Immunoreactivity to molluskan neuropeptides in the central and stomatogastric nervous systems of the earthworm, Lumbricus terrestris L

Comparative and Evolutionary Aspects of the Digestive System and Its Enteric Nervous System Control

All life forms must gain nutrients from the environment and from single cell organisms to mammals a digestive system is present. Components of the digestive system that are recognized in mammals can be seen in the sea squirt that has had its current form for around 500my. Nevertheless, in mammals, the organ system that is most varied is the digestive system, its architecture being related to the dietary niche of each species. Forms include those of foregut or hindgut fermenters, single or multicompartment stomachs and short or capacious large intestines. Dietary niches include nectarivores, folivores, carnivores, etc. The human is exceptional in that, through food preparation (>80% of human consumption is prepared food in modern societies), humans can utilize a wider range of foods than other species. They are cucinivores, food preparers. In direct descendants of simple organisms, such as sponges, there is no ENS, but as the digestive tract becomes more complex, it requires integrated control of the movement and assimilation of its content. This is achieved by the nervous system, notably the enteric nervous system (ENS) and an array of gut hormones. An ENS is first observed in the phylum cnidaria, exemplified by hydra. But hydra has no collections of neurons that could in any way be regarded as a central nervous system. All animals more complex than hydra have an ENS, but not all have a CNS. In mammals, the ENS is extensive and is necessary for control of movement, enteric secretions and local blood flow, and regulation of the gut immune system. In animals with a CNS, the ENS and CNS have reciprocal connections. From hydra to human, an ENS is essential to life.

The first brain: Species comparisons and evolutionary implications for the enteric and central nervous systems

BACKGROUND

The enteric nervous system (ENS) and the central nervous system (CNS) of mammals both contain integrative neural circuitry and similarities between them have led to the ENS being described as the brain in the gut.

PURPOSE

To explore relationships between the ENS and CNS across the animal kingdom. We found that an ENS occurs in all animals investigated, including hydra, echinoderms and hemichordates that do not have a CNS. The general form of the ENS, which consists of plexuses of neurons intrinsic to the gut wall and an innervation that controls muscle movements, is similar in species as varied and as far apart as hydra, sea cucumbers, annelid worms, octopus and humans. Moreover, neurochemical similarities across phyla imply a common origin of the ENS. Investigation of extant species suggests that the ENS developed in animals that preceded the division that led to cnidaria (exemplified by hydra) and bilateria, which includes the vertebrates. The CNS is deduced to be a bilaterian development, later than the divergence from cnidaria. Consistent with the ENS having developed independent of the CNS, reciprocal connections between ENS and CNS occur in mammals, and separate neurons of ENS and CNS origin converge on visceral organs and prevertebral ganglia. We conclude that an ENS arose before and independently of the CNS. Thus the ENS can be regarded as the first brain.

Family of CNP neuropeptides: common morphology in various invertebrates

Neuropeptides expressed in the command neurons for withdrawal behavior were originally detected in the the central nervous system (CNS) of the terrestrial snail Helix (command neurons peptides, CNP). The family of CNP-like neuropeptides bears a C-terminal signature sequence Tyr-Pro-Arg-X. Using antisera against two of them, we have studied the CNS of various invertebrates belonging to the phyla of mollusks, annelids and insects. The immunoreactive neurons were detected in all studied species. Stained neurons were either interneurons projecting along the CNS ganglia chain, or sensory neurons, or neurohormonal cells. Beyond common morphological features, the immunoreactive cells had another similarity: the level of CNP expression depended on the functional state of the animal. Thus, the homologous neuropeptides in evolutionary distant invertebrate species possess some common morphological and functional features.

Morphology of neuropeptide CNP2 modulation of heart activity in terrestrial snail

A family of neuropeptides called Command Neuron Peptides (CNPs) was described ten years ago as the protein products of the gene HCS2, specifically expressed in the identified interneurons of the nervous system of terrestrial snail (Helix lucorum L. and H. pomatia L.). Recently, the CNP-like peptides have been detected by immunochemistry and immunoblotting in nervous systems of representatives of different invertebrate phyla (Mollusca, Annelida, and Insecta). Still, the function of these peptides remains largely unknown. In Helix it is shown that CNPs: modulate the electrical activity of unidentified central neurons, modulate the pneumostome motoneurons, stimulate neural cones growth in neural cultures. Here, we describe for the first time the CNPs-immunoreactive neural fibers in walls of both auricle and ventricle of the snail heart. We show that application of the synthetic neuropeptide CNP2 (DYPRLamide) in perfusion saline affects heart rate and magnitude of beats in isolated snail heart. The results suggest that in Helix the Command Neuron Peptides could participate in neural modulation of cardiovascular system.

Identification of novel neuropeptides in the ventral nerve cord ganglia and their targets in an annelid worm, Eisenia fetida

Periviscerokinins (PVKs) and pyrokinins (PKs) are neuropeptides known in several arthropod species. Sequence homology of these peptides with the molluscan small cardioactive peptides reveals that the occurrence of PVKs and PKs is not restricted to arthropods. Our study focuses on the biochemical and immunocytochemical identification of neuropeptides with sequence homology to PVKs and PKs in the central and peripheral nervous system of the earthworm Eisenia fetida. By means of affinity chromatography, nanoflow liquid chromatography, and high accuracy mass spectrometry, six peptides, SPFPR(L/I)amide, APFPR(L/I)amide, SPLPR(L/I)amide, SFVR(L/I)amide, AFVR(L/I)amide, and SPAFVR(L/I)amide, were identified in the central nervous system with the common -XR(L/I)amide C-terminal sequence. The exact anatomical position of 13 labeled XR(I/L)amide expressing neuron groups and numerous peptide-containing fibers were determined by means of immunocytochemistry and confocal laser scanning microscopy in whole-mount preparations of ventral nerve cord ganglia. The majority of the stained neurons were interneurons with processes joining the distinct fine-fibered polysegmental tracts in the central neuropil. Some stained fibers were seen running in each segmental nerve that innervated metanephridia and body wall. Distinct groups of neurosecretory cells characterized by small round soma and short processes were also identified. Based on immunoelectron microscopy six different types of labeled cells were described showing morphological heterogeneity of earthworm peptides containing elements. Our findings confirm that the sequence of the identified earthworm neuropeptides homologous to the insect PVKs and PKs suggesting that these peptides are phylogenetically conservative molecules and are expressed in sister-groups of animals such as annelids, mollusks, and insects.

Primary sensory neurons containing command neuron peptide constitute a morphologically distinct class of sensory neurons in the terrestrial snail

In the central nervous system of the terrestrial snail Helix, the gene HCS2, which encodes several neuropeptides of the CNP (command neuron peptide) family, is mostly expressed in cells related to withdrawal behavior. In the present work, we demonstrate that a small percentage (0.1%) of the sensory cells, located in the sensory pad and in the surrounding epithelial region ("collar") of the anterior and posterior tentacles, is immunoreactive to antisera raised against the neuropeptides CNP2 and CNP4, encoded by the HCS2 gene. No CNP-like-immunoreactive neurons have been detected among the tentacular ganglionic interneurons. The CNP-like-immunoreactive fiber bundles enter the cerebral ganglia within the nerves of the tentacles (tentacular nerve and medial lip nerve) and innervate the metacerebral lobe, viz., the integrative brain region well-known as the target area for many cerebral ganglia nerves. The procerebral lobe, which is involved in the processing of olfactory information, is not CNP-immunoreactive. Our data suggest that the sensory cells, which contain the CNP neuropeptides, belong to a class of sensory neurons with a specific function, presumably involved in the withdrawal behavior of the snail.

Neuropeptides of Drosophila related to molluscan neuropeptides: dependence of the immunoreactivity pattern on the ontogenetic stage and functional state

The CNP neuropeptides (Command Neuron Peptides) were first found in the command neurons for withdrawal behavior in the terrestrial snail. Given the fact that certain peptides can be found in various invertebrates, we examined Drosophila brains to determine if CNP-like peptides were present. Two types of antisera were used: one against CNP2, which was expected to recognize peptide products of the genes "hugin", "capa", CG6371, and a second antiserum against CNP4, which was expected to recognize neuropeptides encoded by the gene "capa" only. In larvae, both antibodies stained the abdominal perisympathetic organ, and several groups of neurons in the suboesophageal ganglia, while two neuronal clusters in the protocerebrum were stained with CNP2 antibody exclusively. The set of peptidergic neurons was conserved throughout all larval development. In the suboesophageal ganglia, the number of immunoreactive neurons apparently decreased at the pupae stage, and only one pair of large neurons in the caudal part of the suboesophageal ganglia was detected in adults. Experimental body injury led in the adult fruit flies to appearance of additional immunoreactive neurons, the pattern of staining becoming similar to that in larval CNS. The study demonstrates that the number of neurons expressing CNP-like immunoreactivity depends on the developmental stage and functional state of the animal, and that the CNP2-like and CNP4-like neuropeptides are colocalized in some neurons. We conclude that the family of CNP-like neuropeptides seems to be common for various invertebrate phyla.