Inhibitors of the GABA uptake systems

Structural insights into GABA transport inhibition using an engineered neurotransmitter transporter

γ-aminobutyric acid (GABA) is the major inhibitory neurotransmitter, and its levels in the synaptic space are controlled by the GABA transporter isoforms (GATs). GATs are structurally related to biogenic amine transporters but display interactions with distinct inhibitors used as anti-epileptics. In this study, we engineer the binding pocket of Drosophila melanogaster dopamine transporter to resemble GAT1 and determine high-resolution X-ray structures of the modified transporter in the substrate-free state and in complex with GAT1 inhibitors NO711 and SKF89976a that are analogs of tiagabine, a medication prescribed for the treatment of partial seizures. We observe that the primary binding site undergoes substantial shifts in subsite architecture in the modified transporter to accommodate the two GAT1 inhibitors. We also observe that SKF89976a additionally interacts at an allosteric site in the extracellular vestibule, yielding an occluded conformation. Interchanging SKF89976a interacting residue in the extracellular loop 4 between GAT1 and dDAT suggests a role for this motif in the selective control of neurotransmitter uptake. Our findings, therefore, provide vital insights into the organizational principles dictating GAT1 activity and inhibition.

Application of the concept of oxime library screening by mass spectrometry (MS) binding assays to pyrrolidine-3-carboxylic acid derivatives as potential inhibitors of γ-aminobutyric acid transporter 1 (GAT1)

In the present study, the concept of oxime library screening by MS Binding Assays was successfully extended to N-substituted lipophilic pyrrolidine-3-carboxylic acid derivatives in the pursuit of varying the amino acid motif in order to identify new inhibitors for GAT1 and to broaden structure-activity-relationships for this target, the most abundant GABA transporter in the central nervous system. For the screening, 28 different oxime sub-libraries were employed that were generated by simple condensation reaction of an excess of pyrrolidine-3-carboxylic acid derivatives carrying a hydroxylamine functionality with various sub-libraries each assembled of eight aldehydes with broadly varying chemical structures and functionalities. The compounds responsible for the activity of an oxime sub-library were identified by deconvolution experiments performed by employing single oximes. Binding affinities of the oxime hits were confirmed in full-scale competitive MS Binding Assays. Thereby, oxime derivatives with a 1,1'-biphenyl moiety were found as the first inhibitors of mGAT1 comprising a pyrrolidine-3-carboxylic acid motif with affinities in the submicromolar range.

Structural and molecular aspects of betaine-GABA transporter 1 (BGT1) and its relation to brain function

ɣ-aminobutyric-acid (GABA) functions as the principal inhibitory neurotransmitter in the central nervous system. Imbalances in GABAergic neurotransmission are involved in the pathophysiology of various neurological diseases such as epilepsy, Alzheimer's disease and stroke. GABA transporters (GATs) facilitate the termination of GABAergic signaling by transporting GABA together with sodium and chloride from the synaptic cleft into presynaptic neurons and surrounding glial cells. Four different GATs have been identified that all belong to the solute carrier 6 (SLC6) transporter family: GAT1-3 (SLC6A1, SLC6A13, SLC6A11) and betaine/GABA transporter 1 (BGT1, SLC6A12). BGT1 has emerged as an interesting target for treating epilepsy due to animal studies that reported anticonvulsant effects for the GAT1/BGT1 selective inhibitor EF1502 and the BGT1 selective inhibitor RPC-425. However, the precise involvement of BGT1 in epilepsy remains elusive because of its controversial expression levels in the brain and the lack of highly selective and potent tool compounds. This review gathers the current structural and functional knowledge on BGT1 with emphasis on brain relevance, discusses all available compounds, and tries to shed light on the molecular determinants driving BGT1 selectivity. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Studies on the Activity of Selected Highly Lipophilic Compounds toward hGAT1 Inhibition. Part II

In this paper, we describe the latest results involving molecular modeling and pharmacodynamic studies of the selected highly lipophilic compounds acting by human GABA transporter 1 (hGAT1) inhibition. The chemical interaction of 17 GABA analogues with a model of hGAT1 is described using the molecular docking method. The biological role of GAT1 is related to the regulation of GABA level in the central nervous system and GAT1 inhibition plays an important role in the control of seizure threshold. To confirm that GAT1 can be also a molecular target for drugs used to treat other neurological and psychiatric diseases (e.g., pain and anxiety), in the in vivo part of this study, potential antinociceptive and anxiolytic-like properties of tiagabine, a selective GAT1 inhibitor, are described.

Docking and pharmacodynamic studies on hGAT1 inhibition activity in the presence of selected neuronal and astrocytic inhibitors. Part I

Inhibition of 4-aminobutanoic acid (GABA) uptake is a strategy for enhancing GABA transmission. The utility of this approach is demonstrated by the successful development of such agents for the treatment of epilepsy and pain. Existing reports on acute brain slice preparations indicate the intersecting of complementary channels and receptors sets between astrocytes and neurons cells. Thorough analysis of astroglial cells by means of molecular and functional studies demonstrated their active modulatory role in intercellular communication. The chemical interactions between sixteen GABA analogues and isoform of hGAT1 is outlined in the light of molecular docking results. In the in vivo part antinociceptive properties of racemic nipecotic acid, its R and S enantiomers and isonipecotic acid, each administered intraperitoneally at 3 fixed doses (10, 30 and 100 mg/kg), were assessed in a thermally-induced acute pain model i.e. the mouse hot plate test. Docking analyses provided complex binding energies, specific h-bond components, and h-bond properties, such as energies, distances and angles. In vivo tests revealed statistically significant antinociceptive properties of isonipecotic acid (10 and 30 mg/kg), R-nipecotic acid (30 and 100 mg/kg) and S-nipecotic acid (100 mg/kg) in mice. The docking data endorse the hypothesis of correlation between the strength of their chemical interactions with hGAT1 and analgesic action of studied compounds.

Astrocytic GABA Transporters: Pharmacological Properties and Targets for Antiepileptic Drugs

Inactivation of GABA-mediated neurotransmission is achieved by high-affinity transporters located at both GABAergic neurons and the surrounding astrocytes. Early studies of the pharmacological properties of neuronal and glial GABA transporters suggested that different types of transporters might be expressed in the two cell types, and such a scenario was confirmed by the cloning of four distinctly different GABA transporters from a number of different species. These GABA-transport entities have been extensively characterized using a large number of GABA analogues of restricted conformation, and several of these compounds have been shown to exhibit pronounced anticonvulsant activity in a variety of animal seizure models. As proof of concept of the validity of this drug development approach, one GABA-transport inhibitor, tiagabine, has been developed as a clinically active antiepileptic drug. This review provides a detailed account of efforts to design new subtype-selective GABA-transport inhibitors aiming at identifying novel antiepileptic drug candidates.

Manganese exposure inhibits the clearance of extracellular GABA and influences taurine homeostasis in the striatum of developing rats

Manganese (Mn) accumulation in the brain has been shown to alter the neurochemistry of the basal ganglia. Mn-induced alterations in dopamine biology are fairly well understood, but recently more evidence has emerged characterizing the role of γ-aminobutyric acid (GABA) in this dysfunction. The purpose of this study was to determine if the previously observed Mn-induced increase in extracellular GABA (GABA(EC)) was due to altered GABA transporter (GAT) function, and whether Mn perturbs other amino acid neurotransmitters, namely taurine and glycine (known modulators of GABA). Extracellular GABA, taurine, and glycine concentrations were collected from the striatum of control (CN) or Mn-exposed Sprague-Dawley rats using in vivo microdialysis, and the GAT inhibitor nipecotic acid (NA) was used to probe GAT function. Tissue and extracellular Mn levels were significantly increased, and the Fe:Mn ratio was decreased 36-fold in the extracellular space due to Mn-exposure. NA led to a 2-fold increase in GABA(EC) of CNs, a response that was attenuated by Mn. Taurine responded inversely to GABA, and a novel 10-fold increase in taurine was observed after the removal of NA in CNs. Mn blunted this response and nearly abolished extracellular taurine throughout collection. Striatal taurine transporter (Slc6a6) mRNA levels were significantly increased with Mn-exposure, and Mn significantly increased (3)H-Taurine uptake after 3-min exposure in primary rat astrocytes. These data suggest that Mn increases GABA(EC) by inhibiting the function of GAT, and that perturbed taurine homeostasis potentially impacts neural function by jeopardizing the osmoregulatory and neuromodulatory functions of taurine in the brain.

GABAergic afferents activate both GABAA and GABAB receptors in mouse substantia nigra dopaminergic neurons in vivo

Most in vivo electrophysiological studies of substantia nigra have used rats. With the recent proliferation of the use of mice for in vitro neurophysiological studies because of the availability of various genetically modified strains to identify the roles of various channels and proteins in neuronal function, it is crucial to obtain data on in vivo responses in mice to verify that the in vitro results reflect functioning of systems comparable with those that have been well studied in rat. Inhibitory responses of rat nigral dopaminergic neurons by stimulation of afferents from striatum, globus pallidus, or pars reticulata have been shown to be mediated predominantly or exclusively by GABA(A) receptors. This is puzzling given the substantial expression of GABA(B) receptors and the ubiquitous appearance of GABA(B) synaptic responses in rat dopaminergic neurons in vitro. In the present study, we studied electrically evoked GABAergic inhibition in nigral dopaminergic neurons in C57BL/6J mice. Stimulation of the three major GABAergic inputs elicited stronger and longer-lasting inhibitory responses than those seen in rats. The early inhibition was GABA(A) mediated, whereas the later component, absent in rats, was GABA(B) mediated and selectively enhanced by GABA uptake inhibition. Striatal-evoked inhibition exhibited a slower onset and a weaker initial component compared with inhibition from globus pallidus or substantia nigra pars reticulata. These results are discussed with respect to differences in the size and neuronal density of the rat and mouse brain and the different sites of synaptic contact of the synapses from the three GABAergic afferents.

Release of GABA from sensory neurons transduced with a GAD67-expressing vector occurs by non-vesicular mechanisms

We have demonstrated that dorsal root ganglion neurons transduced with a recombinant replication-defective herpes simplex virus vector coding for glutamic acid decarboxylase (QHGAD67) release GABA to produce an analgesic effect in rodent models of pain. In this study, we examined the mechanism of transgene-mediated GABA release from dorsal root ganglion neurons in vitro and in vivo. Release of GABA from dorsal root ganglion neurons transduced with QHGAD67 was not increased by membrane depolarization induced by 60 mM extracellular K+ nor reduced by the removal of Ca2+ from the medium. Release of GABA from transduced dorsal root ganglion neurons was, however, blocked in a dose-dependent manner by NO-711, a selective inhibitor of the GABA transporter-1. The amount of GABA released from a spinal cord slice preparation, prepared from animals transduced by subcutaneous inoculation of QHGAD67 in the hind paws, was substantially increased compared to animals transduced with control vector Q0ZHG or normal animals, but the amount of GABA released was not changed by stimulation of the dorsal roots at either low (0.1 mA, 0.5-ms duration) or high (10 mA, 0.5-ms duration) intensity. We conclude that QHGAD67-mediated GABA release from dorsal root ganglion neurons is non-vesicular, independent of electrical depolarization, and that this efflux is mediated through reversal of the GABA transporter.

GABAergic mechanisms in regulating the activity state of substantia nigra pars reticulata neurons

Substantia nigra reticulata is the major output structure of the basal ganglia involved in somatosensory integration and organization of movement. While previous work in vitro and in anesthetized animal preparations suggests that these neurons are autoactive and points to GABA as a primary input regulating their activity, single-unit recording coupled with iontophoresis was used in awake, unrestrained rats to further clarify the role of tonic and phasic GABA input in maintenance and fluctuations of substantia nigra reticulata neuronal activity under physiologically relevant conditions. In contrast to glutamate, which was virtually ineffective at stimulating substantia nigra reticulata neurons in awake rats, all substantia nigra reticulata neurons tested were inhibited by iontophoretic GABA and strongly excited by bicuculline, a GABA-A receptor blocker. The GABA-induced inhibition had short onset and offset latencies, a fading response pattern (a rapid decrease in rate followed by its relative restoration), and was independent of basal discharge rate. The bicuculline-induced excitation was inversely related to discharge rate and current (dose)-dependent in individual units. However, the average discharge rate during bicuculline applications at different currents increased to a similar plateau ( approximately 60 impulses/s), which was about twice the mean basal rates. The excitatory effects of bicuculline were phasically inhibited or completely blocked by brief GABA applications and generally mimicked by gabazine, another selective GABA antagonist. These data as well as neuronal inhibitions induced by nipecotic acid, a selective GABA uptake inhibitor, suggest that substantia nigra reticulata neurons in awake, quietly resting conditions are under tonic, GABA-mediated inhibition. Therefore, because of inherent autoactivity and specifics of afferent inputs, substantia nigra reticulata neurons are very sensitive to phasic alterations in GABA input, which appears to be the primary factor determining fluctuations in their activity states under physiological conditions. While these cells are relatively insensitive to direct activation by glutamate, and resistant to a continuous increase in GABA input, they appear to be very sensitive to a diminished GABA input, which may release them from tonic inhibition and determine their functional hyperactivity.

Benzodiazepine-sensitive GABA(A) receptors in the commissural subnucleus of the NTS are involved in the carotid chemoreceptor reflex in rats

We studied the role of benzodiazepine (BDZ) receptors in the commissural subnucleus of the nucleus tractus solitarius (commNTS) in chemoreceptor reflex in urethane-anesthetized, pancronium-immobilized, artificially ventilated and bilaterally vagotomized rats. A BDZ agonist, diazepam (1-4 micromol/kg), administered intravenously reduced resting phrenic nerve activity (PNA) and blood pressure (BP). Stimulation of carotid chemoreceptors induced an increase in PNA and an increase in BP. Diazepam inhibited this chemoreceptor reflex. The effects of intravenous injection of diazepam (4 micromol/kg) on the chemoreceptor reflex were antagonized by microinjection of the BDZ antagonist flumazenil (100 pmol) into the commNTS. Microinjection of flumazenil (100 pmol) alone had no effect on the basal PNA and BP, and the chemoreceptor reflex. These results suggest that BDZ receptors are present in the carotid chemoreceptor reflex pathway in the commNTS and potentiate GABA(A) transmission.

GABAergic modulation of motor-driven behaviors in juvenileDrosophila and evidence for a nonbehavioral role for GABA transport

We have identified specific GABAergic-modulated behaviors in the juvenile stage of the fruit fly, Drosophila melanogaster via systemic treatment of second instar larvae with the potent GABA transport inhibitor DL-2,4-diaminobutyric acid (DABA). DABA significantly inhibited motor-controlled body wall and mouth hook contractions and impaired rollover activity and contractile responses to touch stimulation. The perturbations in locomotion and rollover activity were reminiscent of corresponding DABA-induced deficits in locomotion and the righting reflex observed in adult flies. The effects were specific to these motor-controlled behaviors, because DABA-treated larvae responded normally in olfaction and phototaxis assays. Recovery of these behaviors was achieved by cotreatment with the vertebrate GABA(A) receptor antagonist picrotoxin. Pharmacological studies performed in vitro with plasma membrane vesicles isolated from second instar larval tissues verified the presence of high-affinity, saturable GABA uptake mechanisms. GABA uptake was also detected in plasma membrane vesicles isolated from behaviorally quiescent stages. Competitive inhibition studies of [3H]-GABA uptake into plasma membrane vesicles from larval and pupal tissues with either unlabeled GABA or the transport inhibitors DABA, nipecotic acid, or valproic acid, revealed differences in affinities. GABAergic-modulation of motor behaviors is thus conserved between the larval and adult stages of Drosophila, as well as in mammals and other vertebrate species. The pharmacological studies reveal shared conservation of GABA transport mechanisms between Drosophila and mammals, and implicate the involvement of GABA and GABA transporters in regulating physiological processes distinct from neurotransmission during behaviorally quiescent stages of development.

Partial hippocampal kindling increases GABAB receptor-mediated postsynaptic currents in CA1 pyramidal cells

In previous studies, we showed that partial hippocampal kindling decreased the efficacy of the presynaptic GABAB receptors on both GABAergic and glutamatergic terminals of CA1 neurons in hippocampal slices in vitro. In this study, GABAB receptor-mediated inhibitory postsynaptic currents (GABAB-IPSCs) were assessed by whole-cell recordings in CA1 pyramidal neurons in hippocampal slices of male Long-Evans rats. The peak GABAB-IPSC evoked by a brief train of supramaximal stratum radiatum stimuli (20 pulses of 300 Hz) in the presence of picrotoxin (0.1 mM) and kynurenic acid (1 mM) was larger in neurons of kindled (65.9 +/- 5.2 pA, N=42 cells) than control (45.8 +/- 4.8 pA, N=32 cells) rats (P<0.01). Adding GABA uptake blocker nipecotic acid (1 mM) or GABAB receptor agonist baclofen (0.01 mM) in the perfusate induced outward currents that were blocked by GABAB receptor antagonist CGP 55845A (1 microM). The peak outward current induced by nipecotic acid was larger in neurons of the kindled (55.4 +/- 5.7 pA, N=30) than the control group (39.8 +/- 4.5 pA, N=28) (P<0.05). However, the magnitude of the baclofen-induced current was not different between kindled (90.8 +/- 6.9 pA, N=29) and control (87.2 +/- 5.9 pA, N=21) groups (P>0.05). We concluded that partial hippocampal kindling increased GABAB-IPSCs in hippocampal CA1 pyramidal cells via multiple presynaptic mechanisms.

IL-4 increases GABAergic phenotype in rat retinal cell cultures: involvement of muscarinic receptors and protein kinase C

Interleukin-4 (IL-4) is an anti-inflammatory cytokine. During injuries, infections and neurodegenerative diseases, high levels of this molecule are expressed in the brain. In the present work, we investigated the effect of IL-4 on GABAergic differentiation of retinal cells kept in vitro. We analyzed either the uptake of [3H]-gamma-aminobutyric acid (GABA) or the expression of glutamic acid decarboxylase (GAD-67) following IL-4 treatment. We have also investigated the pharmacological modulation of the [3H]-GABA uptake by cholinergic activation. Our results demonstrate that IL-4 increases the uptake of [3H]-GABA after 48 h in culture in a dose-dependent manner (0.5-100 U/ml). The maximal effect was obtained with 5 U/ml (75% increase). This effect was blocked by 1 mM of nipecotic acid, demonstrating the involvement of the GAT-1 subtype of GABA transporter. The IL-4 effect depends on M1 muscarinic activity, an increase in intracellular calcium levels, tyrosine kinase activity and protein kinase C (PKC) activity. Treatment with IL-4 for 48 h induced an increase of 90% in the number of GAD- and GABA-immunoreactive cells when compared with control cultures. Our results indicate that IL-4 modulates the GABAergic phenotype of retinal cells in culture. This result can suggest an important role for this cytokine either during the normal development of retinal circuitry or during neuroprotection after injuries.

Differential ontogeny of GABA(B)-receptor-mediated pre- and postsynaptic modulation of GABA and glycine transmission in respiratory rhythm-generating network in mouse

Rhythm generation in mature respiratory networks is influenced strongly by synaptic inhibition. In early neonates, GABA(A)-receptor- and glycine-receptor-mediated inhibition is not present, thus the question arises as to whether GABA(B)-receptor-mediated inhibition plays an important role. Using brainstem slices of neonatal mice (postnatal day, P0-P15), we analysed the role of GABA(B)-mediated modulation of GABA and glycine synaptic transmission in the respiratory network. Blockade of GABA uptake by nipecotic acid (0.25-2 mM) reduced the respiratory frequency. This reduction was prevented by the selective GABA(B) receptor antagonist CGP55845A (CGP) alone at P0-P3, but by bicuculline as well as CGP at P7-P15. Blockade of GABA(B) receptors by CGP increased the respiratory frequency at P0-P3, whereas it caused a reduction of frequency in older animals. The effect of CGP on respiratory frequency was diminished in the presence of bicuculline and strychnine in older but not in younger animals. The relative contribution of GABA(B)-receptor-mediated pre- and postsynaptic modulation was examined by analysing the effect of GABA(B) receptors on spontaneous and miniature IPSCs. In younger animals (P0-P3), the GABA(B) receptor agonist baclofen had no detectable effect on IPSC frequency, but caused a significant decrease in the amplitude. In older animals (P7-P15), baclofen decreased both the frequency and amplitude of spontaneous and miniature IPSCs. These results demonstrate that GABA(B)-receptor-mediated postsynaptic modulation plays an important role in the respiratory network from P0 on. GABA(B)-receptor-mediated presynaptic modulation develops with a longer postnatal latency, and becomes predominant within the first postnatal week.

Negative allosteric modulators of AMPA-preferring receptors inhibit [(3)H]GABA release in rat striatum

The effect of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a selective glutamate receptor agonist, on the release of previously incorporated [(3)H]GABA was examined in superfused striatal slices of the rat. The slices were loaded with [(3)H]GABA in the presence of beta-alanine (1 mM) and superfused with Krebs-bicarbonate buffer containing nipecotic acid (0.1 mM) and aminooxyacetic acid (0.1 mM) to inhibit GABA uptake and metabolism. AMPA (0.01 to 3 mM) increased basal [(3)H]GABA outflow and nipecotic acid potentiated this effect. The [(3)H]GABA releasing effect of AMPA was an external Ca(2+)-dependent process in the absence but not in the presence of nipecotic acid. Cyclothiazide (0.03 mM), a positive modulator of AMPA receptors, failed to evoke [(3)H]GABA release by itself, but it dose-dependently potentiated the [(3)H]GABA releasing effect of AMPA. The AMPA (0.3 mM)-induced [(3)H]GABA release was antagonized by NBQX (0.01 mM) in a competitive fashion (pA(2) 5.08). The negative modulator of AMPA receptors, GYKI-53784 (0.01 mM) reversed the AMPA-induced [(3)H]GABA release by a non-competitive manner (pD'(2) 5.44). GYKI-53784 (0. 01-0.1 mM) also decreased striatal [(3)H]GABA outflow on its own right, this effect was stereoselective and was not influenced by concomitant administration of 0.03 mM cyclothiazide. GYKI-52466 (0. 03-0.3 mM), another negative modulator at AMPA receptors, also inhibited basal [(3)H]GABA efflux whereas NBQX (0.1 mM) by itself was ineffective in alteration of [(3)H]GABA outflow. The present data indicate that AMPA evokes GABA release from the vesicular pool in neostriatal GABAergic neurons. They also confirm that multiple interactions may exist between the agonist binding sites and the positive and negative modulatory sites but no such interaction was detected between the positive and negative allosteric modulators. Since GYKI-53784, but not NBQX, inhibited [(3)H]GABA release by itself, AMPA receptors located on striatal GABAergic neurons may be in sensitized state and phasically controlled by endogenous glutamate. It is also postulated that these AMPA receptors are located extrasynaptically on GABAergic striatal neurons.

Application of a combined "effect compartment/indirect response model" to the central nervous system effects of tiagabine in the rat

Pharmacological inhibition of GABA uptake transporters provides a mechanism for increasing GABAergic transmission, which may be useful in the treatment of various neurological disorders. The purpose of our investigations was to develop an integrated pharmacokinetic-pharmacodynamic (PK/PD) model for the characterization of the pharmacological effect of tiagabine, R-N-(4,4-di-(3-methylthien-2-yl)but-3-enyl)nipecotic acid, in individual rats in vivo. The tiagabine-induced increase in the amplitude of the EEG 11.5-30 Hz frequency band (beta), was used as pharmacodynamic endpoint. Chronically instrumented male Wistar rats were randomly allocated to four groups which received an infusion of 3, 10, or 30 mg kg-1 of tiagabine or vehicle over 10 min. The EEG was continuously recorded in conjunction with frequent arterial blood sampling. The pharmacokinetics of tiagabine could be described by a biexponential equation. The pharmacokinetics of tiagabine were not dose dependent, and the pooled values for clearance, volume of distribution at steady state and terminal half-life were (mean +/- SE, n 23) 96 +/- 9 ml min-1 kg-1, 1.5 +/- 0.1 L kg-1 and 20 +/- 0.2 min. A time delay was observed between the occurrence of maximum plasma drug concentrations and maximal response. A physiological PK/PD model has been used to account for this time delay, in which a biophase was postulated to account for tiagabine available to the GABA uptake carriers in the synaptic cleft and the increase in EEG effect was considered an indirect response due to inhibition of GABA uptake carriers. The population values for the pharmacodynamic parameters characterizing the delay in pharmacological response relative to plasma concentrations were keo = 0.030 min-1 and kout = 81 min-1, respectively. Because of the large difference in these values the PK/PD model was simplified to the effect compartment model. Population estimates (mean +/- SE) were E0 = 155 +/- 6 microV, Emax = 100 +/- 5 microV, EC50 = 287 +/- 7 ng ml-1, Hill factor = 1.8 +/- 0.2 and keo = 0.030 +/- 0.002 min-1. The results of this analysis show that for tiagabine the combined "effect compartment-indirect response" model can be simplified to the classical "effect compartment" model.

Veratridine- and glutamate-induced release of [3H]-GABA from cultured chick retina cells: possible involvement of a GAT-1-like subtype of GABA transporter

Four subtypes of GABA carriers (GAT1-GAT4) that transport GABA in a sodium-dependent manner were identified so far. In this report, the sodium-dependent release of GABA was investigated in cultured chick retinal cells. Opening of voltage-sensitive sodium channels by veratridine or activation of non-NMDA glutamate receptors induced the release of GABA from cultured cells. The release of GABA was calcium-independent, but could be completely prevented by the substitution of sodium chloride by lithium or choline chloride in the extracellular medium, suggesting that GABA release could be triggered by multiple mechanisms that led to the flux of sodium into these cells. Pharmacological experiments revealed that, while GABA uptake was almost completely inhibited by the GAT-1 blockers NNC-711 (50 microM) or nipecotic acid (1 mM), the release of this amino acid was inhibited by NNC-711, but not by nipecotic acid. The incubation with beta-alanine (10 mM), a GAT-2/GAT-3 inhibitor, blocked 50% of GABA uptake but had no effect on the release. Our data suggest that sodium-dependent GABA release from cultured chick retina cells is mediated by a GAT-1 like transporter that shows some, but not all, the pharmacological properties of the GAT-1 carrier.

Carrier-mediated GABA release activates GABA receptors on hippocampal neurons

gamma-Aminobutyric acid (GABA) transporters are electrogenic and sodium-dependent and can operate in reverse when cells are depolarized or when there is reversal of the inward sodium gradient. However, the functional relevance of this phenomenon is unclear. We have examined whether depolarization induced by a physiologically relevant increase in extracellular [K+] leads to sufficient amounts of carrier-mediated GABA release to activate GABAA receptors on neurons. Patch-clamp recordings were made from rat hippocampal neurons in culture with solutions designed to isolate chloride currents in the recorded neuron. Pressure microejection was used to increase extracellular [K+] from 3 to 12 mM. After blockade of vesicular GABA release by removal of extracellular calcium, this stimulus induced a large conductance increase in hippocampal neurons [18.9 +/- 6.8 (SD) nS; n = 16]. This was blocked by the GABAA receptor antagonists picrotoxin and bicuculline and had a reversal potential that followed the Nernst potential for chloride, indicating that it was mediated by GABAA receptor activation. Similar responses occurred after block of vesicular neurotransmitter release by tetanus toxin. GABAA receptors also were activated when an increase in extracellular [K+] (from 3 to 13 mM) was combined with a reduction in extracellular [Na+] or when cells were exposed to a decrease in extracellular [Na+] alone. These results indicate that depolarization and/or reversal of the Na+ gradient activated GABA receptors via release of GABA from neighboring cells. We found that the GABA transporter antagonists 1-(4, 4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid hydrochloride (SKF89976A; 20-100 microM) and 1-(2-([(diphenylmethylene)amino]oxy)ethyl) -1, 2, 5, 6 - tetrahydro - 3 - pyridine - carboxylic acid hydrochloride (NO-711; 10 microM) both decreased the responses, indicating that the release of GABA resulted from reversal of the GABA transporter. We propose that carrier-mediated GABA release occurs in vivo during high-frequency neuronal firing and seizures, and dynamically modulates inhibitory tone.

Glial cell line-derived neurotrophic factor attenuates the excitotoxin-induced behavioral and neurochemical deficits in a rodent model of Huntington's disease

The present study determined the effects of intraventricularly administered glial cell line-derived neurotrophic factor on the behavioral and neurochemical sequelae of unilateral excitotoxic lesions of the striatum. Distinct asymmetrical rotational behavior in response to peripheral administration of amphetamine (5 mg/kg) was noted one and two weeks following injections of quinolinic acid (200 nmol) into two sites in the left striatum. In rats given a single intraventricular injection of glial cell line-derived neurotrophic factor (10-1000 micrograms) 30 min before the toxin, amphetamine-induced rotational behavior was significantly attenuated. Analysis of Nissl-stained coronal sections showed marked neuronal loss in the striatum ipsilateral to the quinolinic acid injections, which was at least partially prevented by glial cell line-derived neurotrophic factor D1 and D2 dopamine binding sites in the striatum, the majority of which are localized to subpopulations of GABAergic neurons, were decreased to a similar extent by quinolinic acid. Moreover, the reduction was attenuated by glial cell line-derived neurotrophic factor treatment to a similar degree, suggesting that the two subpopulations of GABAergic striatal output neurons are equally vulnerable to excitotoxic damage. Concomitant changes in neurotransmitter function as a result of the lesion were also observed: [3H]GABA uptake into striatal target tissues (globus pallidus and substantia nigra) was considerably reduced in the lesioned compared to the contralateral unlesioned tissues, as were [3H]choline and [3H]dopamine uptake into striatal synaptosomes. Similarly, striatal choline acetyltransferase activity was decreased by the lesion. Decrements in neuropeptide levels of similar magnitude were evident ipsilateral to the lesion; substance P, met-enkephalin and dynorphin A contents in the globus pallidus and substantia nigra were significantly reduced. Striatal somatostatin and neuropeptide Y levels were not altered. All of the neurochemical deficits induced by striatal quinolinic acid lesions were attenuated by intraventricular delivery of glial cell line-derived neurotrophic factor. Continuous intraventricular infusion of this trophic factor (10 micrograms/day) over a two-week period did not afford notable improvement compared to the single injection of 10 micrograms. In contrast, continuous infusion of brain-derived neurotrophic factor (10 micrograms/day) directly into the striatum did not affect any of the neurochemical parameters studied. However, neurotrophin-3 (10 micrograms/day) delivery into the striatum significantly increased [3H]GABA uptake, but only modestly affected [3H]choline uptake. The results indicate that glial cell line-derived neurotrophic factor counteracts neuronal damage induced by a striatal excitotoxic insult and support its potential use as a treatment for central nervous system disorders that may be a consequence of excitotoxic processes, such as Huntington's disease.

Pharmacological characterization of pre- and postsynaptic GABAB receptors in the deep nuclei of rat cerebellar slices

Whole-cell current-and voltage-clamp recordings were made from deep nuclear neurons in cerebellar slices from seven- to nine-day-old rats. Baclofen, a GABAB agonist, produced a slow postsynaptic hyperpolarization associated with a decrease in input resistance. The hyperpolarization was G-protein-dependent, blocked by intracellular Cs+ and antagonized by CGP 35348, a GABAB antagonist. In dialysed neurons recorded with Cs+ -containing pipettes, baclofen suppressed deep nuclear neuronal inhibitory postsynaptic potentials and inhibitory postsynaptic currents evoked by electrical stimulations of the Purkinje cell axons. This effect was blocked by CGP 35348, indicating that the suppressions were mediated by presynaptic GABAB receptors. The inability of CGP 35348 or uptake inhibitors (nipecotic acid and NO-711) to alter the decay of inhibitory postsynaptic currents evoked by maximal stimulation suggested that GABAB receptors are not activated by the stimulation of the GABAergic input. Paired-pulse depression of inhibitory postsynaptic currents was not blocked by CGP 35348. Moreover, neither uptake inhibitors nor CGP 35348 produced any significant changes to the whole-cell current produced by a tetanic stimulation of Purkinje cell axons, suggesting that GABAB autoreceptors were also not activated by endogenous GABA release. Our findings indicate that while pre- and postsynaptic GABAB receptors are present in the deep nuclei of the rat cerebellum, they are not activated by electrical stimulation of the Purkinje cell axons.

Acute effects of gamma-vinyl GABA (vigabatrin) on hippocampal GABAergic inhibition in vitro

The acute effects of gamma-vinyl-GABA (GVG) on GABAergic inhibition were investigated in the hippocampal slice preparation using the paired-pulse test of inhibition during extracellular recordings. Superfusion of GVG (100-500 microM) for 60 min resulted in a concentration-dependent decrease in GABAergic inhibition. Slices superfused with higher concentrations of GVG (0.5-1 mM) were hyperexcitable as demonstrated by the appearance of multiple spikes. Binding studies showed that GVG (1 mM) had no effect on the binding of [3H]flunitrazepam or [3H]TBOB and displaced no more than 15% of specific [3H]GABA binding, which indicates that GVG-induced disinhibition is not mediated through an action at the GABAA receptor complex. Consistent with this suggestion is the finding that GVG (500 microM) had little effect on the inhibition of the orthodromically evoked CA1 population spike produced by the GABAA receptor agonist muscimol (10 microM), whereas this inhibition was considerably attenuated by the GABAA receptor antagonist, bicuculline methiodide (5 microM). The results of this study suggest that the acute actions of GVG on the GABAergic neurotransmitter system are not involved in its anticonvulsant effect.

GABA uptake and release by a mammalian cell line stably expressing a cloned rat brain GABA transporter

In order to facilitate study of the neuronal GABA transporter and provide a convenient system for potential drug screening, we have established a CHO cell line, designated 1F9, which stably expresses the cloned GABA transporter from rat brain (GAT-1). 1F9 cells transport GABA at levels approximately 300-fold higher than untransfected CHO cells, and GABA transport in these cells has the following properties: (1) a dependence on sodium and chloride ions; (2) higher sensitivity to neuronal subtype uptake inhibitors (DABA and ACHC) than to glial subtype inhibitors (beta-alanine and THPO); and (3) Km (2.5 microM) and IC50 values for various competitive ligands that are comparable with values determined in synaptosomes and brain slices. Given the fidelity with which the 1F9 cell line expresses these characteristics of the native neuronal GABA transporter, we have used it to further address GABA transporter activity. [3H]GABA uptake by 1F9 cells is inhibited approximately 50% by the chloride transport blockers DIDS and SITS. The GABA receptor agonists muscimol and baclofen also inhibit GABA transport; however, the receptor antagonists bicuculline and phaclofen have no effect. 1F9 cells also show release of [3H]GABA release is calcium independent, and is differentially affected by changes in the ion gradient, as well as by the presence of external substrates and uptake blockers. These experiments indicate that 1F9 cells provide a convenient system for the screening of GABA transport inhibitors.

Enhanced gamma-aminobutyric acid-mediated responses in nucleus tractus solitarius of hypertensive rats

Previous studies demonstrated that stimulation of type B gamma-aminobutyric acid (GABAB) receptors but not type A (GABAA) receptors in the nucleus tractus solitarius of spontaneously hypertensive rats elicited a larger increase in arterial pressure compared with control rats of the Wistar-Kyoto strain. The present studies extended that observation by examining the cardiovascular response to injection into the nucleus tractus solitarius of a selective GABAB receptor antagonist, CGP 35348, in these strains as well as examining the cardiovascular responses to stimulation or blockade of GABAB receptors in the nucleus tractus solitarius in another model of hypertension, the rat treated with deoxycorticosterone acetate and salt. In both groups of hypertensive rats the pressor response to injection into the nucleus tractus solitarius of the GABA uptake blocking drug nipecotic acid was significantly greater compared with control rats (P < .01 in each model). Similarly, in both models of hypertension, the depressor response elicited by blockade of GABAB receptors in the nucleus tractus solitarius by injection of CGP 35348 was approximately 75% greater compared with control rats (P < .05 in each model). These results suggest that alterations in GABAB-mediated neural transmission in the nucleus tractus solitarius may contribute to the elevated arterial pressure observed in these models of hypertension.

Frequency-dependent enhancement of hippocampal inhibition by GABA uptake blockers

The effects of GABA uptake inhibitors, SKF 89976A and SKF 100330A, on recurrent inhibition were studied in the rat hippocampal slice preparation by the antidromic-orthodromic stimulation test. Population spikes evoked orthodromically by stimulation of the stratum radiatum and recorded in the CA1 pyramidal cell body layer were inhibited antidromically by stimulation of the alveus by a single pulse or by a train of pulses, either at low or at high frequency. Low frequency train conditioning produced less inhibition than a single pulse. The uptake blockers had no effect or slightly enhanced the inhibition produced by single stimuli or low frequency trains. High frequency train conditioning produced more and much longer inhibition than a single pulse. This inhibition was further substantially enhanced and prolonged by the drugs. Frequency-dependent enhancement of inhibition may be responsible for suppression of epileptiform discharges by GABA uptake blockers.

Extracellular gamma-aminobutyric acid levels in the rat caudate-putamen: monitoring the neuronal and glial contribution by intracerebral microdialysis

Intracerebral microdialysis with high pressure liquid chromatography (HPLC) coupled to electrochemical detection was employed to characterize gamma-aminobutyric acid (GABA) release and the effects induced by a preceding neuron-depleting ibotenic acid (IBO) lesion in the rat caudate-putamen (CPu). Extracellular GABA overflow was monitored in the intact and excitotoxically lesioned CPu, either 7-10 days (acute) or more than 3 months post-lesioning (chronic), using loop type dialysis probes perfused at a rate of 2 microliters/min. In the intact CPuu, basal GABA levels were 0.97 pmol/30 microliters of dialysate in the awake animals and 0.76 pmol/30 microliters under halothane anaesthesia. In both the acute and chronic IBO lesioned CPu the extracellular GABA levels were reduced by 80% and 67%, respectively, under halothane anaesthesia. KCl added to the perfusion fluid at a concentration of 100 mM resulted in dramatic increases in GABA overflow from baseline levels in the intact CPu (60- to 70-fold), which were almost totally abolished (> 95%) in the excitotoxically lesioned CPu. Veratridine administered at 75 microM, produced a 45-fold increase in GABA overflow in the intact CPu, but failed to produce any effect in the lesioned CPu. The addition of nipecotic acid (0.5 mM), a GABA uptake blocker, increased basal extracellular GABA levels 6-15-fold in the intact CPu, while GABA overflow in either the acute or chronic lesioned CPu was not significantly altered. Although Ca(2+)-free conditions (with 20 mM Mg2+ added) or tetrodotoxin (TTX, 1 microM) did not alter the basal GABA overflow in the intact CPU under halothane anaesthesia, the omission of Ca2+ resulted in a 47% reduction in basal extracellular GABA levels in awake, freely moving animals. Nipecotic acid-induced GABA overflow was reduced by 22% under Ca(2+)-free conditions, and by 33% in the presence of 1 microM TTX. Moreover, KCl-evoked GABA overflow was reduced by 86% in Ca(2+)-free conditions and by 40% when administered in the presence of 1 microM TTX. These results indicate that the extracellular GABA levels recorded by intracerebral microdialysis in the CPu are derived predominantly from neuronal sources. Under baseline resting conditions only a small fraction (up to 20-30%) of the neuronal release was Ca(2+)-dependent and TTX-sensitive (i.e. possessing the characteristics of impulse-dependent vesicular release).(ABSTRACT TRUNCATED AT 400 WORDS)

Hypothermia induced by infusion of methionine sulfoximine into the dorsal raphe nucleus of the rat: involvement of 5-HT1A and GABAB receptors

L-Methionine-D,L-sulfoximine (MSO) (25 micrograms) infused locally into the dorsal raphe nucleus (DRN) induced a progressive decrease in body temperature in conscious restrained rats kept at an ambient temperature of 23 degrees C. Pretreatment with (+/-)-pindolol (3 mg/kg s.c.) significantly attenuated MSO-induced hypothermia, but pretreatment with ketanserin (0.5 mg/kg i.p.) did not alter either the magnitude or the time course of the decrease in body temperature after intra-DRN infusion of MSO. Local accumulation of gamma-aminobutyric acid (GABA) after infusion of gamma-vinylGABA (10 micrograms) and (+/-)-nipecotic acid (6 micrograms) inhibited MSO-induced hypothermia. Blockade of GABAA receptors by infusion of (-)-bicuculline (25 ng) had no effect on the decrease in body temperature elicited by MSO, but blockade of GABAB receptors by infusion of 2-OH-saclofen (13.3 ng) significantly attenuated MSO-induced hypothermia. In conclusion, local infusion of MSO into the DRN must have slowed down the rate of 5-HT turnover in serotonergic neurons and decreased the release and synthesis of GABA. 5-HT1A somato-dendritic autoreceptors and GABAB postsynaptic receptors both appear to be involved in these neurocytochemical processes leading to hypothermia.

Postsynaptic action of endogenous GABA released by nipecotic acid in the hippocampus

Intracellular and whole-cell recording from CA1 pyramidal cells and dentate granule cells was used to study the release of endogenous GABA by nipecotic acid. Local application of nipecotic acid produced responses that could be entirely blocked by a combination of the GABAA receptor antagonist picrotoxin and the GABAB receptor antagonist CGP 35348. These responses were due to the heteroexchange release of endogenous GABA because they were blocked by low Na+ which blocks the GABA transporter and by SKF 89976 which is a competitive antagonist of the GABA transporter. Local application of nipecotic acid could, depending on the location, evoke pure GABAA or pure GABAB responses supporting proposals that GABAA and GABAB receptors can be segregated at separate inhibitory synapses.

Functional expression and CNS distribution of a beta-alanine-sensitive neuronal GABA transporter

The synaptic action of gamma-aminobutyric acid (GABA) is terminated by high affinity, Na(+)-dependent transport processes in both neurons and glia. We have isolated a novel GABA transporter cDNA, GAT-B, which encodes a high affinity (Km = 2.3 microM), Na(+)- and Cl(-)-dependent GABA transport protein that is potently blocked by beta-alanine, a compound generally considered a selective inhibitor of glial transport. However, in situ hybridization studies indicate that GAT-B mRNA is expressed predominantly within neurons. These data indicate that the neuronal-glial distinction of GABA transporters based on inhibitor sensitivities must be reconsidered and suggest a greater diversity of GABA transporters than has been predicted by previous pharmacologic studies.

Inward current caused by sodium-dependent uptake of GABA in the crayfish stretch receptor neurone

A two-microelectrode current-voltage clamp and Cl(-)-selective microelectrodes were used to examine the effects of gamma-aminobutyric acid (GABA) on membrane potential, current and intracellular Cl- activity (aiCl) in the crayfish stretch receptor neurone. All experimental solutions were CO2-HCO3- free. 2. GABA (500 microM) produced a mono- or biphasic depolarization (amplitude < or = 10 mV), often with a prominent initial depolarizing component followed by a transient shift to a more negative level. In some neurones, an additional depolarizing phase was seen upon washout of GABA. Receptor desensitization, being absent, played no role in the multiphasic actions of GABA. 3. The pronounced increase in membrane conductance evoked by GABA (500 microM) was associated with an increase in aiCl which indicates that the depolarizing action was not due to a current carried by Cl- ions. 4. The currents activated by GABA under voltage clamp conditions were inwardly directed when recorded at the level of the resting membrane potential, and they often revealed a biphasic character. The reversal potential of peak currents activated by pulses of 500 microM-GABA (EGABA) was 9-12 mV more positive than the reversal potential of the simultaneously measured net Cl- flux (ECl). ECl was 2-7 mV more negative than the resting membrane potential. 5. EGABA (measured using pulses of 500 microM-GABA) was about 10 mV more positive than the reversal potential of the current activated by 500 microM-muscimol, a GABA agonist that is a poor substrate of the Na(+)-dependent GABA uptake system. 6. In the absence of Na+, the depolarization and inward current caused by 500 microM-GABA were converted to a hyperpolarization and to an outward current. Muscimol produced an immediate outward current both in the presence and absence of Na+. 7. Following block of the inhibitory channels by picrotoxin (100-200 microM), the depolarizing effect of 500 microM-GABA was enhanced and the transient hyperpolarizing shifts were abolished. 8. In the presence of picrotoxin, GABA (> or = 2 microM) produced a concentration-dependent monophasic inward current which had a reversal potential of +30 to +60 mV. This current was inhibited in the absence of Na+ and by the GABA uptake blocker, nipecotic acid. Unlike the channel-mediated current, the picrotoxin-insensitive current was activated without delay also at low (2-10 microM) concentrations of GABA.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of mammalian gamma-aminobutyric acid receptors with distinct pharmacology in Xenopus oocytes

Gamma-Aminobutyric acid (GABA), the major inhibitory neurotransmitter in mammalian brain, is known to interact with two classes of GABA receptors denoted GABAA and GABAB. Using Xenopus oocytes, we compared the electrical and pharmacological properties of GABA receptors expressed by poly(A)+ RNA isolated from mammalian brain and retina. RNA from cerebral cortex expressed GABA responses with features characteristic of currents mediated by GABAA receptors. In contrast, RNA from retina expressed responses mediated by GABAA receptors and, in addition, GABA responses that were insensitive to the GABAA antagonist bicuculline and the GABAB agonist baclofen and showed no modulation by barbiturates or benzodiazepines. The bicuculline/baclofen-insensitive GABA response was a Cl- current that was blocked by picrotoxin but showed little desensitization or outward rectification. Our results suggest that mammalian retina contains RNAs encoding GABA receptors with distinct pharmacology.

Distinct, developmentally regulated brain mRNAs direct the synthesis of neurotransmitter transporters

The Xenopus laevis oocyte expression system was utilized to define developmental and structural properties of neurotransmitter transporter mRNAs and the pharmacological characteristics of encoded carriers independent of the complexities of brain tissue preparations. Poly(A)+ RNA from dissected brain regions of neonatal and adult rats was microinjected into Xenopus oocytes and the expression of Na(+)-dependent neurotransmitter transporters determined 48 h later. Transport studies conducted with oocytes injected with RNAs derived from juvenile rat tissues indicate a region- and transporter-specific, postnatal increase in mRNA abundance as a major factor in the developmental changes observed for brain high-affinity amino acid uptake systems. Both L-glutamic acid (Glu) and gamma-aminobutyric acid (GABA) uptake systems were detectable by day 3 in postnatal forebrain mRNA and became progressively enriched during the next 2 weeks of forebrain development. In contrast, brainstem Glu and GABA transporter enrichment was 60-70% of adult values by day 3 and exceeded adult levels by day 10. Parallel determinations of L-glutamic acid decarboxylase mRNA abundance during development argue for distinct regulatory influences on mRNAs directing transmitter synthesis and reuptake. Glycine uptake could not be detected at any point of forebrain development and exhibited a gradual postnatal rise to adult levels over the first 3 postnatal weeks of brainstem development. Uptake studies conducted with well-characterized inhibitors of Glu, GABA, dopamine, and choline transport (D-aspartate, nipecotic acid, nomifensine, and hemicholinium-3, respectively) revealed that oocyte transporters encoded by adult rat brain mRNAs retained antagonist sensitivities exhibited by in vitro brain preparations. In addition, a differential regional sensitivity to the Glu transport antagonist dihydrokainate (1 mM) was observed, lending support to previous reports of region-specific Glu transporter subtypes. To determine the structural diversity present among brain transporter mRNAs, poly(A)+ RNA was size-fractionated on linear (10-31%) sucrose density gradients prior to oocyte injection. These experiments revealed two mRNA size classes (2.4-3.0 kb, 4.0-4.5 kb) independently capable of directing the synthesis of Glu, GABA, and glycine transporters. In regions other than the cerebellum, Glu and GABA transporter activities migrated as single, yet distinct, peaks of 4.0-4.5 kb. In contrast, both Glu and GABA transporters exhibited major peaks of activity at 2.5-3.0 kb with size-fractionated cerebellar mRNA. Brainstem glycine uptake exhibited a broad sedimentation profile, with peaks apparent at 2.4 and 4.0 kb. Taken together, these findings indicate previously unappreciated complexity in mRNA structure and regulation which underlies the expression of amino acid neurotransmitter uptake systems in the rodent CNS.

Release of gamma-amino[3H]butyric acid from cultured amacrine-like neurons mediated by different excitatory amino acid receptors

The release of preaccumulated gamma-amino[3H]butyric acid ([3H]GABA) from putative GABAergic amacrine cells was studied in neuronal monolayer cultures made from embryonic chick retina. Release was specifically stimulated by excitatory amino acid agonists. N-Methyl-D-aspartate (NMDA; EC50, 19.1 +/- 5.0 microM), kainic acid (EC50, 15.6 +/- 2.3 microM), and the presumptive endogenous ligand glutamate (EC50, 3.6 +/- 0.5 microM) showed the same efficacy. Quisqualic acid, although the most potent agonist (EC50, 0.56 +/- 0.12 microM), was only half as efficacious. The time course of [3H]GABA release and autoradiographic visualization of responsive GABA-accumulating cells suggest that approximately 50% of the [3H]GABA-accumulating cells possess no or very low responsiveness to quisqualic acid. Depolarization (56 mM KCl)-induced release was fivefold lower than the maximal effect elicited by excitatory amino acids. Release of [3H]GABA and of endogenous GABA was entirely independent of extracellular Ca2+ but was completely abolished after replacement of Na+ by choline or Li+. The effects of NMDA and low concentrations of glutamate (up to 10 microM) were blocked by 2-amino-5-phosphonovaleric acid, by MK 801, and (in a voltage-dependent manner) by Mg2+. The reduction of NMDA responses by kynurenic acid was reversed by D-serine, and quisqualic acid competitively inhibited kainic acid-evoked release. Our results show that the cultured [3H]GABA-accumulating neurons, which probably represent the in vitro counterparts of GABAergic amacrine cells, express at least two types of excitatory amino acid receptors (of the NMDA and non-NMDA type), both of which can mediate a Ca2(+)-independent but Na2(+)-dependent release of GABA.

Substance P-evoked release of GABA from isolated spinal cord of the newborn rat

Isolated spinal cords of newborn rats were perfused with artificial cerebrospinal fluid and the effects of substance P and its analogs on the release of endogenous GABA were examined. Application of substance P evoked a dose-dependent release of GABA from spinal cords. The threshold concentration of substance P for induction of a significant increase in the GABA release was 3 microM. The substance P-evoked GABA release was neither blocked by removal of Ca2+ from perfusion medium nor by tetrodotoxin. In contrast, the GABA release evoked by high K+ (90 mM) was abolished in Ca(2+)-free medium, and the GABA release evoked by veratridine (5 microM) was suppressed by tetrodotoxin (1 microM). A GABA uptake inhibitor, cis-4-hydroxynipecotic acid, markedly augmented the GABA release induced by high K+, but not that induced by substance P or veratridine. These results suggest the possibility that a carrier-mediated mechanism might be involved in the GABA release induced by substance P, as well as by veratridine, in the newborn rat spinal cord. Two N-terminal fragments of substance P, substance P free acid and substance P1-10 amide, as well as [D-Arg1,D-Trp7,9,Leu11]substance P (spantide), evoked an increase in the GABA release, whereas substance P1-6, and a C-terminal fragment, substance P5-11 were inactive. Somatostatin and compound 48/80 also evoked a GABA release, which was independent of external Ca2+ and resistant to tetrodotoxin. [D-Pro4,D-Trp7,9,10]substance P4-11 (10-15 microM) inhibited the GABA release evoked by substance P, somatostatin and compound 48/80.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-independent gamma-aminobutyric acid release from growth cones: role of gamma-aminobutyric acid transport

Neuronal growth cones isolated in bulk from neonatal rat forebrain have uptake and K(+)-stimulated release mechanisms for gamma-aminobutyric acid (GABA). Up to and including postnatal day 5, the K(+)-stimulated release of [3H]GABA and endogenous GABA is Ca2+ independent. At these ages, isolated growth cones neither contain synaptic vesicles nor stain for synaptic vesicle antigens. Here we examined the possibility that the release mechanism underlying Ca2(+)-independent GABA release from isolated growth cones is by reversal of the plasma membrane GABA transporter. The effects of two GABA transporter inhibitors, nipecotic acid and an analogue of nipecotic acid, SKF 89976-A, on K(+)-stimulated release of [3H]GABA from superfused growth cones were examined. Nipecotic acid both stimulated basal [3H]GABA release and enhanced K(+)-stimulated release of [3H]GABA, which indicates that this agent can stimulate GABA release and is, therefore, not a useful inhibitor with which to test the role of the GABA transporter in K(+)-stimulated GABA release from growth cones. In contrast, SKF 89976-A profoundly depressed both basal and K(+)-stimulated [3H]GABA release. This occurred at similar concentrations at which uptake was blocked. These observations provide evidence for a major role of the GABA transporter in GABA release from neuronal growth cones.

Endogenous GABA acts on GABAB receptors in nucleus tractus solitarius to increase blood pressure

Previous studies found that injection of the GABA uptake inhibitor nipecotic acid into the nucleus tractus solitarius (NTS) increases arterial pressure. This effect of nipecotic acid was not antagonized by the selective GABAA receptor blocking agent bicuculline, suggesting that the action of nipecotic acid was mediated through an action of GABA on GABAB receptors in the NTS. The present studies examined this issue using a newly described GABAB antagonist, phaclofen. Injection of phaclofen (4 nmol in 100 nl artificial CSF) into the NTS of chloralose-anesthetized rats produced a slight decrease in arterial pressure (-8 +/- 2 mmHg) lasting less than 1 min. Smaller doses had no effect. Phaclofen antagonized in a dose-dependent (0.5-4 nmol) manner the increase in arterial pressure produced by injection into the NTS of the GABAB agonist baclofen (5-100 pmol). In contrast, phaclofen had no effect on the pressor response elicited by injection into the NTS of the GABAA agonist muscimol. Phaclofen (4 nmol) injected into the NTS totally reversed the increase in blood pressure elicited by injection into the NTS of a maximally effective dose of nipecotic acid (10 nmol). Phaclofen also inhibited the pressor response elicited by injection into the NTS of another indirectly acting GABA agonist, gamma-vinylGABA (GVG). Although GVG is an effective inhibitor of GABA transaminase, the enzyme involved in the metabolism of GABA, the time course of inhibition of GABA transaminase evoked by GVG was not consistent with the pressor response being produced by this mechanism. However, the pressor response elicited by GVG is consistent with its reported ability to inhibit GABA uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of two lipophilic gamma-aminobutyric acid uptake blockers in CA1 of the rat hippocampal slice

1. Drugs that increase inhibitory synaptic transmission in the central nervous system may be valuable tools in the treatment of seizures. Theoretically, substances that block the uptake of inhibitory transmitters such as gamma-aminobutyric acid (GABA) into intracellular compartments should also increase inhibition and therefore have potential value as antiepileptic drugs. However, most of these substances penetrate the blood-brain barrier poorly and have therefore until now had limited value. NO-05-0328 and NO-05-0329 are two new lipophilic GABA uptake inhibitors that readily enter the CNS from the blood. 2. We have investigated the effect of these two uptake inhibitors on the responses to exogenous GABA and on GABA-mediated inhibitory synaptic potentials in pyramidal neurones of the CA1 region in the rat hippocampal slice. 3. We found that both drugs increased the amplitude and duration of responses to exogenous GABA. Furthermore, the inhibitory synaptic potentials increased in amplitude. This increase was seen in both early and late phases of the synaptic potentials. We conclude that NO-05-0328 and NO-05-0329, at least in vitro, are more effective than older GABA uptake inhibitors such as nipecotic acid and they therefore deserve consideration for clinical use.