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J. Cui, Z.-H. Pan; Inhibitory Synaptic Inputs at the Axon Terminals of ON-Type Cone Bipolar Cells in the Rat Retina . Invest. Ophthalmol. Vis. Sci. 2007;48(13):4594.
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GABA and glycine are the two major inhibitory neurotransmitters in the retina. Less is know about the properties of inhibitory synaptic inputs to the axon terminals of cone bipolar cells (CBCs). Here we investigated the properties of GABA and glycine receptors and their synaptic inputs at the axon terminals of ON-type CBCs in the rat retina.
whole-cell patch-clamp recordings were performed in retinal slice preparations from rats. The morphology of the recorded CBCs was identified by filling fluorescent dye, Alexa 488 or 568. After recordings, slices were stained with antibody to ChAT or calretinin to determine the terminal stratification level of the recorded cells.
Most of our recorded ON-type CBCs were type-5 or type-8 CBCs. For type-5 CBCs, application of GABA at the axon terminals consistently elicited a relatively large current. The majority of the GABA-evoked current was blocked by bicuculline. The magnitude of the glycine-evoked current was variable among recorded cells. Puffing kainite (KA) onto the inner plexiform layer (IPL) elicited a relatively small current. The majority of the KA-evoked current was blocked by the combination of bicuculline and TPMPA. For the majority of the recorded cells, application of bicuculline alone partially blocked the current. For the remaining cells, however, application of bicuculline either showed no effects or enhanced the current. A small component of the current was blocked by strychnine. In some cells, after the blockade of GABA and glycine receptors, a small current still remained. This current could be blocked by gap junction blocker, meclofenamic acid. For type-8 CBCs, applying KA evoked a much larger current than that observed in type-5 CBCs. Similarly, the majority of the current was blocked by bicuculline plus TPMPA. A small component of the current also remained after the blockade of GABA and glycine receptors.
This study provides evidence for the existence of functional GABAergic and glycinergic synaptic inputs at the axon terminals of ON-type CBCs. For the ON-type CBCs we studied, the majority of the inhibitory synaptic inputs were mediated by GABA receptors.
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