In either the vertebrate nose or the insect antenna, most olfactory receptor neurons (ORNs) respond to multiple odors. However, some ORNs respond to just a single odor, or at most to a few highly related odors. It has been hypothesized that narrowly-tuned ORNs project to narrowly-tuned neurons in the brain, and that these dedicated circuits mediate innate behavioral responses to a particular ligand. Here we have investigated neural activity and behavior downstream from two narrowly-tuned ORN types in Drosophila. We found that genetically ablating either of these ORN types impairs innate behavioral attraction to their cognate ligand. Neurons in the antennal lobe postsynaptic to one of these ORN types are, like their presynaptic ORNs, narrowly tuned to a pheromone. However, neurons postsynaptic to the second ORN type are broadly tuned. These results demonstrate that some narrowly-tuned ORNs project to dedicated central circuits, ensuring a tight connection between stimulus and behavior, whereas others project to central neurons which participate in the ensemble representations of many odors.
GABAb receptor (GABAbR)-mediated suppression of glutamate release is critical for limiting glutamatergic transmission across the central nervous system. Here we show that, upon tetanic stimulation of afferents to lateral amygdala, presynaptic GABAbR-mediated inhibition only occurs in glutamatergic inputs to principle neurons (PNs), but not to interneurons (INs), despite the presence of GABAbR in terminals to both types of neurons. The selectivity is caused by differential local GABA accumulation; it requires GABA reuptake, and parallels distinct spatial distributions of presynaptic GABAbR in terminals to PNs and INs. Moreover, GABAbR-mediated suppression of theta-burst induced long-term potentiation (LTP) occurs only in the inputs to PNs, but not to INs. Thus, target cell-specific control of glutamate release by presynaptic GABAbR orchestrates the inhibitory dominance inside amygdala and may contribute to prevention of non-adaptive defensive behaviors.
Internal physiological states influence behavioral decisions. We have investigated the underlying cellular and molecular mechanisms at the first olfactory synapse for starvation modulation of food search behavior in Drosophila. We found that a local signal by short neuropeptide F (sNPF) and a global metabolic cue by insulin are integrated at specific odorant receptor neurons (ORNs) to modulate olfactory sensitivity. Results from two-photon calcium imaging show that starvation increases presynaptic activity via intraglomerular sNPF signaling. Expression of sNPF and its receptor (sNPFR1) in Or42b neurons is necessary for starvation-induced food search behavior. Presynaptic facilitation in Or42b neurons is sufficient to mimic starvation-like behavior in fed flies. Furthermore, starvation elevates the transcription level of sNPFR1 but not that of sNPF, and insulin signaling suppresses sNPFR1 expression. Thus, starvation increases expression of sNPFR1 to change the odor map, resulting in more robust food search behavior.
Drosophila; olfaction; sNPF; gain control; feeding behavior; two-photon imaging; Insulin; PI3K; wortmannin; LY294002
Psychostimulants induce neuroadaptations in excitatory and fast inhibitory transmission in the ventral tegmental area (VTA). Mechanisms underlying drug-evoked synaptic plasticity of slow inhibitory transmission mediated by GABAB receptors and G protein-gated inwardly rectifying potassium (GIRK/Kir3) channels, however, are poorly understood. Here, we show that one day after methamphetamine (METH) or cocaine exposure, both synaptically-evoked and baclofen-activated GABABR-GIRK currents were significantly depressed in VTA GABA neurons, and remained depressed for 7 days. Presynaptic inhibition mediated by GABABRs on GABA terminals was also weakened. Quantitative immunoelectron microscopy revealed internalization of GABAB1R and GIRK2, which occurred coincident with dephosphorylation of Ser783 in GABAB2R, a site implicated in regulating GABABR surface expression. Inhibition of protein phosphatases recovered GABABR-GIRK currents in VTA GABA neurons of METH-injected mice. This psychostimulant-evoked impairment in GABABR signaling removes an intrinsic brake on GABA neuron spiking, which may augment GABA transmission in the mesocorticolimbic system.
Synaptic transmission depends on the regulated surface expression of neurotransmitter receptors, but many of the cellular processes required to achieve this remain poorly understood. To better define specific mechanisms for the GABAB receptor (GABABR) trafficking, we screened for proteins that bind to the carboxy-terminus of the GABAB1 subunit. We report the identification and characterization of a novel 130-kDa protein, GPCR interacting scaffolding protein (GISP), that interacts directly with the GABAB1 subunit via a coiled-coil domain. GISP co-fractionates with GABABR and with the postsynaptic density and co-immunoprecipitates with GABAB1 and GABAB2 from rat brain. In cultured hippocampal neurons, GISP displays a punctate dendritic distribution and has an overlapping localization with GABABRs. When co-expressed with GABABRs in human embryonic kidney cells, GISP promotes GABABR surface expression and enhances both baclofen-evoked extracellular signal-regulated kinase (ERK) phosphorylation and G-protein inwardly rectifying potassium channel (GIRK) currents. These results suggest that GISP is involved in the forward trafficking and stabilization of functional GABABRs.
A-kinase anchoring protein; cultured neurons; GABAB receptor; GPCR interacting scaffolding protein; hippocampus; receptor trafficking
Conflicting views exist of how circuits of the antennal lobe, the insect equivalent of the olfactory bulb, translate input from olfactory receptor neurons (ORNs) into projection neuron (PN) output. Synaptic connections between ORNs and PNs are one-to-one, yet PNs are more broadly tuned to odors than ORNs. The basis for this difference in receptive range remains unknown. Analyzing a Drosophila mutant lacking ORN input to one glomerulus, we show that some of the apparent complexity in the antennal lobe’s output arises from lateral, interglomerular excitation of PNs. We describe a previously unidentified population of cholinergic local neurons (LNs) with multiglomerular processes. These excitatory LNs respond broadly to odors but exhibit little glomerular specificity in their synaptic output, suggesting that PNs are driven by a combination of glomerulus-specific ORN afferents and diffuse LN excitation. Lateral excitation may boost PN signals and enhance their transmission to third-order neurons in a mechanism akin to stochastic resonance.
Here we describe several fundamental principles of olfactory processing in the Drosophila antennal lobe (the analog of the vertebrate olfactory bulb), based on a systematic analysis of input and output spike trains of seven identified glomeruli. Repeated presentations of the same odor elicit more reproducible responses in second-order projection neurons (PNs) than in their presynaptic olfactory receptor neurons (ORNs). PN responses rise and accommodate rapidly, emphasizing odor onset. Furthermore, weak ORN inputs are amplified in the PN layer but strong inputs are not. This nonlinear transformation broadens PN tuning, and produces more uniform distances between odor representations in PN coding space. Additionally, a portion of a PN’s odor response profile is not systematically related to its direct ORN inputs, likely reflecting lateral connections between glomeruli. Finally, we show that a linear discriminator classifies odors more accurately using PN spike trains as compared to an equivalent number of ORN spike trains.
Presynaptic GABAB-receptors (GABABR) control glutamate and GABA release at many synapses in the nervous system. In the present study we used whole-cell patch clamp recordings of spontaneous excitatory and inhibitory synaptic currents in the presence of TTX to monitor glutamate and GABA release from synapses in layer II and V of the rat entorhinal cortex (EC) in vitro. In both layers the release of both transmitters was reduced by application of GABABR agonists. Quantitatively, the depression of GABA release in layer II and layer V, and of glutamate release in layer V was similar, but glutamate release in layer II was depressed to a greater extent. The data suggest that the same GABABR may be present on both GABA and glutamate terminals in the EC, but that the heteroreceptor may show a greater level of expression in layer II. Studies with GABABR antagonists suggested that neither the auto- nor the heteroreceptor was consistently tonically activated by ambient GABA in the presence of TTX. Studies in EC slices from rats made chronically epileptic using a pilocarpine model of temporal lobe epilepsy revealed a reduced effectiveness of both auto- and heteroreceptor function in both layers. This could suggest that enhanced glutamate and GABA release in the EC may be associated with the development of the epileptic condition.
GABAB receptors; entorhinal cortex; glutamate release; GABA release; epilepsy
Individual olfactory receptor neurons (ORNs) selectively express one or a small number of odor receptors from among a large receptor repertoire. The expression of an odor receptor dictates the odor response spectrum of the ORN. The process of receptor gene choice relies in part on a combinatorial code of transcription factors. In Drosophila, the POU domain transcription factor Acj6 is one element of the transcription factor code. In acj6 null mutants, many ORNs do not express an appropriate odor receptor gene and thus are not correctly specified. We find that acj6 is alternatively spliced to yield many structurally distinct transcripts in the olfactory organs. We generate flies that express single splice forms of acj6 in an acj6− background. We find that different splice forms are functionally distinct; they differ in their abilities to specify ORN identities. Some individual splice forms can fully rescue the specification of some ORNs. Individual splice forms can function both positively and negatively in receptor gene regulation. ORNs differ in their requirements for splice forms; some are not fully rescued by any single splice form tested, suggesting that some ORNs may require the combinatorial action of multiple splice forms. Late expression of some acj6 splice forms is sufficient to rescue some ORN classes, consistent with a direct role for Acj6 isoforms in receptor gene expression. The results indicate that alternative splicing may add another level of richness to the regulatory code that underlies the process of odor receptor gene choice.
olfaction; Drosophila; Acj6; POU-domain; odor receptor; splicing
GABAB receptors (GABABRs) have been linked to a wide range of physiological and cognitive processes and are of interest for treating a number of neurodegenerative and psychiatric disorders. As many of these diseases are associated with advanced age, it is important to understand how the normal aging process impacts GABABR expression and signaling. Thus, we investigated GABABR expression and function in the prefrontal cortex (PFC) and hippocampus of young and aged rats characterized in a spatial learning task. Baclofen-stimulated GTP-binding and GABABR1 and GABABR2 proteins were reduced in the PFC of aged rats but these reductions were not associated with spatial learning abilities. In contrast, hippocampal GTP-binding was comparable between young and aged rats but reduced hippocampal GABABR1 expression was observed in aged rats with spatial learning impairment. These data demonstrate marked regional differences in GABABR complexes in the adult and aged brain and could have implications for both understanding the role of GABAergic processes in normal brain function and the development of putative interventions that target this system.
Aging; Baclofen; Spatial learning; GABABR; PFC; Memory
Diverse sensory organs, including mammalian taste buds and insect chemosensory sensilla, show a striking compartmentalization of receptor cells. However, the functional impact of this organization remains unclear. Here we show that compartmentalized Drosophila olfactory receptor neurons (ORNs) communicate with each other directly. The sustained response of one ORN is inhibited by the transient activation of a neighboring ORN. Mechanistically, such lateral inhibition does not depend on synapses and is likely mediated by ephaptic coupling. Moreover, lateral inhibition in the periphery can modulate olfactory behavior. Together, the results show that integration of olfactory information can occur via lateral interactions between ORNs. Inhibition of a sustained response by a transient response may provide a means of encoding salience. Finally, a CO2-sensitive ORN in the malaria mosquito Anopheles can also be inhibited by excitation of an adjacent ORN, suggesting a broad occurrence of lateral inhibition in insects and possible applications in insect control.
Several experiments indicate that there exists substantial synaptic-depression at the synapses between olfactory receptor neurons (ORNs) and neurons within the drosophila antenna lobe (AL). This synaptic-depression may be partly caused by vesicle-depletion, and partly caused by presynaptic-inhibition due to the activity of inhibitory local neurons within the AL. While it has been proposed that this synaptic-depression contributes to the nonlinear relationship between ORN and projection neuron (PN) firing-rates, the precise functional role of synaptic-depression at the ORN synapses is not yet fully understood. In this paper we propose two hypotheses linking the information-coding properties of the fly AL with the network mechanisms responsible for ORNAL synaptic-depression. Our first hypothesis is related to variance coding of ORN firing-rate information — once stimulation to the ORNs is sufficiently high to saturate glomerular responses, further stimulation of the ORNs increases the regularity of PN spiking activity while maintaining PN firing-rates. The second hypothesis proposes a tradeoff between spike-time reliability and coding-capacity governed by the relative contribution of vesicle-depletion and presynaptic-inhibition to ORNAL synaptic-depression. Synaptic-depression caused primarily by vesicle-depletion will give rise to a very reliable system, whereas an equivalent amount of synaptic-depression caused primarily by presynaptic-inhibition will give rise to a less reliable system that is more sensitive to small shifts in odor stimulation. These two hypotheses are substantiated by several small analyzable toy models of the fly AL, as well as a more physiologically realistic large-scale computational model of the fly AL involving glomerular channels.
Understanding the intricacies of sensory processing is a major scientific challenge. In this paper we examine the early stages of the olfactory system of the fruit-fly. Many experiments have revealed a great deal regarding the architecture of this system, including the types of neurons within it, as well as the connections those neurons make amongst one another. In this paper we examine the potential dynamics produced by this neuronal network. Specifically, we construct a computational model of this early olfactory system and study the effects of synaptic-depression within this system. We find that the dynamics and coding properties of this system depend strongly on the strength, and sources of, synaptic-depression. This work has ramifications for understanding the coding properties of other insect olfactory systems, and perhaps even other sensory modalities in other animals.
Accumulating evidence indicate that GABA regulates activity-dependent development of inhibitory synapses in the vertebrate brain, but the underlying mechanisms remain unclear. Here we combined live imaging of cortical GABAergic axons with single cell genetic manipulation to dissect the role of presynaptic GABAB receptors (GABABRs) in inhibitory synapse formation in mouse. Developing GABAergic axons form a significant number of transient boutons but only a subset was stabilized. Synaptic vesicles in these nascent boutons are often highly mobile in the course of tens of minutes. Activation of presynaptic GABABRs stabilized mobile vesicles in nascent boutons through the local enhancement of actin polymerization. Inactivation of GABABRs in developing basket interneurons resulted in aberrant pattern of bouton size distribution, reduced bouton density and reduced axon branching, as well as reduced frequency of miniature inhibitory currents in postsynaptic pyramidal neurons. These results suggest that GABABRs along developing inhibitory axons act as a local sensor of GABA release and promote presynaptic maturation through increased recruitment of mobile vesicle pools. Such release-dependent validation and maturation of nascent terminals is well suited to sculpt the pattern of synapse formation and distribution along axon branches.
GABAB receptor; synaptic vesicle dynamics; live cell imaging; actin polymerization; FRET; inhibitory synapses; activity-dependent development
The Drosophila antennal lobe is subdivided into multiple glomeruli, each of which represents a unique olfactory information processing channel. In each glomerulus, feedforward input from olfactory receptor neurons (ORNs) is transformed into activity of projection neurons (PNs), which represent the output. Recent investigations have indicated that lateral presynaptic inhibitory input from other glomeruli controls the gain of this transformation. Here, we address why this gain control acts “pre”-synaptically rather than “post”-synaptically. Postsynaptic inhibition could work similarly to presynaptic inhibition with regard to regulating the firing rates of PNs depending on the stimulus intensity. We investigate the differences between pre- and postsynaptic gain control in terms of odor discriminability by simulating a network model of the Drosophila antennal lobe with experimental data. We first demonstrate that only presynaptic inhibition can reproduce the type of gain control observed in experiments. We next show that presynaptic inhibition decorrelates PN responses whereas postsynaptic inhibition does not. Due to this effect, presynaptic gain control enhances the accuracy of odor discrimination by a linear decoder while its postsynaptic counterpart only diminishes it. Our results provide the reason gain control operates “pre”-synaptically but not “post”-synaptically in the Drosophila antennal lobe.
Drosophila; antennal lobe; odor discriminability; presynaptic inhibition; postsynaptic inhibition; gain control; decorrelation; concentration invariance
Many species of mosquitoes, including the major malaria vector Anopheles gambiae, utilize carbon dioxide (CO2) and 1-octen-3-ol as olfactory cues in host-seeking behaviors that underlie their vectorial capacity. However, the molecular and cellular basis of such olfactory responses remains largely unknown.
Here, we use molecular and physiological approaches coupled with systematic functional analyses to define the complete olfactory sensory map of the An. gambiae maxillary palp, an olfactory appendage that mediates the detection of these compounds. In doing so, we identify three olfactory receptor neurons (ORNs) that are organized in stereotyped triads within the maxillary-palp capitate-peg-sensillum population. One ORN is CO2-responsive and characterized by the coexpression of three receptors that confer CO2 responses, whereas the other ORNs express characteristic odorant receptors (AgORs) that are responsible for their in vivo olfactory responses.
Our results describe a complete and highly concordant map of both the molecular and cellular olfactory components on the maxillary palp of the adult female An. gambiae mosquito. These results also facilitate the understanding of how An. gambiae mosquitoes sense olfactory cues that might be exploited to compromise their ability to transmit malaria.
We investigated the cellular mechanism underlying presynaptic regulation of olfactory receptor neuron (ORN) input to the mouse olfactory bulb using optical-imaging techniques that selectively report activity in the ORN pre-synaptic terminal. First, we loaded ORNs with calcium-sensitive dye and imaged stimulus-evoked calcium influx in a slice preparation. Single olfactory nerve shocks evoked rapid fluorescence increases that were largely blocked by the N-type calcium channel blocker ω-conotoxin GVIA. Paired shocks revealed a long-lasting suppression of calcium influx with ~40% suppression at 400-ms interstimulus intervals and a recovery time constant of ~450 ms. Blocking activation of postsynaptic olfactory bulb neurons with APV/CNQX reduced this suppression. The GABAB receptor agonist baclofen inhibited calcium influx, whereas GABAB antagonists reduced paired-pulse suppression without affecting the response to the conditioning pulse. We also imaged transmitter release directly using a mouse line that expresses synaptopHluorin selectively in ORNs. We found that the relationship between calcium influx and transmitter release was superlinear and that paired-pulse suppression of transmitter release was reduced, but not eliminated, by APV/CNQX and GABAB antagonists. These results demonstrate that primary olfactory input to the CNS can be presynaptically regulated by GABAergic interneurons and show that one major intracellular pathway for this regulation is via the suppression of calcium influx through N-type calcium channels in the pre-synaptic terminal. This mechanism is unique among primary sensory afferents.
Neurons in the nucleus laminaris (NL) of birds act as coincidence detectors and encode interaural time difference to localize the sound source in the azimuth plane. GABAergic transmission in a number of CNS nuclei including the NL is subject to a dual modulation by presynaptic GABAB receptors (GABABRs) and metabotropic glutamate receptors (mGluRs). Here, using in vitro whole-cell patch clamp recordings from acute brain slices of the chick, we characterized the following important but unknown properties pertaining to such a dual modulation: (1) emergence of functional GABA synapses in NL neurons; (2) the temporal onset of neuromodulation mediated by GABABRs and mGluRs; and (3) the physiological conditions under which GABABRs and mGluRs are activated by endogenous transmitters. We found that (1) GABAAR-mediated synaptic responses were observed in about half of the neurons at embryonic day 11 (E11); (2) GABABR-mediated modulation of the GABAergic transmission was detectable at E11, whereas the modulation by mGluRs did not emerge until E15; and (3) endogenous activity of GABABRs was induced by both low- (5 or 10 Hz) and high-frequency (200 Hz) stimulation of the GABAergic pathway, whereas endogenous activity of mGluRs was induced by high- (200 Hz) but not low-frequency (5 or 10 Hz) stimulation of the glutamatergic pathway. Furthermore, the endogenous activity of mGluRs was mediated by group II but not group III members. Therefore, autoreceptor-mediated modulation of GABAergic transmission emerges at the same time when the GABA synapses become functional. Heteroreceptor-mediated modulation appears at a later time and is receptor type dependent in vitro.
DEET is the most widely used insect repellent worldwide. In Drosophila olfactory receptor neurons (ORNs), DEET is detected through a mechanism that employs the olfactory receptor, OR83b. However, it is controversial as to whether ORNs respond directly to DEET or whether DEET blocks the response to attractive odors. Here, we showed that DEET suppressed feeding behavior in Drosophila and this effect was mediated by gustatory receptor neurons (GRNs). DEET was potent in suppressing feeding as <0.1% DEET elicited aversive behavior. Inhibition of feeding by DEET required multiple gustatory receptors (GRs), which were expressed in inhibitory GRNs. DEET stimulated action potentials in GRNs that respond to aversive compounds, and this response was lost in Gr32a, Gr33a and Gr66a mutants. Since 0.02% DEET elicited action potentials, we conclude that DEET directly activates of GRNs. We suggest that the effectiveness of DEET in pest control owes to its dual action in inducing avoidance simultaneously via GRNs and ORNs.
Olfactory stimulation induces an odor-guided crawling behavior of Drosophila melanogaster larvae characterized by either an attractive or a repellent reaction. In order to understand the underlying processes leading to these orientations we stimulated single olfactory receptor neurons (ORNs) through photo-activation within an intact neuronal network. Using the Gal4-UAS system two light inducible proteins, the light-sensitive cation channel channelrhodopsin-2 (ChR-2) or the light-sensitive adenylyl cyclase (Pacα) were expressed in all or in individual ORNs of the larval olfactory system. Blue light stimulation caused an activation of these neurons, ultimately producing the illusion of an odor stimulus. Larvae were tested in a phototaxis assay for their orientation toward or away from the light source. Here we show that activation of Pacα expressing ORNs bearing the receptors Or33b or Or45a in blind norpA mutant larvae induces a repellent behavior away from the light. Conversely, photo-activation of the majority of ORNs induces attraction towards the light. Interestingly, in wild type larvae two ligands of Or33b and Or45a, octyl acetate and propionic ethylester, respectively, have been found to cause an escape reaction. Therefore, we combined light and odor stimulation to analyze the function of Or33b and Or45a expressing ORNs. We show that the larval olfactory system contains a designated neuronal pathway for repellent odorants and that activation of a specific class of ORNs already determines olfactory avoidance behavior.
Drosophila; olfaction; photo-activation; optogenetics; olfactory behavior; electrophysiology; channelrhodopsin-2; photo-activated adenylyl cyclase
Each odorant receptor gene defines a unique type of olfactory receptor neuron (ORN) and a corresponding type of second-order neuron. Because each odor can activate multiple ORN types, information must ultimately be integrated across these processing channels to form a unified percept. Here we show that, in Drosophila, integration begins at the level of second-order projection neurons (PNs). We genetically silence all the ORNs that normally express a particular odorant receptor, and find that PNs postsynaptic to the silent glomerulus receive substantial lateral excitatory input from other glomeruli. Genetically confining odor-evoked ORN input to just one glomerulus reveals that most PNs postsynaptic to other glomeruli receive indirect excitatory input from the single ORN type that is active. Lateral connections between identified glomeruli vary in strength, and this pattern of connections is stereotyped across flies. Thus, a dense network of lateral connections distributes odor-evoked excitation between channels in the first brain region of the olfactory processing stream.
Olfaction depends on the differential activation of olfactory receptor neurons (ORNs) and on the proper transmission of their activities to the brain. ORNs select individual receptors to express, and they send axons to particular targets in the brain. Little is known about the molecular mechanisms underlying either process. We have identified a new Drosophila POU gene, pdm3, that is expressed in ORNs. Genetic analysis shows that pdm3 is required for odor response in one class of ORNs. We find that pdm3 acts in odor receptor expression in this class, and that the odor response can be rescued by the receptor. Another POU gene, acj6, is required for receptor expression in the same class, and we find a genetic interaction between the two POU genes. The results support a role for a POU gene code in receptor gene choice. pdm3 is also expressed in other ORN classes in which it is not required for receptor expression. For two of these classes pdm3 is required for normal axon targeting. Thus this mutational analysis, the first for a POU class VI gene, demonstrates a role for pdm3 in both of the processes that define the functional organization of ORNs in the olfactory system.
POU gene; odor receptor; axon targeting; Drosophila; maxillary palp; antenna
Olfactory receptor neurons (ORNs) must select—from a large repertoire—which odor receptors to express. In Drosophila, most ORNs express one of 60 Or genes, and most Or genes are expressed in a single ORN class in a process that produces a stereotyped receptor-to-neuron map. The construction of this map poses a problem of receptor gene regulation that is remarkable in its dimension and about which little is known. By using a phylogenetic approach and the genome sequences of 12 Drosophila species, we systematically identified regulatory elements that are evolutionarily conserved and specific for individual Or genes of the maxillary palp. Genetic analysis of these elements supports a model in which each receptor gene contains a zip code, consisting of elements that act positively to promote expression in a subset of ORN classes, and elements that restrict expression to a single ORN class. We identified a transcription factor, Scalloped, that mediates repression. Some elements are used in other chemosensory organs, and some are conserved upstream of axon-guidance genes. Surprisingly, the odor response spectra and organization of maxillary palp ORNs have been extremely well-conserved for tens of millions of years, even though the amino acid sequences of the receptors are not highly conserved. These results, taken together, define the logic by which individual ORNs in the maxillary palp select which odor receptors to express.
Odors are detected by olfactory receptor neurons (ORNs). Which odor an individual neuron detects is dictated by the odor receptors it expresses. Odor receptors are encoded by large families of genes, and an individual neuron must thus select the gene it expresses from among many possibilities. The mechanism underlying this choice is largely unknown. We have examined the problem of receptor gene choice in the fruit fly Drosophila, whose maxillary palp contains six functional classes of ORNs, each expressing different odor receptor genes. By comparing the DNA sequences flanking these genes in 12 different species of Drosophila, we have identified regulatory elements that are evolutionarily conserved and specific to each odor receptor. Genetic analysis of these elements showed that some act positively to dictate expression in a subset of ORNs, while others act negatively to restrict the expression of a receptor gene to a particular ORN class. We identified a transcription factor, Scalloped, that mediates repression. We were surprised to find that the odor response spectra of these neurons have been well-conserved for tens of millions of years, even though the amino acid sequences of their receptors have diverged considerably.
How does an olfactory receptor neuron select which odor receptor to express? A computational analysis of 12Drosophila genomes combined with mutational analysis identifies conservedcis elements and defines a regulatory code.
In many regions of the visual system, the activity of a neuron is normalized by the activity of other neurons in the same region. Here we show that a similar normalization occurs during olfactory processing in the Drosophila antennal lobe. We exploit the orderly anatomy of this circuit to independently manipulate feedforward and lateral input to second-order projection neurons (PNs). Lateral inhibition increases the level of feedforward input needed to drive PNs to saturation, and this normalization scales with the total activity of the olfactory receptor neuron (ORN) population. Increasing total ORN activity also makes PN responses more transient. Strikingly, a model with just two variables (feedforward and total ORN activity) accurately predicts PN odor responses. Finally, we show that discrimination by a linear decoder is facilitated by two complementary transformations: the saturating transformation intrinsic to each processing channel boosts weak signals, while normalization helps equalize responses to different stimuli.
olfaction; neural coding; electrophysiology; computational neuroscience; normalization; lateral inhibition
Oviposition attractants are environmental cues that allow Culex gravid female mosquitoes to locate suitable sites for egg-laying and, therefore, may be exploited for environmentally friendly strategies for controlling mosquito populations. Naturally occurring skatole has been identified as an oviposition attractant for the Southern House mosquito, Culex quinquefasciatus. Previously, we identified in Cx. quinquefasciatus female antennae an olfactory receptor neuron (ORN) highly sensitive to skatole and an odorant-binding protein involved in the detection of this semiochemical. Here, we describe the characterization of an odorant receptor (OR), CquiOR10, which is narrowly tuned to skatole when expressed in the Xenopus oocyte system. Odorant-induced response profiles generated by heterologously expressed CquiOR10 suggest that this OR is expressed in the mosquito ORN sensitive to skatole. However, geranylacetone, which stimulates the antennal ORN, was not detected by CquiOR10-expressing oocytes, thus raising interesting questions about reception of oviposition attractants in mosquitoes.
Odorant receptor; CquiOR10; Culex quinquefasciatus; Xenopus oocyte expression system; 3-Methylindole; 2-Methylphenol
Phosphatidylinositol 3-kinase (PI3K)-dependent signaling couples to receptors for many different ligands in diverse cellular systems. Recent findings suggest that PI3K-dependent signaling also mediates inhibition of odorant responses in rat olfactory receptor neurons (ORNs). Here, we present evidence that murine ORNs show PI3K-dependent calcium responses to odorant stimulation, they express 2 G protein-coupled receptor (GPCR)-activated isoforms of PI3K, PI3Kβ and PI3Kγ, and they exhibit odorant-induced PI3K activity. These findings support our use of a transgenic mouse model to begin to investigate the mechanisms underlying PI3K-mediated inhibition of odorant responses in mammalian ORNs. Mice deficient in PI3Kγ, a class IB PI3K that is activated via GPCRs, lack detectable odorant-induced PI3K activity in their olfactory epithelium and their ORNs are less sensitive to PI3K inhibition. We conclude that odorant-dependent PI3K signaling generalizes to the murine olfactory system and that PI3Kγ plays a role in mediating inhibition of odorant responses in mammalian ORNs.
calcium imaging; complex odorants; ELISA; inhibitory input; transgenic