PHOX2B is a transcription factor essential for the development of different classes of neurons in the central and peripheral nervous system. Heterozygous mutations in the PHOX2B coding region are responsible for the occurrence of Congenital Central Hypoventilation Syndrome (CCHS), a rare neurological disorder characterised by inadequate chemosensitivity and life-threatening sleep-related hypoventilation. Animal studies suggest that chemoreflex defects are caused in part by the improper development or function of PHOX2B expressing neurons in the retrotrapezoid nucleus (RTN), a central hub for CO2 chemosensitivity. Although the function of PHOX2B in rodents during development is well established, its role in the adult respiratory network remains unknown. In this study, we investigated whether reduction in PHOX2B expression in chemosensitive neuromedin-B (NMB) expressing neurons in the RTN altered respiratory function. Four weeks following local RTN injection of a lentiviral vector expressing the short hairpin RNA (shRNA) targeting Phox2b mRNA, a reduction of PHOX2B expression was observed in Nmb neurons compared to both naive rats and rats injected with the non-target shRNA. PHOX2B knockdown did not affect breathing in room air or under hypoxia, but ventilation was significantly impaired during hypercapnia. PHOX2B knockdown did not alter Nmb expression but it was associated with reduced expression of both Task2 and Gpr4, two CO2/pH sensors in the RTN. We conclude that PHOX2B in the adult brain has an important role in CO2 chemoreception and reduced PHOX2B expression in CCHS beyond the developmental period may contribute to the impaired central chemoreflex function.

The Olfr78 gene encodes a G-protein-coupled olfactory receptor that is expressed in several ectopic sites. Olfr78 is one of the most abundant mRNA species in carotid body (CB) glomus cells. These cells are the prototypical oxygen (O2) sensitive arterial chemoreceptors, which, in response to lowered O2 tension (hypoxia), activate the respiratory centers to induce hyperventilation. It has been proposed that Olfr78 is a lactate receptor and that glomus cell activation by the increase in blood lactate mediates the hypoxic ventilatory response (HVR). However, this proposal has been challenged by several groups showing that Olfr78 is not a physiologically relevant lactate receptor and that the O2-based regulation of breathing is not affected in constitutive Olfr78 knockout mice. In another study, constitutive Olfr78 knockout mice were reported to have altered systemic and CB responses to mild hypoxia. To further characterize the functional role of Olfr78 in CB glomus cells, we here generated a conditional Olfr78 knockout mouse strain and then restricted the knockout to glomus cells and other catecholaminergic cells by crossing with a tyrosine hydroxylase-specific Cre driver strain (TH-Olfr78 KO mice). We find that TH-Olfr78 KO mice have a normal HVR. Interestingly, glomus cells of TH-Olfr78 KO mice exhibit molecular and electrophysiological alterations as well as a reduced dopamine content in secretory vesicles and neurosecretory activity. These functional characteristics resemble those of CB neuroblasts in wild-type mice. We suggest that, although Olfr78 is not essential for CB O2 sensing, activation of Olfr78-dependent pathways is required for maturation of glomus cells.

The house fly, Musca domestica L., is a significant human and livestock pest. Experiments used female adult house flies glued onto toothpicks for controlled exposure of their tarsi alone (tarsal assay) or their tarsi and proboscis (proboscis assay) with a sucrose solution containing imidacloprid at either a low (10 µg/mL) or high (4000 µg/mL) concentration. Proboscis extension response (PER) assays were used to characterize the response of imidacloprid-susceptible and behaviorally resistant house fly strains to contact with sucrose solutions containing either a low or high concentration of imidacloprid. In each assay, 150 female flies from each fly strain were individually exposed to sucrose solutions containing either a low or high concentration of imidacloprid by deliberate contact of the fly tarsi to the test solution. The PER for each fly was subsequently recorded at 0, 2, and 10 s following the initial tarsal contact. A significant and rapid reduction in PER was observed only for the behaviorally resistant fly strain and only following contact by the flies\' proboscis with the sucrose solution containing the high imidacloprid concentration. The results suggest that chemoreceptors on the fly labellum or internally on the pharyngeal taste organs are involved in the detection of imidacloprid and discrimination of the concentration, resulting in an avoidance behavior (proboscis retraction) only when imidacloprid is at sufficient concentration. Further research is needed to identify the specific receptor(s) responsible for imidacloprid detection.

The infective juveniles (IJs) of entomopathogenic nematode (EPN) Heterorhabditis bacteriophora find and infect their host insects in heterogeneous soil ecosystems by sensing a universal host cue (CO2) or insect/plant-derived odorants, which bind to various sensory receptors, including G protein-coupled receptors (GPCRs). Nematode chemosensory GPCRs (NemChRs) bind to a diverse set of ligands, including odor molecules. However, there is a lack of information on the NemChRs in EPNs. Here we identified 21 GPCRs in the H. bacteriophora genome sequence in a triphasic manner, combining various transmembrane detectors and GPCR predictors based on different algorithms, and considering inherent properties of GPCRs. The pipeline was validated by reciprocal BLAST, InterProscan, GPCR-CA, and NCBI CDD search. Functional classification of predicted GPCRs using Pfam revealed the presence of four NemChRs. Additionally, GPCRs were classified into various families based on the reciprocal BLAST approach into a frizzled type, a secretin type, and 19 rhodopsin types of GPCRs. Gi/o is the most abundant kind of G-protein, having a coupling specificity to all the fetched GPCRs. As the 21 GPCRs identified are expected to play a crucial role in the host-seeking behavior, these might be targeted to develop novel insect-pest management strategies by tweaking EPN IJ behavior, or to design novel anthelminthic drugs. Our new and stringent GPCR detection pipeline may also be used to identify GPCRs from the genome sequence of other organisms.

Sensory perception of chemical threats coming from an organism\'s environment relies on the coordination of numerous receptors and cell types. In many cases, the physiological processes responsible for driving behavioral responses to chemical cues are poorly understood. Here, we investigated the physiological response of fish to an unpalatable compound, formoside, which is employed as a chemical defense by marine sponges. Construction of fluorescent probe derivatives of formoside allowed visualization of this chemical defense molecule in vivo, interacting with the cells and tissues of the early larvae of a model predator, the zebrafish (Danio rerio). This revealed the precise chemosensory structures targeted by formoside to be in the taste buds and olfactory epithelium of developing zebrafish. Mechanosensory neuromasts were also targeted. This study supports the involvement of a previously identified co-receptor in detection of the chemical defense and provides a springboard for the long-term goal of identification of the cellular receptor of formoside. Extension of this approach to other predators and chemical defenses may provide insight into common mechanisms of chemoreception by predators as well as common strategies of chemical defense employed by prey.

Chemical communication is widespread among insects and exploited to adjust their behavior, such as food and habitat seeking and preferences, recruitment, defense, and mate attraction. Recently, many studies have revealed that microbial symbionts could regulate host chemical communication by affecting the synthesis and perception of insect semiochemicals. In this paper, we review recent studies of the influence of microbial symbionts on insect chemoreception. Microbial symbionts may influence insect sensitivity to semiochemicals by regulating the synthesis of odorant-binding proteins or chemosensory proteins and olfactory or gustatory receptors and regulating host neurotransmission, thereby adjusting insect behavior. The manipulation of insect chemosensory behavior by microbial symbionts is conducive to their proliferation and dispersal and provides the impetus for insects to change their feeding habits and aggregation and dispersal behavior, which contributes to population differentiation in insects. Future research is necessary to reveal the material and information exchange between both partners to improve our comprehension of the evolution of chemoreception in insects. Manipulating insect chemoreception physiology by inoculating them with microbes could be utilized as a potential approach to managing insect populations.

The sensing of chemical cues is essential for several aspects of bivalve biology, such as the detection of food and pheromones. However, little is known about chemical communication systems in bivalves or the possible role of the osphradium as a chemosensory organ. To address this, we adapted an electrophysiological technique extensively used in vertebrates-the electro-olfactogram-to record from the osphradium in the Pacific oyster, Magallana gigas. This technique was validated using amino acids as stimulants. The osphradium proved to be sensitive to most proteinogenic L-amino acids tested, evoking tonic, negative, concentration-dependent \'electro-osphradiogram\' (EOsG) voltage responses, with thresholds of detection in the range of 10- 6 to 10- 5 M. Conversely, it was insensitive to L-arginine and L-glutamic acid. The current study supports the hypothesis that the osphradium is, indeed, a chemosensory organ. The \'electro-osphradiogram\' may prove to be a powerful tool in the isolation and characterization of pheromones and other important chemical cues in bivalve biology.

Epilepsy is one of the most common chronic neurologic diseases, with a prevalence of 1% in the US population. Many people with epilepsy live normal lives, but are at risk of sudden unexpected death in epilepsy (SUDEP). This mysterious comorbidity of epilepsy causes premature death in 17%-50% of those with epilepsy. Most SUDEP occurs after a generalized seizure, and patients are typically found in bed in the prone position. Until recently, it was thought that SUDEP was due to cardiovascular failure, but patients who died while being monitored in hospital epilepsy units revealed that most SUDEP is due to postictal central apnea. Some cases may occur when seizures invade the amygdala and activate projections to the brainstem. Evidence suggests that the pathophysiology is linked to defects in the serotonin system and central CO2 chemoreception, and that there is considerable overlap with mechanisms thought to be involved in sudden infant death syndrome (SIDS). Future work is needed to identify biomarkers for patients at highest risk, improve ascertainment, develop methods to alert caregivers when SUDEP is imminent, and find effective approaches to prevent these fatal events.

Despite the large body of work describing vertebrate ventilatory responses to hypoxia, remarkably little is known about the receptors and afferent pathways mediating these responses in fishes. In this review, we aim to summarize all receptor types to date implicated in the neurotransmission or neuromodulation associated with O2 sensing in the gills of fish. This includes serotonergic, cholinergic, purinergic, and dopaminergic receptor subtypes. Recent transcriptomic analysis of the gills of zebrafish using single-cell RNA sequencing has begun to elucidate specific receptor targets in the gill; however, the absence of receptor characterization at the cellular level in the gill remains a major limitation in understanding the neurochemical control of hypoxia signalling.

Histaminergic neurons of the tuberomammillary nucleus (TMN) are pH sensitive and contribute to CO2/H+-dependent behaviors including arousal and respiratory activity. TMN neurons project to several respiratory centers including the ventral parafacial region (pF), where the chemosensitive retrotrapezoid (RTN) neurons are located, and since RTN neurons are an important source of CO2/H+-dependent respiratory drive, we wondered whether histamine contributes to RTN chemoreception. To test this, we characterized effects of histamine on mean arterial pressure (MAP) and diaphragm muscle activity (DIAEMG) in urethane-anesthetized, vagotomized, and artificially ventilated male Wistar rats. Unilateral injection of histamine in the pF (25 mM) increased DIAEMG amplitude without changing DIAEMG frequency and MAP. Bilateral injections of the H1 receptor antagonist diphenhydramine hydrochloride (DPH; 0.5 mM) into the pF decreased baseline DIAEMG amplitude and frequency and MAP. Despite the strong inhibitory effect of DPH on baseline breathing, the hypercapnic ventilatory response was preserved under these experimental conditions. At the cellular level, chemosensitive RTN neurons showed a dose-dependent excitatory response to histamine that was blunted by DPH and mimicked by H1 receptor agonist 2-pyridylethylamine dihydrochloride (2PYEA) both under control conditions and when fast neurotransmitter receptors were blocked. We also tested effects of 2PYEA in the presence of serotonin, another wake-on neurotransmitter that activates RTN chemoreceptors partly by activation of Gq-coupled receptors. We found that the response to 2PYEA was diminished in serotonin, suggesting that RTN neurons have a limited capacity to respond to multiple Gq-coupled modulators. These results suggest that histamine can modulate breathing at the pF level by a mechanism involving H1 receptors.NEW & NOTEWORTHY Histamine/H1 receptor signaling activates retrotrapezoid (RTN) neurons under control conditions and to a lesser extent in the presence of serotonin. These results suggest that RTN neurons have a limited capacity to respond to simultaneous activation of multiple Gq-coupled receptors.