Effects of vomeronasal organ removal on individual odor discrimination, sex-odor preference, and scent marking by female hamsters

Meadow voles differentiate between scents of different sources

Scent marks are an important means of transmitting information between rodents, and they can be produced from several body sources. Previous studies have shown that scents from multiple sources can convey the same information; female meadow voles, for example, have three scent sources that communicate sex. However, possessing three separate sources that convey the same information is likely costly due to the metabolic energy required to produce these signals and the increased chance that eavesdropping individuals may intercept information present in these signals. In this study, we investigated if these scent sources could communicate other information, in addition to scent donor sex, by determining if male meadow voles could distinguish scent marks taken from different sources of a single female scent donor. This was accomplished with a habituation-test method, that allowed us to compare how male meadow voles differently investigate scent from a familiar and novel source of a female scent donor. Male meadow voles could distinguish between faeces and urine scent marks of a female, but could only distinguish mouth from urine and faeces scent marks when first familiarized with mouth scent marks. Our findings suggest that mouth, urine, and faeces scent marks of female meadow voles produce both redundant and distinct information. The overlap in information between scent marks produced from separate sources may be needed to provide social context, which allows receiving individuals to accurately weigh the tradeoffs associated with responding to an olfactory cue. While this overlap in information remains costly, this cost may be minimized by the different fade-out times of scent marks from distinct body sources, which may limit the amount of time information in a scent mark is available to a time period where this information is socially relevant.

Female Chemical Signalling Underlying Reproduction in Mammals

Chemical communication plays many key roles in mammalian reproduction, although attention has focused particularly on male scent signalling. Here, we review evidence that female chemical signals also play important roles in sexual attraction, in mediating reproductive competition and cooperation between females, and in maternal care, all central to female reproductive success. Female odours function not only to advertise sexual receptivity and location, they can also have important physiological priming effects on male development and sperm production. However, the extent to which female scents are used to assess the quality of females as potential mates has received little attention. Female investment in scent signalling is strongly influenced by the social structure and breeding system of the species. Although investment is typically male-biased, high competition between females can lead to a reversed pattern of female- biased investment. As among males, scent marking and counter-marking are often used to advertise territory defence and high social rank. Female odours have been implicated in the reproductive suppression of young or subordinate females across a range of social systems, with females of lower competitive ability potentially benefiting by delaying reproduction until conditions are more favourable. Further, the ability to recognise individuals, group members and kin through scent underpins group cohesion and cooperation in many social species, as well as playing an important role in mother-offspring recognition. However, despite the diversity of female scent signals, chemical communication in female mammals remains relatively understudied and poorly understood. We highlight several key areas of future research that are worthy of further investigation.

Development of the main olfactory system and main olfactory epithelium-dependent male mating behavior are altered in Go-deficient mice

In mammals, initial detection of olfactory stimuli is mediated by sensory neurons in the main olfactory epithelium (MOE) and the vomeronasal organ (VNO). The heterotrimeric GTP-binding protein Go is widely expressed in the MOE and VNO of mice. Early studies indicated that Go expression in VNO sensory neurons is critical for directing social and sexual behaviors in female mice [Oboti L, et al. (2014) BMC Biol 12:31]. However, the physiological functions of Go in the MOE have remained poorly defined. Here, we examined the role of Go in the MOE using mice lacking the α subunit of Go Development of the olfactory bulb (OB) was perturbed in mutant mice as a result of reduced neurogenesis and increased cell death. The balance between cell types of OB interneurons was altered in mutant mice, with an increase in the number of tyrosine hydroxylase-positive interneurons at the expense of calbindin-positive interneurons. Sexual behavior toward female mice and preference for female urine odors by olfactory sensory neurons in the MOE were abolished in mutant male mice. Our data suggest that Go signaling is essential for the structural and functional integrity of the MOE and for specification of OB interneurons, which in turn are required for the transmission of pheromone signals and the initiation of mating behavior with the opposite sex.

Vomeronasal organ lesion disrupts social odor recognition, behaviors and fitness in golden hamsters

Most studies support the viewpoint that the vomeronasal organ has a profound effect on conspecific odor recognition, scent marking and mating behavior in the golden hamster (Mesocricetus auratus). However, the role of the vomeronasal organ in social odor recognition, social interaction and fitness is not well understood. Therefore, we conducted a series of behavioral and physiological tests to examine the referred points in golden hamster. We found that male hamsters with vomeronasal organ lesion showed no preference between a predator odor (the anal gland secretion of the Siberian weasels (Mustela sibirica) and putative female pheromone components (myristic acid and palmitic acid), but were still able to discriminate between these 2 kinds of odors. In behavioral tests of anxiety, we found that vomeronasal organ removal causes female hamsters to spend much less time in center grids and to cross fewer center grids and males to make fewer crossings between light and dark boxes than sham-operated controls. This indicates that a chronic vomeronasal organ lesion induced anxious responses in females. In aggressive behavioral tests, we found that a chronic vomeronasal organ lesion decreased agonistic behavior in female hamsters but not in males. The pup growth and litter size show no differences between the 2 groups. All together, our data suggested that vomeronasal organ ablation disrupted the olfactory recognition of social chemosignals in males, and induced anxiety-like and aggressive behavior changes in females. However, a vomeronasal organ lesion did not affect the reproductive capacity and fitness of hamsters. Our studies may have important implications concerning the role of the vomeronasal organ in golden hamsters and also in rodents.

Chemosensory cues affect amygdaloid neurogenesis and alter behaviors in the socially monogamous prairie vole

The current study examined the effects of pheromonal exposure on adult neurogenesis and revealed the role of the olfactory pathways on adult neurogenesis and behavior in the socially monogamous prairie vole (Microtus ochrogaster). Subjects were injected with a cell proliferation marker [5-bromo-2'-deoxyuridine (BrdU)] and then exposed to their own soiled bedding or bedding soiled by a same- or opposite-sex conspecific. Exposure to opposite-sex bedding increased BrdU labeling in the amygdala (AMY), but not the dentate gyrus (DG), of female, but not male, voles, indicating a sex-, stimulus-, and brain region-specific effect. The removal of the main olfactory bulbs or lesioning of the vomeronasal organ (VNOX) in females reduced BrdU labeling in the AMY and DG, and inhibited the male bedding-induced BrdU labeling in the AMY, revealing the importance of an intact olfactory pathway for amygdaloid neurogenesis. VNOX increased anxiety-like behavior and altered social preference, but it did not affect social recognition memory in female voles. VNOX also reduced the percentage of BrdU-labeled cells that co-expressed the neuronal marker TuJ1 in the AMY, but not the DG. Together, our data indicate the importance of the olfactory pathway in mediating brain plasticity in the limbic system as well as its role in behavior.

Chemosignals and hormones in the neural control of mammalian sexual behavior

Males and females of most mammalian species depend on chemosignals to find, attract and evaluate mates and, in most cases, these appetitive sexual behaviors are strongly modulated by activational and organizational effects of sex steroids. The neural circuit underlying chemosensory-mediated pre- and peri-copulatory behavior involves the medial amygdala (MA), the bed nucleus of the stria terminalis (BNST), medial preoptic area (MPOA) and ventromedial hypothalamus (VMH), each area being subdivided into interconnected chemoreceptive and hormone-sensitive zones. For males, MA-BNST connections mediate chemoinvestigation whereas the MA-MPOA pathway regulates copulatory initiation. For females, MA-MPOA/BNST connections also control aspects of precopulatory behavior whereas MA-VMH projections control both precopulatory and copulatory behavior. Significant gaps in understanding remain, including the role of VMH in male behavior and MPOA in female appetitive behavior, the function of cortical amygdala, the underlying chemical architecture of this circuit and sex differences in hormonal and neurochemical regulation of precopulatory behavior.

Endogenous oxytocin is necessary for preferential Fos expression to male odors in the bed nucleus of the stria terminalis in female Syrian hamsters

Successful reproduction in mammals depends on proceptive or solicitational behaviors that enhance the probability of encountering potential mates. In female Syrian hamsters, one such behavior is vaginal scent marking. Recent evidence suggests that the neuropeptide oxytocin (OT) may be critical for regulating this behavior. Blockade of OT receptors in the bed nucleus of the stria terminalis (BNST) or the medial preoptic area (MPOA) decreases vaginal marking responses to male odors; lesion data suggest that BNST, rather than MPOA, mediates this effect. However, how OT interacts with sexual odor processing to drive preferential solicitation is not known. To address this issue, intact female Syrian hamsters were exposed to male or female odors and their brains processed for immunohistochemistry for Fos, a marker of recent neuronal activation, and OT. Additional females were injected intracerebroventricularly (ICV) with an oxytocin receptor antagonist (OTA) or vehicle, and then tested for vaginal marking and Fos responses to sexual odors. Colocalization of OT and Fos in the paraventricular nucleus of the hypothalamus was unchanged following exposure to male odors, but decreased following exposure to female odors. Following injections of OTA, Fos expression to male odors was decreased in BNST, but not in MPOA or the medial amygdala (MA). Fos expression in BNST may be functionally relevant for vaginal marking, given that there was a positive correlation between Fos expression and vaginal marking for BNST, but not MPOA or MA. Together, these data suggest that OT facilitation of neuronal activity in BNST underlies the facilitative effects of OT on solicitational responses to male odors.

Chemosignals, hormones and mammalian reproduction

Many mammalian species use chemosignals to coordinate reproduction by altering the physiology and behavior of both sexes. Chemosignals prime reproductive physiology so that individuals become sexually mature and active at times when mating is most probable and suppress it when it is not. Once in reproductive condition, odors produced and deposited by both males and females are used to find and select individuals for mating. The production, dissemination and appropriate responses to these cues are modulated heavily by organizational and activational effects of gonadal sex steroids and thereby intrinsically link chemical communication to the broader reproductive context. Many compounds have been identified as "pheromones" but very few have met the expectations of that term: a unitary, species-typical substance that is both necessary and sufficient for an experience-independent behavioral or physiological response. In contrast, most responses to chemosignals are dependent or heavily modulated by experience, either in adulthood or during development. Mechanistically, chemosignals are perceived by both main and accessory (vomeronasal) olfactory systems with the importance of each system tied strongly to the nature of the stimulus rather than to the response. In the central nervous system, the vast majority of responses to chemosignals are mediated by cortical and medial amygdala connections with hypothalamic and other forebrain structures. Despite the importance of chemosignals in mammals, many details of chemical communication differ even among closely related species and defy clear categorization. Although generating much research and public interest, strong evidence for the existence of a robust chemical communication among humans is lacking.

The medial preoptic area is necessary for sexual odor preference, but not sexual solicitation, in female Syrian hamsters

Precopulatory behaviors that are preferentially directed towards opposite-sex conspecifics are critical for successful reproduction, particularly in species wherein the sexes live in isolation, such as Syrian hamsters (Mesocricetus auratus). In females, these behaviors include sexual odor preference and vaginal scent marking. The neural regulation of precopulatory behaviors is thought to involve a network of forebrain areas that includes the medial amygdala (MA), the bed nucleus of the stria terminalis (BNST), and the medial preoptic area (MPOA). Although MA and BNST are necessary for sexual odor preference and preferential vaginal marking to male odors, respectively, the role of MPOA in odor-guided female precopulatory behaviors is not well understood. To address this issue, female Syrian hamsters with bilateral, excitotoxic lesions of MPOA (MPOA-X) or sham lesions (SHAM) were tested for sexual odor investigation, scent marking, and lordosis. MPOA-X females did not investigate male odors more than female odors in an odor preference test, indicating that MPOA may be necessary for normal sexual odor preference in female hamsters. This loss of preference cannot be attributed to a sensory deficit, since MPOA-X females successfully discriminated male odors from female odors during an odor discrimination test. Surprisingly, no deficits in vaginal scent marking were observed in MPOA-X females, although these females did exhibit decreased overall levels of flank marking compared to SHAM females. Finally, all MPOA-X females exhibited lordosis appropriately. These results suggest that MPOA plays a critical role in the neural regulation of certain aspects of odor-guided precopulatory behaviors in female Syrian hamsters.

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.

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.

Wired for behaviors: from development to function of innate limbic system circuitry

The limbic system of the brain regulates a number of behaviors that are essential for the survival of all vertebrate species including humans. The limbic system predominantly controls appropriate responses to stimuli with social, emotional, or motivational salience, which includes innate behaviors such as mating, aggression, and defense. Activation of circuits regulating these innate behaviors begins in the periphery with sensory stimulation (primarily via the olfactory system in rodents), and is then processed in the brain by a set of delineated structures that primarily includes the amygdala and hypothalamus. While the basic neuroanatomy of these connections is well-established, much remains unknown about how information is processed within innate circuits and how genetic hierarchies regulate development and function of these circuits. Utilizing innovative technologies including channel rhodopsin-based circuit manipulation and genetic manipulation in rodents, recent studies have begun to answer these central questions. In this article we review the current understanding of how limbic circuits regulate sexually dimorphic behaviors and how these circuits are established and shaped during pre- and post-natal development. We also discuss how understanding developmental processes of innate circuit formation may inform behavioral alterations observed in neurodevelopmental disorders, such as autism spectrum disorders, which are characterized by limbic system dysfunction.

The bed nucleus of the stria terminalis is critical for sexual solicitation, but not for opposite-sex odor preference, in female Syrian hamsters

Successful reproduction in vertebrates depends critically upon a suite of precopulatory behaviors that occur prior to mating. In Syrian hamsters (Mesocricetus auratus), these behaviors include vaginal scent marking and preferential investigation of male odors. The neural regulation of vaginal marking and opposite-sex odor preference likely involves an interconnected set of steroid-sensitive nuclei that includes the medial amygdala (MA), the bed nucleus of the stria terminalis (BNST), and the medial preoptic area (MPOA). For example, lesions of MA eliminate opposite-sex odor preference and reduce overall levels of vaginal marking, whereas lesions of MPOA decrease vaginal marking in response to male odors. Although BNST is densely interconnected with both MA and MPOA, little is known about the role of BNST in female precopulatory behaviors. To address this question, females received either bilateral, excitotoxic lesions of BNST (BNST-X) or sham lesions (SHAM), and were tested for scent marking and for investigatory responses to male and female odors. Whereas SHAM females vaginal marked more to male odors than female odors on two days of the estrous cycle, BNST-X females marked at equivalent levels to both odors. This deficit is not due to alterations in social odor investigation, as both BNST-X and SHAM females investigated male odors more than female odors. Finally, BNST lesions did not generally disrupt the cyclic changes in reproductive behaviors that occur across the estrous cycle. Taken together, these results demonstrate that BNST is critical for the normal expression of solicitational behaviors by females in response to male odor stimuli.

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.

Effects of neonatal treatment with valproic acid on vasopressin immunoreactivity and olfactory behaviour in mice

Recent findings demonstrate that epigenetic modifications are required for the sexual differentiation of the brain. For example, neonatal administration of the histone deacetylase inhibitor, valproic acid, blocks masculinisation of cell number in the principal nucleus of the bed nucleus of the stria terminalis (BNST). In the present study, we examined the effects of valproic acid on neurochemistry and behaviour, focusing on traits that are sexually dimorphic and linked to the BNST. Newborn mice were treated with saline or valproic acid and the effect on vasopressin immunoreactivity and olfactory preference behaviour was examined in adulthood. As expected, males had more vasopressin immunoreactive fibres than females in the lateral septum and medial dorsal thalamus, which are two projection sites of BNST vasopressin neurones. Neonatal valproic acid increased vasopressin fibre density specifically in females in the lateral septum, thereby reducing the sex difference, and increased vasopressin fibres in both sexes in the medial dorsal thalamus. The effects were not specific to BNST vasopressin projections, however, because valproic acid also significantly increased vasopressin immunoreactivity in the anterior hypothalamic area in both sexes. Subtle sex-specific effects of neonatal valproic acid treatment were observed on olfactory behaviour. As predicted, males showed a preference for investigating female-soiled bedding, whereas females showed a preference for male-soiled bedding. Valproic acid did not significantly alter olfactory preference, per se, although it increased the number of visits females made to female-soiled bedding and the overall time females spent investigating soiled versus clean bedding. Taken together, these results suggest that a transient disruption of histone deacetylation at birth does not have generalised effects on sexual differentiation, although it does produce lasting effects on brain neurochemistry and behaviour.

Blocking oxytocin receptors inhibits vaginal marking to male odors in female Syrian hamsters

In Syrian hamsters (Mesocricetus auratus), precopulatory behaviors such as vaginal scent marking are essential for attracting a suitable mate. Vaginal marking is dependent on forebrain areas implicated in the neural regulation of reproductive behaviors in rodents, including the medial preoptic/anterior hypothalamus (MPOA-AH). Within MPOA-AH, the neuropeptide oxytocin (OT) acts to facilitate copulation (lordosis), as well as ultrasonic vocalizations towards males. It is not known, however, if OT in this area also facilitates vaginal marking. In the present study, a specific oxytocin receptor antagonist (OTA) was injected into MPOA-AH of intact female Syrian hamsters to determine if oxytocin receptor-dependent signaling is critical for the normal expression of vaginal marking elicited by male, female, and clean odors. OTA injections significantly inhibited vaginal marking in response to male odors compared with vehicle injections. There was no effect of OTA on marking in response to either female or clean odors. When injected into the bed nucleus of the stria terminalis (BNST), a nearby region to MPOA-AH, OTA was equally effective in decreasing marking. Finally, the effects of OTA appear to be specific to vaginal marking, as OTA injections in MPOA-AH or BNST did not alter general locomotor activity, flank marking, or social odor investigation. Considered together, these results suggest that OT in MPOA-AH and/or BNST normally facilitates male odor-induced vaginal marking, providing further evidence that OT generally supports prosocial interactions among conspecifics.

Lesions that functionally disconnect the anterior and posterodorsal sub-regions of the medial amygdala eliminate opposite-sex odor preference in male Syrian hamsters (Mesocricetus auratus)

In many rodent species, such as Syrian hamsters, reproductive behavior requires neural integration of chemosensory information and steroid hormone cues. The medial amygdala (MA) processes both of these signals through anatomically distinct sub-regions; the anterior region (MeA) receives substantial chemosensory input, but contains few steroid receptor-labeled neurons, whereas the posterodorsal region (MePD) receives less chemosensory input, but contains a dense population of steroid receptors. Importantly, these sub-regions have considerable reciprocal connections, and the goal of this experiment was therefore to determine whether interactions between MeA and MePD are required for male hamsters' preference to investigate female over male odors. To functionally disconnect MeA and MePD, males received unilateral lesions of MeA and MePD within opposite brain hemispheres. Control males received either unilateral lesions of MeA and MePD within the same hemisphere or sham surgery. Odor preferences were measured using a 3-choice apparatus, which simultaneously presented female, male and clean odor stimuli; all tests were done under conditions that either prevented or allowed contact with the odor sources. Under non-contact conditions, males with asymmetrical lesions investigated female and male odors equally, whereas males in both control groups preferred to investigate female odors. Under contact conditions, all groups investigated female odors longer than male odors, although males with asymmetrical lesions displayed decreased investigation of female odors compared to sham males. These data suggest that MeA-MePD interactions are critical for processing primarily the volatile components of social odors and highlight the importance of input from the main olfactory system (MOS) to these nuclei in the regulation of reproductive behavior. More broadly, these results support the role of the MA in integrating chemosensory and hormone information, a process that may underlie social odor processing in a variety of behavioral contexts.

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.

Neural mechanisms of individual and sexual recognition in Syrian hamsters (Mesocricetus auratus)

Recognizing the individual and sexual identities of conspecifics is critical for adaptive social behavior and, in most mammals this information is communicated primarily by chemosensory cues. Due to its heavy reliance on odor cues, we have used the Syrian hamster as our model species for investigating the neural regulation of social recognition. Using lesion, electrophysiological and immunocytochemical techniques, separate neural pathways underlying recognition of individual odors and guidance of sex-typical responses to opposite-sex odors have been identified in both male and female hamsters. Specifically, we have found that recognition of individual odor identity requires olfactory bulb connections to entorhinal cortex (ENT) rather than other chemoreceptive brain regions. This kind of social memory does not appear to require the hippocampus and may, instead, depend on ENT connections with piriform cortex. In contrast, sexual recognition, through either differential investigation or scent marking toward opposite-sex odors, depends on both olfactory and vomeronasal system input to the corticomedial amygdala. Preference for investigating opposite-sex odors requires primarily olfactory input to the medial amygdala (ME) whereas appropriately targeted scent marking responses require vomeronasal input to ME as well as to other structures. Within the ME, the anterior section (MEa) appears important for evaluating or classifying social odors whereas the posterodorsal region (MEpd) may be more involved in generating approach to social odors. Evidence is presented that analysis of social odors may initially be done in MEa and then communicated to MEpd, perhaps through micro-circuits that separately process male and female odors.

Sexual differentiation of pheromone processing: links to male-typical mating behavior and partner preference

Phoenix et al. (Phoenix, C., Goy, R., Gerall, A., Young, W., 1959. Organizing actions of prenatally administered testosterone propionate on the tissues mediating mating behavior in the female guinea pig. Endocrinology 65, 369-382.) were the first to propose an essential role of fetal testosterone exposure in the sexual differentiation of the capacity of mammals to display male-typical mating behavior. In one experiment control male and female guinea pigs as well as females given fetal testosterone actually showed equivalent levels of mounting behavior when gonadectomized and given ovarian steroids prior to adult tests with a stimulus female. This finding is discussed in the context of a recent, high-profile paper by Kimchi et al. (Kimchi, T., Xu, J., Dulac, C., 2007. A functional circuit underlying male sexual behaviour in the female mouse brain. Nature 448, 1009-1014.) arguing that female rodents possess the circuits that control the expression of male-typical mating behavior and that their function is normally suppressed in this sex by pheromonal inputs that are processed via the vomeronasal organ (VNO)-accessory olfactory nervous system. In another Phoenix et al. experiment, significantly more mounting behavior was observed in male guinea pigs and in females given fetal testosterone than in control females following adult gonadectomy and treatment with testosterone. Literature is reviewed that attempts to link sex differences in the anatomy and function of the accessory versus the main olfactory projections to the amygdala and hypothalamus to parallel sex differences in courtship behaviors, including sex partner preference, as well as the capacity to display mounting behavior.

A direct main olfactory bulb projection to the 'vomeronasal' amygdala in female mice selectively responds to volatile pheromones from males

The main olfactory system, like the accessory olfactory system, responds to pheromones involved in social communication. Whereas pheromones detected by the accessory system are transmitted to the hypothalamus via the medial ('vomeronasal') amygdala, the pathway by which pheromones are detected and transmitted by the main system is not well understood. We examined in female mice whether a direct projection from mitral/tufted (M/T) cells in the main olfactory bulb (MOB) to the medial amygdala exists, and whether medial amygdala-projecting M/T cells are activated by volatile urinary odors from conspecifics or a predator (cat). Simultaneous anterograde tracing using Phaseolus vulgaris leucoagglutinin and Fluoro-Ruby placed in the MOB and accessory olfactory bulb (AOB), respectively, revealed that axons of MOB M/T cells projected to superficial laminae of layer Ia in anterior and posterodorsal subdivisions of the medial amygdala, whereas projection neurons from the AOB sent axons to non-overlapping, deeper layer Ia laminae of the same subdivisions. Placement of the retrograde tracer cholera toxin B into the medial amygdala labeled M/T cells that were concentrated in the ventral MOB. Urinary volatiles from male mice, but not from female conspecifics or cat, induced Fos in medial amygdala-projecting MOB M/T cells of female subjects, suggesting that information about male odors is transmitted directly from the MOB to the 'vomeronasal' amygdala. The presence of a direct MOB-to-medial amygdala pathway in mice and other mammals could enable volatile, opposite-sex pheromones to gain privileged access to diencephalic structures that control mate recognition and reproduction.

A centrifugal pathway to the mouse accessory olfactory bulb from the medial amygdala conveys gender-specific volatile pheromonal signals

We previously found that female mice exhibited Fos responses in the accessory olfactory bulb (AOB) after exposure to volatile opposite-sex, but not same-sex, urinary odours. This effect was eliminated by lesioning the main olfactory epithelium, raising the possibility that the AOB receives information about gender via centrifugal inputs originating in the main olfactory system instead of from the vomeronasal organ. We asked which main olfactory forebrain targets send axonal projections to the AOB, and whether these input neurons express Fos in response to opposite-sex urinary volatiles. Female mice received bilateral injections of the retrograde tracer cholera toxin B (CTB), into the AOB, and were exposed to either same- or opposite-sex volatile urinary odours 1 week later. We found CTB-labeled cell bodies in several forebrain sites including the bed nucleus of the accessory olfactory tract, the rostral portion of the medial amygdala (MeA) and the posteromedial cortical nucleus of the amygdala. A significant increase in the percentage of CTB/Fos co-labeled cells was seen only in the MeA of female subjects exposed to male but not to female urinary volatiles. In Experiment 2, CTB-injected females were later exposed to volatile odours from male mouse urine, food, or cat urine. Again, a significant increase in the percentage of CTB/Fos co-labeled cells was seen in the MeA of females exposed to male mouse urinary volatiles but not to food or predator odours. Main olfactory-MeA-AOB signaling may motivate approach behaviour to opposite-sex pheromonal signals that ensure successful reproduction.

Olfactory experience and the development of odor preference and vaginal marking in female Syrian hamsters

Rodent reproductive behavior relies heavily on odor processing, and evidence suggests that many odor-guided sexual behaviors are shaped by prior experience. We sought to determine if exposure to male odors during development is required for the adult expression of proceptive sexual behavior toward male odors in female Syrian hamsters. Exposure to male odors was restricted in naïve subjects by removing all male siblings from the litter at three to five days of age. Control litters were also culled, but included equal numbers of male and female pups. As adults, naïve females displayed investigatory preferences toward male odors in a Y-maze that were comparable to control females; this preference was observed whether contact with the odor stimuli was prevented of allowed. In contrast, naïve females vaginal scent-marked equally toward male and female volatile odors, suggesting an inability to target behavior toward sexually relevant odors. However, naïve females marked preferentially toward male odors when allowed to contact the odor stimuli. These results provide evidence for the experience-dependent development of vaginal marking behavior toward volatile components of sexual odors. Furthermore, they suggest that distinct mechanisms regulate the development of odor preferences and vaginal marking behavior in this species.

Olfactory responses of neotropical short-tailed singing mice, Scotinomys teguina, to odors of the mid-ventral sebaceous gland: discrimination of conspecifics, gender, and female reproductive condition

We examined the olfactory communication of the Neotropical short-tailed singing mouse, Scotinomys teguina, by investigating whether S. teguina responded to odors produced by the mid-ventral sebaceous gland of conspecifics. Females spent significantly more time investigating male odor than an odorless stimulus or a female odor. Males spent significantly more time investigating female odor than an odorless stimulus, but not that of a male odor. This latter result does not seem to be explained by differences in age or sexual experience of test subjects, but may be influenced by reproductive condition of the female odor donor. Male S. teguina spent significantly more time (1) investigating and (2) in total proximity to odors of estrous than non-estrous females. Males spent more time (1) investigating and (2) in total proximity to odors of males than non-estrous females. Finally, given the choice between odor of males vs proestrous females, males showed no preference. Thus, the mid-ventral gland in S. teguina seems to convey information about conspecifics, sex, and female reproductive condition. Male odor, compared with proestrous female odor, is equally interesting to males, suggesting that the gland also plays an important role in male-male communication. Sexual dimorphism in the size of the gland and in the amount of secretion produced by the gland may be related to either male-male competition or female choice.

Sexually dimorphic activation of the accessory, but not the main, olfactory bulb in mice by urinary volatiles

Previous research suggests that volatile body odourants detected by the main olfactory epithelium (MOE) are processed mainly by the main olfactory bulb (MOB) whereas nonvolatile body odourants detected by the vomeronasal organ (VNO) are processed via the accessory olfactory bulb (AOB). We asked whether urinary volatiles from males and females differentially activate the AOB in addition to the MOB in gonadectomized mice of either sex. Exposure to urinary volatiles from opposite-sex but not same-sex conspecifics augmented the number of Fos-immunoreactive mitral and granule cells in the AOB. Volatile urinary odours from male as well as female mice also stimulated Fos expression in distinct clusters of MOB glomeruli in both sexes. Intranasal administration of ZnSO(4), intended to disrupt MOE function, eliminated the ability of volatile urinary odours to stimulate Fos in both the MOB and AOB. In ovariectomized, ZnSO(4)-treated females a significant, though attenuated, AOB Fos response occurred after direct nasal exposure to male urine plus soiled bedding, suggesting that VNO signaling remained partially functional in these mice. Future studies will determine whether MOE or VNO signaling, or both types of input, drive the sexually dimorphic response of the AOB to volatile opposite-sex odours and whether this AOB response contributes to reproductive success.

Androgen receptor is essential for sexual differentiation of responses to olfactory cues in mice

During sexual differentiation males and females are exposed to different levels of testosterone, which promotes sex differences in the adult brain and in behavior. Testosterone can act after aromatization or reduction via a number of steroid hormone receptors. Here we provide new evidence that the androgen receptor (AR) is essential for sexual differentiation in mice. We used mice carrying the testicular feminization (Tfm) mutation of the AR. Adult Tfm males, wild-type male and female littermates were gonadectomized and given subcutaneous estradiol implants. In all sexually dimorphic traits, Tfm males had responses equivalent to females and different from males. In simultaneous choice tests, males spent significantly more time investigating female-soiled bedding, whereas females and Tfm males preferred to investigate male-soiled bedding. Tfm males and females did not have a partner preference in tests with awake stimulus animals, whereas males showed a preference for females over males. Exposure to male-soiled, but not clean, bedding produced a significant increase in c-Fos-immunoreactive cells in the medial preoptic area and bed nucleus of the stria terminalis in Tfm males and females, no increase was noted in males. Masculine sexual behavior (mounting and thrusting) was not sexually dimorphic, and all groups displayed these behaviors. Our results support data collected in humans suggesting a role for the androgen receptor in sexual differentiation of social preferences and neural responses to pheromones.

Effect of vomeronasal organ removal from male mice on their preference for and neural Fos responses to female urinary odors

Four experiments were conducted to determine whether vomeronasal organ (VNO) inputs in male mice mediate the rewarding properties of estrous female urinary odors. Sexually naive male mice with either an intact (VNOi) or lesioned (VNOx) VNO preferred to investigate female urine over water in Y-maze tests. Subsequently, VNOi males ran significantly more quickly and remained in nasal contact longer with estrous female urine than with male urine, whereas VNOx males investigated these odors equally. In home-cage habituation-dishabituation tests, VNOi males also investigated female urine significantly longer than did VNOx males, although both groups investigated female urine longer than other non-body odors. Finally, female urinary odors induced Fos in the nucleus accumbens core of VNOi males but not of VNOx males. Our results suggest that female urinary odors retain some incentive value in VNOx males. However, once direct nasal contact is made with female urine, VNO inputs further activate forebrain mechanisms that amplify the reward salience of this stimulus for the male mouse.

Neural pathways involved in the endocrine response of anestrous ewes to the male or its odor

During the non-breeding season, anestrous ewes do not experience ovarian cycles but exposure to a ram or its odor results in the activation of the luteinizing hormone secretion leading to ovulation. The aim of our work was to identify the neural pathways involved in this phenomenon. Using Fos immunocytochemistry, we examined the brain areas activated by the male or its fleece, in comparison with ewes exposed to the female fleece or the testing room (control group). In comparison with the control group, the male or its odor significantly increases Fos neuronal expression in the main and accessory olfactory bulbs, anterior olfactory nucleus, cortical and basal amygdala, dentate gyrus, ventromedial nucleus of the hypothalamus, piriform and orbitofrontal cortices. The main olfactory bulb, the cortical amygdala and the dentate gyrus are specifically more activated by the male odor than the female odor. Using a procedure of double labeling for Fos and gonadotropin-releasing hormone, we also compared the number of gonadotropin-releasing hormone neurons activated in the four groups of females. The male or its odor significantly increases the number and the proportion of gonadotropin-releasing hormone cells expressing Fos-immunoreactivity in the preoptic area and the organum vasculosum of the lamina terminalis, whereas no such induction of Fos-immunoreactivity was found in gonadotropin-releasing hormone neurons of ewes exposed to the female odor or the testing room. These findings emphasize the role of the main olfactory system in the detection and the integration of the ram odor, and also suggest the participation of the accessory olfactory system. Numerous structures widely distributed seem involved in the processing of the male olfactory cue to reach the gonadotropin-releasing hormone neurons.

The vomeronasal organ is required for the expression of lordosis behaviour, but not sex discrimination in female mice

The role of the vomeronasal organ (VNO) in mediating neuroendocrine responses in female mice is well known; however, whether the VNO is equally important for sex discrimination is more controversial as evidence exists for a role of the main olfactory system in mate recognition. Therefore, we studied the effect of VNO removal (VNOx) on the ability of female mice to discriminate between volatile and non-volatile odours of conspecifics of the two sexes and in different endocrine states using Y-maze tests. VNOx female mice were able to reliably distinguish between male and female or male and gonadectomized (gdx) male volatile odours. However, when subjects had to discriminate between male and female or gdx male non-volatile odours, VNOx females were no longer able to discriminate between sex or different endocrine status. These results thus show that the VNO is primarily involved in the detection and processing of non-volatile odours, and that female mice can use volatile odours detected and processed by the main olfactory system for mate recognition. However, VNO inputs are needed to promote contact with the male, including facilitation of lordosis responses to his mounts. A single subcutaneous injection with gonadotropin-releasing hormone (GnRH) partially reversed the deficit in lordosis behaviour observed in VNOx females suggesting that VNO inputs may reach hypothalamic GnRH neurons to influence the display of sexual behaviour.

Recognition of familiar individuals in golden hamsters: a new method and functional neuroanatomy

The ability to recognize individuals is essential for many aspects of social interaction and social organization, yet we know relatively little about the neural mechanisms underlying this ability. Most laboratory studies of individual recognition in rodents have studied differential responses to familiar versus unfamiliar individuals rather than differential responses to equally well known individuals having different significance for the subject. In experiment 1, we use a new method for studying true individual recognition in which male hamsters first had different experiences with two stimulus males (exposures to one male across a wire-mesh barrier and fights with another male). One day later, losers of fights were tested in a Y-maze for reactions to one of the two familiar males. Subjects tested with the familiar winner avoided this stimulus male, but subjects tested with the familiar, neutral male were attracted to him. Immunohistochemistry for c-Fos and Egr-1 implicate several areas of the brain in individual recognition, particularly the anterior piriform cortex, the CA1 and CA3 regions of anterior dorsal hippocampus, anterior and posterior dentate gyrus, and perirhinal cortex. In experiment 2, temporary inactivation of the CA1 region of anterior dorsal hippocampus by microinfusion of lidocaine eliminated the avoidance of the familiar winner, but a saline control injection had no effect. These results are the first to use a rodent model to characterize neural circuits involved in the recognition of equally well known individuals and the corresponding emotional responses to them.

Olfactory sex discrimination persists, whereas the preference for urinary odorants from estrous females disappears in male mice after vomeronasal organ removal

Based on observed changes in the social context for the display of ultrasonic vocalizations, scent marking, aggression, and mounting behavior by male mice with a null mutation of the transient receptor potential 2 ion channel, it was proposed recently that a primary function of the mouse vomeronasal organ (VNO)/accessory olfactory system is sex discrimination. We tested this hypothesis directly by studying the ability of male mice to discriminate between urinary odors of conspecifics of the two sexes and in different endocrine states using habituation-dishabituation tests. Male mice from which the VNO had been surgically removed (VNOx) resembled sham-operated controls (VNOi) in their ability to discriminate between volatile urinary odors from estrous females versus gonadally intact males, as well as between urinary odors from estrous versus ovariectomized females and from gonadally intact versus castrated males. When physical access to stimuli was permitted, VNOi control males strongly preferred to investigate volatile and nonvolatile urinary odorants from estrous females as opposed to intact males, whereas VNOx males showed no such preference. Mating performance in tests with estrous females was equivalent in VNOi and VNOx subjects. Both groups of males preferred to mount an estrous female instead of a castrated male. Our results suggest that the VNO is not required for sex discrimination but instead detects the nonvolatile components of opposite-sex urine that may be used to help prolong contact with individuals that produce these chemosignals.

Effects of vomeronasal organ removal on olfactory sex discrimination and odor preferences of female ferrets

Previous research suggests that body odorants, including anal scents and urinary odors, contribute to sex discrimination and mate identification in European ferrets of both sexes. We assessed the possible role of the vomeronasal organ (VNO) in these functions by surgically removing the organ bilaterally in sexually experienced female ferrets. Lesioned (VNOx) and sham-operated control (VNOi) females reliably discriminated between male- and female-derived anal scent gland as well as fresh urinary odors in habituation/dishabituation tests. However, VNOi females spent significantly more time than VNOx subjects investigating male urinary odors in these tests. Also, VNOi females, but not VNOx subjects, preferred to investigate day-old male versus female urine spots as well as wooden blocks that had previously been soiled by male versus female ferrets. Both groups of female ferrets preferred to approach volatile odors from a breeding male instead of an estrous female in Y-maze tests and both groups showed similar levels of receptive sexual behavior in response to a male's neck grip. The VNO is apparently not required for olfactory sex discrimination or mate recognition in this carnivore, but instead may play a role in promoting continued contact with nonvolatile body odors previously deposited by opposite-sex conspecifics during territorial scent marking.

Modification of Odor Investigation and Discrimination in Female Opossums (Monodelphis domestica) Following the Ablation of the Accessory Olfactory Bulbs

To determine whether the vomeronasal system of the Brazilian short-tailed opossum (Monodelphis domestica) is important to the response to conspecific chemical signals, the authors tested female opossums with conspecific odors, before and after ablation of their accessory olfactory bulbs (AOBs). Anesthesia and sham treatments did not modify females' discrimination of conspecific odors when tested against water, between male and female odors, or between different odors from the same male donors. Odor investigation was partially diminished following partial ablation of the AOB, and complete ablation of the AOBs further impaired the ability of females to discriminate between certain odors. These findings provide the first evidence for the importance of the vomeronasal system in the detection of chemosignals of known origin in opossums.

Differential distribution of olfactory receptor neurons in goldfish: structural and molecular correlates

The olfactory system of many terrestrial vertebrates comprises a main olfactory organ and a vomeronasal organ each containing a morphologically distinct type of olfactory receptor neuron (ORN). The two cell types also differ in the expression of G-proteins and odorant receptor molecules. Fish do not have a vomeronasal organ, and their olfactory neurons-three different morphological types-are contained in one epithelium. The olfactory organ of goldfish appears as a rosette, with the sensory epithelium lying along the proximal portion of each lamella, where it attaches to the midline raphe. Using immunocytochemistry, in situ hybridization, and electron microscopy, we tested whether a correlation exists between receptor cell morphology, distribution of cell type within the sensory epithelium, and expression of odorant receptors and G-proteins. A strong correlation exists between ORN morphology, type of odorant receptor and G-protein expressed and the distribution of sensory cells within the olfactory epithelium. The Buck and Axel type of odorant receptor and Galpha(olf) are expressed in tall ciliated ORNs distributed homogenously across the entire sensory epithelium. In contrast, microvillous ORNs expressing V2R-like receptors, and Galpha(o), Galpha(q), or Galpha(i-3), and crypt type ORNs expressing Galpha(o) and Galpha(q), are preferentially located along the dorsal margin of the epithelium and near the midline raphe. V2R- and OR-type receptor molecules do not colocalize in one cell, and only crypt-type ORNs express more than one G-protein.

Discrimination of social odors and their locations: role of lateral entorhinal area

Discrimination of individual conspecifics by their odors has been reported for many mammalian species, but little information is available on the brain mechanisms underlying such discrimination. A previous study reported that large parahippocampal lesions, centered on entorhinal cortex but extending into adjacent areas of the brain, eliminated female hamsters' ability to discriminate the flank gland odors of different individuals, as tested with habituation-dishabituation methods. The current study examined the effects of lesions restricted to the lateral entorhinal area on such discriminations. Female hamsters were tested in several types of habituation procedure that differed across a sequence of trials in the locations of familiar and novel social odors. Discrimination of two individuals' odors depended on the sequences of locations of the odors, indicating that odor identity and location were simultaneously salient to female hamsters. Lesions of lateral entorhinal area interfered with this spatial-olfactory discrimination. When confounding spatial cues were eliminated, hamsters did discriminate between novel and familiar odors, and lesions in the entorhinal area did not eliminate this ability. Thus, although the lateral entorhinal area is not necessary for individual odor discrimination, it is involved in processing odor-place combinations.

Mice (Mus musculus) lacking a vomeronasal organ can discriminate MHC-determined odortypes

Major histocompatibility complex (MHC) genes in mammals (H-2 in mice) play a major role in regulating immune function. They also bestow individuality in the form of a chemical signature or odortype. At present, the respective contributions of the olfactory epithelium and the vomeronasal organ (VNO) in the recognition of individual odortypes are not well defined. We examined a possible role for the VNO in the recognition of MHC odortypes in mice by first removing the organ (VNX) and then training the mice to distinguish the odors of two congenic strains of mice that differed only in their MHC type. C57BL/6J mice (bb at H-2) and C57BL/6J-H-2(k) (kk at H-2) provided urine for sensory testing. Eight VNX and six sham-operated mice were trained to make the discrimination. Neither the number of training trials-to-criterion nor the rate of learning differed significantly for VNX and sham-operated mice. We conclude that the VNO is not necessary for learning to discriminate between MHC odortypes.

Sexual incentive motivation, olfactory preference, and activation of the vomeronasal projection pathway by sexually relevant cues in non-copulating and naive male rats

There are some apparently healthy male rats that fail to mate after repeated testing with receptive females. We have previously shown that these "non-copulator (NC)" males show no partner preference for a receptive female when given the opportunity to physically interact with a sexually receptive female or a sexually active male. We also demonstrated that although NC males prefer odors from estrous females to odors from anestrous females, this preference is significantly reduced in comparison to the preference displayed by copulating (C) males. The aim of the present study was to evaluate in NC males sexual incentive motivation, that is, the approach behavior of male rats to either a sexually receptive female or a sexually active male in a test where the subjects can smell, hear, and see the stimulus animal but prevents their physical interaction. In addition, we determined whether NC rats have alterations in their ability to detect odors from conspecifics or odors related to food. In the detection of odors from conspecifics, we determined if these NC males are sexually attracted toward odors from receptive females or sexually active males. For food-related odors, we quantified the time it took the subjects to locate a hidden a piece of apple. Finally, using the induction of Fos-immunoreactivity (Fos-IR) as an index of neuronal activation, we compared the response of the vomeronasal projection pathway (VN pathway) of C and NC male rats exposed to estrous bedding. Males without sexual experience (WSE) were included in all experiments to determine the importance of previous heterosexual experience in the different behavioral tests and in the activity of the VN pathway. In the sexual incentive motivation test, we found that C and WSE male rats have a clear preference for estrous females over sexually active males, whereas NC male rats showed no preference. In odor tests, our results showed that C males had a clear preference for odors from estrous females as opposed to odors from sexually active males. Although NC and WSE male rats showed a preference for estrous female odors, this preference was significantly reduced compared to that shown by C males. No differences were found between WSE, C, and NC males in the detection of stimuli associated with food-related odors. A significant increase in Fos-IR was observed in the mitral cell layer of the accessory olfactory bulb in all groups when exposed to estrous bedding. However, only the C male rats exposed to estrous female bedding showed an increase Fos-IR in all structures of the VN pathway. An increase in Fos-IR was observed in the medial preoptic area (MPOA) of WSE males exposed to estrous bedding. No increases in Fos-IR were detected along the VN pathway in NC male rats. We proposed that NC male rats do not display sexual behavior due to a reduced sexual motivation that could be caused by alterations in the neuronal activity of the VN pathway during the processing of estrous odors.

Sexual differentiation of the neuroendocrine mechanisms regulating mate recognition in mammals

When in breeding condition, male and female mammals seek out and mate with opposite-sex conspecifics. The neural mechanisms controlling mate recognition and heterosexual partner preference are sexually differentiated by the perinatal actions of sex steroid hormones. Many mammalian species use odours to identify potential mates. Thus, sex differences in partner preference may actually reflect sex differences in how male and female mammals perceive socially relevant odours. Two olfactory systems have evolved in vertebrates that differ considerably in their anatomy and function. It is generally believed that the main olfactory system is used to detect a wide variety of volatile odours derived from food prey among many sources, whereas the accessory olfactory system has evolved to detect and process primarily nonvolatile odours shown to influence reproductive behaviours and neuroendocrine functions. Some recent results obtained in oestradiol-deficient aromatase knockout (ArKO) mice that provide evidence for a developmental role of oestradiol in olfactory investigation of volatile body odours are discussed, suggesting that: (i) oestrogens contribute to the development of the main olfactory system and (ii) mate recognition is mediated by the main as opposed to the accessory olfactory system. Thus, sex differences in mate recognition and sexual partner preference may reflect sex differences in the perception of odours by the main olfactory system.

Vomeronasal organ detects odorants in absence of signaling through main olfactory epithelium

It is commonly assumed that odorants are detected by the main olfactory epithelium (MOE) and pheromones are sensed through the vomeronasal organ (VNO). The complete loss of MOE-mediated olfaction in type-3 adenylyl cyclase knockout mice (AC3-/-) allowed us to examine chemosensory functions of the VNO in the absence of signaling through the MOE. Here we report that AC3-/- mice are able to detect certain volatile odorants via the VNO. These same odorants elicited electro-olfactogram transients in the VNO and MOE of wild-type mice, but only VNO responses in AC3-/- mice. This indicates that some odorants are detected through an AC3-independent pathway in the VNO.

The perirhinal–entorhinal cortex, but not the hippocampus, is critical for expression of individual recognition in the context of the Coolidge effect

The Coolidge effect is a phenomenon in which males show renewed sexual interest in a novel female following copulation to satiety with another female. In golden hamsters, this phenomenon depends on the ability to recognize conspecifics using chemosensory cues processed through the main olfactory system. Here we tested whether olfactory targets in the hippocampal system support this natural form of recognition memory. Male hamsters received ibotenic acid lesions of the perirhinal-entorhinal cortex (PR-ENT) or hippocampus (H) and were allowed to copulate to satiety with a female conspecific, then were presented with two anesthetized females, the familiar mate and an unfamiliar female that copulated with another male. Sham-operated and H-lesioned subjects preferentially investigated the novel female, indicating intact recognition of individual identity. By contrast, PR-ENT-lesioned males failed to discriminate familiar and novel females, and this deficit could not be attributed to abnormal copulatory behavior during mating. All subjects were able to detect and discriminate between female odors when presented in isolation during a habituation-discrimination test, indicating that behavioral deficits shown by PR-ENT males were not due to anosmia or a general investigatory deficit. Thus, the perirhinal-entorhinal cortex, but not the hippocampus, is critical for the recognition of familiar conspecifics in this naturalistic situation. This study reveals an essential role for the perirhinal-entorhinal cortex, but not the hippocampus, in a natural form of recognition memory within the social behavior of hamsters. The findings show a strikingly similar pattern to the effects of selective damage to the same brain regions on performance in standard recognition memory tasks by rats and monkeys. Therefore, the present data extend our understanding of the differential role of structures of the hippocampal memory system, showing continuity across species and between formal laboratory tests and the function of memory in natural social behavior.

Attractive properties of sexual pheromones in mice: innate or learned?

It is generally assumed that chemical signals (sexual pheromones) constitute the primary stimulus for sexual attraction in many mammals. However, it is unclear whether these pheromones are volatile or nonvolatile and which sensory systems are involved in their detection (vomeronasal and/or olfactory). Moreover, it has been demonstrated that experience influences the behavioral response to sexual pheromones and the sensory systems implicated. In order to clarify this issue, the attractive properties of volatile and nonvolatile components of the male-soiled bedding have been analyzed in female mice that had no previous experience with adult male-derived chemical signals (chemically naïve females) using two-choice preference tests. The results indicate that some nonvolatile male-derived substances exert an innate attraction to females, but volatiles derived from male-soiled bedding do not attract chemically nai;ve females. Therefore, the primary attractive sexual pheromone includes a nonvolatile compound (e.g. major urinary proteins, MUPs). On the other hand, male-derived volatiles become attractive to females because of repeated exposure to male-soiled bedding. This represents a Pavlovian-like associative learning in which previously neutral volatiles (very likely odorants) acquire attractive properties by association with the nonvolatile, innately attractive pheromone(s). These findings indicate that not only the sexual but also the 'chemical' experience (previous experience with sexual pheromones) has to be taken into account to interpret the role of chemicals as releaser or primer pheromones. The sensory systems involved in the detection of these stimuli and the neural basis of the odor-pheromone association are discussed.

Recognition of conspecific odors by laboratory rats (Rattus norvegicus) does not show context specificity

This experiment investigated how contextual cues affect recognition of conspecific odors in laboratory rats (Rattus norvegicus). Rats received 5 encounters with the same odor in the same context. For the 6th test encounter, all rats received a simultaneous presentation of the original odor and a novel odor. The authors tested 1 group of rats (context same) in the same context as before. For the remaining 2 groups, the test encounter was in a different context that 1 group (context different) had experienced but that 1 group (context novel) had not. A significant preference to investigate the novel odor by context-same and context-different rats, but not by context-novel rats, suggests that odor recognition can occur following transfer to a different, but familiar, test context, indicating a lack of context specificity.

Fear and power-dominance drive motivation: neural representations and pathways mediating sensory and mnemonic inputs, and outputs to premotor structures

Based on the available literature on activation of brain structures by fear- and anger-inducing stimuli, on the effects of electrical and chemical stimulation and lesions of candidate structures, and on connectional data, we propose that both the fear and power-dominance drives are represented in four distinct locations: the medial hypothalamus, lateral/dorsolateral periaqueductal gray, midline thalamic nuclei, and medial prefrontal cortex. The hypothalamic fear representation is located in the dorsomedial and posterior hypothalamic nuclei, the midbrain representation in the caudal part of the lateral/dorsolateral periaqueductal gray, the thalamic representation primarily in parts of the paraventricular and reuniens thalamic nuclei, and the cortical representation in prelimbic cortex. The hypothalamic power-dominance representation is located in the anterior hypothalamic nucleus, dorsomedial aspect of the ventromedial nucleus, and in adjacent parts of the medial preoptic area. The corresponding midbrain representation occurs in rostral part of the lateral/dorsolateral periaqueductal gray, and the thalamic representation in parts of the paraventricular, parataenial, and reuniens thalamic nuclei. We discuss sensory/mnemonic inputs to these representations, and outputs to premotor structures in the medulla, caudate-putamen, and cortex, and their differential contributions to involuntary, learned sequential, and voluntary motor acts. We examine potential contributions of neuronal activities in these representations to the subjective awareness of fear and anger.

The influence of sex on the olfactory function in healthy subjects

The sense of smell is the least examined of all senses. The significance of the organs of smell is in their influence on the mental state as well as on the vegetative, visceral and sexual functions. The objective of this experiment was to define the influence of sex on the olfactory function. It was performed on 120 subjects (60 females and 60 males) divided into three age groups (20 - 30; 31 - 40; 41 - 50 years of age). The experiment was carried out by the Fortunato-Niccolini olfactometric method using six odorous experimental substances: A - anethol, PH - phenyl-ethyl-alcohol, C citral, M - menthol, V- vanillin and P - pyridine, the thresholds of perception (TP) and identification (TI) being defined for each odorous substance. The examined females had slightly lower thresholds of perception (TP) and identification (TI) in relation to the males of the same age group. However, the differences were not statistically significant except for the group of subjects between 41 and 50 years of age where the females, being in the pre-menopause, had significantly better olfactory functions. The results can be explained by the weakening of the olfactory power as a result of ageing in both sexes, however, the females still experienced the protective role of sex hormones.

The vomeronasal organ is involved in discrimination of individual odors by males but not by females in golden hamsters

The vomeronasal organ (VNO) has a wide variety of functions in terrestrial vertebrates, some of which involve responses to classical pheromones whereas others do not. We examined the role of the VNO in discrimination of individual differences in odors of male and female golden hamsters using a habituation paradigm. Removal of the VNO resulted in elimination of the ability of male hamsters to discriminate between some individually distinctive odors (e.g., flank gland secretion), but not others (e.g., urine). In females, such lesions had no effect. The type of test trial also influenced the results; in test trials employing a single, novel odor, removal of the VNO in males did have an effect but in test trials in which both the novel and the familiar odor were presented, VNO removal had no effect. It is concluded that (a) there is a sex difference in the role of the VNO in the discrimination of individual odors, (b) the role of the VNO in discrimination of individual odors varies from odor to odor, and (c) deficits due to VNO removal are more readily observed in more difficult tasks.