GABA-mediated component in the feedback response of turtle retinal cones
The negative feedback from horizontal cells to cone photoreceptors contributes to the formation of the receptive-field surround in cone photoreceptors. Recently, studies on the modulation of voltage-gated Ca2+ currents in cone photoreceptors have led to great progress in our understanding of the mechanism of horizontal-cone feedback. Another highly probable hypothesis is that GABA mediates this feedback. This hypothesis is supported by the facts that cone photoreceptors respond to GABA and that horizontal cells release GABA. However, GABA-mediated synaptic inputs from horizontal cells to cone photoreceptors have not been demonstrated. In the present study, we examined whether cone photoreceptors receive GABAergic inputs from horizontal cells using a slice patch technique in the turtle retina. When 1 mM of GABA was applied to the cone photoreceptors, GABA-induced currents were activated. GABA-induced currents reversed their polarity at the equilibrium potential of Cl−. The application of 30 [mu]M of SR95531, an antagonist of GABAA receptors, alone did not produce any change in the holding currents. When 200 [mu]M of pentobarbital was introduced to potentiate the GABAergic inputs to the cone photoreceptors, however, the inhibitory action of SR95531 on GABAergic inputs became detectable. The amplitude of the GABAergic inputs, potentiated by pentobarbital, increased when the horizontal cells were depolarized by the application of 20 [mu]M of kainate, while the amplitude decreased when the horizontal cells were hyperpolarized by the application of 10 [mu]M of CNQX. When the cone photoreceptors were voltage clamped at a potential at which the voltage-gated Ca2+ current was inactive, horizontal-cone feedback was not observed. However, the horizontal-cone feedback became detectable when the GABAergic inputs to the cone photoreceptors were potentiated by pentobarbital. We concluded that the contribution of GABAergic inputs from horizontal cells to cone pedicles in the formation of the receptive-field surround in cone photoreceptors is very limited but that the modulation of voltage-gated Ca2+ currents in cone photoreceptors is a physiologically relevant mechanism for horizontal-cone feedback.(Received August 9 2004)
(Accepted March 7 2005)
Key Words: GABA; Cone photoreceptor; Horizontal cell; Retina; Turtle.
c1 Address correspondence and reprint requests to: M. Kaneda, Department of Physiology, Keio University School of Medicine, Tokyo 160-8582, Japan. E-mail: email@example.com
p1 Present address of T. Tatsukawa: Laboratory of Cellular Neurobiology School of Life Science, Tokyo University of Pharmacy and Life Science, 1432-1 Horinouchi, Hachioji-shi, Tokyo 192-0392, Japan
p2 Present address of H. Hirasawa: Department of Neurobiology, Harvard Medical School, Boston, MA 02115, USA
p3 Present address of A. Kaneko: Seijoh University School of Rehabilitation, 2-172 Fukinodai, Tokai-shi, Aichi 476-8588, Japan