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P Meyrand

Publications and source records attributed to P Meyrand.

31 records · Page 2Linked to original sources
PubMed1998

Activation of a lobster motor rhythm-generating network by disinhibition of permissive modulatory inputs.

Rhythm generation by the gastric motor network in the stomatogastric ganglion (STG) of the lobster Homarus gammarus is controlled by modulatory projection neurons from rostral commissural ganglia (CoGs); blocking action potential conduction in these inputs to the STG of a stomatogastric nervous system in vitro rapidly renders the gastric network silent. However, exposure of the CoGs to low Ca2+ saline to block chemical synapses activates a spontaneously silent gastric network or enhances an ongoing gastric rhythm. A similar permissive effect was observed when picrotoxin was also superfused on these ganglia. We conclude that in the CoGs continuous synaptic inhibition is exerted on modulatory projection neuron(s) and that release from this inhibition allows strong activation of the gastric network.

Action Potentials↗
PubMed1996

Modulation of a neural network by physiological levels of oxygen in lobster stomatogastric ganglion.

Although a large body of literature has been devoted to the role of O2 in the CNS, how neural networks function during long-term exposures to low but physiological O2 partial pressure (PO2) has never been studied. We addressed this issue in crustaceans, where arterial blood PO2 is set in the 1-3 kPa range, a level that is similar to the most frequently measured tissue PO2 in the vertebrate CNS. We demonstrate that over its physiological range, O2 can reversibly modify the activity of the pyloric network in the lobster Homarus gammarus. This network is composed of 12 identified neurons that spontaneously generate a triphasic rhythmic motor output in vitro as well as in vivo. When PO2 decreased from 20 to 1 kPa, the pyloric cycle period increased by 30-40%, and the neuronal pattern was modified. These effects were all dose- and state-dependent. Specifically, we found that the single lateral pyloric (LP) neuron was responsible for the O2-mediated changes. At low PO2, the LP burst duration increased without change in its intraburst firing frequency. Because LP inhibits the pyloric pacemaker neurons, the increased LP burst duration delayed the onset of each rhythmic pacemaker burst, thereby reducing significantly the cycling frequency. When we deleted LP, the network was no longer O2-sensitive. In conclusion, we propose that (1) O2 has specific neuromodulator-like actions in the CNS and that (2) the physiological role of this reduction of activity and energy expenditure could be a key adaptation for tolerating low but physiological PO2 in sensitive neural networks.

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PubMed1995

A switch between two modes of synaptic transmission mediated by presynaptic inhibition.

Presynaptic inhibition reduces chemical synaptic transmission in the central nervous system between pairs of neurons, but its role(s) in shaping the multisynaptic interactions underlying neural network activity are not well studied. We therefore used the crustacean stomatogastric nervous system to study how presynaptic inhibition of the identified projection neuron, modulatory commissural neuron 1 (MCN1), influences the MCN1 synaptic effects on the gastric mill neural network. Tonic MCN1 discharge excites gastric mill network neurons and activates the gastric mill rhythm. One network neuron, the lateral gastric (LG) neuron, presynaptically inhibits MCN1 and is electrically coupled to its terminals. We show here that this presynaptic inhibition selectively reduces or eliminates transmitter-mediated excitation from MCN1 without reducing its electrically mediated excitatory effects, thereby switching the network neurons excited by MCN1. By switching the type of synaptic output from MCN1 and, hence, the activated network neurons, this presynaptic inhibition is pivotal to motor pattern generation.

Acetates↗
PubMed1995

Modulation and dynamic specification of motor rythm-generating circuits in crustacea.

The operation of central pattern generators (CPGs), oscillatory neural circuits responsible for rhythmic motor behavior, is now known to depend both on the synaptic interactions between constituent neurons and their intrinsic membrane properties (oscillatory, plateauing, etc). Moreover, these synaptic and cellular properties are not invariant, but are subject to a wide range of neuromodulatory influences that, by modifying the bioelectrical character of individual neurons and/or the strength of their synapses, are able to adapt the output of a given CPG circuit to the changing needs of the animal. Despite this ability to produce different functional configurations, however, the assumption remains of a CPG as a predefined assemblage of interconnected neurons dedicated to a particular behavior and functionally distinguishable from other circuits responsible for other tasks. However, our recent studies on the stomatogastric nervous system (STNS) of crustacea have begun to question this concept of the CPG as a discrete and identifiable entity within the central nervous system. Here we review evidence showing that under neuromodulatory instruction, individual neurons can participate in different oscillatory motor circuits and hence more than one rhythmic behaviour, and even more profoundly, preexisting networks can be dismantled to specify dynamically a new circuit for an entirely different behaviour. This de novo network construction is achieved again by neuromodulatory-induced alterations in the oscillatory and synaptic properties of individual target neurons. On this basis, therefore, a functional CPG network must be seen in a more dynamic context than previously thought since it may exist only in a particular behavioural situation dictated by modulatory influences.

Animals↗
PubMed1995

Functional differentiation of adult neural circuits from a single embryonic network.

The stomatogastric nervous system (STNS) of adult lobsters and crabs generates a number of different rhythmic motor patterns which control different regional movements of the foregut. Since these output patterns are generated by discrete neural networks that, in the adult, are well characterized in terms of synaptic and cellular properties, this system constitutes an ideal model for exploring the mechanisms underlying the ontogeny of neural network organization. The foregut and its rhythmic motor patterns were studied in in vitro STNS nerve-muscle preparations of the embryo and different larval stages of the lobster Homarus gammarus. The development of Homarus comprises a long embryonic stage in ovo followed by three pelagic larval stages prior to the onset of benthic life. During these stages the foregut itself develops slowly from a simple ectodermal invagination that occurs in the embryo. During successive larval stages it progressively acquires all the specialized structures and shape of the adult foregut. In contrast, the STNS is morphologically recognizable at early embryonic stages. In all recorded stages the STNS spontaneously expresses rhythmic motor activity. During development, this activity is progressively restructured, beginning with a single rhythmic motor pattern in the embryo where all the stomodeal muscles are strongly coordinated. In subsequent stages, however, this single pattern is progressively subdivided to give rise eventually to the three discrete rhythmic motor patterns characteristic of the adult STNS. Our data suggest that rather than a dismantling of redundant embryonic and larval neural networks, the different adult networks emerge as a progressive partitioning of discrete circuits from a single embryonic network.

Animals↗
PubMed1994

Dynamic construction of a neural network from multiple pattern generators in the lobster stomatogastric nervous system.

In the stomatogastric nervous system (STNS) of the lobster Homarus gammarus, the rhythmic discharge of a pair of identified modulatory neurons (PS cells) is able to construct de novo a functional network from neurons otherwise belonging to other functional networks. The PS interneurons are electrically coupled and possess endogenous oscillatory properties that can be activated synaptically by stimulation of an identified sensory pathway. PS neurons themselves project synaptically onto the three major neural networks (esophageal, gastric mill, and pyloric) of the STNS. When a PS is rhythmically active in vitro, either spontaneously (rarely) or in response to direct stimulation, it dramatically restructures the otherwise independent activity patterns of all three target networks. This functional reconfiguration elicited by a single cell does not rely on changes in neuronal allegiance to pre-existing circuits, or on a simple merger of these different circuits. Rather, PS is responsible for the creation of an entirely new motor rhythm in that, via its widespread synaptic connections, the interneuron is able to subjugate the ongoing activity of the three STNS circuits and selectively appropriate individual elements to its own intrinsic rhythm. In addition, PS excites motor neurons that innervate dilator muscles of a valve situated between the esophagus and the stomach. The reorganization of the regional foregut motor rhythms by the interneuron is therefore coordinated to the opening of this valve, which itself carries sensory receptors that have been found to activate bursting in PS. Our data suggest that the role of PS in massively restructuring stomatogastric output is to generate a unique motor pattern appropriate for swallowing-like behavior. In a wider context, moreover, the results demonstrate that a neural network may not exist as a predefined entity within the CNS, but may be dynamically assembled according to changing behavioral circumstances.

Animals↗
PubMed1992

Multiple axonal spike initiation zones in a motor neuron: serotonin activation.

The lateral gastric (LG) motor neuron of the stomatogastric nervous system of the crab Cancer borealis has a large soma in the stomatogastric ganglion (STG). The LG motor neuron makes inhibitory synaptic connections within the neuropil of the STG, and also projects to the periphery, where it innervates a series of muscles that control the movements of the lateral teeth of the gastric mill. The LG motor neuron has a spike initiation zone close to its neuropilar integrative regions, from which spikes propagate orthodromically to the muscles. Additionally, under certain conditions, the LG neuron can initiate spikes at peripheral axonal sites that can be 0.5-2.0 cm from the STG. Peripherally initiated spikes propagate antidromically into the STG and also propagate to the muscle. The peripheral spike initiation zones are often active in combined preparations in which the muscles are left attached. When the muscles are removed, depolarization of the LG soma together with 5-HT applied to the motor nerve also evokes peripheral spike initiation. At a given 5-HT concentration, the duration of the trains of antidromic spikes can be controlled by current injection into the soma, suggesting the presence of a slow voltage-dependent conductance in the LG axon. The antidromic spikes contribute to lengthening of the duration of contraction in some of the muscles innervated by the LG, but do not evoke IPSPs onto LG follower neurons. Thus, the LG neuron can send different signals to its peripheral and central targets.

Animals↗
PubMed1991

A method for the determination of projection areas of GABA immunoreactive neurons in the invertebrate nervous system.

Axonal transport of metallic salts (nickel or cobalt chloride) has been widely used for the anatomical mapping of neural pathways. We show here that when nickel is introduced into GABAergic neurons it completely eliminates GABA immunolabelling. We have used this property to determine the axonal projections of GABAergic neurons in the stomatogastric system of Crustacea. For example, following nickel backfills from either cut axons or from terminals, GABA immunostaining labels only those GABA-immunoreactive neurons which had not been retrogradely labelled with nickel and hence did not project in the cut nerve or to the neuropile uptake site. By comparing such immunolabelled preparations with those not pretreated with nickel the projection patterns of all the GABA immunoreactive neurons in a given system can be revealed. This effect of nickel appears to be selective for GABA immunostaining, insofar as it does not interfere with the immunodetection of either the peptide proctolin or a FMRFamide-like peptide. This method may prove to be a useful tool for analyzing GABAergic neuronal pathways in the nervous systems of invertebrates.

Animals↗
PubMed1991

Neurons that form multiple pattern generators: identification and multiple activity patterns of gastric/pyloric neurons in the crab stomatogastric system.

1. The stomatogastric ganglion (STG) of decapod crustaceans has been characterized by its production of two motor patterns, the gastric mill rhythm and the pyloric rhythm. The period of the gastric rhythm is typically 5-10 s, whereas the period of the pyloric rhythm is approximately 1 s. 2. In the STG of the crab, Cancer borealis, we find routinely that many motor neurons are active in time with both the pyloric and gastric rhythms. We rigorously identified the motor neurons according to the muscles they innervate. Some neurons usually classified as members of the pyloric network can be active in time with the gastric rhythm. All of the gastric motor neurons except the dorsal gastric (DG) neuron can generate pyloric-timed firing patterns. 3. Two motor neurons innervate muscles found in several different regions of the stomach. The inferior cardiac (IC) neuron, usually considered part of the pyloric network, innervates cardiac sac, gastric mill, and pyloric muscles. The lateral posterior gastric (LPG) neurons innervate muscles of both the gastric mill and the pyloric chamber. 4. These data show that the gastric and pyloric networks in the crab are not separate groups of neurons that independently generate two different rhythmic behaviors. Rather, these neurons together provide a synaptically connected pool of neurons from which many different pattern-generating circuits can be assembled, under different physiological conditions.

Action Potentials↗
PubMed1991

Matching neural and muscle oscillators: control by FMRFamide-like peptides.

Stomatogastric nervous systems of the shrimp, Palaemon serratus, were stained with antisera raised against the peptide FMRFamide. FMRFamide-like immunoreactivity was found in fibers in the input nerve to the stomatogastric ganglion (STG), in several STG somata, in dense neuropil in the STG, in the motor nerves that innervate the dilator muscles of the pyloric region, but not in the pyloric dilator (PD) motor neurons. FMRFamide and several FMRFamide-like peptides elicited sequences of rhythmic depolarizations and contractions of the pyloric dilator muscle. As peptide concentrations were increased, a discrete threshold for contraction was found, above which contractions were initiated with a decreasing latency in an all-or-none fashion. Muscles stopped rhythmically contracting after many seconds to several minutes of activity; the duration of spontaneous oscillatory activity in peptide was proportional to the concentration of applied peptide. In the absence of peptide, each motor neuron discharge evoked small graded muscle contractions. During peptide-induced oscillations, motor neuron activity did not always entrain muscle oscillations. After spontaneous oscillations had stopped, when the motor neurons were stimulated in the presence of the peptide, each motor neuron burst evoked large amplitude contractions as a result of the peptide-induced regenerative properties of the muscle membrane.

Animals↗
PubMed1990

Identification of all GABA-immunoreactive neurons projecting to the lobster stomatogastric ganglion.

The stomatogastric ganglion of lobsters (Homarus or Jasus) contains a large number of gamma-aminobutyric acid-immunoreactive processes originating from ten fibres in the single input nerve, the stomatogastric nerve. The cell bodies and axonal pathways of these ten fibres have been identified using gamma-aminobutyric acid immunohistochemistry in combination with Lucifer Yellow staining (double labelling) and nickel chloride backfilling (selective gamma-aminobutyric acid immunoinhibition). It is shown that eight gamma-aminobutyric acid-immunoreactive neurons project to the stomatogastric ganglion: gamma-aminobutyric acid neurons 1 and 2, found posterior to the oesophageal ganglion, entering the stomatogastric nerve via the oesophageal nerve as well as sending an axonal branch into each superior oesophageal nerve; gamma-aminobutyric acid neurons 3 and 4, found anterior to the oesophageal ganglion, each sending an axonal branch into each inferior oesophageal nerve to reach the stomatogastric nerve via the commissural ganglion and the superior oesophageal nerve; and gamma-aminobutyric acid neurons 5 and 6, found in each commissural ganglion, projecting into the stomatogastric nerve via the inferior oesophageal nerve, the oesophageal ganglion and the oesophageal nerve. These gamma-aminobutyric acid-immunoreactive neurons were also characterized by electrophysiological methods coupled with Lucifer Yellow labelling, and their picrotoxin-sensitive effects on several stomatogastric ganglion neurons were demonstrated. The present results provide a firm basis for further studies concerning the physiological significance of one class of neurochemically-defined input neurons to stomatogastric ganglion networks.

Animals↗