Effect of putative neuromodulators on rhythmic buccal motor output in Lymnaea stagnalis

The effects of a variety of neuromodulator substances on rhythmic motor output and activity in neurons in the feeding circuitry of Lymnaea stagnalis were examined. Each neuromodulator produced a unique combination of effects at different levels in the network: i.e., pattern-generating interneurons (N1, N2, and N3), an identified higher-order interneuron (cerebral giant cell, CGC), and buccal motoneurons. 5-Hydroxytryptamine, acetylcholine, and FMRFamide all inhibited rhythmic motor activity. However, this was achieved in different ways. Dopamine changed the nature of rhythmic activity from one in which N2 interneuronal activity was predominant ("N2 rhythm") to a feeding rhythm. Dopamine was the only substance capable of activating the feeding rhythm. Activity in the CGC was increased by 5-hydroxytryptamine, dopamine, and acetylcholine and reduced by FMRFamide. Differential responses in buccal motoneurons were also observed. The results are discussed in relation to previous work on other species and also in terms of the selection of different patterns of motor output by neuromodulators.     

Met-enkephalin modulates rhythmic activity in central neurons of Lymnaea stagnalis

1. The effects of met-enkephalin (10⁻⁶-10⁻⁴m) on electrical activity of identified neurons in the isolated CNS and semi-intact preparations of Lymnaea stagnalis have been investigated.2. Met-enkephalin (in concentrations up to 10⁻⁴M) induced very weak hyperpolarisation or depolarisation (1–4 mV) on the majority of neurons tested here.3. Met-enkephalin inhibited the 5-HT-induced respiratory rhythm during the first few minutes of its action.4. Met-enkephalin later (5–30 min after its administration) induced slow oscillations of the membrane potential in central neurons related to respiratory and locomotory programmes as well as in electrically coupled neurosecretory cells.     

Effect of a toxicant on phagocytosis pathways in the freshwater snail Lymnaea stagnalis

The disturbance of plasma membrane carbohydrates and of lipopolysaccharide (LPS) ligands in relation to cytoskeletal transformations of haemocytes has been investigated after chronic exposure of pond snails (Lymnaea stagnalis) to the peroxidizing toxicant fomesafen. Neither of the two lectins used (concanavalin A and wheat germ agglutinin) showed any binding modification after incubation of the snails in the presence of the toxicant. However, after exposure of the snails to fomesafen, a clear and persistent reduction in LPS labelling of haemocytes occurred. The actin cytoskeleton of the same cells also appeared to be sensitive to the toxicant. The reduction in LPS-binding sites was related to actin staining, leading to the hypothesis that LPS ligands and actin could be similarly modulated by the toxicant. Damaged cells showed non-adherent membrane portions with reduced filopodial extrusions, exhibiting a smooth surface free of microvilli. These changes could lower the spreading and adhesion of the cells and could therefore account for the loss in their phagocytic capabilities.     

Effect of acute temperature change on lung respiration of the mollusc Lymnaea stagnalis

• 1.Temperature-dependent effects on respiratory behaviour as well as the corresponding temperature-dependent activities of identified neurons within the respiratory network of the pulmonate snail Lymnaea stagnalis were investigated.
• 2.Lymnaea lung ventilation terminated at low temperatures (under 10 °C) while temperature elevation increased ventilation rates. The respiratory central pattern generator (CPG) functioning was relatively quiescent at temperatures under 12.5±0.44 °C.
• 3.Identified CPG neurons (RPeD1, VD4, VD1/RPaD2) and the respiratory network motor neurons (Vi- and RPa-cells) were found to exhibit varied temperature-dependent electrophysiological parameters (action potential frequency and amplitude, resting potential value) between cell types.
• 4.The observed alterations in the electrical activity of the Lymnaea respiratory network neurons underlie the marked changes of respiratory behaviour observed in the intact animal during temperature changes.

     

Distribution of serotonin and FMRF-amide in the brain of Lymnaea stagnalis with respect to the visual system

Despite serotonin’s and FMRF-amide’s wide distribution in the nervous system of invertebrates and their importance as neurotransmitters,the exact roles they play in neuronal networks leaves many questions.We mapped the presence of serotonin and FMRF-amide-immunoreactivity in the central nervous system and eyes of the pond snail Lymnaea stagnalis and interpreted the results in connection with our earlier findings on the central projections of different peripheral nerves.Since the chemical nature of the intercellular connections in the retina of L.stagnalis is still largely unknown,we paid special attention to clarifying the role of serotonin and FMRF-amide in the visual system of this snail and compared our findings with those reported from other species.At least one serotonin-and one FMRF-amidergic fibre were labeled in each optic nerve,and since no cell bodies in the eye showed immunoreactivity to these neurotransmitters,we believe that efferent fibres with somata located in the central ganglia branch at the base of the eye and probably release 5HT and FMRF-amide as neuro-hormones.Double labelling revealed retrograde transport of neurobiotin through the optic nerve,allowing us to conclude that the central pathways and serotonin-and FMRF-amideimmunoreactive cells and fibres have different locations in the CNS in L.stagnalis.The chemical nature of the fibres,which connect the two eyes in L.stagnalis,is neither serotoninergic nor FMRF-amidergic.     

Respiratory behavior in the pond snail Lymnaea stagnalis

Summary This study describes the neural basis of respiratory behavior in a pulmonate mollusc, Lymnaea stagnalis. We describe and identify muscles of the respiratory orifice (pneumostome) and mantle cavity as well as relevant motor neurons innervating these muscles. All of these identified motor neurons are active during spontaneously occurring respiratory behavior and a sporadically occurring synaptic input, termed Input 3, controls the activities of these motor neurons. This spontaneous input can also be recorded from isolated brain preparations, suggesting that the respiratory motor program is generated centrally. However, evidence is also presented that in semi-intact preparations the role of peripheral feedback is important for the initiation and termination of respiratory behavior in Lymnaea.     

Fixed phagocytes in the freshwater snail Lymnaea stagnalis

Morphological and histochemical examination of the blood and connective tissue of the freshwater snail Lymnaea stagnalis injected with various types of foreign particulate materials has shown the existence of free as well as fixed phagocytic cells. The morphology of the fixed phagocytes is described, and the phagocytic system of the snail is compared with that of other molluscan species.     

Dynamics of milk ejection reflex during continuous rhythmic stimulation of areola-nipple complex of the mammary gland

In continuous 10-minute simultaneous action of the vacuum and compression stimuli at a 1 cycle/second frequency, the rate of the milk ejection from the breast was periodically changing in lactating women. The milk flow first peaks latency was 0.5-3.0 minutes, the number of peaks being 2-7 per minute with the inter-peak interval 2-4 minutes during a 10-minute milk ejection. The women revealed an individual pattern of the milk flow peaks. The findings suggest that a central gate mechanism intervenes to set the periodicity of the milk ejection.     

Chemosensory afference in the tentacle nerve of Lymnaea stagnalis

Although the neural control of behavior has been extensively studied in gastropods, basic gaps remain in our understanding of how sensory stimuli are processed. In particular, there is only patchy evidence regarding the functional roles of sense organs and the extensive peripheral nervous system they contain. Our goal was to use extracellular electrophysiological recordings to confirm the chemosensory role of the tentacles in the great pond snail, Lymnaea stagnalis. Employing a special twin-channel suction electrode to improve signal-to-noise ratio, we applied three food odors (derived from earthworm-based food pellets, algae-based pellets, and fresh lettuce) to a reduced preparation of the tentacle while recording neuronal activity in the tentacle nerve. Responses were assessed by comparing average spike frequencies produced in response to saline flow with and without odors. We report stronger neuronal responses to earthworm-based food odors and weaker responses to algae-based food odors. There were no clear neuronal responses produced when lettuce food odor or control saline was applied to the tentacle. Overall, our results provide strong evidence for the chemosensory role of the tentacles in navigation behavior by L. stagnalis. Although it is unclear whether the differences in neuronal responses to different odors are a technical consequence of our recording system or a genuine feature of the snail sensory system, these results are a useful foundation for further study of peripheral nervous system function in gastropods.     

Gene expression in early development of Lymnaea stagnalis

Pond snails (Lymnaea stagnalis) were irradiated with doses of 2760 r or 5520 r in order to induce lethal factors in the germ cells. The descendants of these irradiated snails were individually reared to adulthood, when by self-fertilization they produced egg masses. The occurrence of lethal factors was studied in these egg masses and the stage at which they manifested themselves was determined. No developmental disturbance was found during cleavage, but gastrulation was affected. This indicates that gastrulation in Lymnaea is controlled by the genome of the embryo. The stage at which most genes interfere with development is the early trochophore stage. At later stages the number of genes, which for the first time are active in development, declines. Since the late veliger stage was never affected, all genes essential for development, are apparently functioning before this stage.     

Comparative study of visuo-vestibular conditioning in Lymnaea stagnalis

In this review, we compare the current understanding of visuo-vestibular conditioning in Hermissenda crassicornis and Lymnaea stagnalis on the basis of behavioral, electrophysiologic, and morphologic studies. Paired presentation of a photic conditioned stimulus (CS) and an orbital rotation unconditioned stimulus (US) results in conditioned escape behavior in both species. In Hermissenda, changes in excitability of type B photoreceptors and morphologic modifications at the axon terminals follow conditioning. Caudal hair cells, which detect mechanical turbulence, have reciprocal inhibition with type B photoreceptors. In Lymnaea, the interaction between photoreceptors and hair cells is dependent on statocyst location. Furthermore, the organization of the Lymnaea eye is complex, with more than 100 photoreceptors distributed in a uniquely folded retina. Although the optimal conditions to produce long-term memory (memory persistent for >1 week) are almost identical in Hermissenda and Lymnaea, physiologic and morphologic differences suggest that the neuronal mechanisms underlying learning and memory are distinct.