The Grueneberg ganglion: a novel sensory system in the nose

Early life oxytocin treatment improves thermo-sensory reactivity and maternal behavior in neonates lacking the autism-associated gene Magel2

Atypical responses to sensory stimuli are considered as a core aspect and early life marker of autism spectrum disorders (ASD). Although recent findings performed in mouse ASD genetic models report sensory deficits, these were explored exclusively during juvenile or adult period. Whether sensory dysfunctions might be present at the early life stage and rescued by therapeutic strategy are fairly uninvestigated. Here we found that under cool environment neonatal mice lacking the autism-associated gene Magel2 present pup calls hypo-reactivity and are retrieved with delay by their wild-type dam. This neonatal atypical sensory reactivity to cool stimuli was not associated with autonomic thermoregulatory alteration but with a deficit of the oxytocinergic system. Indeed, we show in control neonates that pharmacogenetic inactivation of hypothalamic oxytocin neurons mimicked atypical thermosensory reactivity found in Magel2 mutants. Furthermore, pharmacological intranasal administration of oxytocin to Magel2 neonates was able to rescue both the atypical thermosensory response and the maternal pup retrieval. This preclinical study establishes for the first-time early life impairments in thermosensory integration and suggest a therapeutic potential benefit of intranasal oxytocin treatment on neonatal atypical sensory reactivity for autism.

The Grueneberg ganglion: signal transduction and coding in an olfactory and thermosensory organ involved in the detection of alarm pheromones and predator-secreted kairomones

In numerous mammalian species, the nose harbors several compartments populated by chemosensory cells. Among them, the Grueneberg ganglion (GG) located in the anterior nasal region comprises sensory neurons activated by given substances. In rodents, in which the GG has been best studied, these chemical cues mainly include heterocyclic compounds released by predators or by conspecifics. Since some of these substances evoke fear- or stress-associated responses, the GG is considered as a detector for alerting semiochemicals. In fact, certain behavioral and physiological reactions to alarm pheromones and predator-secreted kairomones are attenuated in the absence of a functional GG. Intriguingly, GG neurons are also stimulated by cool temperatures. Moreover, ambient temperatures modulate olfactory responsiveness in the GG, indicating that cross-talks exist between the transduction pathways mediating chemo- and thermosensory signaling in this organ. In this context, exploring the relevant molecular cascades has demonstrated that some chemosensory transduction elements are also crucial for thermosensory signaling in the GG. Finally, for further processing of sensory information, axons of GG neurons project to the olfactory bulb of the brain where they innervate distinct glomerular structures belonging to the enigmatic necklace glomeruli. In this review, the stimuli activating GG neurons as well as the underlying transduction pathways are summarized. Because these stimuli do not exclusively activate GG neurons but also other sensory cells, the biological relevance of the GG is discussed, with a special focus on the role of the GG in detecting alarm signals.

Main Olfactory and Vomeronasal Epithelium Are Differently Affected in Niemann-Pick Disease Type C1

INTRODUCTION

Olfactory impairment is one of the earliest symptoms in neurodegenerative disorders that has also been documented in Niemann-Pick disease type C1 (NPC1). NPC1 is a very rare, neurovisceral lipid storage disorder, characterized by a deficiency of Npc1 gene function that leads to progressive neurodegeneration. Here, we compared the pathologic effect of defective Npc1 gene on the vomeronasal neuroepithelium (VNE) with that of the olfactory epithelium (OE) in an NPC1 mouse model.

METHODS

Proliferation in the VNE and OE was assessed by applying a bromodeoxyuridine (BrdU) protocol. We further compared the immunoreactivities of anti-olfactory marker protein (OMP), and the lysosomal marker cathepsin-D in both epithelia. To investigate if degenerative effects of both olfactory systems can be prevented or reversed, some animals were treated with a combination of miglustat/allopregnanolone/2-hydroxypropyl-cyclodextrin (HPβCD), or a monotherapy with HPβCD alone.

RESULTS

Using BrdU to label dividing cells of the VNE, we detected a proliferation increase of 215% ± 12% in Npc1-/- mice, and 270% ± 10% in combination- treated Npc1-/- animals. The monotherapy with HPβCD led to an increase of 261% ± 10.5% compared to sham-treated Npc1-/- mice. Similar to the OE, we assessed the high regenerative potential of vomeronasal progenitor cells. OMP reactivity in the VNE of Npc1-/- mice was not affected, in contrast to that observed in the OE. Concomitantly, cathepsin-D reactivity in the VNE was virtually absent. Conclusion: Vomeronasal receptor neurons are less susceptible against NPC1 pathology than olfactory receptor neurons. Compared to control mice, however, the VNE of Npc1^-/- mice displays an increased neuroregenerative potential, indicating compensatory cell renewal.

In-vivo activation of vomeronasal neurons shows adaptive responses to pheromonal stimuli

In most mammals, the vomeronasal system has a pivotal role in mediating socio-sexual behaviours. The vomeronasal organ senses pheromones through the activation of specific receptors. Pheromone binding to cognate receptors activates Ca-influx via the gating of a cation channel that generates membrane depolarisation. The ex-vivo activation of vomeronasal neurons (VSNs) by pheromonal stimuli has been largely investigated by electrophysiological and imaging techniques; however, few studies have been carried out to determine the physiological responses of VSNs, in-vivo. By tracking the phosphorylation of S6 ribosomal protein as a marker of neuronal activity, we show that S6 becomes phosphorylated (pS6) in mouse VSNs stimulated by intraspecific and heterospecific pheromonal cues. We observed that female scent induces pS6 immunoreactivity in the apical VSNs of male vomeronasal epithelium, whereas male cues stimulate S6 phosphorylation in both the basal and apical VSNs of females. We also show that this dimorphic pattern of pS6 immunoreactivity is reproduced when heterospecific stimuli are used. Moreover, we found that a consistent proportion of VSNs is activated by both heterospecific and intraspecific pheromones. Additionally, we have evidence of adaptive responses to S6 phosphorylation when stimulation with cues of the same and opposite sex and of different species is sustained.

Regulatory Features for Odorant Receptor Genes in the Mouse Genome

The odorant receptor genes, seven transmembrane receptor genes constituting the vastest mammalian gene multifamily, are expressed monogenically and monoallelicaly in each sensory neuron in the olfactory epithelium. This characteristic, often referred to as the one neuron-one receptor rule, is driven by mostly uncharacterized molecular dynamics, generally named odorant receptor gene choice. Much attention has been paid by the scientific community to the identification of sequences regulating the expression of odorant receptor genes within their loci, where related genes are usually arranged in genomic clusters. A number of studies identified transcription factor binding sites on odorant receptor promoter sequences. Similar binding sites were also found on a number of enhancers that regulate in cis their transcription, but have been proposed to form interchromosomal networks. Odorant receptor gene choice seems to occur via the local removal of strongly repressive epigenetic markings, put in place during the maturation of the sensory neuron on each odorant receptor locus. Here we review the fast-changing state of art for the study of regulatory features for odorant receptor genes.

A 3.7 kb fragment of the mouse Scn10a gene promoter directs neural crest but not placodal lineage EGFP expression in a transgenic animal

Under physiological conditions, the voltage-gated sodium channel Nav1.8 is expressed almost exclusively in primary sensory neurons. The mechanism restricting Nav1.8 expression is not entirely clear, but we have previously described a 3.7 kb fragment of the Scn10a promoter capable of recapitulating the tissue-specific expression of Nav1.8 in transfected neurons and cell lines (Puhl and Ikeda, 2008). To validate these studies in vivo, a transgenic mouse encoding EGFP under the control of this putative sensory neuron specific promoter was generated and characterized in this study. Approximately 45% of dorsal root ganglion neurons of transgenic mice were EGFP-positive (mean diameter = 26.5 μm). The majority of EGFP-positive neurons bound isolectin B4, although a small percentage (∼10%) colabeled with markers of A-fiber neurons. EGFP expression correlated well with the presence of Nav1.8 transcript (95%), Nav1.8-immunoreactivity (70%), and TTX-R INa (100%), although not all Nav1.8-expressing neurons expressed EGFP. Several cranial sensory ganglia originating from neurogenic placodes, such as the nodose ganglion, failed to express EGFP, suggesting that additional regulatory elements dictate Scn10a expression in placodal-derived sensory neurons. EGFP was also detected in discrete brain regions of transgenic mice. Quantitative PCR and Nav1.8-immunoreactivity confirmed Nav1.8 expression in the amygdala, brainstem, globus pallidus, lateral and paraventricular hypothalamus, and olfactory tubercle. TTX-R INa recorded from EGFP-positive hypothalamic neurons demonstrate the usefulness of this transgenic line to study novel roles of Nav1.8 beyond sensory neurons. Overall, Scn10a-EGFP transgenic mice recapitulate the majority of the Nav1.8 expression pattern in neural crest-derived sensory neurons.

Identification of pyridine analogs as new predator-derived kairomones

In the wild, animals have developed survival strategies relying on their senses. The individual ability to identify threatening situations is crucial and leads to increase in the overall fitness of the species. Rodents, for example have developed in their nasal cavities specialized olfactory neurons implicated in the detection of volatile cues encoding for impending danger such as predator scents or alarm pheromones. In particular, the neurons of the Grueneberg ganglion (GG), an olfactory subsystem, are implicated in the detection of danger cues sharing a similar chemical signature, a heterocyclic sulfur- or nitrogen-containing motif. Here we used a “from the wild to the lab” approach to identify new molecules that are involuntarily emitted by predators and that initiate fear-related responses in the recipient animal, the putative prey. We collected urines from carnivores as sources of predator scents and first verified their impact on the blood pressure of the mice. With this approach, the urine of the mountain lion emerged as the most potent source of chemical stress. We then identified in this biological fluid, new volatile cues with characteristic GG-related fingerprints, in particular the methylated pyridine structures, 2,4-lutidine and its analogs. We finally verified their encoded danger quality and demonstrated their ability to mimic the effects of the predator urine on GG neurons, on mice blood pressure and in behavioral experiments. In summary, we were able to identify here, with the use of an integrative approach, new relevant molecules, the pyridine analogs, implicated in interspecies danger communication.

Morphological and physiological species-dependent characteristics of the rodent Grueneberg ganglion

In the mouse, the Grueneberg ganglion (GG) is an olfactory subsystem implicated both in chemo- and thermo-sensing. It is specifically involved in the recognition of volatile danger cues such as alarm pheromones and structurally-related predator scents. No evidence for these GG sensory functions has been reported yet in other rodent species. In this study, we used a combination of histological and physiological techniques to verify the presence of a GG and investigate its function in the rat, hamster, and gerbil comparing with the mouse. By scanning electron microscopy (SEM) and transmitted electron microscopy (TEM), we found isolated or groups of large GG cells of different shapes that in spite of their gross anatomical similarities, display important structural differences between species. We performed a comparative and morphological study focusing on the conserved olfactory features of these cells. We found fine ciliary processes, mostly wrapped in ensheating glial cells, in variable number of clusters deeply invaginated in the neuronal soma. Interestingly, the glial wrapping, the amount of microtubules and their distribution in the ciliary processes were different between rodents. Using immunohistochemistry, we were able to detect the expression of known GG proteins, such as the membrane guanylyl cyclase G and the cyclic nucleotide-gated channel A3. Both the expression and the subcellular localization of these signaling proteins were found to be species-dependent. Calcium imaging experiments on acute tissue slice preparations from rodent GG demonstrated that the chemo- and thermo-evoked neuronal responses were different between species. Thus, GG neurons from mice and rats displayed both chemo- and thermo-sensing, while hamsters and gerbils showed profound differences in their sensitivities. We suggest that the integrative comparison between the structural morphologies, the sensory properties, and the ethological contexts supports species-dependent GG features prompted by the environmental pressure.

Olfactory systems and neural circuits that modulate predator odor fear

When prey animals detect the odor of a predator a constellation of fear-related autonomic, endocrine, and behavioral responses rapidly occur to facilitate survival. How olfactory sensory systems process predator odor and channel that information to specific brain circuits is a fundamental issue that is not clearly understood. However, research in the last 15 years has begun to identify some of the essential features of the sensory detection systems and brain structures that underlie predator odor fear. For instance, the main (MOS) and accessory olfactory systems (AOS) detect predator odors and different types of predator odors are sensed by specific receptors located in either the MOS or AOS. However, complex predator chemosignals may be processed by both the MOS and AOS, which complicate our understanding of the specific neural circuits connected directly and indirectly from the MOS and AOS to activate the physiological and behavioral components of unconditioned and conditioned fear. Studies indicate that brain structures including the dorsal periaqueductal gray (DPAG), paraventricular nucleus (PVN) of the hypothalamus, and the medial amygdala (MeA) appear to be broadly involved in predator odor induced autonomic activity and hypothalamic-pituitary-adrenal (HPA) stress hormone secretion. The MeA also plays a key role in predator odor unconditioned fear behavior and retrieval of contextual fear memory associated with prior predator odor experiences. Other neural structures including the bed nucleus of the stria terminalis and the ventral hippocampus (VHC) appear prominently involved in predator odor fear behavior. The basolateral amygdala (BLA), medial hypothalamic nuclei, and medial prefrontal cortex (mPFC) are also activated by some but not all predator odors. Future research that characterizes how distinct predator odors are uniquely processed in olfactory systems and neural circuits will provide significant insights into the differences of how diverse predator odors activate fear.

The thermosensitive potassium channel TREK-1 contributes to coolness-evoked responses of Grueneberg ganglion neurons

Neurons of the Grueneberg ganglion (GG) residing in the vestibule of the murine nose are activated by cool ambient temperatures. Activation of thermosensory neurons is usually mediated by thermosensitive ion channels of the transient receptor potential (TRP) family. However, there is no evidence for the expression of thermo-TRPs in the GG, suggesting that GG neurons utilize distinct mechanisms for their responsiveness to cool temperatures. In search for proteins that render GG neurons responsive to coolness, we have investigated whether TREK/TRAAK channels may play a role; in heterologous expression systems, these potassium channels have been previously found to close upon exposure to coolness, leading to a membrane depolarization. The results of the present study indicate that the thermosensitive potassium channel TREK-1 is expressed in those GG neurons that are responsive to cool temperatures. Studies analyzing TREK-deficient mice revealed that coolness-evoked responses of GG neurons were clearly attenuated in these animals compared with wild-type conspecifics. These data suggest that TREK-1 channels significantly contribute to the responsiveness of GG neurons to cool temperatures, further supporting the concept that TREK channels serve as thermoreceptors in sensory cells. Moreover, the present findings provide the first evidence of how thermosensory GG neurons are activated by given temperature stimuli in the absence of thermo-TRPs.

Mammalian pheromones

Mammalian pheromones control a myriad of innate social behaviors and acutely regulate hormone levels. Responses to pheromones are highly robust, reproducible, and stereotyped and likely involve developmentally predetermined neural circuits. Here, I review several facets of pheromone transduction in mammals, including (a) chemosensory receptors and signaling components of the main olfactory epithelium and vomeronasal organ involved in pheromone detection; (b) pheromone-activated neural circuits subject to sex-specific and state-dependent modulation; and (c) the striking chemical diversity of mammalian pheromones, which range from small, volatile molecules and sulfated steroids to large families of proteins. Finally, I review (d) molecular mechanisms underlying various behavioral and endocrine responses, including modulation of puberty and estrous; control of reproduction, aggression, suckling, and parental behaviors; individual recognition; and distinguishing of own species from predators, competitors, and prey. Deconstruction of pheromone transduction mechanisms provides a critical foundation for understanding how odor response pathways generate instinctive behaviors.

Neural mechanisms of alarm pheromone signaling

Alarm pheromones are important semiochemicals used by many animal species to alert conspecifics or other related species of impending danger. In this review, we describe recent developments in our understanding of the neural mechanisms underlying the ability of fruit flies, zebrafish and mice to mediate the detection of alarm pheromones. Specifically, alarm pheromones are detected in these species through specialized olfactory subsystems that are unique to the chemosensitive receptors, second messenger-signaling and physiology. Thus, the alarm pheromones appears to be detected by signaling mechanisms that are distinct from those seen in the canonical olfactory system.

Mutual influences between the main olfactory and vomeronasal systems in development and evolution

The sense of smell plays a crucial role in the sensory world of animals. Two chemosensory systems have been traditionally thought to play-independent roles in mammalian olfaction. According to this, the main olfactory system (MOS) specializes in the detection of environmental odorants, while the vomeronasal system (VNS) senses pheromones and semiochemicals produced by individuals of the same or different species. Although both systems differ in their anatomy and function, recent evidence suggests they act synergistically in the perception of scents. These interactions include similar responses to some ligands, overlap of telencephalic connections and mutual influences in the regulation of olfactory-guided behavior. In the present work, we propose the idea that the relationships between systems observed at the organismic level result from a constant interaction during development and reflects a common history of ecological adaptations in evolution. We review the literature to illustrate examples of developmental and evolutionary processes that evidence these interactions and propose that future research integrating both systems may shed new light on the mechanisms of olfaction.