GABA-ergic neurons in the crayfish nervous system: an immunocytochemical census of the segmental ganglia and stomatogastric system

Localization of neurons expressing choline acetyltransferase, serotonin and/or FMRFamide in the central nervous system of the decapod shore crab Hemigrapsus sanguineus

Although it is now established that neurons in crustacea contain multiple transmitter substances, little is know about patterns of expression and co-expression or about the functional effects of such co-transmission. The present study was designed to characterize the distributions and potential colocalization of choline acetyltransferase (ChAT), serotonin (5-HT) and neuropeptide H-Phe-Met-Arg-Phe-NH2 (FMRFamide) in the central nervous system (CNS) of the Asian shore crab, Hemigrapsus sanguineus using immunohistochemical analyses in combination with laser scanning confocal microscopy. ChAT was found to be expressed by small, medium-sized, and large neurons in all regions of the brain and ventral nerve cord (VNC). For the most part, ChAT, FMRFamide, and 5-HT are expressed in different neurons, although some colocalization of ChAT- with FMRFamide- or 5-HT-LIR is observed in small and medium-sized cells, mostly neurons that immunostain only weakly. In the brain, such double immunolabeling is observed primarily in neurons of the protocerebrum and, to a particularly great extent, in local olfactory interneurons of the deutocerebrum. The clusters of neurons in the VNC that stain most intensely for ChAT, FMRFamide, and 5-HT, with colocalization in some cases, are located in the subesophageal ganglia. This colocalization appears to be related to function, since it is present in regions of the CNS characterized by multiple afferent projections and outputs to a variety of functionally related centers involved in various physiological and behavioral processes. Further elucidation of the functional significance of these neurons and of the widespread process of co-transmission in the crustaceans should provide fascinating new insights.

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.

Proprioceptive feedback modulates coordinating information in a system of segmentally distributed microcircuits

The system of modular neural circuits that controls crustacean swimmerets drives a metachronal sequence of power-stroke (PS, retraction) and return-stroke (RS, protraction) movements that propels the animal forward efficiently. These neural modules are synchronized by an intersegmental coordinating circuit that imposes characteristic phase differences between these modules. Using a semi-intact preparation that left one swimmeret attached to an otherwise isolated central nervous system (CNS) of the crayfish, Pacifastacus leniusculus, we investigated how the rhythmic activity of this system responded to imposed movements. We recorded extracellularly from the PS and RS nerves that innervated the attached limb and from coordinating axons that encode efference copies of the periodic bursts in PS and RS axons. Simultaneously, we recorded from homologous nerves in more anterior and posterior segments. Maintained retractions did not affect cycle period but promptly weakened PS bursts, strengthened RS bursts, and caused corresponding changes in the strength and timing of efference copies in the module's coordinating axons. Changes in these efference copies then caused changes in the phase and duration, but not the strength, of PS bursts in modules controlling neighboring swimmerets. These changes were promptly reversed when the limb was released. Each swimmeret is innervated by two nonspiking stretch receptors (NSSRs) that depolarize when the limb is retracted. Voltage clamp of an NSSR changed the durations and strengths of bursts in PS and RS axons innervating the same limb and caused corresponding changes in the efference copies of this motor output.

Mechanisms of coordination in distributed neural circuits: decoding and integration of coordinating information

We describe the synaptic connections through which information required to coordinate limb movements reaches the modular microcircuits that control individual limbs on different abdominal segments of the crayfish, Pacifastacus leniusculus. In each segmental ganglion, a local commissural interneuron, ComInt 1, integrates information about other limbs and transmits it to one microcircuit. Five types of nonspiking local interneurons are components of each microcircuit's pattern-generating kernel (Smarandache-Wellmann et al., 2013). We demonstrate here, using paired microelectrode recordings, that the pathway through which information reaches this kernel is an electrical synapse between ComInt 1 and one of these five types, an IRSh interneuron. Using single-electrode voltage clamp, we show that brief changes of ComInt 1's membrane potential affect the timing of its microcircuit's motor output. Changing ComInt 1's membrane potential also changes the phase, duration, and strengths of bursts of spikes in its microcircuit's motor neurons and corresponding changes in its efferent coordinating neurons that project to other ganglia. These effects on coordinating neurons cause changes in the phases of motor output from other microcircuits in those distant ganglia. ComInt 1s function as hub neurons in the intersegmental circuit that synchronizes distributed microcircuits. The synapse between each ComInt 1 and its microcircuit's IRSh neuron completes a five synapse pathway in which analog information is encoded as a digital signal by efference-copy neurons and decoded from digital to analog form by ComInt 1. The synaptic organization of this pathway provides a cellular explanation of this nervous system's key dynamic properties.

Five types of nonspiking interneurons in local pattern-generating circuits of the crayfish swimmeret system

We conducted a quantitative analysis of the different nonspiking interneurons in the local pattern-generating circuits of the crayfish swimmeret system. Within each local circuit, these interneurons control the firing of the power-stroke and return-stroke motor neurons that drive swimmeret movements. Fifty-four of these interneurons were identified during physiological experiments with sharp microelectrodes and filled with dextran Texas red, Neurobiotin, or both. Five types of neurons were identified on the basis of combinations of physiological and anatomical characteristics. Anatomical categories were based on 16 anatomical parameters measured from stacks of confocal images obtained from each neuron. The results support the recognition of two functional classes: inhibitors of power stroke (IPS) and inhibitors of return stroke (IRS). The IPS class of interneuron has three morphological types with similar physiological properties. The IRS class has two morphological types with physiological properties and anatomical features different from the IPS neurons but similar within the class. Three of these five types have not been previously identified. Reviewing the evidence for dye coupling within each type, we conclude that each type of IPS neuron and one type of IRS neuron occur as a single copy in each local pattern-generating circuit. The last IRS type includes neurons that might occur as a dye-coupled pair in each local circuit. Recognition of these different interneurons in the swimmeret pattern-generating circuits leads to a refined model of the local pattern-generating circuit that includes synaptic connections that encode and decode information required for intersegmental coordination of swimmeret movements.

Neurobiology of the crustacean swimmeret system

The crustacean swimmeret system includes a distributed set of local circuits that individually control movements of one jointed limb. These modular local circuits occur in pairs in each segmental ganglion, and normally operate synchronously to produce smoothly coordinated cycles of limb movements on different body segments. The system presents exceptional opportunities for computational and experimental investigation of neural mechanisms of coordination because: (a) The system will express in vitro the periodic motor pattern that normally drives cycles of swimmeret movements during forward swimming. (b) The intersegmental neurons which encode information that is necessary and sufficient for normal coordination have been identified, and their activity can be recorded. (c) The local commissural neurons that integrate this coordinating information and tune the phase of each swimmeret are known. (d) The complete set of synaptic connections between coordinating neurons and these commissural neurons have been described. (e). The synaptic connections onto each local pattern-generating circuit through which coordinating information tunes the circuit's phase have been discovered. These factors make possible for the first time a detailed, comprehensive cellular and synaptic explanation of how this neural circuit produces an effective, behaviorally significant output. This paper is the first comprehensive review of the system's neuroanatomy and neurophysiology, its local and intersegmental circuitry, its transmitter pharmacology, its neuromodulatory control mechanisms, and its interactions with other motor systems. Each of these topics is covered in detail in an attempt to provide a complete review of the literature as a foundation for new research. The series of hypotheses that have been proposed to account for the system's properties are reviewed critically in the context of experimental tests of their validity.

State-changes in the swimmeret system: a neural circuit that drives locomotion

The crayfish swimmeret system undergoes transitions between a silent state and an active state. In the silent state, no patterned firing occurs in swimmeret motor neurons. In the active state, bursts of spikes in power stroke motor neurons alternate periodically with bursts of spikes in return stroke motor neurons. In preparations of the isolated crayfish central nervous system (CNS), the temporal structures of motor patterns expressed in the active state are similar to those expressed by the intact animal. These transitions can occur spontaneously, in response to stimulation of command neurons, or in response to application of neuromodulators and transmitter analogues. We used single-electrode voltage clamp of power-stroke exciter and return-stroke exciter motor neurons to study changes in membrane currents during spontaneous transitions and during transitions caused by bath-application of carbachol or octopamine (OA). Spontaneous transitions from silence to activity were marked by the appearance of a standing inward current and periodic outward currents in both types of motor neurons. Bath-application of carbachol also led to the development of these currents and activation of the system. Using low Ca(2+)-high Mg(2+) saline to block synaptic transmission, we found that the carbachol-induced inward current included a direct response by the motor neuron and an indirect component. Spontaneous transitions from activity to silence were marked by disappearance of the standing inward current and the periodic outward currents. Bath-application of OA led promptly to the disappearance of both currents, and silenced the system. OA also acted directly on both types of motor neurons to cause a hyperpolarizing outward current that would contribute to silencing the system.

The GABAergic-like system in the marine demosponge Chondrilla nucula

Gamma-amino butyric acid (GABA) is believed to be the principal inhibitory neurotransmitter in the mammalian central nervous system, a function that has been extended to a number of invertebrate systems. The presence of GABA in the marine demosponge Chondrilla nucula was verified using immunofluorescence detection and high-pressure liquid chromatography. A strong GABA-like immunoreactivity (IR) was found associated with choanocytes, exopinacocytes, endopinacocytes lining inhalant, and exhalant canals, as well as in archaeocytes scattered in the mesohyl. The capacity to synthesize GABA from glutamate and to transport it into the vesicles was confirmed by the presence in C. nucula of glutamate decarboxylase (GAD) and vesicular GABA transporters (vGATs), respectively. GAD-like and vGAT-like IR show the same distribution as GABA-like IR. Supporting the similarity between sponge and mammalian proteins, bands with an apparent molecular weight of about 65-67 kDa and 57 kDa were detected using antibodies raised against mammalian GAD and vGAT, respectively. A functional metabotropic GABA(B)-like receptor is also present in C. nucula. Indeed, both GABA(B) R1 and R2 isoforms were detected by immunoblot and immunofluorescence. Also in this case, IR was found in choanocytes, exopinacocytes, and endopinacocytes. The content of GABA in C. nucula amounts to 1225.75 +/- 79 pmol/mg proteins and GABA is released into the medium when sponge cells are depolarized. In conclusion, this study is the first indication of the existence of the GABA biosynthetic enzyme GAD and of the GABA transporter vGAT in sponges, as well as the first demonstration that the neurotransmitter GABA is released extracellularly.

Immunocytochemical mapping and quantification of expression of a putative type 1 serotonin receptor in the crayfish nervous system

Serotonin is an important neurotransmitter that is involved in modulation of sensory, motor, and higher functions in many species. In the crayfish, which has been developed as a model for nervous system function for over a century, serotonin modulates several identified circuits. Although the cellular and circuit effects of serotonin have been extensively studied, little is known about the receptors that mediate these signals. Physiological data indicate that identified crustacean cells and circuits are modulated via several different serotonin receptors. We describe the detailed immunocytochemical localization of the crustacean type 1 serotonin receptor, 5-HT1crust, throughout the crayfish nerve cord and on abdominal superficial flexor muscles. 5-HT1crust is widely distributed in somata, including those of several identified neurons, and neuropil, suggesting both synaptic and neurohormonal roles. Individual animals show very different levels of 5-HT1crust immunoreactivity (5-HT(1crust)ir) ranging from preparations with hundreds of labeled cells per ganglion to some containing only a handful of 5-HT(1crust)ir cells in the entire nerve cord. The interanimal variability in 5-HT(1crust)ir is great, but individual nerve cords show a consistent level of labeling between ganglia. Quantitative RT-PCR shows that 5-HT1crust mRNA levels between animals are also variable but do not directly correlate with 5-HT(1crust)ir levels. Although there is no correlation of 5-HT1crust expression with gender, social status, molting or feeding, dominant animals show significantly greater variability than subordinates. Functional analysis of 5-HT1crust in combination with this immunocytochemical map will aid further understanding of this receptor's role in the actions of serotonin on identified circuits and cells.

Distribution of glutamatergic immunoreactive neurons in the terminal abdominal ganglion of the crayfish

Using an antiserum directed against glutamate, we have analyzed the distribution of glutamate-like immunoreactive neurons in the terminal abdominal ganglion of the crayfish Procambarus clarkii. Approximately 160 central neurons (157 +/- 8; mean +/- SEM, n = 8) showed positive glutamate-like immunoreactivity, which represents approximately 25% of the total number of neurons in the terminal ganglion. Using a combination of intracellular staining with the marker Lucifer yellow and immunocytochemical staining has shown that most excitatory motor neurons are glutamatergic and that glutamate acts as an excitatory transmitter at peripheral neuromuscular junctions. Seven of 10 identified spiking local interneurons and only 2 of 19 identified ascending interneurons, showed positive immunoreactivity. Our observation that inhibitory spiking interneurons were immunopositive, whereas excitatory ascending interneurons were immunonegative, indicates that glutamate is likely to act as an inhibitory neurotransmitter within the central nervous system. Local pressure injection of L-glutamate into the neuropil of the ganglion caused a hyperpolarization of the membrane potentials of many interneurons. gamma-Aminobutyric acid (GABA)ergic posterolateral nonspiking interneurons and the bilateral nonspiking interneuron LDS showed no glutamate-like immunoreactivity, whereas non-GABAergic anterolateral III nonspiking interneurons showed glutamate-like immunoreactivity. Thus, not only GABA but also glutamate are used in parallel as inhibitory neurotransmitters at central synapses.

During fictive locomotion, graded synaptic currents drive bursts of impulses in swimmeret motor neurons

During forward swimming, motor neurons that innervate each crayfish swimmeret fire periodic coordinated bursts of impulses. These bursts occur simultaneously in neurons that are functional synergists but alternate with bursts in their antagonists. These impulses ride on periodic oscillations of membrane potential that occur simultaneously in neurons of each type. A model of the local circuit that generates this motor pattern has been proposed. In this model, each motor neuron is driven alternately by excitatory and inhibitory synaptic currents from nonspiking local interneurons. I tested this model by perturbing individual interneurons and recording synaptic currents and changes in input resistance from each class of motor neuron. I also simulated the synaptic currents that would be observed in a cell subject to different patterns of presynaptic input. When the CNS was actively expressing the swimming motor pattern, changes in the membrane potential of individual local interneurons controlled firing of whole sets of motor neurons. Membrane currents in these motor neurons oscillated in phase with the motor output from their own local circuit. The phases of these oscillations differed in different functional classes of motor neurons. In neurons that could be clamped at the reversal potential of their outward currents, the model predicted that large periodic inward currents would be recorded. I observed no signs of periodic inward currents, even when the outward currents clearly had reversed. These results permit a simplification of the cellular model. They are discussed in the context of neural control of locomotion in crustacea and insects.

Peripheral targets of centrally located putative accessory neurons of MRO in the isopod Ligia exotica

The three centrally located putative accessory neurons of the muscle receptor organ (MRO) of the isopod Ligia exotica were identified to the third segmental nerve (N3) of the thoracic ganglion by backfilling with Lucifer Yellow. These neurons were then studied intracellularly and extracellularly to determine whether they suppressed the stretch-activated responses of thoracic stretch receptors. Intracellular injection of depolarizing currents into these three putative accessory neurons revealed that only neuron #3 had an inhibitory effect, suggesting that it is an inhibitory accessory neuron related to thoracic stretch receptors. We searched for the peripheral targets of neurons #1 and #2 by intracellular filling with Lucifer Yellow or by recording of junctional potentials in extensor muscles, and show that they are motor neurons that innervate the deep extensor and superficial extensor muscles, respectively.

Architectonics of crayfish ganglia

The central nervous system of crayfish consists of a chain of segmental ganglia that are linked by cables of intersegmental axons. Each ganglion contains a highly-ordered core of longitudinal tracts, vertical tracts, commissures, and synaptic neuropils. We review from a technical perspective the history of the description of these ganglia, and recognize four episodes of progress. Each major innovation in anatomical methods has led to new insight into the structure and function of this nervous system, and new awareness of the structural patterns that are common to the CNS of all arthropods. Ganglia in different segments of the body differ in size, and appear to differ in anatomy. From a comparison of the structures of the cores of abdominal, thoracic, and subesophageal ganglia, we argue that this apparent difference is illusory. Rather, each of these ganglia is organized on the same plan, a plan also found in insect segmental ganglia. The apparent differences follow from longitudinal compression during development and from allometric growth of particular neuropils associated with innervation of the walking legs. Different authors have described the internal organization of ganglia in different segments, so we provide a cross-reference to the nomenclatures they have introduced. We compare the locations of cell bodies of motor neurons and accessory neurons that innervate different peripheral structures, and demonstrate double-labeling of certain GABAergic peripheral inhibitory neurons. Finally, we describe the construction of digital movies of serial sections of these ganglia, and discuss their utility.

Evolution of the arthropod neuromuscular system. 2. Inhibitory innervation of the walking legs of a scorpion: Vaejovis spinigerus (Wood, 1863), Vaejovidae, Scorpiones, Arachnida

Inhibitory motoneurons which supply the leg musculature are identified and characterized in the scorpion, Vaejovis spinigerus (Wood, 1863) (Vaejovidae, Scorpiones, Arachnida). (1) Successive intracellular muscle fiber recordings from antagonists, and correlation of the monitored inhibitory postsynaptic potentials with spikes in motor nerves, suggest supply of the scorpion leg musculature by common inhibitory motoneurons. (2) Anti-GABA immunohistochemistry is combined with transmission electron microscopy to estimate the number of inhibitory motor axons present in the main leg nerve. The number of immunoreactive axons decreases toward more distal leg segments, from 14 to 18 in the basis to 6-8 in the tibia. No immunoreactive axons are detected beyond the tibia. (3) The distribution of putative inhibitory neurons in the subesophageal ganglion mass is determined by anti-GABA immunohistochemistry, revealing notable similarities to the situation in pterygote insects. This provides a framework for the characterization of the inhibitory motoneurons. (4) Backfills from leg nerves are combined with anti-GABA immunocytochemistry to identify inhibitory motoneurons in the central nervous system. Putative inhibitory motoneurons occur in three clusters per hemi-segment. Two clusters are located near the posterior edge of the neuromere, one lateral, the other more medial, and both contain ca. 8-10 cell bodies. The third cluster consists of two somata located contralaterally, just off the ganglion midline.

GABA enhances transmission at an excitatory glutamatergic synapse

GABA mediates both presynaptic and postsynaptic inhibition at many synapses. In contrast, we show that GABA enhances transmission at excitatory synapses between the lateral gastric and medial gastric motor neurons and the gastric mill 6a and 9 (gm6a, gm9) muscles and between the lateral pyloric motor neuron and pyloric 1 (p1) muscles in the stomach of the lobster Homarus americanus. Two-electrode current-clamp or voltage-clamp techniques were used to record from muscle fibers. The innervating nerves were stimulated to evoke excitatory junctional potentials (EJPs) or excitatory junctional currents. Bath application of GABA first decreased the amplitude of evoked EJPs in gm6a and gm9 muscles, but not the p1 muscle, by activating a postjunctional conductance increase that was blocked by picrotoxin. After longer GABA applications (5-15 min), the amplitudes of evoked EJPs increased in all three muscles. This increase persisted in the presence of picrotoxin. beta-(Aminomethyl)-4-chlorobenzenepropanoic acid (baclofen) was an effective agonist for the GABA-evoked enhancement but did not increase the postjunctional conductance. Muscimol activated a rapid postsynaptic conductance but did not enhance the amplitude of the nerve-evoked EJPs. GABA had no effect on iontophoretic responses to glutamate and decreased the coefficient of variation of nerve-evoked EJPs. In the presence or absence of tetrodotoxin, GABA increased the frequency but not the amplitude of miniature endplate potentials. These data suggest that GABA acts presynaptically via a GABA(B)-like receptor to increase the release of neurotransmitter.

Dual and opposing modulatory effects of serotonin on crayfish lateral giant escape command neurons

Serotonin modulates afferent synaptic transmission to the lateral giant neurons of crayfish, which are command neurons for escape behavior. Low concentrations, or high concentrations reached gradually, are facilitatory, whereas high concentrations reached rapidly are inhibitory. The modulatory effects rapidly reverse after brief periods of application, whereas longer periods of application are followed by facilitation that persists for hours. These effects of serotonin can be reproduced by models that involve multiple interacting intracellular signaling systems that are each stimulated by serotonin. The dependence of the neuromodulatory effect on dose, rate, and duration of modulator application may be relevant to understanding the effects of natural neuromodulation on behavior and cognition and to the design of drug therapies.

Crab stomach pyloric muscles display not only excitatory but inhibitory and neuromodulatory nerve terminals

Movements of the foregut in crustaceans are produced by striated muscles that are innervated by motor neurons in the stomatogastric ganglion (STG). Firing of the STG motor neurons generates excitatory junctional potentials (EJPs) in the stomach muscles. We now provide evidence for the existence of separate inhibitory and neuromodulatory innervations of some pyloric muscles in the foregut of several crabs, Callinectes sapidus, Cancer magister, and Cancer borealis. Electron microscopic examination of several pyloric muscles revealed three distinct types of nerve terminals. Excitatory terminals were readily identified by the spherical shape of their small, clear synaptic vesicles. These terminals also housed a few large dense core vesicles. Inhibitory nerve terminals were recognized by the elliptical shape of their small, clear synaptic vesicles, and contacted the muscles at well-defined synapses equipped with dense bar active zones. Bath application of GABA reduced the amplitudes of EJPs in a pyloric muscle of C. borealis, consistent with the presence of GABAergic inhibitory innervation. Neuromodulatory terminals were characterized by their predominant population of large dense and dense core vesicles. These terminals formed synapses with presynaptic dense bars on the muscle, as well as on the excitatory and inhibitory nerve terminals. The presence of the inhibitory and neuromodulatory terminals creates a functional context for previously described reports of neuromodulatory actions on stomach muscles and suggests that the transfer function from STG motor patterns to pyloric movement may be orchestrated by a complex innervation from sources outside of the STG itself.

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

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

Localization of GABA-like immunoreactivity in the central nervous system of Aplysia californica

Gamma-aminobutyric acid (GABA) is present in the central nervous system of Aplysia californica (Gastropoda, Opisthobranchia) where its role as a neurotransmitter is supported by pharmacological, biochemical, and anatomical investigations. In this study, the distribution of GABA-immunoreactive (GABAi) neurons and fiber systems in Aplysia was examined by using wholemount immunohistochemistry and nerve backfill methods. GABAi neurons were located in the buccal, cerebral, and pedal ganglia. Major commissural fiber systems were present in each of these ganglia, whereas more limited fiber systems were observed in the ganglionic connectives. Some of the interganglionic fibers were found to originate from two unpaired GABAi neurons, one in the buccal ganglion and one in the right pedal ganglion, each of which exhibited bilateral projections. No GABAi fibers were found in the nerves that innervate peripheral sensory, motor, or visceral organs. Although GABAi cells were not observed in the pleural or abdominal ganglia, these ganglia did receive limited projections of GABAi fibers originating from neurons in the pedal ganglia. The distribution of GABAi neurons suggests that this transmitter system may be primarily involved in coordinating certain bilateral central pattern generator (CPG) systems related to feeding and locomotion. In addition, the presence of specific interganglionic GABAi projections also suggests a role in the regulation or coordination of circuits that produce components of complex behaviors.

Peripheral synapses at identified mechanosensory neurons in spiders: three-dimensional reconstruction and GABA immunocytochemistry

The mechanosensory organs of arachnids receive diverse peripheral inputs. Little is known about the origin, distribution, and function of these chemical synapses, which we examined in lyriform slit sense organ VS-3 of the spider Cupiennius salei. The cuticular slits of this organ are each associated with two large bipolar mechanosensory neurons with different adaptation rates. With intracellular recording, we have now been able to correlate directly the staining intensity of a neuron for acetylcholinesterase with its adaptation rate, thus allowing us simply to stain a neuron to identify its functional type. All rapidly adapting neurons stain more heavily than slowly adapting neurons. Immunostaining of whole-mount preparations reveals GABA-like immunoreactive fibers forming numerous varicosities at the surface of all sensory neurons in VS-3; peripheral GABA-like immunoreactive somata are lacking. Sectioning the leg nerve procures rapid degeneration of most fiber profiles, confirming that the fibers are efferent. Punctate synapsin-like immunoreactivity colocalizes to these varicosities, although some synapsin-like immunoreactive puncta are GABA-immunonegative. Fibers with similar immunoreactivities are also associated with trichobothria, tactile hairs, internal joint receptors, i.e. other types of spider mechanosensory organs. In organ VS-3, immunoreactivity is most dense across the initial axon segment. The exact distribution of peripheral synapses was reconstructed from a 10-microm-long electron micrograph series of the dendritic, somatic, and initial axon regions of acetylcholinesterase-stained VS-3 neurons. These reveal a pattern similar to that of the synapsin-like immunoreactivity. Two different types of synapse were distinguished on the basis of their presynaptic vesicle populations. Many peripheral synapses thus appear to derive from efferent GABA-like immunoreactive fibers and probably provide centrifugal inhibitory control of primary mechanosensory activities.

Distribution of GABA-like immunoreactive neurons in insects suggests lineage homology

Gamma-aminobutyric acid (GABA) is an important inhibitory neurotransmitter in vertebrates and invertebrates (Sattelle [1990] Adv. Insect Physiol. 22:1-113). The GABA phenotype is lineally determined in postembryonic neurons in the tobacco hawkmoth, Manduca sexta (Witten and Truman, [1991] J. Neurosci. 11:1980-1989) and is restricted to six identifiable postembryonic lineages in the moth's thoracic hemiganglia. We used a comparative approach to determine whether this distinct clustering of GABAergic neurons is conserved in Insecta. In the nine orders of insects surveyed (Thysanura, Odonata, Orthoptera, Isoptera, Hemiptera, Coleoptera, Diptera, Lepidoptera, and Hymenoptera), GABA-like immunoreactive neurons within a thoracic hemiganglion were clustered into six distinct groups that occupied positions similar to the six postembryonic lineages in Manduca. On the basis of cell body position and axon trajectories, we suggest that these are indeed homologous lineage groups and that the lineal origins of the GABAergic cells have been very conservative through insect evolution. The distinctive clustering of GABA-positive cells is shared with crustaceans (Mulloney and Hall [1990] J. Comp. Neurol. 291:383-394; Homberg et al. [1993] Cell Tissue Res. 271:279-288) but is not found in the centipede Lithobius forficulatus. There is a two- to threefold increase in numbers of thoracic neurons between the flightless Thysanura and the most advanced orders of insects. Using the GABA clusters as indicators of specific lineages, we find that only selected lineages have significantly contributed to this increase in neuronal numbers.

The ontogeny of GABA- and glutamate-like immunoreactivity in the embryonic Australian freshwater crayfish, Cherax destructor

The distribution and ontogeny of GABA- and glutamate-like immunoreactivity in embryos of the Australian freshwater crayfish Cherax destructor were investigated over the period from 30% development until hatching. GABA-like immunoreactive cells and fibres appeared first in the brain at 40-45% development. By 70% development, GABA-like immunoreactive cells were present in almost all ganglia, and GABA-like immunoreactive fibres were distributed extensively throughout the neuropil, commissures and connectives of the central nervous system, and were also found in peripheral nerve roots supplying the appendages and the abdominal musculature. In contrast, glutamate-like immunoreactivity did not appear in the central nervous system until 60-65% development. By the time of hatching, the distribution of glutamate-like immunoreactivity was restricted to discrete regions of neuropil and fibre staining in the thoracic and abdominal nerve cord, the abdominal musculature and the appendages. The precocious establishment of the extensive distribution of GABA-like immunoreactive neurons in the developing crayfish embryo is consistent with the possibility that these neurons play a trophic role in controlling or modulating the development of the nervous system.

Distribution of GABAergic premotor nonspiking local interneurones in the terminal abdominal ganglion of the crayfish

The inhibitory neurotransmitter of premotor nonspiking local interneurones in the crayfish terminal abdominal ganglion was investigated physiologically and immunocytochemically. Depolarization of a nonspiking interneurone evoked a hyperpolarization in a uropod motor neurone. The amplitude of hyperpolarization in the motor neurone was gradually decreased under low-calcium/high-magnesium saline. Local pressure injection of gamma-aminobutyric acid (GABA) into the neuropil caused a similar hyperpolarization of the motor neurone. These physiological studies suggested a GABAergic inhibitory interaction between nonspiking interneurones and the motor neurones. Premotor nonspiking interneurones are classified into two subgroups ofposterolateral (PL) and anterolateral (AL) interneurones, and AL interneurones are further divided into three subtypes. A combination of intracellular staining from nonspiking local interneurones with Lucifer yellow and immunocytochemical staining with an antiserum directed against GABA revealed that all the PL interneurones sampled in this study showed GABA-like immunoreactivity. A population of cell bodies (n = 6-11) with a small diameter (15-30 microm) packed together forming a cluster showed GABA-like immunoreactivity, and the cell bodies of most PL interneurones were found in this cluster. To compare the number and the pattern of main branches of PL interneurones, cells were classified into three identifiable sets of interneurones, called PL-1, PL-2, and PL-3. By contrast, about one-half of AL interneurones, especially the third subtype of AL interneurones, which have cell bodies located ventrolaterally in the ganglion, did not show GABA-like immunoreactivity. Furthermore, the position of cell bodies of GABA-immunoreactive AL interneurones was scattered compared to that of PL interneurones.

Modulation of force during locomotion: differential action of crustacean cardioactive peptide on power-stroke and return- stroke motor neurons

Crustacean cardioactive peptide (CCAP) elicited expression of the motor pattern that drives coordinated swimmeret beating in crayfish and modulated this pattern in a dose-dependent manner. In each ganglion that innervates swimmerets, neurons with CCAP-like immunoreactivity sent processes to the lateral neuropils, which contain branches of swimmeret motor neurons and the local pattern-generating circuits. CCAP affected each of the four functional groups of motor neurons, power-stroke excitors (PSE), return-stroke excitors (RSE), power-stroke inhibitors (PSI), and return-stroke inhibitors (RSI), that innervate each swimmeret. When CCAP was superfused, the membrane potentials of these neurons began to oscillate periodically about their mean potentials. The mean potentials of PSE and RSI neurons depolarized, and some of these neurons began to fire during each depolarization. Both intensity and durations of PSE bursts increased significantly. The mean potentials of RSE and PSI neurons hyperpolarized, and these neurons were less likely to fire during each depolarization. When CCAP was superfused in a low Ca2+ saline that blocked chemical transmission, these changes in mean potential persisted, but the periodic oscillations disappeared. These results are evidence that CCAP acts at two levels: activation of local premotor circuits and direct modulation of swimmeret motor neurons. The action on motor neurons is differential; PSEs and RSIs are excited, but RSEs and PSIs are inhibited. The consequences of this selectivity are to increase intensity of bursts of impulses that excite power-stroke muscles.

Passive properties of swimmeret motor neurons

Four different functional types of motor neurons innervate each swimmeret: return-stroke excitors (RSEs), power-stroke excitors (PSEs), return-stroke inhibitors (RSIs), and power-stroke inhibitors (PSIs). We studied the structures and passive electrical properties of these neurons, and tested the hypothesis that different types of motor neurons would have different passive properties that influenced generation of the swimmeret motor pattern. Cell bodies of neurons innervating one swimmeret were clustered in two anatomic groups in the same ganglion. The shapes of motor neurons in both groups were similar, despite the differences in locations of their cell bodies and in their functions. Diameters of their axons in the swimmeret nerve ranged from <2 to approximately 35 microm. Resting membrane potentials, input resistances, and membrane time constants were recorded with microelectrodes in the processes of swimmeret motor neurons in isolated abdominal nerve cord preparations. Membrane potentials had a median of -59 mV, with 25th and 75th percentiles of -66.0 and -53 mV. The median input resistance was 6.4 M omega, with 25th and 75th percentiles of 3.4 and 13.7 M omega. Membrane time constants had a median of 9.3 ms, with 25th and 75th percentiles of 5.7 and 15.0 ms. Excitatory and inhibitory motor neurons had similar passive properties. RSE motor neurons were typically more depolarized than the other types, but the passive properties of RSE, PSE, RSI, and PSI neurons were not significantly different. Membrane time constants measured from cell bodies were briefer than those measured from neuropil processes, but membrane potentials and input resistances were not significantly different. The relative sizes of different motor neurons were measured from the sizes of their impulses recorded extracellularly from the swimmeret nerve. Smaller motor neurons had lower membrane potentials and were more likely to be active in the motor pattern than were large motor neurons. Motor neurons of different sizes had similar input resistances and membrane time constants. Motor neurons that were either oscillating or oscillating and firing in phase with the swimmeret motor pattern had lower average membrane potentials and longer time constants than those that were not oscillating. When the state of the swimmeret system changed from quiescence to continuous production of the motor pattern, the resting potentials, input resistances, and membrane time constants of individual swimmeret motor neurons changed only slightly. On average, both input resistance and membrane time constant increased. These similarities are considered in light of the functional task each motor neuron performs, and a hypothesis is developed that links the brief time constants of these neurons and graded synaptic transmission by premotor interneurons to control of the swimmeret muscles and the performance of the swimmeret system.

Two types of identified ascending interneurons with distinct GABA receptors in the crayfish terminal abdominal ganglion

More than half of the identified ascending interneurons originating in the terminal abdominal ganglion of the crayfish received inhibitory sensory inputs from hair afferents innervating the tailfan on the side contralateral to their main branches. Biochemical aspects of this transverse lateral inhibition of ascending interneurons were examined by the use of neurophysiological and pharmacological techniques. Local application of gamma-aminobutyric acid (GABA) and its agonist muscimol into the neuropil induced membrane hyperpolarization of identified ascending interneurons with an increase in membrane conductance. Because the reversal potential of inhibitory postsynaptic potential (IPSPs) in ascending interneurons elicited by the sensory stimulation and GABA injection was similar, and the sensory-stimulated IPSPs of the interneurons were blocked by GABA and muscimol application, this study strongly suggests a GABAergic nature for transverse lateral inhibition of ascending interneurons. According to the response to the GABAA antagonists bicuculline and picrotoxin, ascending interneurons were classified into two types, picrotoxin-sensitive and picrotoxin-insensitive interneurons. Identified ascending interneurons VE-1 and RO-4 showed a pharmacological profile similar to that of the classical GABAA receptor of the vertebrates. Bath application of both bicuculline and picrotoxin reversibly reduced the amplitudes of IPSPs. The other identified ascending interneurons CA-1, RO-1, and RO-2 were not affected significantly by the bath application of GABAA and GABAB antagonists, although bath application of low-chloride saline reversed the sensory-stimulated IPSPs. IPSPs of the picrotoxin-sensitive interneurons had a rather faster time course and shorter duration in comparison with those of the picrotoxin-insensitive interneurons.

Acetylcholinesterase activity in neurons of crayfish abdominal ganglia

Acetylcholine is known to be a neurotransmitter in crustacean central nervous systems, but the numbers and distribution of cholinergic neurons in the segmental ganglia have not been described. To begin a census of cholinergic neurons in these ganglia, we used a histochemical assay for acetylcholinesterase to map neurons that contained this enzyme in the six abdominal ganglia of crayfish. In each abdominal ganglion, about 47 cell bodies were stained. The distributions of these stained cells in individual ganglia were similar, and the numbers were not significantly different. None of these stained cell bodies could be identified from their structures or locations as previously identified motor neurons or sensory neurons with central cell bodies. The process of one unpaired midline neuron that occurred only in the first three abdominal ganglia divided to send a pair of axons anteriorly into both halves of the connective. The central projections of afferent axons from many peripheral sensory neurons stained clearly as they entered each ganglion. Terminals of these axons were heavily stained in the horseshoe neuropil and the lateral neuropils. We labeled both gamma-aminobutyric acid (GABA) and acetylcholinesterase in individual ganglia. Only a few neurons in each ganglion were double-labeled. The unpaired midline neurons in the three anterior ganglia that stained for acetylcholinesterase did not show GABA-like immunoreactivity, but cells with similar shapes did label with the GABA antiserum. Acetylcholinesterase is not a definitive marker of cholinergic neurons, but its presence is often associated with the cholinergic phenotype. These stained cells should be considered as putative cholinergic neurons.

Crustacean cardioactive peptide-immunoreactive neurons in the ventral nervous system of crayfish

Crustacean cardioactive peptide-immunoreactive neurons have been mapped in whole-mount preparations and sections of the ventral nervous system of the crayfish Astacus astacus and Orconectes limosus. Based on their morphology, projection patterns, and staining characteristics, two types of contralaterally projecting neurons are individually identifiable. In both species, these neurons occur in all neuromers as apparent serial homologs. In adult specimens, one type of cell has a small, densely stained dorsal lateral perikaryon, and a descending axon, and appears to be an interneuron. Each neuromer contains a single pair of these cells. Only in maxillary ganglia, these cells may have an additional ascending projection. The other type, a neurosecretory cell, has a larger, weakly stained perikaryon and a projection to the segmental third root of the next anterior neuromer. All neuromers contain a single pair of these neurons adjacent to the interneurons except for the abdominal neuromers, which contain two pairs of the neurosecretory cells. Central arborizations and varicose processes toward the surface of the third roots and within the perineural sheath of the ventral nerve cord arise from these neurons. Electron microscopy of granule-containing terminals substantiated that these newly discovered extensive neurohemal areas are release sites for the peptide. In young immature specimens, the perikarya of both neuron types do not differ in size. Additional weakly stained small perikarya occur in all neuromers of Astacus astacus. These two types of crayfish neurons and other comparable aminergic and peptidergic neurons of crayfish and lobster are differentially distributed in the ventral cord. Furthermore, comparison of similar neuron types in crab, locust, meal worm, and moth species indicates intra- and interphyletic structural homologies.

Proctolin and excitation of the crayfish swimmeret system

The ventral nerve cord of crayfish contains axons of five pairs of excitatory interneurons, each of which can activate the swimmeret system. Perfusion of the ventral nerve cord with the neuropeptide proctolin also activates the swimmeret system. The experiments reported here were conducted to test the hypothesis that one or more of these excitatory interneurons uses proctolin as a transmitter. Each of the five excitatory axons was located and stimulated separately in an individual crayfish, and similar motor activity was elicited by stimulating each of them. Quantitative comparison of spontaneous swimmeret motor patterns with activity caused by stimulating one of these excitatory axons, EC, or by perfusing with proctolin solutions showed that the motor patterns produced under these three conditions were not significantly different (P > 0.05). By using a new, affinity-purified proctolin antiserum, we labeled axons in the connective tissue between the last thoracic and first abdominal ganglion and compared the positions of labeled axons with the previously described positions of the excitatory axons. About 0.3% of the axons in these connective tissues showed proctolin-like immunoreactivity, but heavily labeled pairs of axons did occur bilaterally in the regions of excitatory swimmeret axons. The projections of these labeled axons into the abdominal ganglia were traced in serial plastic sections. Labeled processes were abundant in the lateral neuropils, the loci of the swimmeret pattern-generating circuitry. From this evidence, we propose that three of these excitatory swimmeret interneurons use proctolin as a transmitter, but that a fourth does not. The evidence for the fifth axon is ambiguous.

The inhibitory motoneurons of crayfish thoracic limbs: identification, structures, and homology with insect common inhibitors

The three inhibitory motoneurons supplying crayfish thoracic limbs were identified, stained, and compared structurally. The inhibitors to the walking leg muscles (in Orconectes) were identified anatomically by a combination of immunocytochemical staining for gamma-aminobutyric acid (GABA) or glutamate decarboxylase and differential backfill staining with nickel and cobalt ions. The cheliped inhibitors were identified intracellularly and injected with Lucifer Yellow (Pacifastacus) or cobalt (Procambarus). The common inhibitor (CI) in each thoracic segment has a medial or slightly contralateral soma near the ganglion's posterior boundary, a gently curving primary neurite, an extensive ipsilateral dendritic tree, and an axon emerging through the anterior root. The stretcher-closer inhibitor (SI) has a soma slightly anterior and ipsilteral to the CI's a sharply bent proximal neurite, a smaller dendritic tree, and an axon in the posterior root. The opener inhibitor (OI) lies more laterally and often posterior to the CI; its diagnoally directed neurite enters the posterior root. The inhibitors' structures were related to major neuroanatomical landmarks within the ganglion, to soma positions of excitatory motoneurons revealed by backfilling, and to soma locations of inhibitory interneurons revealed by GABA-like immunoreactivity. In their peripheral distributions to the leg muscles and in their central structures, these crayfish limb inhibitors show striking similarities with those of the locust. Crayfish and locust thoracic ganglia also show more general neuroanatomical similarities. These observations suggest that the crayfish CI, SI, and OI are, respectively, homologous with the locust CI1, CI2, and CI3. The implications of such a homology for arthropod phylogeny are discussed.

Neurons with histaminelike immunoreactivity in the segmental and stomatogastric nervous systems of the crayfish Pacifastacus leniusculus and the lobster Homarus americanus

We used a polyclonal antiserum against histamine to map histaminelike immunoreactivity (HLI) in whole mounts of the segmental ganglia and stomatogastric ganglion of crayfish and lobster. Carbodiimide fixation permitted both HRP-conjugated and FITC-conjugated secondary antibodies to be used effectively to visualize HLI in these whole mounts. Two interneurons that send axons through the inferior ventricular nerve (ivn) and the stomatogastric nerve to the stomatogastric ganglion had strong HLI, both in crayfish and in lobster. These ivn interneurons were known from other evidence to be histaminergic. The neuropil of the stomatogastric ganglion in both crayfish and lobster contained brightly labeled terminals of axons that entered the ganglion from the stomatogastric nerve. No neuronal cell bodies in this ganglion had HLI. Each segmental ganglion contained at least one pair of neurons with HLI. Some neurons in the subesophageal ganglion and in each thoracic ganglion labeled very brightly. Axons of projection interneurons with strong HLI occurred in the dorsal lateral tracts of each segmental ganglion, and sent branches to the lateral neurophils and tract neurophils of each ganglion. All the labeled neurons were interneurons; no HLI was observed in peripheral nerves.

A method for the determination of projection areas of GABA immunoreactive neurons in the invertebrate nervous system

Axonal transport of metallic salts (nickel or cobalt chloride) has been widely used for the anatomical mapping of neural pathways. We show here that when nickel is introduced into GABAergic neurons it completely eliminates GABA immunolabelling. We have used this property to determine the axonal projections of GABAergic neurons in the stomatogastric system of Crustacea. For example, following nickel backfills from either cut axons or from terminals, GABA immunostaining labels only those GABA-immunoreactive neurons which had not been retrogradely labelled with nickel and hence did not project in the cut nerve or to the neuropile uptake site. By comparing such immunolabelled preparations with those not pretreated with nickel the projection patterns of all the GABA immunoreactive neurons in a given system can be revealed. This effect of nickel appears to be selective for GABA immunostaining, insofar as it does not interfere with the immunodetection of either the peptide proctolin or a FMRFamide-like peptide. This method may prove to be a useful tool for analyzing GABAergic neuronal pathways in the nervous systems of invertebrates.

GABA mediated inhibition of abdominal postural flexion in lobster

Swimmeret mechanostimulation initiates an abdominal extension program which includes flexion inhibition. Agonists and antagonists were used to examine the GABAergic nature of inhibitory responses recorded intracellularly from a flexion producing interneuron (FPI 303) and flexor motor neuron (f3) pair, and extracellularly from the other flexor efferents. The GABA antagonist picrotoxin (PTX) enhanced spontaneous flexion. As PTX levels increased, the swimmeret evoked response shifted from inhibition of flexion (less than 10 microM), to inhibition followed by excitation (10-30 microM), to flexion excitation (greater than or equal to 50 microM). The irreversibility of PTX effects, and the absence of bicuculline or baclofen induced changes in flexion activity, suggests that the receptors differ from mammalian GABA receptors. Both GABA and its agonist muscimol suppressed flexion activity and reduced intracellular potential amplitudes. Proof that PTX acts by binding the GABA receptor was obtained by observing that the addition of GABA or muscimol to preparations pretreated with PTX did not affect either spontaneous or swimmeret evoked activities, or intracellular potential amplitudes. These results imply involvement of GABAergic interneurons in the abdominal motor programs which inhibit flexion.

The effect of various neurotransmitters and some of their agonists and antagonists on the crayfish abdominal positioning system

1. Crayfish abdominal nerve cords were perfused with selected transmitters or their agonists or antagonists. Motor activity underlying abdominal positioning behavior was monitored. 2. All the neurotransmitters except glycine had a measurable effect on this system. 3. Acetylcholine and its agonists were slightly stimulatory. Both muscarinic and nicotinic receptors were indicated. 4. GABA was weakly inhibitory. Picrotoxin was strongly stimulatory, perhaps as a result of its known ability to block GABA and inhibitory acetylcholine receptors. 5. Histamine was strongly inhibitory. Both H1 and H2 receptors were indicated. 6. Glutamate was found to be slightly inhibitory while its agonist, NMDA, showed no effect. 7. Finally, L-Dopa was stimulatory, but only at a high concentration.