Chloride equilibrium potential in salamander cones.
BACKGROUND
GABAergic inhibition and effects of intracellular chloride ions on calcium channel activity have been proposed to regulate neurotransmission from photoreceptors. To assess the impact of these and other chloride-dependent mechanisms on release from cones, the chloride equilibrium potential (ECl) was determined in red-sensitive, large single cones from the tiger salamander retinal slice.


RESULTS
Whole cell recordings were done using gramicidin perforated patch techniques to maintain endogenous Cl- levels. Membrane potentials were corrected for liquid junction potentials. Cone resting potentials were found to average -46 mV. To measure ECl, we applied long depolarizing steps to activate the calcium-activated chloride current (ICl(Ca)) and then determined the reversal potential for the current component that was inhibited by the Cl- channel blocker, niflumic acid. With this method, ECl was found to average -46 mV. In a complementary approach, we used a Cl-sensitive dye, MEQ, to measure the Cl- flux produced by depolarization with elevated concentrations of K+. The membrane potentials produced by the various high K+ solutions were measured in separate current clamp experiments. Consistent with electrophysiological experiments, MEQ fluorescence measurements indicated that ECl was below -36 mV.


CONCLUSIONS
The results of this study indicate that ECl is close to the dark resting potential. This will minimize the impact of chloride-dependent presynaptic mechanisms in cone terminals involving GABAa receptors, glutamate transporters and ICl(Ca).
Thoreson
Wallace B
WB
Department of Ophthalmology, University of Nebraska Medical Center, Omaha, NE, USA. wbthores@unmc.edu
Bryson
Eric J
EJ
D000557:Ambystoma; D000818:Animals; D018118:Chloride Channels / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism*; D004553:Electric Conductivity; D008564:Membrane Potentials; D018408:Patch-Clamp Techniques; D017949:Retinal Cone Photoreceptor Cells / Q000378:metabolism / Q000502:physiology*
D018118:Chloride Channels; D002712:Chlorides