, a new database, OdorMapDB, is developed to provide a repository for the generated odor maps. Web interfaces to the database are provided for the data entry, modification and retrieval. OdorMapDB is based on the EAV/CR (entity-attribute-value with classes and relationships) architecture and it is integrated with two other SenseLab olfactory databases: the olfactory receptor and odor databases. Both OdorMapBuilder and OdorMapDB should serve as useful tools and resources for the field and help facilitate experimental research in understanding the olfactory system and the mechanism for smell perception.

Topographic representation of odorant molecular features in the rat olfactory bulb

Individual glomeruli in the mammalian olfactory bulb (OB) most probably represent a single odorant receptor (OR). The assembly of glomeruli thus forms the maps of ORs. How is the approximately 1,000 ORs represented spatially in the glomerular map? Using the method of optical imaging of intrinsic signals and systematic panels of stimulus odorants, we recorded odorant-induced glomerular activity from the dorsal and dorsolateral areas of the rat OB, and examined the molecular receptive range (MRR) of individual glomeruli. We then deduced the characteristic molecular features that were shared by odorants effective in activating individual glomeruli. Analysis of the spatial representation of the MRR showed that glomeruli with similar MRRs gathered in close proximity and formed molecular feature clusters and subclusters. Although the shape of the clusters varied among different OBs, the clusters were arranged at stereotypical positions in relation to the zonal organization of the OB. Examination of the spatial representation of the characteristic molecular features of odorants using structurally semirigid aromatic compounds suggest a systematic and gradual change in the characteristic molecular features according to the position of subclusters in the map. The topographic map of the characteristic molecular features may reflect a systematic spatial representation of the ORs and may participate in the neural bases for the odorant structure-odor quality relationship.

Olfactory processing in a changing brain

The perception of odorant molecules provides the essential information that allows animals to explore their surrounding. We describe here how the external world of scents may sculpt the activity of the first central relay of the olfactory system, i.e., the olfactory bulb. This structure is one of the few brain areas to continuously replace one of its neuronal populations: the local GABAergic interneurons. How the newly generated neurons integrate into a pre-existing neural network and how basic olfactory functions are maintained when a large percentage of neurons are subjected to continuous renewal, are important questions that have recently received new insights. Furthermore, we shall see how the adult neurogenesis is specifically subjected to experience-dependent modulation. In particular, we shall describe the sensitivity of the bulbar neurogenesis to the activity level of sensory inputs from the olfactory epithelium and, in turn, how this neurogenesis may adjust the neural network functioning to optimize odor information processing. Finally, we shall discuss the behavioral consequences of the bulbar neurogenesis and how it may be appropriate for the sense of smell. By maintaining a constitutive turnover of bulbar interneurons subjected to modulation by environmental cues, we propose that adult ongoing neurogenesis in the olfactory bulb is associated with improved olfactory memory. These recent findings not only provide new fuel for the molecular and cellular bases of sensory perception but should also shed light onto cellular bases of learning and memory.

Olfactory learning modifies the expression of odour-induced oscillatory responses in the gamma (60-90 Hz) and beta (15-40 Hz) bands in the rat olfactory bulb

This study addressed the question of the possible functional relevance of two different oscillatory activities, beta and gamma (15-40 and 60-90 Hz, respectively) for perception and memory processes in olfactory areas of mammals. Local field potentials were recorded near relay olfactory bulb neurons while rats performed an olfactory discrimination task. Signals reflected the mass activity from this region and characteristics of oscillatory activities were used as an index of local synchrony. Beta and gamma oscillatory activities were quantified by time-frequency methods before during and after odour sampling. In rats early in their training, olfactory sampling was associated with a significant decrease in power in the gamma band in parallel with a weak but significant increase in the beta band (centred on 27 Hz). Several days later, in well-trained rats, the gamma oscillatory depression was significantly enhanced both in duration and amplitude. It appeared within the 500 ms time period preceding odour onset and was further reduced during the odour period. Concurrently the beta oscillatory response (now centred on 24 Hz) during odour sampling was amplified by a twofold factor. The beta band response was modulated according to the chemical nature of the stimuli and rat's behavioural response. This study showed for the first time that odour sampling in behaving animals is associated with a clear shift in the olfactory bulb neuronal activity from a gamma to a beta oscillatory regime. Moreover, the data stress the importance of studying the odour-induced beta activity and its relation to perception and memory.

Functional mapping of the rat olfactory bulb using diverse odorants reveals modular responses to functional groups and hydrocarbon structural features

In an effort to understand the olfactory code of rats, we collected more than 1,500,000 measurements of glomerular activity in response to 54 odorants selected to provide differences in functional groups and hydrocarbon structure. Each odorant evoked a unique response pattern by differentially stimulating clusters of glomeruli, called modules. Odorants sharing specific aspects of their structure activated the same modules, allowing us to relate responses to structure across approximately 80% of the glomerular layer. The most obvious relationship was between the presence of particular oxygen-containing functional groups and the activity of glomeruli within dorsal modules. Functional group-specific responses were observed for odorants possessing a wide range of hydrocarbon structure, including aliphatic, cyclic, and aromatic features. Even formic acid and acetone, the simplest odorants possessing acid or ketone functional groups, respectively, stimulated modules specific for these functional groups. At the same time, quantitative analysis of pattern similarities revealed relationships in activation patterns between odorants of similar hydrocarbon structure. The odorant responses were reliable enough to allow us to predict accurately specific aspects of odorant molecular structure from the evoked glomerular activity pattern, as well as predicting the location of glomerular activity evoked by novel odorants.

Altered odor-induced expression of c-fos and arg 3.1 immediate early genes in the olfactory system after familiarization with an odor

In adult rats, repeated exposure to an odorant, in absence of any experimentally delivered reinforcement, leads to a drastic decrease in mitral/tufted (M/T) cell responsiveness, not only for the familiar odor but also for other novel odors. In the present study, using two different and complementary in situ hybridization methods, we analyzed the effect of familiarization with an odorant on c-fos and arg 3.1 mRNA expression levels, and we examined the odor specificity of this effect. Odor exposure induces a specific increase in c-fos and arg 3.1 expression in some particular olfactory bulb quadrants. Previous familiarization with the test odor results in a decreased expression of both IEGs in these quadrants, leading to the alteration of the odor-specific pattern of c-fos and arg 3.1 expression. In contrast, this odor-specific pattern is not affected when different odors are used for familiarization and test. Similarly, an odor-specific familiarization effect leading to a reduced c-fos and arg 3.1 expression was also detected in the cingulate cortex and in the anterior piriform cortex. These results support our hypothesis that the decrease in M/T cell responsiveness following a preceding familiarization with an odorant may be related to a particular form of synaptic plasticity involving changes at the genomic level, and reveals further insight in olfactory information processing and the cellular mechanisms underlying familiarization in the olfactory system.

Video-microscopic analysis of dye coupling in the salamander olfactory bulb

Cells in the mitral cell (MCL) and granule cell (GCL) layers of the olfactory bulb shape the representation of odor information in the brain. After intracellular Lucifer Yellow (LY) injections into lightly fixed olfactory bulb slices, clusters of dye coupled cells were previously observed in the MCL and GCL, but the relative extent of coupling in the two layers was unknown in adults. In the present study, the time course of LY coupling in the adult salamander olfactory bulb was quantified using video-microscopic methods. Analysis of fluorescent cell body counts showed that the incidence and the extent of LY coupling are greater in the GCL than in the MCL. With optimal low-current injection procedures, 97% of the injections into the GCL exhibited at least one coupled cell, and on average groups of six to eight cells were counted. Fewer injections into the MCL exhibited only one to three coupled cells. Some of these coupled cells were clearly mitral cells. No staining of cells was observed after extracellular LY injections, and intracellular injections of dextran dyes stained single cells, providing evidence that the LY coupled cells were stained through an intercellular route, presumably gap junctions. In live intact preparations, rapid LY staining of cell clusters was also observed using patch pipettes. Together, these results provide evidence that robust coupling occurs among olfactory bulb neurons in adults, which could have functional significance.

Molecular-feature domains with posterodorsal-anteroventral polarity in the symmetrical sensory maps of the mouse olfactory bulb: mapping of odourant-induced Zif268 expression

Individual glomeruli in the mammalian olfactory bulb presumably represent a single type of odourant receptor. Thus, the glomerular sheet provides odourant receptor maps at the surface of the olfactory bulb. To understand the basic spatial organization of the olfactory sensory maps, we first compared the spatial distribution of odourant-induced responses measured by the optical imaging of intrinsic signals with that detected immunohistochemically by expressions of Zif268, one of the immediate early gene products in juxtaglomerular cells. In the dorsal surface of the bulb, we detected a clear correlation in the spatial pattern between these responses. In addition, the molecular-feature domains and their polarities (spatial shifts of responses with an increase in carbon chain length) that were defined by the optical imaging method could be also detected by the Zif268 mapping method. We then mapped the Zif268 signals over the entire olfactory bulb using a homologous series of fatty acids and aliphatic alcohols as stimulus odourants. We superimposed the Zif268 signals onto the standard unrolled map with the help of cell adhesion molecule compartments. Each odourant typically elicited two pairs of clusters of dense Zif268 signals. The results showed that molecular-feature domains and their polarities were arranged symmetrically at stereotypical positions in a mirror-image fashion between the lateral and the medial sensory maps. The polarity of each domain was roughly in parallel with the posterodorsal-anteroventral axis that was defined by the cell adhesion molecule compartments. These results suggest that the molecular-feature domain with its fixed polarity is one of the basic structural units in the spatial organization of the odourant receptor maps in the olfactory bulb.

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.

Odorant molecular length: one aspect of the olfactory code

Organic acid odorants of differing carbon number produce systematically different spatial patterns of [(14)C]2-deoxyglucose uptake in the glomerular layer of the olfactory bulb. Because increasing carbon number correlates with progressive increases in several molecular features, including hydrophobicity, length, and volume, we determined which of these properties was most associated with systematic changes in the location of an anterior, dorsomedial module responding to fatty acids. We exposed groups of rats to two series of organic acids that each had the same number of carbons, but differed in their hydrocarbon structures. These straight-chained, branched, cyclic, and double-bonded molecules differed independently in hydrophobicity, length, and volume. The only molecular property that was strongly correlated with the location of the module was molecular length, suggesting that this molecular feature is the principal determinant of the chemotopic organization of glomeruli within the module. We also found that distinct hydrocarbon structures produced large differences in spatial patterns of 2-deoxyglucose uptake in posterior parts of the bulb. Even subtly distinct structural isomers evoked posterior responses that differed greatly. The odorant 2-methylbutyric acid evoked much greater uptake in the posterior bulb than did its structural isomer 3-methylbutyric acid (isovaleric acid). These data suggest that posterior portions of the bulb may encode specific steric features of odorant molecules and that some odorant features may have an inherent or acquired greater representation than do others.

Modular representations of odorants in the glomerular layer of the rat olfactory bulb and the effects of stimulus concentration

To study the mechanism whereby odorants are encoded in the nervous system, we studied the glomerular-layer activity patterns in the rat olfactory bulb evoked by closely related odorants from different chemical families. These odorants had a common straight-chain hydrocarbon structure, but differed systematically in their functional groups. Neural activity was mapped across the entire glomerular layer by using the ¿(14)C2-deoxyglucose method. Group responses were averaged and compared by using data matrices. The glomerular activity patterns that resulted from this analysis were comprised of modules. Unique combinations of modules were activated by each odorant, demonstrating what may be part of the neural code for odorants. Most of the modules were clustered together in the bulb, perhaps providing for enhanced contrast between related chemicals by means of lateral inhibition. We also determined whether changes in odorant concentration would affect spatial patterns of glomerular activity. Two odorants, pentanal and 2-hexanone, evoked different patterns at increased concentrations, with additional glomeruli being recruited at a great distance from glomeruli in which activity was evoked at lower concentrations. Humans report that both of these odorants change in perceived odor with increasing concentration. Three other odorants (pentanoic acid, methyl pentanoate, and pentanol) did not recruit new areas of glomerular activation with increasing concentration, and humans do not report a changed odor across concentrations of these odorants. The results suggest that changes in modular glomerular activity patterns could underlie altered odor perception across odorant concentrations, and they provide additional support for a combinatorial, spatially based code in the olfactory system.

Olfaction in rats with extensive lesions of the olfactory bulbs: implications for odor coding

Rats were initially trained on a series of odor detection tasks and then received a unilateral olfactory bulbectomy and removal of different parts of the contralateral bulb. After postoperative recovery they were tested for detection of different concentrations of four odors, on a series of odor discrimination tasks and for their ability to acquire a relatively easy and a more difficult odor mixture discrimination task. Groups were formed based on which region of the bulb was intact (regional savings score) and on amount of bulb intact (bulbar savings score). In general, only rats with bulbar savings scores of less than 21% had deficits in detection or discrimination tasks but most performed as well as controls in most tasks. Correlations between bulbar savings scores and error scores were relatively low across all rats but, within the subgroup with the largest lesions (bulbar savings scores <21%), high correlations between these variables were obtained. There was no evidence for a specific anosmia in any group or individual rat and, except for the more difficult odor mixture discrimination, no one task proved difficult for any subgroup. The present results demonstrate that rats with relatively small remnants of one olfactory bulb can perform a variety of odor detection and discrimination tasks as well or nearly as well as controls. These outcomes provide no support for localization of function within the olfactory bulb but are in accord with recent proposals that odors may be coded by a highly distributed pattern of bulbar input.

Spatial coding of odorant features in the glomerular layer of the rat olfactory bulb

In order to determine whether molecular features of odorants are represented spatially in the glomerular layer of the olfactory bulb, we used metabolic mapping of [14C] 2-deoxyglucose uptake in rats exposed to equal vapor concentrations of odorants differing systematically in chemical structure. The odorants were ethyl acetate, ethyl butyrate, isoamyl acetate, and isoamyl butyrate. Statistical analysis of anatomically standardized arrays of uptake revealed that each ester produced a characteristic spatial pattern of activity in the glomerular layer. The patterns were similar in different rats exposed to the same odorant, and their complexity increased with increasing odorant carbon number. This finding suggests that the presence of more potentially recognized molecular features is associated with a greater number of activated receptors. Individual regions of the glomerular layer responded specifically to isoamyl esters, and other regions preferred ethyl esters. Regions of similar specificity occurred in lateral and medial aspects of the bulb, the medial representation being more caudal and ventral than the lateral one. This pattern correlates with projections of olfactory sensory neurons expressing the same putative olfactory receptor gene. The patterns overlapped greatly in the posterolateral and posteromedial glomerular layer, a finding one should predict, given the large overlap in chemical structure across the aliphatic esters. Thus, molecular features appear to be encoded spatially in the glomerular layer, and the identity of the odorant may be determined by a subsequent decoding of the combination of molecular features represented in the glomerular layer.

Odor exposure reveals non-uniform expression profiles of c-Jun protein in rat olfactory bulb neurons

In the main olfactory bulb, neurons are arranged strategically in distinct layers among which translaminar synaptic transmission can be made from the superficial, sensory to the deep, output layers that account for the processing of olfactory information. To search for stimulus-transcription coupling thought to be operated differentially in several cell types, c-Jun expression was examined immunohistochemically in rat olfactory bulb following 30-min odor stimulation with acetic acid and 1-butanol. c-Jun was rapidly induced in neuronal cell nuclei belonging to periglomerular, tufted, mitral and granule cells. The disappearance of c-Jun, however, differed between each cell type. In the glomerular layer, the glomeruli composed of c-Jun-expressing periglomerular cells were seen. Different odors led to labeling of different sets of glomeruli. The labeled periglomerular cells disappeared within 2 h. In all the deeper layers, however, a rather homogeneous label was noted for the tufted, mitral and granule cells present throughout the olfactory bulb, regardless of the difference in odor. In tufted and mitral cells, the c-Jun expression persisted for 4 days after odor stimulation. In the granule cell layer, numerous granule cells increased c-Jun immunoreactivity which lasted for 1 day following odor application. In control rats which were given clean air, the basal amount of c-Jun expression was seen confined to scattered granule cells. The results suggest that c-Jun is expressed in a variety of odorant-stimulated bulb neurons with a time course being dependent on cell type.

Detection and discrimination of propionic acid after removal of its 2-DG identified major focus in the olfactory bulb: a psychophysical analysis

Prior 2-deoxyglucose and c-fos studies have demonstrated increased metabolic activity in a rostral dorsomedial area of the olfactory bulb in response to the vapor of propionic acid. We used psychophysical tests to assess the effect of removing this area of the bulb on odor sensitivity and discrimination. Normal rats, those with lesions of the rostral dorsomedial bulb or with control lesions of the lateral olfactory bulb were tested for propionic acid absolute detection and intensity difference thresholds and ability to discriminate propionic acid from other odors. There were no differences among groups for absolute or intensity difference threshold or on simple 2-odor discrimination tests but both groups with bulbar lesions made more errors than controls on a relatively difficult odor-mixture task. The results demonstrate that removal of an area of the bulb identified as responsive to propionic acid is essentially without effect on sensitivity to that odor or ability to discriminate it from other odors.

A stochastic model for interconnected neurons

A model is proposed to describe the collective behavior of a biologically plausible neural network, composed of interconnected spiking neurons which separately receive external stationary stimulations. The spiking dynamics of each neuron is represented by an hourglass metaphor. This network model was first studied in a special case where the connections are only inhibitory (Cottrell, 1988, 1992). We study the network dynamics as a function of the parameters which quantify the strengths of both inhibitory and excitatory connections. We show that the model exhibits two kinds of limit states. In the first states (convergent case), the system is ergodic and all neurons have a positive mean firing rate. In the other states (divergent case), some neurons become definitively inactive while the sub-network of the active neurons is ergodic. The patterns which result from these divergent states can be seen as a neural coding of the external stimulation by the network. This property is applied to the olfactory system to produce a code for an odor. The role of inhibitory connections in odor discrimination is studied.

Spatial distribution of [14C]2-deoxyglucose uptake in the glomerular layer of the rat olfactory bulb following early odor preference learning

Previous work has shown that odors induce focal uptake of [14C]2-deoxyglucose (2-DG) within the glomerular layer of the main olfactory bulb and that the amount of 2-DG accumulated in these foci increases after early odor learning. To determine if learning-associated changes in 2-DG uptake occur across the entire glomerular layer, we have mapped uptake throughout the layer at fixed angles in coronal sections through the bulb. Resulting arrays for individual bulbs were corrected for differing bulb size and averaged across experimental groups to address the spatial distribution of uptake. The average arrays revealed at least three discrete fields of uptake in naive, peppermint-exposed rats at postnatal day 19 that were not seen in air-exposed littermates. In agreement with previous studies, early preference training with peppermint odor given on postnatal days 1-18 increased 2-DG uptake at postnatal day 19 within odor-dependent patches of uptake in the posterior half of the midlateral bulb, whereas odor-dependent, ventrolateral patches of uptake did not increase to the same extent. In addition, early preference learning was associated with significantly increased 2-DG uptake average over the entire analyzed glomerular layer. These increases were smaller than those within odor-dependent foci and were distributed widely across the glomerular layer, showing low overlap between trained and control rats in anterior regions where peppermint odor did not stimulate 2-DG uptake. The widely distributed increases in 2-DG uptake after learning may reflect changed activity of centrifugal projections that diffusely innervate the glomerular layer.

Investigation of the role of interneurons and their modulation by centrifugal fibers in a neural model of the olfactory bulb

Olfactory bulb processing results from the interaction of relay neurons with two main categories of interneurons which mediate inhibition in two distinct layers: periglomerular cells and granule cells. We present here a neural model of the mammalian olfactory bulb which allows to separately investigate the functional consequences of the two types of interneurons onto the relay neurons responsiveness to odors. The model, although built with simplified representations of neural elements generates various aspects of neural dynamics from the cellular to the populational level. We propose that the combined action of centrifugal control at two different layers of processing is complementary: reduction of the number of active relay neurons responding to a given odorant through increased activity of periglomerular cells, and an increase of response intensity of active mitral cells through decrease of granule cell inhibition.

A field potential analysis on the effects of lamotrigine, GP 47779, and felbamate in neocortical slices

We studied the action of the new antiepileptic drugs lamotrigine (LTG), GP 47779 (the active metabolite of oxcarbazepine), and felbamate (FBM) on stimulus-evoked field potentials recorded from rat prefrontal and frontal cortical slices. In the presence of physiologic concentrations of extracellular magnesium (1.2 mM) the field potential amplitude was not affected by the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, 2-amino-5-phosphonovalerate (APV), while it was blocked by the non-NMDA glutamate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). When magnesium was removed from the bathing medium, there was a significant NMDA-mediated component of the field potential. LTG and GP 47779 decreased, in a dose-dependent manner, the field potential amplitude under both experimental conditions. FBM caused a dose-related decrease of the field potential amplitude only in the absence of external magnesium, suggesting a selective interaction with an NMDA-mediated component of this potential. These findings indicate that the reduction of cortical excitatory transmission might represent a common target for new antiepileptic drugs.

Odour-induced c-fos expression in the rat olfactory bulb: involvement of centrifugal afferents

Expression of the proto-oncogene c-fos is known to increase in granule cells of the olfactory bulb following a sustained olfactory stimulation. Most granule cells displaying high levels of Fos accumulation are located in the bulbar columns defined by the odour-induced foci of high 2-deoxyglucose glomerular uptake. The present studies were undertaken in order to assess the possible involvement of centrifugal afferents in the modulation of odour-induced patterns of either 2-deoxyglucose accumulation or c-fos expression in the olfactory bulb. A unilateral olfactory peduncle section had no effect on the odour-induced 2-deoxyglucose foci but induced a significant decrease in the number of Fos-containing neurons in odour-selective areas of both olfactory bulbs, ipsilateral and contralateral to the lesion. This suppressive effect was much more pronounced in the side ipsilateral to the peduncle section. It is concluded that c-fos expression induced by a sustained stimulation with propionic acid vapours is not only determined by the olfactory peripheral input but also by afferents of central origin. In order to estimate the contingent involvements of the cholinergic and noradrenergic afferents in this control of c-fos expression, we attempted to mimic the effects of the surgical deafferentation on odour-induced c-fos expression by using a pharmacological approach with selective cholinergic and noradrenergic antagonists. The beta-adrenergic antagonist propanolol induced a suppression of the odour-related patterns of Fos accumulation similar to the one caused by the surgical deafferentation of the olfactory bulb. The muscarinic antagonist scopolamine did not alter c-fos expression in the odour-selective area but increased significantly Fos labelling in the other bulbar aspects. Pharmacological investigations indicate that the noradrenergic and cholinergic centrifugal systems are likely involved in the central modulation of c-fos expression in the OB. The Fos protein could be expressed as an early nuclear signal triggering further long-term modifications of the neuronal phenotype, in certain conditions of sensory stimulation involving the activation of centrifugal systems.

Spatial patterning and information coding in the olfactory system

The ability of mammals to discriminate thousands of structurally diverse odorants appears to derive from the existence of a multigene family that encodes approximately 1000 different odorant receptors. Recent studies have used this family to explore how the olfactory system organizes sensory information. These studies reveal striking patterns of organization suggesting that incoming sensory information is first broadly organized in the nose and is then transformed in the olfactory bulb into a stereotyped and highly organized spatial map.

Focal 2-DG uptake persists following olfactory bulb lesions

Exposure of rats to different odors produces spatially distinct patterns of 14C-2-deoxyglucose uptake (2-DG) in the glomerular layer of the main olfactory bulb. However, lesions of specific regions of the bulb that reliably contain 2-DG foci reportedly do not impair the ability of rats to perform olfactory-guided behaviors, suggesting that the lesioned olfactory bulb retains odor-responsiveness. Because the absence of focal 2-DG incorporation in lesioned olfactory bulbs has not been verified by 2-DG autoradiography, it cannot be concluded that focal responses in the olfactory bulb do not contribute to the encoding of olfactory information. To examine the effects of bulb lesions on 2-DG uptake in the olfactory bulb, we placed lesions in specific regions of the bulb that reliably contain 2-DG foci. We then exposed rats to odors 3 or 6 weeks later to determine if the lesions effectively eliminated focal 2-DG uptake in these bulbs. The results indicate that lesioned olfactory bulbs contain focal regions of 2-DG uptake in response to odor stimulation.

Topographic organization of sensory projections to the olfactory bulb

The detection of odorant receptor mRNAs within the axon terminals of sensory neurons has permitted us to ask whether neurons expressing a given receptor project their axons to common glomeruli within the olfactory bulb. In situ hybridization with five different receptor probes demonstrates that axons from neurons expressing a given receptor converge on one, or at most, a few glomeruli within the olfactory bulb. Moreover, the position of specific glomeruli is bilaterally symmetric and is constant in different individuals within a species. These data support a model in which exposure to a given odorant may result in the stimulation of a spatially restricted set of glomeruli, such that the individual odorants would be associated with specific topographic patterns of activity within the olfactory bulb.

Small subclass of rat olfactory neurons with specific bulbar projections is reactive with monoclonal antibodies to the HSP70 heat shock protein

As part of a study of turnover of rat olfactory receptor neurons we have been examining immunohistochemical expression of members of the 70 kD heat shock protein (HSP70) family in the olfactory epithelium. Expression of HSP70 family members is up-regulated in many cells following exposure to physiologically stressing conditions. Because dying neurons are likely to undergo some sort of physiological stress before the onset of frank degeneration, we hoped that anti-HSP70 monoclonal antibodies would prove to be useful markers for early stages of olfactory neuron cell death. Two anti-human HSP70 monoclonal antibodies were used, Mabs 2A4 and 3a3. Two-dimensional gel electrophoresis/western blot analysis indicates that these Mabs are reactive with the HSC70 and HSP70 members of the rat HSP70 family. Immunohistological observations show that both Mabs are strongly reactive with a widely dispersed subpopulation of olfactory receptor neurons. Morphological, immunohistological, and autoradiographic birthdating analyses demonstrate that reactive cells are fully mature receptor neurons. Their reactivity, however, does not appear to be stress-related. More significantly, axons of reactive neurons show intense anti-2A4 reactivity. This has allowed us to trace these axons to their target glomeruli in the olfactory bulb, demonstrating that the reactive neurons project to just one to two glomeruli on either side of each bulb via consistent and predictable pathways. This is the first subpopulation of olfactory receptor neurons to be traced to such a small number of glomeruli. Given this extremely small number, it seems likely that the reactive receptor cell subpopulation serves some specific olfactory function. In addition, axonal 2A4 reactivity should also prove useful in defining the relative roles of receptor neurons and glomeruli in the establishment of epithelial-glomerular connections.

Spatial segregation of odorant receptor expression in the mammalian olfactory epithelium

The signal elicited by the interaction of odorous ligands with receptors on olfactory sensory neurons must be decoded by the brain to determine which of the numerous receptors have been activated. We have examined the patterns of odorant receptor expression in the rat olfactory epithelium to determine whether the mammalian olfactory system employs spatial segregation of sensory input to encode the identity of an odorant stimulus. In situ hybridization experiments with probes for 11 different odorant receptors demonstrate that sensory neurons expressing distinct receptors are topologically segregated into a small number of broad, yet circumscribed, zones within the olfactory epithelium. Within a given zone, however, olfactory neurons expressing a specific receptor appear to be randomly distributed, rather than spatially localized. The complex mammalian olfactory system may therefore compartmentalize the epithelium into anatomically and functionally discrete units, such that each zone expresses only a subset of the entire receptor repertoire.

Are there structural and functional modules in the vertebrate olfactory bulb?

A number of different recording methods have shown that odorants elicit patterns of neuronal activity widely distributed across cells of the olfactory receptor epithelium, olfactory bulb, and piriform cortex in the vertebrate olfactory system. These findings suggest that the physicochemical properties of odorant molecules are processed by distributed coding mechanisms activated in parallel in olfactory circuits in order to characterize a single, "monomolecular" odorant. These findings also suggest that the response patterns seen at higher levels are set up by differential responses in peripheral receptor cells of the olfactory epithelium. One requirement for understanding the details of this proposed encoding scheme is correlation of odor-generated patterns with the components of these circuits. In this paper, results from 2-deoxyglucose and voltage-sensitive dye studies suggest that certain components of these responses may relate to patterns established in reproducibly identifiable aggregates of bulbar cells. These findings are consistent with previous observations suggesting that columnar groups of periglomerular, mitral/tufted and granule cells, oriented perpendicular to the laminae of the bulb, are functionally related to one another. Such cell groups or modules, when activated in parallel, could serve as building block components of the complete ensemble response. According to this hypothesis, different sets of such modules would be activated with different odorant stimuli and modules could be shared to the degree to which the physicochemical properties of the different stimuli overlap.

Neural circuit computation: complex patterns in the olfactory bulb

The olfactory bulb principal neurons show complex responses during olfactory stimulation that are characterized by periods of profound suppression, temporal patterns of activity, and nonmonotonic intensity response functions. The model presented here suggests that these features may arise from changing spatial patterns of activity across the bulbar surface due to restricted (chemotopic) activation in bulbar glomeruli. The model is based on a simplified version of the bulbar neural circuit as it is presently known from anatomical and physiological investigations and provides explanations for several anomalous bulbar response patterns.

Odor-induced fos-like immunoreactivity in the rat olfactory bulb

We here report odor-induced mapping patterns of c-fos-like protein (Fos) immunoreactivity in the rat olfactory bulb under urethane anesthesia. Regional patterns of cells with nuclei expressing Fos, plotted on a spread-out reconstruction of the mitral cell layer, were strikingly comparable to data from the 2-deoxyglucose method, by which different foci of the most-labelled glomeruli of metabolic activity were demonstrated using different odors. Cells in the glomerular, external plexiform, and granule cell layers were also labelled.

Sensory processing in the main and accessory olfactory systems: comparisons and contrasts

The vomeronasal organ (VNO) and accessory olfactory system (AOS) are present in most terrestrial vertebrates except birds and higher primates. The receptor neurons of the AOS are sequestered inside the VNO, away from the main airflow to the main olfactory receptor neurons. Mechanisms of stimulus access to the sensory neurons vary across species but in most cases there is a system for delivering stimuli faster than would be possible by diffusion. Vomeronasal (VN) receptor neurons typically lack cilia, the site of most of the transduction apparatus in the main olfactory receptors. The VN receptor neurons have a restricted but privileged pathway to the areas of the brain concerned with reproduction and social behavior. In contrast, the main olfactory neurons have a broad pathway to wide areas of the brain, including the neocortex. Experiments where the VNOs or other parts of the accessory olfactory pathway were ablated indicate that the system is important in many behavioral and physiological responses to pheromones (chemical signals carrying information about gender or reproductive or dominance status), some of which may be proteins. VN sensory neurons respond to both volatile and non-volatile stimuli. There is no evidence in the vertebrate AOS for the extreme sensitivity or selectivity characteristic of insect pheromone detectors, but this has not been adequately tested. There is some evidence for learning, possibly by synaptic modification at the second-order neuron level. Social and reproductive cues stimulating the AOS often elicit an intracerebral release of LHRH--which may act at receptors different from those of the pituitary to facilitate behavior. Whether the LHRH release is necessary for AOS-mediated behavioral response is not yet clear.

Analysis of 2-DG autoradiograms using image-averaging and image-differencing procedures for systems-level description of neurobehavioral function

Computer assisted 2-deoxyglucose (2-DG) autoradiography has been used to provide functional maps of areas of altered neural activity related to changes in an animal's behavior or state. The standard procedure for comparison of autoradiograms between different treatment groups has been to take measurement samples from predefined neuroanatomical regions and to average these across brains to attain statistical sensitivity for detecting treatment effects. Unfortunately, when sampling is restricted to predefined areas, important topographic information is lost along with the ability to reveal an unexpected change in neural activity. To preserve the rich topographical detail of metabolic information and to enhance the capacity to uncover novel areas of altered metabolic activity, we have developed a system for averaging entire images from 2-DG autoradiograms and for comparing the average images from two experimental groups by creating an image of differences. This procedure does not rely on sampling only preselected regions, but still allows statistical comparisons between experimental groups. The procedures we describe can be readily and inexpensively adapted for use in individual laboratories and are based on modifications of preexisting image analysis software. We show that, when average and difference images are created using standardized protocols for sectioning brain tissue and editing section images, they are impressively resolved and realistic and can serve as effective topographic descriptions of group differences in neural activity of functional and behavioral relevance.

The Projections of Mitral Cells from Small Local Regions of the Olfactory Bulb: An Anterograde Tracing Study Using PHA-L (Phaseolus vulgaris Leucoagglutinin)

Numerous anatomical and electrophysiological studies have demonstrated a lack of simple point-to-point topographical relationships between the olfactory bulb and primary olfactory projection areas. They reveal instead, a complex pattern of divergence and convergence. Furthermore, several authors reported that a single mitral cell could project onto different widely spaced cortical regions of the olfactory cortex. In the present study, we attempted to label the projections of a few mitral cells so close together so that they might be assumed to be connected to the same glomerulus, and to determine if these cells had similar patterns of axonal projections. For this purpose small Phaseolus vulgaris leucoagglutinin (PHA-L) injections were performed in the olfactory bulb of adult rats. We found that labelling two to five mitral cells, lying close together in the mitral cell layer, resulted in well-delineated patches of labelled fibres in the cortex. The number of patches was not related to the number of labelled mitral cells but the fibre density in each patch increased with the number of PHA-L filled somata in the olfactory bulb. We conclude that mitral cells lying close together in the mitral cell layer have similar patterns of axonal projections. Functional implications of such an organization in olfactory coding is discussed.

Contributions of topography and parallel processing to odor coding in the vertebrate olfactory pathway

Odor information appears to be encoded by activity distributed across many neurons at each level in the olfactory pathway. Thus olfactory circuits function as parallel distributed processors. New methods for observing distributed activity in such systems permit computer simulations to be constructed that are constrained by patterns of activity observed in the real system. Analysis of the system using a combination of physiological measurements and computational approaches might elucidate the principles by which odors are discriminated.

Identification of a new non-neuronal cell type in rat olfactory epithelium

We have examined adult and embryonic rat olfactory epithelia by immunohistochemical techniques using the monoclonal antibody 1A-6, which was raised against embryonic rat olfactory epithelia. A heretofore unidentified cell type, reactive with the monoclonal antibody 1A-6, was observed scattered within the epithelium. The 1A-6 reactivity of these cells is most intense on the microvilli projecting from the luminal cell surfaces. For several reasons, we believe these cells are not neurons but a distinct subpopulation of supporting cells or some other sort of non-neuronal cells. (1) They have no identifiable axonal process, are not reactive with an antibody against olfactory marker protein, and are not in juxtaposition with trigeminal axons. (2) They survive ablation of the olfactory bulb. (3) Their nuclei lie within the supporting cell layer, and they resemble supporting cells morphologically and in their [3H]thymidine birthdating and turnover characteristics. However, the 1A-6-positive cells fail to react with the general supporting cell-specific monoclonal antibody SUS-1 [see Hempstead J. L. and Morgan J. I. (1983) Brain Res. 188, 289-295] a finding which suggests that they are not typical supporting cells. Immunoreactivity to 1A-6 is developmentally regulated. Immunohistochemical preparations of almost all tissues we examined showed widespread reactivity in the embryo but a much more restricted pattern in the adult. In the olfactory epithelium of the fetus, the luminal surfaces of all cells, including supporting cells and olfactory receptor cells and cilia, are reactive, while in the adult only the non-neuronal cell subpopulation shows this reactivity. We also found that during the reconstitution of olfactory epithelium which occurs in response to olfactory bulbectomy-induced neuronal degeneration, fetal patterns of 1A-6 reactivity are not re-expressed, i.e. the only 1A-6-positive cells are the non-neuronal cells seen in unperturbed adult olfactory epithelium. Preliminary biochemical analyses of membrane fractions from E19 brain and from adult olfactory mucosa indicate that the 1A-6 reactivity is associated with two bands, having molecular weights of 42,000 and 46,000 on Western blots.

Evidence for the Involvement of Rat Olfactory Bulb in Processes Supporting Long-Term Olfactory Memory

Current advances in the neurobiology of learning and memory suggest the existence of experience-induced plasticity in sensorial pathways conveying relevant information to higher integrative brain structures. For instance, olfactory learning is known to induce long-lasting modifications of neural activity at the level of the first relay structure of the olfactory system, the olfactory bulb. The observed forms of plasticity depend on the action exerted during learning by ascending neuromodulatory systems, such as the noradrenergic (NA) system originating from the locus ceruleus. This study was aimed at investigating the importance of olfactory bulb plasticity in learning and retention of an olfactory task. In a daily training schedule animals had to learn to use multi-site electrical stimulation patterns of the olfactory bulb as discriminative cues for choosing between a palatable and a nonpalatable solution. We first examined the effects of a continuous intrabulbar infusion of propranolol (a beta-NA receptor antagonist) carried out during the learning period. We found that this treatment neither impaired the retention of a previously learned task nor the learning of a new task. However, the animals presented a severe deficit in long-term retention (>5 days) of the task learned under perfusion. Unexpectedly, this effect cannot be ascribed to a selective blockade of beta-NA receptors since infusion of the drug vehicle (saline-ascorbate) produced exactly the same deficit while a saline solution remained without effect. A final experiment showed that the selective deficit in long-term retention was not observed when the infusion of the saline-ascorbate solution started on the day following completion of learning. Taken together, these results suggest that ascorbate-sensitive neural processes occurring within the olfactory bulb during learning are of functional importance for long-term storage of olfactory information.