GABA-mediated horizontal cell signaling switches back and forth between cones at night and ON-cone bipolar cells in the day

Abstract

Purpose : Horizontal cells (HCs) can signal cone bipolar cells (cBCs) directly and indirectly by providing feedback inhibition to cones. Although HCs evoke cBC surround responses, the identity of the HC transmitter and whether the two HC pathways have similar functions remain unclear. My lab has shown that dopamine D₁Rs (on ON-cBC dendrites) and D₂Rs (on cones), by modulating intracellular PKA, increase the expression and activity of GABA_ARs on 1) ON-cBC dendrites in the day following maintained (>30 min) illumination (Chaffiol et al, submitted) and 2) cone terminals at night following maintained darkness (Mangel et al, 2015, ARVO). We thus studied whether HC signaling to ON-cBCs and to cones depend on GABA_ARs, D₁Rs, D₂Rs, and the time of day or night.

Methods : The effects of artificially polarizing HCs on nearby ON-cBCs in rabbit retinal slices and on nearby cones in intact goldfish retinas were studied in the day and night in the presence and absence of gabazine (GBZ, GABA_AR antagonist), SCH23390 (SCH, D₁R antagonist), spiperone (SP, D₂R antagonist) and MFA (gap junction blocker) (HCs: sharp pipettes; ON-cBCs, cones: patch pipettes).

Results : Preliminary rabbit HC/ON-cBC paired recordings revealed that in the day 1) artificially hyperpolarizing HCs reduced ON-cBC surround responses (as did GBZ or SCH), and 2) in the absence of light stimuli and with APB in the Ringer, depolarizing and hyperpolarizing HCs depolarized and hyperpolarized ON-cBCs, respectively (i.e. sign-conserved signaling), effects that were blocked by GBZ or SCH. Preliminary fish HC/cone recordings revealed that in the absence of light stimuli and with MFA in the Ringer HC polarizations altered cone voltage in a sign-inverted manner at night in the dark and in the day following SP (effects that were blocked by GBZ), but not in the day without SP.

Conclusions : The results suggest that 1) in the day when the D₁Rs of ON-cBCs are activated HCs provide a GABA_AR-mediated sign-conserving (excitatory) signal to ON-cBCs that contributes to surround responses, and 2) at night when the D₂Rs of cones are NOT activated HCs provide a GABA_AR-mediated sign-inverting (inhibitory) signal to cones that decreases cone light responses. Because cones and ON-cBCs do not exhibit surrounds at night, the findings also suggest that in addition to operating at different times, HC signaling to cones and to ON-cBCs have different functions.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.