Simultaneous activation of mouse main and accessory olfactory bulbs by odors or pheromones

Olfactory systems and neural circuits that modulate predator odor fear

When prey animals detect the odor of a predator a constellation of fear-related autonomic, endocrine, and behavioral responses rapidly occur to facilitate survival. How olfactory sensory systems process predator odor and channel that information to specific brain circuits is a fundamental issue that is not clearly understood. However, research in the last 15 years has begun to identify some of the essential features of the sensory detection systems and brain structures that underlie predator odor fear. For instance, the main (MOS) and accessory olfactory systems (AOS) detect predator odors and different types of predator odors are sensed by specific receptors located in either the MOS or AOS. However, complex predator chemosignals may be processed by both the MOS and AOS, which complicate our understanding of the specific neural circuits connected directly and indirectly from the MOS and AOS to activate the physiological and behavioral components of unconditioned and conditioned fear. Studies indicate that brain structures including the dorsal periaqueductal gray (DPAG), paraventricular nucleus (PVN) of the hypothalamus, and the medial amygdala (MeA) appear to be broadly involved in predator odor induced autonomic activity and hypothalamic-pituitary-adrenal (HPA) stress hormone secretion. The MeA also plays a key role in predator odor unconditioned fear behavior and retrieval of contextual fear memory associated with prior predator odor experiences. Other neural structures including the bed nucleus of the stria terminalis and the ventral hippocampus (VHC) appear prominently involved in predator odor fear behavior. The basolateral amygdala (BLA), medial hypothalamic nuclei, and medial prefrontal cortex (mPFC) are also activated by some but not all predator odors. Future research that characterizes how distinct predator odors are uniquely processed in olfactory systems and neural circuits will provide significant insights into the differences of how diverse predator odors activate fear.

Alarm pheromone is detected by the vomeronasal organ in male rats

It is widely known that a stressed animal releases specific pheromones, possibly for alarming nearby conspecifics. We previously investigated an alarm pheromone in male rats and found that this alarm pheromone evokes several responses, including increases in the defensive and risk assessment behaviors in a modified open-field test, and enhancement of the acoustic startle reflex. However, the role of the vomeronasal organ in these pheromone effects remains unclear. To clarify this point, vomeronasal organ-excising or sham surgeries were performed in male rats for use in 2 experimental models, after which they were exposed to alarm pheromone. We found that the vomeronasal organ-excising surgery blocked the effects of this alarm pheromone in both the modified open-field test and acoustic startle reflex test. In addition, the results of habituation/dishabituation test and soybean agglutinin binding to the accessory olfactory bulb suggested that the vomeronasal organ-excising surgery completely ablated the vomeronasal organ while preserving the functioning of the main olfactory system. From the above results, we showed that the vomeronasal organ plays an important role in alarm pheromone effects in the modified open-field test and acoustic startle reflex test.

Nasopalatine ducts and flehmen behavior in the mandrill: reevaluating olfactory communication in Old World primates

Compared to other modes of communication, chemical signaling between conspecifics generally has been overlooked in Old World primates, despite the presence in this group of secretory glands and scent-marking behavior, as well as the confirmed production and perception of olfactory signals. In other mammalian species, flehmen is a behavior thought to transport primarily nonvolatile, aqueous-soluble odorants via specialized ducts to the vomeronasal organ (VNO). By contrast, Old World primates are traditionally thought to lack a functional VNO, relying instead on the main olfactory system to process volatile odorants from their environment. Here, in the mandrill (Mandrillus sphinx), we document unusual morphological and behavioral traits that typically are associated with the uptake of conspecific chemical cues for processing by an accessory olfactory system. Notably, we confirmed that both sexes possess open nasopalatine ducts and, in response to the presentation of conspecific odorants, we found that both sexes showed stereotyped behavior consistent with the flehmen response. If, as in other species, flehmen in the mandrill serves to mediate social or reproductive information, we expected its occurrence to vary with characteristics of either the signaler or receiver. Flehmen, particularly in a given male, occurred most often in response to odorants derived from male, as opposed to female, conspecifics. Moreover, odorants derived during the breeding season elicited more flehmen responses than did odorants collected during the birthing season. Lastly, odorants from reproductively cycling females also elicited more responses than did odorants from contracepted females. Although confirming a link between the nasopalatine ducts, flehmen behavior, and olfactory processing in mandrills would require further study, our observations provide new information to suggest anatomical variability within Old World primates, calling further attention to the underappreciated role of chemical communication in this lineage.

2-Heptanone increases the firing rate of the basal amygdala: role of anterior olfactory epithelial organs

Wistar rats subjected to physical stress release a urine alarm pheromone (2-heptanone) that produces signs of anxiety and despair in receptor rats not subjected to physical stress. However, unknown are the effects of 2-heptanone on the firing rate of the basal amygdala, a structure that participates in the expression of fear, and the participation of anterior olfactory epithelial organs, namely the septal organ and vomeronasal organ (SO-VNO). We explored the effects of 2-heptanone applied near the nostrils on single-unit extracellular recordings from the basal amygdala in a sham-operated group and rats that underwent removal of the SO-VNO. The firing rate of basal amygdala neurons in the SO-VNO removal group was significantly higher than in the sham-operated group. In both groups, recordings were classified according to the responses to 2-heptanone (i.e., increased firing rate, decreased firing rate, and no response). SO-VNO removal was associated with an increased firing rate in the three types of neurons. A similar number of neurons increased their firing rate during and after 2-heptanone stimulation in both groups, but such an increase in firing rate was longer in the group of rats subjected to SO-VNO removal. The results indicate that the SO-VNO is not essential for the effect of 2-heptanone on the firing rate of basal amygdala neurons. SO-VNO ablation did not block but rather accentuated the response of amygdala neurons to 2-heptanone.

One nose, one brain: contribution of the main and accessory olfactory system to chemosensation

The accessory olfactory system is present in most tetrapods. It is involved in the perception of chemical stimuli, being implicated also in the detection of pheromones. However, it is sensitive also to some common odorant molecules, which have no clear implication in intraspecific chemical communication. The accessory olfactory system may complement the main olfactory system and may contribute different perceptual features to the construction of a unitary representation, which merges the different chemosensory qualities. Crosstalk between the main and accessory olfactory systems occurs at different levels of central processing, in brain areas where the inputs from the two systems converge. Interestingly, centrifugal projections from more caudal brain areas are deeply involved in modulating both main and accessory sensory processing. A high degree of interaction between the two systems may be conceived and partial overlapping appears to occur in many functions. Therefore, the central chemosensory projections merge inputs from different organs to obtain a complex chemosensory picture.

Visualization of odor-induced neuronal activity by immediate early gene expression

Background

Sensitive detection of sensory-evoked neuronal activation is a key to mechanistic understanding of brain functions. Since immediate early genes (IEGs) are readily induced in the brain by environmental changes, tracing IEG expression provides a convenient tool to identify brain activity. In this study we used in situ hybridization to detect odor-evoked induction of ten IEGs in the mouse olfactory system. We then analyzed IEG induction in the cyclic nucleotide-gated channel subunit A2 (Cnga2)-null mice to visualize residual neuronal activity following odorant exposure since CNGA2 is a key component of the olfactory signal transduction pathway in the main olfactory system.

Results

We observed rapid induction of as many as ten IEGs in the mouse olfactory bulb (OB) after olfactory stimulation by a non-biological odorant amyl acetate. A robust increase in expression of several IEGs like c-fos and Egr1 was evident in the glomerular layer, the mitral/tufted cell layer and the granule cell layer. Additionally, the neuronal IEG Npas4 showed steep induction from a very low basal expression level predominantly in the granule cell layer. In Cnga2-null mice, which are usually anosmic and sexually unresponsive, glomerular activation was insignificant in response to either ambient odorants or female stimuli. However, a subtle induction of c-fos took place in the OB of a few Cnga2-mutants which exhibited sexual arousal. Interestingly, very strong glomerular activation was observed in the OB of Cnga2-null male mice after stimulation with either the neutral odor amyl acetate or the predator odor 2, 3, 5-trimethyl-3-thiazoline (TMT).

Conclusions

This study shows for the first time that in vivo olfactory stimulation can robustly induce the neuronal IEG Npas4 in the mouse OB and confirms the odor-evoked induction of a number of IEGs. As shown in previous studies, our results indicate that a CNGA2-independent signaling pathway(s) may activate the olfactory circuit in Cnga2-null mice and that neuronal activation which correlates to behavioral difference in individual mice is detectable by in situ hybridization of IEGs. Thus, the in situ hybridization probe set we established for IEG tracing can be very useful to visualize neuronal activity at the cellular level.

Odor memory stability after reinnervation of the olfactory bulb

The olfactory system, particularly the olfactory epithelium, presents a unique opportunity to study the regenerative capabilities of the brain, because of its ability to recover after damage. In this study, we ablated olfactory sensory neurons with methimazole and followed the anatomical and functional recovery of circuits expressing genetic markers for I7 and M72 receptors (M72-IRES-tau-LacZ and I7-IRES-tau-GFP). Our results show that 45 days after methimazole-induced lesion, axonal projections to the bulb of M72 and I7 populations are largely reestablished. Furthermore, regenerated glomeruli are re-formed within the same areas as those of control, unexposed mice. This anatomical regeneration correlates with functional recovery of a previously learned odorant-discrimination task, dependent on the cognate ligands for M72 and I7. Following regeneration, mice also recover innate responsiveness to TMT and urine. Our findings show that regeneration of neuronal circuits in the olfactory system can be achieved with remarkable precision and underscore the importance of glomerular organization to evoke memory traces stored in the brain.

The main but not the accessory olfactory system is involved in the processing of socially relevant chemosignals in ungulates

Ungulates like sheep and goats have, like many other mammalian species, two complementary olfactory systems. The relative role played by these two systems has long been of interest regarding the sensory control of social behavior. The study of ungulate social behavior could represent a complimentary alternative to rodent studies because they live in a more natural environment and their social behaviors depend heavily on olfaction. In addition, the relative size of the main olfactory bulb (MOB) [in comparison to the accessory olfactory bulb (AOB)] is more developed than in many other lissencephalic species like rodents. In this review, we present data showing a clear involvement of the main olfactory system in two well-characterized social situations under olfactory control in ungulates, namely maternal behavior and offspring recognition at birth and the reactivation of the gonadotropic axis of females exposed to males during the anestrous season. In conclusion, we discuss the apparent discrepancy between the absence of evidence for a role of the vomeronasal system in ungulate social behavior and the existence of a developed accessory olfactory system in these species.

Plant odour stimuli reshape pheromonal representation in neurons of the antennal lobe macroglomerular complex of a male moth

Male moths are confronted with complex odour mixtures in a natural environment when flying towards a female-emitted sex pheromone source. Whereas synergistic effects of sex pheromones and plant odours have been observed at the behavioural level, most investigations at the peripheral level have shown an inhibition of pheromone responses by plant volatiles, suggesting a potential role of the central nervous system in reshaping the peripheral information. We thus investigated the interactions between sex pheromone and a behaviourally active plant volatile, heptanal, and their effects on responses of neurons in the pheromone-processing centre of the antennal lobe, the macroglomerular complex, in the moth Agrotis ipsilon. Our results show that most of these pheromone-sensitive neurons responded to the plant odour. Most neurons responded to the pheromone with a multiphasic pattern and were anatomically identified as projection neurons. They responded either with excitation or pure inhibition to heptanal, and the response to the mixture pheromone + heptanal was generally weaker than to the pheromone alone, showing a suppressive effect of heptanal. However, these neurons responded with a better resolution to pulsed stimuli. The other neurons with either purely excitatory or inhibitory responses to all three stimuli did not exhibit significant differences in responses between stimuli. Although the suppression of the pheromone responses in AL neurons by the plant odour is counter-intuitive at first glance, the observed better resolution of pulsed stimuli is probably more important than high sensitivity to the localization of a calling female.

The functional upregulation of piriform cortex is associated with cross-modal plasticity in loss of whisker tactile inputs

BACKGROUND

Cross-modal plasticity is characterized as the hypersensitivity of remaining modalities after a sensory function is lost in rodents, which ensures their awareness to environmental changes. Cellular and molecular mechanisms underlying cross-modal sensory plasticity remain unclear. We aim to study the role of different types of neurons in cross-modal plasticity.

METHODOLOGY/PRINCIPAL FINDINGS

In addition to behavioral tasks in mice, whole-cell recordings at the excitatory and inhibitory neurons, and their two-photon imaging, were conducted in piriform cortex. We produced a mouse model of cross-modal sensory plasticity that olfactory function was upregulated by trimming whiskers to deprive their sensory inputs. In the meantime of olfactory hypersensitivity, pyramidal neurons and excitatory synapses were functionally upregulated, as well as GABAergic cells and inhibitory synapses were downregulated in piriform cortex from the mice of cross-modal sensory plasticity, compared with controls. A crosswire connection between barrel cortex and piriform cortex was established in cross-modal plasticity.

CONCLUSION/SIGNIFICANCE

An upregulation of pyramidal neurons and a downregulation of GABAergic neurons strengthen the activities of neuronal networks in piriform cortex, which may be responsible for olfactory hypersensitivity after a loss of whisker tactile input. This finding provides the clues for developing therapeutic strategies to promote sensory recovery and substitution.

Second-order input to the medial amygdala from olfactory sensory neurons expressing the transduction channel TRPM5

Recent anatomical tracing experiments in rodents have established that a subset of mitral cells in the main olfactory bulb (MOB) projects directly to the medial amygdala (MeA), traditionally considered a target of the accessory olfactory bulb. Neurons that project from the MOB to the MeA also show activation in response to conspecific (opposite sex) volatile urine exposure, establishing a direct role of the MOB in semiochemical processing. In addition, olfactory sensory neurons (OSNs) that express the transient receptor potential M5 (TRPM5) channel innervate a subset of glomeruli that respond to putative semiochemical stimuli. In this study, we examined whether the subset of glomeruli targeted by TRPM5-expressing OSNs is innervated by the population of mitral cells that projects to the MeA. We injected the retrograde tracer cholera toxin B (CTB) into the MeA of mice in which the TRPM5 promoter drives green fluorescent protein (GFP). We found overlapping clusters of CTB-labeled mitral cell dendritic branches (CTB(+) ) in TRPM5-GFP(+) glomeruli at significantly greater frequency than expected by chance. Despite the significant degree of colocalization, some amygdalopetal mitral cells extended dendrites to non-TRPM5-GFP glomeruli and vice versa, suggesting that, although significant overlapping glomerular innervation is observed between these two features, it is not absolute.

A putative functional vomeronasal system in anuran tadpoles

We investigated the occurrence and anatomy of the vomeronasal system (VNS) in tadpoles of 13 different anuran species. All of the species possessed a morphologically fully developed VNS with a highly conserved anatomical organisation. We found that a bean-shaped vomeronasal organ (VNO) developed early in the tadpoles, during the final embryonic stages, and was located in the anteromedial nasal region. Histology revealed the presence of bipolar chemosensory neurones in the VNO that were immunoreactive for the Gαo protein. Tract-tracing experiments demonstrated that chemosensory neurones from the VNO reach specific areas in the brain, where a discernible accessory olfactory bulb (AOB) could be observed. The AOB was located in the ventrolateral side of the anterior telencephalon, somewhat caudal to the main olfactory bulb. Synaptophysin-like immunodetection revealed that synaptic contacts between VNO and AOB are established during early larval stages. Moreover, using lectin staining, we identified glomerular structures in the AOB in most of the species that we examined. According to our findings, a significant maturation in the VNS is achieved in anuran larvae. Recent published evidence strongly suggests that the VNS appeared early in vertebrate evolution and was already present in the aquatic last common ancestor of lungfish and tetrapods. In this context, tadpoles may be a good model in which to investigate the anatomical, biochemical and functional aspects of the VNS in an aquatic environment.

Contribution of pheromones processed by the main olfactory system to mate recognition in female mammals

Until recently it was widely believed that the ability of female mammals (with the likely exception of women) to identify and seek out a male breeding partner relied on the detection of non-volatile male pheromones by the female's vomeronasal organ (VNO) and their subsequent processing by a neural circuit that includes the accessory olfactory bulb (AOB), vomeronasal amygdala, and hypothalamus. Emperical data are reviewed in this paper that demonstrate the detection of volatile pheromones by the main olfactory epithelium (MOE) of female mice which, in turn, leads to the activation of a population of glomeruli and abutting mitral cells in the main olfactory bulb (MOB). Anatomical results along with functional neuroanatomical data demonstrate that some of these MOB mitral cells project to the vomeronasal amygdala. These particular MOB mitral cells were selectively activated (i.e., expressed Fos protein) by exposure to male as opposed to female urinary volatiles. A similar selectivity to opposite sex urinary volatiles was also seen in mitral cells of the AOB of female mice. Behavioral data from female mouse, ferret, and human are reviewed that implicate the main olfactory system, in some cases interacting with the accessory olfactory system, in mate recognition.

From genes to social communication: molecular sensing by the vomeronasal organ

The ability to distinguish molecular cues emitted by other individuals is a fundamental feature of social interactions such as finding and identifying a mate, establishing social hierarchies, and initiating interspecies defensive behaviors. In rodents, this ability involves the vomeronasal organ (VNO), a distinct chemoreceptive structure that is part of the olfactory system. Recent insights have led to unprecedented progress in identifying ligand and receptor families underlying vomeronasal recognition, characterizing the behavioral consequences caused by VNO activation, and defining higher neural circuits underlying the initiation of instinctive behaviors such as aggression. Here, we review such findings and discuss future areas for investigation, including large-scale mapping studies, immune system-VNO interactions, in vivo recording of neural activity, and optogenetic alteration of sexual and social behaviors.

Centrifugal telencephalic afferent connections to the main and accessory olfactory bulbs

Parallel to the olfactory system, most mammals possess an accessory olfactory or vomeronasal system. The olfactory and vomeronasal epithelia project to the main and accessory olfactory bulbs, which in turn project to adjacent areas of the telencephalon, respectively. New data indicate that projections arising from the main and accessory olfactory bulbs partially converge in the rostral telencephalon and are non-overlapping at caudal telencephalic levels. Therefore, the basal telencephalon should be reclassified in olfactory, vomeronasal, and mixed areas. On the other hand, it has been demonstrated that virtually all olfactory- and vomeronasal-recipient structures send reciprocal projections to the main and accessory olfactory bulbs, respectively. Further, non-chemosensory recipient structures also projects centrifugally to the olfactory bulbs. These feed-back projections appear to be essential modulating processing of chemosensory information. The present work aims at characterizing centrifugal projections to the main and accessory olfactory bulbs arising from olfactory, vomeronasal, mixed, and non-chemosensory recipient telencephalic areas. This issue has been addressed by using tracer injections in the rat and mouse brain. Tracer injections were delivered into the main and accessory olfactory bulbs as well as in olfactory, vomeronasal, mixed, and non-chemosensory recipient telencephalic structures. The results confirm that olfactory- and vomeronasal-recipient structures project to the main and accessory olfactory bulbs, respectively. Interestingly, olfactory (e.g., piriform cortex), vomeronasal (e.g., posteromedial cortical amygdala), mixed (e.g., the anterior medial amygdaloid nucleus), and non-chemosensory-recipient (e.g., the nucleus of the diagonal band) structures project to the main and to the accessory olfactory bulbs thus providing the possibility of simultaneous modulation and interaction of both systems at different stages of chemosensory processing.

Postnatal characterization of cells in the accessory olfactory bulb of wild type and reeler mice

Olfaction is the most relevant chemosensory sense of the rodents. General odors are primarily detected by the main olfactory system while most pheromonal signals are received by the accessory olfactory system. The first relay in the brain occurs in the olfactory bulb, which is subdivided in the main and accessory olfactory bulb (MOB/AOB). Given that the cell generation time is different between AOB and MOB, and the cell characterization of AOB remains limited, the goal of this work was first, the definition of the layering of AOB/MOB and second, the determination of cellular phenotypes in the AOB in a time window corresponding to the early postnatal development. Moreover, since reelin (Reln) deficiency has been related to olfactory learning deficits, we analyzed reeler mice. First, we compared the layering between AOB and MOB at early embryonic stages. Then, cell phenotypes were established using specific neuronal and glial markers as well as the Reln adaptor protein Dab1 to analyse differences in both genetic backgrounds. There was no apparent difference in the cell phenotypes among AOB and MOB or between wild type (wt) and reeler animals. However, a disruption in the granular cell layer of reeler with respect to wt mice was observed. In conclusion, the AOB in Reln-deficient mice showed similar neuronal and glial cell types being only affected the organization of granular neurons.

Differential interactions of sex pheromone and plant odour in the olfactory pathway of a male moth

Most animals rely on olfaction to find sexual partners, food or a habitat. The olfactory system faces the challenge of extracting meaningful information from a noisy odorous environment. In most moth species, males respond to sex pheromone emitted by females in an environment with abundant plant volatiles. Plant odours could either facilitate the localization of females (females calling on host plants), mask the female pheromone or they could be neutral without any effect on the pheromone. Here we studied how mixtures of a behaviourally-attractive floral odour, heptanal, and the sex pheromone are encoded at different levels of the olfactory pathway in males of the noctuid moth Agrotis ipsilon. In addition, we asked how interactions between the two odorants change as a function of the males' mating status. We investigated mixture detection in both the pheromone-specific and in the general odorant pathway. We used a) recordings from individual sensilla to study responses of olfactory receptor neurons, b) in vivo calcium imaging with a bath-applied dye to characterize the global input response in the primary olfactory centre, the antennal lobe and c) intracellular recordings of antennal lobe output neurons, projection neurons, in virgin and newly-mated males. Our results show that heptanal reduces pheromone sensitivity at the peripheral and central olfactory level independently of the mating status. Contrarily, heptanal-responding olfactory receptor neurons are not influenced by pheromone in a mixture, although some post-mating modulation occurs at the input of the sexually isomorphic ordinary glomeruli, where general odours are processed within the antennal lobe. The results are discussed in the context of mate localization.

Odor and pheromone sensing via chemoreceptors

Evolutionally, chemosensation is an ancient but yet enigmatic sense. All organisms ranging from the simplest unicellular form to the most advanced multicellular creature possess the capability to detect chemicals in the surroundings. Conversely, all living things emit some forms of smells, either as communicating signals or as by-products of metabolism. Many species (from worms, insects to mammals) rely on the olfactory systems which express a large number of chemoreceptors to locate food and mates and to avoid danger. Most chemoreceptors expressed in olfactory organs are G-protein coupled receptors (GPCRs) and can be classified into two major categories: odorant receptors (ORs) and pheromone receptors, which principally detect general odors and pheromones, respectively. In vertebrates, these two types of receptors are often expressed in two distinct apparatuses: The main olfactory epithelium (MOE) and the vomeronasal organ (VNO), respectively. Each olfactory sensory neuron (OSN) in the MOE typically expresses one type of OR from a large repertoire. General odors activate ORs and their host OSNs (ranging from narrowly- to broadly-tuned) in a combinatorial manner and the information is sent to the brain via the main olfactory system leading to perception of smells. In contrast, pheromones stimulate relatively narrowly-tuned receptors and their host VNO neurons and the information is sent to the brain via the accessory olfactory system leading to behavioral and endocrinological changes. Recent studies indicate that the functional separation between these two systems is blurred in some cases and there are more subsystems serving chemosensory roles. This chapter focuses on the molecular and cellular mechanisms underlying odor and pheromone sensing in rodents, the best characterized vertebrate models.

The post-natal chemosensory environment induces epigenetic changes in vomeronasal receptor gene expression and a bias in olfactory preference

Vomeronasal stem cells are generated throughout the life of a mouse and differentiate into neurons that express one vomeronasal type 1 (V1r), one or two vomeronasal type 2 (V2r), or one olfactory receptor. Vomeronasal stem cells can be induced to differentiate into neurons by treatment with lipocalins from mouse urine or by epigenetic modification following treatment with histone deacetylase inhibitors. An important question is, do chemosensory signals, modify the detection capabilities of the vomeronasal organ and affect behaviour. Rearing mice in the presence of urine (and its pheromonal signals) derived from a different mouse strain, affected the behavioural preference for non-kin which were accompanied by changes in vomeronasal receptor expression. Significant changes in the expression of vomeronasal V1r, V2r and olfactory receptors, major urinary proteins, and a number of genes thought to be involved in transcriptional regulation were also observed following urine treatment. These results suggest that modification of a mouse's urinary environment may exert epigenetic effects on developing vomeronasal neurons, which modify the type of vomeronasal receptors that are expressed. This may provide a mechanism by which environmental changes are able to modify the detection capabilities of the vomeronasal organ to respond optimally to the most likely social environment that a mouse will encounter when mature.

Odors from proximal species reverse the stress-decreased neurogenesis via main olfactory processing

Unconditioned foot shock followed by restraint in water was used as a stress regimen to induce decreases in neurogenesis in mouse dentate gyrus (DG). Presence of conspecific odors has been known to reverse the stress-induced decrease in DG neurogenesis. In this study, we found that the conspecific odors did not produce these protective effects in mice whose MOE was impaired by nasal zinc sulfate lavage. Moreover, we observed that the presence of odors from rats, hamsters, and guinea pigs throughout the stress procedure reversed the stress-induced decrease in cell proliferation and neurogenesis in mouse dentate gyrus, while these odors alone did not affect mouse dentate cell proliferation or neurogenesis. In contrast, the presence of rabbit, sugar glider, hedgehog, beetle odors did not affect cell proliferation, neurogenesis, the stress-decreased cell proliferation or neurogenesis in DG. Finally, the presence of fox urine odors decreased mouse dentate cell proliferation and neurogenesis but did not affect the stress-induced decrease in cell proliferation or neurogenesis. Taken together, we conclude that olfactory processing via activation of sensory neurons in MOE is responsible for the conspecific odor-produced protective effect against the stress-decreased cell proliferation and neurogenesis. Phylogenetic distances of the odor-generating species and mice might contribute to the odors' protective effects against the stress-induced decreases in cell proliferation and neurogenesis.

Genetics of canine olfaction and receptor diversity

Olfaction is a particularly important sense in the dog. Humans selected for this capacity during the domestication process, and selection has continued to be employed to enhance this ability. In this review we first describe the different olfactory systems that exist and the different odorant receptors that are expressed in those systems. We then focus on the dog olfactory receptors by describing the olfactory receptor gene repertoire and its polymorphisms. Finally, we discuss the different uses of dog olfaction and the questions that still need to be studied.

Both olfactory epithelial and vomeronasal inputs are essential for activation of the medial amygdala and preoptic neurons of male rats

Chemosensory inputs signaling volatile and nonvolatile molecules play a pivotal role in sexual and social behavior in rodents. We have demonstrated that olfactory preference in male rats, that is, attraction to receptive female odors, is regulated by the medial amygdala (MeA), the cortical amygdala (CoA), and the preoptic area (POA). In this paper, we investigated the involvement of two chemosensory organs, the olfactory epithelium (OE) and the vomeronasal organ (VNO), in olfactory preference and copulatory behavior in male rats. We found that olfactory preferences were impaired by zinc sulfate lesion of the OE but not surgical removal of the VNO. Copulatory behaviors, especially intromission frequency and ejaculation, were also suppressed by zinc sulfate treatment. Neuronal activation in the accessory olfactory bulb (AOB), the MeA, the CoA, and the POA was analyzed after stimulation by airborne odors or soiled bedding of estrous females using cFos immunohistochemistry. Although the OE and VNO belong to different neural systems, the main and accessory olfactory systems, respectively, both OE lesion and VNO removal almost equally suppressed the number of cFos-immunoreactive cells in those areas that regulate olfactory preference. These results suggest that signals received by the OE and VNO interact and converge in the early stage of olfactory processing, in the AOB and its targets, although they have distinct roles in the regulation of social behaviors.

Newborn interneurons in the accessory olfactory bulb promote mate recognition in female mice

In the olfactory bulb of adult rodents, local interneurons are constantly replaced by immature precursors derived from the subventricular zone. Whether any olfactory sensory process specifically relies on this cell renewal remains largely unclear. By using the well known model of mating-induced imprinting to avoid pregnancy block, which requires accessory olfactory bulb (AOB) function, we demonstrate that this olfactory memory formation critically depends on the presence of newborn granule neurons in this brain region. We show that, in adult female mice, exposure to the male urine compounds involved in mate recognition increases the number of new granule cells surviving in the AOB. This process is modulated by male signals sensed through the vomeronasal organ and, in turn, changes the activity of the downstream amygdaloid and hypothalamic nuclei involved in the pregnancy block response. Chemical depletion of newly generated bulbar interneurons causes strong impairment in mate recognition, thus resulting in a high pregnancy failure rate to familiar mating male odors. Taken together, our results indicate that adult neurogenesis is essential for specific brain functions such as persistent odor learning and mate recognition.

Olfactory and tissue markers of fear in mammals including humans

Pheromones are a mysterious world of chemical signals involved in conspecific communication. They play a number of key functions important for preservation of life of individual organisms, for their defence, survival of offspring and preservation of species. The best-known groups of pheromones include: trail pheromones, territorial pheromones, sex pheromones, aggregation pheromones, dispersion pheromones, repellent pheromones, social pheromones and alarm pheromones. Alarm pheromones are pheromones that are emitted by animals in threatening situations and inform members of the same species of danger. The identified alarm pheromones are synthesised by insects and aquatic organisms. Also humans are able to emit and perceive pheromones. Although alarm pheromones have not been isolated and identified in man so far, there is presumably evidence for their presence in humans. Pinpointing human alarm pheromones, determinants of experienced stress and inductors of provoked fear could have widespread consequences. Their identification could also be of significant importance for the practical utilisation of results by institutions responsible for safety and defence as well as law enforcement/crime detection and antiterrorist activities.

Brain processing of the mammary pheromone in newborn rabbits

Chemosignals strongly contribute to social interactions in mammals, including mother-young relationships. In the European rabbit, a volatile compound emitted by lactating females in milk, the 2-methylbut-2-enal, has been isolated. Carrying the properties of a pheromone, in particular the spontaneous ability to release critical sucking-related movements in newborns, it has been called the mammary pheromone (MP). Lesion of the vomeronasal organ and preliminary 2-deoxyglucose data suggested that the MP could be processed by the main olfactory system. However, the neuronal substrate that sustains the MP-induced response of neonates remained unknown. Here, we evaluated Fos expression in 4-day-old-rabbits exposed to the MP (in comparison with control neonates exposed to non-relevant odorant, no odorant or unmanipulated pups) both at the level of the olfactory bulb and central brain regions. Evidence of high and widespread Fos immunoreactivity in the main olfactory bulb appear in MP pups while the accessory olfactory bulb exhibits a negligible staining. However, no obvious bulbar pattern of Fos expression is observed, when in contrast a certain pattern emerges with the neutral odorant. Compared to this latter, the MP exposure increases Fos expression in the anterior piriform cortex, the organum vasculosum of the lamina terminalis and the habenula, with a tendency in the lateral preoptic region. For the first time, a pheromone essential for mother-young interaction is thus highlighted for its processing by the main olfactory system, the whole olfactory bulb, and by brain regions involved in osmoregulation, thirst and motivation-guided motor responses.

Biomimetic chemical sensors using nanoelectronic readout of olfactory receptor proteins

We have designed and implemented a practical nanoelectronic interface to G-protein coupled receptors (GPCRs), a large family of membrane proteins whose roles in the detection of molecules outside eukaryotic cells make them important pharmaceutical targets. Specifically, we have coupled olfactory receptor proteins (ORs) with carbon nanotube transistors. The resulting devices transduce signals associated with odorant binding to ORs in the gas phase under ambient conditions and show responses that are in excellent agreement with results from established assays for OR-ligand binding. The work represents significant progress on a path toward a bioelectronic nose that can be directly compared to biological olfactory systems as well as a general method for the study of GPCR function in multiple domains using electronic readout.

Paced-mating increases the number of adult new born cells in the internal cellular (granular) layer of the accessory olfactory bulb

The continuous production and addition of new neurons during life in the olfactory bulb is well accepted and has been extensively studied in rodents. This process could allow the animals to adapt to a changing environment. Olfactory neurogenesis begins in the subventricular zone where stem cells proliferate and give rise to young undifferentiated neuroblasts that migrate along the rostral migratory stream to the olfactory bulb (OB). Olfaction is crucial for the expression of sexual behavior in rodents. In female rats, the ability to control the rate of sexual interactions (pacing) has important physiological and behavioral consequences. In the present experiment we evaluated if pacing behavior modifies the rate of new cells that reach the main and accessory olfactory bulb. The BrdU marker was injected before and after different behavioral tests which included: females placed in a mating cage (control), females allowed to pace the sexual interaction, females that mated but were not able to control the rate of the sexual interaction and females exposed to a sexually active male. Subjects were sacrificed fifteen days after the behavioral test. We observed a significant increase in the density of BrdU positive cells in the internal cellular layer of the accessory olfactory bulb when females paced the sexual interaction in comparison to the other 3 groups. No differences in the cell density in the main olfactory bulb were found. These results suggest that pacing behavior promotes an increase in density of the new cells in the accessory olfactory bulb.

Regulation of adult neurogenesis by behavior and age in the accessory olfactory bulb

The vomeronasal system (VNS) participates in the detection and processing of pheromonal information related to social and sexual behaviors. Within the VNS, two different populations of sensory neurons, with a distinct pattern of distribution, line the epithelium of the vomeronasal organ (VNO) and give rise to segregated sensory projections to the accessory olfactory bulb (AOB). Apical sensory neurons in the VNO project to the anterior AOB (aAOB), while basal neurons project to the posterior AOB (pAOB). In the AOB, the largest population of neurons are inhibitory, the granule and periglomerular cells (GCs and PGs) and remarkably, these neurons are continuously born and functionally integrated in the adult brain, underscoring their role on olfactory function. Here we show that behaviors mediated by the VNS differentially regulate adult neurogenesis across the anterior-posterior axis of the AOB. We used immunohistochemical labeling of newly born cells under different behavioral conditions in mice. Using a resident-intruder aggression paradigm, we found that subordinate mice exhibited increased neurogenesis in the aAOB. In addition, in sexually naive adult females exposed to soiled bedding odorized by adult males, the number of newly born cells was significantly increased in the pAOB; however, neurogenesis was not affected in females exposed to female odors. In addition, we found that at two months of age adult neurogenesis was sexually dimorphic, with male mice exhibiting higher levels of newly born cells than females. Interestingly, adult neurogenesis was greatly reduced with age and this decrease correlated with a decrease in progenitor cells proliferation but not with an increase in cell death in the AOB. These results indicate that the physiological regulation of adult neurogenesis in the AOB by behaviors is both sex and age dependent and suggests an important role of newly born neurons in sex dependent behaviors mediated by the VNS.

Cladistic analysis of olfactory and vomeronasal systems

Most tetrapods possess two nasal organs for detecting chemicals in their environment, which are the sensory detectors of the olfactory and vomeronasal systems. The seventies’ view that the olfactory system was only devoted to sense volatiles, whereas the vomeronasal system was exclusively specialized for pheromone detection was challenged by accumulating data showing deep anatomical and functional interrelationships between both systems. In addition, the assumption that the vomeronasal system appeared as an adaptation to terrestrial life is being questioned as well. The aim of the present work is to use a comparative strategy to gain insight in our understanding of the evolution of chemical “cortex.” We have analyzed the organization of the olfactory and vomeronasal cortices of reptiles, marsupials, and placental mammals and we have compared our findings with data from other taxa in order to better understand the evolutionary history of the nasal sensory systems in vertebrates. The olfactory and vomeronsasal cortices have been re-investigated in garter snakes (Thamnophis sirtalis), short-tailed opossums (Monodelphis domestica), and rats (Rattus norvegicus) by tracing the efferents of the main and accessory olfactory bulbs using injections of neuroanatomical anterograde tracers (dextran-amines). In snakes, the medial olfactory tract is quite evident, whereas the main vomeronasal-recipient structure, the nucleus sphaericus is a folded cortical-like structure, located at the caudal edge of the amygdala. In marsupials, which are acallosal mammals, the rhinal fissure is relatively dorsal and the olfactory and vomeronasal cortices relatively expanded. Placental mammals, like marsupials, show partially overlapping olfactory and vomeronasal projections in the rostral basal telencephalon. These data raise the interesting question of how the telencephalon has been re-organized in different groups according to the biological relevance of chemical senses.

Olfactory responses to natal stream water in sockeye salmon by BOLD fMRI

Many studies have shown that juvenile salmon imprint olfactory memory of natal stream odors during downstream migration, and adults recall this stream-specific odor information to discriminate their natal stream during upstream migration for spawning. The odor information processing of the natal stream in the salmon brain, however, has not been clarified. We applied blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging to investigate the odor information processing of the natal stream in the olfactory bulb and telencephalon of lacustrine sockeye salmon (Oncorhynchus nerka). The strong responses to the natal stream water were mainly observed in the lateral area of dorsal telencephalon (Dl), which are homologous to the medial pallium (hippocampus) in terrestrial vertebrates. Although the concentration of L-serine (1 mM) in the control water was 20,000-times higher than that of total amino acid in the natal stream water (47.5 nM), the BOLD signals resulting from the natal stream water were stronger than those by L-serine in the Dl. We concluded that sockeye salmon could process the odor information of the natal stream by integrating information in the Dl area of the telencephalon.

A novel neural substrate for the transformation of olfactory inputs into motor output

It is widely recognized that animals respond to odors by generating or modulating specific motor behaviors. These reactions are important for daily activities, reproduction, and survival. In the sea lamprey, mating occurs after ovulated females are attracted to spawning sites by male sex pheromones. The ubiquity and reliability of olfactory-motor behavioral responses in vertebrates suggest tight coupling between the olfactory system and brain areas controlling movements. However, the circuitry and the underlying cellular neural mechanisms remain largely unknown. Using lamprey brain preparations, and electrophysiology, calcium imaging, and tract tracing experiments, we describe the neural substrate responsible for transforming an olfactory input into a locomotor output. We found that olfactory stimulation with naturally occurring odors and pheromones induced large excitatory responses in reticulospinal cells, the command neurons for locomotion. We have also identified the anatomy and physiology of this circuit. The olfactory input was relayed in the medial part of the olfactory bulb, in the posterior tuberculum, in the mesencephalic locomotor region, to finally reach reticulospinal cells in the hindbrain. Activation of this olfactory-motor pathway generated rhythmic ventral root discharges and swimming movements. Our study bridges the gap between behavior and cellular neural mechanisms in vertebrates, identifying a specific subsystem within the CNS, dedicated to producing motor responses to olfactory inputs.

High convergence of olfactory and vomeronasal influence in the telencephalon of the terrestrial salamander Plethodon shermani

Previous work suggested that the telencephalic pathways of the main olfactory and vomeronasal systems of vertebrates are mostly isolated from each other, with the possible exception of convergence of the two systems into a small part of the olfactory amygdala. We tested the hypothesis of convergence between the main olfactory and vomeronasal systems by investigating the physiology of telencephalic olfactory responses in an in vitro brain preparation of the salamander Plethodon shermani. This animal was chosen because its olfactory and vomeronasal nerves can be separated and stimulated independently. The nerves were stimulated by short current pulses delivered through suction electrodes. Evoked field potentials and intracellular responses were systematically recorded in the telencephalon. The results showed an abundant overlap of olfactory and vomeronasal nerve-evoked field potentials in the ipsilateral lateral telencephalon and the amygdala. Single neurons receiving bimodal main olfactory and vomeronasal input were found in the dorsolateral telencephalon and amygdala. A classification of response latencies suggested that a subset of these neurons received direct input from both the main and accessory olfactory bulbs. Unimodal excitatory main olfactory responses were mostly found in neurons of the caudal telencephalic pole, but were also present in the striato-pallial transition area/lateral pallium region and striatum. Unimodal excitatory vomeronasal responses were found in neurons of the striato-pallial transition area, vomeronasal amygdala, and caudal amygdala. We conclude that the main olfactory and vomeronasal systems are extensively integrated within the salamander telencephalon and probably act in concert to modulate behavior.

Different importance of the volatile and non-volatile fractions of an olfactory signature for individual social recognition in rats versus mice and short-term versus long-term memory

When tested in the olfactory cued social recognition/discrimination test, rats and mice differ in their retention of a recognition memory for a previously encountered conspecific juvenile: Rats are able to recognize a given juvenile for approximately 45 min only whereas mice show not only short-term, but also long-term recognition memory (≥ 24 h). Here we modified the social recognition/social discrimination procedure to investigate the neurobiological mechanism(s) underlying the species differences. We presented a conspecific juvenile repeatedly to the experimental subjects and monitored the investigation duration as a measure for recognition. Presentation of only the volatile fraction of the juvenile olfactory signature was sufficient for both short- and long-term recognition in mice but not rats. Applying additional volatile, mono-molecular odours to the "to be recognized" juveniles failed to affect short-term memory in both species, but interfered with long-term recognition in mice. Finally immunocytochemical analysis of c-Fos as a marker for cellular activation, revealed that juvenile exposure stimulated areas involved in the processing of olfactory signals in both the main and the accessory olfactory bulb in mice. In rats, we measured an increased c-Fos synthesis almost exclusively in cells of the accessory olfactory bulb. Our data suggest that the species difference in the retention of social recognition memory is based on differences in the processing of the volatile versus non-volatile fraction of the individuals' olfactory signature. The non-volatile fraction is sufficient for retaining a short-term social memory only. Long-term social memory - as observed in mice - requires a processing of both the volatile and non-volatile fractions of the olfactory signature.

Memória de reconhecimento social em ratos

O paradigma intruso-residente vem sendo intensamente empregado em estudos para avaliar a memória de reconhecimento social em roedores. Tipicamente, ratos adultos (residentes) são expostos a dois encontros de 5 minutos cada com um mesmo intruso juvenil ou com juvenis diferentes; o intervalo entre encontros é usualmente 30 minutos. A quantidade de comportamentos sociais do residente, no segundo encontro, em relação a um intruso familiar é substancialmente menor do que o observado no primeiro encontro, o que não ocorre quando o segundo encontro envolve um juvenil novo; esse resultado caracteriza memória de reconhecimento social. Neste estudo discutimos achados recentes sobre os tipos de comportamentos usualmente incluídos nas categorias social e não-social, a influência da fase temporal, a interferência de rotinas laboratoriais na memória de reconhecimento social, modalidades sensoriais usualmente empregadas por roedores no processamento de informações na memória social e alternativas adicionais para o estudo da socialidade em roedores.

Muscarinic receptor type 1 (M1) stimulation, probably through KCNQ/Kv7 channel closure, increases spontaneous GABA release at the dendrodendritic synapse in the mouse accessory olfactory bulb

Cholinergic modulation of spontaneous GABAergic currents (mIPSC) was studied using whole-cell patch methods in mouse accessory olfactory bulb slices. Carbachol (above 100 microM) administration produced an increase in the mIPSC frequency in mitral cells, but did not affect the responses of mitral cells to GABA. The carbachol effect persisted in the presence of combined ionotropic and metabotropic glutamatergic receptor antagonists. The carbachol effect was reduced by the muscarinic receptor type-1 and -4 (M1 and M4) antagonist pirenzepine (10 microM), but not by the M2 and M4 antagonist himbacine (10 microM). The KCNQ/Kv7 potassium channel openers retigabine (80 microM) and diclofenac (300 microM) blocked the carbachol action, while the KCNQ potassium channel blocker XE-911 (20 microM) increased the mIPSC frequency. XE-911's action persisted in the presence of glutamate receptor blockers. In the presence of carbachol, mIPSCs were abolished by Ni (200 microM), while being insensitive to the calcium channel blocker nimodipine (30 microM), suggesting a role for R-type calcium channels in the GABA release. These results suggest that carbachol closed KCNQ channels by stimulating M1 receptors on granule cell dendrites, and the resulting depolarized and unstable membrane promoted calcium influx, thus increasing the GABA release. The possible role of acetylcholine in facilitating formation of a pheromone memory in mice is also discussed.

Accessory olfactory bulb function is modulated by input from the main olfactory epithelium

Although it is now established that sensory neurons in both the main olfactory epithelium and the vomeronasal organ may be activated by both general and pheromonal odorants, it remains unclear what initiates sampling by the vomeronasal organ. Anterograde transport of wheat germ agglutinin-horseradish peroxidase was used to determine that adequate intranasal syringing with zinc sulfate interrupted all inputs to the main olfactory bulb but left intact those to the accessory olfactory bulb. Adult male treated mice were frankly anosmic when tested with pheromonal and non-pheromonal odors and failed to engage in aggressive behavior. Treated juvenile females failed to show puberty acceleration subsequent to exposure to bedding from adult males. Activation of the immediate early gene c-Fos and electrovomeronasogram recording confirmed the integrity of the vomeronasal system in zinc sulfate-treated mice. These results support the hypothesis that odor detection by the main olfactory epithelium is required to initiate sampling by the vomeronasal system.

Distinct patterns of neuronal inputs and outputs of the juxtaparaventricular and suprafornical regions of the lateral hypothalamic area in the male rat

We have analyzed at high resolution the neuroanatomical connections of the juxtaparaventricular region of the lateral hypothalamic area (LHAjp); as a control and in comparison to this, we also performed a preliminary analysis of a nearby LHA region that is dorsal to the fornix, namely the LHA suprafornical region (LHAs). The connections of these LHA regions were revealed with a coinjection tract-tracing technique involving a retrograde (cholera toxin B subunit) and anterograde (Phaseolus vulgaris leucoagglutinin) tracer. The LHAjp and LHAs together connect with almost every major division of the cerebrum and cerebrospinal trunk, but their connection profiles are markedly different and distinct. In simple terms, the connections of the LHAjp indicate a possible primary role in the modulation of defensive behavior; for the LHAs, a role in the modulation of ingestive behavior is suggested. However, the relation of the LHAjp and LHAs to potential modulation of these behaviors, as indicated by their neuroanatomical connections, appears to be highly integrative as it includes each of the major functional divisions of the nervous system that together determine behavior, i.e., cognitive, state, sensory, and motor. Furthermore, although a primary role is indicated for each region with respect to a particular mode of behavior, intermode modulation of behavior is also indicated. In summary, the extrinsic connections of the LHAjp and LHAs (so far as we have described them) suggest that these regions have a profoundly integrative role in which they may participate in the orchestrated modulation of elaborate behavioral repertoires.

Characterization of TRPC2, an essential genetic component of VNS chemoreception, provides insights into the evolution of pheromonal olfaction in secondary-adapted marine mammals

Pheromones are chemical cues released and sensed by individuals of the same species, which are of major importance in regulating reproductive and social behaviors of mammals. Generally, they are detected by the vomeronasal system (VNS). Here, we first investigated and compared an essential genetic component of vomeronasal chemoreception, that is, TRPC2 gene, of four marine mammals varying the degree of aquatic specialization and related terrestrial species in order to provide insights into the evolution of pheromonal olfaction in the mammalian transition from land to water. Our results based on sequence characterizations and evolutionary analyses, for the first time, show the evidence for the ancestral impairment of vomeronasal pheromone signal transduction pathway in fully aquatic cetaceans, supporting a reduced or absent dependence on olfaction as a result of the complete adaptation to the marine habitat, whereas the amphibious California sea lion was found to have a putatively functional TRPC2 gene, which is still under strong selective pressures, reflecting the reliance of terrestrial environment on chemical recognition among the semiadapted marine mammals. Interestingly, our study found that, unlike that of the California sea lion, TRPC2 genes of the harbor seal and the river otter, both of which are also semiaquatic, are pseudogenes. Our data suggest that other unknown selective pressures or sensory modalities might have promoted the independent absence of a functional VNS in these two species. In this respect, the evolution of pheromonal olfaction in marine mammals appears to be more complex and confusing than has been previously thought. Our study makes a useful contribution to the current understanding of the evolution of pheromone perception of mammals in response to selective pressures from an aquatic environment.

The effect of vapor of propylene glycol on rats

Propylene glycol (PG) is commonly used as a solvent for odorous chemicals employed in studies of the olfactory system because PG has been considered to be odorless for humans and other animals. However, if laboratory rats can detect the vapor of PG and if exposure to this influences behaviors, such effects might confound data obtained from experiments exposing conscious rats to odorants dissolved in PG. Therefore, we examined this issue using differences in the acoustic startle reflex (ASR) as an index. We also conducted a habituation/dishabituation test to assess the ability of rats to detect the vapor of PG. In addition, we observed Ca(2+) responses of vomeronasal neurons (VNs) in rats exposed to PG using the confocal Ca(2+)-imaging approach. Pure PG vapor significantly enhanced the ASR at a dose of 1 x 10(-4) M, which was much lower than the dose for efficiently detecting. In Ca(2+) imaging, VNs were activated by PG at a dose of 1 x 10(-4) M or lower. These results suggest that PG vapor acts as an aversive stimulus to rats at very low doses, even lower than those required for its detection, indicating that we should consider such effect of PG when it is employed as a solvent for odorants in studies using conscious rats. In addition, our study suggests that some non-pheromonal volatile odorants might affect animal behaviors via the vomeronasal system.

Olfactory perception: receptors, cells, and circuits

Remarkable advances in our understanding of olfactory perception have been made in recent years, including the discovery of new mechanisms of olfactory signaling and new principles of olfactory processing. Here, we discuss the insight that has been gained into the receptors, cells, and circuits that underlie the sense of smell.

Accessory and main olfactory systems influences on predator odor-induced behavioral and endocrine stress responses in rats

Exposures to predator odors are very effective methods to evoke a variety of stress responses in rodents. We have previously found that ferret odor exposure leads to changes in endocrine hormones (corticosterone and ACTH) and behavior. To distinguish the contributions of the main and accessory olfactory systems in these responses, studies were designed to interfere with these two systems either independently, or simultaneously. Male Sprague-Dawley rats were treated with 10% zinc sulfate (ZnSO(4)), which renders rodents anosmic (unable to smell) while leaving the accessory olfactory areas intact, or saline, in Experiment 1. In Experiment 2, the vomeronasal organs of rats were surgically removed (VNX) to block accessory olfactory processing, while leaving the main olfactory system intact. And in the third experiment both the main and accessory olfactory areas were disrupted by combining the two procedures in the same rats. Neither ZnSO(4) treatment nor VNX alone reliably reduced the increased corticosterone response to ferret odor compared to strawberry odor, but in combination, they did. This suggests that processing through the main or the accessory olfactory system can elicit the endocrine stress response to ferret odor. VNX alone also did not affect the behavioral responses to the ferret odor. ZnSO(4) treatment, alone and in combination with VNX, led to changes in behavior in response to both ferret and strawberry odor, making the behavioral results less clearly interpretable. Overall these studies suggest that both the main and accessory olfactory systems mediate the neuroendocrine response to predator odor.

Subsystem organization of the mammalian sense of smell

The mammalian olfactory system senses an almost unlimited number of chemical stimuli and initiates a process of neural recognition that influences nearly every aspect of life. This review examines the organizational principles underlying the recognition of olfactory stimuli. The olfactory system is composed of a number of distinct subsystems that can be distinguished by the location of their sensory neurons in the nasal cavity, the receptors they use to detect chemosensory stimuli, the signaling mechanisms they employ to transduce those stimuli, and their axonal projections to specific regions of the olfactory forebrain. An integrative approach that includes gene targeting methods, optical and electrophysiological recording, and behavioral analysis has helped to elucidate the functional significance of this subsystem organization for the sense of smell.

From Pheromones to Behavior

In recent years, considerable progress has been achieved in the comprehension of the profound effects of pheromones on reproductive physiology and behavior. Pheromones have been classified as molecules released by individuals and responsible for the elicitation of specific behavioral expressions in members of the same species. These signaling molecules, often chemically unrelated, are contained in body fluids like urine, sweat, specialized exocrine glands, and mucous secretions of genitals. The standard view of pheromone sensing was based on the assumption that most mammals have two separated olfactory systems with different functional roles: the main olfactory system for recognizing conventional odorant molecules and the vomeronasal system specifically dedicated to the detection of pheromones. However, recent studies have reexamined this traditional interpretation showing that both the main olfactory and the vomeronasal systems are actively involved in pheromonal communication. The current knowledge on the behavioral, physiological, and molecular aspects of pheromone detection in mammals is discussed in this review.

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.

The main and the accessory olfactory systems interact in the control of mate recognition and sexual behavior

In the field of sensory perception, one noticeable fact regarding olfactory perception is the existence of several olfactory subsystems involved in the detection and processing of olfactory information. Indeed, the vomeronasal or accessory olfactory system is usually conceived as being involved in the processing of pheromones as it is closely connected to the hypothalamus, thereby controlling reproductive function. By contrast, the main olfactory system is considered as a general analyzer of volatile chemosignals, used in the context of social communication, for the identification of the status of conspecifics. The respective roles played by the main and the accessory olfactory systems in the control of mate recognition and sexual behavior are at present still controversial. We summarize in this review recent results showing that both the main and accessory olfactory systems are able to process partially overlapping sets of sexual chemosignals and that both systems support complimentary aspects in mate recognition and in the control of sexual behavior.

Increased apoptosis during neonatal brain development underlies the adult behavioral deficits seen in mice lacking a functional paternally expressed gene 3 (Peg3)

Inactivation of the maternally imprinted, paternally expressed gene 3 (Peg3) induces deficits in olfactory function, sexual and maternal behaviors, oxytocin neuron number, metabolic homeostasis and growth. Peg3 is expressed in a number of developing hypothalamic and basal forebrain structures and is a component of the P53 apoptosis pathway. Peg3 inactivation in neuronal cell culture lines inhibits P53 mediated apoptosis, which is important in the early postnatal development and sexual differentiation of the brain. In this study, we investigated the effect of inactivating the Peg3 gene on the incidence of caspase 3 positive cells (a marker of apoptosis) in 4- and 6-day postpartum mouse brain. Inactivating the Peg3 gene resulted in an increase in the incidence of total forebrain caspase 3 positive cells at 4 and 6 days postpartum. Increases in specific neuroanatomical regions including the bed nucleus of the stria terminalis, nucleus accumbens, caudate putamen, medial pre-optic area, arcuate nucleus, medial amygdala, anterior cortical and posteriodorsal amygdaloid nuclei, were also observed. In wild-type mice, sex differences in the incidence of caspase 3 positive cells in the medial amygdala, bed nucleus of the stria terminalis, nucleus accumbens, arcuate nucleus and the M2 motor cortex, were also observed. This neural sex difference was ameliorated in the Peg-3 mutant. These findings suggest that the neuronal and behavioral deficits seen in mice lacking a functional Peg3 gene are mediated by increases in the incidence of early neonatal apoptosis in neuroanatomical regions important for reproductive behavior, olfactory and pheromonal processing, thermoregulation and reward.