The nature of surround-induced depolarizing responses in goldfish cones

Cones in the vertebrate retina project to horizontal and bipolar cells and the horizontal cells feedback negatively to cones. This organization forms the basis for the center/surround organization of the bipolar cells, a fund amental step in the visual signal processing. Although the surround respons es of bipolar cells have been recorded on many occasions, surprisingly the underlying surround-induced responses in cones are not easily detected. In this paper, the nature of the surround-induced responses in cones is studie d. Horizontal cells feed back to cones by shifting the activation function of the calcium current in cones to more negative potentials. This shift inc reases the calcium influx, which increases the neurotransmitter. release of the cone. In this paper, we will show that under certain conditions, in ad dition to this increase of neurotransmitter release, a calcium-dependent ch loride current will be activated, which polarizes the cone membrane potenti al. The question is, whether the modulation of the calcium current ol the p olarization of the cone membrane potential is the major determinant for fee dback-mediated responses in second-order neurons. Depolarizing light respon ses of biphasic horizontal cells are generated by feedback from monophasic horizontal cells to conies, It was found that niflumic acid blocks the feed back-induced depolarizing responses in cones, while the shift of the calciu m current activation function and the depolarizing biphasic horizontal cell responses remain intact. This shows that horizontal cells can feed back to cones, without inducing major changes in the cone membrane potential. This makes the feedback synapse from horizontal cells to cones a unique synapse . Polarization of the presynaptic (horizontal) cell leads to calcium influx in the postsynaptic cell (cone), but due to the combined activity of the c alcium current and the calcium-dependent chloride current, the membrane pot ential of the postsynaptic cell will be hardly modulated, whereas the outpu t of the postsynaptic cell will be strongly modulated. Since no polarizatio n of the postsynaptic cell is needed for these feedback-mediated responses, this mechanism of synaptic transmission call modulate the neurotransmitter release in single synaptic terminals without affecting the membrane potent ial of the entire cell.