Saturation analysis of ligand binding using a centrifugation procedure.

The use of filtration assays is often unsuitable for radioligands with rapid dissociation rates. Rapid dissociation may result in the loss of significant amounts of bound ligand during the separation of bound from free ligand associated with filtration. The primary advantage of the centrifugation assay for characterizing the binding of a radioligand is that the bound ligand is separated from free ligand without initiating significant ligand dissociation from its receptor. Nonetheless, the small volume of supernatant that remains trapped within the membrane pellet (even after a superficial rinsing of the pellet surface) often results in an increased nonspecific binding compared to filtration techniques. The unit presents a protocol for the binding of [(3)H]glycine to the glycine recognition site of the NMDA receptor complex. This assay is commonly employed because of the current lack of high-affinity agonist ligands that interact with this recognition site. The use of other radioligands may require different tissue preparations, buffer systems, and assay conditions, but the basic steps involving sedimentation of the pellet, removal of supernatant, superficial washing of the pellet surface, and solubilization of the pellet will remain the same.

Effects of conantokins on L-3,4-dihydroxyphenylalanine-induced behavior and immediate early gene expression.

Conantokins, peptides from Conus snails, are N-methyl-D-aspartate (NMDA) receptor antagonists. NMDA receptor antagonists potentiate L-3,4-dihydroxyphenylalanine (L-DOPA)-induced rotation in 6-hydroxydopamine-treated rodents, an index of anti-Parkinsonian potential. This study examined the effects of conantokin-G, conantokin-T(G), CGS 19755, and ifenprodil on L-DOPA-induced contralateral rotation and immediate early gene (IEG) expression in 6-hydroxydopamine-treated rats. Rats received unilateral infusions of 6-hydroxydopamine into the medial forebrain bundle. Three weeks later, rats were treated with an NMDA receptor antagonist, followed by an injection of L-DOPA. Contralateral rotations were recorded for 2 h. In addition, the expression of zif268 and c-fos were examined. Conantokin-G, conantokin-T(G), and CGS 19755 potentiated L-DOPA-induced rotation. Conantokin-G and ifenprodil had no effect on L-DOPA-induced IEG expression, whereas conantokin-T(G) and CGS 19755 attenuated expression. These data suggest that conantokins may be useful in treating Parkinson's disease. Furthermore, different NMDA receptor antagonists have distinct effects on striatal gene expression.

Isolation and characterization of a novel conus peptide with apparent antinociceptive activity.

Cone snails are tropical marine mollusks that envenomate prey with a complex mixture of neuropharmacologically active compounds. We report the discovery and biochemical characterization of a structurally unique peptide isolated from the venom of Conus marmoreus. The new peptide, mr10a, potently increased withdrawal latency in a hot plate assay (a test of analgesia) at intrathecal doses that do not produce motor impairment as measured by rotarod test. The sequence of mr10a is NGVCCGYKLCHOC, where O is 4-trans-hydroxyproline. This sequence is highly divergent from all other known conotoxins. Analysis of a cDNA clone encoding the toxin, however, indicates that it is a member of the recently described T-superfamily. Total chemical synthesis of the three possible disulfide arrangements of mr10a was achieved, and elution studies indicate that the native form has a disulfide connectivity of Cys1-Cys4 and Cys2-Cys3. This disulfide linkage is unprecedented among conotoxins and defines a new family of Conus peptides.

Felbamate increases [3H]glycine binding in rat brain and sections of human postmortem brain.

The anticonvulsant compound felbamate (2-phenyl-1,3-propanediol dicarbamate; FBM) appears to inhibit the function of the N-methyl-D-aspartate (NMDA) receptor complex through an interaction with the strychnine-insensitive glycine recognition site. Since we have demonstrated previously that FBM inhibits the binding of [3H]5, 7-dichlorokynurenic acid (DCKA), a competitive antagonist at the glycine site, we assessed the ability of FBM to modulate the binding of an agonist, [3H]glycine, to rat forebrain membranes and human brain sections. In contrast to its ability to inhibit [3H]5,7-DCKA binding, FBM increased [3H]glycine binding (20 nM; EC50 = 485 microM; Emax = 211% of control; nH = 1.8). FBM, but not carbamazepine, phenytoin, valproic acid or phenobarbital, also increased [3H]glycine binding (50 nM; EC50 = 142 microM; Emax = 157% of control; nH = 1.6) in human cortex sections. Autoradiographic analysis of human brain slices demonstrated that FBM produced the largest increases in [3H]glycine binding in the cortex, hippocampus and the parahippocampal gyrus. Because various ions can influence the binding of glycine-site ligands, we assessed their effects on FBM-modulation of [3H]glycine binding. FBM-enhanced [3H]glycine binding was attenuated by Zn++ and not inhibited by Mg++ in human brain. These results suggest that FBM increases [3H]glycine binding in a manner sensitive to ions which modulate the NMDA receptor. These data support the hypothesis that FBM produces anticonvulsant and neuroprotective effects by inhibiting NMDA receptor function, likely through an allosteric modulation of the glycine site.

Structure--activity studies for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropanoic acid receptors: acidic hydroxyphenylalanines.

Antagonists of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropanoic acid (AMPA) receptors may have therapeutic potential as psychotropic agents. A series of mononitro- and dinitro-2- and 3-hydroxyphenylalanines was prepared, and their activity compared with willardiine, 5-nitrowillardiine, AMPA, and 2,4,5-trihydroxyphenylalanine (6-hydroxydopa) as inhibitors of specific [3H]AMPA and [3H]kainate binding in rat brain homogenates. The most active compounds were highly acidic (pKa 3-4), namely, 2-hydroxy-3,5-dinitro-DL-phenylalanine (13; [3H]AMPA IC50 approximately equal to 25 microM) and 3-hydroxy-2,4-dinitro-DL-phenylalanine (19; [3H]AMPA IC50 approximately equal to 5 microM). Two other dinitro-3-hydroxyphenylalanines, and 3,5-dinitro-DL-tyrosine, were considerably less active. Various mononitrohydroxyphenylalanines, which are less acidic, were also less active or inactive, and 2- and 3-hydroxyphenylalanine (o- and m-tyrosine) were inactive. Compounds 13 and 19, DL-willardiine (pKa 9.3, [3H]AMPA IC50 = 2 microM), and 5-nitro-DL-willardiine (pKa 6.4, [3H]AMPA IC50 = 0.2 microM) displayed AMPA >> kainate selectivity in binding studies. Compound 19 was an AMPA-like agonist, but 13 was an antagonist in an AMPA-evoked norepinephrine release assay in rat hippocampal nerve endings. Also, compound 13 injected into the rat ventral pallidum antagonized the locomotor activity elicited by systemic amphetamine.

Chronic antidepressant treatments increase cytochrome b mRNA levels in mouse cerebral cortex.

Antidepressant therapies include drugs with a remarkable structural diversity and non-pharmacological interventions, such as electroconvulsive shock. Although the primary neurochemical effects of these treatments may differ, the > or = 2- to 3-wk lag in therapeutic onset has led to the hypothesis that adaptive changes in a final common pathway are required for an antidepressant action. Based on this hypothesis, we sought to identify and characterize common changes in gene expression following chronic antidepressant treatments. We utilized a differential display strategy to identify genes that were differentially expressed in mice following chronic treatment with imipramine and electroconvulsive shock. Differential display PCR followed by subcloning, screening by reverse Northern blot, and confirmation by Northern blot revealed a significant increase in the expression of one gene candidate from mouse cortex following antidepressant treatments. The sequence of this 193-bp gene candidate was an exact match to the DNA sequence of mouse mitochondrial cytochrome b. In contrast to the increased mRNA levels of cytochrome b found in cortex, chronic treatment with these antidepressants did not alter mRNA levels in hippocampus, cerebellum, or liver. Moreover, no differences in cortical levels of cytochrome b mRNA were observed after either acute antidepressant treatments or chronic treatment with nonantidepressant drugs (haloperidol and morphine). The observation that chronic, but not acute treatment with imipramine and electroconvulsive shock produces a region-specific change in the levels of mRNA encoding cytochrome b suggests that this enzyme may be involved in the sequence of events resulting in an antidepressant action.

Structurally modified ibogaine analogs exhibit differing affinities for NMDA receptors.

Based on both preclinical findings and anecdotal evidence in man, the psychoactive indole alkaloid ibogaine has been suggested to have anti-addictive properties. Previous studies indicate that blockade of NMDA receptors may mediate at least some of the putative anti-addictive actions of ibogaine. The potencies of a series of ibogaine analogs to inhibit (+)-[3-3H]5-methyl-10,11-dihydro-5H-dibenzo-[a,d]cyclohepten-5,10- imine ([3H]MK-801) binding to NMDA receptors were examined. This series of analogs included the putative ibogaine metabolite O-desmethylibogaine, its metabolism resistant analog O-t-butyl-O-desmethylibogaine, the iboga alkaloids (+/-)-ibogamine, (+/-)-coronaridine, tabernanthine, harmaline, and the indolotropanes endo-3-(1-methylindol-2-yl)-8-methyl-8-azabicyclo[3.2.1]loctane (RS 075194-190), exo-3-(1-methylindol-2-yl)-8-methyl-8-azabicyclo[3.2.1]octane (RS 075237-190), and endo-3-(indol-2-yl)-8-methyl-8-azabicyclo[3.2.1]octane (RS 025989-190). Among these compounds, ibogaine was the most potent inhibitor of [3H]MK-801 binding (Ki = approximately 1.2 microM), whilst the compounds with the greatest structural similarity to ibogaine, O-desmethylibogaine and O-t-butyl-O-desmethylibogaine were less potent (Ki = approximately 5.5 and 179.0 microL, respectively). In morphine-dependent mice, ibogaine, but not O-desmethylibogaine or O-t-butyl-O-desmethylibogaine, attenuated naloxone precipitated withdrawal jumping. These findings are consistent with the hypothesis that inhibition of the expression of morphine dependence by ibogaine is related to its NMDA receptor antagonist properties.

Adaptation of N-methyl-D-aspartate (NMDA) receptors following antidepressant treatment: implications for the pharmacotherapy of depression.

NMDA antagonists mimic the effects of clinically effective antidepressants in both preclinical tests predictive of antidepressant action and procedures designed to model aspects of depressive symptomatology. These findings led to experiments demonstrating that chronic administration of NMDA antagonists to rodents results in a downregulation of cortical beta-adrenoceptors, a phenomenon also observed following chronic treatment with many antidepressants. These neurochemical and behavioral similarities between antidepressants and NMDA antagonists prompted us to examine the impact of chronic antidepressant treatment on NMDA receptors. Chronic (14 days) but not acute (1 day) administration of seventeen different antidepressants to mice produced adaptive changes in radioligand binding to NMDA receptors. Detailed studies with three antidepressants (imipramine, citalopram, and electroconvulsive shock) show that these changes develop slowly, persist for some time after cessation of treatment, and (for imipramine and citalopram) are dose dependent. Moreover, following chronic treatment with imipramine, these changes in radioligand binding to NMDA receptors appear restricted to the cerebral cortex. Based on the consistency of these effects across antidepressant treatments, we propose that adaptive changes in NMDA receptors may be the final common pathway for antidepressant action. The recent demonstration (Nowak et al., 1995) that radioligand binding to NMDA receptors is altered in frontal cortex of suicide victims (compared to age and post-mortem interval matched controls) is consistent with the hypothesis (Trullas and Skolnick, 1990) that this family of ligand gated ion channels is involved in the pathophysiology of depression.

NMDA antagonist properties of the putative antiaddictive drug, ibogaine.

Both anecdotal reports in humans and preclinical studies indicate that ibogaine interrupts addiction to a variety of abused substances including alcohol, opiates, nicotine and stimulants. Based on the similarity of these therapeutic claims to recent preclinical studies demonstrating that N-methyl-D-aspartate (NMDA) antagonists attenuate addiction-related phenomena, we examined the NMDA antagonist properties of ibogaine. Pharmacologically relevant concentrations of ibogaine produce a voltage-dependent block of NMDA receptors in hippocampal cultures (Ki, 2.3 microM at -60 mV). Consistent with this observation, ibogaine competitively inhibits [3H]1-[1-(2-thienyl)-cyclohexyl]piperidine binding to rat forebrain homogenates (Ki, 1.5 microM) and blocks glutamate-induced cell death in neuronal cultures (IC50, 4.5 microM). Moreover, at doses previously reported to interfere with drug-seeking behaviors, ibogaine substitutes as a discriminative stimulus (ED50, 64.9 mg/kg) in mice trained to discriminate the prototypic voltage-dependent NMDA antagonist, dizocilpine (0.17 mg/kg), from saline. Consistent with previous reports, ibogaine reduced naloxone-precipitated jumping in morphine-dependent mice (ED50, 72 mg/kg). Although pretreatment with glycine did not affect naloxone-precipitated jumping in morphine-dependent mice, it abolished the ability of ibogaine to block naloxone-precipitated jumping. Taken together, these findings link the NMDA antagonist actions of ibogaine to a putative "antiaddictive" property of this alkaloid, its ability to reduce the expression of morphine dependence.

Antidepressant-like actions of the polyamine site NMDA antagonist, eliprodil (SL-82.0715).

Functional N-methyl-D-aspartate (NMDA) antagonists including competitive antagonists, glycine partial agonists, and use-dependent channel blockers exhibit antidepressant-like actions in preclinical models. The present study examined the effects of eliprodil (SL-82.0715), an NMDA antagonist acting at polyamine sites, in behavioral and neurochemical tests predictive of antidepressant activity. In mice, eliprodil produced a dose-dependent reduction in immobility in the forced swim test, but was inactive in the tail suspension test. Chronic treatment with eliprodil produced both a significant downregulation of beta-adrenoceptors and a reduction in the potency of glycine to inhibit [3H]5,7-dichlorokynurenic acid binding to strychnine-insensitive glycine receptors in neocortical membranes. In toto, these data indicate that like other NMDA antagonists, eliprodil possesses antidepressant-like actions in preclinical tests predictive of clinical efficacy.

The putative anti-addictive drug ibogaine is a competitive inhibitor of [3H]MK-801 binding to the NMDA receptor complex.

Ibogaine is a putative anti-addictive drug with potential efficacy for the treatment of opiate, stimulant, and alcohol abuse. We now report ibogaine is a competitive inhibitor (Ki, 1.01 +/- 0.1 microM) of [3H]MK-801 binding to N-methyl-D-aspartate (NMDA) receptor coupled cation channels. Since MK-801 can attenuate the development of tolerance to morphine and alcohol as well as sensitization to stimulants in preclinical studies, the reported ability of ibogaine to modify drug-seeking behavior in man may be attributable to a blockade of NMDA receptor coupled cation channels.

Adaptation of the N-methyl-D-aspartate receptor complex following chronic antidepressant treatments.

Chronic (14 day) but not acute (1 day) treatment of mice with clinically active antidepressants produces a significant (approximately 1.8-4.3 fold) reduction in the potency of glycine to inhibit [3H]-5,7-dichlorkynurenic acid (5,7-DCKA) binding to strychnine-insensitive glycine receptors in neocortical membranes. Moreover, these effects were not observed following chronic treatment with a variety of nonantidepressant drugs such as D-deprenyl, chlorpromazine, salbutamol, scopolamine and chlordiazepoxide. The time course and dose-response relationships for this effect were examined after treatment with two representative antidepressant drugs (imipramine and citalopram) and electriconvulsive shock (ECS). Increases in the IC50 of glycine to inhibit [3H]-5,7-DCKA binding were observed after treatment for 7 days with ECS, 10 days with citalopram and 14 days with imipramine, respectively, and were no longer apparent by the 10th day after cessation of treatment. These findings indicate that the antidepressant-induced reduction in the IC50 of glycine to inhibit [3H]-5,7-DCKA binding is: 1) a slowly developing, adaptive phenomenon; 2) remarkably persistent after cessation of treatment; and 3) a significantly better predictor of antidepressant activity (22 of 23 drugs) than either beta adrenoceptor down-regulation (15 of 23 drugs) or efficacy in the forced swim test (13 of 23 drugs) [P < .01 vs. each measure, Fisher's Exact Test]. The ability of antidepressants drawn from every principal therapeutic class to effect adaptive changes in the N-methyl-D-aspartate receptor complex is consistent with the hypothesis that this ligand-gated ion channel serves as a final common pathway of antidepressant action and indicates that glutamatergic pathways may be involved in the pathophysiology of depression.

Adaptation of the NMDA receptor in rat cortex following chronic electroconvulsive shock or imipramine.

Chronic (14 days) administration of either imipramine or electroconvulsive shock effected significant changes in the ligand binding properties of the NMDA (N-methyl-D-aspartate) receptor complex in rat cortex. These changes were manifested as: (1) a reduction in the potency of glycine to inhibit the binding of 5,7-dichloro[3H]kynurenic acid to strychnine-insensitive glycine receptors; and (2) a reduction in the proportion of high affinity, glycine-displaceable [3H]CGP-39653 binding to NMDA receptors. Chronic electroconvulsive shock, but not imipramine treatment also reduced the density of [3H]CGP-39653 binding sites in cortical membranes. These findings demonstrate that the ability of chronic antidepressant treatments to induce adaptive changes in the glycine and glutamate regulatory sites of the NMDA receptor is not species specific, since it obtains in rats as well as mice.

MK-801, but not drugs acting at strychnine-insensitive glycine receptors, attenuate methamphetamine nigrostriatal toxicity.

Repeated administration of methamphetamine (METH) results in damage to nigrostriatal dopaminergic neurons. Both competitive N-methyl-D-aspartate (NMDA) receptor antagonists and use-dependent cation channel blockers attenuate METH-induced damage. The objectives of the present study were to examine whether comparable reductions in METH-induced damage could be obtained by compounds acting at strychnine-insensitive glycine receptors on the NMDA receptor complex. Four injections of METH (5 mg/kg i.p.) resulted in a approximately 70.9% depletion of striatal dopamine (DA) and approximately 62.7% depletion of dihydroxyphenylacetic acid (DOPAC) content, respectively. A significant protection against METH-induced DA and DOPAC depletion was afforded by the use-dependent channel blocker, MK-801. The competitive glycine antagonist 7-chlorokynurenic acid (7-Cl-KA), the low efficacy glycine partial agonist (+)-3-amino-1-hydroxy-2-pyrrolidone ((+)-HA-966), and the high efficacy partial glycine agonist 1-aminocyclopropane-carboxylic acid (ACPC) were ineffective against METH-induced toxicity despite their abilities to attenuate glutamate-induced neurotoxicity under both in vivo and in vitro conditions. These results indicate that glycinergic ligands do not possess the same broad neuroprotective spectrum as other classes of NMDA antagonists.

Effects of the AMPA/kainate receptor antagonist DNQX in the nucleus accumbens on drug-induced conditioned place preference.

Activation of AMPA/kainate glutamatergic receptors in the nucleus accumbens may be a component of the mechanism of drug induced reward. To test this hypothesis, 6,7-dinitroquinoxaline-2,3-dione (DNQX), an alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA)/kainate glutamatergic receptor anatagonist, was injected into the nucleus accumbens before the administration of amphetamine or morphine during the training phase (acquisition) of a conditioned place preference paradigm. Rats were then tested for place preference in the absence of drugs. In other experiments, DNQX was given before testing for place preference (expression) but not during the training phase. Bilateral injection of DNQX (1 microgram/0.5 microliters/side) inhibited acquisition of place preference to amphetamine (1 mg/kg) but not morphine (10 mg/kg). During acquisition, DNQX marginally attenuated the locomotor stimulation elicited by amphetamine during the first but not subsequent training sessions, while the combination of morphine plus DNQX produced marked akinesia during each training session. When given prior to testing, DNQX inhibited the expression of place preference induced by amphetamine and morphine but did not affect locomotor activity. The results suggest that activation of AMPA/kainate receptors is involved in the primary reward stimulation (acquisition of place preference) of amphetamine but not morphine and in behaviors elicited by memory of primary reward stimulation (expression of place preference) for both drugs. Furthermore, locomotor activity during conditioning is not necessary for acquisition of place preference.

Adaptive changes in the N-methyl-D-aspartate receptor complex after chronic treatment with imipramine and 1-aminocyclopropanecarboxylic acid.

Chronic (14 daily injections) treatment of mice with the prototypic tricyclic antidepressant imipramine significantly alters ligand binding to the N-methyl-D-aspartate (NMDA) receptor complex. These effects were compared to a chronic regimen of 1-aminocyclopropanecarboxylic acid, a high-affinity partial agonist at strychnine-insensitive glycine receptors which mimics the effects of imipramine in preclinical models predictive of antidepressant action. Changes in the NMDA receptor complex after chronic, but not acute treatment with imipramine were manifested as: 1) a reduction in the potency of glycine to inhibit [3H]5,7-dichlorokynurenic acid binding to strychnine-insensitive glycine receptors; 2) a decrease in the proportion of high-affinity glycine sites inhibiting [3H]CGP 39653 binding to NMDA receptors; and 3) a decrease in basal [3H]MK-801 binding (under nonequilibrium conditions) to sites within NMDA receptor-coupled cation channels which was reversible by the addition of glutamate. These effects were observed in cerebral cortex, but not in hippocampus, striatum or basal forebrain. Chronic treatment with 1-aminocyclopropanecarboxylic acid resulted in changes which paralleled those of imipramine on ligand binding to the NMDA receptor complex, but the reduction in basal [3H]MK-801 binding did not achieve statistical significance. These findings indicate that adaptive changes in the NMDA receptor complex could be a feature common to chronic treatment with structurally unrelated antidepressants.

The NMDA receptor antagonist MK-801 elicits conditioned place preference in rats.

(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate, (MK-801) a potent noncompetitive antagonist of central NMDA receptors, has been hypothesized to have rewarding properties indicative of abuse potential. To test this hypothesis, the effects of MK-801 on the acquisition of a conditioned place preference and on locomotor activity were assessed and compared with d-amphetamine. Both MK-801 (0.03 and 0.1 mg/kg, SC) and d-amphetamine (1.0 mg/kg, SC) administration resulted in the acquisition of a conditioned place preference. However, while both amphetamine and the higher dose of MK-801 produced a behavioral activation during the training period the lower dose of MK-801 did not. These results suggest that MK-801, at doses that produce behavioral activation and below, is rewarding and therefore may have abuse potential.

Stimulation of locomotor activity by intra-accumbens AMPA is not inhibited by neonatal 6-hydroxydopamine-induced lesions.

The involvement of dopamine in the hypermotility responses to amphetamine s.c. or AMPA injected into the nucleus accumbens was evaluated in adult rats depleted of dopamine as neonates with 6-hydroxydopamine. The hypermotility response to amphetamine was markedly inhibited in the lesioned animals, while that to AMPA was enhanced. In addition, the hypermotility produced by AMPA in these rats was not inhibited by sulpiride+SCH-23390; however, it was inhibited completely by alpha-methyl-p-tyrosine. These results suggest that the hypermotility produced by AMPA requires endogenous dopamine, but is mediated by a different mechanism than that produced by amphetamine.

Effect of serotonergic agonists in the nucleus accumbens on d-amphetamine-stimulated locomotion.

Serotonergic projections from the raphe nuclei are thought to modulate locomotor activity in the rat, and serotonin injection into the nucleus accumbens attenuates the hypermotility elicited by amphetamine. The purpose of the present study was to characterize the effects of various classes of serotonergic agonists administered into the nucleus accumbens on d-amphetamine-stimulated locomotor activity in order to determine which serotonin receptor subtypes are involved. Administration of the nonselective 5-HT agonist quipazine, the 5-HT-1 agonist mCPP, the 5-HT-1a agonist 8-OH-DPAT, the 5-HT-1b agonist CGS-12066B, and the 5HT-1c/2 agonist DOI did not inhibit d-amphetamine-stimulated locomotor activity. Pronounced lateral head weaving was noted after 8-OH-DPAT administration. The combination of the 5-HT-1a agonist 8-OH-DPAT and the 5-HT-1b agonist CGS-12066B, however, did inhibit d-amphetamine-stimulated locomotor activity. In contrast, the 5-HT-3 agonist 1-phenylbiguanide enhanced the locomotor effect of d-amphetamine. This effect was partially reversed by the 5-HT-3 antagonist MDL-7222. These studies suggest that serotonin has complex and multiple effects on the regulation of locomotor activity within the nucleus accumbens.