Cells showing immunoreactivity for calcitonin or calcitonin gene-related peptide (CGRP) in the central nervous system of some invertebrates

Sensory cells and the organization of the peripheral nervous system of the siboglinid Oligobrachia haakonmosbiensis Smirnov, 2000

Background

The nervous system of siboglinids has been studied mainly in Osedax and some Vestimentifera, while data in Frenulata – one of the four pogonophoran main branches – is still fragmentary. In most of the studies, the focus is almost always on the central nervous system, while the peripheral nervous system has traditionally received little attention. In contrast to other annelids, the structure and diversity of sensory structures in siboglinids are still quite undescribed. Meanwhile, the peripheral nervous system, as well as sensory elements, are extremely evolutionarily labile, and information about their organization is of high importance to understand lifestyles and behavior as well as main trends that lead siboglinids to their peculiar organization.

Results

The structure of the peripheric nervous system, sensory elements, and neuromuscular relationships of Oligobrachia haakonmosbiensis were studied using both scanning electron and confocal laser microscopy. A significant number of monociliary sensory cells, as well as sensory complexes located diffusely in the epithelium of the whole body were revealed. The latter include the cephalic tentacles, sensory cells accumulations along the dorsal furrow and ciliary band, areas of the openings of the tubiparous glands, and papillae. The oval ciliary spot located on the cephalic lobe at the base of the tentacles can also be regarded as a sensory organ. Most of the detected sensory cells show immunoreactivity to substance P and/or acetylated α-tubulin. FMRFamide- and serotonin-like immunoreactivity are manifested by neurons that mainly innervate tentacles, muscles, body wall epithelium, skin glands, tubiparous glands, and papillae. In the larva of O. haakonmosbiensis, monociliary sensory elements were revealed in the region of the apical organ, along the body, and on the pygidium.

Conclusions

The diversity of sensory structures in O. haakonmosbiensis comprises epidermal solitary sensory cells, sensory spots around tubiparous glands openings, and putative sensory organs such as cephalic tentacles, an oval ciliary spot on the cephalic lobe, the dorsal furrow, and papillae. Sensory structures associated with papillae and tubiparous glands play presumable mechanosensory functions and are associated with regulation of tube building as well as anchorage of the worm inside the tube. Sensory structures of the dorsal furrow are presumably engaged in the regulation of reproductive behavior. An overall low level of morphological differentiation of O. haakonmosbiensis peripheral nervous system is not typical even for annelids with the intraepithelial nervous system. This can be considered as a plesiomorphic feature of its peripheral plexus’s organization, or as evidence for the neotenic origin of Siboglinidae.

Evolution of the CT/CGRP family: comparative study with new data from models of teleosts, the eel, and cephalopod molluscs, the cuttlefish and the nautilus

In mammals, alternative splicing of the calcitonin gene generates two distinct peptides: calcitonin (CT), synthesised in the thyroid C cells and involved in the regulation of calcium metabolism, and calcitonin gene-related peptide (CGRP), brain neuromediator synthesised in the peripheral and central nerves. CGRP is well represented and molecularly conserved during evolution whereas CT has not been detected in any of the invertebrates analysed so far. In order to better understand the evolution of this CT/CGRP peptide family we reviewed the major data concerning its evolution from the literature and our recent data obtained in models of teleosts and cephalopod molluscs. The presence of both CGRP-like molecules and its specific bindings sites in the central nervous system of eel, cuttlefish and nautilus, suggests that the brain neurotransmitter role of CGRP could represent an ancient role in metazoa, already present in cephalopods and conserved among vertebrates, as still observed in mammals. In contrast, the presence of CGRP specific binding sites, and not the peptide itself, in the gills suggests an endocrine role for CGRP, in cephalopods and teleosts, that may have been lost during the evolution of the tetrapod lineage. These data, and the absence of CT-like molecules that we observed in cephalopods, support the hypothesis that CGRP represents the ancestral molecule of the CT/CGRP family, appeared in metazoa before the vertebrate emergence. The distinction between CT and CGRP receptors appears to be an event posterior to the emergence of ecdysozoan and lophotrochozoan protostomes, probably in relation to the CT appearance. The evolution of the CT/CGRP peptide family is probably similar to the evolution of the CT/CGRP receptor family. In fact, the genic duplication that induced the appearance of the two separate molecules, CT and CGRP, may constitute an event close to that, which induced the appearance of the two specific receptors. These events remain to be further studied in order to better understand the peptide and receptor evolution of the CT/CGRP family.

Centrifugal neurons of the octopus optic lobe cortex are immunopositive for calcitonin gene-related peptide

Distribution of calcitonin gene-related peptide (CGRP)-like substance in the optic lobe cortex and retina of the octopus was examined immunohistochemically. Wheat germ agglutinin (WGA), a retrograde-transporting marker, was also used to label the centrifugal neurons. CGRP-immunoreactive (CGRP-IR) somata were seen in the inner granular cell layer, but not in the outer granular cell layer or the retina. CGRP-IR fibers were seen not only in the optic lobe cortex, but also in the retinal nerve plexus. Retrogradely labeled somata were seen in the inner granular cell layer, but not in the outer granular cell layer. Immunohistochemical double staining indicated that WGA-labeled centrifugal neurons were immunopositive for CGRP. These results suggested that the centrifugal neurons in the octopus optic lobe cortex are CGRP-like peptide-containing neurons, and that the peptide may modulate photoreceptor cell functions.

Effect of calcitonin gene related peptide on the adenylate cyclase activity in abalone gill membranes

The aim of the present study was to investigate and to compare the effect of calciotropic hormones, human calcitonin gene related peptide (CGRP) I and II, salmon calcitonin (CT) and human amylin on the adenylate cyclase activity in abalone gill membranes. In addition to human CGRPI, human CGRPII and salmon CT stimulated the adenylate cyclase activity. No effect was observed with amylin. The higher effect was observed with human CGRPI and II that induced a 160-170% increase of the enzyme activity. Fifty percent of the maximal activity was observed with 3 and 8 nM of CGRP I and II, respectively. Salmon CT induced a lower effect: the maximal activity was obtained with a hormone concentration of 266 nM and represented a 130% stimulation of the basal activity. In the presence of CGRP 8-37, a competitive antagonist of CGRP action, the stimulation observed with CGRPI was abolished and returned to the basal level. This study points out that, in invertebrates, CGRP receptors present in gill membranes are linked to an adenylate cyclase system similar to that described in vertebrates. In addition, these data are in favour of a role for CGRP in branchial function both in non-mammalian vertebrates and invertebrates

Calcitonin cells in the intestine of goldfish and a comparison of the number of cells among saline-fed, soup-fed, or high Ca soup-fed fishes

Calcitonin-immunoreactive cells were found in the intestine of goldfish. These cells were distributed mainly in the anterior part of the intestine, dispersed in the intestinal epithelium. The nucleus was located in the basal portion of the serosal side, and the cytoplasm was elongated to the luminal side. From the anterior part of the intestine, cDNA fragments with the same nucleotide sequence as that of the goldfish calcitonin gene were amplified by RT-PCR method. After administration of one of three kinds of solutions (saline, consommé soup, or high Ca consommé soup) into the digestive tract of the goldfish, the number of those cells was the largest in the consommé group at 6 h after ingestion, although blood Ca levels were the highest in the high Ca consommé group. The function of calcitonin cells in the intestine may be to restrain the acute absorption of nutrients and not to control blood Ca levels.

Characterization of binding sites for calcitonin gene related peptide in abalone gill

Target organs for calcitonin gene related peptide were investigated in the abalone. To elucidate the function of this neuropeptide in the biomineralization process, we have localized, in different tissues from abalone, specific binding sites for human calcitonin gene related peptide (hCGRP). Highest binding was observed in gill membranes where two classes of affinity components were identified. The affinity constants and the number of binding sites per mg of proteins for the site I were 5 X 10(9)M(-1)and 8.8 x 10(10). For the site II, the affinity constant was 1.34 X 10(7) M(-1)and the number of binding sites per mg of proteins was 1.1 x 10(12). In contrast, no specific calcitonin binding could be detected in every tested tissue, but the similar displacement of the 125I-labeled CGRP binding with unlabeled hCGRP and sCT suggested that, in the abalone, the identified receptor could belong to a third class receptor subtype, that recognize both hCGRPalpha and sCT. These data suggest that, during evolution, the role of CGRP in gill function is particularly well conserved and that this neuropeptide is likely to participate in the control of hydromineral metabolism in aquatic species.

Calcitropic peptides: neural perspectives

In mammals and higher vertebrates, calcitropic peptides are produced by peripheral endocrine glands: the parathyroid gland (PTH), thyroid or ultimobranchial gland (calcitonin) and the anterior pituitary gland (growth hormone and prolactin). These hormones are, however, also found in the neural tissues of lower vertebrates and invertebrates that lack these endocrine organs, suggesting that neural tissue may be an ancestral site of calcitropic peptide synthesis. Indeed, the demonstration of CNS receptors for these calcitropic peptides and their induction of neurological actions suggest that these hormones arose as neuropeptides. Neural and neuroendocrine roles of some of these calcitropic hormones (calcitonin and parathyroid hormone) and related peptides (calcitonin gene related peptide, stanniocalcin and parathyroid hormone related peptide) are thus the focus of this review.

Selective regulation of T-cell development and function by calcitonin gene-related peptide in thymus and spleen. An example of differential regional regulation of immunity by the neuroendocrine system

In the course of our studies, we have shown the presence of calcitonin gene related peptide (CGRP) by immunocytochemistry in cell bodies and nerve fibers of the murine thymus and in a sparse innervation of the spleen. Receptors for CGRP have been characterized within these glands, and their activation by physiological levels of CGRP was found to suppress Con A-stimulated proliferation of thymocytes and splenic T cells as well as antigen-specific T-cell proliferation. This suppression is blocked by the antagonist for CGRP (CGRP 8-37). Within the thymus cultures, the antagonist CGRP (8-37) alone enhanced proliferation of thymocytes during Con A stimulation, most likely by inhibiting the endogenous release of CGRP into the culture medium by resident thymocytes. Some of the CGRP-induced suppression of mitogenic stimulation of thymocytes, but not of splenocytes, was due to apoptosis. The antagonist, CGRP(8-37), did not block apoptosis caused by Con A or CGRP but rather enhanced it. Flow cytometric analysis of CGRP-treated cultures using antibodies to cluster determinates (CD) showed that the majority of thymocytes undergoing apoptosis induced by CGRP were of the CD4/CD8 double-positive type. These data indicate that apoptosis in the thymocytes is mediated by a CGRP receptor not sensitive to the antagonist CGRP(8-37). Because proliferation of thymocytes and splenocytes induced by Con A is blocked by this antagonist and splenocytes are refractory to CGRP induced apoptosis, CGRP appears to mediate at least two separate functions on subpopulations of thymocytes and T cells via two different CGRP receptors within the gland. These effects of a neuropeptide exemplify the phenomenon of differential regional regulation of immunity by the autonomic and neuroendocrine systems.

Purification of calcitonin-like peptides from rat brain and pituitary

Accumulating evidence supports the existence of nonthyroidal calcitonin (CT)-like peptides, more similar to fish CTs, which may act as endogenous regulators of CT receptors in brain and other tissues. In this study, we have carried out large-scale extractions from Sprague-Dawley rat brain diencephalon and pituitary, and purified a novel, biologically active, CT-like peptide from pituitary. Monitoring of the calcitonin-like activity of the peptides from rat brain and pituitary required different detection systems. While the brain CT cross-reacted with C-terminally directed salmon CT-specific antisera, the pituitary CT did not. However, the pituitary CT was biologically active, exhibiting specific interaction with CT receptors to activate adenylate cyclase. Conventional chromatographic techniques were employed to purify the CT-like peptides. Although the brain CT was not purified to homogeneity, size exclusion chromatography revealed the presence of multiple molecular weight forms of immunoreactive CT. Of these, only the lowest molecular weight form was biologically active. Purification from the pituitary resulted in the isolation of a biologically active peptide with a mass of 3267 Da. This mass differs from the mass of both salmon and thyroid-derived rat CT. Initial amino acid sequencing of the pituitary CT indicated that it was N-terminally blocked. Following aminopeptidase digestion, a unique six amino acid sequence, EKSQSP, was identified. Elucidation of the amino acid composition provided supporting evidence that the peptide was novel and was consistent with a full length peptide of approximately 30 amino acids. These data support the existence of novel, nonthyroidal, CTs which are potential regulators of CT receptor-mediated functions.

Cockroach allatostatin-like immunoreactivity in the central nervous system of the freshwater snails Bulinus globosus (Planorbidae) and Stagnicola elodes (Lymnaeidae)

Allatostatin-like immunoreactivity was demonstrated in the central nervous system (CNS) neurons of the freshwater pulmonate snails Bulinus globosus (Planorbidae) and Stagnicola elodes (Lymnaeidae). Immunopositive neurons were localized using a monoclonal antibody highly specific to allatostatin I (APSGAQRLYGFGL-amide) from the cockroach Diploptera punctata. All CNS ganglia in both snail species contained immunopositive neurons. The pedal ganglia showed large numbers (80-100) of neurons in Bulinus and Stagnicola. Large numbers of immunopositive cells were also found in the cerebral (60) and buccal ganglia (20) of Bulinus. Other ganglia contained fewer immunopositive cells, but these cells were most concentrated in the cerebral (Stagnicola) and left parietal (Bulinus) ganglia and the visceral ganglia of both species. The high concentration of immunopositive cells in the pedal ganglia and axons demonstrable in the pedal nerves suggests that one possible function for a molluscan allatostatin-like peptide would be to modulate muscular function. Extract of Stagnicola CNS effected 50% inhibition of juvenile hormone synthesis by corpora allata of D. punctata at between 15 and 30 CNS equivalents, providing further evidence that the molluscan immunoreactive material is a peptide, or peptides, with sequence similarity to the active part of the D. punctata allatostatins.

Characterization of a calcitonin gene related peptide-like molecule in the abalone, Haliotis tuberculata

Immunoreactive related CGRP molecules (ir-CGRP) were identified in the abalone, Haliotis tuberculata, mainly in mantle and cephalic part extracts. Ir-CGRP in both tissues accounted for 461 and 455.6 pg per mg of proteins, respectively. These CGRP-immunoreactive molecules were further analyzed for their ability to interact with the CGRP radioreceptor assay. In specific target tissues for CGRP (rat liver membranes), 50% inhibition of 125I-labeled CGRP specific binding was observed with 4.7 micrograms and 21.1 micrograms of proteins from mantle and cephalic part extract, respectively. These molecules were submitted to gel-filtration chromatography on a Sephacryl S-100 column and were further analyzed in the radioreceptor assay specific for CGRP. The elution position of these molecules suggested a molecular weight close to that of synthetic salmon calcitonin.

Characterization of binding sites for calcitonin gene-related peptide in the mollusc gill

Target organs for calcitonin gene-related peptide (CGRP) were investigated in Pecten maximus using 125I-labelled human CGRP. CGRP was shown to interact specifically with mantle and gill tissue. Receptor studies using branchial membrane preparations indicated that the binding was time dependent. Scatchard analysis of binding data showed that there was a single class of binding sites. The affinity constant was found to be 0.7.10(8) M-1 and the number of binding sites 2600.10(8)/mg of protein. Salmon CT inhibited the binding of 125I-labelled CGRP to branchial membranes with a lesser efficiency than that of the unlabelled hormone. A 40% inhibition of the 125I-labelled CGRP binding was observed in the presence of 2.6 and 26 nM CGRP and salmon CT, respectively. In addition, 200 nM human CGRP inhibited 25 and 10% of the basal branchial and mantle adenylate cyclase activity, respectively. These data suggest that CGRP participates in the regulation of the branchial function in molluscs probably via a vasoconstrictor role.

Diversity and modulation of ionic conductances in leech neurons

A complete understanding of animal behavior at the cellular level requires detailed information on the intrinsic biophysical properties of neurons, muscles, and the synaptic connections they make. In the past 10 to 15 years, electrophysiological studies of leech neurons have revealed a diverse array of voltage-gated ionic conductances distinguished by their pharmacological sensitivity to classic ion channel blockers. Voltage-clamp studies have provided new information about the kinetics and voltage-dependence of Na+ conductances, several K+ currents, including IA, IK and IK(Ca.), and high- and low-voltage-gated Ca2+ conductances. These studies showed that the action potentials of most leech neurons result from the usual sequence of permeability changes to Na+, K+, and Ca2+ ions. They also added insight as to the role played by particular combinations of conductances in providing individual neurons with electrical properties appropriate for the particular information they encode. Evidence is accumulating on the modulatory actions fo endogenous neurotransmitters such as FMRFamide, serotonin, and octopamine on motor behaviors in the animal. Parallel studies suggest that changes in behavior can be explained, at least in part, by the alteration of firing patterns of selected neurons and muscles resulting from modulation of multiple ion conductances. This makes the leech exceptionally attractive for neuroethological studies because it is one of the simplest organisms in which the methods of psychology and neurobiology can be combined. Information gathered from this animal will therefore increase our understanding regarding general principles underlying the cellular basis of behavior.

Inhibition of the calcitonin-induced outward current in identified Aplysia neurons by interleukin-1 and interleukin-2

1. The effects of bath-applied recombinant human interleukin-1 (rhIL-1) and interleukin-2 (rhIL-2) on the calcitonin (CT)-induced outward current recorded from identified neurons (R9-R12) of Aplysia kurodai were investigated with conventional voltage-clamp and pressure ejection techniques. 2. Micropressure ejection of CT onto the soma of the neuron induced a slow outward current [Io(CT); 4-6 nA in amplitude, 30-40 sec in duration] associated with a decrease in input membrane conductance. 3. Io(CT) was increased by hyperpolarization. 4. The extrapolated reversal potential was +10 mV. Additionally, Io(CT) was sensitive to changes in (Na+)o but not to changes in (K+)o, (Ca2+)o, and (Cl-)o. 5. Micropressure-ejected forskolin produced a slow outward current similar to that induced by CT. 6. Bath-applied rhIL-1 and rhIL-2 (10-40 U/ml) reduced the CT-induced current in identified Aplysia neurons without affecting the resting membrane conductance or the holding current. 7. The inhibitory effects of both cytokines on the current were completely reversible. Heat-inactivated rhIL-1 and rhIL-2 were without effect. 8. These results suggest that the immunomodulators, IL-1 and IL-2, can modulate the CT-induced outward current associated with a decrease in Na+ conductance in the nervous system of Aplysia. Therefore, the study suggests that these cytokines may also serve as neuromodulators.

Calcitonin induces a decreased Na+ conductance in identified neurons of Aplysia

The ionic mechanism of the effect of extracellularly ejected calcitonin (CT) on the membrane of identified neurons R9 and R10 of Aplysia was investigated with voltage-clamp, micropressure ejection, and ion substitution techniques. Micropressure-ejected CT caused a marked hyperpolarization in the unclamped neuron. Heat-inactivated CT was without effect. Clamping the same neuron at its resting potential level (-60 mV) and re-ejecting CT with the same dose produced a slow outward current (Io(CT), 30-40 sec in duration, 4-6 nA in amplitude) associated with a decrease in input membrane conductance. Io(CT) was decreased by depolarization and increased by hyperpolarization. The extrapolated reversal potential of Io(CT) was approximately +10 mV. Io(CT) was sensitive to changes in the external Na+ concentration but not to changes in K+, Ca2+, and Cl- concentrations. Micropressure-ejected forskolin produced a slow outward current, which, like the current to CT, was associated with a decrease in input membrane conductance, and was sensitive to changes in the external Na+ concentration. Io(CT) was prolonged by bath-applied isobutylmethylxanthine (IBMX) but was not affected by 1-oleoyl-2-acetylglycerol (OAG) and calphostin C. Neither superfusion of the neuron with nordihydroguaiaretic acid (NDGA) nor superfusion with indomethacin caused any changes in Io(CT). These results suggest that extracellular CT can induce a slow outward current associated with a decrease in Na+ conductance, mediated by a receptor-controlled increase in intracellular cyclic adenosine 3',5'-monophosphate.