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.     

Morphometric modeling of olfactory circuits in the insect antennal lobe: I. Simulations of spiking local interneurons.

Inhibitory local interneurons (LNs) play a critical role in shaping the output of olfactory glomeruli in both the olfactory bulb of vertebrates and the antennal lobe of insects and other invertebrates. In order to examine how the complex geometry of LNs may affect signaling in the antennal lobe, we constructed detailed multi-compartmental models of single LNs from the sphinx moth, Manduca sexta, using morphometric data from confocal-microscopic images. Simulations clearly revealed a directionality in LNs that impeded the propagation of injected currents from the sub-micron-diameter glomerular dendrites toward the much larger-diameter integrating segment (IS) in the coarse neuropil. Furthermore, the addition of randomly-firing synapses distributed across the LN dendrites (simulating the noisy baseline activity of afferent input recorded from LNs in the odor-free state) led to a significant depolarization of the LN. Thus the background activity typically recorded from LNs in vivo could influence synaptic integration and spike transformation in LNs through voltage-dependent mechanisms. Other model manipulations showed that active currents inserted into the IS can help synchronize the activation of inhibitory synapses in glomeruli across the antennal lobe. These data, therefore, support experimental findings suggesting that spiking inhibitory LNs can operate as multifunctional units under different ambient odor conditions. At low odor intensities, (i.e. subthreshold for IS spiking), they participate in local, mostly intra-glomerular processing. When activated by elevated odor concentrations, however, the same neurons will fire overshooting action potentials, resulting in the spread of inhibition more globally across the antennal lobe. Modulation of the passive and active properties of LNs may, therefore, be a deciding factor in defining the multi-glomerular representations of odors in the brain.     

Transmission of olfactory information between three populations of neurons in the antennal lobe of the fly

Three classes of neurons form synapses in the antennal lobe of Drosophila, the insect counterpart of the vertebrate olfactory bulb: olfactory receptor neurons, projection neurons, and inhibitory local interneurons. We have targeted a genetically encoded optical reporter of synaptic transmission to each of these classes of neurons and visualized population responses to natural odors. The activation of an odor-specific ensemble of olfactory receptor neurons leads to the activation of a symmetric ensemble of projection neurons across the glomerular synaptic relay. Virtually all excited glomeruli receive inhibitory input from local interneurons. The extent, odor specificity, and partly interglomerular origin of this input suggest that inhibitory circuits assemble combinatorially during odor presentations. These circuits may serve as dynamic templates that extract higher order features from afferent activity patterns.     

Local peptidergic signaling in the antennal lobe shapes olfactory behavior

Transfer and processing of olfactory information in the antennal lobe of Drosophila relies primarily on neurotransmitters such as acetylcholine and GABA, but novel studies also implicated a neuropeptide: the Drosophila tachykinin (DTK). DTK is expressed in local interneurons that innervate the glomeruli of the antennal lobe with varicose processes. Recently, DTK was shown to mediate presynaptic inhibition of olfactory sensory neurons by physiological and behavioral analysis (Ignell et al. 2009, PNAS). That study drew our attention to the issue of alternative targets of DTK in the antennal lobe. Hence, in the present study, we interfered with DTK peptide and DTK receptor (DTKR) expression in local interneurons of the antennal lobe and studied the behavioral outcome of these manipulations. We show that the DTKR is expressed not only in olfactory sensory neurons, but most likely also in local interneurons. The behavioral consequences of interfering with postsynaptic peptide receptors are different from presynaptic peptide receptor interference. We discuss the possibility that the sum of pre- and postsynaptic interactions may be to modulate the dynamic range in odor sensitivity.     

Integration of temperature and olfactory information in cockroach antennal lobe glomeruli

Individual neurons in the antennal lobe of the cockroach not only respond to warming, cooling and the odor of lemon oil but they also integrate the responses to simultaneously occurring temperature and olfactory stimuli. This integration results in an increase or decrease of the neuron's activity as compared to its responses to the temperature stimuli presented alone. The mean gain for a change in temperature in the warm and cold direction is 9.5 (imp s(-1)) degrees C(-1) and 10.2 (imp s(-1)) degrees C(-1), respectively. Thus, the average neuron elevates its impulse frequency by 1 imp s(-1) when temperature is increased by 0.1 degree C or decreased by 0.09 degree C. Examination of response scatter reveals that the difference required between two warm or two cold stimuli to be discriminated is 0.5 degree C. Similar values for gain and resolving power are obtained for the enhanced responses to the warm-odor and the cold-odor stimulus combinations. The neurons described are: (1) local interneurons innervating a number of glomeruli distributed within the antennal lobe, and (2) projection neurons collecting information from single glomeruli at 140-280 microm from the surface of the antennal lobe and providing links with the calyces of the mushroom bodies and the lateral lobe of the protocerebrum.     

Spiking patterns and their functional implications in the antennal lobe of the tobacco hornworm Manduca sexta

Bursting as well as tonic firing patterns have been described in various sensory systems. In the olfactory system, spontaneous bursts have been observed in neurons distributed across several synaptic levels, from the periphery, to the olfactory bulb (OB) and to the olfactory cortex. Several in vitro studies indicate that spontaneous firing patterns may be viewed as "fingerprints" of different types of neurons that exhibit distinct functions in the OB. It is still not known, however, if and how neuronal burstiness is correlated with the coding of natural olfactory stimuli. We thus conducted an in vivo study to probe this question in the OB equivalent structure of insects, the antennal lobe (AL) of the tobacco hornworm Manduca sexta. We found that in the moth's AL, both projection (output) neurons (PNs) and local interneurons (LNs) are spontaneously active, but PNs tend to produce spike bursts while LNs fire more regularly. In addition, we found that the burstiness of PNs is correlated with the strength of their responses to odor stimulation--the more bursting the stronger their responses to odors. Moreover, the burstiness of PNs was also positively correlated with the spontaneous firing rate of these neurons, and pharmacological reduction of bursting resulted in a decrease of the neurons' responsiveness. These results suggest that neuronal burstiness reflects a physiological state of these neurons that is directly linked to their response characteristics.     

Offset response of the olfactory projection neurons in the moth antennal lobe

We investigated a population activity of central olfactory neurons after the termination of odor input. Olfactory response of projection neurons in the moth primary olfactory center was characterized using in vivo intracellular recording and staining techniques. The population activity changed rapidly to the different states after the stimulus offset. The response after stimulus offset represents information regarding odor identity. We analyzed the spatial distribution of offset-activated glomeruli in a virtual neuronal population that was reconstructed using accumulated individual recordings obtained from different specimens. The offset-activated glomeruli tended to be widely distributed, whereas the onset-activated glomeruli were relatively clustered. These results suggest the importance of lateral interaction in shaping the offset olfactory response.     

Glomerular interactions in olfactory processing channels of the antennal lobes

An open question in olfactory coding is the extent of interglomerular connectivity: do olfactory glomeruli and their neurons regulate the odorant responses of neurons innervating other glomeruli? In the olfactory system of the moth Manduca sexta, the response properties of different types of antennal olfactory receptor cells are known. Likewise, a subset of antennal lobe glomeruli has been functionally characterized and the olfactory tuning of their innervating neurons identified. This provides a unique opportunity to determine functional interactions between glomeruli of known input, specifically, (1) glomeruli processing plant odors and (2) glomeruli activated by antennal stimulation with pheromone components of conspecific females. Several studies describe reciprocal inhibitory effects between different types of pheromone-responsive projection neurons suggesting lateral inhibitory interactions between pheromone component-selective glomerular neural circuits. Furthermore, antennal lobe projection neurons that respond to host plant volatiles and innervate single, ordinary glomeruli are inhibited during antennal stimulation with the female’s sex pheromone. The studies demonstrate the existence of lateral inhibitory effects in response to behaviorally significant odorant stimuli and irrespective of glomerular location in the antennal lobe. Inhibitory interactions are present within and between olfactory subsystems (pheromonal and non-pheromonal subsystems), potentially to enhance contrast and strengthen odorant discrimination.     

Combined effects of olfactory and mechanical inputs in antennal lobe neurons of the cockroach

Although it has been known that olfactory and mechanical inputs from the antenna converge in the antennal lobe of the deutocerebrum of the American cockroach, the capacity of antennal lobe neurons to integrate cues from these modalities was never examined. In the present study, neurons responsive to both the odor of lemon oil and to lateral displacement of the antenna were used to compare the effects of unimodal and bimodal stimulation. The combination of olfactory and mechanical stimuli produced increases over unimodal olfactory responses in 61% (30/49) of the neurons. In the remaining neurons the response either decreased (20%; 10/49), or no bimodal interaction was apparent (19%; 9/49). Dye injection (lucifer yellow) following physiological characterization revealed that these bimodal neurons are local neurons or projection neurons. The antennal lobe links the inputs from olfactory and mechanosensory systems and provides a substrate through which olfactory and mechanical stimuli influence one another's effects. Accepted: 29 September 1997     

A neural network model of general olfactory coding in the insect antennal lobe.

A central problem in olfaction is understanding how the quality of olfactory stimuli is encoded in the insect antennal lobe (or in the analogously structured vertebrate olfactory bulb) for perceptual processing in the mushroom bodies of the insect protocerebrum (or in the vertebrate olfactory cortex). In the study reported here, a relatively simple neural network model, inspired by our current knowledge of the insect antennal lobes, is used to investigate how each of several features and elements of the network, such as synapse strengths, feedback circuits and the steepness of neural activation functions, influences the formation of an olfactory code in neurons that project from the antennal lobes to the mushroom bodies (or from mitral cells to olfactory cortex). An optimal code in these projection neurons (PNs) should minimize potential errors by the mushroom bodies in misidentifying the quality of an odor across a range of concentrations while maximizing the ability of the mushroom bodies to resolve odors of different quality. Simulation studies demonstrate that the network is able to produce codes independent or virtually independent of concentration over a given range. The extent of this range is moderately dependent on a parameter that characterizes how long it takes for the voltage in an activated neuron to decay back to its resting potential, strongly dependent on the strength of excitatory feedback by the PNs onto antennal lobe intrinsic neurons (INs), and overwhelmingly dependent on the slope of the activation function that transforms the voltage of depolarized neurons into the rate at which spikes are produced. Although the code in the PNs is degraded by large variations in the concentration of odor stimuli, good performance levels are maintained when the complexity of stimuli, as measured by the number of component odorants, is doubled. When excitatory feedback from the PNs to the INs is strong, the activity in the PNs undergoes transitions from initial states to stimulus-specific equilibrium states that are maintained once the stimulus is removed. When this PN-IN feedback is weak the PNs are more likely to relax back to a stimulus-independent equilibrium state, in which case the code is not maintained beyond the application of the stimulus. Thus, for the architecture simulated here, strong feedback from the PNs onto the INs, together with step-like neuronal activation functions, could well be important in producing easily discriminable odor quality codes that are invariant over several orders of magnitude in stimulus concentration.     

Smelling excitement in the antennal lobe

In the fly antennal lobe projection neurons receive odor information from olfactory sensory neurons and transmit it to higher brain centers. However, projection neurons respond differently to odors than sensory neurons, despite the fact that they appear to have one-to-one connectivity. Shang et al. (2007) now describe the existence of excitatory neurons within the antennal lobe that may account for some of these unexplained differences.     

Oscillations and slow patterning in the antennal lobe

Odor presentation generates both fast oscillations and slow patterning in the spiking activity of the projection neurons (PNs) in the antennal lobe (AL) of locusts, moths and bees. Experimental results indicate that the oscillations are the result of the interaction between the PNs and the inhibitory local neurons (LNs) in the AL; e.g., blocking inhibition by application of GABA-receptor antagonists abolishes these oscillations. The slow patterning, on the other hand, was shown to be somewhat resistant to such blockage. In a H-H model, we reproduce both the oscillations and the slow patterning. As previously suggested, the oscillations are the result of the interaction between the PNs and LNs. We suggest that calcium and calcium-dependent potassium channels (found in PNs of bees and moths) are sufficient to account for the slow patterning resistant to the application of GABA-receptor antagonists. The intrinsic bursting property of the PNs, resulting from these additional modeled currents, give rise to another network feature that was seen experimentally in locusts: A relatively small increase in the number of additional generated PN action potentials when LN input is blocked. Consequently, the major effect of network inhibition is to redistribute the action potentials of the PNs from bursting to one action potential per cycle of the oscillations. Action Editor: Christiane Linster     

Differences of Ca2+ handling properties in identified central olfactory neurons of the antennal lobe

Information processing in neurons depends on highly localized Ca²⁺ signals. The spatial and temporal dynamics of these signals are determined by a variety of cellular parameters including the calcium influx, calcium buffering and calcium extrusion. Our long-term goal is to better understand how intracellular Ca²⁺ dynamics are controlled and contribute to information processing in defined interneurons of the insect olfactory system. The latter has served as an excellent model to study general mechanisms of olfaction. Using patch-clamp recordings and fast optical imaging in combination with the ‘added buffer approach’, we analyzed the Ca²⁺ handling properties of different identified neuron types in Periplaneta americana's olfactory system. Our focus was on two types of local interneurons (LNs) with significant differences in intrinsic electrophysiological properties: (1) spiking LNs that generate ‘normal’ Na⁺ driven action potentials and (2) non-spiking LNs that do not express voltage-activated Na⁺ channels. We found that the distinct electrophysiological properties from different types of central olfactory interneurons are strongly correlated with their cell specific calcium handling properties: non-spiking LNs, in which Ca²⁺ is the only cation that enters the cell to contribute to membrane depolarization, had the highest endogenous Ca²⁺ binding ratio and Ca²⁺ extrusion rate.     

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.     

Projections to higher olfactory centers from subdivisions of the antennal lobe macroglomerular complex of the male silkmoth

The macroglomerular complex (MGC) is the first-order center for synaptic processing of olfactory information about the female sex pheromone in the male moth brain. We have investigated the MGC subdivisions of the male silkmoth Bombyx mori by use of three-dimensional reconstruction of the MGC from sequential series of confocal slice images. The B. mori MGC consists of three subdivisions similar to those of Manduca sexta: the cumulus, toroid and horseshoe. Intracellular recording and staining revealed that responses of MGC projection neurons to pheromonal stimulation correlate with their dendritic arborizations in the subdivisions of the MGC (the cumulus, toroid and horseshoe) and each subdivision specific projection neuron transmits information to different regions in the calyces of the mushroom body and the inferior lateral protocerebrum. We revealed that major pheromone component information is transferred to the medial part of the inferior lateral protocerebrum through three different antennocerebral pathways. Although it is generally accepted that the calyces of the mushroom body and the inferior lateral protocerebrum are the target sites for pheromone information from the MGC in moths, our results suggest that the medial part of the inferior lateral protocerebrum may be a more important processing site for major pheromonal information in B. mori.     

Neuronal architecture of the antennal lobe in Drosophila melanogaster

Summary Computer reconstruction of the antennal lobe of Drosophila melanogaster has revealed a total of 35 glomeruli, of which 30 are located in the periphery of the lobe and 5 in its center. Several prominent glomeruli are recognizable by their location, size, and shape; others are identifiable only by their positions relative to prominent glomeruli. No obvious sexual dimorphism of the glomerular architecture was observed. Golgi impregnations revealed: (1) Five of the glomeruli are exclusive targets for ipsilateral antennal input, whereas all others receive afferents from both antennae. Unilateral amputation of the third antennal segment led to a loss of about 1000 fibers in the antennal commissure. Hence, about 5/6 of the approximately 1200 antennal afferents per side have a process that extends into the contralateral lobe. (2) Afferents from maxillary palps (most likely from basiconic sensilla) project into both ipsi-and contralateral antennal lobes, yet their target glomeruli are apparently not the same as those of antennal basiconic sensilla. (3) Afferents in the antennal lobe may also stem from pharyngeal sensilla. (4) The most prominent types of interneurons with arborizations in the antennal lobe are: (i) local interneurons ramifying in the entire lobe, (ii) unilateral relay interneurons that extend from single glomeruli into the calyx and the lateral protocerebrum (LPR), (iii) unilateral interneurons that connect several glomeruli with the LPR only, (iv) bilateral interneurons that link a small number of glomeruli in both antennal lobes with the calyx and LPR, (v) giant bilateral interneurons characterized by extensive ramifications in both antennal lobes and the posterior brain and a cell body situated in the midline of the suboesophageal ganglion, and (vi) a unilateral interneuron with extensive arborization in one antennal lobe and the posterior brain and a process that extends into the thorax. These structural results are discussed in the context of the available functional and behavioral data.Abbreviations AC antennal commissure - AMMC antennal mechanosensory and motor center - iACT, mACT, oACT inner/middle/outer antenno-cerebral tract - bACTI, uACTI bilateral/unilateral ACT relay interneuron - AN antennal nerve - AST antenno-suboesophageal tract - FAI fine arborization relay interneuron - GSI giant symmetric relay interneuron - LI local interneuron - LPR lateral protocerebrum - SOG suboesophageal ganglion - TI thoracic relay interneuron - bVI bilateral V-relay interneuron     

Genetic and functional subdivision of the Drosophila antennal lobe

Olfactory systems confer the recognition and discrimination of a large number of structurally distinct odor molecules. Recent molecular analysis of odorant receptor (OR) genes and circuits has led to a model of odor coding in which a population of olfactory sensory neurons (OSNs) expressing a single OR converges upon a unique olfactory glomerulus. Activation of the OR can thus be read out by the activation of its cognate glomerulus. Drosophila is a powerful system in which to test this model because the entire repertoire of 62 ORs can be manipulated genetically. However, a complete understanding of how fly olfactory circuits are organized is lacking. Here, we present a nearly complete map of OR projections from OSNs to the antennal lobe (AL) in the fly brain. Four populations of OSNs coexpress two ORs along with Or83b, and a fifth expresses one OR and one gustatory receptor (GR) along with Or83b. One glomerulus receives coconvergent input from two separate populations of OSNs. Three ORs label sexually dimorphic glomeruli implicated in sexual courtship and are thus candidate Drosophila pheromone receptors. This olfactory sensory map provides an experimental framework for relating ORs to glomeruli and ultimately behavior.     

Maturation of odor representation in the honeybee antennal lobe

The antennal lobe (AL) is the first center for processing odors in the insect brain, as is the olfactory bulb (OB) in vertebrates. Both the AL and the OB have a characteristic glomerular structure; odors sensed by olfactory receptor neurons are represented by patterns of glomerular activity. Little is known about when and how an odor begins to be perceived in a developing brain. We address this question by using calcium imaging to monitor odor-evoked neural activity in the ALs of bees of different ages. We find that odor-evoked neural activity already occurs in the ALs of bees as young as 1 or 2 days. In young bees, the responses to odors are relatively weak and restricted to a small number of glomeruli. However, different odors already evoke responses in different combinations of glomeruli. In mature bees, the responses are stronger and are evident in more glomeruli, but continue to have distinct odor-dependent signatures. Our findings indicate that the specific glomerular patterns for odors are conserved during the development, and that odor representations are fully developed in the AL during the first 2 weeks following emergence.     

Odor reception in antenna and antennal lobe of Drosophila

In early years of neurogenetics of Drosophila, most of us were inclined to believe that behavior of the fruit fly is largely stereotyped and hard-wired. This, at least, was a common prejudice when genetic analysis of olfaction began. We now know that Drosophila like other insects is capable of several types of learning or experience-dependent modification of behavior.