Axonal mapping of the giant peptidergic neurons VD1 and RPD2 located in the CNS of the pond snail Lymnaea stagnalis, with particular reference to the innervation of the auricle of the heart

The Great Pond Snail (Lymnaea stagnalis) as a Model of Aging and Age-Related Memory Impairment: An Overview

With the increase of life span, normal aging and age-related memory decline are affecting an increasing number of people; however, many aspects of these processes are still not fully understood. Although vertebrate models have provided considerable insights into the molecular and electrophysiological changes associated with brain aging, invertebrates, including the widely recognized molluscan model organism, the great pond snail (Lymnaea stagnalis), have proven to be extremely useful for studying mechanisms of aging at the level of identified individual neurons and well-defined circuits. Its numerically simpler nervous system, well-characterized life cycle, and relatively long life span make it an ideal organism to study age-related changes in the nervous system. Here, we provide an overview of age-related studies on L. stagnalis and showcase this species as a contemporary choice for modeling the molecular, cellular, circuit, and behavioral mechanisms of aging and age-related memory impairment.

A Comparative Study of Cell Specific Effects of Systemic and Volatile Anesthetics on Identified Motor Neurons and Interneurons of Lymnaea stagnalis (L.), Both in the Isolated Brain and in Single Cell Culture

1. A comparative descriptive analysis of systemic (sodium pentobarbital, sodium thiopentone, ketamine) and volatile (halothane, isoflurane, enflurane) general anesthetics revealed important differences in the neuronal responses of identified motor neurons and interneurons in the isolated central nervous system (CNS) and cultured identified neurons in single cell culture of Lymnaea stagnalis (L.). 2. At high enough concentrations all anesthetics eventually caused cessation of spontaneous or evoked action potentials, but volatile anesthetics were much faster acting. Halothane at low concentrations caused excitation, thought to be equivalent to the early excitatory phase of anesthesia. Strong synaptic inputs were not always abolished by pentobarbital. 3. There were cell specific concentration-dependent responses to halothane and pentobarbital in terms of membrane potential, action potential characteristics, the after hyperpolarization and patterned activity. Individual neurons generated specific responses to the applied anesthetics. 4. The inhalation anesthetics, enflurane, and isoflurane, showed little concentration dependence of effect, in contrast to results obtained with halothane. Enflurane was faster acting than halothane and isoflurane was particularly different, producing quiescence in all cells types studied at all concentrations studied. 5. Halothane, enflurane, the barbiturate general anesthetics, pentobarbital, and sodium thiopentone and the dissociative anesthetic ketamine, produced two distinctly different effects which could be correlated with cell type and their location in the isolated brain: either a decline in spontaneous and evoked activity prior to quiescence in interneurons or paroxysmal depolarizing shifts (PDS) in motor neurons, again prior to quiescence, which were reversed when the anesthetic was eliminated from the bath. In the strongly electrically coupled motor neurons, VD1 and RPD2, both types of response were observed, depending on the anesthetic used. Thus, with the exception isoflurane, all the motor neurons subjected to the anesthetic agents studied here were capable of generating PDS in situ, but the interneurons did not do so. 6. The effects of halothane on isolated cultured neurons indicates that PDS can be generated by single identified neurons in the absence of synaptic inputs. Further, many instances of PDS in neurons that do not generate it in situ have been found in cultured neurons. The nature of PDS is discussed.

Role for electrical synapses in shaping the output of coupled peptidergic neurons from Lymnaea

Electrically coupled neurons communicate through channel assemblies called gap junctions, which mediate the transfer of current from one cell to another. Electrical synapses ensure spike synchronization and reliable transmission, which influences bursting patterns and firing frequency. The present study concerns an electrically coupled two-neuron network in the gastropod mollusc, Lymnaea stagnalis. The neurons, designated Visceral Dorsal 1 (VD1) and Right Parietal Dorsal 2 (RPD2), are peptidergic, innervate aspects of the cardio-respiratory system, and show strong coupling, such that they fire synchronously. Using dual sharp-electrode current-clamp recording and morphological staining in isolated brain preparations, the hypothesis that the electrical synapse is necessary for accurate network output was tested. We found that both cells make extensive projections within and out of the brain, including across the visceral-parietal connective, which links VD1 and RPD2. Cutting this connective uncoupled the neurons and disrupted the firing rate and pattern of RPD2 more than VD1, consistent with VD1 being the master and RPD2 the follower. The electrical synapse was inhibited by select gap junction blockers, with niflumic acid and 5-nitro-2-(3-phenylpropylamino) benzoic acid decreasing the VD1→RPD2 and RPD2→VD1 coupling coefficients, whereas carbenoxolone, α-glycyrrhetinic acid, meclofenamic acid, and quinine were ineffective. There was little-to-no impact on VD1↔RPD2 firing synchrony or frequency when coupling was reduced pharmacologically. However, in the presence of gap junction blockers, suppressing the activity of VD1 by prolonged hyperpolarization revealed a distinct, low-frequency firing pattern in RPD2. This suggests that strong electrical coupling is key to maintaining a synchronous output and proper firing rate.

Immunocytochemical studies reveal novel neural structures in nemertean pilidium larvae and provide evidence for incorporation of larval components into the juvenile nervous system

Introduction

Nemertea is one of the least studied lophotrochozoan phyla concerning neurogenesis. The sparse data available do not unambiguously allow for answering questions with respect to the neural groundplan of the phylum or the fate of larval neural structures during metamorphosis. In order to contribute to this issue, we studied neurotransmitter distribution during development of the pilidiophoran Lineus albocinctus Verrill, 1900.

Results

Two serotonin-like immunoreactive (lir) neurons are present in the anterior part of the apical plate. They send numerous processes into the four lobes of the pilidium larva, where they form a complex subepithelial nerve net. All four larval lobes are underlain by a marginal neurite bundle, which is associated with numerous serotonin-lir monociliated perikarya. A serotonin-lir oral nerve ring encircles the stomach sphincter and is associated with few serotonin-lir conical cells. Two suboral neurites descend from the oral nerve ring and merge with the marginal neurite bundle. The oral nerve ring and the suboral neurites contain the mollusk-specific VD1/RPD2 α-neuropeptide. The lateral lobes of the larva have three and the anterior and the posterior lobes two VD1/RPD2-lir marginal neurite bundles. The lobar FMRFamide-lir plexus of Lineus albocinctus is much more complex than previously described for any pilidium larva. It includes a circumesophageal neurite that descends along each side of the larval esophagus and together with the inner marginal neurite bundle gives rise to the lobar plexus of the lateral lobes. An outer FMRFamide-lir marginal neurite bundle with numerous associated FMRFamide-lir marginal sensory cells surrounds all four lobes. FMRFamide-lir structures are absent in the larval apical region. The oral nerve ring and the two suboral serotonin-lir neurites are incorporated into the juvenile nervous system.

Conclusion

Our study confirms the presence of serotonin-lir components in the apical region of the pilidium larva of Lineus albocinctus and thus contradicts an earlier study on the same species. We show that the nervous system of pilidium larvae, especially the FMRFamide-lir components, is much more complex than previously assumed. The presence of the VD1/RPD2-α-neuropeptide indicates that this compound may have been part of the lophotrochozoan neural groundplan.

The VD1/RPD2 α1-neuropeptide is highly expressed in the brain of cephalopod mollusks

In certain gastropod mollusks, the central neurons VD(1) and RPD(2) express a distinct peptide, the so-called VD(1)/RPD(2) α1-neuropeptide. In order to test whether this peptide is also present in the complex cephalopod central nervous system (CNS), we investigated several octopod and squid species. In the adult decapod squid Idiosepius notoides the α1-neuropeptide is expressed throughout the CNS, with the exception of the vertical lobe and the superior and inferior frontal lobes, by very few immunoreactive elements. Immunoreactive cell somata are particularly abundant in brain lobes and associated organs unique to cephalopods such as the subvertical, optic, peduncle, and olfactory lobes. The posterior basal lobes house another large group of immunoreactive cell somata. In the decapod Idiosepius notoides, the α1-neuropeptide is first expressed in the olfactory organ, while in the octopod Octopus vulgaris it is first detected in the olfactory lobe. In prehatchlings of the sepiolid Euprymna scolopes as well as the squids Sepioteuthis australis and Loligo vulgaris, the α1-neuropeptide is expressed in the periesophageal and posterior subesophageal mass. Prehatchlings of L. vulgaris express the α1-neuropeptide in wide parts of the CNS, including the vertical lobe. α1-neuropeptide expression in the developing CNS does not appear to be evolutionarily conserved across various cephalopod taxa investigated. Strong expression in different brain lobes of the adult squid I. notoides and prehatching L. vulgaris suggests a putative role as a neurotransmitter or neuromodulator in these species; however, electrophysiological evidence is still missing.

The neuronal control of cardiac functions in Molluscs

In this manuscript, I review the current and relevant classical studies on properties of the Mollusca heart and their central nervous system including ganglia, neurons, and nerves involved in cardiomodulation. Similar to mammalian brain hemispheres, these invertebrates possess symmetrical pairs of ganglia albeit visceral (only one) ganglion and the parietal ganglia (the right ganglion is bigger than the left one). Furthermore, there are two major regulatory drives into the compartments (pericard, auricle, and ventricle) and cardiomyocytes of the heart. These are the excitatory and inhibitory signals that originate from a few designated neurons and their putative neurotransmitters. Many of these neurons are well-identified, their specific locations within the corresponding ganglion are mapped, and some are termed as either heart excitatory (HE) or inhibitory (HI) cells. The remaining neurons are classified as cardio-regulatory, and their direct and indirect actions on the heart's function have been documented. The cardiovascular anatomy of frequently used experimental animals, Achatina, Aplysia, Helix, and Lymnaea is relatively simple. However, as in humans, it possesses all major components including even trabeculae and atrio-ventricular valves. Since the myocardial cells are enzymatically dispersible, multiple voltage dependent cationic currents in isolated cardiomyocytes are described. The latter include at least the A-type K(+), delayed rectifier K(+), TTX-sensitive Na(+), and L-type Ca(2+) channels.

Acetylcholine binding protein of mollusks is unlikely to act as a regulator of cholinergic neurotransmission at neurite-neurite synaptic sites in vivo

A population of glial cells in the central nervous system of the gastropod mollusk Lymnaea stagnalis produces a soluble protein that specifically binds acetylcholine. This protein is named the acetylcholine binding protein (AChBP). Experiments performed in vitro indicated that AChBP inactivates released acetylcholine at cholinergic synapses. On the basis of these observations, a similar in vivo role for AChBP was hypothesized. To fulfill this function, AChBP-expressing glia ought to be located in close proximity to cholinergic synapses in vivo. To examine this, we have analyzed the cellular and subcellular expression of AChBP in the intact CNS. Using a variety of molecular techniques, we demonstrate here that AChBP expression is confined to a subpopulation of glial cells located within the peripheral zone of each of the ganglia constituting the CNS. This zone contains the cell bodies of neurons, but few synapses. Conversely, glial cells that do not express the AChBP are predominantly located in the synapse-rich central neuropile zone but are rare in the cell body zone. Thus, our findings are not compatible with the previous conclusions drawn from in vitro studies and suggest that AChBP functions in vivo as a regulator of nonsynaptic cholinergic transmission.

Peptidomics of a single identified neuron reveals diversity of multiple neuropeptides with convergent actions on cellular excitability

In contrast to classical transmitters, the detailed structures and cellular and synaptic actions of neuropeptides are less well described. Peptide mass profiling of single identified neurons of the mollusc Lymnaea stagnalis indicated the presence of 17 abundant neuropeptides in the cardiorespiratory neuron, visceral dorsal 1 (VD1), and a subset of 14 peptides in its electrically coupled counterpart, right parietal dorsal 2. Altogether, based on this and previous work, we showed that the high number of peptides arises from the expression and processing of four distinct peptide precursor proteins, including a novel one. Second, we established a variety of posttranslational modifications of the generated peptides, including phosphorylation, disulphide linkage, glycosylation, hydroxylation, N-terminal pyroglutamylation, and C-terminal amidation. Specific synapses between VD1 and its muscle targets were formed, and their synaptic physiology was investigated. Whole-cell voltage-clamp analysis of dissociated heart muscle cells revealed, as tested for a selection of representative family members and their modifications, that the peptides of VD1 exhibit convergent activation of a high-voltage-activated Ca current. Moreover, the differentially glycosylated and hydroxylated alpha2 peptides were more potent than the unmodified alpha2 peptide in enhancing these currents. Together, this study is the first to demonstrate that single neurons exhibit such a complex pattern of peptide gene expression, precursor processing, and differential peptide modifications along with a remarkable degree of convergence of neuromodulatory actions. This study thus underscores the importance of a detailed mass spectrometric analysis of neuronal peptide content and peptide modifications related to neuromodulatory function.

Giant multimodal heart motoneurons of Achatina fulica: a new cardioregulatory input in pulmonates

The regulation of the heartbeat by the two largest neurons, d-VLN and d-RPLN, on the dorsal surface of visceral and right parietal ganglia of Giant African snail, Achatina fulica, was examined. Using the new method of animal preparation, for the first time, discrete biphasic inhibitory-excitatory junction potentials (I-EJPs) in the heart and several muscles of the visceral sac were recorded. The duration of hyperpolarizing phase (H-phase) of biphasic I-EJPs was 269+/-5.6 ms (n=5), which is 2-3 times less than that of the cholinergic inhibitory JPs (682+/-68.5 ms, n=5). The H-phase of I-EJPs was not altered by the application of atropine, picrotoxine, succinylcholinchloride, D-tubocurarine and tetraethylammonium or substitution of Cl(-) ions. Even the low-frequency neuronal discharges (1-2 imp/s) evoked significant facilitation and potentiation of the H-phase. Between the multimodal neurons d-VLN/d-RPLN and mantle or visceral organs there is evidence of direct synaptic connections. These neurons were found to have no axonal branches in the intestinal nerve as once suspected but reach the heart through several other nerves. New giant heart motoneurons do not interact with previously identified cardioregulatory neurons.

Characterization of cDNA and expression of mRNA encoding an Achatina cardioexcitatory RFamide peptide

Achatina cardioexcitatory peptide-1 (ACEP-1) is an RFamide family peptide isolated from the atria of the African giant snail, Achatina fulica. In this report, we describe an identification of the ACEP-1 cDNA sequence and localizations of the ACEP-1 mRNA. Southern blot analysis revealed that the ACEP-1 mRNA was present in the atrium as well as in the central nervous system. Furthermore, in situ hybridization revealed the localizations of the ACEP-1 mRNA in small neurons of the cerebral and pedal ganglia and a few large neurons of the right parietal and visceral ganglia.

Partial characterization of a novel cardioinhibitory peptide from the brain of the snail Helix aspersa

1. We report the isolation of a peptide from the brain of the snail Helix aspersa by radioimmunoassay using an antisomatostatin. 2. The sequencing of an immunopositive fraction showed the presence of a new tridecapeptide, termed Helix cardioinhibitory peptide (HCIP), with the following primary structure: H-Val-Phe-Gln-Asn-Gln-Phe-Lys-Gly-Ile-Gln-Gly-Arg-Phe-NH2. It is structurally related to the Achatina cardioexcitatory peptide (ACEP-1) and the terminal-amino acid sequence of HCIP is identical to that of FMRFamide family peptides. 3. The synthetic HCIP was tested on heart and neuronal activities and it was found to have inhibitory actions not only on the ventricle but also on visceral neurons of the central nervous system of Helix. Immunocytochemical investigation indicates its presence in visceral and parietal ganglia, in which cells taking part in the regulation of the heartbeat have been previously identified.

Identification and localization of a [Met5]-enkephalin-like peptide in the mollusc, Lymnaea stagnalis

The goal of this study was to determine whether [Met5]-enkephalin, or an analog, is present in identified neurons in the central nervous system (CNS) of the freshwater snail, Lymnaea stagnalis. High performance liquid chromatography and radioimmunoassay of CNS tissue homogenates revealed both [Met5]-enkephalin and oxidized [Met5]-enkephalin. NO [Leu5]-enkephalin, [Met5]-enkephalin-Arg6-Phe7 or [Met5]-enkephalin-Arg6-Gly7-Leu8 were detected. Quantification of [Met5]-enkephalin, by radioimmunoassay, revealed that the Lymnaea CNS contains approximately 2.2 fmol/CNS (undigested tissue) and 4.5 fmol/CNS (tissue enzymatically digested with trypsin and carboxypeptidase B). The increased amount of [Met5]-enkephalin following tissue digestion indicates the presence of as yet unidentified extended forms of [Met5]-enkephalin in Lymnaea. Using indirect immunocytochemistry, a [Met5]-enkephalin-like peptide was localized to individual cells and cell clusters within the CNS, as well as to fibers in the atrium of the heart. A neuronal map depicting [Met5]-enkephalin-like immunoreactive cells was produced. Among the immunoreactive neurons were four identified, well-characterized, giant cells: VD1, RPD2, LB1 and RB1. Identifiable [Met5]-enkephalin-like immunoreactive neurons were characterized electrophysiologically and morphologically. Additionally, neurons VD1 and RPD2 were confirmed to be immunoreactive to Lymnaea alpha-peptide. The lack of both cross reactivity and sequence homology between alpha-peptide and [Met5]-enkephalin suggests that a [Met5]-enkephalin-like peptide and alpha-peptide are co-localized within these neurons.

Electrical and mechanical characteristics of the atrium of the whelk Busycon canaliculatum

1. The mean resting potential of 22 atrial preparations of Busycon heart was 42.5 mV, examined by the sucrose gap technique. Spontaneous action potentials of 8-18 mV amplitude occurred in repeated cycles of burst activity, generating burst patterned phasic contractile activity. 2. Isolated ventricles showed slow (1-3 beats min-1) constant myogenic contractile activity, suggesting that the primary driving pacemaker may reside in the atrium. 3. The atrial electrocardiogram commenced with a small prepotential leading to a plateau-like phase and terminated with a sharp spike potential. 4. Acetylcholine (ACh) at high concentrations depolarised the atrium by 5-8 mV and induced strong tonic contractures while suppressing spontaneous action potentials, suggesting an overall inhibitory role in downregulating cardiac intrinsic myogenic rhythms. 5. Serotonin (5-hydroxytryptamine, 5HT) was consistently excitatory, enhancing both action potential amplitude and rhythmic contractions by up to 50% at concentrations of 5 x 10(-7) to 10(-5)M. Neither methysergide nor metoclopramide affected atrial responses to 5HT and the 5HT1 antagonist metitipine simply increased action potential discharge in the rhythmic cycle. The vertebrate 5HT1-3 receptor classification is inappropriate to this molluscan preparation. 6. The atrium was very sensitive to the tetrapeptides FMRF- and FLRFamide, but the enhanced phasic contractions were not accompanied by alteration of resting potential or action potential amplitude, suggestive of neuromodulatory upregulation involving a secondary messenger. The related peptide SCP-B was without effect on the preparation, but GAPFLRFamide was excitatory, although much less so than FMRF- and FLRFamide. 7. Neither adenosine and ATP nor guanosine and GTP affected intrinsic atrial electrical or mechanical activity, suggesting that there was no noncholinergic, nonaminergic element to cardiac neuromodulation in this species. Only ACh, 5HT and FMRF/FLRFamide could be assigned clear roles in this respect.

Ionic dependency of membrane potential and autorhythmicity in the atrium of the whelk Busycon canaliculatum

1. Calcium-free media usually caused a cessation of all electrical and mechanical activity of the Busycon atrium. Where any electrical activity survived, the action potential consisted of a pre- and plateau-like potential devoid of the usual terminal spike. 2. High Ca salines induced tonic force, membrane depolarization and reduction in generation of spontaneous action potentials. The Ca ionophore A23187 enhanced contractions and the SR CaATPase inhibitor cyclopiazonic acid induced slight depolarization, tonic contractures and increased action potential firing. 3. The inorganic Ca antagonist Co2+ was without effect on the preparations, although the lanthanide Gd3+ inhibited contractions and spontaneous action potentials as well as inducing membrane potential depolarization. 4. The organic Ca entry-blocker nifedipine enhanced both spontaneous action potential amplitude and the phasic contractions they generated. 5. High K salines considerably depolarized atrial preparations with accompanying large tonic contractures and suppression of action potentials. The K channel-blocker 4AP enhanced action potential amplitude with slight increase in contractions, and TEA depolarized the atrium, and enhanced action potentials and rhythmic contractions. 6. Sodium-free salines strongly hyperpolarized atrial preparations and abolished spontaneous action potentials and, on washout, the membrane potential became temporarily unstable. In 2 preparations, low chloride and chloride-free media induced significant membrane potential hyperpolarization. 7. It is concluded that, in the atrium, the resting membrane potential is largely determined by the transmembrane K gradient, but with significant conductances to Na and Cl though probably not Ca. The action potential spike appears to be a Ca-dependent event and the plateau-like phase may be a Na-dependent event.

Localization of FMRFamide- and ACEP-1-like immunoreactivities in the nervous system and heart of a pulmonate mollusc, Achatina fulica

Immunohistochemical localization of two neuropeptides possibly involved in the regulation of cardiac activity in a pulmonate mollusc, Achatina fulica Férussac, was studied. On the ventral surface of the right cerebral ganglion, more than 50 neurons with diameters of 30-50 microns showed immunoreactivity to the antiserum of the neuropeptide FMRFamide. Many were also immunoreactive to an antiserum raised against Achatina cardio-excitatory peptide-1 (ACEP-1). Although FMRFamide-like immunoreactive neurons occurred in all components of the subesophageal ganglia, identifiable ACEP-1-like immunoreactive neurons were located only in the visceral ganglion and the right parietal ganglion. In the heart, FMRFamide- and ACEP-1-like immunoreactive fibers were restricted to the atrium and the aortic end of the ventricle, consistent with morphological observations of cardiac innervation. The present results suggest that FMRFamide- and ACEP-1-like peptides are involved in regulating the heart beat of this snail.

Axonal localization of neuropeptide-encoding mRNA in identified neurons of the snail Lymnaea stagnalis

mRNA transcripts encoding neuropeptides were detected, by means of in situ hybridization, in the axonal compartments of different types of identified neurons in the central nervous system of the pond snail Lymnaea stagnalis. All cell types studied contained axonal mRNA although the relative intensities of the hybridization signals (i.e., the intensity of the signal over the cell body versus that over the axonal compartment of a particular cell) varied greatly between the different cell types studied. Strong signals over the axonal compartment were obtained with an oligonucleotide probe specific for the molluscan insulin-related peptide gene III mRNA, whereas low signals were obtained, e.g., with a probe for the mRNA encoding the neuropeptide APG-Wamide. Furthermore, some neurons are known to express more than one neuropeptide gene, e.g., the molluscan insulin-related peptide-producing light green cells and the egg-laying hormone-producing caudo-dorsal cells; these cell types express 4 and 2 related neuropeptide genes, respectively. The results may indicate that the different neuropeptide transcripts within a neuron are transported selectively to the axonal compartment.

Processing, axonal transport and cardioregulatory functions of peptides derived from two related prohormones generated by alternative splicing of a single gene in identified neurons VD1 and RPD2 of Lymnaea

The VD1/RPD2 mRNA precursor in identified neurons VD1 and RPD2 of the freshwater snail Lymnaea stagnalis is alternatively spliced to yield two related variants encoding two distinct yet related preprohormones, named the VD1/RPD2-A and -B preprohormones. Here, we report the isolation and structural characterization of alpha 1, alpha 2 and beta peptides from dissected neurons VD1 and RPD2. The alpha 1 and alpha 2 peptides are derived from VD1/RPD2-A and B prohormones, respectively, whereas beta peptide is identical for both prohormones. In addition, we report the isolation and structural characterization of the alpha 2 peptide from the heart, demonstrating that the mature peptides are transported and released in the heart. The pharmacological actions of synthetic alpha 1 and alpha 2 peptides on isolated auricle preparations of the Lymnaea heart were examined. The two alpha peptides have similar excitatory effects on beat rate and beat amplitude, while their potencies differed considerably, indicating that alternative splicing results in structurally and functionally overlapping, through non-identical, sets of peptides.

Effects of met-enkephalin on electrical coupling between identified neurons in the pulmonate snails, Helix and Lymnaea

1. The effects of met-enkephalin on electrical coupling between molluscan neurons have been investigated using the isolated brains of Helix pomatia and Lymnaea stagnalis. 2. In the presence of both serotonin and met-enkephalin, non-rectifying electrical coupling is strongly facilitated between identified respiratory neurons in Helix, whilst coupling between putative, serotonin-containing, ciliomotoneurons in Lymnaea is facilitated by met-enkephalin alone. 3. Facilitation of coupling by met-enkephalin is weaker in the strongly coupled neurons, VD1/RPaD2 of Lymnaea. 4. These data suggest that met-enkephalin can modulate different groups of electrically coupled cells and may be involved in coordination of motor patterns.

Neurons in a variety of molluscs react to antibodies raised against the VD1/RPD2 alpha-neuropeptide of the pond snail Lymnaea stagnalis

The VD1 and RPD2 neurons of Lymnaea stagnalis innervate other central neurons, certain skin areas, the pneumostome area, and the auricle of the heart. Recently, a set of four (delta, epsilon, alpha, beta) neuropeptides produced by these giant neurons and by certain other central neurons has been characterized. Although alternative splicing of the preprohormone of these neurons yields at least 10 different alpha neuropeptides, an affinity-purified antiserum directed against a domain common to all alpha neuropeptides has previously been shown to be highly selective in staining VD1, RPD2 and other neurons that produce the preprohormone. Since the gene encoding the neuropeptides is structurally similar to that expressed in R15 of the marine opisthobranch Aplysia californica, we have used the affinity purified antiserum as a marker for VD1/RPD2-related systems in other molluscs. Immunopositive neurons and fibers are observed in the central nervous systems of all species studied (Achatina fulica, Anodonta sp., Aplysia brasiliana, A. californica, Bulinus truncatus, Cepea sp., Eobania vermiculata, Helix aspersa, H. pomatia, Limax maximus, Mytilus edulis, Nassarius reticulatus, Viviparus viviparus). Several medium-sized and small neurons and 1-4 giant neurons are found in the pulmonates and opisthobranchs. The giant neurons in pulmonates have locations in the subesophageal ganglion, axonal branching patterns, and terminal arborizations in the auricle of the heart; all these characteristics are similar to those of VD1 and RPD2. Double-labelling (Lucifer yellow injection, immunocytochemistry) confirms that the two giant neurons in Helix pomatia are Br and Br'. The immunoreactive cells in A. fulica appear to include the VIN and PON neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptides regulate the cardiac activity of a prosobranch mollusc, Rapana thomasiana

Involvement of neuropeptides in the regulation of cardiac activity in a prosobranch mollusc, Rapana thomasiana, was studied physiologically as well as immunohistochemically. A catch-relaxing peptide (CARP) showed strong inhibitory effects on the heart with a lower threshold than acetylcholine. The action of CARP was in contrast to that of another neuropeptide, FMRFamide, which has previously been shown to enhance the heart beat. Benzoquinonium blocked the effects of acetylcholine and stimulation of right cardiac nerves 1 and 3 b, but not those of CARP, suggesting that the effects of nerve stimulation are mainly due to the release of acetylcholine. Immunohistochemical examinations demonstrated that FMRFamide-like and CARP-like immunoreactive neurons are distributed in the visceral ganglia. Although a neuron appeared to show weak immunoreactivity to both antisera, evidence for the coexistence of peptides in a single neuron was not exhibited. Positive immunoreactivity to FMRFamide and CARP antisera also appeared in right cardiac nerves 1 and 3. In the heart, FMRFamide- and CARP-like immunoreactive fibers were restricted to the atrium and the aortic end of the ventricle, consistent with the morphological observation of innervation. The present results suggest that FMRFamide- and CARP-like peptides are involved in regulating the heart beat.

The VD1/RPD2 neuronal system in the central nervous system of the pond snail Lymnaea stagnalis studied by in situ hybridization and immunocytochemistry

VD1 and RPD2 are two giant neuropeptidergic neurons in the central nervous system (CNS) of the pond snail Lymnaea stagnalis. We wished to determine whether other central neurons in the CNS of L. stagnalis express the VD1/RPD2 gene. To this end, in situ hybridization with the cDNA probe of the VD1/RPD2 gene and immunocytochemistry with antisera specific to VD1 and RPD2 (the alpha 1-antiserum, Mab4H5 and ALMA 6) and to R15 (the alpha 1 and 16-mer antisera) were performed on alternate tissue sections. A VD1/RPD2 neuronal system comprising three classes of neurons (A1-A3) was found. All neurons of the system express the gene. Division into classes is based on immunocytochemical characteristics. Class A1 neurons (VD1 and RPD2) immunoreact with the alpha 1-antiserum, Mab4H5 and ALMA 6. Class A2 neurons (1-5 small and 1-5 medium sized neurons in the visceral and right parietal ganglion, and two clusters of small neurons and 5 medium-sized neurons in the cerebral ganglia) immunoreact with the alpha 1-antiserum and Mab4H5, but not with ALMA 6. Class A3 neurons (3-4 medium-sized neurons and a cluster of 4-5 small neurons located in the pedal ganglion) immunoreact with the alpha 1-antiserum only. All neurons of the system are immunonegative to the R15 antisera. The observations suggest that the neurons of the VD1/RPD2 system produce different sets of neuropeptides. A group of approximately 15 neurons (class B), scattered in the ganglia, immunostained with one or more of the antisera, but did not react with the cDNA probe in in situ hybridization.

The role of pacemaker properties and synaptic input in generation and modulation of spiking activity in a pair of electrically coupled peptidergic neurons

The origin of patterned electrical activity in two electronically coupled peptidergic neurons, VD1 and RPD2, in the CNS of Lymnaea stagnalis was investigated. VD1 proved to have intrinsic beating pacemaker properties. Hybrid current/voltage clamp experiments demonstrated that in the intact CNS generation of spike activity in the coupled cell system is dominated by VD1. Modulation of spiking activity of VD1/RPD2 appears to originate mainly from chemical synaptic input. The electrical coupling of VD1 and RPD2 proved essential for spike synchronization between the cells.

Characterization and evolutionary aspects of a transcript encoding a neuropeptide precursor of Lymnaea neurons, VD1 and RPD2

We isolated and characterized a cDNA clone encoding the major prohormone of VD1 and RPD2, two electrotonically coupled identified neurons in the central nervous system of the freshwater snail, Lymnaea stagnalis. The VD1/RPD2 prohormone may be cleaved to generate a set of 4 different neuropeptides, called epsilon, delta, alpha 1 and beta peptides, as well as a single aspartate. Since VD1 and RPD2 probably are involved in O2 perception and modulation of cardio-respiratory functions, it is thought that the neuropeptides synthesized and released by these neurons coordinate the adaptive physiological and behavioural processes that occur in response to changes in O2 availability. Comparison of the Lymnaea VD1/RPD2 precursor with two related precursors, prohormones R15-1 and R15-2, identified from neuron R15 in the marine mollusc Aplysia californica revealed a similar pattern of organization of the preprohormones. The overall homology is rather low, however, detailed comparisons show a highly differential pattern of conservation of peptide regions on the precursors.