Effect of auditory deafferentation on the synaptic connectivity of a pair of identified interneurons in adult field crickets

Transcriptional expression changes during compensatory plasticity in the terminal ganglion of the adult cricket Gryllus bimaculatus

BACKGROUND

Damage to the adult central nervous system often leads to long-term disruptions in function due to the limited capacity for neurological recovery. The central nervous system of the Mediterranean field cricket, Gryllus bimaculatus, shows an unusual capacity for compensatory plasticity, most obviously in the auditory system and the cercal escape system. In both systems, unilateral sensory disruption leads the central circuitry to compensate by forming and/or strengthening connections with the contralateral sensory organ. While this compensatory plasticity in the auditory system relies on robust dendritic sprouting and novel synapse formation, the compensatory plasticity in the cercal escape circuitry shows little obvious dendritic sprouting and instead may rely on shifts in excitatory and inhibitory synaptic strength.

RESULTS

In order to better understand what types of molecular pathways might underlie this compensatory shift in the cercal system, we used a multiple k-mer approach to assemble a terminal ganglion transcriptome that included ganglia collected one, three, and 7 days after unilateral cercal ablation in adult, male animals. We performed differential expression analysis using EdgeR and DESeq2 and examined Gene Ontologies to identify candidates potentially involved in this plasticity. Enriched GO terms included those related to the ubiquitin-proteosome protein degradation system, chromatin-mediated transcriptional pathways, and the GTPase-related signaling system.

CONCLUSION

Further exploration of these GO terms will provide a clearer picture of the processes involved in compensatory recovery of the cercal escape system in the cricket and can be compared and contrasted with the distinct pathways that have been identified upon deafferentation of the auditory system in this same animal.

De novo assembly of a transcriptome for the cricket Gryllus bimaculatus prothoracic ganglion: An invertebrate model for investigating adult central nervous system compensatory plasticity

The auditory system of the cricket, Gryllus bimaculatus, demonstrates an unusual amount of anatomical plasticity in response to injury, even in adults. Unilateral removal of the ear causes deafferented auditory neurons in the prothoracic ganglion to sprout dendrites across the midline, a boundary they typically respect, and become synaptically connected to the auditory afferents of the contralateral ear. The molecular basis of this sprouting and novel synaptogenesis in the adult is not understood. We hypothesize that well-conserved developmental guidance cues may recapitulate their guidance functions in the adult in order to facilitate this compensatory growth. As a first step in testing this hypothesis, we have generated a de novo assembly of a prothoracic ganglion transcriptome derived from control and deafferented adult individuals. We have mined this transcriptome for orthologues of guidance molecules from four well-conserved signaling families: Slit, Netrin, Ephrin, and Semaphorin. Here we report that transcripts encoding putative orthologues of most of the candidate developmental ligands and receptors from these signaling families were present in the assembly, indicating expression in the adult G. bimaculatus prothoracic ganglion.

Quantification of dendritic and axonal growth after injury to the auditory system of the adult cricket Gryllus bimaculatus

Dendrite and axon growth and branching during development are regulated by a complex set of intracellular and external signals. However, the cues that maintain or influence adult neuronal morphology are less well understood. Injury and deafferentation tend to have negative effects on adult nervous systems. An interesting example of injury-induced compensatory growth is seen in the cricket, Gryllus bimaculatus. After unilateral loss of an ear in the adult cricket, auditory neurons within the central nervous system (CNS) sprout to compensate for the injury. Specifically, after being deafferented, ascending neurons (AN-1 and AN-2) send dendrites across the midline of the prothoracic ganglion where they receive input from auditory afferents that project through the contralateral auditory nerve (N5). Deafferentation also triggers contralateral N5 axonal growth. In this study, we quantified AN dendritic and N5 axonal growth at 30 h, as well as at 3, 5, 7, 14, and 20 days after deafferentation in adult crickets. Significant differences in the rates of dendritic growth between males and females were noted. In females, dendritic growth rates were non-linear; a rapid burst of dendritic extension in the first few days was followed by a plateau reached at 3 days after deafferentation. In males, however, dendritic growth rates were linear, with dendrites growing steadily over time and reaching lengths, on average, twice as long as in females. On the other hand, rates of N5 axonal growth showed no significant sexual dimorphism and were linear. Within each animal, the growth rates of dendrites and axons were not correlated, indicating that independent factors likely influence dendritic and axonal growth in response to injury in this system. Our findings provide a basis for future study of the cellular features that allow differing dendrite and axon growth patterns as well as sexually dimorphic dendritic growth in response to deafferentation.

Developmental and activity-dependent plasticity of filiform hair receptors in the locust

A group of wind sensitive filiform hair receptors on the locust thorax and head makes contact onto a pair of identified interneuron, A4I1. The hair receptors' central nervous projections exhibit pronounced structural dynamics during nymphal development, for example, by gradually eliminating their ipsilateral dendritic field while maintaining the contralateral one. These changes are dependent not only on hormones controlling development but on neuronal activity as well. The hair-to-interneuron system has remarkably high gain (close to 1) and makes contact to flight steering muscles. During stationary flight in front of a wind tunnel, interneuron A4I1 is active in the wing beat rhythm, and in addition it responds strongly to stimulation of sensory hairs in its receptive field. A role of the hair-to-interneuron in flight steering is thus suggested. This system appears suitable for further study of developmental and activity-dependent plasticity in a sensorimotor context with known connectivity patterns.

Bilateral consequences of chronic unilateral deafferentation in the auditory system of the cricket Gryllus bimaculatus

The auditory system of the cricket has the unusual ability to respond to deafferentation by compensatory growth and synapse formation. Auditory interneurons such as ascending neuron 2 (AN-2) in the cricket Gryllus bimaculatus possess a dendritic arbor that normally grows up to, but not over, the midline of the prothoracic ganglion. After chronic deafferentation throughout larval development, however, the AN-2 dendritic arbor changes dramatically, and medial dendrites sprout across the midline where they form compensatory synapses with the auditory afferents from the contralateral ear. We quantified the extent of the effects of chronic, unilateral deafferentation by measuring several cellular parameters of 3 different neuronal components of the auditory system: the deafferented AN-2, the contralateral (or nondeafferented) AN-2 and the contralateral auditory afferents. Neuronal tracers and confocal microscopy were used to visualize neurons, and double-label experiments were performed to examine the cellular relationship between pairs of cells. Dendritic complexity was quantified using a modified Sholl analysis, and the length and volume of processes and presynaptic varicosities were assessed under control and deafferented conditions. Chronic deafferentation significantly influenced the morphology of all 3 neuronal components examined. The overall dendritic complexity of the deafferented AN-2 dendritic arbor was reduced, while both the contralateral AN-2 dendritic arbor and the remaining, intact, auditory afferents grew longer. We found no significant changes in the volume or density of varicosities after deafferentation. These complex cellular changes after deafferentation are interpreted in the light of the reported differential regulation of vesicle-associated membrane protein and semaphorin 2a.

Contralateral Deafferentation Does Not Affect Regeneration Processes in the Auditory System of Schistocerca gregaria (Orthoptera)

The auditory system of locusts has high regeneration capacity following injury of the peripheral afferents. Regenerating auditory afferents can re-innervate their target areas even after changed neuronal pathways. Here, possible influences of contralateral deafferentation on regenerating afferents were investigated. Contralateral deafferentation was performed at different stages of the regeneration. Regeneration was triggered by crushing the tympanal nerve. The regenerated fibers showed aberrant fiber outgrowth, reduced density of terminations in the target area, the auditory neuropile and collateral sprouts crossing the midline. However, these results were not significantly influenced by the contralateral deafferentation. Therefore the bilateral symmetrical systems seem to be largely independent from each other.

Morphological and physiological regeneration in the auditory system of adult Mecopoda elongata (Orthoptera: Tettigoniidae)

Orthopterans are suitable model organisms for investigations of regeneration mechanisms in the auditory system. Regeneration has been described in the auditory systems of locusts (Caelifera) and of crickets (Ensifera). In this study, we comparatively investigate the neural regeneration in the auditory system in the bush cricket Mecopoda elongata. A crushing of the tympanal nerve in the foreleg of M. elongata results in a loss of auditory information transfer. Physiological recordings of the tympanal nerve suggest outgrowing fibers 5 days after crushing. An anatomical regeneration of the fibers within the central nervous system starts 10 days after crushing. The neuronal projection reaches the target area at day 20. Threshold values to low frequency airborne sound remain high after crushing, indicating a lower regeneration capability of this group of fibers. However, within the central target area the low frequency areas are also innervated. Recordings of auditory interneurons show that the regenerating fibers form new functional connections starting at day 20 after crushing.

Differential gene expression during compensatory sprouting of dendrites in the auditory system of the cricket Gryllus bimaculatus

Neurones that lose their presynaptic partners because of injury usually retract or die. However, when the auditory interneurones of the cricket Gryllus bimaculatus are denervated, dendrites respond by growing across the midline and forming novel synapses with the opposite auditory afferents. Suppression subtractive hybridization was used to detect transcriptional changes 3 days after denervation. This is a stage at which we demonstrate robust compensatory dendritic sprouting. Whereas 49 unique candidates were down-regulated, no sufficiently up-regulated candidates were identified at this time point. Several candidates identified in this study are known to influence the translation and degradation of proteins in other systems. The potential role of these factors in the compensatory sprouting of cricket auditory interneurones in response to denervation is discussed.

Long-term neuromodulatory regulation of a motor pattern-generating network: maintenance of synaptic efficacy and oscillatory properties

Rhythm generation by the pyloric motor network in the stomatogastric ganglion (STG) of the spiny lobster requires permissive neuromodulatory inputs from other central ganglia. When these inputs to the STG are suppressed by cutting the single, mainly afferent stomatogastric nerve (stn), pyloric neurons cease to burst and the network falls silent. However, as shown previously, if such a decentralized quiescent ganglion is maintained in organ culture, pyloric network rhythmicity returns after 3-4 days and, although slower, is similar to the motor pattern expressed when the stn is intact. Here we use current- and voltage-clamp, primarily of identified pyloric dilator (PD) neurons, to investigate changes in synaptic and cellular properties that underlie this transition in network behavior. Although the efficacy of chemical synapses between pyloric neurons decreases significantly (by <or=50%) after STG decentralization, the fundamental change leading to rhythm recovery occurs in the voltage-dependent properties of the neurons themselves. Whereas pyloric neurons, including the PD, lateral pyloric, and pyloric cell types, are unable to generate burst-producing membrane potential oscillations in the short-term absence of extrinsic modulatory inputs, in long-term decentralized ganglia, the same cells are able to oscillate spontaneously, even after experimental isolation in situ from all other elements in the pyloric network. In PD neurons this reacquisition of rhythmicity is associated with a net reduction in outward tetraethylammonium-sensitive ionic currents that include a delayed-rectifier type potassium current (I(Kd)) and a calcium-dependent K(+) current, I(KCa). By contrast, long-term STG decentralization caused enhancement of a hyperpolarization-activated inward current that resembles I(h). These results are consistent with the hypothesis that modulatory inputs sustain the modulation-dependent rhythmogenic character of the pyloric network by continuously regulating the balance of membrane conductances that underlie neuronal oscillation.

Auditory symmetry analysis

The study of biological symmetry continues to be an important and active area of research, yet in the hearing sciences there are no established quantitative methods for measuring auditory asymmetries and dissimilarities in threshold tuning curves (i.e. audiograms). Using a paired design and adopting methods from the analysis of fluctuating asymmetry, we describe methods for auditory researchers interested in delineating auditory asymmetries and comparing tuning curves, behavioral or neural. We illustrate the methods using audiograms of the prothoracic T-cell interneuron in a nocturnal katydid (Neoconocephalus ensiger). The results show that 87–92 % of T-cells had right-minus-left threshold asymmetries no larger than expected from measurement error alone. Thus, apart from small random fluctuating asymmetries, T-cell pairs in N. ensiger showed no sensory bias and were bilaterally symmetrical from 5 to 100 kHz. The sensitivity of the methods for detecting tuning curve dissimilarities was confirmed in a sound lateralization paradigm by comparing the ‘symmetry’ (i.e. similarity) of T-cell tuning curves measured at 0 degrees stimulation with tuning curves measured at 90 degrees stimulation for the same T-cell. The results show that T-cell thresholds measured frontally (0 degrees) were significantly higher than those measured laterally (90 degrees), particularly for ultrasonic frequencies. Statistically, the directional shift (increase) in auditory thresholds was detected as a directional asymmetry in T-cell tuning, whose origin and functional significance to an insect behaving normally are discussed. The paper discusses practical considerations for detecting auditory asymmetries and tuning curve dissimilarities in general, and closes by questioning the relevance of auditory symmetry for sound localization in both vertebrates and insects.

Septal cholinergic deafferentation of the dentate gyrus results in a loss of a subset of neuropeptide Y somata and an increase in synaptic area on remaining neuropeptide Y dendrites

Removal of cholinergic septal inputs using the immunotoxin 192 IgG-saporin reduces the number of interneurons containing neuropeptide Y (NPY)-immunoreactivity in the rat dentate gyrus by approximately 30% [Milner et al., J. Comp. Neurol. 386 (1997) 48-59]. The goal of the present study was to determine if NPY-containing neurons that survive deafferentation have any distinguishing morphological and/or microenvironmental features. For this, 2 or 24 weeks after intracerebroventricular injections of 192 IgG-saporin, NPY-immunolabeled neurons in the hilus of the dentate gyrus were examined by electron microscopy. Neither the size nor morphological traits of NPY-labeled perikaryal or dendritic profiles from lesioned compared to control rats at either time-point differed significantly. However, at both time-points, NPY-containing somatal profiles from immunolesioned rats compared to controls had a reduced percentage of their plasmalemmal surface apposed to unmyelinated axon profiles and an increased percentage of their surface occupied by astrocytic profiles. At the 24 week time-point, these differences were statistically significant. The primary contributing factor for these changes was the absence of a subgroup of NPY-labeled somatal profiles in lesioned rats compared to controls which was: (a) distinguished by frequent appositions of unmyelinated axons (from 15 to 35%) to the plasmalemmal surface; and (b) located primarily in the central hilar region. Unlike NPY-containing somata, changes associated with NPY-labeled dendritic profiles were exclusively related to associated presynaptic profiles at the 24 week time-point. In lesioned rats compared to controls at this time-point, NPY-containing dendritic profiles had a concurrent increase in the percentage of the plasmalemmal surface occupied by active zones and the size of terminals contacting them. The present results combined with those of our earlier study suggest that septal cholinergic deafferentation results in: (a) the loss of a distinct subpopulation of hippocampal NPY-containing neurons; and (b) an increase in total active zone area suggesting a strengthening of synaptic connections to the surviving population of NPY-containing neurons in the long term.

Neuronal supernumerary and dendritic sprouting of the nucleus ambiguus after chronic alteration of peripheral targets in cats

Anatomic changes of neuronal profiles in response to chronic alteration of peripheral targets were investigated in the nucleus ambiguus (NA) of cats. Unilateral vagal-hypoglossal nerve anastomosis was performed by suturing the transected proximal stump of the vagus nerve to the transected distal stump of the hypoglossal nerve. After comparing horseradish peroxidase (HRP)-labeled neurons on the ipsilateral operated side of the NA with the contralateral unoperated NA and the NA following transection and reuniting to the vagus itself, a remarkable ramification and elongation of the dendritic trees was observed in the HRP-positive neurons on the ipsilateral NA. Quantitative analysis of neuronal profiles revealed that the number of the medium and large neurons on the ipsilateral NA was greater than the contralateral NA and the NA following autologous suturing of the vagus. Comparisons of variable dendritic lengths of the medium and large neurons on the ipsilateral NA revealed longer distances and more branches of the tertiary and perisomatic dendrites than those of the contralateral NA and the NA ipsilateral to autologous reunion. Our results suggest that remarkable sprouting and elongation of the dendritic trees as well as cell supernumerary occurred in the dominant NA motoneurons ipsilateral to the nerve anastomosis. In conclusion, there is a trophic influence in the tongue musculature, which was retrogradely transported to the NA neurons via the regenerating axons and caused the morphological changes in the NA in response to the rerouting of efferents from the vagus nerve to the hypoglossal nerve to innervate intimate tongue musculature.

Removing antennae and maxillae has little effect on feeding on normal host plants by two species of caterpillar

Models of feeding regulation postulate that chemosensory information from available food both initiates and maintains feeding. However, we find that removing antennae and maxillae (AM) from Manduca sexta and Diacrisia virginica larvae has little effect on amounts eaten, patterns of feeding, and the microstructure (each bite and pause) of feeding, as quickly as two days after the operation. However, there was a small change in the microstructure of feeding of AM D. virginica. Bite frequency of AM D. virginica was significantly lower than for controls when larvae began meals without exploring the food first. Exploring was also followed by longer chewing bouts. Acute effects of the ablation were tested using only Manduca. Six hours after the operation larvae ate less than most controls by eating fewer meals and by biting more slowly. Unilateral ablates also ate less 6h post-operatively, by eating fewer meals; their bite frequency was not low. These observations suggest that chemosensory input affects bite frequency but not other aspects of feeding. As quickly as 24h post-operatively other sensory input to the CNS of AM larvae may compensate for the reduction in chemosensory information, but overall, chemosensory input appears relatively unimportant in non-choice situations.

Neurogenesis in adult insect mushroom bodies

The occurrence of neurogenesis in mushroom bodies of adult insects belonging to several orthopteroid and coleopteran families is described. Using injections of 5-bromo, T2'-deoxyuridine, we showed that neuroblasts, which are progenitors of Kenyon cells during preimaginal instars, continue to divide in adult Acheta domesticus. Their progeny constitute a central column in mushroom body cortices of 3-week-old females. Other Gryllidae, Gryllus bimaculatus and Gryllomorpha dalmatina, show the same pattern of neuroblast activity and migration of their progeny. Immunocytochemical staining of glial cells failed to reveal any immunoreactivity, either in proliferating regions or in the resulting cells. In another orthopteran, Locusta migratoria, discrete clusters of cells, located dorsolateral to the Kenyon cells, incorporated 5-bromo, 2'-deoxyuridine, but we could not detect any neuronal progeny migrating to the mushroom body cortices. These cells were strongly labeled with an antiglial antibody, indicating that the replicating cells are glioblasts rather than neuroblasts. In Periplaneta americana (Dictyoptera), cells replicating their DNA were similarly shown to immunoreact with glial antibodies. In contrast, three coleopterans (Tenebrio molitor, Zophobas species, Harmonia axyridis) have two large neuroblasts located in the middle of the mushroom body cortices. These produce cells which migrate within the group of Kenyon cells, their nuclei having the same shape and size as those of surrounding Kenyon cells. In adult insects, neurogenesis in mushroom bodies occurs in Gryllidae and several coleopteran families, but could not be demonstrated in Dictyoptera and Acrididae. Its occurrence and distribution raise the issue of unexpected plasticity in the adult insect brain.

Reorganization of sensory regulation of locust flight after partial deafferentation

Previous investigations have shown that the flight motor pattern of the mature locust (Locusta migratoria L.) relies heavily on the input of the hindwing tegulae. Removal of the hindwing tegulae results in an immediate change in the motor pattern: the wingbeat frequency (WBF) decreases and the interval between the activity of depressor and elevator muscles (D-E interval) increases. In contrast, removal of the forewing tegulae has little effect on the motor pattern. Here we report adaptive modifications in the flight system that occur after the removal of the hindwing tegulae. Over a period of about 2 weeks following hindwing tegula removal, the flight motor pattern progressively returned towards normal, and in about 80% of the animals recovery of the flight motor pattern was complete. We describe the changes in the activity pattern of flight muscles and in the patterns of depolarizations in flight motoneurons and flight interneurons associated with this recovery. In contrast to the situation in the intact animal, the activity of the forewing tegulae is necessary in recovered animals for the generation of the motor pattern. Removal of the forewing tegulae in recovered animals resulted in similar changes in the flight motor pattern as were observed in intact animals after the removal of the hindwing tegulae. Furthermore, electrical stimulation of forewing tegula afferents in recovered animals produced similar resetting effects on the motor pattern as electrical stimulation of the hindwing tegulae afferents in intact animals. From these observations we conclude that recovery is due to the functional replacement of the removed hindwing tegulae by input from the forewing tegulae.

Connections of the forewing tegulae in the locust flight system and their modification following partial deafferentation

The flight motor pattern of the adult locust (Locusta migratoria L.) is able to recover from the loss of the hindwing tegulae. This recovery is due to a functional substitution of the hindwing tegulae by the forewing tegulae (Büschges, Ramirez, and Pearson, 1992). To assess changes in the pathways from the forewing tegulae in the flight system, we investigated the pathways of the forewing tegula in intact locusts and in animals 2 weeks after hindwing tegula removal. The following physiological alterations in these pathways were found to be associated with the recovery: (1) In the intact locusts, the connections of forewing tegula afferents to flight interneurons are variable but this variability did not occur in recovered animals, and (2) larger numbers of forewing tegula afferents connect to interneurons that excite elevator motoneurons (interneurons 566 and 567) and to an interneuron that inhibits depressor motoneurons (interneuron 511). The size of unitary excitatory postsynaptic potentials (EPSPs) evoked by signal forewing tegula afferents was found not to be altered in recovered animals. The changes in connectivity of forewing tegula afferents are correlated with morphological alterations in the structure of the terminal processes of the afferents and with sprouting of some branches of interneurons receiving input from these afferents.

Regeneration of the projection and synaptic connections of tympanic receptor fibers of Locusta migratoria (Orthoptera) after axotomy

The tergite nerve N6 of the first abdominal segment of the locust Locusta migratoria contains receptor fibers, from the tympanic organ, and hair sensilla as well as motoric axons. The nerve was axotomized in nymphal instars or adults, and the regeneration of nerve fibers was studied. The sensory fibers regrow and regenerate their projection pattern within the central nervous system. They recognize their specific neuropile areas even after entering the ganglion through different pathways. The receptor fibers of the tympanic organ reestablish synaptic connections to auditory interneurons, even though the physiological characteristics of the interneurons are not fully restored. This regenerative capability contrasts with the lack of regeneration of peripheral structures in locusts, but supports the described plasticity in the auditory system of monaural locusts (Lakes, Kalmring, and Engelhard, 1990). The motor fibers do not regenerate nerves innervating muscles of the body wall.

Reorganization of persistent motoneurons in a metamorphosing insect (Tenebrio molitor L., Coleoptera)

The present analysis outlines how the shape of motoneurons which persist through metamorphosis in the beetle Tenebrio molitor is regulated by cellular interactions. This study focused on the structural changes of prothoracic leg motoneurons. The fate of these neurons is described in normal metamorphic development, so as to provide a basis for experimental analysis. In a first experiment the prothoracic leg imaginal discs or part of these were extirpated in the prepupa or early pupa. In a second experiment the leg imaginal discs were rotated by 180 degrees in early larval instars of Tenebrio; the procedure caused a threefold leg anlage. Thereafter, the treated individuals continued to develop. In both experiments the effect of the operation on the structure of the dendritic trees of the persisting motoneurons was analyzed at the imaginal stage. In the first experiment the dendritic tree of the motoneurons is locally deleted. In the second experiment the branching index (quantitative description of dendritic arborization pattern) of the dendritic tree of the persisting motoneurons increased. Both experiments provided evidence that the shape of persistent leg motoneurons is stabilized and even regulated by cellular interactions during metamorphosis. Evidence is presented that sensory neurons are effective both in stabilization and regulation of the shape of persistent motoneurons.

Ultrasound sensitive neurons in the cricket brain

1. The aim of this study was to identify neurons in the brain of the cricket, Teleogryllus oceanicus, that are tuned to high frequencies and to determine if these neurons are involved in the pathway controlling negative phonotaxis. In this paper we describe, both morphologically and physiologically, 20 neurons in the cricket brain which are preferentially tuned to high frequencies. 2. These neurons can be divided into two morphological classes: descending brain interneurons (DBINs) which have a posteriorly projecting axon in the circumesophageal connective and local brain neurons (LBNs) whose processes reside entirely within the brain. All the DBINs and LBNs have processes which project into one common area of the brain, the ventral brain region at the border of the protocerebrum and deutocerebrum. Some of the terminal arborizations of Int-1, an ascending ultrasound sensitive interneuron which initiates negative phonotaxis, also extend into this region. 3. Physiologically, ultrasonic sound pulses produce 3 types of responses in the DBINs and LBNs. (1) Seven DBINs and 6 LBNs are excited by ultrasound. (2) Ongoing activity in one DBIN and 5 LBNs is inhibited by ultrasound, and (3) one cell, (LBN-ei), is either excited or inhibited by ultrasound depending on the direction of the stimulus. 4. Many of the response properties of both the DBINs and LBNs to auditory stimuli are similar to those of Int-1. Specifically, the strength of the response, either excitation or inhibition, to 20 kHz sound pulses increases with increasing stimulus intensity, while the response latency generally decreases. Moreover, the thresholds to high frequencies are much lower than to low frequencies. These observations suggest that the DBINs and LBNs receive a majority of their auditory input from Int-1. However, the response latencies and directional sensitivity of only LBN-ei suggest that it is directly connected to Int-1. 5. The response of only one identified brain neuron, DBIN8, which is inhibited by 20 kHz sound pulses, is facilitated during flight compared to its response at rest. This suggests that suppression of activity in DBIN8 may be associated with ultrasound-induced negative phonotactic steering responses in flying crickets. The other DBINs and LBNs identified in this paper may also play a role in negative phonotaxis, and possibly in other cricket auditory behaviors influenced by ultrasonic frequencies.