Protease-sensitive urinary pheromones induce region-specific Fos-expression in rat accessory olfactory bulb

The Bruce effect in Norway rats

Intrauterine implantation of fertilized ova can be blocked by exposing recently inseminated females with an unfamiliar male. This selective pregnancy failure, designated as the Bruce effect (Bruce, Nature 1959; 184:105), is well studied in laboratory mice and has been confirmed in several other rodent species. However, no clear information exists concerning this phenomenon in the laboratory rat. The present study was conducted to investigate whether or not the Bruce effect exists in the rat. Females of two F1 hybrid strains (n(total) = 354) with different MHC genotypes (F344BNF1, RT1(lv1/n), and LEWPVGF1, RT1(l/c)) were mated with males of their own strain and subsequently exposed during the first 4 days postcoitus either to a male of the other hybrid strain or to an unfamiliar male of the same strain as the stud. The litter rate of each treatment group was determined. As a control, mated females of both strains were reexposed to the stud male to determine baseline litter rates. Female rats of both F1 hybrid strains showed a significantly lower litter rate when exposed to males of a different strain than their stud male, compared to the expected values of birth rates observed in control females (F344BNF1: P = 0.017; LEWPVGF1: P = 0.019). In contrast, there was no difference between expected and observed litter rates in females of both F1 hybrid strains after exposure to an unfamiliar male of the same strain as their stud. Our results demonstrate for the first time that the Bruce effect, well documented in mice, occurs in the Norway rat.

Changes in Fos expression in the accessory olfactory bulb of sexually experienced male rats after exposure to female urinary pheromones

We studied Fos-immunoreactive (Fos-ir) structures in the accessory olfactory bulb (AOB) of rats after the vomeronasal organ was exposed to urine. Exposure of the vomeronasal organ of male Wistar rats to oestrous and dioestrous female Wistar urine led to the appearance of many Fos-ir cells in the rostral region of the periglomerular cell (PGC) layer, but induced few Fos-ir cells in the caudal region. These results suggest that the regionalization of Fos-ir cells after exposure to female urine is remarkable in the PGC layer of the AOB. Sexually experienced male rats have been shown to prefer oestrous to dioestrous female urine, while sexually inexperienced males do not exhibit these preferences. In the present study, we compared the expression of Fos-ir cells in the AOB of sexually experienced and sexually inexperienced male rats following exposure to oestrous and dioestrous urine. In the localized region (lateral and rostral sectors) of the PGC layer, many more Fos-ir cells were expressed in the sexually experienced rats than in the inexperienced rats. These results suggest that sexual experience in males enhances the transmission of reproductively salient information concerning potential oestrous status to a specific PGC region of the AOB.

Involvement of the calcium channel beta3 subunit in olfactory signal transduction

Despite the expression of voltage-dependent Ca2+ channels in nasal turbinate epithelium, their role in odorant chemosensation has remained obscure. Therefore, we investigated olfactory neurotransduction in beta3-deficient mice. RT-PCR and Western blots confirmed the expression of various types of Ca2+ channels in the nasal turbinate. Electrophysiological evaluations revealed that beta3-null mice had a 60% reduction in the high-voltage-dependent Ca2+ currents in olfactory receptor neurons due to reduced N- and L-type channel currents. The beta3-null mice showed increased olfactory neuronal activity to triethylamine, and this effect was mimicked by the perfusion of the specific N-type Ca2+ channel inhibitor omega-conotoxin GVIA in the electro-olfactogram. Diluted male urine odors induced higher Fos immunoreactivity in the main olfactory bulbs of beta3-deficient mice, indicating enhanced signal transduction of odor information in these mice. Our data indicate the involvement of voltage-dependent Ca2+ channels and importance of the beta3 subunit in olfactory signal transduction.

Decreases in urinary pheromonal activities in male mice after exposure to 3-methylchoranthrene

Many classes of environmental pollutants, which are found at significant levels in the environment, affect the reproductive functions. The gonadal functions of various animals are regulated by pheromones excreted from mating partners. Pheromones in male urine play essential roles in the sexual maturation of female mice. Pheromones are received by sensory neurons in the vomeronasal organ, which innervate to the accessory olfactory bulb (AOB). The effects of a typical aromatic environmental pollutant (3-methylchoranthrene) on excretion of pheromones from male mice were explored based on neuronal Fos responses of the AOB of female mice. On days 1 and 3 after intraperitoneal administration of 3-methylchoranthrene (3-MC), the density of Fos-immunoreactive (Fos-ir) cells in the AOB of female mice after exposure to urine excreted from the administered males was lower than that after exposure to urine from non-administered males. These results suggest that 3-MC blocks chemical communication from male to female mice by reducing pheromonal activities.

Species-specific chemosignals evoke delayed excitation of the vomeronasal amygdala in freely-moving female rats

Male rat chemosignals attract females and influence their reproductive status. Through the accessory olfactory bulb and its projection target, the posteromedial cortical nucleus of the amygdala (PMCo), species-specific chemosignals detected by the vomeronasal organ (VNO) may reach the hypothalamus. To test this hypothesis in vivo, behavioural activation and neurotransmitter release in the PMCo were simultaneously monitored in freely moving female oestrus rats exposed to either rat or mouse urinary stimuli, or to odorants. Plasma levels of the luteinizing hormone were subsequently monitored. All stimuli induced an immediate behavioural activation, but only species-specific chemosignals led to a delayed behavioural activation. This biphasic behavioural activation was accompanied by a VNO-mediated release of the excitatory amino acids, aspartate and glutamate, in the PMCo. The late behavioural and neurochemical activation was followed by an increase in the levels of circulating luteinizing hormone. In conclusion, these data show that only species-specific chemosignals induce a delayed behavioural activation and excitatory activation of the PMCo, which is dependent on an intact VNO.

Decreases in pheromonal responses at the accessory olfactory bulb of mice with a deficiency of the alpha1B or beta3 subunits of voltage-dependent Ca2+-channels

Pheromones affect gonadal functions and sexual behaviors. Information in regard to pheromones is received by the vomeronasal organ (VNO) and transmitted to the accessory olfactory bulb (AOB). We investigated the physiological role of the alpha1B and beta3 subunits of the N (neuronal)-type voltage-dependent Ca2+ channel in the neurotransduction in the accessory olfactory (vomeronasal) system using alpha1B-deficient mice and beta3-deficient mice. RT-PCR studies showed the existence of beta1, beta2, beta3, beta4, alpha1A, alpha1B, and alpha1C subunits of voltage-dependent Ca2+ channels in the mouse VNO. Immunohistochemical studies showed that the alpha1A, alpha1B, and alpha1C subunits of voltage-dependent Ca2+ channels exist in the sensory neurons and supporting cells of the mouse VNO. Exposure of the VNO to urine samples excreted from male mice induced lower Fos-immunoreactivity in the periglomerular (PG) cells of the AOBs in alpha1B-deficient female mice than in those of wild mice. The density of Fos-immunoreactive (Fos-ir) cells after exposure to female urine samples at the periglomerular cell layer of alpha1B-deficient male mice was lower than that of wild mice. Exposure of the VNO of beta3-deficient female mice to male urine samples also induced low Fos-ir cells in the periglomerular cell layer of the AOB. These data suggest the importance of the alpha1B and beta3 subunits of the N-type voltage-dependent Ca2+ channel for the pheromone signal transduction system.

Increases in plasma concentration of progesterone by protease-sensitive urinary pheromones in female rats

Plasma progesterone concentrations in female Wistar rats after exposure to urine preparations with and without protease-treatment were measured to explore the effects of protease-sensitive pheromones on the endocrine state. Exposure to crude urine excreted from male rats induced an increase in the plasma progesterone concentration in female rats. The progesterone concentration of oestrous females increased with an increase in the protein concentration in urine samples. Exposure of females in the oestrous state to urine preparations treated with protease did not induce increases in plasma progesterone. These results suggest that the presence of a protease-sensitive component in male urine exerts an influence on the endocrine state of oestrous females.

Increases in Fos-immunoreactivity after exposure to a combination of two male urinary components in the accessory olfactory bulb of the female rat

Exposure to either the dialyzed urine preparation (<500 Da) or the remaining substances (>500 Da) did not induce expression of Fos-immunoreactive cells in the mitral/tufted cell layer of the accessory olfactory bulb (AOB), whereas exposure to a mixture of these preparation did induce expression. These results suggest that a combination of low and high molecular weight substances is necessary for the increases in Fos-immunoreactivity in the AOB.

Sensory coding of pheromone signals in mammals

The vomeronasal organ (VNO) of mammals plays an essential role in the detection of pheromones, chemical cues secreted by animals that elicit genetically programmed sexual and aggressive behaviors among conspecifics. The recent characterization of genes encoding molecular components of the VNO sensory response suggests that VNO neurons express a unique set of molecules to recognize and translate pheromone signals into neuronal electrical activity. Identification of these genes, which include putative pheromone receptor genes, has offered a new opportunity to uncover basic principles of pheromone sensory processing and important aspects of vomeronasal development.