Role of astrocytic transport processes in glutamatergic and GABAergic neurotransmission

The fine tuning of both glutamatergic and GABAergic neurotransmission is to a large extent dependent upon optimal function of astrocytic transport processes. Thus, glutamate transport in astrocytes is mandatory to maintain extrasynaptic glutamate levels sufficiently low to prevent excitotoxic neuronal damage. In GABA synapses hyperactivity of astroglial GABA uptake may lead to diminished GABAergic inhibitory activity resulting in seizures. As a consequence of this the expression and functional activity of astrocytic glutamate and GABA transport is regulated in a number of ways at transcriptional, translational and post-translational levels. This opens for a number of therapeutic strategies by which the efficacy of excitatory and inhibitory neurotransmission may be manipulated.

GABA transporters and GABA-transaminase as drug targets

The fine-tuning and homeostatic balance of the GABAergic inhibitory tone in the central nervous system (CNS) is a prerequisite for controlling the excitatory neurotransmission. This principal mechanism for controlling excitation is inhibition which has been the topic of intensive research covering all known functional entities of the GABAergic synapse. The therapeutical scope for targeting the GABA system covers a large number of neurological and psychiatric disorders. This review focuses on the major inactivation systems for GABAergic neurotransmission, the GABA transporters (GATs) and the GABA catabolic enzyme GABA -transaminase (GABA-T) as drug targets. Tiagabin and Vigabatrin, two anti-epileptic drugs on the market today, specifically inhibit GABA transport and metabolism, respectively. However, previous and recent evidence has clearly demonstrated the importance and differential functional roles of glial and neuronal GABA uptake and the metabolic fate of the sequestered neurotransmitter GABA in these cells. Moreover, the diverse expression patterns of the GABA transporters, in combination with development of GAT inhibitors with novel pharmacological profiles may initiate a renaissance for these inactivation systems as drugs targets. In particular, further research to elucidate the specialized physiological function of the GATs combined with their differential spatial expression could be of fundamental importance for the understanding of concerted action with regard to the fine-tuning of the GABAergic inhibitory tone. As such, selective targeting and modulation of GABA transporter subtypes and cell-specific GABA uptake and metabolism is of therapeutical interest in GABA-related CNS disorders, including epilepsy.

Neurotoxic and neuroprotective effects of the glutamate transporter inhibitor DL-threo-beta-benzyloxyaspartate (DL-TBOA) during physiological and ischemia-like conditions

Maintenance of low extracellular glutamate ([Glu](O)) preventing excitotoxic cell death requires fast removal of glutamate from the synaptic cleft. This clearance is mainly provided by high affinity sodium-dependent glutamate transporters. These transporters can, however, also be reversed and release glutamate to the extracellular space in situations with energy failure. In this study the cellular localisation of the glutamate transporters GLAST and GLT-1 in organotypic hippocampal slice cultures was studied by immunofluorescence confocal microscopy, under normal culture conditions, and after a simulated ischemic insult, achieved by oxygen and glucose deprivation (OGD). In accordance with in vivo findings, GLAST and GLT-1 were primarily expressed by astrocytes under normal culture conditions, but after OGD some damaged neurons also expressed GLAST and GLT-1. The potential damaging effect of inhibition of the glutamate transporters by DL-threo-beta-benzyloxyaspartate (DL-TBOA) was studied using cellular uptake of propidium iodide (PI) as a quantitative marker for the cell death. Addition of DL-TBOA for 48 h was found to induce significant cell death in all hippocampal regions, with EC(50) values ranging from 38 to 48 microM for the different hippocampal subregions. The cell death was prevented by addition of the glutamate receptor antagonists NBQX and MK-801, together with an otherwise saturating concentration of DL-TBOA (100 microM). Finally, the effect of inhibition of glutamate release, via reverse operating transporters during OGD, was investigated. Addition of a sub-toxic (10 microM) dose of DL-TBOA during OGD, but not during the subsequent 48 h recovery period, significantly reduced the OGD-induced PI uptake. It is concluded: (1) that the cellular expression of the glutamate transporters GLAST and GLT-1 in hippocampal slice cultures in general corresponds to the expression in vivo, (2) that inhibition of the glutamate transporters induces cell death in the slice cultures, and (3) that partial inhibition during simulation of ischemia by OGD protects against the induced PI uptake, most likely by blocking the reverse operating transporters otherwise triggered by the energy failure.

GABA transporters as drug targets for modulation of GABAergic activity

The identification and subsequent development of the GABA transport inhibitor tiagabine has confirmed the important role that GABA transporters play in the control of CNS excitability. Tiagabine was later demonstrated to be a selective inhibitor of the GABA transporter GAT1. Although selective for GAT1, tiagabine lacks cell type selectivity and is an equipotent inhibitor of neuronal and glial GAT1. To date, four GABA transporters have been cloned, i.e., GAT1-4. The finding that some of these display differential cellular and regional expression patterns suggests that drugs targeting GABA transporters other than GAT1 might offer some therapeutic advantage over GAT1 selective inhibitors. Furthermore, it is particularly interesting that several recently defined GABA transport inhibitors have been demonstrated to display a preferential selectivity for the astrocytic GAT1 transporter. That cellular heterogeneity of GAT1 plays a role in the control of CNS function is confirmed by the demonstration that inhibition of astrocytic GABA uptake is highly correlated to anticonvulsant activity. At the present time, a functional role for the other GABA transporters is less well defined. However, recent findings have suggested a role for the mouse GAT2 (homologous to the human betaine transporter) in the control of seizure activity. In these studies, the non-selective GAT1 and mouse GAT2 transport inhibitor EF1502 (N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-hydroxy-4-(methylamino)-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol) was found to exert a synergistic anticonvulsant action when tested in combination with the GAT1 selective inhibitors tiagabine and LU-32-176B (N-[4,4-bis(4-fluorophenyl)-butyl]-3-hydroxy-4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol). Additional studies will be required to define a role for the other GABA transporters and to further identify the functional importance of their demonstrated cellular and regional heterogeneity. A summary of these and other issues are discussed in this brief review.

First demonstration of a functional role for central nervous system betaine/{gamma}-aminobutyric acid transporter (mGAT2) based on synergistic anticonvulsant action among inhibitors of mGAT1 and mGAT2

In a recent study, EF1502 [N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-hydroxy-4-(methylamino)-4,5,6,7-tetrahydrobenzo [d]isoxazol-3-ol], which is an N-substituted analog of the GAT1-selective GABA uptake inhibitor exo-THPO (4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol), was found to inhibit GABA transport mediated by both GAT1 and GAT2 in human embryonic kidney (HEK) cells expressing the mouse GABA transporters GAT1 to 4 (mGAT1-4). In the present study, EF1502 was found to possess a broad-spectrum anticonvulsant profile in animal models of generalized and partial epilepsy. When EF1502 was tested in combination with the clinically effective GAT1-selective inhibitor tiagabine [(R)-N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]nipecotic acid] or LU-32-176B [N-[4,4-bis(4-fluorophenyl)-butyl]-3-hydroxy-4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol], another GAT1-selective N-substituted analog of exo-THPO, a synergistic rather than additive anticonvulsant interaction was observed in the Frings audiogenic seizure-susceptible mouse and the pentylenetetrazol seizure threshold test. In contrast, combination of the two mGAT1-selective inhibitors, tiagabine and LU-32-176B, resulted in only an additive anticonvulsant effect. Importantly, the combination of EF1502 and tiagabine did not result in a greater than additive effect in the rotarod behavioral impairment test. In subsequent in vitro studies conducted in HEK-293 cells expressing the cloned mouse GAT transporters mGAT1 and mGAT2, EF1502 was found to noncompetitively inhibit both mGAT1 and the betaine/GABA transporter mGAT2 (K(i) of 4 and 5 muM, respectively). Furthermore, in a GABA release study conducted in neocortical neurons, EF1502 did not act as a substrate for the GABA carrier. Collectively, these findings support a functional role for mGAT2 in the control of neuronal excitability and suggest a possible utility for mGAT2-selective inhibitors in the treatment of epilepsy.

Selective inhibitors of GABA uptake: synthesis and molecular pharmacology of 4-N-methylamino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol analogues

A series of lipophilic diaromatic derivatives of the glia-selective GABA uptake inhibitor (R)-4-amino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol [(R)-exo-THPO, 4] were synthesized via reductive amination of 3-ethoxy-4,5,6,7-tetrahydrobenzo[d]isoxazol-4-one (9) or via N-alkylation of O-alkylatedracemic 4. The effects of the target compounds on GABA uptake mechanisms in vitro were measured using a rat brain synaptosomal preparation or primary cultures of mouse cortical neurons and glia cells (astrocytes), as well as HEK cells transfected with cloned mouse GABA transporter subtypes (GAT1-4). The activity against isoniazid-induced convulsions in mice after subcutaneous administration of the compounds was determined. All of the compounds were potent inhibitors of synaptosomal uptake the most potent compound being (RS)-4-[N-(1,1-diphenylbut-1-en-4-yl)amino]-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol (17a, IC50 = 0.14 microM). The majority of the compounds showed a weak preference for glial, as compared to neuronal, GABA uptake. The highest degree of selectivity was 10-fold corresponding to the glia selectivity of (R)-N-methyl-exo-THPO (5). All derivatives showed a preference for the GAT1 transporter, as compared with GAT2-4, with the exception of (RS)-4-[N-[1,1-bis(3-methyl-2-thienyl)but-1-en-4-yl]-N-methylamino]-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol (28d), which quite surprisingly turned out to be more potent than GABA at both GAT1 and GAT2 subtypes. The GAT1 activity was shown to reside in (R)-28d whereas (R)-28d and (S)-28d contributed equally to GAT2 activity. This makes (S)-28d a GAT2 selective compound, and (R)-28d equally effective in inhibition of GAT1 and GAT2 mediated GABA transport. All compounds tested were effective as anticonvulsant reflecting that these compounds have blood-brain barrier permeating ability.

Correlation between anticonvulsant activity and inhibitory action on glial gamma-aminobutyric acid uptake of the highly selective mouse gamma-aminobutyric acid transporter 1 inhibitor 3-hydroxy-4-amino-4,5,6,7-tetrahydro-1,2-benzisoxazole and its N-alkylated analogs

The inhibitory effect of 3-hydroxy-4-amino-4,5,6,7-tetrahydro-1,2-benzisoxazole (exo-THPO) and its N-methylated (N-methyl-exo-THPO) and N-ethylated (N-ethyl-exo-THPO) analogs, derived from gamma-aminobutyric acid (GABA) and 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol (THPO) on GABA transport was investigated using cultured neocortical neurons (GABA-ergic) and astrocytes and cloned mouse GABA transporters GAT1-4 expressed in human embryonic kidney (HEK) 293 cells. Anticonvulsant activity was assessed after i.c.v. administration to Frings audiogenic seizure-susceptible mice. Anticonvulsant activity of the O-pivaloyloxymethyl prodrug of N-methyl-exo-THPO was assessed after i.p. administration. Results from these studies were compared with those obtained from similar studies with the novel anticonvulsant drug tiagabine, which acts via inhibition of GABA transport. exo-THPO and its N-alkyl analogs inhibited neuronal, astrocytic, and GAT1-mediated GABA transport but not GABA uptake mediated by GAT2-4. N-Methyl-exo-THPO was 8-fold more potent as an inhibitor of astrocytic versus neuronal GABA uptake. The IC(50) value for inhibition of GABA uptake by GAT1 closely reflected its IC(50) value for inhibition of neuronal uptake. Tiagabine was approximately 1000-fold more potent than exo-THPO and its alkyl derivatives as an inhibitor of GABA uptake in cultured neural cells and GAT1-expressing HEK 293 cells. exo-THPO, its alkylated analogs, and tiagabine displayed a time- and dose-dependent inhibition of audiogenic seizures after i.c.v. administration. N-Methyl-exo-THPO was the most potent anticonvulsant among the exo-THPO compounds tested and only slightly less potent than tiagabine. The findings suggest a correlation between anticonvulsant efficacy and selective inhibition of astroglial GABA uptake. Furthermore, results obtained with the N-methyl-exo-THPO prodrug demonstrate the feasibility of developing a glial-selective GABA uptake inhibitor with systemic bioavailability.

GDNF pre-treatment aggravates neuronal cell loss in oxygen-glucose deprived hippocampal slice cultures: a possible effect of glutamate transporter up-regulation

Besides its neurotrophic and neuroprotective effects on dopaminergic neurons and spinal motoneurons, glial cell line-derived neurotrophic factor (GDNF) has potent neuroprotective effects in cerebral ischemia. The protective effect has so far been related to reduced activation of N-methyl-D-aspartate receptors (NMDAr). This study tested the effects of GDNF on glutamate transporter expression, with the hypothesis that modulation of glutamate transporter activity would affect the outcome of cerebral ischemia. Organotypic hippocampal slice cultures, derived from 1-week-old rats, were treated with 100 ng/ml GDNF for either 2 or 5 days, followed by Western blot analysis of NMDAr subunit 1 (NR1) and two glutamate transporter subtypes, GLAST and GLT-1. After 5-day exposure to GDNF, expression of GLAST and GLT-1 was up-regulated to 169 and 181% of control values, respectively, whereas NR1 was down-regulated to 64% of control. However, despite these changes that potentially would support neuronal resistance to excitotoxicity, the long-term treatment with GDNF was found to aggravate the neuronal damage induced by oxygen-glucose deprivation (OGD). The increased cell death, assessed by propidium iodide (PI) uptake, occurred not only among the most susceptible CA1 pyramidal cells, but also in CA3 and fascia dentata. Given that glutamate transporters are able to release glutamate by reversed action during energy failure, it is suggested that the observed increase in OGD-induced cell death in the GDNF-pretreated cultures was caused by the build-up of excitotoxic concentrations of extracellular glutamate released through the glutamate transporters, which were up-regulated by GDNF. Although the extent and consequences of glutamate release via reversal of GLAST and GLT-1 transporters seem to vary in different energy failure models, the present findings should be taken into account in clinical trials of GDNF.

Effects of 3-hydroxy-4-amino-4,5,6,7-tetrahydro-1,2-benzisoxazol (exo-THPO) and its N-substituted analogs on GABA transport in cultured neurons and astrocytes and by the four cloned mouse GABA transporters

The system of GABA transporters in neural cells constitutes an efficient mechanism for terminating inhibitory GABAergic neurotransmission. As such these transporter are important therapeutical targets in epilepsy and potentially other neurological diseases related to the GABA system. In this study a number of analogs of 3-hydroxy-4-amino-4,5,6,7-tetrahydro-1,2-benzisoxazol (exo-THPO), a promising lead structure for inhibitors of GABA uptake were investigated. It was found that the selectivity of N-acetyloxyethyl-exo-THPO for inhibition of the astroglial GABA uptake system was 10-fold as compared to inhibition of the neuronal GABA uptake system. Selectivity in this magnitude may provide potent anti-convulsant activity as has recently been demonstrated with the likewise glia-selective GABA uptake inhibitor, N-methyl-exo-THPO. In contrast to the competitive inhibition of GABA uptake exhibited by N-substituted analogs of 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol (THPO), nipecotic acid, and guvacine, N-4,4-diphenyl-3-butenyl(DPB)-N-methyl-exo-THPO and 4-phenylbutyl-exo-THPO exhibited non-competitive type inhibition kinetics. The lipophilic character of a number of GABA analogs was concluded by far to constitute the determining factor for the potency of these compounds as inhibitors of GAT1-mediated uptake of GABA. This finding underscores the complexity of the pharmacology of the GABA transport system, since these non-competitive inhibitors are structurally very similar to some competitive GABA uptake inhibitors. Whether these structure-activity relationships for inhibition of GABA uptake may provide sufficient information for the development of new structural leads and to what extent these compounds may be efficient as therapeutical anti-convulsant agents remain to be elucidated.

Effect of Hyperosmotic Conditions on the Expression of the Betaine-GABA-Transporter (BGT-1) in Cultured Mouse Astrocytes

The adaptation of cells to hyperosmotic conditions involves accumulation of organic osmolytes to achieve osmotic equilibrium and maintenance of cell volume. The Na+ and Cl(-)-coupled betaine/GABA transporter, designated BGT-1, is responsible for the cellular accumulation of betaine and has been proposed to play a role in osmoregulation in the brain. BGT-1 is also called GAT2 (GABA transporter 2) when referring to the mouse transporter homologue. Using Western Blotting the expression of the mouse GAT2 protein was investigated in astrocyte primary cultures exposed to a growth medium made hyperosmotic (353+/-2.5 mosmol/kg) by adding sodium chloride. A polyclonal anti-BGT-1 antibody revealed the presence of two characteristic bands at 69 and 138 kDa. When astrocytes were grown for 24 h under hyperosmotic conditions GAT2 protein was up-regulated 2-4-fold compared to the level of the isotonic control. Furthermore, the expected dimer of GAT2 was also up-regulated after 24 h under the hyperosmotic conditions. The [3H]GABA uptake was examined in the hyperosmotic treated astrocytes, and characterized using different selective GABA transport inhibitors. The up-regulation of GAT2 protein was not affecting total GABA uptake but the hyperosmotic condition did change total GABA uptake possibly involving GAT1. Immunocytochemical studies revealed cell membrane localization of GAT2 throughout astroglial processes. Taken together, these results indicate that astroglial GAT2 expression and function may be regulated by hyperosmolarity in cultured mouse astrocytes, suggesting a role of GAT2 in osmoregulation in neural cells.

Role of the betaine/GABA transporter (BGT-1/GAT2) for the control of epilepsy

Inactivation of gamma-aminobutric acid (GABA) as a neurotransmitter is mediated by diffusion in the synaptic cleft followed by binding to transporter sites and translocation into the intracellular compartment. The GABA transporters of which four subtypes have been cloned (GAT1-4) are distributed at presynaptic nerve endings as well as extrasynaptically on astrocytic and neuronal elements. This anatomical arrangement of the transporters appears to be of critical functional importance for the maintenance of GABAergic neurotransmission. Pharmacological characterization of the GABA transporters using a large number of GABA analogs having restricted conformation and lipophilic character has been of instrumental importance for elucidation of the functional importance of the different transporters. One such analog EF1502 (N-[4,4-bis(3-methyl-2-thienyl)-3-butenyl]-3-hydroxy-4-methylamino-4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol) has been shown to selectively inhibit GAT1 (GABA transporter 1) and GAT2/BGT-1 (betaine/GABA transporter). Moreover, this GABA analog exhibits an unusually high efficiency as an anticonvulsant suggesting a novel role of the betaine/GABA transporter in epileptic seizure control. It is hypothesized that extrasynaptic actions of GABA may be involved in this phenomenon.

Effect of topiramate and dBcAMP on expression of the glutamate transporters GLAST and GLT-1 in astrocytes cultured separately, or together with neurons

The mechanism of the antiepileptic drug topiramate is not fully understood, but interaction with the excitatory neurotransmission, e.g. glutamate receptors, is believed to be part of its anticonvulsant effect. The glutamate transporters GLAST and GLT-1 are responsible for the inactivation of glutamate as a neurotransmitter and it was therefore investigated if topiramate might affect the expression of GLAST and GLT-1 in astrocytes cultured separately or together with neurons. Since expression and membrane trafficking of glutamate transporters are affected by the protein kinase C system as well as by dBcAMP it was also investigated if these signalling pathways might play a role. In astrocyte cultures expressing mainly GLAST treatment with dBcAMP (0.25 mM) led to an increased expression of the total amount of GLAST as well as of its membrane association. The enhanced expression in the membrane was particularly pronounced for the oligomeric form of GLAST. No detectable effect on the expression of GLAST in astrocytes treated with topiramate in the presence and absence of protein kinase C activators or inhibitors was observed. Astrocytes co-cultured with neurons expressed both GLAST and GLT-1. In these cultures prolonged exposure to 30 muM topiramate (10 days) led to a statistically significant increase (P<0.025) in the membrane expression of GLAST. In case of GLT-1, culture in the presence of 30 microM topiramate for 1 and 10 days led to alterations in the total, cytoplamic and membrane expression of the oligomeric form of the transporter.

Psychological assessment of blood related renal donors

The impact of kidney donation on the psychological health of 31 living related donors was assessed by administering certain psychological tests before and after the operation (for donating the kidney). The results indicated a significant rise in the somatization subscale of the Middlesex Hospital Questionnaire (MHQ) from a mean of 1.61 to 3.23. There was no significant change in the other variables of these instruments or in the locus of control score. Only about one-fourth of the donors had prior knowledge of renal transplant. In almost all cases, the decision to donate had been voluntary and immediate, motivated by a concern for the recipient; there was virtually no second thoughts or regrets subsequently, which was apparently partly related to the opinions of other relatives who positively valued the act of donation.