Phylogenetic conservation of 4-aminobutyric acid (GABA) transporter isoforms. Cloning and pharmacological characterization of a GABA/beta-alanine transporter from Torpedo

Gene expression profiling as a tool to investigate the molecular machinery activated during hippocampal neurodegeneration induced by trimethyltin (TMT) administration

Trimethyltin (TMT) is an organotin compound exhibiting neurotoxicant effects selectively localized in the limbic system and especially marked in the hippocampus, in both experimental animal models and accidentally exposed humans. TMT administration causes selective neuronal death involving either the granular neurons of the dentate gyrus or the pyramidal cells of the Cornu Ammonis, with a different pattern of localization depending on the different species studied or the dosage schedule. TMT is broadly used to realize experimental models of hippocampal neurodegeneration associated with cognitive impairment and temporal lobe epilepsy, though the molecular mechanisms underlying the associated selective neuronal death are still not conclusively clarified. Experimental evidence indicates that TMT-induced neurodegeneration is a complex event involving different pathogenetic mechanisms, probably acting differently in animal and cell models, which include neuroinflammation, intracellular calcium overload, and oxidative stress. Microarray-based, genome-wide expression analysis has been used to investigate the molecular scenario occurring in the TMT-injured brain in different in vivo and in vitro models, producing an overwhelming amount of data. The aim of this review is to discuss and rationalize the state-of-the-art on TMT-associated genome wide expression profiles in order to identify comparable and reproducible data that may allow focusing on significantly involved pathways.

Cloning, immunolocalization, and functional expression of a GABA transporter from the retina of the skate

Termination of GABA signals within the retina occurs through high-affinity reuptake of the released neurotransmitter by GABA transporters (GATs) present in neurons and glia surrounding the release site. In the present work, we have cloned a novel GAT from the retina of the skate (Raja erinacea). The clone codes for a 622 amino acid protein whose sequence has highest similarity to the GABA/β-alanine transporter of the electric ray (Torpedo marmorata) (88% identity) and the GAT-3 isolated from rat brain (75% identity). The protein was expressed inXenopusoocytes and characterized using the two-electrode voltage-clamp technique. Application of GABA induced a dose-dependent inward current, with 8 μM GABA producing a half-maximal response. The current required the presence of extracellular sodium and was unaffected by the GABA receptor blocker picrotoxin or the GAT-1 specific antagonist NO-711. The high homology between the cloned skate GABA transporter and the GAT-3 equivalents of other species, coupled with the strikingly similar pharmacological profile to GAT-3s of other species, lead us to conclude that we had cloned the GAT-3 homologue for the skate. Polyclonal antibodies specific to GAT-3 and the previously cloned skate GAT-1 transporter were used to examine the distribution of GAT-3 and GAT-1 immunoreactivity in the retina and in isolated cells of the skate. Antibodies for both transporters showed labeling in the outer and inner plexiform layers, and staining extended from the outer to inner limiting membranes. Both GAT-1 and GAT-3 antibodies labeled enzymatically isolated Müller cells, while bipolar cells and horizontal cells did not appear to express either transporter. These results imply that GAT-1 and GAT-3 are both present in Müller cells of the skate retina where they are likely involved in regulating extracellular concentrations of GABA.

A novel approach to the psychopharmacologic treatment of insomnia in depression

BACKGROUND

Depressed patients who respond to anti-depressant drugs but have persisting insomnia can be hypothesized as having one foot on the bipolar spectrum. In this hypothesis, the insomnia would respond to augmentation of the anti-depressants with anti-kindling agents (i.e., anti-convulsant drugs for the purposes of this paper) in the same way that many anti-convulsants are effective in bipolar disorders as mood-stabilizers from above (anti-manic agents) or below (anti-depressants).

CASE REPORTS

Cases supporting this augmentation strategy are presented. The patients met DSM-IV criteria for recurrent major depression, depression with psychotic features, and depression NOS. There was no discernible history of mania or hypomania and all had insomnia that did not respond or partially responded to anti-depressant drugs singly or in combinations. All responded to the addition of anti-convulsant drugs.

DISCUSSION

This augmentation strategy seems to be an effective treatment for chronic insomnia in depression, but controlled studies are needed to quantify and qualify these effects. This strategy's effectiveness may be due to the anti-kindling or inhibitory effects of anti-convulsant drugs, which are often GABAergic. Since it is becoming clear that bipolar spectrum disorders are misdiagnosed as unipolar depression at an alarming rate, and since anti-depressants often have deleterious effects on the course of bipolar illness, perhaps most depressed patients should be protected with an anti-convulsant whenever they are started on an anti-depressant.

Postsynaptic localization of ?-aminobutyric acid transporters and receptors in the outer plexiform layer of the goldfish retina: An ultrastructural study

The gamma-aminobutyric acid (GABA)-ergic system in the outer plexiform layer (OPL) of the goldfish retina was studied via light and electron immunohistochemistry. The subcellular distributions of immunoreactivity (-IR) of plasma membrane GABA transporters GAT2 and GAT3, the alpha1 and alpha3 subunits of the ionotropic GABA(A) receptor, and the rho1 subunit of the ionotropic GABA(C) receptor were determined. The localization of the GAT2-IR and GAT3-IR to horizontal cell dendrites at the base of the cone synaptic complex was the main characteristic at the ultrastructural level. Very rarely, GAT2-IR and GAT3-IR were found in horizontal cell dendrites innervating rod spherules. alpha1-IR and alpha3-IR were seen in wide bands in the OPL, whereas rho1-IR appeared as a narrow band in the OPL. Most alpha1-IR was intracellular in rod and cone terminals. Membrane-associated alpha1-IR was observed in cone pedicles but not in rod spherules; postsynaptic elements were also labeled. alpha3-IR was concentrated in the lateral elements of horizontal cell dendrites in cone pedicles. In contrast, rho1-IR was found mainly on the spinules of the horizontal cell dendrites in cone pedicles. In addition, in another type of cone pedicle, rho1-IR was found at the position of OFF-bipolar cell dendrites. alpha3-IR and rho1-IR were rarely found in horizontal cell dendrites innervating rods. We suggest that two GABAergic pathways exist in the outer retina- first, a GABAergic positive loop with GABA receptors mainly on the horizontal cell dendrites and spinules and, second, a GABAergic feedback pathway involving GABA receptors on cone pedicles and GABA transporters on horizontal cells and that this pathway presumably modulates feedback strength from horizontal cells to cones.

Distribution of muscimol, QNB, and 5HT binding in the vertebrate diencephalon: A comparative study of eight mammals and three non-mammals

The distribution of muscimol, quinuclidinyl benzilate (QNB), and serotonin (5HT)-bound receptors in the diencephalon was examined by conventional receptor-binding methods in 11 species of amniotes including 2 reptiles, 1 bird, and 8 mammals, selected mostly on the basis of their differing last common ancestor with Anthropoids. We found that receptor binding can help define major subdivisions of the forebrain. The results show that in each of these species, the distribution of muscimol and QNB binding across the four major subdivisions of the diencephalon was consistent; densest in the dorsal thalamus, with hypothalamus and then either ventral thalamus or epithalamus with successively lesser amounts. However, the binding of serotonin (5HT) was most prevalent in the hypothalamus with equivalent amounts in the other diencephalic subdivisions. Myelin- and cell-stained materials showed that the pattern of high-density binding probably is not the secondary result of non-neurochemical factors such as differences in cell or neuropil density or in total available membrane. Perhaps more importantly, the receptor distributions suggest functional roles for major subdivisions across taxa. Results show that GABA-A and muscaranic Ach receptors are common in the dorsal diencephalon across vertebrate species and, therefore, are probably responsible for the gating of information to the cortex. Results show that serotonin is predominant in the hypothalamus. The lack of it in the dorsal thalamus indicates that it is probably not responsible for gating of information to the cortex. Results also show that in nonmammals the amount of GABA-A and muscaranic Ach differs from that found in mammals. For muscaranic Ach, the labeling in marsupials differs from that in placentals. Primates differ from other species (nonmammals and mammals combined) in the amount of 5HT found in the ventral diencephalon and the hypothalamus.

GABA and GABA-transporter (GAT-1) immunoreactivities in the retina of the salmon (Salmo salar L.)

Putative GABAergic elements in the retina of the Atlantic salmon have been identified by immunohistochemistry, utilising polyclonal antisera against gamma-aminobutyric acid (GABA) and the GABA transporter GAT-1. Cell types immunoreactive (ir) for GABA comprise horizontal cells, amacrine cells, displaced amacrine cells in the ganglion cell layer, displaced amacrine cells in the inner plexiform layer (interstitial cells), and Müller cells. In addition, a GABA-immunonegative type of interstitial cell was also identified. In the inner plexiform layer, GABAir fibres were organised in sublayers that were strikingly similar to the sublayering of GAT-1ir fibres. GAT-1ir cell bodies comprise amacrine cells and displaced amacrine cells that may represent a subpopulation of the GABAir ones. In view of the very similar sublayering of GABAir and GAT-ir fibres in the IPL we suggest that a similar type of GABA transporter, that can be recognised with antibodies against rat GAT-1, is present at least in the dendrites of all GABAir amacrine cells but is not expressed in the cell bodies of all GABAir cells.

GABA transporters in Drosophila melanogaster: molecular cloning, behavior, and physiology

Molecular cloning of GABA transporter-homologous cDNAs from a Drosophila melanogaster head-specific library was accomplished using a conserved oligomer from a highly conserved domain within the mammalian GABA transporters. Partial DNA sequencing of these cDNAs demonstrated homology with the mammalian transporters, indicating these are ancient, evolutionarily conserved molecules. Although the Drosophila cDNAs had distinct restriction enzyme patterns, they recognized the same locus in Drosophila genomic DNA, suggesting that the multiple isoforms might arise via alternative splicing. Antibodies specific for the mammalian GABA transporters GAT 1, GAT 2 and GAT 3 recognized non-overlapping and developmentally distinct patterns of expression in Drosophila neuronal tissues. Treatment of larval instars with nipecotic acid, a generalized GABA reuptake inhibitor, revealed specific, dose-dependent alterations in behavior consistent with the presence of multiple transporter molecules with differing affinities for this drug. Synaptic current recordings revealed that nipecotic acid treated larvae have an increase in latency jitter of evoked quantal release, resulting in a broader average excitatory junctional current which was manifested in a broader EJP. These results imply that alterations in the development of the CNS occur if GABAergic neurotransmission is potentiated during development. The data suggest that, as in mammals, there are multiple GABA transporters in Drosophila whose expression is differentially regulated.

Immunocytochemical localization of gamma-aminobutyric acid plasma membrane transporters in the tiger salamander retina

Gamma-aminobutyric acid (GABA) plasma membrane transporters (GATs) play an important role in regulating GABA neurotransmission in the nervous system. The distribution of two GATs, GAT 1 and GAT 3, in salamander retina was investigated by using affinity-purified polyclonal antisera directed to the predicted C-terminals of rat GAT 1 and rat GAT 3. GAT 1-immunoreactivity (-IR) was found in type IB and IIB orthotopic bipolar cells (BCs) located in the distal and middle of the inner nuclear layer (INL), respectively; in type IIA and IA amacrine cells (ACs) located in the middle and proximal INL, respectively; and in interplexiform cells and cells in the ganglion cell layer (GCL). No detectable staining was found in horizontal cells (HCs) or in structures resembling Müller cells. GAT 1-immunoreactive fibers were present in the outer plexiform layer (OPL) and inner plexiform layer (IPL) in three bands corresponding to the three bands previously reported to be GABA-IR. GAT 3 antibodies labeled fewer cells and cell types than GAT 1 antibodies. GAT 3-IR was localized to type IIA and IA ACs and cells in the GCL, but not to BCs, HCs, or Müller cell-like structures. There was weak labeling of the OPL and stronger labeling of the IPL, with three distinct bands at the same depth as observed with GAT 1-IR. Double-labeling showed that the majority of GAT 1-IR BCs (88%), ACs (88%), and cells in the GCL (78%) colocalized with GABA-IR. The present study provides the first direct evidence of the expression of two GAT subtypes in neurons of nonmammalian retinas. These transporters could regulate GABA neurotransmission by reuptake and termination of GABA's action and, perhaps, by GABA release mechanisms. The presence of GAT 1-IR/GABA-IR bipolar cells further supports our earlier observations that a subgroup of orthotopic bipolar cells are likely to be GABAergic.