PCP Binding Site Research Articles

Stereoselective Synthesis of 1-Substi- tuted 1,2,3,4-Tetrahydro-b-carbolines by Asymmetric Electrophilic a-Amido- alkylation Reactions

An efficient procedure for the asymmetric synthesis of 1-substituted 1,2,3,4-tetrahydro-9-methyl-β-carbolines based on asymmetric electrophilic α-amidoalkylation reactions is described. Stereoselective addition of variousorganometallic reagents to a chiral N-acyl-β-carbolinium ion gave the corresponding 1-substituted 1,2-dihydro-β-carbolines in high yields and very good to excellent diastereomeric ratios. Catalytic hydrogenation followed by the removal of the chiral auxiliary via reductive cleavage of the amide bond proceeded with complete conservation of the absolute configuration at the newly created stereocenter leading to 1-substituted 1,2,3,4-tetrahydro-9-methyl-β-carboline derivatives in high yields. Their absolute stereochemistry was proven by X-Ray analysis. The 1-phenyl-substituted 1,2,3,4-tetrahydro-β-carbolines were evaluated for their affinity to the PCP binding site of the NMDA receptor.

Synthesis of [11C] N‐(2‐chloro‐5‐thiomethylphenyl)‐N′‐(3‐methoxyphenyl)‐N′‐methylguanidine ([11C]GMOM): a candidate PET tracer for imaging the PCP site of the NMDA ion channel

AbstractThe N‐methyl‐D‐aspartate (NMDA) ion channel plays an important role in a number of neurodegenerative disorders including stroke, Parkinson's disease, Huntington's Chorea, Alzheimer's disease, schizophrenia and epilepsy. To provide effective radioligands for imaging the PCP binding site of the NMDA ion channel, we synthesized and characterized in vitro the candidate PCP site ligand N‐(2‐chloro‐5‐thiomethylphenyl)‐N′‐(3‐methoxyphenyl)‐N′‐methylguanidine (GMOM: Ki = 5.2 ± 0.3 nM, log P = 2.34). The corresponding PET radiotracer [11C]GMOM was synthesized with a radiochemical yield of 8.4 ± 3.2% EOS and with a specific activity of 1.23 ± 0.25 Ci/μmol EOS (n = 5). The average time required for synthesis, purification and formulation was 52 ± 5 min. The final product was prepared in a sterile saline solution suitable for in vivo use. Copyright © 2002 John Wiley & Sons, Ltd.

Syntheses of fluorinated phencyclidine analogs

Syntheses of several fluorinated phencyclidine (PCP) analogs are described. These compounds are being used to probe PCP binding sites on N-methyl-d-aspartate (NMDA) receptors. The compounds were prepared in good yields by Grignard reaction of appropriate fluorine substituted bromobenzene with carbonitrile intermediates. Syntheses of the known compound 1-[1-(3-fluorophenyl)cyclohexyl]piperidine, and the novel compounds 1-[1-(4-fluorophenyl)cyclohexyl]piperidine, 1-[1-(3-fluorophenyl) cyclohexyl]pyrrolidine, and 1-[1-(4-fluorophenyl)cyclohexyl]pyrrolidine are reported.

Inhibition by sigma receptor ligand, MS-377, of N-methyl- D-aspartate-induced currents in dopamine neurons of the rat ventral tegmental area.

MS-377 [( R)-(+)-1-(4-chlorophenyl)-3-[4-(2-methoxyethyl) piperazin-1-yl]methyl-2-pyrrolidinone L-tartrate] is a novel anti-psychotic drug candidate with high affinity for sigma receptors but devoid of binding affinity for PCP binding site of NMDA receptor/ion channel complex. The effects of MS-377 on NMDA receptor and/or its ion channel complex were examined to elucidate the antipsychotic properties of MS-377. We examined the effect of MS-377 on NMDA ( N-methyl- D-aspartate)-induced current in acutely dissociated dopamine neurons of rat ventral tegmental area (VTA) using patch clamp whole cell recording. MS-377 applied in a bath inhibited the peak current evoked by NMDA applied via the U-tube method for 2 s in a concentration-dependent manner. Other sigma receptor ligands, BD-1063 (1-[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine), NE-100 ( N, N-dipropyl-2-[4-methoxy-3-(2-phenylenoxy)-phenyl]-ethylamine monohydrochloride) and haloperidol also inhibited NMDA-induced current in a concentration-dependent manner. Interestingly, concomitant application of MS-377 with BD-1063, NE-100 or haloperidol at concentrations that had no effects on NMDA-induced current, potentiated the MS-377-induced inhibition. The results suggest that MS-377, as well as other sigma receptor ligands, indirectly acts on the sigma receptor to inhibit glutaminergic transmission mediated by NMDA receptor/ion channel complex in VTA dopamine neurons, thereby inhibiting dopamine release in target VTA areas.

SYNTHESIS AND IN VITRO AND IN VIVO EVALUATION OF [11C]METHYL-BIII277CL FOR IMAGING THE PCP-BINDING SITE OF THE NMDA RECEPTOR BY PET

ABSTRACTA new benzomorphane derivative, [11C]methyl-BIII277CL, was evaluated as a potential radiotracer for visualizing the PCP-binding site of the N-methyl-D-aspartate (NMDA) receptor by positron emission tomography (PET). Methyl-BIII277CL was prepared by reacting the desmethyl compound (BIII277CL) with dimethylsulfate. The pharmacological profile of methyl-BIII277CL was determined by in vitro receptor-screening assays. At a concentration of 100 nM, methyl-BIII277CL showed a significant interaction with the PCP-binding site of the NMDA receptor (79% inhibition of specific binding) and the σ1-binding site (46% inhibition). In displacement assays using mice cortical membranes, methyl-BIII277CL displayed a high affinity at the PCP-binding site of the NMDA receptor (Ki = 49 ± 14 nmol/L) and a 130-fold lower interaction with the σ1-binding site (Ki = 6.35 ± 0.26 µmol/L). For saturation experiments and in vivo studies, methyl-BIII277CL was radiolabeled with 11C at the O-position of the desmethyl precursor (BIII277CL) using [11C]methyliodide with a specific activity of 35–70 GBq/µmol at the end of synthesis (EOS). In saturation assays using rat whole brain membranes [11C]methyl-BIII277CL showed a Kd of 6 ± 1 nmol/L and a Bmax of 670 ± 154 fmol/mg protein. Biodistribution and PET studies in rats and pigs, however, indicated a lack of specific binding and unfavorable pharmacokinetics. Kinetic modeling using the 1-tissue compartment model demonstrated for [11C]methyl-BIII277CL a low distribution volume (Dv = 0.98 mL/mLtissue) and very high values for the kinetic parameters K1 and k2 (K1 = 0.36 mL/mLtissue/min and k2 = 0.37 min−1) in pig cortex. [11C]methyl-BIII277CL, due to the lack of specificity in vivo, may not be a candidate for imaging the PCP-binding site of the NMDA receptor.

Phencyclidine-induced discriminative stimulus is mediated via phencyclidine binding sites on the N-methyl- d-aspartate receptor-ion channel complex, not via sigma 1 receptors

The effects of several N-methyl- d-aspartate (NMDA) receptor- and sigma receptor-related compounds on the discriminative stimulus effects of phencyclidine (PCP) were examined in rats trained to discriminate PCP (1.5 mg/kg, i.p.) from saline under a two-lever fixed ratio 20 schedule of food reinforcement. PCP produced a dose-dependent increase in PCP-appropriate responding. A non-competitive NMDA receptor antagonist, dizocilpine (0.2 mg/kg, i.p.) and a putative sigma 1 receptor agonist, (+)-SKF-10047 (10 mg/kg, i.p.) fully substituted for PCP in every rat tested. Neither a competitive NMDA receptor antagonist, CGS-19755 (0.1–3 mg/kg, i.p.), sigma 1 receptor agonist, (+)-pentazocine (10–30 mg/kg, i.p.) nor dextromethorphan (10–20 mg/kg, i.p.) produced PCP-like discriminative stimulus effects. The discriminative stimulus effects of PCP (1.5 mg/kg, i.p.), dizocilpine (0.2 mg/kg, i.p.) and (+)-SKF-10047 (10 mg/kg, i.p.) were significantly attenuated by CGS-19755 (1 mg/kg, i.p.), but not by sigma 1 receptor antagonist BMY-14802 (10 mg/kg, i.p.) and NE-100 (5 mg/kg, i.p.). These results suggest that the discriminative stimulus effects of PCP are predominantly mediated via PCP binding sites on the NMDA receptor-ion channel complex, not via sigma 1 receptors. In addition, the PCP-like discriminative stimulus effects of (+)-SKF-10047 were demonstrated to be mediated via PCP binding sites.

Synthesis and structure–affinity relationships of 1,3,5-alkylsubstituted cyclohexylamines binding at NMDA receptor PCP site

A series of 1,3,5-alkylsubstituted cyclohexylamines 2 were synthesized as ligands for the N-methyl-D-aspartate (NMDA) receptor phencyclidine (PCP) binding site. Pure diastereomers with defined configuration of amino group 2 -ax and 2- eq were obtained. The optimal size of 1,3,5-substituents was determined for cyclohexylamines 2 with an equatorial amino group in the lowest energy conformation using Hansch analysis. According to the data, the lipophilic part of cyclohexylamines 2 does not discriminate between hydrophobic regions of the PCP binding site but rather recognizes this site as a whole lipophilic pocket.

Crystal structure of the surfactin synthetase-activating enzyme Sfp: a prototype of the 4'-phosphopantetheinyl transferase superfamily

The Bacillus subtilis Sfp protein activates the peptidyl carrier protein (PCP) domains of surfactin synthetase by transferring the 4'-phosphopantetheinyl moiety of coenzyme A (CoA) to a serine residue conserved in all PCPs. Its wide PCP substrate spectrum renders Sfp a biotechnologically valuable enzyme for use in combinatorial non-ribosomal peptide synthesis. The structure of the Sfp-CoA complex determined at 1.8 A resolution reveals a novel alpha/beta-fold exhibiting an unexpected intramolecular 2-fold pseudosymmetry. This suggests a similar fold and dimerization mode for the homodimeric phosphopantetheinyl transferases such as acyl carrier protein synthase. The active site of Sfp accommodates a magnesium ion, which is complexed by the CoA pyrophosphate, the side chains of three acidic amino acids and one water molecule. CoA is bound in a fashion that differs in many aspects from all known CoA-protein complex structures. The structure reveals regions likely to be involved in the interaction with the PCP substrate.

5-Doxylstearate-Induced Displacement of Phencyclidine from Its Low-Affinity Binding Sites on the Nicotinic Acetylcholine Receptor

Fatty acids as well as phencyclidine (PCP) inhibit the ion channel activity of the nicotinic acetylcholine receptor (AChR) by a noncompetitive mechanism. However, the exact localization of the fatty acid binding sites is unknown and, thus, the noncompetitive inhibitory mechanism for these endogenous modulators remains to be elucidated. In an attempt to determine the location of the fatty acid binding sites, we study the mutually exclusive action between 5-doxylstearate (5-SASL), a derivative of the endogenous noncompetitive antagonist (NCA) stearic acid, and other exogenous NCAs. For this purpose, both equilibrium and competitive binding assays using fluorescent and radiolabeled ligands were performed on desensitized AChRs. More specifically, we determined: (i) the effect of 5-SASL on the binding of the exogenous NCA [3H]PCP; (ii) the effect of 5-SASL on the binding of either quinacrine or ethidium, two fluorescent NCAs from exogenous origin; and (iii) the PCP-induced displacement of quinacrine and ethidium from their respective high-affinity binding sites. Our first target (i) is carried out by measuring the [3H]PCP binding in the absence or in the presence of increasing concentrations of 5-SASL. We found that 5-SASL displaces PCP from its low-affinity binding sites. The low-affinity PCP binding sites were pharmacologically characterized by an apparent dissociation contant (Kd) of 6.1 ± 5.0 μM and a stoichiometry of 3.7 ± 1.5 sites per AChR. The fact that 5-SASL increased the apparent Kd without changing the number of sites per AChR is indicative of a mutually exclusive action. From these results, an apparent inhibition constant (Ki) of 75 ± 31 μM for 5-SASL was calculated. In addition, 5-SASL affected neither the apparent Kd (0.46 ± 0.37 μM) nor the stoichiometry (1.07 ± 0.57 sites per AChR) of the high-affinity PCP binding site. The second objective (ii) is achieved by titrating either quinacrine or ethidium into AChR native membranes in the absence or in the presence of increasing concentrations of 5-SASL. These experiments showed that 5-SASL efficiently increased the apparent Kd of quinacrine without perturbing the interaction of ethidium with its high-affinity locus. Considering that (a) 5-SASL effectively quenched the AChR-bound quinacrine fluorescence (H. R. Arias, Biochim. Biophys. Acta 1347, 9–22, 1997) and (b) fluorescence-quenching is a short-range process, it is possible to suggest that 5-SASL displaces quinacrine from its high-affinity binding site by a steric mechanism. In this regard, a Ki of 38 ± 5 μM for 5-SASL was calculated. Concerning the last objective (iii), AChR-bound quinacrine or ethidium was back titrated with PCP. Two PCP Ki values were obtained by fitting the displacement plots by nonlinear regression with two components. The lowest Ki values obtained for either quinacrine (0.86 ± 0.37 μM) or ethidium (0.29 ± 0.23 μM) displacement from their respective high-affinity binding sites coincide with the previously determined high-affinity [3H]PCP Kd. In addition, the highest Ki values obtained for either NCA displacement are in the same concentration range as the observed low-affinity [3H]PCP Kd. Taking into account all experimental data, we reached the following conclusions: (i) fatty acid molecules, or at least 5-SASL, sterically interact with both the PCP low-affinity and the quinacrine high-affinity binding sites; (ii) the low-affinity PCP binding sites, as well as the high-affinity quinacrine locus, are located at the nonannular lipid domain of the AChR; and, finally, (iii) fatty acid molecules are not accessible to the lumen of the ion channel, indicating an allosteric mode of action for fatty acids to inhibit ion flux. Thus, the 5-SASL, the quinacrine high-affinity, and the PCP low-affinity binding sites are all located at overlapping nonannular loci on the muscle-type AChR.

Memantine does not substantially affect brain stimulation reward: comparison with MK-801

The non-competitive N-methyl- d-aspartate (NMDA)-receptor antagonist MK-801 (dizocilpine) has been shown to potentiate brain stimulation reward (BSR). Memantine (1-amino-3,5-dimethyladamantane) also binds to the PCP binding site of the NMDA receptor but with markedly different kinetics and affinity than MK-801. Here, we examined the effects of memantine on BSR and compared its effects to those of MK-801. MK-801 (0.05 mg/kg–0.4 mg/kg) produced clear, dose-dependent decreases in threshold frequency, manifest in clear leftward shifts of the function relating stimulation frequency to response rate. Memantine (1 mg/kg–17.5 mg/kg) had only small effects on threshold frequencies and only at high doses, manifest in only small shifts in the frequency–response function. The highest dose of each drug also produced a decrease in maximum response rate. This study shows that memantine failed to substantially influence BSR at low to intermediate doses, suggesting that this substance is likely to be largely devoid of rewarding effects in a therapeutic dose range.

Simulation of psychosis by continuous delivery of phencyclidine from controlled-release polymer implants

To simulate psychosis in rats we have developed a method for the continuous delivery of phencyclidine (PCP) using implantable controlled-release polymers. PCP polymer implants produced deficits in latent inhibition which do not occur after repeated bolus injections. PCP implanted rats were also devoid of any anxiogenic signs, motoric hyperactivity and learning acquisition which can be seen in rats receiving daily bolus injections of a comparable PCP dose. This behavioral double-dissociation of the two modes of PCP application was accompanied by respective neurochemical changes. PCP binding sites were reduced in both striatum and hippocampus, but in the hippocampus, loss of PCP binding sites was more severe following pulsatile PCP administration. Morphological assessment revealed a significant shrinkage of the CA3 region in hippocampus in both groups. Pharmacokinetic analysis showed that the maximum PCP concentration in the brain after bolus injections was 10-fold above the PCP implants.

Synthesis and brain distribution of carbon-11 labeled analogs of antagonists for the NMDA receptor coupled PCP-binding site

Two phencyclidine (PCP) analogs, 3 and 4, and one thienylcyclohexyl-piperidine (TCP) analog, (±)6, were labeled with positron emitter carbon-11. These compounds displayed higher in vitro binding affinities than PCP itself for the PCP-binding site located inside the ion channel on the N-methyl-D-aspartate (NMDA) receptors. Brain distribution studies in mice showed different uptake characteristics between the PCP and TCP analogs, indicating their different in vivo interactions with the brain components including the PCP-binding site probably due to the different physicochemical properties of the molecules. © 1998 John Wiley & Sons, Ltd.

Effect of Ibogaine on the Various Sites of the NMDA Receptor Complex and Sigma Binding Sites in Rat Braina.

Although the alkaloid ibogaine is a potent hallucinogenic agent some indications suggest that it may be useful for the treatment of opioid and cocaine addiction. The neurochemical mechanism(s) underlying ibogaine effects remain unclear. In the present study we investigated the interaction of ibogaine with the phencyclidine (PCP) site located in the ionophore of the N-methyl-d-aspartate (NMDA) receptor complex, with the NMDA receptor binding site, and with sigma binding sites. In well-washed membrane preparations of rat cortex and cerebellum, the PCP sites were labeled with [3H]MK-801 or [3H]1-[1(2-theinyl)-cyclohexyl]-piperidine ([3H]TCP), and the NMDA receptor with [3H]-CGP 39653. The sigma-1 and sigma-2 binding site in rat cortex and cerebellum were labeled with [3H]pentazocine and [3H]1,3-di-o-tolyl-guanidine ([3H]DTG), respectively. Results indicated that ibogaine interacts with high- and low-affinity PCP binding sites in the cortex: Ki(H) = 0.01-0.05 μM; Ki(L) = 2-4μM, and only with low-affinity sites in the cerebellum: Ki = 2-4, μM. In contrast, ibogaine (>100 μM) had no affinity for [3H]-CGP 39653 binding sites (cortex and cerebellum). The affinity of ibogaine for sigma-1 and -2 binding sites in cortex and cerebellum ranged from 1.5-3μM. Since NMDA receptor antagonists (e.g., MK-801) are thought to attenuate opioid withdrawal symptoms and cocaine sensitization, it is possible that binding of ibogaine to the PCP sites contributes to its potential 'endabuse' properties. In turn, ibogaine interaction with sigma binding sites may be associated with its adverse effects.

Ethanol and N -methyl- D -aspartate receptor complex interactions: a detailed drug discrimination study in the rat

The discriminative stimulus properties of compounds that interact with the NMDA receptor complex were investigated in rats trained to discriminate ethanol from saline. Male Wistar rats were trained in a two-lever operant drug discrimination paradigm to make differential responses [fixed ratio 10 (FR10)] for food after ethanol (1 g/kg i.p.; 12% v/v ethanol solution) and saline vehicle injections. Drug effects were assessed by means of generalization and antagonism tests. In the generalization tests, the noncompetitive NMDA antagonists acting at the ion channel dizocilpine, memantine, phencyclidine (PCP) and the sigma1 receptor agonists (+)-pentazocine and (+)-N-allyl-normetazocine (NANM) dose-dependently generalized for ethanol, whereas the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) antagonist GYKI 52466, the glycine antagonists L-701,324 and MRZ 2/502, the polyamine site antagonist arcaine and the polyamine site ligand spermidine, did not. Our results show that the noncompetitive NMDA antagonists fully substitute dose-dependently for ethanol in a drug-discrimination task. The ethanol-like discriminative stimulus effects of PCP, pentazocine and NANM, which are also sigma receptor ligands, are likely to be attributed to their activity at NMDA receptors. We therefore assume that some of the acute effects of ethanol are mediated via NMDA receptor antagonism at the PCP binding site.

Differential psychopathology and patterns of cerebral glucose utilisation produced by ( S)- and ( R)-ketamine in healthy volunteers using positron emission tomography (PET)

Until recently, racemic ketamine ( S-ketamine/ R-ketamine=50:50) has been used to study NMDA receptor hypofunction in relation to pathophysiological models of schizophrenia. Ketamine given to normal humans in subanesthetic doses produces a model psychosis including both positive and negative symptoms of schizophrenia. More recently it has been shown that at subanesthetic doses the pure ( S)- and ( R)-ketamine enantiomeres interact differently with the NMDA and sigma receptor sites in human brain. It was found that ( S)-ketamine binds with a 3–4 time higher affinity to the PCP binding site of the NMDA receptor than ( R)-ketamine, and that at these concentrations ( R)-ketamine interacts also weakly with the sigma receptor sites, where ( S)-ketamine binds only negligibly. To further investigate the role of NMDA-receptor mediated neurotransmission in schizophrenic psychosis, the effects of pure ( S)- and ( R)-ketamine enantiomeres on brain energy metabolism in normal humans using positron emission tomography and [ 18F]fluorodeoxyglucose (FDG) are reported here. Psychotomimetic doses of ( S)-ketamine increased cerebral metabolic rates of glucose (CMRglu) markedly in the frontal cortex including the anterior cingulate, parietal and left sensorimotor cortex, and in the thalamus. The metabolic changes in the frontal and left temporal cortex correlated with ego-disintegration and hallucinatory phenomena. Equimolar doses of ( R)-ketamine tended to decrease CMRglu across brain regions and significantly suppressed CMRglu in the temporomedial cortex and left insula. ( R)-ketamine did not produce psychotic symptoms, but a state of relaxation. The ( S)-ketamine-induced metabolic hyperfrontality appears to parallel similar metabolic findings in acute psychotic schizophrenic patients and encourages further investigations of glutamatergic disturbances in schizophrenia.

Psychotogenicity and N-methyl-D-aspartate receptor antagonism: Implications for neuroprotective pharmacotherapy

The development of neuroprotective agents for the prevention of neuronal loss in acute conditions such as stroke and epilepsy or chronic neurodegenerative disorders including Parkinson's disease, Alzheimer's disease, Huntington's chorea, and motor neuron disease is currently focusing on drugs that inhibit excitatory amino acid neurotransmission or exhibit antioxidant properties. Unfortunately, potent antagonists at the N-methyl-D-aspartate (NMDA) type glutamate receptor, which is thought to mediate excitotoxic neuronal injury, e.g., MK-801 or phencyclidine (PCP), share a high probability of inducing psychotomimetic side effects. Further, these drugs have been associated with acute neurotoxicity in vitro and in vivo, precluding their clinical use. In contrast, low affinity NMDA receptor antagonists like amantadine and its dimethyl derivative, memantine, have been administered clinically for the management of Parkinson's disease, dementia, neuroleptic drug-induced side effects, and spasticity. These agents have only rarely induced significant psychotomimetic side effects. Recent pharmacologic advances have helped to elucidate how high drug affinity for the PCP binding site of the NMDA receptor may enhance psychotogenicity. Low affinity and associated fast voltage-dependent channel unblocking kinetics are likely to be responsible for the better tolerance of amantadine and memantine compared with MK-801 and PCP. Further factors apparently modulating psychotogenicity of glutamate receptor antagonists include differential actions on neuronal populations in various brain regions and interactions with neurotransmitter receptors other than the NMDA type glutamate receptor.

In vivo functional interaction between phencyclidine binding sites and σ receptors to produce head-weaving behavior in rats

To investigate the in vivo functional interaction between phencyclidine (1-(1-phenylcyclohexyl)piperidine; PCP) binding sites and σ receptors, we examined the effects of σ receptor ligands on stereotyped head-weaving behavior induced by PCP, a putative PCP/σ receptor ligand, and (+)-5-methyl-10,11-dihydroxy-5 H-dibenzo( a,d)cyclo-hepten-5,10-imine ((+)-MK-801; dizocilpine), a selective PCP binding site ligand, in rats. PCP (7.5 mg/kg, i.p.)-induced head-weaving behavior was inhibited by both N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)-phenyl]-ethylamine (NE-100; 0.03–1.0 mg/kg, p.o.), a selective σ 1 receptor ligand, and α-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperidine butanol (BMY-14802; 3 and 10 mg/kg, p.o.), a prototype σ receptor ligand, in a dose-dependent manner, whereas NE-100 (0.1–1.0 mg/kg, p.o.) and BMY-14802 (3 and 10 mg/kg, p.o.) did not inhibit dizocilpine (0.25 mg/kg, s.c.)-induced head-weaving behavior. These results suggest that NE-100 and BMY-14802 act via σ receptors. Dizocilpine-induced head-weaving behavior was potentiated by 1,3-di- o-tolyl-guanidine (DTG; 0.03–0.3 μg/kg, i.v.) and (+)-3-(3-hydroxyphenyl)- N-(1-propyl)piperidine ((+)-3-PPP; 3 and 6 mg/kg, i.p.), σ 1 σ 2 receptor ligands, as well as by (+)- N-allyl-normetazocine ((+)-SKF-10,047; 8 mg/kg, i.p.), a σ 1 receptor ligand, while DTG (0.3 μg/kg, i.v.), (+)-3-PPP (6 mg/kg, i.p.) and (+)-SKF-10,047 (8 mg/kg, i.p.) did not induce this behavior. Potentiation of dizocilpine-induced head-weaving behavior by DTG (0.3 μg/kg, i.v.), (+)-3-PPP (6 mg/kg, i.p.) and (+)-SKF-10,047 (8 mg/kg, i.p.) was completely blocked by NE-100 (0.1 mg/kg, p.o.) and BMY-14802 (10 mg/kg, p.o.). These results suggest that PCP binding sites and σ receptors are involved in PCP-induced head weaving behavior, and that σ 1 receptors play an important role in modulation of the head-weaving behavior.

Autoradiographic study on the pharmacological characteristics of [ 3H]3-OH-PCP binding sites in rat brain

The pharmacological characteristics and the regional distribution of [ 3H]3-OH-PCP (1-[1(3-hydroxyphenyl)-cyclohexyl]piperidine) binding were investigated in rat brain by quantitative autoradiography. Kinetic analysis of [ 3H]3-OH-PCP binding revealed fast and slow components, in the association and dissociation studies. The regional distribution of binding closely corresponded to those of binding sites labeled by [ 3H] N-[1-(2-thienyl)-cyclohexyl]3,4-piperidine (TCP) and [ 3H](+)-5-methyl-10,11-dihydro-5 H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK 801). High densities of [ 3H]3-OH-PCP binding sites were found in the stratum radiatum and oriens of field CA1 in the hippocampus and in the outer layers of cerebral cortices. In contrast, low levels of binding were seen in the brain stem and the granular cell layer of the cerebellum. [ 3H]3-OH-PCP binding was strongly inhibited by MK 801 and 3-OH-PCP, while the potency of (+)-SKF 10047 in inhibiting [ 3H]3-OH-PCP binding was less in the cerebral cortex and hippocampus. The antagonists for the glutamate, glycine and polyamine recognition sites at the NMDA/PCP receptor complex displaced [ 3H]3-OH-PCP binding sites with a potency similar to that of [ 3H]MK 801. These findings suggest that the [ 3H]3-OH-PCP binding site is similar or identical to the PCP binding site labeled by [ 3H]TCP and [ 3H]MK 801.

Cerebrospinal fluid and serum concentrations of the N-methyl-d-aspartate (NMDA) receptor antagonist memantine in man

Memantine is a uncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist with therapeutic potential in dementia, spasticity and Parkinson's disease. The Ki-value of memantine at the phencyclidine (PCP) binding site of the NMDA receptor is 0.5 μM in human frontal cortex. We investigated whether concentrations of mementine in cerebrospinal fluid (CSF) and serum samples under therapeutic conditions are in the range of its Ki-value at the PCP binding site. The serum levels ranged from 0.025 to 0.529 μM with daily doses between 5 and 30 mg. CSF levels were highly correlated to serum levels and were below serum levels in each patient with a mean CSF/serum ratio of 0.52. Serum and CSF levels were correlated to the daily dose, but not to the duration of treatment. At the concentrations reported here, memantine is expected to specifically interact with the PCP binding site of the NMDA receptor.

Therapeutic brain concentration of the NMDA receptor antagonist amantadine

Amantadine (1-amino-adamantane) is clinically used for the management of Parkinson's disease and drug-induced extrapyramidal symptoms. It has previously been shown that amantadine is a low-affinity uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist with rapid blocking and unblocking channel kinetics (K i-value at the PCP binding site = 10 μM). The aim of the present studies was to estimate concentrations of amantadine in the central nervous system under therapeutic conditions. In homogenates of postmortem human brain tissue the amantadine concentration appeared to be homogeneously distributed over a wide range of brain areas. Amantadine concentration increased with duration of treatment and decreased with drug-free time. When the duration of treatment was ≥ 10 days and drug-free time ≤ 3 days, mean amantadine concentrations in postmortem brain tissue ranged from 48.2 to 386 μM. In contrast to brain tissue, amantadine concentration in cerebrospinal fluid (CSF) and serum was in the low micromolar range (< 17 μM). CSF and serum total values were highly correlated to each other and were always lower in CSF. The mean CSF/serum ratio for total amantadine was 0.76. To further estimate the extracellular concentration, amantadine was determined in microdialysates in the rat striatum. At behaviorally active doses, amantadine concentration in striatal microdialysates ranged between 6 and 21 μM. These results indicate that extracellular concentrations of amantadine (CSF and serum values in patients, striatal microdialysates in the rat) are in the range of its Ki-value at the PCP binding site. Amantadine concentrations in brain tissue are much higher, probably due to intralysosomal accumulation.