Research article

Chloride equilibrium potential in salamander cones

Wallace B Thoreson and Eric J Bryson

Departments of Ophthalmology and Pharmacology, University of Nebraska Medical Center, Omaha, NE, USA

author email corresponding author email

BMC Neuroscience 2004, 5:53doi:10.1186/1471-2202-5-53

Published:	5 December 2004

Abstract

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 (E_Cl) 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 E_Cl, we applied long depolarizing steps to activate the calcium-activated chloride current (I_Cl(Ca)) and then determined the reversal potential for the current component that was inhibited by the Cl^- channel blocker, niflumic acid. With this method, E_Cl 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 E_Cl was below -36 mV.

Conclusions

The results of this study indicate that E_Cl is close to the dark resting potential. This will minimize the impact of chloride-dependent presynaptic mechanisms in cone terminals involving GABA_a receptors, glutamate transporters and I_Cl(Ca).