In goldfish, negative feedback from horizontal cells to cones shifts
the activation function of the Ca2+ current of the cones to
more negative potentials. This shift increases the amount of
Ca2+ flowing into the cones, resulting in an increase in
glutamate release. The increased glutamate release forms the basis of
the feedback-mediated responses in second-order neurons, such as the
surround-induced responses of bipolar cells and the spectral coding of
horizontal cells. Low concentrations of Co2+ block these
feedback-mediated responses in turtle retina. The mechanism by which
this is accomplished is unknown. We studied the effects of
Co2+ on the cone/horizontal network of goldfish retina
and found that Co2+ greatly reduced the feedback-mediated
responses in both cones and horizontal cells in a GABA-independent way.
The reduction of the feedback-mediated responses is accompanied by a
small shift of the Ca2+ current of the cones to positive
potentials. We have previously shown that hemichannels on the tips of
the horizontal cell dendrites are involved in the modulation of the
Ca2+ current in cones. Both the absence of this
Co2+-induced shift of the Ca2+ current in the
absence of a hemichannel conductance and the sensitivity of Cx26
hemichannels to low concentrations of Co2+ are consistent
with a role for hemichannels in negative feedback from horizontal cells
to cones.