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R.F. Nelson, V.P. Connaughton; Glutamate transporter drives the b–wave in zebrafish retina. . Invest. Ophthalmol. Vis. Sci. 2004;45(13):815.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To identify components contributing to the zebrafish retinal b–wave and to determine the neurotransmitter receptors and mechanisms generating these components. Methods: Photopic electroretinograms (ERG’s), evoked by photostimulation from a broad spectrum Xenon source, were recorded in an oxygenated, MEM perfused, in vitro eyecup preparation, before, during, and after treatments with neurotransmitter antagonists. ERG b– and d–waves could be stably observed over recording periods as long as 10 hours in this preparation. Antagonist–blocked ERG components were computed by subtracting treated waveforms from control waveforms. Results: DL–TBOA (DL–threo–beta–Benzyloxyaspartic acid), an excitatory amino acid transport (EAAT) blocker, abolished the zebrafish b–wave, leaving a corneal negative response at stimulus onset, and a corneal positive (d–wave) response at stimulus offset. At 4uM a component peaking at shorter latency than the composite b–wave was blocked; at 100uM all b–wave components were blocked. At all concentrations the d–wave increased in amplitude, peaked at longer latencies, and broadened in duration. DL–AP4, a blocker of metabotropic ON bipolar responses (150–950 µM), reduced but never blocked b–wave responses while increasing d–wave amplitudes. Conclusions: The massed activity of ON–type retinal bipolar cells likely generates the corneal positive b–wave at stimulus onset, as well as a corneal negative response at stimulus offset. In patch recordings of zebrafish ON bipolar cells, Connaughton and Nelson (2000) found that ∼80% were driven only by a transporter–like glutamate gated Cl– mechanism. In this mechanism transport of photoreceptor–released glutamate activates a Cl– channel hyperpolarizing ON bipolar cells in darkness. The sensitivity of the zebrafish b–wave to the glutamate transporter blocker TBOA confirms the predominance of transporter–like mechanisms for zebrafish ON system responses, and suggests that in this species, b–waves may be generated in major part by Cl– currents circulating around ON bipolar cells. Cationic currents generated by ON–bipolar–cell metabotropic (DL–AP4) receptors are only a minor component. Blocker–induced increases in d–wave amplitudes at stimulus offset may result in part from the loss of a negative OFF component also generated by ON bipolar cells. TBOA appears to be a suitable agent for separating ON and OFF system responses in the zebrafish ERG.
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