Descending protocerebral neurons related to the mating dance of the male silkworm moth

fruitless is sex-differentially spliced and is important for the courtship behavior and development of silkmoth Bombyx mori

Sexual dimorphisms of the brain play essential roles in successful reproduction. Silkmoth Bombyx mori exhibits extensive sexual differences in sexual behavior, as well as their morphology. Although the neural circuits that transmit information about sex pheromone in the male brain are extensively analyzed, the molecular mechanisms that regulate their development are still elusive. In the present study, we focused on the silkmoth ortholog of fruitless (fru) as a candidate gene that regulates sexual dimorphisms of the brain. fru transcripts were expressed from multiple promoters in various tissues, and brain-specific transcripts were sex-specifically spliced, in a manner similar to Drosophila. Interestingly, fru was highly expressed in the adult female brain and the male larval testis. Analysis of CRISPR/Cas9-mediated fru knockout strains revealed that fru plays important roles in survival during late larval and pupal stages, testis development, and adult sexual behavior. fru mutant males exhibited highly reduced levels of courtship and low copulation rate, indicating that fru plays significant roles in the sexual behavior of silkmoths, although it is not absolutely necessary for copulation. In the fru mutant males, sexually dimorphic pattern of the odorant receptor expression was impaired, possibly causing the defects in courtship behavior. These results provide important clues to elucidate the development of sexual dimorphisms of silkmoth brains, as well as the evolution of fruitless gene in insects.

Parallel Processing of Olfactory and Mechanosensory Information in the Honey Bee Antennal Lobe

In insects, neuronal responses to clean air have so far been reported only episodically in moths. Here we present results obtained by fast two-photon calcium imaging in the honey bee Apis mellifera, indicating a substantial involvement of the antennal lobe, the first olfactory neuropil, in the processing of mechanical stimuli. Clean air pulses generate a complex pattern of glomerular activation that provides a code for stimulus intensity and dynamics with a similar level of stereotypy as observed for the olfactory code. Overlapping the air pulses with odor stimuli reveals a superposition of mechanosensory and odor response codes with high contrast. On the mechanosensitive signal, modulations were observed in the same frequency regime as the oscillatory motion of the antennae, suggesting a possible way to detect odorless airflow directions. The transduction of mechanosensory information via the insect antennae has so far been attributed primarily to Johnston's organ in the pedicel of the antenna. The possibility that the antennal lobe activation by clean air originates from Johnston's organ could be ruled out, as the signal is suppressed by covering the surfaces of the otherwise freely moving and bending antennae, which should leave Johnston's organ unaffected. The tuning curves of individual glomeruli indicate increased sensitivity at low-frequency mechanical oscillations as produced by the abdominal motion in waggle dance communication, suggesting a further potential function of this mechanosensory code. The discovery that the olfactory system can sense both odors and mechanical stimuli has recently been made also in mammals. The results presented here give hope that studies on insects can make a fundamental contribution to the cross-taxa understanding of this dual function, as only a few thousand neurons are involved in their brains, all of which are accessible by in vivo optical imaging.

Identification and characterization of sexually dimorphic neurons that express the sex-determining gene doublesex in the brain of silkmoth Bombyx mori

Sexual differences in behavior are generated by sexually dimorphic neural circuits in animals. In insects, a highly conserved sex-determining gene doublesex (dsx) plays essential roles in the development of sexual dimorphisms. In the present study, to elucidate the neural basis of sexual differences in behaviors of silkmoth Bombyx mori, we investigated Bombyx mori dsx (Bmdsx) expression in the brains through development. In the brain, Bmdsx was differentially expressed in sex- and developmental stage-dependent manners. BmDSX protein-expressing cells were located in the dorsomedial region of the pupal and adult brains, and constituted two and one neural clusters in males and females, respectively. The number of BmDSX-positive cells was developmentally regulated and peaked at the early to middle pupal stages, suggesting that the sexually dimorphic neural circuits are established during this period. The detection of a neural activity marker protein BmHR38 suggested that the BmDSX-positive cells are not active during sexual behavior in both male and female moths, even though the cells in the vicinity of the BmDSX-positive cell clusters are active. These results imply that Bmdsx plays roles in the development of sexually dimorphic neural circuits, but the neural circuits are not related to sexual behavior in silkmoths.

Descending neurons from the lateral accessory lobe and posterior slope in the brain of the silkmoth Bombyx mori

A population of descending neurons connect the brain and thoracic motor center, playing a critical role in controlling behavior. We examined the anatomical organization of descending neurons (DNs) in the brain of the silkmoth Bombyx mori. Moth pheromone orientation is a good model to investigate neuronal mechanisms of behavior. Based on mass staining and single-cell staining, we evaluated the anatomical organization of neurite distribution by DNs in the brain. Dense innervation was observed in the posterior-ventral part of the brain called the posterior slope (PS). We analyzed the morphology of DNs innervating the lateral accessory lobe (LAL), which is considered important for moth olfactory behavior. We observed that all LAL DNs also innervate the PS, suggesting the integration of signals from the LAL and PS. We also identified a set of DNs innervating the PS but not the LAL. These DNs were sensitive to the sex pheromone, suggesting a role of the PS in motor control for pheromone processing. Here we discuss the organization of descending pathways for pheromone orientation.

A novel method for full locomotion compensation of an untethered walking insect

In this study, we developed a novel unfixed-type experimental system that we call a '3-DOF servosphere.' This system comprises one sphere and three omniwheels that support the sphere. The measurement method is very simple. An experimental animal is placed on top of the sphere. The position and heading angle of the animal are observed by using a high-speed camera installed above the sphere. Because the system can rotate the sphere with three degrees of freedom (DOFs) independently, the position and heading angle at the origin can be maintained without fixing the body. This system can be used to measure an animal's natural behavior while simultaneously providing it with precise stimuli. Moreover, electrodes can be inserted at specific sites to measure biosignals with locomotion. Therefore, this system can simultaneously measure the stimulus input-internal state-locomotion output of an animal. In this study, we focused on the chemical plume tracing (CPT) behavior of the Bombyx mori male silkworm moth in order to identify its CPT algorithm for mounting on a robot. In an experiment, we simultaneously measured the stimulus input, flight muscle electromyogram (EMG), and CPT behavior by using the 3-DOF servosphere to verify the system. We elucidated the relationship between the CPT behavior and flight muscle EMG.

Establishment of tools for neurogenetic analysis of sexual behavior in the silkmoth, Bombyx mori

Background

Silkmoth, Bombyx mori, is an ideal model insect for investigating the neural mechanisms underlying sex pheromone-induced innate behavior. Although transgenic techniques and the GAL4/UAS system are well established in the silkmoth, genetic tools useful for investigating brain function at the neural circuit level have been lacking.

Results

In the present study, we established silkmoth strains in which we could visualize neural projections (UAS-mCD8GFP) and cell nucleus positions (UAS-GFP.nls), and manipulate neural excitability by thermal stimulation (UAS-dTrpA1). In these strains, neural projections and nucleus position were reliably labeled with green fluorescent protein in a GAL4-dependent manner. Further, the behavior of silkworm larvae and adults could be controlled by GAL4-dependent misexpression of dTrpA1. Ubiquitous dTrpA1 misexpression led both silkmoth larvae and adults to exhibit seizure-like phenotypes in a heat stimulation-dependent manner. Furthermore, dTrpA1 misexpression in the sex pheromone receptor neurons of male silkmoths allowed us to control male sexual behavior by changing the temperature. Thermally stimulated male silkmoths exhibited full sexual behavior, including wing-flapping, orientation, and attempted copulation, and precisely approached a thermal source in a manner similar to male silkmoths stimulated with the sex pheromone.

Conclusion

These findings indicate that a thermogenetic approach using dTrpA1 is feasible in Lepidopteran insects and thermogenetic analysis of innate behavior is applicable in the silkmoth. These tools are essential for elucidating the relationships between neural circuits and function using neurogenetic methods.

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

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

Visualization of neural activity in insect brains using a conserved immediate early gene, Hr38

Many insects exhibit stereotypic instinctive behavior [1-3], but the underlying neural mechanisms are not well understood due to difficulties in detecting brain activity in freely moving animals. Immediate early genes (IEGs), such as c-fos, whose expression is transiently and rapidly upregulated upon neural activity, are powerful tools for detecting behavior-related neural activity in vertebrates [4, 5]. In insects, however, this powerful approach has not been realized because no conserved IEGs have been identified. Here, we identified Hr38 as a novel IEG that is transiently expressed in the male silkmoth Bombyx mori by female odor stimulation. Using Hr38 expression as an indicator of neural activity, we mapped comprehensive activity patterns of the silkmoth brain in response to female sex pheromones. We found that Hr38 can also be used as a neural activity marker in the fly Drosophila melanogaster. Using Hr38, we constructed a neural activity map of the fly brain that partially overlaps with fruitless (fru)-expressing neurons in response to female stimulation. These findings indicate that Hr38 is a novel and conserved insect neural activity marker gene that will be useful for a wide variety of neuroethologic studies.

Descending brain neurons in the cricket Gryllus bimaculatus (de Geer): auditory responses and impact on walking

The activity of four types of sound-sensitive descending brain neurons in the cricket Gryllus bimaculatus was recorded intracellularly while animals were standing or walking on an open-loop trackball system. In a neuron with a contralaterally descending axon, the male calling song elicited responses that copied the pulse pattern of the song during standing and walking. The accuracy of pulse copying increased during walking. Neurons with ipsilaterally descending axons responded weakly to sound only during standing. The responses were mainly to the first pulse of each chirp, whereas the complete pulse pattern of a chirp was not copied. During walking the auditory responses were suppressed in these neurons. The spiking activity of all four neuron types was significantly correlated to forward walking velocity, indicating their relevance for walking. Additionally, injection of depolarizing current elicited walking and/or steering in three of four neuron types described. In none of the neurons was the spiking activity both sufficient and necessary to elicit and maintain walking behaviour. Some neurons showed arborisations in the lateral accessory lobes, pointing to the relevance of this brain region for cricket audition and descending motor control.

Sex-linked transcription factor involved in a shift of sex-pheromone preference in the silkmoth Bombyx mori

In the sex-pheromone communication systems of moths, odorant receptor (Or) specificity as well as higher olfactory information processing in males should be finely tuned to the pheromone of conspecific females. Accordingly, male sex-pheromone preference should have diversified along with the diversification of female sex pheromones; however, the genetic mechanisms that facilitated the diversification of male preference are not well understood. Here, we explored the mechanisms involved in a drastic shift in sex-pheromone preference in the silkmoth Bombyx mori using spli mutants in which the genomic structure of the gene Bmacj6, which encodes a class IV POU domain transcription factor, is disrupted or its expression is repressed. B. mori females secrete an ∼11:1 mixture of bombykol and bombykal. Bombykol alone elicits full male courtship behavior, whereas bombykal alone shows no apparent activity. In the spli mutants, the behavioral responsiveness of males to bombykol was markedly reduced, whereas bombykal alone evoked full courtship behavior. The reduced response of spli males to bombykol was explained by the paucity of bombykol receptors on the male antennae. It was also found that, in the spli males, neurons projecting into the toroid, a compartment in the brain where bombykol receptor neurons normally project, responded strongly to bombykal. The present study highlights a POU domain transcription factor, Bmacj6, which may have caused a shift of sex-pheromone preference in B. mori through Or gene choice and/or axon targeting.

Identification and expression analysis of nervous wreck, which is preferentially expressed in the brain of the male silkworm moth, Bombyx mori

Sexually dimorphic neural circuits are essential for reproductive behaviour. The molecular basis of sexual dimorphism in the silkworm moth (Bombyx mori) brain, however, is unclear. We conducted cDNA subtraction screening and identified nervous wreck (Bmnwk), a synaptic growth regulatory gene, whose expression is higher in the male brain than in the female brain of the silkworm. Bmnwk was preferentially expressed in the brain at the late pupae and adult stages. In situ hybridization revealed that Bmnwk is highly expressed in the optic lobe of the male moth brain. These findings suggest that Bmnwk has a role in the development and/or maintenance of the optic lobe in the male silkworm brain.

Processing of species-specific auditory patterns in the cricket brain by ascending, local, and descending neurons during standing and walking

The recognition of the male calling song is essential for phonotaxis in female crickets. We investigated the responses toward different models of song patterns by ascending, local, and descending neurons in the brain of standing and walking crickets. We describe results for two ascending, three local, and two descending interneurons. Characteristic dendritic and axonal arborizations of the local and descending neurons indicate a flow of auditory information from the ascending interneurons toward the lateral accessory lobes and point toward the relevance of this brain region for cricket phonotaxis. Two aspects of auditory processing were studied: the tuning of interneuron activity to pulse repetition rate and the precision of pattern copying. Whereas ascending neurons exhibited weak, low-pass properties, local neurons showed both low- and band-pass properties, and descending neurons represented clear band-pass filters. Accurate copying of single pulses was found at all three levels of the auditory pathway. Animals were walking on a trackball, which allowed an assessment of the effect that walking has on auditory processing. During walking, all neurons were additionally activated, and in most neurons, the spike rate was correlated to walking velocity. The number of spikes elicited by a chirp increased with walking only in ascending neurons, whereas the peak instantaneous spike rate of the auditory responses increased on all levels of the processing pathway. Extra spiking activity resulted in a somewhat degraded copying of the pulse pattern in most neurons.

Neurons associated with the flip-flop activity in the lateral accessory lobe and ventral protocerebrum of the silkworm moth brain

The lateral accessory lobe (LAL) and the ventral protocerebrum (VPC) are a pair of symmetrical neural structures in the insect brain. The LAL-VPC is regarded as the major target of olfactory responding neurons as well as the control center for olfactory-evoked sequential zigzag turns. Previous studies of the silkworm moth Bombyx mori showed that these turns are controlled by long-lasting anti-phasic activities of the flip-flopping descending neurons with dendrites in the LAL-VPC. To elucidate the neural mechanisms underlying the generation of this alternating activity between the LAL-VPC units of both hemispheres, we first analyzed the detailed neural architecture of the LAL-VPC and identified five subregions. We then investigated the morphology and physiological responses of the LAL-VPC neurons by intracellular recording and staining and morphologically identified three types of bilateral neurons and three types of unilateral neurons. Bilateral neurons showed either brief or cyclic long-lasting responses. At least some neurons of the latter type produced gamma-aminobutyric acid (GABA). Unilateral neurons linking the LAL and VPC, in contrast, showed long-lasting or quick alternating activity. Timing analysis of the activity onset of each neural type suggests that quick reciprocal neural transmission between unilateral neurons would be responsible for the generation of long-lasting activity in one LAL-VPC unit, which lasts for up to a few seconds. Reciprocal inhibition and excitation by the bilateral neurons with long-lasting activities would mediate the alternating long-lasting activity between both LAL-VPC units, which might last for up to 20 seconds.

Spatio-temporal activities in the antennal lobe analyzed by an optical recording method in the male silkworm moth Bombyx mori

Optical recordings with a voltage-sensitive dye showed that the spatio-temporal pattern of depolarizing responses evoked by electrical stimulation of antennal nerve (AN) was non-homologously distributed in the antennal lobe (AL) of the male silkworm moth, Bombyx mori. Time courses of postsynaptic activities and GABAergic inhibitory potentials of AL neurons were individually demonstrated by pharmacological experiments, i.e. Ca2+ free and bicuculline conditions. GABAergic inhibitory potentials began with a ca. 3 ms delay from the beginning of the postsynaptic activities. Intensity of the postsynaptic activities and GABAergic inhibitory potentials were non-homologously distributed in the AL. Relatively strong postsynaptic activities and GABAergic inhibitory potentials were consistently observed in some parts of the macroglomerular complex (MGC) and/or in some ordinary glomeruli (Gs) in the medial and ventral part of the AL.

Long-lasting excitation of protocerebral bilateral neurons in the pheromone-processing pathways of the male moth Bombyx mori

Intracellular recording and staining with Lucifer yellow were used to characterize the responses and structure of pheromone-processing bilateral neurons in the protocerebrum of the brain of the male silkworm moth Bombyx mori. Numerous olfactory bilateral neurons innervated a particular neuropil region lateral to the central body, the lateral accessory lobe (LAL). The LALs are linked to each other by bilateral neurons with arborizations in each LAL. The LAL appears to be important for collecting the olfactory information from both sides of the brain. Many of the bilateral neurons showed a characteristic long-lasting excitation (LLE) that outlasted the olfactory stimuli (1.5 s). In some preparations, the LLE lasted more than 20 s and the firing gradually decreased to the background level.

Physiology and morphology of protocerebral olfactory neurons in the male moth Manduca sexta

1. We have used intracellular recording and staining with Lucifer Yellow, followed by reconstruction from serial sections, to characterize the responses and structure of olfactory neurons in the protocerebrum (PC) of the brain of the male sphinx moth Manduca sexta. 2. Many olfactory protocerebral neurons (PCNs) innervate a particular neuropil region lateral to the central body, the lateral accessory lobe (LAL), which appears to be important for processing olfactory information. 3. Each LAL is linked by its constituent neurons to the ipsilateral lateral PC, where projection neurons from the antennal lobe terminate, as well as to other regions of the PC. The LALs are also linked to each other by bilateral neurons with arborizations in each LAL. 4. Some PC neurons showed long-lasting excitation (LLE) that outlasted the olfactory stimuli by greater than or equal to 1 s, and as long as 30 s in some preparations. LLE was more frequently elicited by the sex-pheromone blend than by individual pheromone components. All bilateral neurons that showed LLE had arborizations in the LALs. LLE responses were also recorded in a single local neuron innervating the mushroom body. 5. In some other PC neurons, pheromonal stimuli elicited brief excitations that recovered to background firing rates less than 1 s after stimulation.

Descending neurons supplying the neck and flight motor of Diptera: physiological and anatomical characteristics

In Diptera, dorsal neuropils of the pro-, meso-, and metathoracic ganglia supply motor neurons to neck and flight muscles. Motor circuits are supplied by more than 50 pairs of descending neurons (DNs) whose dendritic trees in the brain are restricted to dorsal neuropils of the deutocerebrum where they are grouped together into discrete clusters. Each cluster is visited by wide-field motion-sensitive neurons and by morphologically small-field retinotopic elements. This organization suggests that flight descending neurons should respond to complex stimuli reflecting panoramic movement and small-field motion. Intracellular recordings, combined with dye filling, confirm this. Certain descending neurons responding to visual flow fields terminate bilaterally in superficial pterothoracic neuropils, at the level of indirect (power) flight muscle motor neurons. Other DNs terminate laterally, and provide segmental collaterals to areas containing neck and direct (steering) flight muscle motor neurons. Such DNs are activated by wide-field directional stimuli corresponding to pitch, roll, or yaw, and to small-field stimuli. Appropriate directional mechanosensory stimuli also activate dorsal descending neurons. The significance of dorsal descending neurons for the control of flight is discussed and compared with studies on course deviation neurons in other insects. It is suggested that, in Diptera, dorsal descending neurons may separately be involved in the control of velocity, stabilization, and steering manoeuvres.

Olfactory interneurons in the brain of the larval sphinx moth Manduca sexta

1. The physiology and morphology of olfactory interneurons in the brain of larval Manduca sexta were studied using intracellular recording and staining techniques. Antennal olfactory receptors were stimulated with volatile substances from plants and with pure odorants. Neurons responding to the stimuli were investigated further to reveal their response specificities, dose-response characteristics, and morphology. 2. We found no evidence of specific 'labeled-lines' among the odor-responsive interneurons, as none responded exclusively to one plant odor or pure odorant; most olfactory interneurons were broadly tuned in their response spectra. This finding is consistent with an 'across-fiber' pattern of odor coding. 3. Mechanosensory and olfactory information are integrated at early stages of central processing, appearing in the responses of some local interneurons restricted to the primary olfactory nucleus in the brain, the larval antennal center (LAC). 4. The responses of LAC projection neurons and higher-order protocerebral interneurons to a given odor were more consistent than the responses of LAC local interneurons. 5. The LAC appears to be functionally subdivided, as both local and projection neurons had arborizations in specific parts of the LAC, but none had dendrites throughout the LAC. 6. The mushroom bodies and the lateral protocerebrum contain neurons that respond to olfactory stimulation.

Physiology and morphology of projection neurons in the antennal lobe of the male moth Manduca sexta

1. We have used intracellular recording and staining, followed by reconstruction from serial sections, to characterize the responses and structure of projection neurons (PNs) that link the antennal lobe (AL) to other regions of the brain of the male sphinx moth Manduca sexta. 2. Dendritic arborizations of the AL PNs were usually restricted either to ordinary glomeruli or to the male-specific macroglomerular complex (MGC) within the AL neuropil. Dendritic fields in the MGC appeared to belong to distinct partitions within the MGC. PNs innervating the ordinary glomeruli had arborizations in a single glomerulus (uniglomerular) or in more than one ordinary glomerulus of one AL (multiglomerular) or in one case, in single glomeruli in both ALs (bilateral-uniglomerular). One PN innervated the MGC and many or all ordinary glomeruli of the AL. 3. PNs with dendritic arborizations in the ordinary glomeruli and PNs associated with the MGC typically projected both to the calyces of the ipsilateral mushroom body and to the lateral protocerebrum, but some differences in the patterns of termination in those regions have been noted for the two classes of PNs. One PN conspicuously lacked branches in the calyces but did project to the lateral protocerebrum. The PN innervating the MGC and many ordinary glomeruli projected to the calyces of the ipsilateral mushroom body and the superior protocerebrum. 4. Crude sex-pheromone extracts excited all neurons with arborizations in the MGC, although some were inhibited by other odors. One P(MGC) was excited by crude sex-pheromone extract and by a mimic of one component of the pheromone blend but was inhibited by another component of the blend. 5. PNs with dendritic arborizations in ordinary glomeruli were excited or inhibited by certain non-pheromonal odors. Some of these PNs also responded to mechanosensory stimulation of the antennae. 6. The PN with dendritic arborizations in the MGC and many ordinary glomeruli was excited by crude sex-pheromone extracts and non-pheromonal odors and also responded to mechanosensory stimulation of the antenna.

Anatomy of antenno-cerebral pathways in the brain of the sphinx moth Manduca sexta

In the moth Manduca sexta, the number and morphology of neuronal connections between the antennal lobes and the protocerebrum were examined. Cobalt injections revealed eight morphological types of neurons with somata adjacent to the AL neuropil that project in the inner, middle, and outer antenno-cerebral tracts to the proto-cerebrum. Neurons innervating the macroglomerular complex and many neurons with fibers in the inner antenno-cerebral tract have uniglomerular antennal-lobe arborizations. Most neurons in the middle and outer antenno-cerebral tracts, on the other hand, seem to innervate more than one glomerulus. Protocerebral areas receiving direct input from the antennal lobe include the calyces of the mushroom bodies, and circumscribed areas termed "olfactory foci" in the lateral horn of the protocerebrum and several other regions, especially areas in close proximity to the mushroom bodies. Fibers in the inner antenno-cerebral tract that innervate the male-specific macroglomerular complex have arborizations in the protocerebrum that are distinct from the projections of sexually non-specific neurons. Protocerebral neurons projecting into the antennal lobe are much less numerous than antennal-lobe output cells. Most of these protocerebral fibers enter the antennal lobe in small fiber tracts that are different from those described above. In the protocerebrum, these centrifugal cells arborize in olfactory foci and also in the inferior median protocerebrum and the lateral accessory lobes. The morphological diversity of connections between the antennal lobes and the protocerebrum, described here for the first time on a single-cell level, suggests a much greater physiological complexity of the olfactory system than has been assumed so far.

Male-specific, sex pheromone-selective projection neurons in the antennal lobes of the moth Manduca sexta

A subset of olfactory projection neurons in the brain of male Manduca sexta is described, and their role in sex pheromone information processing is examined. These neurons have extensive arborizations in the macroglomerular complex (MGC), a distinctive and sexually dimorphic area of neuropil in the antennal lobe (AL), to which the axons of two known classes of antennal pheromone receptors project. Each projection neuron sends an axon from the AL into the protocerebrum. Forty-one projection neurons were characterized according to their responses to electrical stimulation of the antennal nerve as well as olfactory stimulation of antennal receptors. All neurons exhibited strong selectivity for female sex pheromones. Other behaviorally relevant odors, such as plant volatiles, had no obvious effect on the activity of these neurons. Two broad physiological categories were found: cells that were excited by stimulation of the ipsilateral antenna with pheromones (29 out of 41), and cells that received a mixed input (inhibition and excitation) from pheromone pathways (12 out of 41). Of the cells in the first category, 13 out of 29 were equally excited in response to stimulation of the antenna with either the principal natural pheromone (bombykal) or a mimic of a second unidentified pheromone ('C-15') and were similarly excited by the natural pheromone blend. The remaining 16 out of 29 cells responded selectively, and in some cases, in a dose-dependent manner, to stimulation of the antenna with bombykal or C-15, but not both. Some of these neurons had dendritic arborizations restricted to only a portion of the MGC neuropil, whereas most had arborizations throughout the MGC. Of the cells in the second category, 9 out of 12 were excited by bombykal, inhibited by C-15, and showed a mixed response to the natural pheromone blend. For the other 3 out of 12 cells, the response polarity was reversed for the two chemically-identified odors. Two additional neurons, which were not tested with olfactory stimuli, were tonically inhibited in response to electrical stimulation of the ipsilateral antennal nerve. These observations suggest that some of the male-specific projection neurons may signal general pheromone-triggered arousal, whereas a smaller number can actively integrate inputs from the two know receptor classes (Bal- and C-15-selective) and may operate as 'mixture detectors' at this level of the olfactory subsystem that processes information about sex pheromones.