Electrical feedback in the cone pedicle: a computational analysis

One of the fundamental principles of neuroscience is that direct electrical interactions between neurons are not possible without specialized electrical contacts, gap junctions, because the transmembrane resistance of neurons is typically much higher than the resistance of the adjacent extracellular space. However it has been proposed that in the retina direct electrical interactions between cones and second-order neurons occur due to the specific morphology of the cone synaptic terminal. This electrical mechanism could potentially explain the phenomenon of "negative feedback" from horizontal cells to cones and the recent finding that the tips of horizontal cell dendrites contain hemichannels has rekindled interest in the idea. We quantitatively evaluated the possibility that hemichannels and/or glutamate channels mediate electrical feedback from horizontal cells to cones. The calculations show that it is unlikely that an electrical mechanism plays a significant functional role because 1) the necessity of preserving adequate cone-to-horizontal-cell synaptic transmission limits the extracellular space resistance and the horizontal-cell dendritic transmembrane resistances to values at which the effectiveness of electrical feedback is very low and its electrical effect on the cone presynaptic membrane is negligible, 2) electrical feedback is most effective in the dark and weaker during light adaptation, which contradicts the experimental data, and 3) electrical negative feedback is associated with much stronger electrical positive feedback from horizontal cells to cones, a phenomenon that has never been reported. Therefore it is likely that negative feedback from horizontal cells to cones is chemical in nature.