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

Actions of kinin peptides in the stomatogastric ganglion of the crab Cancer borealis

To fully understand neuronal network operation, the influence of all inputs onto that network must be characterized. As in most systems, many neuronal and hormonal pathways influence the multifunctional motor circuits of the crustacean stomatogastric ganglion (STG), but the actions of only some of them are known. Therefore, we characterized the influence of the kinin peptide family on the gastric mill (chewing) and pyloric (filtering of chewed food) motor circuits in the STG of the crab Cancer borealis. The kinins are myoactive in arthropods and they occur within the arthropod central nervous system (CNS), but their CNS actions are not well characterized in any species. The pevkinins were first identified in the shrimp Penaeus vannamei, but they have yet to be studied in the STG of any species. We identified kinin-like immunolabeling (KLI) in the pericardial organs (POs) in C. borealis, but there was no KLI within the STG. The POs are a major source of hormonal influence on the STG. Pevkinin peptides activated the pyloric circuit and they caused a modest increase in the speed of ongoing pyloric rhythms. This modest influence on cycle speed resulted in part from pevkinin excitation of the lateral pyloric neuron, whose strengthened inhibitory synapse onto the pyloric pacemaker neurons limited the pevkinin-mediated increase in cycle speed. The pevkinin excitation of the pyloric rhythm was not strong enough to interfere with the previously documented, gastric mill rhythm-mediated weakening of the pyloric rhythm. Pevkinin also had little influence on the gastric mill rhythm. These results indicate that the kinin peptides have distinct and selective modulatory actions on the pyloric rhythm.

Distribution of FMRFamide-like immunoreactivity in the alimentary tract and hindgut ganglia of the barnacle Balanus amphitrite (Cirripedia, Crustacea)

In this study, the presence and distribution of FMRFamide-like immunoreactivity in the alimentary tract of barnacle Balanus amphitrite were investigated. A net of nerve fibers strongly immunoreactive to FMRFamide-like molecules was localized in the posterior midgut and hindgut. Positive varicose nerve terminals were also localized close to the circular muscle cells and, in the hindgut, close to the radial muscular fibers. Besides this nerve fibers network, one pair of contralateral ganglia was localized in the hindgut, each of them constituted by two strongly FMRFamide-labeled neurons and one nonlabeled neuron. Their immunoreactive axons directed toward the hindgut and posterior midgut suggest an involvement of FMRFamide-like substances in adult B. amphitrite gut motility. The hindgut associated ganglia of barnacles seem to correspond to the terminal abdominal ganglia of the other crustaceans. Since they are the only residual gut ganglia in the barnacle's reduced nervous system, we can hypothesize that gut motility needs a nervous system regulation partially independent of the central nervous system.

In situ tissue analysis of neuropeptides by MALDI FTMS in-cell accumulation

Herein we report the first application of Fourier transform mass spectrometry for the analysis of neuropeptides directly from neuronal tissues. Sample preparation protocols and instrumentation conditions are developed to allow in situ neuropeptide analysis of the neuroendocrine organs freshly isolated from a marine organism Cancer borealis. The utility of a previously developed in-cell accumulation (ICA) technique is extended for peptide analysis in complex tissue samples. With the ICA procedure, ion signals from multiple laser shots are accumulated in the analyzer cell prior to detection. This procedure allows the accumulation of ion signals without accumulating noise, thus improving the signal-to-noise ratio and enhancing the sensitivity for the detection of trace-level endogenous neuropeptides. De novo sequencing of peptides directly from tissue samples becomes more feasible through this improvement. Additionally, an integrated pulse sequence is constructed to cover a wide mass range from m/z 215 to 9000 by centering quadrupole collection of ions at different masses for successive laser shots. Finally, improved mass measurement accuracy (2 ppm) for tissue peptide analysis is achieved using ICA by incorporating calibrants on a separate spot from the sample of interest without premixing calibration standards with the analytes.

Allatostatin-like immunoreactivity in the abdomen of the locust Schistocerca gregaria

A polyclonal antibody against allatostatin 1 (AST-1) of cockroach Diploptera punctata was used to investigate the distribution of AST-like immunoreactivity within the abdomen of the locust, Schistocerca gregaria. In the abdominal ganglia, AST-like immunoreactivity was found in both cell bodies and neuropile. In ganglia 6 and 7, staining was found in serial homologous cell bodies in anterior dorsolateral and dorsomedial, and posterior ventrolateral and ventromedial locations. In the terminal ganglion, the numerous immunoreactive somata were smaller in size than those in the unfused ganglia. The combination of backfill experiments with immunocytochemistry showed that, in abdominal ganglion 7, one neuron of the ventromedian cell body cluster and two of the ventrolateral cluster innervated the oviduct, which itself was covered with a dense mesh of AST-immunoreactive varicosities. Combining electron microscopy with immunohistochemistry revealed AST-like immunoreactivity in dense-core vesicles located in neurohaemal-like terminals lacking structures normally found in synapses. A mesh of AST-immunoreactive varicosities was also found on the muscles of the spermatheca and the spermathecal duct. In addition, a mesh of strongly stained varicosities was present in the distal perisympathetic organs (neurohaemal organs in the abdomen) and on the lateral heart nerves (a putative neurohaemal release zone). These data indicate that AST is an important neuroactive substance that is probably involved in multiple tasks in the control of the locust abdomen.

Mass spectrometric characterization and physiological actions of GAHKNYLRFamide, a novel FMRFamide-like peptide from crabs of the genus Cancer

The stomatogastric ganglion (STG) and the cardiac ganglion (CG) of decapod crustaceans are modulated by neuroactive substances released locally and by circulating hormones released from neuroendocrine structures including the pericardial organs (POs). Using nanoscale liquid chromatography electrospray ionization quadrupole-time-of-flight tandem mass spectrometry and direct tissue matrix-assisted laser desorption/ionization Fourier transform mass spectrometry we have identified and sequenced a novel neuropeptide, GAHKNYLRFamide (previously misassigned as KHKNYLRFamide in a study that did not employ peptide derivatization), from the POs and/or the stomatogastric nervous system (STNS) of the crabs, Cancer borealis, Cancer productus and Cancer magister. In C. borealis, exogenous application of GAHKNYLRFamide increased the burst frequency and number of spikes per burst of the isolated CG and re-initiated bursting activity in non-bursting ganglia, effects also elicited by the FMRFamide-like peptides (FLPs) SDRNFLRFamide and TNRNFLRFamide. In the intact STNS (which contains the STG), exogenous application of GAHKNYLRFamide increased the frequency of the pyloric rhythm and activated the gastric mill rhythm, effects also similar to those elicited by SDRNFLRFamide and TNRNFLRFamide. FLP-like immunoreactivity in the POs and the STNS was abolished by pre-adsorption with the synthetic GAHKNYLRFamide. Different members of the FLP family exhibited differential degradation in the presence of extracellular peptidases. Taken collectively, the amino acid sequence of GAHKNYLRFamide, the blocking of FLP-like immunostaining, and its physiological effects on the CG and STNS suggest that this peptide is a novel member of the FLP superfamily.

Metabotropic glutamate receptor agonists modify the pyloric output of the crustacean stomatogastric ganglion

We have studied the effects of groups I, II, and III metabotropic glutamate receptor (mGluR) agonists and antagonists on pyloric activity in the stomatogastric ganglion (STG) of the Caribbean spiny lobster Panulirus argus. We have found that agonists for all three groups of mGluRs modify the pyloric output. The group I agonist, l-quisqualic acid (l-QA), activated the pyloric central pattern generator (CPG). When the pyloric rhythm was partially suppressed by sucrose-block of input fibers in the stomatogastric nerve (stn), l-QA accelerated the rhythmic activity. In addition, the number of spike discharges was increased in pyloric motoneurons: pyloric (PY), and lateral pyloric (LP). In completely blocked preparations, a slow pyloric rhythm was initiated by l-QA. Groups II and III agonists exerted an inhibitory effect on pyloric activity. The group II agonist, (2S,1'S,2'S)-2-(Carboxycyclopropyl)glycine (L-CCG-I), decreased both the frequency of the pyloric rhythm and the number of spike discharges in the motoneurons: ventricular dilator (VD), PY, and LP. The effects of L-CCG-I were dose-dependent. The group III agonist, l-(+)-2-Amino-4-phosphonobutyric acid (l-AP4), slightly decreased the frequency of the pyloric rhythm and suppressed spike discharges in the VD neuron. All effects of mGluR agonists were reversible. The effect of l-QA was blocked by the broad spectrum mGluR antagonist (S)-Methyl-4-carboxyphenylglycine (MCPG). The inhibitory effect of L-CCG-I was prevented by MCPG and by the group II/III mGluR antagonist (RS)-alpha-Methyl-4-phosphonophenylglycine (MPPG), and was partially blocked by the group II mGluR antagonist (RS)-1-amino-5-phosphonoindan-1-carboxylic acid (APICA). The inhibitory effect of l-AP4 was blocked by MPPG and partially blocked by APICA.

Identification of neuropeptides from the decapod crustacean sinus glands using nanoscale liquid chromatography tandem mass spectrometry

Neurosecretory systems are known to synthesize and secrete a diverse class of peptide hormones which regulate many physiological processes. The crustacean sinus gland (SG) is a well-defined neuroendocrine site that produces numerous hemolymph-borne agents including the most complex class of endocrine signaling molecules--neuropeptides. As an ongoing effort to define the peptidome of the crustacean SG, we determine the neuropeptide complements of the SG of the Jonah crab, Cancer borealis, and the Maine lobster, Homarus americanus, using nanoflow liquid chromatography electrospray ionization quadrupole time-of-flight (ESI-QTOF) MS/MS. Numerous neuropeptides were identified, including orcokinins, orcomyotropin, crustacean hyperglycemic hormone (CHH), CHH precursor-related peptides (CPRPs), red pigment concentrating hormone (RPCH), beta-pigment dispersing hormone (beta-PDH), proctolin and HL/IGSL/IYRamide. Among them, two novel orcokinins were de novo sequenced from the SG of H. americanus. Three CPRPs including a novel isoform were sequenced in H. americanus. Four new CPRPs were sequenced from the SG of C. borealis. Our results show that structural polymorphisms in CPRPs (and thus the CHH precursors) are common in Dendrobranchiata as well as in Pleocyemata. The evolutionary relationship between the CPRPs is also discussed.

Matrix-Assisted Laser Desorption/Ionization Fourier Transform Mass Spectrometry for the Identification of Orcokinin Neuropeptides in Crustaceans Using Metastable Decay and Sustained Off-Resonance Irradiation

Vacuum UV matrix-assisted laser desorption/ionization (MALDI) Fourier transform ion cyclotron resonance mass spectrometry (FTMS) has been applied to the direct analysis of crustacean neuronal tissues using in-cell accumulation techniques to improve sensitivity. In an extension of previous work by Li and co-workers (Kutz, K. K.; Schmidt, J. J.; Li, L. Anal. Chem. 2004, 76, 5630-5640), and with a focus on the Maine lobster, Homarus americanus, we report that many peaks appearing in direct tissue spectra from crustaceans result from the metastable decay of aspartate-containing neuropeptides with localized protonation sites. We report on mass spectral characteristics of crustacean neuropeptides under MALDI-FTMS conditions and show how fragments formed by Asp-Xxx cleavages can be used to advantage for the identification of orcokinin peptides, a ubiquitous family of crustacean neuropeptides with a highly conserved N-terminus sequence. We show that predicted fragment ion fingerprints (FIFs) can be used to screen internally calibrated direct tissue spectra to provide high-confidence identification of previously identified orcokinin peptides. We use FIFs, identified based upon characteristic neutral losses, to screen for new members of the orcokinin family. Sustained off-resonance irradiation of y-series fragment ions is used to sequence the variable C-terminus. We apply these techniques to the analysis of CoG tissues from Cancer borealis and Panulirus interruptus and show that orcokinins in P. interruptus were misidentified in a previous MALDI-TOF study.

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

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

The anterior cardiac plexus: an intrinsic neurosecretory site within the stomatogastric nervous system of the crab Cancer productus

The stomatogastric nervous system (STNS) of decapod crustaceans is modulated by both locally released and circulating substances. In some species, including chelate lobsters and freshwater crayfish, the release zones for hormones are located both intrinsically to and at some distance from the STNS. In other crustaceans, including Brachyuran crabs, the existence of extrinsic sites is well documented. Little, however, is known about the presence of intrinsic neuroendocrine structures in these animals. Putative intrinsic sites have been identified within the STNS of several crab species, though ultrastructural confirmation that these structures are in fact neuroendocrine in nature remains lacking. Using a combination of anatomical techniques, we demonstrate the existence of a pair of neurosecretory sites within the STNS of the crab Cancer productus. These structures, which we have named the anterior cardiac plexi (ACPs), are located on the anterior cardiac nerves (acns), which overlie the cardiac sac region of the foregut. Each ACP starts several hundred micro m from the origin of the acn and extends distally for up to several mm. Transmission electron microscopy done on these structures shows that nerve terminals are present in the peripheral portion of each acn, just below a well defined epineurium. These terminals contain dense-core and, occasionally, electron-lucent vesicles. In many terminals, morphological correlates of hormone secretion are evident. Immunocytochemistry shows that the ACPs are immunopositive for FLRFamide-related peptide. All FLRFamide labeling in the ACPs originates from four axons, which descend to these sites through the superior oesophageal and stomatogastric nerves. Moreover, these FLRFamide-immunopositive axons are the sole source of innervation to the ACPs. Collectively, our results suggest that the STNS of C. productus is not only a potential target site for circulating hormones, but also serves as a neuroendocrine release center itself.

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

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

Stereotyped neuropil branching of an identified stomatogastric motor neuron

Anatomical studies of the crab stomatogastric ganglion (STG) have suggested only minimal organization within the neuropil of this structure. Here, we present evidence that, for at least one intrinsic neuron type, the ventricular dilator (VD) neuron, a highly organized and stereotyped branching structure exists within the stomatogastric neuropil. Specifically, we show the morphology of the VD neuron consists of a single primary neurite that projects from the soma into the neuropil and bifurcates into a pair of subprimary neurites, which in turn exit the neuropilar region, one entering the left and the other the right medial ventricular nerve. Nearly all secondary neurite branching of the VD neuron is from the subprimary neurites. There are approximately 22 secondary branches/neuron (range 14-28), with no significant difference between the number of secondary branches off the right vs. the left subprimary neurite, although the ratio of secondary branches between subprimaries varies (range 0.4-1.6). The fine neurites that branch from the secondary processes segregate hemispherically within the neuropil, based on the subprimary neurite of origin. Within this hemispherical organization, another level of fine neurite segregation is present, namely, the fine neurites derived from each secondary branch are restricted to discrete regions of the hemisphere with only minimal overlap with those derived from other secondary branches. Monte Carlo simulations show that this segregation differs significantly from a random distribution. The organization of branching seen in the VD neuron may play a critical role in the electrotonic and local computational organization of this neuron and sets the stage for physiological experimentation addressing these issues.

Immunocytochemical evidence for nitric oxide- and carbon monoxide-producing neurons in the stomatogastric nervous system of the crayfishCherax quadricarinatus

Nitric oxide (NO) and carbon monoxide (CO) have been shown to serve neuromodulatory roles in both vertebrates and invertebrates. Here, we use antibodies to their respective biosynthetic enzymes, nitric oxide synthase (NOS) and heme oxygenase 2 (HO-2), to map the distribution of putative gas-producing neurons in the stomatogastric nervous system (STNS) of the crayfish Cherax quadricarinatus. In this species, NOS immunolabeling is found in the neuropil of the stomatogastric ganglion (STG). This staining originates from two immunopositive axons that project to the STG through the superior oesophageal and stomatogastric nerves, presumably from cell bodies located in the commissural ganglia (CoGs). HO-2 immunoreactivity is present in small diameter fibers and varicosities in the periphery of nerves located in the anterior portion of the STNS. This labeling originates from approximately 12 somata in each CoG. Transmission electron microscopy done on the nerves of the anterior STNS shows they contain a neuroendocrine plexus. Collectively, our results indicate that NO- and CO-producing neurons are likely to exist in the crayfish STNS. Moreover, these gases appear to be produced by distinct subsets of the neurons present there. The localization of NO to the STG neuropil suggests that it serves as a locally released modulator or is involved in the local release of other substances within this ganglion. The presence of CO in the neurohemal plexus of the anterior STNS suggests that it serves as a circulating hormone or is involved in the control of neuroendocrine release from this plexus.

Mass spectrometric investigation of the neuropeptide complement and release in the pericardial organs of the crab, Cancer borealis

The crustacean stomatogastric ganglion (STG) is modulated by both locally released neuroactive compounds and circulating hormones. This study presents mass spectrometric characterization of the complement of peptide hormones present in one of the major neurosecretory structures, the pericardial organs (POs), and the detection of neurohormones released from the POs. Direct peptide profiling of Cancer borealis PO tissues using matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) revealed many previously identified peptides, including proctolin, red pigment concentrating hormone (RPCH), crustacean cardioactive peptide (CCAP), several orcokinins, and SDRNFLRFamide. This technique also detected corazonin, a well-known insect hormone, in the POs for the first time. However, most mass spectral peaks did not correspond to previously known peptides. To characterize and identify these novel peptides, we performed MALDI postsource decay (PSD) and electrospray ionization (ESI) MS/MS de novo sequencing of peptides fractionated from PO extracts. We characterized a truncated form of previously identified TNRNFLRFamide, NRNFLRFamide. In addition, we sequenced five other novel peptides sharing a common C-terminus of RYamide from the PO tissue extracts. High K+ depolarization of isolated POs released many peptides present in this tissue, including several of the novel peptides sequenced in the current study.

Neuropeptides in the crayfish stomatogastric nervous system

Neuropeptides are peptides with profound effects on the nervous system. The function of neuropeptides can be studied in detail in the stomatogastric nervous system (STNS). Neuropeptides are ubiquitously distributed in the STNS and it contains well-studied neural circuits that are strongly modulated by neuropeptides. The STNS controls the movements of the foregut in crustaceans and has been studied intensively in a variety of decapod crustaceans including crayfish. This article reviews our knowledge of neuropeptides in the crayfish STNS. Within crayfish, peptides reach the circuits of the STNS as neurohormones released by neurohaemal organs or by putative neurohemal zones located within the STNS. As transmitters, neuropeptides are present in identified motoneurons, interneurons, and sensory neurons (mainly shown by immunocytochemistry), indicating a multiple role of peptides in the plasticity of neural networks. Neuropeptides are not only present in varicosities within the neuropil of ganglia, but also in varicosities on muscles and within small neuropil patches along nerves. This suggests that the muscles of the stomach are under a more direct modulatory control than previously thought, and that information processing can also occur within nerves. In addition to anatomical studies, biochemical and electrophysiological methods were used. For example, MALDI-TOF MS (matrix-assisted laser desorption ionization time of flight mass spectrometry) revealed the presence of four different peptides of the orcokinin family within a single neuron, and electrophysiological experiments demonstrated that the networks of the STNS are not only under excitatory but also inhibitory peptidergic influence. Comparing the similarities and differences between the STNS of crayfish and that of other decapod crustaceans has already contributed to our knowledge about peptides and will further help to unravel peptide function in the plasticity of neural circuits. For example, the identified neurons in the STNS can be used to study co-transmission because neuropeptides are co-localized with classical transmitters, biogenic amines, or other peptides in these neurons.

Physiological functions of FMRFamide-like peptides (FLPs) in crustaceans

Neuropeptides play important roles in chemical signalling in the central and peripheral nervous systems. One of the largest families of neuropeptides is that of the FMRFamide-like peptides (FLPs). This paper reviews what is known about the physiological functions of FLPs in crustaceans, focussing on the cardiovascular, digestive and neuromuscular systems.

Putative neurohemal release zones in the stomatogastric nervous system of decapod crustaceans

The stomatogastric nervous system (STNS) of decapod crustaceans has long been used to study the modulation of small neural circuits. Profiles in the sheath of the nerves and ganglia of the STNS, which contain only dense-core vesicles, have been described in electron microscopical studies (Friend [1976] Cell Tissue Res. 175:369-380; Kilman and Marder [1997] Soc Neurosci Abstr. 23:477; Skiebe and Ganeshina [2000] J Comp Neurol 420:373-397). These profiles resemble those found in neurohemal organs and suggest the presence of neurohemal release zones in the STNS. To map these putative neurohemal release zones, a combination of two antibodies was used in the present study. A synapsin antibody recognizing vesicle proteins of clear vesicles was combined with a synaptotagmin antibody recognizing vesicle proteins of clear and dense-core vesicles. Exclusive synaptotagmin-like staining, therefore, indicated the regions with only dense-core vesicles. Such a staining was found in a mesh in the perineural sheath of nerves in the STNS of all three species investigated. In the crayfish Cherax destructor and the lobster Homarus americanus, the stained mesh was located in the sheath of nerves connecting all four ganglia of the STNS, whereas in the crab Cancer pagurus it was found on different nerves, which are more directly exposed to the hemolymph in this species. Exclusive synaptotagmin-like staining was also found in a putative neurohemal release zone in the sheath of the circumoesophageal connectives and the postoesophageal commissure in C. destructor. These data suggest that an important source of modulation of the networks and the muscles of the stomach is a compartmentalized release of neurohormones from zones in the STNS.

A small-systems approach to motor pattern generation

How neuronal networks enable animals, humans included, to make coordinated movements is a continuing goal of neuroscience research. The stomatogastric nervous system of decapod crustaceans, which contains a set of distinct but interacting motor circuits, has contributed significantly to the general principles guiding our present understanding of how rhythmic motor circuits operate at the cellular level. This results from a detailed documentation of the circuit dynamics underlying motor pattern generation in this system as well as its modulation by individual transmitters and neurons.

Identification of orcokinins in single neurons in the stomatogastric nervous system of the crayfish, Cherax destructor

The orcokinins are a highly conserved family of crustacean peptides that enhance hindgut contractions in the crayfish Orconectes limosus (Stangier et al. [1992] Peptides 13:859-864). By combining immunocytochemical and mass spectrometrical analysis of the stomatogastric nervous system (STNS) in the crayfish Cherax destructor, we show that multiple orcokinins are synthesized in single neurons. Immunocytochemistry demonstrated orcokinin-like immunoreactivity in all four ganglia of the STNS and in the pericardial organs, a major neurohaemal organ. Identified neurons in the STNS were stained, including a pair of modulatory interneurons (inferior ventricular nerve neuron, IVN), a neuron with its cell body in the stomatogastric ganglion that innervates cardiac muscle c6 via the anterior median nerves (AM-c6), and a sensory neuron (anterior gastric receptor neuron). Five orcokinin-related peptides were identified by matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS) post source decay fragmentation in samples of either the stomatogastric ganglion or the pericardial organs. Four of these peptides are identical to peptides derived from the cloned Procambarus clarkii precursor (Yasuda-Kamatani and Yasuda [2000] Gen. Comp. Endocrinol. 118:161-172), including the original [Asn(13)]-orcokinin (NFDEIDRSGFGFN, [M+H](+) = 1,517.7 Da), [Val(13)]-orcokinin ([M+H](+) = 1,502.7 Da), [Thr(8)-His(13)]-orcokinin ([M+H](+) = 1,554.8 Da), and FDAFTTGFGHS ([M+H](+) = 1,186.5 Da). The fifth peptide is a hitherto unknown orcokinin variant: [Ala(8)-Ala(13)]-orcokinin ([M+H](+) = 1,458.7 Da). The masses of all five peptides were also detected in the inferior ventricular nerve of C. destructor, which contains the cell bodies and axons of the IVNs as well as the axons of two other orcokinin-like immunoreactive neurons. In the oesophageal nerve, in which all the orcokinin-like immunoreactivity derives from the IVNs, at least two of the orcokinins were detected, indicating that multiple orcokinins are synthesized in these neurons. Similarly, all four orcokinin masses were detected in the anterior median nerves, in which all the orcokinin-like immunoreactivity derives from the AM-c6 neuron. This study therefore lays the groundwork to investigate the function of the orcokinin peptide family using single identified neurons in a well-studied system.

Central pattern generators and the control of rhythmic movements

Central pattern generators are neuronal circuits that when activated can produce rhythmic motor patterns such as walking, breathing, flying, and swimming in the absence of sensory or descending inputs that carry specific timing information. General principles of the organization of these circuits and their control by higher brain centers have come from the study of smaller circuits found in invertebrates. Recent work on vertebrates highlights the importance of neuro-modulatory control pathways in enabling spinal cord and brain stem circuits to generate meaningful motor patterns. Because rhythmic motor patterns are easily quantified and studied, central pattern generators will provide important testing grounds for understanding the effects of numerous genetic mutations on behavior. Moreover, further understanding of the modulation of spinal cord circuitry used in rhythmic behaviors should facilitate the development of new treatments to enhance recovery after spinal cord damage.