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

Early Metazoan Origin and Multiple Losses of a Novel Clade of RIM Presynaptic Calcium Channel Scaffolding Protein Homologs

The precise localization of Ca_V2 voltage-gated calcium channels at the synapse active zone requires various interacting proteins, of which, Rab3-interacting molecule or RIM is considered particularly important. In vertebrates, RIM interacts with Ca_V2 channels in vitro via a PDZ domain that binds to the extreme C-termini of the channels at acidic ligand motifs of D/E-D/E/H-WC-_COOH, and knockout of RIM in vertebrates and invertebrates disrupts Ca_V2 channel synaptic localization and synapse function. Here, we describe a previously uncharacterized clade of RIM proteins bearing domain architectures homologous to those of known RIM homologs, but with some notable differences including key amino acids associated with PDZ domain ligand specificity. This novel RIM emerged near the stem lineage of metazoans and underwent extensive losses, but is retained in select animals including the early-diverging placozoan Trichoplax adhaerens, and molluscs. RNA expression and localization studies in Trichoplax and the mollusc snail Lymnaea stagnalis indicate differential regional/tissue type expression, but overlapping expression in single isolated neurons from Lymnaea. Ctenophores, the most early-diverging animals with synapses, are unique among animals with nervous systems in that they lack the canonical RIM, bearing only the newly identified homolog. Through phylogenetic analysis, we find that Ca_V2 channel D/E-D/E/H-WC-_COOH like PDZ ligand motifs were present in the common ancestor of cnidarians and bilaterians, and delineate some deeply conserved C-terminal structures that distinguish Ca_V1 from Ca_V2 channels, and Ca_V1/Ca_V2 from Ca_V3 channels.

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.

Neuroeffector connections of giant multimodal neurons in the African snail Achatina fulica

A new method of making preparations was used to analyse the neuroeffector connections of the paired giant neurons of the African snail Achatina fulica. These neurons were found to induce postsynaptic potentials in the muscles of the mantle, heart, the wall of the pulmonary cavity, and the muscular elements of the renal complex, the pericardium, the sexual apparatus, the walls of the cerebral arteries, the filaments of the columellar muscles, the wall of the abdomen, and the tentacle retractor muscles. Rhythmic neuron activity led to the development of marked facilitation and long-term potentiation of synaptic potentials. The possible significance of the multiple neuroeffector connections of giant neurons is discussed.

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.

Detection of APGWamide-like immunoreactivity in the sea scallop, Placopecten magellanicus

In pulmonate gastropods, the peptide, Ala-Pro-Gly-Trp-NH2 (APGWamide), appears to be located nearly exclusively in the neural circuitry controlling the male reproductive organs. This neuropeptide and related neuropeptides are also present and apparently bioactive in bivalve molluscs, although their physiological role in these latter animals is unknown. The present report uses immunohistochemistry to examine the distribution of APGWamide and/or related peptides within the tissues of the deep sea scallop, Placopecten magellanicus. Much of the APGWamide-like immunoreactivity (APGWa-LIR) was detected in the central nervous system (CNS) of both juvenile and adult scallops, where it was concentrated in the cerebral, pedal and parietovisceral ganglia, particularly in the cortex of ganglionic cells and their axons which extend into the central neuropilar region. APGWa-LIR was also detected in the nerves ramifying from these ganglia. In addition, strong APGWa-LIR was localized in what appeared to be axonal terminals within peripheral tissues including the striated adductor muscle, foot, gills, labial palps, lips, tentacles and gonads of the juvenile scallops. The presence of APGWa-LIR was also confirmed in the gonads of adults of both sexes. Cursory examinations of the CNS of the mussel, Mytilus edulis, and the oyster, Crassostrea virginica, revealed the presence of APGWa-LIR in cell bodies and processes in these bivalve species as well. It is concluded that APGWamide and/or related peptides are probably important neurotransmitters and/or neuromodulators of several central and peripheral functions in P. magellanicus and other bivalves. Future work must focus on the possible roles for APGWamide in the physiological processes of these and other bivalve species.

Detection of FMRFamide-like immunoreactivities in the sea scallop Placopecten magellanicus by immunohistochemistry and western blot analysis

FMRFamide-like immunoreactivity was detected histochemically in the sea scallop Placopecten magellanicus. Most immunoreactivity was concentrated in the cerebral, pedal, and parietovisceral ganglia, particularly in the cortical cell bodies and in their fibers which extend into the central neuropile. Whole-mount immunofluorescence studies were used to localize concentrations of immunoreactive cells on the dorsal and ventral surfaces of each ganglion. Immunoreactivity was also detected in nerves emanating from the ganglia. Strong immunoreactivity was localized in peripheral organs, including the gut and gills of juvenile and adult scallops. Weak immunoreactivity was detected in the gonads, heart, and adductor muscle of the adults. A broad FMRFamide-like immunoreactive band of 2.5-8.2 kDa was detected by Western blotting of acetone extracts of the parietovisceral ganglia. In the presence of protease inhibitors, two FMRFamide-like immunoreactive bands (7.2-8.2 kDa and > 17 kDa) were obtained. Neither of these bands comigrated with the FMRFamide standard. It is concluded that peptides of the FMRFamide family are probably regulators of numerous central and peripheral functions in P. magellanicus.