Selective suppression of rod signal transmission by cobalt ions of low levels in carp retina.

Selective suppression of rod signal transmission by cobalt ions was reported in carp retina. Using 10 munol/L Co(2+), rod-driven horizontal cells were hyperpolarized and light responses were completely suppressed in superfused, isolated retina, while cone-driven horizontal cells were almost unaffected. Similarly, scotopic electroretinographic bwave was suppressed by 10 munol/L Co(2+), while the photopic b-wave remained unaffected. Furthermore, the glutamate-isolated receptor potential (PIII) was not altered by low Co(2+) under dark-adapted conditions. Other divalent ions with high affinity to calcium channels, such as cadmium and manganese ions, did not show similar suppressive effect on the rod horizontal cells. When rod horizontal cells were hyperpolarized by 10 munol/L Co(2+), the use of 3 mmol/L glutamate caused a significant depolarization of the cells, indicating that Co(2+) application did not impair the ability of these cells to respond to glutamate. On the other hand, application of 200 munol/L beta-hydroxyaspartate, a glutamate transport blocker, mimicked the effect of low Co(2+), suggesting a possibility that the low Co(2+) effect might be related to a blockade of glutamate uptake by rods.