Unitary response of mouse olfactory receptor neurons

The sense of smell begins with odorant molecules binding to membrane receptors on the cilia of olfactory receptor neurons (ORNs), thereby activating a G protein, G(olf), and the downstream effector enzyme, an adenylyl cyclase (ACIII). Recently, we have found in amphibian ORNs that an odorant-binding event has a low probability of activating sensory transduction at all; even when successful, the resulting unitary response apparently involves a single active Gα(olf)-ACIII molecular complex. This low amplification is in contrast to rod phototransduction in vision, the best-quantified G-protein signaling pathway, where each photoisomerized rhodopsin molecule is well known to produce substantial amplification by activating many G-protein, and hence effector-enzyme, molecules. We have now carried out similar experiments on mouse ORNs, which offer, additionally, the advantage of genetics. Indeed, we found the same low probability of transduction, based on the unitary olfactory response having a fairly constant amplitude and similar kinetics across different odorants and randomly encountered ORNs. Also, consistent with our picture, the unitary response of Gα(olf)(+/-) ORNs was similar to WT in amplitude, although their Gα(olf)-protein expression was only half of normal. Finally, from the action potential firing, we estimated that ≤19 odorant-binding events successfully triggering transduction in a WT mouse ORN will lead to signaling to the brain.     

Post-eclosion odor experience modifies olfactory receptor neuron coding in Drosophila

Olfactory responses of Drosophila undergo pronounced changes after eclosion. The flies develop attraction to odors to which they are exposed and aversion to other odors. Behavioral adaptation is correlated with changes in the firing pattern of olfactory receptor neurons (ORNs). In this article, we present an information-theoretic analysis of the firing pattern of ORNs. Flies reared in a synthetic odorless medium were transferred after eclosion to three different media: (i) a synthetic medium relatively devoid of odor cues, (ii) synthetic medium infused with a single odorant, and (iii) complex cornmeal medium rich in odors. Recordings were made from an identified sensillum (type II), and the Jensen–Shannon divergence (D_JS) was used to assess quantitatively the differences between ensemble spike responses to different odors. Analysis shows that prolonged exposure to ethyl acetate and several related esters increases sensitivity to these esters but does not improve the ability of the fly to distinguish between them. Flies exposed to cornmeal display varied sensitivity to these odorants and at the same time develop greater capacity to distinguish between odors. Deprivation of odor experience on an odorless synthetic medium leads to a loss of both sensitivity and acuity. Rich olfactory experience thus helps to shape the ORNs response and enhances its discriminative power. The experiments presented here demonstrate an experience-dependent adaptation at the level of the receptor neuron.     

Olfactory regulation by dopamine and DRD2 receptor in the nose

Olfactory behavior is important for animal survival, and olfactory dysfunction is a common feature of several diseases. Despite the identification of neural circuits and circulating hormones in olfactory regulation, the peripheral targets for olfactory modulation remain relatively unexplored. In analyzing the single-cell RNA sequencing data from mouse and human olfactory mucosa (OM), we found that the mature olfactory sensory neurons (OSNs) express high levels of dopamine D2 receptor (Drd2) rather than other dopamine receptor subtypes. The DRD2 receptor is expressed in the cilia and somata of mature OSNs, while nasal dopamine is mainly released from the sympathetic nerve terminals, which innervate the mouse OM. Intriguingly, genetic ablation of Drd2 in mature OSNs or intranasal application with DRD2 antagonist significantly increased the OSN response to odorants and enhanced the olfactory sensitivity in mice. Mechanistic studies indicated that dopamine, acting through DRD2 receptor, could inhibit odor-induced cAMP signaling of olfactory receptors. Interestingly, the local dopamine synthesis in mouse OM is down-regulated during starvation, which leads to hunger-induced olfactory enhancement. Moreover, pharmacological inhibition of local dopamine synthesis in mouse OM is sufficient to enhance olfactory abilities. Altogether, these results reveal nasal dopamine and DRD2 receptor as the potential peripheral targets for olfactory modulation.

Olfactory behavior is important for food seeking and animal survival. On the other hand, olfactory dysfunction is a common feature of several diseases such as psychiatric disorders, neurodegeneration, and COVID-19 (1–3). Interestingly, the olfactory ability can be regulated by feeding status and external environments (4, 5). Recent studies have made progress in identifying the neural circuits and circulating hormones in olfactory regulation (6–11). However, the peripheral targets modulating olfactory ability remain relatively unexplored (12).Dopamine (DA) is a monoamine neurotransmitter (13, 14), which plays important roles in a variety of brain functions. DA is released by dopaminergic neurons in the central nervous system. In addition, DA can be released by sympathetic nerves in the peripheral tissues including the olfactory mucosa (OM) (15–18). The sympathetic innervation of rodent OM originates predominantly from the superior cervical ganglion (SCG) (17). Tyrosine hydroxylase (TH) is the rate-limiting enzyme for DA synthesis (19). Intriguingly, the Th mRNA is locally translated in the sympathetic nerve axons, which facilitates local DA synthesis (20, 21).There are two types of DA receptors based on sequence homology and function: The excitatory D1-like receptors (DRD1 and DRD5) and inhibitory D2-like receptors (DRD2–DRD4) (22). Activation of DRD2, a Gα_i/o-coupled receptor, can reduce the intracellular levels of cyclic adenosine monophosphate (cAMP). Drd2 is associated with several neuropsychiatric diseases and is the target of some antipsychotic drugs (23–28). In the central nervous system including the olfactory bulb (OB), DA-DRD2 signaling plays important roles in regulating synaptic transmission and plasticity (29–33). However, the function and regulation of DA-DRD2 signaling in the peripheral tissues are relatively less understood.Here we show that DRD2 is expressed in the cilia and somata of mature olfactory sensory neurons (OSNs) in mice. We provide evidence that DA-DRD2 signaling has a tonic inhibition on OSN activity and olfactory function in mice. Intriguingly, hunger greatly reduces the N4-acetylcytidine (ac⁴C) modification of Th mRNA and local DA synthesis in mouse OM, which causes the olfactory enhancement during starvation. We further show that inhibition of local DA synthesis or DRD2 receptor in mouse OM recapitulates enhanced olfactory abilities during starvation. Collectively, these results reveal nasal DA and DRD2 receptor as the potential peripheral targets for olfactory regulation.     

Sugar-regulated cation channel formed by an insect gustatory receptor

Insects sense the taste of foods and toxic compounds in their environment through the gustatory system. Genetic studies using fruit flies have suggested that putative seven-transmembrane gustatory receptors (Grs) expressed in gustatory sensory neurons are required for responses to specific tastants. We reconstituted sugar responses of Bombyx mori Gr-9 (BmGr-9), a silkworm Gr, in two heterologous expression systems. Xenopus oocytes or HEK293T cells expressing BmGr-9 selectively responded to D-fructose with an influx of extracellular Ca(2+) and a nonselective cation current conductance in a G protein-independent manner. Outside-out patch-clamp recording of BmGr-9-expressing cell membranes provides evidence supporting the hypothesis that BmGr-9 constitutes a ligand-gated ion channel. The fructose-activated current associated with BmGr-9 was suppressed by other hexoses, including glucose and sorbose. The activation and inhibition of insect Gr ion channels may be the molecular basis for the decoding system that discriminates subtle differences in sweet taste. Finally, Drosophila melanogaster Gr43a (DmGr43a), a BmGr-9 ortholog, also responded to D-fructose, suggesting that DmGr43a relatives appear to compose the family of fructose receptors.     

Dedicated olfactory neurons mediating attraction behavior to ammonia and amines in Drosophila

Animals across various phyla exhibit odor-evoked innate attraction behavior that is developmentally programmed. The mechanism underlying such behavior remains unclear because the odorants that elicit robust attraction responses and the neuronal circuits that mediate this behavior have not been identified. Here, we describe a functionally segregated population of olfactory sensory neurons (OSNs) and projection neurons (PNs) in Drosophila melanogaster that are highly specific to ammonia and amines, which act as potent attractants. The OSNs express IR92a, a member of the chemosensory ionotropic receptor (IR) family and project to a pair of glomeruli in the antennal lobe, termed VM1. In vivo calcium-imaging experiments showed that the OSNs and PNs innervating VM1 were activated by ammonia and amines but not by nonamine odorants. Flies in which the IR92a⁺ neurons or IR92a gene was inactivated had impaired amine-evoked physiological and behavioral responses. Tracing neuronal pathways to higher brain centers showed that VM1-PN axonal projections within the lateral horn are topographically segregated from those of V-PN and DC4-PN, which mediate innate avoidance behavior to carbon dioxide and acidity, respectively, suggesting that these sensory stimuli of opposing valence are represented in spatially distinct neuroanatomic loci within the lateral horn. These experiments identified the neurons and their cognate receptor for amine detection, and mapped amine attractive olfactory inputs to higher brain centers. This labeled-line mode of amine coding appears to be hardwired to attraction behavior.     

昆虫气味受体及其介导的嗅觉信号转导途径

减少媒介昆虫的叮咬是控制虫媒病的重要手段。然而,传统防制手段的弊端已逐渐暴露,因此,研制新型防制方法迫在眉睫。昆虫寻找宿主和吸血等行为在很大程度上是由其嗅觉系统控制的,因此,通过干扰昆虫嗅觉系统进行防制成为新的虫媒病控制手段。在昆虫通过嗅觉系统感受环境中众多气味分子的过程中,昆虫气味受体的作用尤为重要。本文就昆虫气味受体及其介导的嗅觉信号转导等方面取得的研究进展作一简要综述。     

Detection and avoidance of a carnivore odor by prey

Predator-prey relationships provide a classic paradigm for the study of innate animal behavior. Odors from carnivores elicit stereotyped fear and avoidance responses in rodents, although sensory mechanisms involved are largely unknown. Here, we identified a chemical produced by predators that activates a mouse olfactory receptor and produces an innate behavioral response. We purified this predator cue from bobcat urine and identified it to be a biogenic amine, 2-phenylethylamine. Quantitative HPLC analysis across 38 mammalian species indicates enriched 2-phenylethylamine production by numerous carnivores, with some producing >3,000-fold more than herbivores examined. Calcium imaging of neuronal responses in mouse olfactory tissue slices identified dispersed carnivore odor-selective sensory neurons that also responded to 2-phenylethylamine. Two prey species, rat and mouse, avoid a 2-phenylethylamine odor source, and loss-of-function studies involving enzymatic depletion of 2-phenylethylamine from a carnivore odor indicate it to be required for full avoidance behavior. Thus, rodent olfactory sensory neurons and chemosensory receptors have the capacity for recognizing interspecies odors. One such cue, carnivore-derived 2-phenylethylamine, is a key component of a predator odor blend that triggers hard-wired aversion circuits in the rodent brain. These data show how a single, volatile chemical detected in the environment can drive an elaborate danger-associated behavioral response in mammals.     

Environmental CO2 inhibits Caenorhabditis elegans egg-laying by modulating olfactory neurons and evokes widespread changes in neural activity

Carbon dioxide (CO₂) gradients are ubiquitous and provide animals with information about their environment, such as the potential presence of prey or predators. The nematode Caenorhabditis elegans avoids elevated CO₂, and previous work identified three neuron pairs called “BAG,” “AFD,” and “ASE” that respond to CO₂ stimuli. Using in vivo Ca²⁺ imaging and behavioral analysis, we show that C. elegans can detect CO₂ independently of these sensory pathways. Many of the C. elegans sensory neurons we examined, including the AWC olfactory neurons, the ASJ and ASK gustatory neurons, and the ASH and ADL nociceptors, respond to a rise in CO₂ with a rise in Ca²⁺. In contrast, glial sheath cells harboring the sensory endings of C. elegans’ major chemosensory neurons exhibit strong and sustained decreases in Ca²⁺ in response to high CO₂. Some of these CO₂ responses appear to be cell intrinsic. Worms therefore may couple detection of CO₂ to that of other cues at the earliest stages of sensory processing. We show that C. elegans persistently suppresses oviposition at high CO₂. Hermaphrodite-specific neurons (HSNs), the executive neurons driving egg-laying, are tonically inhibited when CO₂ is elevated. CO₂ modulates the egg-laying system partly through the AWC olfactory neurons: High CO₂ tonically activates AWC by a cGMP-dependent mechanism, and AWC output inhibits the HSNs. Our work shows that CO₂ is a more complex sensory cue for C. elegans than previously thought, both in terms of behavior and neural circuitry.Most living matter creates temporal or spatial gradients of carbon dioxide (CO₂). Animals across phylogeny use such gradients to help detect food, conspecifics, or predators (1, 2). The ubiquity of CO₂ suggests that the ecologically relevant information it communicates will depend on the dynamics of the CO₂ stimulus and on context. CO₂-responsive excitable cells have been identified in mammals (3), arthropods (4), and nematodes (5, 6). However, the number of CO₂-responsive neurons that are functional in vivo, how they are embedded in neural circuits, and how they shape behavior is unclear.CO₂ crosses membranes readily and dissolves to generate CO₂(aq), H⁺, and HCO₃⁻. Many proteins whose activity is modified by CO₂ or its solvation products have been identified. pH changes can modulate G protein-coupled receptors (7), Ca²⁺-activated K⁺ channels (8), inwardly rectifying K⁺ channels (9), two pore domain K⁺ channels (10), transient receptor potential (TRP) channels (11, 12), acid-sensing ion channels (ASICs) (13, 14), and Pyk2 and ErbB1/2 kinases (15). HCO₃⁻ modulates soluble adenylate cyclase (16) and transmembrane guanylate cyclases (17); and CO₂(aq) has been proposed to regulate transmembrane guanylate cyclases (18) and connexin 26 (19) directly. Cells expressing any of these proteins potentially could transduce changes in CO₂/H⁺, raising the question: Do animals use a few specific sensory channels or a large distributed set to respond to ecologically meaningful fluctuations in CO₂? If there are many responsive neurons, how does each contribute to altered behavior or physiology?In mammals, CO₂ levels are tightly controlled to ensure that blood pH remains stable. Peripheral sensors in the carotid bodies and incompletely defined central chemoreceptors respond to small changes in CO₂/H⁺ by homeostatically altering the breathing rate (3, 20). In concert, pH and HCO₃⁻ sensors in the kidneys regulate H⁺ and HCO₃⁻ excretion (21, 22). These mechanisms keep human blood pH close to 7.4, and in healthy individuals neurons experience only limited fluctuation in CO₂/H⁺. In contrast, in small invertebrates such as nematodes that breathe by diffusion through a gas-permeable skin and have limited buffering capacity, CO₂ levels in body fluid probably vary more widely, according to ambient CO₂ levels.The nematode Caenorhabditis elegans avoids environments with elevated CO₂ (23, 24), and high CO₂ can adversely effect its development, mobility, fertility, and aging (25). Three neurons that respond robustly to CO₂ have been identified thus far in this animal: BAG neurons that also respond to O₂, the thermosensory AFD neurons, and the gustatory ASE neurons (5, 6). CO₂ responses in BAG neurons are mediated by a receptor-type guanylate cyclase, GCY-9, that signals via a cGMP-gated ion channel encoded by the tax-2 and tax-4 genes (6). The CO₂ responsiveness of AFD thermosensors is sculpted by previous acclimation temperature, suggesting experience-dependent cross-modulation between temperature- and CO₂-sensing mechanisms in this neuron (26). Acute changes in O₂ also alter C. elegans’ CO₂ responsiveness: CO₂ avoidance is suppressed when O₂ approaches 21%, because of tonic signaling by the O₂-sensing neuron URX (26, 27).Here, we identify many additional C. elegans cells that respond to CO₂, including both neurons and glia. Some of these cells probably are intrinsically CO₂ sensitive. We show that elevated CO₂ inhibits egg-laying, and tonically represses the hermaphrodite-specific neurons (HSNs) critical for normal egg-laying behavior. CO₂ inhibition of egg-laying involves the AWC olfactory neurons, which are persistently stimulated at high CO₂ by a cGMP-dependent mechanism.     

Reproduction phase-related expression of GnRH-like immunoreactivity in the olfactory receptor neurons,their projections to the olfactory bulb and in the nervus terminalis in the female Indian major carp Cirrhinus mrigala (Ham.)

The reproductive biology of the Indian major carp Cirrhinus mrigala is tightly synchronized with the seasonal changes in the environment. While the ovaries show growth from February through June, the fish spawn in July-August to coincide with the monsoon; thereafter the fish pass into the postspawning and resting phases. We investigated the pattern of GnRH immunoreactivity in the olfactory system at regular intervals extending over a period of 35 months. Although no signal was detected in the olfactory organ of fish collected from April through February following year, distinct GnRH-like immunoreactivity appeared in the fish collected in March. Intense immunoreactivity was noticed in several olfactory receptor neurons (ORNs) and their axonal fibers as they extend over the olfactory nerve, spread in the periphery of the olfactory bulb (OB), and terminate in the glomerular layer. Strong immunoreactivity was seen in some fascicles of the medial olfactory tracts extending from the OB to the telencephalon. Some neurons of the ganglion cells of nervus terminalis showed GnRH immunostaining during March; no immunoreactivity was detected at other times of the year.Plexus of GnRH immunoreactive fibers extending throughout the bulb represented a different component of the olfactory system; the fiber density showed a seasonal pattern that could be related to the status of gonadal maturity. While it was highest in the prespawning phase, significant reduction in the fiber density was noticed in the fish of spawning and the following regressive phases. Taken together the data suggest that the GnRH in the olfactory system of C. mrigala may play a major role in translation of the environmental cues and influence the downstream signals leading to the stimulation of the brain-pituitary-ovary axis.     

Distribution of the P2X2 receptor and chemical coding in ileal enteric neurons of obese male mice (ob/ob)

AIM: To investigate the colocalization, density and profile of neuronal areas of enteric neurons in the ileum of male obese mice.METHODS: The small intestinal samples of male mice in an obese group (OG) (C57BL/6J ob/ob) and a control group (CG) (+/+) were used. The tissues were analyzed using a double immunostaining technique for immunoreactivity (ir) of the P2X2 receptor, nitric oxide synthase (NOS), choline acetyl transferase (ChAT) and calretinin (Calr). Also, we investigated the density and profile of neuronal areas of the NOS-, ChAT- and Calr-ir neurons in the myenteric plexus. Myenteric neurons were labeled using an NADH-diaphorase histochemical staining method.RESULTS: The analysis demonstrated that the P2X2 receptor was expressed in the cytoplasm and in the nuclear and cytoplasmic membranes only in the CG. Neuronal density values (neuron/cm²) decreased 31% (CG: 6579 ± 837; OG: 4556 ± 407) and 16.5% (CG: 7796 ± 528; OG: 6513 ± 610) in the NOS-ir and calretinin-ir neurons in the OG, respectively (P < 0.05). Density of ChAT-ir (CG: 6200 ± 310; OG: 8125 ± 749) neurons significantly increased 31% in the OG (P < 0.05). Neuron size studies demonstrated that NOS, ChAT, and Calr-ir neurons did not differ significantly between the CG and OG groups. The examination of NADH-diaphorase-positive myenteric neurons revealed an overall similarity between the OG and CG.CONCLUSION: Obesity may exert its effects by promoting a decrease in P2X2 receptor expression and modifications in the density of the NOS-ir, ChAT-ir and CalR-ir myenteric neurons.