Neurochemical characterization of nervous elements innervating the body wall of earthworms (Lumbricus, Eisenia): immunohistochemical and pharmacological studies

Co-created in vivo pharmacology practical classes using the novel organism Lumbriculus variegatus

Co-creation within higher education emphasizes learner empowerment to promote collaboration between the students and staff, enabling students to become active participants in their learning process and the construction of resources with academic staff. Concurrently, a diminishing number of higher education institutions offer in vivo practical classes, resulting in an in vivo skills shortage. To address this, and to actively engage students in their own learning, we describe the co-creation of a student-led drug trial using Lumbriculus variegatus. Under blinded conditions, final-year undergraduate biomedical science students, under the tutelage of academic staff and fellow students, were involved in the co-creation of an in vivo practical class to determine the effects of histamine and histamine receptor inverse agonists mepyramine and loratadine. Throughout this process, undergraduate- and masters-level students played key roles in every aspect of practical delivery and data analysis. Herein, students demonstrated the test compounds, both in isolation and in combination, resulted in reduced stereotypical movements of L. variegatus (p < .05, n ≥ 6). 15% of students in the class responded to a feedback survey (n = 8) after the class. Students reported the class provided "real life" insights into in vivo research and enabled the development of hands-on skills which would be useful in applying in their future careers. All students reported that they enjoyed the class with 25% (n = 2) reporting concerns about animal use in research, enabling useful discussions about animals in research. Moreover, these student-led in vivo trials add to the pharmacological knowledge of L. variegatus promoting education-led research.

The Protective Effect of Lithium Against Rotenone may be Evolutionarily Conserved: Evidence from Eisenia fetida, a Primitive Animal with a Ganglionic Brain

Chronic exposure to stress is a non-adaptive situation that is associated with mitochondrial dysfunction and the accumulation of reactive oxygen species (ROS), especially superoxide anion (SA). This accumulation of ROS produces damage-associated molecular patterns (DAMPs), which activate chronic inflammatory states and behavioral changes found in several mood disorders. In a previous study, we observed that an imbalance of SA triggered by rotenone (Ro) exposure caused evolutionarily conserved oxi-inflammatory disturbances and behavioral changes in Eisenia fetida earthworms. These results supported our hypothesis that SA imbalance triggered by Ro exposure could be attenuated by lithium carbonate (LC), which has anti-inflammatory properties. The initial protocol exposed earthworms to Ro (30 nM) and four different LC concentrations. LC at a concentration of 12.85 mg/L decreased SA and nitric oxide (NO) levels and was chosen to perform complementary assays: (1) neuromuscular damage evaluated by optical and scanning electron microscopy (SEM), (2) innate immune inefficiency by analysis of Eisenia spp. extracellular neutrophil traps (eNETs), and (3) behavioral changes. Gene expression was also evaluated involving mitochondrial (COII, ND1), inflammatory (EaTLR, AMP), and neuronal transmission (nAchR α5). LC attenuated the high melanized deposits in the circular musculature, fiber disarrangement, destruction of secretory glands, immune inefficiency, and impulsive behavior pattern triggered by Ro exposure. However, the effects of LC and Ro on gene expression were more heterogeneous. In summary, SA imbalance, potentially associated with mitochondrial dysfunction, appears to be an evolutionary component triggering oxidative, inflammatory, and behavioral changes observed in psychiatric disorders that are inhibited by LC exposure.

GABA has a presynaptic inhibitory effect at Lumbricus terrestris body wall muscle synapses

Earthworm body wall muscle synapses have been suggested to contain both excitatory and inhibitory inputs, and therefore allow for investigation of excitatory/inhibitory signaling in an easily accessible model system. While previous studies have focused on postsynaptic GABAergic inhibitory mechanisms, this study investigated the hypothesis that GABAergic signaling also has presynaptic inhibitory function. This hypothesis was tested by loading synaptogreen C4 dye (also called FM1-43) into presynaptic vesicles in the presence of GABA at Lumbricus terrestris longitudinal muscle synapses. GABA treatment significantly reduced the fluorescence intensity observed at these synapses, suggesting that GABAergic signaling does indeed have a presynaptic inhibitory mechanism.

Phylogenetic analyses of 5-hydroxytryptamine 3 (5-HT3) receptors in Metazoa

The 5-hydroxytrptamine 3 (5-HT3) receptor is a member of the 'Cys-loop' family and the only pentameric ligand gated ion channel among the serotonin receptors. 5-HT3 receptors play an important role in controlling growth, development, and behaviour in animals. Several 5-HT3 receptor antagonists are used to treat diseases (e.g., irritable bowel syndrome, nausea and emesis). Humans express five different subunits (A-E) enabling a variety of heteromeric receptors to form but all contain 5HT3A subunits. However, the information available about the 5-HT3 receptor subunit occurrence among the metazoan lineages is minimal. In the present article we searched for 5-HT3 receptor subunit homologs from different phyla in Metazoa. We identified more than 1000 5-HT3 receptor subunits in Metazoa in different phyla and undertook simultaneous phylogenetic analysis of 526 5HT3A, 358 5HT3B, 239 5HT3C, 70 5HT3D, and 173 5HT3E sequences. 5-HT3 receptor subunits were present in species belonging to 11 phyla: Annelida, Arthropoda, Chordata, Cnidaria, Echinodermata, Mollusca, Nematoda, Orthonectida, Platyhelminthes, Rotifera and Tardigrada. All subunits were most often identified in Chordata phylum which was strongly represented in searches. Using multiple sequence alignment, we investigated variations in the ligand binding region of the 5HT3A subunit protein sequences in the metazoan lineage. Several critical amino acid residues important for ligand binding (common structural features) are commonly present in species from Nematoda and Platyhelminth gut parasites through to Chordata. Collectively, this better understanding of the 5-HT3 receptor evolutionary patterns raises possibilities of future pharmacological challenges facing Metazoa including effects on parasitic and other species in ecosystems that contain 5-HT3 receptor ligands.

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.

Neuromuscular transmitter candidates of a centipede (Lithobius forficatus, Chilopoda)

Background

The neuromuscular junction is the chemical synapse where motor neurons communicate with skeletal muscle fibers. Whereas vertebrates and many invertebrates use acetylcholine as transmitter at the neuromuscular junction, in those arthropods examined up to now, glutamate and GABA are used instead. With respect to taxon sampling in a phylogenetic context, there is, however, only a limited amount of data available, focusing mainly on crustaceans and hexapods, and neglecting other, arthropod groups. Here we investigate the neurotransmitter equipment of neuromuscular synapses of a myriapod, Lithobius forficatus, using immunofluorescence and histochemical staining methods.

Results

Glutamate and GABA could be found colocalised with synapsin in synaptic boutons of body wall and leg muscles of Lithobius forficatus. Acetylcholinesterase activity as a marker for cholinergic synapses was found abundantly in the central nervous system and also in some peripheral nerves, but not at neuromuscular junctions. Furthermore, a large number of leg sensory neurons displayed GABA-immunofluorescence and was also labeled with an antiserum against the GABA-synthesizing enzyme, glutamate decarboxylase.

Conclusions

Our data indicate that glutamate and GABA are neurotransmitters at Lithobius forficatus neuromuscular junctions, whereas acetylcholine is very unlikely to play a role here. This is in line with the concept of glutamate as excitatory and GABA as the main inhibitory neuromuscular transmitters in euarthropods. Furthermore, we have, to our knowledge for the first time, localized GABA in euarthropod leg sensory neurons, indicating the possibility that neurotransmitter panel in arthropod sensory systems may be far more extensive than hitherto assumed.

Identification of oscillatory firing neurons associated with locomotion in the earthworm through synapse imaging

We used FM imaging to identify neurons that receive sensory feedback from the body wall in a circuit for octopamine (OA)-evoked rhythmic locomotion in the earthworm, Eisenia fetida. We visualized synapses in which postsynaptic neurons receive the sensory feedback, by using FM1-43 dye to label the synapses of both motor and sensory pathways that are associated with locomotion, then clearing the motor pathway synapse labeling, and finally identifying the target synapses by distinguishing physiologically functional synapses through destaining using a high-K(+) solution. A pair of synaptic regions associated with the sensory feedback was found to be located two or three cell body-widths away from the midline, between the anterior parts of the roots of the second lateral nerves (LNs) at the segmental ganglia (SGs). Using conventional intracellular recording and dye loading of the cell bodies surrounding these synaptic regions, we identified a pair of bilateral neurons with cell bodies larger than those of other cells in these regions, and named them "Oscillatory firing neurons Projecting to Peripheral nerves" (OPPs). These had a bipolar shape and projected neurites to the ipsilateral first and third LNs, fired rhythmically, and had a burst timing synchronized with the motor pattern bursts from the ipsilateral first LNs. Current injection into an OPP caused firing in the ipsilateral first LNs, supporting the hypothesis that OPPs functionally project to the peripheral nerves. OPPs also sent neurites to the adjacent anterior and posterior SGs, suggesting connections with the adjacent segments. We conclude that FM imaging can be used to identify neurons involved in specific functions, and that OPPs are the first neurons to be associated with OA-induced locomotion in the earthworm.

Organization of the sensory system of the earthworm Lumbricus terrestris (Annelida, Clitellata) visualized by DiI

The anatomical organization of the peripheral and central sensory structures of the earthworm Lumbricus terrestris was investigated applying a fluorescent carbocyanine dye (DiI) as a neuronal tracer. Using whole-mount preparations and confocal laser scanning microscopy, the pattern of primary sensory cells and pathways of their processes were traced and reconstructed in three-dimensions. Our study shows that a ventral nerve cord ganglion receives sensory fibers from at least two adjacent segments suggesting that the peripheral nervous system is not segmental in its arrangement and the receptive-fields of the body wall overlap in earthworms. Furthermore, our result suggests an integrative function of the basiepidermal plexus consists of sensory and motor fibers.

Gene expression analysis of CL-20-induced reversible neurotoxicity reveals GABA(A) receptors as potential targets in the earthworm Eisenia fetida

The earthworm Eisenia fetida is one of the most used species in standardized soil ecotoxicity tests. End points such as survival, growth, and reproduction are eco-toxicologically relevant but provide little mechanistic insight into toxicity pathways, especially at the molecular level. Here we apply a toxicogenomic approach to investigate the mode of action underlying the reversible neurotoxicity of hexanitrohexaazaisowurtzitane (CL-20), a cyclic nitroamine explosives compound. We developed an E. fetida-specific shotgun microarray targeting 15119 unique E. fetida transcripts. Using this array we profiled gene expression in E. fetida in response to exposure to CL-20. Eighteen earthworms were exposed for 6 days to 0.2 μg/cm(2) of CL-20 on filter paper, half of which were allowed to recover in a clean environment for 7 days. Nine vehicle control earthworms were sacrificed at days 6 and 13, separately. Electrophysiological measurements indicated that the conduction velocity of earthworm medial giant nerve fiber decreased significantly after 6-day exposure to CL-20, but was restored after 7 days of recovery. Total RNA was isolated from the four treatment groups including 6-day control, 6-day exposed, 13-day control, and 13-day exposed (i.e., 6-day exposure followed by 7-day recovery), and was hybridized to the 15K shotgun oligo array. Statistical and bioinformatic analyses suggest that CL-20 initiated neurotoxicity by noncompetitively blocking the ligand-gated GABA(A) receptor ion channel, leading to altered expression of genes involved in GABAergic, cholinergic, and Agrin-MuSK pathways. In the recovery phase, expression of affected genes returned to normality, possibly as a result of autophagy and CL-20 dissociation/metabolism. This study provides significant insights into potential mechanisms of CL-20-induced neurotoxicity and the recovery of earthworms from transient neurotoxicity stress.

The neuropeptide Gonadotropin-releasing hormone modifies the spontaneous muscular contraction in the earthworm: Eisenia foetida

We investigated whether the Gonadotropin-releasing hormone affects the spontaneous muscular contraction in the earthworm Eisenia foetida. In addition, we investigated the presence of Gonadotropin-releasing hormone receptor in ventral nerve cord by immunohistochemistry and polymerase chain reaction. Gonadotropin-releasing hormone induced a significant increase on both amplitude and muscular tone and decrease in the frequency of spontaneous muscular contraction. We found the presence of immunoreactive material to Gonadotropin-releasing hormone receptor in the ventral nerve cord, likewise the Gonadotropin-releasing hormone receptor mRNA expression. In conclusion, the Gonadotropin-releasing hormone modifies the spontaneous muscular contraction in E. foetida and these effects can be due to the activation of the Gonadotropin-releasing hormone receptor.

Serotonin modulates muscle function in the medicinal leech Hirudo verbana

The body wall muscles of sanguivorous leeches power mechanically diverse behaviours: suction feeding, crawling and swimming. These require longitudinal muscle to exert force over an extremely large length range, from 145 to 46 per cent of the mean segmental swimming length. Previous data, however, suggest that leech body wall muscle has limited capacity for force production when elongated. Serotonin (5-HT) alters the passive properties of the body wall and stimulates feeding. We hypothesized that 5-HT may also have a role in allowing force production in elongated muscle by changing the shape of the length-tension relationship (LTR). LTRs were measured from longitudinal muscle strips in vitro in physiological saline with and without the presence of 10 µM 5-HT. The LTR was much broader than previously measured for leech muscle. Rather than shifting the LTR, 5-HT reduced passive muscle tonus and increased active stress at all lengths. In addition to modulating leech behaviour and passive mechanical properties, 5-HT probably enhances muscle force and work production during locomotion and feeding.

Mechanisms of carbacholine and GABA action on resting membrane potential and Na+/K+-ATPase of Lumbricus terrestris body wall muscles

This work was aimed to identify the action of several ion channel and pump inhibitors as well as nicotinic, GABAergic, purinergic and serotoninergic drugs on the resting membrane potential (RMP) and assess the role of cholinergic and GABAergic sensitivity in earthworm muscle electrogenesis. The nicotinic agonists acetylcholine (ACh), carbacholine (CCh) and nicotine depolarize the RMP at concentrations of 5 μM and higher. The nicotinic antagonists (+)tubocurarine, α-bungarotoxin, muscarinic antagonists atropine and hexamethonium do not remove or prevent the CCh-induced depolarization. Verapamil, tetrodotoxin, removal of Cl(-) and Ca(2+) from the solution also cannot prevent the depolarization by CCh. In a Na(+)-free medium, however, CCh lost this depolarization ability and this indicates that the drug opens the sodium permeable pathway. Serotonin, glutamate, glycine, adenosine triphosphate (ATP) and cis-4-aminocrotonic acid (GABA(C) receptor antagonist) had no effect on the RMP. On the other hand, isoguvacin, γ-aminobutyric acid (GABA) and baclofen (GABA(B) receptor agonist) hyperpolarized the RMP. Ouabain, bicucullin (GABA(A) antagonist) and phaclofen (GABA(B) antagonist), as well as the removal of Cl(-), suppressed the effect of GABA and baclofen. CCh did not enhance the depolarization generated by ouabain but, on the other hand, hindered the hyperpolarizing activity of baclofen both in the absence and presence of atropine and (+)tubocurarine. The long-term application of CCh depolarizes the RMP primarily by inhibiting the Na(+)/K(+)-ATPase. The muscle membrane also contains A and B type GABA binding sites, the activation of which increases the RMP at the expense of increasing the action of ouabain- and Cl(-) -sensitive electrogenic pumps.

Avoidance behavior of earthworms under exposure to pesticides: is it always chemosensorial?

Soil avoidance by earthworms has been generally considered a relevant and sensitive endpoint for assessing soil contamination by xenobiotics. However, when pesticide ecotoxicological assessment is concerned, the sensitivity of the recently standardized avoidance assay has been questioned. We hypothesized that this controversy may be due to the specific pesticide mode of action of the chemicals used rather than reveal inconsistencies in the test feasibility, i.e. provided that no pesticides interfering with neuronal pathways are tested, this bioassay should keep expected high levels of sensitivity. In this study, the avoidance behaviour of the earthworm Eisenia andrei under exposure to the carbamate insecticide methomyl [S-methyl N-(methylcarbamoyloxy)thioacetimidate] was linked to the corresponding acetylcholinesterase (AChE) inhibition. Significant AChE inhibition occurred at lower concentrations (from 0.86 mg Kg(-1) onwards) than significant avoidance of spiked soil (from 5.62 mg Kg(-1) onwards). This indicates that assessments regarding pesticides that have neurotoxic activity may be biased if behavioral endpoints are selected. Despite theoretical hypothesis that have been raised, this should be the first study providing preliminary experimental evidence on such a link between avoidance behavior and neuronal impairment levels in earthworms. Further studies are ongoing that should refine conclusions of this study.

A review of FMRFamide- and RFamide-like peptides in metazoa

Neuropeptides are a diverse class of signalling molecules that are widely employed as neurotransmitters and neuromodulators in animals, both invertebrate and vertebrate. However, despite their fundamental importance to animal physiology and behaviour, they are much less well understood than the small molecule neurotransmitters. The neuropeptides are classified into families according to similarities in their peptide sequence; and on this basis, the FMRFamide and RFamide-like peptides, first discovered in molluscs, are an example of a family that is conserved throughout the animal phyla. In this review, the literature on these neuropeptides has been consolidated with a particular emphasis on allowing a comparison between data sets in phyla as diverse as coelenterates and mammals. The intention is that this focus on the structure and functional aspects of FMRFamide and RFamide-like neuropeptides will inform understanding of conserved principles and distinct properties of signalling across the animal phyla.

Correlative morphometric and electrochemical measurements of serotonin content in earthworm muscles

Distribution of serotonin (5-HT) content of nervous fibers in both the somatic and the visceral muscle of Eisenia fetida have been investigated using immunocytochemical staining and voltammetric measurements. The somatic muscles in the body wall are richer innervated with serotoninergic fibers than the visceral ones in the pharynx and gizzard. The relative density of immunopositive fibers in the circular muscle layer of the body wall was found to be 2.73% while in the prostomium it was 1.02%. In the case of the muscle in pharynx 1.12% and in gizzard 1.28% density values were found. Differential Pulse Voltammetric (DPV) measurements with carbon fiber electrodes in the above mentioned muscle layers gave 272.5 nA, 135.0 nA, 122.5 nA, 137.5 nA peak heights, respectively. In the statistical analysis T-test was used at a confidence level of 95% (p<0.05). DPV current peak (i(p)) values reflect clearly the 5-HT concentration differences. Significant correlation was found between the innervation density and the i(p) values recorded in different areas. The i(p) values recorded at different times in different locations are determined by instantaneous serotonin concentration of the living tissue. As far as we know this is the first report using in vivo voltammetry investigating serotonin content in earthworm, E. fetida.

Embryogenesis of the serotonergic system in the earthwormEisenia fetida (Annelida, Oligochaeta): Immunohistochemical and biochemical studies

Organization of the serotonergic system and changes of the serotonin (5-HT) content were studied during the embryogenesis of the earthworm Eisenia fetida, using immunocytochemistry and HPLC. A gradual emergence of 5-HT immunoreactive (IR) cells and their axon projections in the several ganglia of the central (CNS) and peripheral nervous system are described in the context of a staged time-scale of development. The first 5-HT-IR neurons appear in the subesophageal ganglion at an early embryonic stage (E2), followed by neurons in some rostrally located ventral ganglia. In the cerebral ganglion, 5-HT-IR cells can be detected only from stage E5. The number of labeled cells in each ganglion of the embryo increases until hatching, when it is still considerably lower than that observed in adults. This shows that the development of the 5-HTergic system is far from complete by the end of embryogenesis. Organization of 5-HT-IR innervation of the body wall starts by stages E3 to E4. In the stomatogastric nervous system the first 5-HT-IR fibers can be detected by stage E5. By stage E9 5-HT immunopositive neurons can be observed in both the stomatogastric ganglia and the enteric plexus. Both 5-HT levels and the numbers of the labeled cells show a significant increase before hatching, which indicate a functional maturation of the 5-HTergic system. Based on the early appearance of 5-HT, we suppose that it may play a regulatory role in both the gangliogenesis and the maturation of peripheral functions necessary during postembryonic life.