Architecture of the primary taste center ofDrosophila melanogasterlarvae

A simple nervous system combined with stereotypic behavioral responses to tastants, together with powerful genetic and molecular tools, have turned Drosophila larvae into a very promising model for studying gustatory coding. Using the Gal4/UAS system and confocal microscopy for visualizing gustatory afferents, we provide a description of the primary taste center in the larval central nervous system. Essentially, gustatory receptor neurons target different areas of the subesophageal ganglion (SOG), depending on their segmental and sensory organ origin. We define two major and two smaller subregions in the SOG. One of the major areas is a target of pharyngeal sensilla, the other one receives inputs from both internal and external sensilla. In addition to such spatial organization of the taste center, circumstantial evidence suggests a subtle functional organization: aversive and attractive stimuli might be processed in the anterior and posterior part of the SOG, respectively. Our results also suggest less coexpression of gustatory receptors than proposed in prior studies. Finally, projections of putative second-order taste neurons seem to cover large areas of the SOG. These neurons may thus receive multiple gustatory inputs. This suggests broad sensitivity of secondary taste neurons, reminiscent of the situation in mammals.

Excitatory interactions between olfactory processing channels in the Drosophila antennal lobe

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.

Humidity-Dependent Cold Cells on the Antenna of the Stick Insect

We present the first systematic study of the response of insect “cold cells” to a variation in the partial pressure of water vapor in ambient air. The cold cells on the antenna of the stick insect respond with an increase in activity when either the temperature or the partial pressure of water vapor is suddenly reduced. This double dependency does not in itself constitute bimodality because it could disappear with the proper choice of parameters involving temperature and humidity. In this study, we demonstrate that the evaporation of a small amount of water from the sensillum surface resulting from a drop in the water vapor pressure—leading to a transient drop in temperature and thus to a brief rise in impulse frequency—is the most plausible explanation for this bimodal response. We also show with an order-of-magnitude calculation that this mechanism is plausible and consistent with the amounts of water vapor potentially present on the sensillum. We hypothesize that a film of moisture collects on the hygroscopic sensillum surface at higher humidity and then tends to evaporate when humidity is lowered. The water might even be bound loosely within the cuticular wall, a situation conceivable in a sensillum that contains two hygroreceptive cells in addition to the cold cell.

Electrophysiological characterisation of the infrared organ of the Australian "Little Ash Beetle" Acanthocnemus nigricans (Coleoptera, Acanthocnemidae)

This study characterises the response properties of the sensilla located on the prothoracic disc organ of the beetle Acanthocnemus nigricans, such as intensity response functions and temporal coding properties. Warming the sensilla by a red laser accelerated their ongoing spiking activity, cessation of the stimulus suppressed their firing as revealed by extracellular recordings. Convective heat sources also increased sensillum activity, but stimuli of other modalities failed to elicit responses. The response threshold was between 11 and 25 mW/cm2 and latencies ranged between 20 and 40 ms. Repeating stimuli with frequencies between 5 and 20 Hz were reliably resolved by the sensilla. This temporal resolution enables the disc sensilla to represent behaviourally relevant changes in heat stimuli in a thermally patchy environment. These findings complement our knowledge on the sensory physiology of pyrophilous insects by hinting at two different, elementary orientation strategies evolved in the three pyrophilous beetle species described. A. nigricans seems to be best adapted to short-range orientation on freshly burnt areas.

Mechanotransduction and auditory transduction in Drosophila

Insects are utterly reliant on sensory mechanotransduction, the process of converting physical stimuli into neuronal receptor potentials. The senses of proprioception, touch, and hearing are involved in almost every aspect of an adult insect's complex behavioral repertoire and are mediated by a diverse array of specialized sensilla and sensory neurons. The physiology and morphology of several of these have been described in detail; genetic approaches in Drosophila, combining behavioral screens and sensory electrophysiology with forward and reverse genetic techniques, have now revealed specific proteins involved in their differentiation and operation. These include three different TRP superfamily ion channels that are required for transduction in tactile bristles, chordotonal stretch receptors, and polymodal nociceptors. Transduction also depends on the normal differentiation and mechanical integrity of the modified cilia that form the neuronal sensory endings, the accessory structures that transmit stimuli to them and, in bristles, a specialized receptor lymph and transepithelial potential. Flies hear near-field sounds with a vibration-sensitive, antennal chordotonal organ. Biomechanical analyses of wild-type antennae reveal non-linear, active mechanical properties that increase their sensitivity to weak stimuli. The effects of mechanosensory and ciliary mutations on antennal mechanics show that the sensory cilia are the active motor elements and indicate distinct roles for TRPN and TRPV channels in auditory transduction and amplification.

Olfactory processing and behavior downstream from highly selective receptor neurons

In both the vertebrate nose and 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 melanogaster. 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 that participate in the ensemble representations of many odors.

Glial Nax Channels Control Lactate Signaling to Neurons for Brain [Na+] Sensing

Sodium (Na) homeostasis is crucial for life, and Na levels in body fluids are constantly monitored in the brain. The subfornical organ (SFO) is the center of the sensing responsible for the control of salt-intake behavior, where Na(x) channels are expressed in specific glial cells as the Na-level sensor. Here, we show direct interaction between Na(x) channels and alpha subunits of Na(+)/K(+)-ATPase, which brings about Na-dependent activation of the metabolic state of the glial cells. The metabolic enhancement leading to extensive lactate production was observed in the SFO of wild-type mice, but not of the Na(x)-knockout mice. Furthermore, lactate, as well as Na, stimulated the activity of GABAergic neurons in the SFO. These results suggest that the information on a physiological increase of the Na level in body fluids sensed by Na(x) in glial cells is transmitted to neurons by lactate as a mediator to regulate neural activities of the SFO.

Receptors and neurons for fly odors in Drosophila

Remarkably little is known about the molecular and cellular basis of mate recognition in Drosophila[1]. We systematically examined the trichoid sensilla, one of the three major types of sensilla that house olfactory receptor neurons (ORNs) on the Drosophila antenna, by electrophysiological analysis. We find that none respond strongly to food odors but that all respond to fly odors. Two subtypes of trichoid sensilla contain ORNs that respond to cis-vaccenyl acetate (cVA), an anti-aphrodisiac pheromone transferred from males to females during mating [2-4]. All trichoid sensilla yield responses to a male extract; a subset yield responses to a virgin-female extract as well. Thus, males can be distinguished from virgin females by the activity they elicit among the trichoid ORN population. We then systematically tested all members of the Odor receptor (Or) gene family [5-7] that are expressed in trichoid sensilla [8] by using an in vivo expression system [9]. Four receptors respond to fly odors in this system: Two respond to extracts of both males and virgin females, and two respond to cVA. We propose a model describing how these receptors might be used by a male to distinguish suitable from unsuitable mating partners through a simple logic.

Expression profiling of peroxisome proliferator-activated receptor-delta (PPAR-delta) in mouse tissues using tissue microarray

Peroxisome proliferator-activated receptor-delta (PPAR-delta) is known as a transcription factor involved in the regulation of fatty acid oxidation and mitochondrial biogenesis in several tissues, such as skeletal muscle, liver and adipose tissues. In this study, to elucidate systemic physiological functions of PPAR-delta, we examined the tissue distribution and localization of PPAR-delta in adult mouse tissues using tissue microarray (TMA)-based immunohistochemistry. PPAR-delta positive signals were observed on variety of tissues/cells in multiple systems including cardiovascular, urinary, respiratory, digestive, endocrine, nervous, hematopoietic, immune, musculoskeletal, sensory and reproductive organ systems. In these organs, PPAR-delta immunoreactivity was generally localized on the nucleus, although cytoplasmic localization was observed on several cell types including neurons in the nervous system and cells of the islet of Langerhans. These expression profiling data implicate various physiological roles of PPAR-delta in multiple organ systems. TMA-based immunohistochemistry enables to profile comprehensive protein localization and distribution in a high-throughput manner.

The molecular receptive range of an olfactory receptor in vivo (Drosophila melanogaster Or22a)

Understanding how odors are coded within an olfactory system requires knowledge about its input. This is constituted by the molecular receptive ranges (MRR) of olfactory sensory neurons that converge in the glomeruli of the olfactory bulb (vertebrates) or the antennal lobe (AL, insects). Aiming at a comprehensive characterization of MRRs in Drosophila melanogaster we measured odor-evoked calcium responses in olfactory sensory neurons that express the olfactory receptor Or22a. We used an automated stimulus application system to screen [Ca(2+)] responses to 104 odors both in the antenna (sensory transduction) and in the AL (neuronal transmission). At 10(-2) (vol/vol) dilution, 39 odors elicited at least a half-maximal response. For these odorants we established dose-response relationships over their entire dynamic range. We tested 15 additional chemicals that are structurally related to the most efficient odors. Ethyl hexanoate and methyl hexanoate were the best stimuli, eliciting consistent responses at dilutions as low as 10(-9). Two substances led to calcium decrease, suggesting that Or22a might be constitutively active, and that these substances might act as inverse agonists, reminiscent of G-protein coupled receptors. There was no difference between the antennal and the AL MRR. Furthermore we show that Or22a has a broad yet selective MRR, and must be functionally described both as a specialist and a generalist. Both these descriptions are ecologically relevant. Given that adult Drosophila use approximately 43 ORs, a complete description of all MRRs appears now in reach.

Sensory cell proliferation within the olfactory epithelium of developing adult Manduca sexta (Lepidoptera)

BACKGROUND

Insects detect a multitude of odors using a broad array of phenotypically distinct olfactory organs referred to as olfactory sensilla. Each sensillum contains one to several sensory neurons and at least three support cells; these cells arise from mitotic activities from one or a small group of defined precursor cells. Sensilla phenotypes are defined by distinct morphologies, and specificities to specific odors; these are the consequence of developmental programs expressed by associated neurons and support cells, and by selection and expression of subpopulations of olfactory genes encoding such proteins as odor receptors, odorant binding proteins, and odor degrading enzymes.

METHODOLOGY/PRINCIPAL FINDINGS

We are investigating development of the olfactory epithelium of adult M. sexta, identifying events which might establish sensilla phenotypes. In the present study, antennal tissue was examined during the first three days of an 18 day development, a period when sensory mitotic activity was previously reported to occur. Each antenna develops as a cylinder with an outward facing sensory epithelium divided into approximately 80 repeat units or annuli. Mitotic proliferation of sensory cells initiated about 20-24 hrs after pupation (a.p.), in pre-existing zones of high density cells lining the proximal and distal borders of each annulus. These high density zones were observed as early as two hr. a.p., and expanded with mitotic activity to fill the mid-annular regions by about 72 hrs a.p. Mitotic activity initiated at a low rate, increasing dramatically after 40-48 hrs a.p.; this activity was enhanced by ecdysteroids, but did not occur in animals entering pupal diapause (which is also ecdysteroid sensitive).

CONCLUSIONS/SIGNIFICANCE

Sensory proliferation initiates in narrow zones along the proximal and distal borders of each annulus; these zones rapidly expand to fill the mid-annular regions. These zones exist prior to any mitotic activity as regions of high density cells which form either at or prior to pupation. Mitotic sensitivity to ecdysteroids may be a regulatory mechanism coordinating olfactory development with the developmental choice of diapause entry.

Anatomical organization of antennal lobe projection neurons in the moth Heliothis virescens

A prerequisite for understanding how odor information is coded in the central nervous system is to know the morphology and spatial relationship of the principal neurons forming the olfactory pathways. The present account provides an anatomical description of the morphology of the neuronal connections between the antennal lobe and the protocerebrum in the moth Heliothis virescens, a species used for studies of olfactory processing and learning. Intracellular labeling and antennal lobe focal injections with dextran fluorescent markers were combined with neuropil immunostaining and three-dimensional reconstructions. The experiments revealed four antennocerebral tracts, the inner, middle, outer, and dorsomedial, and eight morphological types of projection neurons in addition to a neuron with an unpaired median soma in the subesophageal ganglion. Multiglomerular projection neurons, present in all but the dorsomedial antennocerebral tract, project in several olfactory foci of the protocerebral neuropil. With few exceptions, these neurons do not innervate the calyces of the mushroom body. Uniglomerular projection neurons appear most numerous in the inner antennocerebral tract but are also present in the outer and dorsomedial tracts. These neurons always ramify in the calyces of the mushroom body and in the lateral horn. The projection areas of the neurons following different tracts are largely separated in the secondary olfactory centers. This is most evident in the lateral horn, whereas, in the calyces, the axonal ramifications are more intermingled. The mushroom body architecture, revealed by neuropil immunolabeling, showed striking similarities to that of other lepidopteran species as well as insects of other taxa.

Proneural basic helix-loop-helix proteins and epidermal growth factor receptor signaling coordinately regulate cell type specification and cdk inhibitor expression during development

Cell differentiation and cell cycle exit are coordinately regulated during development; however, the molecular logic underlying this regulation is not known. The Drosophila cdk inhibitor Dacapo (Dap) is one of the key cell cycle regulators that exhibit dynamic expression during development and contribute to the developmental regulation of the cell cycle. In this study, regulation of Dap expression during cell type specification was investigated. The expression of Dap in the R2 and R5 precursors of the developing eye and in the newly recruited leg disc femoral sense organ precursors was found to be controlled by the epidermal growth factor receptor signaling-regulated transcription factor Pointed (Pnt) and the proneural basic helix-loop-helix proteins Atonal (Ato) and Daughterless (Da). We show that Pnt, Ato, and Da regulate Dap expression directly through their respective binding sites precisely at the time when these transcription factors function to specify neural fates. These results show that Dap expression is directly regulated by developmental mechanisms that simultaneously control cell type specification. This is potentially a general mechanism by which the expression of key cell cycle regulators is coordinated with differentiation during normal development. The direct regulation of key cell cycle regulators by the differentiation factors ensures coordinated regulation of cell cycle and differentiation.

Manganese-enhanced 3D MRI of established and disrupted synaptic activity in the developing insect brain in vivo

The antennal lobe of the sphinx moth Manduca sexta serves as a model for the development of the olfactory system. Here, the establishment of the glomerular synaptic network formed by the olfactory receptor axons and antennal lobe neurons at pupal stage P12 was followed by transection of the right antenna and - within 24 h - by injection of MnCl2 into the hemolymph. In vivo 3D MRI at 100 and 60 microm isotropic resolution was then performed at P13 to P17. Whereas the left antennal lobe revealed a pronounced increase of the signal-to-noise ratio (SNR) reflecting normal synaptic activity, the observation of only a small SNR increase within the right antennal lobe indicated the disruption of pertinent activity after antennal transection. The accumulation of manganese in the intact antennal system became observable within 3 h and lasted for at least 2 days after injection. Intra-individual comparisons between the right and left side yielded a statistically significant differential SNR increase in the left antennal lobe. Because such an effect was not observed in younger animals studied at pupal stages P10/P11, the MRI findings confirm the development of functional synapses in the antennal lobe of Manduca sexta by P13.

Adaptations in the structure and innervation of follicle-sinus complexes to an aquatic environment as seen in the Florida manatee (Trichechus manatus latirostris)

Florida manatees are large-bodied aquatic herbivores that use large tactile vibrissae for several purposes. Facial vibrissae are used to forage in a turbid water environment, and the largest perioral vibrissae can also grasp and manipulate objects. Other vibrissae distributed over the entire postfacial body appear to function as a lateral line system. All manatee vibrissae emanate from densely innervated follicle-sinus complexes (FSCs) like those in other mammals, although proportionately larger commensurate with the caliber of the vibrissae. As revealed by immunofluorescence, all manatee FSCs have many types of C, Adelta and Abeta innervation including Merkel, club, and longitudinal lanceolate endings at the level of the ring sinus, but they lack other types such as reticular and spiny endings at the level of the cavernous sinus. As in non-whisking terrestrial species, the inner conical bodies of facial FSCs are well innervated but lack Abeta-fiber terminals. Importantly, manatee FSCs have two unique types of Abeta-fiber endings. First, all of the FSCs have exceptionally large-caliber axons that branch to terminate as novel, gigantic spindle-like endings located at the upper ring sinus. Second, facial FSCs have smaller caliber Abeta fibers that terminate in the trabeculae of the cavernous sinus as an ending that resembles a Golgi tendon organ. In addition, the largest perioral vibrissae, which are used for grasping, have exceptionally well-developed medullary cores that have a structure and dense small-fiber innervation resembling that of tooth pulp. Other features of the epidermis and upper dermis structure and innervation differ from that seen in terrestrial mammals.

Termination profiles of insect chemosensory afferents in the antennal lobe are dependent on their origin on the flagellum

In cockroach antennae, sensory afferents from sensilla on the anterodorsal surface of the flagellum form the anterior antennal nerve, while afferents from the posteroventral surface form the posterior nerve. Anterograde staining was used to investigate afferent termination profiles in the glomeruli of the antennal lobe. The densities of terminal arborizations from the two nerves differed between glomeruli, with groupings of similar glomeruli evident. Individual glomeruli showed heterogeneous distribution of afferent terminals, with posterior nerve afferent terminals occurring near the nerve/glomeruli interface, and anterior nerve afferent terminals occurring on the opposite side. This study demonstrates, for the first time, a correlation between the distribution of primary afferent terminals in the individual glomeruli, and their origin on the surface of the flagellum.

Physiological changes of premotor nonspiking interneurons in the central compensation of eyestalk posture following unilateral sensory ablation in crayfish

We investigated how the physiological characteristics and synaptic activities of nonspiking giant interneurons (NGIs), which integrate sensory inputs in the brain and send synaptic outputs to oculomotor neurons innervating eyestalk muscles, changed after unilateral ablation of the statocyst in order to clarify neuronal mechanisms underlying the central compensation process in crayfish. The input resistance and membrane time constant in recovered animals that restored the original symmetrical eyestalk posture 2 weeks after operation were significantly greater than those immediately after operation on the operated side whereas in non-recovered animals only the membrane time constant showed a significant increase. On the intact side, both recovered and non-recovered animals showed no difference. The frequency of synaptic activity showed a complex pattern of change on both sides depending on the polarity of the synaptic potential. The synaptic activity returned to the bilaterally symmetrical level in recovered animals while bilateral asymmetry remained in non-recovered ones. These results suggest that the central compensation of eyestalk posture following unilateral impairment of the statocyst is subserved by not only changes in the physiological characteristics of the NGI membrane but also the activity of neuronal circuits presynaptic to NGIs.

A Kinase Gets Caspases into Shape

Two new papers (Kuranaga et al., 2006; Oshima et al., 2006) describe a previously uncharacterized Drosophila kinase (DmIKK epsilon) that regulates the abundance of DIAP1, a protein best known for its ability to inhibit apoptosis. However, DmIKK epsilon-mediated degradation of DIAP1 does not regulate apoptosis as might be predicted but instead regulates actin dynamics, cell morphology, and the differentiation of sensory organ precursor cells.

Drosophila IKK-Related Kinase Regulates Nonapoptotic Function of Caspases via Degradation of IAPs

Caspase activation has been extensively studied in the context of apoptosis. However, caspases also control other cellular functions, although the mechanisms regulating caspases in nonapoptotic contexts remain obscure. Drosophila IAP1 (DIAP1) is an endogenous caspase inhibitor that is crucial for regulating cell death during development. Here we describe Drosophila IKK-related kinase (DmIKKvarepsilon) as a regulator of caspase activation in a nonapoptotic context. We show that DmIKKvarepsilon promotes degradation of DIAP1 through direct phosphorylation. Knockdown of DmIKKvarepsilon in the proneural clusters of the wing imaginal disc, in which nonapoptotic caspase activity is required for proper sensory organ precursor (SOP) development, stabilizes endogenous DIAP1 and affects Drosophila SOP development. Our results demonstrate that DmIKKvarepsilon is a determinant of DIAP1 protein levels and that it establishes the threshold of activity required for the execution of nonapoptotic caspase functions.

Appetitive odor learning does not change olfactory coding in a subpopulation of honeybee antennal lobe neurons

Odors elicit spatio-temporal patterns of activity in the olfactory bulb of vertebrates and the antennal lobe of insects. There have been several reports of changes in these patterns following olfactory learning. These studies pose a conundrum: how can an animal learn to efficiently respond to a particular odor with an adequate response, if its primary representation already changes during this process? In this study, we offer a possible solution for this problem. We measured odor-evoked calcium responses in a subpopulation of uniglomerular AL output neurons in honeybees. We show that their responses to odors are remarkably resistant to plasticity following a variety of appetitive olfactory learning paradigms. There was no significant difference in the changes of odor-evoked activity between single and multiple trial forward or backward conditioning, differential conditioning, or unrewarded successive odor stimulation. In a behavioral learning experiment we show that these neurons are necessary for conditioned odor responses. We conclude that these uniglomerular projection neurons are necessary for reliable odor coding and are not modified by learning in this paradigm. The role that other projection neurons play in olfactory learning remains to be investigated.

Multiple olfactory receptor neurons and their axonal projections in the antennal lobe of the honeybee Apis mellifera

The poreplate sensilla of honeybees are equipped with multiple olfactory receptor neurons (ORNs), which innervate glomeruli of the antennal lobe (AL). We investigated the axonal projection pattern in glomeruli of the AL (glomerular pattern), formed by the multiple ORNs of individual poreplate sensilla. We used the different glomerular patterns to draw conclusions about the equipment of poreplate sensilla with different ORN types. ORNs of single poreplate sensilla were stained and analyzed by laser-scanning confocal microscopy and 3D software (AMIRA). In 13 specimens we found between 7 and 23 ORNs. This is in accordance with data found in the literature (5-35 ORNs) suggesting that all ORNs of the single poreplate sensilla were stained. The ORNs innervate the AL via all four sensory tracts (T1-T4), and glomeruli of the anterior part of the AL are more often innervated. Each ORN innervates a single glomerulus (uniglomerular), and all ORNs of one poreplate sensillum project to different glomeruli. Visual inspection and individual identification of glomeruli, based on the honeybee digital AL atlas, were used to evaluate mapping of glomeruli by a rigid transformation of the experimental ALs onto a reference AL. ORNs belonging to individual poreplate sensilla form variable glomerular patterns, and we did not find a common organization of glomerular patterns. We conclude that poreplate sensilla are equipped with different ORN types but that the same ORN types can be found in different poreplate sensilla. The equipment of poreplate sensilla with ORNs is overlapping. The mapping of glomeruli by rigid transformation is revealed to be a powerful tool for comparative neuroanatomy.

Wiring stability of the adult Drosophila olfactory circuit after lesion

Neuronal wiring plasticity in response to experience or injury has been reported in many parts of the adult nervous system. For instance, visual or somatosensory cortical maps can reorganize significantly in response to peripheral lesions, yet a certain degree of stability is essential for neuronal circuits to perform their dedicated functions. Previous studies on lesion-induced neuronal reorganization have primarily focused on systems that use continuous neural maps. Here, we assess wiring plasticity in a discrete neural map represented by the adult Drosophila olfactory circuit. Using conditional expression of toxins, we genetically ablated specific classes of neurons and examined the consequences on their synaptic partners or neighboring classes in the adult antennal lobe. We find no alteration of connection specificity between olfactory receptor neurons (ORNs) and their postsynaptic targets, the projection neurons (PNs). Ablating an ORN class maintains PN dendrites within their glomerular borders, and ORN axons normally innervating an adjacent target do not expand. Likewise, ablating PN classes does not alter their partner ORN axon connectivity. Interestingly, an increase in the contralateral ORN axon terminal density occurs in response to the removal of competing ipsilateral ORNs. Therefore, plasticity in this circuit can occur but is confined within a glomerulus, thereby retaining the wiring specificity of ORNs and PNs. We conclude that, although adult olfactory neurons can undergo plastic changes in response to the loss of competition, the olfactory circuit overall is extremely stable in preserving segregated information channels in this discrete map.

Activation of epidermal growth factor receptor mediates receptor axon sorting and extension in the developing olfactory system of the moth Manduca sexta

During development of the adult olfactory system of the moth Manduca sexta, olfactory receptor neurons extend axons from the olfactory epithelium in the antenna into the brain. As they arrive at the brain, interactions with centrally derived glial cells cause axons to sort and fasciculate with other axons destined to innervate the same glomeruli. Here we report studies indicating that activation of the epidermal growth factor receptor (EGFR) is involved in axon ingrowth and targeting. Blocking the EGFR kinase domain pharmacologically leads to stalling of many axons in the sorting zone and nerve layer as well as abnormal axonal fasciculation in the sorting zone. We also find that neuroglian, an IgCAM known to activate the EGFR through homophilic interactions in other systems, is transiently present on olfactory receptor neuron axons and on glia during the critical stages of the sorting process. The neuroglian is resistant to extraction with Triton X-100 in the sorting zone and nerve layer, possibly indicating its stabilization by homophilic binding in these regions. Our results suggest a mechanism whereby neuroglian molecules on axons and possibly sorting zone glia bind homophilically, leading to activation of EGFRs, with subsequent effects on axon sorting, pathfinding, and extension, and glomerulus development.

Olfactory receptors on the cockroach antenna signal odour ON and odour OFF by excitation

A morphologically identifiable type of olfactory sensillum on the antenna of the American cockroach contains a pair of ON and OFF cells that responds oppositely to changes in the concentration of fruit odours. The odour of lemon oil was used to study the accuracy with which these cells can discriminate between rapid step-like, ramp-like and oscillating changes in odour concentration. The discharge rates of both cells are not only affected by the actual concentration at particular instants in time (instantaneous concentration) but also by the rate at which concentration changes. The impulse frequency of the fruit odour ON cell is high when odour concentration is high, but higher still when odour concentration is also rising. Conversely, the impulse frequency of the fruit odour OFF cell is high when odour concentration is low and higher still when odour concentration is also falling. Thus, the effect of odour concentration on the responses of both cells is reinforced by the rate of change. Sensitivity to the rate of concentration change becomes greater when the rate is low. Because of the high sensitivity to low rates of change, these cells are optimized to detect fluctuations in fruit odour concentration. Whereas the ON cell signals the arrival and presence of fruit odour, the OFF cell detects its termination and absence. These cells provide excitatory responses for both increase and decrease in fruit odour concentration and may therefore reinforce contrast information.