Distribution of synapses on two ascending interneurones carrying frequency-specific information in the auditory system of the cricket: evidence for GABAergic inputs

Local prothoracic auditory neurons in Ensifera

A new method for individually staining insect neurons with metal ions was described in the late 60s, closely followed by the introduction of the first bright fluorescent dye, Lucifer Yellow, for the same purpose. These milestones enabled an unprecedented level of detail regarding the neuronal basis of sensory processes such as hearing. Due to their conspicuous auditory behavior, orthopterans rapidly established themselves as a popular model for studies on hearing (first identified auditory neuron: 1974; first local auditory interneuron: 1977). Although crickets (Ensifera, Gryllidae) surpassed grasshoppers (Caelifera) as the main model taxon, surprisingly few neuronal elements have been described in crickets. More auditory neurons are described for bush crickets (Ensifera, Tettigoniidae), but due to their great biodiversity, the described auditory neurons in bush crickets are scattered over distantly related groups, hence being confounded by potential differences in the neuronal pathways themselves. Our review will outline all local auditory elements described in ensiferans so far. We will focus on one bush cricket species, Ancistrura nigrovittata (Phaneropterinae), which has the so-far highest diversity of identified auditory interneurons within Ensifera. We will present one novel and three previously described local prothoracic auditory neuron classes, comparing their morphology and aspects of sensory processing. Finally, we will hypothesize about their functions and evolutionary connections between ensiferan insects.

An auditory-responsive interneuron descending from the cricket brain: a new element in the auditory pathway

Crickets receive auditory information from their environment via ears located on the front legs. Ascending interneurons forward auditory activity to the brain, which houses a pattern recognition network for phonotaxis to conspecific calling songs and which controls negative phonotaxis to high-frequency sound pulses. Descending brain neurons, however, which are clearly involved in controlling these behaviors, have not yet been identified. We describe a descending auditory-responsive brain neuron with an arborization pattern that coincides with the ring-like auditory neuropil in the brain formed by the axonal arborizations of ascending and local interneurons, indicating its close link to auditory processing. Spiking activity of this interneuron occurs with a short latency to calling song patterns and the neuron copies the sound pulse pattern. The neuron preferentially responds to short sound pulses, but its activity appears to be independent of the calling song pattern recognition process. It also receives a weaker synaptic input in response to high-frequency pulses, which may contribute to its short latency spiking responses. This interneuron could be a crucial part in the auditory-to-motor transformation of the nervous system and contribute to the motor control of cricket auditory behavior.

Auditory DUM neurons in a bush-cricket: A filter bank for carrier frequency

In bush-crickets the first stage of central auditory processing occurs in the prothoracic ganglion. About 15 to 50 different auditory dorsal unpaired median neurons (DUM neurons) exist but they have not been studied in any detail. These DUM neurons may be classified into seven different morphological types, although, there is only limited correlation between morphology and physiological responses. Ninety seven percent of the stained neurons were local, 3% were intersegmental. About 90% project nearly exclusively into the auditory neuropile, and 45% into restricted areas therein. Lateral extensions overlap with the axons of primary auditory sensory neurons close to their branching point. DUM neurons are typically tuned to frequencies covering the range between 2 and 50 kHz and thereby may establish a filter bank for carrier frequency. Less than 10% of DUM neurons have their branches in adjacent and more posterior regions of the auditory neuropile and are mostly tuned to low frequencies, less sensitive than the other types and respond to vibration. Thirty five percent of DUM show indications of inhibition, either through reduced responses at higher intensities, or by hyperpolarizing responses to sound. Most DUM neurons produce phasic spike responses preferably at higher intensities. Spikes may be elicited by intracellular current injection. Preliminary data suggest that auditory DUM neurons have GABA as transmitter and therefore may inhibit other auditory interneurons. From all known local auditory neurons, only DUM neurons have frequency specific responses which appear suited for local processing relevant for acoustic communication in bush crickets.

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.

Bursting neurons and ultrasound avoidance in crickets

Decision making in invertebrates often relies on simple neural circuits composed of only a few identified neurons. The relative simplicity of these circuits makes it possible to identify the key computation and neural properties underlying decisions. In this review, we summarize recent research on the neural basis of ultrasound avoidance in crickets, a response that allows escape from echolocating bats. The key neural property shaping behavioral output is high-frequency bursting of an identified interneuron, AN2, which carries information about ultrasound stimuli from receptor neurons to the brain. AN2's spike train consists of clusters of spikes - bursts - that may be interspersed with isolated, non-burst spikes. AN2 firing is necessary and sufficient to trigger avoidance steering but only high-rate firing, such as occurs in bursts, evokes this response. AN2 bursts are therefore at the core of the computation involved in deciding whether or not to steer away from ultrasound. Bursts in AN2 are triggered by synaptic input from nearly synchronous bursts in ultrasound receptors. Thus the population response at the very first stage of sensory processing - the auditory receptor - already differentiates the features of the stimulus that will trigger a behavioral response from those that will not. Adaptation, both intrinsic to AN2 and within ultrasound receptors, scales the burst-generating features according to the stimulus statistics, thus filtering out background noise and ensuring that bursts occur selectively in response to salient peaks in ultrasound intensity. Furthermore AN2's sensitivity to ultrasound varies adaptively with predation pressure, through both developmental and evolutionary mechanisms. We discuss how this key relationship between bursting and the triggering of avoidance behavior is also observed in other invertebrate systems such as the avoidance of looming visual stimuli in locusts or heat avoidance in beetles.

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.

Processing of auditory information in insects

Insects exhibit an astonishing diversity in the design of their ears and the subsequent processing of information within their auditory pathways. The aim of this review is to summarize and compare the present concepts of auditory processing by relating behavioral performance to known neuronal mechanisms. We focus on three general aspects, that is frequency, directional, and temporal processing. The first part compares the capacity (in some insects high) for frequency analysis in the ear with the rather low specificity of tuning in interneurons by looking at Q10dB values and frequency dependent inhibition of interneurons. Since sharpening of frequency does not seem to be the prime task of a set of differently tuned receptors, alternative hypotheses are discussed. Moreover, the physiological correspondence between tonotopic projections of receptors and dendritic organization of interneurons is not in all cases strong. The second part is concerned with directional hearing and thus with the ability for angular resolution of insects. The present concepts, as derived from behavioral performances, for angular resolution versus lateralization and serial versus parallel processing of directional and pattern information can be traced to the thoracic level of neuronal processing. Contralateral inhibition, a mechanism for enhancing directional tuning, appears to be most effective in parallel pathways, whereas in serial processing it may have detrimental effects on pattern processing. The third part, after some considerations of signal analysis in the temporal domain, demonstrates that closely related species often use different combinations of temporal parameters in their recognition systems. On the thoracic level, analysis of temporal modulation functions and effects of inhibition on spiking patterns reveals relatively simple processing, whereas brain neurons may exhibit more complex properties.

Contralateral inhibition as a sensory bias: the neural basis for a female preference in a synchronously calling bushcricket, Mecopoda elongata

Imperfect synchrony between male calls occurs widely in acoustically courting crickets and bushcrickets. Males which are able to establish the temporal leadership usually attract more females in choice experiments but the proximate mechanism for this precedence effect is unknown. Here we show that contralateral inhibition, the neural basis for lateral contrast enhancement in the auditory pathways of insects and vertebrates, is also the probable proximate neural mechanism for this female preference. We recorded simultaneously from a pair of identified auditory interneurons in the synchronizing bushcricket Mecopoda elongata. When two identical acoustic stimuli are presented from opposite directions, one preceding the other by 120 ms, the neural representation within the receiver is far stronger for the leader signal. This results from a suppression of the neural response to the follower chirp by reciprocal contralateral inhibition. The advantage of the representation of the leader is 2-3-fold with time delays between 70 and 130 ms; the most clear-cut female preferences have also been found with such delays in previous behavioural experiments. In time-intensity trading experiments, a lead by 120 ms could only be compensated for by increasing the amplitude of the follower signal by 7-11 dB. We discuss contralateral inhibition in auditory systems as a sensory bias that results in female preference for leading signals, with important evolutionary consequences for male calling strategies.

Synaptic structure, distribution, and circuitry in the central nervous system of the locust and related insects

The Orthopteran central nervous system has proved a fertile substrate for combined morphological and physiological studies of identified neurons. Electron microscopy reveals two major types of synaptic contacts between nerve fibres: chemical synapses (which predominate) and electrotonic (gap) junctions. The chemical synapses are characterized by a structural asymmetry between the pre- and postsynaptic electron dense paramembranous structures. The postsynaptic paramembranous density defines the extent of a synaptic contact that varies according to synaptic type and location in single identified neurons. Synaptic bars are the most prominent presynaptic element at both monadic and dyadic (divergent) synapses. These are associated with small electron lucent synaptic vesicles in neurons that are cholinergic or glutamatergic (round vesicles) or GABAergic (pleomorphic vesicles). Dense core vesicles of different sizes are indicative of the presence of peptide or amine transmitters. Synapses are mostly found on small-diameter neuropilar branches and the number of synaptic contacts constituting a single physiological synapse ranges from a few tens to several thousand depending on the neurones involved. Some principles of synaptic circuitry can be deduced from the analysis of highly ordered brain neuropiles. With the light microscope, synaptic location can be inferred from the distribution of the presynaptic protein synapsin I. In the ventral nerve cord, identified neurons that are components of circuits subserving known behaviours, have been studied using electrophysiology in combination with light and electron microscopy and immunocytochemistry of neuroactive compounds. This has allowed the synaptic distribution of the major classes of neurone in the ventral nerve cord to be analysed within a functional context.

GABA and glutamate-like immunoreactivity at synapses on depressor motorneurones of the leg of the crayfish, Procambarus clarkii

To investigate their synaptic relationships, depressor motorneurones of the crayfish leg were impaled with microelectrodes, intracellularly injected with horseradish peroxidase, and prepared for electron microscopy. Post-embedding immunogold labelling with antibodies against gamma-aminobutyric acid (GABA) or glutamate was carried out either alone or together on the same section and allowed the identification of three classes of input synapses: 51% were immunoreactive for glutamate and contained round agranular vesicles, 31% were immunoreactive for GABA and contained pleomorphic agranular vesicles, and the remainder were immunoreactive for neither and also predominantly contained pleomorphic agranular vesicles. Output synapses were abundant in some of the motorneurones but were not seen in others, suggesting that members of the motor pool differ in their connectivity.

Distribution of input and output synapses on the central branches of bushcricket and cricket auditory afferent neurones: immunocytochemical evidence for GABA and glutamate in different populations of presynaptic boutons

In order to investigate the synapses on the terminals of primary auditory afferents in the bushcricket and cricket, these were impaled with microelectrodes and after physiological characterisation, injected intracellularly with horseradish peroxidase. The tissue was prepared for electron microscopy, and immunocytochemistry for gamma-aminobutyric acid (GABA) and glutamate was carried out on ultrathin sections by using a post-embedding immunogold technique. The afferent terminals received many input synapses. Between 60-65% of these were made by processes immunoreactive for GABA and approximately 25% from processes immunoreactive for glutamate. The relative distribution of the different classes of input were analysed from serial section reconstruction of terminal afferent branches. Inputs from GABA and glutamate-immunoreactive processes appeared to be scattered at random over the terminal arborisation of the afferents both with respect to each other and to the architecture of the terminals. They were, however, always found close to the output synapses. The possible roles of presynaptic inhibition in the auditory afferents is discussed in the context of the auditory responses of the animals.

Picrotoxin eliminates frequency selectivity of an auditory interneuron in a bushcricket

AN1, an auditory interneuron in the bushcricket Ancistrura nigrovittata, is narrowly tuned to the male song frequency ( approximately 15 kHz). It receives pronounced inhibitory input at frequencies below and, more prominently, above this fundamental frequency. It is also subject to side-dependent inhibition producing asymmetric response functions for left- and right-side stimulation. In addition, intensity-response functions of AN1 peak as stimulus intensities increase. Application of the GABAA channel-blocker picrotoxin eliminates all subthreshold inhibitory postsynaptic potentials, revealing underlying excitation that is particularly obvious in the high-frequency range. Excitatory thresholds close to the song frequency remain unchanged by picrotoxin. Thus a specifically tuned neuron is shown to become broadly tuned after elimination of frequency-dependent inhibition. Although average maximum response strength is increased by 150% after picrotoxin application, at male song frequencies a slight reduction of the responses is still present at high intensities. Side-dependent inhibition remains largely unaffected by picrotoxin, suggesting that side- and frequency-dependent inhibitions are caused by different transmitters from different neurons.

Ultrastructure of synaptic contacts between identified neurons of the auditory pathway in Gryllus bimaculatus DeGeer

The synaptic contacts between the auditory sensory cells and identified auditory interneurons ON1 and AN2 have been examined at the ultrastructural level by selective electron-dense labeling of two interneurons or of one interneuron and the sensory fibers in the same preparation. The experiments have provided the following information. The auditory afferent fibers have a monosynaptic connection with the lateral inhibitors ON1 and the ascending interneuron AN2, allowing direct activation of these interneurons. Furthermore, our work proves that the paired, lateral, inhibitor ON1 neurons have direct output synapses onto each other. The results also show that the auditory afferent axons, themselves, receive synaptic inputs just before entering the central auditory neuropil. The effects of current injection into the ON1 neuron during auditory processing indicate that these synaptic inputs onto the afferents originate, in part, from the lateral branches of the ipsilateral ON1 neuron and that they have inhibitory function. The significance of these results for auditory processing and a future perspective for electron microscopic analysis of neuropil are discussed.