Olfactory inputs to hypothalamic neurons controlling reproduction and fertility

Cytonemes and tunneling nanotubules in cell-cell communication and viral pathogenesis

Cells use a variety of intercellular structures, including gap junctions and synapses, for cell-cell communication. Here, we present recent advances in the understanding of thin membrane bridges that function in cell-cell signaling and intercellular transport. Cytonemes or filopodial bridges connect neighboring cells via mechanisms of adhesion, which enable ligand-receptor-mediated transfer of surface-associated cargoes from cell to cell. By contrast, tunneling nanotubes establish tubular conduits between cells that provide for the exchange of both cell-surface molecules and cytoplasmic content. We propose models for the biogenesis of both types of membrane bridges and describe how viruses use these structures for the purpose of cell-to-cell spread.

Relaxin-3 stimulates the hypothalamic-pituitary-gonadal axis

The hypothalamus plays a key role in the regulation of both energy homeostasis and reproduction. Evidence suggests that relaxin-3, a recently discovered member of the insulin superfamily, is an orexigenic hypothalamic neuropeptide. Relaxin-3 is thought to act in the brain via the RXFP3 receptor, although the RXFP1 receptor may also play a role. Relaxin-3, RXFP3, and RXFP1 are present in the hypothalamic paraventricular nucleus, an area with a well-characterized role in the regulation of energy balance that also modulates reproductive function by providing inputs to hypothalamic gonadotropin-releasing hormone (GnRH) neurons. Other members of the relaxin family are known to play a role in the regulation of reproduction. However, the effects of relaxin-3 on reproductive function are unknown. We studied the role of relaxin-3 in the regulation of the hypothalamo-pituitary-gonadal (HPG) axis. Intracerebroventricular (5 nmol) and intraparaventricular (540-1,620 pmol) administration of human relaxin-3 (H3) in adult male Wistar rats significantly increased plasma luteinizing hormone (LH) 30 min postinjection. This effect was blocked by pretreatment with a peripheral GnRH antagonist. Central administration of human relaxin-2 showed no significant effect on plasma LH. H3 dose-dependently stimulated the release of GnRH from hypothalamic explants and GT(1)-7 cells, which express RXFP1 and RXFP3, but did not influence LH or follicle-stimulating hormone release from pituitary fragments in vitro. We have demonstrated a novel role for relaxin-3 in the stimulation of the HPG axis, putatively via hypothalamic GnRH neurons. Relaxin-3 may act as a central signal linking nutritional status and reproductive function.

Increased dopamine after mating impairs olfaction and prevents odor interference with pregnancy

In rodents, social odor sensing influences female reproductive status by affecting neuroendocrine cascades. The odor of male mouse urine can induce ovulation or block pregnancy within 3 d post coitus. Females avoid the action of such olfactory stimuli after embryonic implantation. The mechanisms underlying these changes are unknown. Here we report that shortly after mating, a surge in dopamine in the mouse main olfactory bulb impairs the perception of social odors contained in male urine. Treatment of females at 6.5 d post coitus with a dopamine D2 receptor antagonist restores social odor sensing and favors disruption of pregnancy by inhibition of prolactin release, when administered in the presence of alien male urine odors. These results show that an active sensory barrier blocks social olfactory cues detrimental to pregnancy, consistent with the main olfactory bulb being a major relay through which social odor modulates reproductive status.

Impaired sexual behavior in male mice deficient for the beta1-3 N-acetylglucosaminyltransferase-I gene

The beta1-3 N-acetylglucosaminyltransferase-1 (B3gnt1) gene encodes a poly-N-acetyllactosamine synthase which can initiate and extend poly-N-acetyllactosamine chains [Gal(beta1-4)GlcNAc (beta1-3)(n)]. Previous investigations with heterozygous and homozygous null mice for this gene have revealed the importance of poly-N-acetyllactosamine chains for the formation of olfactory axon connections with the olfactory bulb and the migration of gonadotropin releasing hormone neurons to the hypothalamus. The possible long-term effects of these developmental defects, however, has not yet been studied. Here we have examined a reproductive phenotype observed in B3gnt1-null mice. Whereas the B3gnt1 null females were fertile, the B3gnt1 null males were not able to sire litters at the expected rate when mated to either wildtype or B3gnt1-null females. We assessed male sexual behavior as well as male reproduction parameters such as testes size, spermatogenesis, sperm number, morphology, and the development of early embryos in order to identify the source of a reduced rate of reproduction. Our findings show that the B3gnt1 null male reproductive organs were functional and could not account for the lower rate at which they produced offspring with wildtype conspecifics. Hence, we propose that the phenotype observed resulted from an impaired sexual response to female mating partners.

Anatomical, immnunohistochemical and physiological characteristics of the vomeronasal vessels in cows and their possible role in vomeronasal reception

The general morphology of the vomeronasal vessels in adult cows was studied following a classic protocol, including optical, confocal and ultrastructural approaches. This anatomical work was completed immunohistochemically. The vomeronasal organ in cows is well developed, and its vessels are considerable in size. This fact allowed some functional properties of the vomeronasal arteries to be evaluated and, for the first time, their isometric tension to be recorded. Our functional studies were in agreement with the immunohistochemistry, and both corroborated the morphological data on the similarity between the vomeronasal vessels and those of the typical erectile tissue. In consequence, the vasoconstriction and vasodilation of the vomeronasal vessels would facilitate an influx and outflow of fluids in the vomeronasal organ, that is to say, this organ in cows would be able to work as a pump mechanism to send chemical signals to the vomeronasal receptor neurones.

Temporal and spatial regulation of CRE recombinase expression in gonadotrophin-releasing hormone neurones in the mouse

Gonadotrophin-releasing hormone (GnRH) neurones located within the brain are the final neuroendocrine output regulating the reproductive hormone axis. Their small number and scattered distribution in the hypothalamus make them particularly difficult to study in vivo. The Cre/loxP system is a valuable tool to delete genes in specific cells and tissues. We report the production of two mouse lines that express the CRE bacteriophage recombinase in a GnRH-specific manner. The first line, the GnRH-CRE mouse, contains a transgene in which CRE is under the control of the murine GnRH promoter and targets CRE expression specifically to GnRH neurones in the hypothalamus. The second line, the GnRH-CRETeR mouse, uses the same murine GnRH promoter to target CRE expression to GnRH neurones, but is modified to be constitutively repressed by a tetracycline repressor (TetR) expressed from a downstream tetracycline repressor gene engineered within the transgene. GnRH neurone-specific CRE expression can therefore be induced by treatment with doxycycline which relieves repression by TetR. These GnRH-CRE and GnRH-CRETeR mice can be used to study the function of genes expressed specifically in GnRH neurones. The GnRH-CRETeR mouse can be used to study genes that may have distinct roles in reproductive physiology during the various developmental stages.

Site-specific effects of anosmia and cloacal gland anesthesia on Fos expression induced in male quail brain by sexual behavior

In rats, expression of the immediate early gene, c-fos observed in the brain following male copulatory behavior relates mostly to the detection of olfactory information originating from the female and to somatosensory feedback from the penis. However, quail, like most birds, are generally considered to have a relatively poorly developed sense of smell. Furthermore, quail have no intromittent organ (e.g., penis). It is therefore intriguing that expression of male copulatory behavior induces in quail and rats a similar pattern of c-fos expression in the medial preoptic area (mPOA), bed nucleus of the stria terminalis (BSTM) and parts of the amygdala. We analyzed here by immunocytochemistry Fos expression in the mPOA/BSTM/amygdala of male quail that had been allowed to copulate with a female during standardized tests. Before these tests, some of the males had either their nostrils plugged, or their cloacal area anesthetized, or both. A control group was not exposed to females. These manipulations did not affect frequencies of male sexual behavior and all birds exposed to a female copulated normally. In the mPOA, the increased Fos expression induced by copulation was not affected by the cloacal gland anesthesia but was markedly reduced in subjects deprived of olfactory input. Both manipulations affected copulation-induced Fos expression in the BSTM. No change in Fos expression was observed in the amygdala. Thus immediate early gene expression in the mPOA and BSTM of quail is modulated at least in part by olfactory cues and/or somatosensory stimuli originating from the cloacal gland. Future work should specify the nature of these stimuli and their function in the expression of avian male sexual behavior.

Vasoactive intestinal polypeptide can excite gonadotropin-releasing hormone neurons in a manner dependent on estradiol and gated by time of day

A surge of GnRH release signals the LH surge that triggers ovulation. The GnRH surge is dependent on a switch in estradiol feedback from negative to positive and, in rodents, a daily neural signal, likely from the suprachiasmatic nuclei. Vasoactive intestinal polypeptide (VIP) may be involved in suprachiasmatic nuclei-GnRH neuron communication. Here we assessed the effects of acute VIP (5 min treatment) on GnRH neuron function using targeted extracellular recordings of firing activity of GnRH neurons in brain slices. We examined the effect of VIP on firing rate at different times of day using an established ovariectomized, estradiol-treated (OVX+E) mouse model that exhibits daily LH surges timed to the late afternoon. Cells from OVX animals (no estradiol) did not respond to VIP, regardless of time of day. With estradiol, the effect of VIP on GnRH neurons was dependent on the time of recording. During negative feedback, OVX+E cells did not respond. VIP increased firing in cells recorded during surge onset, but this excitatory response was reduced at surge peak. Acute treatment of OVX+E cells during surge peak with a VIP receptor antagonist decreased GnRH neuron firing. This suggests endogenous VIP may both increase GnRH neuron firing during the surge and occlude response to exogenous VIP. These data provide functional evidence for VIP effects on GnRH neurons and indicate that both estradiol and time of day gate the GnRH neuron response to this peptide. VIP may provide an excitatory signal from the circadian clock that helps time the GnRH surge.

Male pheromones initiate prolactin-induced neurogenesis and advance maternal behavior in female mice

Prolactin is required for rapid onset of maternal behavior after parturition, inducing adaptive changes in the maternal brain including enhanced neurogenesis in the subventricular zone during pregnancy. The resultant increase in olfactory interneurons may be required for altered processing of olfactory cues during the establishment of maternal behavior. Pheromones act through olfactory pathways to exert powerful effects on behavior in rodents and also affect prolactin secretion. Hence, this study aimed to investigate the effect of male pheromones on neurogenesis and maternal behavior in female mice. Virgin female mice were housed individually or in split-cages where they had pheromonal but not physical contact with a male. Maternal behavior was assessed in a foster pup retrieval paradigm. Some mice were injected with bromodeoxyuridine, and the labeled cells visualized using immunohistochemistry. The data show that exposure to male pheromones, for a duration equivalent to a murine pregnancy, advanced maternal behavior in both virgin and postpartum female mice. The pheromone action was dependent on prolactin and ovarian steroids, and was associated with increased cell proliferation in the subventricular zone and subsequent increases in new neurons in the olfactory bulb. Moreover, the effect of pheromones on both cell proliferation and maternal behavior could be induced solely through administration of exogenous prolactin to mimic the pheromone-induced changes in prolactin secretion. The data suggest that male pheromones induce a prolactin-mediated increase in neurogenesis in female mice, resulting in advanced maternal behavior.

Encoding olfactory signals via multiple chemosensory systems

Most animals have evolved multiple olfactory systems to detect general odors as well as social cues. The sophistication and interaction of these systems permit precise detection of food, danger, and mates, all crucial elements for survival. In most mammals, the nose contains two well described chemosensory apparatuses (the main olfactory epithelium and the vomeronasal organ), each of which comprises several subtypes of sensory neurons expressing distinct receptors and signal transduction machineries. In many species (e.g., rodents), the nasal cavity also includes two spatially segregated clusters of neurons forming the septal organ of Masera and the Grueneberg ganglion. Results of recent studies suggest that these chemosensory systems perceive diverse but overlapping olfactory cues and that some neurons may even detect the pressure changes carried by the airflow. This review provides an update on how chemosensory neurons transduce chemical (and possibly mechanical) stimuli into electrical signals, and what information each system brings into the brain. Future investigation will focus on the specific ligands that each system detects with a behavioral context and the processing networks that each system involves in the brain. Such studies will lead to a better understanding of how the multiple olfactory systems, acting in concert, offer a complete representation of the chemical world.

Neural mechanisms underlying sex-specific behaviors in vertebrates

From invertebrates to humans, males and females of a given species display identifiable differences in behaviors, mostly but not exclusively pertaining to sexual and social behaviors. Within a species, individuals preferentially exhibit the set of behaviors that is typical of their sex. These behaviors include a wide range of coordinated and genetically pre-programmed social and sexual displays that ensure successful reproductive strategies and the survival of the species. What are the mechanisms underlying sex-specific brain function? Although sexually dimorphic behaviors represent the most extreme examples of behavioral variability within a species, the basic principles underlying the sex specificity of brain activity are largely unknown. Moreover, with few exceptions, the quest for fundamental differences in male and female brain structures and circuits that would parallel that of sexual behaviors and peripheral organs has so far uncovered modest quantitative rather than the expected clear qualitative differences. As will be detailed in this review, recent advances have directly challenged the established notion of the unique role of steroid hormones in organizing and activating male- and female-specific brain circuits and have uncovered new mechanisms underlying the neural control of sex-specific behaviors.

Are pheromones detected through the main olfactory epithelium?

A major sensory organ for the detection of pheromones by animals is the vomeronasal organ (VNO). Although pheromones control the behaviors of various species, the effect of pheromones on human behavior has been controversial because the VNO is not functional in adults. However, recent genetic, biochemical, and electrophysiological data suggest that some pheromone-based behaviors, including male sexual behavior in mice, are mediated through the main olfactory epithelium (MOE) and are coupled to the type 3 adenylyl cyclase (AC3) and a cyclic nucleotide-gated (CNG) ion channel. These recent discoveries suggest the provocative hypothesis that human pheromones may signal through the MOE.

The alpha-herpesviruses: molecular pathfinders in nervous system circuits

Several neuroinvasive viruses can be used to study the mammalian nervous system. In particular, infection by pseudorabies virus (PRV), an alpha-herpesvirus with broad host range, reveals chains of functionally connected neurons in the nervous systems of a variety of mammals. The specificity of PRV trans-neuronal spread has been established in several systems. One attenuated strain, PRV-Bartha, causes a reduced inflammatory response and also spreads only from infected post- to pre-synaptic neurons. We review the basics of PRV tracing and then discuss new developments and novel approaches that have enabled a more detailed understanding of the architecture of the nervous system. As questions and techniques evolve in the field of neuroscience, advances in PRV tracing will certainly follow.

Gonadotropin-releasing hormone-1 neuronal activity is independent of cyclic nucleotide-gated channels

Pulsatile release of GnRH-1 is essential for secretion of gonadotropin hormones. The frequency of GnRH-1 pulses is regulated during the reproductive cycle by numerous neurotransmitters. Cyclic nucleotide-gated (CNG) channels have been proposed as a mechanism to integrate the cAMP signal evoked by many neurotransmitters. This study reports the expression of the CNGA2 subunit in GnRH-1 neurons obtained from mouse nasal explants and shows the ability of GnRH-1 neurons to increase their activity in response to forskolin (activator of adenylyl cyclases), or 3-isobutyl-1-methylxanthine (inhibitor of phosphodiesterases) even after removal of gamma-aminobutyric acid (A)-ergic input. Next, the endogenous activity of adenylyl cyclases was evaluated as a component of the oscillatory mechanism of GnRH-1 neurons. Inhibition of endogenous activity of adenylyl cyclases did not alter GnRH-1 activity. The potential involvement of CNGA2 subunit in basal or induced activity was tested on GnRH-1 neurons obtained from CNGA2-deficient mice. Without up-regulation of CNGA1 or CNGA3, the absence of functional CNGA2 did not alter either the endogenous GnRH-1 neuronal activity or the response to forskolin, negating CNG channels from cAMP-sensitive mechanisms leading to changes in GnRH-1 neuronal activity. In addition, the potential role of CNGA2 subunit in the synchronization of calcium oscillations previously described was evaluated in GnRH-1 neurons from CNGA2-deficient explants. Synchronized calcium oscillations persisted in CNGA2-deficient GnRH-1 neurons. Taken together, these results indicate that CNGA2 channels are not necessary for either the response of GnRH-1 neurons to cAMP increases or the basal rhythmic activity of GnRH-1 neurons.

Pheromone sensing in mice

Beginning with the neuroepithelium of the vomeronasal organ, the accessory olfactory system in rodents runs parallel to the main olfactory system and is specialized in the detection of pheromones. Only a small number of vomeronasal agonists carrying pheromonal information have been identified this far. These structurally diverse classes of chemicals include peptides secreted by exocrine glands and range from small volatile molecules to proteins and fragments thereof present in urine. Most pheromones activate both vomeronasal and main olfactory sensory neurons, making the identification of functionally relevant populations of sensory neurons difficult. Analyses of gene-targeted mice selectively affecting either vomeronasal or main olfactory signaling have attempted to elucidate the functional contribution of the different chemosensory epithelia to pheromone sensing in mice. These mouse models suggest that both the main and the accessory olfactory systems can converge and synergize to express the complex array of stereotyped behaviors and hormonal changes triggered by pheromones.

The combined role of the main olfactory and vomeronasal systems in social communication in mammals

The main olfactory and the vomeronasal systems are the two systems by which most vertebrates detect chemosensory cues that mediate social behavior. Much research has focused on how one system or the other is critical for particular behaviors. This has lead to a vision of two distinct and complexly autonomous olfactory systems. A closer look at research over the past 30 years reveals a different picture however. These two seemingly distinct systems are much more integrated than previously thought. One novel set of chemosensory cues in particular (MHC Class I peptide ligands) can show us how both systems are capable of detecting the same chemosensory cues, through different mechanisms yet provide the same general information (genetic individuality). Future research will need to now focus on how two seemingly distinct chemosensory systems together detect pheromones and mediate social behaviors. Do these systems work independently, synergistically or competitively in communicating between individuals of the same species?

Definition of brainstem afferents to gonadotropin-releasing hormone neurons in the mouse using conditional viral tract tracing

Brainstem monoamines have long been considered to play a role in regulating the activity of GnRH neurons, although their neuroanatomical relationship with these cells has remained unclear. Using a Cre-dependent pseudorabies virus (Ba2001) technique that permits retrograde tracing selectively from GnRH neurons in the mouse, we have examined the organization of brainstem inputs to rostral preoptic area (rPOA) GnRH neurons. Two days after injection of Ba2001 into the rPOA of adult female GnRH-Cre transgenic mice, five to nine GnRH neurons located immediately adjacent to the injection site were found to express green fluorescent protein (GFP), the marker of virus infection, with no GFP expression anywhere else in the brain. In mice killed 24 h later (3 d after injection), GFP-expressing cells were identified (in order of density) in the raphe nuclei, periaqueductal grey, locus coeruleus, nucleus tractus solitarius, and area postrema. This time course is compatible with these neurons representing primary afferent inputs to the GnRH neurons. Four and 6 d after Ba2001 injection, GFP-expressing cells were found in additional brain regions. Dual-label immunofluorescence experiments in 3-d postinjection mice demonstrated that 100% of GFP-expressing neurons in the raphe were positive for tryptophan hydroxylase, whereas 100% and approximately 50% of GFP neurons in the locus coeruleus and nucleus tractus solitarius, respectively, expressed tyrosine hydroxylase. These observations demonstrate that rPOA GnRH neurons receive direct projections from brainstem A2 and A6 noradrenergic neurons and that, surprisingly, the largest afferent input from the brainstem originates from raphe serotonin neurons in the mouse.

Convergence of olfactory and vomeronasal projections in the rat basal telencephalon

Olfactory and vomeronasal projections have been traditionally viewed as terminating in contiguous non-overlapping areas of the basal telencephalon. Original reports, however, described areas such as the anterior medial amygdala where both chemosensory afferents appeared to overlap. We addressed this issue by injecting dextran amines in the main or accessory olfactory bulbs of rats and the results were analyzed with light and electron microscopes. Simultaneous injections of different fluorescent dextran amines in the main and accessory olfactory bulbs were performed and the results were analyzed using confocal microscopy. Similar experiments with dextran amines in the olfactory bulbs plus FluoroGold in the bed nucleus of the stria terminalis indicate that neurons projecting through the stria terminalis could be integrating olfactory and vomeronasal inputs. Retrograde tracing experiments using FluoroGold or dextran amines confirm that areas of the rostral basal telencephalon receive inputs from both the main and accessory olfactory bulbs. While both inputs clearly converge in areas classically considered olfactory-recipient (nucleus of the lateral olfactory tract, anterior cortical amygdaloid nucleus, and cortex-amygdala transition zone) or vomeronasal-recipient (ventral anterior amygdala, bed nucleus of the accessory olfactory tract, and anteroventral medial amygdaloid nucleus), segregation is virtually complete at posterior levels such as the posteromedial and posterolateral cortical amygdalae. This provides evidence that areas so far considered receiving a single chemosensory modality are likely sites for convergent direct olfactory and vomeronasal inputs. Therefore, areas of the basal telencephalon should be reclassified as olfactory, vomeronasal, or mixed chemosensory structures, which could facilitate understanding of olfactory-vomeronasal interactions in functional studies.

Effect of bilateral accessory olfactory bulb lesions on volatile urinary odor discrimination and investigation as well as mating behavior in male mice

Previous research raises the possibility that urinary volatiles from estrous female mice activate mitral cells in the accessory olfactory bulb (AOB) of male mice following detection via the main olfactory epithelium as opposed to the vomeronasal organ. We asked whether bilateral lesions of the AOB would disrupt the ability of male mice to discriminate between urinary volatiles from mice of different sexes or endocrine states, or affect their interest in investigating these odors when they were presented sequentially in home-cage habituation/dishabituation tests. Males with either partial or complete bilateral lesions of the AOB resembled sham-operated control males in their ability to discriminate between ovariectomized and estrous female urinary volatiles as well as between male and estrous female urinary volatiles. However, males with either complete or partial AOB lesions spent significantly less time than sham-operated control males investigating urinary volatiles from estrous females, especially during tests when the alternative stimulus presented was male urine. Placement of AOB lesions failed to disrupt males' mating performance. Our results suggest that the incentive value of opposite-sex (female) volatile urinary odors which are initially detected by the main olfactory system is enhanced when they are further processed by the male's AOB.

A functional circuit underlying male sexual behaviour in the female mouse brain

In mice, pheromone detection is mediated by the vomeronasal organ and the main olfactory epithelium. Male mice that are deficient for Trpc2, an ion channel specifically expressed in VNO neurons and essential for VNO sensory transduction, are impaired in sex discrimination and male-male aggression. We report here that Trpc2-/- female mice show a reduction in female-specific behaviour, including maternal aggression and lactating behaviour. Strikingly, mutant females display unique characteristics of male sexual and courtship behaviours such as mounting, pelvic thrust, solicitation, anogenital olfactory investigation, and emission of complex ultrasonic vocalizations towards male and female conspecific mice. The same behavioural phenotype is observed after VNO surgical removal in adult animals, and is not accompanied by disruption of the oestrous cycle and sex hormone levels. These findings suggest that VNO-mediated pheromone inputs act in wild-type females to repress male behaviour and activate female behaviours. Moreover, they imply that functional neuronal circuits underlying male-specific behaviours exist in the normal female mouse brain.

Mammalian pheromone sensing

The traditional distinction that the mammalian main olfactory system recognizes general odor molecules and the accessory (vomeronasal) system detects pheromones is no longer valid. The emerging picture is that both systems have considerable overlap in terms of the chemosignals they detect and the effects that they mediate. Recent investigations have discovered large families of pheromonal signals together with a rich variety of specific receptor systems and nasal detection pathways. Selective genetic targeting of these subsystems should help to unravel their biological role in pheromone-mediated behavioral responses.

Defining the gonadotrophin-releasing hormone neuronal network: transgenic approaches to understanding neurocircuitry

The gonadotrophin-releasing hormone (GnRH) neurones are the final downstream effector neurones driving the central regulation of reproductive function and fertility in all mammalian species. Although it is abundantly clear that successful fertility relies upon the communication of a variety signals regarding internal and external cues to the GnRH neuronal population, how this is achieved remains poorly understood. A range of technical limitations has posed significant hurdles to defining, with any certainty, the complexities of the synaptic neuronal network regulating GnRH neurones. However, recent advances in transgenic technology have opened up new avenues to reconsider questions aimed at understanding this critical network. This article addresses some of the latest advances that use transgenic mouse models as tools to understand the neuronal circuitry underpinning the regulation of the GnRH neurones. By incorporating standard morphological and viral tract tracing techniques with innovative transgenic tools, recent studies have uncovered previously unappreciated qualities of the GnRH neurone, including extensive dendritic lengths, numerous somal and dendritic spines and plasticity over pubertal development, along with beginning to define the primary and higher-order afferents that make up the GnRH neuronal network.

Male pheromone-stimulated neurogenesis in the adult female brain: possible role in mating behavior

The regulation of female reproductive behaviors may involve memories of male pheromone signatures, formed in part by neural circuitry involving the olfactory bulb and hippocampus. These neural structures are the principal sites of adult neurogenesis; however, previous studies point to their independent regulation by sensory and physiological stimuli. Here we report that the pheromones of dominant (but not subordinate) males stimulate neuronal production in both the olfactory bulb and hippocampus of female mice, which are independently mediated by prolactin and luteinizing hormone, respectively. Neurogenesis induced by dominant-male pheromones correlates with a female preference for dominant males over subordinate males, whereas blocking neurogenesis with the mitotic inhibitor cytosine arabinoside eliminated this preference. These results suggest that male pheromones are involved in regulating neurogenesis in both the olfactory bulb and hippocampus, which may be important for female reproductive success.

Association Study of HumanVN1R1Pheromone Receptor Gene Alleles and Gender

Pheromones are water-soluble chemicals that elicit neuroendocrine and physiological changes, while they also provide information about gender within individuals of the same species. VN1R1 is the only functional pheromone receptor in humans. We have undertaken a large mutation screening approach in 425 adult individuals from the Hellenic population to investigate whether the allelic differences, namely alleles 1a and 1b present in the human VN1R1 gene, are gender specific. Here we show that both VN1R1 1a and 1b alleles are found in chromosomes of both male and female subjects at frequency of 26.35% and 73.65%, respectively. Given the fact that those allelic differences potentially cause minor changes in the protein conformation and its transmembrane domains, as simulated by the TMHMM software, our data suggest that the allelic differences in the human VN1R1 gene are unlikely to be associated with gender and hence to contribute to distinct gender-specific behavior.

Methamphetamine administration causes death of dopaminergic neurons in the mouse olfactory bulb

BACKGROUND

Methamphetamine (METH) is an addictive drug that can cause neurological and psychiatric disorders. In the rodent brain, toxic doses of METH cause damage of dopaminergic terminals and apoptosis of nondopaminergic neurons. The olfactory bulb (OB) is a brain region that is rich with dopaminergic neurons and terminals.

METHODS

Rats were given a single injection of METH (40 mg/kg) and sacrificed at various time points afterward. The toxic effects of this injection on the OB were assessed by measuring monoamine levels, tyrosine hydroxylase (TH) immunocytochemistry, terminal deoxynucleotidyl transferase-mediated deoxyribonucleotide triphosphate (dNTP) nick end labeling (TUNEL) histochemistry, and caspase-3 immunochemistry.

RESULTS

Methamphetamine administration caused marked decreases in dopamine (DA) levels and TH-like immunostaining in the mouse OB. The drug also caused increases in TUNEL-labeled OB neurons, some of which were also positive for TH expression. Moreover, there was METH-induced expression of activated caspase-3 in TH-positive cells. Finally, the METH injection was associated with increased expression of the proapoptotic proteins, Bax and Bid, but with decreased expression of the antideath protein, Bcl2.

CONCLUSIONS

These observations show, for the first time, that METH can cause loss of OB DA terminals and death of DA neurons, in part, via mechanisms that are akin to an apoptotic process.

Local retinal circuits of melanopsin-containing ganglion cells identified by transsynaptic viral tracing

Intrinsically photosensitive melanopsin-containing retinal ganglion cells (ipRGCs) control important physiological processes, including the circadian rhythm, the pupillary reflex, and the suppression of locomotor behavior (reviewed in [1]). ipRGCs are also activated by classical photoreceptors, the rods and cones, through local retinal circuits [2, 3]. ipRGCs can be transsynaptically labeled through the pupillary-reflex circuit with the derivatives of the Bartha strain of the alphaherpesvirus pseudorabies virus(PRV) [4, 5] that express GFP [6-12]. Bartha-strain derivatives spread only in the retrograde direction [13]. There is evidence that infected cells function normally for a while during GFP expression [7]. Here we combine transsynaptic PRV labeling, two-photon laser microscopy, and electrophysiological techniques to trace the local circuit of different ipRGC subtypes in the mouse retina and record light-evoked activity from the transsynaptically labeled ganglion cells. First, we show that ipRGCs are connected by monostratified amacrine cells that provide strong inhibition from classical-photoreceptor-driven circuits. Second, we show evidence that dopaminergic interplexiform cells are synaptically connected to ipRGCs. The latter finding provides a circuitry link between light-dark adaptation and ipRGC function.

Multimodal fast optical interrogation of neural circuitry

Our understanding of the cellular implementation of systems-level neural processes like action, thought and emotion has been limited by the availability of tools to interrogate specific classes of neural cells within intact, living brain tissue. Here we identify and develop an archaeal light-driven chloride pump (NpHR) from Natronomonas pharaonis for temporally precise optical inhibition of neural activity. NpHR allows either knockout of single action potentials, or sustained blockade of spiking. NpHR is compatible with ChR2, the previous optical excitation technology we have described, in that the two opposing probes operate at similar light powers but with well-separated action spectra. NpHR, like ChR2, functions in mammals without exogenous cofactors, and the two probes can be integrated with calcium imaging in mammalian brain tissue for bidirectional optical modulation and readout of neural activity. Likewise, NpHR and ChR2 can be targeted together to Caenorhabditis elegans muscle and cholinergic motor neurons to control locomotion bidirectionally. NpHR and ChR2 form a complete system for multimodal, high-speed, genetically targeted, all-optical interrogation of living neural circuits.

Chemosensory cues from the lacrimal and preputial glands stimulate production of IP3 in the vomeronasal organ and aggression in male mice

The social and reproductive behaviors of most mammals are modulated by chemosensory cues. The perception of some of these cues is mediated by the vomeronasal organ, which is a cartilage-encased elongated organ associated with the vomer bone in the rostral nasal cavity. Several studies have shown that chemosensory cues are present in urine, seminal fluid or vaginal secretions but only a few studies have focused on exocrine glands as a source of chemosensory cues. Here we show that chemosensory cues present in two exocrine glands, i.e., the preputial gland located at the caudal region and the lacrimal gland located at the rostral region, are capable of stimulating aggression in male mice. We further show that these extracts can stimulate the production of inositol-(1,4,5)-trisphosphate in the vomeronasal organ.

Estradiol induces diurnal shifts in GABA transmission to gonadotropin-releasing hormone neurons to provide a neural signal for ovulation

Ovulation is initiated by a surge of gonadotropin-releasing hormone (GnRH) secretion by the brain. GnRH is normally under negative feedback control by ovarian steroids. During sustained exposure to estradiol in the late follicular phase of the reproductive cycle, however, the feedback action of this steroid switches to positive, inducing the surge. Here, we used an established ovariectomized, estradiol-treated (OVX+E) mouse model exhibiting daily surges to investigate the neurobiological mechanisms underlying this switch. Specifically, we examined changes in GABA transmission to GnRH neurons, which can be excited by GABA(A) receptor activation. Spontaneous GABAergic postsynaptic currents (PSCs) were recorded in GnRH neurons from OVX+E and OVX mice in coronal and sagittal slices. There were no diurnal changes in PSC frequency in cells from OVX mice in either slice orientation. In OVX+E cells in both orientations, PSC frequency was low during negative feedback but increased at surge onset. During the surge peak, this increase subsided in coronal slices but persisted in sagittal slices. Comparison of PSCs before and during tetrodotoxin (TTX) treatment showed TTX decreased PSC frequency in OVX+E cells in sagittal slices, but not coronal slices. This indicates estradiol acts on multiple GABAergic afferent populations to increase transmission through both activity-dependent and -independent mechanisms. Estradiol also increased PSC amplitude during the surge. Estradiol and the diurnal cycle thus interact to induce shifts in both GABA transmission and postsynaptic response that would produce appropriate changes in GnRH neuron firing activity and hormone release.

Genetic analysis of brain circuits underlying pheromone signaling

Molecular approaches and genetic manipulations have provided novel insights into the processing of pheromone-mediated information by the olfactory and vomeronasal systems of mammals. We will review and discuss the specific contribution of each of the two chemosensory systems that ensure specific behavioral responses to conspecific animals.

Cell type-specific expression of a genetically encoded calcium indicator reveals intrinsic calcium oscillations in adult gonadotropin-releasing hormone neurons

The gonadotropin-releasing hormone (GnRH) neurons exhibit a unique pattern of episodic activity to control fertility in all mammals. To enable the measurement of intracellular calcium concentration ([Ca2+]i) in adult GnRH neurons in situ, we generated transgenic mice in which the genetically encodable calcium indicator ratiometric Pericam was expressed by approximately 95% of GnRH neurons. Real-time monitoring of [Ca2+]i within adult male GnRH neurons in the acute brain slice revealed that approximately 70% of GnRH neurons exhibited spontaneous, 10-15 s duration [Ca2+]i transients with a mean frequency of 7 per hour. The remaining 30% of GnRH neurons did not exhibit calcium transients nor did a population of non-GnRH cells located within the lateral septum that express Pericam. Pharmacological studies using antagonists to the inositol-1,4,5-trisphosphate receptor (InsP3R) and several calcium channels, demonstrated that [Ca2+]i transients in GnRH neurons were generated by an InsP3R-dependent store-release mechanism and were independent of plasma membrane ligand- or voltage-gated calcium channels. Interestingly, the abolition of action potential-mediated transmission with tetrodotoxin reduced the number of [Ca2+]i transients in GnRH neurons by 50% (p < 0.05), suggesting a modulatory role for synaptic inputs on [Ca2+]i transient frequency. Using a novel transgenic strategy that enables [Ca2+]i to be examined in a specific neuronal phenotype in situ, we provide evidence for spontaneous [Ca2+]i fluctuations in adult GnRH neurons. This represents the initial description of spontaneous [Ca2+]i transients in mature neurons and shows that they arise from an InsP3R-generating mechanism that is further modulated by synaptic inputs.

Olfactory neurons expressing transient receptor potential channel M5 (TRPM5) are involved in sensing semiochemicals

Olfactory sensory neurons (OSNs) in the main olfactory epithelium respond to environmental odorants. Recent studies reveal that these OSNs also respond to semiochemicals such as pheromones and that main olfactory input modulates animal reproduction, but the transduction mechanism for these chemosignals is not fully understood. Previously, we determined that responses to putative pheromones in the main olfactory system were reduced but not eliminated in mice defective for the canonical cAMP transduction pathway, and we suggested, on the basis of pharmacology, an involvement of phospholipase C. In the present study, we find that a downstream signaling component of the phospholipase C pathway, the transient receptor potential channel M5 (TRPM5), is coexpressed with the cyclic nucleotide-gated channel subunit A2 in a subset of mature OSNs. These neurons project axons primarily to the ventral olfactory bulb, where information from urine and other socially relevant signals is processed. We find that these chemosignals activate a subset of glomeruli targeted by TRPM5-expressing OSNs. Our data indicate that TRPM5-expressing OSNs that project axons to glomeruli in the ventral area of the main olfactory bulb are involved in processing of information from semiochemicals.

Conditional gene targeting in the mouse nervous system: Insights into brain function and diseases

Conditional gene knockout represents an extremely powerful approach to study the function of single genes in the nervous system. The Cre-LoxP system is the most advanced technology for spatial and temporal control of genetic inactivation, and there is rapid progress using this methodology in neuroscience research. In this approach, mice with LoxP sites flanking the gene of interest (floxed mice) are bred with transgenic mice expressing Cre recombinase under the control of a selected promoter (Cre mice). This promoter is critical in that it determines the time and site of Cre expression. Cre enzyme, in turn, recombines the floxed gene and produces gene knockout. Here we review Cre mouse lines that have been developed to target either the entire brain, selected brain areas, or specific neuronal populations. We then summarize phenotypic consequences of conditional gene targeting in the brain for more than 40 genes, as reported to date. For many broadly expressed genes, brain-restricted knockout has overcome lethality of conventional knockout (KO) and has highlighted a specific role of the encoded protein in some aspect of brain function. In the case of neural genes, data from null mutants in specific brain sites or neurons has refined our understanding of the role of individual molecules that regulate complex behaviors or synaptic plasticity within neural circuits. Among the many developing functional genomic approaches, conditional gene targeting in the mouse has become an excellent tool to elucidate the function of the approximately 5000 known or unknown genes that operate in the nervous system.

Comparative genomic analysis identifies an evolutionary shift of vomeronasal receptor gene repertoires in the vertebrate transition from water to land

Two evolutionarily unrelated superfamilies of G-protein coupled receptors, V1Rs and V2Rs, bind pheromones and "ordinary" odorants to initiate vomeronasal chemical senses in vertebrates, which play important roles in many aspects of an organism's daily life such as mating, territoriality, and foraging. To study the macroevolution of vomeronasal sensitivity, we identified all V1R and V2R genes from the genome sequences of 11 vertebrates. Our analysis suggests the presence of multiple V1R and V2R genes in the common ancestor of teleost fish and tetrapods and reveals an exceptionally large among-species variation in the sizes of these gene repertoires. Interestingly, the ratio of the number of intact V1R genes to that of V2R genes increased by approximately 50-fold as land vertebrates evolved from aquatic vertebrates. A similar increase was found for the ratio of the number of class II odorant receptor (OR) genes to that of class I genes, but not in other vertebrate gene families. Because V1Rs and class II ORs have been suggested to bind to small airborne chemicals, whereas V2Rs and class I ORs recognize water-soluble molecules, these increases reflect a rare case of adaptation to terrestrial life at the gene family level. Several gene families known to function in concert with V2Rs in the mouse are absent outside rodents, indicating rapid changes of interactions between vomeronasal receptors and their molecular partners. Taken together, our results demonstrate the exceptional evolutionary fluidity of vomeronasal receptors, making them excellent targets for studying the molecular basis of physiological and behavioral diversity and adaptation.

Pheromonal communication in vertebrates

Recent insights have revolutionized our understanding of the importance of chemical signals in influencing vertebrate behaviour. Previously unknown families of pheromonal signals have been identified that are expanding the traditional definition of a pheromone. Although previously regarded as functioning independently, the main olfactory and vomeronasal systems have been found to have considerable overlap in terms of the chemosignals they detect and the effects that they mediate. Studies using gene-targeted mice have revealed an unexpected diversity of chemosensory systems and their underlying cellular and molecular mechanisms. Future developments could show how the functions of the different chemosensory systems are integrated to regulate innate and learned behavioural and physiological responses to pheromones.

Molecular biology of peptide pheromone production and reception in mice

Intraspecies communication via pheromones plays an important role in social and sexual behaviors, which are critical for survival and reproduction in many animal species. In mice, pheromonal signals are processed by the parallel action of two olfactory systems: the main olfactory system and the vomeronasal pathway. Pheromones are recognized by chemosensory receptors expressed in the main olfactory epithelium and by V1R- and V2R-type receptors expressed in the vomeronasal organ (VNO). Mice take advantage of the chemical properties of both types of pheromones (i.e., volatile/nonvolatile) to precisely control the spatial and temporal transmission of their individual signals. The recent discovery of the exocrine gland-secreting peptide (ESP) family, which appears to encode a VNO-specific ligand repertoire, should open a new avenue to understanding peptide pheromone-mediated communication via the vomeronasal pathway in mice. In this chapter, I will review the current knowledge on genetic and molecular aspects of peptide pheromones and their receptors, by focusing primarily on the mouse VNO system. It is also an intriguing aspect to discuss peptide pheromones in the context of the evolutionary importance of species-specific chemical communication.

Diversity of the vomeronasal system in mammals: The singularities of the sheep model

The enormous morphological diversity and heterogeneity of the vomeronasal system (VNS) in mammals--as well as its complete absence in some cases--complicates the extrapolation of data from one species to another, making any physiological and functional conclusions valid for the whole Mammalian Class difficult and risky to draw. Some highly-evolved macrosmatic mammals, like sheep, have been previously used in interesting behavioral studies concerning the main and accessory olfactory systems. However, in this species, certain crucial morphological peculiarities have not until now been considered. Following histological, histochemical and immunohistochemical procedures, we have studied the vomeronasal organ (VNO) and the accessory olfactory bulb (AOB) of adult sheep. We have determined: (1) that all structures which classically define the VNO in mammals are present and well developed, providing the morphological basis for functional activity. (2) that, conversely, there is only a scant population of scattered mitral/tufted cells. One morphological consequence of both details is that the strata of the AOB in adult sheep are not as sharply defined as in other species; moreover, the small number of the mitral/tufted cells in the AOB may imply that the VNS of adult sheep is not capable of functioning in the way a well-developed VNS does in other species. (3) the zone to zone projection from the apical and basal sensory epithelium of the VNO to the anterior and posterior part of the AOB, respectively, typical in rodents, lagomorphs and marsupials, is not present in adult sheep.

Nursing behavior: remembrance of things past

Successful suckling is vital to the survival of mammalian newborns. In many mammals, nursing behavior is triggered by maternally derived odors. Such odors may also promote the learned association of odorant cues present in the environment during nursing.

Identification of a gonadotropin-releasing hormone receptor orthologue in Caenorhabditis elegans

BACKGROUND

The Caenorhabditis elegans genome is known to code for at least 1149 G protein-coupled receptors (GPCRs), but the GPCR(s) critical to the regulation of reproduction in this nematode are not yet known. This study examined whether GPCRs orthologous to human gonadotropin-releasing hormone receptor (GnRHR) exist in C. elegans.

RESULTS

Our sequence analyses indicated the presence of two proteins in C. elegans, one of 401 amino acids [GenBank: NP_491453; WormBase: F54D7.3] and another of 379 amino acids [GenBank: NP_506566; WormBase: C15H11.2] with 46.9% and 44.7% nucleotide similarity to human GnRHR1 and GnRHR2, respectively. Like human GnRHR1, structural analysis of the C. elegans GnRHR1 orthologue (Ce-GnRHR) predicted a rhodopsin family member with 7 transmembrane domains, G protein coupling sites and phosphorylation sites for protein kinase C. Of the functionally important amino acids in human GnRHR1, 56% were conserved in the C. elegans orthologue. Ce-GnRHR was actively transcribed in adult worms and immunoanalyses using antibodies generated against both human and C. elegans GnRHR indicated the presence of a 46-kDa protein, the calculated molecular mass of the immature Ce-GnRHR. Ce-GnRHR staining was specifically localized to the germline, intestine and pharynx. In the germline and intestine, Ce-GnRHR was localized specifically to nuclei as revealed by colocalization with a DNA nuclear stain. However in the pharynx, Ce-GnRHR was localized to the myofilament lattice of the pharyngeal musculature, suggesting a functional role for Ce-GnRHR signaling in the coupling of food intake with reproduction. Phylogenetic analyses support an early evolutionary origin of GnRH-like receptors, as evidenced by the hypothesized grouping of Ce-GnRHR, vertebrate GnRHRs, a molluscan GnRHR, and the adipokinetic hormone receptors (AKHRs) and corazonin receptors of arthropods.

CONCLUSION

This is the first report of a GnRHR orthologue in C. elegans, which shares significant similarity with insect AKHRs. In vertebrates, GnRHRs are central components of the reproductive endocrine system, and the identification of a GnRHR orthologue in C. elegans suggests the potential use of C. elegans as a model system to study reproductive endocrinology.

Definition of estrogen receptor pathway critical for estrogen positive feedback to gonadotropin-releasing hormone neurons and fertility

The mechanisms through which estrogen regulates gonadotropin-releasing hormone (GnRH) neurons to control mammalian ovulation are unknown. We found that estrogen positive feedback to generate the preovulatory gonadotropin surge was normal in estrogen receptor beta knockout (ERbeta) mutant mice, but absent in ERalpha mutant mice. An ERalpha-selective compound was sufficient to generate positive feedback in wild-type mice. As GnRH neurons do not express ERalpha, estrogen positive feedback upon GnRH neurons must be indirect in nature. To establish the cell type responsible, we generated a neuron-specific ERalpha mutant mouse line. These mice failed to exhibit estrogen positive feedback, demonstrating that neurons expressing ERalpha are critical. We then used a GnRH neuron-specific Pseudorabies virus (PRV) tracing approach to show that the ERalpha-expressing neurons innervating GnRH neurons are located within rostral periventricular regions of the hypothalamus. These studies demonstrate that ovulation is driven by estrogen actions upon ERalpha-expressing neuronal afferents to GnRH neurons.

Probing the transcriptome of neuronal cell types

Distinct neuronal cell types acquire and maintain their identity by expressing different genes. Recently it has become feasible to measure this cell type specific expression by isolating and amplifying mRNA from small populations of fluorescently labeled neurons and probing this mRNA with microarrays. Prior to this, most neuronal gene expression studies used tissue homogenates or randomly selected single cells and were, therefore, not well suited to studying transcriptional differences between cell types. Microarray studies of purified cell types have enabled investigators to identify the transcriptional signatures of, for example, subtypes of pyramidal neurons and interneurons in the neocortex, modulatory dopaminergic and serotonergic neurons, and the striatal neurons that form the so-called 'direct' and 'indirect' pathways through the basal ganglia. These studies are opening up new approaches to understanding brain circuitry, plasticity and pathology and are refining the concept of the neuronal cell type.

Sex differences in the human olfactory system

The olfactory system (accessory) implicated in reproductive physiology and behavior in mammals is sexually dimorphic. These brain sex differences present two main characteristics: they are seen in neural circuits related to sexual behavior and sexual physiology and they take one of two opposite morphological patterns (male>female or female>male). The present work reports sex differences in the olfactory system in a large homogeneous sample of men (40) and women (51) using of voxel-based morphology. Gray matter concentration showed sexual dimorphism in several olfactory regions. Women have a higher concentration in the orbitofrontal cortex involving Brodmann's areas 10, 11 and 25 and temporomedial cortex (bilateral hippocampus and right amygdala), as well as their left basal insular cortex. In contrast, men show a higher gray matter concentration in the left entorhinal cortex (Brodmann's area 28), right ventral pallidum, dorsal left insular cortex and a region of the orbitofrontal cortex (Brodmann's area 25). This study supports the hypothesis that the mammalian olfactory system is a sexually dimorphic network and provides a theoretical framework for the morphofunctional approach to sex differences in the human brain.

Sexual experience does not compensate for the disruptive effects of zinc sulfate--lesioning of the main olfactory epithelium on sexual behavior in male mice

Recent studies point to an important role for the main olfactory epithelium (MOE) in regulating sexual behavior in male mice. We asked whether sexual experience could compensate for the disruptive effects of lesioning the MOE on sexual behavior in male mice. Male mice, which were either sexually naive or experienced, received an intranasal irrigation of either a zinc sulfate solution to destroy the MOE or saline. Sexual behavior in mating tests with an estrous female was completely abolished in zinc sulfate-treated male mice regardless of whether subjects were sexually experienced or not before the treatment. Furthermore, zinc sulfate treatment clearly disrupted olfactory investigation of both volatile and nonvolatile odors. Destruction of the MOE by zinc sulfate treatment was confirmed by a significant reduction in the expression of Fos protein in the main olfactory bulb following exposure to estrous female urine. By contrast, vomeronasal function did not seem to be affected by zinc sulfate treatment: nasal application of estrous female urine induced similar levels of Fos protein in the mitral and granule cells of the accessory olfactory bulb (AOB) of zinc sulfate- and saline-treated males. Likewise, the expression of soybean agglutinin, which stains the axons of vomeronasal organ neurons projecting to the glomerular layer of the AOB, was similar in zinc sulfate- and saline-treated male mice. These results show that the main olfactory system is essential for the expression of sexual behavior in male mice and that sexual experience does not overcome the disruptive effects of MOE lesioning on this behavior.

Expression of pheromone receptor gene families during olfactory development in the mouse: expression of a V1 receptor in the main olfactory epithelium

In the mouse, two large gene families, V1R and V2R, encoding putative pheromone receptors have been described. Studies have suggested a homotypic recognition role for V1Rs and V2Rs during development in the targeting of vomeronasal axons to specific sets of glomeruli in the accessory olfactory bulb (AOB). Analysis of the onset of expression of the V1R and V2R gene families in developing vomeronasal neurons using polymerase chain reaction and in situ hybridization now suggests that a role for these receptors in the organization of axon projections is only likely at the final stages of targeting within the AOB. Surprisingly, our studies reveal expression of a V1Rd receptor in scattered cells within the main olfactory epithelium, suggesting that limited pheromone detection may also take place in this structure. The pheromone sensory neurons of the vomeronasal system and the neuroendocrine gonadotrophin-releasing hormone (GnRH) neurons that regulate fertility both arise from progenitor cells of the nasal placode. The development of these two cell types is intimately linked, and the GnRH neuron population migrates into the forebrain during embryogenesis in close association with a subset of vomeronasal sensory axons; how GnRH neurons recognize this axon subset is unknown. We report selective expression of a V1Ra gene in the clonal NLT GnRH cell line, raising the possibility of a similar role for V1Rs or V2Rs in the directed migration of GnRH neurons. However, no expression of this gene or of other V1Rs and V2Rs is detectable at the cellular level in migrating GnRH neurons in the mouse.

A second class of chemosensory receptors in the olfactory epithelium

The mammalian olfactory system detects chemicals sensed as odours as well as social cues that stimulate innate responses. Odorants are detected in the nasal olfactory epithelium by the odorant receptor family, whose approximately 1,000 members allow the discrimination of a myriad of odorants. Here we report the discovery of a second family of receptors in the mouse olfactory epithelium. Genes encoding these receptors, called 'trace amine-associated receptors' (TAARs), are present in human, mouse and fish. Like odorant receptors, individual mouse TAARs are expressed in unique subsets of neurons dispersed in the epithelium. Notably, at least three mouse TAARs recognize volatile amines found in urine: one detects a compound linked to stress, whereas the other two detect compounds enriched in male versus female urine-one of which is reportedly a pheromone. The evolutionary conservation of the TAAR family suggests a chemosensory function distinct from odorant receptors. Ligands identified for TAARs thus far suggest a function associated with the detection of social cues.

A multireceptor genetic approach uncovers an ordered integration of VNO sensory inputs in the accessory olfactory bulb

Pheromone detection by the vomeronasal organ (VNO) is thought to rely on activation of specific receptors from the V1R and V2R gene families, but the central representation of pheromone receptor activation remains poorly understood. We generated transgenic mouse lines in which projections from multiple populations of VNO neurons, each expressing a distinct V1R, are differentially labeled with fluorescent proteins. This approach revealed that inputs from neurons expressing closely related V1Rs intermingle within shared, spatially conserved domains of the accessory olfactory bulb (AOB). Mitral cell-glomerular connectivity was examined by injecting intracellular dyes into AOB mitral cells and monitoring dendritic contacts with genetically labeled glomeruli. We show that individual mitral cells extend dendrites to glomeruli associated with different, but likely closely related, V1Rs. This organization differs from the labeled line of OR signaling in the main olfactory system and suggests that integration of information may already occur at the level of the AOB.