Transporter-mediated GABA responses in horizontal and bipolar cells of zebrafish retina

Controlling Horizontal Cell-Mediated Lateral Inhibition in Transgenic Zebrafish Retina with Chemogenetic Tools

Horizontal cells (HCs) form reciprocal synapses with rod and cone photoreceptors, an arrangement that underlies lateral inhibition in the retina. HCs send negative and positive feedback signals to photoreceptors, but how HCs initiate these signals remains unclear. Unfortunately, because HCs have no unique neurotransmitter receptors, there are no pharmacological treatments for perturbing membrane potential specifically in HCs. Here we use transgenic zebrafish whose HCs express alien receptors, enabling cell-type-specific control by cognate alien agonists. To depolarize HCs, we used the Phe-Met-Arg-Phe-amide (FMRFamide)-gated Na⁺ channel (FaNaC) activated by the invertebrate neuropeptide FMRFamide. To hyperpolarize HCs we used a pharmacologically selective actuator module (PSAM)-glycine receptor (GlyR), an engineered Cl^– selective channel activated by a synthetic agonist. Expression of FaNaC or PSAM-GlyR was restricted to HCs with the cell-type selective promoter for connexin-55.5. We assessed HC-feedback control of photoreceptor synapses in three ways. First, we measured presynaptic exocytosis from photoreceptor terminals using the fluorescent dye FM1-43. Second, we measured the electroretinogram (ERG) b-wave, a signal generated by postsynaptic responses. Third, we used Ca²⁺ imaging in retinal ganglion cells (RGCs) expressing the Ca²⁺ indicator GCaMP6. Addition of FMRFamide significantly decreased FM1-43 destaining in darkness, whereas the addition of PSAM-GlyR significantly increased it. However, both agonists decreased the light-elicited ERG b-wave and eliminated surround inhibition of the Ca²⁺ response of RGCs. Taken together, our findings show that chemogenetic tools can selectively manipulate negative feedback from HCs, providing a platform for understanding its mechanism and helping to elucidate its functional roles in visual information processing at a succession of downstream stages.

Optical measurement of neuronal activity in the developing cerebellum of zebrafish using voltage-sensitive dye imaging

Voltage-sensitive dye (VSD) imaging enables fast, direct, and simultaneous detection of membrane potentials from a population of neurons forming neuronal circuits. This enables the detection of hyperpolarization together with depolarization, whose balance plays a pivotal role in the function of many brain regions. Among these is the cerebellum, which contains a significant number of inhibitory neurons. However, the mechanism underlying the functional development remains unclear. In this study, we used a model system ideal to study neurogenesis by applying VSD imaging to the cerebellum of zebrafish larvae to analyze the neuronal activity of the developing cerebellum, focusing on both excitation and inhibition. We performed in-vivo high-speed imaging of the entire cerebellum of the zebrafish, which was stained using Di-4-ANEPPS, a widely used VSD. To examine whether neuronal activity in the zebrafish cerebellum could be detected by this VSD, we applied electrical stimulation during VSD imaging, which showed that depolarization was detected widely in the cerebellum upon stimulation. These responses mostly disappeared following treatment with tetrodotoxin, indicating that Di-4-ANEPPS enabled optical measurement of neuronal activity in the developing cerebellum of zebrafish. Moreover, hyperpolarizing signals were also detected upon stimulation, but these were significantly reduced by treatment with picrotoxin, a GABAA receptor inhibitor, indicating that these responses represent inhibitory signals. This approach will enable a detailed analysis of the spatiotemporal dynamics of the excitation and inhibition in the cerebellum along its developmental stages, leading to a deeper understanding of the functional development of the cerebellum in vertebrates.

Characterization of the γ-aminobutyric acid signaling system in the zebrafish (Danio rerio Hamilton) central nervous system by reverse transcription-quantitative polymerase chain reaction

In the vertebrate brain, inhibition is largely mediated by γ-aminobutyric acid (GABA). This neurotransmitter comprises a signaling machinery of GABA_A, GABA_B receptors, transporters, glutamate decarboxylases (gads) and 4-aminobutyrate aminotransferase (abat), and associated proteins. Chloride is intimately related to GABA_A receptor conductance, GABA uptake, and GADs activity. The response of target neurons to GABA stimuli is shaped by chloride-cation co-transporters (CCCs), which strictly control Cl^- gradient across plasma membranes. This research profiled the expression of forty genes involved in GABA signaling in the zebrafish (Danio rerio) brain, grouped brain regions and retinas. Primer pairs were developed for reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The mRNA levels of the zebrafish GABA system share similarities with that of mammals, and confirm previous studies in non-mammalian species. Proposed GABA_A receptors are α₁β₂γ₂, α₁β₂δ, α_2bβ₃γ₂, α_2bβ₃δ, α₄β₂γ₂, α₄β₂δ, α_6bβ₂γ₂ and α_6bβ₂δ. Regional brain differences were documented. Retinal hetero- or homomeric ρ-composed GABA_A receptors could exist, accompanying α₁β_yγ₂, α₁β_yδ, α_6aβ_yγ₂, α_6aβ_yδ. Expression patterns of α_6a and α_6b were opposite, with the former being more abundant in retinas, the latter in brains. Given the stoichiometry α_6wβ_yγ_z, α_6a- or α_6b-containing receptors likely have different regulatory mechanisms. Different gene isoforms could originate after the rounds of genome duplication during teleost evolution. This research depicts that one isoform is generally more abundantly expressed than the other. Such observations also apply to GABA_B receptors, GABA transporters, GABA-related enzymes, CCCs and GABA_A receptor-associated proteins, whose presence further strengthens the proof of a GABA system in zebrafish.

GABA and Glutamate Uptake and Metabolism in Retinal Glial (Müller) Cells

Müller cells, the principal glial cells of the retina, support the synaptic activity by the uptake and metabolization of extracellular neurotransmitters. Müller cells express uptake and exchange systems for various neurotransmitters including glutamate and γ-aminobutyric acid (GABA). Müller cells remove the bulk of extracellular glutamate in the inner retina and contribute to the glutamate clearance around photoreceptor terminals. By the uptake of glutamate, Müller cells are involved in the shaping and termination of the synaptic activity, particularly in the inner retina. Reactive Müller cells are neuroprotective, e.g., by the clearance of excess extracellular glutamate, but may also contribute to neuronal degeneration by a malfunctioning or even reversal of glial glutamate transporters, or by a downregulation of the key enzyme, glutamine synthetase. This review summarizes the present knowledge about the role of Müller cells in the clearance and metabolization of extracellular glutamate and GABA. Some major pathways of GABA and glutamate metabolism in Müller cells are described; these pathways are involved in the glutamate-glutamine cycle of the retina, in the defense against oxidative stress via the production of glutathione, and in the production of substrates for the neuronal energy metabolism.

Bipolar cells in the zebrafish retina

Zebrafish are an existing model for genetic and developmental studies due to their rapid external development and transparent embryos, which allow easy manipulation and observation of early developmental stages. The application of the zebrafish model to vision research has allowed for examination of retinal development and the characteristics of different retinal cell types, including bipolar cells. In particular, bipolar cell development, including differentiation, maturation, and gene expression, has been documented, as has physiological properties, such as voltage- and ligand-gated currents, and neurotransmitter receptor and ion channel expression. Mutant strains and transgenic lines have been used to document how bipolar cell connections and/or development may be altered, and toxicological studies examining how environmental factors may impact bipolar cell activity have been performed. The purpose of this paper was to review the existing literature on zebrafish bipolar cells, to provide a comprehensive overview of current information pertaining to this retinal cell type.

Immunocytochemical evidence for SNARE protein-dependent transmitter release from guinea pig horizontal cells

Horizontal cells are lateral interneurons that participate in visual processing in the outer retina but the cellular mechanisms underlying transmitter release from these cells are not fully understood. In non-mammalian horizontal cells, GABA release has been shown to occur by a non-vesicular mechanism. However, recent evidence in mammalian horizontal cells favors a vesicular mechanism as they lack plasmalemmal GABA transporters and some soluble NSF attachment protein receptor (SNARE) core proteins have been identified in rodent horizontal cells. Moreover, immunoreactivity for GABA and the molecular machinery to synthesize GABA have been found in guinea pig horizontal cells, suggesting that if components of the SNARE complex are expressed they could contribute to the vesicular release of GABA. In this study we investigated whether these vesicular and synaptic proteins are expressed by guinea pig horizontal cells using immunohistochemistry with well-characterized antibodies to evaluate their cellular distribution. Components of synaptic vesicles including vesicular GABA transporter, synapsin I and synaptic vesicle protein 2A were localized to horizontal cell processes and endings, along with the SNARE core complex proteins, syntaxin-1a, syntaxin-4 and synaptosomal-associated protein 25 (SNAP-25). Complexin I/II, a cytosolic protein that stabilizes the activated SNARE fusion core, strongly immunostained horizontal cell soma and processes. In addition, the vesicular Ca(2+)-sensor, synaptotagmin-2, which is essential for Ca(2+)-mediated vesicular release, was also localized to horizontal cell processes and somata. These morphological findings from guinea pig horizontal cells suggest that mammalian horizontal cells have the capacity to utilize a regulated Ca(2+)-dependent vesicular pathway to release neurotransmitter, and that this mechanism may be shared among many mammalian species.

Electrophysiological evidence of GABAA and GABAC receptors on zebrafish retinal bipolar cells

To refine inhibitory circuitry models for ON and OFF pathways in zebrafish retina, GABAergic properties of zebrafish bipolar cells were studied with two techniques: whole cell patch responses to GABA puffs in retinal slice, and voltage probe responses in isolated cells. Retinal slices documented predominantly axon terminal responses; isolated cells revealed mainly soma-dendritic responses. In the slice, GABA elicited a conductance increase, GABA responses were more robust at axon terminals than dendrites, and Erev varied with [Cl(-)]in. Axon terminals of ON- and OFF-type cells were similarly sensitive to GABA (30-40 pA peak current); axotomized cells were unresponsive. Bicuculline-sensitive, picrotoxin-sensitive, and picrotoxin-insensitive components were identified. Muscimol was as effective as GABA; baclofen was ineffective. Isolated bipolar cells were either intact or axotomized. Even in cells without an axon, GABA or muscimol (but not baclofen) hyperpolarized dendritic and somatic regions, suggesting significant distal expression. Median fluorescence change for GABA was -0.22 log units (approximately -16 mV); median half-amplitude dose was 0.4 microM. Reduced [Cl(-)]out blocked GABA responses. GABA hyperpolarized isolated ON-bipolar cells; OFF-cells were either unresponsive or depolarized. Hyperpolarizing GABA responses in isolated cells were bicuculline and TPMPA insensitive, but blocked or partially blocked by picrotoxin or zinc. In summary, axon terminals contain bicuculline-sensitive GABAA receptors and both picrotoxin-sensitive and insensitive GABAC receptors. Dendritic processes express zinc- and picrotoxin-sensitive GABAC receptors.