Differential modulation of GABAA receptor-channel complex by polyvalent cations in rat dorsal root ganglion neurons

The physiological and pathophysiological roles of copper in the nervous system

Copper is a critical trace element in biological systems due the vast number of essential enzymes that require the metal as a cofactor, including cytochrome c oxidase, superoxide dismutase and dopamine-β-hydroxylase. Due its key role in oxidative metabolism, antioxidant defence and neurotransmitter synthesis, copper is particularly important for neuronal development and proper neuronal function. Moreover, increasing evidence suggests that copper also serves important functions in synaptic and network activity, the regulation of circadian rhythms, and arousal. However, it is important to note that because of copper's ability to redox cycle and generate reactive species, cellular levels of the metal must be tightly regulated to meet cellular needs while avoiding copper-induced oxidative stress. Therefore, it is essential that the intricate system of copper transporters, exporters, copper chaperones and copper trafficking proteins function properly and in coordinate fashion. Indeed, disorders of copper metabolism such as Menkes disease and Wilson disease, as well as diseases linked to dysfunction of copper-requiring enzymes, such as SOD1-linked amyotrophic lateral sclerosis, demonstrate the dramatic neurological consequences of altered copper homeostasis. In this review, we explore the physiological importance of copper in the nervous system as well as pathologies related to improper copper handling.

Copper in the suprachiasmatic circadian clock: A possible link between multiple circadian oscillators

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) is very robust, able to coordinate our daily physiological and behavioral rhythms with exquisite accuracy. Simultaneously, the SCN clock is highly sensitive to environmental timing cues such as the solar cycle. This duality of resiliency and sensitivity may be sustained in part by a complex intertwining of three cellular oscillators: transcription/translation, metabolic/redox, and membrane excitability. We suggest here that one of the links connecting these oscillators may be forged from copper (Cu). Cellular Cu levels are highly regulated in the brain and peripherally, and Cu affects cellular metabolism, redox state, cell signaling, and transcription. We have shown that both Cu chelation and application induce nighttime phase shifts of the SCN clock in vitro and that these treatments affect glutamate, N-methyl-D-aspartate receptor, and associated signaling processes differently. More recently we found that Cu induces mitogen-activated protein kinase-dependent phase shifts, while the mechanisms by which Cu removal induces phase shifts remain unclear. Lastly, we have found that two Cu transporters are expressed in the SCN, and that one of these transporters (ATP7A) exhibits a day/night rhythm. Our results suggest that Cu homeostasis is tightly regulated in the SCN, and that changes in Cu levels may serve as a time cue for the circadian clock. We discuss these findings in light of the existing literature and current models of multiple coupled circadian oscillators in the SCN.

Copper signalling: causes and consequences

Copper-containing enzymes perform fundamental functions by activating dioxygen (O2) and therefore allowing chemical energy-transfer for aerobic metabolism. The copper-dependence of O2 transport, metabolism and production of signalling molecules are supported by molecular systems that regulate and preserve tightly-bound static and weakly-bound dynamic cellular copper pools. Disruption of the reducing intracellular environment, characterized by glutathione shortage and ambient Cu(II) abundance drives oxidative stress and interferes with the bidirectional, copper-dependent communication between neurons and astrocytes, eventually leading to various brain disease forms. A deeper understanding of of the regulatory effects of copper on neuro-glia coupling via polyamine metabolism may reveal novel copper signalling functions and new directions for therapeutic intervention in brain disorders associated with aberrant copper metabolism.

Loss of divalent metal transporter 1 function promotes brain copper accumulation and increases impulsivity

The divalent metal transporter 1 (DMT1) is a major iron transporter required for iron absorption and erythropoiesis. Loss of DMT1 function results in microcytic anemia. While iron plays an important role in neural function, the behavioral consequences of DMT1 deficiency are largely unexplored. The goal of this study was to define the neurobehavioral and neurochemical phenotypes of homozygous Belgrade (b/b) rats that carry DMT1 mutation and explore potential mechanisms of these phenotypes. The b/b rats (11-12 weeks old) and their healthy littermate heterozygous (+/b) Belgrade rats were subject to elevated plus maze tasks. The b/b rats spent more time in open arms, entered open arms more frequently and traveled more distance in the maze than +/b controls, suggesting increased impulsivity. Impaired emotional behavior was associated with down-regulation of GABA in the hippocampus in b/b rats. Also, b/b rats showed increased GABAA receptor α1 and GABA transporter, indicating altered GABAergic function. Furthermore, metal analysis revealed that b/b rats have decreased total iron, but normal non-heme iron, in the brain. Interestingly, b/b rats exhibited unusually high copper levels in most brain regions, including striatum and hippocampus. Quantitative PCR analysis showed that both copper importer copper transporter 1 and exporter copper-transporting ATPase 1 were up-regulated in the hippocampus from b/b rats. Finally, b/b rats exhibited increased 8-isoprostane levels and decreased glutathione/glutathione disulfide ratio in the hippocampus, reflecting elevated oxidative stress. Combined, our results suggest that copper loading in DMT1 deficiency could induce oxidative stress and impair GABA metabolism, which promote impulsivity-like behavior. Iron-copper model: Mutations in the divalent metal transporter 1 (DMT1) decrease body iron status and up-regulate copper absorption, which leads to copper loading in the brain and consequently increases metal-induced oxidative stress. This event disrupts GABAergic neurotransmission and promotes impulsivity-like behavior. Our model provides better understanding of physiological risks associated with imbalanced metal metabolism in mental function and, more specifically, the interactions with GABA and redox control in the treatment of emotional disorders.

Redox and trace metal regulation of ion channels in the pain pathway

Given the clinical significance of pain disorders and the relative ineffectiveness of current therapeutics, it is important to identify alternative means of modulating nociception. The most obvious pharmacological targets are the ion channels that facilitate nervous transmission from pain sensors in the periphery to the processing regions within the brain and spinal cord. In order to design effective pharmacological tools for this purpose, however, it is first necessary to understand how these channels are regulated. A growing area of research involves the investigation of the role that trace metals and endogenous redox agents play in modulating the activity of a diverse group of ion channels within the pain pathway. In the present review, the most recent literature concerning trace metal and redox regulation of T-type calcium channels, NMDA (N-methyl-D-aspartate) receptors, GABA_A (γ-aminobutyric acid A) receptors and TRP (transient receptor potential) channels are described to gain a comprehensive understanding of the current state of the field as well as to provide a basis for future thought and experimentation.

Copper at synapse: Release, binding and modulation of neurotransmission

Over the last decade, a piece of the research studying copper role in biological systems was devoted to unravelling a still elusive, but extremely intriguing, aspect that is the involvement of copper in synaptic function. These studies were prompted to provide a rationale to the finding that copper is released in the synaptic cleft upon depolarization. The copper pump ATP7A, which mutations are responsible for diseases with a prominent neurodegenerative component, seems to play a pivotal role in the release of copper at synapses. Furthermore, it was found that, when in the synaptic cleft, copper can control, directly or indirectly, the activity of the neurotransmitter receptors (NMDA, AMPA, GABA, P2X receptors), thus affecting excitability. In turn, neurotransmission can affect copper trafficking and delivery in neuronal cells. Furthermore, it was reported that copper can also modulate synaptic vesicles trafficking and the interaction between proteins of the secretory pathways. Interestingly, proteins with a still unclear role in neuronal system though associated with the pathogenesis of neurodegenerative diseases (the amyloid precursor protein, APP, the prion protein, PrP, α-synuclein, α-syn) show copper-binding domains. They may act as copper buffer at synapses and participate in the interplay between copper and the neurotransmitters receptors. Given that copper dysmetabolism occurs in several diseases affecting central and peripheral nervous system, the findings on the contribution of copper in synaptic transmission, beside its more consolidate role as a neuronal enzymes cofactor, may open new insights for therapy interventions.

Interaction of metal ions with neurotransmitter receptors and potential role in neurodiseases

There is increasing evidence that toxic metals play a role in diseases of unknown etiology. Their action is often mediated by membrane proteins, and in particular neurotransmitter receptors. This brief review will describe recent findings on the direct interaction of metal ions with ionotropic γ-aminobutyric acid (GABAA) and glutamate receptors, the main inhibitory and excitatory neurotransmitter receptors in the mammalian central nervous system, respectively. Both hyper and hypo function of these receptors are involved in neurological and psychotic syndromes and modulation by metal ions is an important pharmacological issue. The focus will be on three xenobiotic metals, lead (Pb), cadmium (Cd) and nickel (Ni) that have no biological function and whose presence in living organisms is only detrimental, and two trace metals, zinc (Zn) and copper (Cu), which are essential for several enzymatic functions, but can mediate toxic actions if deregulated. Despite limited access to the brain and tight control by metalloproteins, exogenous metals interfere with receptor performances by mimicking physiological ions and occupying one or more modulatory sites on the protein. These interactions will be discussed as a potential cause of neuronal dysfunction.

Copper chelation and exogenous copper affect circadian clock phase resetting in the suprachiasmatic nucleus in vitro

Light stimulates specialized retinal ganglion cells to release glutamate (Glu) onto circadian clock neurons of the suprachiasmatic nucleus (SCN). Glu resets the phase of the SCN circadian clock by activating N-methyl-d-aspartate receptors (NMDAR) causing either delays or advances in the clock phase, depending on early- or late-night stimulation, respectively. In addition, these Glu-induced phase shifts require tropomyosin receptor kinase B (TrkB) receptor activity. Previous studies show that copper (Cu) released at hippocampal synapses can inhibit NMDAR activity, and application of exogenous Cu likewise inhibits NMDAR activity. We investigated the effects of Cu in acute SCN brain slices prepared from C57BL/6Nhsd adult, male mice using treatments that decrease or increase available Cu levels in vitro and recorded neuronal activity on the following day. When bath-applied for 10 min at zeitgeber time (ZT) 16 (where ZT0=lights-on in the donor animal colony), the Cu-specific chelators tetrathiomolybdate (TTM) and bathocuproine disulfonate each induce ∼2.5-3-h phase delays in circadian neuronal activity rhythms, similarly to Glu-induced phase delays. Co-application of 10 μM CuCl2, but not 10 μM CoCl₂ blocks TTM-induced phase delays. Furthermore, TTM causes phase advances when applied at ZT23. At both application times, TTM-induced phase shifts are blocked by NMDA or TrkB receptor antagonists. Surprisingly, bath-application of 10 μM Cu alone also induces phase shifts in analogous experiments at ZT16 and ZT23. Inhibiting NMDAR does not block Cu-induced phase shifts. TrkB inhibition blocks Cu-induced phase delays but not phase advances. Thus, increasing and decreasing Cu availability appear to shift the SCN clock phase through different mechanisms, at least at the receptor level. We propose that Cu plays a role in the SCN circadian clock by modulating Glu signaling.

Copper signaling in the mammalian nervous system: synaptic effects

Copper is an essential metal present at high levels in the CNS. Its role as a cofactor in mitochondrial ATP production and in essential cuproenzymes is well defined. Menkes and Wilson's diseases are severe neurodegenerative conditions that demonstrate the importance of Cu transport into the secretory pathway. In the brain, intracellular levels of Cu, which is almost entirely protein bound, exceed extracellular levels by more than 100-fold. Cu stored in the secretory pathway is released in a Ca(2+)-dependent manner and can transiently reach concentrations over 100 μM at synapses. The ability of low micromolar levels of Cu to bind to and modulate the function of γ-aminobutyric acid type A (GABA(A)) receptors, N-methyl-D-aspartate (NMDA) receptors, and voltage-gated Ca(2+) channels contributes to its effects on synaptic transmission. Cu also binds to amyloid precursor protein and prion protein; both proteins are found at synapses and brain Cu homeostasis is disrupted in mice lacking either protein. Especially intriguing is the ability of Cu to affect AMP-activated protein kinase (AMPK), a monitor of cellular energy status. Despite this, few investigators have examined the direct effects of Cu on synaptic transmission and plasticity. Although the variability of results demonstrates complex influences of Cu that are highly method sensitive, these studies nevertheless strongly support important roles for endogenous Cu and new roles for Cu-binding proteins in synaptic function/plasticity and behavior. Further study of the many roles of Cu in nervous system function will reveal targets for intervention in other diseases in which Cu homeostasis is disrupted.

Dopamine and serotonin modulate human GABAρ1 receptors expressed in Xenopus laevis oocytes

GABAρ1 receptors are highly expressed in bipolar neurons of the retina and to a lesser extent in several areas of the central nervous system (CNS), and dopamine and serotonin are also involved in the modulation of retinal neural transmission. Whether these biogenic amines have a direct effect on ionotropic GABA receptors was not known. Here, we report that GABAρ1 receptors, expressed in X. laevis oocytes, were negatively modulated by dopamine and serotonin and less so by octopamine and tyramine. Interestingly, these molecules did not have effects on GABA(A) receptors. 5-Carboxamido-tryptamine and apomorphine did not exert evident effects on any of the receptors. Schild plot analyses of the inhibitory actions of dopamine and serotonin on currents elicited by GABA showed slopes of 2.7 ± 0.3 and 6.1 ± 1.8, respectively, indicating a noncompetitive mechanism of inhibition. The inhibition of GABAρ1 currents was independent of the membrane potential and was insensitive to picrotoxin, a GABA receptor channel blocker and to the GABAρ-specific antagonist (1,2,5,6-tetrahydropyridine-4-yl)methyl phosphinic acid (TPMPA). Dopamine and serotonin changed the sensitivity of GABAρ1 receptors to the inhibitory actions of Zn(2+). In contrast, La(3+) potentiated the amplitude of the GABA currents generated during negative modulation by dopamine (EC(50) 146 μM) and serotonin (EC(50) 196 μM). The functional role of the direct modulation of GABAρ receptors by dopamine and serotonin remains to be elucidated; however, it may represent an important modulatory pathway in the retina, where GABAρ receptors are highly expressed and where these biogenic amines are abundant.

Metal toxicity at the synapse: presynaptic, postsynaptic, and long-term effects

Metal neurotoxicity is a global health concern. This paper summarizes the evidence for metal interactions with synaptic transmission and synaptic plasticity. Presynaptically metal ions modulate neurotransmitter release through their interaction with synaptic vesicles, ion channels, and the metabolism of neurotransmitters (NT). Many metals (e.g., Pb(2+), Cd(2+), and Hg(+)) also interact with intracellular signaling pathways. Postsynaptically, processes associated with the binding of NT to their receptors, activation of channels, and degradation of NT are altered by metals. Zn(2+), Pb(2+), Cu(2+), Cd(2+), Ni(2+), Co(2+), Li(3+), Hg(+), and methylmercury modulate NMDA, AMPA/kainate, and/or GABA receptors activity. Al(3+), Pb(2+), Cd(2+), and As(2)O(3) also impair synaptic plasticity by targeting molecules such as CaM, PKC, and NOS as well as the transcription machinery involved in the maintenance of synaptic plasticity. The multiple effects of metals might occur simultaneously and are based on the specific metal species, metal concentrations, and the types of neurons involved.

Functional impact of serial deletions at the C-terminus of the human GABArho1 receptor

GABArho1 receptors are formed by homopentameric assemblies that gate a chloride ion-channel upon activation by the neurotransmitter. Very little is known about the structural and functional roles played by the different domains that form each subunit; but one of them, the fourth transmembrane segment (TM4), is known to form a hydrophobic bundle together with three other TM segments that are necessary to stabilize the structure of the receptor. In this study we progressively removed amino acid residues from the C-terminus of the human GABArho1 and studied the functional properties of the receptor mutants expressed in X. laevis oocytes. We found that deletions of up to the last four residues gave rise to receptors that were still functional, generating currents of 3.92 microA for the wt, 5.75 microA for S479X, 1.82 microA for F478X, 0.52 microA for I477X and 0.27 microA for S476X when exposed to 5 microM GABA; surprisingly, the mutant with one residue removed resulted more sensitive to the agonists. Further deletions, up to residue W475, resulted in receptors that did not gate an ion-channel. In addition, deleting the signal sequence, from R2-A15, in the N-terminus produced non-functional receptors. This study reveals that GABArho1 can tolerate removal of several residues that form the fourth transmembrane segment up to a critical point, signaled by W475, beyond which the mutant protein is translated but does not form functional receptors. A comparative study is presented of some electrophysiological and pharmacological properties of the deletion mutants that were able to generate GABA currents.

Inhibition of chloride outward transport by gadolinium in cultured rat spinal cord neurons

Gadolinium is a rare-earth lanthanide metal ion and is used as organic gadolinium complexes in magnetic resonance imaging (MRI). Although gadolinium-based MRI agents are thought to be safe in clinical use, the in vivo release of the toxic free inorganic gadolinium (Gd3+) has been reported in some patients with kidney disease. In central nervous system neurons, the inhibitory action of GABA is a consequence of relatively hyperpolarized Cl- equilibrium potential (ECl), which results from the activity of K+-Cl- co-transporter (KCC). The lanthanide ions are reported to affect GABAA receptors. However, little is known about the effect of Gd3+ on GABAA receptor function with intact intracellular Cl- concentration. In the present study, we investigated the effect of Gd3+ on GABAA receptor-mediated currents using gramicidin perforated patch recording method in cultured rat spinal cord neurons. The application of muscimol, a GABAA receptor agonist, caused outward current at a holding potential of -50 mV. Gd3+ inhibited the muscimol-induced outward current in a concentration-dependent and reversible manner. Gd3+ inhibited the maximum muscimol response but had no effect on the half-maximum concentration. The Gd3+ inhibition was accompanied by a depolarizing shift of the reversal potential. The Gd3+ action was blocked by furosemide, a blocker of both KCC and Na+-K+-Cl- co-transporter (NKCC), but not bumetanide, a specific blocker of NKCC. Gd3+ failed to inhibit the muscimol-induced outward currents recorded by conventional whole-cell patch-clamp method which cannot retain intact intracellular Cl- concentration. These results suggest that Gd3+ inhibits a KCC function and gives rise to increase in intracellular Cl- concentration. The reduction of outward chloride transport could be related to the neurotoxic effects of Gd3+.

Dissecting the facilitator and inhibitor allosteric metal sites of the P2X4 receptor channel: critical roles of CYS132 for zinc potentiation and ASP138 for copper inhibition

Zinc and copper are atypical modulators of ligand-gated ionic channels in the central nervous system. We sought to identify the amino acids of the rat P2X4 receptor involved in trace metal interaction, specifically in the immediate linear vicinity of His140, a residue previously identified as being critical for copper-induced inhibition of the ATP-evoked currents. Site-directed mutagenesis replaced conspicuous amino acids located within the extracellular domain region between Thr123 and Thr146 for alanines. cDNAs for the wild-type and the receptor mutants were expressed in Xenopus laevis oocytes and examined by the two-electrode technique. Cys132, but not Cys126, proved crucial for zinc-induced potentiation of the receptor activity, but not for copper-induced inhibition. Zinc inhibited in a concentration-dependent manner the ATP-gated currents of the C132A mutant. Likewise, Asp138, but not Asp131 was critical for copper and zinc inhibition; moreover, mutant D138A was 20-fold more reactive to zinc potentiation than wild-type receptors. Asp129, Asp131, and Thr133 had minor roles in metal modulation. We conclude that this region of the P2X4 receptor has a pocket for trace metal coordination with two distinct and separate facilitator and inhibitor metal allosteric sites. In addition, Cys132 does not seem to participate exclusively as a structural receptor channel folding motif but plays a role as a ligand for zinc modulation highlighting the role of trace metals in neuronal excitability.

Potentiation of inhibitory amino acid receptors-mediated responses by lanthanum in rat sacral dorsal commissural neurons

Lanthanum is one of rare earth cations with extremely active chemical property and has been reported to influence neuronal transmitter systems. To date, little attention has been directed towards the sacral dorsal commissural nucleus (SDCN), which serves as a relay of sensory information from the pelvic viscera in the spinal cord. Therefore, the effect of lanthanum on the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) and glycine (Gly) responses in neurons acutely dissociated from the rat SDCN was investigated using the nystatin-perforated patch-recording configuration under voltage-clamp conditions. At a holding potential of -40 mV, La(3+) reversibly potentiated GABA (3 microM)-activated currents (I(GABA)) in a concentration-dependent manner over the concentration range of 10 microM to 30 mM, with the EC(50) value of 67.3+/-16.4 microM. Similarly, La(3+) reversibly potentiated glycine (10 microM)-activated currents (I(Gly)) in a concentration-dependent manner over the concentration range of 1 microM to 1 mM, with the EC(50) value of 52.3+/-10.9 microM. The effects of La(3+) on I(GABA) and I(Gly) were voltage-independent. Moreover, both of the potentiations were not use-dependent and were overcome by increasing the concentration of agonist. Our results indicate that La(3+) potentiates the inhibitory amino acid receptors-mediated responses in SDCN, which may reduce the transmission of the pelvic visceral information. The information provided by this work may help to elucidate the mechanisms and effects of lanthanum on brain functions.

Analysis of macroscopic ionic currents mediated by GABArho1 receptors during lanthanide modulation predicts novel states controlling channel gating

Lanthanide-induced modulation of GABA(C) receptors expressed in Xenopus oocytes was studied. We obtained two-electrode voltage-clamp recordings of ionic currents mediated by recombinant homomeric GABArho(1) receptors and performed numerical simulations of kinetic models of the macroscopic ionic currents.GABA-evoked chloride currents were potentiated by La(3+), Lu(3+) and Gd(3+) in the micromolar range. Lanthanide effects were rapid, reversible and voltage independent. The degree of potentiation was reduced by increasing GABA concentration.Lu(3+) also induced receptor desensitization and decreased the deactivation rate of GABArho(1) currents. In the presence of 300 microM Lu(3+), dose-response curves for GABA-evoked currents showed a significant enhancement of the maximum amplitude and an increase of the apparent affinity. The rate of onset of TPMPA and picrotoxin antagonism of GABArho(1) receptors was modulated by Lu(3+). These results suggest that the potentiation of the anionic current was the result of a direct lanthanide-receptor interaction at a site capable of allosterically modulating channel properties. Based on kinetic schemes, which included a second open state and a nonconducting desensitized state that closely reproduced the experimental results, two nonexclusive probable models of GABArho(1) channels gating are proposed.

Benzodiazepine modulation of partial agonist efficacy and spontaneously active GABA(A) receptors supports an allosteric model of modulation

Benzodiazepines (BZDs) have been used extensively for more than 40 years because of their high therapeutic index and low toxicity. Although BZDs are understood to act primarily as allosteric modulators of GABA(A) receptors, the mechanism of modulation is not well understood. The applicability of an allosteric model with two binding sites for gamma-aminobutyric acid (GABA) and one for a BZD-like modulator was investigated. This model predicts that BZDs should enhance the efficacy of partial agonists. Consistent with this prediction, diazepam increased the efficacy of the GABA(A) receptor partial agonist kojic amine in chick spinal cord neurons. To further test the validity of the model, the effects of diazepam, flurazepam, and zolpidem were examined using wild-type and spontaneously active mutant alpha1(L263S)beta3gamma2 GABA(A) receptors expressed in HEK-293 cells. In agreement with the predictions of the allosteric model, all three modulators acted as direct agonists for the spontaneously active receptors. The results indicate that BZD-like modulators enhance the amplitude of the GABA response by stabilizing the open channel active state relative to the inactive state by less than 1 kcal, which is similar to the energy of stabilization conferred by a single hydrogen bond.

Heavy metals modulate the activity of the purinergic P2X4 receptor

To further characterize the nature of the regulatory metal-binding sites of the rat P2X(4) receptor, several transition heavy metals were tested to examine their ability to mimic the facilitator action of zinc or the inhibitory action of copper. cDNA coding for the rat P2X(4) receptor was injected into Xenopus laevis oocytes; the two-electrode voltage-clamp technique was used to measure and quantify the ATP-evoked currents in the absence or presence of the metals. Cadmium facilitated the ATP-gated currents in a reversible and voltage-independent manner; maximal potentiation occurred within less than 1 min. Cadmium displaced leftward, in a concentration-dependent manner, the ATP concentration-response curve. In contrast, mercury reduced the ATP-gated currents in a reversible, time, and concentration manner. Maximal inhibition occurred after about 5 min of metal application. Cobalt also augmented the ATP-evoked currents, but its action was long lasting and did not reverse even after 45 min of metal washout. Other metals such as lead, nickel, manganese, silver, or gallium did not significantly alter the ATP-gated currents. The co-application of cadmium plus zinc or mercury plus copper caused additive effects. Mutation of H140 by alanine (H140A) augmented both the cadmium-induced facilitation and the mercury-induced inhibition. In contrast, the H241A mutant showed characteristics indistinguishable from the wild type. The H286A mutant showed a normal cadmium-induced potentiation, but an increased mercury inhibition. Out of the metals examined, only cadmium mimicked closely the action of zinc, evidencing commonalities. While mercury mimicked the action of copper, both metals apparently interact at distinct metal-binding sites. The present findings allow us to infer that heavy metals modulate the P2X(4) receptor by acting in at least three separate metal-binding sites.

Copper-induced changes in locomotor behaviors and neuronal physiology of the freshwater oligochaete, Lumbriculus variegatus

The behavioral and neurotoxic effects of copper exposure were examined in the freshwater oligochaete, Lumbriculus variegatus. The 24 h LC50 for worms exposed to copper sulfate in an artificial pond water was 0.45 microM. Almost all animals that died due to copper exposure died during the first day of exposure. Immersion in water containing 0.2 or 0.4 microM copper produced time- and concentration-dependent reductions in the ability of tactile stimulation to evoke two stereotyped locomotory behaviors, body reversal and helical swimming. Helical swimming was more severely affected by copper exposure than was body reversal behavior. Upon return to clean water, both behaviors returned to normal levels within 1-2 days. Noninvasive electrophysiological testing indicated that copper exposure produced time- and concentration-dependent reductions in the conduction velocities of the medial and lateral giant nerve fibers. An 8 h exposure to 0.2 microM copper produced significant reductions in giant fiber conduction velocities that returned to normal levels within 3 days of return to clean water. It is likely that copper exposure can significantly degrade the ability of aquatic oligochaetes to avoid predators.

An N-Terminal Histidine Is the Primary Determinant of α Subunit-Dependent Cu2+Sensitivity of αβ3γ2L GABAAReceptors

Copper (Cu2+) is a physiologically important cation and is released from nerve terminals. Cu2+ modulates GABAA receptor currents in an alpha subunit subtype-dependent manner; alpha1beta3gamma2L receptors are more sensitive to Cu2+ than alpha6beta3gamma2L receptors. We compared the effect of Cu2+ on alphabeta3gamma2L receptors containing each of the six alpha subtypes and generated alpha1/alpha6 chimeras and mutants to determine the functional domain(s) and specific residues responsible for alpha subtype-dependent differences in Cu2+ sensitivity. Whole-cell GABAA receptor currents were obtained from L929 fibroblasts coexpressing wild-type, chimeric and mutant alpha subunits with beta3 and gamma2L subunits. Maximal Cu2+ inhibition of alpha1beta3gamma2L and alpha2beta3gamma2L receptor currents was larger (52.2 +/- 3.0 and 59.0 +/- 2.5%, respectively) than maximal inhibition of alpha3beta3gamma2L, alpha4beta3gamma2L, alpha5beta3gamma2L, and alpha6beta3gamma2L receptor currents (22.6 +/- 3.1, 19.2 +/- 3.4, 20.2 +/- 4.8, and 21.2 +/- 3.6%, respectively). Receptors containing chimeric constructs with alpha1 subtype N-terminal sequence between residues 127 and 232 were inhibited by Cu2+ to an extent similar to those with alpha1 subtypes, suggesting that this N-terminal region (127-232) contains a major determinant for high Cu2+ sensitivity. alpha1 subtype residues V134, R135, and H141 in a VRAECPMH motif (VQAECPMH in the alpha2 subtype) conferred higher Cu2+ sensitivity, and the H141 residue was the major determinant in the motif. The beta3 subtype M2 domain residue H267, which is a major determinant of Zn2+ inhibition, and alpha6 subtype M2-M3 loop residue H273, which is responsible for the increased Zn2+ sensitivity of the alpha6 subtype, also seemed to contribute to Cu2+ inhibition. These data suggest that the N-terminal VR(Q)AECPMH motif in alpha1 and alpha2 subtypes is the major determinant of increased subtype-dependent inhibition by Cu2+, that residue H141 is the major determinant in that motif, and that Cu2+ may also interact with GABAA receptors at sites similar to or overlapping Zn2+ sites.

Cu2+, Co2+, and Mn2+ modify the gating kinetics of high-voltage-activated Ca2+ channels in rat palaeocortical neurons

The effects of three divalent metal cations (Mn2+, Co2+, and Cu2+) on high-voltage-activated (HVA) Ca2+ currents were studied in acutely dissociated pyramidal neurons of rat piriform cortex using the patch-clamp technique. Cu2+, Mn2+, and Co2+ blocked HVA currents conducted by Ba2+ ( IBa) with IC50 of approximately 920 nM, approximately 58 micro M, and approximately 65 micro M, respectively. Additionally, after application of non-saturating concentrations of the three cations, residual currents activated with substantially slower kinetics than control IBa. As a consequence, the current fraction abolished by the blocking cations typically displayed, in its early phase, an unusually fast-decaying transient. The latter phenomenon turned out to be a subtraction artifact, since none of the pharmacological components (L-, N-, P/Q-, and R-type) that constitute the total HVA currents under study showed a similarly fast early decay: hence, the slow activation kinetics of residual currents was not due to the preferential inhibition of a fast-activating/inactivating component, but rather to a true slowing effect of the blocker cations. The percent IBa-amplitude inhibition caused by Mn2+, Co2+, and Cu2+ was voltage-independent over the whole potential range explored (up to +30 mV), hence the slowing of IBa activation kinetics was not due to a mechanism of voltage- and time-dependent relief from block. Moreover, Mn2+, Co2+, and Cu2+ significantly reduced I(Ba) deactivation speed upon repolarization, which also is not compatible with a depolarization-dependent unblocking mechanism. The above results show that 1) Cu2+ is a particularly potent HVA Ca2+-channel blocker in rat palaeocortical neurons; and 2) Mn2+, Co2+, and Cu2+, besides exerting a blocking action on HVA Ca2+-channels, also modify Ca2+-current activation and deactivation kinetics, most probably by directly interfering with channel-state transitions.

Histidine 140 plays a key role in the inhibitory modulation of the P2X4 nucleotide receptor by copper but not zinc

To elucidate the role of extracellular histidines in the modulation of the rat P2X4 receptor by trace metals, we generated single, double, and triple histidine mutants for residues 140, 241, and 286, replacing them with alanines. cDNAs for the wild-type and receptor mutants were expressed in Xenopus laevis oocytes and in human embryonic kidney 293 cells and examined by the two electrode and patch clamp techniques, respectively. Whereas copper inhibited concentration-dependently the ATP-gated currents in the wild-type and in the single or double H241A and H286A receptor mutants, all receptors containing H140A were insensitive to copper in both cell systems. The characteristic bell-shaped concentration-response curve of zinc observed in the wild-type receptor became sigmoid in both oocytes and human embryonic kidney cells expressing the H140A mutant; in these mutants, the zinc potentiation was 2.5-4-fold larger than in the wild-type. Results with the H140T and H140R mutants further support the importance of a histidine residue at this position. We conclude that His-140 is critical for the action of copper, indicating that this histidine residue, but not His-241 or His-286, forms part of the inhibitory allosteric metal-binding site of the P2X4 receptor, which is distinct from the putative zinc facilitator binding site.

Copper blocks quinolinic acid neurotoxicity in rats: contribution of antioxidant systems

Reactive oxygen species and oxidative stress are involved in quinolinic acid (QUIN)-induced neurotoxicity. QUIN, a N-methyl-D-aspartate receptor (NMDAr) agonist and prooxidant molecule, produces NMDAr overactivation, excitotoxic events, and direct reactive oxygen species formation. Copper is an essential metal exhibiting both modulatory effects on neuronal excitatory activity and antioxidant properties. To investigate whether this metal is able to counteract the neurotoxic and oxidative actions of QUIN, we administered copper (as CuSO(4)) intraperitoneally to rats (2.5, 5.0, 7.5, and 10.0 mg/kg) 30 min before the striatal infusion of 1 microliter of QUIN (240 nmol). A 5.0 mg/kg CuSO(4) dose significantly increased the copper content in the striatum, reduced the neurotoxicity measured both as circling behavior and striatal gamma-aminobutyric acid (GABA) depletion, and blocked the oxidative injury evaluated as striatal lipid peroxidation (LP). In addition, copper reduced the QUIN-induced decreased striatal activity of Cu,Zn-dependent superoxide dismutase, and increased the ferroxidase activity of ceruloplasmin in cerebrospinal fluid from QUIN-treated rats. However, copper also produced significant increases of plasma lactate dehydrogenase activity and mortality at the highest doses employed (7.5 and 10.0 mg/kg). These results show that at low doses, copper exerts a protective effect on in vivo QUIN neurotoxicity.

Neuromodulator role of zinc and copper during prolonged ATP applications to P2X4 purinoceptors

To further elucidate the modulator role of trace metals such as zinc and copper on the activity of nucleotide purinoceptors, the action of these metals was assessed during prolonged ATP applications to rat P2X(4) purinoceptors expressed in Xenopus laevis oocytes. Application of ATP for 3 min resulted in a biphasic effect; a fast transient peak was followed by a slower stable current component with similar pharmacological and biophysical characteristics. The application of 1-300 microM Cu(2+) inhibited both current components to a comparable extent; likewise, Zn(2+) facilitated to a similar degree the transient and the slower stable current components. Carnosine (Car), cysteine (Cys), histidine (His), and the metal chelator, penicillamine, prevented the inhibitory action of Cu(2+); the Zn(2+) facilitation was not prevented by neither Car nor His but by either bathophenantroline or Cys, revealing metal selectivity. While the noncompetitive Cu(2+) inhibition appears to decrease channel conductance, Zn(2+) likely increases ATP affinity independently of the activation state of the purinoceptor. These results strongly support the notion that trace metals modulate the activity of the P2X(4) purinoceptor and could become relevant during continual activity of a P2X(4) purinoceptor-containing synapse.

Cu(2+) inhibition of glycine-activated currents in rat sacral dorsal commissural neurons

The effect of Cu(2+) on glycine (Gly) response was examined in neurons acutely dissociated from the rat sacral dorsal commissural nucleus (SDCN) using the nystatin perforated patch clamp recording configuration under voltage-clamp conditions. Cu(2+), in the concentration range 10-1000 microM, reversibly inhibited chloride current activated by 30 microM Gly at a holding potential of -40 mV with an IC(50) of 88.4 microM. Cu(2+) shifted the Gly concentration response curve to the right in a parallel manner, which indicated that Cu(2+) decreased the apparent affinity of the receptor for Gly. Cu(2+) suppression of Gly-activated current was independent of membrane potential between -60 and +60 mV and did not involve a shift in the reversal potential of the current. Furthermore, Cu(2+) antagonized the inhibitory action of Zn(2+) in a concentration-dependent manner, suggesting a common site or mechanism of action of Cu(2+) and Zn(2+) on Gly receptors. The results show that Cu(2+) is a potent inhibitor of Gly receptor-mediated responses in rat spinal neurons.

A histidine residue in the extracellular N-terminal domain of the GABA(A) receptor alpha5 subunit regulates sensitivity to inhibition by zinc

The divalent cation zinc is abundant in the brain, particularly in the mossy fibers of the hippocampus. Recent evidence suggests that zinc is packaged into some synaptic vesicles in this region and can be co-released with neurotransmitter. Zinc inhibits the activity of GABA(A) receptors and the sensitivity of the receptor to zinc is influenced by its alpha subunit subtype composition. The alpha4, alpha5 and alpha6 subunits confer greater sensitivity to zinc than receptors containing other alpha subunits. The alpha4 and alpha5 subunits are highly expressed in hippocampal neurons, and likely mediate any effects of zinc on GABAergic neurotransmission in this area. The alpha5 subunit contains a unique histidine residue in the N-terminal extracellular domain while the other alpha subunits have an aspartate residue in this location. Point mutations were created to exchange the histidine and aspartate residues of the alpha1 and alpha5 subunits. Receptors containing the mutated alpha5((H195D)) subunit had reduced sensitivity to zinc, while alpha1((D191H))beta3gamma2L receptors had increased sensitivity to zinc, similar to the alpha5beta3gamma2L wild type receptors. These findings indicate that histidine195 of the alpha5 subunit plays an important role in determining the sensitivity of recombinant GABA(A) receptors to zinc.

Interaction between copper and zinc at GABA(A) receptors in acutely isolated cerebellar Purkinje cells of the rat

Nanomolar concentrations of Cu(2+) induce a slowly reversible block of GABA(A) receptor-mediated currents which can be removed by chelating substances. The possible interaction of Cu(2+) with the Zn(2+) binding site on the GABA(A) receptor complex was studied in acutely isolated Purkinje cells using whole-cell recording and a fast drug application system. When Zn(2+) was applied together with 2 microM GABA, the Zn(2+)-induced block of GABA-mediated currents was not additive to the Cu(2+)-induced block. In the presence of 0.1 microM Cu(2+) in the bath solution the degree of inhibition of GABA-mediated responses by Zn(2+) was strongly attenuated. Preapplication of 100 microM Zn(2+) during 10 s, terminated 1 s before exposure to 2 microM GABA did not affect the GABA current in Cu(2+)-free solution, but relieved its block by 0.1 microM Cu(2+). This effect of Zn(2+) was concentration-dependent with an EC(50) of 72 microM. When the Cu(2+)-induced block was removed by histidine, preapplication of Zn(2+) did not increase the GABA current, indicating that the relief of Cu(2+) block by Zn(2+) is the result of its ability to actively remove Cu(2+) from the GABA receptor complex. It is proposed that the inhibitory effects of Zn(2+) and Cu(2+) on GABA-induced currents result from an action of these metal ions at distinct, but conformationally linked sites on the GABA(A) receptor protein. Under physiological conditions Zn(2+) would liberate Cu(2+) from the GABA(A) receptor, thus facilitating Cu(2+) turnover and its binding by other endogenous chelating molecules.

Tonic benzodiazepine-sensitive GABAergic inhibition in cultured rodent cerebellar granule cells

Recent studies have demonstrated that granule cells in rat cerebellar slices exhibit a tonic form of GABAergic inhibition. The presence of a similar constitutive GABAergic conductance was investigated in synaptically coupled cultures of neonatal rat cerebellum. In cells exhibiting spontaneous inhibitory postsynaptic currents (IPSCs), application of the GABA(A) receptor antagonist bicuculline (10 microM) eliminated the IPSCs and also produced a significant decrease in holding current. This latter effect was lacking in cells that did not exhibit IPSCs. Application of TTX (1 microM) and Cd(2+) (100 microM) decreased the IPSC frequency and also produced a change in holding current; these effects were eliminated by the prior application of bicuculline. In the presence of TTX, application of the benzodiazepine (BDZ) Flunitrazepam (1 microM) caused a 85+/-15% increase in the component of holding current that arose from GABA(A) receptor activity. Noise analysis indicated that the GABA(A) receptors underlying this tonic form of GABAergic inhibition exhibited a mean single channel conductance close to 14 pS, a value similar to that seen for somatic GABA(A) receptors in these cells. Thus, like their counterparts in cerebellar slices, cerebellar granule cells in culture exhibit a background GABAergic conductance. The most likely source of this tonic current is GABA spilt over from active inhibitory synapses. As this conductance was sensitive to benzodiazepine receptor agonists it is unlikely to arise entirely from GABA(A) receptors containing the alpha6 subunit.

Anisatin modulation of the gamma-aminobutyric acid receptor-channel in rat dorsal root ganglion neurons

1. Anisatin, a toxic, insecticidally active component of Sikimi plant, is known to act on the GABA system. In order to elucidate the mechanism of anisatin interaction with the GABA system, whole-cell and single-channel patch clamp experiments were performed with rat dorsal root ganglion neurons in primary culture. 2. Repeated co-applications of GABA and anisatin suppressed GABA-induced whole-cell currents with an EC50 of 1.10 microM. No recovery of currents was observed after washout with anisatin-free solution. 3. However, pre-application of anisatin through the bath had no effect on GABA-induced currents. The decay phase of currents was accelerated by anisatin. These results indicate that anisatin suppression of GABA-induced currents requires opening of the channels and is use-dependent. 4. Anisatin suppression of GABA-induced currents was not voltage dependent. 5. Picrotoxinin attenuated anisatin suppression of GABA-induced currents. [3H]-EBOB binding to rat brain membranes was competitively inhibited by anisatin. These data indicated that anisatin bound to the picrotoxinin site. 6. At the single-channel level, anisatin did not alter the open time but prolonged the closed time. The burst duration was reduced and channel openings per burst were decreased indicating that anisatin decreased the probability of openings.

Lanthanum-mediated modification of GABAA receptor deactivation, desensitization and inhibitory synaptic currents in rat cerebellar neurons

1. We investigated La3+ effects on recombinant and native gamma-aminobutyric acid A (GABAA) receptors using rapid agonist applications and on inhibitory synaptic currents (IPSCs) in granule and stellate neurons of rat cerebellar slices. 2. Rapid desensitization of currents elicited by 200 ms pulses of 1 mM GABA to small lifted cells transfected with alpha1beta3gamma2 cDNAs was greatly decreased by the coapplication of 100 microM LaCl3. 3. GABA responses were unaffected when coapplication lasted only 2 ms. In contrast, with LaCl3 pre-perfusion, a significant slowing of deactivation in response to 2 ms applications was observed. LaCl3 pre-perfusion also prolonged the duration of responses to 20 mM taurine. 4. Outside-out patches excised from cells transfected with alpha1beta3gamma2 subunit cDNAs were briefly exposed to a saturating concentration of GABA, eliciting a transient activation of single channel currents with a main conductance of 30 pS. Opening and burst durations increased by pre-equilibration of patches with LaCl3. 5. LaCl3 depressed the peak amplitude without affecting the slow deactivation and desensitization of GABA responses in cells transfected with alpha6beta3gamma2 and alpha6beta3delta cDNAs. No significant difference in La3+ modulation of GABA-gated currents was observed between alpha1beta3gamma2 and alpha1beta3delta receptors. 6. The effects of LaCl3 on deactivation and desensitization of GABA responses observed in nucleated patches excised from rat cerebellar granule and stellate neurons were comparable to those in the cells transfected with alpha1beta3gamma2 cDNAs. In addition, La3+ clearly prolonged the spontaneous IPSC time course without changing the amplitude. 7. Our results indicate that La3+ has a dual action on GABA-gated currents: it decreases desensitization and increases channel opening duration. These actions depend on receptor subunit composition and contribute to the prolongation of IPSCs.

Selective modulation of GABAA receptors by aluminum

Aluminum has been implicated in several neurodegenerative conditions including Alzheimer's disease. Because the mammalian olfactory system has an unusual capacity for the uptake and transneuronal spread of inhaled substances such as aluminum, whole cell recording techniques were used to examine the actions of aluminum on basic membrane properties and amino acid receptors on rat olfactory bulb mitral/tufted (M/T) neurons in culture. Aluminum had little direct effects on M/T neurons. Aluminum (100 microM) did not evoke a membrane current or alter action-potential shape or duration. Aluminum also had no marked effects on the family of voltage-gated membrane currents evoked by a series of 10-mV, 50-ms depolarizing steps. However, aluminum dramatically potentiated the current evoked by 30 microM gamma-aminobutyric acid (GABA) at concentrations <100 microM. Conversely, higher concentrations of aluminum blocked the GABA-evoked current. The effects of aluminum on GABA-evoked currents were not voltage dependent. Aluminum (100 microM) equally potentiated both inward currents at -30 mV and outward currents at + 30 mV. At 300 microM, aluminum blocked both inward and outward currents to a similar extent. In some neurons, aluminum only blocked the current and potentiation was not observed. The biphasic action of aluminum on GABA-evoked currents suggests separate binding sites: a high-affinity potentiating site and a low-affinity inhibiting site. Despite its effects on GABA-evoked currents, aluminum did not alter membrane currents evoked by glutamate, N-methyl-D-aspartate, kainate, or glycine. Aluminum also did not reduce spontaneous excitatory synaptic activity, suggesting little, if any, effect on glutamate release. Although a causal role for aluminum in Alzheimer's disease and other neuropathological conditions remains controversial, it is clear that elevated aluminum concentrations in the brain are associated with a variety of cognitive impairments. The present results indicate that aluminum can alter the function of GABAA receptors and may suggest that aluminum can contribute to cognitive impairment through disruption of inhibitory circuits.

Alcohol modulation of single GABA(A) receptor-channel kinetics

Alcohol modulation of single-channel kinetics of GABA(A) receptor currents was studied with rat dorsal root ganglion neurons using the excised outside-out patch clamp technique. GABA (1 microM) alone or GABA (1 microM) plus ethanol (30-300 mM) or n-Octanol (30-300 microM) were applied by pressure ejection to evoke single-channel currents. The main single-channel conductance was not changed by either ethanol or n-Octanol at 25 pS. Both alcohols exerted the same effects on the single-channel kinetics, although n-Octanol was more potent than ethanol. The frequency of openings, the mean open time, the percentage of open time, the frequency of bursts, and the mean burst duration were all increased, but the mean closed time was decreased. These changes in channel kinetics account for the increase in whole-cell current amplitude caused by ethanol and n-Octanol.

The role of an alpha subtype M2-M3 His in regulating inhibition of GABAA receptor current by zinc and other divalent cations

Sensitivity of GABAA receptors (GABARs) to inhibition by zinc and other divalent cations is influenced by the alpha subunit subtype composition of the receptor. For example, alpha6beta3gamma2L receptors are more sensitive to inhibition by zinc than alpha1beta3gamma2L receptors. We examined the role of a His residue located in the M2-M3 extracellular domain (rat alpha6 H273) in the enhanced zinc sensitivity conferred by the alpha6 subtype. The alpha1 subtype contains an Asn (N274) residue in the equivalent location. GABA-activated whole-cell currents were obtained from L929 fibroblasts after transient transfection with expression vectors containing GABAA receptor cDNAs. Mutation of alpha1 (alpha1(N274H)) or alpha6 (alpha6(H273N)) subtypes did not alter the GABA EC50 of alphabeta3gamma2L receptors. alpha1(N274H)beta3gamma2L receptor currents were as sensitive to zinc as alpha6beta3gamma2L receptor currents, although alpha6(H273N)beta3gamma2L receptor currents had the reduced zinc sensitivity of alpha1beta3gamma2L receptor currents. We also examined the activity of other inhibitory divalent cations with varying alpha subtype dependence: nickel, cadmium, and copper. alpha6beta3gamma2L receptor currents were more sensitive to nickel, equally sensitive to cadmium, and less sensitive to copper than alpha1beta3gamma2L receptor currents. Studies with alpha1 and alpha6 chimeric subunits indicated that the structural dependencies of the activity of some of these cations were different from zinc. Compared with alpha6beta3gamma2L receptor currents, alpha6(H273N)beta3gamma2L receptor currents had reduced sensitivity to cadmium and nickel, but the sensitivity to copper was unchanged. Compared with alpha1beta3gamma2L receptor currents, alpha1(N274H)beta3gamma2L receptor currents had increased sensitivity to nickel, but the sensitivity to cadmium and copper was unchanged. These findings indicate that H273 of the alpha6 subtype plays an important role in determining the sensitivity of recombinant GABARs to the divalent cations zinc, cadmium, and nickel, but not to copper. Our results also suggest that the extracellular N-terminal domain of the alpha1 subunit contributes to a regulatory site(s) for divalent cations, conferring high sensitivity to inhibition by copper and cadmium.

High-affinity copper block of GABA(A) receptor-mediated currents in acutely isolated cerebellar Purkinje cells of the rat

The actions of Cu2+ ions on GABAA receptor-mediated currents in acutely isolated Purkinje cells from rat cerebellum were studied using the whole-cell patch-clamp technique and a rapid perfusion system. Bath application of Cu2+ reduced currents induced by 2 microM gamma-aminobutyric acid (GABA) in a concentration-dependent manner with an IC50 of 35 nM. The Cu2+-induced block of GABA responses was not voltage-dependent. Increasing the GABA concentration (from 2 to 50 microM) decreased the blocking effect of Cu2+. Dose-response analysis for activation of GABAA receptors revealed a twofold decrease in apparent affinity for GABA in the presence of 0.1 microM Cu2+. Recovery from the block required several minutes after removal of Cu2+ from the medium. The block was removed by histidine, which preferentially forms complexes with Cu2+, or by other chelating substances. Application of 10 microM histidine immediately before application of 2 microM GABA completely relieved the block of GABA responses produced by 0.1 microM Cu2+. The effect of histidine was concentration-dependent with an EC50 of 0.75 microM. The results demonstrate that Cu2+ is a potent inhibitor of GABA-evoked responses in rat Purkinje cells. Copper may be an endogenous synaptic modulating factor. Cu2+ toxicity, notably in Wilson's disease, could result to some extent from chronic GABAA receptor blockade.

Suppression of long-term potentiation in hippocampal slices by copper

Cu(2+)-ions are known to interfere with gamma-aminobutyric acid (GABA)- and glutamate-operated ion channels from experiments with isolated neurons. Such actions are likely involved in the pathophysiology of Wilson's disease. We have now studied the effects of Cu2+ in the CA1 region of hippocampal slices. Field excitatory postsynaptic potential (EPSP) slopes in the CA1 region were unaffected by 1 microM Cu2+ but were depressed by 10 microM (to 85%) and 100 microM (to 50%). A paired-pulse test revealed no difference in facilitation in the presence or absence of Cu2+, indicating a postsynaptic action. A late component of intracellularly registered EPSPs in Mg(2+)-free solution was also reduced by Cu2+. The N-methyl-D-aspartate (NMDA) component of the field EPSP, isolated by adding CNQX and bicuculline in Mg(2+)-free solution, was reduced to 69% of control by 1 microM and to 50% of control by 10 microM Cu2+. Long-term potentiation, evoked by 3 x 50 pulses at 100 Hz, 20 s interval amounted to 132 +/- 11% 90 min after tetanization under control conditions but was absent in the presence of 1 microM Cu2+ in the bath. Thus low concentrations of copper can selectively reduce NMDA-mediated potentials and synaptic plasticity.

Identification of a Zn2+ binding site on the murine GABAA receptor complex: dependence on the second transmembrane domain of beta subunits

1. Whole-cell currents were recorded from Xenopus laevis oocytes expressing wild-type and mutant recombinant GABAA receptors to locate a binding site for Zn2+ ions in the beta 3 subunit. 2. The Cl(-)-selective current, spontaneously gated by beta 3 subunit homomers, was enhanced by pentobarbitone and inhibited by picrotoxinin. The potencies of these agents were minimally affected by mutating histidine (H) 292 to alanine (A) in the second transmembrane domain (TM2). 3. Zn2+ inhibited the beta 3 subunit-gated conductance (IC50, 0.31 microM); the inhibition was voltage insensitive. The H292A mutation in beta 3 subunits caused a 1000-fold reduction in Zn2+ potency (IC50, 307 microM). 4. GABA-activated responses recorded from heteromeric alpha 1 beta 3 GABAA receptors were also inhibited by Zn2+ (IC50, 0.11 microM). This inhibition was reduced by mutating H292A in the beta 3 subunit (IC50, 22.8 microM). 5. H292 in TM2 of the beta 3 subunit is an important determinant of a Zn2+ binding site on the GABAA receptor. Its location in the presumed ion channel lining suggests that Zn2+ can penetrate into an anion-selective channel and that the ionic selectivity filter and channel gate are located beyond H292.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

Contrasting actions of lanthanum on different recombinant gamma-aminobutyric acid receptor isoforms expressed in L929 fibroblasts

Functional studies have indicated that, unlike most divalent cations, lanthanum increases both native and recombinant gamma-aminobutyric acid (GABA) receptor (GABAR) currents. In the present study, we have examined whether lanthanum shows subunit-dependent selectivity for modification of currents from different GABAR isoforms. The effects of lanthanum on three different GABAR isoforms, alpha1beta3gamma2L, alpha6beta3gamma2L, and alpha6beta3delta, were determined by transient expression of combinations of alpha1, alpha6, beta3, gamma2L, and delta subunit cDNAs in L929 fibroblasts. Whole-cell recording was used to determine the concentration-response curves for lanthanum for the three different isoforms at submaximal concentrations of GABA. Lanthanum displayed strong potentiation of alpha1beta3gamma2L GABAR currents consistent with earlier reports of potentiation of GABAR currents by lanthanum in neurons and recombinant GABAR isoforms. However, in contrast to the potentiation of alpha1beta3gamma2L GABAR currents by lanthanum, alpha6beta3delta GABAR currents were strongly inhibited and alpha6beta3gamma2L GABAR currents were weakly inhibited by lanthanum. Interaction of lanthanum with GABAR isoforms was competitive, with lanthanum decreasing the EC50 value for GABA of alpha1beta3gamma2L GABARs without changing the maximum current and increasing the EC50 value for GABA of alpha6beta3delta and alpha6beta3gamma2L GABAR currents (greater shift in EC50 value in the alpha6beta3delta compared with the alpha6beta3gamma2L GABARs) without changing the maximum GABAR current. Neither potentiation nor inhibition of GABAR currents by lanthanum showed any voltage dependence. These results suggest that 1) changing the alpha-subunit subtype from alpha1 to alpha6 altered the effect of lanthanum from potentiation to inhibition, 2) changing the gamma2L subunit to the delta-subunit changed the level of maximal inhibition of alpha6 subtype-containing GABAR currents by lanthanum, and 3) the site for interaction with lanthanum probably was on the extracellular surface of GABARs.

Dual, non-competitive interaction of lead with neuronal nicotinic acetylcholine receptors in N1E-115 neuroblastoma cells

In cultured mouse neuroblastoma N1E-115 cells, inorganic lead (Pb2+) affects inward currents induced by activation of neuronal type nicotinic acetylcholine receptors (nAChR) in a biphasic manner. At nanomolar concentrations a blocking action is observed, while at submillimolar concentrations this blocking effect is reversed, resulting in a U-shaped concentration-effect curve. Maximal block by 90% is observed at 1-3 microM Pb2+. The interactions of Pb2+ with nAChR were examined at the blocking concentration of 10 nM Pb2+ and at 10 microM Pb2+, presenting the reversal of block. The fitted Emax for nAChR receptor activation by ACh was attenuated at both high and low Pb2+ concentrations by 24% and 54%, respectively. The EC50 values of the activation curves were not significantly altered; amounting to 53 microM, 64 microM and 86 microM ACh in the control situation and in the presence of 10 nM and 10 microM Pb2+, respectively. Further, receptor desensitization and ion channel block by ACh were also not affected by Pb2+. The results indicate that Pb2+ affects nAChR in a dual manner that involves inhibition and potentiation, both by non-competitive interactions. Neuronal nAChR expressed in N1E-115 cells resemble a combination of alpha 4 and beta 2 subunits, that constitute the predominant subunits in the central nervous system. The differential inhibition and potentiation of nAChR, together with the high sensitivity, are of interest with respect to Pb2+ neurotoxicity.

The effects of copper ions on glutamate receptors in cultured rat cortical neurons

Copper plays an important role in the function of many physiological processes and can affect different neurotransmitter systems. In this study, we used the patch-clamp technique to investigate the effect of copper ions on glutamate receptors in cultured rat cortical neurons. Cu2+ inhibited (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors with an IC50 of 4.3 +/- 0.6 microM (with 100 microM kainate, holding potential -60 mV). The concentration-response could be best described by a two-site binding model. Moreover, copper reduced the efficacy of kainate at the AMPA receptor: in the presence of 30 microM Cu2+, the EC50 of kainate was shifted from 100.3 +/- 2.0 microM to 329.9 +/- 31.4 microM. The block by copper ions was not use-dependent. Complete recovery only occurred after the application of a high agonist concentration, or in the presence of the antioxidant dithiotreitol (DTT). A high concentration of histidine, a physiological ligand for Cu2+, did not augment the recovery. The kinetics of block were compared to those induced by 2,3-dihydro-6-nitro-7-sulfamoyl-benz(F)quinoxaline (NBQX), a well-described competitive antagonist of AMPA receptors. The onset, as well as the offset of block by NBQX could be well approximated by single exponential functions with time constants of 0.28 +/- 0.02 and 0.87 +/- 0.09 s, respectively. Within seconds of wash-out of the antagonist, the response to kainate completely recovered. The kinetics of copper block were more complex: the block developed more slowly, and the onset, as well as the offset could be described by two exponential functions with quite different time constants (tau(on1), 0.8 +/- 0.13 s; tau(on2), 8.32 +/- 1.13 s; tau(off1), 0.17 +/- 0.01 s; tau(off2), 69 +/- 36.3 s). In addition to the described effects, Cu2+ also blocked currents induced by the application of N-methyl-D-aspartate (IC50: 15.0 +/- 2.6 microM with 50 microM NMDA). Based on these findings, a modulatory role of copper ions on the neurotransmission by excitatory amino acids is discussed.

Intracerebroventricular La3+ but not Gd3+ inhibits cocaine-induced motor activation in rats

In the present work the effects of i.c.v. administration of La3+ and Gd3+ on the motor stimulant effect of cocaine in rats were studied. Both La3+ and Gd3+ failed to influence basal motor activity. However, the two metal ions differ in modulation of cocaine-induced activation of motor activity. While Gd3+ (10.0, 20.0 and 40.0 mM/1 ml) did not influence significantly the cocaine effect, La3+ (0.1, 1.0 and 10 mM/1 ml) inhibited cocaine-induced motor activation in a dose-dependent manner. The results suggest the possible involvement of La(3+)-but not Gd(3+)-sensitive calcium channels in the locomotor stimulant effect of cocaine.

Lack of correlation between the molecular size and the efficacy of lanthanides for potentiating GABAA currents in rat septal cholinergic neurons in culture

Actions of lanthanides were examined on GABAA receptor-activated responses in rat septal neurons in culture by using the whole-cell voltage-clamp technique. GABA (10 microM) currents were enhanced by these ions at 100 microM with an efficacy sequence of Pr3+ (1.11) > or = Lu3+ (1.07) > or = Gd3+ (1.02) > or = Er3+ (1.01) > or = La3+ (1: 40% increase) > or = Ce3+ (0.980) > or = Nd3+ (0.932) > or = Yb3+ (0.913) > or = Tb3+ (0.912), indicating the lack of correlation between the potentiating efficacy and ionic size of lanthanides.

Lanthanum and zinc sensitivity of GABAA-activated currents in adult medial septum/diagonal band neurons from ethanol dependent rats

The impact of chronic ethanol treatment, sufficient to induce tolerance and physical dependence, on GABAA receptor function was studied in acutely isolated neurons from the medial septum/nucleus diagonal band (MS/nDB) of adult rats using whole cell, patch-clamp recordings. In ethanol-naive Controls, GABA (0.3-300 microM) induced concentration-dependent increases in Cl- current with a threshold of 0.3-1 microM, a mean maximal current of 7645 +/- 2148 pA at 100-300 microM, an EC50 of 11.3 +/- 1.3 microM and a slope of 1.53 +/- 0.07. GABA-activated currents in neurons from animals receiving two weeks of ethanol liquid diet treatment did not differ significantly on any of these measures. The rate of GABAA receptor desensitization (t1/2 = 6.49 +/- 1.19 s) estimated as the time required for loss of 50% of peak current during sustained application of 10 microM GABA, as well as the residual steady state current remaining following complete desensitization for controls was unchanged by chronic ethanol. The impact of chronic ethanol treatment on the GABAA receptor modulation by lanthanum and zinc which act as positive and negative allosteric modulators, respectively, was also evaluated. Test pulses of 3 microM GABA in control neurons showed maximal potentiation by 141 +/- 30% at approximately 1000 microM lanthanum with an EC50 of 107 +/- 34 microM and a slope of approximately 1. Lanthanum potentiation remained the same following chronic ethanol treatment. Initial estimates based on fitted concentration response curves suggested that maximal inhibition of 3 microM GABA responses by zinc at the level of 70.2 +/- 8.5% in control cells was significantly increased by chronic ethanol treatment to 95.3 +/- 2.5%, although the IC50 of 60.2 +/- 25 microM was not changed. However, this difference was not supported by direct tests of maximal 3-10 mM zinc concentrations. These results suggest that chronic ethanol treatment, sufficient to induce tolerance and physical dependence, probably does not lead to readily detectible changes in GABAA receptor function in MS/nDB neurons.

Extensive heterogeneity of recombinant gamma-aminobutyric acidA receptors expressed in alpha 4 beta 3 gamma 2-transfected human embryonic kidney 293 cells

Human embryonic kidney 293 cells transiently transfected with alpha 4-, beta 3- and gamma 2-subunits of gamma-aminobutyric acidA (GABAA) receptors from the rat exhibited specific high affinity binding sites for [3H]muscimol, [3H]Ro 15-4513 and [35S]t-butylbicyclophosphorothionate (TBPS). Bmax values obtained, however, were dramatically different for these compounds. In addition, GABA was able to inhibit only 20% of specific [35S]TBPS binding to membranes from alpha 4 beta 3 gamma 2-transfected cells. In order to investigate possible receptor heterogeneity, receptors were extracted from alpha 4 beta 3 gamma 2-transfected cells and were fractionated by chromatography on an anti-gamma 2-, followed by an anti-alpha 4- and an anti-beta 3-immunoaffinity column. Western blot analysis of the column eluates indicated the separate existence of GABAA receptors consisting of alpha 4 beta 3 gamma 2-, alpha 4 beta 3- or beta 3-subunits in alpha 4 beta 3 gamma 2-transfected cells. This, and the finding that only alpha 4 beta 3 gamma 2- but not alpha 4 beta 3- or beta 3-receptors possess high affinity binding sites for all three radiolabeled ligands investigated, combined with the observation that [35S]TBPS binding to receptors consisting of beta 3-subunits cannot be inhibited by GABA, can explain most of the binding data obtained. The present results suggest an inefficient assembly of gamma 2- with alpha 4- and/or beta 3-subunits under the conditions used, and indicate that recombinant receptors expressed in HEK cells are not necessarily homogeneous.

Effects of the nootropic drug nefiracetam on the GABAA receptor-channel complex in dorsal root ganglion neurons

The effects of nefiracetam on GABA-induced chloride currents were studied with rat dorsal root ganglion neurons in primary culture using the whole-cell patch-clamp technique. The dose-response curve for GABA-induced currents was shifted by 16 microM to lower concentrations by 10 microM nefiracetam while the maximal response was reduced by 22.84 +/- 0.68%. Thus at a low concentration (10 microM) of GABA, the chloride currents were potentiated by nefiracetam in a concentration-dependent manner. With 10 microM nefiracetam, the potentiation occurred slowly and the recovery after washout was also slow. The desensitization of the GABAA receptor at high concentration (100 microM) of GABA was accelerated by nefiracetam. The recovery process of chloride currents from desensitization was not affected by nefiracetam. KT 5720 (0.56 microm), a specific protein kinase A (PKA) inhibitor, blocked the transient potentiation of GABA-activated currents by nefiracetam, but did not affect the acceleration of desensitization. Nefiracetam suppression of GABA-induced currents was also abolished by KT 5720 or the pertussis toxin. Thus, nefiracetam may inhibit Gi/G(o) proteins leading to a cascade of events that increase the intracellular cAMP level, activate the PKA system, and suppress GABA-induced currents. Nefiracetam-induced transient potentiation and acceleration of desensitization of GABA-induced currents may involve other pathways. The nefiracetam modulation of the GABAA receptor function will result in a nootropic effect on the central nervous system through modification of synaptic transmission.

GABAA receptor pharmacology

gamma-Aminobutyric acid (GABA)A receptors for the inhibitory neurotransmitter GABA are likely to be found on most, if not all, neurons in the brain and spinal cord. They appear to be the most complicated of the superfamily of ligand-gated ion channels in terms of the large number of receptor subtypes and also the variety of ligands that interact with specific sites on the receptors. There appear to be at least 11 distinct sites on GABAA receptors for these ligands.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.