In situ hybridization analysis showed that OCT3 is expressed in mouse RPE and in several cell types of the neural retina, including photoreceptor, ganglion, amacrine, and horizontal cells. The expression of OCT3 in RPE was confirmed by Northern blot analysis and RT-PCR. The characteristics of MPP( +) uptake in cultured ARPE-19 cells included the stimulation of transport by alkaline pH, high affinity (K(t) = 28 +/- 4 microM), competition with several cationic drugs and monoamine neurotransmitters and sensitivity to steroids. In addition, the uptake of MPP(+) in RPE cells was inhibited by dopamine and histamine with IC(50) values (concentration needed for 50% inhibition) of 637 +/- 84 microM and 150 +/- 20 microM, respectively. CONCLUSIONS. This study provides the first report on the expression and function of an organic cation transporter, OCT3, in the eye and in particular the RPE. The data have physiological and pharmacological relevance as it is likely that OCT3 participates in the clearance of dopamine and histamine from the subretinal space and may also play a key role in the disposition of the retinal neurotoxin MPP(+).

In situ hybridization analysis showed that OCT3 is expressed in mouse RPE and in several cell types of the neural retina, including photoreceptor, ganglion, amacrine, and horizontal cells. The expression of OCT3 in RPE was confirmed by Northern blot analysis and RT-PCR. The characteristics of MPP( +) uptake in cultured ARPE-19 cells included the stimulation of transport by alkaline pH, high affinity (K(t) = 28 +/- 4 microM), competition with several cationic drugs and monoamine neurotransmitters and sensitivity to steroids. In addition, the uptake of MPP(+) in RPE cells was inhibited by dopamine and histamine with IC(50) values (concentration needed for 50% inhibition) of 637 +/- 84 microM and 150 +/- 20 microM, respectively. CONCLUSIONS. This study provides the first report on the expression and function of an organic cation transporter, OCT3, in the eye and in particular the RPE. The data have physiological and pharmacological relevance as it is likely that OCT3 participates in the clearance of dopamine and histamine from the subretinal space and may also play a key role in the disposition of the retinal neurotoxin MPP(+).