Feedback-induced glutamate spillover enhances negative feedback from horizontal cells to cones

KEY POINTS: In the retina, horizontal cells feed back negatively to cone photoreceptors. Glutamate released from cones can spill over to neighbouring cones. Here we show that cone glutamate release induced by negative feedback can also spill over to neighbouring cones. This glutamate activates the glutamate transporter-associated chloride current in these neighbouring cones, which leads to a change in their membrane potential and thus modulates their output. In this way, feedback-induced glutamate spillover enhances negative feedback from horizontal cells to cones, thus forming an additional feedback pathway. This effect will be particularly prominent in cones that are strongly hyperpolarized by light.

ABSTRACT: Inhibition in the outer retina functions via an unusual mechanism. When horizontal cells hyperpolarize the activation potential of the Ca(2+) current of cones shifts to more negative potentials. The underlying mechanism consists of an ephaptic component and a Panx1/ATP-mediated component. Here we identified a third feedback component, which remains active outside the operating range of the Ca(2+) current. We show that the glutamate transporters of cones can be activated by glutamate released from their neighbours. This pathway can be triggered by negative feedback from horizontal cells to cones, thus providing an additional feedback pathway. This additional pathway is mediated by a Cl(-) current, can be blocked by either removing the gradient of K(+) or by adding the glutamate transporter blocker TBOA, or low concentrations of Zn(2+) . These features point to a glutamate transporter-associated Cl(-) current. The pathway has a delay of 4.7 ± 1.7 ms. The effectiveness of this pathway in modulating the cone output depends on the equilibrium potential of Cl(-) (ECl ) and the membrane potential of the cone. Because estimates of ECl show that it is around the dark resting membrane potential of cones, the activation of the glutamate transporter-associated Cl(-) current will be most effective in changing the membrane potential during strong hyperpolarization of cones. This means that negative feedback would particularly be enhanced by this pathway when cones are hyperpolarized. Spatially, this pathway does not reach further than the direct neighbouring cones. The consequence is that this feedback pathway transmits information between cones of different spectral type.