Giant Interneurons Research Articles

Central projections of cercal giant interneurons in the adult field cricket, Gryllus bimaculatus.

The structures of neurons, such as dendrites and axonal projections, are closely related to their response properties and their specific functions in neural circuits. Identified neurons, having genetically determined morphological features and pre- and postsynaptic partners, play significant roles in specific behaviors. Giant interneurons (GIs) are identified in the terminal abdominal ganglion of the cricket as mechanosensory projection neurons and are sensitive to airflow stimulation of the cerci. GIs are classified into ventral GIs (vGIs) or dorsal GIs (dGIs) depending on the location of their axons running within the connective nerve cord. Based on their response properties to airflow, vGIs are presumed to be involved in triggering the wind-elicited escape response, whereas dGIs are thought to be airflow direction-encoding neurons. The previous findings regarding airflow sensitivity point to possible differences in the morphology of the central projections that may correspond to their neural functions. However, the detailed morphologies of the GIs in the cephalic and thoracic ganglia of adult crickets remain unclear. In this study, we stained six GIs, namely, GI 8-1 (medial giant interneuron, MGI), 9-1 (lateral giant interneuron, LGI), 9-2, 9-3, 10-2, and 10-3, using intracellular iontophoretic or pressure injection of dyes. Staining revealed remarkable differences in the axonal branching patterns between vGIs and dGIs. The dGIs were further divided into subgroups based on the profiles of their axon collaterals and projection sites in the brain. The anatomical differences between the GIs' central projections seemed to be related to their information encodement and behavioral functions.

Cellular interactions between social experience, alcohol sensitivity, and GABAergic inhibition in a crayfish neural circuit.

We report here that prior social experience modified the behavioral responses of adult crayfish to acute alcohol exposure. Animals housed individually for 1 wk before alcohol exposure were less sensitive to the intoxicating effects of alcohol than animals housed in groups, and these differences are based on changes in the nervous system rather than differences in alcohol uptake. To elucidate the underlying neural mechanisms, we investigated the neurophysiological responses of the lateral giant (LG) interneurons after alcohol exposure. Specifically, we measured the interactions between alcohol and different GABAA-receptor antagonists and agonists in reduced crayfish preparations devoid of brain-derived tonic GABAergic inhibition. We found that alcohol significantly increased the postsynaptic potential of the LG neurons, but contrary to our behavioral observations, the results were similar for isolated and communal animals. The GABAA-receptor antagonist picrotoxin, however, facilitated LG postsynaptic potentials more strongly in communal crayfish, which altered the neurocellular interactions with alcohol, whereas TPMPA [(1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid], an antagonist directed against GABAA-receptors with ρ subunits, did not produce any effects. Muscimol, an agonist for GABAA-receptors, blocked the stimulating effects of alcohol, but this was independent of prior social history. THIP [4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol], an agonist directed against GABAA-receptors with δ subunits, which were not previously known to exist in the LG circuit, replicated the suppressing effects of muscimol. Together, our findings provide strong evidence that alcohol interacts with the crayfish GABAergic system, and the interplay between prior social experience and acute alcohol intoxication might be linked to changes in the expression and function of specific GABAA-receptor subtypes.NEW & NOTEWORTHY The complex interactions between alcohol and prior social experience are still poorly understood. Our work demonstrates that socially isolated crayfish exhibit lower neurobehavioral sensitivity to acute ethanol compared with communally housed animals, and this socially mediated effect is based on changes in the nervous systems rather than on differences in uptake or metabolism. By combining intracellular neurophysiology and neuropharmacology, we investigated the role of the main inhibitory neurotransmitter GABA, and its receptor subtypes, in shaping this process.

Not so fast: giant interneurons control precise movements of antennal scales during escape behavior of crayfish.

High-speed video recordings of escape responses in freely behaving crayfish revealed precisely coordinated movements of conspicuous head appendages, the antennal scales, during tail-flips that are produced by giant interneurons. For tail-flips that are generated by the medial giants (MG) in response to frontal attacks, the scales started to extend immediately after stimulation and extension was completed before the animal began to propel backwards. For tail-flips that are elicited by caudal stimuli and controlled by the lateral giants (LG), scale extensions began with significant delay after the tail-flip movement was initiated, and full extension of the scales coincided with full flexion of the tail. When we used implanted electrodes and stimulated the giant neurons directly, we observed the same patterns of scale extensions and corresponding timing. In addition, single action potentials of MG and LG neurons evoked with intracellular current injections in minimally restrained preparations were sufficient to activate scale extensions with similar delays as seen in freely behaving animals. Our results suggest that the giant interneurons, which have been assumed to be part of hardwired reflex circuits that lead to caudal motor outputs and stereotyped behavior, are also responsible for activating a pair of antennal scales with high temporal precision.

Identification of Immediate Early Genes in the Nervous System of Snail Helix lucorum.

Immediate early genes (IEGs) are useful markers of neuronal activation and essential components of neuronal response. While studies of gastropods have provided many insights into the basic learning and memory mechanisms, the genome-wide assessment of IEGs has been mainly restricted to vertebrates. In this study, we identified IEGs in the terrestrial snail Helix lucorum. In the absence of the genome, we conducted de novo transcriptome assembly using reads with short and intermediate lengths cumulatively covering more than 98 billion nucleotides. Based on this assembly, we identified 37 proteins corresponding to contigs differentially expressed (DE) in either the parietal ganglia (PaG) or two giant interneurons located within the PaG of the snail in response to the neuronal stimulation. These proteins included homologues of well-known mammalian IEGs, such as c-jun/jund, C/EBP, c-fos/fosl2, and Egr1, as well as homologues of genes not yet implicated in the neuronal response.

Effects of Ethanol on Sensory Inputs to the Medial Giant Interneurons of Crayfish.

Crayfish are capable of two rapid, escape reflexes that are mediated by two pairs of giant interneurons, the lateral giants (LG) and the medial giants (MG), which respond to threats presented to the abdomen or head and thorax, respectively. The LG has been the focus of study for many decades and the role of GABAergic inhibition on the escape circuit is well-described. More recently, we demonstrated that the LG circuit is sensitive to the acute effects of ethanol and this sensitivity is likely mediated by interactions between ethanol and the GABAergic system. The MG neurons, however, which receive multi-modal sensory inputs and are located in the brain, have been less studied despite their established importance during many naturally occurring behaviors. Using a combination of electrophysiological and neuropharmacological techniques, we report here that the MG neurons are sensitive to ethanol and experience an increase in amplitudes of post-synaptic potentials following ethanol exposure. Moreover, they are affected by GABAergic mechanisms: the facilitatory effect of acute EtOH can be suppressed by pretreatment with a GABA receptor agonist whereas the inhibitory effects resulting from a GABA agonist can be occluded by ethanol exposure. Together, our findings suggest intriguing neurocellular interactions between alcohol and the crayfish GABAergic system. These results enable further exploration of potentially conserved neurochemical mechanisms underlying the interactions between alcohol and neural circuitry that controls complex behaviors.

Silencing giant cholinergic interneurons in the nucleus accumbens with DREADDS reduces nicotine self-administration in rats

Silencing giant cholinergic interneurons in the nucleus accumbens with DREADDS reduces nicotine self-administration in rats

Complete identification of four giant interneurons supplying mushroom body calyces in the cockroach Periplaneta americana.

Global inhibition is a fundamental physiological mechanism that has been proposed to shape odor representation in higher-order olfactory centers. A pair of mushroom bodies (MBs) in insect brains, an analog of the mammalian olfactory cortex, are implicated in multisensory integration and associative memory formation. With the use of single/multiple intracellular recording and staining in the cockroach Periplaneta americana, we succeeded in unambiguous identification of four tightly bundled GABA-immunoreactive giant interneurons that are presumably involved in global inhibitory control of the MB. These neurons, including three spiking neurons and one nonspiking neuron, possess dendrites in termination fields of MB output neurons and send axon terminals back to MB input sites, calyces, suggesting feedback roles onto the MB. The largest spiking neuron innervates almost exclusively the basal region of calyces, while the two smaller spiking neurons and the second-largest nonspiking neuron innervate more profusely the peripheral (lip) region of the calyces than the basal region. This subdivision corresponds well to the calycal zonation made by axon terminals of two populations of uniglomerular projection neurons with dendrites in distinct glomerular groups in the antennal lobe. The four giant neurons exhibited excitatory responses to every odor tested in a neuron-specific fashion, and two of the neurons also exhibited inhibitory responses in some recording sessions. Our results suggest that two parallel olfactory inputs to the MB undergo different forms of inhibitory control by the giant neurons, which may, in turn, be involved in different aspects of odor discrimination, plasticity, and state-dependent gain control. J. Comp. Neurol. 525:204-230, 2017. © 2016 Wiley Periodicals, Inc.

Spatial dynamics of action potentials estimated by dendritic Ca2+ signals in insect projection neurons

The spatial dynamics of action potentials, including their propagation and the location of spike initiation zone (SIZ), are crucial for the computation of a single neuron. Compared with mammalian central neurons, the spike dynamics of invertebrate neurons remain relatively unknown. Thus, we examined the spike dynamics based on single spike-induced Ca2+ signals in the dendrites of cricket mechanosensory projection neurons, known as giant interneurons (GIs). The Ca2+ transients induced by a synaptically evoked single spike were larger than those induced by an antidromic spike, whereas subthreshold synaptic potentials caused no elevation of Ca2+. These results indicate that synaptic activity enhances the dendritic Ca2+ influx through voltage-gated Ca2+ channels. Stimulation of the presynaptic sensory afferents ipsilateral to the recording site evoked a dendritic spike with higher amplitude than contralateral stimulation, thereby suggesting that alteration of the spike waveform resulted in synaptic enhancement of the dendritic Ca2+ transients. The SIZ estimated from the spatial distribution of the difference in the Ca2+ amplitude was distributed throughout the right and left dendritic branches across the primary neurite connecting them in GIs.

Direction-Specific Adaptation in Neuronal and Behavioral Responses of an Insect Mechanosensory System.

Stimulus-specific adaptation (SSA) is considered to be the neural underpinning of habituation and novelty detection. We found that crickets exhibit stimulus-direction-specific adaptation in wind-elicited avoidance behavior. Repetitive air currents inducing this behavioral adaptation altered the directional selectivity of wind-sensitive giant interneurons (GIs) via direction-specific adaptation mediated by dendritic Ca(2+) elevation. The GIs for which directional tuning was altered displayed heterogeneous direction selectivity in their Ca(2+) dynamics and the transient increase in Ca(2+) evoked by the repeated puffs was restricted to a specific area of dendrites. These results suggest that depression induced by local Ca(2+) accumulation at repetitively activated synapses of key neurons underlies direction-specific behavioral adaptation. Our findings elucidate the subcellular mechanism underlying SSA-like neuronal plasticity related to behavioral adaptation.

Motor neurons in the escape response circuit of white shrimp (Litopenaeus setiferus).

Many decapod crustaceans perform escape tailflips with a neural circuit involving giant interneurons, a specialized fast flexor motor giant (MoG) neuron, populations of larger, less specialized fast flexor motor neurons, and fast extensor motor neurons. These escape-related neurons are well described in crayfish (Reptantia), but not in more basal decapod groups. To clarify the evolution of the escape circuit, I examined the fast flexor and fast extensor motor neurons of white shrimp (Litopenaeus setiferus; Dendrobranchiata) using backfilling. In crayfish, the MoGs in each abdominal ganglion are a bilateral pair of separate neurons. In L. setiferus, the MoGs have massive, possibly syncytial, cell bodies and fused axons. The non-MoG fast flexor motor neurons and fast extensor motor neurons are generally found in similar locations to where they are found in crayfish, but the number of motor neurons in both the flexor and extensor pools is smaller than in crayfish. The loss of fusion in the MoGs and increased number of fast motor neurons in reptantian decapods may be correlated with an increased reliance on non-giant mediated tailflipping.

Habituation of LG-mediated tailflip in the crayfish.

Crayfish escape from threatening stimuli by tailflipping. If a stimulus is applied to the rear, crayfish escape up and forwards in a summersault maneuver that is mediated by the activation of lateral giant (LG) interneurons. The occurrence probability of LG-mediated tailflip, however, diminishes and habituates if a stimulus is repeatedly applied. Since crayfish have a relatively simple CNS with many identifiable neurons, crayfish represent a good animal to analyze the cellular basis of habituation. A reduction in the amplitude of the EPSP in the LGs, caused by direct chemical synaptic connection from sensory afferents by repetitive stimulations, is essential to bring about an inactivation of the LGs. The spike response of the LGs recovers within several minutes of habituation, but the LGs subsequently fail to spike when an additional stimulus is applied after specific periods following habituation. These results indicate that a decline in synaptic efficacy from the mechanosensory afferents recovers readily after a short delay, but then the excitability of the LGs themselves decreases. Furthermore, the processes underlying habituation are modulated depending on a social status. When two crayfish encounter each other, a winner-loser relationship is established. With a short interstimulus interval of 5s, the rate of habituation of the LG in both socially dominant and subordinate crayfish becomes lower than in socially isolated animals. Serotonin and octopamine affect this social status-dependent modulation of habituation by means of activation of downstream second messenger system of cAMP and IP3 cascades, respectively.

Ca2+ imaging of cricket protocerebrum responses to air current stimulation

Crickets (Gryllus bimaculatus) use the cercal sensory system at the rear of the abdomen to detect air currents and direct predator avoidance behavior. Sensory information regarding the direction and dynamic properties of air currents is processed within the terminal abdominal ganglion, and conveyed by ascending giant interneurons (GIs) to higher centers including the brain. However, the brain region responsible for decoding cercal sensory information has not yet been identified, nor the response properties within the brain characterized. In this study, we performed in vivo Ca2+ imaging to investigate wind-evoked neural activities within the cricket protocerebrum. Ca2+ responses to air current stimuli were observed at peripheral regions of the ventrolateral neuropile (VLNP) where projection of GIs’ axon terminals has been observed in larvae. The wind-evoked Ca2+ response had temporal dynamics and directional sensitivity that varied with different recorded regions displaying transient or sustained Ca2+ increases. Individual cells showed Ca2+ elevation in response to air currents from a specific angle, while stimuli from a different angle evoked decreased signals. Removing the antennae reduced the air-current-evoked responses in VLNP, suggesting contribution of sensory inputs from antennae in addition to the cercal inputs. The VLNP is presumably an integrative center for mechanosensory processing from antennae and cerci where directional information is primarily decoded by protocerebral neurons.

Corollary discharge inhibition of wind-sensitive cercal giant interneurons in the singing field cricket.

Crickets carry wind-sensitive mechanoreceptors on their cerci, which, in response to the airflow produced by approaching predators, triggers escape reactions via ascending giant interneurons (GIs). Males also activate their cercal system by air currents generated due to the wing movements underlying sound production. Singing males still respond to external wind stimulation, but are not startled by the self-generated airflow. To investigate how the nervous system discriminates sensory responses to self-generated and external airflow, we intracellularly recorded wind-sensitive afferents and ventral GIs of the cercal escape pathway in fictively singing crickets, a situation lacking any self-stimulation. GI spiking was reduced whenever cercal wind stimulation coincided with singing motor activity. The axonal terminals of cercal afferents showed no indication of presynaptic inhibition during singing. In two ventral GIs, however, a corollary discharge inhibition occurred strictly in phase with the singing motor pattern. Paired intracellular recordings revealed that this inhibition was not mediated by the activity of the previously identified corollary discharge interneuron (CDI) that rhythmically inhibits the auditory pathway during singing. Cercal wind stimulation, however, reduced the spike activity of this CDI by postsynaptic inhibition. Our study reveals how precisely timed corollary discharge inhibition of ventral GIs can prevent self-generated airflow from triggering inadvertent escape responses in singing crickets. The results indicate that the responsiveness of the auditory and wind-sensitive pathway is modulated by distinct CDIs in singing crickets and that the corollary discharge inhibition in the auditory pathway can be attenuated by cercal wind stimulation.

Netrin and Frazzled regulate presynaptic gap junctions at a Drosophila giant synapse.

Netrin and its receptor, Frazzled, dictate the strength of synaptic connections in the giant fiber system (GFS) of Drosophila melanogaster by regulating gap junction localization in the presynaptic terminal. In Netrin mutant animals, the synaptic coupling between a giant interneuron and the "jump" motor neuron was weakened and dye coupling between these two neurons was severely compromised or absent. In cases in which Netrin mutants displayed apparently normal synaptic anatomy, half of the specimens exhibited physiologically defective synapses and dye coupling between the giant fiber (GF) and the motor neuron was reduced or eliminated, suggesting that gap junctions were disrupted in the Netrin mutants. When we examined the gap junctions with antibodies to Shaking-B (ShakB) Innexin, they were significantly decreased or absent in the presynaptic terminal of the mutant GF. Frazzled loss of function mutants exhibited similar defects in synaptic transmission, dye coupling, and gap junction localization. These data are the first to show that Netrin and Frazzled regulate the placement of gap junctions presynaptically at a synapse.

Neural Basis of Stimulus-Angle-Dependent Motor Control of Wind-Elicited Walking Behavior in the Cricket Gryllus bimaculatus

Crickets exhibit oriented walking behavior in response to air-current stimuli. Because crickets move in the opposite direction from the stimulus source, this behavior is considered to represent ‘escape behavior’ from an approaching predator. However, details of the stimulus-angle-dependent control of locomotion during the immediate phase, and the neural basis underlying the directional motor control of this behavior remain unclear. In this study, we used a spherical-treadmill system to measure locomotory parameters including trajectory, turn angle and velocity during the immediate phase of responses to air-puff stimuli applied from various angles. Both walking direction and turn angle were correlated with stimulus angle, but their relationships followed different rules. A shorter stimulus also induced directionally-controlled walking, but reduced the yaw rotation in stimulus-angle-dependent turning. These results suggest that neural control of the turn angle requires different sensory information than that required for oriented walking. Hemi-severance of the ventral nerve cords containing descending axons from the cephalic to the prothoracic ganglion abolished stimulus-angle-dependent control, indicating that this control required descending signals from the brain. Furthermore, we selectively ablated identified ascending giant interneurons (GIs) in vivo to examine their functional roles in wind-elicited walking. Ablation of GI8-1 diminished control of the turn angle and decreased walking distance in the initial response. Meanwhile, GI9-1b ablation had no discernible effect on stimulus-angle-dependent control or walking distance, but delayed the reaction time. These results suggest that the ascending signals conveyed by GI8-1 are required for turn-angle control and maintenance of walking behavior, and that GI9-1b is responsible for rapid initiation of walking. It is possible that individual types of GIs separately supply the sensory signals required to control wind-elicited walking.

Short communications

This issue of JEB sees the first three articles in a new journal section: Short Communications. Introduced in response to author feedback, this brief (<2500 words) style of article aims to provide a venue for high-quality, hypothesis-driven, self-contained pieces of original research. The articles should not be preliminary reports or merely incremental studies but should augment the body of knowledge and be of significant and broad interest to the field of comparative physiology. Additionally, they may be opinionated and challenge existing theories and/or propose new theories or concepts based on existing research.The three Short Communications in this issue cover the breadth of fields published in the journal, ranging from the repeatability of metabolic rate to the way ants use visual information to navigate to the role of insulin-related peptides in memory formation.In the first of this new style of report, Craig White, Natalie Schimpf and Phillip Casey (p. 1763) conduct a meta-analysis to compare the contrasting results of two previously published JEB papers on metabolic measurements. In the first study, Nespolo and Franco (J. Exp. Biol. 210, 2000-2005) concluded that metabolic rate is generally a repeatable trait and that repeatability is not affected by the time between measurements. However, in a later study, Norin and Malte (J. Exp. Biol. 214, 1668-1675) found that repeatability of metabolic rate for a single species (brown trout) was high in the short term but declined to essentially nothing in the long term. White and colleagues carried out their own meta-analysis of all data for metabolic rate published to date, demonstrating that the repeatability of metabolic rate decreases with time for both ectotherms and endotherms. Confirming the generality of Norin and Malte's findings, this analysis has implications for studies testing for phenotypic associations among metabolic rate and other traits, because variation among individuals is the raw material on which natural selection acts, and inter-individual differences must be both consistent and genetically based for traits to respond to selection.Challenging the perceived dogma about the information that ants store about their surroundings, Antoine Wystrach and colleagues propose an alternative method by which ants use visual information to navigate complex idiosyncratic routes to a discrete goal. Since the seminal work on ant navigation published by Cartwright and Collett in 1983 (J. Comp. Physiol. A 151, 521-543), the commonly held view is that ants rely on visual cues and store multiple ‘snapshot memories’ at discrete locations that are retrieved sequentially to break routes into sections. However, in their article on p. 1766, Wystrach and colleagues employ a computational model they have recently developed (Baddeley et al. PLoS Comput. Biol. 8, e1002336) that uses a neural network to gather information from all experienced views and compile it into a single ‘holistic memory’, which can then be used to determine the most familiar heading at a given location. By applying this familiarity-based algorithm, the team was able to replicate behaviours observed in key behavioural studies and thus show that whatever information-processing architecture insects use for navigation, it does not require the tricky problem of learning, storing and using discrete snapshots sequentially.In the last of our inaugural Short Communication studies, Etsuro Ito and colleagues continue their analysis of the role of insulin-related peptides in memory formation. Using the ability of the pond snail Lymnaea to learn taste aversion and to consolidate this learning into long-term memory (conditioned taste aversion), the international team has previously shown that molluscan insulin-related peptides (MIPs) are upregulated in snails exhibiting conditioned taste aversion and that exogenous application of MIPs to the isolated central nervous system evokes long-term synaptic enhancement at the synaptic connection between the cerebral giant interneuron and the B1 buccal motor neuron – key neurons in memory formation. Following on from this work, Jun Murakami and co-workers (p. 1771) now show that the synaptic enhancement – hypothesized to be the neural correlate of behavioural memory – is due to changes on the postsynaptic B1 neuron, emphasizing that both postsynaptic and presynaptic changes needs to be considered when studying the neuronal basis of memory formation in invertebrates. In addition, the study demonstrates that the cognitive enhancements brought about by insulin can be studied at the level of single neurons in invertebrates, which may help to further understand the function of insulin in the more complex mammalian brain.Having launched this new article type for the communication of concise, self-contained studies, we look forward to receiving more submissions in this area. Further information about submitting a Short Communication to JEB can be found at http://jeb.biologists.org/site/author/article_types.xhtml#short.

Pretreatment of the cockroach cercal afferent/giant interneuron synapses with nicotinoids and neonicotinoids differently affects acetylcholine and nicotine-induced ganglionic depolarizations

We have recently demonstrated that neonicotinoid insecticides were able to act as agonists of postsynaptic nicotinic acetylcholine receptors (nAChRs) expressed at the synapse between the cercal nerve XI and the giant interneurons, in the sixth abdominal ganglion. In this work, we demonstrated that nicotinoids such as nornicotine acted as an agonist of nicotinic acetylcholine receptors expressed at cercal afferent/giant interneurons while cotinine was a poor agonist. Indeed, nornicotine induced a ganglionic depolarization which was blocked by the nicotinic antagonist mecamylamine. In addition, we found that pretreatment of the sixth abdominal ganglion with 1 and 10μM nornicotine and cotinine had no significant effect on acetylcholine and nicotine-induced depolarization. But pretreatment with 1 and 10μM acetamiprid and imidacloprid had a strong effect. 1 and 10μM acetamiprid completely blocked acetylcholine-induced depolarization, whereas imidacloprid had a partial effect. The present work therefore suggests, in agreement with previous studies, that nornicotine and cotinine bind to distinct cockroach postsynaptic nAChRs, whereas acetamiprid and imidacloprid have competitive effects with acetylcholine and nicotine on ganglionic depolarization.

Increase in cyclic AMP concentration in a cerebral giant interneuron mimics part of a memory trace for conditioned taste aversion of the pond snail.

Conditioned taste aversion (CTA) can be classically conditioned in the pond snail Lymnaea stagnalis and subsequently be consolidated into long-term memory (LTM). The neural trace that subserves CTA-LTM can be summarized as follows: A polysynaptic inhibitory postsynaptic potential recorded in the neuron 1 medial (N1M) cell in the conditioned snails as a result of activation of the cerebral giant cell (CGC) is larger and lasts longer than that in control snails. The N1M cell is ultimately activated by the CGC via the neuron 3 tonic (N3t) cell. That is, the inhibitory monosynaptic inputs from the N3t cell to the N1M cell are facilitated. The N1M and N3t cells are the members of feeding central pattern generator, whereas the CGC is a multimodal interneuron thought to play a key role in feeding behavior. Here we examined the involvement of a second messenger, cAMP, in the establishment of the memory trace. We injected cAMP into the CGC and monitored the potentials of the B3 motor neuron activated by the CGC. B3 activity is used as an index for the synaptic inputs from the N3t cell to the N1M cell. We found that the B3 potentials were transiently enlarged. Thus, when the cAMP concentration is increased in the CGC by taste aversion training, cAMP-induced changes may play a key role in the establishment of a memory trace in the N3t cell.

Responses of cricket cercal interneurons to realistic naturalistic stimuli in the field

The ability of the insect cercal system to detect approaching predators has been studied extensively in the laboratory and in the field. Some previous studies have assessed the extent to which sensory noise affects the operational characteristics of the cercal system, but these studies have only been carried out in laboratory settings using white noise stimuli of unrealistic nature. Using a piston mimicking the natural airflow of an approaching predator, we recorded the neural activity through the abdominal connectives from the terminal abdominal ganglion of freely moving wood crickets (Nemobius sylvestris) in a semi-field situation. A cluster analysis of spike amplitudes revealed six clusters, or 'units', corresponding to six different subsets of cercal interneurons. No spontaneous activity was recorded for the units of larger amplitude, reinforcing the idea they correspond to the largest giant interneurons. Many of the cercal units are already activated by background noise, sometimes only weakly, and the approach of a predator is signaled by an increase in their activity, in particular for the larger-amplitude units. A scaling law predicts that the cumulative number of spikes is a function of the velocity of the flow perceived at the rear of the cricket, including a multiplicative factor that increases linearly with piston velocity. We discuss the implications of this finding in terms of how the cricket might infer the imminence and nature of a predatory attack.

IP3-mediated octopamine-induced synaptic enhancement of crayfish LG neurons

The biogenic amines, octopamine and serotonin, modulate the synaptic activity of the lateral giant interneuron (LG) circuitry of the crayfish escape behavior. Bath application of both octopamine and serotonin enhances the synaptic responses of LG to sensory stimulation. We have shown previously (Araki et al. J Neurophysiol 94:2644-2652, 2005) that a serotonin-induced enhancement of the LG response was mediated by an increase in cAMP levels following activation of adenylate cyclase; however, octopamine acts independently. Here, we clarify how octopamine enhances the LG response during sensory stimulation using physiological and pharmacological analyses. When phospholipase C inhibitor U-73122 was directly injected into the LG before biogenic amine application, it abolished the enhancing effect of octopamine on direct sensory input to the LG, but did not block indirect input via sensory interneurons or the effect of serotonin. Direct injection of IP(3), and its analogue adenophostin A, into the LG increased the synaptic response of the LG to sensory stimulation. Thus, IP(3) mediates octopamine-induced synaptic enhancement of the LG, but serotonin acts independently. These results indicate that both octopamine and serotonin enhance the synaptic responses of the LG to sensory stimulation, but that they activate two different signaling cascades in the LG.