Excitatory amino acid-evoked release of gamma-[3H]aminobutyric acid from striatal neurons in primary culture

Cell Death Induction and Protection by Activation of Ubiquitously Expressed Anion/Cation Channels. Part 1: Roles of VSOR/VRAC in Cell Volume Regulation, Release of Double-Edged Signals and Apoptotic/Necrotic Cell Death

Cell volume regulation (CVR) is essential for survival and functions of animal cells. Actually, normotonic cell shrinkage and swelling are coupled to apoptotic and necrotic cell death and thus called the apoptotic volume decrease (AVD) and the necrotic volume increase (NVI), respectively. A number of ubiquitously expressed anion and cation channels are involved not only in CVD but also in cell death induction. This series of review articles address the question how cell death is induced or protected with using ubiquitously expressed ion channels such as swelling-activated anion channels, acid-activated anion channels and several types of TRP cation channels including TRPM2 and TRPM7. The Part 1 focuses on the roles of the volume-sensitive outwardly rectifying anion channels (VSOR), also called the volume-regulated anion channel (VRAC), which is activated by cell swelling or reactive oxygen species (ROS) in a manner dependent on intracellular ATP. First we describe phenotypical properties, the molecular identity, and physical pore dimensions of VSOR/VRAC. Second, we highlight the roles of VSOR/VRAC in the release of organic signaling molecules, such as glutamate, glutathione, ATP and cGAMP, that play roles as double-edged swords in cell survival. Third, we discuss how VSOR/VRAC is involved in CVR and cell volume dysregulation as well as in the induction of or protection from apoptosis, necrosis and regulated necrosis under pathophysiological conditions.

GABA transporter 1 tunes GABAergic synaptic transmission at output neurons of the mouse neostriatum

GABAergic medium-sized striatal output neurons (SONs) provide the principal output for the neostriatum. In vitro and in vivo data indicate that spike discharge of SONs is tightly controlled by effective synaptic inhibition. Although phasic GABAergic transmission critically depends on ambient GABA levels, the role of GABA transporters (GATs) in neostriatal GABAergic synaptic transmission is largely unknown. In the present study we aimed at elucidating the role of GAT-1 in the developing mouse neostriatum (postnatal day (P) 7-34). We recorded GABAergic postsynaptic currents (PSCs) using the whole-cell patch-clamp technique. Based on the effects of NO-711, a specific GAT-1 blocker, we demonstrate that GAT-1 is operative at this age and influences GABAergic synaptic transmission by presynaptic and postsynaptic mechanisms. Presynaptic GABA(B)R-mediated suppression of GABA release was found to be functional at all ages tested; however, there was no evidence for persistent GABA(B)R activity under control conditions, unless GAT-1 was blocked (P12-34). In addition, whereas no tonic GABA(A)R-mediated conductances were detected in SONs until P14, application of a specific GABA(A)R antagonist caused distinct tonic outward currents later in development (P19-34). In the presence of NO-711, tonic GABA(A)R-mediated currents were also observed at P7-14 and were dramatically increased at more mature stages. Furthermore, GAT-1 block reduced the median amplitude of GABAergic miniature PSCs indicating a decrease in quantal size. We conclude that in the murine neostriatum GAT-1 operates in a net uptake mode. It prevents the persistent activation of presynaptic GABA(B)Rs (P12-34) and prevents (P7-14) or reduces (P19-34) tonic postsynaptic GABA(A)R activity.

Induction of GABAergic phenotype in a neural stem cell line for transplantation in an excitotoxic model of Huntington's disease

The implementation of cell replacement therapies for Huntington's disease using multipotent neural stem cells (NSCs) requires the specific differentiation into gamma-aminobutyric acid (GABA) neuronal subtype before transplantation. Here we present an efficient culture procedure that induces stable GABAergic neurons from the immortalized striatal neural stem cell line ST14A. This process requires sequential retinoic acid treatment and KCl depolarization. Initial addition of 10 microM retinoic acid increased cell survival and promoted neuronal differentiation. Subsequent stimulation with 40 mM KCl induced specific differentiation into GABAergic neurons, yielding 74% of total cultured cells. KCl-evoked Ca(2+) influx reduced cell proliferation and nestin expression, and induced neurite outgrowth and GABAergic markers as well as GABA contents, release, and uptake. Characterization of the integration, survival, and phenotype of these predifferentiated GABAergic neurons following transplantation into the adult brain in a model of Huntington's disease revealed long-term survival in quinolinate-lesioned striata. Under these conditions, cells maintained their GABAergic phenotype and elaborated neurite processes with synaptic contacts with endogenous neurons. In conclusion, we have generated a homogeneous population of functional GABAergic neurons from a neural stem cell line, which survive and maintain their acquired fate in vivo. These data may lend support to the possibility of cell replacement therapies for Huntington's disease using neural stem cells.

Morphine acutely and persistently attenuates nonvesicular GABA release in rat nucleus accumbens

Withdrawal from repeated exposure to morphine causes a long-lasting increase in the reactivity of nucleus accumbens nerve terminals towards excitation. The resulting increase in action potential-induced exocytotic release of neurotransmitters, associated with behavioral sensitization, is thought to contribute to its addictive properties. We recently showed that activation of N-methyl-D-aspartate (NMDA) as well as dopamine (DA) D1 receptors in rat striatum causes tetrodotoxin-insensitive transporter-dependent GABA release. Since sustained changes in extracellular GABA levels may play a role in drug-induced neuronal hyperresponsiveness, we examined the acute and long-lasting effect of morphine on this nonvesicular GABA release in rat nucleus accumbens slices. The present study shows that morphine, through activation of mu-opioid receptors, reduces nonvesicular NMDA-induced [(3)H]GABA release in superfused nucleus accumbens slices. Moreover, prior repeated morphine treatment of rats (10 mg/kg, sc, 14 days) caused a reduction in NMDA-stimulated [(3)H]GABA release in vitro until at least 3 weeks after morphine withdrawal. This persistent neuroadaptive effect was not observed studying dopamine D1 receptor-mediated [(3)H]GABA release in nucleus accumbens slices. Moreover, this phenomenon appeared to be absent in slices of the caudate putamen. Interestingly, even a single exposure of rats to morphine (>2 mg/kg) caused a long-lasting inhibition of NMDA-induced release of GABA in nucleus accumbens slices. These data suggest that a reduction in nonvesicular GABA release within the nucleus accumbens, by enhancing the excitability of input and output neurons of this brain region, may contribute to the acute and persistently enhanced exocytotic release of neurotransmitters from nucleus accumbens neurons in morphine-exposed rats.

Brain-derived neurotrophic factor (BDNF) mediates bone morphogenetic protein-2 (BMP-2) effects on cultured striatal neurones

Bone morphogenetic proteins are members of the transforming growth factor-beta superfamily that have multiple functions in the developing nervous system. One of them, bone morphogenetic protein-2 (BMP-2), promotes the differentiation of cultured striatal neurones, enhancing dendrite growth and calbindin-positive phenotype. Bone morphogenetic proteins have been implicated in cooperative interactions with other neurotrophic factors. Here we examined whether the effects of BMP-2 on cultured striatal neurones are mediated or enhanced by other neurotrophic factors. BMP-2 had a cooperative effect with low doses of brain-derived neurotrophic factor or neurotrophin-3 (but not with other neurotrophic factors such as glial cell line-derived neurotrophic factor, neurturin or transforming growth factor-beta 2) on the number of calbindin-positive striatal neurones. Moreover, BMP-2 induced phosphorylated Trk immunoreactivity in cultured striatal neurones, suggesting that neurotrophins are involved in BMP-2 neurotrophic effects. The addition of TrkB-IgG or antibodies against brain-derived neurotrophic factor abolished the effects of BMP-2 on the number and degree of differentiation of calbindin-positive striatal neurones. Indeed, BMP-2 treatment increased brain-derived neurotrophic factor protein levels in treated cultures media and BDNF immunocytochemistry revealed that this neurotrophin was produced by neuronal cells. Taken together, these results indicate that brain-derived neurotrophic factor mediates the effects of BMP-2 on striatal neurones.

Involvement of intrinsic cholinergic and GABAergic innervation in the effect of NMDA on striatal dopamine efflux and metabolism as assessed by microdialysis studies in freely moving rats

Microdialysis perfusion was used to study the participation of striatal cholinergic and gamma-aminobutyric acid-ergic (GABAergic) neurotransmission in basal and N-methyl-D-aspartate (NMDA) receptor-modulated dopamine release and metabolism in the striatum of the freely moving rat. Reverse dialysis of atropine (1-50 microM) induced a concentration-related increase in dopamine efflux and decrease in 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) efflux. (+)-Bicuculline (10-100 microM) similarly increased dopamine efflux, but was without consistent effect on metabolite efflux. Reverse dialysis of NMDA (1 mM) evoked an approximately twofold increase in dopamine efflux and decreased DOPAC and HVA efflux to 30-40% of basal levels. The effect of NMDA on dopamine efflux was completely abolished by coadministration of tetrodotoxin (TTX; 1 microM) or atropine (10 microM), and markedly potentiated (approximately fourfold) by coadministration of (+)-bicuculline (50 microM). The NMDA-induced decrease in dopamine metabolite efflux was inhibited by coadministration of TTX or (+)-bicuculline, but was unaffected by atropine. Our data suggest that dopamine release in the striatum is subject to both cholinergic and GABAergic tonic inhibitory mechanisms mediated through muscarinic and GABAA receptors, respectively. Furthermore, NMDA-stimulated dopamine release also involves obligatory cholinergic facilitation and an inhibitory GABAergic component mediated through these respective receptors.

Neuroprotection of striatal neurons against kainate excitotoxicity by neurotrophins and GDNF family members

Neurotrophic factors are regarded as potential therapeutic tools in neurodegenerative disorders. Here, we analysed the protective effects of brain-derived neurotrophic factor, neurotrophin-3, glial cell line-derived neurotrophic factor and neurturin against the excitotoxic damage induced by kainate in striatal neurons in vitro and in vivo. Our results show that the decrease in the number of cultured striatal calbindin-positive neurons induced by kainate was prevented by treatment with any of these factors. To characterize their protective effects in vivo, cell lines overexpressing brain-derived neurotrophic factor, neurotrophin-3, glial cell line-derived neurotrophic factor or neurturin were grafted into the striatum. We found that the numbers of striatal projection neurons (calbindin-positive) and striatal interneurons (parvalbumin- or choline acetyltransferase-positive) were differentially decreased after kainate lesion. These neurotrophic factors prevented the loss of striatal projection neurons and interneurons with differing efficiency: brain-derived neurotrophic factor was the most efficient, whereas neurturin was the least. Our findings show that brain-derived neurotrophic factor, neurotrophin-3, glial cell line-derived neurotrophic factor and neurturin have specific neuroprotective profiles in striatal neurons and indicate that they are specific modulators of the survival of distinct subsets of striatal neurons in pathophysiological conditions.

Bone morphogenetic protein-2, but not bone morphogenetic protein-7, promotes dendritic growth and calbindin phenotype in cultured rat striatal neurons

Bone morphogenetic proteins are members of the transforming growth factor-beta superfamily. They are widely expressed in the mammalian nervous system, where they exert trophic effects on several neuronal populations. We studied the neurotrophic activity of bone morphogenetic protein-2 and bone morphogenetic protein-7 (also called osteogenic protein-1) on cultured striatal cells, previously shown to express bone morphogenetic protein ligands and receptors. Our results indicate that only bone morphogenetic protein-2 promoted the differentiation of GABAergic neurons, especially of the calbindin-positive subpopulation, the subset of projecting striatal neurons that degenerates in Huntington's disease. Bone morphogenetic protein-2 increased the area, perimeter and degree of arborization of GABAergic neurons, promoting calbindin phenotype without altering proliferation or apoptosis. In contrast, neither bone morphogenetic protein-2 nor -7 affected striatal cholinergic interneurons. However, they both increased the number of glial fibrillary acidic protein-positive cells. Suppression of glial proliferation with 5-fluorodeoxyuridine did not abolish bone morphogenetic protein-2 effects on the differentiation of striatal neurons, ruling out an indirect mechanism through astrocytes. In conclusion, our results show that bone morphogenetic protein-2 promotes the differentiation of cultured GABAergic striatal neurons, suggesting that bone morphogenetic proteins are involved in the development of the striatum.

Glutamic acid decarboxylase-expressing astrocytes exhibit enhanced energetic metabolism and increase PC12 cell survival under glucose deprivation

Astrocytes play a key role by catabolizing glutamate from extracellular space into glutamine and tricarboxylic acid components. We previously produced an astrocytic cell line that constitutively expressed glutamic acid decarboxylase (GAD67), which converts glutamate into GABA to increase the capacity of astrocytes to metabolize glutamate. In this study, GAD-expressing astrocytes in the presence of glutamate were shown to have increased energy metabolism, as determined by a moderate increase of 3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction, by an increased ATP level, and by enhanced lactate release. These changes were due to GAD transgene expression because transient expression of a GAD antisense plasmid resulted in partial suppression of the ATP level increase. These astrocytes had an increased survival in response to glucose deprivation in the presence of glutamate compared with the parental astrocytes, and they were also able to enhance survival of a neuronal-like cell line (PC12) under glucose deprivation. This protection may be partially due to the increased lactate release by GAD-expressing astrocytes because PC12 cell survival was enhanced by lactate and pyruvate under glucose deprivation. These results suggest that the establishment of GAD expression in astrocytes enhancing glutamate catabolism could be an interesting strategy to increase neuronal survival under hypoglycemia conditions.

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.

Synergistically interacting dopamine D1 and NMDA receptors mediate nonvesicular transporter-dependent GABA release from rat striatal medium spiny neurons

Given the complex interactions between dopamine D1 and glutamate NMDA receptors in the striatum, we investigated the role of these receptors in transporter-mediated GABA release from cultured medium spiny neurons of rat striatum. Like NMDA receptor-mediated [(3)H]-GABA release, that induced by prolonged (20 min) dopamine D1 receptor activation was enhanced on omission of external calcium, was action potential-independent (tetrodotoxin-insensitive), and was diminished by the GABA transporter blocker nipecotic acid, indicating the involvement of transporter-mediated release. Interestingly, lowering the external sodium concentration only reduced the stimulatory effect of NMDA. Blockade of Na(+)/K(+)-ATPase by ouabain enhanced NMDA-induced but abolished dopamine-induced release. Moreover, dopamine appeared to potentiate the effect of NMDA on [(3)H]-GABA release. These effects of dopamine were mimicked by forskolin. mu-Opioid receptor-mediated inhibition of adenylyl cyclase by morphine reduced dopamine- and NMDA-induced release. These results confirm previous studies indicating that NMDA receptor activation causes a slow action potential-independent efflux of GABA by reversal of the sodium-dependent GABA transporter on sodium entry through the NMDA receptor channel. Moreover, our data indicate that activation of G-protein-coupled dopamine D1 receptors also induces a transporter-mediated increase in spontaneous GABA release, but through a different mechanism of action, i.e., through cAMP-dependent inhibition of Na(+)/K(+)-ATPase, inducing accumulation of intracellular sodium, reversal of the GABA carrier, and potentiation of NMDA-induced release. These receptor interactions may play a crucial role in the behavioral activating effects of psychostimulant drugs.

The release of amino acids from rat neostriatum and substantia nigra in vivo: a dual microdialysis probe analysis

It has previously been demonstrated, in dual probe microdialysis studies, that stimulation of the neostriatum with kainic acid causes the release of GABA both locally within the neostriatum and distally in the substantia nigra, observations that are consistent with the known anatomy of the basal ganglia. The object of the present study was to further examine the characteristics of GABA release and to determine whether taurine, which has been proposed to be present in striatonigral neurons, has similar characteristics of release, and to examine the release of excitatory amino acids under the same conditions. To this end, dual probe microdialysis studies were carried out on freely-moving rats. The application of kainic acid to neostriatum enhanced the release of GABA, taurine, aspartate and glutamate locally in the neostriatum and distally in the substantia nigra. The distal release of each amino acid in the substantia nigra was sensitive to the administration of 6,7-dinitroquinoxaline-2,3-dione and tetrodotoxin to the neostriatum. Similarly the local release of GABA, aspartate and glutamate but not taurine was sensitive to the intrastriatal application of 6,7-dinitroquinoxaline-2,3-dione or tetrodotoxin. It is concluded that the release of taurine from the substantia nigra has similar characteristics to that of GABA and may be released from the terminals of striatonigral neurons following the stimulation of their cell bodies in the neostriatum. The release of taurine in the neostriatum however, is likely to be mediated mainly by different mechanisms and not related to neuronal activity. The release of excitatory amino acids is likely to involve indirect effects in the neostriatum and polysynaptic pathways in the substantia nigra.

Novel synthesis and release of GABA in cerebellar granule cell cultures after infection with defective herpes simplex virus vectors expressing glutamic acid decarboxylase

The inhibitory amino acid neurotransmitter gamma-aminobutyric acid (GABA) is synthesized from glutamate in a single step by the enzyme glutamatic acid decarboxylase (GAD). We sought to determine whether viral vectors containing GAD cDNA could be used to enhance synthesis and stimulation-evoked release of GABA in cultures of CNS neurons. For this purpose, we generated double-cassette defective herpes simplex virus (HSV) vectors that expressed one of the two GAD isoforms (GAD65 or GAD67), and Escherichia coli LacZ. Infection of cerebellar granule cell (CGC) cultures with vectors containing GAD cDNA resulted in a significant increase in isoform-specific expression of GAD, synthesis of GABA, and stimulation-evoked GABA release. GAD65 and GAD67 vector-infected neurons exhibited a comparable profile of GABA levels, synthesis and release, as well as GAD protein distribution. In CGCs cultured for 6 days in vitro (DIV), GABA synthesized after vector-derived GAD expression was released by treatment with glutamate or veratridine, but only in a Ca2+-independent fashion. In more mature (10 DIV) cultures, both Ca2+-dependent, K+ depolarization-induced, as well as Ca2+-independent, veratridine-induced, GABA release was significantly enhanced by GAD vector infection. Treatment of CGCs with kainic acid, which destroys most of the GABAergic neurons (<1% remaining), did not prevent vector-derived expression of GAD nor synthesis of GABA. This suggests that defective HSV vector-derived GAD expression can be used to increase GABA synthesis and release in CNS tissue, even in the relative absence of GABAergic neurons. The use of such GAD vectors in the CNS has potential therapeutic value in neurologic disorders such as epilepsy, chronic pain, Parkinson's and Huntington's disease.

Activity at the GABA transporter contributes to acute cellular swelling produced by metabolic impairment in retina

The role of the GABA transporter in acute toxicity in chick retina due to metabolic inhibition was investigated by the use of several substrate (nipecotic acid, THPO) and nonsubstrate (SKF 89976A, NO711) GABA transport inhibitors. Metabolic stress-induced acute toxicity in the retina is characterized by swelling of distinct populations of retinal neurons and selective release of GABA into the medium. Inhibitor concentrations were based on that needed to attenuate 14C-GABA uptake at its approximate KM concentration by > or = 70%. Under basal conditions, substrate, but not nonsubstrate, inhibitors increased extracellular GABA, but did not cause histological swelling per se. Under conditions of glycolytic inhibition, nonsubstrate, but not substrate, inhibitors significantly attenuated acute toxicity. Metabolic stress-induced acute toxicity was not altered by the GABA agonist muscimol, nor did muscimol reverse the protective effects of nonsubstrate transport inhibitors, suggesting that an increase in extracellular GABA during metabolic stress was not a component of the acute phase of toxicity. The results indicate that during metabolic inhibition, activity at the GABA transporter contributes to acute cellular swelling.

Stimulatory effect of arachidonic acid on the release of GABA in matrix-enriched areas from the rat striatum

Arachidonic acid was shown to stimulate the release of preloaded [3H]GABA from microdiscs of tissue punched out in matrix-enriched areas of the rat striatum. This effect, which was calcium- and dose-dependent, persisted in the presence of inhibitors of arachidonic acid catabolism. Other fatty acids were less or not effective. Arachidonic acid also inhibited [3H]GABA uptake into purified striatal synaptosomes, however the arachidonic acid-evoked release of [3H]GABA persisted following inhibition of the GABA neuronal uptake process. The stimulatory effect of arachidonic acid on GABA release may largely result from the activation of a protein kinase C since the arachidonic acid response was reduced by several protein kinase C inhibitors. Arachidonic acid also dose-dependently stimulated the release of preloaded [3H]GABA from purified striatal synaptosomes. Similar results were obtained when synaptosomes were previously incubated with [3H]glutamine to study the release of endogenously synthesized [3H]GABA. Further indicating a direct action of the fatty acid on GABAergic neurons, the arachidonic acid-induced release of [3H]GABA from microdiscs was not modified in the presence of the D1 dopaminergic antagonist SCH23390 or of glutamatergic antagonists. Finally, the release of [3H]GABA evoked by the combined application of NMDA and carbachol (a treatment known to markedly stimulate arachidonic acid formation) was reduced by inhibitors of phospholipase A2 further indicating that endogenously formed arachidonic acid significantly facilitates the release of GABA in the striatum.

Attenuation of excitotoxic cell swelling and GABA release by the GABA transport inhibitor SKF 89976A

Acute excitotoxicity in the chick retina is characterized by cellular swelling and the subsequent selective release of GABA. In order to understand the source of GABA release, embryonic day 15 retina were incubated with 1 mM glutamate for 30 min in the presence or absence of the GABA transport inhibitor SKF 89976A (1-100 microM). SKF 89976A dose-dependently attentuated glutamate-induced GABA release (IC50, 39 microM). Histological examination of retina showed that SKF 89976A greatly reduced cellular swelling caused by glutamate exposure. Interaction of SKF 89976A with glutamate receptors was ruled out as a possible reason for protection vs acute glutamate excitotoxicity, since SKF 89976A had no effect on glutamate receptor-induced 22Na+ influx. In contrast, the NMDA antagonist, MK-801, significantly blocked glutamate-evoked 22NA+ uptake. These studies indicate that reversal of the GABA transporter contributes to the bulk of GABA release during acute excitotoxicity in retina. Further, a net effect of the presence of SKF 89976A during glutamate exposure is reduction in cellular swelling. It is not clear at present if attenuation of swelling is mediated specifically by an interaction with the GABA transporter or by a nonspecific or indirect effect of SKF 89976A.

Sodium-dependent GABA-induced currents in GAT1-transfected HeLa cells

1. HeLa cells were infected with recombinant vaccinia virus containing the T7 RNA polymerase gene and transfected with the cDNA for a rat GABA transporter, GAT1, cloned downstream of a T7 RNA polymerase promoter. Six to sixteen hours after transfection, whole-cell recording with a voltage ramp in the range -90 to 50 mV revealed GABA-induced currents (approximately -100 pA at -60 mV in 100 microM GABA, 16 h after transfection at room temperature). No GABA-induced currents were observed in parental HeLa cells or in mock-transfected cells. 2. GABA-induced currents were suppressed by extracellular perfusion with GABA-free solutions or addition of GAT1 inhibitors SKF89976-A or SKF100330-A. At fixed voltage the GABA dependence of the inward current fitted the Michaelis-Menten equation with a Hill coefficient, n, near unity and an equilibrium constant, K(m), near 3 microM. The Na+ dependence of the inward currents fitted the Michaelis-Menten equation with n approximately equal to 2 and K(m) approximately equal to 10 mM. The constants n and K(m) for GABA and Na+ were independent of voltage in the range -90 to -30 mV. 3. GABA-induced currents reverse direction in the range 5-10 mV. The implication of this result is that GAT1 can mediate electrogenic (electrophoretic) influx or efflux of GABA depending on the membrane voltage. The presence of an outward current in our experiments is consistent with radioactive-labelled flux data from resealed vesicle studies. However, it is inconsistent with frog oocyte expression experiments using the sample clone. In oocytes, GAT1 generates no outward current in a similar voltage range. Smaller intracellular volume or higher turnover rates in the mammalian expression system may explain the outward currents. 4. External GABA induces inward current, and internal GABA induces outward current. However, in cells initially devoid of internal GABA, external GABA can also facilitate an outward current. This GAT1-mediated outward current occurs only after applying negative potentials to the cell. These data are consistent with the concept that negative potentials drive GABA and Na+ into the cell, which then leads to electrogenic efflux through GAT1 at positive voltages. 5. Assuming coupled transport, we estimate the number of transporters, N, times the turnover rate, r, to be Nr approximately 10(9) s-1 under nominal conditions (V = -60 mV, 30 microM GABA, 130 mM Na+ and room temperature). This indicates either very high levels of expression (approximately 10(4) microns-2), assuming published turnover rates (approximately 10 s-1), or turnover rates that are significantly greater than previously reported. As an alternative, a channel may exist in the GAT1 protein that is gated by GABA and Na+ and blocked by GAT1 antagonists. The channel mode of conduction would exist in addition to the coupled, fixed-stoichiometry transporter mode of conduction.

Glutamate receptor activation induces carrier mediated release of endogenous GABA from rat striatal slices

The regulation of striatonigral and striatopallidal GABAergic neurons by glutamatergic afferents is thought to play a critical role in normal basal ganglia function. Here we report that in striatal slices about 17% of K(+)-induced endogenous GABA release was Ca(2+)-independent and this could be blocked by a GABA transport inhibitor. Activation of N-methyl-D-aspartate (NMDA)- and quisqualate-sensitive receptors induced endogenous GABA efflux only in the presence of a GABA transaminase inhibitor; this efflux was inhibited by 60-80% with a GABA transport inhibitor. NMDA-induced GABA release was blocked by phencyclidine, Mg2+ and CGS 19755. Quisqualate-induced GABA release was blocked completely by a combination of the metabotropic antagonist, L-AP3 and CNQX, a non-NMDA receptor antagonist. These data indicate that excitatory amino acid agonists-induced GABA release is distinct from that induced by high K+ depolarization.

Effects of AMPA receptor modulators on the production of arachidonic acid from striatal neurons

The abilities of different compounds acting at alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors to modulate the overflow of [3H]arachidonic acid from rat striatal neurons were examined. The combination of AMPA (0.1 mM) and carbachol (1mM) stimulated [3H]arachidonic acid production, this effect could be dose-dependently enhanced by the newly discovered allosteric modulator of AMPA receptors: cyclothiazide. Competitive (6-cyano-7-nitroquinoxaline-2,3-dione [CNQX] and 6-(1-imidazolyl)-7-nitroquinazoline-2,3-dione [YM 900]) and non-competitive antagonists, like 1-(amino)-phenyl)-4-methyl -7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466), antagonized the responses induced by either AMPA + carbachol or AMPA + carbachol + cyclothiazide. In order to appreciate the respective part of AMPA-versus kainate-preferring receptors experiments were performed with kainic acid (0.1 mM) and the more specific kainate agonist domoic acid (0.1 mM). Kainic acid behaves like AMPA, but the response induced by the combination domoic acid + carbachol could not be potentiated by cyclothiazide. On the contrary, concanavalin A potentiated the responses evoked by kainic acid or domoic acid (in combination with carbachol) but did not enhance the AMPA-evoked response. It could be concluded that both AMPA- and kainate-preferring receptors are present in cultured rat striatal neurons and that these two types of receptors were involved together with muscarinic receptors in the overflow of [3H]arachidonic acid.

The effect of phencyclidine on the basal and high potassium evoked extracellular GABA levels in the striatum of freely-moving rats: an in vivo microdialysis study

The effect of phencyclidine (PCP) on the gamma-aminobutyric acid-ergic (GABAergic) transmission in the striatum of freely-moving rats was investigated using an in vivo microdialysis. The high potassium (100 mM) increased the extracellular GABA level to 4000% of the basal level. Although the basal GABA level in the striatal dialysate did not show either calcium dependency or tetrodotoxin (TTX) sensitivity, the high potassium evoked GABA level was reduced by 82% under calcium-free conditions (with 12.5 mM magnesium) and by 54% in the presence of 10 microM TTX. The systemic administration of PCP (7.5 mg/kg) or the local perfusion of PCP (100 microM and 1 mM) significantly inhibited the high potassium evoked GABA release in the rat striatum. The local perfusion of MK-801 (10 microM and 100 microM), a more potent and selective N-methyl-D-aspartate (NMDA) receptor antagonist, also inhibited the high potassium evoked striatal GABA release. These drugs did not show any significant effect on the basal extracellular GABA level. NMDA (1 mM) either partly or completely blocked the effect of PCP (1 mM) or MK-801 (100 microM) on the high potassium evoked striatal GABA release. On the other hand, nomifensine (100 microM), a dopamine uptake blocker, did not show any effect on the high potassium evoked GABA release. These results suggest that PCP inhibited the striatal GABAergic neuronal transmission through its antagonism of the NMDA receptor.

Effects of the non-NMDA antagonists NBQX and the 2,3-benzodiazepine GYKI 52466 on different seizure types in mice: comparison with diazepam and interactions with flumazenil

1. GYKI 52466 is a benzodiazepine derivative that has muscle relaxant and anticonvulsant properties thought to be mediated by highly selective, noncompetitive antagonism of non-NMDA receptors. However, recent electrophysiological data showed that, in addition to non-NMDA receptors, the GABAA-receptor associated benzodiazepine site is involved in the depressant effect of GYKI 52466 on spinal reflex transmission. In view of the structural similarities between the 2,3 benzodiazepine derivative GYKI 52466 and 1,4-benzodiazepines such as diazepam, the benzodiazepine site of GABAA receptor complex could also be involved in the anticonvulsant activity of GYKI 52466, which has not yet been proven. This prompted us to study the effect of the benzodiazepine receptor antagonist, flumazenil, on anticonvulsant and adverse effects of GYKI 52466 in different seizure models in mice. The non-NMDA antagonist, NBQX and diazepam were used for comparison. 2. Seizure threshold models for different types of generalized seizures were used. The threshold for maximal (tonic) electroshock seizures (MES) was significantly increased by GYKI 52466 (10-20 mg kg-1), NBQX (80-120 mg kg-1) and diazepam (5 mg kg-1) shortly after i.p. drug administration. The same dose-range of the non-NMDA antagonists also significantly increased the threshold for myoclonic and clonic seizures induced by i.v. infusion of pentylenetetrazol (PTZ), although the magnitude of threshold increases obtained with the respective drugs, differed, at least in part, from that seen in the MES experiments. GYKI 52466 was clearly less potent in increasing PTZ thresholds for myoclonic and clonic seizures than on the MES threshold, while NBQX exerted about the same potency in both models. In contrast to the non-NMDA antagonists, diazepam was capable of increasing themyoclonic and clonic PTZ seizure threshold at much lower doses than the MES threshold. The PTZ threshold for tonic seizures was markedly increased by GYKI 52466, while NBQX and diazepam were clearly less potent in this respect.3. With respect to adverse effects, GYKI 52466 and NBQX induced significant seizure threshold increases in the different seizure models only at doses which caused sedation and ataxia, while diazepam increased the myoclonic and clonic PTZ seizure threshold at doses below those inducing motor impairment.4. Flumazenil (5-20 mg kg-1) antagonized the anticonvulsant and adverse effects of diazepam but not GYKI 52466. Instead, the anticonvulsant effect of GYKI 52466 was potentiated by flumazenil in some experiments. The anticonvulsant activity of NBQX was slightly reduced by flumazenil in the MES model but not in the PTZ test.5. The data indicate that the GABAA receptor-associated benzodiazepine site is not critically involved in anticonvulsant or adverse effects of GYKI 52466. However, both GYKI 52466 and NBQX were unable to increase seizure thresholds at doses below those inducing sedation and motor impairment,thus demonstrating that non-NMDA antagonists lack a selective anticonvulsant action in standard models of generalized seizures.

Distinct sites of dopaminergic and glutamatergic regulation of haloperidol-induced catalepsy within the rat caudate-putamen

Previous studies have indicated that corticostriatal glutamatergic pathways are implicated in the regulation of neuroleptic catalepsy. To obtain a better understanding of the way in which dopamine (DA) and glutamate interact within the caudate-putamen (CP) in the development of catalepsy, we investigated the regional distribution within the rat CP of the cataleptogenic effect of haloperidol and its antagonism by D(-)-2-amino-5-phosphonopentanoic acid (D(-)AP5), a selective antagonist of the N-methyl-D-aspartate (NMDA) glutamate receptor subtype. Bilateral injections of haloperidol (3 micrograms/side) into the rostral ventromedial (VM) CP induced potent catalepsy with a short latency after the injection. In contrast, only a weak cataleptic response, of slower onset, was observed after haloperidol injections into the rostral ventrolateral (VL), rostral dorsomedial (DM), or rostral dorsolateral (DL) CP, or into the nucleus accumbens. D(-)AP5 (5 micrograms/side) injected bilaterally into the dorsorostral CP (DM and DL) strongly inhibited the catalepsy induced by systemic haloperidol (1 mg/kg, i.p.), and this effect lasted longer when the drug was injected into the DM than when it was injected into the DL. D(-)AP5 did not affect haloperidol-induced catalepsy when injected into the ventrorostral (VM and VL) or intermediate dorsal CP. D(-)AP5 injected into the DM, the region most sensitive to the anticataleptic effect of the drug, had no effect on basal levels of DA and its metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, or on the modification of these levels by haloperidol in either the DM or VM. These findings suggest that, while the catalepsy resulting from DA receptor blockade by haloperidol originates mainly from the VM, the expression of this phenomenon depends on an intact glutamatergic transmission within the dorsorostral CP. In the development of neuroleptic catalepsy, the mesencephalostriatal DAergic and corticostriatal glutamatergic pathways seem to be functionally linked through an indirect, rather than a direct, interaction.

Domoic acid induced release of [3H]GABA in cultured chick retina cells

The effect of the neurotoxin domoic acid (DOM), a structural analogue of kainic acid, on the release of [3H]gamma-aminobutyric acid (GABA) and on the [Ca2+]i was studied in cultured chick retina cells. DOM stimulated dose-dependently the release of [3H]GABA with an EC50 of 2.5 microM. In Ca(2+)-containing medium (1 mM), DOM (5 microM) increased the [Ca2+]i by about 190 nM and evoked the release of 11.8 +/- 1.3% of the intracellular [3H]GABA, while in the absence of extracellular Ca2+ DOM induced the release of only 7.9 +/- 1.4% of the accumulated [3H]GABA. The Ca(2+)-independent release of [3H]GABA was blocked by the non-competitive inhibitor of the GABA carrier 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-py ridine- carboxylic acid hydrochloride (NNC-711), but a component of Ca(2+)-dependent release remains. DOM evoked Ca(2+)-independent release of [3H]GABA was significantly depressed in the absence of external Na+ and completely blocked by the non-selective antagonist of the non-NMDA glutamate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Similarly, CNQX decreased the [Ca2+]i response to DOM, whereas L(+)-2-amino-3-phosphonopropionic acid (L-AP3), an antagonist of the metabotropic glutamate receptors, was without effect. MK-801 did not affect the release of [3H]GABA stimulated by DOM. Taken together our results indicate that DOM evokes both Ca(2+)-dependent and Ca(2+)-independent release of [3H]GABA, most likely by activating kainate receptors.

Does bicuculline antagonize NMDA receptors? Further evidence in the rat striatum

In two areas of the rat striatum, the in vitro N-methyl-D-aspartate (NMDA, 50 microM)-evoked release of [3H]dopamine was studied in the presence of bicuculline (5 and 50 microM), an antagonist of GABAA receptors. The responses observed with the higher concentration (50 microM) is compatible with an antagonistic activity of bicuculline on NMDA receptor, as recently reported by Wright and Nowak.

Repeated NMDA receptor activation induces distinct intracellular calcium changes in subpopulations of striatal neurons in vitro

The mechanisms underlying long-term calcium changes evoked by excitatory amino acids have not been previously examined in striatal neurons. Fura-2 fluorescence measurements were used to examine intracellular calcium concentration ([Ca2+]i) changes due to repeated N-methyl-D-aspartate (NMDA) receptor activation, in primary cultures of murine striatal neurons. Three applications of 200 microM NMDA (for 2 min, each application separated by 7 min), in 0 magnesium-containing artificial cerebral spinal fluid, elicited three distinct responses. In 50 +/- 8% of the NMDA-responsive neurons, no persistent increases in [Ca2+]i (final [Ca2+]i < or = 150% baseline) were observed, while in 33 +/- 7% and 17 +/- 3% of the cells, sustained (peak response > final [Ca2+]i > 150% baseline) and uncontrolled increases (final [Ca2+]i > or = peak response) were observed, respectively. NMDA-responsive neurons that were intensely immunoreactive for the calcium binding protein calbindin-D28k never exhibited uncontrolled increases in [Ca2+]i. Removal of extracellular Ca2+ significantly attenuated sustained, but not uncontrolled, increases in [Ca2+]i; sustained increases in some neurons were also attenuated by application of verapamil (100 microM) or MK-801 (1 microM). Pre-treatment of striatal neurons with the protein kinase C blocker sphingosine (20 microM), virtually eliminated the development of sustained or uncontrolled increases in [Ca2+]i. These findings suggest that specific intracellular mechanisms regulate the distinct [Ca2+]i responses of subpopulations of striatal neurons to repeated NMDA receptor activation.

Local GABAergic regulation of the N-methyl-D-aspartate-evoked release of dopamine is more prominent in striosomes than in matrix of the rat striatum

Using an in vitro microsuperfusion device we have previously demonstrated that in the absence of magnesium, the N-methyl-D-aspartate-evoked release of [3H]dopamine (continuously synthesized from [3H]tyrosine) is more prominent in matrix- than in striosome-enriched areas of the rat striatum and that in the matrix, the response is partially tetrodotoxin-sensitive. Since the medium-sized GABAergic neurons are the main targets of the corticostriatal glutamatergic fibers, the involvement of local GABAergic regulation in the N-methyl-D-aspartate-evoked release of [3H]dopamine was investigated in both striatal compartments using the same experimental approach. Firstly, bicuculline alone (5 microM, 25-min application) was shown to enhance the release of [3H]dopamine similarly in both compartments revealing the existence of a tonic GABAergic control of the spontaneous release of [3H]dopamine. Secondly, the N-methyl-D-aspartate (50 microM, 25-min application)-evoked release of [3H]dopamine was markedly amplified in the presence of bicuculline (5 microM, continuous delivery). This effect being more important in striosome- than in matrix-enriched areas (5.5- and two-times the N-methyl-D-aspartate-evoked response observed in the absence of the GABAA antagonist, respectively). Thirdly, the tetrodotoxin (1 microM, continuous delivery)-resistant N-methyl-D-aspartate-evoked responses were also enhanced in the presence of bicuculline, but in this case, the amplification of the N-methyl-D-aspartate-evoked release of [3H]dopamine was less marked than in the absence of tetrodotoxin and identical in both compartments (about two-times the tetrodotoxin-resistant N-methyl-D-aspartate-evoked responses observed in the absence of bicuculline). Altogether, these results indicate that GABAergic neurons exert locally an important inhibitory regulation of the N-methyl-D-aspartate-evoked release of dopamine and that this effect is more prominent in the striosome-enriched area. Both tetrodotoxin-sensitive (striosome) and tetrodotoxin-resistant (striosome and matrix) processes intervene in this inhibitory GABAergic presynaptic regulation of dopamine release.

Interleukin-2 modulates evoked release of [3H]dopamine in rat cultured mesencephalic cells

Mesencephalic cell cultures were used as a model to investigate the effects of interleukin-2 (IL-2) on evoked release of [3H]dopamine ([3H]DA) and gamma-[3H]-aminobutyric acid ([3H]GABA). At low concentrations (10(-13)-10(-12) M), IL-2 potentiated [3H]DA release evoked by the excitatory amino acids N-methyl-D-aspartate (NMDA) and kainate, whereas higher IL-2 concentrations (10(-9)-10(-8) M) had no effect. IL-2 (10(-14)-10(-8) M) modulated K(+)-evoked [3H]DA release in a biphasic manner, with low concentrations (10(-12)-10(-11) M) of IL-2 potentiating and higher concentrations (10(-9)-10(-8) M) inhibiting K(+)-induced [3H]DA release. IL-2 (10(-14)-10(-8) M) by itself failed to alter spontaneous [3H]DA release. The inhibition by IL-2 of K(+)-evoked [3H]DA release was reversible and not due to neurotoxicity, as preexposure to IL-2 (10(-8) M) had no significant effect on the subsequent ability of dopaminergic cells to take up and to release [3H]DA. Under our experimental conditions, IL-2 (10(-8) M) did not alter Ca(2+)-independent [3H]GABA release evoked by either K+ or NMDA. The results of this study indicate that IL-2 is able to potentiate [3H]DA release evoked by a number of different stimuli, including K+ depolarization and activation of both NMDA and non-NMDA receptor subtypes in mesencephalic cell cultures. IL-2 is active at very low concentrations, a finding that indicates a potent effect of IL-2 on dopaminergic neurons and implicates a physiological role for this cytokine in the modulation of DA release.

Excitatory amino acid response in cultured rat striatal neurons results in a developmentally regulated cGMP formation

Glutamate and its analogues play a central role in excitatory neurotransmission throughout the brain. Their signal in the postsynaptic cells can be transduced by several second messengers. Here we show that in primary cultures of embryonic rat striatum, excitatory amino acid receptor stimulation increases cyclic GMP intracellular concentration and the magnitude of this response depends upon the time in culture. Formation of cyclic GMP appears to be mediated by both N-methyl-D-aspartate (NMDA) and non-NMDA type excitatory amino acid receptors, it is blocked by specific excitatory amino acid antagonists and requires extracellular Ca++. The effect mediated via the NMDA receptor is also regulated by extracellular Mg++. These results show that excitatory amino acids make use of cyclic GMP for signal transduction in striatal neurons in vitro. We suggest that cyclic GMP may be an independent second messenger possibly important in the development of a defined population of striatal neurons.

Depolarization by K+ and glutamate activates different neurotransmitter release mechanisms in GABAergic neurons: vesicular versus non-vesicular release of GABA

Neurotransmitter release and changes in the concentration of intracellular free calcium ([Ca++]i) were studied in cultured GABAergic cerebral cortical neurons, from mice, upon depolarization with either an unphysiologically high potassium concentration (55 mM) or the physiological excitatory neurotransmitter glutamate (100 microM). Both depolarizing stimuli exerted prompt increases in the release of preloaded [3H]GABA as well as in [Ca++]i. However, the basic properties of transmitter release and the increase in [Ca++]i under a variety of conditions were different during stimulation with K+ or glutamate. Potassium-evoked release of [3H]GABA consisted of two phases, a rapid, large and transient phase followed by a smaller, more persistent second phase. The rapid phase was inhibited (60%) by nocodazole which reduced the number of vesicles in the neurites by 80%. This rapid phase of the GABA release was also reduced by organic (verapamil) and inorganic (Co++) Ca++ channel blockers but was insensitive to the GABA transport inhibitor SKF 89976A. In contrast, the second phase was less sensitive to nocodazole and Ca++ channel antagonists but could be inhibited by SKF 89976A. The glutamate-induced [3H]GABA release, which was mainly mediated by N-methyl-D-aspartate receptors, consisted of a single, sustained phase. This was insensitive to nocodazole, partly inhibited by verapamil and could be blocked by Co++ as well as SKF 89976A. The action of Co++ could be attributed to a block of N-methyl-D-aspartate-associated ion channels. These findings strongly suggest that the majority of the K(+)-stimulated GABA release is dependent upon vesicles whereas the glutamate induced release is non-vesicular and mediated by a depolarization-dependent reversal of the direction of high-affinity GABA transport. The basic differences in the mode of action of the two depolarizing stimuli were reflected in the properties of the increase in [Ca++]i elicited by 55 mM K+ and 100 microM glutamate, respectively. The K(+)-induced increase in [Ca++]i was reduced by both verapamil and Ca(++)-free media whereas the corresponding glutamate response was only sensitive to Ca(++)-free conditions. Exposure of the cells to nocodazole or SKF 89976A had no effect on the ability of K+ or glutamate to increase [Ca++]i. Altogether, the results clearly demonstrate that K(+)-induced transmitter release from these GABAergic neurons is vesicular in nature whereas that induced by the neurotransmitter glutamate is not.

Glutamate increases the [Ca2+]i but stimulates Ca(2+)-independent release of [3H]GABA in cultured chick retina cells

The effect of glutamate on [Ca2+]i and on [3H] gamma-aminobutyric acid (GABA) release was studied on cultured chick embryonic retina cells. It was observed that glutamate (100 microM) increases the [Ca2+]i by Ca2+ influx through Ca2+ channels sensitive to nitrendipine, but not to omega-conotoxin GVIA (omega-Cg Tx) (50%), and by other channels insensitive to either Ca2+ channel blocker. Mobilization of Ca2+ by glutamate required the presence of external Na+, suggesting that Na+ mobilization through the ionotropic glutamate receptors is necessary for the Ca2+ channels to open. The increase in [Ca2+]i was not related to the release of [3H]GABA induced by glutamate, suggesting that the pathway for the entry of Ca2+ triggered by glutamate does not lead to exocytosis. In fact, the glutamate-induced release of [3H]GABA was significantly depressed by Ca(2+)o, but it was dependent on Na(+)o, just as was observed for the [3H]GABA release induced by veratridine (50 microM). The veratridine-induced release could be fully inhibited by TTX, but this toxin had no effect on the glutamate-induced [3H]GABA release. Both veratridine- and glutamate-induced [3H]GABA release were inhibited by 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-py ridine- carboxylic acid (NNC-711), a blocker of the GABA carrier. Blockade of the NMDA and non-NMDA glutamate receptors with MK-801 and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), respectively, almost completely blocked the release of [3H]GABA evoked by glutamate.(ABSTRACT TRUNCATED AT 250 WORDS)

N-methyl-D-aspartic acid differentially regulates extracellular dopamine, GABA, and glutamate levels in the dorsolateral neostriatum of the halothane-anesthetized rat: an in vivo microdialysis study

The effects of local perfusion with the glutamate receptor agonist NMDA and the noncompetitive NMDA receptor antagonist dizolcipine (MK-801) on extracellular dopamine (DA), GABA, and glutamate (Glu) levels in the dorsolateral striatum were monitored using in vivo microdialysis in the halothane-anesthetized rat. In addition, the sensitivity of both the basal and NMDA-induced increases in levels of these neurotransmitter substances to perfusion with tetrodotoxin (TTX: 10(-5) M) and a low Ca2+ concentration (0.1 mM) was studied. The results show that the local perfusion (10 min) with both the 10(-3) and 10(-4) M dose of NMDA increased striatal DA and GABA outflow, whereas only the (10(-3) M) dose of NMDA was associated with a small and delayed increase in extracellular Glu levels. The NMDA-induced effects were dose-dependently counteracted by simultaneous perfusion with MK-801 (10(-6) and 10(-5) M). Both the basal and NMDA (10(-3) M)-induced increase in extracellular striatal DA content was reduced in the presence of TTX and a low Ca2+ concentration, whereas both basal and NMDA-stimulated GABA levels were unaffected by these treatments. Both the basal and NMDA-stimulated Glu levels were enhanced following TTX treatment, whereas perfusion with a low Ca2+ concentration reduced basal Glu levels and enhanced and prolonged the NMDA-induced stimulation. These data support the view that NMDA receptor stimulation plays a role in the regulation of extracellular DA, GABA, and Glu levels in the dorso-lateral neostriatum and provide evidence for a differential effect of NMDA receptor stimulation on these three striatal neurotransmitter systems, possibly reflecting direct and indirect actions mediated via striatal NMDA receptors.

Amino acid neurotransmitter transporters: structure, function, and molecular diversity

Many biologically active compounds including neurotransmitters, metabolic precursors, and certain drugs are accumulated intracellularly by transporters that are coupled to the transmembrane Na+ gradient. Amino acid neurotransmitter transporters play a key role in the regulation of extracellular amino acid concentrations and termination of neurotransmission in the CNS section. Transporters for the major amino acid neurotransmitters glutamate, GABA, and glycine are found in both neurons and glial cells. Recent work has resulted in the identification of cDNAs encoding several amino acid neurotransmitter transport proteins, all of which belong to the Na(+)- and Cl(-)-dependent transporter gene family. The diversity of this family suggests a degree of transporter heterogeneity that is greater than that indicated by biochemical and pharmacological studies.

Amino acid levels and gamma-aminobutyric acidA receptors in rat neostriatum, cortex, and thalamus after neonatal 6-hydroxydopamine lesion

The amino acid gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in brain, and GABAergic neurons have been proposed to play a major role in basal ganglia physiology. In the neostriatum (caudate putamen), medium-sized aspiny interneurons, as well as neostriatal output neurons that project to several brain regions, use GABA as their neurotransmitter. Dopamine fibers arising from the substantia nigra represent a major input to the neostriatum where, besides their classic neurotransmitter role, they are seemingly involved in the regulation of amino acid neurotransmitter release. To further characterize the nature of some of the amino acid/dopamine interactions, selective dopaminergic deafferentations were produced in neonatal rats (3 days postnatal) by intraventricular administration of the neurotoxin 6-hydroxydopamine (6-OHDA); the noradrenergic neurons were protected by prior administration of desmethylimipramine. After a 3-month survival, levels of catecholamines, indoleamines, and amino acids were determined in cingulate cortex, thalamus, and neostriatum. In addition, GABAA receptors were measured in membrane preparations from these three regions, using the specific agonist [3H]muscimol. In the 6-hydroxydopamine-lesioned rats, levels of dopamine and its metabolites homovanillic acid, 3,4-dihydroxyphenylacetic acid, and 3-methoxytyramine were decreased, as expected, in cortex and neostriatum, but remained unmodified in thalamus. In all three regions, serotonin content was increased; its metabolite, 5-hydroxyindole-3-acetic acid, was also elevated, but only in cortex and neostriatum. The levels of GABA were increased in neostriatum and thalamus, but remained unmodified in cortex. Glycine was increased in all three regions examined. There were also increases of phosphatidylethanolamine and serine in thalamus, and of aspartic acid and alanine in neostriatum. The density of GABAA binding sites was increased in neostriatum, but remained unchanged in cortex and thalamus. The changes in amino acid levels and [3H]muscimol binding sites induced by a neonatal 6-hydroxydopamine treatment differ from those found after similar lesions in adult animals, possibly because of the plastic and synaptic rearrangements that can still occur during early postnatal development. The present results also demonstrate that adaptations occur in response to a dopaminergic deafferentation at an early age and that these exhibit a regional specificity.

Cytotoxic actions and effects on intracellular Ca2+ and cGMP concentrations of sulphur-containing excitatory amino acids in cultured cerebral cortical neurons

Effects of the sulphur-containing acidic amino acids (SAAs) cysteic acid (CA), homocysteic acid (HCA), cysteine sulphinic acid (CSA), homocysteine sulphinic acid (HCSA), and S-sulphocysteine (SC) on intracellular concentrations of Ca2+ ([Ca2+]i) and cGMP ([cGMP]i) as well as their cytotoxic actions were investigated in cultured cerebral cortical neurons. The glutamate receptor subtype selective antagonists APV (D-(-)-2-amino-5-phosphonopentanoate) acting on N-methyl-D-aspartate (NMDA) receptors and DNQX (6,7-dinitroquinoxaline-2,3-dione) acting on non-NMDA receptors were employed to obtain information about the involvement of glutamate receptor subtypes in these actions of the SAAs. It was found that all SAAs exerted a cytotoxic action on the neurons. The ED50 values for CSA, CA, HCSA, and HCA were around 30 to 50 microM and that for SC was about 150 microM. The glutamate transport blocker L-aspartate-beta-hydroxamate increased the efficacy of CSA and CA but had no effect on the cytotoxic actions of the remaining SAAs. In case of CA, HCA, and SC the cytotoxicity could be prevented by APV alone and for HCSA, DNQX could block the toxic action. DNQX reduced the toxicity of HCA somewhat but the presence of APV was required for complete protection. CSA toxicity could only be blocked by the combination of APV and DNQX. All SAAs induced an increase in [cGMP]i and [Ca2+]i and with regard to [Ca2+]i SC was the most potent and CA the least potent SAA. The effect of all SAAs on [cGMP]i could be blocked by APV alone whereas DNQX had no effect except in the case of HCSA where the response was blocked completely and HCA where the response was inhibited by 75%.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of GABA release by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and N-methyl-D-aspartate receptors in matrix-enriched areas of the rat striatum

Using a new in vitro superfusion device, the release of preloaded [3H]GABA was examined in microdiscs of tissues taken from sagittal slices in matrix-enriched areas of the rat striatum. Potassium (9 mM, 15 mM) stimulated the release of [3H]GABA in a concentration- and calcium-dependent manner and the veratridine (1 microM)-evoked release of [3H]GABA was completely abolished in the presence of tetrodotoxin (1 microM). The selective glutamatergic agonist alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (1 mM) enhanced the potassium-evoked release of [3H]GABA as well as the basal outflow of [3H]GABA. This latter effect was found to be calcium-dependent, partially diminished by tetrodotoxin (1 microM), completely blocked by 6,7-dinitro-quinoxaline-2,3-dione (0.1 mM), which is generally used as an antagonist of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors, but not affected by (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK801, 10 microM), a specific antagonist of N-methyl-D-aspartate receptors. Similarly, N-methyl-D-aspartate (1 mM) enhanced both the potassium (9 mM) and the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (1 mM)-evoked release of [3H]GABA but when used alone, due to the presence of magnesium in the superfusion medium, was ineffective on the basal efflux of [3H]GABA. A stimulatory effect of N-methyl-D-aspartate (1 mM) on the basal outflow of [3H]GABA was observed, however, when magnesium was omitted from the superfusion medium. The stimulatory effect of N-methyl-D-aspartate (1 mM) observed in the presence of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate was not potentiated by glycine (1 microM, in the presence of strychnine 1 microM) and the N-methyl-D-aspartate-evoked response seen in the absence of magnesium was not enhanced by D-serine (1 mM), suggesting that endogenous glycine is already acting on N-methyl-D-aspartate receptors. In fact, in the absence of magnesium, 7-chloro-kynurenate (1 mM) completely abolished the stimulatory effect of N-methyl-D-aspartate on the release of [3H]GABA confirming that under our conditions, the glycine site of the N-methyl-D-aspartate receptor is saturated. N-methyl-D-aspartate-evoked responses were all blocked by MK801 (10 microM). Finally, the N-methyl-D-aspartate-evoked response seen in the absence of magnesium was markedly reduced in the presence of tetrodotoxin (1 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Ca(2+)-dependent release of [3H]GABA in cultured chick retina cells

Depolarization by K+ (50 mM) of cultured chick retina cells released 1.14 +/- 0.28% of the accumulated [3H] gamma-aminobutyric acid (GABA) in the absence of Ca2+, but when 1.0 mM Ca2+ was present, the internal free calcium ion concentration [Ca2+]i rose by about 750 nM and the [3H]GABA release about doubled to a value of 2.22 +/- 0.2% of the total [3H]GABA. Nitrendipine (0.1 microM), a blocker of the L-type Ca2+ channels, blocked the [Ca2+]i response to K+ depolarization by about 65%, and the omega-Conotoxin GVIA (omega-CgTx) (0.5 microM), a blocker of the N-type of Ca2+ channels, inhibited by 27% the [Ca2+]i rise due to K+ depolarization. Parallel experiments showed that nitrendipine inhibits [3H]GABA release to the level observed in the absence of Ca2+, whereas omega-CgTx did not inhibit significantly the release of [3H]GABA. The results also show that the release of [3H]GABA due to K(+)-depolarization in the absence of Ca2+ can be totally blocked by 1-(2-(((Diphenylmethylene) amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridine-carboxylic acid hydrochloride (NNC-711), an inhibitor of the GABA carrier. However, in the presence of Ca2+, NNC-711 blocks the release only by about 66%, corresponding to the Ca(2+)-independent release. Thus, it is concluded that [3H]GABA is released in chick retina cells by the exocytotic mechanism, which is Ca(2+)-dependent, and by reversal of the carrier, which is Ca(2+)-independent, in much the same way as has been found for other GABAergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Cortical input to parvalbumin-immunoreactive neurones in the putamen of the squirrel monkey

The cortex projects heavily to the striatum and makes asymmetrical synaptic contact mainly with the spines of medium-sized densely spiny neurones. The possibility exists that corticostriatal terminals also make synaptic contact with classes of striatal interneurones. The primary objective of the present experiment was to determine whether parvalbumin-immunoreactive neurones, which represent a class of GABAergic interneurones in the striatum, also receive a direct synaptic input from corticostriatal fibres. The anterograde tracer biocytin was injected into the motor and premotor cortices of the squirrel monkey (Saimiri sciureus). Following perfuse-fixation, sections of the striatum were processed histochemically to reveal the transported biocytin using an avidin-biotin-peroxidase complex and diaminobenzidine as the chromogen. They were then immunostained to reveal parvalbumin using benzidine dihydrochloride as the chromogen. In both the light and electron microscopes, the morphological features and the afferent synaptic input of the parvalbumin-immunoreactive neurones were similar to those observed in other species. Similarly, the morphology and postsynaptic targets of the corticostriatal terminals were similar to those described in other species. Light microscopic examination revealed that the anterogradely labelled corticostriatal terminals were often in close apposition to the parvalbumin-positive neurones. At the electron microscopic level the biocytin-positive corticostriatal terminals were found to make asymmetrical synaptic contacts mainly with spines. The parvalbumin-positive neurones were seen to have an invaginated nucleus, extensive cytoplasm and relatively few spines. Parvalbumin-immunoreactive dendrites received a dense synaptic input consisting mainly of asymmetric synapses and only a few symmetric synapses. Biocytin-labelled corticostriatal terminals were often seen in asymmetrical synaptic contact with parvalbumin-immunoreactive dendrites. These results show that GABAergic interneurones identified on the basis of parvalbumin immunoreactivity, in addition to the projection neurones of the striatum, are under the direct influence of the cerebral cortex.

Striatal N-methyl-D-aspartate receptors in haloperidol-induced catalepsy

Bilateral ablation of the frontal cortex of rats markedly reduced the catalepsy induced by haloperidol (1 mg/kg i.p.). Similarly, the selective antagonist of N-methyl-D-aspartate (NMDA) receptors, D(-)-2-amino-5-phosphonopentanoic acid (10 micrograms/side), injected bilaterally into the rostral part of the caudate-putamen (CP) reduced haloperidol-induced catalepsy whereas its injection into the intermediate part of the CP was ineffective. The quisqualate receptor antagonist, L-glutamic acid diethyl ester (100 micrograms/side), did not affect haloperidol-induced catalepsy when injected into the rostral part of the CP. On the other hand, NMDA (1 micrograms/side) injected bilaterally into the rostral part of the CP was able to restore haloperidol-induced catalepsy in frontally decorticated rats without any notable cataleptic effect of its own. These findings suggest that a certain degree of tonic stimulatory effect of corticostriatal glutamatergic pathways on NMDA receptors within the rostral part of the CP is a prerequisite for the expression of the cataleptogenic action of haloperidol.

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.

N-methyl-D-aspartate evokes the release of somatostatin from striatal interneurons in primary culture

Indirect immunocytochemistry of striatal neurons in primary culture, generated from the embryonic mouse brain, suggested that 2-4% of the neurons contained somatostatin-like immunoreactivity; the majority of these cells also contained neuropeptide Y immunoreactivity, characteristic of a subset of striatal interneurons. Although 10-15% of cultured striatal neurons showed moderate or intense immunoreactivity for calbindin-D28k, the majority of neurons with somatostatin-like immunoreactivity did not contain calbindin-D28k-like immunoreactivity; parvalbumin immunoreactivity was absent from the culture preparation. A highly sensitive radioimmunoassay was used to examine the actions of depolarizing agents and excitatory amino acids on the release of endogenous somatostatin-like immunoreactivity from striatal interneurons. During a 15 min incubation period, 47 +/- 10 fmol of somatostatin-like immunoreactivity were released from 14 days in vitro striatal neurons, cultured in 35 mm dishes. Depolarization with 56 mM KCl or 10 micrograms/ml veratrine resulted in an additional 105 +/- 9 and 56 +/- 5 fmol, respectively, of somatostatin-like immunoreactivity released; the release evoked by veratrine was blocked by 1 microM tetrodotoxin. In the presence of 100 microM N-methyl-D-aspartate, 112 +/- 21 fmol of somatostatin-like immunoreactivity (above basal) were released (+238%); the N-methyl-D-aspartate-evoked release was dose-dependent (EC50, 20 microM), attenuated in the absence of added Ca2+, potentiated in the absence of added Mg2+ and unaffected by the presence of 1 microM tetrodotoxin. The selective antagonists 2-amino-5-phosphonovalerate (100 microM) and MK-801 (1 microM) blocked the N-methyl-D-aspartate-evoked release of somatostatin-like immunoreactivity; KCl-evoked release was unaffected. Kainate was slightly more effective, yet five-fold less potent (EC50, 100 microM), than N-methyl-D-aspartate in evoking somatostatin-like immunoreactivity release; quisqualate was marginally effective. The results of this study suggest that N-methyl-D-aspartate and kainate receptors are present on striatal somatostatinergic interneurons in primary culture.

Quisqualate agonists occlude kainate-induced current in cultured striatal neurons

We employed the whole cell patch-clamp technique to examine the ionic currents induced via activation of kainate/quisqualate receptors on striatal neurons in primary culture when N-methyl-D-aspartate receptors were blocked by selective antagonists. Bath perfusion of 10 microM-1 mM each of quisqualate, glutamate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (a selective quisqualate agonist) or kainate, induced only a sustained current, but more rapid application by pressure ejection of each of the first three agonists (but not kainate) also activated a rapidly desensitizing current. The current induced by a near-saturating concentration of kainate (1 mM) was, on average, 16-fold larger than the maximum sustained current induced by quisqualate (10 microM), or 7.5-fold larger than that induced by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (100 microM) or glutamate (100 microM). When kainate (100 microM-10 mM) was co-applied with each of the agonists (1 microM-1 mM), the sustained current was not the algebraic sum of the currents activated by kainate or the other agonist alone; rather, the kainate-induced current was increasingly occluded by co-application with increasing concentrations of another agonist. The potency to occlude kainate-induced current had a rank order of quisqualate greater than alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate approximately glutamate; although at sufficiently high concentrations all three agonists could occlude the kainate-induced current completely. When kainate and quisqualate were co-applied during the continued presence of quisqualate, the onset of the kainate-induced sustained current was dramatically slowed. However, the steady-state occlusion by quisqualate could be abolished when the ratio kainate to quisqualate was raised to 100:1; therefore, the occlusion appears to involve a competition between kainate and quisqualate at some shared receptor binding sites which have a higher affinity for quisqualate than kainate.

Striatal proenkephalin turnover and gene transcription are regulated by cyclic AMP and protein kinase C-related pathways

Preproenkephalin metabolism, in the rat, was studied in primary striatal neurons maintained in a chemically defined medium. Acute treatment (30 min) with forskolin (10(-5) M) or phorbol 12 myristate 13 acetate (10(-7) M) resulted, respectively, in a two- and seven-fold increase in methionine-enkephalin secretion. Chronic treatment with forskolin or phorbol 12 myristate 13 acetate (24 h) induced a 100% increase in methionine-enkephalin content (forskolin) and on the other hand a 50% decrease in methionine-enkephalin (phorbol 12 myristate 13 acetate). Both treatments increased preproenkephalin mRNA levels in a time-dependent manner, this augmentation being observable after 180 min by Northern blot analysis and in situ hybridization. These data indicate that under chronic stimulation, with either forskolin or phorbol 12 myristate 13 acetate, proenkephalin turnover is accelerated. However, after stimulation with phorbol 12 myristate 13 acetate, the more potent methionine-enkephalin secretagogue, increased peptide synthesis is not sufficient to replenish methionine-enkephalin intracellular stores. Preproenkephalin gene transcription was analysed by introducing the preproenkephalin gene promoter fused to the bacterial acetyl chloramphenicol transferase reporter gene into primary neurons. Chronic stimulation (48 h) by forskolin (10(-5) M) or phorbol 12 myristate 13 acetate (10(-7) M) of striatal neurons transfected with this fusion gene increased chloramphenicol acetyltransferase activity six-fold and the two effects were additive. These data suggest that the cyclic AMP and the protein kinase C pathways directly activate preproenkephalin gene transcription.

Stimulation of dopamine release from cultured rat mesencephalic cells by naturally occurring excitatory amino acids: involvement of both N-methyl-D-aspartate (NMDA) and non-NMDA receptor subtypes

In rat mesencephalic cell cultures, L-glutamate at concentrations ranging from 100 microM to 1 mM stimulated release of [3H]dopamine that was attenuated by the non-N-methyl-D-aspartate (non-NMDA) receptor antagonist 6,7-dinitroquinoxalinedione, but not by the selective NMDA receptor antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801; 10 microM) and 3-(2-carboxypiperazine-4-yl)propyl-1-phosphonate (300 microM). Even at 1 mM glutamate, this release was Ca2+ dependent. These observations suggest that the release was mediated by a non-NMDA receptor. Only release stimulated by a lower concentration (10 microM) of glutamate was inhibited by MK-801 (10 microM), indicating that glutamate at this concentration activates the NMDA receptor. By contrast, L-aspartate at concentrations of 10 microM to 1 mM evoked [3H]dopamine release that was completely inhibited by MK-801 (10 microM) and was also Ca2+ dependent (tested at 1 and 10 mM aspartate). Thus, effects of aspartate involved activation of the NMDA receptor. Sulfur-containing amino acids (L-homocysteate, L-homocysteine sulfinate, L-cysteate, L-cysteine sulfinate) also evoked [3H]dopamine release. Release evoked by submillimolar concentrations of these amino acids was attenuated by MK-801 (10 microM), indicating involvement of the NMDA receptor.(ABSTRACT TRUNCATED AT 250 WORDS)