Odor-induced metabolic activity in the olfactory bulb of rats trained to detect propionic acid vapor

The pattern of glucose metabolism in the glomerular layer of the main olfactory bulb was studied in rats trained to sample brief odor stimuli. After injection with [¹⁴C]2-deoxyglucose (2-DG), 5 rats were tested for discrimination of propionic acid from air. Over the 45 min test period rats sampled the stimulus for 0.5–0.8 s during each trial and their total exposure to the stimulus was 53–147 s. A discrete focus of increased glucose metabolism was found in the central dorsomedial sector of the glomerular layer in each animal. The position of this focus and the overall pattern of glomerular layer activity was essentially identical to that obtained in 4 control rats which were exposed passively to alternating 5 min periods of the odor and clean air for 45 min. The size of the primary focus was only slightly smaller in the trained rats, despite the large difference in total exposure time. The absence of olfactory adaptation during the behavioral tests and the similar pattern of 2-DG uptake in controls and trained animals indicate that adaptation does not play a significant role in the patterns of glucose metabolism induced in the glomeruli of the olfactory bulb by extended exposure to an odor.     

Spatial coding of olfactory information in the antennal lobe of Drosophila melanogaster

[3H]2-deoxyglucose autoradiography has revealed foci in the antennal lobe of Drosophila melanogaster that are active during olfactory stimulation. Labelling in the receptor axons and the lobe interneurons can be observed. Each class of odors stimulates activity in a specific subset of antennal glomeruli. This defines the activity domain, which is recognizably distinct, although overlapping, for different classes of volatile chemicals. Unilateral stimulation with attractants predominantly results in excitation of the ipsilateral glomeruli. Collaterals of the receptor neurons to the contralateral lobe stimulate no visible postsynaptic activity. On the other hand, unilateral stimulation with repellent odors, such as benzaldehyde, results in an equally strong activity in both antennal lobes.     

Integration of olfactory information in the Colorado potato beetle brain

The processing of olfactory information in the Colorado potato beetle, Leptinotarsa decemlineata Say, was studied by recording responses of olfactory neurones intracellularly in the deutocerebrum. Response characteristics of neurones in this first relay station of the olfactory pathway were measured when the antennae were stimulated with five general green leaf volatiles, i.e. cis-3-hexen-1-ol, trans-2-hexenal, cis-3-hexenyl acetate, trans-2-hexen-1-ol and 1-hexanol. These compounds are part of the so-called green odour of potato, whose defined composition is essential for the beetle's host plant finding. The response spectra of deutocerebral neurones can be divided roughly into two classes: one class containing neurones which are not very specific for the tested compounds, and another class with highly specialized neurones. Their different responses to a potato leaf extract suggest two channels for the processing of olfactory information in the antennal lobe: one channel for the detection of the presence of green leaf odour components, and another one for an evaluation of the component ratios.     

Detection of propionic acid vapor by rats with lesions of olfactory bulb areas associated with high 2-DG uptake

This study tested the functional significance of recent 2-deoxyglucose (2-DG) results demonstrating a discrete focus of activity in the olfactory bulb of rats exposed to the vapor of propionic acid. Rats with lesions that destroyed this area of the olfactory bulb performed as well as did sham operated and lesioned controls in detection of amyl acetate, butanol, geraniol, propionic acid, and on a test for propionic acid threshold. Our results demonstrate that an area of the olfactory bulb containing a major focus of metabolic activity induced by exposure to an odor can be removed without producing a deficit in the detection of that odor.     

[H]2-Deoxyglucose mapping of odor-induced neuronal activity in the antennal lobes of Drosophila melanogaster

Olfactory stimultion results in an enhanced uptake of [³H]2-deoxyglucose in specific glomeruli in the antennal lobes of Drosophila melanogaster. Unilateral stimulation induces activity in receptor axons and lobe interneurons on the ipsilateral side. Collaterals from the receptor axons to the contralateral lobe are also active but stimulate either weak or no postsynaptic activity. This difference in signal transfer properties could be relevant to odor detection by flies.     

Colocalization of NADPH-diaphorase and GABA-immunoreactivity in the olfactory and visual system of the locust

Nitric oxide synthesizing neurons of the locust CNS have been identified by NADPH-diaphorase staining. However, the conventional transmitters of these neurons are unknown. Here we use double labelling for NADPH-diaphorase and GABA-immunofluorescence on sections of the brain to investigate a potential coexpression of both markers. The antennal lobe is innervated by a cluster of about 45–50 NADPH-diaphorase positive local interneurons which express GABA-immunofluorescence. The mushroom bodies are a higher order olfactory center which receive an extrinsic innervation from GABA-immunoreactive and NADPH-diaphorase positive fiber systems. Each optic lobe contains about 4500 GABA-immunoreactive cell bodies. In the visual system, identifyable GABA-immunoreactive neurons arborize in the external plexiform layer of the lamina, in several strata of the medulla, and in the lobula complex. A survey of all NADPH-diaphorase positive cell groups detected a colocalization of GABA-immunoreactivity in a small subpopulation of somata along the anterior rim of the medulla. These cytochemical findings suggest that nitric oxide may be a characteristic cotransmitter of GABAergic circuits of the antennal lobe, while in mushroom bodies and the visual system the majority of nitric oxide and GABA releasing neurons are distinct populations.     

Searching for anatomical correlates of olfactory lateralization in the honeybee antennal lobes: a morphological and behavioural study

The honeybee, Apis mellifera L. (Hymenoptera: Apidae), has recently become a model for studying brain asymmetry among invertebrates. A strong lateralization favouring the right antenna was discovered in odour learning and short-term memory recall experiments, and a lateral shift favouring the left antenna for long-term memory recall. Corresponding morphological asymmetries have been found in the distribution of olfactory sensilla between the antennae and confirmed by electrophysiological odour response measurements in isolated right and left antennae. The aim of this study was to investigate whether a morphological asymmetry can be observed in the volume of the primary olfactory centres of the central nervous system, the antennal lobes (ALs). Precise volume measurements of a subset of their functional units, the glomeruli, were performed in both sides of the brain, exploiting the advantages of two-photon microscopy. This novel method allowed minimal invasive acquisition of volume images of the ALs, avoiding artefacts from brain extraction and dehydration. The study was completed by a series of behavioural experiments in which response asymmetry in odour recall following proboscis extension reflex conditioning was assessed for odours, chosen to stimulate strong activity in the same glomeruli as in the morphological study. The volumetric measurements found no evidence of lateralization in the investigated glomeruli within the experimental limits. Instead, in the behavioural experiments, a striking odour dependence of the lateralization was observed. The results are discussed on the basis of recent neurophysiological and ethological experiments in A. mellifera.     

Olfactory coding in the mammalian olfactory bulb

There have been a number of recent approaches to the study of olfactory coding, each of which has its advantages and disadvantages. In the present review, we discuss our own work on this topic, which has involved mapping uptake of [¹⁴C]2-deoxyglucose across the entire glomerular layer of the rat main olfactory bulb in response to systematically selected pure odorant molecules. Our strategy to understand the olfactory code has involved four approaches. In the first, we determined whether the system encodes odorants in their entirety, or whether it encodes odorants by representing combinations of molecular features that add together to comprise a neural picture of each odorant. Multiple odorant features appeared to be coded by multiple receptors. Our second strategy examined the ways that such features are represented. We stimulated rats with odorants that differed greatly in their molecular structure to be able to identify a set of odorant feature response domains. Our third approach asked how odorants with very small differences in molecular structure are coded, and we found systematic differences in the representation of such features within response domains. Finally, we were able to predict odor perception from the neural representations of odorants that differed in only a single aspect of their structure. Using these strategies, we have been able to learn some of the rules by which the olfactory code operates. These rules have allowed us to predict where previously unmapped molecules would be represented and how differences in molecular representations affect olfactory perceptions.     

Monoclonal antibodies to synaptic macromolecules of Drosophila melanogaster

We have obtained monoclonal antibodies to Drosophila acetylcholinesterase, glutamate dehydrogenase as well as other unknown macromolecules which may have some relevance in synaptic function. The majority of antibodies against acetylcholinesterase recognised common epitopes on all four subunits--but one (MA2) was specific to a 110 kDa dimer. Antibodies to unknown synaptic macromolecules were identified by their selective staining in immunofluorescence studies. F2A3 stains sensory neurons and their synapses in the visual and olfactory systems.     

Spatially resolved time‐frequency analysis of odour coding in the insect antennal lobe

Antennal lobes constitute the first neurophils in the insect brain involved in coding and processing of olfactory information. With their stereotyped functional and anatomical organization, they provide an accessible model with which to investigate information processing of an external stimulus in a neural network in vivo. Here, by combining functional calcium imaging with time‐frequency analysis, we have been able to monitor the oscillatory components of neural activity upon olfactory stimulation. The aim of this study is to investigate the presence of stimulus‐induced oscillatory patterns in the honeybee antennal lobe, and to analyse the distribution of those patterns across the antennal lobe glomeruli. Fast two‐photon calcium imaging reveals the presence of low‐frequency oscillations, the intensity of which is perturbed by an incoming stimulus. Moreover, analysis of the spatial arrangement of this activity indicates that it is not homogeneous throughout the antennal lobe. On the contrary, each glomerulus displays an odorant‐specific time‐frequency profile, and acts as a functional unit of the oscillatory activity. The presented approach allows simultaneous recording of complex activity patterns across several nodes of the antennal lobe, providing the means to better understand the network dynamics regulating olfactory coding and leading to perception.     

Preservation of olfaction in surgery of olfactory groove meningiomas

Introduction

Olfaction is commonly considered as secondary among the sensory functions, perhaps reflecting a lack of interest in sparing olfaction after surgery for the olfactory groove meningiomas (OGM). However, considering the repercussions of olfaction for the quality of life, the assessment of post-operative olfaction should be necessary. We retrospectively reviewed the olfactory outcome in patients with OGM and investigated the factors associated with sparing the post-operative olfaction.

Methods

Between 1993 and 2012, 40 patients with OGM underwent surgical resection and estimated the olfactory function using the Korean version of “Sniffin'Sticks” test (KVSS). Variable factors, such as tumor size, degree of preoperative edema, tumor consistency, preoperative olfactory function, surgical approaches, patient's age, and gender were analyzed with attention to the post-operative olfactory function.

Results

Anatomical and functional preservation of olfactory structures were achieved in 26 patients (65%) and 22 patients (55%), respectively. Among the variable factors, size of tumor was significant related to the preservation of post-operative olfaction. (78.6% in size <4 cm and 42.3% in size >4 cm, p = 0.035). Sparing the olfaction was significantly better in patients without preoperative olfactory dysfunction (84.6%) compared with ones with preoperative olfactory dysfunction (40.7%, p = 0.016). The frontolateral approach achieved much more excellent post-operative olfactory function (71.4%) than the bifrontal approach (36.8%, p = 0.032).

Conclusions

If the tumor was smaller than 4 cm and the patients did not present olfactory dysfunction preoperatively, the possibility of sparing the post-operative olfaction was high. Among the variable surgical approaches, frontolateral route may be preferable sparing the post-operative olfaction.     

Multiple olfactory receptor neurons and their axonal projections in the antennal lobe of the honeybee Apis mellifera

The poreplate sensilla of honeybees are equipped with multiple olfactory receptor neurons (ORNs), which innervate glomeruli of the antennal lobe (AL). We investigated the axonal projection pattern in glomeruli of the AL (glomerular pattern), formed by the multiple ORNs of individual poreplate sensilla. We used the different glomerular patterns to draw conclusions about the equipment of poreplate sensilla with different ORN types. ORNs of single poreplate sensilla were stained and analyzed by laser-scanning confocal microscopy and 3D software (AMIRA). In 13 specimens we found between 7 and 23 ORNs. This is in accordance with data found in the literature (5-35 ORNs) suggesting that all ORNs of the single poreplate sensilla were stained. The ORNs innervate the AL via all four sensory tracts (T1-T4), and glomeruli of the anterior part of the AL are more often innervated. Each ORN innervates a single glomerulus (uniglomerular), and all ORNs of one poreplate sensillum project to different glomeruli. Visual inspection and individual identification of glomeruli, based on the honeybee digital AL atlas, were used to evaluate mapping of glomeruli by a rigid transformation of the experimental ALs onto a reference AL. ORNs belonging to individual poreplate sensilla form variable glomerular patterns, and we did not find a common organization of glomerular patterns. We conclude that poreplate sensilla are equipped with different ORN types but that the same ORN types can be found in different poreplate sensilla. The equipment of poreplate sensilla with ORNs is overlapping. The mapping of glomeruli by rigid transformation is revealed to be a powerful tool for comparative neuroanatomy.     

Sensillum-specific, topographic projection patterns of olfactory receptor neurons in the antennal lobe of the cockroach Periplaneta americana

In vertebrates and many invertebrates, olfactory signals detected by peripheral olfactory receptor neurons (ORNs) are conveyed to a primary olfactory center with glomerular organization in which odor-specific activity patterns are generated. In the cockroach, Periplaneta americana, ORNs in antennal olfactory sensilla project to 205 unambiguously identifiable antennal lobe (AL) glomeruli that are classified into 10 glomerular clusters (T1-T10 glomeruli) innervated by distinct sensory tracts. In this study we employed single sensillum staining techniques and investigated the topographic projection patterns of individual ORNs to elucidate the relationship between sensillum types and glomerular organization in the AL. Axons of almost all ORNs projected to individual glomeruli. Axons of ORNs in perforated basiconic sensilla selectively innervated the anterodorsal T1-T4 glomeruli, whereas those in trichoid and grooved basiconic sensilla innervated the posteroventral T5-T9 glomeruli. About 90% of stained ORNs in trichoid sensilla sent axons to the T5 glomeruli and more than 90% of ORNs in grooved basiconic sensilla innervated the T6, T8, and T9 glomeruli. The T5 and T9 glomeruli exclusively receive sensory inputs from the trichoid and grooved basiconic sensilla, respectively. All investigated glomeruli received convergent input from a single type of sensillum except F11 glomerulus in the T6 glomeruli, which was innervated from both trichoid and grooved basiconic sensilla. These results suggest that ORNs in distinct sensillum types project to glomeruli in distinct glomerular clusters. Since ORNs in distinct sensillum types are each tuned to distinct subsets of odorant molecules, the AL is functionally compartmentalized into groups of glomeruli.     

Spatial coding and invariance in object-selective cortex

The present study examined the coding of spatial position in object selective cortex. Using functional magnetic resonance imaging (fMRI) and pattern classification analysis, we find that three areas in object selective cortex, the lateral occipital cortex area (LO), the fusiform face area (FFA), and the parahippocampal place area (PPA), robustly code the spatial position of objects. The analysis further revealed several anisotropies (e.g., horizontal/vertical asymmetry) in the representation of visual space in these areas. Finally, we show that the representation of information in these areas permits object category information to be extracted across varying locations in the visual field; a finding that suggests a potential neural solution to accomplishing translation invariance.     

Neural correlates of odor learning in the honeybee antennal lobe

Extracellular spiking activity and local field potentials (LFP) were recorded via tetrodes at the output of the antennal lobe (AL) in the honeybee brain during olfactory conditioning. Odors induce reliable rate responses that consist of either phasic‐tonic responses, or complex responses with odor‐specific profiles. In addition, odors evoke consistent responses of LFP oscillations in the 50‐Hz band during the phasic ON‐response to odor stimulation, and variable LFP responses at other frequency bands during the sustained response. A principal component analysis of the ensemble activity during differential conditioning consistently indicates the largest changes in response to the learned odor (conditioned stimulus; CS+). Relative LFP power increases for CS+ in the 15–40‐Hz frequency band during the sustained response, and decreases for frequencies above 45 Hz. To quantify the relationship between these population responses given by the ensemble spiking activity and LFP, we show that for CS+ the learning‐related changes in the degree of the phase‐locked spiking activity correlate with the power changes in the corresponding frequency bands. Our results indicate associative plasticity in the AL of the bee leading to both enhancement and decrease of neuronal response rates. LFP power changes and the correlated changes in the locking between spikes and LFP at different frequencies observed for the learned odor serve as further evidence for a learning‐induced restructuring of temporal ensemble representations.     

Olfactory bulb transplantation into the olfactory bulb of neonatal rats: an autoradiographic study

Tritiated thymidine prelabeled presumptive olfactory bulbs (E15–E17, and E19) were homotopically transplanted in unilaterally partially or totally bulbectomized neonatal rats (P1–P5). [³H]thymidine was injected to pregnant rats at the time when the large neurons of the bulb were undergoing cellular division. After postoperative survival times from 20 days to 7 months, the animals were sacrificed and processed for histological, immunohistochemical and autoradiographic observations. The nuclear autoradiographic label allowed easy recognition of the transplanted tissue in totally bulbectomized animals after short survival and in partially bulbectomized animals after long survival. The autoradiographic label was strictly confined to the transplanted tissue and intermingling of host and donor neurons was never observed. The reliability of the autoradiographic technique in our study will enable us to mark those neurons whose axons can be demonstrated, by retrograde tracing methods, to establish connections with the host brain.     

Dopaminergic periglomerular cells in the turtle olfactory bulb

With the indirect immunofluorescence technique using antisera to three catecholamine synthesizing enzymes, labeled periglomerular cells as well as their intraglomerular processes were observed in the turtle olfactory bulb. These cells could also be recognized in the EPL and the glomerular layer. Unlabeled periglomerular cells were also seen. Thick labeled processes (presumably dendrites) entered the glomerular neuropil, and there formed a dense network, with numerous terminal varicosities. These results support the existence of a unique, homologous dopaminergic subdivision of the periglomerular interneurons throughout classes of vertebrates. In addition, a second type of weakly tyrosine hydroxylase immunoreactive neurons was observed in the outer part of the granule layer. Dopamine beta-hydroxylase positive fibers were seen in the granule, mitral and external plexiform layers.     

Differential expression of G proteins in the mouse olfactory system

Transmembrane signaling events at the dendrites and axons of olfactory receptor neurons mediate distinct functions. Whereas odorant recognition and chemosensory transduction occur at the dendritic membranes of olfactory neurons, signal propagation, axon sorting and target innervation are functions of their axons. The roles of G proteins in transmembrane signaling at the dendrites have been studied extensively, but axonal G proteins have not been investigated in detail. We used immunohistochemistry to visualize expression of alpha subunits of G(o) and G(i2) in the mouse olfactory system. G(o) is expressed ubiquitously on axons of olfactory receptor neurons throughout the olfactory neuroepithelium and in virtually all glomeruli in the main olfactory bulb. In contrast, expression of G(i2) is restricted to a sub-population of olfactory neurons, along the dorsal septum and the dorsal recess of the nasal cavity, which projects primarily to medial regions of the olfactory bulb, with the exception of glomeruli adjacent to the pathway of the vomeronasal nerve. In contrast to the overlapping expression patterns of G(o) and G(i2) in the main olfactory system, neurons expressing G(o) and those expressing G(i2) in the accessory olfactory bulb are more clearly separated, in agreement with previous studies. Vomeronasal axons terminating in glomeruli in the rostral region of the accessory olfactory bulb express G(i2), whereas those projecting to the caudal region express G(o). Characterization of the expression patterns of G(i2) and G(o) in the olfactory projection is essential for future studies aimed at relating transmembrane signaling events to signal propagation, axon sorting and target innervation.     

Neural representation of olfactory mixtures in the honeybee antennal lobe

Natural olfactory stimuli occur as mixtures of many single odors. We studied whether the representation of a mixture in the brain retains single-odor information and how much mixture-specific information it includes. To understand mixture representation in the honeybee brain, we used in vivo calcium imaging at the level of the antennal lobe, and systematically measured odor-evoked activity in 24 identified glomeruli in response to four single odorants and all their possible binary, ternary and quaternary mixtures. Qualitatively, mixture-induced activity patterns always contained glomeruli belonging to the pattern of at least one of the components, suggesting a high conservation of component information in olfactory mixtures. Quantitatively, glomerular activity saturated quickly and increasing the number of components resulted in an increase of cases in which the response of a glomerulus to the mixture was lower than that to the strongest component ('suppression'). This shows global inhibition in the antennal lobe, probably acting as overall gain control. Single components were not equally salient (in terms of number of active glomeruli) and mixture activity patterns were always more similar to the more salient components, in a way that could be predicted linearly. Thus, although a gain control system in the honeybee antennal lobe prevents saturation of the olfactory system, mixture representation follows essentially elemental rules.     

Lesion of the main olfactory epithelium facilitates maternal behavior in virgin rabbits

Maternal behavior is induced in virgin female rabbits (normally unresponsive to foster pups) by removing the accessory olfactory bulbs. To determine if the main olfactory system (MOS) plays a similar inhibitory role in the present work we investigated the effect of lesioning the olfactory epithelium with a ZnSO₄ spray on the facilitation of maternal behavior in New Zealand white virgin rabbits. Four days after the chemical lesion 40% of females showed behaviors indistinguishable from those of normal mothers, i.e.: rapid entrance into the nest box containing the pups, adoption of a crouching posture over them, acceptance of suckling, and exit from the nest box after ca. 3 min. The proportion of females showing these behaviors rose to 70% by day 14 post-lesion. Ovariectomized rabbits sprayed with ZnSO₄ or animals sprayed with NaCl did not behave maternally. ZnSO₄ also provoked a transient reduction in olfactory perception: before the lesion animals from all groups directed significantly more sniffs to a flask containing male urine than to one containing water. This difference was abolished in ZnSO₄-sprayed females (intact and ovariectomized) for 3–6 days post-lesion and was re-established by 7–9 days. NaCl did not provoke such transitory hyposmia. ZnSO₄ lesions did not provoke malaise in the animals, as determined by food intake and the frequency of scent-marking and ambulation. Results suggest that olfactory cues from the pups are aversive to virgin rabbits and that a transitory reduction in their perception (accompanied by the action of ovarian secretions) is enough to facilitate maternal responsiveness.