Effects of Extracellular Chloride Ions on the Catfish Retinal Neurons

ABSTRACT

The catfish (Ictalurus punctatus) retinal neurons were investigated by using the intracellular recording techniques to analyze the function of the chloride ions in the light responses and the ionic mechanisms of the depolarizing actions by GABA. Experiments were performed in the superfused retina-eyecup preparation. The retina was exposed by exicising the cornea, iris, and vitreous. A piece of absorbent tissue with a hole large enough to expose the retina was centered over the eyecup to serve as a wick to draw off the superfusate. Diffuse light stimuli were generated by light-emitting diode positioned above the eyecup. The recordings were made with the use of borosilicate glass micropipettes fashioned from' omega dot' capillary tubing filled with 2 M potassium acetate. Voltage recordings were obtained using an amplifier and amplified signals were recorded on a storage oscillocope, penwriter, and a data recorder. In the catfish retina, the dark membrane potentials were depolarized and the light evoked responses were enhanced in the chloride"-free medium on the catfish horizontal cells. The amplitude of the light evoked potentials were increased by chloride free Ringer's solution on the ON- and OFF-bipolar cells. But the dark membrane potentials were hyperpolarized on the ON-bipolar cell and depolarized on the OFF-bipolar cells in the chloride free medium. The chloride free Ringer's solution changed the light response from ON-sustained to OFF-sustained without any change in amplitude on the ON-sustained cell. The depolarizing actions by GABA on the horizontal cells were maintained in chloride-free environment. But GABA did not abolished the light evoked potentials of the horizontal cell and the ON-sustained cell under the chloride free environment. The results suggest that chloride ion has important roles on the signal transmission of the dark periods in the catfish retina and the depolarizing actions by GABA on the neurons in the catfish retina might be chloride dependent.