Stomatogastric Nervous System Research Articles

Distribution of Neuropeptide F-Like Immunoreactivity in the Eastern Subterranean Termite,Reticulitermes flavipes

The nervous system and gut of worker, soldier and alate castes of the eastern subterranean termite, Reticulitermes flavipes Kollar (Isoptera: Rhinotermitidae) were examined for immunoreactivity to an antiserum to Helicoverpa zea (Boddie) (Leipidoptera: Noctuidae) MP-I (QAARPRF-NH2), a truncated form of neuropeptide F. More than 145 immunostained axons and cell bodies were seen in the brain and all ganglia of the ventral nerve cord. Immunoreactive axons exiting the brain projected anteriorly to the frontal ganglion and posteriorly to the corpora cardiaca and corpora allata. In the stomatogastric nervous system, immunoreactive axons were observed over the surface of the foregut, salivary glands, midgut and rectum. These axons originated in the brain and from 15–25 neurosecretory cells on the foregut. Staining patterns were consistent between castes, with the exception of immunostaining observed in the optic lobes of alates. At least 600 immunoreactive endocrine cells were evenly distributed in the midguts of all castes with higher numbers present in the worker caste. Immunostaining of cells in the nervous system and midgut was blocked by preabsorption of the antiserum with Hez MP-I but not by a peptide having only the RF-NH2 in common. This distribution suggests NPF-like peptides coordinate feeding and digestion in all castes of this termite species.

FMRFamide-like immunoreactivity in the central nervous system and alimentary tract of the non-hematophagous blow fly, Phormia regina, and the hematophagous horse fly, Tabanus nigrovittatus.

FMRFamide-related peptides (FaRPs) are a diverse and physiologically important class of neuropepeptides in the metazoa. In insects, FaRPs function as brain-gut neuropeptides and have been immunolocalized throughout the nervous system and alimentary tract where they have been shown to affect feeding behavior. The occurrence of FMRFamide-like immunoreactivity (FLI) was examined in the central nervous system and alimentary tract of non-hematophagous blow fly, Phormia regina Meigen (Diptera: Calliphoridae), and the hematophagous horse fly, Tabanus nigrovittatus Macquart (Diptera:Tabanidae). Although the central nervous system and alimentary anatomy differ between these two dipteran species, many aspects of FLI remain similar. FLI was observed throughout the central and stomatogastric nervous systems, foregut, and midgut in both flies. In the central nervous system, cells and processes with FLI occurred in the brain, subesophageal ganglion, and ventral nerve cord. FLI was associated with neurohemal areas of the brain and ventral nerve cord. A neurohemal plexus of fibers with FLI was present on the dorsal region of the thoracic central nervous system in both species. In the gut, processes with FLI innervated the crop duct, crop and anterior midgut. Endocrine cells with FLI were present in the posterior midgut. The distribution of FLI in these two flies, in spite of their different feeding habits, further supports the role of FaRPs as important components of the braingut neurochemical axis in these insects and implicates FaRPs as regulators of insect feeding physiology among divergent insect taxa.

Developmental Regulation of Neuromodulator Function in the Stomatogastric Ganglion of the Lobster,Homarus americanus

Neuromodulatory substances have profound effects on the two motor patterns generated by the adult crustacean stomatogastric ganglion (STG), the gastric mill rhythm and the pyloric rhythm. Developmentally regulated changes in the modulatory functions of neuromodulators could therefore play an important role in the maturation of the output from the developing STG. We compared the effects of neuromodulators on isolated embryonic and adult STG of the lobster, Homarus americanus. Bath application of Val(1)-SIFamide, a peptide whose expression is different in embryos and adults, activated different neuron classes in embryos and adults. Cancer borealis tachykinin-related peptide 1a, a peptide that does not appear in the terminals of modulatory neurons in the STG until after embryonic development, also produced different motor patterns in embryos and adults. In contrast, red pigment concentrating hormone, a peptide with a similar distribution in the STNS across development, produced similar motor patterns in embryonic and adult STG. Proctolin, serotonin, and allatostatin were also physiologically active on the isolated embryonic STG. Together, these results demonstrate that receptors to many neuromodulators are present and functional on STG neurons before the motor patterns of the stomatogastric nervous system are mature. Moreover, neuromodulator responses change during development, perhaps contributing to the maturation of the output from the stomatogastric nervous system.

State-Dependent Presynaptic Inhibition Regulates Central Pattern Generator Feedback to Descending Inputs

Central pattern generators (CPGs) provide feedback to their projection neuron inputs. However, it is unknown whether this feedback is regulated and how that might shape CPG output. We are studying feedback from the pyloric CPG to identified projection neurons that regulate the gastric mill CPG, in the crab stomatogastric nervous system. Both CPGs are located in the stomatogastric ganglion (STG) and are influenced by projection neurons originating in the paired commissural ganglia (CoGs). Two extrinsic inputs [ventral cardiac neurons (VCNs) and postoesophageal commissure (POC) neurons] trigger distinct gastric mill rhythms despite acting via the same projection neurons [modulatory commissural neuron 1 (MCN1); commissural projection neuron 2 (CPN2)]. These projection neurons receive feedback inhibition from the pyloric CPG interneuron anterior burster (AB), resulting in their exhibiting pyloric-timed activity during the retraction phase of the VCN- and POC-triggered gastric mill rhythms. However, during the gastric mill protraction phase, MCN1/CPN2 exhibit pyloric-timed activity during the POC-triggered rhythm but fire tonically during the VCN-triggered rhythm. Here, we show that the latter, tonic activity pattern results from the elimination of AB inhibition of MCN1/CPN2, despite persistent AB actions within the STG and AB action potentials still propagating into each CoG. This loss of pyloric-timed AB input likely results from presynaptic inhibition of AB in each CoG because, when a secondary rhythmic AB burst initiation zone in the CoG is activated, the associated action potentials are selectively suppressed during the VCN protraction phase. Thus, rhythmic CPG feedback can be locally regulated, in a state-dependent manner, enabling the same projection neurons to drive multiple motor patterns from the same neuronal circuit.

Multiple Mechanisms for Integrating Proprioceptive Inputs That Converge on the Same Motor Pattern-Generating Network

Movement-derived sensory feedback adapts centrally generated motor programs to changing behavioral demands. Motor circuit output may also be shaped by distinct proprioceptive systems with different central actions, although little is known about the integrative processes by which such convergent sensorimotor regulation occurs. Here, we explore the combined actions of two previously identified proprioceptors on the gastric mill motor network in the lobster stomatogastric nervous system. Both mechanoreceptors [anterior gastric receptor (AGR) and posterior stomach receptor (PSR)] access the gastric circuit via the same pair of identified projection interneurons that either excite [commissural gastric (CG)] or inhibit [gastric inhibitor (GI)] different subsets of gastric network neurons. Mechanosensory information from the two receptors is integrated upstream to the gastric circuit at two levels: (1) postsynaptically, where both receptors excite the GI neuron while exerting opposing effects on the CG neuron, and (2) presynaptically, where PSR reduces AGR's excitation of the CG projection neuron. Concomitantly PSR selectively enhances AGR's activation of the GI neuron, possibly also via a presynaptic action. PSR's influences also far outlast its transient synaptic effects, indicating the additional involvement of modulatory processes. Consequently, PSR activation causes parallel input from AGR to be conveyed preferentially via the GI interneuron, resulting in a prolonged switch in the pattern of gastric circuit output. Therefore, via a combination of short- and long-lasting, presynaptic and postsynaptic actions, one proprioceptive system is able to promote its impact on a target motor network by biasing the access of a different sensory system to the same circuit.

Characteristic differences in modulation of stomatogastric musculature by a neuropeptide in three species of Cancer crabs

Stomatogastric musculature from crabs in the genus Cancer provides a system in which modulatory roles of peptides from the FLRFamide family can be compared. The anterior cardiac plexus (ACP) is a neuroendocrine release site within the Cancer stomatogastric nervous system that is structurally identical in C. borealis, C. productus, and C. magister but that appears to contain FLRFamide-like peptide(s) only in C. productus. We measured the effect of TNRNFLRFamide on nerve-evoked contractions of muscles that were nearby, an intermediate distance, or far from the ACP. We found the spatial pattern of FLRFamidergic modulation of muscles in C. productus to be qualitatively different than in C. borealis or C. magister. In C. productus, muscles proximal to the ACP were more responsive than distal muscles. In C. borealis, FLRFamidergic response was less dependent on muscle location. These results suggest that functionally different roles of FLRFamides in modulating stomatogastric muscle movements may have evolved in different Cancer species.

Neuropeptides associated with the regulation of feeding in insects

The stomatogastric nervous system plays a pivotal role in feeding behaviour. Central to this system is the frontal ganglion, which is responsible for foregut motor activity, and hence the passage of food through the gut. Many insect peptides, which exhibit myoactivity on the visceral muscles of the gut in vitro, have been detected in the stomatogastric nervous system by immunochemical or mass spectrometric techniques. This localisation of myoactive peptides, particularly in the frontal ganglion, implies roles for these peptides in the neural control and modulation of feeding in insects. Insect sulfakinins, tachykinins, allatotropin and proctolin have all been shown to stimulate the foregut muscles, whereas myosuppressins, myoinhibitory peptides and allatostatins all inhibited spontaneous contractions of the foregut in a variety of insects. Some of these peptides, when injected, inhibited feeding in vivo. Both the A-type and B-type allatostatins suppressed feeding activity when injected into the cockroach, Blattella germanica and the Manduca sexta C-type allatostatin and allatotropin inhibited feeding when injected into the larvae of two noctuid moths, Lacanobia oleracea and Spodoptera frugiperda, respectively. Injection of sulfakinins into the fly Phormia regina, the locust Schistocera gregaria and the cockroach B. germanica also suppressed feeding, whereas silencing the sulfakinin gene through the injection of double stranded RNA resulted in an increase in food consumption in the cricket Gryllus bimaculatus. The regulation of feeding in insects is clearly very complex, and involves the interaction of a number of mechanisms, one of which is the release, either centrally or locally, of neuropeptides. However, the role of neuropeptides, their mechanisms of action, interactions with each other, and their release are still poorly understood. It is also unclear why insects possess such a number of different peptides, some with multiples copies or homologues, which stimulate or inhibit gut motility, and how their release, sometimes from the same neurone, is regulated. These neuropeptides may also act at sites other than visceral muscles, such as centrally through the brain or on gut stretch receptors.

Characterization of a descending pathway: activation and effects on motor patterns in the brachyuran crustacean stomatogastric nervous system

The regulation of motor patterns by higher-order neuronal centers ensures appropriate motor function and behavior, but only a few studies have characterized this regulation at the cellular level. Here, we address motor pattern regulation in the stomatogastric nervous system (STNS) of the crab Cancer pagurus. This easily accessible model system is an extension of the central nervous system and contains the motor circuits that generate the rhythmic motor patterns for ingestion (esophageal rhythm) and processing of food (gastric mill and pyloric rhythms). We have documented the actions of two identified neurons located in the brain on the STNS motor circuits. We show that these neurons provide exteroceptive chemosensory information to the motor circuits and we outline their axonal projection patterns, their firing activity and their effects on three motor patterns. Backfill stainings and activity measurements in vivo and in vitro show that two neurons located in cluster 17 of the brain project via the inferior ventricular (IV) nerve to the STNS. These IV neurons started to burst rhythmically when chemosensory stimuli were applied to the first antennae. When rhythmically activated in vitro, gastric mill rhythms were elicited or, if already active, entrained by the IV neuron activity. In addition, IV neuron stimulation excited the esophageal motor neuron and inhibited several pyloric neurons such that the timing of the IV neuron activity was imposed on all motor rhythms. The IV neurons were thus capable of synchronizing the activities of different motor circuits, which demonstrates the regulation of motor patterns by higher-order neuronal centers.

Regulation of motor pattern frequency by reversals in proprioceptive feedback

Proprioceptive sensory feedback has important functions for motor pattern generation in which phasic negative and positive feedback is used to coordinate neural and musculoskeletal dynamics. Whether and how feedback sign regulates the motor patterns in behaviorally relevant closed-loop conditions has not been fully elucidated. We characterized the feedback provided by the anterior gastric receptor (AGR), a muscle tendon organ in the stomatogastric nervous system of the crab Cancer pagurus, to the gastric mill motor pattern in intact animals. AGR innervates the protractor muscles and was activated either during the protraction or retraction phase of the rhythm. Experiments with neuromuscular preparations imply that this was due to isometric contractions of the protractor muscles and their passive stretch by the antagonistic retractor muscles. As AGR excited the protractors and inhibited the retractors independently of the timing of its activation, the timing switch changed AGR feedback from positive to negative. We tested the effects of this change in feedback sign on the motor pattern in the isolated nervous system by activating AGR at the corresponding phases of the rhythm, using intracellular current injection. When AGR was activated during the protractor phase and provided positive feedback, it prolonged the burst activities of protractor and retractor neurons and slowed ongoing rhythms. When activated during the retraction phase and thus provided negative feedback, burst durations decreased and the rhythm cycle frequency increased. Our study thus shows that the cycle frequency of centrally generated activity patterns can be regulated by switching the sign of phasic proprioceptive feedback.

The pyloric neural circuit of the herbivorous crabPugettia productashows limited sensitivity to several neuromodulators that elicit robust effects in more opportunistically feeding decapods

Modulation of neural circuits in the crustacean stomatogastric nervous system (STNS) allows flexibility in the movements of the foregut musculature. The extensive repertoire of such resulting motor patterns in dietary generalists is hypothesized to permit these animals to process varied foods. The foregut and STNS of Pugettia producta are similar to those of other decapods, but its diet is more uniform, consisting primarily of kelp. We investigated the distribution of highly conserved neuromodulators in the stomatogastric ganglion (STG) and neuroendocrine organs of Pugettia, and documented their effects on its pyloric rhythm. Using immunohistochemistry, we found that the distributions of Cancer borealis tachykinin-related peptide I (CabTRP I), crustacean cardioactive peptide (CCAP), proctolin, red pigment concentrating hormone (RPCH) and tyrosine hydroxylase (dopamine) were similar to those of other decapods. For all peptides except proctolin, the isoforms responsible for the immunoreactivity were confirmed by mass spectrometry to be the authentic peptides. Only two modulators had physiological effects on the pyloric circuit similar to those seen in other species. In non-rhythmic preparations, proctolin and the muscarinic acetylcholine agonist oxotremorine consistently initiated a full pyloric rhythm. Dopamine usually activated a pyloric rhythm, but this pattern was highly variable. In only about 25% of preparations, RPCH activated a pyloric rhythm similar to that seen in other species. CCAP and CabTRP I had no effect on the pyloric rhythm. Thus, whereas Pugettia possesses all the neuromodulators investigated, its pyloric rhythm, when compared with other decapods, appears less sensitive to many of them, perhaps because of its limited diet.

Spectral Analyses Reveal the Presence of Adult-Like Activity in the Embryonic Stomatogastric Motor Patterns of the Lobster,Homarus americanus

The stomatogastric nervous system (STNS) of the embryonic lobster is rhythmically active prior to hatching, before the network is needed for feeding. In the adult lobster, two rhythms are typically observed: the slow gastric mill rhythm and the more rapid pyloric rhythm. In the embryo, rhythmic activity in both embryonic gastric mill and pyloric neurons occurs at a similar frequency, which is slightly slower than the adult pyloric frequency. However, embryonic motor patterns are highly irregular, making traditional burst quantification difficult. Consequently, we used spectral analysis to analyze long stretches of simultaneous recordings from muscles innervated by gastric and pyloric neurons in the embryo. This analysis revealed that embryonic gastric mill neurons intermittently produced pauses and periods of slower activity not seen in the recordings of the output from embryonic pyloric neurons. The slow activity in the embryonic gastric mill neurons increased in response to the exogenous application of Cancer borealis tachykinin-related peptide 1a (CabTRP), a modulatory peptide that appears in the inputs to the stomatogastric ganglion (STG) late in larval development. These results suggest that the STG network can express adult-like rhythmic behavior before fully differentiated adult motor patterns are observed, and that the maturation of the neuromodulatory inputs is likely to play a role in the eventual establishment of the adult motor patterns.

A newly identified extrinsic input triggers a distinct gastric mill rhythmviaactivation of modulatory projection neurons

Neuronal network flexibility enables animals to respond appropriately to changes in their internal and external states. We are using the isolated crab stomatogastric nervous system to determine how extrinsic inputs contribute to network flexibility. The stomatogastric system includes the well-characterized gastric mill (chewing) and pyloric (filtering of chewed food) motor circuits in the stomatogastric ganglion. Projection neurons with somata in the commissural ganglia (CoGs) regulate these rhythms. Previous work characterized a unique gastric mill rhythm that occurred spontaneously in some preparations, but whose origin remained undetermined. This rhythm includes a distinct protractor phase activity pattern, during which a key gastric mill circuit neuron (LG neuron) and the projection neurons MCN1 and CPN2 fire in a pyloric rhythm-timed activity pattern instead of the tonic firing pattern exhibited by these neurons during previously studied gastric mill rhythms. Here we identify a new extrinsic input, the post-oesophageal commissure (POC) neurons, relatively brief stimulation (30 s) of which triggers a long-lasting (tens of minutes) activation of this novel gastric mill rhythm at least in part via its lasting activation of MCN1 and CPN2. Immunocytochemical and electrophysiological data suggest that the POC neurons excite MCN1 and CPN2 by release of the neuropeptide Cancer borealis tachykinin-related peptide Ia (CabTRP Ia). These data further suggest that the CoG arborization of the POC neurons comprises the previously identified anterior commissural organ (ACO), a CabTRP Ia-containing neurohemal organ. This endocrine organ thus appears to also have paracrine actions, including activation of a novel and lasting gastric mill rhythm.

Antifeeding properties of myosuppressin in a generalist phytophagous leafworm, Spodoptera littoralis (Boisduval)

Insect myosuppressins are a family of peptides with a characteristic HV/SFLRFamide carboxy terminus. They are expressed in brain, neurohemal organs, stomatogastric nervous system, and in midgut endocrine cells. From a functional point of view, myosuppressins inhibit contractions of different visceral muscles, stimulate certain skeletal muscles and activate enzyme secretion from the gut. Moreover, in the omnivorous cockroach Blattella germanica, myosuppressin inhibits food intake. Based on these results, we studied the antifeeding activity of myosuppressin in the phytophagous leafworm Spodoptera littoralis. Firstly, we isolated the cDNA corresponding to the S. littoralis myosuppressin precursor encoding the typical myosuppressin peptide of lepidopterans: pQDVVHSFLRFamide. Then, we determined the expression patterns (in terms of mRNA and peptide) of myosuppressin in brain and midgut, and peptide levels in the haemolymph. Myosuppressin patterns in the brain and haemolymph were similar, and symmetrical to that of food consumption, thus suggesting that myosuppressin might inhibit feeding in S. littoralis. Moreover, synthetic myosuppressin effectively inhibited food intake in non-choice antifeeding tests. Taken together, the obtained results point to the hypothesis that myosuppressin represses feeding in S. littoralis.

Crustacean dopamine receptors: localization and G protein coupling in the stomatogastric ganglion

Neuromodulators, such as dopamine (DA), control motor activity in many systems. To begin to understand how DA modulates motor behaviors, we study a well-defined model: the crustacean stomatogastric nervous system (STNS). The spiny lobster STNS receives both neuromodulatory and neurohormonal dopaminergic input, and extensive background information exists on the cellular and network effects of DA. However, there is a void of information concerning the mechanisms of DA signal transduction in this system. In this study, we show that Gs, Gi, and Gq are activated in response to DA in STNS membrane preparations from five crustacean species representing distant clades in the order Decapoda. Three evolutionarily conserved DA receptors mediate this response in spiny lobsters: D(1alphaPan), D(1betaPan) and D(2alphaPan). G protein coupling for these receptors can vary with the cell type. In the native membrane, the D(1alphaPan) receptor couples with Gs and Gq, the D(1betaPan) receptor couples with Gs, and the D(2alphaPan) receptor couples with Gi. All three receptors are localized exclusively to the synaptic neuropil and most likely generate global biochemical signals that alter ion channels in distant compartments, as well as local signals.

SIFamide peptides in clawed lobsters and freshwater crayfish (Crustacea, Decapoda, Astacidea): A combined molecular, mass spectrometric and electrophysiological investigation

Recently, we identified the peptide VYRKPPFNGSIFamide (Val1-SIFamide) in the stomatogastric nervous system (STNS) of the American lobster Homarus americanus using matrix-assisted laser desorption/ionization-Fourier transform mass spectrometry (MALDI-FTMS). Given that H. americanus is the only species thus far shown to possess this peptide, and that a second SIFamide isoform, Gly1-SIFamide, is broadly conserved in other decapods, including another astacidean, the crayfish Procambarus clarkii, we became interested both in confirming our identification of Val1-SIFamide via molecular methods and in determining the extent to which this isoform is conserved within other members of the infraorder Astacidea. Here, we present the identification and characterization of an H. americanus prepro-SIFamide cDNA that encodes the Val1 isoform. Moreover, we demonstrate via MALDI-FTMS the presence of Val1-SIFamide in a second Homarus species, Homarus gammarus. In contrast, only the Gly1 isoform was detected in the other astacideans investigated, including the lobster Nephrops norvegicus, a member of the same family as Homarus, and the crayfish Cherax quadricarinatus, P. clarkii and Pacifastacus leniusculus, which represent members of each of the extant families of freshwater astacideans. These results suggest that Val1-SIFamide may be a genus (Homarus)-specific isoform. Interestingly, both Val1- and Gly1-SIFamide possess an internal dibasic site, Arg3-Lys4, raising the possibility of the ubiquitously conserved isoform PPFNGSIFamide. However, this octapeptide was not detected via MALDI-FTMS in any of the investigated species, and when applied to the isolated STNS of H. americanus possessed little bioactivity relative to the full-length Val1 isoform. Thus, it appears that the dodeca-variants Val1- and Gly1-SIFamide are the sole bioactive isoforms of this peptide family in clawed lobsters and freshwater crayfish.

Mechanosensory Gating of Proprioceptor Input to Modulatory Projection Neurons

Sensorimotor gating commonly occurs at sensory neuron synapses onto motor circuit neurons and motor neurons. Here, using the crab stomatogastric nervous system, we show that sensorimotor gating also occurs at the level of the projection neurons that activate motor circuits. We compared the influence of the gastro-pyloric receptor (GPR) muscle stretch-sensitive neuron on two projection neurons, modulatory commissural neuron 1 (MCN1) and commissural projection neuron 2 (CPN2), with and without a preceding activation of the mechanosensory ventral cardiac neurons (VCNs). MCN1 and CPN2 project from the paired commissural ganglia (CoGs) to the stomatogastric ganglion (STG), where they activate the gastric mill (chewing) motor circuit. When stimulated separately, the GPR and VCN neurons each elicit the gastric mill rhythm by coactivating MCN1 and CPN2. When GPR is instead stimulated during the VCN-gastric mill rhythm, it slows this rhythm. This effect results from a second GPR synapse onto MCN1 that presynaptically inhibits its STG terminals. Here, we show that, during the VCN-triggered rhythm, the GPR excitation of MCN1 and CPN2 in the CoGs is gated out, leaving only its influence in the STG. This gating effect appears to occur within the CoG and does not result from a ceiling effect on projection neuron firing frequency. Additionally, this gating action enables GPR to either activate rhythmic motor activity or act as a phasic sensorimotor feedback system. These results also indicate that the site of sensorimotor gating can occur at the level of the projection neurons that activate a motor circuit.

Characterization of neuropeptide F-like immunoreactivity in the blood-feeding hemipteran, Rhodnius prolixus

The invertebrate neuropeptide Y (NPY) homolog, neuropeptide F (NPF), has been characterized for a wide range of invertebrate phyla, including platyhelminthes, molluscs, and arthropods. Current hypotheses suggest that NPF may be capable of regulating responses to diverse external cues related to nutritional status and feeding. The qualitative and quantitative distribution of an NPF-like peptide in fifth instar Rhodnius prolixus was undertaken using an antiserum raised against Drosophila NPF. Immunohistochemistry reveals NPF-like immunoreactive neurons and processes in the central nervous system, stomatogastric nervous system and peripheral nervous system. The distribution of NPF-like immunoreactivity within the medial neurosecretory cells of the brain and neurohemal areas of the corpus cardiacum and dorsal vessel, suggests NPF may act as a neurohormone. Immunoreactive processes are present over the surface of the hindgut and the immunoreactivity in these processes is greatly reduced in intensity 24 h post-feeding. The quantification of partially purified NPF-like material in the CNS of R. prolixus was conducted by HPLC fractionation and radioimmunoassay. The results suggest that NPF-like material is present in fifth instar R. prolixus and likely released into the hemolymph following a blood meal.

Long-term exposure to histamine induces the expression of an embryonic-like motor pattern in an adult nervous system

Neuromodulatory inputs play important roles in shaping the outputs of neural networks. While the actions of neuromodulatory substances over the short term (seconds, minutes) have been examined in detail, far less is known about the possible longer-term (hours) effects of these substances. To investigate this issue, we used the stomatogastric nervous system (STNS) of the lobster to examine the short- and long-term effects of histamine on rhythmic network activity. The application of histamine to the entire STNS had strong inhibitory effects on all three of the STNS networks, observable within minutes. In contrast, longer-term (> 1 h) application of histamine induced the expression of a single, unified rhythm involving neurons from all three networks. Selective application of histamine to different regions of the STNS demonstrated that a unified rhythm arises following the long-term application of histamine to the commissural ganglia (CoGs; modulatory centres), but not the stomatogastric ganglion (site of neural networks). Strikingly, the single rhythm observed following the long-term application of histamine to the CoGs exhibits many similarities with the single rhythm expressed by the embryonic STNS. Together, these results demonstrate that histamine has markedly different short- and long-term effects on network activity; short-term effects arising through direct actions on the networks and long-term effects mediated by actions on modulatory neurons. Furthermore, they indicate that histamine is able to induce the expression of an embryonic-like rhythm in an adult system, suggesting that long-term actions of histamine may play key roles in the development of the STNS networks.

Anatomy of the stomatogastric nervous system associated with the foregut in Drosophila melanogaster and Calliphora vicina third instar larvae

The stomatogastric nervous system (SNS) associated with the foregut was studied in 3rd instar larvae of Drosophila melanogaster and Calliphora vicina (blowfly). In both species, the foregut comprises pharynx, esophagus, and proventriculus. Only in Calliphora does the esophagus form a crop. The position of nerves and neurons was investigated with neuronal tracers in both species and GFP expression in Drosophila. The SNS is nearly identical in both species. Neurons are located in the proventricular and the hypocerebral ganglion (HCG), which are connected to each other by the proventricular nerve. Motor neurons for pharyngeal muscles are located in the brain not, as in other insect groups, in the frontal ganglion. The position of the frontal ganglion is taken by a nerve junction devoid of neurons. The junction is composed of four nerves: the frontal connectives that fuse with the antennal nerves (ANs), the frontal nerve innervating the cibarial dilator muscles and the recurrent nerve that innervates the esophagus and projects to the HCG. Differences in the SNS are restricted to a crop nerve only present in Calliphora and an esophageal ganglion that only exists in Drosophila. The ganglia of the dorsal organs give rise to the ANs, which project to the brain. The extensive conformity of the SNS of both species suggests functional parallels. Future electrophysiological studies of the motor circuits in the SNS of Drosophila will profit from parallel studies of the homologous but more accessible structures in Calliphora.

Comparative immunohistochemistry of the cerebral ganglion in Gastrotricha: an analysis of FMRFamide-like immunoreactivity in Neodasys cirritus (Chaetonotida), Xenodasys riedli and Turbanella cf. hyalina (Macrodasyida)

The neuropeptide FMRFamide (Phe–Met–Arg–Phe–NH2) is part of a large and diverse family of peptidergic neurotransmitters present throughout the animal kingdom. To date, no such neuropeptides have been demonstrated in gastrotrichs despite their presence in closely related invertebrates such as nematodes. Here, the FMRFamidergic nervous system of three marine gastrotrichs is investigated with immunofluorescence, CLSM, and 3D computer imaging to gain insight into structure of the cerebral ganglion and test various phylogenetic hypotheses on its organization. Results reveal that FMRFamide-like immunoreactivity (IR) is present throughout the nervous systems of three species: Neodasys cirritus (Chaetonotida), Xenodasys riedli and Turbanella cf. hyalina (Macrodasyida). Both macrodasyidans possess FMRFamide-like IR in the central, peripheral- and stomatogastric-nervous systems, while FMRFamide-like IR is restricted to the CNS in N. cirritus. In all three species, the cerebral ganglion is dumbbell-shaped and bordered bilaterally by cerebral perikarya: numerous perikarya are present in X. riedli and N. cirritus, while few perikarya are present in T. cf. hyalina. Cerebral perikarya flank the nerve ring neuropil, which contains IR fibers in the supra- and subpharyngeal commissures of both macrodasyidans, but in N. cirritus, only contains IR fibers in the suprapharyngeal commissure. Together, these results confirm the peripharyngeal nature of the gastrotrich cerebral ganglion, but are equivocal on hypotheses of its tripartite structure. Still, the neural organization of gastrotrichs, in particular, the architecture of the cerebral ganglion, is expected to hold valuable information for future assessments of gastrotrich phylogeny, and may yet provide key insights into the evolution of this enigmatic taxon.