Vomeronasal organ ablation elicits chemosensory dysfunction and abnormal behavior in mice

Tph2-/- female mice restore socio-sexual recognition through upregulating ERα and OTR genes in the amygdala

The central 5-hydroxytryptamine system impairs sociosexual behaviors and olfaction preferences in sexually naive mice. However, it remains unknown whether reproductive experiences impart an effect on the sexual olfactory preferences of female mice lacking central serotonin. Here, we aimed at examining such effects and the underlying mechanisms using Tph2 knockout female mice. Sexually naive Tph2^−/− female mice failed to recognize olfactory cues regarding sex, genetic relatedness, and social hierarchy despite exhibiting normal olfactory discrimination. However, reproduction-experienced Tph2^−/− female mice recovered sexual olfactory preferences, as did sexually naive Tph2^+/+ females. Meanwhile, both the estrogen receptor α and oxytocin receptor in the amygdala of reproduction-experienced Tph2^−/− females presented upregulated expression at the mRNA level and an upward tendency at the protein level vs. sexually naive Tph2^−/− females. Intracerebroventricular administration of a combination of estrogen receptor α and oxytocin receptor agonists, but not either agent alone, could restore the sexual olfactory preferences of sexually naive Tph2^−/− female mice to some degree. We speculate that estrogen receptor α and oxytocin receptor activation in the amygdala after reproductive experiences restores sexual olfactory recognition in Tph2^−/− female mice.

Chronic Co-species Housing Mice and Rats Increased the Competitiveness of Male Mice

Rats are predators of mice in nature. Nevertheless, it is a common practice to house mice and rats in a same room in some laboratories. In this study, we investigated the behavioral and physiological responsively of mice in long-term co-species housing conditions. Twenty-four male mice were randomly assigned to their original raising room (control) or a rat room (co-species-housed) for more than 6 weeks. In the open-field and light-dark box tests, the behaviors of the co-species-housed mice and controls were not different. In a 2-choice test of paired urine odors [rabbit urine (as a novel odor) vs. rat urine, cat urine (as a natural predator-scent) vs. rabbit urine, and cat urine vs. rat urine], the co-species-housed mice were more ready to investigate the rat urine odor compared with the controls and may have adapted to it. In an encounter test, the rat-room-exposed mice exhibited increased aggression levels, and their urines were more attractive to females. Correspondingly, the levels of major urinary proteins were increased in the co-species-housed mouse urine, along with some volatile pheromones. The serum testosterone levels were also enhanced in the co-species-housed mice, whereas the corticosterone levels were not different. The norepinephrine, dopamine, and 5-HT levels in the right hippocampus and striatum were not different between the 2. Our findings indicate that chronic co-species housing results in adaptation in male mice; furthermore, it appears that long-term rat-odor stimuli enhance the competitiveness of mice, which suggests that appropriate predator-odor stimuli may be important to the fitness of prey animals.

Conditioned odor aversion induces social anxiety towards females in wild-type and TrpC2 knockout male mice

Female-emitted pheromonal inputs possess an intrinsic rewarding value for conspecific males, promoting approach and investigation of the potential mating partner. In mice these inputs are detected mainly by the vomeronasal organ (VNO) and the main olfactory epithelium (MOE). We investigated the role of VNO-mediated inputs in experience-dependent plasticity of reproductive responses. We applied a sex-specific conditioned odor aversion (COA) paradigm on adult, wild-type (WT) male mice and on male mice impaired in VNO-mediated signal transduction (TrpC2^-/- ). We found that WT males, which underwent COA to female-soiled bedding, lost their innate preference to female odors and presented lower motivation to approach a sexually receptive female. COA also abolished the testosterone surge normally seen following exposure to female odors. Moreover, the conditioned males displayed impairments in copulatory behaviors, which lasted for several weeks. Surprisingly, these males also exhibited phobic behaviors towards receptive females, including freezing and fleeing responses. In contrast, WT males which underwent COA specifically to male pheromones showed no change in olfactory preference and only a marginally significant elevation in intermale aggression. Finally, we show that TrpC2^-/- males were able to acquire aversion to female-soiled bedding and presented similar behavioral alterations following COA in their responses to female cues. Our results demonstrate that the intrinsic rewarding value of female pheromones can be overridden through associative olfactory learning, which occurs independently of VNO inputs, probably through MOE signaling.

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.

Inactivation of the bed nucleus of the stria terminalis suppresses the innate fear responses of rats induced by the odor of cat urine

In this study, we investigated whether two brain regions, the bed nucleus of the stria terminalis (BNST) and the basolateral amygdala (BLA), affected male rats' (Rattus norvigicus) ability to innately discriminate between a predator odor (cat urine) and female rat urine. Muscimol, a GABAa receptor agonist, was bilaterally microinjected into either the BNST or BLA of rats through implanted stainless-steel guide cannulas to temporarily inactivate these brain nuclei. The behavioral responses of the treated rats to female rat urine and cat urine were then tested in an experimental arena. Compared to a saline infusion control, the injection of muscimol into the BNST strongly reversed the innate aversion of rats to cat urine but the injection of muscimol into the BLA had no effect. Furthermore, intra-BNST infusion of muscimol caused rats to be equally attracted to urine from cats and female rats but intra-BLA infusion did not stop rats manifesting fear on exposure to cat urine and exploratory behavior on exposure to female rat urine. We conclude that the BNST plays a more crucial role in modulating innate fear responses in rats than the BLA.