The uptake and radioautographical localization in the frog retina of [3H](+/-)-aminocyclohexane carboxylic acid, a selective inhibitor of neuronal GABA transport

Abstract

The accumulation of [3H](+/-)-cis-3-aminocyclohexane carboxylic acid ([3H] ACHC) in frog retinae in vitro was highly localized in horizontal cells and their processes. [3H]GABA was also mainly accumulated within horizontal cells, but [3H]L-2,4-diaminobutyric acid ([3H]DABA) was taken up predominantly by the neuroglial Müller cells, whilst [3H]beta-alanine was localised largely within the photoreceptors. The uptake of [3H]ACHC (4.2 microM) was almost linear for 30 min and after 60 min a tissue/medium ratio of 5.25 was achieved. The uptake process was temperature sensitive highly dependent on sodium ions, and appeared to be mediated by a saturable transport process with an IC50 value of 0.83 mM. The accumulation of [3H]ACHC was inhibited by GABA and DABA (IC50 = 0.32 mM and 0.23 mM, respectively) whilst beta-alanine was a relatively weak inhibitor (IC50 = 9 mM) of ACHC uptake. In agreement with these results, the efflux of [3H]ACHC from the retina was increased by exposure to ACHC, GABA and DABA but not beta-alanine. In contrast, the efflux of [3H]DABA from the retina was not increased by GABA or ACHC, although DABA itself and potassium depolarization stimulated the release of [3H]DABA. These results strongly suggest that ACHC is accumulated in the frog retina by the same neuronal transport process as GABA. In contrast, the high affinity sites for DABA are localized mainly in glia, although inhibitor and release studies suggest that, at high concentration, DABA also interacts with the neuronal GABA (ACHC) transport process.