Action potentials and chemosensitive conductances in the dendrites of olfactory neurons suggest new features for odor transduction

A Comparison of the Primary Sensory Neurons Used in Olfaction and Vision

Vision, hearing, smell, taste, and touch are the tools used to perceive and navigate the world. They enable us to obtain essential resources such as food and highly desired resources such as mates. Thanks to the investments in biomedical research the molecular unpinning’s of human sensation are rivaled only by our knowledge of sensation in the laboratory mouse. Humans rely heavily on vision whereas mice use smell as their dominant sense. Both modalities have many features in common, starting with signal detection by highly specialized primary sensory neurons—rod and cone photoreceptors (PR) for vision, and olfactory sensory neurons (OSN) for the smell. In this chapter, we provide an overview of how these two types of primary sensory neurons operate while highlighting the similarities and distinctions.

Electrical properties of cells from human olfactory epithelium

OBJECTIVE

The electrical properties of olfactory cells (OCs) are typically examined using animals such as newts, mice, and frogs, with few studies on human OCs. This study investigated the electrical properties of human cells from olfactory epithelium (hCOEs) obtained from subjects of olfactory epithelium showing no clinical symptoms during endoscopic sinus surgery.

METHODS

hCOEs were isolated by collagenase treatment for whole-cell patch clamp recording. The identity of the cells was confirmed by immunohistochemistry with an antibody against olfactory maker protein. Under the voltage clamp with the whole-cell recording configuration, the voltage-gated currents of isolated hCOEs were recorded when the membrane potential was depolarized from a holding potential of -100 mV in a stepwise manner between -90 mV and + 40 mV.

RESULTS

Only one of 14 hCOE samples expressed a transient inward current at the depolarizing voltage step that was activated by depolarization beyond -40 mV and reached a peak at -30 mV. Delayed and sustained outward currents (444 ± 106 pA at + 40 mV pulse; n = 20) were suppressed by tetraethyl ammonium (n = 3), which is consistent with the properties of newt OCs.

CONCLUSIONS

Most hCOEs did not exhibit the transient inward current observed in animal models. These findings provide insight into the physiological basis of the unique aspects of human olfactory signal transduction.

Antigenic homogeneity of male Müllerian gland (MG) secretory proteins of a caecilian amphibian with secretory proteins of the mammalian prostate gland and seminal vesicles: evidence for role of the caecilian MG as a male accessory reproductive gland

Whereas in all other vertebrates the Müllerian ducts of genetic males are aborted during development, under the influence of Müllerian-inhibiting substance, in the caecilian amphibians they are retained as a pair of functional glands. It has long been speculated that the Müllerian gland might be the male accessory reproductive gland but there has been no direct evidence to this effect. The present study was undertaken to determine whether the caecilian Müllerian gland secretory proteins would bear antigenic similarity to secretory proteins of the prostate gland and/or the seminal vesicles of a mammal. The secretory proteins of the Müllerian gland of Ichthyophis tricolor were evaluated for cross-reactivity with antisera raised against rat ventral prostate and seminal vesicle secretory proteins, adopting SDS-PAGE, two-dimensional electrophoresis and immunoblot techniques. Indeed there was a cross-reaction of five Müllerian gland secretory protein fractions with prostatic protein antiserum and of three with seminal vesicle protein antiserum. A potential homology exists because in mammals the middle group of the prostate primordia is derived from a diverticulum of the Müllerian duct. Thus this study, by providing evidence for expression of prostatic and seminal vesicle proteins in the Müllerian gland, substantiates the point that in caecilians the Müllerian glands are the male accessory reproductive glands.

Suppression and recovery of voltage-gated currents after cocaine treatments of olfactory receptor cells

OBJECTIVE

Cocaine (1-5% concentrations) is commonly used as a local anesthetic for the otorhinolaryngeal surgery of the nasal cavity. Recent reports indicate that some patients complain of olfactory deficits after surgery, and decreased olfaction is found in cocaine abusers. In spite of these reports, the effects of cocaine on the olfactory receptor cells are unknown.

METHODS

Effect of cocaine was examined in olfactory receptor cells isolated from the newt. Under the voltage clamp with the whole-cell recording configuration, the voltage-gated currents were recorded when the membrane potential was depolarized from a holding potential of -100 mV in a step wise between -90 mV and +40 mV.

RESULTS

When cocaine was applied by a puff pressure (5%) and the extracellular solution, the voltage-gated currents, including inward and outward components, were significantly reduced. The dose-suppression curves of cocaine for sodium and potassium currents could be fitted by the Hill equation. Half-blocking concentration of sodium and potassium currents were 43 μM and 557 μM; Hill coefficient was 1.1 and 0.9, respectively.

CONCLUSION

This rapid and complete recovery from the suppression was confirmed even after the treatments with the high concentration cocaine. This fact implies that cocaine does not affect olfactory ability after locally high dose treatments of nasal cavity in surgical operation.

Molecular components of signal amplification in olfactory sensory cilia

The mammalian olfactory system detects an unlimited variety of odorants with a limited set of odorant receptors. To cope with the complexity of the odor world, each odorant receptor must detect many different odorants. The demand for low odor selectivity creates problems for the transduction process: the initial transduction step, the synthesis of the second messenger cAMP, operates with low efficiency, mainly because odorants bind only briefly to their receptors. Sensory cilia of olfactory receptor neurons have developed an unusual solution to this problem. They accumulate chloride ions at rest and discharge a chloride current upon odor detection. This chloride current amplifies the receptor potential and promotes electrical excitation. We have studied this amplification process by examining identity, subcellular localization, and regulation of its molecular components. We found that the Na(+)/K(+)/2Cl(-) cotransporter NKCC1 is expressed in the ciliary membrane, where it mediates chloride accumulation into the ciliary lumen. Gene silencing experiments revealed that the activity of this transporter depends on the kinases SPAK and OSR1, which are enriched in the cilia together with their own activating kinases, WNK1 and WNK4. A second Cl(-) transporter, the Cl(-)/HCO(3)(-) exchanger SLC4A1, is expressed in the cilia and may support Cl(-) accumulation. The calcium-dependent chloride channel TMEM16B (ANO2) provides a ciliary pathway for the excitatory chloride current. These findings describe a specific set of ciliary proteins involved in anion-based signal amplification. They provide a molecular concept for the unique strategy that allows olfactory sensory neurons to operate as efficient transducers of weak sensory stimuli.

Gonadotropin-releasing hormone modulates voltage-activated sodium current and odor responses in Necturus maculosus olfactory sensory neurons

The terminal nerve (nervus terminalis) extends from the basal forebrain to the nasal cavity and has been shown to contain gonadotropin-releasing hormone (GnRH). The specific function of the terminal nerve is unknown, but it has been hypothesized that it modulates the function of olfactory neurons. To examine the effects of GnRH on isolated Necturus maculosus olfactory sensory neurons (OSNs), we used the perforated configuration of the patch clamp technique to record current responses. GnRH had no effect on the membrane current at any holding potential but did modulate voltage-activated TTX-sensitive sodium current (INa). Within 1 min of applying GnRH, approximately 60% of the OSNs showed a decrease in the magnitude of INa. Initial responses to GnRH were inhibitory, although in one group of cells the initial inhibitory response was followed by a potentiation of INa with continual application (approximately 5 min). The time course of the GnRH response suggested that a second messenger pathway mediated the response. Inhibitors of PKC, tyrosine kinase, and PI3K were all able to inhibit the INa, but none of them could prevent the GnRH response. Application of a cAMP analog mimicked the effects of GnRH, and only inhibitors of PKA and PKG could prevent GnRH-induced inhibition of INa. This suggests that the modulation of voltage-activated sodium currents by GnRH involve a cyclic nucleotide pathway. In addition, GnRH modulated the odor responses of OSNs. Our data suggest the release of GnRH, presumably from the terminal nerve, can serve to modulate olfactory sensory neurons.

Fast adaptation in mouse olfactory sensory neurons does not require the activity of phosphodiesterase

Vertebrate olfactory sensory neurons rapidly adapt to repetitive odorant stimuli. Previous studies have shown that the principal molecular mechanisms for odorant adaptation take place after the odorant-induced production of cAMP, and that one important mechanism is the negative feedback modulation by Ca²⁺-calmodulin (Ca²⁺-CaM) of the cyclic nucleotide-gated (CNG) channel. However, the physiological role of the Ca²⁺-dependent activity of phosphodiesterase (PDE) in adaptation has not been investigated yet. We used the whole-cell voltage-clamp technique to record currents in mouse olfactory sensory neurons elicited by photorelease of 8-Br-cAMP, an analogue of cAMP commonly used as a hydrolysis-resistant compound and known to be a potent agonist of the olfactory CNG channel. We measured currents in response to repetitive photoreleases of cAMP or of 8-Br-cAMP and we observed similar adaptation in response to the second stimulus. Control experiments were conducted in the presence of the PDE inhibitor IBMX, confirming that an increase in PDE activity was not involved in the response decrease. Since the total current activated by 8-Br-cAMP, as well as that physiologically induced by odorants, is composed not only of current carried by Na⁺ and Ca²⁺ through CNG channels, but also by a Ca²⁺-activated Cl⁻ current, we performed control experiments in which the reversal potential of Cl⁻ was set, by ion substitution, at the same value of the holding potential, −50 mV. Adaptation was measured also in these conditions of diminished Ca²⁺-activated Cl⁻ current. Furthermore, by producing repetitive increases of ciliary's Ca²⁺ with flash photolysis of caged Ca²⁺, we showed that Ca²⁺-activated Cl⁻ channels do not adapt and that there is no Cl⁻ depletion in the cilia. All together, these results indicate that the activity of ciliary PDE is not required for fast adaptation to repetitive stimuli in mouse olfactory sensory neurons.

Calmodulin contributes to gating control in olfactory calcium-activated chloride channels

In sensory neurons of the peripheral nervous system, receptor potentials can be amplified by depolarizing Cl currents. In mammalian olfactory sensory neurons (OSNs), this anion-based signal amplification results from the sequential activation of two distinct types of transduction channels: cAMP-gated Ca channels and Ca-activated Cl channels. The Cl current increases the initial receptor current about 10-fold and leads to the excitation of the neuron. Here we examine the activation mechanism of the Ca-dependent Cl channel. We focus on calmodulin, which is known to mediate Ca effects on various ion channels. We show that the cell line Odora, which is derived from OSN precursor cells in the rat olfactory epithelium, expresses Ca-activated Cl channels. Single-channel conductance, ion selectivity, voltage dependence, sensitivity to niflumic acid, and Ca sensitivity match between Odora channels and OSN channels. Transfection of Odora cells with CaM mutants reduces the Ca sensitivity of the Cl channels. This result points to the participation of calmodulin in the gating process of Ca-ativated Cl channels, and helps to understand how signal amplification works in the olfactory sensory cilia. Calmodulin was previously shown to mediate feedback inhibition of cAMP-synthesis and of the cAMP-gated Ca channels in OSNs. Our results suggest that calmodulin may also be instrumental in the generation of the excitatory Cl current. It appears to play a pivotal role in the peripheral signal processing of olfactory sensory information. Moreover, recent results from other peripheral neurons, as well as from smooth muscle cells, indicate that the calmodulin-controlled, anion-based signal amplification operates in various cell types where it converts Ca signals into membrane depolarization.

Cyclic AMP cascade mediates the inhibitory odor response of isolated toad olfactory receptor neurons

Odor stimulation may excite or inhibit olfactory receptor neurons (ORNs). It is well established that the excitatory response involves a cyclic AMP (cAMP) transduction mechanism that activates a nonselective cationic cyclic nucleotide-gated (CNG) conductance, accompanied by the activation of a Ca2+-dependent Cl(-) conductance, both causing a depolarizing receptor potential. In contrast, odor inhibition is attributed to a hyperpolarizing receptor potential. It has been proposed that a Ca2+-dependent K+ (K(Ca)) conductance plays a key role in odor inhibition, both in toad and rat isolated olfactory neurons. The mechanism underlying odor inhibition has remained elusive. We assessed its study using various pharmacological agents and caged compounds for cAMP, Ca2+, and inositol 1,4,5-triphosphate (InsP3) on isolated toad ORNs. The odor-triggered K(Ca) current was reduced on exposing the cell either to the CNG channel blocker LY83583 (20 microM) or to the adenylyl cyclase inhibitor SQ22536 (100 microM). Photorelease of caged Ca2+ activated a Cl- current sensitive to niflumic acid (10 microM) and a K+ current blockable by charybdotoxin (20 nM) and iberiotoxin (20 nM). In contrast, photoreleased Ca2+ had no effect on cells missing their cilia, indicating that these conductances are confined to the cilia. Photorelease of cAMP induced a charybdotoxin-sensitive K+ current in intact ORNs. Photorelease of InsP3 did not increase the membrane conductance of olfactory neurons, arguing against a direct role of InsP3 in chemotransduction. We conclude that a cAMP cascade mediates the activation of the ciliary Ca2+-dependent K+ current and that the Ca2+ ions that activate the inhibitory current enter the cilia through CNG channels.

Chloride accumulation in mammalian olfactory sensory neurons

The generation of an excitatory receptor current in mammalian olfactory sensory neurons (OSNs) involves the sequential activation of two distinct types of ion channels: cAMP-gated Ca(2+)-permeable cation channels and Ca(2+)-gated Cl(-) channels, which conduct a depolarizing Cl(-) efflux. This unusual transduction mechanism requires an outward-directed driving force for Cl(-), established by active accumulation of Cl(-) within the lumen of the sensory cilia. We used two-photon fluorescence lifetime imaging microscopy of the Cl(-)-sensitive dye 6-methoxy-quinolyl acetoethyl ester to measure the intracellular Cl(-) concentration in dendritic knobs of OSNs from mice and rats. We found a uniform intracellular Cl(-) concentration in the range of 40-50 mm, which is indicative of active Cl(-) accumulation. Functional assays and PCR experiments revealed that NKCC1-mediated Cl(-) uptake through the apical membrane counteracts Cl(-) depletion in the sensory cilia, and thus maintains the responsiveness of OSNs to odor stimulation. To permit Cl(-) accumulation, OSNs avoid the "chloride switch": they do not express KCC2, the main Cl(-) extrusion cotransporter operating in neurons of the adult CNS. Cl(-) accumulation provides OSNs with the driving force for the depolarizing Cl(-) current that is the basis of the low-noise receptor current in these neurons.

Contribution of the secretory material of caecilian (amphibia: Gymnophiona) male Mullerian gland to motility of sperm: A study inUraeotyphlus narayani

Caecilians are a unique group of limbless burrowing amphibians with discontinuous distribution. Several caecilian species are viviparous, and all practice internal fertilization. In amniotic vertebrates the sperm undergo post-testicular physiological maturation when they are initiated into motility under the influence of an epididymal secretion. Further, during ejaculation mammalian sperm are suspended in a fluid secreted by the male accessory sex glands, viz., prostate gland and seminal vesicles. Caecilians lack comparable glands, but still practice internal fertilization. Uniquely, male caecilians retain the Mullerian ducts in the adults as a pair of functional glands. It has long been hypothesized, based on indirect evidence, that the Mullerian gland would be a male accessory sex gland, secreting a fluid in which sperm are suspended during ejaculation and which would also provide nutritional support to the ejaculated sperm. In the present study, the secretory material of the Mullerian gland of Uraeotyphlus narayani was mixed with sperm obtained from the testis, and the changes in motility were recorded. Uraeotyphlus narayani sperm possess a perforatorium of the acrosome proceeding deep into the endonuclear canal of the nucleus. The midpiece is characterized by closely applied centrioles, the anterior ends of the axoneme and axial fiber, and a mitochondrial sheath. The long tail has an undulating membrane on one side, supported by the axoneme and an axial fiber. The live sperm possess a mitochondrial vesicle, also known as the cytoplasmic droplet, anywhere along the head and the midpiece, as in anuran sperm, which is shed from sperm that have ceased motility. Uraeotyphlus narayani sperm are motile the moment they are released directly from the testis, indicating that the sperm do not require post-testicular physiological maturation. On being mixed with the secretory material of the Mullerian gland, the spermatozoa are enhanced in speed as well as duration of motility. Therefore, the caecilian male Mullerian gland is considered to be the male accessory sex gland.

Pheromones cause disease: pheromone/odourant transduction

This paper compares two models of the sense of smell and demonstrates that the new model has advantages over the accepted model with implications for medical research. The accepted transduction model had an odourant or pheromone contacting an aqueous sensory lymph then movement through it to a receptor membrane beneath. If the odourant or pheromone were non-soluble, the odourant/pheromone supposedly would be bound to a soluble protein in the lymph to be carried across. Thus, an odourant/carrier protein complex physically moved through the receptor lymph/mucus to interact with a membrane bound receptor. After the membranous receptor interaction, the molecule would be deactivated and any odourant/pheromone-binding protein recycled. This new electrical chemosensory model being proposed here has the pheromone or other odourant generating an electrical event in the extra-cellular mucus. Before the pheromone arrives, proteins of the 'carrier class' dissolved in the receptor mucus slowly and continuously sequester ions. A sensed pheromonal chemical species sorbs to the mucus and immediately binds to the now ion-holding dissolved protein. The binding of the pheromone to the protein causes a measurable conformational change in the pheromone/odourant-binding protein, desequestering ions. Releasing the bound ions changes the potential differences across a nearby super-sensitive dendritic membrane resulting in dendrite excitation. Pheromones will be implicated in the aetiology of the infectious, psychiatric and autoimmune diseases. This is the third article in a series of twelve to systematically explore this contention (see references 1-9).

A common inhibitory binding site for zinc and odorants at the voltage-gated K(+) channel of rat olfactory receptor neurons

This study compared the effects of zinc and odorants on the voltage-gated K(+) channel of rat olfactory neurons. Zinc reduced current magnitude, depolarized the voltage activation curve and slowed activation kinetics without affecting inactivation or deactivation kinetics. Zinc inhibition was potentiated by the NO compound, S-nitroso-cysteine. The pH- and diethylpyrocarbonate-dependence of zinc inhibition suggested that zinc acted by binding to histidine residues. Cysteine residues were eliminated as contributing to the zinc-binding site. The odorants, acetophenone and amyl acetate, also reduced current magnitude, depolarized the voltage activation curve and selectively slowed activation kinetics. Furthermore, the diethylpyrocarbonate- and pH-dependence of odorant inhibition implied that the odorants also bind to histidine residues. Zinc inhibitory potency was dramatically diminished in the presence of odorants, implying competition for a common binding site. These observations indicate that the odorants and zinc share a common inhibitory binding site on the external surface of the voltage-gated K(+) channel.

Responses to prolonged odour stimulation in frog olfactory receptor cells

1. The suction pipette technique was used to record receptor current and spiking responses from isolated frog olfactory receptor cells during prolonged odour stimuli. 2. The majority (70 %) of cells displayed 'oscillatory' responses, consisting of repeated bursts of spikes accompanied by regular increases in receptor current. The period of this oscillation varied from 3.5 to 12 s in different cells. The remaining cells responded either with a 'transient' burst of spikes at the onset of stimulation (10 %), or by 'sustained' firing throughout the odour stimulus (20 %). 3. In cells with oscillatory responses, the Ca(2+)-activated Cl(-) channel blocker niflumic acid prolonged the period of oscillation only slightly, despite a 3.8-fold decrease in the receptor current. A 3-fold reduction in the external Cl(-) concentration nearly doubled the receptor current, but had little effect on the oscillation period. These results imply that the majority of the receptor current underlying these oscillatory responses is carried by the Ca(2+)-activated Cl(-) conductance, suggesting that the intracellular Ca(2+) concentration oscillates also. 4. In cells with oscillatory responses, the period of oscillation was prolonged 1.5-fold when stimulated in a low-Na(+) solution designed to incapacitate Na(+)-Ca(2+) exchange, irrespective of whether Na(+) was replaced by permeant Li(+) or impermeant choline. The dependence of the oscillation period upon external Na(+) suggests that it may be governed by the dynamics of Ca(2+) extrusion via Na(+)-Ca(2+) exchange. 5. Exposure to the membrane-permeable cyclic nucleotide analogue CPT-cAMP evoked a sustained rather than an oscillatory response even in cells with oscillatory responses to odour. The inability of CPT-cAMP to evoke an oscillatory response suggests that the cAMP concentration is likely to oscillate also. 6. Perforated-patch recordings revealed that oscillatory responses could only be evoked when the membrane potential was free to change, but not when it was clamped near the resting potential. Since substantial changes in Ca(2+)-activated Cl(-) current, and hence odour-induced depolarisation, had little effect upon the period of oscillation, changes in membrane potential are suggested to play only a permissive role in these oscillatory responses. 7. These results are interpreted in terms of the coupled oscillation of Ca(2+) and cyclic nucleotide concentrations within the olfactory cilia during prolonged odour stimulation.

Noninvasive measurement of chloride concentration in rat olfactory receptor cells with use of a fluorescent dye

Inwardly directed Ca(2+)-dependent chloride currents are thought to prolong and boost the odorant-induced transient receptor currents in olfactory cilia. Cl(-) inward current, of course, requires a sufficiently high intracellular Cl(-) concentration ([Cl(-)](i)). In previous measurements using a fluorescent Cl(-) probe, N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE), [Cl(-)](i) of newt olfactory cells was estimated to be only 40 mM. This low value led us to reexamine the [Cl(-)](i) by an improved procedure. When isolated rat olfactory neurons were bathed in Tyrode's solution (150 mM Cl(-)) at room temperature, the [Cl(-)] was 81.5 +/- 13.5 mM (mean +/- SE) in the tip of the dendrite (olfactory knob) and 81.8 +/- 10.2 mM (mean +/- SE) in the soma. The corresponding Cl(-) equilibrium potentials were -15.4 and -15.3 mV, respectively. Therefore, at resting potentials in the range of -90 to -50 mV, Cl(-) currents are predicted to be inward and capable of contributing to the depolarization induced by odorants. Yet, if the cell was depolarized beyond -15 mV, somal Cl(-) currents would be outward and facilitate repolarization during excitation. The measured [Cl(-)] in soma and knob are of interest, because in the cilia the chloride content may be expected to equilibrate with that of the knob in the resting state. They provide a starting point for the decrease in ciliary [Cl(-)] predicted to occur during transduction.

Modulation by cGMP of the voltage-gated currents in newt olfactory receptor cells

Effects of cGMP on voltage-gated currents in the somatic membrane of isolated newt olfactory receptor cells were investigated using the whole-cell mode of the patch-clamp technique. Under voltage clamp, membrane depolarization generated time- and voltage-dependent current responses, a transient inward current and a sustained outward current. When cGMP or a membrane permeant analog of cGMP, 8-p-chlorophenylthio-cGMP (CPT-cGMP), was applied to the recorded cell, the amplitude of the transient inward current increased markedly, but that of the sustained outward current did not change significantly. When each current was isolated by pharmacological agents, 0.1 mM CPT-cGMP increased the peak amplitude of a Na(+) current (I(Na)) by approximately 40%, a T-type Ca(2+) current (I(Ca,T)) by approximately 40%, and an L-type Ca(2+)current (I(Ca,L)) by approximately 10%; however it did not change significantly the amplitude of a delayed rectifier K(+) current (I(K)). A selective cGMP-dependent protein kinase inhibitor, KT5823, blocked the enhancement by cGMP of I(Na) and I(Ca,T), suggesting that cGMP increases these currents via cGMP-dependent phosphorylation. Under current-clamp conditions, application of CPT-cGMP lowered the current threshold of action potentials induced by current injection, and increased the maximum spike frequency in response to strong stimuli. We suggest that cGMP may lower the threshold in olfactory perception by decreasing the current threshold to generate spikes, and also prevent the saturation of odor signals by increasing the maximum spike frequency.

Cellular and molecular constituents of olfactory sensation in vertebrates

Since the discovery of odorant-activated adenylate cyclase in the olfactory receptor cilia, research into the olfactory perception of vertebrates has rapidly expanded. Recent studies have shown how the odor discrimination starts at the receptor level: each of 700-1000 types of the olfactory neurons in the neural olfactory epithelium contains a single type of odor receptor protein. Although the receptors have relatively low specific affinities for odorants, excitation of different types of receptors forms an excitation pattern specific to each odorant in the glomerular layer of the olfactory bulb. It was demonstrated that adenosine 3',5'-cyclic monophosphate (cAMP) is very likely the sole second messenger for olfactory transduction. It was also demonstrated that the affinity of the cyclic nucleotide-gated channel for cAMP regulated by Ca(2+)/calmodulin is solely responsible for the adaptation of the cell. However, many other regulatory components were found in the transduction cascade. Regulated by Ca(2+) and/or the protein-phosphorylation, many of them may serve for the adaptation of the cell, probably on a longer time scale. It may be important to consider the resensitization as a part of this adaptation, as well as to collect kinetic data of each reaction to gain further insight into the olfactory mechanism.

Neuronal Ca2+ -activated Cl- channels--homing in on an elusive channel species

Ca2+ -activated Cl- channels control electrical excitability in various peripheral and central populations of neurons. Ca2+ influx through voltage-gated or ligand-operated channels, as well as Ca2+ release from intracellular stores, have been shown to induce substantial Cl- conductances that determine the response to synaptic input, spike rate, and the receptor current of various kinds of neurons. In some neurons, Ca2+ -activated Cl- channels are localized in the dendritic membrane, and their contribution to signal processing depends on the local Cl- equilibrium potential which may differ considerably from those at the membranes of somata and axons. In olfactory sensory neurons, the channels are expressed in ciliary processes of dendritic endings where they serve to amplify the odor-induced receptor current. Recent biophysical studies of signal transduction in olfactory sensory neurons have yielded some insight into the functional properties of Ca2+ -activated Cl- channels expressed in the chemosensory membrane of these cells. Ion selectivity, channel conductance, and Ca2+ sensitivity have been investigated, and the role of the channels in the generation of receptor currents is well understood. However, further investigation of neuronal Ca2+ -activated Cl- channels will require information about the molecular structure of the channel protein, the regulation of channel activity by cellular signaling pathways, as well as the distribution of channels in different compartments of the neuron. To understand the physiological role of these channels it is also important to know the Cl- equilibrium potential in cells or in distinct cell compartments that express Ca2+ -activated Cl- channels. The state of knowledge about most of these aspects is considerably more advanced in non-neuronal cells, in particular in epithelia and smooth muscle. This review, therefore, collects results both from neuronal and from non-neuronal cells with the intent of facilitating research into Ca2+ -activated Cl- channels and their physiological functions in neurons.

The contribution of a Ca(2+)-activated Cl(−) conductance to amino-acid-induced inward current responses of ciliated olfactory neurons of the rainbow trout

To determine whether amino-acid-induced inward currents of ciliated olfactory receptor neurons (ORNs) in rainbow trout (Oncorhynchus mykiss) include a Ca(2+)-activated Cl(−) conductance, we first studied changes in reversal potential and the current/voltage relationships of the responses of ORNs to an amino acid mixture (l-alanine, l-arginine, l-glutamate and l-norvaline; all 10 mmol l(−)(1)) with different concentrations of Na(+) and Cl(−) in the perfusion and recording pipette solutions. We also examined the effects of six different Cl(−) channel blockers on the responses of ORNs using a conventional whole-cell voltage-clamp technique. The amino acid mixture and one blocker were applied focally to the cilia of ORNs using a double-barrelled micropipette and a pressure ejection system. The expected shifts in reversal potential, indicating the contribution of the Ca(2+)-activated Cl(−) conductance, occurred in both positive and negative directions depending on the external and internal Na(+) and Cl(−) concentrations. Niflumic acid, flufenamic acid, NPPB [5-nitro-2-(3-phenylpropylamino)-benzonate] and DCDPC (3′, 5-dichlorodiphenylamine-2-carboxylate), at 0.5 mmol l(−)(1), reversibly blocked both the amino-acid-induced inward currents and the background activity in most ORNs. The effectiveness of these blocking agents varied from 77 to 91 % for ORNs perfused externally with standard Ringer's solution. SITS (4-acetamido-4′-isothiocyanatostilbene-2,2′-disulphonate), at 5.0 mmol l(−)(1), irreversibly inhibited the physiological response (100 % inhibition), whereas DIDS (4,4′-diisothiocyanatostilbene-2, 2′-disulphonate), at 5.0 mmol l(−)(1), had the smallest effect (45 %) of the inhibitors tested. The dose of niflumic acid inducing 50 % inhibition (IC(50)), determined specifically for the current component of the Ca(2+)-activated Cl(−) channels, was 70 μmol l(−)(1). Our results suggest that these blockers are not specific for Ca(2+)-activated Cl(−) channels and that the density of these channels varies between individual ORNs. Our results also show that the Ca(2+)-activated Cl(−) conductance plays an important role in olfactory transduction and allows fishes to adapt to various ionic environments.

Odorants suppress T- and L-type Ca2+ currents in olfactory receptor cells by shifting their inactivation curves to a negative voltage

Mechanisms underlying suppression of T- and L-type Ca2+ currents (I(Ca,T) and I(Ca,L)) by odorants were investigated in newt olfactory receptor cells (ORCs) using the whole-cell version of the patch-clamp technique. Under voltage clamp, odorants (amyl acetate, limonene and acetophenone) reversibly suppressed I(Ca,T) and I(Ca, L). These currents disappeared completely within 150 ms following amyl acetate puffs, and recovered in approximately 1 s after the washout. Hyperpolarization of the membrane greatly relieved the odorant block of I(Ca,T) and I(Ca,L). The activation curves of both currents were not changed significantly by odorants, while their inactivation curves were shifted to negative voltages. Half-inactivation voltages of I(Ca,T) were - 66 mV (control), - 102 mV (amyl acetate), - 101 mV (limonene) and - 105 mV (acetophenone) (all 0.3 mM); those of I(Ca,L) were -33 mV (control), - 61 mV (amyl acetate), - 59 mV (limonene), and - 63 mV (acetophenone) (all 0.3 mM). These phenomena are similar to the effects of local anesthetics on I(Ca) in various preparations and also similar to the effects of odorants on I(Na) in ORCs, suggesting that these types of suppression are caused by the same mechanism.

Adaptation of the odour-induced response in frog olfactory receptor cells

1. Receptor current and spiking responses were recorded simultaneously from isolated frog olfactory receptor cells using the suction pipette technique. Cells were stimulated with the odour cineole by rapid exchange of the solution bathing the olfactory cilia. 2. The receptor current response to a 1 s odour stimulus increased in a graded manner over a 300-fold range of odour concentration without clear saturation, and was accompanied by a train of action potentials. As the concentration of the odour stimulus increased, the frequency of firing increased also, until it saturated at the highest concentrations. The number of spikes evoked by the stimulus first increased and then decreased with increasing concentration, reaching a maximum at intermediate odour concentrations. The dose-response relation for spike firing rose at lower odour concentrations than the dose-response relation for the receptor current response. 3. Adaptation to steady odour stimuli was investigated by exposing the cilia to a 4 s odour pre-pulse and then to a 1 s odour test pulse. As the pre-pulse concentration was increased the dose-response relations derived from the receptor current and spiking responses shifted to higher absolute test pulse concentrations. However the number of spikes fired in response to a given test pulse was little affected by the pre-pulse until, at the highest pre-pulse concentrations spike firing was abolished despite the continued presence of a receptor current response. 4. The sensitivity of the receptor-current response to incremental stimuli fell with increasing pre-pulse concentration, declining with a limiting slope of 2.4 in double logarithmic co-ordinates. The sensitivity determined from the spiking responses declined to zero at a lower pre-pulse concentration, reflecting the abolition of spike firing at pre-pulse concentrations which still evoked a graded receptor-current response.

Cyclic AMP diffusion coefficient in frog olfactory cilia

Cyclic AMP (cAMP) is one of the intracellular messengers that mediate odorant signal transduction in vertebrate olfactory cilia. Therefore, the diffusion coefficient of cAMP in olfactory cilia is an important factor in the transduction of the odorous signal. We have employed the excised cilium preparation from the grass frog (Rana pipiens) to measure the cAMP diffusion coefficient. In this preparation an olfactory cilium is drawn into a patch pipette and a gigaseal is formed at the base of the cilium. Subsequently the cilium is excised, allowing bath cAMP to diffuse into the cilium and activate the cyclic nucleotide-gated channels on the plasma membrane. In order to estimate the cAMP diffusion coefficient, we analyzed the kinetics of the currents elicited by step changes in the bath cAMP concentration in the absence of cAMP hydrolysis. Under such conditions, the kinetics of the cAMP-activated currents has a simple dependence on the diffusion coefficient. From the analysis we have obtained a cAMP diffusion coefficient of 2.7 +/- 0.2. 10(-6) cm2 s-1 for frog olfactory cilia. This value is similar to the expected value in aqueous solution, suggesting that there are no significant diffusional barriers inside olfactory cilia. At cAMP concentrations higher than 5 microM, diffusion slowed considerably, suggesting the presence of buffering by immobile cAMP binding sites. A plausible physiological function of such buffering sites would be to prolong the response of the cell to strong stimuli.

Inhibitory and excitatory responses of olfactory receptor neurons of xenopus laevis tadpoles to stimulation with amino acids

Recordings were made from olfactory receptor neurons of Xenopus laevis tadpoles using the patch-clamp technique to investigate the responses of these cells to odorants. Four amino acids (glutamate, methionine, arginine and alanine) both individually and as a mixture were used as stimuli. Of the 156 olfactory neurons tested, 43 showed a response to at least one of the stimuli. Of the cells tested, 19 % responded to glutamate, 16 % to methionine, 12 % to arginine and 10 % to alanine. Each amino acid was able to induce both excitatory and inhibitory responses, although these occurred in different cells. Each amino acid produced approximately equal numbers of inhibitory and excitatory responses. Inhibitory responses could best be observed in the perforated-patch configuration using gramicidin as an ionophore and a recording configuration that is a current-clamp for fast signals and a voltage-clamp for slow signals. The diversity of the odorant responses, in particular the existence of excitatory and inhibitory responses, is not consistent with a single transduction pathway in olfactory neurons of Xenopus laevis tadpoles.

Lysophosphatidic acid stimulates neurotransmitter-like conductance changes that precede GABA and L-glutamate in early, presumptive cortical neuroblasts

During neurogenesis in the embryonic cerebral cortex, the classical neurotransmitters GABA and L-glutamate stimulate ionic conductance changes in ventricular zone (VZ) neuroblasts. Lysophosphatidic acid (LPA) is a bioactive phospholipid producing myriad effects on cells including alterations in membrane conductances (for review, see Moolenaar et al., 1995). Developmental expression patterns of its first cloned receptor gene, lpA1/vzg-1 (Hecht et al., 1996; Fukushima et al., 1998) in the VZ suggested that functional LPA receptors were synthesized at these early times, and thus, LPA could be an earlier stimulus to VZ cells than the neurotransmitters GABA and L-glutamate. To address this possibility, primary cultures of electrically coupled, presumptive cortical neuroblast clusters were identified by age, morphology, electrophysiological profile, BrdU incorporation, and nestin immunostaining. Single cells from cortical neuroblast cell lines were also examined. Whole-cell variation of the patch-clamp technique was used to record from nestin-immunoreactive cells after stimulation by local administration of ligands. After initial plating at embryonic day 11 (E11), cells responded only to LPA but not to GABA or L-glutamate. Continued growth in culture for up to 12 hr produced more LPA-responsive cells, but also a growing population of GABA- or L-glutamate-responsive cells. Cultures from E12 embryos showed LPA as well as GABA and L-glutamate responses, with LPA-responsive cells still representing a majority. Overall, >50% of cells responded to LPA with depolarization mediated by either chloride or nonselective cation conductances. These data implicate LPA as the earliest reported extracellular stimulus of ionic conductance changes for cortical neuroblasts and provide evidence for LPA as a novel, physiological component in CNS development.

Lysophosphatidic acid stimulates neurotransmitter-like conductance changes that precede GABA and L-glutamate in early, presumptive cortical neuroblasts

During neurogenesis in the embryonic cerebral cortex, the classical neurotransmitters GABA and L-glutamate stimulate ionic conductance changes in ventricular zone (VZ) neuroblasts. Lysophosphatidic acid (LPA) is a bioactive phospholipid producing myriad effects on cells including alterations in membrane conductances (for review, see Moolenaar et al., 1995). Developmental expression patterns of its first cloned receptor gene, lpA1/vzg-1 (Hecht et al., 1996; Fukushima et al., 1998) in the VZ suggested that functional LPA receptors were synthesized at these early times, and thus, LPA could be an earlier stimulus to VZ cells than the neurotransmitters GABA and L-glutamate. To address this possibility, primary cultures of electrically coupled, presumptive cortical neuroblast clusters were identified by age, morphology, electrophysiological profile, BrdU incorporation, and nestin immunostaining. Single cells from cortical neuroblast cell lines were also examined. Whole-cell variation of the patch-clamp technique was used to record from nestin-immunoreactive cells after stimulation by local administration of ligands. After initial plating at embryonic day 11 (E11), cells responded only to LPA but not to GABA or L-glutamate. Continued growth in culture for up to 12 hr produced more LPA-responsive cells, but also a growing population of GABA- or L-glutamate-responsive cells. Cultures from E12 embryos showed LPA as well as GABA and L-glutamate responses, with LPA-responsive cells still representing a majority. Overall, >50% of cells responded to LPA with depolarization mediated by either chloride or nonselective cation conductances. These data implicate LPA as the earliest reported extracellular stimulus of ionic conductance changes for cortical neuroblasts and provide evidence for LPA as a novel, physiological component in CNS development.

Adrenaline enhances odorant contrast by modulating signal encoding in olfactory receptor cells

Olfactory perception is influenced by hormones. Here we report that adrenaline can directly affect the signal encoding of olfactory receptor cells. Application of adrenaline suppressed action potentials near threshold and increased their frequency in response to strong stimuli, resulting in a narrower dynamic range. Under voltage-clamp conditions, adrenaline enhanced sodium current and reduced T-type calcium current. Because sodium current is the major component of spike generation and T-type calcium current lowers the threshold in olfactory receptor cells, the effects of adrenaline on these currents are consistent with the results obtained under current-clamp conditions. Both effects involved a common cytoplasmic pathway, cAMP-dependent phosphorylation. We suggest that adrenaline may enhance contrast in olfactory perception by this mechanism.

Involvement of genes encoding a K+ channel (ether a go-go) and a Na+ channel (smellblind) in Drosophila olfaction

We have investigated the roles of the putative cyclic nucleotide-modulated K+ channel subunit encoded by the ether a go-go (eag) gene and a voltage-gated Na+ channel, smellblind (sbl), encoded by the paralytic (para) locus in odorant responsiveness and cell excitability in Drosophila melanogaster. Three independent mutant alleles of eag revealed reduced antennal responsiveness in adult flies to a subset of odorants, all having short aliphatic side chains: ethyl butyrate (EB), propionic acid, 2-butanone and ethyl acetate (manuscript submitted). Loose patch recordings revealed that significantly fewer eag antennal neurons responded to EB compared to control neurons. As expected if Eag were involved in odor transduction, fewer EB-induced inhibitory responses were observed in eag mutants and focal application of high K+ saline to sensillae altered the excitability of the majority of neurons from wild-type, but not eag, antennae. Interestingly, there were fewer excitatory odorant responses dependent on extracellular Ca2+ in eag neurons. In contrast to the involvement of Eag in adult olfactory neuron odorant transduction, we found no evidence that adult sbl and allelic olfactory D (olfD) gene mutants were defective in their behavioral response to a complex attractive odor. Furthermore, electrophysiological analyses of adult sbl and olfD mutants revealed normal electroantennogram responses to a broad range of individual pure odorants and no changes in the excitable properties of olfactory neurons as determined by loose patch recordings.

Na+-dependent Ca2+ extrusion governs response recovery in frog olfactory receptor cells

To study the mechanism by which Ca2+, which enters during the odor response, is extruded during response recovery, recordings were made from isolated frog olfactory receptor cells using the suction pipette technique, while superfusing the olfactory cilia with solutions of modified ionic composition. When external Na+ was substituted with another cation, the response to odor was greatly prolonged. This prolongation of the response was similar irrespective of whether Na+ was replaced with Li+, which permeates the cyclic nucleotide-gated conductance, or choline, which does not. The prolonged current was greatly reduced by exposure to 300 microM niflumic acid, a blocker of the calcium-activated chloride channel, indicating that it is carried by this conductance, and abolished if Ca2+ was omitted from the external solution, demonstrating that Ca2+ influx is required for its generation. When the cilia were exposed to Na+-free solution after odor stimulation, the recovery of the response to a second stimulus from the adaptation induced by the first was greatly reduced. We conclude that a Na+-dependent Ca2+ extrusion mechanism is present in frog olfactory cilia and that it serves as the main mechanism that returns cytoplasmic Ca2+ concentration to basal levels after stimulation and mediates the normally rapid recovery of the odor response and the restoration of sensitivity after adaptation.

A depolarizing chloride current contributes to chemoelectrical transduction in olfactory sensory neurons in situ

Recent biophysical investigations of vertebrate olfactory signal transduction have revealed that Ca2+-gated Cl- channels are activated during odorant detection in the chemosensory membrane of olfactory sensory neurons (OSNs). To understand the role of these channels in chemoelectrical signal transduction, it is necessary to know the Cl--equilibrium potential that determines direction and size of Cl- fluxes across the chemosensory membrane. We have measured Cl-, Na+, and K+ concentrations in ultrathin cryosections of rat olfactory epithelium, as well as relative element contents in isolated microsamples of olfactory mucus, using energy-dispersive x-ray microanalysis. Determination of the Cl- concentrations in dendritic knobs and olfactory mucus yielded an estimate of the Cl--equilibrium potential ECl in situ. With Cl- concentrations of 69 mM in dendritic knobs and 55 mM in olfactory mucus, we obtained an ECl value of +6 +/- 12 mV. This indicates that Ca2+-gated Cl- channels in olfactory cilia conduct inward currents in vivo carried by Cl- efflux into the mucus. Our results show that rat OSNs are among the few known types of neurons that maintain an elevated level of cytosolic Cl-. In these cells, activation of Cl- channels leads to depolarization of the membrane voltage and can induce electrical excitation. The depolarizing Cl- current in mammalian OSNs appears to contribute a major fraction to the receptor current and may sustain olfactory function in sweet-water animals.

A depolarizing chloride current contributes to chemoelectrical transduction in olfactory sensory neurons in situ

Recent biophysical investigations of vertebrate olfactory signal transduction have revealed that Ca2+-gated Cl- channels are activated during odorant detection in the chemosensory membrane of olfactory sensory neurons (OSNs). To understand the role of these channels in chemoelectrical signal transduction, it is necessary to know the Cl--equilibrium potential that determines direction and size of Cl- fluxes across the chemosensory membrane. We have measured Cl-, Na+, and K+ concentrations in ultrathin cryosections of rat olfactory epithelium, as well as relative element contents in isolated microsamples of olfactory mucus, using energy-dispersive x-ray microanalysis. Determination of the Cl- concentrations in dendritic knobs and olfactory mucus yielded an estimate of the Cl--equilibrium potential ECl in situ. With Cl- concentrations of 69 mM in dendritic knobs and 55 mM in olfactory mucus, we obtained an ECl value of +6 +/- 12 mV. This indicates that Ca2+-gated Cl- channels in olfactory cilia conduct inward currents in vivo carried by Cl- efflux into the mucus. Our results show that rat OSNs are among the few known types of neurons that maintain an elevated level of cytosolic Cl-. In these cells, activation of Cl- channels leads to depolarization of the membrane voltage and can induce electrical excitation. The depolarizing Cl- current in mammalian OSNs appears to contribute a major fraction to the receptor current and may sustain olfactory function in sweet-water animals.

Transduction mechanisms in vertebrate olfactory receptor cells

Considerable progress has been made in the understanding of transduction mechanisms in olfactory receptor neurons (ORNs) over the last decade. Odorants pass through a mucus interface before binding to odorant receptors (ORs). The molecular structure of many ORs is now known. They belong to the large class of G protein-coupled receptors with seven transmembrane domains. Binding of an odorant to an OR triggers the activation of second messenger cascades. One second messenger pathway in particular has been extensively studied; the receptor activates, via the G protein Golf, an adenylyl cyclase, resulting in an increase in adenosine 3',5'-cyclic monophosphate (cAMP), which elicits opening of cation channels directly gated by cAMP. Under physiological conditions, Ca2+ has the highest permeability through this channel, and the increase in intracellular Ca2+ concentration activates a Cl- current which, owing to an elevated reversal potential for Cl-, depolarizes the olfactory neuron. The receptor potential finally leads to the generation of action potentials conveying the chemosensory information to the olfactory bulb. Although much less studied, other transduction pathways appear to exist, some of which seem to involve the odorant-induced formation of inositol polyphosphates as well as Ca2+ and/or inositol polyphosphate -activated cation channels. In addition, there is evidence for odorant-modulated K+ and Cl- conductances. Finally, in some species, ORNs can be inhibited by certain odorants. This paper presents a comprehensive review of the biophysical and electrophysiological evidence regarding the transduction processes as well as subsequent signal processing and spike generation in ORNs.

Direct measurement of the chloride concentration in newt olfactory receptors with the fluorescent probe

Chloride (Cl-) current in the olfactory cell has been proposed to be excitatory and amplifying the receptor current. As an intracellular concentration of Cl- ([Cl-]i) is critically important for the proposed function, we tried to measure [Cl-]i of the isolated newt olfactory receptors using chloride-sensitive fluorescent dye, N-(6-methoxyquinolyl)-acetoethyl ester (MQAE). We found that the average of Cl- concentration through the cell is about 40 mM. This value is fairly lower than 120 mM that was suggested from reversal potential of tail component of the olfactory response in the low Cl- bath solution. Because the reversal potential for the present [Cl-]i is above the resting potential, opening of the Cl- channels may serve as a booster for the depolarizing odor-response.

Urine-derived compound evokes membrane responses in mouse vomeronasal receptor neurons

Sensory neurons of the vomeronasal organ (VNO) are thought to detect species-specific chemical signals important for reproductive function. The electrical properties of VNO neurons have begun to be characterized in a variety of species; however, the response of VNO neurons to possible physiological ligands has not yet been reported. One physiological effector, dehydro-exo-brevicomin (DHB), is found in the urine of intact male mice and affects the estrous cycle of female mice. In the present study, dissociated VNO neurons were voltage- or current-clamped and their response to DHB was determined. Approximately 26% of VNO neurons responded to DHB with an outward current at negative holding potentials; the current reversed at approximately +4 mV. Application of DHB in current-clamp mode produced membrane hyperpolarization and/or a reduction in the firing of action potentials. Because membrane conductance was shown to be decreased during application of DHB, the results suggest that the outward current associated with DHB application is a reflection of a reduction in inward current caused by closing an ion channel. This study provides the first evidence that a compound found in male urine directly affects VNO neurons.

Odorant-induced currents in intact patches from rat olfactory receptor neurons: theory and experiment

Odorant-induced currents in mammalian olfactory receptor neurons have proved difficult to obtain reliably using conventional whole-cell recording. By using a mathematical model of the electrical circuit of the patch and rest-of-cell, we demonstrate how cell-attached patch measurements can be used to quantitatively analyze responses to odorants or a high (100 mM) K+ solution. High K+ induced an immediate current flux from cell to pipette, which was modeled as a depolarization of approximately 52 mV, close to that expected from the Nernst equation (56 mV), and no change in the patch conductance. By contrast, a cocktail of cAMP-stimulating odorants induced a current flux from pipette into cell following a significant (4-10 s) delay. This was modeled as an average patch conductance increase of 36 pS and a depolarization of 13 mV. Odorant-induced single channels had a conductance of 16 pS. In cells bathed with no Mg2+ and 0.25 mM Ca2+, odorants induced a current flow from cell to pipette, which was modeled as a patch conductance increase of approximately 115 pS and depolarization of approximately 32 mV. All these results are consistent with cAMP-gated cation channels dominating the odorant response. This approach, which provides useful estimates of odorant-induced voltage and conductance changes, is applicable to similar measurements in any small cells.

Nonselective suppression of voltage-gated currents by odorants in the newt olfactory receptor cells

Effects of odorants on voltage-gated ionic channels were investigated in isolated newt olfactory receptor cells by using the whole cell version of the patch-clamp technique. Under voltage clamp, membrane depolarization to voltages between -90 mV and +40 mV from a holding potential (Vh) of -100 mV generated time- and voltage-dependent current responses; a rapidly (< 15 ms) decaying initial inward current and a late outward current. When odorants (1 mM amyl acetate, 1 mM acetophenone, and 1 mM limonene) were applied to the recorded cell, the voltage-gated currents were significantly reduced. The dose-suppression relations of amyl acetate for individual current components (Na+ current: I(Na), T-type Ca2+ current: I(Ca), T, L-type Ca2+ current: I(Ca), L, delayed rectifier K+ current: I(KV) and Ca2(+)-activated K+ current: IK(Ca)) could be fitted by the Hill equation. Half-blocking concentrations for each current were 0.11 mM (INa), 0.15 mM (ICa,T), 0.14 mM (ICa,L), 1.7 mM (IKV), and 0.17 mM (IK(Ca)), and Hill coefficient was 1.4 (INa), 1.0 (ICa,T), 1.1 (ICa,L), 1.0 (IKV), and 1.1 (IK(Ca)), suggesting that the inward current is affected more strongly than the outward current. The activation curve of INa was not changed significantly by amyl acetate, while the inactivation curve was shifted to negative voltages; half-activation voltages were -53 mV at control, -66 mV at 0.01 mM, and -84 mV at 0.1 mM. These phenomena are similar to the suppressive effects of local anesthetics (lidocaine and benzocaine) on INa in various preparations, suggesting that both types of suppression are caused by the same mechanism. The nonselective blockage of ionic channels observed here is consistent with the previous notion that the suppression of the transduction current by odorants is due to the direst blockage of transduction channels.

Voltage-activated and odor-modulated conductances in olfactory neurons of Drosophila melanogaster

Voltage-activated currents and odor-modulated conductances were studied in cells in semi-intact Drosophila third antennal segments (the main olfactory organ) using patch-clamp techniques. All neurons expressed outward currents, and most expressed labile fast transient inward currents with kinetics similar to Na+ currents in other systems. Action potentials were detected as bipolar capacitative current transients in cell-attached or loose patches from the soma of both odor-sensitive (97%) and insensitive neurons. A mixture of odorants from five chemical classes caused an increase (approximately 70%), decrease (approximately 10%), or no effect on firing frequency in pharate adult neurons. The development of chemosensitivity was examined and odor-induced changes in action potential firing frequency were recorded in pupal antennal neurons as early as P8, a stage after completion of sensillar development. The character of odor-induced responses was more profound and complex later in development; small, tonic increases in firing frequency were observed at pupal stages P8 through P11 (ii), while in older pupae and young adults approximately 25% of the increased responses were phasic-tonic. The apical dendrite was the site of odor modulation in approximately 90% and 100% of responsive adult and early pupal neurons, respectively. Whole-cell recordings revealed that apparent nonselective cation and chloride conductances were modulated by a mixture of odorants in separate antennal neurons.

A putative cyclic nucleotide-gated channel is required for sensory development and function in C. elegans

In vertebrate visual and olfactory systems, a cyclic nucleotide-gated channel couples receptor activation to electrical activity of the sensory neurons. The Caenorhabditis elegans tax-2 gene is required for some forms of olfaction, for chemosensation of salts, and for thermosensation. We show here that tax-2 encodes a predicted subunit of a cyclic nucleotide-gated channel that is expressed in olfactory, gustatory, and thermosensory neurons, implicating this channel in multiple sensory modalities. Some sensory neurons display axon outgrowth defects in tax-2 mutants. Thus, the channel has an unexpected role in sensory neuron development in addition to its role in sensation. Consistent with this proposed dual function, a Tax-2::GFP fusion protein is present both in sensory cilia and in sensory axons.