GABA and responses to GABA in the stomatogastric ganglion of the crab Cancer borealis

Hibernation reduces GABA signaling in the brainstem to enhance motor activity of breathing at cool temperatures

BACKGROUND

Neural circuits produce reliable activity patterns despite disturbances in the environment. For this to occur, neurons elicit synaptic plasticity during perturbations. However, recent work suggests that plasticity not only regulates circuit activity during disturbances, but these modifications may also linger to stabilize circuits during future perturbations. The implementation of such a regulation scheme for real-life environmental challenges of animals remains unclear. Amphibians provide insight into this problem in a rather extreme way, as circuits that generate breathing are inactive for several months during underwater hibernation and use compensatory plasticity to promote ventilation upon emergence.

RESULTS

Using ex vivo brainstem preparations and electrophysiology, we find that hibernation in American bullfrogs reduces GABAA receptor (GABAAR) inhibition in respiratory rhythm generating circuits and motor neurons, consistent with a compensatory response to chronic inactivity. Although GABAARs are normally critical for breathing, baseline network output at warm temperatures was not affected. However, when assessed across a range of temperatures, hibernators with reduced GABAAR signaling had greater activity at cooler temperatures, enhancing respiratory motor output under conditions that otherwise strongly depress breathing.

CONCLUSIONS

Hibernation reduces GABAAR signaling to promote robust respiratory output only at cooler temperatures. Although frogs do not ventilate lungs during underwater hibernation, we suggest this would be beneficial for stabilizing breathing when the animal passes through a large temperature range during emergence in the spring. More broadly, these results demonstrate that compensatory synaptic plasticity can increase the operating range of circuits in harsh environments, thereby promoting adaptive behavior in conditions that suppress activity.

UBR-1 ubiquitin ligase regulates the balance between GABAergic and glutamatergic signaling

Excitation/inhibition (E/I) balance is carefully maintained by the nervous system. The neurotransmitter GABA has been reported to be co-released with its sole precursor, the neurotransmitter glutamate. The genetic and circuitry mechanisms to establish the balance between GABAergic and glutamatergic signaling have not been fully elucidated. Caenorhabditis elegans DVB is an excitatory GABAergic motoneuron that drives the expulsion step in the defecation motor program. We show here that in addition to UNC-47, the vesicular GABA transporter, DVB also expresses EAT-4, a vesicular glutamate transporter. UBR-1, a conserved ubiquitin ligase, regulates DVB activity by suppressing a bidirectional inhibitory glutamate signaling. Loss of UBR-1 impairs DVB Ca2+ activity and expulsion frequency. These impairments are fully compensated by the knockdown of EAT-4 in DVB. Further, glutamate-gated chloride channels GLC-3 and GLC-2/4 receive DVB's glutamate signals to inhibit DVB and enteric muscle activity, respectively. These results implicate an intrinsic cellular mechanism that promotes the inherent asymmetric neural activity. We propose that elevated glutamate in ubr-1 mutants, being the cause of the E/I shift, potentially contributes to Johanson Blizzard syndrome.

Neural circuit regulation by identified modulatory projection neurons

Rhythmic behaviors (e.g., walking, breathing, and chewing) are produced by central pattern generator (CPG) circuits. These circuits are highly dynamic due to a multitude of input they receive from hormones, sensory neurons, and modulatory projection neurons. Such inputs not only turn CPG circuits on and off, but they adjust their synaptic and cellular properties to select behaviorally relevant outputs that last from seconds to hours. Similar to the contributions of fully identified connectomes to establishing general principles of circuit function and flexibility, identified modulatory neurons have enabled key insights into neural circuit modulation. For instance, while bath-applying neuromodulators continues to be an important approach to studying neural circuit modulation, this approach does not always mimic the neural circuit response to neuronal release of the same modulator. There is additional complexity in the actions of neuronally-released modulators due to: (1) the prevalence of co-transmitters, (2) local- and long-distance feedback regulating the timing of (co-)release, and (3) differential regulation of co-transmitter release. Identifying the physiological stimuli (e.g., identified sensory neurons) that activate modulatory projection neurons has demonstrated multiple “modulatory codes” for selecting particular circuit outputs. In some cases, population coding occurs, and in others circuit output is determined by the firing pattern and rate of the modulatory projection neurons. The ability to perform electrophysiological recordings and manipulations of small populations of identified neurons at multiple levels of rhythmic motor systems remains an important approach for determining the cellular and synaptic mechanisms underlying the rapid adaptability of rhythmic neural circuits.

Functional Characteristics of the Lepidopteran Ionotropic GABA Receptor 8916 Subunit Interacting with the LCCH3 or the RDL Subunit

The ionotropic γ-aminobutyric acid (iGABA) receptor is commonly considered as a fast inhibitory channel and is an important insecticide target. Since 1990, RDL, LCCH3, and GRD have been successively isolated and found to be potential subunits of the insect iGABA receptor. More recently, one orphan gene named 8916 was found and considered to be another potential iGABA receptor subunit according to its amino acid sequence. However, little information about 8916 has been reported. Here, the 8916 subunit from Chilo suppressalis was studied to determine whether it can form part of a functional iGABA receptor by co-expressing this subunit with CsRDL1 or CsLCCH3 in the Xenopus oocyte system. Cs8916 or CsLCCH3 did not form functional ion channels when expressed alone. However, Cs8916 was able to form heteromeric ion channels when expressed with either CsLCCH3 or CsRDL1. The recombinant heteromeric Cs8916/LCCH3 channel was a cation-selective channel, which was sensitive to GABA or β-alanine. The current of the Cs8916/LCCH3 channel was inhibited by dieldrin, endosulfan, fipronil, or ethiprole. In contrast, fluralaner, broflanilide, and avermectin showed little effect on the Cs8916/LCCH3 channel (IC₅₀s > 10 000 nM). The Cs8916/RDL1 channel was sensitive to GABA, but was significantly different in EC₅₀ and I_max for GABA to those of homomeric CsRDL1. Fluralaner, fipronil, or dieldrin showed antagonistic actions on Cs8916/RDL1. In conclusion, Cs8916 is a potential iGABA receptor subunit, which can interact with CsLCCH3 to generate a cation-selective channel that is sensitive to several insecticides. Also, as Cs8916/RDL1 has a higher EC₅₀ than homomeric CsRDL1, Cs8916 may affect the physiological functions of CsRDL1 and therefore play a role in fine-tuning GABAergic signaling.

Dual modulatory effects on feedback from a proprioceptor in the crustacean stomatogastric nervous system

Neuromodulatory actions that change the properties of proprioceptors or the muscle movements to which they respond necessarily affect the feedback provided to the central network. Here we further characterize the responses of the gastropyloric receptor 1 (GPR1) and gastropyloric receptor 2 (GPR2) neurons in the stomatogastric nervous system of the crab Cancer borealis to movements and contractions of muscles, and we report how neuromodulation modifies those responses. We observed that the GPR1 response to contractions of the gastric mill 4 muscle (gm4) was absent, or nearly so, when the neuron was quiescent but robust when it was spontaneously active. We also found that the effects of four neuromodulatory substances (GABA, serotonin, proctolin, and TNRNFLRFamide) on the GPR1 response to muscle stretch were similar to those previously reported for GPR2. Finally, we showed that an excitatory action on gm4 due to proctolin combined with an inhibitory action on GPR2 due to GABA can allow for larger muscle contractions without increased proprioceptive feedback.NEW & NOTEWORTHY We report that the combination of GABA and the peptide proctolin increases contraction of a stomatogastric muscle while decreasing the corresponding response of the proprioceptor that reports on it. These results suggest a general mechanism by which muscle movements can be modified while sensory feedback is conserved, one that may be particularly well suited for providing flexibility to central pattern generator networks.

GABA as a Neurotransmitter in Gastropod Molluscs

The neurotransmitter gamma-aminobutyric acid (GABA) is widely distributed in the mammalian central nervous system, where it acts as a major mediator of synaptic inhibition. GABA also serves as a neurotransmitter in a range of invertebrate phyla, including arthropods, echinoderms, annelids, nematodes, and platyhelminthes. This article reviews evidence supporting the neurotransmitter role of GABA in gastropod molluscs, with an emphasis on its presence in identified neurons and well-characterized neural circuits. The collective findings indicate that GABAergic signaling participates in the selection and specification of motor programs, as well as the bilateral coordination of motor circuits. While relatively few in number, GABAergic neurons can influence neural circuits via inhibitory, excitatory, and modulatory synaptic actions. GABA's colocalization with peptidergic and classical neurotransmitters can broaden its integrative capacity. The functional properties of GABAergic neurons in simpler gastropod systems may provide insight into the role of this neurotransmitter phenotype in more complex brains.

Neuronal morphologies built for reliable physiology in a rhythmic motor circuit

It is often assumed that highly-branched neuronal structures perform compartmentalized computations. However, previously we showed that the Gastric Mill (GM) neuron in the crustacean stomatogastric ganglion (STG) operates like a single electrotonic compartment, despite having thousands of branch points and total cable length >10 mm (Otopalik et al., 2017a; 2017b). Here we show that compact electrotonic architecture is generalizable to other STG neuron types, and that these neurons present direction-insensitive, linear voltage integration, suggesting they pool synaptic inputs across their neuronal structures. We also show, using simulations of 720 cable models spanning a broad range of geometries and passive properties, that compact electrotonus, linear integration, and directional insensitivity in STG neurons arise from their neurite geometries (diameters tapering from 10-20 µm to < 2 µm at their terminal tips). A broad parameter search reveals multiple morphological and biophysical solutions for achieving different degrees of passive electrotonic decrement and computational strategies in the absence of active properties.

Profiling neurotransmitters in a crustacean neural circuit for locomotion

Locomotor systems are widely used to study rhythmically active neural networks. These networks have to be coordinated in order to produce meaningful behavior. The crayfish swimmeret system is well suited to investigate such coordination of distributed neural oscillators because the neurons and their connectivity for generating and especially for coordinating the motor output are identified. The system maintains a fixed phase lag between the segmental oscillators, independent of cycle period. To further the understanding of the system’s plasticity for keeping the phase lag fixed, we profiled the neurotransmitters used by the Coordinating Neurons, which are necessary and sufficient for coordination of the segmental oscillators. We used a combination of electrophysiological, immunohistochemical, and mass spectrometric methods. This arrangement of methods ensured that we could screen for several specific neurotransmitters, since a single method is often not suitable for all neurotransmitters of interest. In a first step, to preselect neurotransmitter candidates, we investigated the effect of substances known to be present in some swimmeret system neurons on the motor output and coordination. Subsequently, we demonstrated electrophysiologically that the identified synapse between the Coordinating Neurons and their target is mainly chemical, but neither glutamate antagonist nor γ-aminobutyric acid antagonist application affected this synapse. With immunohistochemical experiments, we provide strong evidence that the Coordinating Neurons are not serotonergic. Single-cell MALDI-TOF mass spectrometry with subsequent principal component analysis identified acetylcholine as the putative neurotransmitter for both types of Coordinating Neurons.

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.

The Gastric Ganglion of Octopus vulgaris: Preliminary Characterization of Gene- and Putative Neurochemical-Complexity, and the Effect of Aggregata octopiana Digestive Tract Infection on Gene Expression

The gastric ganglion is the largest visceral ganglion in cephalopods. It is connected to the brain and is implicated in regulation of digestive tract functions. Here we have investigated the neurochemical complexity (through in silico gene expression analysis and immunohistochemistry) of the gastric ganglion in Octopus vulgaris and tested whether the expression of a selected number of genes was influenced by the magnitude of digestive tract parasitic infection by Aggregata octopiana. Novel evidence was obtained for putative peptide and non-peptide neurotransmitters in the gastric ganglion: cephalotocin, corticotrophin releasing factor, FMRFamide, gamma amino butyric acid, 5-hydroxytryptamine, molluscan insulin-related peptide 3, peptide PRQFV-amide, and tachykinin-related peptide. Receptors for cholecystokininA and cholecystokininB, and orexin2 were also identified in this context for the first time. We report evidence for acetylcholine, dopamine, noradrenaline, octopamine, small cardioactive peptide related peptide, and receptors for cephalotocin and octopressin, confirming previous publications. The effects of Aggregata observed here extend those previously described by showing effects on the gastric ganglion; in animals with a higher level of infection, genes implicated in inflammation (NFκB, fascin, serpinB10 and the toll-like 3 receptor) increased their relative expression, but TNF-α gene expression was lower as was expression of other genes implicated in oxidative stress (i.e., superoxide dismutase, peroxiredoxin 6, and glutathione peroxidase). Elevated Aggregata levels in the octopuses corresponded to an increase in the expression of the cholecystokininA receptor and the small cardioactive peptide-related peptide. In contrast, we observed decreased relative expression of cephalotocin, dopamine β-hydroxylase, peptide PRQFV-amide, and tachykinin-related peptide genes. A discussion is provided on (i) potential roles of the various molecules in food intake regulation and digestive tract motility control and (ii) the difference in relative gene expression in the gastric ganglion in octopus with relatively high and low parasitic loads and the similarities to changes in the enteric innervation of mammals with digestive tract parasites. Our results provide additional data to the described neurochemical complexity of O. vulgaris gastric ganglion.

Functional consequences of neuropeptide and small-molecule co-transmission

Colocalization of small-molecule and neuropeptide transmitters is common throughout the nervous system of all animals. The resulting co-transmission, which provides conjoint ionotropic ('classical') and metabotropic ('modulatory') actions, includes neuropeptide- specific aspects that are qualitatively different from those that result from metabotropic actions of small-molecule transmitter release. Here, we focus on the flexibility afforded to microcircuits by such co-transmission, using examples from various nervous systems. Insights from such studies indicate that co-transmission mediated even by a single neuron can configure microcircuit activity via an array of contributing mechanisms, operating on multiple timescales, to enhance both behavioural flexibility and robustness.

Circuit feedback increases activity level of a circuit input through interactions with intrinsic properties

Central pattern generator (CPG) motor circuits underlying rhythmic behaviors provide feedback to the projection neuron inputs that drive these circuits. This feedback elicits projection neuron bursting linked to CPG rhythms. The brief periodic interruptions in projection neuron activity in turn influence CPG output, gate sensory input, and enable coordination of multiple target CPGs. However, despite the importance of the projection neuron activity level for circuit output, it remains unknown whether feedback also regulates projection neuron intraburst firing rates. I addressed this issue using identified neurons in the stomatogastric nervous system of the crab, Cancer borealis, a small motor system controlling chewing and filtering of food. Mechanosensory input triggers long-lasting activation of two projection neurons to elicit a chewing rhythm, during which their activity is patterned by circuit feedback. Here I show that feedback increases the intraburst firing rate of only one of the two projection neurons (commissural projection neuron 2: CPN2). Furthermore, this is not a fixed property because the CPN2 intraburst firing rate is decreased instead of increased by feedback when a chewing rhythm is activated by a different modulatory input. I establish that a feedback pathway that does not impact the CPN2 activity level in the control state inhibits CPN2 sufficiently to trigger postinhibitory rebound following mechanosensory stimulation. The rebound increases the CPN2 intraburst firing rate above the rate due only to mechanosensory activation of CPN2. Thus in addition to patterning projection neuron activity, circuit feedback can adjust the intraburst firing rate, demonstrating a novel functional role for circuit feedback to central projection neurons.NEW & NOTEWORTHY Feedback from central pattern generator (CPG) circuits patterns activity of their projection neuron inputs. However, whether the intraburst firing rate between rhythmic feedback inhibition is also impacted by CPG feedback was not known. I establish that CPG feedback can alter the projection neuron intraburst firing rate through interactions with projection neuron intrinsic properties. The contribution of feedback to projection neuron activity level is specific to the modulatory condition, demonstrating a state dependence for this novel role of circuit feedback.

Deep sequencing of transcriptomes from the nervous systems of two decapod crustaceans to characterize genes important for neural circuit function and modulation

BACKGROUND

Crustaceans have been studied extensively as model systems for nervous system function from single neuron properties to behavior. However, lack of molecular sequence information and tools have slowed the adoption of these physiological systems as molecular model systems. In this study, we sequenced and performed de novo assembly for the nervous system transcriptomes of two decapod crustaceans: the Jonah crab (Cancer borealis) and the American lobster (Homarus americanus).

RESULTS

Forty-two thousand, seven hundred sixty-six and sixty thousand, two hundred seventy-three contigs were assembled from C. borealis and H. americanus respectively, representing 9,489 and 11,061 unique coding sequences. From these transcripts, genes associated with neural function were identified and manually curated to produce a characterization of multiple gene families important for nervous system function. This included genes for 34 distinct ion channel types, 17 biogenic amine and 5 GABA receptors, 28 major transmitter receptor subtypes including glutamate and acetylcholine receptors, and 6 gap junction proteins - the Innexins.

CONCLUSION

With this resource, crustacean model systems are better poised for incorporation of modern genomic and molecular biology technologies to further enhance the interrogation of fundamentals of nervous system function.

An extrasynaptic GABAergic signal modulates a pattern of forward movement in Caenorhabditis elegans

As a common neurotransmitter in the nervous system, γ-aminobutyric acid (GABA) modulates locomotory patterns in both vertebrates and invertebrates. However, the signaling mechanisms underlying the behavioral effects of GABAergic modulation are not completely understood. Here, we demonstrate that a GABAergic signal in C. elegans modulates the amplitude of undulatory head bending through extrasynaptic neurotransmission and conserved metabotropic receptors. We show that the GABAergic RME head motor neurons generate undulatory activity patterns that correlate with head bending and the activity of RME causally links with head bending amplitude. The undulatory activity of RME is regulated by a pair of cholinergic head motor neurons SMD, which facilitate head bending, and inhibits SMD to limit head bending. The extrasynaptic neurotransmission between SMD and RME provides a gain control system to set head bending amplitude to a value correlated with optimal efficiency of forward movement.

DOI:http://dx.doi.org/10.7554/eLife.14197.001

Differential effects of GABA in modulating nociceptive vs. non-nociceptive synapses

GABA (γ-amino-butyric acid) -mediated signaling is normally associated with synaptic inhibition due to ionotropic GABA receptors that gate an inward Cl(-) current, hyperpolarizing the membrane potential. However, there are also situations where ionotropic GABA receptors trigger a Cl(-) efflux that results in depolarization. The well-characterized central nervous system of the medicinal leech was used to study the functional significance of opposing effects of GABA at the synaptic circuit level. Specifically, we focused on synapses made by the nociceptive N cell and the non-nociceptive P (pressure) cell that converge onto a common postsynaptic target. It is already known that GABA hyperpolarizes the P cell, but depolarizes the N cell and that inhibition of ionotropic GABA receptors by bicuculline (BIC) has opposing effects on the synapses made by these two inputs; enhancing P cell synaptic transmission, but depressing N cell synapses. The goal of the present study was to determine whether the opposing effects of GABA were due to differences in Cl(-) homeostasis between the two presynaptic neurons. VU 0240551 (VU), an inhibitor of the Cl(-) exporter K-Cl co-transporter isoform 2 (KCC2), attenuated GABA-mediated hyperpolarization of the non-nociceptive afferent while bumetanide (BUM), an inhibitor of the Cl(-) importer Na-K-Cl co-transporter isoform 1 (NKCC1), reduced GABA-mediated depolarization of the nociceptive neuron. VU treatment also enhanced P cell synaptic signaling, similar to the previously observed effects of BIC and consistent with the idea that GABA inhibits synaptic signaling at the presynaptic level. BUM treatment depressed N cell synapses, again similar to what is observed following BIC treatment and suggests that GABA has an excitatory effect on these synapses. The opposing effects of GABA could also be observed at the behavioral level with BIC and VU increasing responsiveness to non-nociceptive stimulation while BIC and BUM decreased responsiveness to nociceptive stimulation. These findings demonstrate that distinct synaptic inputs within a shared neural circuit can be differentially modulated by GABA in a functionally relevant manner.

Comparison of two voltage-sensitive dyes and their suitability for long-term imaging of neuronal activity

One of the key approaches for studying neural network function is the simultaneous measurement of the activity of many neurons. Voltage-sensitive dyes (VSDs) simultaneously report the membrane potential of multiple neurons, but often have pharmacological and phototoxic effects on neuronal cells. Yet, to study the homeostatic processes that regulate neural network function long-term recordings of neuronal activities are required. This study aims to test the suitability of the VSDs RH795 and Di-4-ANEPPS for optically recording pattern generating neurons in the stomatogastric nervous system of crustaceans with an emphasis on long-term recordings of the pyloric central pattern generator. We demonstrate that both dyes stain pyloric neurons and determined an optimal concentration and light intensity for optical imaging. Although both dyes provided sufficient signal-to-noise ratio for measuring membrane potentials, Di-4-ANEPPS displayed a higher signal quality indicating an advantage of this dye over RH795 when small neuronal signals need to be recorded. For Di-4-ANEPPS, higher dye concentrations resulted in faster and brighter staining. Signal quality, however, only depended on excitation light strength, but not on dye concentration. RH795 showed weak and slowly developing phototoxic effects on the pyloric motor pattern as well as slow bleaching of the staining and is thus the better choice for long-term experiments. Low concentrations and low excitation intensities can be used as, in contrast to Di-4-ANEPPS, the signal-to-noise ratio was independent of excitation light strength. In summary, RH795 and Di-4-ANEPPS are suitable for optical imaging in the stomatogastric nervous system of crustaceans. They allow simultaneous recording of the membrane potential of multiple neurons with high signal quality. While Di-4-ANEPPS is better suited for short-term experiments that require high signal quality, RH795 is a better candidate for long-term experiments since it has only minor effects on the motor pattern.

Small organic solutes in sticky droplets from orb webs of the spider Zygiella atrica (Araneae; Araneidae): β-alaninamide is a novel and abundant component

In northeastern North America, Zygiella atrica often build their orb webs near the ocean. We analyzed individual field-built Z. atrica webs to determine if organic low-molecular-mass solutes (LMM) in their sticky droplets showed any unusual features not previously seen in orb webs of other species living in less salty environments. While two of the three most abundant organic LMM (putrescine (butane-1,4-diamine) and GABamide (4-aminobutanamide)) are already well-known from webs of inland spiders, the third major LMM, β-alaninamide (3-aminopropanamide), a homolog of GABamide, has not been detected in sticky droplets from any other araneoid spiders (27 species). It remains to be established, however, whether or not use of β-alaninamide is related to proximity to saltwater. We observed variability in organic LMM composition in Z. atrica webs that appeared to be influenced more by an undetermined factor associated with different collecting locations and/or collection dates than by different genders or instars. Shifts in composition when adult females were transferred from the field to the laboratory were also observed. Structural similarities and inverse correlations among β-alaninamide, GABamide, and N-acetylputrescine suggest that they may form a series of LMM fulfilling essentially the same, as yet unknown, role in the webs of those species in which they occur.

Distribution and physiological effects of B-type allatostatins (myoinhibitory peptides, MIPs) in the stomatogastric nervous system of the crab Cancer borealis

The crustacean stomatogastric ganglion (STG) is modulated by a large number of amines and neuropeptides that are found in descending pathways from anterior ganglia or reach the STG via the hemolymph. Among these are the allatostatin (AST) B types, also known as myoinhibitory peptides (MIPs). We used mass spectrometry to determine the sequences of nine members of the AST-B family of peptides that were found in the stomatogastric nervous system of the crab Cancer borealis. We raised an antibody against Cancer borealis allatostatin-B1 (CbAST-B1; VPNDWAHFRGSWa) and used it to map the distribution of CbAST-B1-like immunoreactivity (-LI) in the stomatogastric nervous system. CbAST-B1-LI was found in neurons and neuropil in the commissural ganglia (CoGs), in somata in the esophageal ganglion (OG), in fibers in the stomatogastric nerve (stn), and in neuropilar processes in the STG. CbAST-B1-LI was blocked by preincubation with 10(-6) M CbAST-B1 and was partially blocked by lower concentrations. Electrophysiological recordings of the effects of CbAST-B1, CbAST-B2, and CbAST-B3 on the pyloric rhythm of the STG showed that all three peptides inhibited the pyloric rhythm in a state-dependent manner. Specifically, all three peptides at 10(-8) M significantly decreased the frequency of the pyloric rhythm when the initial frequency of the pyloric rhythm was below 0.6 Hz. These data suggest important neuromodulatory roles for the CbAST-B family in the stomatogastric nervous system.

Identifiable cells in the crustacean stomatogastric ganglion

Neural circuits rely on slight physiological differences between the component cells for proper function. When any circuit is analyzed, it is important to characterize the features that distinguish one cell type from another. This review describes the methods used to identify the neurons of the crustacean stomatogastric ganglion.

Enhancement of muscle contraction in the stomach of the crab Cancer borealis: a possible hormonal role for GABA

Gamma-aminobutyric acid (GABA) is best known as an inhibitory neurotransmitter in the mammalian central nervous system. Here we show, however, that GABA has an excitatory effect on nerve-evoked contractions and on excitatory junctional potentials (EJPs) of the gastric mill 4 (gm4) muscle from the stomach of the crab Cancer borealis. The threshold concentration for these effects was between 1 and 10 micromol l(-1). Using immunohistochemical techniques, we found that GABA is colocalized with the vesicle-associated protein synapsin in nearby nerves and hence is presumably released there. However, since these nerves do not innervate the muscle directly, we conclude that these release sites are not the likely source of the GABA responsible for muscle modulation. We also extracted hemolymph from the crab pericardial cavity, which contains the pericardial organs, a major neurosecretory structure. Through reversed-phase liquid chromatography-mass spectrometry analysis we determined the concentration of GABA in the hemolymph to be 3.3 +/- 0.7 micromol l(-1), high enough to modulate the muscle. These findings suggest that the gm4 muscle could be modulated by GABA produced by and released from a distant neurohemal organ.

Molecular characterization of agonists that bind to an insect GABA receptor

Ionotropic GABA receptors are widely distributed throughout the vertebrate and invertebrate central nervous system (CNS) where they mediate inhibitory neurotransmission. One of the most widely studied insect GABA receptors is constructed from RDL (resistance to dieldrin) subunits from Drosophila melanogaster. The aim of this study was to determine critical features of agonists binding to RDL receptors using in silico and experimental data. Partial atomic charges and dipole separation distances of a range of GABA analogues were calculated, and the potency of the analogues was determined using RDL receptors expressed in Xenopus oocytes. These data revealed functional agonists require an ammonium group and an acidic group with an optimum separation distance of ∼5 Å. To determine how the agonists bind to the receptor, a homology model of the extracellular domain was generated and agonists were docked into the binding site. The docking studies support the requirements for functional agonists and also revealed a range of potential interactions with binding site residues, including hydrogen bonds and cation−π interactions. We conclude that the model and docking procedures yield a good model of the insect GABA receptor binding site and the location of agonists within it.

Cys-Loop Ligand-Gated Chloride Channels in Dorsal Unpaired Median Neurons of Locusta migratoria

In insects, inhibitory neurotransmission is generally associated with members of the cys-loop ligand-gated anion channels, such as the glutamate-gated chloride channel (GluCl), the GABA-gated chloride channels (GABACl), and the histamine-gated chloride channels (HisCl). These ionotropic receptors are considered established target sites for the development of insecticides, and therefore it is necessary to obtain a better insight in their distribution, structure, and functional properties. Here, by combining electrophysiology and molecular biology techniques, we identified and characterized GluCl, GABACl, and HisCl in dorsal unpaired median (DUM) neurons of Locust migratoria. In whole cell patch-clamp recordings, application of glutamate, GABA, or histamine induced rapidly activating ionic currents. GluCls were sensitive to ibotenic acid and blocked by picrotoxin and fipronil. The pharmacological profile of the L. migratoria GABACl fitted neither the vertebrate GABAA nor GABAC receptor and was similar to the properties of the cloned Drosophila melanogaster GABA receptor subunit (Rdl). The expression of Rdl-like subunit-containing GABA receptors was shown at the molecular level using RT-PCR. Sequencing analysis indicated that the orthologous GABACl of D. melanogaster CG10357-A is expressed in DUM neurons of L. migratoria. Histamine-induced currents exhibited a fast onset and desensitized completely on continuous application of histamine. In conclusion, within the DUM neurons of L. migratoria, we identified three different cys-loop ligand-gated anion channels that use GABA, glutamate, or histamine as their neurotransmitter.

The evolution of pentameric ligand-gated ion channels

Fast, ionotropic neurotransmission mediated by ligand-gated ion channels is essential for timely behavioral responses in multicellular organisms. Metazoa employ more ionotropic neurotransmitters in more types of synapses, inhibitory or excitatory, than is generally appreciated. It is becoming increasingly clear that the adaptability of a single neurotransmitter receptor superfamily, the pentameric ligand-gated ion channels (pLGICs), makes the diversity in ionotropic neurotransmission possible. Modification ofa common pLGIC structure generates channels that are gated by ligands as different as protons, histamine or zinc and that pair common neurotransmitters with both cation and anion permeability. A phylogeny of the pLGIC gene family from representative metazoa suggests that pLGIC diversity is ancient and evolution of contemporary phyla was characterized by a surprising loss of pLGIC diversity. The pLGIC superfamily reveals aspects of early metazoan evolution, may help us identify novel neurotransmitters and can inform our exploration of structure/function relationships.

Modulation of stomatogastric rhythms

Neuromodulation by peptides and amines is a primary source of plasticity in the nervous system as it adapts the animal to an ever-changing environment. The crustacean stomatogastric nervous system is one of the premier systems to study neuromodulation and its effects on motor pattern generation at the cellular level. It contains the extensively modulated central pattern generators that drive the gastric mill (chewing) and pyloric (food filtering) rhythms. Neuromodulators affect all stages of neuronal processing in this system, from membrane currents and synaptic transmission in network neurons to the properties of the effector muscles. The ease with which distinct neurons are identified and their activity is recorded in this system has provided considerable insight into the mechanisms by which neuromodulators affect their target cells and modulatory neuron function. Recent evidence suggests that neuromodulators are involved in homeostatic processes and that the modulatory system itself is under modulatory control, a fascinating topic whose surface has been barely scratched. Future challenges include exploring the behavioral conditions under which these systems are activated and how their effects are regulated.

Activity and neuromodulatory input contribute to the recovery of rhythmic output after decentralization in a central pattern generator

Central pattern generators (CPGs) are neuronal networks that control vitally important rhythmic behaviors including breathing, heartbeat, and digestion. Understanding how CPGs recover activity after their rhythmic activity is disrupted has important theoretical and practical implications. Previous experimental and modeling studies indicated that rhythm recovery after central neuromodulatory input loss (decentralization) could be based entirely on activity-dependent mechanisms, but recent evidence of long-term conductance regulation by neuromodulators suggest that neuromodulator-dependent mechanisms may also be involved. Here we examined the effects of altering activity and the neuromodulatory environment before decentralization of the pyloric CPG in Cancer borealis on the initial phase of rhythmic activity recovery after decentralization. We found that pretreatments altering the network activity through shifting the ionic balance or the membrane potential of pyloric pacemaker neurons reduced the delay of recovery initiation after decentralization, consistent with the recovery process being triggered already during the pretreatment period through an activity-dependent mechanism. However, we observed that pretreatment with neuromodulators GABA and proctolin, acting via metabotropic receptors, also affected the initial phase of the recovery of pyloric activity after decentralization. Their distinct effects appear to result from interactions of their metabotropic effects with their effects on neuronal activity. Thus we show that the initial phase of the recovery process can be accounted for by the existence of distinct activity-and neuromodulator-dependent pathways. We propose a computational model that includes activity- and neuromodulator-dependent mechanisms of the activity recovery process, which successfully explains the experimental observations and predicts the results of key biological experiments.

Contributions of Voltage- and Ca2+-Activated Conductances to GABA-Induced Depolarization in Spider Mechanosensory Neurons

Activation of ionotropic γ-aminobutyric acid type A (GABAA) receptors depolarizes neurons that have high intracellular [Cl−], causing inhibition or excitation in different cell types. The depolarization often leads to inactivation of voltage-gated Na channels, but additional ionic mechanisms may also be affected. Previously, a simulated model of spider VS-3 mechanosensory neurons suggested that although voltage-activated Na+current is partially inactivated during GABA-induced depolarization, a slowly activating and inactivating component remains and may contribute to the depolarization. Here, we confirmed experimentally, by blocking Na channels prior to GABA application, that Na+current contributes to GABA-induced depolarization in VS-3 neurons. Ratiometric Ca2+imaging experiments combined with intracellular recordings revealed a significant increase in intracellular [Ca2+] when GABAAreceptors were activated, synchronous with the depolarization and probably due to Ca2+influx via low-voltage–activated (LVA) Ca channels. In contrast, GABAB-receptor activation in these neurons was previously shown to inhibit LVA current. Blockade of voltage-gated K channels delayed membrane repolarization, extending GABA-induced depolarization. However, inhibition of Ca channels significantly increased the amplitude of GABA-induced depolarization, indicating that Ca2+-activated K+current has an even stronger repolarizing effect. Regulation of intracellular [Ca2+] is important for many cellular processes and Ca2+control of K+currents may be particularly important for some functions of mechanosensory neurons, such as frequency tuning. These data show that GABAA-receptor activation participates in this regulation.

Divergent co-transmitter actions underlie motor pattern activation by a modulatory projection neuron

Co-transmission is a common means of neuronal communication, but its consequences for neuronal signaling within a defined neuronal circuit remain unknown in most systems. We are addressing this issue in the crab stomatogastric nervous system by characterizing how the identified modulatory commissural neuron (MCN)1 uses its co-transmitters to activate the gastric mill (chewing) rhythm in the stomatogastric ganglion (STG). MCN1 contains gamma-aminobutyric acid (GABA) plus the peptides proctolin and Cancer borealis tachykinin-related peptide Ia (CabTRP Ia), which it co-releases during the retractor phase of the gastric mill rhythm to influence both retractor and protractor neurons. By focally applying each MCN1 co-transmitter and pharmacologically manipulating each co-transmitter action during MCN1 stimulation, we found that MCN1 has divergent co-transmitter actions on the gastric mill central pattern generator (CPG), which includes the neurons lateral gastric (LG) and interneuron 1 (Int1), plus the STG terminals of MCN1 (MCN1(STG)). MCN1 used only CabTRP Ia to influence LG, while it used only GABA to influence Int1 and the contralateral MCN1(STG). These MCN1 actions caused a slow excitation of LG, a fast excitation of Int1 and a fast inhibition of MCN1(STG). MCN1-released proctolin had no direct influence on the gastric mill CPG, although it likely indirectly regulates this CPG via its influence on the pyloric rhythm. MCN1 appeared to have no ionotropic actions on the gastric mill follower motor neurons, but it did use proctolin and/or CabTRP Ia to excite them. Thus, a modulatory projection neuron can elicit rhythmic motor activity by using distinct co-transmitters, with different time courses of action, to simultaneously influence different CPG neurons.

Peptide hormone modulation of a neuronally modulated motor circuit

Rhythmically active motor circuits are influenced by neuronally released and circulating hormone modulators, but there are few systems in which the influence of a peptide hormone modulator on a neuronally modulated motor circuit has been determined. We performed such an analysis in the isolated crab stomatogastric nervous system by assessing the influence of the hormone crustacean cardioactive peptide (CCAP) on the gastric mill (chewing) rhythm elicited by identified modulatory projection neurons. The gastric mill circuit is located in the stomatogastric ganglion. In situ, this ganglion is located within the ophthalmic artery and thus is in the path of circulating hormones such as CCAP. Focally-applied CCAP directly excited some gastric mill neurons, including the gastric mill central pattern generator neurons LG and Int1, but it did not elicit a sustained gastric mill rhythm. At concentrations as low as 10(-10) M, however, CCAP did influence gastric mill rhythms elicited by coactivating the projection neurons MCN1 and CPN2 and by selectively stimulating MCN1. In both cases, CCAP slowed this rhythm by selectively prolonging the protraction phase, although its influence on the MCN1-elicited rhythm was limited to those with relatively brief cycle periods. Interestingly, CCAP also reduced the threshold MCN1 firing frequency for activating the gastric mill rhythm. Last, the gastric mill neurons that exhibited altered activity during these CCAP-influenced rhythms did not correspond completely to the set of CCAP-responsive neurons. These results highlight the ability of hormonal modulation to enhance the flexibility provided by the neuronal modulation of rhythmically active motor circuits.

Understanding Circuit Dynamics Using the Stomatogastric Nervous System of Lobsters and Crabs

Studies of the stomatogastric nervous systems of lobsters and crabs have led to numerous insights into the cellular and circuit mechanisms that generate rhythmic motor patterns. The small number of easily identifiable neurons allowed the establishment of connectivity diagrams among the neurons of the stomatogastric ganglion. We now know that (a) neuromodulatory substances reconfigure circuit dynamics by altering synaptic strength and voltage-dependent conductances and (b) individual neurons can switch among different functional circuits. Computational and experimental studies of single-neuron and network homeostatic regulation have provided insight into compensatory mechanisms that can underlie stable network performance. Many of the observations first made using the stomatogastric nervous system can be generalized to other invertebrate and vertebrate circuits.

The cys-loop ligand-gated ion channel superfamily of the honeybee, Apis mellifera

Members of the cys-loop ligand-gated ion channel (cys-loop LGIC) superfamily mediate neurotransmission in insects and are targets of successful insecticides. We have described the cys-loop LGIC superfamily of the honeybee, Apis mellifera, which is an important crop pollinator and a key model for social interaction. The honeybee superfamily consists of 21 genes, which is slightly smaller than that of Drosophila melanogaster comprising 23 genes. As with Drosophila, the honeybee possesses ion channels gated by acetylcholine, gamma-amino butyric acid, glutamate and histamine as well as orthologs of the Drosophila pH-sensitive chloride channel (pHCl), CG8916, CG12344 and CG6927. Similar to Drosophila, honeybee cys-loop LGIC diversity is broadened by differential splicing which may also serve to generate species-specific receptor isoforms. These findings on Apis mellifera enhance our understanding of cys-loop LGIC functional genomics and provide a useful basis for the development of improved insecticides that spare a major beneficial insect species.

Central Pattern Generating Neurons Simultaneously Express Fast and Slow Rhythmic Activities in the Stomatogastric Ganglion

Neuronal firing patterns can contain different temporal information. It has long been known that the fast pyloric and the slower gastric motor patterns in the stomatogastric ganglion of decapod crustaceans interact. However, the bidirectional influences between the pyloric rhythm and the gastric mill rhythm have not been quantified in detail from preparations that spontaneously express both patterns in vitro. We found regular and stable spontaneous gastric and pyloric activity in 71% of preparations of the isolated stomatogastric nervous system of the lobster, Homarus americanus. The gastric [cycle period: 10.96 ± 2.67 (SD) s] and pyloric (cycle period: 1.35 ± 0.18 s) patterns showed bidirectional interactions and coordination. Gastric neuron firing showed preferred phases within the reference frame of the pyloric cycle. The relative timing and burst parameters of the pyloric neurons systematically changed within the reference frame of the gastric cycle. The gastric rhythm showed a tendency to run at cycle periods that were integer multiples of the pyloric periods, but coupling and coordination between the two rhythms were variable. We used power spectra to quantify the gastric and pyloric contributions to the firing pattern of each individual neuron. This provided us with a way to analyze the firing pattern of each gastric and pyloric neuron type individually without reference to either gastric or pyloric phase. Possible functional consequences of these network interactions for motor output are discussed.

Invertebrate central pattern generation moves along

Central pattern generators (CPGs) are circuits that generate organized and repetitive motor patterns, such as those underlying feeding, locomotion and respiration. We summarize recent work on invertebrate CPGs which has provided new insights into how rhythmic motor patterns are produced and how they are controlled by higher-order command and modulatory interneurons.

Bistable Behavior Originating in the Axon of a Crustacean Motor Neuron

Both vertebrate and invertebrate motor neurons can display bistable behavior in which self-sustained tonic firing results from a brief excitatory stimulus. Induction of the bistability is usually dependent on activation of intrinsic conductances located in the somatodendritic area and is commonly sensitive to action of neuromodulators. We have observed bistable behavior in a neuromuscular preparation from the foregut of the crab Cancer borealis that consists of the gastric mill 4 (gm4) muscle and the nerve that innervates it, the dorsal gastric nerve ( dgn). Nerve-evoked contractions of enhanced amplitude and long duration (>30 s) were induced by extracellular stimulation when the stimulus voltage was above a certain threshold. Intracellular and extracellular recordings showed that the large contractions were accompanied by persistent firing of the dorsal gastric (DG) motor neuron that innervates gm4. The persistent firing could be induced only by stimulating a specific region of the axon and could not be triggered by depolarizing the soma, even at current amplitudes that induced high-frequency firing of the neuron. The bistable behavior was abolished in low-Ca2+saline or when nicardipine or flufenamic acid, blockers of L-type Ca2+and Ca2+-activated nonselective cation currents, respectively, was applied to the axonal stimulation region of the dgn. Negative immunostaining for synapsin and synaptotagmin argued against the presence of synaptic/modulatory neuropil in the dgn. Collectively, our results suggest that bistable behavior in a motor neuron can originate in the axon and may not require the action of a locally released neuromodulator.

Identification and characterization of a tachykinin-containing neuroendocrine organ in the commissural ganglion of the crab Cancer productus

A club-shaped, tachykinin-immunopositive structure first described nearly two decades ago in the commissural ganglion (CoG) of three species of decapod crustaceans has remained enigmatic, as its function is unknown. Here, we use a combination of anatomical, mass spectrometric and electrophysiological techniques to address this issue in the crab Cancer productus. Immunohistochemistry using an antibody to the vertebrate tachykinin substance P shows that a homologous site exists in each CoG of this crab. Confocal microscopy reveals that its structure and organization are similar to those of known neuroendocrine organs. Based on its location in the anterior medial quadrant of the CoG, we have named this structure the anterior commissural organ (ACO). Matrix-assisted laser desorption/ionization Fourier transform mass spectrometry shows that the ACO contains the peptide APSGFLGMRamide, commonly known as Cancer borealis tachykinin-related peptide Ia (CabTRP Ia). Using the same technique, we show that CabTRP Ia is also released into the hemolymph. As no tachykinin-like labeling is seen in any of the other known neuroendocrine sites of this species (i.e. the sinus gland, the pericardial organ and the anterior cardiac plexus), the ACO is a prime candidate to be the source of CabTRP Ia present in the circulatory system. Our electrophysiological studies indicate that one target of hemolymph-borne CabTRP Ia is the foregut musculature. Here, no direct CabTRP Ia innervation is present, yet several gastric mill and pyloric muscles are nonetheless modulated by hormonally relevant concentrations of the peptide. Collectively, our findings show that the C. productus ACO is a neuroendocrine organ providing hormonal CabTRP Ia modulation to the foregut musculature. Homologous structures in other decapods are hypothesized to function similarly.

Hormone complement of theCancer productus sinus gland and pericardial organ: An anatomical and mass spectrometric investigation

In crustaceans, circulating hormones influence many physiological processes. Two neuroendocrine organs, the sinus gland (SG) and the pericardial organ (PO), are the sources of many of these compounds. As a first step in determining the roles played by hemolymph-borne agents in the crab Cancer productus, we characterized the hormone complement of its SG and PO. We show via transmission electron microscopy that the nerve terminals making up each site possess dense-core and/or electron-lucent vesicles, suggesting diverse complements of bioactive molecules for both structures. By using immunohistochemistry, we show that small molecule transmitters, amines and peptides, are among the hormones present in these tissues, with many differentially distributed between the two sites (e.g., serotonin in the PO but not the SG). With several mass spectrometric (MS) methods, we identified many of the peptides responsible for the immunolabeling and surveyed the SG and PO for peptides for which no antibodies exist. By using MS, we characterized 39 known peptides [e.g., beta-pigment-dispersing hormone (beta-PDH), crustacean cardioactive peptide, and red pigment-concentrating hormone] and de novo sequenced 23 novel ones (e.g., a new beta-PDH isoform and the first B-type allatostatins identified from a non-insect species). Collectively, our results show that diverse and unique complements of hormones, including many previously unknown peptides, are present in the SG and PO of C. productus. Moreover, our study sets the stage for future biochemical and physiological studies of these molecules and ultimately the elucidation of the role(s) they play in hormonal control in C. productus.

Coordinated Excitatory Effect of GABAergic Interneurons on Three Feeding Motor Programs in the MolluskClione limacina

Coordination between different motor centers is essential for the orderly production of all complex behaviors. Understanding the mechanisms of such coordination during feeding behavior in the carnivorous mollusk Clione limacina is the main goal of the current study. A bilaterally symmetrical interneuron identified in the cerebral ganglia and designated Cr-BM neuron produced coordinated activation of neural networks controlling three main feeding structures: prey capture appendages called buccal cones, chitinous hooks used for prey extraction from the shell, and the toothed radula. The Cr-BM neuron produced strong excitatory inputs to motoneurons controlling buccal cone protraction. It also induced a prominent activation of the neural networks controlling radula and hook rhythmic movements. In addition to the overall activation, Cr-BM neuron synaptic inputs to individual motoneurons coordinated their activity in a phase-dependent manner. The Cr-BM neuron produced depolarizing inputs to the radula protractor and hook retractor motoneurons, which are active in one phase, and hyperpolarizing inputs to the radula retractor and hook protractor motoneurons, which are active in the opposite phase. The Cr-BM neuron used GABA as its neurotransmitter. It was found to be GABA-immunoreactive in the double-labeling experiments. Exogenous GABA mimicked the effects produced by Cr-BM neuron on the postsynaptic neurons. The GABA antagonists bicuculline and picrotoxin blocked Cr-BM neuron-induced PSPs. The prominent coordinating effect produced by the Cr-BM neuron on the neural networks controlling three major elements of the feeding behavior in Clione suggests that this interneuron is an important part of the higher-order system for the feeding behavior.

The action of spike frequency adaptation in the postural motoneurons of hermit crab abdomen during the first phase of reflex activation

Cuticular strain associated with support of the shell of the hermit crab, Pagurus pollicarus, by its abdomen activates mechanoreceptors that evoke a stereotyped reflex in postural motoneurons. This reflex consists of three phases: a brief high-frequency burst of motoneuron spikes, a pause, and a much longer duration but lower frequency period of spiking. These phases are correlated with a rapid increase in muscle force followed by a slight decline to a level of tone that is greater than that at rest but less than maximal. The present experiments address the mechanisms underlying the transition from the first to second phase of the reflex and their role in force generation. Although centrally generated inhibitory post-synaptic potentials (IPSPS) are present during the pause period of the reflex, intracellular current injection of motoneurons reveals a spike frequency adaptation that rapidly and substantially reduces motoneuron firing frequency and is unchanged in saline that reduces synaptic transmission. The adaptation is voltage sensitive and persists for several hundred milliseconds upon repolarization. Hyperpolarization partially restores the initial response of the motoneuron to depolarizing current. Spike frequency adaptation and synaptic inhibition are important mechanisms in the generation of force that maintains abdominal stiffness at a constant, submaximal level.

Dendritic excitability and localization of GABA-mediated inhibition in spider mechanoreceptor neurons

GABAergic inhibition of mechanosensory afferent axon terminals is a widespread phenomenon in vertebrates and invertebrates. Spider mechanoreceptor neurons receive efferent innervation on their peripherally located axons, somata and sensory dendrites, and the dendrites have recently been shown to be excitable. Excitability of the spider sensory neurons is inhibited by muscimol and GABA, agonists of ionotropic GABA receptors. Here we asked where in the neurons this inhibition occurs. We found no evidence for inhibition of action potentials in the sensory dendrites, but axonal action potentials were rapidly suppressed by both agonists. Earlier work showed that metabotropic GABA(B) receptors are located on the dendrites and distal somata of the spider sensory neurons, where they modulate voltage-activated conductances and may provide slower, prolonged inhibition. Therefore, GABA released from single peripheral efferents may activate both ionotropic and metabotropic receptor types, providing rapid suppression of axonal activity followed by slower inhibition that eventually prevents action potential initiation in the distal dendrites.

Drosophila melanogaster GRD and LCCH3 subunits form heteromultimeric GABA-gated cation channels

In addition to its action as a fast inhibitory neurotransmitter, gamma-aminobutyric acid (GABA) is thought to mediate excitatory action by activating cation currents in some cell types in invertebrates. However, to date no GABA receptor capable of mediating such action has been identified at the molecular level in insects. Using a systematic expression screening approach, we found that the Drosophila ligand-gated ion channel subunits GRD and LCCH3 combine to form cation-selective GABA-gated ion channels when coexpressed in Xenopus laevis oocytes. The heteromultimeric receptor is activated by GABA (EC50=4.5 microm), muscimol (EC50=4.8 microm) and trans-4-aminocrotonic acid (EC50=104.5 microm), and partially by cis-4-aminocrotonic acid (EC50=106.3 microm). Picrotoxin effectively blocked the GABA-gated channel (IC50=0.25 microm), but bicuculline, TPMTA, dieldrin and lindane did not. The benzodiazepines flunitrazepam and diazepam did not potentiate the GABA-evoked current. Our data suggest that heteromultimeric channels composed of GRD and LCCH3 subunits form GABA-gated cation channels in insects.

Actions of a histaminergic/peptidergic projection neuron on rhythmic motor patterns in the stomatogastric nervous system of the crab Cancer borealis

Histamine is a neurotransmitter with actions throughout the nervous system of vertebrates and invertebrates. Nevertheless, the actions of only a few identified histamine-containing neurons have been characterized. Here, we present the actions of a histaminergic projection neuron on the rhythmically active pyloric and gastric mill circuits within the stomatogastric ganglion (STG) of the crab Cancer borealis. An antiserum generated against histamine labeled profiles throughout the C. borealis stomatogastric nervous system. Labeling occurred in several somata and neuropil within the paired commissural ganglia as well as in neuropil within the STG and at the junction of the superior oesophageal and stomatogastric nerves. The source of all histamine-like immunolabeling in the STG neuropil was one pair of neuronal somata, the previously identified inferior ventricular (IV) neurons, located in the supraoesophageal ganglion. These neurons also exhibited FLRFamide-like immunoreactivity. Activation of the IV neurons in the crab inhibited some pyloric and gastric mill neurons and, with inputs from the commissural ganglia eliminated, terminated both rhythms. Focal application of histamine had comparable effects. The actions of both applied histamine and IV neuron stimulation were blocked, reversibly, by the histamine type-2 receptor antagonist cimetidine. With the commissural ganglia connected to the STG, IV neuron stimulation elicited a longer-latency activation of commissural projection neurons which in turn modified the pyloric rhythm and activated the gastric mill rhythm. These results support the hypothesis that the histaminergic/peptidergic IV neurons are projection neurons with direct and indirect actions on the STG circuits of the crab C. borealis.

Differential and history-dependent modulation of a stretch receptor in the stomatogastric system of the crab, Cancer borealis

Neuromodulators can modify the magnitude and kinetics of the response of a sensory neuron to a stimulus. Six neuroactive substances modified the activity of the gastropyloric receptor 2 (GPR2) neuron of the stomatogastric nervous system (STNS) of the crab Cancer borealis during muscle stretch. Stretches were applied to the gastric mill 9 (gm9) and the cardio-pyloric valve 3a (cpv3a) muscles. SDRNFLRFamide and dopamine had excitatory effects on GPR2. Serotonin, GABA, and the peptide allatostatin-3 (AST) decreased GPR2 firing during stretch. Moreover, SDRNFLRFamide and TNRNFLRFamide increased the unstimulated spontaneous firing rate, whereas AST and GABA decreased it. The actions of AST and GABA were amplitude- and history-dependent. In fully recovered preparations, AST and GABA decreased the response to small-amplitude stretches proportionally more than to those evoked by large-amplitude stretches. For large-amplitude stretches, the effects of AST and GABA were more pronounced as the number of recent stretches increased. The modulators that affected the stretch-induced GPR2 firing rate were also tested when the neuron was operating in a bursting mode of activity. Application of SDRNFLRFamide increased the bursting frequency transiently, whereas high concentrations of serotonin, AST, and GABA abolished bursting altogether. Together these data demonstrate that the effects of neuromodulators depend on the previous activity and current state of the sensory neuron.

EXP-1 is an excitatory GABA-gated cation channel

Gamma-aminobutyric acid (GABA) mediates fast inhibitory neurotransmission by activating anion-selective ligand-gated ion channels. Although electrophysiological studies indicate that GABA may activate cation-selective ligand-gated ion channels in some cell types, such a channel has never been characterized at the molecular level. Here we show that GABA mediates enteric muscle contraction in the nematode Caenorhabditis elegans via the EXP-1 receptor, a cation-selective ligand-gated ion channel. The EXP-1 protein resembles ionotropic GABA receptor subunits in almost all domains. In the pore-forming domain of EXP-1, however, the residues that confer anion selectivity are exchanged for those that specify cation selectivity. When expressed in Xenopus laevis oocytes, EXP-1 forms a GABA receptor that is permeable to cations and not anions. We conclude that some of the excitatory functions assigned to GABA are mediated by cation channels rather than by anion channels.

GABA-like immunoreactivity in nonspiking interneurons of the locust metathoracic ganglion

Nonspiking interneurons are important components of the premotor circuitry in the thoracic ganglia of insects. Their action on postsynaptic neurons appears to be predominantly inhibitory, but it is not known which transmitter(s) they use. Here, we demonstrate that many but not all nonspiking local interneurons in the locust metathoracic ganglion are immunopositive for GABA (γ-aminobutyric acid). Interneurons were impaled with intracellular microelectrodes and were shown physiologically to be nonspiking. They were further characterized by defining their effects on known leg motor neurons when their membrane potential was manipulated by current injection. Lucifer Yellow was then injected into these interneurons to reveal their cell bodies and the morphology of their branches. Some could be recognised as individuals by comparison with previous detailed descriptions. Ganglia were then processed for GABA immunohistochemistry. Fifteen of the 17 nonspiking interneurons studied were immunopositive for GABA, but two were not. The results suggest that the majority of these interneurons might exert their well-characterized effects on other neurons through the release of GABA but that some appear to use a transmitter other than GABA. These nonspiking interneurons are therefore not an homogeneous population with regard to their putative transmitter.

Cellular, synaptic and network effects of neuromodulation

All network dynamics emerge from the complex interaction between the intrinsic membrane properties of network neurons and their synaptic connections. Nervous systems contain numerous amines and neuropeptides that function to both modulate the strength of synaptic connections and the intrinsic properties of network neurons. Consequently network dynamics can be tuned and configured in different ways, as a function of the actions of neuromodulators. General principles of the organization of modulatory systems in nervous systems include: (a) many neurons and networks are multiply modulated, (b) there is extensive convergence and divergence in modulator action, and (c) some modulators may be released extrinsically to the modulated circuit, while others may be released by some of the circuit neurons themselves, and act intrinsically. Some of the computational consequences of these features of modulator action are discussed.

Neuropeptides are ubiquitous chemical mediators: Using the stomatogastric nervous system as a model system

The stomatogastric nervous system (STNS) controls the movements of the foregut and the oesophagus of decapod crustaceans and is a good example for demonstrating that peptides are ubiquitously distributed chemical mediators in the nervous system. The stomatogastric ganglion (STG), one of the four ganglia of the STNS, contains the most intensively investigated neuronal circuits. The other ganglia, including the two commissural ganglia (CoGs) and the oesophageal ganglion (OG), are thought to be modulatory control centres. Peptides reach the STNS either as neurohormones or are released as transmitters. Peptide neurohormones can be released either from neurohaemal organs or from local neurohaemal release zones located on the surface of nerves and connectives. There were thought to be no peptidergic neurones with cell bodies in the STG itself. However, some have recently been described in adults of four species, in addition to a transient expression of peptides during development in two species. None of these peptidergic neurones has been investigated physiologically, in contrast to peptidergic neurones that project to the STG and have cell bodies in either the CoGs or the OG. It has been shown that neurones containing the same peptide elicit different motor patterns, that the peptide transmitter and the classical transmitter are not necessarily co-released and that the effect of a peptidergic neurone depends on its firing frequency and on which other modulatory neurones are co-active. The activity of modulatory projection neurones can be elicited by sensory neurones, and their activity can depend on the firing frequency of the sensory neurone. In addition to being found within the neuropile of ganglia, peptides are present in neuropile patches located within the nerves of the STNS, suggesting that these nerves can integrate as well as transfer information. Furthermore, sensory neurones and muscles exhibit peptide-like immunoreactivity and are modulated by peptides. Bath-applied peptides elicit peptide-specific motor patterns within the STG by targeting subsets of neurones. This divergence is contrasted by a convergence at the level of currents: five different peptides modulate a single current. Peptides not only induce motor patterns but can also switch the alliance of neurones from one network to another or are able to fuse different networks. In general, peptides are the most abundant group of modulators within the STNS; they are ubiquitously present, indicating that they play multiple roles in the plasticity of neural networks.

Increasing sensor flexibility through neuromodulation

Both biological and man-made motor control networks require input from sensors to allow for modification of the motor program. Real sensory neurons are more flexible than typical robotic sensors because they are dynamic rather than static. The membrane properties of neurons and hence their excitability can be modified by the presence of neuromodulatory substances. In the case of a sensory neuron, this can change, in a functionally significant way, the code used to describe a stimulus. For instance, extension of the neuron's dynamic range or modification of its filtering characteristics can result. This flexibility has an apparent cost. The code used may be situation-dependent and hence difficult to interpret. To address this issue and to understand how neuromodulation is used effectively in a motor control network, I am studying the GPR2 stretch receptor in the crustacean stomatogastric nervous system. Several different neuromodulatory substances can modify its encoding properties. Comparisons of physiological and anatomical evidence suggest that neuromodulation can be effected both by GPR2 itself and by other neurons in the network. These results suggest that the analog of neuromodulation might be useful for improving sensor performance in an artificial motor control system.