One of the best documented examples of feedback is that which governs the center-surround receptive field organization of retinal neurons. There is abundant evidence that the earliest stage at which feedback occurs is at the first synapse in the visual pathway, i.e., between the axon terminals of photoreceptors and the dendritic processes of horizontal cells both for cones and for rods. Although extensively investigated in many vertebrate species, there is still no general agreement as to the mechanism that mediates feedback. Nevertheless, two appealing, but very different, views on how feedback modulates the Ca2+current in cones have emerged from recent studies on lower vertebrates. According to the Lomitapide Mesylate hemichannel hypothesis, surround illumination causes 
the horizontal cell to hyperpolarize, thus leading to an increase in the current flowing through both the hemichannels and the glutamategated channels. This current flow produces a voltage drop along the high resistance path of the synaptic cleft, thereby shifting the Ca2+ current in the cones to more negative potentials and enhances glutamate release from the cone terminal. An alternative view of the mechanism that mediates feedback in the distal retina was put forth by Hirasawa and Kaneko. They found that the feedback-induced shift of the Ca2+ current in cones was inhibited when extracellular proton fluctuations were stabilized by a high concentration of the pH buffer HEPES; the high concentration of HEPES also reduced light-induced surround effects in bipolar cells. These data together with evidence that increasing extracellular pH shifts the L-type Ca2+ current toward negative potentials led them to conclude that protons regulate the horizontal cell-to-cone feedback pathway. In the present study, we performed a series of electrophysiological experiments designed to test the pH-mediated feedback mechanism and to determine whether the effects of extracellular pH buffering can also be evaluated in terms of the ephaptic feedback hypothesis. A major feature of these experiments is the use of a series of pharmacological agents that change the intracellular pH or the extracellular pH buffering. Our 3,4,5-Trimethoxyphenylacetic acid experimental findings suggest that artificial pH buffers, on which several key arguments supporting the pH hypothesis are based, induce intracellular acidification in addition to clamping the extracellular pH. Because hemichannels are inhibited by intracellular acidification, these experiments do not seem to discriminate between a pH-mediated or a hemichannel-mediated mechanism. Independent experiments to further test the pH hypothesis failed to generate support for the pH-mediated mechanism. Moreover, the experimental results concerning the endogenous pH buffering system, and the computational analysis, are in line with an ephaptic feedback pathway that operates through both hemichannels and glutamate-gated channels. The fact that increasing the pH buffering capacity of the extracellular milieu with HEPES suppressed the light-induced feedback signal raises the question of whether the feedback or the feedforward signal was affected. Any decrease in the feedforward signal will obviously lead to a decrease in the light-induced feedback-signal, because the feedback pathway will receive less input. Because our findings indicate that HEPES reduced the light response of horizontal cells.