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E.M. Lasater, K. Rapp, J. Vigh; Metabotropic Glutamate Receptor Activation Inhibits GABAA Current in Isolated Wide-Field Amacrine Cells . Invest. Ophthalmol. Vis. Sci. 2003;44(13):5141.
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Purpose: Activation of mGluRs has modulatory effects in the CNS including the modulation of receptor-gated ion-channels. In a previous study we showed that caffeine inhibited the GABAA current (IGABA) of teleost wide-field amacrine cells (WFAC) by elevating [Ca++]i and triggering a Ca++→ Ca++/calmodulin→ calcineurin cascade. Here we tested whether or not the elevation of inositol 1,4,5-triphosphate (IP3) resulting from mGluR activation triggers the Ca++→ Ca++/calmodulin→ calcineurin cascade to modulate IGABA. Methods: Amacrine cells were isolated from white bass retina and maintained in culture. They were recorded from using standard whole-cell voltage clamp techniques. Test agents were applied either by local superfusion or through the recording pipette. Results: We found that pre-exposure to glutamate (Glu, 100 µM) and to the non-selective mGluR 1/mGluR 5 agonist (RS) DHPG (100 µM) reversibly inhibited IGABA by about 63%. Interestingly, the inhibition of IGABA by Glu depended on the pre-exposure time: maximal inhibition required at least 12 seconds of Glu or DHPG application. Both the effects of DHPG and Glu could be blocked by equimolar AP-3 or by including heparin in the pipette solution, verifying the involvement of mGluR 1 activation and IP3 production in the inhibition. It also eliminated the ionotropic GluRs as a possible source of [Ca++]i elevation. Including the protein phosphatase inhibitor okadaic acid or the calcineurin specific blocker calmidazolium in the pipette solution also eliminated the DHPG effect. Conclusions: Long exposure to Glu reversibly inhibited IGABA in WFACs through mGluR 1 receptor activation and consequent IP3 generation which elevated [Ca++]i and in turn activated Ca++-dependent protein phosphatases. Dephosphorylation of the GABAA receptor or a regulatory complex reduced the GABA mediated current. Thus the mGluR 1 pathway could provide a "timed" disinhibitory mechanism by suppressing GABA-mediated conductances as a function of the duration of presynaptic excitatory (Glu) input. In a local retinal circuit this may selectively accentuate prolonged excitatory inputs to the wide-field amacrine cells causing specific inputs to "stand out". This is similar to some forms of long term potentiation in the central nervous system.
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