Retinal uptake and release of [3H]DABA

Guinea pig horizontal cells express GABA, the GABA-synthesizing enzyme GAD 65, and the GABA vesicular transporter

Gamma-aminobutyric acid (GABA) is likely expressed in horizontal cells of all species, although conflicting physiological findings have led to considerable controversy regarding its role as a transmitter in the outer retina. This study has evaluated key components of the GABA system in the outer retina of guinea pig, an emerging retinal model system. The presence of GABA, its rate-limiting synthetic enzyme glutamic acid decarboxylase (GAD(65) and GAD(67) isoforms), the plasma membrane GABA transporters (GAT-1 and GAT-3), and the vesicular GABA transporter (VGAT) was evaluated by using immunohistochemistry with well-characterized antibodies. The presence of GAD(65) mRNA was also evaluated by using laser capture microdissection and reverse transcriptase-polymerase chain reaction. Specific GABA, GAD(65), and VGAT immunostaining was localized to horizontal cell bodies, as well as to their processes and tips in the outer plexiform layer. Furthermore, immunostaining of retinal whole mounts and acutely dissociated retinas showed GAD(65) and VGAT immunoreactivity in both A-type and B-type horizontal cells. However, these cells did not contain GAD(67), GAT-1, or GAT-3 immunoreactivity. GAD(65) mRNA was detected in horizontal cells, and sequencing of the amplified GAD(65) fragment showed approximately 85% identity with other mammalian GAD(65) mRNAs. These studies demonstrate the presence of GABA, GAD(65), and VGAT in horizontal cells of the guinea pig retina, and support the idea that GABA is synthesized from GAD(65), taken up into synaptic vesicles by VGAT, and likely released by a vesicular mechanism from horizontal cells.

Multiple gamma-Aminobutyric acid plasma membrane transporters (GAT-1, GAT-2, GAT-3) in the rat retina

gamma-Aminobutyric acid (GABA) plasma membrane transporters (GATs) influence synaptic neurotransmission by high-affinity uptake and release of GABA. The distribution and cellular localization of GAT-1, GAT-2, and GAT-3 in the rat retina have been evaluated by using affinity-purified polyclonal antibodies directed to the C terminus of each of these GAT subtypes. Small GAT-1-immunoreactive cell bodies were located in the proximal inner nuclear layer (INL) and ganglion cell layer (GCL), and processes were distributed to all laminae of the interplexiform layer (IPL). Varicose processes were in the optic fiber layer (OFL) and the outer plexiform layer (OPL). Weak GAT-1 immunostaining surrounded cells in the INL and GCL, and it was found in the OFL and OPL and in numerous processes in the outer nuclear layer (ONL) that ended at the outer limiting membrane. GAT-1 is therefore strongly expressed by amacrine, displaced amacrine, and interplexiform cells and weakly expressed by Müller cells. GAT-2 immunostaining was observed in the retina pigment epithelium and the nonpigmented ciliary epithelium. GAT-3 immunoreactivity was distributed to the OFL, to all laminae of the IPL, GCL and INL, and to processes in the ONL that ended at the outer limiting membrane. Small GAT-3-immunoreactive cell bodies were in the proximal INL and GCL. GAT-3 is therefore strongly expressed by Müller cells, and by some amacrine and displaced amacrine cells. Together, these observations demonstrate a heterologous distribution of GATs in the retina. These transporters are likely to take up GABA from, and perhaps release GABA into, the synaptic cleft and extracellular space. This suggests that GATs regulate GABA levels in these areas and thus influence synaptic neurotransmission.

Expression of GAT-1, a high-affinity gamma-aminobutyric acid plasma membrane transporter in the rat retina

Gamma-aminobutyric acid (GABA) plasma membrane transporters influence synaptic transmission by high-affinity, Na(+)-dependent transport processes. The cDNA clone, GAT-1, encodes a high-affinity Na(+)- and Cl(-)-dependent GABA plasma membrane transporter, which has kinetic and pharmacological properties similar to those of high-affinity GABA uptake systems associated with neurons. The present study evaluates the distribution and cellular localization of this putative neuronal GABA transporter by RNA blot hybridization and in situ hybridization histochemistry in the rat retina. Northern blot hybridization analysis of total retinal and cerebellar RNA extracts demonstrated a single band of hybridization at 4.2 kilobases. GABA transporter mRNA is expressed by numerous cells that are distributed to the proximal inner nuclear layer and the ganglion cell layer and by a few cells located in the inner plexiform layer. Double label studies combining the retrograde transport of the fluorescent dye Fluorogold from the superior colliculus to identify ganglion cells and in situ hybridization histochemistry demonstrated that most GAT-1 mRNA-containing cells in the ganglion cell layer are displaced amacrine cells, although some ganglion cells containing GAT-1 mRNA were visualized. In freshly dissociated retinal cell preparations, the GAT-1 RNA signal is strong in neurons and weak to moderate in specialized glial cells called Müller cells. Müller cells were identified by both their morphology and the presence of the selective Müller cell marker cellular retinaldehyde-binding protein. Only background labeling is seen with the sense GAT-1 RNA probe in both tissue sections and dissociated retinal cell preparations. These findings demonstrate that GAT-1 mRNA is expressed in both the retina and brain. In the retina, this transporter is predominantly localized to amacrine, displaced amacrine and interplexiform cells, and some ganglion cells. This transporter mRNA is also expressed by Müller cells but at a lower level than by neurons. These observations indicate that GABA transport by GAT-1 plasma membrane transporters in the retina is mediated by both neurons and glia cells.

Retinal GABA neuron labelling with [3H]isoguvacine in different species

Retinas from goldfish, chicken, rate, guinea-pig, rabbit and humans were exposed to [3H]isoguvacine either by intravitreal injection in vivo or by incubation in a balanced salt solution. The distribution of radioactivity was then studied by autoradiography. The substance labelled a set of presumed amacrine cells in all types of animals. The inner plexiform layer was well demarcated and a variable number of ganglion cells was marked. In goldfish and chicken, radioactivity could also be seen in horizontal cells, particularly when the retina had detached from the pigment epithelium. Even 24 hr after an intraocular injection there was a significant amount of radioactivity left in nerve cells. Only a little glial labelling could be seen at any time. Since the distribution of labelled neurones was similar to that of GABA neurones and since isoguvacine is a potent GABA agonist, it seems reasonable to presume that [3H]isoguvacine labels GABA neurones. The uptake is predominantly neuronal with strong binding to presumed GABA storage sites.

(3H)-muscimol, (3H)-nipecotic acid and (3H)-isoguvacine as autoradiographic markers for GABA neurotransmission

Retinas from rabbit, goldfish and guinea-pig were exposed to (3H) GABA, (3H)-nipecotic acid and (3H)-isoguvacine either by intravitreal injection in vivo or by incubations in a balanced salt solution and the distribution of radioactivity was then studied with autoradiography. All substances labelled a similar set of presumed amacrine cells. Incubating at 0 degrees C, in 10-(5)M ouabain, or in 10-(3)M GABA inhibited the labelling by (3H)-muscimol whereas bicuculline (10-(4)M), and glycine (10-(3)M) were less efficient blockers. The result is interpreted as a neuronal uptake of (3H)-muscimol rather than as a GABA receptor binding. All the substances except (3H)-isoguvacine also labelled glia to such a degree that neuronal labelling was often disguised in rabbits and goldfish. Glial labelling by muscimol was less pronounced in guinea-pig. (3H)-isoguvacine (tested only in rabbits) gave a strong labelling of cells with the distribution of GABA neurons and only little glial labelling.

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.

Effect of pentobarbitone on the spontaneous efflux of gamma-amino acids from rabbit retina

The effect of pentobarbitone on the spontaneous release of radioactivity from rabbit retinas preloaded with [3H]GABA, [3H]DABA, [3H]beta-alanine and [3H]glycine was studied. In high concentration of pentobarbitone (10(-3)M) the rate of spontaneous efflux of neuronal [3H]GABA, [3H]DABA and [3H]beta-alanine was reduced but not that of [3H]glycine. Glial release of [3H]GABA was much less influenced. At lower concentrations (10(-4)-10(-5)M) there was an initial increase in the release of [3H]GABA, [3H]DABA and [oH]beta-alanine followed by a decrease, indicating several components in the release system for the amino acids.