Evidence for the heterogeneity of L-2,4-diaminobutyric acid uptake in rat cortex

GABA 'desensitization' of frog primary afferent fibers

GABA (gamma-aminobutyric acid) depolarizes the terminals of primary afferent fibers of the in vitro hemisected frog spinal cord. During sustained or repetitive exposure to GABA or to muscimol, the amplitude of the depolarization is characterized by a rapid and exponential decline to a steady plateau level (desensitization). Desensitization to muscimol was eliminated by removal of Ca2+ and addition of Mn2+ to the superfusate--a finding consistent with the presence of 'receptor' ('true') desensitization (i.e., receptor inactivation). GABA desensitization was significantly reduced by exposure of the cord to either low Na+, low temperature, ouabain, dinitrophenol, (+/-)-nipecotic acid, or cis-1,3-aminocyclohexanecarboxylic acid. These treatments also significantly decreased the high affinity uptake of GABA when the latter process was studied by incubating frog spinal slices in Ringer's solution containing a low concentration of [3H]GABA. These results suggest that cellular transport processes can influence the form of GABA responses and indicate that neuronal removal of GABA is responsible in part for GABA desensitization.

Inhibitors of the GABA uptake systems

This review describes a novel class of heterocyclic GABA uptake inhibitor with no affinity for the GABA receptors. The parent compound nipecotic acid is a potent inhibitor of neuronal and glial GABA uptake, and nipecotic acid is a substrate for the transport carriers concerned. The structurally related cyclic amino acids guvacine and cis-4-hydroxynipecotic acid are also potent inhibitors of both GABA transport systems. Even minor structural alterations of these compounds result in considerable or complete loss of activity. Whereas homonipecotic acid is a weak but selective inhibitor of glial GABA uptake, homoguvacine is virtually inactive. Similarly the lower homologues of nipecotic acid and guvacine, beta-proline and 3-pyrroline-3-carboxylic acid, respectively, show some selectivity with respect to inhibition of glial GABA uptake, but these compounds are much weaker than the parent compounds. The bicyclic compounds THPO and THAO, in which the carboxyl groups of nipecotic acid and homonipecotic acid have been replaced by 3-isoxazolol units are moderately potent and practically specific inhibitors of glial GABA uptake. cis-4-Mercaptonipecotic acid is considerably weaker than the closely related analogue cis-4-hydroxynipecotic acid, but the former compound may interact irreversibly with the GABA transport carriers. The results demonstrate a pronounced substrate specificity of the glial and in particular the neuronal GABA transport system. It is evident that the GABA molecule is transported in a conformation different from that, in which it activates its receptors. These findings are of importance for the development of drugs for selective pharmacological regulation of the functions of central GABA-mediated synapses in certain neurological diseases.

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

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.