Excitation, inhibition, and suppression by odors in isolated toad and rat olfactory receptor neurons

Vertebrate olfactoryreceptor neurons (ORNs) exhibit odor-induced increases in actionpotential firing rate due to an excitatory cAMP-dependent current. Fishand amphibian ORNs also give inhibitory odor responses, manifested asdecreases in firing rate, but the underlying mechanism is poorlyunderstood. In the toad, an odor-induced Ca²⁺-activatedK⁺ current is responsible for the hyperpolarizing receptorpotential that causes inhibition. In isolated ORNs, a third manner bywhich odors affect firing is suppression, a direct and nonspecificreduction of voltage-gated and transduction conductances. Here we showthat in whole cell voltage-clamped toad ORNs, excitatory or inhibitory currents were not strictly associated to a particular odorant mixture.Occasionally, both odor effects, in addition to suppression, wereconcurrently observed in a cell. We report that rat ORNs also exhibitodor-induced inhibitory currents, due to the activation of aK⁺ conductance closely resembling that in the toad,suggesting that this conductance is widely distributed amongvertebrates. We propose that ORNs operate as complex integrator unitsin the olfactory epithelium, where the first events in the process ofodor discrimination take place.

     

Ion conductances related to development of repetitive firing in mouse retinal ganglion neurons in situ

ABSTRACT: In the retina, the ability to encode graded depolarizations into spike trains of variable frequency appears to be a specific property of retinal ganglion neurons (RGNs). To deduce the developmental changes in ion conductances underlying the transition from single to repetitive firing, patch-clamp recordings were performed in the isolated mouse retina between embryonic day 15 (E15) and postnatal day 5 (P5). Immature neurons of the E15 retina were selected according to their capacity to generate voltage-activated Na+ currents (I(Na)(v)). Identification of P5 RGNs was based on retrograde labeling, visualization of the axon, or the amplitude of I(Na)(v). At E15, half of the cells were excitable but none of them generated more than one spike. At P5, all cells were excitable and a majority discharged in tonic fashion. Ion conductances subserving maintenance of repetitive discharge were identified at P5 by exposure to low extracellular Ca2+, Cd2+, and charybdotoxin, all of which suppressed repetitive discharge. omega-Conotoxin GVIA and nifedipine had no effect. We compared passive membrane properties and a variety of voltage-activated ion channels at E15 and P5. It was found that the density of high voltage-activated (HVA) Ca2+ currents increased in parallel with the development of repetitive firing, while the density of Ni2+-sensitive low voltage-activated (LVA) Ca2+ currents decreased. Changes in density and activation kinetics of tetrodotoxin-sensitive Na+ currents paralleled changes in firing thresholds and size of action potentials, but seemed to be unrelated to maintenance of repetitive firing. Densities of A-type K+ currents and delayed rectifier currents did not change. The results suggest that HVA Ca2+ channels, and among them a toxin-resistant subtype, are specifically engaged in activation of Ca2+-sensitive K+ conductance and thereby account for frequency coding in postnatal RGNs.     

Action potentials and chemosensitive conductances in the dendrites of olfactory neurons suggest new features for odor transduction

Odors affect the excitability of an olfactory neuron by altering membrane conductances at the ciliated end of a single, long dendrite. One mechanism to increase the sensitivity of olfactory neurons to odorants would be for their dendrites to support action potentials. We show for the first time that isolated olfactory dendrites from the mudpuppy Necturus maculosus contain a high density of voltage-activated Na+ channels and produce Na-dependent action potentials in response to depolarizing current pulses. Furthermore, all required steps in the transduction process beginning with odor detection and culminating with action potential initiation occur in the ciliated dendrite. We have previously shown that odors can modulate Cl- and K+ conductances in intact olfactory neurons, producing both excitation and inhibition. Here we show that both conductances are also present in the isolated, ciliated dendrite near the site of odor binding, that they are modulated by odors, and that they affect neuronal excitability. Voltage- activated Cl- currents blocked by 4,4'-diisothiocyanatostilbene-2,2' disulfonic acid and niflumic acid were found at greater than five times higher average density in the ciliated dendrite than in the soma, whereas voltage-activated K+ currents inhibited by intracellular Cs+ were distributed on average more uniformly throughout the cell. When ciliated, chemosensitive dendrites were stimulated with the odorant taurine, the responses were similar to those seen in intact cells: Cl- currents were increased in some dendrites, whereas in others Cl- or K+ currents were decreased, and responses washed out during whole-cell recording. The Cl- equilibrium potential for intact neurons bathed in physiological saline was found to be -45 mV using an on-cell voltage- ramp protocol and delayed application of channel blockers. We postulate that transduction of some odors is caused by second messenger-mediated modulation of the resting membrane conductance (as opposed to a specialized generator conductance) in the cilia or apical region of the dendrite, and show how this could alter the firing frequency of olfactory neurons.     

Differentiation of insect sensory neurons in the absence of their normal synaptic targets.

Sensory neurons in the antenna of the moth, Manduca sexta, arise and differentiate during the 18 days of metamorphosis from pupa to adult, sending axons to the brain. To assess the trophic dependence of developing antennal neurons on their targets, we studied antennae from surgically debrained animals. If the brain is removed 1 to 45 hr after pupal ecdysis (before and during the period when antennal neurons arise by cell divisions), adult development can be triggered by injection of β-ecdysone; if the brain is removed 50 to 60 hr after pupal ecdysis (before antennal axons contact the brain), metamorphosis proceeds spontaneously. Neurons proliferate normally and differentiate extensively in the antennae of debrained animals. They acquire a characteristic size and shape, elaborate axons and dendrites, migrate to appropriate positions in the sensilla, accumulate components of a neurotransmitter system (acetylcholine, choline acetyltransferase, and acetylcholinesterase), and generate electrical responses to olfactory and mechanical stimuli. Antennal sensory neurons thus differ from a variety of vertebrate neurons, which fail to mature when deprived of their synaptic targets.     

Elemental and configural olfactory coding by antennal lobe neurons of the honeybee (<Emphasis Type="Italic">Apis mellifera</Emphasis>)

When smelling an odorant mixture, olfactory systems can be analytical (i.e. extract information about the mixture elements) or synthetic (i.e. creating a configural percept of the mixture). Here, we studied elemental and configural mixture coding in olfactory neurons of the honeybee antennal lobe, local neurons in particular. We conducted intracellular recordings and stimulated with monomolecular odorants and their coherent or incoherent binary mixtures to reproduce a temporally dynamic environment. We found that about half of the neurons responded as ‘elemental neurons’, i.e. responses evoked by mixtures reflected the underlying feature information from one of the components. The other half responded as ‘configural neurons’, i.e. responses to mixtures were clearly different from responses to their single components. Elemental neurons divided in late responders (above 60 ms) and early responder neurons (below 60 ms), whereas responses of configural coding neurons concentrated in-between these divisions. Latencies of neurons with configural responses express a tendency to be faster for coherent stimuli which implies employment in different processing circuits.     

Study of nicotinic acetylcholine receptors on cultured antennal lobe neurones from adult honeybee brains

In insects, acetylcholine (ACh) is the main neurotransmitter, and nicotinic acetylcholine receptors (nAChRs) mediate fast cholinergic synaptic transmission. In the honeybee, nAChRs are expressed in diverse structures including the primary olfactory centres of the brain, the antennal lobes (AL) and the mushroom bodies. Whole-cell, voltage-clamp recordings were used to characterize the nAChRs present on cultured AL cells from adult honeybee, Apis mellifera. In 90% of the cells, applications of ACh induced fast inward currents that desensitized slowly. The classical nicotinic agonists nicotine and imidacloprid elicited respectively 45 and 43% of the maximum ACh-induced currents. The ACh-elicited currents were blocked by nicotinic antagonists methyllycaconitine, dihydroxy-β-erythroidine and α-bungarotoxin. The nAChRs on adult AL cells are cation permeable channels. Our data indicate the existence of functional nAChRs on adult AL cells that differ from nAChRs on pupal Kenyon cells from mushroom bodies by their pharmacological profile and ionic permeability, suggesting that these receptors could be implicated in different functions.     

Expression patterns of two putative odorant-binding proteins in the olfactory organs of Drosophila melanogaster have different implications for their functions

The aqueous medium bathing the dendrites of olfactory neurons contains high concentrations of odorant-binding proteins (OBPs) whose role is still unclear. OBPs may facilitate interactions between odorants and their membrane-bound receptors, perhaps by increasing the water solubility of hydrophobic molecules. Alternatively, OBPs may be involved in the inactivation of odorants and other volatile molecules, preventing desensitization and/or protecting olfactory neurons from toxic chemicals. We report here novel features of the localization of two putative OBPs, PBPRP2 and PBPRP5, that have important and different implications for their role in olfaction. Unlike several other putative OBPs of Drosophila melanogaster that are only found in adult olfactory organs, PBPRP5 is also expressed in the larval olfactory organs, suggesting that it plays a common role in olfaction at both stages. In the adult, PBPRP5 expression is restricted to the sensillum lymph that bathes the olfactory dendrites of a subset of olfactory hairs, the basiconic sensilla. Since individual basiconic sensilla differ in olfactory specificity, PBPRP5 may be able to bind to and mediate olfactory responses to a wide range of odorants. In contrast, PBPRP2 is present in the space immediately below the antennal cuticle and in the outer cavity of approximately 30% of the double-walled coeloconic sensilla on the antennal surface. In neither case is PBPRP2 in contact with the dendritic membranes of olfactory neurons, making a carrier function unlikely for this protein. Instead, PBPRP2 may act as a sink, binding to odorants and other volatile chemicals and limiting their interactions with olfactory neurons.     

Responses of protocerebral neurons in Manduca sexta to sex-pheromone mixtures

Male Manduca sexta moths are attracted to a mixture of two components of the female’s sex pheromone at the natural concentration ratio. Deviation from this ratio results in reduced attraction. Projection neurons innervating prominent male-specific glomeruli in the male’s antennal lobe produce maximal synchronized spiking activity in response to synthetic mixtures of the two components centering around the natural ratio, suggesting that behaviorally effective mixture ratios are encoded by synchronous neuronal activity. We investigated the physiological activity and morphology of downstream protocerebral neurons that responded to antennal stimulation with single pheromone components and their mixtures at various concentration ratios. Among the tested neurons, only a few gave stronger responses to the mixture at the natural ratio whereas most did not distinguish among the mixtures that were tested. We also found that the population response distinguished among the two pheromone components and their mixtures, prior to the peak population response. This observation is consistent with our previous finding that synchronous firing of antennal-lobe projection neurons reaches its maximum before the firing rate reaches its peak. Moreover, the response patterns of protocerebral neurons are diverse, suggesting that the representation of olfactory stimuli at the level of protocerebrum is complex.     

Transplant Antennae and Host Brain Interact to Shape Odor Perceptual Space in Male Moths

Behavioral responses to odors rely first upon their accurate detection by peripheral sensory organs followed by subsequent processing within the brain’s olfactory system and higher centers. These processes allow the animal to form a unified impression of the odor environment and recognize combinations of odorants as single entities. To investigate how interactions between peripheral and central olfactory pathways shape odor perception, we transplanted antennal imaginal discs between larval males of two species of moth Heliothis virescens and Heliothis subflexa that utilize distinct pheromone blends. During metamorphic development olfactory receptor neurons originating from transplanted discs formed connections with host brain neurons within olfactory glomeruli of the adult antennal lobe. The normal antennal receptor repertoire exhibited by males of each species reflects the differences in the pheromone blends that these species employ. Behavioral assays of adult transplant males revealed high response levels to two odor blends that were dissimilar from those that attract normal males of either species. Neurophysiological analyses of peripheral receptor neurons and central olfactory neurons revealed that these behavioral responses were a result of: 1. the specificity of H. virescens donor olfactory receptor neurons for odorants unique to the donor pheromone blend and, 2. central odor recognition by the H. subflexa host brain, which typically requires peripheral receptor input across 3 distinct odor channels in order to elicit behavioral responses.     

Developmental changes of the antenna and its neurons in the silkworm,Bombyx mori,with special regard to larval-pupal transformation

The larval antenna of Bombyx mori has 13 sensilla and about 52 sensory neurons in its distal portion. The axons form two nerve cords which unite in the cranial hemocoel to supply the brain as the olfactory nerve. The antennal imaginal disc, which is a thick pseudostratified epithelium continuous with the antennal epidermis, thickens markedly during the 5th instar by rapid cell proliferation. At the prepupal stage cell proliferation ceases and the disc everts to form a large pupal antenna. Simultaneously, an extensive cell rearrangement occurs in the antennal epidermis and the disc tissue becomes much thinner because of the abrupt expansion of antennal surface area. The two larval nerve cords thin down markedly by degeneration of axons, but they do not disintegrate totally even after the onset of pupation. The epidermis of the larval antenna forms the distal portion of the pupal antenna, while the imaginal disc forms the more basal portion. Development to the adult antenna occurs almost immediately after the onset of pupation; many adult neurons appear in the simple epidermis facing toward the thick outer side of the newly formed pupal cuticle. By 12 hours after the onset of pupation, these neurons align themselves in many transverse rows which are the first sign of the adult antennal configuration. Addition of these neuronal axons to the once-thinned nerve cords causes resumed thickening of the cords during the first 24 hours and thereafter. Differentiation of adult sensilla begins in the next 24 hours and is almost completed at the third day of pupation, which requires a total of 10 days.     

Dendritic pattern development of the honeybee antennal lobe neurons: a laser scanning confocal microscopic study

The processing of odorant signals is performed, in the olfactory bulb of vertebrates or in the antennal lobe of insects, by different types of neurons which display specific morphological and functional features. The present work characterizes the morphogenesis of the main neuronal types which participate in olfactory discrimination in the adult honeybee (Apis mellifera). Neurons were stained intracellularly with Lucifer yellow at different stages of pupal development and in the adult, and imaged by laser scanning confocal microscopy. Attending to branching patterns, all pupal neurons could be attributed to morphological types previously established in the adult. Given the functional importance of intraglomerular dendritic arbors in the processing of olfactory information, the study focused on their development. The two main classes, dense and sparse intraglomerular arbors, display adultlike features as early as the second day of pupal development. However, morphometric measurements and confocal observations show that their general pattern undergoes continuous maturation processes until late pupal stages and after emergence of the adult. Among these, the results point out a pruning of dendritic branches in sparse arbors, but not in dense arbors. © 1999 John Wiley & Sons, Inc. J Neurobiol 39: 461–474, 1999     

Effects of olfactory peduncle sectioning on the single unit responses of olfactory bulb neurons to odor presentation in awake rabbits

The influences of centrifugal inputs to the olfactory bulb werestudied by recording singlecell responses evoked by olfactorystimuli in intact and peduncle-sectioned bulbs of awake freebreathingrabbits. Responses of intact animals were mainly characterizedby a temporal reorganization of the single unit discharge -responsive second order neurons increased their firing activityduring inspirations and were silent during expirations. Thissynchronization of firing discharge with respiration occurredin the absence of any significant change in the overall firingactivity measured over intervals which included both the inspiratoryand expiratory phases of the respiratory cycle. By contrast,neurons recorded in isolated olfactory bulbs exhibited eithera significant increase or a decrease in firing activity duringodor presentation, and, furthermore, the synchronization ofthese units to the respiratory cycle was markedly reduced comparedwith that in intact animals. Comparison of cell responsivenessbetween intact and isolated olfactory bulbs indicated that thelesion increased the number of odors which induced a response,but did not change the percentage of cells which failed to respondto any of the 5 odorants used in this study. The cell responsivenessincreased for camphor and isoamyl acetate, and to a lesser extentfor food odor. The results indicate that high order nervousstructures exert a powerful inhibitory influence on the responsesof olfactory bulb second-order neurons to odor stimuli. Theyalso suggest that, in intact rabbits, centrifugal inputs playa role in the odor-induced synchronization of the single unitactivity with respiration.     

Temporal Features of Spike Trains in the Moth Antennal Lobe Revealed by a Comparative Time-Frequency Analysis

The discrimination of complex sensory stimuli in a noisy environment is an immense computational task. Sensory systems often encode stimulus features in a spatiotemporal fashion through the complex firing patterns of individual neurons. To identify these temporal features, we have developed an analysis that allows the comparison of statistically significant features of spike trains localized over multiple scales of time-frequency resolution. Our approach provides an original way to utilize the discrete wavelet transform to process instantaneous rate functions derived from spike trains, and select relevant wavelet coefficients through statistical analysis. Our method uncovered localized features within olfactory projection neuron (PN) responses in the moth antennal lobe coding for the presence of an odor mixture and the concentration of single component odorants, but not for compound identities. We found that odor mixtures evoked earlier responses in biphasic response type PNs compared to single components, which led to differences in the instantaneous firing rate functions with their signal power spread across multiple frequency bands (ranging from 0 to 45.71 Hz) during a time window immediately preceding behavioral response latencies observed in insects. Odor concentrations were coded in excited response type PNs both in low frequency band differences (2.86 to 5.71 Hz) during the stimulus and in the odor trace after stimulus offset in low (0 to 2.86 Hz) and high (22.86 to 45.71 Hz) frequency bands. These high frequency differences in both types of PNs could have particular relevance for recruiting cellular activity in higher brain centers such as mushroom body Kenyon cells. In contrast, neurons in the specialized pheromone-responsive area of the moth antennal lobe exhibited few stimulus-dependent differences in temporal response features. These results provide interesting insights on early insect olfactory processing and introduce a novel comparative approach for spike train analysis applicable to a variety of neuronal data sets.     

Odor suppression of voltage-gated currents contributes to the odor-induced response in olfactory neurons

Olfactorychemotransduction involves a signaling cascade. In addition totriggering transduction, odors suppress ion conductances. Bystimulating with brief odorant pulses, we observed a current associatedwith odor-induced suppression of voltage-gated conductances and studiedits time dependence. We characterized this suppression current inisolated Caudiverbera caudiverberaolfactory neurons. All four voltage-gated currents are suppressed byodor pulses in almost every neuron, and suppression is caused by odorsinducing excitation and by those inducing inhibition, indicating anonselective phenomenon, in contrast to transduction. Suppression has a10-fold shorter latency than transduction. Suppression was morepronounced when odors were applied to the soma than to the cilia,opposite to transduction. Suppression was also present in rat olfactory neurons. Furthermore, we could induce it inDrosophila photoreceptor cells,demonstrating its independence from the chemotransduction cascade. Weshow that odor concentrations causing suppression are similar to thosetriggering chemotransduction and that both suppression and transductioncontribute to the odor response in isolated olfactory neurons.Furthermore, suppression affects spiking, implying a possiblephysiological role in olfaction.

     

Pharmacological characterization of ionic currents that regulate high‐frequency spontaneous activity of electromotor neurons in the weakly electric fish,Apteronotus leptorhynchus

The neural circuit that controls the electric organ discharge (EOD) of the brown ghost knifefish (Apteronotus leptorhynchus) contains two spontaneous oscillators. Both pacemaker neurons in the medulla and electromotor neurons (EMNs) in the spinal cord fire spontaneously at frequencies of 500–1000 Hz to control the EOD. These neurons continue to fire in vitro at frequencies that are highly correlated with in vivo EOD frequency. Previous studies used channel blocking drugs to pharmacologically characterize ionic currents that control high‐frequency firing in pacemaker neurons. The goal of the present study was to use similar techniques to investigate ionic currents in EMNs, the other type of spontaneously active neuron in the electromotor circuit. As in pacemaker neurons, high‐frequency firing of EMNs was regulated primarily by tetrodotoxin‐sensitive sodium currents and by potassium currents that were sensitive to 4‐aminopyridine and κA‐conotoxin SIVA, but resistant to tetraethylammonium. EMNs, however, differed from pacemaker neurons in their sensitivity to some channel blocking drugs. Alpha‐dendrotoxin, which blocks a subset of Kv1 potassium channels, increased firing rates in EMNs, but not pacemaker neurons; and the sodium channel blocker μO‐conotoxin MrVIA, which reduced firing rates of pacemaker neurons, had no effect on EMNs. These results suggest that similar, but not identical, ionic currents regulate high‐frequency firing in EMNs and pacemaker neurons. The differences in the ionic currents expressed in pacemaker neurons and EMNs might be related to differences in the morphology, connectivity, or function of these two cell types. © 2005 Wiley Periodicals, Inc. J Neurobiol, 2006     

Dendritic pattern development of the honeybee antennal lobe neurons: a laser scanning confocal microscopic study.

The processing of odorant signals is performed, in the olfactory bulb of vertebrates or in the antennal lobe of insects, by different types of neurons which display specific morphological and functional features. The present work characterizes the morphogenesis of the main neuronal types which participate in olfactory discrimination in the adult honeybee (Apis mellifera). Neurons were stained intracellularly with Lucifer yellow at different stages of pupal development and in the adult, and imaged by laser scanning confocal microscopy. Attending to branching patterns, all pupal neurons could be attributed to morphological types previously established in the adult. Given the functional importance of intraglomerular dendritic arbors in the processing of olfactory information, the study focused on their development. The two main classes, dense and sparse intraglomerular arbors, display adultlike features as early as the second day of pupal development. However, morphometric measurements and confocal observations show that their general pattern undergoes continuous maturation processes until late pupal stages and after emergence of the adult. Among these, the results point out a pruning of dendritic branches in sparse arbors, but not in dense arbors.     

Tonic and phasic receptor neurons in the vertebrate olfactory epithelium

Olfactory receptor neurons (ORNs) respond to odorants with characteristic patterns of action potentials that are relevant for odor coding. Prolonged odorant exposures revealed three populations of dissociated toad ORNs, which were mimicked by depolarizing currents: tonic (TN, displaying sustained firing, 49% of 102 cells), phasic (PN, exhibiting brief action potential trains, 36%) and intermediate neurons (IN, generating trains longer than PN, 15%). We studied the biophysical properties underlying the differences between TNs and PNs, the most extreme cases among ORNs. TNs and PNs possessed similar membrane capacitances (approximately 4 pF), but they differed in resting potential (-82 versus -64 mV), input resistance (4.2 versus 2.9 G(Omega)) and unspecific current, I(u) (TNs: 0 < I(u) 1 pA/pF). Firing behavior did not correlate with differences in voltage-gated conductances. We developed a mathematical model that accurately simulates tonic and phasic patterns. Whole cell recordings from rat ORNs in fragments (approximately 4 mm(2)) of olfactory epithelium showed that such a tissue normally contains tonic and phasic receptor neurons, suggesting that this feature is common across a wide range of vertebrates. Our findings show that the individual passive electrical properties can govern the firing patterns of ORNs.     

Transient dynamics versus fixed points in odor representations by locust antennal lobe projection neurons

Projection neurons (PNs) in the locust antennal lobe exhibit odor-specific dynamic responses. We studied a PN population, stimulated with five odorants and pulse durations between 0.3 and 10 s. Odor representations were characterized as time series of vectors of PN activity, constructed from the firing rates of all PNs in successive 50 ms time bins. Odor representations by the PN population can be described as trajectories in PN state space with three main phases: an on transient, lasting 1-2 s; a fixed point, stable for at least 8 s; and an off transient, lasting a few seconds as activity returns to baseline. Whereas all three phases are odor specific, optimal stimulus separation occurred during the transients rather than the fixed points. In addition, the PNs' own target neurons respond least when their PN-population input stabilized at a fixed point. Steady-state measures of activity thus seem inappropriate to understand the neural code in this system.     

Pheromone-evoked potentials and oscillations in the antennal lobes of the sphinx moth Manduca sexta

Using intra- and extracellular recording methods, we studied the activity of pheromone-responsive projection neurons in the antennal lobe of the moth Manduca sexta. Intracellularly recorded responses of neurons to antennal stimulation with the pheromone blend characteristically included both inhibitory and excitatory stages of various strengths. To observe the activity of larger groups of neurons, we recorded responses extracellularly in the macroglomerular complex of the antennal lobe. The macroglomerular complex is part of a specialized olfactory subsystem and the site of first-order central processing of sex-pheromonal information. Odors such as the pheromone blend and host-plant (tobacco) volatiles gave rise to evoked potentials that were reproducible upon repeated antennal stimulation. Evoked potentials showed overriding high-frequency oscillations when the antenna was stimulated with the pheromone blend or with either one of the two key pheromone components. The frequency of the oscillations was in the range of 30–50 Hz. Amplitude and frequency of the oscillations varied during the response to pheromonal stimulation. Recording intracellular and extracellular activity simultaneously revealed phase-locking of action potentials to potential oscillations. The results suggest that the activity of neurons of the macroglomerular complex was temporally synchronized, potentially to strengthen the pheromone signal and to improve olfactory perception. Accepted: 19 December 1997     

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.Abbreviations AL antennal lobe - EPSP excitatory postsynaptic potential - GABA -aminobutyric acid - IPSP inhibitory postsynaptic potential - LN local interneuron - MGC macroglomerular complex - OB olfactory bulb - PN projection neuron - TES N-tris[hydroxymethyl]methyl-2-aminoethane-sulfonic acid