Effects of gamma-aminobutyric acid on isolated cone photoreceptors of the turtle retina.

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RESUMO

Isolated cones dissociated from the retina of the freshwater turtle were voltage clamped using a single 'patch' pipette electrode. gamma-Aminobutyric acid (GABA) applied ionophoretically to the axon terminal evoked an inward current in cells held at -66 mV when they were recorded with patch pipettes filled with the 'control' pipette solution containing 120 mM-Cl-. Polarity of the GABA-induced current reversed near 0 mV when examined with the pipette filled with the control pipette solution. The reversal potential depended strongly on both external and intrapipette Cl- concentrations ([Cl-]o, and [Cl-]p). The reversal potential agreed with the equilibrium potential for Cl- calculated by the Nernst equation with given [Cl-]o and [Cl-]p. The reversal potential was not affected by concentrations of either external Na or K ions. Voltage responses evoked by GABA were hyperpolarizing from its resting level of about -50 mV immediately after the rupture of the patch membrane. The response polarity reversed into depolarization in a few seconds when [Cl-]p was greater than 24 mM, while hyperpolarizing responses persisted when [Cl-]p was less than 12 mM. Thus, the intracellular Cl- concentration of undisturbed isolated cones was estimated to be between 12 and 24 mM. Cones were desensitized to GABA in the presence of GABA (greater than 100 nM) in the medium, or by a prolonged ionophoretic application. The maximum reduction in response amplitude was about 70% in both experiments. Muscimol was as potent as GABA, while beta-p-chlorophenyl-GABA (baclofen) was ineffective even at 100 microM. GABA was antagonized by bicuculline competitively, and by picrotoxin non-competitively. These observations suggest that turtle cones have GABAA receptors which associate with chloride channels. The present results suggest that GABA, presumably released continuously from monophasic horizontal cells in the dark, would exert a tonic hyperpolarization in red-sensitive and green-sensitive cones. Suppression by light of tonic GABA release would depolarize these types of cones by disinhibition. Disinhibitory depolarization in cones may contribute to the centre surround antagonism in retinal neurones, and to the biphasic colour responses recorded in a subtype of horizontal cells.

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