Chemoreceptor Cells Research Articles

Fatty acid acylation of dopamine in the carotid body

In this article, we put forward a hypothesis concerning the assembling and storage of dopamine molecules in the dense-core vesicles of the carotid body chemoreceptor cell. We posit that dopamine molecules are packed and sustained in the vesicular form due to the formation of N-acyldopamine, a condensation product of fatty acid acyl chain and dopamine at the amino group of the latter. N-acyldopamine would then be stored in a micelle-like supramolecular structure formed due to self-association through the hydrophilic dopamine headgroups. This hypothesis may help explain the perennial problem of the role of dopamine in chemoception. It also draws attention to the possibility of the existence of neurotransmitters in the N-acylated form. This could lead to the design of acylated compounds that would play a role of prodrugs slow-releasing active substances by hydrolysis into the desired environment.

Role of acetylcholine receptors and dopamine transporter in regulation of extracellular dopamine in rat carotid body cultures grown in chronic hypoxia or nicotine.

Using dissociated rat carotid body (CB) cultures, we compared levels of extracellular dopamine (DA) around oxygen-sensitive glomus cells grown for approximately 12 days in normoxia (Nox; 20% O2), chronic hypoxia (CHox; 6% O2), or chronic nicotine (CNic; 10 microM nicotine, 20% O2), with or without acetylcholine (ACh) receptor (AChR) agonists/antagonists and blockers of DA uptake. In Nox cultures, extracellular DA, determined by HPLC and normalized to the number of tyrosine hydroxylase-positive glomus cells present, was augmented by acute (approximately 15-min) exposure to hypoxia (5% O2; approximately 6x basal), high extracellular K+ (30 mM; approximately 10x basal), nomifensine (1 microM; a selective DA uptake inhibitor; approximately 3x basal), and nicotine (100 microM; approximately 5x basal), but not methylcholine (300 microM; a specific muscarinic agonist). In contrast, in CHox cultures where basal DA release is markedly elevated (approximately 9x control), the stimulatory effect of high K+ (3-4x basal) and acute hypoxia (approximately 2x basal) on DA release persisted, but nicotine and nomifensine were no longer effective and methylcholine had a partial inhibitory effect. In CNic cultures, basal DA levels were also elevated (approximately 9x control), similar to that in CHox cultures; however, although acute hypoxia had a stimulatory effect on DA release (approximately 2x basal), nicotine, nomifensine, and high K+ were ineffective. The elevated basal DA in both CHox and CNic cultures was attenuated by acute or chronic treatment with mecamylamine (100 microM), a nicotinic AChR (nAChR) antagonist. In addition, long-term (16-h), but not acute (15-min), treatment with the muscarinic antagonist atropine (1 microM) produced an additional enhancement of basal DA levels in CHox cultures. Thus, after chronic hypoxia or nicotine in vitro, extracellular DA levels around CB chemoreceptor cell clusters appear to be set by a variety of factors including released ACh, positive and negative feedback regulation via nAChRs and muscarinic AChRs, respectively, and modulation of DA transporters. These results provide insight into roles of endogenous transmitters in the adaptation of CB chemoreceptors to chronic hypoxia and suggest pathways by which neuroactive drugs, e.g., nicotine, can interfere with the protective chemoreflex response against hypoxia.

Amiloride high- and low-sensitivity, as well as insensitive sites in the blowfly: implications for sugar, water and anion taste reception mechanisms

Among chemoreceptor cells of the blowfly ( Protophormia terraenovae , the so-called 'sugar' and 'water' cells respond to bovine serum albumin (BSA) and L -alanine (the C-terminal amino acid of the BSA molecule) with the highest spike frequency. In both cell types the response to either BSA or L -alanine was unaffected by 0.1 mM amiloride. However, 0.5 mM amiloride decreased the response of the 'water' cell to both BSA and L-alanine significantly, and similarly inhibited the response of the 'sugar' cell to L -alanine, but in contrast to the 'water' cell, 0.5 mM amiloride did not affect the 'sugar' cell response to BSA. The 'sugar' cell responses to both BSA and L-alanine were unaffected by the putative cell second messengers, cAMP and cGMP. The so-called 'anion' cell unexpectedly gave responses to BSA and L-alanine that were enhanced by amiloride at both concentrations. However, like the 'sugar' cell, the 'anion' cell was also unaffected by cAMP and cGMP.We conclude that the 'sugar' cell must have at least three types of receptor sites: the previously described 'F' (amiloride sensitive) and the 'P' (amiloride insensitive) sites, and a low-sensitivity 'T' site that mediates, at least in part, the response to BSA and L -alanine. The effects of amiloride on the responses of the 'water' and 'anion' cells are more difficult to interpret because fundamental information on their chemoreception mechanisms is still lacking.

Comparative aspects of central CO 2 chemoreception

We compare and contrast the putative mechanisms underlying CO 2 chemoreceptor function in air breathing vertebrates and terrestrial pulmonate snails. We discuss the role of intracellular pH (pHi) in central respiratory responses to CO 2 and describe a variety of patterns of pHi regulation in chemosensory areas. One pattern, in which pHi retains a fixed relationship to the CO 2 stimulus over time, seems well suited to chemoreceptor cells. Alphastat regulation of ventilation is apparent in both air breathing vertebrates and terrestrial pulmonate snails. Diethyl pyrocarbonate inhibits respiratory responses to hypercapnia in both groups of animals. The neuronal basis of chemosensitivity is similar, in that putative chemoreceptor cells depolarize during hypercapnic stimulation, but the ionic basis of excitability appears to be a potassium conductance in the vertebrates studied to date and a calcium conductance in the snails. Despite divergent evolutionary histories, chemosensory responses and mechanisms are remarkably similar in air breathing vertebrates and terrestrial pulmonate snails.

Correlated evolution of prey chemical discrimination with foraging, lingual morphology and vomeronasal chemoreceptor abundance in lizards

Comparative data from ten families of lizards suggest that correlated evolution has occurred between the ability to identify prey chemicals and several aspects of lingual function and morphology, abundance of vomeronasal chemoreceptor cells, and foraging behavior. Ability to discriminate prey chemicals from control substances was measured experimentally and correlated with other variables by Felsenstein's method. This ability increased with evolutionary increases in degree of lingual protrusion during tongue-flicking, which may reflect the tongue's ability to reach substrates to be sampled. It increased with deepened lingual forking and greater lingual elongation, which may be important for scent-trailing and sampling ability, respectively. Discriminatory ability also increased with abundance of vomerolfactory chemoreceptors, which presumably reflects some aspects of analytical capacities of the vomeronasal system. Prey chemical discrimination increased with degree of active foraging. Natural selection for improved vomerolfactory sampling and analysis of prey chemicals by active, but not ambush, foragers appears to account for the observed relationships. In active foragers that use vomerolfaction to locate prey, natural selection favors increased abilities to lingually sample chemicals from environmental substrates, analyze the samples for prey chemicals, and respond appropriately if prey chemicals or possible prey chemicals are detected. Such selection can account for the observed relationships among the sampling device and its movements, the sense, the discriminations, and variations in foraging ecology.

Carbon monoxide inhibits hypoxic pulmonary vasoconstriction in rats by a cGMP-independent mechanism.

Hypoxia activates erythropoietin-producing cells, chemoreceptor cells of the carotid body and pulmonary artery smooth muscle cells (PSMC) with a comparable arterial PO2 threshold of some 70 mmHg. The inhibition by CO of the hypoxic responses in the two former cell types has led to the proposal that a haemoprotein is involved in the detection of the PO2 levels. Here, we report the effect of CO on the hypoxic pulmonary vasoconstriction (HPV). Pulmonary arterial pressure (PAP) was measured in an in situ, blood-perfused lung preparation. PAP in normoxia (20% O2, 5% CO2) was 15.2+/-1.8 mmHg, and hypoxia (2% O2, 5% CO2) produced a DeltaPAP of 6.3+/-0.4 mmHg. Addition of 8% or 15% CO to the hypoxic gas mixture reduced the DeltaPAP by 88.3+/-2.7% and 78.2+/-6.1% respectively. The same levels of CO did not affect normoxic PAP nor reduced the DeltaPAP produced by angiotensin II. The effect of CO was studied after inhibition of the NO-cyclic guanosine monophosphate (cGMP) cascade with N-methyl-l-arginine (5.10(-5) M) or methylene blue (1.4.10(-4) M). It was found that both inhibitors more than doubled the hypoxic DeltaPAP without altering the effectiveness of CO to inhibit the HPV. In in vitro experiments we verified the inhibition of guanylate cyclase by measuring the levels of cGMP in segments of the pulmonary artery. Cyclic GMP levels were 1.4+/-0.2 (normoxia), 2.5+/-0.3 (hypoxia) and 3.3+/-0.5 pmole/mg tissue (hypoxia plus 8% CO); sodium nitroprusside increased normoxic cGMP levels about fourfold. Methylene blue reduced cGMP levels to less than 10% in all cases, and abolished the differences among normoxic, hypoxic and hypoxic plus CO groups. It is concluded that CO inhibits HPV by a NO-cGMP independent mechanism and it is proposed that a haemoprotein could be involved in O2-sensing in PSMC.

Chemoreception by the red‐jointed fiddler crab Uca minax (Le Conte): Spectral tuning properties of the walking legs

The red‐jointed fiddler crab Uca minax is one of the most abundant macroinvertebrates inhabiting the temperate western Atlantic salt marshes, along the eastern and southern coasts of the United States. Dactyl chemoreception is the primary sensory modality involved in food detection. Ninety‐six chemoreceptor cells from 69 male and female second and third legs were tested with 20 compounds known to be stimulatory in other decapods. Each compound was tested as 1 s pulses at 10‐3M. Overall, chemoreceptor cells on the dactyls responded strongest to glutamate and ammonium chloride followed by citric acid. Glutamate‐ and ammonium chloride‐best cells formed the most prominent cell populations and were relatively narrowly tuned. Individual cells exhibited a range of tuning breadths based on responses to single compounds. Amines were moderately stimulatory. Surprisingly, hexose sugars which cause strong behavioural responses in U. minax elicited only weak physiological responses. Glutamate sensitivity separates U. minax from other species of fiddler crabs. The results indicate that the chemical response spectrum of U. minax includes compounds that occur naturally in salt marshes as algal and animal constituents, exudates, and decomposition products.

Immunocytochemical localization on O2-sensing protein (NADPH oxidase) in chemoreceptor cells.

A potential candidate for an oxygen-sensing protein in chemoreceptor cells is a heme-linked multicomponent NADPH oxidase, originally described in neutrophils. The postulated function for the oxidase in chemoreceptor cells is to signal changes in oxygen levels (either in the blood or in the airway lumen) via changes in oxygen metabolite production. An alteration in either superoxide (or dismuted hydrogen peroxide) production may affect the gating properties of the O2-sensitive K+ channels. We have previously reported immunohistochemical localization of gp91 glycoprotein component of the oxidase to the plasma membrane of pulmonary neuroepithelial body (NEB) cells. In this study we have investigated the immunocytochemical localization of the other polypeptide components of the oxidase in NEB cells and in the glomus cells of the carotid body. Cultures of dissociated fetal rabbit NEB cells and newborn rat glomus cells were immunostained with specific antibodies recognizing the various polypeptide subunits of the oxidase using indirect immunofluorescence methods. Immunostaining with the anti-oxidase antibodies reveal strong positive reaction in both NEB and glomus cell clusters while other cells were unstained. The positive reaction product was localized to the plasma membrane and/or cytoplasm and no nuclear staining was observed. Live cell labelling studies with anti-p22 antibody showed positive immunofluorescence on the surface of NEB cells, suggesting that this component of the oxidase is also associated with the plasma membrane. In glomus cells, similar strongly positive immunofluorescence signal was observed for p22 and gp91 in paraformaldehyde-fixed cultures, regardless whether they were permeabilized or not. Taken together, our findings of cell surface localization of gp91 and p22 components of the oxidase in chemoreceptive cells suggests that the heme-linked cytochrome b558 component is associated with the plasma membrane. This association allows for direct interaction with the O2-sensitive K+ channel thus forming the molecular complex of membrane bound O2 sensor.

Evidence for two types of nicotinic receptors in the cat carotid body chemoreceptor cells

Current concepts on the location and functional significance of nicotinic receptors in the carotid body rest on α-bungarotoxin binding and autoradiographic studies. Using an in vitro preparation of the cat carotid body whose catecholamine deposits have been labeled by prior incubation with the tritiated natural precursor [ 3H]tyrosine, we have found that nicotine induces release of [ 3H]catecholamines in a dose-dependent manner (IC 50=9.81 μM). We also found that mecamylamine (50 μM) completely abolished the nicotine-induced release, while α-bungarotoxin (100 nM; ≈20 times its binding K d) only reduced the release by 56%. These findings indicate that chemoreceptor cells, and perhaps other carotid body structures, contain nicotinic receptors that are not sensitive to α-bungarotoxin and force a revision of the current concepts on cholinergic mechanisms in the carotid body chemoreception.

Immunocytochemical localization of O2‐sensing protein (NADPH oxidase) in chemoreceptor cells

Immunocytochemical localization of O2‐sensing protein (NADPH oxidase) in chemoreceptor cells

Role of Basic FGF and Oxygen in Control of Proliferation, Survival, and Neuronal Differentiation in Carotid Body Chromaffin Cells

Crest-derived glomus cells of the carotid body (CB) are O2-sensitive chemoreceptors, which resemble sympathoadrenal (SA) chromaffin cells. In this study, we tested whether perinatal rat glomus cells are sensitive to basic fibroblast growth factor (bFGF)in vitroand whether their sensitivity is regulated by oxygen. In chemically defined medium, bFGF (1–100 ng/ml) caused a significant, dose-dependent increase in the number of surviving tyrosine hydroxylase-positive (TH+) glomus cells in embryonic (E17–E19) CB cultures, following a 48-hr exposure. Though basic FGF (10 ng/ml) appeared mitogenic for these cells, based on stimulation of bromodeoxyuridine (BrdU) uptake, it supported survival of only ∼60% of the initial TH+population, suggesting that significant cell death was occurring. This apparent cell loss in E17 cultures could be largely prevented by combined treatment with bFGF and low oxygen (6% O2). In contrast, in early postnatal (P1) cultures, glomus cell number was relatively unchanged over 48 hr under control conditions or in presence of mitogenic activity from either bFGF or low oxygen. However, combined treatment with both bFGF and low oxygen stimulated proliferation of P1 glomus cells such that by 48 hr the TH+population had increased to ∼1.5× the initial density. Basic FGF (10 ng/ml) also stimulated neurite outgrowth and neurofilament expression in E18–E19, but not P1–P3, glomus cells. In contrast to bFGF, treatment with nerve growth factor was ineffective. Taken together, these results suggest that bFGF and low oxygen are mitogens for perinatal CB chromaffin cells and interact cooperatively as survival factors. It is plausible that these mechanisms may operate to regulate chemoreceptor cell density, during the animal's transition fromin utero(hypoxic) toex utero(normoxic) life.

Properties of ionic currents from isolated adult rat carotid body chemoreceptor cells: effect of hypoxia.

1. The electrical properties of chemoreceptor cells from neonatal rat and adult rabbit carotid bodies (CBs) are strikingly different. These differences have been suggested to be developmental and/or species related. To distinguish between the two possibilities, the whole-cell configuration of the patch-clamp technique was used to characterize the ionic currents present in isolated chemoreceptor cells from adult rat CBs. Since hypoxia-induced inhibition of O2-sensitive K+ currents is considered a crucial step in O2 chemoreception, the effect of hypoxia on the adult rat chemoreceptor cell currents was also studied. 2. Outward currents were carried mainly by K+, and two different components could be distinguished: a Ca(2+)-dependent K+ current (IK(Ca)) sensitive to Cd2+ and charybdotoxin (CTX), and a Ca(2+)-insensitive, voltage-dependent K+ current (IK(V)). IK(V) showed a slow voltage-dependent activation (time constant (tau) of 87.4 ms at -20 mV and 8.8 ms at +60 mV) and a very slow inactivation, described by the sum of two exponentials (tau 1 = 684 +/- 150 ms and tau 2 = 4.96 +/- 0.76 s at + 30 mV), that was almost voltage insensitive. The kinetic and pharmacological properties of IK(V) are typical of a delayed rectifier K+ channel. 3. Voltage-dependent Ca2+ currents (ICa) were present in nineteen of twenty-seven cells. TTX-sensitive Na+ currents were also observed in about 10% of the cells. 4. Low PO2 (< 10 mmHg) reduced the whole outward current amplitude by 22.17 +/- 1.96% (n = 27) at +20 mV. This effect was absent in the presence of Cd2+. Since low PO2 did not affect ICa, we conclude that hypoxia selectively blocks IK(Ca). 5. The properties of the currents recorded in adult rat chemoreceptor cells, including the specific inhibition of IK(Ca) by hypoxia, are similar to those reported in neonatal rat CB cells, implying that the differences between rat and rabbit chemoreceptor cells are species related.

Spectral tuning of chemoreceptor cells in the lateral antennules of the American lobster, Homarus Americanus

Chemoreceptor cells in the lateral flagellum of the first antennae (lateral antennule) of the American lobster, Homarus americanus, serve, among other functions, in long distance orientation and mediate chemical recognition of food and chemical information used in social interactions. Based upon extracellular recordings of action potentials, we report on 60 cells identified with a 15‐compound equimolar mixture of mostly amino acids and a few other compounds used in previous studies of lobster chemoreceptors. Subsequently, all cells were tested with each compound separately. Forty‐three percent of all cells responded strongest to hydroxyproline, 13% to taurine, and 10% to glutamate. These cells were generally narrowly tuned and had no consistent second best stimulus. Other cells were more broadly tuned and responded strongest to valine, arginine, leucine, glutamine, serine, glycine, alanine and ammonium. Most chemoreceptor cells responded less to a mixture containing their best compound than to the best compound alone, though a few gave stronger responses to the mixture. At a low concentration (10‐6 M) the mixture components enhanced the response to Hydroxyproline (Hyp); at medium and higher concentrations (10‐5 M, 10‐4 M) they suppressed the response to Hyp, resulting in a shallow, extended range of the stimulus‐response function for the mixture.

Mechanisms of alpha2-adrenoceptor-mediated inhibition in rabbit carotid body.

We have used the in vitro preparation of the intact carotid body (CB) and isolated chemoreceptor cells to elucidate the distribution and function of alpha2-adrenoreceptors. The significance of the study lies in the fact that norepinephrine (NE), being the neurotransmitter of the sympathetic innervation to the CB, is also abundant in chemoreceptor cells. In intact CB whose catecholamine (CA) deposits had been labeled by prior incubation with the CA precursor [3H]tyrosine, the alpha2-antagonist yohimbine (10 microM) potentiated the low-PO2 (33 and 60 mmHg)-induced release of [3H]CA by 100 and 53%, respectively. Yohimbine (10 microM) and SKF-86466 (50 microM; another alpha2-antagonist) reversed the inhibition of the release of [3H]CA produced by the alpha2-receptor agonists clonidine and UK-14304 (10 microM). The increase in adenosine 3',5'-cyclic monophosphate produced by low PO2 was further augmented by yohimbine and nearly halved by UK-14304 and clonidine. In isolated chemoreceptor cells, UK-14304 and NE inhibited voltage-dependent Ca2+ currents by 28 and 32%, respectively. These results indicate that alpha2-receptors are present in chemoreceptor cells, where they reduce the release of [3H]CA. Inhibition of adenylate cyclase(s) and Ca2+ channels may be involved in this effect. Using intact CB from normal and chronically sympathectomized animals, we demonstrated a specific accumulation of [3H]NE in intraglomic sympathetic endings. Hypoxia (PO2 approximately 33 mmHg) did not elicit release of [3H]NE from the sympathetic endings, but high extracellular K+ (K+(e)) induced a release of [3H]NE that was inhibited by alpha2-agonists and augmented by alpha2-antagonists. These findings demonstrate that alpha2-receptors are also present in the sympathetic endings of the CB, where they modulate the release of NE. As a whole, this work provides a more detailed understanding of the role of the sympathetic innervation in the control of the CB chemoreceptor function, including the cellular mechanisms of the action of NE.

Identification of CO2Chemoreceptors inHelix pomatia

SYNOPSIS. Gas exchange in pulmonate snails of the family Helicidae occurs through a highly vascularized diffusion lung known as the mantle. The extent of ventilation of the mantle depends upon the duration and size of opening of an occlusible pore known as the pneumostome. In Helix aspersa and Helix pomatia , pneumostomal size and frequency of opening are exquisitely sensitive to CO2. Respiratory CO2 chemosensitivity resides in a discrete region of the subesophageal ganglia. The discharge pattern of many neurons in the chemoreceptor area changes during stimulation with CO2. However, the electrophysiological response to CO, stimulation alone does not discriminate between CO2 chemoreceptor cells and CO2-insensitive neurons active in the pneumostomal response to CO2. We identified a subset of CO2-sensitive neurons from the larger population of neurons active during CO2 stimulation. The action potential discharge frequency of CO2 chemosensory neurons increased in response to CO2 stimulation. An increased discharge frequency of CO2-sensitive neurons was associated with increased pneumostomal opening, and both the size and the frequency of pneumostomal opening increased during CO2 stimulation. Injecting depolarizing current into individual chemosensory neurons elicited opening of the pneumostome in the absence of CO2. Action potential generation in response to CO2 was independent of synaptic transmission. Removal of individual CO2-sensitive cells or inhibition of action potential generation in CO2-sensitive cells reduced or eliminated pneumostomal responses to CO2. CO2 sensitivity in chemoreceptor cells required extracellular calcium, but not sodium. Substituting barium for calcium supported chemoreceptor activity. In summary, we have identified respiratory related, chemosensory neurons that are CO2 sensitive in the absence of synaptic input.

Arterial and venous paragangliomas: the value of a preoperative pacemaker and a multidisciplinary approach

Non-chromaffin paragangliomas are very rare tumours. They arise from chemoreceptor cells at multiple sites throughout the body. They are usually very vascular and most arise in intimate contact with vital structures making their excision a surgical challenge. We describe in this paper four cases of paragangliomas in the carotid body and the jugular bulb. Their presentation was unusual. Two cases underwent excisional surgery and two had radiation treatment because they were not fit for surgery. We had to use a temporary pacemaker for preoperative preparation of the carotid body because of an associated hypersensitive carotid sinus syndrome. The clinical presentation and the surgical management will be described in this paper.

Independent evolution of squamate olfaction and vomerolfaction and correlated evolution of vomerolfaction and lingual structure

AbstractThe comparative abundances of chemoreceptor cells for olfaction and vomerolfaction, the sense mediated by the vomeronasal organs, were studied in numerous squamate families, with emphasis on lizards, and compared with abundance of lingual taste buds and aspects of lingual structure likely to be related to chemosensory sampling for vomerolfaction. Abundances of vomerolfactory receptors vary greatly among, but little within squamate families. Using Felsenstein's method of independent contrasts, the abundance of vomeronasal receptors is significantly correlated with size of lingual tines, degree of lingual elongation, and condition of sampling surfaces (pallets) on the ventral side of the tongue. This indicates that correlated evolution has occurred. An intimate relationship exists between the chemosensory organ and its primary sampling device, highlighting a lingual-vomeronasal complex that functions to sample and analyze chemical stimuli from external environmental surfaces. Attributes enhancing lingual facility for chemical sampling are forking, which permits scent-trailing by tropotaxis, and possibly elongation, which may permit greater extension beyond the mouth or enhance maneuverability. Ventral lingual pallets, surfaces that directly contact substrates during sampling, are large in forms having low abundances of vomerolfactory receptors, but are reduced and are eventually lost with progressive increases in vomerolfactory receptor abundance. Pallet condition is negatively correlated with tine size and lingual elongation. Large tines appear to take over the sampling function in forms lacking pallets. Lingual taste bud abundance is negatively correlated with vomerolfactory receptor abundance, forking, elongation, and positively with pallet condition. The negative correlation between the abundance of vomerolfactory receptors and lingual taste buds may be attributable to physical incompatibility between taste buds and specialized modifications of the foretongue for vomerolfactory sampling or to loss of opportunity for lingual taste buds to function in vomerolfactory specialists that lack lingual functions such as prey prehension, manipulation, and transport and in which the tongues are ensheathed while in the mouth. The abundances of olfactory and vomeronasal chemoreceptor cells are uncorrelated using Felsenstein's method. Thus, olfaction appears to have evolved independently of vomerolfaction.

Evolution of taste and solitary chemoreceptor cell systems.

Vertebrates possess four distinct chemosensory systems distinguishable on the basis of structure, innervation and utilization: olfaction, taste, solitary chemoreceptor cells (SCC) and the common chemical sense (free nerve endings). Of these, taste and the SCC sense rely on secondary receptor cells situated in the epidermis and synapsing on sensory nerve fibers innervating them near their base. The SCC sense occurs in anamniote aquatic craniates, including hagfish, and may be used for feeding or predator avoidance. The sense of taste occurs only in vertebrates and is always utilized for feeding. The SCC system achieves a high degree of specialization in two teleosts: sea robins (Prionotus) and rocklings (Ciliata). In sea robins, SCCs are abundant on the three anterior fin rays of the pectoral fin which are free of fin webbing and are used in active exploration of the substrate. Behavioral and physiological studies show that this SCC system responds to feeding cues and drives feeding behavior. It is connected centrally like a somatosensory system. In contrast, the specialized SCC system of rocklings occurs on the anterior dorsal fin which actively samples the surrounding water. This system responds to mucus substances and may serve as a predator detector. The SCC system in rocklings is connected centrally like a gustatory system. Taste buds contain multiple receptor cell types, including a serotonergic Merkel-like cell. Taste receptor cells respond to nutritionally relevant substances. Due to similarities between SCCs and one type of taste receptor cell, the suggestion is made that taste buds may be compound sensory organs that include some cells related to SCCs and others related to cutaneous Merkel cells. The lack of taste buds in hagfish and their presence in all vertebrates may indicate that the phylogenetic development of taste buds coincided with the elaboration of head structures at the craniate-vertebrate transition.

Temporal resolution in olfaction II: time course of recovery from adaptation in lobster chemoreceptor cells.

1. Adaptation and disadaptation rates determine the temporal response properties of sensory receptor cells. In chemoreception, temporal filter properties of receptor cells are poorly understood. We studied the time course of disadaptation in lobster antennular chemoreceptor cells by using in situ high-resolution stimulus measurement and extracellularly recorded spike responses. Fifteen receptor cells were each tested with two series (one at 10 microM, one at 100 microM) of three odor (hydroxyproline) pulses: a 200-ms test pulse, a 5-s adapting pulse, and a 200-ms probe pulse after time intervals ranging from 1 to 60 s. After complete adaptation by the adapting pulse, individual cells recovered at different rates. After 1 s, a third of the cells responded with a mean response of 3 spikes/cell, representing approximately 20% recovery. All cells fully recovered between 10 and 30 s. Mean full recovery was within 25 s, with a time constant of 14 s, independent of stimulus concentration.

Temporal resolution in olfaction: stimulus integration time of lobster chemoreceptor cells

The stimulus integration time of lobster olfactory receptor cells in situ was determined using extracellularly recorded spiking responses from receptor cells and on-line high-resolution measurement of odor square pulses. At a fixed odor concentration, odor steps of 200 ms duration elicited maximum responses; shorter odor steps did not drive the cells to their maximum response and longer odor steps added spikes but did not result in higher firing rates. Excitatory processes peaked within 220 ms of stimulus onset. At 160&shy;300 ms, stimulus intensity discrimination was most evident. Adaptation processes reduced response magnitude to near-zero levels within 1000 ms of stimulus onset. Olfactory receptor cells thus resolve odor peak onsets within the first few hundred milliseconds: this time window corresponds to the 4&shy;5 Hz frequency of olfactory sampling (i.e. 'sniffing') as well as the rapid fluctuations in odor concentration that are common in natural odor plumes. The stimulus integration time of 200 ms may play a role in the filtering of information used by lobsters to orient to distant odor sources.