Serotonin enhances central olfactory neuron responses to female sex pheromone in the male sphinx moth manduca sexta

Serotonergic modulation of olfactory processing in locust antennae

Insects have sophisticated olfactory systems that enable them to detect and respond to complex exogenous chemical cues. The encoding mechanisms of these chemical signals have been studied both in their peripheral and central nervous systems (CNS). While various neuromodulators have been shown to play significant roles in olfactory processing within the antennal lobes of the brain, their roles in peripheral olfactory sensory systems, such as the antennae, are less understood. This review focuses on the role of serotonin (5-HT) receptor in the locust antenna, specifically the modulatory function of the serotonin receptor2 on odour inputs. We also review recent studies on the modulation of olfaction in the peripheral nervous systems of insects and discuss potential directions for future research on the role of neuromodulators in insect peripheral olfactory systems.

Anatomic and neurochemical analysis of the palpal olfactory system in the red flour beetle Tribolium castaneum, HERBST

The paired antennal lobes were long considered the sole primary processing centers of the olfactory pathway in holometabolous insects receiving input from the olfactory sensory neurons of the antennae and mouthparts. In hemimetabolous insects, however, olfactory cues of the antennae and palps are processed separately. For the holometabolous red flour beetle Tribolium castaneum, we could show that primary processing of the palpal and antennal olfactory input also occurs separately and at distinct neuronal centers. While the antennal olfactory sensory neurons project into the antennal lobes, those of the palps project into the paired glomerular lobes and the unpaired gnathal olfactory center. Here we provide an extended analysis of the palpal olfactory pathway by combining scanning electron micrographs with confocal imaging of immunohistochemical staining and reporter expression identifying chemosensory and odorant receptor-expressing neurons in the palpal sensilla. In addition, we extended the anatomical characterization of the gnathal olfactory center by 3D reconstructions and investigated the distribution of several neuromediators. The similarities in the neuromediator repertoire between antennal lobes, glomerular lobes, and gnathal olfactory center underline the role of the latter two as additional primary olfactory processing centers.

The modulatory effects of biogenic amines on male mating performance in Bactrocera dorsalis

In insects, the emergence of mating behavior requires the interplay among sex-determination hierarchy mechanisms that regulate sex-specific differentiation, perception and integration of different sensory cues, and precisely patterned behavioral outputs. Biogenic amines, including octopamine (OA), dopamine (DA), tyramine (TA), serotonin and histamine, have been identified and proposed as putative neurotransmitters, neurohormones and/or neuromodulators in the central nervous system of insects to influence multiple physiologies and behaviors. The current study provides the physiological roles and pharmacology of these biogenic amines in the mating performance of Bactrocera dorsalis. Silencing gene expressions coding for biosynthetic enzymes of DA and serotonin in male flies could decrease mating rates, while OA, TA and histamine had no such effects on mating. Furthermore, injection of DA or the DA receptor antagonist chlorpromazine could affect mating rate, as well as injection of serotonin. Pharmacological treatments with other biogenic amines or their receptor antagonists in male flies have no roles in regulating mating performance. We conclude that DA and its receptors are involved in regulating male mating behaviors in B. dorsalis, while changes in serotonin levels in male flies could also affect mating rates. In the current study, the modulatory effects of these biogenic amines on mating performance were investigated, and these results will be helpful in providing a new strategy for controlling B. dorsalis.

Task-specific roles of local interneurons for inter- and intraglomerular signaling in the insect antennal lobe

Local interneurons (LNs) mediate complex interactions within the antennal lobe, the primary olfactory system of insects, and the functional analog of the vertebrate olfactory bulb. In the cockroach Periplaneta americana, as in other insects, several types of LNs with distinctive physiological and morphological properties can be defined. Here, we combined whole-cell patch-clamp recordings and Ca²⁺ imaging of individual LNs to analyze the role of spiking and nonspiking LNs in inter- and intraglomerular signaling during olfactory information processing. Spiking GABAergic LNs reacted to odorant stimulation with a uniform rise in [Ca²⁺]_i in the ramifications of all innervated glomeruli. In contrast, in nonspiking LNs, glomerular Ca²⁺ signals were odorant specific and varied between glomeruli, resulting in distinct, glomerulus-specific tuning curves. The cell type-specific differences in Ca²⁺ dynamics support the idea that spiking LNs play a primary role in interglomerular signaling, while they assign nonspiking LNs an essential role in intraglomerular signaling.

Antennal-lobe neurons in the moth Helicoverpa armigera: Morphological features of projection neurons, local interneurons, and centrifugal neurons

The relatively large primary olfactory center of the insect brain, the antennal lobe (AL), contains several heterogeneous neuronal types. These include projection neurons (PNs), providing olfactory information to higher‐order neuropils via parallel pathways, and local interneurons (LNs), which provide lateral processing within the AL. In addition, various types of centrifugal neurons (CNs) offer top‐down modulation onto the other AL neurons. By performing iontophoretic intracellular staining, we collected a large number of AL neurons in the moth, Helicoverpa armigera, to examine the distinct morphological features of PNs, LNs, and CNs. We characterize 190 AL neurons. These were allocated to 25 distinct neuronal types or sub‐types, which were reconstructed and placed into a reference brain. In addition to six PN types comprising 15 sub‐types, three LN and seven CN types were identified. High‐resolution confocal images allowed us to analyze AL innervations of the various reported neurons, which demonstrated that all PNs innervating ventroposterior glomeruli contact a protocerebral neuropil rarely targeted by other PNs, that is the posteriorlateral protocerebrum. We also discuss the functional roles of the distinct CNs, which included several previously uncharacterized types, likely involved in computations spanning from multisensory processing to olfactory feedback signalization into the AL.

Social isolation causes downregulation of immune and stress response genes and behavioural changes in a social insect

Humans and other social mammals experience isolation from their group as stressful, triggering behavioural and physiological anomalies that reduce fitness. While social isolation has been intensely studied in social mammals, it is less clear how social insects, which evolved sociality independently, respond to isolation. Here we examined whether the typical mammalian responses to social isolation, e.g., an impaired ability to interact socially and immune suppression are also found in social insects. We studied the consequences of social isolation on behaviour and brain gene expression in the ant Temnothorax nylanderi. Following isolation, workers interacted moderately less with adult nestmates, increased the duration of brood contact, and reduced the time spent self-grooming, an important sanitary behaviour. Our brain transcriptome analysis revealed that only a few behaviour-related genes had altered their expression with isolation time. Rather, many genes linked to immune system functioning and stress response had been downregulated. This probably sensitizes isolated individuals to various stressors, in particular because isolated workers exhibit reduced sanitary behaviour. We provide evidence of the diverse consequences of social isolation in social insects, some of which resemble those found in social mammals, suggesting a general link between social well-being, stress tolerance, and immune competence in social animals.

Plasticity and modulation of olfactory circuits in insects

Olfactory circuits change structurally and physiologically during development and adult life. This allows insects to respond to olfactory cues in an appropriate and adaptive way according to their physiological and behavioral state, and to adapt to their specific abiotic and biotic natural environment. We highlight here findings on olfactory plasticity and modulation in various model and non-model insects with an emphasis on moths and social Hymenoptera. Different categories of plasticity occur in the olfactory systems of insects. One type relates to the reproductive or feeding state, as well as to adult age. Another type of plasticity is context-dependent and includes influences of the immediate sensory and abiotic environment, but also environmental conditions during postembryonic development, periods of adult behavioral maturation, and short- and long-term sensory experience. Finally, plasticity in olfactory circuits is linked to associative learning and memory formation. The vast majority of the available literature summarized here deals with plasticity in primary and secondary olfactory brain centers, but also peripheral modulation is treated. The described molecular, physiological, and structural neuronal changes occur under the influence of neuromodulators such as biogenic amines, neuropeptides, and hormones, but the mechanisms through which they act are only beginning to be analyzed.

Serotonergic modulation of visual neurons in Drosophila melanogaster

Sensory systems rely on neuromodulators, such as serotonin, to provide flexibility for information processing as stimuli vary, such as light intensity throughout the day. Serotonergic neurons broadly innervate the optic ganglia of Drosophila melanogaster, a widely used model for studying vision. It remains unclear whether serotonin modulates the physiology of interneurons in the optic ganglia. To address this question, we first mapped the expression patterns of serotonin receptors in the visual system, focusing on a subset of cells with processes in the first optic ganglion, the lamina. Serotonin receptor expression was found in several types of columnar cells in the lamina including 5-HT2B in lamina monopolar cell L2, required for spatiotemporal luminance contrast, and both 5-HT1A and 5-HT1B in T1 cells, whose function is unknown. Subcellular mapping with GFP-tagged 5-HT2B and 5-HT1A constructs indicated that these receptors localize to layer M2 of the medulla, proximal to serotonergic boutons, suggesting that the medulla neuropil is the primary site of serotonergic regulation for these neurons. Exogenous serotonin increased basal intracellular calcium in L2 terminals in layer M2 and modestly decreased the duration of visually induced calcium transients in L2 neurons following repeated dark flashes, but otherwise did not alter the calcium transients. Flies without functional 5-HT2B failed to show an increase in basal calcium in response to serotonin. 5-HT2B mutants also failed to show a change in amplitude in their response to repeated light flashes but other calcium transient parameters were relatively unaffected. While we did not detect serotonin receptor expression in L1 neurons, they, like L2, underwent serotonin-induced changes in basal calcium, presumably via interactions with other cells. These data demonstrate that serotonin modulates the physiology of interneurons involved in early visual processing in Drosophila.

Serotonergic neurons innervate the Drosophila melanogaster eye, but it was not known whether serotonin signaling could induce acute physiological responses in visual interneurons. We found serotonin receptors expressed in all neuropils of the optic lobe and identified specific neurons involved in visual information processing that express serotonin receptors. Activation of these receptors increased intracellular calcium in first order interneurons L1 and L2 and may enhance visually induced calcium transients in L2 neurons. These data support a role for the serotonergic neuromodulation of interneurons in the Drosophila visual system.

Serotonergic modulation across sensory modalities

The serotonergic system has been widely studied across animal taxa and different functional networks. This modulatory system is therefore well positioned to compare the consequences of neuromodulation for sensory processing across species and modalities at multiple levels of sensory organization. Serotonergic neurons that innervate sensory networks often bidirectionally exchange information with these networks but also receive input representative of motor events or motivational state. This convergence of information supports serotonin's capacity for contextualizing sensory information according to the animal's physiological state and external events. At the level of sensory circuitry, serotonin can have variable effects due to differential projections across specific sensory subregions, as well as differential serotonin receptor type expression within those subregions. Functionally, this infrastructure may gate or filter sensory inputs to emphasize specific stimulus features or select among different streams of information. The near-ubiquitous presence of serotonin and other neuromodulators within sensory regions, coupled with their strong effects on stimulus representation, suggests that these signaling pathways should be considered integral components of sensory systems.

Differential modulation of courtship behavior and subsequent aggression by octopamine, dopamine and serotonin in male crickets

Aggression is a behavioral strategy for securing limited resources and its expression is strongly influenced by their presence and value. In particular, males are generally thought to guard females after mating to ward off other males, but the underlying control mechanisms are unknown. Here, we investigated the role of amines on male courtship behavior and its subsequent effect on male-male aggression in crickets (Gryllus bimaculatus). Contrary to the guarding hypothesis, female presence alone had no immediate effect on male-male aggression. Furthermore, confirming studies on other species, prior female contact, but not necessarily courtship or copulation, promoted subsequent male-male aggression in subordinate, but not socially naive crickets. This promoting effect of female contact is transient and slowly wanes after her removal. Selective aminergic receptor antagonists revealed that the promoting effect of prior female contact on male-male aggression is mediated by octopamine (OA), as well as by serotonin (5HT) acting most likely via 5HT₁ and/or 5HT₇ like receptors. This contrasts the role of 5HT₂-like receptors in maintaining reduced aggressiveness after social defeat. Furthermore, while dopamine (DA) is necessary for the recovery of aggression in subordinates after defeat, it appears to play no part in female induced aggression. Male courtship, on the other hand, is selectively promoted by DA and 5HT, again most likely via 5HT₁ and/or 5HT₇ like receptors, but not by OA. We conclude that OA, DA and 5HT each differentially modulate different aspects of courtship and aggressive behavior in a context specific fashion.

Analysis of Single Neurons by Perforated Patch Clamp Recordings and MALDI-TOF Mass Spectrometry

Single-cell mass spectrometry has become an established technique to study specific molecular properties such as the neuropeptide complement of identified neurons. Here, we describe a strategy to characterize, by MALDI-TOF mass spectrometry, neurochemical composition of neurons that were identified by their electrophysiological and neuroanatomical characteristics. The workflow for the first time combined perforated patch clamp recordings with dye loading by electroporation for electrophysiological and neuroanatomical characterization as well as chemical profiling of somata by MALDI-TOF mass spectrometry with subsequent immunohistochemistry. To develop our protocol, we used identified central olfactory neurons from the American cockroach Periplaneta americana. First, the combined approach was optimized using a relative homogeneous, well-characterized neuron population of uniglomerular projection neurons, which show acetylcholine esterase immunoreactivity. The general applicability of this approach was verified on local interneurons, which are a diverse neuron population expressing highly differentiated neuropeptidomes. Thus, this study shows that the newly established protocol is suitable to comprehensively analyze electrophysiological, neuroanatomical, and molecular properties of single neurons. We consider this approach an important step to foster single-cell analysis in a wide variety of neuron types.

Nosema ceranae parasitism impacts olfactory learning and memory and neurochemistry in honey bees (Apis mellifera)

Nosema sp. is an internal parasite of the honey bee, Apis mellifera, and one of the leading contributors to colony losses worldwide. This parasite is found in the honey bee midgut and has profound consequences for the host's physiology. Nosema sp. impairs foraging performance in honey bees, yet, it is unclear whether this parasite affects the bee's neurobiology. In this study, we examined whether Nosema sp. affects odor learning and memory and whether the brains of parasitized bees show differences in amino acids and biogenic amines. We took newly emerged bees and fed them with Nosema ceranae At approximate nurse and forager ages, we employed an odor-associative conditioning assay using the proboscis extension reflex and two bioanalytical techniques to measure changes in brain chemistry. We found that nurse-aged bees infected with N. ceranae significantly outperformed controls in odor learning and memory, suggestive of precocious foraging, but by forager age, infected bees showed deficits in learning and memory. We also detected significant differences in amino acid concentrations, some of which were age specific, as well as altered serotonin, octopamine, dopamine and l-dopa concentrations in the brains of parasitized bees. These findings suggest that N. ceranae infection affects honey bee neurobiology and may compromise behavioral tasks. These results yield new insight into the host-parasite dynamic of honey bees and N. ceranae, as well as the neurochemistry of odor learning and memory under normal and parasitic conditions.

Intrinsic and Extrinsic Neuromodulation of Olfactory Processing

Neuromodulation is a ubiquitous feature of neural systems, allowing flexible, context specific control over network dynamics. Neuromodulation was first described in invertebrate motor systems and early work established a basic dichotomy for neuromodulation as having either an intrinsic origin (i.e., neurons that participate in network coding) or an extrinsic origin (i.e., neurons from independent networks). In this conceptual dichotomy, intrinsic sources of neuromodulation provide a “memory” by adjusting network dynamics based upon previous and ongoing activation of the network itself, while extrinsic neuromodulators provide the context of ongoing activity of other neural networks. Although this dichotomy has been thoroughly considered in motor systems, it has received far less attention in sensory systems. In this review, we discuss intrinsic and extrinsic modulation in the context of olfactory processing in invertebrate and vertebrate model systems. We begin by discussing presynaptic modulation of olfactory sensory neurons by local interneurons (LNs) as a mechanism for gain control based on ongoing network activation. We then discuss the cell-class specific effects of serotonergic centrifugal neurons on olfactory processing. Finally, we briefly discuss the integration of intrinsic and extrinsic neuromodulation (metamodulation) as an effective mechanism for exerting global control over olfactory network dynamics. The heterogeneous nature of neuromodulation is a recurring theme throughout this review as the effects of both intrinsic and extrinsic modulation are generally non-uniform.

Using insects to drive mobile robots - hybrid robots bridge the gap between biological and artificial systems

The use of mobile robots is an effective method of validating sensory-motor models of animals in a real environment. The well-identified insect sensory-motor systems have been the major targets for modeling. Furthermore, mobile robots implemented with such insect models attract engineers who aim to avail advantages from organisms. However, directly comparing the robots with real insects is still difficult, even if we successfully model the biological systems, because of the physical differences between them. We developed a hybrid robot to bridge the gap. This hybrid robot is an insect-controlled robot, in which a tethered male silkmoth (Bombyx mori) drives the robot in order to localize an odor source. This robot has the following three advantages: 1) from a biomimetic perspective, the robot enables us to evaluate the potential performance of future insect-mimetic robots; 2) from a biological perspective, the robot enables us to manipulate the closed-loop of an onboard insect for further understanding of its sensory-motor system; and 3) the robot enables comparison with insect models as a reference biological system. In this paper, we review the recent works regarding insect-controlled robots and discuss the significance for both engineering and biology.

Transient voltage-activated K+ currents in central antennal lobe neurons: cell type-specific functional properties

In this study we analyzed transient voltage-activated K+ currents (IA) of projection neurons and local interneurons in the antennal lobe of the cockroach Periplaneta americana The antennal lobe is the first synaptic processing station for olfactory information in insects. Local interneurons are crucial for computing olfactory information and form local synaptic connections exclusively in the antennal lobe, whereas a primary task of the projection neurons is the transfer of preprocessed olfactory information from the antennal lobe to higher order centers in the protocerebrum. The different physiological tasks of these neurons require specialized physiological and morphological neuronal phenotypes. We asked if and how the different physiological phenotypes are reflected in the functional properties of IA, which is crucial for shaping intrinsic electrophysiological properties of neurons. Whole cell patch-clamp recordings from adult male P. americana showed that all their central antennal lobe neurons can generate IA The current exhibited marked cell type-specific differences in voltage dependence of steady-state activation and inactivation, and differences in inactivation kinetics during sustained depolarization. Pharmacological experiments revealed that IA in all neuron types was partially blocked by α-dendrotoxin and phrixotoxin-2, which are considered blockers with specificity for Shaker- and Shal-type channels, respectively. These findings suggest that IA in each cell type is a mixed current generated by channels of both families. The functional role of IA was analyzed in experiments under current clamp, in which portions of IA were blocked by α-dendrotoxin or phrixotoxin-2. These experiments showed that IA contributes significantly to the intrinsic electrophysiological properties, such as the action potential waveform and membrane excitability.NEW & NOTEWORTHY In the insect olfactory system, projection neurons and local interneurons have task-specific electrophysiological and morphological phenotypes. Voltage-activated potassium channels play a crucial role in shaping functional properties of these neurons. This study revealed marked cell type-specific differences in the biophysical properties of transient voltage-activated potassium currents in central antennal lobe neurons.

Development of the Neurochemical Architecture of the Central Complex

The central complex represents one of the most conspicuous neuroarchitectures to be found in the insect brain and regulates a wide repertoire of behaviors including locomotion, stridulation, spatial orientation and spatial memory. In this review article, we show that in the grasshopper, a model insect system, the intricate wiring of the fan-shaped body (FB) begins early in embryogenesis when axons from the first progeny of four protocerebral stem cells (called W, X, Y, Z, respectively) in each brain hemisphere establish a set of tracts to the primary commissural system. Decussation of subsets of commissural neurons at stereotypic locations across the brain midline then establishes a columnar neuroarchitecture in the FB which is completed during embryogenesis. Examination of the expression patterns of various neurochemicals in the central complex including neuropeptides, a neurotransmitter and the gas nitric oxide (NO), show that these appear progressively and in a substance-specific manner during embryogenesis. Each neuroactive substance is expressed by neurons located at stereotypic locations in a given central complex lineage, confirming that the stem cells are biochemically multipotent. The organization of axons expressing the various neurochemicals within the central complex is topologically related to the location, and hence birthdate, of the neurons within the lineages. The neurochemical expression patterns within the FB are layered, and so reflect the temporal topology present in the lineages. This principle relates the neuroanatomical to the neurochemical architecture of the central complex and so may provide insights into the development of adaptive behaviors.

Functional integration of a serotonergic neuron in the Drosophila antennal lobe

Serotonin plays a critical role in regulating many behaviors that rely on olfaction and recently there has been great effort in determining how this molecule functions in vivo. However, it remains unknown how serotonergic neurons that innervate the first olfactory relay respond to odor stimulation and how they integrate synaptically into local circuits. We examined the sole pair of serotonergic neurons that innervates the Drosophila antennal lobe (the first olfactory relay) to characterize their physiology, connectivity, and contribution to pheromone processing. We report that nearly all odors inhibit these cells, likely through connections made reciprocally within the antennal lobe. Pharmacological and immunohistochemical analyses reveal that these neurons likely release acetylcholine in addition to serotonin and that exogenous and endogenous serotonin have opposing effects on olfactory responses. Finally, we show that activation of the entire serotonergic network, as opposed to only activation of those fibers innervating the antennal lobe, may be required for persistent serotonergic modulation of pheromone responses in the antennal lobe.

DOI:http://dx.doi.org/10.7554/eLife.16836.001

Properties and physiological function of Ca2+-dependent K+ currents in uniglomerular olfactory projection neurons

Ca(2+)-activated potassium currents [IK(Ca)] are an important link between the intracellular signaling system and the membrane potential, which shapes intrinsic electrophysiological properties. To better understand the ionic mechanisms that mediate intrinsic firing properties of olfactory uniglomerular projection neurons (uPNs), we used whole cell patch-clamp recordings in an intact adult brain preparation of the male cockroach Periplaneta americana to analyze IK(Ca) In the insect brain, uPNs form the principal pathway from the antennal lobe to the protocerebrum, where centers for multimodal sensory processing and learning are located. In uPNs the activation of IK(Ca) was clearly voltage and Ca(2+) dependent. Thus under physiological conditions IK(Ca) is strongly dependent on Ca(2+) influx kinetics and on the membrane potential. The biophysical characterization suggests that IK(Ca) is generated by big-conductance (BK) channels. A small-conductance (SK) channel-generated current could not be detected. IK(Ca) was sensitive to charybdotoxin (CTX) and iberiotoxin (IbTX) but not to apamin. The functional role of IK(Ca) was analyzed in occlusion experiments under current clamp, in which portions of IK(Ca) were blocked by CTX or IbTX. Blockade of IK(Ca) showed that IK(Ca) contributes significantly to intrinsic electrophysiological properties such as the action potential waveform and membrane excitability.

The anatomical basis for modulatory convergence in the antennal lobe of Manduca sexta

The release of neuromodulators by widely projecting neurons often allows sensory systems to alter how they process information based on the physiological state of an animal. Neuromodulators alter network function by changing the biophysical properties of individual neurons and the synaptic efficacy with which individual neurons communicate. However, most, if not all, sensory networks receive multiple neuromodulatory inputs, and the mechanisms by which sensory networks integrate multiple modulatory inputs are not well understood. Here we characterized the relative glomerular distribution of two extrinsic neuromodulators associated with distinct physiological states, serotonin (5-HT) and dopamine (DA), in the antennal lobe (AL) of the moth Manduca sexta. By using immunocytochemistry and mass dye fills, we characterized the innervation patterns of both 5-HT- and tyrosine hydroxylase-immunoreactive processes relative to each other, to olfactory receptor neurons (ORNs), to projection neurons (PNs), and to several subsets of local interneurons (LNs). 5-HT immunoreactivity had nearly complete overlap with PNs and LNs, yet no overlap with ORNs, suggesting that 5-HT may modulate PNs and LNs directly but not ORNs. TH immunoreactivity overlapped with PNs, LNs, and ORNs, suggesting that dopamine has the potential to modulate all three cell types. Furthermore, the branching density of each neuromodulator differed, with 5-HT exhibiting denser arborizations and TH-ir processes being sparser. Our results suggest that 5-HT and DA extrinsic neurons target partially overlapping glomerular regions, yet DA extends further into the region occupied by ORNs.

Firing and intrinsic properties of antennal lobe neurons in the Noctuid moth Agrotis ipsilon

The antennal lobe (AL) of the Noctuid moth Agrotis ipsilon has emerged as an excellent model for studying olfactory processing and its plasticity in the central nervous system. Odor-evoked responses of AL neurons and input-to-output transformations involved in pheromone processing are well characterized in this species. However, the intrinsic electrical properties responsible of the firing of AL neurons are poorly known. To this end, patch-clamp recordings in current- and voltage-clamp mode from neurons located in the two main clusters of cell bodies in the ALs were combined with intracellular staining on A. ipsilon males. Staining indicated that the lateral cluster (LC) is composed of 85% of local neurons (LNs) and 15% of projection neurons (PNs). The medial cluster (MC) contains only PNs. Action potentials were readily recorded from the soma in LNs and PNs located in the LC but not from PNs in the MC where recordings showed small or no action potentials. In the LC, the spontaneous activity of about 20% of the LNs presented irregular bursts while being more regular in PNs. We also identified a small population of LNs lacking voltage-gated Na(+) currents and generating spikelets. We focused on the firing properties of LNs since in about 60% of LNs, but not in PNs, action potentials were followed by depolarizing afterpotentials (DAPs). These DAPs could generate a second action potential, so that the activity was composed of action potential doublets. DAPs depended on voltage, Ca(2+)-channels and possibly on Ca(2+)-activated non-specific cationic channels. During steady state current injection, DAPs occurred after each action potential and did not require high-frequency firing. The amplitude of DAPs increased when the interspike interval was small, typically within bursts, likely arising from a Ca(2+) build up. DAPs were more often found in bursting than in non-bursting LNs but do not support bursting activity. DAPs and spike doublets also occurred during odor-evoked activity suggesting that they can mediate olfactory integration in the AL.

Identification, functional characterization, and pharmacological profile of a serotonin type-2b receptor in the medically important insect, Rhodnius prolixus

In the Chagas disease vector, Rhodnius prolixus, two diuretic hormones act synergistically to dramatically increase fluid secretion by the Malpighian tubules (MTs) during the rapid diuresis that is initiated upon engorgement of vertebrate blood. One of these diuretic hormones is the biogenic amine, serotonin (5-hydroxytryptamine, 5-HT), which controls a variety of additional activities including cuticle plasticization, salivary gland secretion, anterior midgut absorption, cardioacceleratory activity, and myotropic activities on a number of visceral tissues. To better understand the regulatory mechanisms linked to these various physiological actions of serotonin, we have isolated and characterized a serotonin type 2b receptor in R. prolixus, Rhopr5HTR2b, which shares sequence similarity to the vertebrate serotonin type 2 receptors. Rhopr5HTR2b transcript is enriched in well-recognized physiological targets of serotonin, including the MTs, salivary glands and dorsal vessel (i.e., insect heart). Notably, Rhopr5HTR2b was not enriched in the anterior midgut where serotonin stimulates absorption and elicits myotropic control. Using a heterologous functional receptor assay, we examined Rhopr5HTR2b activation characteristics and its sensitivity to potential agonists, antagonists, and other biogenic amines. Rhopr5HTR2b is dose-dependently activated by serotonin with an EC50 in the nanomolar range. Rhopr5HTR2b is sensitive to alpha-methyl serotonin and is inhibited by a variety of serotonin receptor antagonists, including propranolol, spiperone, ketanserin, mianserin, and cyproheptadine. In contrast, the cardioacceleratory activity of serotonin revealed a unique pharmacological profile, with no significant response induced by alpha-methyl serotonin and insensitivity to ketanserin and mianserin. This distinct agonist/antagonist profile indicates that a separate serotonin receptor type may mediate cardiomodulatory effects controlled by serotonin in R. prolixus.

Modeling the cellular mechanisms and olfactory input underlying the triphasic response of moth pheromone-sensitive projection neurons

In the antennal lobe of the noctuid moth Agrotis ipsilon, most pheromone-sensitive projection neurons (PNs) exhibit a triphasic firing pattern of excitation (E₁)-inhibition (I)-excitation (E₂) in response to a pulse of the sex pheromone. To understand the mechanisms underlying this stereotypical discharge, we developed a biophysical model of a PN receiving inputs from olfactory receptor neurons (ORNs) via nicotinic cholinergic synapses. The ORN is modeled as an inhomogeneous Poisson process whose firing rate is a function of time and is fitted to extracellular data recorded in response to pheromone stimulations at various concentrations and durations. The PN model is based on the Hodgkin-Huxley formalism with realistic ionic currents whose parameters were derived from previous studies. Simulations revealed that the inhibitory phase I can be produced by a SK current (Ca²⁺-gated small conductance K⁺ current) and that the excitatory phase E₂ can result from the long-lasting response of the ORNs. Parameter analysis further revealed that the ending time of E₁ depends on some parameters of SK, Ca²⁺, nACh and Na⁺ currents; I duration mainly depends on the time constant of intracellular Ca²⁺ dynamics, conductance of Ca²⁺ currents and some parameters of nACh currents; The mean firing frequency of E₁ and E₂ depends differentially on the interaction of various currents. Thus it is likely that the interplay between PN intrinsic currents and feedforward synaptic currents are sufficient to generate the triphasic firing patterns observed in the noctuid moth A. ipsilon.

Colocalization of allatotropin and tachykinin-related peptides with classical transmitters in physiologically distinct subtypes of olfactory local interneurons in the cockroach (Periplaneta americana)

In the insect antennal lobe different types of local interneurons mediate complex excitatory and inhibitory interactions between the glomerular pathways to structure the spatiotemporal representation of odors. Mass spectrometric and immunohistochemical studies have shown that in local interneurons classical neurotransmitters are likely to colocalize with a variety of substances that can potentially act as cotransmitters or neuromodulators. In the antennal lobe of the cockroach Periplaneta americana, gamma-aminobutyric acid (GABA) has been identified as the potential inhibitory transmitter of spiking type I local interneurons, whereas acetylcholine is most likely the excitatory transmitter of nonspiking type IIa1 local interneurons. This study used whole-cell patch clamp recordings combined with single-cell labeling and immunohistochemistry to test if the GABAergic type I local interneurons and the cholinergic type IIa1 local interneurons express allatotropin and tachykinin-related neuropeptides (TKRPs). These are two of the most abundant types of peptides in the insect antennal lobe. GABA-like and choline acetyltransferase (ChAT)-like immunoreactivity were used as markers for GABAergic and cholinergic neurons, respectively. About 50% of the GABA-like immunoreactive (-lir) spiking type I local interneurons were allatotropin-lir, and ∼ 40% of these neurons were TKRP-lir. About 20% of nonspiking ChAT-lir type IIa1 local interneurons were TKRP-lir. Our results suggest that in subpopulations of GABAergic and cholinergic local interneurons, allatotropin and TKRPs might act as cotransmitters or neuromodulators. To unequivocally assign neurotransmitters, cotransmitters, and neuromodulators to identified classes of antennal lobe neurons is an important step to deepen our understanding of information processing in the insect olfactory system.

The role of nitric oxide in memory is modulated by diurnal time

Nitric oxide (NO) is thought to play an important neuromodulatory role in the olfactory system. This modulation has been suggested to be particularly important for olfactory learning and memory in the antennal lobe (the primary olfactory network in invertebrates). We are using the hawkmoth, Manduca sexta, to further investigate the role of NO in olfactory memory. Recent findings suggest that NO affects short-term memory traces and that NO concentration fluctuates with the light cycle. This gives rise to the hypothesis that NO may be involved in the connection between memory and circadian rhythms. In this study, we explore the role of diurnal time and NO in memory by altering the time of day when associative-olfactory conditioning is performed. We find a strong effect of NO on short-term memory, and two surprising effects of diurnal time. We find that (1) at certain time points, NO affects longer traces of memory in addition to short-term memory; and (2) when conditioning is performed close to the light cycle switches—both from light to dark and dark to light—NO does not significantly affect memory at all. These findings suggest an intriguing functional role for NO in olfactory conditioning that is modulated as a function of diurnal time.

Choline acetyltransferase-like immunoreactivity in a physiologically distinct subtype of olfactory nonspiking local interneurons in the cockroach (periplaneta americana)

Behavioral and physiological studies have shown that local interneurons are pivotal for processing odor information in the insect antennal lobe. They mediate inhibitory and excitatory interactions between the glomerular pathways and ultimately shape the tuning profile of projection neurons. To identify putative cholinergic local interneurons in the antennal lobe of Periplaneta americana, an antibody raised against the biosynthetic enzyme choline acetyltransferase (ChAT) was applied to individual morphologically and electrophysiologically characterized local interneurons. In nonspiking type IIa1 local interneurons, which were classified in this study, we found ChAT-like immunoreactivity suggesting that they are most likely excitatory. This is a well-defined population of neurons that generates Ca(2+) -driven spikelets upon depolarization and stimulation with odorants, but not Na(+) -driven action potentials, because they lack voltage-activated transient Na(+) currents. The nonspiking type IIa2 and type IIb local interneurons, in which Ca(2+) -driven spikelets were absent, had no ChAT-like immunoreactivity. The GABA-like immunoreactive, spiking type I local interneurons had no ChAT-like immunoreactivity. In addition, we showed that uniglomerular projection neurons with cell bodies located in the ventral portion of the ventrolateral somata group and projections along the inner antennocerebral tract exhibited ChAT-like immunoreactivity. Assigning potential transmitters and neuromodulators to distinct morphological and electrophysiological types of antennal lobe neurons is an important prerequisite for a detailed understanding of odor information processing in insects.

Postsynaptic odorant concentration dependent inhibition controls temporal properties of spike responses of projection neurons in the moth antennal lobe

Although odorant concentration-response characteristics of olfactory neurons have been widely investigated in a variety of animal species, the effect of odorant concentration on neural processing at circuit level is still poorly understood. Using calcium imaging in the silkmoth (Bombyx mori) pheromone processing circuit of the antennal lobe (AL), we studied the effect of odorant concentration on second-order projection neuron (PN) responses. While PN calcium responses of dendrites showed monotonic increases with odorant concentration, calcium responses of somata showed decreased responses at higher odorant concentrations due to postsynaptic inhibition. Simultaneous calcium imaging and electrophysiology revealed that calcium responses of PN somata but not dendrites reflect spiking activity. Inhibition shortened spike response duration rather than decreasing peak instantaneous spike frequency (ISF). Local interneurons (LNs) that were specifically activated at high odorant concentrations at which PN responses were suppressed are the putative source of inhibition. Our results imply the existence of an intraglomerular mechanism that preserves time resolution in olfactory processing over a wide odorant concentration range.

A characterization of the Manduca sexta serotonin receptors in the context of olfactory neuromodulation

Neuromodulation, the alteration of individual neuron response properties, has dramatic consequences for neural network function and is a phenomenon observed across all brain regions and taxa. However, the mechanisms underlying neuromodulation are made complex by the diversity of neuromodulatory receptors expressed within a neural network. In this study we begin to examine the receptor basis for serotonergic neuromodulation in the antennal lobe of Manduca sexta. To this end we cloned all four known insect serotonin receptor types from Manduca (the Ms5HTRs). We used phylogenetic analyses to classify the Ms5HTRs and to establish their relationships to other insect serotonin receptors, other insect amine receptors and the vertebrate serotonin receptors. Pharmacological assays demonstrated that each Ms5HTR was selective for serotonin over other endogenous amines and that serotonin had a similar potency at all four Ms5HTRs. The pharmacological assays also identified several agonists and antagonists of the different Ms5HTRs. Finally, we found that the Ms5HT1A receptor was expressed in a subpopulation of GABAergic local interneurons suggesting that the Ms5HTRs are likely expressed heterogeneously within the antennal lobe based on functional neuronal subtype.

Insect-machine hybrid system for understanding and evaluating sensory-motor control by sex pheromone in Bombyx mori

To elucidate the dynamic information processing in a brain underlying adaptive behavior, it is necessary to understand the behavior and corresponding neural activities. This requires animals which have clear relationships between behavior and corresponding neural activities. Insects are precisely such animals and one of the adaptive behaviors of insects is high-accuracy odor source orientation. The most direct way to know the relationships between neural activity and behavior is by recording neural activities in a brain from freely behaving insects. There is also a method to give stimuli mimicking the natural environment to tethered insects allowing insects to walk or fly at the same position. In addition to these methods an 'insect-machine hybrid system' is proposed, which is another experimental system meeting the conditions necessary for approaching the dynamic processing in the brain of insects for generating adaptive behavior. This insect-machine hybrid system is an experimental system which has a mobile robot as its body. The robot is controlled by the insect through its behavior or the neural activities recorded from the brain. As we can arbitrarily control the motor output of the robot, we can intervene at the relationship between the insect and the environmental conditions.

Mixture and odorant processing in the olfactory systems of insects: a comparative perspective

Natural olfactory stimuli are often complex mixtures of volatiles, of which the identities and ratios of constituents are important for odor-mediated behaviors. Despite this importance, the mechanism by which the olfactory system processes this complex information remains an area of active study. In this review, we describe recent progress in how odorants and mixtures are processed in the brain of insects. We use a comparative approach toward contrasting olfactory coding and the behavioral efficacy of mixtures in different insect species, and organize these topics around four sections: (1) Examples of the behavioral efficacy of odor mixtures and the olfactory environment; (2) mixture processing in the periphery; (3) mixture coding in the antennal lobe; and (4) evolutionary implications and adaptations for olfactory processing. We also include pertinent background information about the processing of individual odorants and comparative differences in wiring and anatomy, as these topics have been richly investigated and inform the processing of mixtures in the insect olfactory system. Finally, we describe exciting studies that have begun to elucidate the role of the processing of complex olfactory information in evolution and speciation.

A multisensory centrifugal neuron in the olfactory pathway of heliothine moths

We have characterized, by intracellular recording and staining, a unique type of centrifugal neuron in the brain olfactory center of two heliothine moth species; one in Heliothis virescens and one in Helicoverpa armigera. This unilateral neuron, which is not previously described in any moth, has fine processes in the dorsomedial region of the protocerebrum and extensive neuronal branches with blebby terminals in all glomeruli of the antennal lobe. Its soma is located dorsally of the central body close to the brain midline. Mass-fills of antennal-lobe connections with protocerebral regions showed that the centrifugal neuron is, in each brain hemisphere, one within a small group of neurons having their somata clustered. In both species the neuron was excited during application of non-odorant airborne signals, including transient sound pulses of broad bandwidth and air velocity changes. Additional responses to odors were recorded from the neuron in Heliothis virescens. The putative biological significance of the centrifugal antennal-lobe neuron is discussed with regard to its morphological and physiological properties. In particular, a possible role in multisensory processes underlying the moth's ability to adapt its odor-guided behaviors according to the sound of an echo-locating bat is considered.

Circadian rhythms and endocrine functions in adult insects

Many behavioral and physiological processes in adult insects are influenced by both the endocrine and circadian systems, suggesting that these two key physiological systems interact. We reviewed the literature and found that experiments explicitly testing these interactions in adult insects have only been conducted for a few species. There is a shortage of measurements of hormone titers throughout the day under constant conditions even for the juvenile hormones (JHs) and ecdysteroids, the best studied insect hormones. Nevertheless, the available measurements of hormone titers coupled with indirect evidence for circadian modulation of hormone biosynthesis rate, and the expression of genes encoding proteins involved in hormone biosynthesis, binding or degradation are consistent with the hypothesis that the circulating levels of many insect hormones are influenced by the circadian system. Whole genome microarray studies suggest that the modulation of farnesol oxidase levels is important for the circadian regulation of JH biosynthesis in honey bees, mosquitoes, and fruit flies. Several studies have begun to address the functional significance of circadian oscillations in endocrine signaling. The best understood system is the circadian regulation of Pheromone Biosynthesis Activating Neuropeptide (PBAN) titers which is important for the temporal organization of sexual behavior in female moths. The evidence that the circadian and endocrine systems interact has important implications for studies of insect physiology and behavior. Additional studies on diverse species and physiological processes are needed for identifying basic principles underlying the interactions between the circadian and endocrine systems in insects.

The spike generator in the labellar taste receptors of the blowfly is differently affected by 4-aminopyridine and 5-hydroxytryptamine

In taste chemoreception of invertebrates the interaction of taste stimuli with specific membrane receptors and/or ion channels located in the apical membrane of taste receptor cells results in the generation of a receptor potential which, in turn, activates the 'encoder' region to produce action potentials which propagate to the CNS. This study investigates, in the labellar chemosensilla of the blowfly, Protophormia terraenovae, the voltage-gated K(+) currents involved in the action potential repolarization and repetitive firing of the neurons by way of the K(v) channel inhibitors, 4-aminopyridine and 5-hydroxytryptamine. The receptor potential and the spike activity were simultaneously recorded from the 'salt', 'sugar' and 'deterrent' cells, by means of the extracellular side-wall technique, in response to 150 mM NaCl, 100 mM sucrose and 1 mM quinine HCl, before, 0÷10 min after apical administration of 4-AP (0.01-10 mM) or 5-HT (0.1-100 mM). The results show that the receptor potential in all three cells is neither affected by 4-AP nor by 5-HT. Instead, spike activity is significantly decreased, by way of blocking different K(v) channel types: an inactivating A-type K(+) current (KA) modulating repetitive firing of the cells and responsible for the after hyperpolarization, and a sustained K(+) current that resembles the delayed rectifier (DKR) and contributes to action potential repolarization.

Olfactory modulation by dopamine in the context of aversive learning

The need to detect and process sensory cues varies in different behavioral contexts. Plasticity in sensory coding can be achieved by the context-specific release of neuromodulators in restricted brain areas. The context of aversion triggers the release of dopamine in the insect brain, yet the effects of dopamine on sensory coding are unknown. In this study, we characterize the morphology of dopaminergic neurons that innervate each of the antennal lobes (ALs; the first synaptic neuropils of the olfactory system) of the moth Manduca sexta and demonstrate with electrophysiology that dopamine enhances odor-evoked responses of the majority of AL neurons while reducing the responses of a small minority. Because dopamine release in higher brain areas mediates aversive learning we developed a naturalistic, ecologically inspired aversive learning paradigm in which an innately appetitive host plant floral odor is paired with a mimic of the aversive nectar of herbivorized host plants. This pairing resulted in a decrease in feeding behavior that was blocked when dopamine receptor antagonists were injected directly into the ALs. These results suggest that a transient dopaminergic enhancement of sensory output from the AL contributes to the formation of aversive memories. We propose a model of olfactory modulation in which specific contexts trigger the release of different neuromodulators in the AL to increase olfactory output to downstream areas of processing.

Presynaptic modulation of early olfactory processing in Drosophila

Most animals are endowed with an olfactory system that is essential for finding foods, avoiding predators, and locating mating partners. The olfactory system must encode the identity and intensity of behaviorally relevant stimuli in a dynamic environmental landscape. How is olfactory information represented? How is a large dynamic range of odor concentrations represented in the olfactory system? How is this representation modulated to meet the demands of different internal physiological states? Recent studies have found that sensory terminals are important targets for neuromodulation. The emerging evidence suggests that presynaptic inhibition scales with sensory input and thus provides a mechanism to increase dynamic range of odor representation. In addition, presynaptic facilitation could be a mechanism to alter behavioral responses in hungry animals. This review will focus on the GABA(B) (gamma-aminobutyric acid) receptor-mediated presynaptic inhibition, and neuropeptide-mediated presynaptic modulation in Drosophila.

Serotonin modulates olfactory processing in the antennal lobe of Drosophila

Sensory systems must be able to extract features of environmental cues within the context of the different physiological states of the organism and often temper their activity in a state-dependent manner via the process of neuromodulation. We examined the effects of the neuromodulator serotonin on a well-characterized sensory circuit, the antennal lobe of Drosophila melanogaster, using two-photon microscopy and the genetically expressed calcium indicator, G-CaMP. Serotonin enhances sensitivity of the antennal lobe output projection neurons in an odor-specific manner. For odorants that sparsely activate the antennal lobe, serotonin enhances projection neuron responses and causes an offset of the projection neuron tuning curve, most likely by increasing projection neuron sensitivity. However, for an odorant that evokes a broad activation pattern, serotonin enhances projection neuron responses in some, but not all, glomeruli. Further, serotonin enhances the responses of inhibitory local interneurons, resulting in a reduction of neurotransmitter release from the olfactory sensory neurons via GABA(B) receptor-dependent presynaptic inhibition, which may be a mechanism underlying the odorant-specific modulation of projection neuron responses. Our data suggest that the complexity of serotonin modulation in the antennal lobe accommodates coding stability in a glomerular pattern and flexible projection neuron sensitivity under different physiological conditions.

The neurobiology of insect olfaction: sensory processing in a comparative context

The simplicity and accessibility of the olfactory systems of insects underlie a body of research essential to understanding not only olfactory function but also general principles of sensory processing. As insect olfactory neurobiology takes advantage of a variety of species separated by millions of years of evolution, the field naturally has yielded some conflicting results. Far from impeding progress, the varieties of insect olfactory systems reflect the various natural histories, adaptations to specific environments, and the roles olfaction plays in the life of the species studied. We review current findings in insect olfactory neurobiology, with special attention to differences among species. We begin by describing the olfactory environments and olfactory-based behaviors of insects, as these form the context in which neurobiological findings are interpreted. Next, we review recent work describing changes in olfactory systems as adaptations to new environments or behaviors promoting speciation. We proceed to discuss variations on the basic anatomy of the antennal (olfactory) lobe of the brain and higher-order olfactory centers. Finally, we describe features of olfactory information processing including gain control, transformation between input and output by operations such as broadening and sharpening of tuning curves, the role of spiking synchrony in the antennal lobe, and the encoding of temporal features of encounters with an odor plume. In each section, we draw connections between particular features of the olfactory neurobiology of a species and the animal's life history. We propose that this perspective is beneficial for insect olfactory neurobiology in particular and sensory neurobiology in general.

Serotonin regulates voltage-dependent currents in type I(e(A)) and I(i) interneurons of Hermissenda

Serotonin (5-HT) has both direct and modulatory actions on central neurons contributing to behavioral arousal and cellular-synaptic plasticity in diverse species. In Hermissenda, 5-HT produces changes in intrinsic excitability of different types of identified interneurons in the circumesophageal nervous system. Using whole cell patch-clamp techniques we have examined membrane conductance changes produced by 5-HT that contribute to intrinsic excitability in two identified classes of interneurons, types I(i) and I(eA). Whole cell currents were examined before and after 5-HT application to the isolated nervous system. A 4-aminopyridine-sensitive transient outward K(+) current [I(K(A))], a tetraethylammonium-sensitive delayed rectifier K(+) current [I(K(V))], an inward rectifier K(+) current [I(K(IR))], and a hyperpolarization-activated current (I(h)) were characterized. 5-HT decreased the amplitude of I(K(A)) and I(K(V)) in both type I(i) and I(eA) interneurons. However, differences in 5-HT's effects on the activation-inactivation kinetics were observed in different types of interneurons. 5-HT produced a depolarizing shift in the activation curve of I(K(V)) and a hyperpolarizing shift in the inactivation curve of I(K(A)) in type I(i) interneurons. In contrast, 5-HT produced a depolarizing shift in the activation curve and a hyperpolarizing shift in the inactivation curve of both I(K(V)) and I(K(A)) in type I(eA) interneurons. In addition, 5-HT decreased the amplitude of I(K(IR)) in type I(i) interneurons and increased the amplitude of I(h) in type I(eA) interneurons. These results indicate that 5-HT-dependent changes in I(K(A)), I(K(V)), I(K(IR)), and I(h) contribute to multiple mechanisms that synergistically support modulation of increased intrinsic excitability associated with different functional classes of identified type I interneurons.

Developmental expression of neuromodulators in the central complex of the grasshopper Schistocerca gregaria

The central complex is a major integrative region within the insect brain with demonstrated roles in spatial orientation, the regulation of locomotor behavior, and sound production. In the hemimetabolous grasshopper, the central complex comprises the protocerebral bridge, central body (CB), ellipsoid body, noduli, and accessory lobes, and this modular organization develops entirely during embryogenesis. From a biochemical perspective, a range of neuroactive substances has been demonstrated in these modules of the adult central complex, but little is known about their developmental expression. In this study, we use matrix-assisted laser desorption/ionization-imaging mass spectrometry on single brain slices to confirm the presence of several peptide families (tachykinin, allatostatin, periviscerokinin/pyrokinin, FLRFamide, and neuropeptide F) in the adult central complex and then use immunohistochemistry and histology to examine their developmental expression, together with that of the indolamin serotonin, and the endogenous messenger nitric oxide (NO; via its synthesizing enzyme). We find that each neuromodulator is expressed according to a unique, stereotypic, pattern within the various modules making up the central complex. Neuropeptides such as tachykinin (55%) and allatostatin (65%), and the NO-synthesizing enzyme diaphorase (70%), are expressed earlier during embryonic development than the biogenic amine serotonin (80%), whereas periviscerokinin-like peptides and FLRFamide-like peptides begin to be expressed only postembryonically. Within the CB, these neuroactive substances are present in tangential projection neurons before they appear in columnar neurons. There is also no colocalization of serotonin-positive and peptide-positive projections up to the third larval instar during development, consistent with the clear dorsoventral layering of the neuropil we observe. Our results provide the first neurochemical fingerprint of the developing central complex in an hemimetabolous insect.

Mating-induced differential coding of plant odour and sex pheromone in a male moth

Innate behaviours in animals can be influenced by several factors, such as the environment, experience, or physiological status. This behavioural plasticity originates from changes in the underlying neuronal substrate. A well-described form of plasticity is induced by mating. In both vertebrates and invertebrates, males experience a post-ejaculatory refractory period, during which they avoid new females. In the male moth Agrotis ipsilon, mating induces a transient inhibition of responses to the female-produced sex pheromone. To understand the neural bases of this inhibition and its possible odour specificity, we carried out a detailed analysis of the response characteristics of the different neuron types from the periphery to the central level. We examined the response patterns of pheromone-sensitive and plant volatile-sensitive neurons in virgin and mated male moths. By using intracellular recordings, we showed that mating changes the response characteristics of pheromone-sensitive antennal lobe (AL) neurons, and thus decreases their sensitivity to sex pheromone. Individual olfactory receptor neuron (ORN) recordings and calcium imaging experiments indicated that pheromone sensory input remains constant. On the other hand, calcium responses to non-pheromonal odours (plant volatiles) increased after mating, as reflected by increased firing frequencies of plant-sensitive AL neurons, although ORN responses to heptanal remained unchanged. We suggest that differential processing of pheromone and plant odours allows mated males to transiently block their central pheromone detection system, and increase non-pheromonal odour detection in order to efficiently locate food sources.

The organization of the antennal lobe correlates not only with phylogenetic relationship, but also life history: a Basal hymenopteran as exemplar

The structure of the brain is a consequence of selective pressures and the ancestral brain structures modified by those pressures. The Hymenoptera are one of the most behaviorally complex insect orders, and the olfactory system of honeybees (one of the most derived members) has been extensively studied. To understand the context in which the olfactory system of the Hymenoptera evolved, we performed a variety of immunocytochemical and anatomical labeling techniques on the antennal lobes (ALs) of one of its most primitive members, the sawflies, to provide a comparison between the honeybee and other insect model species. The olfactory receptor neurons project from the antennae to fill the entire glomerular volume but do not form distinct tracts as in the honeybee. Labeling of projection neurons revealed 5 output tracts similar to those in moths and immunolabeling for several transmitters revealed distinct populations of local interneurons and centrifugal neurons that were also similar to moths. There were, however, no histaminergic or dopaminergic AL neurons. The similarities between sawflies and moths suggest that along with the great radiation and increased complexity of behavioral repertoire of the Hymenoptera, there were extensive modifications of AL structure.

Multiple neuropeptides in the Drosophila antennal lobe suggest complex modulatory circuits

The fruitfly, Drosophila, is dependent on its olfactory sense in food search and reproduction. Processing of odorant information takes place in the antennal lobes, the primary olfactory center in the insect brain. Besides classical neurotransmitters, earlier studies have indicated the presence of a few neuropeptides in the olfactory system. In the present study we made an extensive analysis of the expression of neuropeptides in the Drosophila antennal lobes by direct profiling using matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometry and immunocytochemistry. Neuropeptides from seven different precursor genes were unambiguously identified and their localization in neurons was subsequently revealed by immunocytochemistry. These were short neuropeptide F, tachykinin related peptide, allatostatin A, myoinhibitory peptide, SIFamide, IPNamide, and myosuppressin. The neuropeptides were expressed in subsets of olfactory sensory cells and different populations of local interneurons and extrinsic (centrifugal) neurons. In some neuron types neuropeptides were colocalized with classical neurotransmitters. Our findings suggest a huge complexity in peptidergic signaling in different circuits of the antennal lobe.

Structural long-term changes at mushroom body input synapses

How does the sensory environment shape circuit organization in higher brain centers? Here we have addressed the dependence on activity of a defined circuit within the mushroom body of adult Drosophila. This is a brain region receiving olfactory information and involved in long-term associative memory formation. The main mushroom body input region, named the calyx, undergoes volumetric changes correlated with alterations of experience. However, the underlying modifications at the cellular level remained unclear. Within the calyx, the clawed dendritic endings of mushroom body Kenyon cells form microglomeruli, distinct synaptic complexes with the presynaptic boutons of olfactory projection neurons. We developed tools for high-resolution imaging of pre- and postsynaptic compartments of defined calycal microglomeruli. Here we show that preventing firing of action potentials or synaptic transmission in a small, identified fraction of projection neurons causes alterations in the size, number, and active zone density of the microglomeruli formed by these neurons. These data provide clear evidence for activity-dependent organization of a circuit within the adult brain of the fly.

Mating-induced transient inhibition of responses to sex pheromone in a male moth is not mediated by octopamine or serotonin

In the male moth, Agrotis ipsilon, mating induces a transient inhibition of behavioural and central nervous responses to sex pheromone. Newly mated males are not attracted to sex pheromone, and the sensitivity of their antennal lobe (AL) neurons is lower than in virgin males. This rapid transient olfactory inhibition prevents them from re-mating unsuccessfully until they have refilled their sex glands. We hypothesized that this olfactory 'switch off' might be controlled by neuromodulators such as biogenic amines. To test our hypothesis, we studied the effects of octopamine (OA) and serotonin (5-hydroxytryptamine, 5-HT) on the coding properties of pheromone-sensitive AL neurons in virgin and newly mated males. We show that AL neuron sensitivity increased in newly mated males after injection of OA or 5-HT, but only OA treatment affected certain response characteristics of AL neurons in virgin males. Whereas all measured AL neuron response characteristics were different between virgin and newly mated males, amine treatment in newly mated males restored only the latency and spike frequency, but not the duration of excitatory and inhibitory phases, which were initially found in virgin males. Additionally, we investigated the behavioural effects of OA and 5-HT treatments in virgin and mated males. Although OA and 5-HT enhanced the general flight activity of newly mated males, amine treatments did not restore the behavioural pheromone response of mated moths. Altogether, these results show that, although biogenic amines modulate the olfactory system in moths, OA and 5-HT are probably not involved in the post-mating inhibition of responses to sex pheromone in A. ipsilon males.

Carbonic anhydrase I, II, and VI, blood plasma, erythrocyte and saliva zinc and copper increase after repetitive transcranial magnetic stimulation

INTRODUCTION

Repetitive transcranial magnetic stimulation (rTMS) has been used to treat symptoms from many disorders; biochemical changes occurred with this treatment. Preliminary studies with rTMS in patients with taste and smell dysfunction improved sensory function and increased salivary carbonic anhydrase (CA) VI and erythrocyte CA I, II. To obtain more information about these changes after rTMS, we measured changes in several CA enzymes, proteins, and trace metals in their blood plasma, erythrocytes, and saliva.

METHODS

Ninety-three patients with taste and smell dysfunction were studied before and after rTMS in an open clinical trial. Before and after rTMS, we measured erythrocyte CA I, II and salivary CA VI, zinc and copper in parotid saliva, blood plasma, and erythrocytes, and appearance of novel salivary proteins by using mass spectrometry.

RESULTS

After rTMS, CA I, II and CA VI activity and zinc and copper in saliva, plasma, and erythrocytes increased with significant sensory benefit. Novel salivary proteins were induced at an m/z value of 21.5K with a repetitive pattern at intervals of 5K m/z.

CONCLUSIONS

rTMS induced biochemical changes in specific enzymatic activities, trace metal concentrations, and induction of novel salivary proteins, with sensory improvement in patients with taste and smell dysfunction. Because patients with several neurologic disorders exhibit taste and smell dysfunction, including Parkinson disease, Alzheimer disease, and multiple sclerosis, and because rTMS improved their clinical symptoms, the biochemical changes we observed may be relevant not only in our patients with taste and smell dysfunction but also in patients with neurologic disorders with these sensory abnormalities.

Parallel olfactory systems in insects: anatomy and function

A striking commonality across insects and vertebrates is the recurring presence of parallel olfactory subsystems, suggesting that such an organization has a highly adaptive value. Conceptually, two different categories of parallel systems must be distinguished. In one, specific sensory organs or processing streams analyze different chemical stimuli (segregate parallel systems). In the other, similar odor stimuli are processed but analyzed with respect to different features (dual parallel systems). Insects offer many examples for both categories. For example, segregate parallel systems for different chemical stimuli are realized in specialized neuronal streams for processing sex pheromones and CO(2). Dual parallel streams related to similar or overlapping odor stimuli are prominent in Hymenoptera. Here, a clear separation of sensory tracts to higher-order brain centers is present despite no apparent differences regarding the classes or categories of olfactory stimuli being processed. In this paper, we review the situation across insect species and offer hypotheses for the function and evolution of parallel olfactory systems.

Distinct Electrophysiological Properties in Subtypes of Nonspiking Olfactory Local Interneurons Correlate With Their Cell Type–Specific Ca2+ Current Profiles

A diverse population of local interneurons (LNs) helps to process, structure, and spatially represent olfactory information in the insect antennal lobe. In Periplaneta americana, we identified two subtypes of nonspiking local interneurons (type II LNs) by their distinct morphological and intrinsic electrophysiological properties. As an important step toward a better understanding of the cellular mechanisms that mediate odor information processing, we present a detailed analysis of their distinct voltage-activated Ca2+ currents, which clearly correlated with their distinct intrinsic electrophysiological properties. Both type II LNs did not posses voltage-activated Na+ currents and apparently innervated all glomeruli including the macroglomerulus. Type IIa LNs had significant longer and thicker low-order neurites and innervated each glomerulus entirely and homogeneously, whereas type IIb LNs innervated only parts of each glomerulus. All type II LNs were broadly tuned and responded to odorants of many chemical classes with graded changes in the membrane potential. Type IIa LNs responded with odor-specific elaborate patterns of excitation that could also include “spikelets” riding on the depolarizations and periods of inhibition. In contrast, type IIb LNs responded mostly with sustained, relatively smooth depolarizations. Consistent with the strong active membrane properties of type IIa LNs versus type IIb LNs, the voltage-activated Ca2+ current of type IIa LNs activated at more hyperpolarized membrane potentials and had a larger transient component.

Intrinsic Membrane Properties and Inhibitory Synaptic Input of Kenyon Cells as Mechanisms for Sparse Coding?

The insect mushroom bodies (MBs) are multimodal signal processing centers and are essential for olfactory learning. Electrophysiological recordings from the MBs' principal component neurons, the Kenyon cells (KCs), showed a sparse representation of olfactory signals. It has been proposed that the intrinsic and synaptic properties of the KC circuitry combine to reduce the firing of action potentials and to generate relatively brief windows for synaptic integration in the KCs, thus causing them to operate as coincidence detectors. To better understand the ionic mechanisms that mediate the KC intrinsic firing properties, we used whole cell patch-clamp recordings from KCs in the adult, intact brain of Periplaneta americana to analyze voltage- and/or Ca2+-dependent inward ( ICa, INa) and outward currents [ IA, IK(V), IK,ST, IO(Ca)]. In general the currents had properties similar to those of currents in other insect neurons. Certain functional parameters of ICaand IO(Ca), however, had unusually high values, allowing them to assist sparse coding. ICahad a low-activation threshold and a very high current density compared with those of ICain other insect neurons. Together these parameters make ICasuitable for boosting and sharpening the excitatory postsynaptic potentials as reported in previous studies. IO(Ca)also had a large current density and a very depolarized activation threshold. In combination, the large ICaand IO(Ca)are likely to mediate the strong spike frequency adaptation. These intrinsic properties of the KCs are likely to be supported by their tonic, inhibitory synaptic input, which was revealed by specific GABA antagonists and which contributes significantly to the hyperpolarized membrane potential at rest.