Description:

The studies undertaken in this work focus on the cellular and molecular mechanisms used by cells in the vertebrate retina to process visual information. In particular, we have been examining the hypothesis that hydrogen ion release regulated by horizontal cells might play an important role in modulating neuronal signals in the outer retina. Horizontal cells are second order neurons that receive direct connections from photoreceptors and are thought to send inhibitory feedback signals onto photoreceptor synaptic terminals and feed forward inhibitory signals onto the dendrites of bipolar cells. The molecular mechanisms used by horizontal cells to provide this inhibition remain under debate. One hypothesis that has received recent experimental support suggests that horizontal cells release H⁺ which then bind to calcium channels on photoreceptor synaptic terminals, leading to the closure of voltage-gated calcium channels and a consequent reduction in the release of neurotransmitter.

Progress:

To address this issue, we have used self-referencing pH-selective microelectrodes to measure H⁺ flux from horizontal cells isolated from the retina of the skate. We have found a continuous outward flux of H⁺ from horizontal cells, and also have found that glutamate, the photoreceptor neurotransmitter, produces an alteration in this proton flux. We have further determined that calcium influx initiated by glutamate is key in inducing this change in proton flux. Pharmacological tools allow us to conclude that it is activation of AMPA/kainate receptors that is key in initiating this increase in intracellular calcium. Additional experiments suggest that the glutamate-induced alteration in proton flux is mediated by activation of a Ca²⁺ H⁺ ATPase (the plasmalemma calcium ATPase, PMCA).

We are also focusing on the mechanisms regulating pH fluxes from isolated horizontal cells. We have previously published data showing that the addition of the neurotransmitter glutamate leads to a marked decrease in proton flux around the extracellular surface of the rod-driven horizontal cells of the skate. This is in sharp contrast to the rise in proton flux that has been suggested to take place in cone-driven horizontal cells in order to mediate center-surround interactions in the retina. We are currently examining the effects of glutamate on proton flux from the cone-driven horizontal cells of catfish and goldfish in order to resolve this controversy. Our preliminary data suggest that, as for the skate, proton flux from the cone-driven horizontal cells of goldfish and catfish goes down upon application of glutamate. These results will have a significant impact on our understanding of the molecular mechanisms used by horizontal cells to process visual information in the outer retina.