Neuroplasticity and phonotaxis in monaural adult female crickets (Gryllus bimaculatus de Geer)

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.

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.

Electromagnetic field and TGF-β enhance the compensatory plasticity after sensory nerve injury in cockroach Periplaneta americana

Recovery of function after sensory nerves injury involves compensatory plasticity, which can be observed in invertebrates. The aim of the study was the evaluation of compensatory plasticity in the cockroach (Periplaneta americana) nervous system after the sensory nerve injury and assessment of the effect of electromagnetic field exposure (EMF, 50 Hz, 7 mT) and TGF-β on this process. The bioelectrical activities of nerves (pre-and post-synaptic parts of the sensory path) were recorded under wind stimulation of the cerci before and after right cercus ablation and in insects exposed to EMF and treated with TGF-β. Ablation of the right cercus caused an increase of activity of the left presynaptic part of the sensory path. Exposure to EMF and TGF-β induced an increase of activity in both parts of the sensory path. This suggests strengthening effects of EMF and TGF-β on the insect ability to recognize stimuli after one cercus ablation. Data from locomotor tests proved electrophysiological results. The takeover of the function of one cercus by the second one proves the existence of compensatory plasticity in the cockroach escape system, which makes it a good model for studying compensatory plasticity. We recommend further research on EMF as a useful factor in neurorehabilitation.

Central pattern generating networks in insect locomotion

Central pattern generators (CPGs) are neural circuits that based on their connectivity can generate rhythmic and patterned output in the absence of rhythmic external inputs. This property makes CPGs crucial elements in the generation of many kinds of rhythmic motor behaviors in insects, such as flying, walking, swimming, or crawling. Arguably representing the most diverse group of animals, insects utilize at least one of these types of locomotion during one stage of their ontogenesis. Insects have been extensively used to study the neural basis of rhythmic motor behaviors, and particularly the structure and operation of CPGs involved in locomotion. Here, we review insect locomotion with regard to flying, walking, and crawling, and we discuss the contribution of central pattern generation to these three forms of locomotion. In each case, we compare and contrast the topology and structure of the CPGs, and we point out how these factors are involved in the generation of the respective motor pattern. We focus on the importance of sensory information for establishing a functional motor output and we indicate behavior-specific adaptations. Furthermore, we report on the mechanisms underlying coordination between different body parts. Last but not least, by reviewing the state-of-the-art knowledge concerning the role of CPGs in insect locomotion, we endeavor to create a common ground, upon which future research in the field of motor control in insects can build.

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.

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.

Sex and the single (-eared) female: leg function, limb autotomy and mating history trade-offs in field crickets (Gryllus bimaculatus)

Both male and female field crickets (Gryllus bimaculatus) autotomize front (tympanal) limbs more slowly than hind limbs. Arguably, this pattern could reflect possible differences in the mechanism of limb autotomy. However, we demonstrate that, for females, limb autotomy is also dependent on their mating status: virgin females autotomize front legs significantly more slowly than mated females. This response suggests a central control for leg autotomy in these animals, and less readiness to autotomize a front leg, possibly because the tympanum is crucial for mate location.

Plasticity of synaptic connections in sensory-motor pathways of the adult locust flight system

We investigated possible roles of retrograde signals and competitive interactions in the lesion-induced reorganization of synaptic contacts in the locust CNS. Neuronal plasticity is elicited in the adult flight system by removal of afferents from the tegula, a mechanoreceptor organ at the base of the wing. We severed one hindwing organ and studied the resulting rearrangement of synaptic contacts between flight interneurons and afferent neurons from the remaining three tegulae (2 forewing, 1 hindwing). This was done by electric stimulation of afferents and intracellular recording from interneurons (and occasionally motoneurons). Two to three weeks after unilateral tegula lesion, connections between tegula afferents and flight interneurons were altered in the following way. 1) Axons from the forewing tegula on the operated side had established new synaptic contacts with metathoracic elevator interneurons. In addition, the amplitude of compound excitatory postsynaptic potentials elicited by electric stimulation was increased, indicating that a larger number of afferents connected to any given interneuron. 2) On the side contralateral to the lesion, connectivity between axons from the forewing tegula and elevator interneurons was decreased. 3) The efficacy of the (remaining) hindwing afferents appeared to be increased with regard to both synaptic transmission to interneurons and impact on flight motor pattern. 4) Flight motoneurons, which are normally restricted to the ipsilateral hemiganglion, sprouted across the ganglion midline after unilateral tegula removal and apparently established new synaptic contacts with tegula afferents on that side. The changes on the operated side are interpreted as occupation of synaptic space vacated on the interneurons by the severed hindwing afferents. On the contralateral side, the changes in synaptic contact must be elicited by retrograde signals from bilaterally arborizing flight interneurons, because tegula projections remain strictly ipsilateral. The pattern of changes suggests competitive interactions between forewing and hindwing afferents. The present investigation thus presents evidence that the CNS of the mature locust is capable of extensive synaptic rearrangement in response to injury and indicates for the first time the action of retrograde signals from interneurons.

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.