Distribution of serotonin-, Gamma-Aminobutyric Acid- and substance P-like immunoreactivity in the central and peripheral nervous system of Mytilus galloprovincialis

Role of the Neuroendocrine System of Marine Bivalves in Their Response to Hypoxia

Mollusks comprise one of the largest phylum of marine invertebrates. With their great diversity of species, various degrees of mobility, and specific behavioral strategies, they haveoccupied marine, freshwater, and terrestrial habitats and play key roles in many ecosystems. This success is explained by their exceptional ability to tolerate a wide range of environmental stresses, such as hypoxia. Most marine bivalvemollusksare exposed to frequent short-term variations in oxygen levels in their marine or estuarine habitats. This stressfactor has caused them to develop a wide variety of adaptive strategies during their evolution, enabling to mobilize rapidly a set of behavioral, physiological, biochemical, and molecular defenses that re-establishing oxygen homeostasis. The neuroendocrine system and its related signaling systems play crucial roles in the regulation of various physiological and behavioral processes in mollusks and, hence, can affect hypoxiatolerance. Little effort has been made to identify the neurotransmitters and genes involved in oxygen homeostasis regulation, and the molecular basis of the differences in the regulatory mechanisms of hypoxia resistance in hypoxia-tolerant and hypoxia-sensitive bivalve species. Here, we summarize current knowledge about the involvement of the neuroendocrine system in the hypoxia stress response, and the possible contributions of various signaling molecules to this process. We thusprovide a basis for understanding the molecular mechanisms underlying hypoxic stress in bivalves, also making comparisons with data from related studies on other species.

Histamine and gamma-aminobutyric acid in the nervous system of Pygospio elegans (Annelida: Spionidae): structure and recovery during reparative regeneration

Background

In recent two decades, studies of the annelid nervous systems were revolutionized by modern cell labeling techniques and state-of-the-art microscopy techniques. However, there are still huge gaps in our knowledge on the organization and functioning of their nervous system. Most of the recent studies have focused on the distribution of serotonin and FMRFamide, while the data about many other basic neurotransmitters such as histamine (HA) and gamma-aminobutyric acid (GABA) are scarce.

Results

Using immunohistochemistry and confocal microscopy we studied the distribution of histamine and gamma-aminobutyric acid in the nervous system of a spionid annelid Pygospio elegans and traced their redevelopment during reparative regeneration. Both neurotransmitters show specific patterns in central and peripheral nervous systems. HA-positive cells are concentrated mostly in the brain, while GABA-positive cell somata contribute equally to brain and segmental ganglia. Some immunoreactive elements were found in peripheral nerves. Both substances were revealed in high numbers in bipolar sensory cells in the palps. The first signs of regenerating HAergic and GABAergic systems were detected only by 3 days after the amputation. Further redevelopment of GABAergic system proceeds faster than that of HAergic one.

Conclusions

Comparisons with other annelids and mollusks examined in this respect revealed a number of general similarities in distribution patterns of HAergic and GABAergic cells in different species. Overall, the differences in the full redevelopment of various neurotransmitters correlate with neuronal development during embryogenesis. Our results highlight the importance of investigating the distribution of different neurotransmitters in comparative morphological and developmental studies.

Effect of Air Exposure-Induced Hypoxia on Neurotransmitters and Neurotransmission Enzymes in Ganglia of the Scallop Azumapecten farreri

The nervous system expresses neuromolecules that play a crucial role in regulating physiological processes. Neuromolecule synthesis can be regulated by oxygen-dependent enzymes. Bivalves are a convenient model for studying air exposure-induced hypoxia. Here, we studied the effects of hypoxia on the expression and dynamics of neurotransmitters, and on neurotransmitter enzyme distribution, in the central nervous system (CNS) of the scallop Azumapecten farreri. We analyzed the expression of the neurotransmitters FMRFamide and serotonin (5-HT) and the choline acetyltransferase (CHAT) and universal NO-synthase (uNOS) enzymes during air exposure-induced hypoxia. We found that, in early-stage hypoxia, total serotonin content decreased in some CNS regions but increased in others. CHAT-lir cell numbers increased in all ganglia after hypoxia; CHAT probably appears de novo in accessory ganglia. Short-term hypoxia caused increased uNOS-lir cell numbers, while long-term exposure led to a reduction in their number. Thus, hypoxia weakly influences the number of FMRFamide-lir neurons in the visceral ganglion and does not affect peptide expression in the pedal ganglion. Ultimately, we found that the localization and level of synthesis of neuromolecules, and the numbers of cells expressing these molecules, vary in the scallop CNS during hypoxia exposure. This indicates their possible involvement in hypoxia resistance mechanisms.

Distribution of Molecules Related to Neurotransmission in the Nervous System of the Mussel Crenomytilus grayanus

In bivalves neurotransmitters are involved in a variety of behaviors, but their diversity and distribution in the nervous system of these organisms remains somewhat unclear. Here, we first examined immunohistochemically the distributions of neurons containing different neurotransmitters, neuropeptides, and related enzymes, as well as the proliferative status of neurons in the ganglia of the mussel Crenomytilus grayanus. H-Phe-Met-Arg-Phe-NH2 (FMRFamide), choline acetyltransferase (ChAT), γ-aminobutyric acid (GABA) and tyrosine hydroxylase (TH) were found to be expressed by neurons in all the ganglia, whereas serotonin (5-HT) neurons were found only in the cerebropleural and pedal, but not visceral ganglia. Moreover, incubation of living mussels in the presence of a 5-HT precursor (5-HTP) confirmed the absence of 5-HT-containing neurons from the visceral ganglia, indicating that the "serotonin center" of the visceral nervous system is located in the cerebral ganglia. Furthermore, immunostaining of molecules related to neurotransmission together with α-acetylated tubulin demonstrated that this cytoskeletal protein may be a potential pan-neuronal marker in bivalves. Adult mussel neurons do not proliferate, but a population of proliferating PCNA-LIP cells which do not express any of the neurotransmitters examined, perhaps glia cells, was detected in the ganglia. These novel findings suggest that the nervous system of bivalves contains a broad variety of signal molecules most likely involved in the regulation of different physiological and behavioral processes. In addition, proliferating cells may maintain and renew glial cells and neurons throughout the lives of bivalves.

Motility of the Labial Palps in Feeding Behavior and its Innervation in the Marine Mussel, Mytilus galloprovincialis

The labial palps of bivalves are thought to be involved in suspension feeding. However, the function of their muscular movements and neural regulation are still unclear. In semi-intact preparations of Mytilus, in which one valve was removed, suspended particles were removed from the labial palps following two kinds of compound movements: torsional and rotational. Both of these compound movements are therefore thought to function in rejection during feeding. These movements were observed in reduced preparations of isolated labial palps with intact cerebral ganglia, and were maintained even after removal of the cerebral ganglia, suggesting that they are generated by the peripheral neural network. Stimulation of the anterior pallial nerve elicited tetanic contraction of the labial palp, followed by secondary responses, including torsional movement. Secondary responses were dramatically reduced by a high concentration of divalent cations, in which polysynaptic pathways were inhibited. Hence, the cerebral ganglia may play an excitatory role within the peripheral neural network and the labial palp musculature via the anterior pallial nerve. Administration of serotonin induced repetitive muscular movements, whereas dopamine did not induce muscular movements. Serotonin-induced muscular movements were not elicited under a high concentration of divalent cation condition. In histochemical experiments, both the serotonergic and dopaminergic neural processes and cell body-like structures were widely observed inside the labial palp, the anterior pallial nerve, and the cerebral ganglia. Serotonin may thus contribute to activation of polysynaptic peripheral pathways, which are involved in regulating compound movements.

Peripheral sensory neurons govern development of the nervous system in bivalve larvae

Recent findings regarding early lophotrochozoan development have altered the conventional model of neurogenesis and revealed that peripheral sensory elements play a key role in the initial organization of the larval nervous system. Here, we describe the main neurogenetic events in bivalve mollusks in comparison with other Lophotrochozoa, emphasizing a novel role for early neurons in establishing larval nervous systems and speculating about the morphogenetic function of the apical organ. We demonstrate that during bivalve development, peripheral sensory neurons utilizing various transmitters differentiate before the apical organ emerges. The first neurons and their neurites serve as a scaffold for the development of the nervous system. During veliger stage, cerebral, pleural, and visceral ganglia form along the lateral (visceral) nerve cords in anterior-to-posterior axis. The pedal ganglia and corresponding ventral (pedal) nerve cords develop much later, after larval settlement and metamorphosis. Pharmacological abolishment of the serotonin gradient within the larval body disrupts the navigation of "pioneer" axons resulting in malformation of the whole nervous system architecture. Comparative morphological data on neurogenetic events in bivalve mollusks shed new light on the origin of the nervous system, mechanisms of early axon navigation, and sequence of the tetraneurous nervous system formation. Furthermore, this information improves our understanding of the basic nervous system architecture in larval Bivalvia and Mollusca.

Neuronal Development in the Larvae of the Invasive Biofouler Dreissena polymorpha (Mollusca: Bivalvia), with Special Attention to Sensory Elements and Swimming Behavior

Although understanding of the neuronal development of Trochozoa has progressed recently, little attention has been paid to freshwater bivalves, including species with a strong ecological impact, such as the zebra mussel (Dreissena polymorpha). Therefore, an important question might concern how the developing nervous system is involved in the formation of the rapid and successful invasive behavior of this species. Our aim was to reveal the neuronal development of trochophore and veliger larvae of Dreissena, with special attention to the organization of sensory structures and their possible involvement in detecting environmental cues. After applying serotonin and FMRFamide immunocytochemistry, the first serotonin immunoreactive sensory elements appeared 16-18 hours after fertilization, whereas the first FMRFamide immunoreactive sensory cell was seen only at 32 hours of development (trochophore stage). Later, sensory elements were found in three parts of the larval body, including the apical organ, the posterior region, and the stomach. Although differences in the timing of appearance and the morphology of cells were observed, the two signaling systems showed basic similarity in their organization pattern until the end of the veliger stage. Pharmacological, physiological, and quantitative immunocytochemical investigations were also performed, suggesting the involvement of both the serotoninergic system and the FMRFamidergic system in sensomotor processes. Manipulation of the serotonin synthesis by para-chloroplenylalanine and 5-hydroxytryptophane, as well as application of increased salinity, influenced larval swimming activity, both accompanied by changes in immunofluorescence intensity. We concluded that these two early sensory systems may play an important role in the development of settlement competency of this biofouling invasive bivalve, Dreissena.

Immunohistochemical localization of serotonin (5-hydroxytryptamine) in the gonad and digestive gland of Mya arenaria (Mollusca: Bivalvia)

Serotonin (5-hydroxytryptamine or 5-HT) C(10)H(12)N(2)O plays a central role in several physiological processes in marine molluscs, especially in reproduction. 5-HT acts as a neurohormone to modulate spawning, parturition and meiosis by reinitiating prophase in arrested oocytes. Preliminary experiments using 10(-5)M 5-HT dissolved in aquarium water showed that 5-HT induced spawning movements in ripe clams and in both sexes of Mya arenaria while only a few males released sperm. The occurrence of serotoninergic fibers was demonstrated by PAP immunohistochemical reaction in the gonad of both sexes during gametogenesis. In an organism infected by the trematode parasite Prosorhynchus squamatus, we showed that serotoninergic innervation completely disappeared around the gonad's follicles. Although the gonad and digestive gland are intertwined, no serotoninergic innervations were found in the digestive gland. These findings suggest, for the first time to our knowledge in the soft shell clam, that serotonin might be involved in the regulation of gametogenesis.

GABA-immunoreactive neurones and interactions of GABA with serotonin and FMRFamide in a peripheral sensory ganglion of the pond snail Lymnaea stagnalis

The osphradium is a putative chemosensory organ of aquatic molluscs. Previously, we identified cells with serotonin (5-HT) and FMRFamide (FMRFa)-like immunoreactivity in the osphradial ganglion of Lymnaea stagnalis. The present investigation has established the presence of cells immunoreactive to gamma-aminobutyric acid (GABA). Some of these cells send processes to the sensory epithelium and are thus considered to be primary sensory neurones. Colocalisation of GABA and FMRFamide-like immunoreactivities was found in some of these and other neurones. The responses of the osphradial output electrical activity to the single and combined application of the above neuroactive substances were examined. 5-HT slightly increased and FMRFa decreased the activity. GABA alone was generally ineffective; however, it had a consistent stimulating effect after pretreatment with 5-HT. In its turn, pretreatment with GABA significantly increased the inhibitory action of FMRFa. Primary sensory neurones giving this kind of responses in the nerve were identified electrophysiologically and morphologically in the osphradial ganglion. Our results indicate that GABA takes part in relay of sensory signals into the central nervous system, and transmitter interactions involving GABA, 5-HT, and FMRFa are considerable for the final output pattern of the osphradial sensory network.

Modulation of serotonin binding sites in Spisula solidissima oocytes by phorbol ester

In Spisula solidissima oocytes, serotonin (5-hydroxytryptamine, 5-HT)-dependent meiosis reinitiation is mediated via specific 5-HT membrane binding sites. This oocyte response is inhibited by the phorbol ester TPA. To assess whether the inhibitory effect of TPA was due to alteration of oocyte membrane binding sites, we studied their characteristics after TPA treatment. [3H]-5HT binding assays revealed that TPA decreased the affinity and, after prolonged treatment, increased the number of oocyte binding sites. Moreover, inhibitory actions of TPA on 5-HT-induced meiosis reinitiation paralleled its inhibitory effects on 5-HT binding site affinity. The inhibitory actions in biological assays were restricted to TPA (an inactive analog of TPA, TPA-met was inefficient) and were completely reversed by staurosporine. Our data thus suggest an inhibitory role for protein kinase C on oocyte 5-HT binding sites under physiological conditions.