Induction of c-fos in hamster accessory olfactory bulbs by natural and cloned aphrodisin

Sex separation unveils the functional plasticity of the vomeronasal organ in rabbits

Chemosensory cues are vital for social and sexual behaviours and are primarily detected and processed by the vomeronasal system (VNS), whose plastic capacity has been investigated in mice. However, studying chemosensory plasticity outside of laboratory conditions may give a more realistic picture of how the VNS adapts to a changing environment. Rabbits are a well-described model of chemocommunication since the discovery of the rabbit mammary pheromone and their vomeronasal organ (VNO) transcriptome was recently characterised, a first step to further study plasticity-mediated transcriptional changes. In this study, we assessed the plastic capacity of the rabbit male and female VNO under sex-separation vs. sex-combined scenarios, including adults and juveniles, to determine whether the rabbit VNO is plastic and, if so, whether such plasticity is already established at early stages of life. First, we characterised the number of differentially expressed genes (DEGs) between the VNO of rabbit male and female under sex-separation and compared it to sex-combined individuals, both in adults and juveniles, finding that differences between male and female were larger in a sex-separated scenario. Secondly, we analysed the number of DEGs between sex-separated and sex-combined scenarios, both in males and females. In adults, both sexes showed a high number of DEGs while in juveniles only females showed differences. Additionally, the vomeronasal receptor genes were strikingly downregulated in sex-separated adult females, whereas in juveniles upregulation was shown for the same condition, suggesting a role of VRs in puberty onset. Finally, we described the environment-modulated plastic capacity of genes involved in reproduction, immunity and VNO functional activity, including G-protein coupled receptors. Our results show that sex-separation induces sex- and stage-specific gene expression differences in the VNO of male and female rabbit, both in adults and juveniles. These results bring out for the first time the plastic capacity of the rabbit VNO, supporting its functional adaptation to specifically respond to a continuous changing environment. Finally, species-specific differences and individual variability should always be considered in VNO studies and overall chemocommunication research.

Effects of Sleep Deprivation by Olfactorily Induced Sexual Arousal Compared to Immobilization Stress and Manual Sleep Deprivation on Neuromessengers and Time Keeping Genes in the Suprachiasmatic Nuclei and Other Cerebral Entities of Syrian Hamsters-An Immunohistochemical Study

We investigated the effects of sexual arousal induced by olfactory stimuli on the expression of neuromodulators, neurotransmitters and sexual steroid receptors in the suprachiasmatic nucleus (SCN, the circadian pacemaker of mammals) and other cerebral entities of Syrian hamsters (Mesocricetus auratus) compared to manual sleep deprivation and immobilization stress. The hamsters kept under a 12:12 hours (h) light:dark cycle were deprived of sleep by sexual stimulation, gentle manual handling or immobilization stress for 1 h at the beginning of the light phase and subsequently sacrificed at zeitgeber time 01:00, respectively; for comparison, hamsters were manually sleep deprived for 6 or 20 h or sacrificed after completing a full sleep phase. As demonstrated by immunohistochemistry, apart from various alterations after manual sleep deprivation, sexual stimulation caused down-regulation of arginine-vasopressin (AVP), vasointestinal peptide (VIP), serotonin (5-HT), substance P (SP), and met-enkephalin (ME) in the SCN. Somatostatin (SOM) was diminished in the medial periventricular nucleus (MPVN). In contrast, an increase in AVP was observed in the PVN, that of oxytocin (OXY) in the supraoptic nucleus (SON), of tyrosine-hydroxylase (TH) in the infundibular nucleus (IN), and dopamine beta-hydroxylase (DBH) in the A7 neuron population of the brain stem (A7), respectively. Testosterone in plasma was increased. The results indicate that sexual arousal extensively influences the neuropeptide systems of the SCN, suggesting an involvement of the SCN in reproductive behavior.

The neuroendocrinology of sexual attraction

Sexual attraction has two components: Emission of sexually attractive stimuli and responsiveness to these stimuli. In rodents, olfactory stimuli are necessary but not sufficient for attraction. We argue that body odors are far superior to odors from excreta (urine, feces) as sexual attractants. Body odors are produced by sebaceous glands all over the body surface and in specialized glands. In primates, visual stimuli, for example the sexual skin, are more important than olfactory. The role of gonadal hormones for the production of and responsiveness to odorants is well established. Both the androgen and the estrogen receptor α are important in male as well as in female rodents. Also in primates, gonadal hormones are necessary for the responsiveness to sexual attractants. In males, the androgen receptor is sufficient for sustaining responsiveness. In female non-human primates, estrogens are needed, whereas androgens seem to contribute to responsiveness in women.

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.

Age-dependent spatial distribution of bromodeoxyuridine-immunoreactive cells in the main olfactory bulb

The subventricular zone (SVZ) generates an immense number of neurons, which migrate to the main olfactory bulb (MOB) and differentiate into granule cells and periglomerular cells in the MOB, even during adulthood. Pheromonal signals, which are mainly received by the vomeronasal organ, provide specific information concerning the reproductive state in a variety of mammal. Vomeronasal sensory neurons project to the accessory olfactory bulb (AOB) located on the dorso-caudal surface of the MOB. In the present study, bromodeoxyuridine-immunoreactive (BrdU-ir) structures in the sagittal section of the MOB including the AOB of young and old male rats were studied to explore the roles of newly generated cells at the region near the rostral end of AOB in the MOB. The density of BrdU-ir cells in the granule cell layer of the MOB of young rats was higher than that of old rats. In young rats, the density of BrdU-ir cells at the region near the rostral end of the AOB was higher than that at the region distant from the AOB. In old rats, the density of BrdU-ir cells at the near region was lower than that at the distant region. The density of BrdU-ir cells at the region near the AOB in the MOB in old rats may be concerned with age-dependent changes in ability of discrimination and memory of general odors and odors related to the reproduction.

A recombinant courtship pheromone affects sexual receptivity in a plethodontid salamander

Pheromones are important chemical signals for many vertebrates, particularly during reproductive interactions. In the terrestrial salamander Plethodon shermani, a male delivers proteinaceous pheromones to the female as part of their ritualistic courtship behavior. These pheromones increase the female's receptivity to mating, as shown by a reduction in courtship duration. One pheromone component in particular is plethodontid receptivity factor (PRF), a 22-kDa protein with multiple isoforms. This protein alone can act as a courtship pheromone that causes the female to be more receptive. We used a bacterial expression system to synthesize a single recombinant isoform of PRF. The recombinant protein was identical to the native PRF, based on mass spectrometry, circular dichroism spectra, and a behavioral bioassay that tested the effects of recombinant PRF (rPRF) on female receptivity (21% reduction in courtship duration). The rPRF appears to mimic the activity of a mixture of PRF isoforms, as well as a mixture of multiple different proteins that comprise the male courtship gland extract. Pheromones that are peptides have been characterized for some vertebrates; to date, however, rPRF is one of only 2 synthesized vertebrate proteins to retain full biological activity.

Is the vomeronasal system really specialized for detecting pheromones?

Many academics, clinicians and lay readers of science incorrectly assume that vomeronasal processing is equivalent to pheromone processing. We review the abundant data concerning the roles of both the olfactory and the vomeronasal systems in the processing of both pheromones and other odorants, demonstrating that this "equivalency hypothesis" is untenable. This conclusion has important implications for the design and interpretation of experiments examining vomeronasal and olfactory system function. We describe some of the problems that arise from assuming that this equivalency holds. Two alternative hypotheses have been offered, but the available data do not enable us to accept or reject either one. Perhaps no single functional description can adequately characterize the role of the vomeronasal system.

Elephant albumin: a multipurpose pheromone shuttle

(Z)-7-dodecenyl acetate (Z7-12:Ac) is present in the urine of female Asian elephants (Elephas maximus) approaching ovulation and functions as a female-to-male sex pheromone. Here we show that a significant fraction of the pheromone in the urine is bound to a protein, elephant serum albumin (ESA), and provide evidence for key physiological functions of urinary ESA. Our biochemical and behavioral experiments suggest a three-fold role of ESA in pheromone signaling: (1) transporting Z7-12:Ac from serum into urine; (2) extending the presence of the pheromone in the environment without hampering detection; and (3) targeting pheromone delivery to chemosensory organs through localized release of the ligand induced by a pH change. The exploitation of albumin in pheromone transport clearly distinguishes the elephant from other mammals studied, and complements the uniqueness of elephant anatomy, physiology, and behavior.

Distribution of methionine and leucine enkephalin neurons within the social behavior circuitry of the male Syrian hamster brain

Enkephalin plays a role in the social behaviors of many species, but no corresponding role for this peptide has been investigated in the male Syrian hamster, a species in which brain nuclei controlling social behaviors have been identified. Previous studies have shown the distribution of dynorphin and beta-endorphin throughout social behavior circuits within the male hamster brain. To date, the only studies of enkephalin in the hamster brain address the distribution of this peptide in the olfactory bulb and hippocampus. The present study provides a complete map of enkephalinergic neurons within the forebrain and midbrain of the male Syrian hamster and addresses the question of whether enkephalin immunoreactive (Enk-ir) cells are found within brain regions relevant to male hamster social behaviors. Following immunocytochemistry for either methionine enkephalin (met-enkephalin) or leucine enkephalin (leu-enkephalin), we observed enkephalin localization consistent with data that have previously been reported in the rat, with notable exceptions including lateral septum, ventromedial nucleus of the hypothalamus and cingulate gyrus. Additionally, met- and leu-enkephalin localization patterns largely overlap. Consistent with the post-translational processing of preproenkephalin, met-enkephalin was more abundant than leu-enkephalin both within individual cells (darker staining), and within given brain nuclei (more met-enkephalin immunoreactive cells). Two exceptions were the posterointermediate bed nucleus of the stria terminalis, containing more neurons heavily labeled for leu-enkephalin, and the main olfactory bulb, where only met-enkephalin was observed. Of most interest for this study was the observation of Enk-ir cells and terminals in areas implicated in both sexual and agonistic behaviors in this species.

Aphrodisin, an aphrodisiac lipocalin secreted in hamster vaginal secretions

Vertebrates communicate through pheromones, which favor biological regulations within each species. Aphrodisin, a protein belonging to the lipocalin superfamily, found in hamster vaginal secretions, is detected by the male accessory olfactory system and induces or facilitates its copulatory behavior. Although much is known about aphrodisin structure, the question of whether aphrodisin bears itself the pheromonal function or is simply a carrier for hydrophobic small pheromones has not been definitely solved. Arguments based on use of recombinant aphrodisin deprived of any natural ligand and its capability to convey hamster pheromonal compounds will be discussed, together with progresses concerning putative natural ligand(s).

Steroid hormone modulation of olfactory processing in the context of socio-sexual behaviors in rodents and humans

Primer pheromones and other chemosensory cues are important factors governing social interactions and reproductive physiology in many species of mammals. Responses to these chemosignals can vary substantially within and between individuals. This variability can stem, at least in part, from the modulating effects steroid and non-steroid hormones exert on olfactory processing. Such modulation frequently augments or facilitates the effects that prevailing social and environmental conditions have on the reproductive axis. The mechanisms underlying the hormonal regulation of responses to chemosensory cues are diverse. They are in part behavioral, achieved through the modulation of chemoinvestigative behaviors, and in part a product of the modulation of the intrinsic responsiveness of the main and accessory olfactory systems to conspecific, as well as other classes, of chemosignals. The behavioral and non-behavioral effects complement one another to ensure that mating and other reproductive processes are confined to reproductively favorable conditions.

NADPH diaphorase activity in olfactory receptor neurons and their axons conforms to a rhinotopically-distinct dorsal zone of the hamster nasal cavity and main olfactory bulb

NADPH diaphorase histochemical protocols were optimized for the histochemical labeling of olfactory receptor neurons (ORNs) in the nasal cavity and their axon terminals in glomeruli of the main olfactory bulb (MOB) in the Syrian hamster. This labeling was then used to map and quantify the spatial distribution of ORNs and their central projections. Diaphorase-positive ORNs were found to be rhinotopically restricted to dorsal-medially situated segments of sensory mucosa associated with central air channels in the nose, together constituting about 25% of the total receptor sheet. This topography closely resembles the zonal expression patterns of putative odorant receptor genes and cell surface glycoconjugates in the nose. Moreover, the projections of ORNs in the diaphorase-positive dorsal/central zone were found to expand onto the entire dorsal half of the MOB, consistent with spatial patterns discerned in retrograde tract-tracing studies. These boundaries indicate that dorsal/central zone ORNs project to a disproportionately larger region of the MOB than do those in the more ventral/peripheral zones. The demonstration of NADPH diaphorase activity in ORNs is inconsistent with the expression of the best-known NADPH-dependent enzymes, such as nitric oxide synthase (neuronal and endothelial isoforms) and NADPH cytochrome P450 oxidoreductase. Understanding the spatial patterning of histochemical labeling in ORNs should facilitate the biochemical identification of this diaphorase.

Olfactory bulb cells generated in adult male golden hamsters are specifically activated by exposure to estrous females

Two experiments were carried out to test whether cells which are born in adulthood and migrate to the olfactory bulb of adult male golden hamsters are activated during sexual behaviors, to determine the time course over which such responsiveness appears, and to ask whether activation is specific to sexual cues. In the first experiment, adult male hamsters were injected with 5'-bromodeoxyuridine (BrdU, 50mg/kg b.w.) 3 times over the course of one week in order to mark dividing cells. Ten days, three weeks, or seven weeks after the first BrdU injection, the animals were allowed to mate with an estrous female for half an hour before being sacrificed. Confocal analysis of fluorescent immunostaining of BrdU and c-Fos first revealed dual labeled cells in the olfactory bulb 3 weeks after injection of the thymidine analog. In order to determine whether the activation of these newly generated cells is specific to sexual cues, we next compared the incidence of c-Fos expression in newborn (BrdU positive) cells among male hamsters exposed to an estrous female, an aggressive male, a cotton swab containing vaginal secretion from an estrous female hamster (FHVS), a cotton swab containing peppermint, or a cotton swab containing distilled water. In the mitral and glomerular layers of the accessory olfactory bulb, animals exposed to an estrous female had significantly more double labeled cells than did those given other treatments (p < 0.01). In the mitral layer of the main bulb, animals exposed to an estrous female had a significantly higher percentage of double labeled cells than those of other groups, except those exposed to an aggressive male (p < 0.05). No double labeled cells were seen in medial preoptic area (MPOA), medial nucleus of the amygdala (Me), the bed nucleus of the stria terminalis (BNST), or the hypothalamus. Our results indicate that cells born in adulthood are more responsive to cues arising from estrous females than other stimuli, and thus may participate in sociosexual behaviors.