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Barnaby W, Dorman Barclay HE, Nagarkar A, Perkins M, Teicher G, Trapani JG, Downes GB. GABAA α subunit control of hyperactive behavior in developing zebrafish. Genetics 2022; 220:6519832. [PMID: 35106556 PMCID: PMC8982038 DOI: 10.1093/genetics/iyac011] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/14/2021] [Accepted: 01/03/2022] [Indexed: 02/04/2023] Open
Abstract
GABAA receptors mediate rapid responses to the neurotransmitter gamma-aminobutyric acid and are robust regulators of the brain and spinal cord neural networks that control locomotor behaviors, such as walking and swimming. In developing zebrafish, gross pharmacological blockade of these receptors causes hyperactive swimming, which is also a feature of many zebrafish epilepsy models. Although GABAA receptors are important to control locomotor behavior, the large number of subunits and homeostatic compensatory mechanisms have challenged efforts to determine subunit-selective roles. To address this issue, we mutated each of the 8 zebrafish GABAA α subunit genes individually and in pairs using a CRISPR-Cas9 somatic inactivation approach and, then, we examined the swimming behavior of the mutants at 2 developmental stages, 48 and 96 h postfertilization. We found that disrupting the expression of specific pairs of subunits resulted in different abnormalities in swimming behavior at 48 h postfertilization. Mutation of α4 and α5 selectively resulted in longer duration swimming episodes, mutations in α3 and α4 selectively caused excess, large-amplitude body flexions (C-bends), and mutation of α3 and α5 resulted in increases in both of these measures of hyperactivity. At 96 h postfertilization, hyperactive phenotypes were nearly absent, suggesting that homeostatic compensation was able to overcome the disruption of even multiple subunits. Taken together, our results identify subunit-selective roles for GABAA α3, α4, and α5 in regulating locomotion. Given that these subunits exhibit spatially restricted expression patterns, these results provide a foundation to identify neurons and GABAergic networks that control discrete aspects of locomotor behavior.
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Affiliation(s)
- Wayne Barnaby
- Neuroscience and Behavior Graduate Program, University of Massachusetts Amherst, Amherst, MA 01003, USA,Biology Department, University of Massachusetts Amherst, Amherst, MA 01003, USA
| | | | - Akanksha Nagarkar
- Biology Department, University of Massachusetts Amherst, Amherst, MA 01003, USA
| | - Matthew Perkins
- Biology Department and Neuroscience Program, Amherst College, Amherst, MA 01002, USA
| | - Gregory Teicher
- Molecular and Cellular Biology Program, University of Massachusetts Amherst, Amherst, MA 01003, USA,Biology Department, University of Massachusetts Amherst, Amherst, MA 01003, USA
| | - Josef G Trapani
- Neuroscience and Behavior Graduate Program, University of Massachusetts Amherst, Amherst, MA 01003, USA,Biology Department and Neuroscience Program, Amherst College, Amherst, MA 01002, USA
| | - Gerald B Downes
- Neuroscience and Behavior Graduate Program, University of Massachusetts Amherst, Amherst, MA 01003, USA,Molecular and Cellular Biology Program, University of Massachusetts Amherst, Amherst, MA 01003, USA,Biology Department, University of Massachusetts Amherst, Amherst, MA 01003, USA,Corresponding author: Biology Department, Neuroscience and Behavior Graduate Program, and Molecular and Cellular Biology Graduate Program, 611 North Pleasant St., Morrill Science Center, Building 4 North, Amherst, MA 01003, USA.
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Monesson-Olson B, McClain JJ, Case AE, Dorman HE, Turkewitz DR, Steiner AB, Downes GB. Expression of the eight GABAA receptor α subunits in the developing zebrafish central nervous system. PLoS One 2018; 13:e0196083. [PMID: 29702678 PMCID: PMC5922542 DOI: 10.1371/journal.pone.0196083] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/11/2018] [Accepted: 04/05/2018] [Indexed: 11/26/2022] Open
Abstract
GABA is a robust regulator of both developing and mature neural networks. It exerts many of its effects through GABAA receptors, which are heteropentamers assembled from a large array of subunits encoded by distinct genes. In mammals, there are 19 different GABAA subunit types, which are divided into the α, β, γ, δ, ε, π, θ and ρ subfamilies. The immense diversity of GABAA receptors is not fully understood. However, it is known that specific isoforms, with their distinct biophysical properties and expression profiles, tune responses to GABA. Although larval zebrafish are well-established as a model system for neural circuit analysis, little is known about GABAA receptors diversity and expression in this system. Here, using database analysis, we show that the zebrafish genome contains at least 23 subunits. All but the mammalian θ and ε subunits have at least one zebrafish ortholog, while five mammalian GABAA receptor subunits have two zebrafish orthologs. Zebrafish contain one subunit, β4, which does not have a clear mammalian ortholog. Similar to mammalian GABAA receptors, the zebrafish α subfamily is the largest and most diverse of the subfamilies. In zebrafish there are eight α subunits, and RNA in situ hybridization across early zebrafish development revealed that they demonstrate distinct patterns of expression in the brain, spinal cord, and retina. Some subunits were very broadly distributed, whereas others were restricted to small populations of cells. Subunit-specific expression patterns in zebrafish resembled were those found in frogs and rodents, which suggests that the roles of different GABAA receptor isoforms are largely conserved among vertebrates. This study provides a platform to examine isoform specific roles of GABAA receptors within zebrafish neural circuits and it highlights the potential of this system to better understand the remarkable heterogeneity of GABAA receptors.
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Affiliation(s)
- Bryan Monesson-Olson
- Neuroscience and Behavior Graduate Program, University of Massachusetts, Amherst, MA, United States of America
- Biology Department, University of Massachusetts, Amherst, MA, United States of America
| | - Jon J. McClain
- Biology Department, University of Massachusetts, Amherst, MA, United States of America
| | - Abigail E. Case
- Biology Department, University of Massachusetts, Amherst, MA, United States of America
| | - Hanna E. Dorman
- Biology Department, University of Massachusetts, Amherst, MA, United States of America
| | - Daniel R. Turkewitz
- Department of Biology and Health Sciences, Pace University, Pleasantville, NY, United States of America
| | - Aaron B. Steiner
- Department of Biology and Health Sciences, Pace University, Pleasantville, NY, United States of America
| | - Gerald B. Downes
- Neuroscience and Behavior Graduate Program, University of Massachusetts, Amherst, MA, United States of America
- Biology Department, University of Massachusetts, Amherst, MA, United States of America
- * E-mail:
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3
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Abstract
The Mauthner cell (M-cell) is one of the few identifiable neurons in the vertebrate central nervous system. The ability to locate the M-cell, along with its inputs and outputs, has resulted in important findings in diverse areas of neurobiology including the molecular biology of neurons, synaptic and systems physiology, behavior, development, and neuroethology. The review provides a brief overview of the M-cell and then focuses on recent studies applying state-of-the-art techniques to address new issues and revisit old ones. One advantage of this preparation is the ability to conduct multidisciplinary studies from the subcellular to behavioral levels. For example, studies of activity-dependent changes in the strength of mixed electrotonic and chemical synapses on the M-cell's lateral dendrite in vivo have been correlated with changes in the probability of eliciting a fast startle response initiated by the M-cell and its associated circuits. Similarly, it is now possible to image the activity of the M-cell and its homologs while observing motor behavior in zebrafish larvae. These approaches will provide direct tests of the functional properties of complex neural networks. Moreover, molecular mechanisms that underlie neuronal development can be tested directly with this neuron and its segmental homologs, because these cells occur in singular pairs at defined locations. Finally, after spinal cord injury, the M-cell's axon regenerates, but does not follow its original course, and the startle response gradually recovers. The accessibility of the M-cell system offers the promise that strategies employed in restoring the function of a neural network will be revealed. Thus, we anticipate that the M-cell system will become a favored preparation for multidisciplinary studies on the neuronal basis of behavior and the recovery of behavior after injury. NEUROSCIENTIST 6:26-38, 2000
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Affiliation(s)
- Steven J. Zottoli
- Department of Biology Williams College Williamstown, Massachusetts (SJZ) Department of Neurobiology and Anatomy MCP Hahnemann University Philadelphia, Pennsylvania (DSF
| | - Donald S. Faber
- Department of Biology Williams College Williamstown, Massachusetts (SJZ) Department of Neurobiology and Anatomy MCP Hahnemann University Philadelphia, Pennsylvania (DSF
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Anadón R, Rodríguez-Moldes I, Adrio F. Glycine-immunoreactive neurons in the brain of a shark (Scyliorhinus caniculaL.). J Comp Neurol 2013; 521:3057-82. [DOI: 10.1002/cne.23332] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/19/2012] [Revised: 03/07/2013] [Accepted: 03/13/2013] [Indexed: 01/12/2023]
Affiliation(s)
- Ramón Anadón
- Department of Cell Biology and Ecology; University of Santiago de Compostela; 15782 Santiago de; Compostela; Spain
| | - Isabel Rodríguez-Moldes
- Department of Cell Biology and Ecology; University of Santiago de Compostela; 15782 Santiago de; Compostela; Spain
| | - Fátima Adrio
- Department of Cell Biology and Ecology; University of Santiago de Compostela; 15782 Santiago de; Compostela; Spain
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Park AY, Chae YS, Lee SH, Kaang BK, Lee S. Presynaptic structure of Aplysia single live neuron by atomic force and confocal laser scanning microscope. J Phys Chem B 2013; 117:4779-88. [PMID: 23594081 DOI: 10.1021/jp3115308] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The structural and functional plasticity of Aplysia mechanosensory presynaptic neurons has been studied in relation with the mechanism underlying learning and memory. Long-term facilitation (LTF), which is a well-known cellular model for long-term memory in Aplysia, is accompanied by new synaptic structural growth or change. We developed a combined atomic force microscope and confocal laser scanning microscope (AFM-CLSM) system integrated with a MATLAB routine for image processing to concurrently obtain high-resolution 3-dimensional (3D) outer-surface morphological images and 3D interior fluorescence images. With our combined AFM-CLSM system, volumetric changes in the presynaptic structures (varicosities) of Aplysia live sensory-motor neuron cocultures were observed. The spatial distribution of synaptic vesicle molecules in the preexisting varicosities was monitored together with a volumetric change in the varicosities. Our combined AFM-CLSM system is successfully adapted for measuring learning-related structural changes and the movement of synaptic molecules in the single live neuron through interaction force and fluorescence imaging.
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Affiliation(s)
- Aee-Young Park
- Molecular Electronics and NanoStructures Laboratory, School of Chemistry, NS60, Seoul National University, Seoul 151-747, Republic of Korea
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Barreiro-Iglesias A, Mysiak KS, Adrio F, Rodicio MC, Becker CG, Becker T, Anadón R. Distribution of glycinergic neurons in the brain of glycine transporter-2 transgenic Tg(glyt2:Gfp) adult zebrafish: Relationship to brain-spinal descending systems. J Comp Neurol 2012; 521:389-425. [DOI: 10.1002/cne.23179] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/06/2011] [Revised: 01/25/2012] [Accepted: 06/21/2012] [Indexed: 12/19/2022]
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Rice C, Ghorai JK, Zalewski K, Weber DN. Developmental lead exposure causes startle response deficits in zebrafish. AQUATIC TOXICOLOGY (AMSTERDAM, NETHERLANDS) 2011; 105:600-8. [PMID: 21955963 PMCID: PMC3207002 DOI: 10.1016/j.aquatox.2011.08.014] [Citation(s) in RCA: 45] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/02/2011] [Revised: 08/09/2011] [Accepted: 08/23/2011] [Indexed: 05/02/2023]
Abstract
Lead (Pb(2+)) exposure continues to be an important concern for fish populations. Research is required to assess the long-term behavioral effects of low-level concentrations of Pb(2+) and the physiological mechanisms that control those behaviors. Newly fertilized zebrafish embryos (<2h post fertilization; hpf) were exposed to one of three concentrations of lead (as PbCl(2)): 0, 10, or 30 nM until 24 hpf. (1) Response to a mechanosensory stimulus: Individual larvae (168 hpf) were tested for response to a directional, mechanical stimulus. The tap frequency was adjusted to either 1 or 4 taps/s. Startle response was recorded at 1000 fps. Larvae responded in a concentration-dependent pattern for latency to reaction, maximum turn velocity, time to reach V(max) and escape time. With increasing exposure concentrations, a larger number of larvae failed to respond to even the initial tap and, for those that did respond, ceased responding earlier than control larvae. These differences were more pronounced at a frequency of 4 taps/s. (2) Response to a visual stimulus: Fish, exposed as embryos (2-24 hpf) to Pb(2+) (0-10 μM) were tested as adults under low light conditions (≈ 60 μW/m(2)) for visual responses to a rotating black bar. Visual responses were significantly degraded at Pb(2+) concentrations of 30 nM. These data suggest that zebrafish are viable models for short- and long-term sensorimotor deficits induced by acute, low-level developmental Pb(2+) exposures.
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Affiliation(s)
- Clinton Rice
- Department of Biology, American University, Washington, DC
| | - Jugal K. Ghorai
- Department of Mathematical Sciences, University of Wisconsin-Milwaukee, Milwaukee, WI
| | - Kathryn Zalewski
- Department of Human Movement Sciences, University of Wisconsin-Milwaukee, Milwaukee, WI
| | - Daniel N. Weber
- Children’s Environmental Health Sciences Center, University of Wisconsin-Milwaukee, Milwaukee, WI
- To Whom Correspondence should be Addressed: Children’s Environmental Health Sciences Center, University of Wisconsin-Milwaukee, 600 E. Greenfield Ave., Milwaukee, WI 53204,
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Ogino K, Ramsden SL, Keib N, Schwarz G, Harvey RJ, Hirata H. Duplicated gephyrin genes showing distinct tissue distribution and alternative splicing patterns mediate molybdenum cofactor biosynthesis, glycine receptor clustering, and escape behavior in zebrafish. J Biol Chem 2010; 286:806-17. [PMID: 20843816 DOI: 10.1074/jbc.m110.125500] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Gephyrin mediates the postsynaptic clustering of glycine receptors (GlyRs) and GABA(A) receptors at inhibitory synapses and molybdenum-dependent enzyme (molybdoenzyme) activity in non-neuronal tissues. Gephyrin knock-out mice show a phenotype resembling both defective glycinergic transmission and molybdenum cofactor (Moco) deficiency and die within 1 day of birth due to starvation and dyspnea resulting from deficits in motor and respiratory networks, respectively. To address whether gephyrin function is conserved among vertebrates and whether gephyrin deficiency affects molybdoenzyme activity and motor development, we cloned and characterized zebrafish gephyrin genes. We report here that zebrafish have two gephyrin genes, gphna and gphnb. The former is expressed in all tissues and has both C3 and C4 cassette exons, and the latter is expressed predominantly in the brain and spinal cord and harbors only C4 cassette exons. We confirmed that all of the gphna and gphnb splicing isoforms have Moco synthetic activity. Antisense morpholino knockdown of either gphna or gphnb alone did not disturb synaptic clusters of GlyRs in the spinal cord and did not affect touch-evoked escape behaviors. However, on knockdown of both gphna and gphnb, embryos showed impairments in GlyR clustering in the spinal cord and, as a consequence, demonstrated touch-evoked startle response behavior by contracting antagonistic muscles simultaneously, instead of displaying early coiling and late swimming behaviors, which are executed by side-to-side muscle contractions. These data indicate that duplicated gephyrin genes mediate Moco biosynthesis and control postsynaptic clustering of GlyRs, thereby mediating key escape behaviors in zebrafish.
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Affiliation(s)
- Kazutoyo Ogino
- Center for Frontier Research, National Institute of Genetics, 1111 Yata, Mishima 411-8540, Japan
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Reynolds A, Brustein E, Liao M, Mercado A, Babilonia E, Mount DB, Drapeau P. Neurogenic role of the depolarizing chloride gradient revealed by global overexpression of KCC2 from the onset of development. J Neurosci 2008; 28:1588-97. [PMID: 18272680 PMCID: PMC6671553 DOI: 10.1523/jneurosci.3791-07.2008] [Citation(s) in RCA: 73] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2007] [Revised: 11/30/2007] [Accepted: 12/29/2007] [Indexed: 11/21/2022] Open
Abstract
GABA- and glycine-induced depolarization is thought to provide important developmental signals, but the role of the underlying chloride gradient has not been examined from the onset of development. We therefore overexpressed globally the potassium-chloride cotransporter 2 (KCC2) in newly fertilized zebrafish embryos to reverse the chloride gradient. This rendered glycine hyperpolarizing in all neurons, tested at the time that motor behaviors (but not native KCC2) first appear. KCC2 overexpression resulted in fewer mature spontaneously active spinal neurons, more immature silent neurons, and disrupted motor activity. We observed fewer motoneurons and interneurons, a reduction in the elaboration of axonal tracts, and smaller brains and spinal cords. However, we observed no increased apoptosis and a normal complement of sensory neurons, glia, and progenitors. These results suggest that chloride-mediated excitation plays a crucial role in promoting neurogenesis from the earliest stages of embryonic development.
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Affiliation(s)
- Annie Reynolds
- Department of Pathology and Cell Biology and Le Groupe de Recherche sur le Système Nerveux Central, Université de Montréal, Montréal, Québec, Canada H3T 1J4
- Centre for Research in Neuroscience, Research Institute of the McGill University Health Centre, Montréal, Québec, Canada H3G 1A4
- Department of Biology, McGill University, Montréal, Québec, Canada H3A 1B1
| | - Edna Brustein
- Department of Pathology and Cell Biology and Le Groupe de Recherche sur le Système Nerveux Central, Université de Montréal, Montréal, Québec, Canada H3T 1J4
- Centre for Research in Neuroscience, Research Institute of the McGill University Health Centre, Montréal, Québec, Canada H3G 1A4
| | - Meijiang Liao
- Department of Pathology and Cell Biology and Le Groupe de Recherche sur le Système Nerveux Central, Université de Montréal, Montréal, Québec, Canada H3T 1J4
- Centre for Research in Neuroscience, Research Institute of the McGill University Health Centre, Montréal, Québec, Canada H3G 1A4
| | - Adriana Mercado
- Renal Division, Brigham and Women's Hospital, Harvard Institutes of Medicine, Boston, Massachusetts 02115, and
| | - Elisa Babilonia
- Renal Division, Brigham and Women's Hospital, Harvard Institutes of Medicine, Boston, Massachusetts 02115, and
| | - David B. Mount
- Renal Division, Brigham and Women's Hospital, Harvard Institutes of Medicine, Boston, Massachusetts 02115, and
- Division of General Internal Medicine, Veterans Affairs Boston Healthcare System, Harvard Medical School, West Roxbury, Massachusetts 02132
| | - Pierre Drapeau
- Department of Pathology and Cell Biology and Le Groupe de Recherche sur le Système Nerveux Central, Université de Montréal, Montréal, Québec, Canada H3T 1J4
- Centre for Research in Neuroscience, Research Institute of the McGill University Health Centre, Montréal, Québec, Canada H3G 1A4
- Department of Biology, McGill University, Montréal, Québec, Canada H3A 1B1
- Department of Neurology and Neurosurgery, McGill University, Montréal, Québec, Canada H3A 2B4
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Stehr CM, Linbo TL, Incardona JP, Scholz NL. The Developmental Neurotoxicity of Fipronil: Notochord Degeneration and Locomotor Defects in Zebrafish Embryos and Larvae. Toxicol Sci 2006; 92:270-8. [PMID: 16611622 DOI: 10.1093/toxsci/kfj185] [Citation(s) in RCA: 138] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Fipronil is a phenylpyrazole insecticide designed to selectively inhibit insect gamma-aminobutyric acid (GABA) receptors. Although fipronil is often used in or near aquatic environments, few studies have assessed the effects of this neurotoxicant on aquatic vertebrates at sensitive life stages. We explored the toxicological effects of fipronil on embryos and larvae using the zebrafish (Danio rerio) experimental model system. Embryos exposed to fipronil at nominal concentrations at or above 0.7 microM (333 mug/l) displayed notochord degeneration, shortening along the rostral-caudal body axis, and ineffective tail flips and uncoordinated muscle contractions along the body axis in response to touch. This phenotype closely resembles zebrafish locomotor mutants of the accordion class and is consistent with loss of reciprocal inhibitory neurotransmission by glycinergic commissural interneurons in the spinal cord. Consistent with the hypothesis that notochord degeneration may be due to abnormal mechanical stress from muscle tetany, the expression patterns of gene and protein markers specific to notochord development were unaffected by fipronil. Moreover, the degenerative effects of fipronil (1.1 microM) were reversed by coexposure to the sodium channel blocker MS-222 (0.6mM). The notochord effects of fipronil were phenocopied by exposure to 70 microM strychnine, a glycinergic receptor antagonist. In contrast, exposure to gabazine, a potent vertebrate GABA(A) antagonist, resulted in a hyperactive touch response but did not cause notochord degeneration. Although specifically developed to target insect GABA receptors with low vertebrate toxicity, our results suggest that fipronil impairs the development of spinal locomotor pathways in fish by inhibiting a structurally related glycine receptor subtype. This represents an unanticipated and potentially novel mechanism for fipronil toxicity in vertebrates.
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Affiliation(s)
- Carla M Stehr
- Ecotoxicology and Environmental Fish Health Program, Northwest Fisheries Science Center, National Oceanic and Atmospheric Administration Fisheries, 2725 Montlake Boulevard East, Seattle, WA 98112, USA
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11
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Brustein E, Drapeau P. Serotoninergic modulation of chloride homeostasis during maturation of the locomotor network in zebrafish. J Neurosci 2006; 25:10607-16. [PMID: 16291933 PMCID: PMC6725851 DOI: 10.1523/jneurosci.2017-05.2005] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
During development, neural networks progress through important functional changes such as the generation of spontaneous activity, the expression of a depolarizing chloride gradient, and the appearance of neuromodulation. Little is known about how these processes are integrated to yield mature behaviors. We showed previously that, during the maturation of the locomotor network of the zebrafish, endogenous serotonin (5HT) increased motor activity by reducing intervals of inactivity, without affecting the active swim periods that are the target of 5HT in other and more mature preparations. Because membrane properties were constant during the rest intervals, we examined here whether 5HT modulates chloride homeostasis. We compared the effects of blocking (inward) chloride cotransport with bumetanide to the effects of 5HT and its antagonists, both behaviorally by video imaging and cellularly by whole-cell and gramicidin-perforated patch recordings. Bumetanide mimicked the effects of 5HT antagonists, by prolonging rest intervals without affecting the properties of swim episodes (duration; frequency; extent of depolarization) either behaviorally or during fictive swimming. Furthermore, bumetanide and 5HT antagonists suppressed the amplitude of depolarizing responses evoked by ionophoresis of glycine onto spinal neurons in the presence of tetrodotoxin and transiently suppressed the amplitude of responses to glycine measured after fictive swimming. The effects of bumetanide contrasted with and occluded the effects of 5HT. We suggest that, during development, endogenous 5HT modulates chloride homeostasis during the quiescent intervals and thereby offsets the long periods of quiescence commonly observed in developing networks to allow expression of sustained and behaviorally relevant activity.
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Affiliation(s)
- Edna Brustein
- Center for Research in Neuroscience, Department of Neurology and Neurosurgery, McGill University, Montréal, Quebec, Canada H3G 1A4
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12
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Abstract
The Mauthner (M) cell is a critical element in a vital escape "reflex" triggered by abrupt or threatening events. Its properties at the molecular and synaptic levels, their various forms of plasticity, and the design of its networks, are all well adapted for this survival function. They guarantee that this behavior is appropriately unilateral, variable, and unpredictable. The M cell sets the behavioral threshold, and, acting in concert with other elements of the brainstem escape network, determines when, where, and how the escape is executed.
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Affiliation(s)
- Henri Korn
- Laboratoire Recepteurs et Cognition, CNRS, URA 2182, Institut Pasteur, 25, rue du Docteur-Roux, 75724 Paris Cedex 15, France
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Müller UK, van Leeuwen JL. Swimming of larval zebrafish: ontogeny of body waves and implications for locomotory development. ACTA ACUST UNITED AC 2004; 207:853-68. [PMID: 14747416 DOI: 10.1242/jeb.00821] [Citation(s) in RCA: 155] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Fish larvae, like most adult fish, undulate their bodies to propel themselves. A detailed kinematic study of the larval body wave is a prerequisite to formulate a set of functional requirements that the locomotor system must fulfil to generate the observed swimming kinematics. Lateral displacement and curvature profiles were obtained for zebrafish (Danio rerio) larvae at 2-21 days post-fertilisation for three swimming behaviours (cyclic swimming, slow starts and fast startle responses) using high-speed video. During cyclic swimming, fish larvae maintain tail beat frequencies of up to 100 Hz. The corresponding longitudinal strains, estimated from the peak curvatures of the midline, reach up to 0.19 in superficial tissue. The strain rate can reach 120 s(-1). The wave of curvature travels along the body at a near-constant rate. Posterior to the stiff head, body-length-specific curvature is high and rises gently along the entire trunk to a maximum value of 6. Burst-and-coast swimming generates similar peak curvatures to cyclic swimming, but curvature rises more steeply from head to tail. Fish larvae exhibit phase shifts of 57-63 degrees between the wave of lateral displacement and the wave of curvature, resulting in a 1:1.2 ratio of body wave length to curvature wave length. During C-starts, muscle strain can reach 0.19 and superficial longitudinal strain rates approach 30 s(-1). Fish larvae do not initiate their escape response with a standing wave of curvature, although their C-starts approach a standing wave as the larvae grow older. The performance demands derived from swimming kinematics suggest that larval axial muscles have very short contraction cycles (10 ms), experience considerable strains (up to 0.2) and strain rates (up to 30 s(-1) in white muscle fibres) yet are able to power swimming for several seconds.
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Affiliation(s)
- Ulrike K Müller
- Wageningen University, Experimental Zoology Group, Marijkeweg 40, 6709 PG Wageningen, The Netherlands.
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14
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Rigo JM, Badiu CI, Legendre P. Heterogeneity of postsynaptic receptor occupancy fluctuations among glycinergic inhibitory synapses in the zebrafish hindbrain. J Physiol 2003; 553:819-32. [PMID: 14500774 PMCID: PMC2343629 DOI: 10.1113/jphysiol.2003.049577] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
The amplitude of glycinergic miniature inhibitory postsynaptic currents (mIPSCs) varies considerably in neurons recorded in the isolated hindbrain of 50-h-old zebrafish larvae. At this age, glycinergic synapses are functionally mature. In order to measure the occupancy level of postsynaptic glycine receptors (GlyRs) and to determine the pre- and/or postsynaptic origin of its variability, we analysed mIPSCs within bursts evoked by alpha-latrotoxin (0.1-1 nM). Two types of burst were observed according to their mIPSC frequencies: 'slow' bursts with clearly spaced mIPSCs and 'fast' bursts characterised by superimposed events. Non-stationary noise analysis of mIPSCs in some 'slow' bursts recorded in the presence or in the absence of Ca2+ denoted that mIPSC amplitude variance did not depend on the quantity of neurotransmitters released (presynaptic origin), but rather on intrinsic stochastic behaviour of the same group of GlyRs (postsynaptic origin). In these bursts, the open probability measured at the peak of the mIPSCs was close to 0.5 while the maximum open probability is close to 0.9 for the synaptic isoform of GlyRs (heteromeric alpha1/beta GlyRs). In 'fast' bursts with superimposed events, a correlation was found between the amplitude of mIPSCs and the basal current level measured at their onset, which could suggest that the same group of GlyRs is activated during such bursts. Altogether, our results indicate that glycine synapses can display different release modes in the presence of alpha-latrotoxin. They also indicate that, in our model, postsynaptic GlyRs cannot be saturated by the release of a single vesicle.
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Affiliation(s)
- Jean-Michel Rigo
- UMR 7102 CNRS, Université Pierre et Marie Curie, Bât B 6ème étage, 7 Quai Saint Bernard, 75252 Paris Cedex 05, France
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Drapeau P, Saint-Amant L, Buss RR, Chong M, McDearmid JR, Brustein E. Development of the locomotor network in zebrafish. Prog Neurobiol 2002; 68:85-111. [PMID: 12450489 DOI: 10.1016/s0301-0082(02)00075-8] [Citation(s) in RCA: 260] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
The zebrafish is a leading model for studies of vertebrate development and genetics. Its embryonic motor behaviors are easy to assess (e.g. for mutagenic screens), the embryos develop rapidly (hatching as larvae at 2 days) and are transparent, permitting calcium imaging and patch clamp recording in vivo. We review primarily the recent advances in understanding the cellular basis for the development of motor activities in the developing zebrafish. The motor activities are generated largely in the spinal cord and hindbrain. In the embryo these segmented structures possess a relatively small number of repeating sets of identifiable neurons. Many types of neurons as well as the two types of muscle cells have been classified based on their morphologies. Some of the molecular signals for cellular differentiation have been identified recently and mutations affecting cell development have been isolated. Embryonic motor behaviors appear in sequence and consist of an early period of transient spontaneous coiling contractions, followed by the emergence of twitching responses to touch, and later by the ability to swim. Coiling contractions are generated by an electrically coupled network of a subset of spinal neurons whereas a chemical (glutamatergic and glycinergic) synaptic drive underlies touch responses and swimming. Swimming becomes sustained in larvae once the neuromodulatory serotonergic system develops. These results indicate many similarities between developing zebrafish and other vertebrates in the properties of the synaptic drive underlying locomotion. Therefore, the zebrafish is a useful preparation for gaining new insights into the development of the neural control of vertebrate locomotion. As the types of neurons, transmitters, receptors and channels used in the locomotor network are being defined, this opens the possibility of combining cellular neurophysiology with forward and reverse molecular genetics to understand the principles of locomotor network assembly and function.
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Affiliation(s)
- Pierre Drapeau
- McGill Centre for Research in Neuroscience and Department of Biology, McGill University, Que., Montreal, Canada.
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Corner MA, van Pelt J, Wolters PS, Baker RE, Nuytinck RH. Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny. Neurosci Biobehav Rev 2002; 26:127-85. [PMID: 11856557 DOI: 10.1016/s0149-7634(01)00062-8] [Citation(s) in RCA: 109] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.
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Affiliation(s)
- M A Corner
- Academic Medical Centre, Meibergdreef 33, Netherlands Institute for Brain Research, 1105 AZ Amsterdam, The Netherlands.
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Imboden M, Devignot V, Korn H, Goblet C. Regional distribution of glycine receptor messenger RNA in the central nervous system of zebrafish. Neuroscience 2001; 103:811-30. [PMID: 11274796 DOI: 10.1016/s0306-4522(00)00576-5] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
Abstract
We report the cloning of the zebrafish beta subunit of the glycine receptor and compare the anatomical distribution of three glycine receptor subunit constituents in adult zebrafish brain (alphaZ1, alphaZ2 and betaZ) to the expression pattern of homologous receptor subunits (alpha1, alpha2 and beta) in the mammalian adult CNS. Non-radioactive hybridization was used to map the distribution of the alphaZ1, alphaZ2 and betaZ glycine receptor subunit messenger RNAs in the adult zebrafish brain. The anterior-posterior expression gradient found in adult zebrafish brain was similar to that reported in mammalian CNS. However, the glycine receptor transcripts, notably the alphaZ1 subunit, were more widely distributed in the anterior regions of the zebrafish than in the adult mammalian brain. The isoform-specific distribution pattern was less regionalized in zebrafish than in the rat mammalian CNS. Nevertheless, there was some regionalization of alphaZ1, alphaZ2 and betaZ transcripts in the diencephalic and mesencephalic nuclei where different sensory and motor centers express either alphaZ1/betaZ or alphaZ2 subunits. In contrast to the widespread distribution of the beta subunit in adult mammalian brain, alphaZ2 messenger RNA presented the widest expression territory of all three glycine receptor subunits tested. alphaZ2 messenger RNA was expressed in the absence of alphaZ1 and betaZ messenger RNA in the outer nuclear layer of the retina, the inferior olive and the raphe of the medulla oblongata, as well as in the nucleus of Cajal of the medulla spinalis. In contrast, an identified central neuron of the reticular formation, the Mauthner cell, expresses all three glycine receptor subunits (alphaZ1, alphaZ2 and betaZ). This report extends the already described glycine receptor expression in the vertebrate CNS and confirms the importance of glycine-mediated inhibition in spinal cord and brainstem.
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Affiliation(s)
- M Imboden
- Laboratoire de Biologie Cellulaire et Moléculaire du Neurone, INSERM U261, Institut Pasteur, 25, Rue du Dr Roux, F-75724, Paris Cedex 15, France.
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18
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Abstract
In vivo recordings from Mauthner cells in adult zebrafish (Danio rerio) and goldfish (Carassius auratus) preparations with potassium chloride filled electrodes revealed a new class of long-lasting synaptic events in these cells. Their decay time constant ranged from 20 to 80ms, which is about 20 times longer than that of previously identified fast glycinergic inhibitory postsynaptic potentials in this neuron. The average time to peak of these slow events ranged from 1 to 6ms. We demonstrated that they are also inhibitory since (i) they were resistant to antagonists of the excitatory glutamatergic receptors; (ii) their amplitude was increased following chloride loading of the Mauthner cell; (iii) their reversal potential was the same as that of fast, glycinergic inhibitory postsynaptic potentials; and (iv) they produced an inhibitory shunt of the cell's membrane resistance. Furthermore, as with the fast inhibitory postsynaptic potentials, the decay time of the slow events is voltage dependent, increasing when the Mauthner cell is depolarized. However, these inhibitory postsynaptic potentials had a different pharmacological profile to the fast glycinergic ones. That is, they persisted in the presence of strychnine at doses that abolished the fast ones and they were more sensitive to bicuculline. These data are compatible with the notion that these inhibitory postsynaptic potentials are mediated by activation of a different inhibitory receptor type, and may be GABAergic. In addition, the decay time constant of the fast inhibitory postsynaptic current was shorter than the first of the two components that contribute to the bi-exponential decay reported previously for miniature inhibitory postsynaptic currents in Mauthner cells of larval zebrafish. This suggests developmental modifications and/or a switch in the assembly of glycine receptor subtypes. While amplitude distributions of the fast miniature inhibitory postsynaptic potentials recorded in the presence of tetrodotoxin generally could fit with a single Gaussian function, the amplitude histograms of slow miniature events were skewed, often with multiple nearly equally spaced peaks, consistent with the synchronous release of several quantal units. These previously undescribed slow unitary inhibitory postsynaptic potentials contribute to inhibitory synaptic noise recorded in the Mauthner cells. Specifically, autocorrelation analysis revealed gamma-like rhythms (30-80Hz) in each of two phases, characterized as "noisy" and "quiet", and dominated by the fast and slow inhibitory postsynaptic potentials, respectively. The major frequencies of these two states were significantly different (i.e. around 90 and 40Hz, respectively), suggesting that the fast and slow inhibitory postsynaptic potentials are derived from different inhibitory networks. Chloride-filled Mauthner cells gradually hyperpolarized in the presence of tetrodotoxin, reflecting the effect of ongoing activity in the interneurons that produce the slow events. We conclude that this new class of inhibitory postsynaptic potentials contributes to the tonic inhibition which controls the Mauthner cell's excitability. In physiological conditions, this regulatory influence is expressed as a continuous shunt of this neuron's input resistance and responsiveness to sensory inputs.
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Affiliation(s)
- K Hatta
- Biologie Cellulaire et Moléculaire du Neurone, INSERM U261, Institut Pasteur, 25, rue du Dr Roux, 75724 Cedex 15, Paris, France
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Ali DW, Drapeau P, Legendre P. Development of spontaneous glycinergic currents in the Mauthner neuron of the zebrafish embryo. J Neurophysiol 2000; 84:1726-36. [PMID: 11024065 DOI: 10.1152/jn.2000.84.4.1726] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We used whole cell and outside-out patch-clamp techniques with reticulospinal Mauthner neurons of zebrafish embryos to investigate the developmental changes in the properties of glycinergic synaptic currents in vivo from the onset of synaptogenesis. Miniature inhibitory postsynaptic currents (mIPSCs) were isolated and recorded in the presence of TTX (1 microM), kynurenic acid (1 mM), and bicuculline (10 microM) and were found to be sensitive to strychnine (1 microM). The mIPSCs were first observed in 26-29 h postfertilization (hpf) embryos at a very low frequency of approximately 0.04 Hz, which increased to approximately 0.5 Hz by 30-40 hpf, and was approximately 10 Hz in newly hatched (>50 hpf) larvae, indicating an accelerated increase in synaptic activity. At all embryonic stages, the amplitudes of the mIPSCs were variable but their means were similar ( approximately 100 pA), suggesting rapid formation of the postsynaptic matrix. The 20-80% rise times of mIPSCs in embryos were longer (0.6-1.2 ms) than in larvae (approximately 0.3 ms), likely due to slower diffusion of glycine at the younger, immature synapses. The mIPSCs decayed with biexponential (tau(off1) and tau(off2)) time courses with a half-width in 26-29 hpf embryos that was longer and more variable than in older embryos and larvae. In 26- to 29-hpf embryos, tau(off1) was approximately 15 ms and tau(off2) was approximately 60 ms, representing events of intermediate duration; but occasionally long mIPSCs were observed in some cells where tau(off1) was approximately 40 ms and tau(off2) was approximately 160 ms. In 30-40 hpf embryos, the events were faster, with tau(off1) approximately 9 ms and tau(off2) approximately 40 ms, and in larvae, events declined somewhat further to tau(off1) approximately 4 ms and tau(off2) approximately 30 ms. Point-per-point amplitude histograms of the decay of synaptic events at all stages resulted in the detection of similar single channel conductances estimated as approximately 45 pS, indicating the presence of heteromeric glycine receptors (GlyRs) from the onset of synaptogenesis. Fast-flow (1 ms) application of a saturating concentration of glycine (3-10 mM) to outside-out patches obtained at 26-29 hpf revealed GlyR currents that decayed biexponentially with time constants resembling the values found for intermediate and long mIPSCs; by 30-40 hpf, the GlyR currents resembled fast mIPSCs. These observations indicate that channel kinetics limited the mIPSC duration. Our data suggest that glycinergic mIPSCs result from the activation of a mixture of fast and slow GlyR subtypes, the properties and proportion of which determine the decay of the synaptic events in the embryos.
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Affiliation(s)
- D W Ali
- Center for Research in Neuroscience, McGill University; and Montreal General Hospital Research Institute, Montreal, Quebec H3G 1A4, Canada
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20
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Abstract
As a first step in the study of the developing motor circuitry of the embryonic zebrafish spinal cord, we obtained patch-clamp recordings in vivo from identified motoneurons in curarized embryos from the onset of the first motor behavior. At an early developmental stage in which embryos showed slow and repetitive spontaneous contractions of the trunk, motoneurons showed periodic depolarizations that triggered rhythmic bursts of action potentials with a frequency and duration that were consistent with those of the spontaneous contractions. The periodic depolarizations were blocked by tetrodotoxin or Cd(2+). Surprisingly, the contractions and periodic depolarizations were insensitive to general blockade of synaptic transmission (by elevated Mg(2+) and reduced Ca(2+), or by Co(2+)) and to selective blockade of the major neurotransmitter receptors of the mature spinal cord (acetylcholine, GABA(A), NMDA, AMPA/kainate, and glycine). The periodic depolarizations were suppressed by heptanol or by intracellular acidification, treatments that are known to uncouple gap junctions, indicating that electrotonic synapses could underlie the earliest motor behavior. A few hours later, most motoneurons already showed a new pattern of repetitive activity consisting of bursts of glycinergic synaptic events, but these were not necessary for the spontaneous contractions. Transecting the spinal cord at the hindbrain border did not affect the rhythmic activity patterns of the motoneurons. We suggest that spontaneous contractions of the zebrafish embryo are mediated by an early spinal circuit that is independent of the main neurotransmitter systems and descending hindbrain projections that are required for locomotion in the mature vertebrate spinal cord.
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Ali DW, Buss RR, Drapeau P. Properties of miniature glutamatergic EPSCs in neurons of the locomotor regions of the developing zebrafish. J Neurophysiol 2000; 83:181-91. [PMID: 10634865 DOI: 10.1152/jn.2000.83.1.181] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
As a first step in understanding the development of synaptic activation in the locomotor network of the zebrafish, we examined the properties of spontaneous, glutamatergic miniature excitatory postsynaptic currents (mEPSCs). Whole cell patch-clamp recordings were obtained from visually identified hindbrain reticulospinal neurons and spinal motoneurons of curarized zebrafish 1-5 days postfertilization (larvae hatch after the 2nd day of embryogenesis). In the presence of tetrodotoxin (TTX) and blockers of inhibitory receptors (strychnine and picrotoxin), we detected fast glutamatergic mEPSCs that were blocked by the AMPA/kainate receptor-selective antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). At positive voltages or in the absence of Mg(2+), a second, slower component of the mEPSCs was revealed that the N-methyl-D-aspartate (NMDA) receptor-selective antagonist DL-2-amino-5-phosphonovalerate (AP-5) abolished. In the presence of both CNQX and AP-5, all mEPSCs were eliminated. The NMDA component of reticulospinal mEPSCs had a large single-channel conductance estimated to be 48 pS. Larval AMPA/kainate and NMDA components of the mEPSCs decayed with biexponential time courses that changed little during development. At all stages examined, approximately one-half of synapses had only NMDA responses (lacking AMPA/kainate receptors), whereas the remainder of the synapses were composed of a mixture of AMPA/kainate and NMDA receptors. There was an overall increase in the frequency and amplitude of mEPSCs with an NMDA component in reticulospinal (but not motoneurons) during development. These results indicate that glutamate is a prominent excitatory transmitter in the locomotor regions of the developing zebrafish and that it activates either NMDA receptors alone at functionally silent synapses or together with AMPA/kainate receptors.
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Affiliation(s)
- D W Ali
- Centre for Research in Neuroscience, Montreal General Hospital Research Institute, Montreal, Quebec H3G 1A4 Canada
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Drapeau P, Ali DW, Buss RR, Saint-Amant L. In vivo recording from identifiable neurons of the locomotor network in the developing zebrafish. J Neurosci Methods 1999; 88:1-13. [PMID: 10379574 DOI: 10.1016/s0165-0270(99)00008-4] [Citation(s) in RCA: 108] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
The zebrafish is a popular model for developmental studies due to its accessibility by cellular, molecular and genetic approaches. As a complement to these other methods, we have devised an exposed hindbrain/spinal cord preparation in the curarized zebrafish embryo and larva that permits intracellular labeling and patch clamp recording from individually identified sensory neurons, motoneurons and interneurons in vivo. Regular bursts of synaptic potentials and action potentials were observed under whole-cell current clamp in embryonic motoneurons and in some identified interneurons. Larval neurons showed prolonged depolarizations with synaptically driven bursts of action potentials. Frequent spontaneous synaptic potentials were observed and synaptic currents were effectively space clamped. It is thus feasible to study in vivo the properties of identifiable neurons of the developing locomotor network in the zebrafish, including their synaptic activity, firing patterns and interconnections.
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Affiliation(s)
- P Drapeau
- Centre for Research in Neuroscience, Montreal General Hospital Research Institute, and Department of Neurology, McGill University, Quebec, Canada.
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A reluctant gating mode of glycine receptor channels determines the time course of inhibitory miniature synaptic events in zebrafish hindbrain neurons. J Neurosci 1998. [PMID: 9526003 DOI: 10.1523/jneurosci.18-08-02856.1998] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Miniature IPSCs (mIPSCs) recorded in the Mauthner (M)-cell of zebrafish larvae have a broad amplitude distribution that is attributable only partly to the functional heterogeneity of postsynaptic glycine receptors (GlyRs). The role of the kinetic properties of GlyRs in amplitude fluctuation was investigated using fast-flow application techniques on outside-out patches. Short applications of a saturating glycine concentration evoked outside-out currents with a biphasic deactivation phase as observed for mIPSCs, and they were consistent with a rapid clearance of glycine from the synaptic cleft. Patch currents declined slowly during continuous applications of 3 mM glycine, but the biphasic deactivation phase of mIPSCs cannot reflect a desensitization process because paired-pulse desensitization was not observed. The maximum open probability (Po) of GlyRs was close to 0.9 with 3 mM glycine. Analyses of the onset of outside-out currents evoked by 0.1 mM glycine are consistent with the presence of two equivalent binding sites with a Kd of O.3-O.4 mM. Activation and deactivation properties of GlyRs were better described with a kinetic model, including two binding states, a doubly liganded open state, and a reluctant gating mode leading to another open state. The 20-80% rise time of mIPSCs was independent of their amplitude and is identical to that of outside-out currents evoked by the applications of a saturating concentration of glycine (>1 mM). These results support the hypothesis that GlyR kinetics determines the time course of synaptic events at M-cell inhibitory synapses and that large mIPSC amplitude fluctuations are mainly of postsynaptic origin.
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