GABA- and Glycine-activated Inhibitory Currents of Rabbit On-cone Bipolar Cell Axon Terminals

Abstract

Abstract: : Purpose: The experimental aim was to characterize the GABA- and glycine-activated currents for the axon terminals of morphologically identified on-cone bipolar cells in the rabbit retina and to determine their differential contribution to the inhibitory input of the cells. Methods: Whole-cell currents were recorded from on-cone bipolar cells in the rabbit retinal slice preparation that was superfused with a Ringer’s solution containing 2 mM Co²⁺ to synaptically isolate the cells. Sulforhodamine B was included in the recording pipettes to label the cells during the recordings in order to correlate the cell’s morphology with its physiology. The on-cone bipolar cells were identified based on the distinct stratification of their axon terminals within sublamina-b of the IPL. GABA and glycine were applied by pressure ejection through a glass pipette positioned above the IPL near the axon terminals; whereas, specific antagonists were applied in the Ringer’s solution as it bathed the retina. Results: Three medium-field and two narrow-field types of on-cone bipolar cells were identified. The application of GABA or glycine activated chloride currents that were differentially blocked by picrotoxin and strychnine, respectively. All types of on-cone bipolar cells displayed larger currents activated by GABA than those activated by glycine. Furthermore, bicuculline, the GABAa antagonist, or TPMPA, the GABAc antagonist, could block different fractions of the GABA-activated currents; however, the ratio of GABAa and GABAc currents was variable among the different types of on-cone bipolar cells. Conclusions: The major inhibitory synaptic input to all on-cone bipolar cell axon terminals appears to be mediated by GABAergic amacrine cells. This is different than our findings reported last year in which only the two narrow-field off-bipolar cells had larger GABA- than glycine-activated currents and the three medium-field off-bipolar cells displayed larger glycine- than GABA-activated currents because of their involvement in the rod pathway with inputs from glycinergic AII amacrine cells. The different contributions of GABAa versus GABAc mediated input may provide the bipolar cell output with different temporal properties.

Keywords: bipolar cells • receptors: pharmacology/physiology • inhibitory neurotransmitters