Stimulation of olfactory receptors alters regulation of [Cai] in olfactory neurons of the catfish (Ictalurus punctatus)

Imaging ensemble activity in arthropod olfactory receptor neurons in situ

We show that lobster olfactory receptor neurons (ORNs), much like their vertebrate counterparts, generate a transient elevation of intracellular calcium (Ca(i)) in response to odorant activation that can be used to monitor ensemble ORN activity. This is done in antennal slice preparation in situ maintaining the polarity of the cells and the normal micro-environment of the olfactory cilia. The Ca(i) signal is ligand-specific and increases in a dose-dependent manner in response to odorant stimulation. Saturating stimulation elicits a robust increase of up to 1 μM free Ca(i) within 1-2s of stimulation. The odor-induced Ca(i) response closely follows the discharge pattern of extracellular spikes elicited by odorant application, with the maximal rise in Ca(i) matching the peak of the spike generation. The Ca(i) signal can be used to track neuronal activity in a functional subpopulation of rhythmically active ORNs and discriminate it from that of neighboring tonically active ORNs. Being able to record from many ORNs simultaneously over an extended period of time not only allows more accurate estimates of neuronal population activity but also dramatically improves the ability to identify potential new functional subpopulations of ORNs, especially those with more subtle differences in responsiveness, ligand specificity, and/or transduction mechanisms.

The peripheral olfactory system of the domestic chicken: physiology and development

Olfaction is a ubiquitous sensory system found in all terrestrial vertebrates. Birds use olfaction for several important activities such as feeding and mating; thus, understanding bird biology would also require the systematic study olfaction. In addition, the olfactory system has several unique features that are useful for the study of nervous system function and development, including a large multigene family for olfactory receptor expression, peripheral neurons that regenerate, and a complex system for sensory innervation of the olfactory bulb. We focused on physiological, anatomical and behavioral approaches to study the chick olfactory neurons and the olfactory bulb. Chick olfactory neurons displayed some properties similar to those found in mature neurons of other vertebrate species, and other properties that were unique. Since information from these neurons is initially processed in the olfactory bulb, we also conducted preliminary studies on the developmental timeline of this structure and showed that glomerular structures are organized in ovo during a critical time period, during which embryonic chicks can form behavioral associations with odorants introduced in ovo. Lastly, we have shown that chick olfactory neurons can grow and mature in vitro, allowing their use in cell culture studies. These results collectively demonstrate some of the features of the olfactory system that are common to all vertebrates, and some that are unique to birds. These highlight the potential for the use of the physiology and development of the olfactory system as a model system for avian brain neurobiology.

Highly specific olfactory receptor neurons for types of amino acids in the channel catfish

Odorant specificity to l-alpha-amino acids was determined electrophysiologically for 93 single catfish olfactory receptor neurons (ORNs) selected for their narrow excitatory molecular response range (EMRR) to only one type of amino acid (i.e., Group I units). These units were excited by either a basic amino acid, a neutral amino acid with a long side chain, or a neutral amino acid with a short side chain when tested at 10(-7) to 10(-5) M. Stimulus-induced inhibition, likely for contrast enhancement, was primarily observed in response to the types of amino acid stimuli different from that which activated a specific ORN. The high specificity of single Group I ORNs to type of amino acid was also previously observed for single Group I neurons in both the olfactory bulb and forebrain of the same species. These results indicate that for Group I neurons olfactory information concerning specific types of amino acids is processed from receptor neurons through mitral cells of the olfactory bulb to higher forebrain neurons without significant alteration in unit odorant specificity.

Novel stimulus-induced calcium efflux in Drosophila mushroom bodies

The mushroom body (MB) is an important part of the Drosophila brain, and is involved in many behaviors, including olfactory learning and memory and some visual cognition. However, the physiological properties of MB neurons remain elusive. Here we used a calcium-imaging technique to study calcium signals in Drosophila MB. We found that, rather than increasing calcium spread, electrical stimuli dramatically decreased calcium signals in the terminals of MB fibers. This novel calcium decrease occurred at all developmental stages from larvae to adults, but was specific for certain regions of the MB neurons. GABA receptor blockade promoted calcium propagation through the MB fibers, but did not disrupt the stimulus-induced decrease in calcium. Furthermore, this decrease in calcium was independent of extracellular calcium concentration and was not due to altered uptake by intracellular calcium stores and mitochondria. Rather, we found that inhibition of sodium-calcium exchangers significantly attenuated the stimulus-induced decrease in calcium, whereas the decrease persisted when membrane calcium pumps were blocked. Our findings indicate that MB neurons exhibit a novel stimulus-induced calcium efflux, which may be importantly regulated by sodium-calcium exchangers in the Drosophila MB.

Characteristics of odorant elicited calcium fluxes in acutely-isolated chick olfactory neurons

To understand avian olfaction, it is important to characterize the peripheral olfactory system of a representative bird species. This study determined the functional properties of olfactory receptor neurons of the chicken olfactory epithelium. Individual neurons were acutely isolated from embryonic day-18 to newborn chicks by dissection and enzymatic dissociation. We tested single olfactory neurons with behaviorally relevant odorant mixtures and measured their responses using ratiometric calcium imaging; techniques used in this study were identical to those used in other studies of olfaction in other vertebrate species. Chick olfactory neurons displayed properties similar to those found in other vertebrates: they responded to odorant stimuli with either decreases or increases in intracellular calcium, calcium increases were mediated by a calcium influx, and responses were reversibly inhibited by 100 microM L: -cis-diltiazem, 1 mM Neomycin, and 20 microM U73122, which are biochemical inhibitors of second messenger signaling. In addition, some cells showed a complex pattern of responses, with different odorant mixtures eliciting increases or decreases in calcium in the same cell. It appears that there are common features of odorant signaling shared by a variety of vertebrate species, as well as features that may be peculiar to chickens.

Evidence for multiple calcium response mechanisms in mammalian olfactory receptor neurons

Olfactory receptor neurons employ a diversity of signaling mechanisms for transducing and encoding odorant information. The simultaneous activation of subsets of receptor neurons provides a complex pattern of activation in the olfactory bulb that allows for the rapid discrimination of odorant mixtures. While some transduction elements are conserved among many species, some species-specificity occurs in certain features that may relate to their particular physiology and ecological niche. However, studies of olfactory transduction have been limited to a relatively small number of vertebrate and invertebrate species. To better understand the diversity and evolution of olfactory transduction mechanisms, we studied stimulus-elicited calcium fluxes in olfactory neurons from a previously unstudied mammalian species, the domestic cat. Isolated cells from cat olfactory epithelium were stimulated with odorant mixtures and biochemical agents, and cell responses were measured with calcium imaging techniques. Odorants elicited either increases or decreases in intracellular calcium; odorant-induced calcium increases were mediated either by calcium fluxes through the cell membrane or by mobilization of intracellular stores. Individual cells could employ multiple signaling mechanisms to mediate responses to different odorants. The physiological features of these olfactory neurons suggest greater complexity than previously recognized in the role of peripheral neurons in encoding complex odor stimuli. The investigation of novel and unstudied species is important for understanding the mechanisms of odorant signaling that apply to the olfactory system in general and suggests both broadly conserved and species-specific evolutionary adaptations.

In situ Ca2+ imaging of odor responses in a coronal olfactory epithelium slice

An in situ Ca2+ -imaging technique was adopted to monitor odorant responses of more than several hundreds of neurons simultaneously in an intact coronal slice of the mouse olfactory epithelium. The sensitivity and resolution of the slice Ca2+ -imaging were high enough to distinguish between olfactory receptor neurons with threshold concentrations in a one-order difference for a particular odorant at the single-cell level. Increasing odorant concentrations resulted in increases in the numbers of odorant-responsive neurons, which were visualized in situ in the coronal slice. The methodology established in this study is a powerful tool to visualize spatial distributions of odorant responsive neurons at a cellular resolution, and to construct odor maps in a coronal view of the olfactory epithelium.

Calcium transients in the garter snake vomeronasal organ

The signaling cascade involved in chemosensory transduction in the VN organ is incompletely understood. In snakes, the response to nonvolatile prey chemicals is mediated by the vomeronasal (VN) system. Using optical techniques and fluorescent Ca(2+) indicators, we found that prey-derived chemoattractants produce initially a transient cytosolic accumulation of [Ca(2+)](i) in the dendritic regions of VN neurons via two pathways: Ca(2+) release from IP(3)-sensitive intracellular stores and, to a lesser extent, Ca(2+) influx through the plasma membrane. Both components seem to be dependent on IP(3) production. Chemoattractants evoke a short-latency Ca(2+) elevation even in the absence of extracellular Ca(2+), suggesting that in snake VN neurons, Ca(2+) release from intracellular stores is independent of a preceding Ca(2+) influx, and both components are activated in parallel during early stages of chemosensory transduction. Once the response develops in apical dendritic segments, other mechanisms can also contribute to the amplification and modulation of these chemoattractant-mediated cytosolic Ca(2+) transients. In regions close to the cell bodies of the VN neurons, the activation of voltage-sensitive Ca(2+) channels and a Ca(2+)-induced Ca(2+) release from intracellular ryanodine-sensitive stores secondarily boost initial cytosolic Ca(2+) elevations increasing their magnitude and durations. Return of intracellular Ca(2+) to prestimulation levels appears to involve a Ca(2+) extrusion mediated by a Na(+)/Ca(2+) exchanger mechanism that probably plays an important role in limiting the magnitude and duration of the stimulation-induced Ca(2+) transients.

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.

Calcium regulation of cyclic nucleotide signaling in lobster olfactory receptor neurons

An elevated free Ca2+ concentration reduces odor-stimulated production of cyclic AMP (cAMP) in the outer dendritic membranes of lobster olfactory receptor neurons in vitro. This effect can occur within 50 ms of odor stimulation. The effect is concentration-dependent at submicromolar concentrations of free Ca2+. An elevated free Ca2+ concentration also reduces basal and forskolin-stimulated cAMP levels in a concentration-dependent manner, suggesting that Ca2+ is not targeting the activation of the odor receptor/G protein complex. The degradation of synthetic cAMP by phosphodiesterases is not enhanced by an increased free Ca2+ concentration, suggesting that Ca2+ acts by down-regulating the olfactory adenylyl cyclase. Western blot analysis of the lobster olfactory sensilla that contain the outer dendrites reveals a protein in the transduction zone with a molecular mass of approximately 138 kDa that is immunoreactive to an antiserum against adenylyl cyclase type III. Given earlier evidence that Ca2+ potentially enters the receptor cell through odor-activated inositol 1,4,5-trisphosphate-gated channels, our results suggest a possible route for cross talk between the cyclic nucleotide and the inositol phospholipid signaling pathways in lobster olfactory receptor neurons.

Combinatorial receptor codes for odors

The discriminatory capacity of the mammalian olfactory system is such that thousands of volatile chemicals are perceived as having distinct odors. Here we used a combination of calcium imaging and single-cell RT-PCR to identify odorant receptors (ORs) for odorants with related structures but varied odors. We found that one OR recognizes multiple odorants and that one odorant is recognized by multiple ORs, but that different odorants are recognized by different combinations of ORs. Thus, the olfactory system uses a combinatorial receptor coding scheme to encode odor identities. Our studies also indicate that slight alterations in an odorant, or a change in its concentration, can change its "code," potentially explaining how such changes can alter perceived odor quality.

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.

Calcium entry through cyclic nucleotide-gated channels in individual cilia of olfactory receptor cells: spatiotemporal dynamics

Transient elevations of intracellular Ca2+ play an important role in regulating the sensitivity of olfactory transduction, but such elevations have not been demonstrated in the olfactory cilia, which are the site of primary odor transduction. To begin to understand Ca2+ signaling in olfactory cilia, we used high-resolution imaging techniques to study the Ca2+ transients that occur in salamander olfactory receptor neurons (ORNs) as a result of cyclic nucleotide-gated (CNG) channel activation. To visualize ciliary Ca2+ signals, we loaded ORNs with the Ca2+ indicator dye Fluo-3 AM and measured fluorescence with a laser scanning confocal microscope. Application of the phosphodiesterase inhibitor IBMX increased fluorescence in the cilia and other neuronal compartments; the ciliary signal occurred first and was more transient. This signal could be abolished by lowering external Ca2+ or by applying LY83583, a potent blocker of CNG channels, indicating that Ca2+ entry through CNG channels was the primary source of fluorescence increases. Direct activation of CNG channels with low levels of 8-Br-cGMP (1 microM) led to tonic Ca2+ signals that were restricted locally to the cilia and the dendritic knob. Elevated external K+, which depolarizes cell membranes, increased fluorescence signals in the cell body and dendrite but failed to increase ciliary Ca2+ fluorescence. The results demonstrate the existence and spatiotemporal properties of Ca2+ transients in individual olfactory cilia and implicate CNG channels as a major pathway for Ca2+ entry into ORN cilia during odor transduction.

Selectivity and response characteristics of human olfactory neurons

Transduction mechanisms were investigated in human olfactory neurons by determining characteristics of odorant-induced changes in intracellular calcium concentration ([Ca2+]i). Olfactory neurons were freshly isolated from nasal biopsies, allowed to attach to coverslips, and loaded with the calcium-sensitive indicator fura-2. Changes in [Ca2+]i were studied in response to exposure to individual odors, or odorant mixtures composed to distinguish between transduction pathways mediated by adenosine 3'5'-monophosphate (cAMP; mix A) or inositol 1,4,5-trisphosphate (InsP3; mix B). Overall, 52% of biopsies produced one or more odorant-responsive olfactory neurons, whereas 24% of all olfactory neurons tested responded to odorant exposure with a change in [Ca2+]i. As in olfactory neurons from other species, the data suggest that odorant exposure elicited calcium influx via second-messenger pathways involving cAMP or InsP3. Unlike olfactory neurons from other species that have been tested, some human olfactory neurons responded to odorants with decreases in [Ca2+]i. Also in contrast with olfactory neurons from other species, human olfactory neurons were better able to discriminate between odorant mixtures in that no neuron responded to more than one type of odor or mixture. These results suggest the presence of a previously unreported type of olfactory transduction mechanism, and raise the possibility that coding of odor qualities in humans may be accomplished to some degree differently than in other vertebrates, with the olfactory neuron itself making a greater contribution to the discrimination process.

G alpha 9/G alpha 11: immunolocalization in the olfactory epithelium of the rat (Rattus rattus) and the channel catfish (Ictalurus punctatus)

The immunohistochemical localization of G alpha 9/G alpha 11 was studied in the olfactory and respiratory epithelium of two representative vertebrates, the rat and the channel catfish. Localization in the rat was found at the apical surface of cells in the epithelium and within nerve tracts in the lamina propria. Immunostaining of neuronal cilia and supporting cell microvilli was confirmed by electron microscopy. Immunoreactivity on the ipsilateral neuroepithelium was abolished five weeks following unilateral bulbectomy. An emergence of patchy immunoreactivity was found, however, after fifteen weeks. In catfish, G alpha 9/G alpha 11 antigenicity was found at the apical surface of cells within the olfactory epithelium, at supranuclear regions within some cell bodies and in basal nerve tracts of the olfactory rosette. Immunoreactivity was removed with unilateral bulbectomy. Specific labelling in both rat and catfish was eliminated by preincubation of the G alpha 9/G alpha 11 antibodies with the cognate peptide. Proteins were extracted from olfactory tissues of both species and solubilized. Using western blotting, bands corresponding in apparent molecular weight to a 38,000 mol. wt protein were found. These data demonstrate the presence of G alpha 9/G alpha 11 in the olfactory tissues of these vertebrates and suggest a role in olfaction for this class of G-protein.

Induction of differentiation of human olfactory neuroblastoma cells into odorant-responsive cells

Olfactory neuroblastoma is a rare malignancy of the olfactory mucosa that may be derived from the olfactory epithelium. To characterize this tumor, we cultured olfactory neuroblastoma cells in the presence or absence of growth factors (transforming growth factor alpha and basic fibroblast growth factor) known to affect olfactory tissue and assessed their responsiveness to known odorants by measuring changes in intracellular calcium. Untreated cells did not respond to odorants. Basic fibroblast growth factor treatment had cytotoxic effects, and treated cells did not respond to odorants. Transforming growth factor alpha treatment resulted in the induction of odor responsiveness in these cells. Cells responded to odorants at 100 nM to 100 microM concentrations and responded with both increases and decreases in intracellular calcium. Increases in intracellular calcium were mediated by a calcium influx and were reversibly blocked by compounds known to inhibit second messenger pathways in olfactory receptor neurons. The calcium responses of the olfactory neuroblastoma cells were thus specific to the odorants and similar to those found in olfactory receptor neurons. The results support the notion that olfactory neuroblastoma cells may be of olfactory origin and thus they can be used as a model cell line to study human olfaction.

Second messenger signaling in olfactory transduction

Olfactory receptor neurons respond to odorants with G-protein mediated increases in the concentration of cyclic adenosine 3',5'-monophosphate (cAMP) and/or inositol 1,4,5-trisphospahte (InsP3). These two second messengers directly regulate opening of cAMP- and InsP3-regulated conductances localized to the apical transduction compartments of the cell (cilia and olfactory knob). In the presence of physiological concentrations of extracellular Ca2+, these second messenger regulated conductances mediate influx of Ca2+ into the olfactory neuron resulting in large, localized increases in intracellular Ca2+ ([Ca2+]i). A significant advance in our understanding of the molecular mechanisms of olfaction is the recent realization that this increase in [Ca2+]i plays an important role as a "third messenger" in olfactory transduction. Second messenger dependent increases in [Ca2+]i cause opening of ciliary Ca(2+)-activated Cl-, cation and/ or K+ channels that can carry a large percentage of the generator current, thus amplifying the signal substantially. As a result of this sequence of events, the generator potential in olfactory neurons can be depolarizing, leading to excitation of the neuron, or hyperpolarizing, leading to suppression of basal action potential firing rate. This dual effect of odorants on olfactory neurons may play an important role in quality coding and in the ability to detect low concentrations of odorants, particularly in complex mixtures.

Calcium controls second-messenger signalling in olfactory cilia

The increase in intracellular calcium concentration elicited by odorant stimulation seems to be involved in down-regulating the responsiveness of olfactory neurons to subsequent stimuli. The present study suggests that this regulatory effect may be due to a calcium-dependent attenuation of the olfactory signalling cascade; the odor-induced cyclic adenosine monophosphate (cAMP) response in olfactory cilia is diminished by calcium in a dose-dependent manner. This reduced cAMP signal is not due to an activation of phosphodiesterases by elevated calcium levels, but rather seems to be mediated by the inhibition of adenylate cyclase by calcium ions.

Phosphoinositide second messengers in olfaction

Olfactory stimuli (odorants) are detected and recognized by binding to receptors belonging to the G-protein-coupled receptor superfamily. The binding of odorants to some receptors stimulates the activity of an odorant-sensitive phospholipase C (PLC) thereby generating the second messengers inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 plays a key role in membrane depolarization by binding to a receptor that is itself a cation channel. The formation of DAG is expected to stimulate the activity of protein kinase C (PKC). PKC, together with G-protein-coupled receptor kinases, mediates signal termination by phosphorylation of odorant receptors and possibly other substrates. This review summarizes recent evidence regarding the role of phosphoinositide-derived second messengers in the molecular events underlying olfactory signaling. In addition, the role of calcium as a "third messenger" that provides a mechanism for interaction between phosphoinositide second messengers and components of the cyclic AMP signaling pathway is also discussed.

Calcium signals in olfactory neurons

Laser scanning confocal microscopy in combination with the fluorescent calcium indicators Fluo-3 and Fura-Red was employed to estimate the intracellular concentration of free calcium ions in individual olfactory receptor neurons and to monitor temporal and spatial changes in the Ca(2+)-level upon stimulation. The chemosensory cells responded to odorants with a significant increase in the calcium concentration, preferentially in the dendritic knob. Applying various stimulation paradigma, it was found that in a population of isolated cells, subsets of receptor neurons display distinct patterns of responsiveness.

Molecular cloning and characterization of a calmodulin-dependent phosphodiesterase enriched in olfactory sensory neurons

The sensing of an odorant by an animal must be a rapid but transient process, requiring an instant response and also a speedy termination of the signal. Previous biochemical and electrophysiological studies suggest that one or more phosphodiesterases (PDEs) may play an essential role in the rapid termination of the odorant-induced cAMP signal. Here we report the molecular cloning, expression, and characterization of a cDNA from rat olfactory epithelium that encodes a member of the calmodulin-dependent PDE family designated as PDE1C. This enzyme shows high affinity for cAMP and cGMP, having a Km for cAMP much lower than that of any other neuronal Ca2+/calmodulin-dependent PDE. The mRNA encoding this enzyme is highly enriched in olfactory epithelium and is not detected in six other tissues tested. However, RNase protection analyses indicate that other alternative splice variants related to this enzyme are expressed in several other tissues. Within the olfactory epithelium, this enzyme appears to be expressed exclusively in the sensory neurons. The high affinity for cAMP of this Ca2+/calmodulin-dependent PDE and the fact that its mRNA is highly concentrated in olfactory sensory neurons suggest an important role for it in a Ca(2+)-regulated olfactory signal termination.

A subtype of the metabotropic glutamate receptor family in the olfactory system of Atlantic salmon

A plasma membrane rich fraction was prepared from olfactory rosettes of Atlantic salmon and used to study binding of L-glutamic acid and activation of phospholipase C (PLC). Glutamate binding was saturable, high affinity, and inhibited by aspartic acid and taurocholate but not by alanine and lysine. Binding of glutamate was potently inhibited by various ligands for rat brain metabotropic glutamate receptors (mGluR) and also by kainate and N-methyl-D-aspartate. Glutamate stimulated phosphatidylinositol 4,5-bisphosphate breakdown consistent with G protein-dependent activation of PLC. Northern blot analyses demonstrated the presence of olfactory rosette RNA that hybridizes with cDNA probes for mGluR1 and mGluR4 under low stringency conditions. The results indicate the salmon olfactory system includes a subtype of the metabotropic glutamate receptor family.

High-affinity Ca2+,Mg(2+)-ATPase in plasma membrane-rich preparations from olfactory epithelium of Atlantic salmon

High-affinity Ca2+,Mg(2+)-ATPase was identified in a plasma membrane-rich fraction of olfactory epithelium from Atlantic salmon (Salmo salar). The enzyme required both Ca2+ and Mg2+ for activation. The apparent Km for Ca2+ was 9.5 nM and Vmax was 0.85 mumol Pi/mg of protein per min. Stimulation by Ca2+ was optimal at 5-100 microM MgCl2. Bovine brain calmodulin had no effect on Ca2+,Mg(2+)-ATPase, even after multiple washes of the membrane preparation with EDTA or EGTA. Endogenous calmodulin was somewhat resistant to removal and could be detected with immunoblotting after multiple washes of the membrane preparation with EDTA or EGTA. This endogenous calmodulin may regulate Ca2+,Mg(2+)-ATPase activity because the activity was inhibited by calmidazolium. Vanadate inhibited Ca2+,Mg(2)-ATPase activity and thapsigargin, a specific inhibitor for Ca2+,Mg(2+)-ATPase of endoplasmic reticulum, had no effect on the enzyme activity. High affinity Ca2+,Mg(2+)-ATPase exists in both ciliary and nonciliary membranes with a similar Km for Ca2+. Ca2+,Mg(2+)-ATPase activity is greater in cilia preparations than in membranes from the deciliated olfactory epithelium. As a putative plasma membrane Ca2+ pump, this high-affinity Ca2+,Mg(2+)-ATPase may play an important role in the regulation of intracellular Ca2+ in olfactory epithelia. In particular, the ciliary membrane may play a prominent role in the removal of Ca2+ from ciliated olfactory receptor cells after odorant stimulation.

Stimulation of Ca(2+)-regulated olfactory phospholipase C by amino acids

L-Amino acids are potent olfactory stimuli for Atlantic salmon. A plasma membrane fraction, previously shown to be rich in amino acid binding sites, was prepared from olfactory rosettes of Atlantic salmon (Salmo salar) and utilized to investigate the role of phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis in olfactory signal transduction. A cocktail of L-amino acids (Ser, Glu, Lys, and Gly) stimulated PIP2 hydrolysis by phospholipase C (PLC) in a dose-dependent manner with half-maximal stimulation when all amino acids were present at approximately 1 microM. Stimulation of PIP2 hydrolysis by amino acids required GTP gamma S, which alone had no effect on PLC activity. Unlike GTP gamma S, AlF4- and Ca2+ stimulated PIP2 breakdown. Preincubation with 1 mM GDP beta S eliminated the effect of amino acids and AlF4- on PIP2 hydrolysis, suggesting the involvement of G protein regulation. The lack of stimulation by GTP gamma S alone suggested that there was negligible exchange of GTP gamma S for GDP in the absence of odorant. There were no significant effects of amino acids on either adenylate cyclase or guanylate cyclase activities in the membrane preparation under these conditions. The effect of the amino acid cocktail was maximal at 1-10 nM free Ca2+. At or above 100 nM free Ca2+, no effect of amino acids on PIP2 hydrolysis was found. However, between 100 nM and 100 microM, Ca2+ directly stimulated PLC activity in a dose-dependent manner. This stimulation by Ca2+ appeared to be G protein independent because it did not require GTP gamma S and was not inhibited by GDP beta S.(ABSTRACT TRUNCATED AT 250 WORDS)

Human olfactory neurons respond to odor stimuli with an increase in cytoplasmic Ca2+

The sense of smell allows terrestrial animals to collect information about the chemical nature of their environment through the detection of airborne molecules. In humans smell is believed to play an important role in protecting the organism from environmental hazards such as fire, gas leaks and spoiled food, in determining the flavor of foods, and perhaps in infant-parent bonding. In addition, the study of human olfaction is relevant to a number of medical problems that result in olfactory dysfunction, which can affect nutritional state, and to the study of the etiology of neurodegenerative diseases which manifest themselves in the olfactory epithelium. Although much is known about behavioral aspects of human olfaction, little is understood about the underlying cellular mechanisms in humans. Here we report that viable human olfactory neurons (HON) can be isolated from olfactory tissue biopsies, and we find that HON respond to odorants with an increase in intracellular calcium concentration ([Cai]).

Electrical signals in olfactory transduction

Olfactory transduction involves a G-protein-coupled second messenger system, which results in the odor-dependent production of cAMP. The direct activation of ion channels in the cilia membrane by cAMP is the final step in producing the slow depolarization that brings the membrane potential to threshold for spike generation. Because of the central role in the transduction cascade occupied by these channels considerable effort has been directed toward understanding their behavior at a molecular level. Alternative second messenger pathways have also been proposed in olfaction, but the physiological evidence for these is less well developed.

Second messenger signalling in olfaction

The primary reactions of the chemo-electrical signal transduction pathway in olfactory receptor neurons are mediated by two alternative second messengers, cAMP and inositol 1,4,5-trisphosphate. The rapid and transient intracellular signalling is terminated by the action of negative-feedback loops which uncouple the reaction cascades (desensitization). Recent evidence suggests that secondary reactions in olfaction (adaptation) may also be controlled by second messengers.

IP3- and cAMP-induced responses in isolated olfactory receptor neurons from the channel catfish

Olfactory receptor neurons enzymatically dissociated from channel catfish olfactory epithelium were depolarized transiently following dialysis of IP3 or cAMP (added to the patch pipette) into the cytoplasm. Voltage and current responses to IP3 were blocked by ruthenium red, a blocker of an IP3-gated Ca(2+)-release channel in sarcoplasmic reticulum. In contrast, the responses to cAMP were not blocked by extracellularly applied ruthenium red, nor by L-cis-diltiazem or amiloride and two of its derivatives. The current elicited by cytoplasmic IP3 in neurons under voltage clamp displayed a voltage dependence different from that of the cAMP response which showed marked outward rectification. A sustained depolarization was caused by increased cytoplasmic IP3 or cAMP when the buffering capacity for Ca2+ of the pipette solution was increased, when extracellular Ca2+ was removed or after addition of 20-200 nM charybdotoxin to the bathing solution, indicating that the repolarization was caused by an increase in [Cai] that opened Ca(2+)-activated K+ channels. The results suggest that different conductances modulated by either IP3 or cAMP are involved in mediating olfactory transduction in catfish olfactory receptor neurons and that Ca(2+)-activated K+ channels contribute to the termination of the IP3 and cAMP responses.