Orexin contributes to eupnea within a critical period of postnatal development

Orexin neurons are active in wakefulness and mostly silent in sleep. In adult rats and humans, orexin facilitates the hypercapnic ventilatory response but has little effect on resting ventilation. The influence of orexin on breathing in the early postnatal period, and across states of vigilance, have not been investigated. This is relevant as the orexin system may be impaired in Sudden Infant Death Syndrome (SIDS) cases. We addressed three hypotheses: 1) orexin provides a drive to breathe in infancy; 2) the effect of orexin depends on stage of postnatal development; and 3) orexin has a greater influence on breathing in wakefulness compared with sleep. Whole body plethysmography was used to monitor breathing of infant rats at three ages: postnatal days (P) 7-8, 12-14, and 17-19. Respiratory variables were analyzed in wakefulness (W), quiet sleep (QS), and active sleep (AS), following suvorexant (5 mg/kg ip), a dual orexin receptor antagonist, or vehicle (DMSO). Effects of suvorexant on ventilatory responses to graded hypercapnia ([Formula: see text] = 0.02, 0.04, 0.06), hypoxia ([Formula: see text] = 0.10), and hyperoxia ([Formula: see text] = 1.0) at P12-14 were also tested. At P12-14, but not at other ages, suvorexant significantly reduced respiratory frequency in all states, reduced the ventilatory equivalent in QW and QS, and increased [Formula: see text] to ∼5 mmHg. Suvorexant had no effect on ventilatory responses to graded hypercapnia or hypoxia. Hyperoxia eliminated the effects of suvorexant on respiratory frequency at P12-14. Our data suggest that orexin preserves eupneic frequency and ventilation in rats, specifically at ∼2 wk of age, perhaps by facilitating tonic peripheral chemoreflex activity.

Effects of acetylcholine on hypoglossal and C4 nerve activity in brainstem-spinal cord preparations from newborn rat

Effects of acetylcholine (ACh) on respiratory activity have been an intriguing theme especially in relation to central chemoreception and the control of hypoglossal nerve activity. We studied the effects of ACh on hypoglossal and phrenic (C4) nerve activities and inspiratory and pre-inspiratory neurons in the rostral ventrolateral medulla in brainstem-spinal cord preparations from newborn rats. ACh application increased respiratory rhythm, decreased inspiratory hypoglossal and C4 nerve burst amplitude, and enhanced pre-inspiratory hypoglossal activity. ACh induced membrane depolarization of pre-inspiratory neurons that might be involved in facilitation of respiratory rhythm by ACh. Effects of ACh on hypoglossal and C4 nerve activity were partially reversed by a nicotinic receptor blocker, mecamylamine. Further application of a muscarinic receptor antagonist, oxybutynin, resulted in slight increase of hypoglossal (but not C4) burst amplitude. Thus, ACh induced different effects on hypoglossal and C4 nerve activity in the brainstem-spinal cord preparation.

Histamine gustatory aversion in Drosophila melanogaster

Many foods and drinks contain histamine; however, the mechanisms that drive histamine taste perception have not yet been investigated. Here, we use a simple model organism, Drosophila melanogaster, to dissect the molecular sensors required to taste histamine. We first investigated histidine and histamine taste perception by performing a binary food choice assay and electrophysiology to identify essential sensilla for histamine sensing in the labellum. Histamine was found to activate S-type sensilla, which harbor bitter-sensing gustatory receptor neurons. Moreover, unbiased genetic screening for chemoreceptors revealed that a gustatory receptor, GR22e and an ionotropic receptor, IR76b are required for histamine sensing. Ectopic expression of GR22e was sufficient to induce a response in I-type sensilla, which normally do not respond to histamine. Taken together, our findings provide new insights into the mechanisms by which insects discriminate between the toxic histamine and beneficial histidine via their taste receptors.

Altered chemosensitivity to CO2 during exercise

The effect of exercise on chemosensitivity to carbon dioxide (CO2 ) has been controversial. Most studies have been based on rebreathing to alter inspired CO2 which is poorly tolerated in exercise. Instead, inhaling a fixed 3% CO2 from rest to moderate exercise was found to be well tolerated by seven normal subjects enabling CO2 chemosensitivity to be studied with minimal negative reaction. Results showed that chemosensitivity to CO2 following 5-6 min of stimulation was significantly enhanced during mild exercise (p < 0.01). This motivated exploring how much of the dynamic ventilatory response to mild exercise breathing air could be predicted by a model with central and peripheral chemosensitivity. Chemoreceptor stimulation combined with hypercapnia has been associated with long-term facilitation of ventilation (LTF). 3% CO2 inhalation during moderate exercise led to ventilation augmentation consistent with LTF following 6 min of exercise in seven normal human subjects (p < 0.01). Increased ventilation could not be attributed to hypercapnia or metabolic changes. Moderate exercise breathing air resulted in significantly less augmentation. In conclusion, both peripheral and central chemosensitivity to CO2 increased in exercise with the peripheral chemoreceptors playing a dominant role. This separation of central and peripheral contributions was not previously reported. This chemoreceptor stimulation can lead to augmented ventilation consistent with LTF.

Effects of chitosan and erythritol on labellar taste neuron activity, proboscis extension reflex, daily food intake, and mortality of male and female spotted-winged drosophila, Drosophila suzukii

In recent years, there has been interest in reduced-risk materials with insecticidal properties for the invasive pest spotted-wing drosophila, Drosophila suzukii. Here, we compared the peripheral sensitivity (via the tip-recording technique, used to monitor the neural activity of gustatory receptor neurons [GRNs]) and palatability (via the Proboscis Extension Reflex [PER]) of chitosan, a polysaccharide derived from chitin, with that of erythritol, a sugar alcohol, to male and female D. suzukii. Because in some insect species it has previously been shown that chitosan has some insecticidal properties, then treatment effects on mortality rates of male and female D. suzukii were quantified. Physiological recordings from the l-type labellar sensilla showed that erythritol evoked responses from one GRN, while chitosan elicited spiked activity from a second one. The first PER bioassay revealed that the level of response to erythritol increased significantly for males and females as the concentrations increased, and the effect of fly sex was non-significant. The second PER bioassay compared the male and female response to chitosan and erythritol each at 0.125, 0.25, 0.5, 1, and 2% concentrations. The overall female PER to erythritol was significantly greater than that exhibited by males, and no differences were noted between sexes when chitosan was evaluated. These results indicate that chitosan alone can elicit PER responses in adult D. suzukii. In the third experiment, chitosan was toxic to D. suzukii. When combined with sucrose (2%), chitosan elicited high levels (80-100%) of mortality of adult D. suzukii within 3 days, particularly in males. The presence of erythritol did not seem to increase the toxic effect of chitosan.

Paraquat herbicide diminishes chemoreflex sensitivity, induces cardiac autonomic imbalance and impair cardiac function in rats

Paraquat (PQT) herbicide is widely used in agricultural practices despite being highly toxic to humans. It has been proposed that PQT exposure may promote cardiorespiratory impairment. However, the physiological mechanisms involved in cardiorespiratory dysfunction following PQT exposure are poorly known. We aimed to determine the effects of PQT on ventilatory chemoreflex control, cardiac autonomic control, and cardiac function in rats. Male Sprague-Dawley rats received two injections/week of PQT (5 mg·kg-1 ip) for 4 wk. Cardiac function was assessed through echocardiography and pressure-volume loops. Ventilatory function was evaluated using whole body plethysmography. Autonomic control was indirectly evaluated by heart rate variability (HRV). Cardiac electrophysiology (EKG) and exercise capacity were also measured. Four weeks of PQT administration markedly enlarged the heart as evidenced by increases in ventricular volumes and induced cardiac diastolic dysfunction. Indeed, end-diastolic pressure was significantly higher in PQT rats compared with control (2.42 ± 0.90 vs. 4.01 ± 0.92 mmHg, PQT vs. control, P < 0.05). In addition, PQT significantly reduced both the hypercapnic and hypoxic ventilatory chemoreflex response and induced irregular breathing. Also, PQT induced autonomic imbalance and reductions in the amplitude of EKG waves. Finally, PQT administration impaired exercise capacity in rats as evidenced by a ∼2-fold decrease in times-to-fatigue compared with control rats. Our results showed that 4 wk of PQT treatment induces cardiorespiratory dysfunction in rats and suggests that repetitive exposure to PQT may induce harmful mid/long-term cardiovascular, respiratory, and cardiac consequences.NEW & NOREWORTHY Paraquat herbicide is still employed in agricultural practices in several countries. Here, we showed for the first time that 1 mo paraquat administration results in cardiac adverse remodeling, blunts ventilatory chemoreflex drive, and promotes irregular breathing at rest in previously healthy rats. In addition, paraquat exposure induced cardiac autonomic imbalance and cardiac electrophysiology alterations. Lastly, cardiac diastolic dysfunction was overt in rats following 1 mo of paraquat treatment.

Oxidative stress in retrotrapezoid nucleus/parafacial respiratory group and impairment of central chemoreception in rat offspring exposed to maternal cigarette smoke

We have reported that smoking during pregnancy is associated with deficit in neonatal central chemoreception. However, the underlying mechanism is not well clarified. In this study, we developed a rat model of maternal cigarette smoke (CS) exposure. Pregnant rats were exposed to CS during gestational day 1-20. Offspring were studied on postnatal day 2. Reactive oxygen species (ROS) content and expressions of antioxidant proteins in retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG) were examined by fluorogenic dye MitoSOX™ Red and Western blotting, respectively. The response of hypoglossal rootlets discharge to acidification was also detected with micro-injection of H2O2 into RTN/pFRG of offspring brainstem slices in vitro. Results showed that maternal CS exposure led to an increase in ROS production, and brought about decreases in mitochondrial superoxide dismutase and Kelch-like ECH-associated protein-1, and an increase in NF-E2-related factor 2 in offspring RTN/pFRG. Catalase and glutathione reductase expressions were not significantly changed. Moreover, oxidative stress induced by micro-injection of H2O2 into RTN/pFRG in vitro inhibited the discharge response of hypoglossal rootlets to acidification. These findings suggest that maternal CS exposure results in oxidative stress in RTN/pFRG of rat offspring, which might play a role in the impairment of central chemoreception.

A Survey of Chemoreceptive Responses on Different Mosquito Appendages

Research on the functions of insect chemoreceptors have primarily focused on antennae (olfactory receptors) and mouthparts (gustatory receptors). However, chemoreceptive sensilla are also present on other appendages, such as the leg tarsi and the anterior wing margin, and their specific roles in chemoreception and mosquito behavior remain largely unknown. In this study, electrophysiological analyses in an electroantennogram recording format were performed on Aedes aegypti (L., Diptera: Culicidae) antennae, mouthparts, tarsi, and wings during exposure to a variety of insect repellent and attractant compounds. The results provide evidence that the tarsi and wings can sense chemicals in a gaseous form, and that the odors produce differing responses on different appendages. The most consistent and strongest response occurred when exposed to triethylamine (TEA). Antennae and mouthparts showed nearly identical responses pattern to all tested compounds, and their rank orders of effectiveness were similar to those of fore- and mid-leg tarsi. Hindleg tarsi only responded to TEA, indicating that the hind legs are not as chemoreceptive. Wings responded to a range of odorants, but with a different rank order and voltage amplitude. Insights gleaned into the function of these appendages in insect chemoreception are discussed.

Lactate does not activate the carotid body of Wistar rat

The carotid body's glomus cells are the primary sensors of hypoxia in mammals. Previous studies suggested that the glomus cells' hypoxia sensitivity is mediated by lactate in mice. This molecule increases the intracellular [Ca²⁺] and induces exocytosis in glomus cells, activating the carotid sinus nerve (the axons of chemoreceptive petrosal neurons). On the other hand, how lactate affects the activity of carotid body of rats is still unknown. We hypothesized that lactate activates the carotid body of rats. In Wistar rats, we measured the changes in the electrical properties of isolated glomus cells and petrosal chemoreceptive neurons in in situ preparations in response to different concentrations of lactate. Superfusion of both physiological and supraphysiological concentrations of lactate did not affect the membrane conductance and potential of glomus cells. Moreover, lactate injected into the carotid body did not activate the anatomically and physiologically identified chemoreceptive petrosal neurons. We conclude that the carotid body of Wistar rats is not sensitive to lactate.

Mitochondrial KATP channels contribute to the protective effects of hydrogen sulfide against impairment of central chemoreception of rat offspring exposed to maternal cigarette smoke

We previously reported that maternal cigarette smoke (CS) exposure resulted in impairment of central chemoreception and induced mitochondrial dysfunction in offspring parafacial respiratory group (pFRG), the kernel for mammalian central chemoreception. We also found that hydrogen sulfide (H₂S) could attenuate maternal CS exposure-induced impairment of central chemoreception in the rat offspring in vivo. Mitochondrial ATP sensitive potassium (mitoK_ATP) channel has been reported to play a significant role in mitochondrial functions and protect against apoptosis in neurons. Thus, we hypothesize here that mitoK_ATP channel plays a role in the protective effects of H₂S on neonatal central chemoreception in maternal CS-exposed rats. Our findings revealed that pretreatment with NaHS (donor of H₂S, 22.4mM) reversed the central chemosensitivity decreased by maternal CS exposure, and also inhibited cell apoptosis in offspring pFRG, however, 5-HD (blocker of mitoK_ATP channels, 19mM) attenuated the protective effects of NaHS. In addition, NaHS declined pro-apoptotic proteins related to mitochondrial pathway apoptosis in CS rat offspring pFRG, such as Bax, Cytochrome C, caspase9 and caspase3. NaHS or 5-HD alone had no significant effect on above indexes. These results suggest that mitoK_ATP channels play an important role in the protective effect of H₂S against impairment of central chemoreception via anti-apoptosis in pFRG of rat offspring exposed to maternal CS.

Isoflurane inhibits a Kir4.1/5.1-like conductance in neonatal rat brainstem astrocytes and recombinant Kir4.1/5.1 channels in a heterologous expression system

All inhalation anesthetics used clinically including isoflurane can suppress breathing; since this unwanted side effect can persist during the postoperative period and complicate patient recovery, there is a need to better understand how isoflurane affects cellular and molecular elements of respiratory control. Considering that astrocytes in a brainstem region known as the retrotrapezoid nucleus (RTN) contribute to the regulation of breathing in response to changes in CO2/H+ (i.e., function as respiratory chemoreceptors), and astrocytes in other brain regions are highly sensitive to isoflurane, we wanted to determine whether and how RTN astrocytes respond to isoflurane. We found that RTN astrocytes in slices from neonatal rat pups (7-12 days postnatal) respond to clinically relevant levels of isoflurane by inhibition of a CO2/H+-sensitive Kir4.1/5.1-like conductance [50% effective concentration (EC50) = 0.8 mM or ~1.7%]. We went on to confirm that similar levels of isoflurane (EC50 = 0.53 mM or 1.1%) inhibit recombinant Kir4.1/5.1 channels but not homomeric Kir4.1 channels expressed in HEK293 cells. We also found that exposure to CO2/H+ occluded subsequent effects of isoflurane on both native and recombinant Kir4.1/5.1 currents. These results identify Kir4.1/5.1 channels in astrocytes as novel targets of isoflurane. These results suggest astrocyte Kir4.1/5.1 channels contribute to certain aspects of general anesthesia including altered respiratory control.NEW & NOTEWORTHY An unwanted side effect of isoflurane anesthesia is suppression of breathing. Despite this clinical significance, effects of isoflurane on cellular and molecular elements of respiratory control are not well understood. Here, we show that isoflurane inhibits heteromeric Kir4.1/5.1 channels in a mammalian expression system and a Kir4.1/5.1-like conductance in astrocytes in a brainstem respiratory center. These results identify astrocyte Kir4.1/5.1 channels as novel targets of isoflurane and potential substrates for altered respiratory control during isoflurane anesthesia.

Arginine vasotocin impacts chemosensory behavior during social interactions of Anolis carolinensis lizards

In reptiles, arginine vasotocin (AVT) impacts the performance of and response to visual social signals, but whether AVT also operates within the chemosensory system as arginine vasopressin (AVP) does in mammals is unknown, despite social odors being potent modifiers of competitive and appetitive behavior in reptiles. Here, we ask whether elevated levels of exogenous AVT impact rates of chemical display behavior (e.g. tongue flicks) in adult males, and whether conspecific males or females can chemically discriminate between competitor males based on differing levels of exogenous AVT in green anoles (Anolis carolinensis). We injected wild-caught green anole males with either AVT (AVT-Males) or a vehicle control (CON-Males) solution, then presented treated males with a conspecific stimulus (Intruder-Male or Intruder-Female) and filmed 30-minute interactions. We found that AVT-Males were faster than CON-Males to perform a tongue flick to conspecifics, and faster to chemically display toward Intruder-Females, suggesting AVT increased male interest in available chemical information during social encounters. Intruders performed more lip smack behavior when interacting with AVT-Males than with CON-Males, and Intruder-Males performed more tongue flick behavior when interacting with AVT-Males than with CON-Males, suggesting anoles can discriminate between conspecifics based on exogenous AVT levels. We also found a reduction in Intruder movement behavior when Intruders were paired with AVT-Males. This study provides empirical support for AVT-mediated chemosensory behavior in reptilian social interactions, in a microsmatic lizard species, suggesting the mechanism by which mammalian AVP and non-mammalian AVT mediate chemosensory behavior during social interactions may be evolutionarily conserved.

ASIC1a plays a key role in evoking the metabolic component of the exercise pressor reflex in rats

The role of the acid-sensing ion channel 1a (ASIC1a) in evoking the exercise pressor reflex is unknown, despite the fact that ASIC1a is opened by decreases in pH in the physiological range. This fact prompted us to test the hypothesis that ASIC1a plays an important role in evoking the exercise pressor reflex in decerebrated rats with freely perfused hindlimb muscles. To test this hypothesis, we measured the effect of injecting two ASIC1a blockers into the arterial supply of the triceps surae muscles on the reflex pressor responses to four maneuvers, namely 1) static contraction of the triceps surae muscles (i.e., the exercise pressor reflex), 2) calcaneal tendon stretch, 3) intra-arterial injection of lactic acid, and 4) intra-arterial injection of diprotonated phosphate. We found that the 2 ASIC1a blockers, psalmotoxin-1 (200 ng/kg) and mambalgin-1 (6.5 μg/kg), decreased the pressor responses to static contraction as well as the peak pressor responses to injection of lactic acid and diprotonated phosphate. In contrast, neither ASIC1a blocker had any effect on the pressor responses to tendon stretch. Importantly, we found that ASIC1a blockade significantly decreased the pressor response to static contraction after a latency of at least 8 s. Our results support the hypothesis that ASIC1a plays a key role in evoking the metabolic component of the exercise pressor reflex.NEW & NOTEWORTHY The role played by acid-sensing ion channel 1a (ASIC1a) in evoking the exercise pressor reflex remains unknown. In decerebrated rats with freely perfused femoral arteries, blocking ASIC1a with psalmotoxin-1 or mambalgin-1 significantly attenuated the pressor response to static contraction, lactic acid, and diprotonated phosphate injection but had no effect on the pressor response to stretch. We conclude that ASIC1a plays a key role in evoking the exercise pressor reflex by responding to contraction-induced metabolites, such as protons.

Physiologic regulation of heart rate and blood pressure involves connexin 36-containing gap junctions

Chronically elevated sympathetic nervous activity underlies many cardiovascular diseases. Elucidating the mechanisms contributing to sympathetic nervous system output may reveal new avenues of treatment. The contribution of the gap junctional protein connexin 36 (Cx36) to the regulation of sympathetic activity and thus blood pressure and heart rate was determined using a mouse with specific genetic deletion of Cx36. Ablation of the Cx36 protein was confirmed in sympathetic preganglionic neurons of Cx36-knockout (KO) mice. Telemetric analysis from conscious Cx36 KO mice revealed higher variance in heart rate and blood pressure during rest and activity compared to wild-type (WT) mice, and smaller responses to chemoreceptor activation when anesthetized. In the working heart-brain stem preparation of the Cx36-KO mouse, respiratory-coupled sympathetic nerve discharge was attenuated and responses to chemoreceptor stimulation and noxious stimulation were blunted compared to WT mice. Using whole cell patch recordings, sympathetic preganglionic neurons in spinal cord slices of Cx36-KO mice displayed lower levels of spikelet activity compared to WT mice, indicating reduced gap junction coupling between neurons. Cx36 deletion therefore disrupts normal regulation of sympathetic outflow with effects on cardiovascular parameters.-Lall, V. K., Bruce, G., Voytenko, L., Drinkhill, M., Wellershaus, K., Willecke, K., Deuchars, J., Deuchars, S. A. Physiologic regulation of heart rate and blood pressure involves connexin 36-containing gap junctions.

Hydrogen sulfide activates TRPA1 and releases 5-HT from epithelioid cells of the chicken thoracic aorta

Epithelioid cells in the chicken thoracic aorta are chemoreceptor cells that release 5-HT in response to hypoxia. It is likely that these cells play a role in chemoreception similar to that of glomus cells in the carotid bodies of mammals. Recently, H2S was reported to be a key mediator of carotid glomus cell responses to hypoxia. The aim of the present study was to reveal the mechanism of action of H2S on 5-HT outflow from chemoreceptor cells in the chicken thoracic aorta. The 5-HT outflow induced by NaHS, an H2S donor, and Na2S3, a polysulfide, was measured by using a HPLC equipped with an electrochemical detector. NaHS (0.3-3mM) caused a concentration-dependent increase in 5-HT outflow, which was significantly inhibited by the removal of extracellular Ca(2+). 5-HT outflow induced by NaHS (0.3mM) was also significantly inhibited by voltage-dependent L- and N-type Ca(2+) channel blockers and a selective TRPA1 channel blocker. Cinnamaldehyde, a TRPA1 agonist, mimicked the secretory response to H2S. 5-HT outflow induced by Na2S3 (10μM) was also inhibited by the TRPA1 channel blocker. Furthermore, the expression of TRPA1 was localized to 5-HT-containing chemoreceptor cells in the aortic wall. These findings suggest that the activation of TRPA1 and voltage-dependent Ca(2+) channels is involved in H2S-evoked 5-HT release from chemoreceptor cells in the chicken aorta.

Pesticides Drive Stochastic Changes in the Chemoreception and Neurotransmission System of Marine Ectoparasites

Scientific efforts to elucidate the mechanisms of chemical communication between organisms in marine environments are increasing. This study applied novel molecular technology to outline the effects of two xenobiotic drugs, deltamethrin (DM) and azamethiphos (AZA), on the neurotransmission system of the copepod ectoparasite Caligus rogercresseyi. Transcriptome sequencing and bioinformatics analyses were conducted to evaluate treatment effects on the glutamatergic synaptic pathway of the parasite, which is closely related to chemoreception and neurotransmission. After drug treatment with DM or AZA, stochastic mRNA expression patterns of glutamatergic synapse pathway components were observed. Both DM and AZA promoted a down-regulation of the glutamate-ammonia ligase, and DM activated a metabotropic glutamate receptor that is a suggested inhibitor of neurotransmission. Furthermore, the delousing drugs drove complex rearrangements in the distribution of mapped reads for specific metabotropic glutamate receptor domains. This study introduces a novel methodological approach that produces high-quality results from transcriptomic data. Using this approach, DM and AZA were found to alter the expression of numerous mRNAs tightly linked to the glutamatergic signaling pathway. These data suggest possible new targets for xenobiotic drugs that play key roles in the delousing effects of antiparasitics in sea lice.

Bitter-sensitive gustatory receptor neuron responds to chemically diverse insect repellents in the common malaria mosquito Anopheles quadrimaculatus

Female mosquitoes feed on blood from animal hosts to obtain nutritional resources used for egg production. These contacts facilitate the spread of harmful human diseases. Chemical repellents are used to disrupt mosquito host-seeking and blood-feeding behaviors; however, little is known about the gustatory sensitivity of mosquitoes to known repellents. Here, we recorded electrical responses from gustatory receptor neurons (GRNs) housed within the labellar sensilla of female Anopheles quadrimaculatus to N,N-diethyl-3-methylbenzamide (DEET), picaridin, IR3535, 2-undecanone, p-menthane-3,8-diol, geraniol, trans-2-hexen-1-ol, quinine, and quinidine. A bitter-sensitive GRN responded to all tested repellents and quinine, a known feeding deterrent. Responses of the bitter-sensitive neuron to quinine and an isomer, quinidine, did not differ. Delayed bursts of electrical activity were observed in response to continuous stimulation with synthetic repellents at high concentrations. Electrophysiological recordings from bitter-sensitive GRNs associated with mosquito gustatory sensilla represent a convenient model to evaluate candidate repellents.

Ibuprofen Blunts Ventilatory Acclimatization to Sustained Hypoxia in Humans

Ventilatory acclimatization to hypoxia is a time-dependent increase in ventilation and the hypoxic ventilatory response (HVR) that involves neural plasticity in both carotid body chemoreceptors and brainstem respiratory centers. The mechanisms of such plasticity are not completely understood but recent animal studies show it can be blocked by administering ibuprofen, a nonsteroidal anti-inflammatory drug, during chronic hypoxia. We tested the hypothesis that ibuprofen would also block the increase in HVR with chronic hypoxia in humans in 15 healthy men and women using a double-blind, placebo controlled, cross-over trial. The isocapnic HVR was measured with standard methods in subjects treated with ibuprofen (400mg every 8 hrs) or placebo for 48 hours at sea level and 48 hours at high altitude (3,800 m). Subjects returned to sea level for at least 30 days prior to repeating the protocol with the opposite treatment. Ibuprofen significantly decreased the HVR after acclimatization to high altitude compared to placebo but it did not affect ventilation or arterial O₂ saturation breathing ambient air at high altitude. Hence, compensatory responses prevent hypoventilation with decreased isocapnic ventilatory O₂-sensitivity from ibuprofen at this altitude. The effect of ibuprofen to decrease the HVR in humans provides the first experimental evidence that a signaling mechanism described for ventilatory acclimatization to hypoxia in animal models also occurs in people. This establishes a foundation for the future experiments to test the potential role of different mechanisms for neural plasticity and ventilatory acclimatization in humans with chronic hypoxemia from lung disease.

The chemosensitivity of labellar sugar receptor in female Phormia regina is paralleled with ovary maturation: Effects of serotonin

Oogenesis in most adult insects is a nutrient-dependent process involving ingestion of both proteins and carbohydrates that ultimately depends on peripheral input from chemoreceptors. The main goal of this study was to characterize, in the female blowfly Phormia regina, the responsive changes of the labellar chemoreceptors to carbohydrates and proteins in relation to four different stages along the ovarian cycle: (1) immature ovaries, (2) mid-mature ovaries, (3) mature ovaries and ready for egg-laying and (4) post egg-laying ovaries. Then, the possible effects exerted by exogenous serotonin on the chemoreceptor sensitivity profiles were investigated. Our results show that ovary length, width and contraction rate progressively increase from stage 1 to 3, when all these parameters reach their maximum values, before declining in the next stage 4. The sensitivity of the labellar "sugar" chemoreceptors to both sucrose and proteins varies during the ovarian maturation stages, reaching a minimum for sucrose in stage 3, while that to proteins begins. Exogenous 5-HT supply specifically increases the chemoreceptor sensitivity to sugar at the stages 3 and 4, while it does not affect that to proteins. In conclusion, our results provide evidence that in female blowflies the cyclic variations in the sensitivity of the labellar chemosensilla to sugars and proteins are time-related to ovarian development and that during the stages 3 and 4 the responsiveness of the sugar cell to sucrose is under serotonergic control.

Impairment of Central Chemoreception in Neonatal Rats Induced by Maternal Cigarette Smoke Exposure during Pregnancy

It has been postulated that prenatal cigarette smoke exposure (CSE) increases the risk for sudden infant death syndrome. The victims of infant death syndrome suffer from respiratory abnormalities, such as central apnea, diminished chemoreflex and alteration in respiratory pattern during sleep. However, no experimental evidence on CSE model exists to confirm whether prenatal CSE gives rise to reduction of neonatal central chemoreception in in vitro preparations in absence of peripheral sensory feedback. The aim of the present study was to test the hypothesis that maternal CSE during pregnancy depresses central chemoreception of the neonatal rats. The pregnant rats were divided into two groups, control (n = 8) and CSE (n = 8). Experiments were performed on neonatal (0-3days) rat pups. Fictive respiratory activity was monitored by recording the rhythmic discharge from the hypoglossal rootlets of the medullary slices obtained from the neonatal rats. The burst frequency (BF) and integrated amplitude (IA) of the discharge were analyzed. Their responses to acidified artificial cerebrospinal fluid (aCSF) were tested to indicate the change of the central chemosensitivity. Under condition of perfusing with standard aCSF (pH 7.4), no significant difference was detected between the two groups in either BF or IA (P>0.05). Under condition of perfusing with acidified aCSF (pH 7.0), BF was increased and IA was decreased in both groups (P<0.01). However, their change rates in the CSE group were obviously smaller than that in the control group, 66.98 ± 10.11% vs. 143.75 ± 15.41% for BF and -22.38 ± 2.51% vs. -44.90 ± 3.92% for IA (P<0.01). In conclusion, these observations, in a prenatal CSE model, provide important evidence that maternal smoking during pregnancy exerts adverse effects on central chemoreception of neonates.

Neural Control of Breathing and CO2 Homeostasis

Recent advances have clarified how the brain detects CO2 to regulate breathing (central respiratory chemoreception). These mechanisms are reviewed and their significance is presented in the general context of CO2/pH homeostasis through breathing. At rest, respiratory chemoreflexes initiated at peripheral and central sites mediate rapid stabilization of arterial PCO2 and pH. Specific brainstem neurons (e.g., retrotrapezoid nucleus, RTN; serotonergic) are activated by PCO2 and stimulate breathing. RTN neurons detect CO2 via intrinsic proton receptors (TASK-2, GPR4), synaptic input from peripheral chemoreceptors and signals from astrocytes. Respiratory chemoreflexes are arousal state dependent whereas chemoreceptor stimulation produces arousal. When abnormal, these interactions lead to sleep-disordered breathing. During exercise, central command and reflexes from exercising muscles produce the breathing stimulation required to maintain arterial PCO2 and pH despite elevated metabolic activity. The neural circuits underlying central command and muscle afferent control of breathing remain elusive and represent a fertile area for future investigation.

Transient receptor potential cation channels in visceral sensory pathways

The extensive literature on this subject is in direct contrast to the limited range of clinical uses for ligands of the transient receptor potential cation channels (TRPs) in diseases of the viscera. TRPV1 is the most spectacular example of this imbalance, as it is in other systems, but it is nonetheless the only TRP target that is currently targeted clinically in bladder sensory dysfunction. It is not clear why this discrepancy exists, but a likely answer is in the promiscuity of TRPs as sensors and transducers for environmental mechanical and chemical stimuli. This review first describes the different sensory pathways from the viscera, and on which nociceptive and non-nociceptive neurones within these pathways TRPs are expressed. They not only fulfil roles as both mechano- and chemo-sensors on visceral afferents, but also form an effector mechanism for cell activation after activation of GPCR and cytokine receptors. Their role may be markedly changed in diseased states, including chronic pain and inflammation. Pain presents the most obvious potential for further development of therapeutic interventions targeted at TRPs, but forms of inflammation are emerging as likely to benefit also. However, despite much basic research, we are still at the beginning of exploring such potential in visceral sensory pathways.

Defining specificity determinants of cGMP mediated gustatory sensory transduction in Caenorhabditis elegans

Cyclic guanosine monophosphate (cGMP) is a key secondary messenger used in signal transduction in various types of sensory neurons. The importance of cGMP in the ASE gustatory receptor neurons of the nematode Caenorhabditis elegans was deduced by the observation that multiple receptor-type guanylyl cyclases (rGCs), encoded by the gcy genes, and two presently known cyclic nucleotide-gated ion channel subunits, encoded by the tax-2 and tax-4 genes, are essential for ASE-mediated gustatory behavior. We describe here specific mechanistic features of cGMP-mediated signal transduction in the ASE neurons. First, we assess the specificity of the sensory functions of individual rGC proteins. We have previously shown that multiple rGC proteins are expressed in a left/right asymmetric manner in the functionally lateralized ASE neurons and are required to sense distinct salt cues. Through domain swap experiments among three different rGC proteins, we show here that the specificity of individual rGC proteins lies in their extracellular domains and not in their intracellular, signal-transducing domains. Furthermore, we find that rGC proteins are also sufficient to confer salt sensory responses to other neurons. Both findings support the hypothesis that rGC proteins are salt receptor proteins. Second, we identify a novel, likely downstream effector of the rGC proteins in gustatory signal transduction, a previously uncharacterized cyclic nucleotide-gated (CNG) ion channel, encoded by the che-6 locus. che-6 mutants show defects in gustatory sensory transduction that are similar to defects observed in animals lacking the tax-2 and tax-4 CNG channels. In contrast, thermosensory signal transduction, which also requires tax-2 and tax-4, does not require che-6, but requires another CNG, cng-3. We propose that CHE-6 may form together with two other CNG subunits, TAX-2 and TAX-4, a gustatory neuron-specific heteromeric CNG channel complex.

Optogenetic stimulation of c1 and retrotrapezoid nucleus neurons causes sleep state-dependent cardiorespiratory stimulation and arousal in rats

C1 catecholaminergic neurons and neurons of the retrotrapezoid nucleus are integrative nodes within the brain stem network regulating cardiorespiratory reflexes elicited by hypoxia and hypercapnia, stimuli that also produce arousal from sleep. In the present study, Channelrhodopsin-2 was selectively introduced into these neurons with a lentiviral vector to determine whether their selective activation also produces arousal in sleeping rats. Sleep stages were identified from electroencephalographic and neck muscle electromyographic recordings. Breathing was measured using unrestrained whole body plethysmography and blood pressure by telemetry. During nonrapid eye movement sleep, unilateral photostimulation of the C1 region caused arousal in 83.0±14.7% of trials and immediate and intense cardiorespiratory activation. Arousal during photostimulation was also observed during rapid eye movement sleep (41.9±5.6% of trials), but less reliably than during nonrapid eye movement sleep. The cardiorespiratory responses elicited by photostimulation were dramatically smaller during rapid eye movement sleep than nonrapid eye movement sleep or wakefulness. Systemic α1-adrenoreceptor blockade reduced the cardiorespiratory effects of photostimulation but had no effect on the arousal caused by photostimulation during nonrapid eye movement sleep. Postmortem histology showed that neurons expressing Channelrhodopsin 2-mCherry were predominantly catecholaminergic (81%). These results show that selective activation of C1 and retrotrapezoid nucleus neurons produces state-dependent arousal and cardiorespiratory stimulation. These neurons, which are powerfully activated by chemoreceptor stimulation, may contribute to the sleep disruption associated with obstructive sleep apnea.

Identification of a Drosophila glucose receptor using Ca2+ imaging of single chemosensory neurons

Evaluation of food compounds by chemosensory cells is essential for animals to make appropriate feeding decisions. In the fruit fly Drosophila melanogaster, structurally diverse chemicals are detected by multimeric receptors composed of members of a large family of Gustatory receptor (Gr) proteins. Putative sugar and bitter receptors are expressed in distinct subsets of Gustatory Receptor Neurons (GRN) of taste sensilla, thereby assigning distinct taste qualities to sugars and bitter tasting compounds, respectively. Here we report a Ca²⁺ imaging method that allows association of ligand-mediated responses to a single GRN. We find that different sweet neurons exhibit distinct response profiles when stimulated with various sugars, and likewise, different bitter neurons exhibit distinct response profiles when stimulated with a set of bitter chemicals. These observations suggest that individual neurons within a taste modality are represented by distinct repertoires of sweet and bitter taste receptors, respectively. Furthermore, we employed this novel method to identify glucose as the primary ligand for the sugar receptor Gr61a, which is not only expressed in sweet sensing neurons of classical chemosensory sensilla, but also in two supersensitive neurons of atypical taste sensilla. Thus, single cell Ca²⁺ imaging can be employed as a powerful tool to identify ligands for orphan Gr proteins.

Arterial chemoreceptor activation reduces the activity of parapyramidal serotonergic neurons in rats

The parapyramidal (ppy) region targets primarily the intermediolateral cell column and is probably involved in breathing and thermoregulation. In the present study, we tested whether ppy serotonergic neurons respond to activation of central and peripheral chemoreceptors. Bulbospinal ppy neurons (n=30) were recorded extracellularly along with the phrenic nerve activity in urethane/α-chloralose-anesthetized, paralyzed, intact (n=7) or carotid body denervated (n=6) male Wistar rats. In intact animals, most of the ppy neurons were inhibited by hypoxia (n=14 of 19) (8% O2, 30s) (1.5 ± 0.03 vs. control: 2.4 ± 0.2 Hz) or hypercapnia (n=15 of 19) (10% CO2) (1.7 ± 0.1 vs. control: 2.2 ± 0.2 Hz), although some neurons were insensitive to hypoxia (n=3 of 19) or hypercapnia (n=4 of 19). Very few neurons (n=2 of 19) were activated after hypoxia, but not after hypercapnia. In carotid body denervated rats, all the 5HT-ppy neurons (n=11) were insensitive to hypercapnia (2.1 ± 0.1 vs. control: 2.3 ± 0.09 Hz). Biotinamide-labeled cells that were recovered after histochemistry were located in the ppy region. Most labeled cells (90%) showed strong tryptophan hydroxylase immunocytochemical reactivity, indicating that they were serotonergic. The present data reveal that peripheral chemoreceptors reduce the activity of the serotonergic premotor neurons located in the ppy region. It is plausible that the serotonergic neurons of the ppy region could conceivably regulate breathing automaticity and be involved in autonomic regulation.

The neuroprotective and lifespan-extension activities of Damnacanthus officinarum extracts in Caenorhabditis elegans

AIM OF THE STUDY

This study was aimed to evaluate the neuroprotective and anti-aging activity of extracts in Caenorhabditis elegans from the roots and leaves of Damnacanthus officinarum Huang to provide the pharmacological basis in traditional medicine.

MATERIALS AND METHODS

Investigations on the neuroprotective and lifespan activity were carried out, which were observed by utilizing the following models: observing the worms' chemosensory behavior test based on the aversion index in the assay plate, neuroprotective activity of nematode by evaluating the ASH neuron survival and lifespan test in C. elegans.

RESULTS

It has been shown that the ethanol, n-butanol and aqueous extracts in the roots possessed significantly neuroprotective effect both in chemosensory behavior test and ASH neuron survival model. The same extracts in the leaves showed similar activities in two models, but have less potency revealing by the data. Four candidate extracts, possessing excellent neuroprotective activity, extend lifespan in C. elegans. The n-butanol extracts in the root part showed best efficacy among them.

CONCLUSION

The results show the n-butanol and aqueous extracts are the major pharmacological plant extracts. Moreover, the neuroprotective and lifespan-extension activity effects of root extracts are superior to leave extracts, supporting the traditional application of above-ground parts of DOH in treating various diseases associated with brain disorders and anti-aging.

Antagonism of progesterone receptor suppresses carotid body responses to hypoxia and nicotine in rat pups

We tested the hypothesis that antagonism of progesterone receptor (PR) in newborn rats alters carotid body and respiratory responses to hypoxia and nicotinic receptor agonists. Rats were treated with the PR antagonist mifepristone (daily oral gavage 40 μg/g/d) or vehicle between postnatal days 3 and 15. In 11-14-day-old rats, we used in vitro carotid body/carotid sinus nerve preparation and whole body plethysmography to assess the carotid body and ventilatory responses to hypoxia (65 mmHg in vitro, 10% O2 in vivo) and to nicotinic receptor agonists (as an excitatory modulator of carotid body activity-nicotine 100 μM for in vitro studies, and epibatidine 5 μg/kg, i.p., which mainly acts on peripheral nicotinic receptors, for in vivo studies). The carotid body responses to hypoxia and nicotine were drastically reduced by mifepristone. Compared with vehicle, mifepristone-treated rats had a reduced body weight. The ventilatory response to epibatidine was attenuated; however, the hypoxic ventilatory response was similar between vehicle and mifepristone-treated pups. Immunohistochemical staining revealed that mifepristone treatment did not change carotid body morphology. We conclude that PR activity is a critical factor ensuring proper carotid body function in newborn rats.

Acute combined exposure to heavy metals (Zn, Cd) blocks memory formation in a freshwater snail

The effect of heavy metals on species survival is well documented; however, sublethal effects on behaviour and physiology are receiving growing attention. Measurements of changes in activity and respiration are more sensitive to pollutants, and therefore a better early indicator of potentially harmful ecological impacts. We assessed the effect of acute exposure (48 h) to two heavy metals at concentrations below those allowable in municipal drinking water (Zn: 1,100 μg/l; Cd: 3 μg/l) on locomotion and respiration using the freshwater snail, Lymnaea stagnalis. In addition we used a novel assessment method, testing the ability of the snail to form memory in the presence of heavy metals in both intact snails, and also snails that had the osphradial nerve severed which connects a chemosensory organ, the osphradium, to the central nervous system. Aerial respiration and locomotion remained unchanged by acute exposure to heavy metals. There was also no effect on memory formation of these metals when administered alone. However, when snails were exposed to these metals in combination memory formation was blocked. Severing the osphradial nerve prevented the memory blocking effect of Zn and Cd, indicating that the snails are sensing these metals in their environment via the osphradium and responding to them as a stressor. Therefore, assessing the ability of this species to form memory is a more sensitive measure of heavy metal pollution than measures of activity, and indicates that the snails' ability to demonstrate behavioural plasticity may be compromised by the presence of these pollutants.

Sleeping birds do not respond to predator odour

BACKGROUND

During sleep animals are relatively unresponsive and unaware of their environment, and therefore, more exposed to predation risk than alert and awake animals. This vulnerability might influence when, where and how animals sleep depending on the risk of predation perceived before going to sleep. Less clear is whether animals remain sensitive to predation cues when already asleep.

METHODOLOGY/PRINCIPAL FINDINGS

We experimentally tested whether great tits are able to detect the chemical cues of a common nocturnal predator while sleeping. We predicted that birds exposed to the scent of a mammalian predator (mustelid) twice during the night would not go into torpor (which reduces their vigilance) and hence would not reduce their body temperature as much as control birds, exposed to the scent of another mammal that does not represent a danger for the birds (rabbit). As a consequence of the higher body temperature birds exposed to the scent of a predator are predicted to have a higher resting metabolic rate (RMR) and to lose more body mass. In the experiment, all birds decreased their body temperature during the night, but we did not find any influence of the treatment on body temperature, RMR, or body mass.

CONCLUSIONS/SIGNIFICANCE

Our results suggest that birds are not able to detect predator chemical cues while sleeping. As a consequence, antipredatory strategies taken before sleep, such as roosting sites inspection, may be crucial to cope with the vulnerability to predation risk while sleeping.

Tarsal taste neurons of Helicoverpa assulta (Guenée) respond to sugars and amino acids, suggesting a role in feeding and oviposition

Helicoverpa assulta and Helicoverpa armigera are sibling species with different host-plant ranges. We have previously reported electrophysiological and behavioral responses of H.armigera to sugars and amino acids. Here we describe a parallel study performed on H. assulta and compare the results obtained with the two species. In females, fourteen gustatory chemosensilla, identified on one ventrolateral side of the fifth tarsomere were stimulated with sucrose, glucose, fructose, maltose, myo-inositol, and the twenty common amino acids, using the tip-recording technique. The taste receptor neurons in eight chemosensilla were identified sensitive to the sugars, myo-inositol, Lys, Glu, Arg, Trp, and Ser which all induced proboscis extension reflex (PER) when tarsi were stimulated. There was a positive correlation between electrophysiological activities and PER responses triggered by sucrose. No stimulatory effect on oviposition was observed with sugar or amino acid mixtures. In males, three chemosensilla showed responses to the four sugars, but generally weaker than in females. The major difference of the two species was the variety of amino acids triggering electrophysiological responses. The stimulatory effect of sugars and amino acids on H.assulta was also generally weaker than that on H. armigera.

Molecular organization of vomeronasal chemoreception

The vomeronasal organ (VNO) has a key role in mediating the social and defensive responses of many terrestrial vertebrates to species- and sex-specific chemosignals. More than 250 putative pheromone receptors have been identified in the mouse VNO, but the nature of the signals detected by individual VNO receptors has not yet been elucidated. To gain insight into the molecular logic of VNO detection leading to mating, aggression or defensive responses, we sought to uncover the response profiles of individual vomeronasal receptors to a wide range of animal cues. Here we describe the repertoire of behaviourally and physiologically relevant stimuli detected by a large number of individual vomeronasal receptors in mice, and define a global map of vomeronasal signal detection. We demonstrate that the two classes (V1R and V2R) of vomeronasal receptors use fundamentally different strategies to encode chemosensory information, and that distinct receptor subfamilies have evolved towards the specific recognition of certain animal groups or chemical structures. The association of large subsets of vomeronasal receptors with cognate, ethologically and physiologically relevant stimuli establishes the molecular foundation of vomeronasal information coding, and opens new avenues for further investigating the neural mechanisms underlying behaviour specificity.

The disruption of central CO2 chemosensitivity in a mouse model of Rett syndrome

People with Rett syndrome (RTT) have breathing instability in addition to other neuropathological manifestations. The breathing disturbances contribute to the high incidence of unexplained death and abnormal brain development. However, the cellular mechanisms underlying the breathing abnormalities remain unclear. To test the hypothesis that the central CO(2) chemoreception in these people is disrupted, we studied the CO(2) chemosensitivity in a mouse model of RTT. The Mecp2-null mice showed a selective loss of their respiratory response to 1-3% CO(2) (mild hypercapnia), whereas they displayed more regular breathing in response to 6-9% CO(2) (severe hypercapnia). The defect was alleviated with the NE uptake blocker desipramine (10 mg·kg(-1)·day(-1) ip, for 5-7 days). Consistent with the in vivo observations, in vitro studies in brain slices indicated that CO(2) chemosensitivity of locus coeruleus (LC) neurons was impaired in Mecp2-null mice. Two major neuronal pH-sensitive Kir currents that resembled homomeric Kir4.1 and heteromeric Ki4.1/Kir5.1 channels were identified in the LC neurons. The screening of Kir channels with real-time PCR indicated the overexpression of Kir4.1 in the LC region of Mecp2-null mice. In a heterologous expression system, an overexpression of Kir4.1 resulted in a reduction in the pH sensitivity of the heteromeric Kir4.1-Kir5.1 channels. Given that Kir4.1 and Kir5.1 subunits are also expressed in brain stem respiration-related areas, the Kir4.1 overexpression may not allow CO(2) to be detected until hypercapnia becomes severe, leading to periodical hyper- and hypoventilation in Mecp2-null mice and, perhaps, in people with RTT as well.

Developmental changes in the magnitude and activation characteristics of Na(+) currents of petrosal neurons projecting to the carotid body

Carotid bodies mediate hypoxia sensing for the respiratory system and increase their sensitivity in the post-natal period. The present study examined the characteristics and developmental change of fast Na(+) currents of chemoreceptor afferent neurons. Rat carotid bodies (P2-P19) were harvested intact with the petrosal ganglia and whole-cell recordings obtained from petrosal somas whose axons projected to the carotid body. The magnitude of Na(+) current increased in the post-natal period in parallel with increased conduction velocity and somal size. Voltage-dependence of activation significantly shifted towards negative potentials but no significant change occurred in the voltage dependence of inactivation or the slope factors for activation or inactivation. The leftward shift in activation increased slowly or non-inactivating currents around resting potential which increases afferent neuron excitability, a result confirmed in current clamp recordings. These results suggest that a developmental shift in Na(+) current activation plays a role in chemoreceptor maturation by enhancing excitability of the afferent neuron.

A synthetic peptide shows retro- and anterograde neuronal transport before disrupting the chemosensation of plant-pathogenic nematodes

Cyst nematodes are a group of plant pathogens each with a defined host range that cause major losses to crops including potato, soybean and sugar beet. The infective mobile stage hatches from dormant eggs and moves a short distance through the soil to plant roots, which it then invades. A novel strategy for control has recently been proposed in which the plant is able to secrete a peptide which disorientates the infective stage and prevents invasion of the pathogen. This study provides indirect evidence to support the mechanism by which one such peptide disrupts chemosensory function in nematodes. The peptide is a disulphide-constrained 7-mer with the amino acid sequence CTTMHPRLC that binds to nicotinic acetylcholine receptors. A fluorescently tagged version of this peptide with both epifluorescent and confocal microscopy was used to demonstrate that retrograde transport occurs from an aqueous environment along bare-ending primary cilia of chemoreceptive sensilla. The peptide is transported to the cell bodies of these neurons and on to a limited number of other neurons to which they connect. It appears to be localised in both neuronal processes and organelles adjacent to nuclei of some neurons suggesting it could be transported through the Golgi apparatus. The peptide takes 2.5 h to reach the neuronal cell bodies. Comparative studies established that similar but less abundant uptake occurs for Caenorhabditis elegans along its well studied dye-filling chemoreceptive neurons. Incubation in peptide solution or root-exudate from transgenic plants that secrete the peptide disrupted normal orientation of infective cyst nematodes to host root diffusate. The peptide probably undergoes transport along the dye-filling non-cholinergic chemoreceptive neurons to their synapses where it is taken up by the interneurons to which they connect. Coordinated responses to chemoreception are disrupted when the sub-set of cholinergic interneurons secrete the peptide at synapses that have post-synaptic nicotinic acetylcholine receptors.

Astrocytes in the retrotrapezoid nucleus sense H+ by inhibition of a Kir4.1-Kir5.1-like current and may contribute to chemoreception by a purinergic mechanism

Central chemoreception is the mechanism by which CO(2)/pH sensors regulate breathing in response to tissue pH changes. There is compelling evidence that pH-sensitive neurons in the retrotrapezoid nucleus (RTN) are important chemoreceptors. Evidence also indicates that CO(2)/H(+)-evoked adenosine 5'-triphosphate (ATP) release in the RTN, from pH-sensitive astrocytes, contributes to chemoreception. However, mechanism(s) by which RTN astrocytes sense pH is unknown and their contribution to chemoreception remains controversial. Here, we use the brain slice preparation and a combination of patch-clamp electrophysiology and immunohistochemistry to confirm that RTN astrocytes are pH sensitive and to determine mechanisms by which they sense pH. We show that pH-sensitive RTN glia are immunoreactive for aldehyde dehydrogenase 1L1, a marker of astrocytes. In HEPES buffer the pH-sensitive current expressed by RTN astrocytes reversed near E(K(+)) (the equilibrium potential for K(+)) and was inhibited by Ba(2+) and desipramine (blocker of Kir4.1-containing channels), characteristics most consistent with heteromeric Kir4.1-Kir5.1 channels. In bicarbonate buffer, the sodium/bicarbonate cotransporter also contributed to the CO(2)/H(+)-sensitive current in RTN astrocytes. To test the hypothesis that RTN astrocytes contribute to chemoreception by a purinergic mechanism, we used fluorocitrate to selectively depolarize astrocytes while measuring neuronal activity. We found that fluorocitrate increased baseline activity and pH sensitivity of RTN neurons by a P2-receptor-dependent mechanism, suggesting that astrocytes may release ATP to activate RTN chemoreceptors. We also found in bicarbonate but not HEPES buffer that P2-receptor antagonists decreased CO(2) sensitivity of RTN neurons. We conclude that RTN astrocytes sense CO(2)/H(+) in part by inhibition of a Kir4.1-Kir5.1-like current and may provide an excitatory purinergic drive to pH-sensitive neurons.

A new tip-recording method to test scorpion pecten chemoresponses to water-soluble stimulants

On the ventral surface of all scorpions are jointed appendages called pectines, which possess thousands of sensory sensilla. Researchers have electrophysiologically examined these peg sensilla in the past, providing evidence for their chemosensitivity and intra-peg synaptic interactions. However, limits to extracellular recording and chemical stimulation have impeded further research. In this study, we develop and apply a new tip-recording technique for stimulating and recording peg neurons. Relative to previous methods in pecten electrophysiology, this technique allows for very fast and efficient data assembly. Using it, we captured sensilla chemoresponses to aqueous stimulants. We see utility in this method for advancing our understanding of sensory physiology; specifically, we suggest this technique may be useful for physiological assays on scorpion and other arthropod chemoreceptors, such as insect and crustacean gustatory sensilla.

The orexin receptor 1 (OX1R) in the rostral medullary raphe contributes to the hypercapnic chemoreflex in wakefulness, during the active period of the diurnal cycle

It has been shown that orexin plays an important role in the hypercapnic chemoreflex during wakefulness, and OX(1)Rs in the retrotrapezoid nucleus (RTN) participate in this mechanism. We hypothesized that OX(1)R in the rostral medullary raphe (MR) also contributes to the hypercapnic chemoreflex. We studied the effects on ventilation in air and in 7% CO(2) of focal antagonism of OX(1)R in the rostral MR by microdialysis of SB-334867 in rats during wakefulness and NREM sleep, under dark and light periods. During wakefulness in the dark period, but not in the light period, SB-334867 caused a 16% reduction of the hyperventilation induced by 7% CO(2) compared with vehicle. There was no significant effect in sleep. The basal ventilation, body temperature and V(O2) were not affected. No effect was observed in a separate group of animals which had the microdialysis probe misplaced (peri-raphe). We conclude that OX(1)R in the rostral medullary raphe contribute to the hypercapnic chemoreflex in wakefulness, during the dark period in rats.

Transduction mechanism(s) of Na-saccharin in the blowfly Protophormia terraenovae: evidence for potassium and calcium conductance involvement

The study on transduction mechanisms underlying bitter stimuli is a particularly intriguing challenge for taste researchers. The present study investigates, in the labellar chemosensilla of the blowfly Protophormia terraenovae, the transduction mechanism by which saccharin evokes the response of the "deterrent" cell, with particular attention to the contribution of K(+) and Ca(2+) current and the role of cyclic nucleotides, since second messengers modulate Ca(2+), Cl(-) and K(+) currents to different extents. As assessed by extracellular single-sensillum recordings, our results show that the addition of a Ca(2+) chelator such as EGTA or the Ca(2+) current blockers SK&F-96365, Mibefradil, Nifedipine and W-7 decrease the response of the "deterrent" cell to saccharin. A similar decreasing effect was also obtained following the addition of 4-aminopyridine, a K(+) current blocker. On the contrary, the membrane-permeable cyclic nucleotide 8-bromoguanosine 3',5'-cyclic monophosphate (8Br-cGMP) activates this cell and shows an additive effect when presented mixed with saccharin. Our results are consistent with the hypothesis that in the labellar chemosensilla of the blowfly both Ca(2+) and K(+) ions are involved in the transduction mechanism of the "deterrent" cell in response to saccharin. Our results also suggest a possible pathway common to saccharin and 8Br-cGMP.

Systemic inhibition of the Na(+)/H (+) exchanger type 3 in intact rats activates brainstem respiratory regions

Selective inhibition of the Na(+)/H(+) exchanger type 3 (NHE3) increases the firing rate of brainstem ventrolateral CO(2)/H(+) sensitive neurons, resembling the responses evoked by hypercapnic stimuli. In anesthetized animals, NHE3 inhibition has also been shown to stimulate the central chemosensitive drive. We aimed to analyze the respiratory-related brainstem regions affected by NHE3 inhibition in anaesthetized spontaneously-breathing rats with intact peripheral afferents. For that, c-Fos immunopositive cells were counted along the brainstem in rats intravenously infused with the selective NHE3 inhibitor AVE1599. A rostral extension of the ventral respiratory column which includes the pre-Bötzinger complex was activated by the NHE3 inhibitor. In addition, the number of c-Fos positive cells resulted significantly increased in the most rostral extension of the retrotrapezoid nucleus/parapyramidal region. In the pons, the intravenous infusion of AVE1599 activated the lateral parabrachial and Kölliker-Fuse nuclei. Thus, selective NHE3 inhibition in anaesthetized rats activates the respiratory network and evokes a pattern of c-Fos expressing cells similar to that induced by hypercapnia.

Chronic hypoxia suppresses the CO2 response of solitary complex (SC) neurons from rats

We studied the effect of chronic hypobaric hypoxia (CHx; 10-11% O(2)) on the response to hypercapnia (15% CO(2)) of individual solitary complex (SC) neurons from adult rats. We simultaneously measured the intracellular pH and firing rate responses to hypercapnia of SC neurons in superfused medullary slices from control and CHx-adapted adult rats using the blind whole cell patch clamp technique and fluorescence imaging microscopy. We found that CHx caused the percentage of SC neurons inhibited by hypercapnia to significantly increase from about 10% up to about 30%, but did not significantly alter the percentage of SC neurons activated by hypercapnia (50% in control vs. 35% in CHx). Further, the magnitudes of the responses of SC neurons from control rats (chemosensitivity index for activated neurons of 166+/-11% and for inhibited neurons of 45+/-15%) were the same in SC neurons from CHx-adapted rats. This plasticity induced in chemosensitive SC neurons by CHx appears to involve intrinsic changes in neuronal properties since they were the same in synaptic blockade medium.

Carotid chemoreceptor reflexes following dietary salt loading in rats

There is a strong association between salt intake and hypertension. Alterations in baroreceptor activity, which precede and contribute to the elevation in blood pressure, have also been shown to affect chemoreceptor reflex response. Dietary salt loading with 8% sodium chloride was carried out in Sprague Dawley rats aged 8 weeks for a period of 5-6 weeks. Blood pressure was thereafter recorded under anaesthesia from the common carotid artery with a Grass Polygraph 7D model, whereas serum Na and K concentrations were measured using a flame photometer. Salt loading resulted in elevated arterial blood pressure as well as hypokalaemia. Stimulation of the carotid chemoreceptor by injection of sodium dithionite resulted in elevated arterial blood pressure, decreased heart rate and hyperventilation in both control and salt-loaded rats. However, the bradycardic response as estimated by the difference in percentage reduction in heart rate was significantly higher in salt rats (36%) than in the control rats (10%). The results indicate that a high-salt diet results in enhanced bradycardic response to carotid chemoreceptor stimulation and that this observation may be related to the attendant hypokalaemia.

Plant insecticide L-canavanine repels Drosophila via the insect orphan GPCR DmX

An orphan G-protein-coupled gustatory receptor mediates detection of the plant poison L-canavanine in fruit flies.

For all animals, the taste sense is crucial to detect and avoid ingesting toxic molecules. Many toxins are synthesized by plants as a defense mechanism against insect predation. One example of such a natural toxic molecule is l-canavanine, a nonprotein amino acid found in the seeds of many legumes. Whether and how insects are informed that some plants contain l-canavanine remains to be elucidated. In insects, the taste sense relies on gustatory receptors forming the gustatory receptor (Gr) family. Gr proteins display highly divergent sequences, suggesting that they could cover the entire range of tastants. However, one cannot exclude the possibility of evolutionarily independent taste receptors. Here, we show that l-canavanine is not only toxic, but is also a repellent for Drosophila. Using a pharmacogenetic approach, we find that flies sense food containing this poison by the DmX receptor. DmXR is an insect orphan G-protein–coupled receptor that has partially diverged in its ligand binding pocket from the metabotropic glutamate receptor family. Blockade of DmXR function with an antagonist lowers the repulsive effect of l-canavanine. In addition, disruption of the DmXR encoding gene, called mangetout (mtt), suppresses the l-canavanine repellent effect. To avoid the ingestion of l-canavanine, DmXR expression is required in bitter-sensitive gustatory receptor neurons, where it triggers the premature retraction of the proboscis, thus leading to the end of food searching. These findings show that the DmX receptor, which does not belong to the Gr family, fulfills a gustatory function necessary to avoid eating a natural toxin.

Plants evolve to fend off the insects that attack them, often by synthesizing compounds toxic to insects. In turn, insects develop strategies to avoid these plants or resist their toxins. Some plant toxins are nonprotein amino acids. For example, seeds from numerous legumes contain high amounts of l-canavanine, a nonprotein amino acid that is structurally related to l-arginine and is highly toxic to most insects. How insects can detect l-canavanine remains to be elucidated. Using pharmacology, genetics, and behavioral approaches, we show that flies sense l-canavanine using the receptor DmX, an orphan G-protein–coupled receptor that has diverged in its ligand binding pocket from metabotropic glutamate receptors. Disruption of the DmXR gene, called mangetout (mtt), suppresses the l-canavanine repellent effect. DmXR is expressed and required in aversive gustatory receptor neurons, where it triggers the premature retraction of the proboscis, thus leading to the end of food searching. Our results indicate a mechanism by which some insects may detect and avoid a plant toxin.

GABA and nitric oxide in the PVN are involved in arterial pressure control but not in the chemoreflex responses in rats

GABAergic, nitrergic and glutamatergic mechanisms in the PVN on the baseline mean arterial pressure (MAP), heart rate (HR) and on the cardiovascular responses to chemoreflex activation in awake rat were evaluated. Chemoreflex was activated with KCN before and after microinjections into the PVN. Bicuculline into the PVN increased baseline MAP (94+/-3 vs 113+/-5 mmHg) and HR (350+/-9 vs 439+/-18 bpm) but had no effect on the pressor (49+/-5 vs 47+/-6 mmHg) or bradicardic (-213+/-23 vs -256+/-42 bpm) responses (n=7). Kynurenic acid into the PVN (n=6) produced no significant changes in the MAP (98+/-3 vs 100+/-3 mmHg), HR (330+/-5 vs 339+/-12 mmHg) or in the pressor (50+/-4 vs 42+/-4 mmHg) and bradicardic (-252+/-4 vs -285+/-16 bpm) responses to chemoreflex. L-NAME into the PVN (n=8) produced increase in the MAP (94+/-3 vs 113+/-5 mmHg) and HR (350+/-9 vs 439+/-18 bpm) but had no effect on the pressor (52+/-5 vs 47+/-6 mmHg) or bradicardic (-253+/-19 vs -320+/-25 bpm) responses to chemoreflex. We conclude that GABA(A) and nitric oxide in the PVN are involved in the maintenance of the baseline MAP but not in the modulation of the responses to chemoreflex. The results also show that Glutamate receptors in the PVN are not involved in maintenance of the baseline MAP, HR or in the cardiovascular responses to chemoreflex in awake rats.

Lethal and sublethal toxicity of the antifoulant compound Irgarol 1051 to the mud snail Ilyanassa obsoleta

Irgarol 1051 is an algistatic compound used in copper-based antifoulant paints. It is a widespread and persistent pollutant of the estuarine environment. Ilyanassa obsoleta, the Eastern mud snail, is a common intertidal gastropod that inhabits mud flats and salt marshes along the east coast of North America. It is an important inhabitant of the estuarine environment; contributing to nutrient regeneration and regulating microbial processes in the sediments. The toxicity of irgarol to estuarine gastropods has not been previously examined, although they have the potential to be exposed to antifoulants through both aqueous and sediment routes. The objectives of this study were to evaluate irgarol's effects on I. obsoleta survival, reproductive status (imposex occurrence and testosterone levels), chemoreceptive function, and cellular respiration (cytochrome-c oxidase activity). Irgarol was moderately toxic to I. obsoleta; adult aqueous 96-h LC(50) = 3.73 mg/L, larval aqueous 96-h LC(50) = 3.16 mg/L, and adult sediment 10-day LC(50) = 12.21 mg/kg. Larval snails were not significantly more sensitive to irgarol than adult snails. A chronic 45-day aqueous irgarol exposure (0.005-2.5 mg/L) did not induce imposex or affect free-testosterone levels. The 45-day chronic LC(50 )of 1.88 mg/L was significantly lower than the 96-h acute value. A 96-h acute aqueous irgarol exposure (0.375-1.5 mg/L) caused a decrease in normal response to chemosensory cues such as the presence of food or predators. There was a significant increase in cytochrome-c oxidase activity at 2.5 mg/L, which might indicate irgarol's disruption of the mitochondrial membrane and subsequently ATP synthesis. Although the toxicity values determined for I. obsoleta exceeded irgarol concentrations measured in surface waters, results from this toxicity assessment will provide valuable information to environmental resource managers faced with decisions regarding the use and regulation of antifoulant paints in the coastal zone.

Heme oxygenase is necessary for the excitatory response of cultured neonatal rat rostral ventrolateral medulla neurons to hypoxia

Heme oxygenase has been linked to the oxygen-sensing function of the carotid body, pulmonary vasculature, cerebral vasculature, and airway smooth muscle. We have shown previously that the cardiorespiratory regions of the rostral ventrolateral medulla are excited by local hypoxia and that heme oxygenase-2 (HO-2) is expressed in the hypoxia-chemosensitive regions of the rostral ventrolateral medulla (RVLM), the respiratory pre-Bötzinger complex, and C1 sympathoexcitatory region. To determine whether heme oxygenase is necessary for the hypoxic-excitation of dissociated RVLM neurons (P1) cultured on confluent medullary astrocytes (P5), we examined their electrophysiological responses to hypoxia (NaCN and low Po(2)) using the whole-cell perforated patch clamp technique before and after blocking heme oxygenase with tin protoporphyrin-IX (SnPP-IX). Following the electrophysiological recording, immunocytochemistry was performed on the recorded neuron to correlate the electrophysiological response to hypoxia with the expression of HO-2. We found that the responses to NaCN and hypoxia were similar. RVLM neurons responded to NaCN and low Po(2) with either depolarization or hyperpolarization and SnPP-IX blocked the depolarization response of hypoxia-excited neurons to both NaCN and low Po(2) but had no effect on the hyperpolarization response of hypoxia-depressed neurons. Consistent with this observation, HO-2 expression was present only in the hypoxia-excited neurons. We conclude that RVLM neurons are excited by hypoxia via a heme oxygenase-dependent mechanism.

Bicarbonate-regulated soluble adenylyl cyclase (sAC) mRNA expression and activity in peripheral chemoreceptors

Peripheral arterial chemoreceptors in the carotid body (CB) are modulated by pH/CO(2). Soluble adenylyl cyclase (sAC) is directly stimulated by bicarbonate ions (HCO(3)). Because CO(2)/HCO(3) mediates depolarization in chemoreceptors, we hypothesized that sAC mRNA would be expressed in the CB, and its expression and function would be regulated by CO(2)/HCO(3).Sprague-Dawley rats at postnatal days 16-17 were used to compare sAC mRNA gene expression between CB and non-chemosensitive tissues: superior cervical (SCG), petrosal (PG) and nodose ganglia (NG) by quantitative real time-PCR. Rat sAC gene expression was standardized to the expression of GAPDH (housekeeping gene) and the data were analyzed with the Pfaffl method. Gene and protein expression, and sAC regulation in the testis was used as a positive control. To determine the regulation of sAC mRNA expression and activity, all tissues were exposed to increasing concentrations of bicarbonate (0, 24, 44 mM, titrated with CO(2) and maintained a constant pH of 7.40).

RESULTS

sAC mRNA expression was between 2-11% of CB expression in the SCG, PG and NG. Furthermore, only in the CB did HCO(3) upregulate sAC gene expression and increase cAMP levels.

CONCLUSION

sAC mRNA and protein expression is present in peripheral arterial chemoreceptors and non-chemoreceptors. In the CB, CO(2)/HCO(3) not only activated sAC but also regulated its expression, suggesting that sAC may be involved in the regulation of cAMP levels in response to hyper/hypocapnia.