Intracellular activity in cricket neurons during generation of song patterns

The Journal of Comparative Physiology A: rooted in great tradition, committed to innovation and discovery

The current volume of the Journal of Comparative Physiology A marks a transition in editorship. This event provides the opportunity to reflect on the 98 years of the history of the Journal; on the impact of its legacy on the evolution of neuroethology and the comparative branches of sensory physiology and neurobiology; and on future changes in editorial organization and content.

Modular timer networks: abdominal interneurons controlling the chirp and pulse pattern in a cricket calling song

Chirping male crickets combine a 30 Hz pulse pattern with a 3 Hz chirp pattern to drive the rhythmic opening-closing movements of the front wings for sound production. Lesion experiments suggest two coupled modular timer-networks located along the chain of abdominal ganglia, a network in A3 and A4 generating the pulse pattern, and a network organized along with ganglia A4–A6 controlling the generation of the chirp rhythm. We analyzed neurons of the timer-networks and their synaptic connections by intracellular recordings and staining. We identified neurons spiking in phase with the chirps and pulses, or that are inhibited during the chirps. Neurons share a similar “gestalt”, regarding the position of the cell body, the dendritic arborizations and the contralateral ascending axon. Activating neurons of the pulse-timer network elicits ongoing motor activity driving the generation of pulses; this activity is not structured in the chirp pattern. Activating neurons of the chirp-timer network excites pulse-timer neurons; it drives the generation of chirps and during the chirps the pulse pattern is produced. Our results support the hypothesis that two modular networks along the abdominal ganglion chain control the cricket calling song, a pattern generating network in the mesothoracic ganglion may not be required.

Reciprocal inhibition and postinhibitory rebound produce reverberation in a locomotor pattern generator

The central pattern generator for swimming in the pteropod mollusk Clione limacina consists of at least four pedal interneurons, two each controlling parapodial upstroke and downstroke. The two sets of antagonistic interneurons are linked by reciprocal monosynaptic inhibitory synapses, and all exhibit apparently strong postinhibitory rebound. This simple neuronal network produces reverberating alternate cyclic activity in the absence of tonic drive or apparent feedback modulation.

Synchrony during acoustic interactions in the bushcricket Mecopoda 'Chirper' (Tettigoniidae:Orthoptera) is generated by a combination of chirp-by-chirp resetting and change in intrinsic chirp rate

In several bushcricket species, individual males synchronise their chirps during acoustic interactions. Synchrony is imperfect with the chirps of one male leading or lagging the other by a few milliseconds. Imperfect synchrony is believed to have evolved in response to female preferences for leading chirps. We investigated the mechanism underlying synchrony in the bushcricket species Mecopoda 'Chirper' from Southern India using playback experiments and simulations of pairwise interactions. We also investigated whether intrinsic chirp period is a good predictor of leading probability during interactions between males. The mechanism underlying synchrony in this species differs from previously reported mechanisms in that it involves both a change in the oscillator's intrinsic rate and resetting on a chirp-by-chirp basis. The form of the phase response curve differs from those of previously reported firefly and bushcricket species including the closely related Malaysian species Mecopoda elongata. Simulations exploring oscillator properties showed that the outcome of pairwise interactions was independent of initial phase and alternation was not possible. Solo intrinsic chirp period was a relatively good predictor of leading probability. However, changing the intrinsic period during interactions could enable males with longer periods to lead during acoustic interactions.

Behavioral genomics and the study of speciation at a porous species boundary

Porous species boundaries are characterized by differential gene flow, where some regions of the genome experience divergent evolution while others experience the homogenizing effects of gene flow. If species can arise or remain distinct despite gene flow between them, speciation can only be understood on a gene by gene level. To understand the genetics of speciation, we therefore must identify the targets of selection that cause divergent evolution and identify the genetic architecture underlying such "speciation phenotypes". This will enable characterization of genomic regions that are "free to flow" between species, and those that diverge in the face of gene flow. We discuss this problem in the genus Laupala, a morphologically cryptic, flightless group of crickets that has radiated in Hawaii. Because songs are used in courtship and always distinguish close relatives of Laupala as well as species in sympatry, we argue that songs in Laupala are speciation phenotypes. Here, we present our approaches to identify the underlying genomic regions and song genes that differentiate closely related species. We discuss what is known about the genetic basis of this species difference derived from classic quantitative genetics and quantitative trait locus mapping experiments. We also present a model of the molecular expression of cricket song to assist in our goal to identify the genes involved in song variation. As most species are sympatric and exchange genes with congeners, we discuss the importance of understanding the genetic and genomic architecture of song as a speciation phenotype that must be characterized to identify differential patterns of gene flow at porous species boundaries.

Identified nerve cells and insect behavior

Studies of insect identified neurons over the past 25 years have provided some of the very best data on sensorimotor integration; tracing information flow from sensory to motor networks. General principles have emerged that have increased the sophistication with which we now understand both sensory processing and motor control. Two overarching themes have emerged from studies of identified sensory interneurons. First, within a species, there are profound differences in neuronal organization associated with both the sex and the social experience of the individual. Second, single neurons exhibit some surprisingly rich examples of computational sophistication in terms of (a) temporal dynamics (coding superimposed upon circadian and shorter-term rhythms), and also (b) what Kenneth Roeder called "neural parsimony": that optimal information can be encoded, and complex acts of sensorimotor coordination can be mediated, by small ensembles of cells. Insect motor systems have proven to be relatively complex, and so studies of their organization typically have not yielded completely defined circuits as are known from some other invertebrates. However, several important findings have emerged. Analysis of neuronal oscillators for rhythmic behavior have delineated a profound influence of sensory feedback on interneuronal circuits: they are not only modulated by feedback, but may be substantially reconfigured. Additionally, insect motor circuits provide potent examples of neuronal restructuring during an organism's lifetime, as well as insights on how circuits have been modified across evolutionary time. Several areas where future advances seem likely to occur include: molecular genetic analyses, neuroecological syntheses, and neuroinformatics--the use of digital resources to organize databases with information on identified nerve cells and behavior.

Control of cricket stridulation by a command neuron: efficacy depends on the behavioral state

Crickets use different song patterns for acoustic communication. The stridulatory pattern-generating networks are housed within the thoracic ganglia but are controlled by the brain. This descending control of stridulation was identified by intracellular recordings and stainings of brain neurons. Its impact on the generation of calling song was analyzed both in resting and stridulating crickets and during cercal wind stimulation, which impaired the stridulatory movements and caused transient silencing reactions. A descending interneuron in the brain serves as a command neuron for calling-song stridulation. The neuron has a dorsal soma position, anterior dendritic processes, and an axon that descends in the contralateral connective. The neuron is present in each side of the CNS. It is not activated in resting crickets. Intracellular depolarization of the interneuron so that its spike frequency is increased to 60-80 spikes/s reliably elicits calling-song stridulation. The spike frequency is modulated slightly in the chirp cycle with the maximum activity in phase with each chirp. There is a high positive correlation between the chirp repetition rate and the interneuron's spike frequency. Only a very weak correlation, however, exists between the syllable repetition rate and the interneuron activity. The effectiveness of the command neuron depends on the activity state of the cricket. In resting crickets, experimentally evoked short bursts of action potentials elicit only incomplete calling-song chirps. In crickets that previously had stridulated during the experiment, short elicitation of interneuron activity can trigger sustained calling songs during which the interneuron exhibits a spike frequency of approximately 30 spikes/s. During sustained calling songs, the command neuron activity is necessary to maintain the stridulatory behavior. Inhibition of the interneuron stops stridulation. A transient increase in the spike frequency of the interneuron speeds up the chirp rate and thereby resets the timing of the chirp pattern generator. The interneuron also is excited by cercal wind stimulation. Cercal wind stimulation can impair the pattern of chirp and syllable generation, but these changes are not reflected in the discharge pattern of the command neuron. During wind-evoked silencing reactions, the activity of the calling-song command neuron remains unchanged, but under these conditions, its activity is no longer sufficient to maintain stridulation. Therefore stridulation can be suppressed by cercal inputs from the terminal ganglia without directly inhibiting the descending command activity.

Temperature coupling in cricket acoustic communication. II. Localization of temperature effects on song production and recognition networks in Gryllus firmus

Acoustic communication in Gryllus firmus is temperature-coupled: temperature induces parallel changes in male calling song temporal pattern, and in female preference for song. Temperature effects on song production and recognition networks were localized by selectively warming head or thorax or both head and thorax of intact crickets, then eliciting aggression song production (males) or phonotaxis to synthetic calling song (females). Because male song is produced by a thoracic central pattern generator (CPG), and because head ganglia are necessary for female song recognition, measurements of female phonotaxis under such conditions may be used to test the following competing hypotheses about organization of the song recognition network: 1. A set of neurons homologous to the male song CPG exist in the female, and are used as a template that determines preferred values of song temporal parameters for song pattern recognition (the common neural elements hypothesis), and 2. temporal pattern preference is determined entirely within the head ganglia. Neither selective warming of the head nor of the thorax was effective in changing female song preference, but simultaneous warming of head and thorax shifted preference toward a faster song in most preparations, as did warming the whole animal by raising ambient temperature. These results suggest that phonotactic preference for song temporal pattern is plurisegmentally determined in field crickets. Selective warming experiments during aggression song production in males revealed that syllable period is influenced but not completely determined by thoracic temperature; head temperature is irrelevant. The song CPG appears to receive some rate-setting information from outside the thoracic central nervous system.

Neural basis of rhythmic behavior in animals

Timing of the repetitive movements that constitute any rhythmic behavior is regulated by intrinsic properties of the central nervous system rather than by sensory feedback from moving parts of the body. Evidence of this permits resolution of the long-standing controversy over the neural basis of rhythmic behavior and aids in the identification of this mechanism as a general principle of neural organization applicable to all animals with central nervous systems.

Temporal Pattern as a Cue for Species-Specific Calling Song Recognition in Crickets

Female crickets can recognize conspecific calling song from its temporal pattern alone. In Teleogryllus oceanicus, the song pattern consists of three classes of interpulse intervals arranged in a stereotyped sequence. Females recognize a model song in which the sequential order of intervals is random. This argues against the hypothesis that recognition results from matching auditory input to an internal template of the song.

Ecdysis: neural orchestration of a complex behavioral performance

Cricket ecdysis (molting) requires continuously changing output in hundreds of motoneurons over a period of several hours, and exhibits considerable plasticity. Despite this complexity, analysis of identified motor units reveals a highly organized three-layered infrastructure, and indicates that the "small system" paradigm currently applied to simple invertebrate motor programs can be extended to much more sophisticated behavioral performances.

Hybrid cricket auditory behavior: evidence for genetic coupling in animal communication

Male field crickets produce species-specific and stereotyped calling songs. Conspecific females are attracted to the call. Reciprocal F1 hybrid females prefer the calls of sibling hybrids to reciprocal hybrids. Discrimination is probably based on temporal pattern and not carrier frequency of the call. The results imply that production of song by males and its detection by females have a common genetic basis.

Electrical responses of insect central neurons: augmentation by nerve section or colchicine

Intracellular recording from the somata of central motor neurons in the cockroach Periplaneta americana normally shows little or no electrical response evoked by soma depolarization or by antidromic stimulation. Within 4 days after either cutting the axon or administration of colchicine, large action potentials can regularly be recorded from cell bodies of metathoracic motor neurons. Each experimental procedure evokes formation of a dense, perinuclear ribonucleic acid ring in the soma of neurons showing augmented electrical responses.

Silk Moth Eclosion: Hormonal Triggering of a Centrally Programmed Pattern of Behavior

The emergence of the adult cecropia silk moth from the pupal skin involves a stereotyped series of abdominal movements-the pre-eclosion behavior. This behavior, triggered by a neurosecretory hormone, consists of three phases that are characterized by the relative frequency and pattern of movements. Electrical recordings from a nerve cord with severed peripheral nerves demonstrate that the pre-eclosion behavior is prepatterned in the abdominal ganglia. In response to the hormone, the entire 1.25-hour behavioral program can be activated and "read off" in the absence of sensory feedback.

Genetic control of an insect neuronal network

Motor activity responsible for the calling song of crickets is generated by a small neuronal network whose output is genetically determined. Genes controlling certain output features are located on the X chromosome. The genetic system involved is polygenic and multichromosomal. In some patterns, genetically derived information is adequate to specify the difference of a single impulse in the output of homologous neurons from different genotypes.

Postembryonic development of adult motor patterns in crickets: a neural analysis

Adult crickets have stereotyped patterns of motor output which are generated by the central nervous system, and which serve as a standard against which emerging nymphal patterns can be measured. The neural circuits generating these patterns are not functional at hatching. The pattern elements appear in an ordered sequence over the course of the last four molts. The circuits are completely functional before the final molt. Circuits which might be prematurely activated are suppressed in the nymph by descending inhibition from the brain.