Dopamine depletion with 6-OHDA enhances dopamine D1-receptor modulation of potassium currents in retinal bipolar cells

Abstract

Ascorbate modulates I_K(V) of ON-type mixed rod/cone bipolar cells (Mb) in the goldfish retinal slice through a dopamine D1/G-protein/PKA-coupled mechanism. We investigated the effects of dopamine depletion with intraocular injections of 6-OHDA on I_K(V) and its modulation by ascorbate over 1–7 weeks following 6-OHDA treatment. Dopamine depletion was verified by tyrosine hydroxylase immunocytochemistry. Slices were perfused in a saline containing 200 μM sodium ascorbate. One-second puffs of ascorbate-free saline (zero [AA]_o), delivered through a 2–3 μm diameter pipette, were directed at the bipolar cells. I_K(V) was recorded by conventional whole-cell patch-clamp methods. In normal retinas, puffs of zero [AA]_o caused a rapid (<100 ms) suppression of I_K(V) of about 50% that lasted for several minutes. This effect was blocked by 1 μM SCH23390 and was unaffected by 2 mM Co²⁺ or 5 μM spiperone. 6-OHDA treatment resulted in major effects. First, I_K(V) was reduced by ∼50% for weeks 1–6, recovering to a 20% reduction by week 7. Second, puffs of zero [AA]_o enhanced I_K(V) rather than suppressed it. The enhancement was blocked by SCH23390 and the PKA inhibitor, Wiptide, but was insensitive to spiperone. Third, all parts of the Mb bipolar cell (except for the axon) were sensitive to puffs of zero [AA]_o in both normal and 6-OHDA-treated retinas. Fourth, bath application of 20 μM dopamine restored the amplitude of I_K(V) but did not reverse the effects of puffed zero [AA]_o. I_K(V) was fit by two exponentials; all of the effects on I_K(V) were on the amplitude of the components and not on the time constants. Chronic dopamine depletion caused reversible changes in the properties of K⁺ channels underlying I_K(V), as well as a long-term change in the intracellular coupling mechanisms between D1-receptor activation and the modulation of I_K(V).