Barium ions can support signal transmission from l-cones to li-horizontal cells in the turtle retina

Purpose: Barium has been used in retinal research to block voltage-dependent potassium conductances and to study the properties of calcium conductances. In this study, we tested the extent to which barium can replace calcium in supporting transmitter release from cone photoreceptors. Methods: The membrane potential and photoresponses of L-cones and LI-horizontal cells were recorded fiom the turtle everted eyecup and isolated retina. The effects of potassium channels Mockers (TEA, 4-AP and cesium ions) and of barium ions were studied. Results: A ocktail containing TEA, 4-AP and cesium chloride produced complex changes in uie dark potential of horizontal cells and in the amplitude and kinetics of their photoresponses indicating that voltage-dependent potassium conductances modulate th< lightinduced activity in these cells. Barium ions (1-2 mM) produced different effects. The horizontal cells depolarized and the photoresponses were slowed down both at light onset and at light offset. Omitting calcium ions from the superfusing solution induced nyperpolarization of horizontal cells and elimination of the photoresponses. These effects could be reversed by adding barium ions. Barium ions exerted no effects on the L-cones. Conclusion: The effects of barium ions on turtle Llhorizontal cells could not be explained by blocking voltage- and time-dependent potassium conductances. Rather, these observations indicated that barium ions permeating through the calcium channels in the cone pedicles could support release of glutamate. Thus, our findings support the hypothesis that glutamate release from L-cones mainly reflects voltage-dependent asynchronous vesicular release.