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Cyclothiazide Decreases [3H]AMPA Binding to Rat Brain Membranes: Evidence That AMPA Receptor Desensitization Increases Agonist Affinity

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Cyclothiazide Decreases [3H]AMPA Binding to Rat Brain Membranes: Evidence That AMPA Receptor Desensitization Increases Agonist Affinity Brain Research, 628 (1993) 345-348 345 Elsevier Science Publishers B.V. BRES 25921 Cyclothiazide decreases [3H]AMPA binding to rat brain membranes: evidence that AMPA receptor desensitization increases agonist affinity Randy A. Hall *, Markus Kessler, Alex Quan, Jose Ambros-Ingerson, Gary Lynch Center for the Neurobiology of Learning and Memory, University of California, lrvine, CA 92717, USA (Accepted 31 August 1993) Key words: Glutamate receptor; Inhibition; Solubilization; Temperature The effects of cyclothiazide, a drug which blocks AMPA (t~-amino-3-hydroxy-5-methyl-4-isoxazolepropionicacid) receptor desensitization, were tested on binding of [3H]AMPA to rat brain membranes. Cyclothiazide reduced [3H]AMPA binding by lowering the apparent affinity of the AMPA receptor. The magnitude of the decrease was temperature dependent and greater for membrane-bound than for solubilized receptors. These data provide evidence that desensitization increases the affinity of the AMPA receptor for agonists and indicate that a significant percentage of AMPA receptors in conventional equilibrium binding assays are in a desensitized state. Physiological experiments indicate that glutamate trolling this conversion are poorly understood, but the receptors of the AMPA (a-amino-3-hydroxy-5-methyl- two affinity states could potentially reflect sensitized 4-isoxazolepropionic acid) subclass rapidly desensitize versus desensitized states of the receptor. If this were following agonist binding; i.e. the receptor channel fails the case, cyclothiazide would be expected to have to open despite the continued presence of appropriate greater effects on solubilized receptors than on mem- ligand 6'9. The diuretic drug cyclothiazide greatly re- brane-bound receptors. duces desensitization and prolongs the duration of Membranes and fractions solubilized with 0.4% Tri- excitatory postsynaptic currents in cultured hippocam- ton X-100 were prepared as previously described 2. The pal neurons 7'12. Physiological experiments utilizing final buffer composition of the samples was 100 mM rapid agonist application have suggested that desensiti- Tris acetate, pH 7.2, with 50 /xM EGTA (with 0.4% zation might increase the affinity of the AMPA recep- Triton X-100 in the case of the soluble samples). tor for agonists 6'9 as in the case for the nicotinic [3H]AMPA binding in the presence of 50 mM potas- cholinergic receptor 5. Estimates of AMPA receptor sium thiocyanate (KSCN) was measured using the cen- affinity derived from equilibrium binding studies might trifugation technique for the membrane samples and thus be distorted by the presence of receptors which the filtration technique for the soluble samples, as have shifted into the desensitized state during incuba- previously described 1,2. Briefly, 80-120 /xg of protein tion of brain membranes with agonists. If this were so, in a final volume of 100 /xl were incubated with and if desensitization was linked to a change in affinity, [3H]AMPA for 40-60 min in either an ice water bath then cyclothiazide would be expected to alter the K d (0°C), a room temperature rack (23°C) or a heated for [3H]AMPA binding without affecting the number water bath (35°C). Membrane samples were cen- of binding sites. The present experiments tested this trifuged at 48,000 × g (at appropriate temperature) for prediction. Tests of the drug's effects were also carried 20 min. The supernatants were then aspirated and the out following solubilization of membrane receptors, a pellets superficially rinsed with ice-cold 0.4 ml 100 mM manipulation which converts low-affinity membrane re- Tris acetate. The pellets were then dissolved in 10 gl ceptors into a higher affinity state 1. The factors con- Beckman Tissue Solubilizer and counted with an effi- * Corresponding author. Fax: (1) (714) 856-8481. 346 ciency of 0.40. Soluble samples were filtered through Whatman GF/B filters (soaked in 0.03% polyethylen- 40 I Membranes A imine for at least 30 min) with 3 washes of 4.0 ml ~oI 100 ~M CYZ ~ 3c buffer (50 mM Tris-HCl, pH 7.2, 100 mM KSCN). Low affinity Bma~ 76 + O3 70 t 09 Results were expressed as specifically bound Ko 660 r 90 305O * 650 High affinity Bma, 045 * 013 041 + 005 [3H]AMPA, which equals the difference between the ~ 2c o KD 23 ' 6 41 * 5 total bound and non-specifically bound ligand. For the © ~2 saturation studies, AMPA concentrations were in- creased between 2 and 3000 nM. Concentrations be- tween 2 and 50 nM were achieved by increasing the concentration of radiolabeled AMPA, while those @I i i i i i i 0 1 2 3 4 !) 6 above 50 nM were produced by adding unlabeled Bound (pmol/mg) AMPA to a fixed [3H]AMPA concentration of 50 nM. 1b () Non-specific binding was defined as the amount of l Soluble B [3H]AMPA bound in the presence of 2.5 mM l-gluta- k ~ ~QQ uM CY-Z mate. For both the membrane and soluble fractions, E Bma~ 35 ± 02 33 ± 02 -~. 100 non-specific binding increased from approximately 10% 0 47,4 E? -- - to 70% of the total binding as AMPA concentration G increased from 2 nM to 3000 nM. Cyclothiazide dilu- 8 tions were made from a stock of cyclothiazide in pure L~ SO z3 dimethyl sulfoxide (DMSO). All samples assayed (both £ © control and cyclothiazide) contained 1% DMSO; this CTD was shown in preliminary experiments to have no effect on [3H]AMPA binding. Two-site analyses of binding 0 ! 2 3 4 Bound (pmol/mg) data were performed by non-linear regression using the Fig. 2. Top: Scatchard transformations of [SH]AMPA binding to Inplot program by GraphPAD Software Inc. Protein membranes in the absence of cyclothiazide (e) and the presence of concentrations were determined with the protein assay 100/xM (o) and 500 ~M (•) cyclothiazide (CYZ). The curves show reagent obtained from Bio-Rad with bovine serum the best two-site fits for the control and 100 ~M cyclothiazide samples. Bottom: Scatchard transformations of [3H]AMPA binding albumin as the standard. [-~H]AMPA was purchased to soluble fractions in the absence (e) and presence (o) of 500/xM from NEN/Dupont and AMPA was obtained from cyclothiazide. Points and error bars indicate the means and S.E.M. for 3 determinations. The inset for the membranes provides Bmax Tocris. and Ka values for two-site regression analyses, while the inset for Fig. 1 shows a dose-response curve for the effects the soluble fractions provides values from linear regression analyses. of cyclothiazide on 50 nM [3H]AMPA binding at 0°C in Bmax values are in pmol/mg, while Ka values are in nM. the presence of 50 mM KSCN for membranes and soluble fractions. The K~ for cyclothiazide in the mem- brane samples was 31 IzM; in the soluble fractions, the K i was 32 txM. The binding appeared to reach a minimum plateau at cyclothiazide concentrations of 500 /xM to 1 mM; however, since 1 mM was approxi- lOC mately the limit of drug solubility, higher concentra- tions could not be tested. 8C The effects of cyclothiazide across a wide range of o 4._ [3H]AMPA concentrations were measured. These data 60 are shown in Fig. 2 for binding to both the membranes < E~ 40 x (top) and soluble fractions (bottom) at 0°C. For the o membrane-binding data, a two-site fit was significantly c{ 20 better (F-squared test, P < 0.05) than a one-site fit both in the presence and absence of 100/xM cyclothia- O I0 I00 1000 zide. In the presence of 500/xM cyclothiazide, specific Log ECyctothiaz,de] (I)M) binding was so low that resolution of two sites was not Fig. 1. Dose-response curves for the effects of cyclothiazide on possible. Cyclothiazide (100 IzM) did not significantly [3H]AMPA binding in membranes (o) and soluble fractions (o). affect Bma x estimates, but there was an approximately Points and error bars indicate the means and S.E.M. for 3 determi- 5-fold decrease in the apparent affinity of the low-af- nations. Two-site fits were not significantly better than one-site fits for either curve (F-squared test, P < 0.05). finity binding site and a 2-fold decrease in the apparent 347 100 3C receptors in the binding assay would remain in the Membranes Soluble sensitized state. The present results satisfy this predic- / _L 25 O_ 075 tion and thus constitute evidence that AMPA receptor 20 oD c desensitization is accompanied by an increase in the affinity of the receptor. ~ o~o [5 Increases in temperature caused a marked reduction !0 ,n in [3H]AMPA binding, as well as a decrease in the O25 relative effects of cyclothiazide. Higher temperatures 0 5 E el_ might increase the off-rate constant of the receptor OOC 3 C and thus the percentage of bound receptors present at O'C 23"C 35"C O'C 23"C any given moment. An increase in the off-rate constant Fig. 3. Binding of 50 nM [3H]AMPA to membranes and soluble fractions at different temperatures. The left bar of each pair repre- would also allow the receptors less time to desensitize sents binding under control conditions, while the right bar represents and thereby would shift the equilibrium of the binding binding in the presence of 500/zM cyclothiazide. The bars and error assay toward the sensitized state. Assuming that the bars represent means and S.E.M. for 3-4 determinations. sensitized state is of lower affinity than the desensi- tized state, this would serve to further decrease the apparent affinity of the receptor, and would also cause affinity of the high-affinity binding site. Bmax and K d drugs which affect desensitization kinetics (like cyclo- values are provided in the insets to Fig.
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