The effects of surgical and chemical lesions on striatal [3H]threo-(+/-)-methylphenidate binding: correlation with [3H]dopamine uptake

The specific binding of [3H]threo-(+/-)-methylphenidate to membranes prepared from rat striatum was significantly reduced following either surgical lesions of the medial forebrain bundle or intracerebroventricular administration of 6-hydroxydopamine. The decrease in the density of [3H]threo-(+/-)-methylphenidate binding sites in striatum following chemical or surgical denervation was highly correlated with the decrease in [3H]dopamine uptake. In contrast, intracerebroventricular administration of 5,7-dihydroxytryptamine, AF64A, or chronic parenteral administration of reserpine did not alter either the number of apparent affinity of [3H]threo-(+/-)-methylphenidate binding sites. These data suggest that the specific binding sites for [3H]-threo-(+/-)-methylphenidate in striatum are localized to dopaminergic nerve terminals, and may be associated with the dopamine transport complex.     

3H]acetylcholine and [3H](-)nicotine label the same recognition site in rat brain

High affinity binding sites for [3H]acetylcholine and [3H](-)nicotine in rat brain were compared with respect to key characteristics, any one of which should distinguish them if they are different. The density of binding sites for each ligand varied approximately 4-fold in five areas of rat forebrain, but in each of these areas and in human cerebral cortex as well, the densities of [3H]acetylcholine- and [3H](-)nicotine-binding sites were indistinguishable. The affinity of [3H](-)nicotine was higher than that of [3H]acetylcholine, but nicotinic cholinergic drugs competed for the sites labeled by the two ligands with similar affinities; and in each case, the site labeled displayed marked stereoselectivity for the enantiomers of nicotine. The binding of [3H]acetylcholine and [3H](-)nicotine was decreased to the same extent by preincubation of tissues with dithiothreitol, and the binding was restored by subsequent treatment with 5,5'-dithiobis-2-nitrobenzoic acid, indicating that a disulfide bond is required at or near the binding site for each ligand. Treatment of rats with nicotine for 10 days increased the density of binding sites for both ligands, and treatment with the cholinesterase inhibitor soman for 9 days decreased the density of binding sites for both ligands. Taken together, these results indicate that [3H]acetylcholine and [3H](-)nicotine bind to the same nicotinic cholinergic recognition site in rat brain.     

Haloperidol competitively inhibits the binding of (+)-[H]SKF-10,047 to σ sites

Scatchard plots of equilibrium saturation binding data revealed that haloperidol inhibits the binding of (+)-[³H]SKF-10,047 to σ sites in a competitive manner. In experiments using membranes from both guinea pig and rat brain the apparent K_d of (+)-[³H]SKF-10,047 for σ sites was significantly increased, whereas the apparent B_max was not altered by the addition of 10 nM haloperidol.     

Characterization of the binding of [3H]Ro 41-1049 to the active site of human monoamine oxidase-A

The novel reversible and selective inhibitor of monoamine oxidase-A (MAO-A) Ro 41-1049 [N-(2-aminoethyl)-5-(m-fluorophenyl)-4-thiazole carboxamide HCl] shows inhibition characteristics similar to those of the structurally related reversible MAO-B inhibitors Ro 16-6491 and Ro 19-6327. In the present study, tritiated Ro 41-1049 was used as a high affinity ligand to study the binding characteristics of this inhibitor to MAO-A and its interactions with the enzyme. An homogeneous population of high affinity binding sites for [3H]Ro 41-1049 was found in membrane preparations from human frontal cortex and placenta (Kd = 16.5 +/- 1.4 and 64.4 +/- 19.2 nM, respectively). In frontal cortex the Bmax value for [3H]Ro 41-1049 (2.6 +/- 0.4 pmol/mg of protein) was about one third of the Bmax calculated for the MAO-B-selective ligand [3H]Ro 16-6491. The density of [3H]Ro 41-1049 binding sites in human placenta varied greatly in the different tissue samples investigated, showing an average Bmax of 101.7 +/- 36.5 pmol/mg of protein. Apparent binding equilibrium was reached after 1 hr of incubation at 37 degrees. At this temperature the binding was reversible, with a dissociation t 1/2 of about 35 min. At lower temperatures the radioactivity dissociation was much slower. Among the various drugs tested, only inhibitors of MAO-A were able to effectively prevent [3H]Ro 41-1049 specific binding. As previously reported for the MAO-B ligands [3H]Ro 16-6491 and [3H]Ro 19-6327, the analysis of the membrane-bound radioactivity showed that [3H]Ro 41-1049 was entirely recovered in the form of its aldehyde derivative, indicating that Ro 41-1049 was deaminated by MAO-A. The existence of a Ro 41-1049 adduct reversibly bound to the enzyme active site might explain the inhibition mechanism of this compound. The exposure of the radioligand-enzyme complex to NaBH3CN at pH 4.5 caused the irreversible covalent incorporation of about 70% of the specifically bound radioactivity into a 60-kDa polypeptide. This incorporation was dependent on the pH and on the amount of NaBH3CN added. The presence of MAO-A- but not MAO-B-selective inhibitors prevented the covalent incorporation of [3H]Ro 41-1049. The present results indicate that [3H]Ro 41-1049 is incorporated into a subunit of MAO-A, in the presence of NaBH3CN, and modifies a protein domain that is essential for the enzyme activity.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cocaine and dopamine differentially protect [3H]mazindol binding sites from alkylation by N-ethylmaleimide.

The binding of cocaine, d-amphetamine and dopamine to the site on the dopamine transporter labeled by [3H]mazindol was investigated in rat striatal membranes. N-Ethylmaleimide inhibited about 95% of the specific binding of 5 nM [3H]mazindol in a concentration-dependent manner. The effect of 10 mM N-ethylmaleimide was completely prevented by cocaine (EC50 of 3 microM), but neither 300 microM dopamine nor d-amphetamine afforded any significant protection. On the other hand, high concentrations of cocaine, d-amphetamine and dopamine provided similar protection against inhibition by 0.1 mM N-ethylmaleimide. Taken together these data support the hypothesis that a significant portion of the cocaine binding domain on the transporter is distinct from that of either dopamine or amphetamine. This distinction may be sufficient to allow properly designed drugs to prevent cocaine binding without inhibiting DA transport.     

Covalent labeling of mu opioid binding site by [3H]beta-funaltrexamine

[3H]beta-funaltrexamine ([3H]beta-FNA) bound irreversibly to bovine striatal membranes. Naloxone inhibited the irreversible binding of 5 nM [3H]beta-FNA in a dose-dependent manner and maximally inhibited this binding at approximately 1 microM. Thus, the specific irreversible binding of [3H]beta-FNA to opioid receptors was defined as that which could be inhibited by 1 microM naloxone. This specific irreversible binding of [3H]beta-FNA was characterized. Exclusion of Na+ from the incubation medium reduced the specific binding of [3H]beta-FNA, and Na+ could be substituted by Li+ but not by K+, Cs+, Mg2+, or guanylylimidodiphosphate. The specific irreversible binding was saturable, time- and temperature-dependent, and was linearly related to tissue mass. Several drugs were used to characterize this specific binding. Levorphanol was 1000 times more potent as an inhibitor than dextrorphan. mu Opioid ligands (sufentanil and morphine) were much better inhibitors than delta (ICI174,864) or kappa (U50,488H) ligands. The potency of [D-Ala2, D-Leu5]enkephalin (DADLE) was between those of sufentanil and ICI174,864. These results demonstrated that under appropriate conditions [3H]beta-FNA specifically and irreversibly bound to the mu opioid binding site. Membrane preparations labeled with [3H]beta-FNA in the presence or absence of 1 microM naloxone or beta-FNA were subjected to polyacrylamide gel electrophoresis under denaturing and reducing conditions. Fluorograms showed that [3H]beta-FNA specifically bound to a protein (most likely the mu opioid binding site) that migrated as a band with a molecular weight range of 68,000-97,000. Such electrophoretic behavior indicates that it is likely to be a glycoprotein. The glycoprotein nature was confirmed by its adsorption onto a wheat germ lectin-Sepharose column after solubilization and subsequent elution by N-acetyl-D-glucosamine. In this lectin column eluate, the mu opioid receptor was the only protein band labeled by [3H]beta-FNA in the total binding preparation, and no labeled protein was observed in the nonspecific binding preparation. When the wheat germ lectin column eluate of the total binding was treated with peptide:N-glycosidase F, the broad labeled band of Mr 68,000-97,000 became a sharp band of Mr 57,000 with high radioactivity and a faintly labeled band of Mr 49,000.     

A binding site for [3H]glipizide in the rat cerebral cortex

[3H]glipizide, a 2nd generation hypoglycemic sulfonylurea, binds specifically to rat cerebral cortex membranes in a time- and temperature-dependent way. The binding is saturable and reversible. The maximal binding capacity is 110 fmol/mg protein and the dissociation constant 1.5 nM. The binding site was destroyed by proteolytic and lipolytic enzymes suggesting a lipoprotein nature. Active analogs of sulfonylureas are characterized by IC50 values in the cerebral cortex which parallel their insulinotropic activity. In the cerebral cortex, adenylate cyclase was not stimulated by glipizide but sulfonylureas could inhibit, at high doses, the cAMP-dependent phosphodiesterase. This central binding site for glipizide displays the characteristics of the recognition moiety of a biological receptor.     

Competitive inhibition of magnesium-induced [3H]N-(1-[thienyl] cyclohexyl)piperidine binding by arcaine: evidence for a shared spermidine-magnesium binding site

The polyamine competitive antagonist arcaine (1,4-diguanidino-butane) produced complete inhibition of basal [3H]N-(1-[thienyl] cyclohexyl)piperidine ([3H]TCP) binding, with an IC50 value of 4.52 +/- 0.93 microM. Arcaine (5 and 10 microM) produced a decrease in the affinity without a significant change in the receptor density of [3H]TCP binding under equilibrium conditions. In addition, arcaine did not alter either N-methyl-D-aspartate-specific [3H] glutamate or strychnine-insensitive [3H]glycine binding. Furthermore, increasing concentrations of arcaine produced parallel rightward shifts in the concentration-response curves for both spermidine- and magnesium-induced [3H]TCP binding, suggesting that arcaine is a competitive inhibitor of both agonists. Similar rightward shifts were observed for barium- and strontium-induced [3H]TCP binding. In contrast, arcaine decreased the efficacy of glutamate- and glycine-induced [3H]TCP binding without changing their EC50 values, indicating a noncompetitive type of inhibition. These results imply that spermidine and certain divalent cations including magnesium share the same mechanism for enhancing [3H]TCP binding, whereas glutamate and glycine have different sites of action. This is further supported by the additive effect of spermidine when tested in the presence of maximal concentrations of glutamate and glycine. On the other hand, spermidine and magnesium were not additive and, in fact, magnesium was able to block the effects of spermidine under certain conditions. The possibility that magnesium is a partial agonist at the polyamine site is discussed.     

Comparison of [3H]YM-09151-2 with [3H]spiperone and [3H]raclopride for dopamine d-2 receptor binding to rat striatum

The Kd value of [3H]YM-09151-2, a potent and highly selective dopamine D-2 antagonist, for binding to rat striatum was about 20 pM (half of that for [3H]spiperone and one-fiftieth of that for [3H]raclopride). The Bmax of [3H]YM-09151-2 binding was about 30% higher than that of [3H]raclopride or [3]spiperone. The ratio (bout 3%) of non-specific to specific binding of [3H]YM-09151-2 was smaller than that of [3H]spiperone and [3H]raclopride. The Hill coefficient values of dopamine D-2 antagonists, SCH23390, mianserin and phentolamine for the inhibition of binding of [3H]YM-09151-2 were near 1.0, and their Ki values with [3H]YM-09151-2 were consistent with those for inhibiting [3H]raclopride and [3H]spiperone binding to D-2 receptors. Thus, [3H]YM-09151-2 may be the most suitable ligand for the labelling of dopamine D-2 receptors in the brain.     

Interactions of spironolactone with (+)-[3H]-isradipine and (-)-[3H]-desmethoxyverapamil binding sites in vascular smooth muscle.

Spironolactone partially inhibited the specific binding of (+)-[3H]-isradipine and (-)-[3H]-desmethoxyverapamil to vascular smooth muscle membranes. It is suggested that spironolactone interacts at a binding site of the calcium channel complex and allosterically modulates ligand binding at receptor sites in the channel.     

Exposure to nicotine enhances the behavioral stimulant effect of nicotine and increases binding of [3H]acetylcholine to nicotinic receptors

Rats were given daily injections of nicotine sulfate in doses ranging from 0.1 to 0.4 mg/kg. The behavioral effect of these injections was measured as locomotor activity in photocell cages. Repeated administration of the same dose to each rat resulted in an enhancement of the stimulant effect of nicotine. This enhanced behavioral effect was quite pronounced within 5 days of repeated injection. Tissue from the cerebral cortex of these rats, exposed to nicotine for 5 days, was assayed for binding of [3H]acetylcholine to nicotinic receptors. These relatively small doses of nicotine resulted in 18-26% increases in cortical nicotinic receptors, compared to saline-treated rats. Rats exposed to 0.2 mg/kg of nicotine for 5 days and then given saline for 7 days still showed an enhanced behavioral response to nicotine on the eighth day after exposure, and nicotinic binding in the cortex was still elevated. However, 21 days after exposure to nicotine both the behavioral response to nicotine and the binding values had returned to the same values as those of saline-treated rats. These data imply that increased binding of [3H]acetylcholine to nicotinic sites and the enhanced behavioral effect of nicotine are functionally linked.     

The binding of [3H]thienyl cyclohexylpiperidine ([3H]TCP) to the NMDA-phencyclidine receptor complex

The high affinity binding of the phencyclidine derivative [3H]TCP to cortical membranes of the rat was investigated. In an extensively washed membrane preparation the binding of [3H]TCP was enhanced in the presence of L-glutamate and NMDA. The stimulation of the binding of [3H]TCP by L-glutamate was inhibited competitively by AP5 and non-competitively by MK801. The binding of [3H]TCP was also enhanced in the presence of glycine; this effect was insensitive to strychnine and inhibited non-competitively by AP5. Saturation experiments demonstrated that MK801 was a competitive inhibitor of the binding of [3H]TCP. These results suggest that [3H]TCP binds to a site similar to that which binds MK801; this site may be associated with the ion channel of the NMDA receptor.     

Acetylcholinesterase potentiates [3H]fluorowillardiine and [3H]AMPA binding to rat cortical membranes

In addition to its action at cholinergic synapses acetylcholinesterase (AChE) has been proposed to modulate neuronal activity by mechanisms unrelated to the hydrolysis of acetylcholine. We have investigated the effects of AChE on the binding of the specific AMPA receptor agonists (S)-[3H]5-fluorowillardiine ([3H]FW) and [3H]AMPA to rat cortical membranes. Pretreatment of membranes with AChE causes a dose-dependent increase in the binding of both radiolabelled agonists with a maximal increase to approximately 60% above control. This increase is completely blocked by the specific AChE inhibitors propidium, physostigmine, DFP and BW 284C51. AChE pretreatment had no effect on [3H]kainate binding. [3H]FW binding to membranes from young (15-day-old) rats is four orders of magnitude more sensitive to AChE modulation than membranes from adult rats (EC50 values of 4x10(-5) and 0.1 unit/ml, respectively) although the total percentage increase in binding is similar. Furthermore, the AChE-induced potentiation of [3H]FW binding is Ca2+ - and temperature-dependent suggesting an enzymatic action for AChE in this system. Saturation binding experiments with [3H]FW to adult membranes reveal high and low affinity binding sites and demonstrate that the main action of AChE is to increase the Bmax of both sites. These findings suggest that modulation of AMPA receptors could provide a molecular mechanism of action for the previously reported effects of AChE in synapse formation, synaptic plasticity and neurodegeneration.     

Characterisation of [3H]gabapentin binding to a novel site in rat brain: homogenate binding studies

The binding characteristics of [³H]gabapentin, the radiolabelled analogue of the novel anticonvulsant gabapentin (1-(aminomethyl)cyclohexaneacetic acid) were studied using purified synaptic plasma membranes prepared from rat cerebral cortex. In 10 mM HEPES buffer [³H]gabapentin bound to a single population of sites with high affinity (K_D = 38 ± 2.8 mM) with a maximum binding capacity of 4.6 ± 0.4 pmol/mg protein, reaching equilibrium after 30 min at 20°C. This novel site was unique to the central nervous system with little or no specific [³H]gabapentin binding being measurable in a range of peripheral tissues. Binding was potently inhibited by a range of gabapentin analogues and 3-alkyl substituted γ-aminobutyric acid (GABA) derivatives although GABA itself and the selective GABA_B receptor ligand baclofen, were only weakly active. Gabapentin itself (IC₅₀ = 80 nM) and 3-isobutyl GABA (IC₅₀ = 80 nM) which also has anticonvulsant properties, showed the highest affinity for the binding site. Of a wide range of other pharmacologically active compounds only the polyamines spermine and spermidine influenced [³H]gabapentin binding, with both compounds producing a maximum of 50% inhibition of specific binding. Magnesium ions produced a similar pattern of inhibition but the effect of the polyamines and magnesium ions were not additive. The data provide evidence for the existence in brain of a novel binding site that may mediate the anticonvulsant effects of gabapentin and other potential anticonvulsant compounds.     

Biochemical and thermodynamic aspects of the binding of [3H]glycine to its strychnine-insensitive recognition site associated with the N-methyl-D-aspartate receptor complex.

The molecular mechanism of interaction between glycine and its strychnine-insensitive binding site linked to the N-methyl-D-aspartate receptor was investigated by examining on the one hand the thermodynamic properties of glycine binding, and, on the other hand, the effects of various functional group modifying agents on ligand binding. Raising the incubation temperature from 0 degrees to 37 degrees resulted in a consistent decrease of glycine binding affinity. Calculation of thermodynamic parameters from the corresponding Van't Hoff plot showed that the binding of glycine was mainly entropy-driven, the change in enthalpy contributing only little (25-30%) to the change in Gibbs free energy. Chemical modification with the sulfhydryl-directed agents p-hydroxy-mercuribenzoate and N-ethyl-maleimide showed free -SH groups to be critical for ligand binding to the receptor site. Furthermore, guanidino groups on arginyl residues, sensitive to 2,3-butanedione, were also found to participate in glycine binding. Both the -SH and the guanidino groups could be protected against their inactivation by co-incubation with glycine, indicating a direct involvement of these functional groups in the binding process. Dithiothreitol, a disulfide-reducing agent, likewise prevented [3H]glycine binding, suggesting that the glycine recognition site is stabilized by at least one disulfide bridge. It is concluded that the binding of glycine probably involves a strong ion-ion interaction between its carboxyl group and a positively charged guanidino group at the receptor site, resulting in a thermodynamically favorable increase in entropy by displacement of water molecules from the latter and a concomitant decrease in enthalpy. Furthermore, at least one free sulfhydryl group seems to participate in the binding process.     

Characterization of the [3H]-desipramine binding site of the bovine adrenomedullary plasma membrane

Summary The specific (i.e. nisoxetine-sensitive) binding of [³H]desipramine was studied in membranes prepared from bovine adrenal medullae. (1) [³H]desipramine bound reversibly and with high affinity (K _D = 2.8 nmol/l) to a single class of non-interacting binding sites (Hill coefficient = 0.96); the maximal number of binding sites (B_max) was 2.1 pmol/mg protein. (2) Binding of [³H]desipramine was dependent on [Na⁺] and [Cl^–]. Increasing the concentrations of these ions increased binding. (3) Substrates and inhibitors of the neuronal noradrenaline transport system (uptake,) inhibited binding of [³H]desipramine with a rank order of potency typical for an interaction with the uptake, carrier.The characteristics of [³H]desipramine binding remained essentially unchanged after solubilization of adrenomedullary membranes with the non-ionic detergent digitonin.The results indicate that the plasma membrane of bovine adreno-medulary cells is endowed with the neuronal uptake₁ transporter. Correspondence to: H. Bönisch     

Interaction of two sulfhydryl reagents with a cation recognition site on the neuronal dopamine carrier evidences small differences between [3H]GBR 12783 and [3H]cocaine binding sites

We have compared the effect of treating rat striatal cell membranes with ionic hydrophilic sulfhydryl reagents on the specific bindings of [³H]cocaine and of [³H]GBR 12783 (1-[2-(diphenylmethoxy)ethyl]4-(3-phenyl-2-[1-³H]propenyl)-piperazine) to the neuronal transporter of dopamine. Treatment with 1 mmol/1 5,5-dithiobis(2-nitrobenzoic acid) (DTNB) resulted in similar time-and concentration-dependent reductions of the specific binding of both radioligands. None of the uptake blockers tested afforded any protection against 1 mmol/1 DTNB. Addition of (sub)millimolar concentrations of CaCl₂ or MgCl₂, or 250 mmol/1 KCl to a treatment medium containing 10 mmol/l Na + significantly increased the DTNB-induced reduction of the specific binding of both radioligands. Cations were likely to be responsible for this effect since ions in combination with DTNB induced similar reductions in binding when either 1 mmol/l CaCl₂ or 50–250 mmol/l NaCl were added. Effects of cations on the DTNB-induced inhibition of binding were generally more marked on [³H]GBR 12783 than on [³H]cocaine binding. When added to a medium containing 10 mmol/1 Na⁺ 1 mmol/1 DTNB induced a reduction in the B_max of the specific binding of both radioligands. Addition of 1 mmol/l Ca²⁺ maintained or increased this B_max reduction and elicited a decrease in affinity which was significant for [³H]GBR 12783 binding.Treatment of membranes with the sodium salt of p-hydroxymercurybenzenesulfonate (pHMBS) induced time-and concentration-dependent decreases in [³H]GBR 12783 binding which were significantly greater than decreases in [³H]cocaine binding. However, 50mol/lpHMBS produced a similar decrease in the B_max of the specific binding of both radioligands. The pHMBS-induced reduction of [³H]GBR 12783 binding was not reversed by drugs whose action is purely that of uptake inhibition or by substrates of the dopamine carrier. Some of these drugs (100 mol/l dopamine, 1 mol/l mazindol or 100 mol/l cocaine) protected the specific binding of [³H]cocaine against the effects of pHMBS, whereas 1 mmol/1 p-tyramine, 10 mol/l nomifensine and 10 nmol/l GBR 12783 were ineffective. Addition of 120 mmol/l Na⁺, 1 mmol/l Ca²⁺ or 10 mmol/l Mg²⁺ to a treatment medium containing 10 mmol/l Na⁺ significantly reduced the effects of pHMBS on the specific binding of both radioligands. When striatal cell membranes were treated in a medium containing 130 mmol/1 Na⁺, there was a general decrease in the effects of ions on the reductions of specific binding produced by DTNB or pHMBS. Cation concentrations which interfered with the actions of DTNB and pHMBS were approximately those which blocked the specific binding of [³H]GBR 12783 when they were present during association of the radioligand (K⁺, Ca ²⁺, Mg²⁺), or, in the case of Na⁺, which are effective in reducing this blockade (Bonnet et al. 1988).The present data are consistent with the existence of mutually exclusive binding sites for [³H]GBR and [³H]cocaine on the neuronal dopamine transporter. The hypothesis of a cation recognition site which could gate admission of uptake inhibitors or carrier substrates to their binding domain on the transporter is discussed.     

High affinity binding of [3H]paroxetine and [3H]imipramine to human platelet membranes

Paroxetine, one of the most potent and specific serotonin uptake inhibitors, was tritiated and used for binding studies with human platelet membranes. Specific, high affinity binding was demonstrated. The binding was compared with [3H]imipramine binding; it was found that the maximal binding (Bmax) was the same for [3H]paroxetine and [3H]imipramine, whereas the affinity was much higher for [3H]paroxetine (KD 0.08 nM and 0.56 nM for paroxetine and imipramine binding, respectively). IC50 was calculated for the inhibition of [3H]paroxetine and [3H]imipramine binding by a number of antidepressants; the corresponding Hill coefficients were also calculated.