Abstract
Abstract: :
Purpose: Recently it has been proposed that horizontal cells modulate the proton concentration in the synaptic cleft of cone photoreceptors, thereby mediating negative feedback from HCs to cones (Hirasawa and Kaneko, 2003). This model opposes the ephaptic, hemichannel mediated feedback model proposed by Kamermans et al (2001). We studied the effect of both extracellular and intracellular pH changes on the light responses of the cone HC network. Since hemichannels could either act as a proton source in the negative feedback pathway from HCs to cones or could be a modulatory target to intracellular pH changes, we especially focused on a possible role of hemichannels in pH modulation in the synaptic cleft. Methods: Voltage clamp and current clamp recordings in the isolated retina of goldfish. Results: Switching from a bicarbonate based Ringer’s solution to one containing 20 mM HEPES strongly reduced feedback mediated responses in both cones and HCs. Our results indicate that the reduction in feedback mediated responses is due to the reduction of HC responsiveness and not due to a block of the feedback pathway. Surprisingly, the results obtained with 20 mM HEPES do not significantly differ from the results obtained with 4 mM HEPES (ARVO 2003) suggesting that 20 mM HEPES does not affect the cone–HC network through the increase in buffer capacity. After superfusion of 20 mM HEPES for at least 30 minutes, light–induced feedback responses appear to be normal again. Pharmacological polarization of HCs in the presence of 20 mM HEPES still shifted the calcium current of the cone, comparable to control conditions. The presence of feedback in Ringer’s solution with high pH buffering suggests that protons are not the neurotransmitter in the negative feedback pathway from HCs to cones in goldfish. However, intracellular acidification by application of Acetate did block hemichannel–mediated feedback, without affecting the ability to elicit feedback through other channels present in the synapse, confirming the ephaptic nature of the feedback mechanism. Conclusions: The light responses of the outer retina seem to be relatively insensitive to activity–dependent changes in extracellular pH whereas it is highly sensitive to changes in intracellular pH
Keywords: retina: distal (photoreceptors, horizontal cells, bipolar cells) • retinal connections, networks, circuitry • PH regulation/protons