The Effects of L(+), D(-), and DL-2-amino-4-phosphonobutyrate (APB) on Electroretinogram and Ganglopn Cell Activity in the Cat Retina
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L(+)-, D(-)-, or DL-2-amino-4-phosphonobutyric acid (APB; 2.5- 16 μmol) were injected into the vitreous body of anesthetized adult cats. The retina was stimulated by diffuse square wave light flashes (10- 60 ms). The flash-induced electroretinogram (ERU) and responses of single retinal ganglion cells (RGC) were recorded simultaneously. Intravitreal injection of L(+)APB led to a decrease in the ERG b-wave amplitude and the unmasking of the a-wave. The magnitude and rate of the b-wave reduction were different for the two enantiomers. The threshold dose of D(-)APB was 6 times higher than for L(+)APB. L(+)APB (8.2 ± 1.6 μmol; n=7) decreased the b-wave with an average time constant r = 88.5 min, D(-)APB (13.2 ± 1.1 μmol; n=6) with r = 357.1 min, and DL-APB (8.35 ± 1.1 μmol; n=5) with r = 101.0 min. Concomitant with the reduction of the b-wave, L(+)APB (2.7μmol) inhibited both the spontaneous and light-evoked firing in ON-center ganglion cells. The threshold doses of L(+), D(-) and DL-APB for inhibition of spontaneous adivity and the light response in ON-center cells paralleled those in reducing the ERU h-wave. Low doses of L(+)APB or DL-APB that were effective in blockng ON-center cell activity caused only very small changes in the activity of OFF-center ganglion cells. However, high doses of L(+)APB (≥8.2μmol) or DL-APB (≥13.7 μmol) also decreased the spontaneous and light-evoked activity in OFF-center RGC and first shortened than prolonged the light-induced inhibition of OFF-center RGC.