Reversed GABA Transport Mediates Starburst Amacrine Cell Directionally–Selective Light Responses

Abstract

Purpose: : Starburst amacrine cells (SACs) synthesize and release GABA and express GABA_A receptors. SAC dendrites are directionally–selective (DS), i.e., they hyperpolarize to centripetal stimulus motion and depolarize to centrifugal motion (Gavrikov et al., 2003). SACs may signal each other by releasing GABA via a Ca²⁺–dependent mechanism at their distal dendritic tips (Zheng et al., 2004) and via Ca²⁺–independent reversed GABA transport (O'Malley et al., 1992). Because GABA uptake by GABA transport can be reversed when the intracellular [Na⁺] is high (Richerson and Wu, 2003), we studied whether GABA transport plays a role in the DS light responses of SAC dendrites by varying the intracellular [Na⁺] of SACs.

Methods: : DAPI–labeled, displaced rabbit SACs were studied using whole–cell patch pipettes or sharp microelectrodes. The superfusate was a modified Ames medium.

Results: : When SACs were recorded with patch pipettes containing a high [Na⁺] (45.6 mM), their resting membrane potential was –50 mV and their dendrites consistently generated DS light responses. Application of gabazine, a specific GABA_A antagonist, eliminated the DS light responses and hyperpolarized the cells by about 20 mV. When SACs were recorded with patch pipettes that contained a low [Na⁺] (5.6 mM), their resting membrane potential was –70 mV and the size of their DS light responses was dramatically reduced. When SACs were recorded with sharp microelectrodes, their resting membrane potential was –50 mV and their dendrites generated DS light responses. SACs, whose somata are 25–50 µm apart with primary dendrites that extend 10–20 µm from the somata before entering the SAC plexus, express GABA_A receptors on their somata and dendrites, suggesting that each SAC itself is the primary source of extracellular GABA close to its soma and adjacent proximal dendrites.

Conclusions: : These results suggest that SACs contain a relatively high level of intracellular Na⁺, and that in the dark autocrine signaling via reversed GABA transport increases the level of extracellular GABA, which depolarizes the somata and proximal dendrites of SACs due to the presence of NKCC2 in these SAC compartments (Mangel et al., 2005, ARVO). Centrifugal stimulus motion further depolarizes the cells and their dendrites. Centripetal stimulus motion hyperpolarizes SAC distal dendrites due to the presence of KCC2 in the distal dendrites and due to the release of GABA from the neighboring processes of other SACs.

Keywords: amacrine cells • neurotransmitters/neurotransmitter systems • ion transporters