Modulation of olfactory information processing in the antennal lobe of Manduca sexta by serotonin.

The nervous system copes with variability in the external and internal environment by using neuromodulators to adjust the efficacy of neural circuits. The role of serotonin (5HT) as a neuromodulator of olfactory information processing in the antennal lobe (AL) of Manduca sexta was examined using multichannel extracellular electrodes to record the responses of ensembles of AL neurons to olfactory stimuli. In one experiment, the effects of 5HT on the concentration-response functions for two essential plant oils across a range of stimulus intensities were examined. In a second experiment, the effect of 5HT on the ability of ensembles to discriminate odorants from different chemical classes was examined. Bath application of 5HT enhanced AL unit responses by increasing response duration and firing rate, which in turn increased the amount of spike time cross-correlation and -covariance between pairs of units. 5HT had the greatest effect on overall ensemble activation at higher odorant concentrations, resulting in an increase in the gain of the dose-response function of individual units. Additionally, response thresholds shifted to lower odorant concentrations for some units, suggesting that 5HT increased their sensitivity. Serotonin enhanced ensemble discrimination of different concentrations of individual odorants as well as discrimination of structurally dissimilar odors at the same concentration. Given the known circadian fluctuations of 5HT in the AL of this species, these findings support the hypothesis that 5HT periodically enhances sensitivity and responsiveness in the AL of Manduca to maximize efficiency when the requirement for olfactory acuity is the greatest.

Representation of binary pheromone blends by glomerulus-specific olfactory projection neurons.

An outstanding challenge in olfactory neurobiology is to explain how glomerular networks encode information about stimulus mixtures, which are typical of natural olfactory stimuli. In the moth Manduca sexta, a species-specific blend of two sex-pheromone components is required for reproductive signaling. Each component stimulates a different population of olfactory receptor cells that in turn target two identified glomeruli in the macroglomerular complex of the male's antennal lobe. Using intracellular recording and staining, we examined how responses of projection neurons innervating these glomeruli are modulated by changes in the level and ratio of the two essential components in stimulus blends. Compared to projection neurons specific for one component, projection neurons that integrated information about the blend (received excitatory input from one component and inhibitory input from the other) showed enhanced ability to track a train of stimulus pulses. The precision of stimulus-pulse tracking was furthermore optimized at a synthetic blend ratio that mimics the physiological response to an extract of the female's pheromone gland. Optimal responsiveness of a projection neuron to repetitive stimulus pulses therefore appears to depend not only on stimulus intensity but also on the relative strength of the two opposing synaptic inputs that are integrated by macroglomerular complex projection neurons.

EGF/ErbB receptor family in ovarian cancer.

In summary, the EGF/ErbB family of receptor tyrosine kinases has been shown to play a key role in normal ovarian follicle development, and cell growth regulation of the ovarian surface epithelium. Disregulation of these normal growth regulatory pathways, including overexpression and/or mutation of EGFR/ErbB receptor family members, as well as elements of their downstream signalling pathways, have been shown to contribute to the etiology and progression of epithelial ovarian cancer. It is, therefore, not surprising that these gene products, and their related soluble receptor isoforms may have clinical utility as tumor and/or serum biomarkers of disease activity. Moreover, since several of these soluble receptor isoforms have potent growth inhibitory activity, and are naturally occurring in the circulation, they are ideal candidates for the development of novel therapeutics for the treatment of ovarian cancer patients.

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.

Odour-plume dynamics influence the brain's olfactory code.

The neural computations used to represent olfactory information in the brain have long been investigated. Recent studies in the insect antennal lobe suggest that precise temporal and/or spatial patterns of activity underlie the recognition and discrimination of different odours, and that these patterns may be strengthened by associative learning. It remains unknown, however, whether these activity patterns persist when odour intensity varies rapidly and unpredictably, as often occurs in nature. Here we show that with naturally intermittent odour stimulation, spike patterns recorded from moth antennal-lobe output neurons varied predictably with the fine-scale temporal dynamics and intensity of the odour. These data support the hypothesis that olfactory circuits compensate for contextual variations in the stimulus pattern with high temporal precision. The timing of output neuron activity is constantly modulated to reflect ongoing changes in stimulus intensity and dynamics that occur on a millisecond timescale.

Multi-unit recordings reveal context-dependent modulation of synchrony in odor-specific neural ensembles.

We used neural ensemble recording to examine odor-evoked ensemble patterns in the moth antennal (olfactory) lobe. Different odors are thought to evoke unique spatiotemporal patterns of glomerular activity, but little is known about the population dynamics underlying formation of these patterns. Using a silicon multielectrode array, we observed dynamic network interactions within and between glomeruli. Whereas brief odor pulses repeatedly triggered activity in the same coding ensemble, the temporal pattern of synchronous activity superimposed on the ensemble was neither oscillatory nor odor specific. Rather, synchrony strongly depended on contextual variables such as odor intensity and intermittency. Also, because of emergent inhibitory circuit interactions, odor blends evoked temporal ensemble patterns that could not be predicted from the responses to the individual odorants. Thus even at this early stage of information processing, the timing of odor-evoked neural representations is modulated by key stimulus factors unrelated to the molecular identity of the odor.

Response characteristics of an identified, sexually dimorphic olfactory glomerulus.

Partitioning of synaptic neuropil into glomeruli is a common feature of primary olfactory centers in most animal species. The functional significance of glomeruli, however, is not yet well understood. The present study is part of our effort to test the hypothesis that each glomerulus is a functional unit dedicated to processing information about a particular odorant or attribute of odor molecules and that the glomerular array constitutes a map of "odor space." We investigated the physiological and morphological features of uniglomerular projection neurons (PNs) associated with an identified glomerulus in each antennal lobe of the female sphinx moth, Manduca sexta. This "lateral large female glomerulus" (latLFG) is sexually dimorphic and therefore may play a female-specific role, such as processing of information about one or more odorants important for orientation of a female to host plants for oviposition. Together with the medial LFG (medLFG), the latLFG resides outside the array of spheroidal ordinary glomeruli, near the entrance of the antennal (olfactory) nerve. Each LFG is innervated by four to five PNs. Using intracellular recording and staining, we examined the responses of latLFG-PNs to odorants that represent major classes of volatiles released by host plants of M. sexta. All latLFG-PNs were excited when the ipsilateral antenna was stimulated with low concentrations of the monoterpenoid linalool. Dose-response analysis showed that neither other monoterpenoids nor representatives of other classes of host plant volatiles were similarly stimulatory to latLFG-PNs. These findings are consistent with the idea that each glomerulus has a characteristic, limited molecular receptive range.

Temporal tuning of odor responses in pheromone-responsive projection neurons in the brain of the sphinx moth Manduca sexta.

By means ofintracellular recording and staining, we studied the ability of several distinct classes of projection (output) neurons, which innervate the sexually dimorphic macroglomerular complex (MGC-PNs) in the antennal lobe of the male moth Manduca sexta, to encode naturally intermittent sex pheromonal stimuli. In many MGC-PNs, antennal stimulation with a blend of the two essential pheromone components evoked a characteristic triphasic response consisting of a brief, hyperpolarizing inhibitory potential (I1) followed by depolarization with firing of action potentials and then a delayed period of hyperpolarization (I2). MGC-PNs described in this study resolved pulsed pheromonal stimuli, up to about five pulses/second, with a distinct burst of action potentials for each pulse of odor. The larger the amplitude of I1, the higher the pulse rate an MGC-PN could follow, illustrating the importance of inhibitory synaptic input in shaping the temporal firing properties of these glomerular output neurons. In some MGC-PNs, triphasic responses were evoked by antennal stimulation with only one of the two key pheromone components. Again, the maximal pulse rate that an MGC-PN could follow with that pheromone component as sole stimulus was high in MGC-PNs that responded with a strong I1. These component-specific MGC-PNs innervated only one of the two principal glomeruli of the MGC, while MGC-PNs that were primarily excited by antennal stimulation with either key pheromone component had arborizations in both major MGC glomeruli. These observations therefore suggest that the population of antennal olfactory receptor cells responding to a single pheromone component is functionally heterogeneous: a subset of these sensory cells activates the excitatory drive to many uniglomerular MGC-PNs, while others feed onto inhibitory circuits that hyperpolarize the same PNs. This convergence of opposing inputs is a circuit property common to uniglomerular MGC-PNs branching in either of the major MGC glomeruli, and it enhances the ability of these glomerular output neurons to resolve intermittent olfactory input. Synaptic integration at the uniglomerular PN level thus contributes to the transmission of behaviorally important temporal information about each key pheromone component to higher centers in the brain.

Combinatorial odor discrimination in the brain: attractive and antagonist odor blends are represented in distinct combinations of uniquely identifiable glomeruli.

The rules governing the central discrimination of odors are complex and poorly understood, but a growing body of evidence supports the hypothesis that olfactory glomeruli may represent functionally distinct coding modules in the brain. Testing this hypothesis requires that both the functional characteristics and the spatial position of the glomerulus under study be uniquely identifiable. To address these questions, we examined a specialized array of glomeruli (the macroglomerular complex; MGC) in the antennal lobe of male moths that receives input from olfactory receptor cells tuned specifically to female-released odorants that either promote upwind flight (conspecific sex pheromones) or inhibit it (interspecific antagonists). By using a three-dimensional reconstruction method based on high-resolution laser-scanning confocal microscopy, we generated precise spatial maps of the MGC glomeruli in two related noctuid species with similar pheromone chemistry, Heliothis virescens and Helicoverpa zea. To determine the breadth of tuning of individual MGC glomeruli in processing information about these social signals, we used intracellular recording and staining methods to examine the responses of projection (output) neurons that innervate MGC glomeruli and that each project an axon to higher integrative centers. In both species, a close correspondence was found between the odor specificity of the projection neurons and the glomerulus (or glomeruli) supplied by them. The binary blend of pheromone components for each species was represented by neural activity in only two distinct glomeruli in both H. virescens and H. zea. Odorants that antagonize upwind flight when they are added to the respective pheromonal blends evoked excitatory activity in output neurons restricted to a third glomerulus in the MGCs of both species. In summary, these results suggest that the selective activation of different combinations of functionally distinct MGC glomeruli is a general means for discriminating these specific attractant and antagonist chemical signals in the brain.

Discrimination of pheromonal cues in fish: emerging parallels with insects.

Many fish species employ hormonal products as sex pheromones, and these cues are often mixtures that are released with a temporal pattern. This behavior is strikingly similar to that of insects, as moths use precise blends of odorants as sex pheromones and are skillful at tracking them in spite of changes in odor intensity associated with aerial dispersal. New studies in both groups of animals suggest many parallels in the functional anatomy of olfactory pathways and the organization of information-coding circuits.

Multitasking in the olfactory system: context-dependent responses to odors reveal dual GABA-regulated coding mechanisms in single olfactory projection neurons.

Studies of olfaction have focused mainly on neural processing of information about the chemistry of odors, but olfactory stimuli have other properties that also affect central responses and thus influence behavior. In moths, continuous and intermittent stimulation with the same odor evokes two distinct flight behaviors, but the neural basis of this differential response is unknown. Here we show that certain projection neurons (PNs) in the primary olfactory center in the brain give context-dependent responses to a specific odor blend, and these responses are shaped in several ways by a bicuculline-sensitive GABA receptor. Pharmacological dissection of PN responses reveals that bicuculline blocks GABAA-type receptors/chloride channels in PNs, and that these receptors play a critical role in shaping the responses of these glomerular output neurons. The firing patterns of PNs are not odor-specific but are strongly modulated by the temporal pattern of the odor stimulus. Brief repetitive odor pulses evoke fast inhibitory potentials, followed by discrete bursts of action potentials that are phase-locked to the pulses. In contrast, the response to a single prolonged stimulus with the same odor is a series of slow oscillations underlying irregular firing. Bicuculline disrupts the timing of both types of responses, suggesting that GABAA-like receptors underlie both coding mechanisms. These results suggest that glomerular output neurons could use more than one coding scheme to represent a single olfactory stimulus. Moreover, these context-dependent odor responses encode information about both the chemical composition and the temporal pattern of the odor signal. Together with behavioral evidence, these findings suggest that context-dependent odor responses evoke different perceptions in the brain that provide the animal with important information about the spatiotemporal variations that occur in natural odor plumes.

GABAergic mechanisms that shape the temporal response to odors in moth olfactory projection neurons.

Mitral/tufted cells in the olfactory bulb and projection neurons (PNs) in the insect antennal lobe are involved in complex synaptic interactions with inhibitory interneurons to help shape their odor-evoked responses. In the moth Manduca sexta, both gamma-aminobutyric acid (GABA) and the GABAA receptor agonist muscimol hyperpolarize and lower input resistance in many PNs, often blocking ongoing spike traffic. The GABA response mimics a short-latency, chloride-mediated inhibitory postsynaptic potential (IPSP) evoked in PNs by electrical or odor stimulation of afferent inputs, and the classical GABAA receptor antagonist bicuculline methiodide (BMI) quickly and reversibly blocks this IPSP. Focal injection of BMI (100 microM) immediately preceding a GABA pulse blocks the hyperpolarization evoked by GABA, but a similar injection of BMI preceding an acetylcholine (ACh) pulse fails to block the depolarization evoked by ACh. Moreover, the temporal pattern of odor-evoked activity in moth PNs is also strongly and reversibly altered by BMI. Importantly, the temporal pattern of the response depends on the temporal characteristics of the stimulus: continuous stimulation evokes more complex, rhythmic responses, whereas a pulsatile stimulus can be copied with a discrete burst of spikes for each pulse. Collectively our results indicate that PNs in the moth antennal lobe possess GABA receptors that share certain characteristics in common with vertebrate GABAA receptors. These receptors are largely responsible for helping PNs integrate information about both the molecular features and the timing of olfactory input to the brain.

A combinatorial model of odor discrimination using a small array of contiguous, chemically defined glomeruli.

The macroglomerular complex (MGC) is a sexually dimorphic array of contiguous glomeruli in male moths, located at the base of the antennal nerve where it enters the primary olfactory processing area of the brain, the antennal lobe. A new three-dimensional reconstruction method was used to map this glomerular array, and images obtained with the laser scanning confocal microscope revealed the precise location and spatial arrangement of the MGC glomeruli in two related species of Heliothine moth. Intracellular recording and staining of projection neurons (PNs) that arborize within the MGC has revealed that information about the two attractive components of the pheromone blend is segregated to different glomeruli in both species. In one species, Helicoverpa zea, a third glomerulus serves as the locus for the processing of odors that antagonize upwind flight. Thus, activation of different but overlapping sets of glomeruli within the MGC may provide a neural substrate for discriminating attractant vs antagonist chemical signals.

Intrapulmonary drug delivery of salmon calcitonin.

Calcitonin (CT) and other bone-active peptides have been restrained in clinical use by the need for parenteral administration. Although nasal and other transmucosal routes can be used for CT treatment, bioavailability and bioactivity of the peptide thus delivered are limited. We have evaluated the intrapulmonary route (IP) for the delivery of salmon calcitonin (SCT) in normal subjects. SCT was administered with a dry powder delivery inhaler. For comparison, each subject also received intramuscular (IM) SCT. Inhaled SCT produced significant hypocalcemia in all subjects as did injected SCT, and the peptide could be readily measured in serum by immunoassay. Compared by dose, IP SCT had 66% of the bioactivity and 28% of the bioavailability of IM SCT. This intrapulmonary route of administration should enhance the clinical acceptability of SCT and could also be applicable to other bone-active peptides.

Coincident stimulation with pheromone components improves temporal pattern resolution in central olfactory neurons.

Male moths must detect and resolve temporal discontinuities in the sex pheromonal odor signal emitted by a conspecific female moth to orient to and locate the odor source. We asked how sensory information about two key components of the pheromone influences the ability of certain sexually dimorphic projection (output) neurons in the primary olfactory center of the male moth's brain to encode the frequency and duration of discrete pulses of pheromone blends. Most of the male-specific projection neurons examined gave mixed postsynaptic responses, consisting of an early suppressive phase followed by activation of firing, to stimulation of the ipsilateral antenna with a blend of the two behaviorally essential pheromone components. Of 39 neurons tested, 33 were excited by the principal (most abundant) pheromone component but inhibited by another, less abundant but nevertheless essential component of the blend. We tested the ability of each neuron to encode intermittent pheromonal stimuli by delivering trains of 50-ms pulses of the two-component blend at progressively higher rates from 1 to 10 per second. There was a strong correlation between 1) the amplitude of the early inhibitory post-synaptic potential evoked by the second pheromone component and 2) the maximal rate of odor pulses that neuron could resolve (r = 0.92). Projection neurons receiving stronger inhibitory input encoded the temporal pattern of the stimulus with higher fidelity. With the principal, excitatory component of the pheromone alone as the stimulus, the dynamic range for encoding stimulus intermittency was reduced in nearly 60% of the neurons tested. The greatest reductions were observed in those neurons that could be shown to receive the strongest inhibitory input from the second behaviorally essential component of the blend. We also tested the ability of these neurons to encode stimulus duration. Again there was a strong correlation between the strength of the inhibitory input to a neuron mediated by the second pheromone component and that neuron's ability to encode stimulus duration. Neurons that were strongly inhibited by the second component could accurately encode pulses of the blend from 50 to 500 ms in duration (r = 0.94), but that ability was reduced in neurons receiving little or no inhibitory input (r = 0.23). This study confirms that certain olfactory projection neurons respond optimally to a particular odor blend rather than to the individual components of the blend. The key components activate opposing synaptic inputs that enable this subset of central neurons to copy the duration and frequency of intermittent odor pulses that are a fundamental feature of airborne olfactory stimuli.

Olfactory information processing in the brain: encoding chemical and temporal features of odors.

A fundamental problem in studying the neural mechanisms of odor recognition and discrimination in the olfactory system lies in determining the features or "primitives" of an odor stimulus that are analyzed by glomerular circuits at the first level of processing in the brain. Several recent studies support the idea that it is not simply the molecular features of odors that contain important information, but also the intermittent pattern of their presentation to the olfactory epithelium that helps determine the behavioral response to odor.

Pheromones as tools for olfactory research.Introduction.

We have long been fascinated by the unique ability of odors to stir our emotions and to evoke long-forgotten memories, but certain odors play a much more fundamental role in that they vastly improve an organism's chances for reproductive success and survival. These odorants are called pheromones, a term commonly applied to semiochemicals that are released by one member of a species and evoke a specific reaction or reactions from members of the same species. Pheromones are known for both the specificity and the potency of their actions, which can be behavioral and/or neuroendocrinological. Pheromones can stimulate individuals to aggregate, to disperse, or to react defensively in the presence of a predator, but they are probably best known for bringing the sexes together. Some pheromones have also been found to trigger a dramatic release of pituitary hormones in several vertebrate species. Although first identified in insects, more recent studies show that sex pheromones influence the lives of a wide range of organisms, from microbes to man. The hormonally-derived sex pheromones in teleost fish, and the airborne pheromones of moths are two systems that illustrate how scientists have used these specialized chemical signals as important tools to investigate the morphology, physiology and biochemistry of olfactory-receptor systems, the mechanisms of odor-information processing in the brain, and the diverse range of behaviors and endocrinological changes associated with pheromonal communication. While our focus is on these two animal models, other examples, including mammalian pheromone systems, are also discussed.

Distinct projections of two populations of olfactory receptor axons in the antennal lobe of the sphinx moth Manduca sexta.

The central projections of olfactory receptor cells associated with two distinct types of antennal sensilla in the sphinx moth Manduca sexta were revealed by anterograde staining. In both sexes, receptor axons that arise from sexually isomorphic, type-II trichoid sensilla (and possibly some basiconic sensilla) project to the spheroidal glomeruli in the ipsilateral antennal lobe. Each axon terminates in one glomerulus. Axons from a limited region of the antenna project to glomeruli throughout the lobe, arguing against strict topographic mapping of antennal receptor cells onto the array of glomeruli. Axons of sex-pheromone-selective receptor cells in the male-specific type-I trichoid sensilla project exclusively to the sexually dimorphic macroglomerular complex (MGC). Axons from sensilla on the dorsal surface of the antenna are biased toward the medial MGC and those from ventral sensilla, toward the lateral MGC. Some receptor-cell axons branch before reaching the MGC, but their terminals are always confined to one of the two main glomerular divisions of the MGC, the cumulus and toroid. These findings confirm that primary-afferent information about pheromonal and non-pheromonal odors is segregated in the antennal lobe and suggest that there is a functional correspondence between particular olfactory receptor cells and specific glomeruli.

Activation of c-fos gene expression by a kinase-deficient epidermal growth factor receptor.

The intrinsic tyrosine kinase activity of the epidermal growth factor receptor (EGFR) has been shown to be responsible for many of the pleiotropic intracellular effects resulting from ligand stimulation [W.S. Chen, C.S. Lazar, M. Poenie, R.Y. Tsien, G.N. Gill, and M.G. Rosenfeld, Nature (London) 328:820-823, 1987; A.M. Honegger, D. Szapary, A. Schmidt, R. Lyall, E. Van Obberghen, T.J. Dull, A. Ulrich, and J. Schlessinger, Mol. Cell. Biol. 7:4568-4571, 1987]. Recently, however, it has been shown that addition of ligand to cells expressing kinase-defective EGFR mutants can result in the phosphorylation of mitogen-activated protein kinase (R. Campos-González and J.R. Glenney, Jr., J. Biol. Chem. 267:14535-14538, 1992; E. Selva, D.L. Raden, and R.J. Davis, J. Biol. Chem. 268:2250-2254, 1993), as well as stimulation of DNA synthesis (K.J. Coker, J.V. Staros, and C.A. Guyer, Proc. Natl. Acad. Sci. USA 91:6967-6971, 1994). Moreover, mitogen-activated protein kinase has been shown to phosphorylate the transcription factor p62TCF in vitro, leading to enhanced ternary complex formation between p62TCF, p67SRF, and the c-fos serum response element (SRE) [H. Gille, A.D. Sharrocks, and P.E. Shaw, Nature (London) 358:414-417, 1992]. On the basis of these observations, we have investigated the possibility that the intrinsic tyrosine kinase activity of the EGFR may not be necessary for transcriptional activation mediated via p62TCF. Here, we demonstrate that a kinase-defective EGFR mutant can signal ligand-induced expression of c-fos protein and that a significant component of this induction appears to be mediated at the transcriptional level. Investigation of transcriptional activation mediated by the c-fos SRE shows that this response is impaired by mutations in the SRE which eliminate binding of p62(TCF). These data indicate that information inherent in the structure of the EGFR can be accessed by ligand stimulation independent of the receptor's catalytic kinase function.

Local interneurons and information processing in the olfactory glomeruli of the moth Manduca sexta.

Intracellular recordings were made from the major neurites of local interneurons in the moth antennal lobe. Antennal nerve stimulation evoked 3 patterns of postsynaptic activity: (i) a short-latency compound excitatory postsynaptic potential that, based on electrical stimulation of the antennal nerve and stimulation of the antenna with odors, represents a monosynaptic input from olfactory afferent axons (71 out of 86 neurons), (ii) a delayed activation of firing in response to both electrical- and odor-driven input (11 neurons), and (iii) a delayed membrane hyperpolarization in response to antennal nerve input (4 neurons). Simultaneous intracellular recordings from a local interneuron with short-latency responses and a projection (output) neuron revealed unidirectional synaptic interactions between these two cell types. In 20% of the 30 pairs studied, spontaneous and current-induced spiking activity in a local interneuron correlated with hyperpolarization and suppression of firing in a projection neuron. No evidence for recurrent or feedback inhibition of projection neurons was found. Furthermore, suppression of firing in an inhibitory local interneuron led to an increase in firing in the normally quiescent projection neuron, suggesting that a disinhibitory pathway may mediate excitation in projection neurons. This is the first direct evidence of an inhibitory role for local interneurons in olfactory information processing in insects. Through different types of multisynaptic interactions with projection neurons, local interneurons help to generate and shape the output from olfactory glomeruli in the antennal lobe.