Antennal lobe interneurons in the desert locust Schistocerca gregaria (Forskal): Processing of aggregation pheromones in adult males and females

Physiological and morphological characteristics of antennal lobe interneurons in male and female Schistocerca gregaria were studied by using intracellular recording and staining techniques. For the first time, the responses of projection neurons to behaviourally active and potential aggregation pheromone compounds and plant odour compounds were recorded in young adult locusts. Excitatory, inhibitory, or combined excitatory/inhibitory responses to the presented odours were detected. The stained neurons had their cell bodies in the frontal cell group, arborized in 10 to 25 glomeruli at equal distances from the central fibre core, and sent their axons to the calyces of the mushroom body and to the lateral protocerebrum. The projection neurons responded to the set of different stimuli with varying specificity. In females, more neurons responded specifically to single plant or aggregation pheromone compounds than in males, where more generalist responses were found. “Blend specialist” neurons, responding only to mixtures of behaviourally active aggregation pheromone compounds, but not to the single compounds, were present in both males and females. Most neurons responded to the behaviourally active aggregation pheromone mixtures and to single compounds present in these mixtures, as well as to plant odours. Fewer neurons responded to the potential aggregation pheromone compounds tested. In several experiments, two spike sizes in the recording were correlated with two stained neurons in the antennal lobe, suggesting electrical coupling of the neurons. No response to any of the stimuli was found in antennal lobe interneurons in old adults. The morphological and physiological features of the projection neurons in S. gregaria are compared with projection neuron characteristics in other insects. © 1996 Wiley-Liss, Inc.     

Connections between the deutocerebrum and the protocerebrum,and neuroanatomy of several classes of deutocerebral projection neurons in the brain of male Periplaneta americana

The topography and neuroanatomy of fibers connecting the deutocerebrum to the protocerebrum in the brain of the American cockroach Periplaneta americana were investigated by staining single or multiple deutocerebral neurons with cobalt, Lucifer Yellow, or biocytin. Five tracts are distinguished on the basis of their routes from origins in the antennal lobe to the protocerebral neuropil: the inner antenno-cerebral tract (IACT); antenno-cerebral tracts II, III, and IV (ACT II, III, IV), and the outer antenno-cereral tract (OACT). These tracts are largely composed of the axons of four classes of deutocerebral projection neurons, which have been identified morphologically; the neuronal arborizations in the glomeruli of the antennal lobe and in the protocerebral projection regions have been examined. Projection neurons with processes in the inner antenno-cerebral tract and in the antenno-cerebral tract II each innervate a single glomerulus in the antennal lobe, and both types have terminals in the calyces of the mushroom bodies and in the lateral lobe of the protocerebrum. The axons of pheromone-sensitive projection neurons with dendritic trees in the male-specific macroglomerulus seem to run exclusively in the inner antenno-cerebral tract. Subgroups of these pheromone sensitive neurons differ in relative sensitivity to the two female attractant components as well as in the arborization pattern of their dendrites in the macroglomerulus. The projection neurons of two other classes each innervate many glomeruli in the antennal lobe, those of one class sending their axons into the protocerebrum in the antenno-cerebral tract IV and the other, in the outer antenno-cerebral tract. The neurons of antenno-cerebral tract IV innervate not only the mushroom body calyces and the lateral lobe but also neuropil regions not previously described in the cockroach. Neurons with axons in the outer antenno-cerebral tract have no terminals in the calyces but innervate the lateral lobe and the neuropil surrounding the tract. The morphological findings presented here show that, in addition to the tracts previously documented in the cockroach brain, there are other, presumably olfactory, connections between the deutocerebrum and the protocerebrum. © 1993 Wiley-Liss, Inc.     

Moth pheromone-selective projection neurons with cell bodies in the antennal lobe lateral cluster exhibit diverse morphological and neurophysiological characteristics

Olfactory projection neurons convey information from the insect antennal lobe (AL) to higher brain centers. Previous reports have demonstrated that pheromone-responsive projection neurons with cell bodies in the moth medial cell cluster (mcPNs) predominantly have dendritic arborizations in the sexually dimorphic macroglomerular complex (MGC) and send an axon from the AL to the calyces of the mushroom body (CA) as well as the lateral horn (LH) of the protocerebrum via the medial AL tract. These neurons typically exhibit a narrow odor tuning range related to the restriction of their dendritic arbors within a single glomerulus (uniglomerular). In this study, we report on the diverse physiological and morphological properties of a group of pheromone-responsive olfactory projection neurons with cell bodies in the AL lateral cell cluster (MGC lcPNs) of two closely related moth species. All pheromone-responsive lcPNs appeared to exhibit “basket-like” dendritic arborizations in two MGC compartments and made connections with various protocerebral targets including ventrolateral and superior neuropils via projections primarily through the lateral AL tract and to a lesser extent the mediolateral antennal lobe tract. Physiological characterization of MGC lcPNs also revealed a diversity of response profiles including those either enhanced by or reliant upon presentation of a pheromone blend. These responses manifested themselves as higher maximum firing rates and/or improved temporal resolution of pulsatile stimuli. MGC lcPNs therefore participate in conveying diverse olfactory information relating to qualitative and temporal facets of the pheromone stimulus to a more expansive number of protocerebral targets than their mcPN counterparts.     

Diversity, variability, and suboesophageal connectivity of antennal lobe neurons in D. melanogaster larvae

Whereas the "vertical" elements of the insect olfactory pathway, the olfactory receptor neurons and the projection neurons, have been studied in great detail, local interneurons providing "horizontal" connections in the antennal lobe were ignored for a long time. Recent studies in adult Drosophila demonstrate diverse roles for these neurons in the integration of odor information, consistent with the identification of a large variety of anatomical and neurochemical subtypes. Here we focus on the larval olfactory circuit of Drosophila, which is much reduced in terms of cell numbers. We show that the horizontal connectivity in the larval antennal lobe differs largely from its adult counterpart. Only one of the five anatomical types of neurons we describe is restricted to the antennal lobe and therefore fits the definition of a local interneuron. Interestingly, the four remaining subtypes innervate both the antennal lobe and the suboesophageal ganglion. In the latter, they may overlap with primary gustatory terminals and with arborizations of hugin cells, which are involved in feeding control. This circuitry suggests special links between smell and taste, which may reflect the chemosensory constraints of a crawling and burrowing lifestyle. We also demonstrate that many of the neurons we describe exhibit highly variable trajectories and arborizations, especially in the suboesophageal ganglion. Together with reports from adult Drosophila, these data suggest that wiring variability may be another principle of insect brain organization, in parallel with stereotypy.     

Functionally distinct subdivisions of the macroglomerular complex in the antennal lobe of the male sphinx moth Manduca sexta.

Each antennal lobe in the brain of a male moth has a distinctive neuropil structure, the macroglomerular complex (MGC), which is specialized for primary processing of information about the conspecific female sex-pheromone blend. Olfactory interneurons with dendritic arborizations in the MGC were examined by means of tandem intracellular recording and staining with Lucifer Yellow. Neurons that responded selectively to stimulation of the antenna with the major pheromone component, (E,Z)-10,12-hexadecadienal, had arborizations that were restricted to a toroidal subdivision (the "toroid") of the MGC. Similarly, neurons that responded selectively to antennal stimulation with (E,Z)-11,13-pentadecadienal, a more stable mimic of a second essential but chemically unstable pheromone component, (E,E,Z)-10,12,14-hexadecatrienal, had arborizations confined to a globular subdivision (the "cumulus") of the MGC situated more proximally to the antennal nerve input. One neuron that responded to both of these stimuli clearly had arborizations in both subdivisions of the MGC. These anatomically distinct subdivisions of the MGC thus appear also to be functionally separate regions of pheromone-processing neuropil.     

Common projection areas of antennal and visual pathways in the honeybee brain, Apis mellifera

The convergence of primary sensory neurons of the antennae, higher order visual interneurons, and antennal motoneurons was analysed with neuroanatomical techniques in the honeybee, Apis mellifera. The different modalities evoke specific antennal responses in this insect. Three different fluorescent dyes were applied successively in the same preparation in order to visualise the various fiber projections from the antennae and the lobula in the brain of the honeybee. Three neuropile areas where sensory fibers of the antennae overlap with visual projection neurons from the lobula were found. Within the posterior-median protocerebrum the antennal tract T6-1 comes in close vicinity to the lobula tract LoT-9 and to some other lobula fibers that cannot be assigned to a special tract. Antennal T6-3 fibers overlap with lobula LoT-7 neurons within the posterior protocerebrum more laterally. Antennal T5 fibers arborise in the dorsal lobe and show common projection sites with lobula LoT-3 neurons. The multimodal convergence in the three common neuropiles demonstrates that these areas are important centers for multimodal information processing between sensory, motor, and descending neurons in insects.     

Neural pathways for the processing of alarm pheromone in the ant brain

Social insects like ants exhibit sophisticated communication by means of pheromones, one example of which is the use of alarm pheromones to alert nestmates for colony defense. In the ant Camponotus obscuripes, we have reported that information about formic acid and n-undecane, alarm pheromone components, is processed in a set of specific glomeruli in the antennal lobe (primary olfactory center). Alarm pheromone signals are then transmitted, mainly via uniglomerular projection neurons (uni-PNs), to the protocerebrum (PR), where sensory signals are integrated to form motor commands for behavioral responses. In this study, we physiologically and morphologically characterized 63 alarm pheromone-sensitive PR neurons in ants by using intracellular recording and staining techniques. Most of the pheromone-sensitive PR neurons had dendrites in the mushroom body (MB), the lateral horn, or the medial PR. Some neurons with dendrites in these areas responded specifically to formic acid or n-undecane and may participate in the control of specific behavioral responses to each pheromone component. Other neurons responded also to non-pheromonal odors, in contrast to uni-PNs, most of which responded specifically to alarm pheromones. Responses to non-pheromonal odors were most prominent in efferent neurons of the MB lobe, suggesting that they may participate in integration of pheromonal and non-pheromonal information. We found a class of PR neurons that receives input in all of these pheromone-processing areas and terminates in a variety of premotor areas. These neurons may participate in the control of pheromone-sensitized aggressive behavior, which is triggered by non-pheromonal sensory stimuli associated with a potential enemy.     

Synaptic organization of the uniglomerular projection neurons of the antennal lobe of the moth Manduca sexta: A laser scanning confocal and electron microscopic study

The detailed branching pattern and synaptic organization of the uniglomerular projection neurons of the antennal lobe, the first processing center of the olfactory pathway, of the moth Manduca sexta were studied with laser scanning confocal microscopy and a technique combining laser scanning confocal microscopy and electron microscopy. Uniglomerular projection neurons, identified electrophysiologically or morphologically, were stained intracellularly with neurobiotin or biocytin. Brains containing the injected neurons were treated with streptavidin-immunogold to label the injected material for electron microscopy and with Cy3-streptavidin to lable the neurons with fluorescence for laser scanning confocal microscopy, and then embedded in Epon. Labeled neurons were imaged and reconstructed with laser scanning confocal microscopy (based on the retained fluorescence of the labeled neuron in the Epon block), and thin sections were cut at selected optical levels for correlation of light microscopic data and electron microscopic detail. Each neuron had a cell body in one of the three cell-body clusters of the antennal lobe, a primary neurite that extended across the coarse neuropil at the center of the antennal lobe and then formed a dense tuft of processes within a single glomerulus, and an axon that emanated from the primary neurite and projected from the antennal lobe via the antenno-cerebral tract to the lateral horn of the ipsilateral protocerebrum and, collaterally, to the calyces of the mushroom body. In the electron microscope, the fine dendritic branches in the apical zones of the glomeruli, where sensory axons terminate, were found to receive many input synapses. In deeper layers across the glomeruli, the processes participated in both input and output synapses, and at the bases of the glomeruli, the most proximal, thickest branches formed output synapses. In both of the protocerebral areas in which axonal branches terminated, those branches formed exclusively output synapses. Our findings indicate that, in addition to conveying olfactory information to the protocerebrum, uniglomerular projection neurons in the antennal lobes of M. sexta participate in local intraglomerular synaptic circuitry. J. Comp. Neurol. 379:2-20, 1997. © 1997 Wiley-Liss, Inc.     

Anatomy and fine structure of neurons in the deutocerebral projection pathway of the crayfish olfactory system.

Golgi impregnation and neurobiotin injection were used to examine details of the neural pathways in the olfactory system of the freshwater crayfish, Procambarus clarkii. Deutocerebral projection neurons (globuli cells) were directly injected with neurobiotin. These neurons have dendritic arborizations in the ipsilateral olfactory and accessory lobes, and they project axons to the lateral protocerebrum, where they terminate in microglomeruli of the hemi-ellipsoid body. The axons of the deutocerebral projection neurons are readily impregnated by Golgi procedures, and they terminate as an expanded membranous knot about 5 microns in diameter. Electron microscopy on Golgi-stained terminals has revealed that each knot makes several hundred synapses with small spine-like or shaft-like processes of postsynaptic neurons. Injection of neurobiotin into local interneurons of the hemi-ellipsoid body and subsequent examination of stained preparations with the electron microscope reveals that these cells are a major postsynaptic target of the deutocerebral projection neurons. Furthermore, the local interneurons make extensive efferent synaptic connections with unidentified neurons in the terminal medulla.     

Immunocytochemistry of dopamine in the brain of the locust Schistocerca gregaria.

Catecholamine-induced histofluorescence studies have suggested a rich innervation of the locust brain by dopamine-containing neurons. To provide a basis for future studies on dopamine action in this insect, the location and morphology of neurons reacting with antisera against dopamine were investigated in the supraoesophageal ganglion of the locust, Schistocerca gregaria. In each brain hemisphere, about 100 interneurons in the midbrain and approximately 3,000 cells in the optic lobe show dopamine-like immunoreactivity. All major areas of the brain except the calyces of the mushroom body, the antennal lobe, large parts of the lobula, and some areas in the inferior lateral protocerebrum contain immunoreactive neuronal processes. The arborization patterns of most dopamine-immunoreactive cell types could be identified through detailed reconstructions. The central body exhibits the most intense immunostaining. It is innervated by at least 40 pairs of dopamine-immunoreactive neurons belonging to three different cell types. Additional arborizations of these neurons are in the superior protocerebrum and in the lateral accessory lobes. A group of 4 immunoreactive neurons with ramifications in the antennal mechanosensory and motor center gives rise to a dense meshwork of varicose fibers in the pedunculus and parts of the alpha- and beta-lobes of the mushroom body. Other cell types innervate the ventrolateral protocerebrum, the inferior protocerebrum and the posterior optic tubercles. Three descending neurons originating in the tritocerebrum exhibit dopamine-like immunoreactivity. In the optic lobe, about 3,000 columnar intrinsic neurons of the medulla and a group of centrifugal tangential cells with arborizations in the medulla and lamina are dopamine-immunoreactive.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regeneration of olfactory afferent axons in the locust brain

The insect olfactory system consists of thousands of sensory neurons on each antenna, which project into the primary olfactory center, the glomerular antennnal lobe. There, they form synapses with local interneurons and projection neurons, which relay olfactory information to the second-order olfactory center, the mushroom body. Olfactory afferents of adult locusts (Locusta migratoria) were axotomized by crushing the base of the antenna. We studied the resulting degeneration and regeneration in the antennal lobe by size measurements, anterograde dye labeling through the antennal nerve, and immunofluorescence staining of cell surface markers. Within 3 days postcrush, the antennal lobe size was reduced by 30% and from then onward regained size back to normal by 2 weeks postinjury. Concomitantly, anterograde labeling revealed regenerating afferents reaching the antennal lobe by day 4 postcrush, and reinnervating the olfactory neuropil almost back to normal within 2 weeks. Regenerated fibers were directed precisely into the antennal lobe, where they reinnervated glomeruli. As a remarkable exception, a few regenerating fibers projected erroneously into the mushroom body on a pathway that is normally chosen by second-order projection neurons. Regenerating afferents expressed the cell surface proteins lachesin and fasciclin I. The antennal lobe neuropil expressed the cell surface marker semaphorin 1a. In conclusion, axonal regeneration in the locust olfactory system appears to be possible, precise, and fast, opening the possibility of future functional and mechanistic studies.     

Optic glomeruli and their inputs in Drosophila share an organizational ground pattern with the antennal lobes

Studying the insect visual system provides important data on the basic neural mechanisms underlying visual processing. As in vertebrates, the first step in visual processing in insects is through a series of retinotopic neurons. Recent studies on flies have found that these converge onto assemblies of columnar neurons in the lobula, the axons of which segregate to project to discrete optic glomeruli in the lateral protocerebrum. This arrangement is much like the fly's olfactory system, in which afferents target uniquely identifiable olfactory glomeruli. Here, whole-cell patch recordings show that even though visual primitives are unreliably encoded by single lobula output neurons because of high synaptic noise, they are reliably encoded by the ensemble of outputs. At a glomerulus, local interneurons reliably code visual primitives, as do projection neurons conveying information centrally from the glomerulus. These observations demonstrate that in Drosophila, as in other dipterans, optic glomeruli are involved in further reconstructing the fly's visual world. Optic glomeruli and antennal lobe glomeruli share the same ancestral anatomical and functional ground pattern, enabling reliable responses to be extracted from converging sensory inputs.     

Pheromone processing center in the protocerebrum of Bombyx mori revealed by nitric oxide-induced anti-cGMP immunocytochemistry

The antennal lobe (AL) of the male silkworm moth Bombyx mori contains 60 +/- 2 ventrally located antennal glomeruli and a dorsal macroglomerular complex (MGC) consisting of three subdivisions. The response patterns of MGC projection neurons (PNs) to pheromonal stimuli correlate with their dendritic arborization in the subdivisions of the MGC. However, the representation of this pheromonal information in the lateral protocerebrum (LPC), which is the target site of the AL PNs, is not well known. We performed nitric oxide (NO)-induced anti-cGMP immunohistochemistry and found that the PNs which respond to the major pheromone component (bombykol) express strong immunoreactivity. They project to a specific area, the delta area in the inferior lateral protocerebrum (DeltaILPC), which clearly represents the processing center for the major pheromone component. Furthermore, to examine the projection sites in the LPC from each subdivision of the MGC, we performed double-labeling of each type of MGC-PNs, combined with NO-induced anti-cGMP immunohistochemistry. We revealed that projections from each subdivision of the MGC overlapped or separated in specific regions of the DeltaILPC. These results suggest that integration and segregation of pheromone information may occur in the DeltaILPC.     

Dye fills reveal additional olfactory tracts in the protocerebrum of wild‐type Drosophila

The antennal lobe (AL) is the primary olfactory center in insect brains. It receives sensory input from the olfactory sensory neurons (OSNs) and sends, through its projection neurons (PNs), reformatted output to secondary olfactory centers, including the mushroom body (MB) calyx and the lateral horn (LH) in the protocerebrum. By injecting dye into the AL of wild‐type Drosophila, we identified previously unknown direct pathways between the AL and the ventrolateral, superior medial, and posterior lateral protocerebra. We found that most of these areas in the protocerebrum are connected with the AL through multiple tracts, suggesting that these areas are sites of convergence for olfactory information. Furthermore, areas such as the superior medial protocerebrum now appear to receive olfactory output both directly from the AL and indirectly from lobes of the MB and the LH, suggesting a degree of functional interaction among these areas. We also analyzed the length and number of fibers in each tract. We compare our results obtained from wild‐type flies with recent results from transgenic strains and discuss how information about odorants is distributed to multiple protocerebral areas. J. Comp. Neurol. 520:4131–4140, 2012. © 2012 Wiley Periodicals, Inc.     

Inventory and distribution of synapses of identified uniglomerular projection neurons in the antennal lobe of Periplaneta americana.

Uniglomerular projection neurons in the antennal lobe of Periplaneta americana, the axons of which connect the lobe to the protocerebrum, were labeled by intracellular injection of Lucifer Yellow or biocytin. The fine structure of individual neurons within the antennal lobe was examined after the injected substances had been converted (by immunohistochemical or histochemical treatment) to electron microscopically visible reaction products. Seven projection neurons were investigated, including attractant neurons, with dendritic arbors in the macroglomerulus, and projection neurons of normal-sized glomeruli. From reconstructions of thin serial sections and examination of additional processes present at various places in the arborization regions, the distribution of synapses within the glomeruli was inferred. Although the projection neurons differ from one another in their glomerular arborization patterns, they are very similar in the spatial segregation of their input and output synapses within the arborization. Output synapses are found on the thick part of the fiber near its site of entry into the glomerulus, as well as in regions within the glomerulus where the neuron has begun to ramify into thinner fibers. In the latter regions, the many output synapses are accompanied by occasional input synapses; hence these are regarded as transitional regions. At the terminal arbors only input synapses were found. This suggests that neurons with dense terminal arborizations receive particularly numerous inputs in these regions. The large number of input synapses reflects the high degree of convergence of afferents onto projection neurons previously demonstrated physiologically. However, the presence of numerous output synapses indicates that projection neurons not only transport sensory information into the protocerebrum but are also a major component of the neuronal circuitry within the antennal lobe.     

Quantitative analysis of olfactory receptor neuron projections in the antennal lobe of the malaria mosquito, Anopheles gambiae

Mosquitoes are highly dependent on the olfactory sense to find their hosts. How olfactory information concerning host odors is represented and processed in the brain to elicit olfactory guided behavior is not known. We present an exploratory analysis of central projections of olfactory receptor neurons originating from antennal and maxillary palp sensilla known to be involved in the detection of host odors in the malaria mosquito, Anopheles gambiae. We developed computational neuroanatomic methods to determine quantitatively the positions of olfactory receptor neuron terminal arborizations and compare them between brains. These quantitative analyses suggested the existence of five nonoverlapping projection zones within the antennal lobe, with one zone receiving exclusive input from maxillary palp sensilla and two zones each receiving exclusive input from trichoid or grooved-peg antennal sensilla. Projection patterns were not found to depend significantly on the odorants used during the staining procedure. The separate zones receiving input from different sensillum types seemed to represent a functional segregation because olfactory receptor neurons present in the different sensilla differed in their response profiles.     

γ-Aminobutyric acid receptor A-mediated inhibition in the honeybee's antennal lobe is necessary for the formation of configural olfactory percepts

Complex odours often possess perceptual qualities that are distinct from their components. Previous studies in humans, rodents, and insects indicate that the perception of complex odour blends depends on the concentration of the components and the mixture's complexity. However, we know relatively little about the way that an odour mixture 'gestalt' is produced by the olfactory system. Here, using an assay for olfactory conditioning in the honeybee (Apis mellifera), we examine the role of γ-aminobutyric acid receptor A (GABA(A) )-ergic inhibition within the olfactory primary relay, the antennal lobe, in the formation of a unique odour percept for complex odours. We found that honeybees perceive odour mixtures as configural stimuli when the mixtures were of low concentration and when they were composed of more than two odorants. When GABA(A) receptors were disrupted using the antagonist, picrotoxin, injected directly into the antennal lobe, we observed that bees no longer perceived the mixture as a configural stimulus. Our results imply that synchronization of antennal lobe projection neurons mediated by GABA(A) receptors is the mechanism responsible for the formation of unique olfactory percepts for complex odours.     

Higher brain centers for social tasks in worker ants, Camponotus japonicus

Ants, eusocial insects, have highly elaborate chemical communication systems using a wide variety of pheromones. In the carpenter ant, Camponotus japonicus, workers and queens have the female-specific basiconic sensilla on antennae. The antennal lobe, the primary processing center, in female carpenter ants contains about 480 glomeruli, which are divided into seven groups (T1–T7 glomeruli) based on sensory afferent tracts. The axons of sensory neurons in basiconic sensilla are thought to project to female-specific T6 glomeruli. Therefore, these sensilla and glomeruli are thought to relate to female-specific social tasks in the ants. By using dye filling into local neurons (LNs) and projection neurons (PNs) in the antennal lobe, we neuroanatomically revealed the existence of an isolated processing system for signals probably relating to social tasks in the worker ant. In the antennal lobe, two categories of glomeruli, T6 glomeruli and non-T6 glomeruli, are clearly segregated by LNs. Furthermore, axon terminals of uniglomerular PNs from the respective categories of glomeruli (T6 uni-PNs and non-T6 uni-PNs) are also segregated in the secondary olfactory centers, the calyces of the mushroom body and the lateral horn: T6 uni-PNs terminate in the outer layers of the basal ring and lip of mushroom body calyces and in the posterior region of the lateral horn, whereas non-T6 uni-PNs terminate in the middle and inner layers of the basal ring and lip and in the anterior region of the lateral horn. These findings suggest that information probably relating to social tasks might be isolated from other olfactory information and processed in a separate subsystem.     

Multimodal efferent and recurrent neurons in the medial lobes of cockroach mushroom bodies

Previous electrophysiological studies of cockroach mushroom bodies demonstrated the sensitivity of efferent neurons to multimodal stimuli. The present account describes the morphology and physiology of several types of efferent neurons with dendrites in the medial lobes. In general, efferent neurons respond to a variety of modalities in a context-specific manner, responding to specific combinations or specific sequences of multimodal stimuli. Efferent neurons that show endogenous activity have dendritic specializations that extend to laminae of Kenyon cell axons equipped with many synaptic vesicles, termed "dark" laminae. Efferent neurons that are active only during stimulation have dendritic specializations that branch mainly among Kenyon cell axons having few vesicles and forming the "pale" laminae. A new category of "recurrent" efferent neuron has been identified that provides feedback or feedforward connections between different parts of the mushroom body. Some of these neurons are immunopositive to antibodies raised against the inhibitory transmitter gamma-aminobutyric acid. Feedback pathways to the calyces arise from satellite neuropils adjacent to the medial lobes, which receive axon collaterals of efferent neurons. Efferent neurons are uniquely identifiable. Each morphological type occurs at the same location in the mushroom bodies of different individuals. Medial lobe efferent neurons terminate in the lateral protocerebrum among the endings of antennal lobe projection neurons. It is suggested that information about the sensory context of olfactory (or other) stimuli is relayed by efferent neurons to the lateral protocerebrum where it is integrated with information about odors relayed by antennal lobe projection neurons.