Differential modulation by divalent cations of [3H]MK-801 binding in brain synaptic membranes

Allosteric modulation of [3H]dizocilpine binding to N-methyl-d-aspartate receptor by an endogenous Na+, K+-ATPase inhibitor: dependence on receptor activation

An endogenous Na(+), K(+)-ATPase inhibitor, termed endobain E, has been isolated from rat brain and proved to decrease [3H]dizocilpine binding to cerebral cortex N-methyl-D-aspartate (NMDA) receptor, an effect independent of sodium pump activity. The purpose of this study was to disclose the mechanism of [3H]dizocilpine binding reduction by endobain E by performing saturation, kinetic and competitive assays. In saturation binding assays, endobain E increased K(d) without modifying B(max) value. To determine whether competitive or allosteric interaction was involved, kinetics of [3H]dizocilpine binding to cerebral cortex membranes was studied. Endobain E increased [3H]dizocilpine dissociation rate constant and induced an initial fast phase, without modifying association rate constant, indicating an allosteric interaction. In competitive [3H]dizocilpine binding assays, no additive effect was observed with endobain E plus competitive antagonists for glutamate or glycine sites (2-amino-5-phosphonopentanoic acid (AP-5) and 7-chlorokynurenic acid, respectively), indicating that coagonist site blockade interferes with endobain E effect. However, the higher glutamate and glycine concentration, the greater its effect. Endobain E binding reduction was partially additive with that induced by ketamine or Mg(2+) (receptor-associated channel blockers). Results suggest that the greater the channel activation by glutamate and glycine, the greater endobain E allosteric effect. Furthermore, as ketamine and Mg(2+) interfere with endobain E effect, this factor most likely binds to the inner surface of the NMDA associated channel.

Inorganics and hormesis

The article is a comprehensive review of the occurrence of hormetic dose-response relationships induced by inorganic agents, including toxic agents, of significant environmental and public health interest (e.g., arsenic, cadmium, lead, mercury, selenium, and zinc). Hormetic responses occurred in a wide range of biological models (i.e., plants, invertebrate and vertebrate animals) for a large and diverse array of endpoints. Particular attention was given to providing an assessment of the quantitative features of the dose-response relationships and underlying mechanisms that could account for the biphasic nature of the hormetic response. These findings indicate that hormetic responses commonly occur in appropriately designed experiments and are highly generalizeable with respect to biological model responses. The hormetic dose response should be seen as a reliable feature of the dose response for inorganic agents and will have an important impact on the estimated effects of such agents on environmental and human receptors.

The effect ofin vitro andin vivo ethylenbis dithiocarbamate fungicides on NMDA receptors in rat brain membranes

To determine whether the ethlenbisdithiocarbamate fungicides, zineb, manzeb and maneb affect the N-methyl-D-aspartate (NMDA) receptor in rat brain membranes, we performed a binding assay using [(3)H]MK-801, a noncompetitive NMDA receptor antagonist. Displacement studies were conducted using well washed membranes to exclude the effect of endogenous acidic amino acids on the binding of [(3)H]MK-801. In both the presence or absence of added glutamate and glycine in the assay buffer, the dose-response curve indicated that zineb enhanced the binding in a concentration range of 100-500 μM. However, the displacement curves indicated that manzeb and maneb inhibited the binding in a concentration range of 10-500 μM. The addition of 50 μM glutamate and glycine to the assay medium increased binding by 5-20% above the control in a concentration range of 0.1-100 μM.No rats injected with zineb, manzeb, maneb (100 mg/kg, ip) showed any characteristic toxic signs or any significant weight changes within 24 hrs. Estimation of [(3)H]MK-801 binding to unwashed membranes from intoxicated rat brains revealed no marked change in Bmax or Kd values for 24 hrs following fungicide administration.

Intrathecal Zn2+ attenuates morphine antinociception and the development of acute tolerance

Vesicular Zn2+, released in the brain and from small dorsal root ganglion neurons, interacts with opioid as well as N-methyl-D-aspartate (NMDA) receptors. We investigated the effect of Zn2+ on morphine antinociception in mice (tail flick assay), as well as acute tolerance and dependence, phenomena associated with NMDA activity. Administered intrathecally (i.t.), Zn2+ inhibited morphine antinociception in a dose-related fashion. Zn2+ also inhibited acute tolerance to morphine antinociception (5 h after 100 mg/kg of morphine). Injection i.t. of di-sodium calcium ethylenediamine tetra acetic acid (Na+Ca2+ EDTA), a chelator of divalent cations, had no effect on analgesia, acute tolerance or acute dependence. However, withdrawal jumps produced by naloxone (1 mg/kg s.c.) in morphine-pellet implanted mice (3 days) were potentiated by injections twice daily of 10 nmol of Na+Ca2+ EDTA, suggesting that endogenous Zn2+ tends to inhibit long-term development of withdrawal. These data suggest that the availability of Zn2+ is an important factor in opioid activity.

Lead inhibits the rat N-methyl-d-aspartate receptor channel by binding to a site distinct from the zinc allosteric site

Due to the importance of the NMDA receptor in cognitive function and in models of synaptic plasticity, the effect of Pb(+2) on this receptor has been one focus of attempts to define the bases of Pb-induced cognitive impairments seen in young children. The following study was performed to identify the effects on access to the NMDA receptor channel of acute exposure to free Pb(+2) in vitro. Cerebrocortical membranes were prepared from adult male Sprague-Dawley rats, and binding was measured in 50 mM Tris-acetate with (3)H-MK-801 in the presence of saturating concentrations of glutamate and glycine. The potency of Pb(+2) to inhibit access to the receptor channel (IC(50) = 0.55 microM) was greater than that of Zn(+2) (IC(50) = 1.30 microM). Dissociation of MK-801 from its binding site exhibited two-component kinetics, and both rate constants were significantly slowed in the presence of Pb(+2) or Zn(+2). To directly address the question of whether Pb(+2) inhibited the receptor channel by binding to the Zn(+2) modulatory site, changes in inhibitory potency for the receptor channel were measured when both metals were present. The results demonstrate that multiple levels of Pb(+2) produce a concentration-dependent downward shift of the Zn(+2) inhibition curve, indicating a noncompetitive inhibition of MK-801 binding by Pb(+2) with respect to that of Zn(+2). Moreover, Zn(+2) IC(50) values significantly decreased as a function of increasing Pb(+2) concentrations. Analogous results were obtained when Pb(+2) inhibition curves were determined in the presence of multiple levels of Zn(+2). These findings indicate that the inhibitory properties of free Pb(+2) and Zn(+2) on the NMDA receptor channel are similar in nature but are exerted via independent allosteric binding sites.

Preventive effects of exogenous phospholipases on inhibition by ferrous ions of [3H]MK-801 binding in rat brain synaptic membranes

Prior treatment with ferrous chloride led to marked inhibition of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801) binding to an open ion channel associated with the N-methyl-D-aspartate (NMDA) receptor in a concentration-dependent manner at concentrations of higher than 1 microM in rat brain synaptic membranes. Both phospholipases A2 and C significantly prevented the inhibition when treated before the treatment with ferrous chloride, while neither superoxide dismutase nor alpha-tocopherol affected the inhibition even when treated simultaneously with ferrous chloride. Of various saturated and unsaturated free fatty acids, moreover, both oleic and arachidonic acids exclusively decreased the potency of ferrous chloride to inhibit binding when membranes were first treated with fatty acids, followed by the second treatment with ferrous chloride. These results suggest that membrane phospholipids may be at least in part responsible for interference by ferrous ions with opening processes of the native NMDA channel through molecular mechanisms associated with the liberation of unsaturated free fatty acids in rat brain.

Differential inhibition by ferrous ions of [3H]MK-801 binding to native N-methyl-D-aspartate channel in neonatal and adult rat brains

In vitro addition or pretreatment with >/=1 microM ferrous chloride markedly inhibited in a concentration-dependent manner [3H]dizocilpine (MK-801) binding to an open ion channel associated with the N-methyl-D-aspartate (NMDA) receptor in rat brain synaptic membranes. The addition of NMDA agonists invariably attenuated the inhibition of [3H]MK-801 binding in hippocampal synaptic membranes previously treated with ferrous chloride, without significantly affecting that in cerebellar synaptic membranes. In the absence of spermidine, ferrous chloride was more potent in inhibiting binding in the cerebral cortex and hippocampus in adult rats than in those in rats at 3 days after birth, while in the striatum [3H]MK-801 binding was 10 times more sensitive to inhibition by added ferrous chloride in neonatal rats than in adult rats. Addition of spermidine significantly attenuated the potency of ferrous chloride to inhibit binding in the cerebral cortex of adult rats, with facilitation of the inhibition in newborn rats. Moreover, spermidine significantly reduced the inhibitory potency of ferrous chloride in neonatal rat striatum, without markedly affecting that in adult rat striatum. These results suggest that ferrous ions may interfere with opening processes of the native NMDA channel through molecular mechanisms peculiar to neuronal development in a manner associated with the polyamine recognition domain.

Nitric oxide-independent inhibition by sodium nitroprusside of the native N-methyl-D-aspartate recognition domain in a manner different from that by potassium ferrocyanide

Binding of [3H](+)-5-methyl-10,11 -dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) was significantly inhibited by the addition of sodium nitroprusside (SNP), a nitric oxide (NO) donor, at a concentration range of 0.1 microM to 0.1 mM in rat brain synaptic membranes. On the contrary, two other NO donors, S-nitroso-N-acetylpenicillamine and S-nitroso-L-glutathione, did not inhibit binding even at 0.1 mM. Similarly potent inhibition of [3H]MK-801 binding was caused by the addition of potassium ferrocyanide, while potassium ferricyanide induced slight inhibition of binding at 0.1 mM. Both SNP and potassium ferrocyanide markedly inhibited binding of [3H]glutamic (Glu) and [3H]D,L-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acids, without significantly affecting that of [3H]glycine and [3H]5,7-dichlorokynurenic acid. Further addition of Glu significantly exacerbated the inhibition by both SNP and potassium ferrocyanide at concentrations of 1-10 microM. Potent inhibition was also induced for [3H]MK-801 binding by the treatment of synaptic membranes with either SNP or potassium ferrocyanide, followed by efficient washing which also inhibited [3H]MK-801 binding due to removal of endogenous agonists. By contrast, dithiothreitol clearly differentiated between inhibitory properties of SNP and potassium ferrocyanide on [3H]MK-801 binding in terms of reversibility of the inhibition following pretreatment and subsequent washing. These results suggest that SNP may interfere with opening processes of the native NMDA channel through molecular mechanisms different from those underlying the inhibition by potassium ferrocyanide at the NMDA recognition domain in a manner independent of the generation of NO radicals.

Modulation by both diphenyliodonium and diphenyleneiodonium of [3H]MK-801 binding to rat brain synaptic membranes

Binding of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801) was significantly inhibited by the addition of several different compounds related to generation of nitric oxide (NO) at 100 microM in rat brain synaptic membranes. These included sodium nitroprusside, diphenyliodonium (DI), diphenyleneiodonium (DPI) and methylene blue. However, neither S-nitroso-N-acetylpenicillamine nor S-nitroso-L-glutathione inhibited binding at 100 microM. Both DI and DPI inhibited binding in a concentration-dependent manner at a concentration range of over 1 microM, while further addition of spermidine (SPD) significantly attenuated the potency of DPI to inhibit binding without affecting that of DI. In contrast, SPD induced significant potentiation of the ability of unlabelled MK-801 to displace [3H]MK-801 binding in a fashion sensitive to antagonism by the novel polyamine antagonist bis-(3-aminopropyl)nonanediamine. This novel polyamine antagonist also prevented the reversing effect of SPD on inhibition by DPI of [3H]MK-801 binding. Moreover, DPI competitively exacerbated the ability of SPD to potentiate [3H]MK-801 binding in the presence of both L-glutamic acid and glycine at maximally effective concentrations. On the other hand, SPD was effective in reversing the inhibition by DPI in cerebellar, but not hippocampal, synaptic membranes. These results suggest that both DI and DPI may modulate synaptic responses mediated by the N-methyl-D-aspartate receptor through inhibition of opening processes of the ion channel in a manner irrespective of generation of NO radicals in particular situations. Possible involvement of the polyamine domain in the inhibition by DPI is also suggested.

Zn2+ inhibition of [3H]MK-801 binding is different in mouse brain and spinal cord: effect of glycine and glutamate

Zn2+ inhibits NMDA-type excitatory amino acid activity by a non-competitive action. Based on regional differences in the central nervous system (CNS) in binding characteristics of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne maleate ([3H]MK-801) and other non-competitive antagonists of NMDA used to label open channels in the receptor complex, we compared the inhibitory influence of Zn2+ on [3H]MK-801 binding in whole mouse brain and spinal cord membranes. Radioligand binding techniques were used in the presence and absence of maximally effective concentrations of glycine and glutamate. Using extensively washed membranes without exogenous glycine and glutamate, Zn2+ was found to be a weaker inhibitor of the [3H]MK-801-labeled site in the spinal cord than in the whole brain. In contrast, exogenous glycine and glutamate decreased the inhibitory effect of Zn2+ in the brain but dramatically increased the inhibitory effect of Zn2+ in the spinal cord. Thus the inhibitory effect of Zn2+ in the spinal cord appears to be magnified by glutamatergic and glycinergic activity while that in the brain is not. The different actions of Zn2+ may be attributable to the differential distribution of NMDA receptor subunits in the mouse brain and spinal cord.

Search for novel ligands selective at a polyamine recognition domain on the N-methyl-D-aspartate receptor complex using membrane binding techniques

Among over 60 polyamine derivatives tested, only N-(3-aminopropyl)octanediamine and bis-(3-aminopropyl)nonanediamine (TE393) markedly inhibited [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801) binding at equilibrium in the presence of added spermidine (SPD) in "non-washed" rat brain synaptic membranes, without affecting that in the absence of added SPD. Although TE393 significantly potentiated [3H]MK-801 binding before equilibrium in the presence of L-glutamic acid (Glu) alone or both Glu and glycine (Gly) added in "Triton-treated" membranes, the putative polyamine antagonists 1,10-decanediamine (DA10) and arcaine invariably inhibited binding irrespective of the addition of agonists. In the absence of added SPD, in addition, TE393 markedly enhanced abilities of both Glu and Gly to potentiate [3H]MK-801 binding before equilibrium. However, TE393 induced a rightward shift of the concentration-response curve of SPD for [3H]MK-801 binding before equilibrium. Moreover, TE393 was effective in potentiating binding of an antagonist but not an agonist radioligand to the NMDA domain and in inhibiting binding of an antagonist but not an agonist radioligand to the Gly domain. The potentiation of NMDA antagonist binding by TE393 occurred in a manner sensitive to prevention by arcaine but not by DA10. These results suggest that TE393 may be a novel ligand at the polyamine domain with an ability to interact with both the NMDA and Gly recognition domains in antagonist-preferring forms.

Potentiation by calcium ions of [3H]MK-801 binding to an ion channel associated with the N-methyl-D-aspartate receptor complex in rat brain

In vitro addition of Ca2+ ions was effective in almost doubling binding of [3H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801) before equilibrium to an ion channel associated with an N-methyl-D-aspartate (NMDA)-sensitive subclass of brain excitatory amino acid receptors. The addition of inhibitors for phospholipase (PLase) A2 markedly reduced binding in the absence of added Ca2+ ions, while Ca2+ ions restored the reduction to the level found in the absence of the inhibitors. Pretreatment with PLases A2 and C but not D was effective in potentiating [3H]MK-801 binding in a biphasic manner at a concentration range of 5 mU/ml-10 U/ml. Moreover, PLases A2 and C at low concentrations not only suppressed the abilities of 3 different agonists to potentiate [3H]MK-801 binding before equilibrium, but also reduced that of Ca2+ ions. These results suggest that Ca2+ ions may potentiate [3H]MK-801 binding to the NMDA channel highly permeable to Ca2+ ions through a mechanism common to that underlying potentiation by exogenous PLase A2 and/or C.

Effect of chemical modification of arginyl and histidyl residues on [3H]MK-801 binding to brain synaptic membranes

Chemical modification of the N-methyl-D-aspartate (NMDA) receptor was investigated in a thoroughly washed Triton-treated pig forebrain membrane preparation. Modification of arginyl residues using phenylglyoxal significantly reduced activation of the NMDA receptor as measured by specific binding of [3H]MK-801 [(+)-5-methyl-10,11-dihydro-dibenzo[a,d]cyclohepten-5,10-imine]. The reduction was due to a decrease in the affinity of MK-801 for its binding site from 3.2 +/- 0.5 nM to 22 +/- 8 nM. Protection studies indicated that alteration of the glutamate or glycine binding sites is not directly responsible for the effect. Diethyl pyrocarbonate treatment also reduced [3H]MK-801 binding, by modification of histidyl residues. The binding affinity was reduced to 8.3 +/- 1.4 nM whereas the Bmax was unchanged. Protection studies indicated that the modified histidine is unlikely to be a component of the glutamate, glycine or MK-801 binding sites. However, the accessibility of the modified histidine seems to be partly dependent on the activation of the receptor.

Differential effects of divalent cations on specific 3H-GR 65630 binding to 5-HT3 receptors in rat cortical membranes

We have examined the ability of divalent cations to inhibit 3H-GR 65630 binding to 5-HT3 receptor recognition sites in rat brain cortical membranes. Among the divalent metal cations tested, Cd2+, Zn2+ and Cu2+ inhibited the specific 3H-GR 65630 binding activity to 5-HT3 receptors at a concentration of 0.1-1 mM. The other divalent metal cations tested (i.e. Co2+, Ni2+, Ba2+, Ca2+, Mg2+ and Mn2+) produced no inhibition of the specific 3H-GR 65630 binding. Cd2+, Zn2+ and Cu2+ did not change the Bmax value of the binding activity, but significantly increased the Kd value. It was suggested that these cations inhibited the binding activity by reducing affinity of the 5-HT3 receptor for the antagonist, resulting in apparent inhibition of the binding activity. As to the binding association rate, Cd2+, Cu2+ and Zn2+ were found to have an inhibitory effect. The binding dissociation rate, however, was shown to be decreased by Cu2+ but not by Cd2+ and Zn2+. Furthermore, the Zn(2+)-induced inhibition of 3H-GR 65630 binding was suggested to be antagonized by both concanavalin A and wheatgerm agglutinin. The Cu(2+)-induced inhibition, however, was not influenced by these lectins, indicating that Cu2+ has a different lectin sensitivity for its inhibitory effect. The different mechanism of action between Cu2+ and Zn2+ was suggested in their inhibitory effect on the specific 3H-GR 65630 binding activity.

Thiocyanate ions selectively antagonize AMPA-evoked responses in Xenopus laevis oocytes microinjected with rat brain mRNA

1 Responses to kainate (KA), willardiine and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were recorded from rat brain mRNA-injected Xenopus laevis oocytes by use of a two-electrode voltage clamp. 2 Thiocyanate (SCN-; 50 microM-4 mM) ions reversibly and selectively inhibited the membrane current responses to AMPA in a non-competitive manner without affecting KA or willardiine-induced responses. 3 The inhibition of AMPA-induced responses by SCN- was dependent on the SCN- concentration with an estimated IC50 of 1 mM. The antagonism was not dependent on the AMPA concentration. 4 The response to a high concentration of AMPA (100-200 microM) exhibited a peak inward current which declined to a steady-state. SCN- inhibited the steady-state current more than the peak response. The inhibition was unaffected by prior incubation with concanavalin-A (Con-A; 10 microM). 5 Responses to KA were antagonized by AMPA in a competitive manner, suggesting that both agonists may activate a common receptor-channel complex. This interaction between two non-NMDA agonists was not affected by the SCN(-)-induced inhibition of the AMPA response. 6 AMPA-induced responses recorded from large cultured cerebellar neurones by whole-cell recording were also inhibited by SCN- in a non-competitive manner. The AMPA-induced peak current was less affected than the steady-state response. 7 We conclude that SCN- can inhibit the response to AMPA in expressed non-NMDA receptors in Xenopus oocytes and also in native receptors in cultured cerebellar neurones. One possible mechanism of action for SCN- inhibition of responses to AMPA may involve a Con-A-insensitive, non-NMDA receptor-mediated desensitization.