Center-surround Antagonism Mediated by Proton Signaling at the Cone Photoreceptor Synapse

The identity of the messenger that carries the inhibitory surround receptive field signal from horizontal cells to cone photoreceptors has eluded retinal neurobiologists for nearly three decades. Encoded in horizontal cell membrane potential, the feedback signal presynaptically inhibits neurotransmitter release at the cone terminal. An interesting collection of candidate mechanisms and messengers have come, and some have gone. In this issue of the Journal of General Physiology, Hirasawa and Kaneko (2003) take a giant step forward in defining the synaptic messenger. The insight their elegant experiments offer is that protons transmit inhibitory surround information from horizontal cells to cones, and that protons subtly modulate presynaptic Ca channel activity to alter neurotransmitter release dependent on illumination of the surround. That such a ubiquitous signal, e.g., pH, is the messenger in one of the most fundamental steps in visual processing is illuminating in its own right....