Figure 2. Blockade of GABAergic inhibition unmasks a robust ON response in OFF α-GCs in the rabbit retina.

A, extracellularly recorded responses of an OFF α-GC consists of a brisk and transient burst of spike activity at stimulus offset. Presentation of the light stimulus is indicated by the stimulus trace at the bottom of the figure. The increase in spike activity at light offset is likely to reflect the removal of a tonic inhibition at light offset. B, blockade of GABAergic inhibition with PTX (100 μm) unmasks a robust brisk and transient ON response at stimulus onset. The increase in spike activity at light offset likely reflects the removal of a tonic inhibition. C, blockade of the ON pathway using the mGluR6 receptor agonist l-AP4 (100 μm) abolishes the unmasked ON response, indicating that the ON component is produced by the ON pathway. The increase in spike activity at stimulus offset is likely to reflect the removal of a crossover inhibition by ON amacrine cells; see text for details. D, after wash, the OFF α-GC once again shows the classic response at stimulus offset. Light intensity of green stimulus for extracellular recordings was 0.21 Rh* rod⁻¹ s⁻¹. E–H, current clamp recordings from an OFF α-GC using the same pharmacological protocol as in A–D. Light intensity for white light stimulus = log –5.5. Maximum irradiance (log 0.0) = 2.37 mW cm⁻².