Chemoreceptor Cells Research Articles

Chemosensory stimuli for the walking legs of the crayfish Procambarus clarkii.

Studies of chemoreception in crustaceans have shown that flesh-eating species can detect amino acids, nucleotides and derivatives, and amines, while most herbivorous and omnivorous species are additionally sensitive to carbohydrates. We used extracellular recording techniques to evaluate the effectiveness of a range of nitrogen-containing compounds (mostly amino acids), bile acids, and carbohydrates in stimulating chemoreceptor cells present in the second and third pereopods of the omnivorous crayfish Procambarus clarkii. When applied at a final concentration of approximately 100 microM, effective stimuli were trehalose, leucine, cellobiose, glycine, sucrose, maltose, and ammonium (from most to least effective). The other 17 compounds tested, many chosen because they are potent stimuli for other crustaceans, were ineffective stimuli for P. clarkii. Concentration-response functions were determined for three single cells that were sensitive to ammonium, for five multiunit fibers sensitive to glycine, and for six multiunit fibers sensitive to leucine. Thresholds ranged from 10 nM to 10 micro, functions were generally linear when plotted against the log of the stimulus concentration, and there was little evidence of saturation. While P. clarkii is sensitive to only seven of 24 compounds tested, the compounds that proved stimulatory should serve as cues for location and identification of food items preferred by this omnivorous crustacean.

Nitric oxide enhances Ca(2+)-dependent K(+) channel activity in rat carotid body cells.

The nitric oxide (NO) donor S-nitroso-acetylpenicillamine (SNAP) enhanced Ca(2+)-dependent K(+) channel activity in rat carotid body chemoreceptor cells. Ca(2+)-dependent K(+) channel activity was enhanced by SNAP in 38% (whole-cell configuration) and 67% (cell-attached mode) of the cells tested and was not affected by intracellular Ca(2+) chelation with BAPTA-AM. Enhancement of Ca(2+)-dependent K(+) channel activity by SNAP was blocked by the cGMP-dependent protein kinase G inhibitor 8-[(4-chlorophenyl)thio]-guanosine 3',5'-cyclic monophosphothioate Rp diastereomer (Rp-8-pCPT-cGMPS). NO thus enhances Ca(2+)-dependent K(+) channel activity through cGMP-dependent protein kinase G. The NO-mediated increase in Ca(2+)-dependent K(+) channel activity is likely to alter the function of carotid body chemoreceptor cells and could explain the decreased chemosensitivity of the carotid body in response to NO released from efferent nerves or vascular endothelial cells.

Chronic hypoxia remodels voltage-gated Ca 2+ entry in a human airway chemoreceptor cell line

Arterial and airway chemoreceptors respond to acute hypoxia by depolarizing, thereby activating voltage-gated Ca 2+ channels and so permitting Ca 2+ entry to trigger transmitter release. Following periods of prolonged hypoxia, these cells undergo a form of remodelling which involves altered expression of ion channels. Here, we use microspectrofluorimetric recordings of voltage-gated Ca 2+ entry (activated by exposure of cells to 50 mM K +) to show that chronic hypoxia suppresses such Ca 2+ entry in model airway chemoreceptor (H146) cells. Furthermore, Ca 2+ entry via L-type channels is suppressed, whilst entry via N-type channels is greatly enhanced. The suppressed response, together with dramatic remodelling of routes available for voltage-gated Ca 2+ entry, is likely to alter significantly the acute O 2 sensing properties of these cells.

Reduced to oxidized glutathione ratios and oxygen sensing in calf and rabbit carotid body chemoreceptor cells.

1. The aim of this work was to test the redox hypotheses of O(2) chemoreception in the carotid body (CB). They postulate that hypoxia alters the levels of reactive oxygen species (ROS) and the ratio of reduced to oxidized glutathione (GSH/GSSG), causing modifications to the sulfhydryl groups/disulfide bonds of K+ channel proteins, which leads to the activation of chemoreceptor cells. 2. We found that the GSH/GSSG ratio in normoxic calf CB (30.14 +/- 4.67; n = 12) and hypoxic organs (33.03 +/- 6.88; n = 10), and the absolute levels of total glutathione (0.71 +/- 0.07 nmol (mg tissue)(-1), normoxia vs. 0.76 +/- 0.07 nmol (mg tissue)(-1), hypoxia) were not statistically different. 3. N-Acetylcysteine (2 mM; NAC), a precursor of glutathione and ROS scavenger, increased normoxic glutathione levels to 1.03 +/- 0.06 nmol (mg tissue)(-1) (P < 0.02) and GSH/GSSG ratios to 59.05 +/- 5.05 (P < 0.001). 4. NAC (20 microM-10 mM) did not activate or inhibit chemoreceptor cells as it did not alter the normoxic or the hypoxic release of (3)H-catecholamines ((3)H-CAs) from rabbit and calf CBs whose CA deposits had been labelled by prior incubation with the natural CA precursor (3)H-tyrosine. 5. NAC (2 mM) was equally ineffective in altering the release of (3)H-CAs induced by stimuli (high external K+ and ionomycin) that bypass the initial steps of the hypoxic cascade of activation of chemoreceptor cells, thereby excluding the possibility that the lack of effect of NAC on normoxic and hypoxic release of (3)H-CAs results from a concomitant alteration of Ca(2+) channels or of the exocytotic machinery. 6. The present findings do not support the contention that O(2) chemoreception in the CB is linked to variations in the GSH/GSSG quotient as the redox models propose.

Anesthetic-sensitive 2P domain K+ channels.

Anesthetic-sensitive 2P domain K+ channels.

A pH-sensitive chloride current in the chemoreceptor cell of rat carotid body.

1. Cardiorespiratory response to acidosis is initiated by the carotid body. 2. The direct effect of extracellular pH (pH(o)) on the chloride currents of isolated chemoreceptor cells of the rat carotid body was investigated using the whole-cell patch-clamp technique. 3. On applying intra- and extracellular solutions with a symmetrical high-Cl(-) content and with the monovalent cations replaced with membrane-impermeant ones, an inwardly rectifying Cl(-) current was found. 4. The current activated slowly and did not display any time-dependent inactivation. Current activation was present at membrane potentials negative to 0 mV (pH(o) = 7.0). 5. The current was activated by extracellular acidosis and inhibited by alkalosis in the physiologically relevant pH range of 7.0-7.8. 6. The current was reduced by 0.1 mM Cd2+ to the level of the leak current and by 1 mM anthracene-9-carboxylic acid (9-AC) to about 40 %, while 0.1 mM Ba2+ had no effect. 7. Application of 1 mM 9-AC caused a slow but statistically significant increase in the resting pH(i) (from a mean of 7.29 to 7.37 in 5 min) in clusters of chemoreceptor cells in CO(2)/HCO3(-)-buffered media as measured with carboxy-SNARF-1. 8. When membrane potential changes were estimated in the cell-attached mode, 1 mM 9-AC hyperpolarized three out of five tested cells (by 14 mV in average) incubated in CO(2)/HCO3(-)-buffered media. 9. In summary, chemoreceptor cells express an inwardly rectifying Cl(-) current, which is directly regulated by pH(o). The current may participate in intracellular acidification and membrane depolarization during acidic challenge.

Gustatory organs of Drosophila melanogaster: fine structure and expression of the putative odorant-binding protein PBPRP2.

In Drosophila, as in most insects, gustation is mediated by sensory hairs located on the external and internal parts of the proboscis and on the legs and wings. We describe in detail the organization and ultrastructure of the gustatory sensilla on the labellum and legs and the distribution of PBPRP2, a putative odorant-binding protein, in the gustatory organs of Drosophila. The labellum carries two kinds of sensilla: taste bristles and taste pegs. The former have the typical morphology of gustatory sensilla and can be further subdivided into three morphological subtypes, each with a stereotyped distribution and innervation. Taste pegs have a unique morphology and are innervated by two receptor cells: one mechanoreceptor and the other a putative chemoreceptor cell. PBPRP2 is abundantly expressed in all adult gustatory organs on labellum, legs, and wings and in the internal taste organs on the proboscis. In contrast to olfactory organs, where PBPRP2 is expressed in the epidermis, this protein is absent from the epidermis of labial palps and legs. In the taste bristles of the labellum and legs, PBPRP2 is localized in the crescent-shaped lumen of the sensilla, and not in the lumen where the dendrites of the gustatory neurons are found, making a function in stimulus transport unlikely in these sensilla. In contrast, PBPRP2 in peg sensilla is expressed in the inner sensillum-lymph cavity and is in contact with the dendrites. Thus, PBPRP2 could be involved as a carrier for hydrophobic ligands, e.g., bitter tastants, in these sensilla.

NADPH Oxidase Does Not Account Fully for O2-Sensing in Model Airway Chemoreceptor Cells

A key feature of O2 sensing by chemoreceptor tissues is the hypoxic inhibition of K+ channels. However, mechanisms coupling a fall of pO2 to channel closure differ between tissues: O2 regulation of K+ channels in chemoreceptive neuroepithelial bodies and their immortal counterparts, H146 cells, involves altered reactive oxygen species generation by NADPH oxidase. In contrast, this enzyme complex is not involved in O2 sensing by the carotid body and pulmonary vasculature. Here, we provide pharmacological evidence to support a role for NADPH oxidase in hypoxic inhibition of K+ currents in H146 cells. Two structurally unrelated NADPH oxidase inhibitors, diphenylene iodonium and phenylarsine oxide, suppressed hypoxic inhibition of K+ currents recorded using the patch-clamp technique. Most importantly, however, neither inhibitor fully blocked this response. Our findings provide the first evidence that multiple mechanisms may coexist within a specific cell type to account for hypoxic suppression of K+ channel activity.

Heterogeneity of neuronal nicotinic acetylcholine receptors in 5-HT-containing chemoreceptor cells of the chicken aorta.

The effects of nicotinic agonists and antagonists on whole-cell currents and 5-hydroxytryptamine (5-HT) release were studied in order to characterize nicotinic ACh receptors on the 5-HT-containing chemoreceptor cells of the chicken aorta. ACh, nicotine and dimethylphenylpiperazinium (DMPP) evoked concentration-dependent inward currents accompanied by increases in current noise at a holding potential of -70 mV. The peak amplitude of the current response to DMPP was 50% larger than that to either nicotine or ACH: Hexamethonium, alpha - bungarotoxin (alpha - BTX) and methyllycaconitine decreased nicotine-induced inward currents in a concentration-dependent manner. Although hexamethonium (0.1 mM) abolished the current response to nicotine (30 microM), a high concentration (1 microM) of alpha - BTX decreased it only by about 30% of the control response. Methyllycaconitine (0.1 microM) decreased the current response to nicotine to the same extent as did alpha - BTX whilst a high concentration (10 microM) abolished the response. ACh, nicotine and DMPP caused concentration-dependent increases in 5-HT output from the thoracic aorta which effect was blocked by hexamethonium (0.1 mM). Pre-treatment with alpha - BTX (1 microM) for 30 min reduced the output of 5-HT induced by ACh to 70% of the control response. It is suggested that neuronal nicotinic ACh receptors, sensitive and insensitive to alpha - BTX, are present on the chemoreceptor cells of the chicken aorta, the activation of which causes the release of 5-HT.

Calcium handling by the cat carotid body - a pyroantimonate study

Subcellular regulation mechanisms of calcium concentrations related to oxygen sensing in the carotid body are unclear. In the present study, we investigated the ultrastructural distribution patterns of calcium in carotid body cells and its changes evoked by hypoxia. Carotid bodies were dissected from anesthetized cats exposed in vivo to normoxic or acute hypoxic conditions. We used the oxalate-pyroantimonate technique that yields an electron-opaque calcium precipitate. X-ray microanalysis and appropriate controls confirmed the presence of calcium in the precipitate. Calcium precipitates were found in all types of cells in carotid body parenchyma: chemoreceptor cells, sustentacular cells, and nerve endings. In normoxic chemoreceptor cells, the precipitate was localized in dense core vesicles, mitochondria, and nuclei, but rarely in the cytoplasm. The most apparent effect of hypoxia was disappearance of the precipitate from dense core vesicles and was associated with its appearance in the cytoplasm. The amount of precipitate throughout the carotid body parenchyma was decreased overall due to hypoxia. These results indicate the involvement of subcellular calcium trafficking in hypoxia-sensing in the carotid body. The redistribution pattern of granular calcium deposits from organelles to the cytoplasm of chemoreceptor cells agrees with biochemical data of calcium release from intracellular stores during hypoxia.

Concomitant effect of dopamine and acetylcholine on carotid body chemosensory response to hypoxia in the cat

Dopamine (DA) and acetylcholine (Ach) are released from the carotid body chemoreceptor cells under basal conditions and stimulation by hypoxia. The role of DA and Ach in the control of the carotid sinus nerve chemosensory discharge (CSND) are still under debate. Here we tested the hypothesis that DA is inhibitory to the CSND and the effect is reversed by Ach. The CSND was recorded in 2 groups of anesthetized and artificially ventilated adult cats. The treatment group received alpha-methyl-paratyrosine (250 mg/kg) and reserpine (5 mg/kg) administered intraperitoneally, respectively 2.5 h and 12 h prior to the experiment in order to minimize the contribution of endogenous DA. The control group was injected with normal saline. CSND and arterial blood pressure (ABP) were recorded 1) at baseline 2) with a continuous iv infusion of DA, 5 μg/kg/min and 3) with a continuous infusion of Ach (25 μg/kg/min), while DA infusion was ongoing. DA & Ach infusions were initiated in room air. At baseline and 10 min into each infusion, the FIO2 was switched from room air to 8% O2 in N2 for 2–3 min and then to 100% O2 for 2–3 min. One min into DA infusion, CSND was decreased from 3.1 ± 0.4–0.8 ± 0.4 imp/s (P < 0.01) in controls and from 3.2 ± 0.8–0.3 ± 0.2 (imp/s, P < 0.01) in the treatment group. Up to 10 min into DA infusion, the CSND was restored in controls but still significantly inhibited in treated cats in room air and hyperoxia. The mean ± SEM CSND (impulses/sec) and ABP (mmHg) at steady state are listed in Table ​Table11 overleaf. Compared with DA alone, DA+ACh appears to increase the CSND in control cats breathing room air as well as in treated cats breathing room air or 100% O2. Compared with DA, ABP was particularly depressed during DA+Ach in the treatment group breathing room air. The CSND in hypoxia was unchanged throughout the experiment. Table 1 It is concluded that endogenous DA regulates the basal CSND and Ach may be capable of restoring the activity previously inhibited by DA. The effect of Ach appears to be independent from ABP changes in room air for control cats and in hyperoxia for treated cats. In the experimental conditions, neither DA nor DA+Ach appear to modify the CSND response to hypoxia.

Nitric oxide and carotid body chemoreception.

Nitric oxide (NO) has been proposed as an inhibitory modulator of carotid body chemosensory responses to hypoxia. It is believed that NO modulates carotid chemoreception by several mechanisms, which include the control of carotid body vascular tone and oxygen delivery and reduction of the excitability of chemoreceptor cells and petrosal sensory neurons. In addition to the well-known inhibitory effect, we found that NO has a dual (dose-dependent) effect on carotid chemoreception depending on the oxygen pressure level. During hypoxia, NO is primarily an inhibitory modulator of carotid chemoreception, while in normoxia NO increased the chemosensory activity. This excitatory effect produced by NO is likely mediated by an impairment of mitochondrial electron transport and oxidative phosphorylation, which increases the chemosensory activity. The recent findings that mitochondria contain an isoform of NO synthase, which produces significant amounts of NO for regulating their own respiration, suggest that NO may be important for the regulation of mitochondrial energy metabolism and oxygen sensing in the CB.

Behavioural and chemoreceptor cell responses of the tick, Ixodes ricinus, to its own faeces and faecal constituents.

Ticks are ectoparasites of vertebrates and utilize a variety of infochemicals for host finding and acceptance as well as for intraspecific aggregation and mating responses. Individual male and female Ixodes ricinus. the vector of Lyme disease in Europe. readily arrest on filter paper strips contaminated with their own faeces. I. ricinus also responds, but to a lesser degree, to faeces-contaminated papers enclosed in metal mesh envelopes. i.e. without directly contacting the faeces, suggesting a role for volatiles in the arrestment response. The faccal constituents guanine. xanthine, uric acid and 8-azaguanine (a bacterial breakdown product of guanine) also caused arrestment of individual I. ricinus males and females. However, mixtures of these products induced arrestment of I. ricinus at doses one hundred fold lower than the lowest active dose of any of them tested singly. Saline extracts of faeces activated receptor cells in terminal pore sensilla on the first leg tarsi of I. ricinus. One cell in these sensilla responded in a similar dose dependent manner to guanine and 8-azaguanine, whereas a second cell was more sensitive to lower doses of 8-azaguanine. The response threshold approached 100 fM for both cells. These findings suggest that faeces and faecal breakdown products are implicated in aggregation responses of I. ricinus. This may account for the clumped distribution of this ectoparasite on the ground and contribute to the high proportion of mated individuals recorded prior to host colonization.

Classical protein kinase C and its hypoxic stimulus-induced translocation in the cat and rat carotid body.

The presence, subcellular distribution, species specificity and possible hypoxic stimulus-induced translocation of classical protein kinase C (cPKC) isozymes were examined in the carotid body. Carotid bodies were dissected from cats exposed in vivo to normoxic or acute hypoxic conditions and from normoxic rats. For immunohistochemistry isoform-specific monoclonal antisera to PKCalpha, PKCbetaI, PKCbetaII and PKCgamma were used. The immunoreactivity was visualized by fluorescein isothiocyanate (FITC) labelling. FITC/Texas red double-labelled specimens for the cPKC isozymes/tyrosine hydroxylase were used to demonstrate the chemoreceptor cell localization of cPKC isozymes. The immunofluorescence was detected using laser scanning confocal image technology. The results showed expression of the PKCalpha and PKCgamma but not PKCbeta isoforms in the cytoplasm of carotid body chemoreceptor cells. The double labelling provided evidence for the chemoreceptor cell localization of the cPKC isoforms detected. The immunostaining was most intense in the periphery of the perikarya, the nuclear envelope and, occasionally, the nucleoplasm. No major differences were found in the immunolocalization of PKCalpha and PKCgamma under normoxic and hypoxic conditions or between species. However, the immunoreactivity tended to accumulate more in the peripheral cytoplasm and away from the nucleus in the hypoxic chemoreceptor cell. This study demonstrates the presence of classical protein kinase C enzymes in chemoreceptor cells. The intensity of the immunoreactivity may suggest a role for the classical protein kinase C signalling pathway in shaping the hypoxic response at the carotid body. However, this study failed to provide firm evidence of this.

Viral Gene Transfer of Dominant-Negative Kv4 Construct Suppresses an O2-Sensitive K+Current in Chemoreceptor Cells

Hypoxia initiates the neurosecretory response of the carotid body (CB) by inhibiting one or more potassium channels in the chemoreceptor cells. Oxygen-sensitive K(+) channels were first described in rabbit CB chemoreceptor cells, in which a transient outward K(+) current was reported to be reversibly inhibited by hypoxia. Although progress has been made to characterize this current with electrophysiological and pharmacological tools, no attempts have been made to identify which Kv channel proteins are expressed in rabbit CB chemoreceptor cells and to determine their contribution to the native O(2)-sensitive K(+) current. To probe the molecular identity of this current, we have used dominant-negative constructs to block the expression of functional Kv channels of the Shaker (Kv1.xDN) or the Shal (Kv4.xDN) subfamilies, because members of these two subfamilies contribute to the transient outward K(+) currents in other preparations. Delivery of the constructs into chemoreceptor cells has been achieved with adenoviruses that enabled ecdysone-inducible expression of the dominant-negative constructs and reporter genes in polycistronic vectors. In voltage-clamp experiments, we found that, whereas adenoviral infections of chemoreceptor cells with Kv1.xDN did not modify the O(2)-sensitive K(+) current, infections with Kv4.xDN suppressed the transient outward current in a time-dependent manner, significantly depolarized the cells, and abolished the depolarization induced by hypoxia. Our work demonstrate that genes of the Shal K(+) channels underlie the transient outward, O(2)-sensitive, K(+) current of rabbit CB chemoreceptor cells and that this current contributes to the cell depolarization in response to low pO(2).

Intracellular Ca(2+) stores in chemoreceptor cells of the rabbit carotid body: significance for chemoreception.

The notion that intracellular Ca(2+) (Ca(i)(2+)) stores play a significant role in the chemoreception process in chemoreceptor cells of the carotid body (CB) appears in the literature in a recurrent manner. However, the structural identity of the Ca(2+) stores and their real significance in the function of chemoreceptor cells are unknown. To assess the functional significance of Ca(i)(2+) stores in chemoreceptor cells, we have monitored 1) the release of catecholamines (CA) from the cells using an in vitro preparation of intact rabbit CB and 2) the intracellular Ca(2+) concentration ([Ca(2+)](i)) using isolated chemoreceptor cells; both parameters were measured in the absence or the presence of agents interfering with the storage of Ca(2+). We found that threshold [Ca(2+)](i) for high extracellular K(+) (K(e)(+)) to elicit a release response is approximately 250 nM. Caffeine (10-40 mM), ryanodine (0.5 microM), thapsigargin (0.05-1 microM), and cyclopiazonic acid (10 microM) did not alter the basal or the stimulus (hypoxia, high K(e)(+))-induced release of CA. The same agents produced Ca(i)(2+) transients of amplitude below secretory threshold; ryanodine (0.5 microM), thapsigargin (1 microM), and cyclopiazonic acid (10 microM) did not alter the magnitude or time course of the Ca(i)(2+) responses elicited by high K(e)(+). Several potential activators of the phospholipase C system (bethanechol, ATP, and bradykinin), and thereby of inositol 1,4,5-trisphosphate receptors, produced minimal or no changes in [Ca(2+)](i) and did not affect the basal release of CA. It is concluded that, in the rabbit CB chemoreceptor cells, Ca(i)(2+) stores do not play a significant role in the instant-to-instant chemoreception process.

Coding of chemosensory information by the nervous system ofDaphnia magna andEnchytraeus albidus under conditions of action of high environmental temperature

Chemical stimuli (glutamate, glycine, cAMP, and AMP) cause slow and pronounced changes in thermostability of the organisms ofDaphnia magna andEnchytraeus albidus, which can be explained by functioning of not adapted or partially adapted chemoreceptive cells. In experiments with action of purines on the thermostability, the presence inE. albidus of two different chemoreceptors, specific accordingly to cAMP and non-cyclic purines, is shown. A complex extremal stimulus—reaction function, with several optimal values of intensity of the chemical stimulus, at its action on theD. magna andE. albidus thermostability, shows that the elements (neurons or their systems) with the extremal input-output function take part in coding the chemosensory information by the nervous system of these animals.

Ascending spinal systems in the fish, Prionotus carolinus.

The fin rays of the pectoral fin of the sea robins (teleostei) are specialized chemosensory organs heavily invested with solitary chemoreceptor cells innervated only by spinal nerves. The rostral spinal cord of these animals is marked by accessory spinal lobes which are unique enlargements of the dorsal horn of the rostral spinal segments receiving input from the fin ray nerves. Horseradish peroxidase (HRP) and 1,1;-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine perchlorate (diI) were used as anterograde and retrograde tracers to examine the connectivity of these accessory lobes and the associated ascending spinal systems in the sea robin, Prionotus carolinus. The majority of dorsal root fibers terminate within the accessory lobes at or nearby their level of entrance into the spinal cord. A few dorsal root axons turn rostrally in the dorsolateral fasciculus to terminate in the lateral funicular complex situated at the spinomedullary junction. The lateral funicular complex also receives a heavy projection from the ipsilateral accessory lobes. In addition, it contains a few large neurons that project back onto the accessory lobes. Injections of either diI or HRP into the lateral funicular complex label fibers of the medial lemniscus which crosses the midline in the caudal medulla to ascend along the ventral margin of the contralateral rhombencephalon. Within the medulla, fibers leave the medial lemniscus to terminate in the inferior olive and in the ventrolateral medullary reticular formation. Upon reaching the midbrain, the medial lemniscus turns dorsally to terminate heavily in a lateral division of the torus semicircularis, in the ventral optic tectum, and in the lateral subnucleus of the nuc. preglomerulosus of the thalamus. Lesser projections also reach the posterior periventricular portion of the posterior tubercle with a few fibers terminating along the ventral, posterior margin of the ventromedial (VM) nucleus of the thalamus. The restricted projection to the ventral tectum is noteworthy in that this part of the tectum maintains the representation of the ventral visual field, that is, the area in which the fin rays lie. A prominent spinocerebellar system is also evident. Both direct and indirect spinocerebellar fibers can be followed through the dorsolateral fasciculus, with or without relay in the lateral funicular nucleus and terminating in a restricted portion of the granule cell layer of the ipsilateral corpus cerebelli. The similarities in connectivity of the spinal cord between the sea robins and other vertebrates are striking. It is especially notable because sea robins utilize the chemosensory input from the fin rays to localize food in the environment. Thus, although these fish use their spinal chemosense as other fishes use their external taste systems, the spinal chemosense apparently relies on the medial lemniscal system to guide this chemically driven feeding behavior.

An oxygen-, acid- and anaesthetic-sensitive TASK-like background potassium channel in rat arterial chemoreceptor cells.

The biophysical and pharmacological properties of an oxygen-sensitive background K+ current in rat carotid body type-I cells were investigated and compared with those of recently cloned two pore domain K+ channels. Under symmetrical K+ conditions the oxygen-sensitive whole cell K+ current had a linear dependence on voltage indicating a lack of intrinsic voltage sensitivity. Single channel recordings identified a K+ channel, open at resting membrane potentials, that was inhibited by hypoxia. This channel had a single channel conductance of 14 pS, flickery kinetics and showed little voltage sensitivity except at extreme positive potentials. Oxygen-sensitive current was inhibited by 10 mM barium (57% inhibition), 200 microM zinc (53% inhibition), 200 microM bupivacaine (55% inhibition) and 1 mM quinidine (105 % inhibition). The general anaesthetic halothane (1.5%) increased the oxygen-sensitive K+ current (by 176%). Halothane (3 mM) also stimulated single channel activity in inside-out patches (by 240%). Chloroform had no effect on background K+ channel activity. Acidosis (pH 6.4) inhibited the oxygen-sensitive background K+ current (by 56%) and depolarised type-I cells. The pharmacological and biophysical properties of the background K+ channel are, therefore, analogous to those of the cloned channel TASK-1. Using in situ hybridisation TASK-1 mRNA was found to be expressed in type-I cells. We conclude that the oxygen- and acid-sensitive background K+ channel of carotid body type-I cells is likely to be an endogenous TASK-1-like channel.

Inhibition of voltage-gated calcium channels by fluoxetine in rat hippocampal pyramidal cells

Fluoxetine, an antidepressant which is used world-wide, is a prominent member of the class of selective serotonin re-uptake inhibitors. Recently, inhibition of voltage-gated Na + and K + channels by fluoxetine has also been reported. We examined the effect of fluoxetine on voltage-gated calcium channels using the patch-clamp technique in the whole-cell configuration. In hippocampal pyramidal cells, fluoxetine inhibited the low-voltage-activated (T-type) calcium current with an IC 50 of 6.8 μM. Fluoxetine decreased the high-voltage-activated (HVA) calcium current with an IC 50 between 1 and 2 μM. Nifedipine and ω-conotoxin GVIA inhibited the HVA current by 24% and 43%, respectively. Fluoxetine (3 μM), applied in addition to nifedipine or ω-conotoxin, further reduced the current. When fluoxetine (3 μM) was applied first neither nifedipine nor ω-conotoxin attenuated the remaining component of the HVA current. This observation indicates that fluoxetine inhibits both L- and N-type currents. In addition, fluoxetine inhibited the HVA calcium current in carotid body type I chemoreceptor cells and pyramidal neurons prepared from prefrontal cortex. In hippocampal pyramidal cells high K +-induced seizure-like activity was inhibited by 1 μM fluoxetine; the mean burst duration was shortened by an average of 44%. These results provide evidence for inhibition of T-, N- and L-type voltage-gated calcium channels by fluoxetine at therapeutically relevant concentrations.