Dextromethorphan attenuates trimethyltin-induced neurotoxicity via sigma1 receptor activation in rats.

We showed that dextromethorphan (DM) provides neuroprotective/anticonvulsant effects and that DM and its major metabolite, dextrorphan, have a high-affinity for sigma(1) receptors, but a low affinity for sigma(2) receptors. In addition, we found that DM has a higher affinity than DX for sigma(1) sites, whereas DX has a higher affinity than DM for PCP sites. We extend our earlier findings by showing that DM attenuated trimethyltin (TMT)-induced neurotoxicity (convulsions, hippocampal degeneration and spatial memory impairment) in rats. This attenuation was reversed by the sigma(1) receptor antagonist BD 1047, but not by the sigma(2) receptor antagonist ifenprodil. DM attenuates TMT-induced reduction in the sigma(1) receptor-like immunoreactivity of the rat hippocampus, this attenuation was blocked by the treatment with BD 1047, but not by ifenprodil. These results suggest that DM prevents TMT-induced neurotoxicity, at least in part, via sigma(1) receptor stimulation.

Inhibition of voltage-gated Ca2+ channels by antazoline.

We showed recently that imidazolines exert neuroprotection against hypoxia and NMDA toxicity in cerebellar and striatal neuronal cultures, through a voltage-dependent blockade of glutamatergic NMDA receptors. Here, we report that in striatal neuronal cultures from mouse embryos the imidazoline compound, antazoline, inhibits voltage-gated Ca2+ channels by acting at a phencyclidine-like site. This effect was fast, fully reversible, voltage-dependent and predominant on P/Q- and N-type Ca2+ channels. Taken together, these results suggest that imidazolines may elicit neuroprotective effects also by decreasing the release of glutamate through inhibition of presynaptic Ca2+ channels.

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.

Differential expression of glutamate receptor subtypes in human brainstem sites involved in perinatal hypoxia-ischemia.

This study delineates the development of N-methyl-D-aspartate (NMDA) and non-NMDA receptor binding in the human brainstem, particularly as it relates to issues of the trophic effects of glutamate, the glutamate-mediated ventilatory response to hypoxia, and regional excitotoxic vulnerability to perinatal hypoxia-ischemia. We used tissue autoradiography to map the development of binding to NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionate (AMPA), and kainate receptors in brainstem sites involved in the glutamate ventilatory response to hypoxia, as well as recognized sites vulnerable to perinatal hypoxia-ischemia. NMDA receptor/channel binding was virtually undetectable in all regions of the human fetal brainstem at midgestation, an unexpected finding given the trophic role for NMDA receptors in early central nervous system maturation in experimental animals. In contrast, non-NMDA (AMPA and kainate) receptor binding was markedly elevated in multiple nuclei at midgestation. Although NMDA binding increased between midgestation and early infancy to moderately high adult levels, AMPA binding dramatically fell over the same time period to low adult levels. High levels of kainate binding did not change significantly between midgestation and infancy, except for an elevation in the infant compared with fetal inferior olive; after infancy, kainate binding decreased to negligible adult levels. Our data further suggest a differential development of components of the NMDA receptor/channel complex. This baseline information is critical in considering glutaminergic mechanisms in human brainstem development, physiology, and pathology.

The low affinity PCP sites in the rat cerebellum not only bind TCP-like but also BTCP-like structures.

Congeners of the potent dopamine (DA) re-uptake inhibitor 1-[1-(2-benzo[b]thiophenyl)cyclohexyl]piperidine (BTCP) are unexpectedly able to bind in the rat cerebellum, although this structure is devoid of dopaminergic nerve endings. In line with previous studies the hypothesis that they bind to low affinity PCP sites labelled with [3H]TCP in the rat cerebellum, even though they do not bind to the high affinity PCP sites in the forebrain, was considered. Analogues of 1-[1-(2-thiophenyl)cyclohexyl]piperidine (TCP) and BTCP with a modified aromatic moiety and with O or S atoms substituted in the cyclohexyl ring were prepared and tested in competition experiments both in rat forebrain and cerebellum membranes labelled with [3H]TCP, and in rat striatum membranes labelled with [3H]BTCP. Results indicated that BTCP and congeners could bind to low affinity PCP sites labelled with [3H]TCP in the rat cerebellum with a decrease of the selectivity for the DA transporter. On the contrary, some TCP analogues displayed a very high selectivity for these low affinity sites; they might be important pharmacological tools to elucidate the nature and function at yet unknown of these sites.

[Atypical antipsychotic profiles of sigma receptor ligands].

This is a review on the recent results of research on sigma-receptor antagonists. NE-100, a selective sigma1-receptor antagonist, shows improvement of abnormal behaviors and cognitive dysfunction induced by phencyclidine (PCP). However, NE-100 does not inhibit dopamine agonist-induced behaviors nor induces catalepsy. The mode of action of NE-100 is estimated to be the indirect modulation of the NMDA/PCP-receptor ion channel complex and the modulation of dopamine release from the dopaminergic nerve terminals. The recently reported MS-355/MS-377, which is also a selective sigma1-receptor antagonist, has a similar pharmacological profiles as NE-100, but in addition, MS-355/MS-377 inhibits methamphetamine-induced formation of reversal tolerance and also inhibits apomorphine-induced climbing behavior like dopamine D2-receptor antagonists. The report on clinical trial targeting schizophrenia shows results on rimcazole, remoxipride, BMY 14802, panamesine (EMD 57445) and SL 82.0715. Rimcazole was effective in the open study, but the double blind trial was discontinued due to seizure induction. Remoxipride showed efficacy different from those of dopamine D2-receptor antagonists (less extrapyramidal adverse effects), but the trial was discontinued due to occurrence of aplastic anemia. Panamesine and SL 82.0714 showed favorable efficacy in the open studies, but BMY 14802 showed no efficacy in clinical trials.

Novel sigma receptor ligands attenuate the locomotor stimulatory effects of cocaine.

Cocaine interacts with sigma receptors, suggesting that these sites are important for many of its behavioral effects. Therefore, two novel sigma receptor ligands, BD1008 (N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1-pyrrolidinyl)ethylamine) and BD1063 (1-[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine), were evaluated for their ability to attenuate cocaine-induced locomotor activity. Receptor binding studies showed that BD1008 and BD1063 have nanomolar affinities for sigma1 and sigma2 sites, but a 250-fold or lower affinity for nine other receptors, making them among the most selective sigma receptor ligands identified. In behavioral studies, pretreatment of mice with BD1008 or BD1063 produced a two-fold increase in the ED50 for the locomotor stimulatory effects of cocaine. These results suggest that sigma receptors are involved in the behavioral effects of cocaine.

Effect of PCP and sigma ligands on both noradrenaline- and electrically-induced contractions and on [3H]-noradrenaline uptake in rat vas deferens.

1. Electrically induced contractions of the epididymal portion of rat vas deferens were potentiated in concentration-dependent manner (0.1-30 microM) by different sigma and PCP receptor ligands (PCP, TCP, (+)-MK-801, dextromethorphan and (+)-3-PPP); dextrorphan did it in a minor extent. 2. Sigma and PCP receptor ligands also potentiated the effect of noradrenaline, inducing a reduction of the noradrenaline EC50 value in the rat vas deferens. The rank order of potencies was: PCP > TCP > (+)-3-PPP > (+)-MK-801 > dextrorphan > > > dextrometorphan. 3. In contrast, haloperidol (1 microM), a sigma receptor ligand, inhibited both the neurogenic and noradrenaline-induced responses in this tissue. 4. The effect of PCP and sigma receptor ligands on noradrenaline uptake was evaluated. All compounds tested, including haloperidol, inhibited the tritiated noradrenaline incorporation to the tissue. IC50 values were in the micromolar range, between 1.09 microM for dextrophan and 18 microM for dextrometorphan. 5. It is concluded that a direct interaction with the noradrenaline uptake system is involved in the potentiating effect of some sigma and PCP receptor ligands in the epididymal portion of rat vas deferens.

The effects of sigma, PCP, and opiate receptor ligands in rats trained with ibogaine as a discriminative stimulus.

Although the mechanism of action of ibogaine, a hallucinogen that may be useful in the treatment of addiction, remains unknown, receptor binding studies suggest that ibogaine produces its effects via interactions with multiple receptor types. In addition to serotonergic receptors, which have been studied previously with respect to ibogaine, likely candidates include opiate, sigma (sigma), and phencyclidine (PCP) binding sites. In an attempt to determine which of these receptor interactions are involved in the in vivo effects of ibogaine, ligands for sigma, PCP, and opiate receptors were assessed for their ability to substitute for or to antagonize the ibogaine-induced discriminative stimulus (10 mg/kg I.P., 60 min presession) in Fischer-344 rats. Intermediate levels of generalization were observed with the subtype nonselective sigma ligands 3-(3-hydroxyphenyl)-N-(1-propyl)-piperidine [(+)-3-PPP] (69.0%) and 1,3-di(2-tolyl)guanidine (DTG) (73.5%) but not with the sigma1-selective agents (+)-N-allylnormetazocine [(+)-SKF 10,047] and (+)-pentazocine. These findings, along with observations that ibogaine has appreciable affinity for sigma2 receptors, suggest that these receptors may be involved in the ibogaine discriminative stimulus. With regard to opiate receptors, neither morphine, the prototypic mu agonist, nor kappa selective agonists (bremazocine,and U-50488) substituted for ibogaine. However, intermediate levels of generalization were observed with the mixed action opiates (-)-SKF 10,047 (78.9%), (+/-)-pentazocine (73.9%), nalorphine (70.4%), and diprenorphine (75.0%) indicating a potential role for opiate receptors in the ibogaine stimulus. Partial substitution was also observed with naltrexone (55.6%) but not with naloxone or the selective kappa antagonist nor-binaltorphimine (nor-BNI). These agents were largely ineffective as antagonists of the ibogaine cue, although naloxone produced a moderate but statistically significant antagonism (69.8%). In addition, naloxone produced complete antagonism of the ibogaine-appropriate responding elicited by both (-)-SKF 10,047 (19.7%) and nalorphine (25.8%), whereas the ibogaine-appropriate responding produced by diprenorphine was only partially antagonized (44.4%). The latter observations taken together with the finding that both nalorphine (>100 microM) and diprenorphine (30 microM) have extremely low affinity for sigma2 receptors, suggest that the ibogaine-appropriate responding produced by these agents is not mediated by sigma2 receptors. These findings imply that opiate effects may be involved in the ibogaine stimulus. In contrast to sigma2 and opiate receptors, ibogaine's reported interactions with NMDA receptors do not appear to be involved in its discriminative stimulus, as neither PCP nor MK-801 produced a significant level of ibogaine-appropriate responding. Thus, the present study offers evidence that unlike NMDA receptors, both sigma2 and opiate receptors may be involved in the ibogaine discriminative stimulus.

Characteristics of the NMDA receptor modulating hypoxia/hypoglycaemia-induced rat striatal dopamine release in vitro.

We investigated the functional characteristics of the NMDA receptor that modulates hypoxia/hypoglycaemia-induced striatal dopamine release. Dopamine release was detected by fast cyclic voltammetry in rat neostriatal slices. Four variables were measured: T(on) -- time from initiation of hypoxia/hypoglycaemia to the onset of dopamine release, Tpk -- time from onset to maximum, deltaDA/delta(t) -- rate of dopamine release and DAmax -- maximum extracellular dopamine concentration. In controls, T(on) = 164.9 +/- 1.7 s, Tpk = 20.9 +/- 0.9 s, deltaDA/delta(t) = 5.31 +/- 0.44 microM/s and DAmax = 79.1 +/- 2.5 microM (means +/- S.E.M., n = 203). Cis-4-(phosphonomethyl)piperidine-2-carboxylic acid (CGS 19755, 20 microM) lengthened, while N-methyl-D-aspartate (NMDA) (100 microM) shortened T(on). (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,1 0-imine hydrogen maleate (MK 801, 1 and 10 microM) and dextromethorphan (10 and 100 microM) increased Tpk and decreased DAmax. Neither glycine (100 microM), 7-chlorokynurenic acid (50 microM) nor 5-nitro-6,7-dichloro-1,4-dihydroquinoxaline-2,3-dione (ACEA 1021, 100 microM) had any effect although 7-chlorokynurenic acid blocked the effect of NMDA. Increasing [Mg2+] from 1.3 to 3.7 mM, increased Tpk and decreased deltaDA/delta(t). Dithiothreitol (1 mM) accelerated T(on) while 5.5-dithio-bis-(2-nitrobenzoic acid) (1 mM) delayed T(on). Neither drug affected Tpk, DAmax or deltaDA/delta(t). Neither spermidine (100 microM) nor arcaine (100 microM) affected T(on), Tpk or deltaDA/delta(t) although arcaine decreased DAmax. In conclusion, hypoxia/hypoglycaemia-induced dopamine release was influenced by an NMDA receptor although modulation of the glycine recognition site of the receptor was ineffective, as were agents acting at polyamine modulatory zones. These findings highlight differences between recombinant and native NMDA receptors and suggest caution in extrapolating molecular biology to functional studies.

Modulation of NMDA and dopaminergic neurotransmissions by sigma ligands: possible implications for the treatment of psychiatric disorders.

Sigma (sigma) receptors, improperly classified as belonging to the opiate receptor family when discovered in 1976, were subsequently confused with phencyclidine binding sites for several years. It's only recently, with the emergence of new selective ligands that their functional significance could be meaningfully addressed. Several subtypes of sigma receptors are present in high densities in the limbic structures as well as in motor-related areas of the CNS. Different lines of evidence suggest that a major role for sigma receptors might be to regulate the activity of the glutamatergic system via the modulation one of its subtype of receptor, the NMDA receptor. This modulation of the glutamatergic system could in turn interfere with the dopaminergic neurotransmission with which, however, sigma ligands could also interact directly. The potential involvement of sigma receptors in schizophrenia has been considered ever since their discovery. The initial suggestion to this respect emerged from the observation that several of the earliest sigma ligands induced psychotomimetic symptoms such as delusions, hallucinations and depersonalization. This link was later reinforced with the demonstration that several neuroleptics, such as haloperidol, have a high affinity for sigma receptors, whereas, some new molecules with a high affinity for sigma receptors, but a low affinity for dopaminergic receptors demonstrated a "neuroleptic-like" pharmacological profile. However, the therapeutic efficacy of selective sigma ligands in schizophrenia has not yet been established and it has even been suggested that sigma receptors might be responsible for some side effects of the classical neuroleptics. The possible implication of sigma receptors in affective disorders has also been suggested by reports showing that some antidepressant drugs have a high affinity for sigma receptors and that long-term treatments with anti- depressant drugs, even with those devoid of affinity for sigma receptors, modify their binding characteristics. In conclusion, indirect evidence suggests possible etiological and/or therapeutic roles for sigma receptors in some psychiatric disorders. However, despite several attempts, no clear indications of a therapeutic efficacy of sigma ligands has yet emerged. More selective ligands and fundamental studies on the respective role of the different subtypes of sigma receptors are needed before clear concepts can be formulated. p3

Anticonvulsant activity of novel derivatives of 2- and 3-piperidinecarboxylic acid in mice and rats.

The relative ability of derivatives of 2-piperidinecarboxylic acid (2-PC; pipecolic acid) and 3-piperidinecarboxylic acid (3-PC; nipecotic acid) to block maximal electroshock (MES)-induced seizures, elevate the threshold for electroshock-induced seizures and be neurotoxic in mice was investigated. Protective index (PI) values, based on the MES test and rotorod performance, ranged from 1.3 to 4.5 for 2-PC benzylamides and from < 1 to > 7.2 for 3-PC derivatives. PI values based on elevation of threshold for electroshock-induced seizures and rotorod performance ranged from > 1.6 to > 20 for both types of derivatives. Since preliminary data indicated that benzylamide derivatives of 2-PC displace [3H]1-[1-(2-thienyl)-cyclohexyl]piperidine (TCP) binding to the phencyclidine (PCP) site of the N-methyl-D-aspartate (NMDA) receptor in the micromolar range and such low affinity uncompetitive antagonists of the NMDA receptor-associated ionophore have been shown to be effective anticonvulsants with low neurological toxicity, the 2-PC derivatives were evaluated in rat brain homogenates for binding affinity to the PCP site. Although all compounds inhibited [3H]TCP binding, a clear correlation between pharmacological activity and binding affinity was not apparent. Select compounds demonstrated minimal ability to protect against pentylenetetrazol-, 4-aminopyridine- and NMDA-induced seizures in mice. Corneal and amygdala kindled rats exhibited different sensitivities to both valproic acid and the nonsubstituted 2-PC benzylamide, suggesting a difference in these two models. Enantiomers of the alpha-methyl substituted benzylamide of 2-PC showed some ability to reduce seizure severity in amygdala kindled rats.

Inhibition by ketamine of muscarinic acetylcholine receptor function.

Although ketamine's primary site of action appears to be the phencyclidine receptor on the N-methyl-D-aspartate (NMDA) receptor complex, additional activity on opiate and quisqualate receptors is suggested. Some phencyclidines have been shown to interact with muscarinic receptors, but this has not been determined for ketamine. We studied the interaction between ketamine and the m1 muscarinic receptor, the most prominent subtype in cortex and hippocampus. Receptors were expressed recombinantly in Xenopus oocytes, and intracellular Ca2+ release in response to the agonist acetyl-beta-methylcholine (MCh, 10(-6)M) was assessed by measuring charge movement through Ca(2+)-activated Cl- channels. Average responses to MCh were 4.1 +/- 0.7 microC. Ketamine inhibited responses to MCh, with complete inhibition at approximately 200 microM ketamine. The IC50 was 5.7 microM, (1.56 micrograms/mL), well within the clinically relevant concentration range. To demonstrate that intracellular signaling pathways and the Ca2+ activated Cl- channel were not affected by ketamine, we tested the effect of ketamine (365 microM) on currents induced by angiotensin II (10(-6) M) in oocytes expressing the AT1A angiotensin receptor. No inhibitory effect was noted. In summary, ketamine profoundly inhibits muscarinic signaling. This effect might explain some of the anticholinergic clinical effects of ketamine, both central (effects on memory and consciousness) and peripheral (prominent sympathetic tone, bronchodilation, mydriasis).

Tricyclic CNS active agents by intramolecular Oxa-Pictet-Spengler reaction.

Mitsunobu inversion of the (S)-configurated lactate (S)-7, which is prepared in four steps starting from (S)-tyrosine, leads to the (R)-configurated lactate (R)-7. The key step in the transformation of the enantiomeric lactates (S)-7 and (R)-7 into the benzomorphan analogous tricycles (R,S)-16a,b, (S,R)-16a,b, (S,S)-22, and (R,R)-22 is an intramolecular Oxa-Pictet-Spengler reaction: The amides (S)-13, (R)-13, (S)-19 and (R)-19, in which the carbonyl moiety-masked as an acetal-is linked to the 2-phenylethanol moiety, are cyclized to give the tricyclic amides (R,S)-15, (S,R)-15, (S,S)-21, and (R,R)-21, respectively. In a concentration of 100 microM both enantiomers of 16a, 16b, and 22 are not able to compete with 3H-(+)-MK 801 for the phencyclidine binding sites of NMDA receptors. In vivo, only (R,S)-16b and (S,S)-22 exhibit weak sedative and analgesic activity.

Alteration of [3H]MK-801 binding associated with the N-methyl-D-Aspartate receptor complex by acute ethanol in rat cortex and hippocampus in vitro.

We investigated the effect of ethanol on specific binding of [3H]MK-801 to the intrachannel phencyclidine (PCP) receptor site, as an index of change in the functional response of the N-methyl-D-Aspartate (NMDA)-associated ion channel. Saturation binding experiments were performed on synaptic membrane homogenates from adult rat cortex and hippocampus. [3H]MK-801 binding assays were conducted under conditions of basal, 10 microM glutamate, or 10 microM glutamate + 30 microM D-serine, with and without 50 or 100 mM ethanol. Association experiments of [3H]MK-801 binding (5 nM) were conducted under conditions of 0 or 10 microM glutamate, with varying concentrations of glycine (0.01, 0.10, and 10 microM) with and without 100 mM ethanol. Ethanol (50 and 100 mM) significantly decreased the percentage of high-affinity (open-channel state) MK-801 receptors with a concomitant increase in percentage of low-affinity receptors, but did not change high- and low-affinity constants of the two binding states. An ethanol-induced increase in the closed-channel receptor density in basal and activated conditions was suggested by the saturation experiments. Association experiments further explained this finding, in that ethanol (100 mM) significantly decreased fast component (open-channel) [3H]MK-801 binding in conditions of glycine (0.01-10 microM) only and activated conditions of glutamate + glycine (0.01-0.10 microM). However, the observed fast and slow kinetic rate constants of [3h]MK-801 binding, as well as total specific binding (fast + slow components), were not altered.(ABSTRACT TRUNCATED AT 250 WORDS)

Distinct neuroprotective profiles for sigma ligands against N-methyl-D-aspartate (NMDA), and hypoxia-mediated neurotoxicity in neuronal culture toxicity studies.

Substantiating evidence has raised the possibility that sigma ligands may have therapeutic potential as neuroprotective agents in brain ischemia. It has been suggested that the neuroprotective capacity of sigma ligands is related primarily to their affinity for the NMDA receptor complex and not to any selective action at the sigma binding site. However, sigma specific ligands, devoid of significant affinity for the NMDA receptor, are also neuroprotective via an inhibition of the ischemic-induced presynaptic release of excitotoxic amino acids. In the present study, we have investigated the potential neuroprotective effect of a comprehensive series of sigma ligands, with either significant (sigma/PCP) or negligible (sigma) affinity for the PCP site of the NMDA receptor, in order to delineate a selective sigma site-dependent neuroprotective effect. For this aim, we have employed two different neuronal culture toxicity paradigms implicating either postsynaptic-mediated neurotoxicity, (brief exposure of cultures to a low concentration of NMDA or Kainate) or pre- and postsynaptic mechanisms (exposure to hypoxic/hypoglycemic conditions). Only sigma ligands with affinity for the NMDA receptor [(+) and (-) cyclazocine, (+) pentazocine, (+) SKF-10047, ifenprodil and haloperidol] were capable of attenuating NMDA-induced toxicity whereas the sigma [(+)BMY-14802, DTG, JO1784, JO1783, and (+)3-PPP] and kappa-opioid [CI-977, U-50488H] ligands, with very low affinity for the NMDA receptor, were inactive. The rank order of potency, based on the 50% protective concentration (PC50) value, of sigma/PCP ligands against NMDA-mediated neurotoxicity correlates with their affinity for the PCP site of the NMDA receptor, and not with their affinity for the sigma site. In addition sigma/PCP, sigma or kappa-opioid ligands failed to attenuate kainate-mediated neurotoxicity. On the other hand, sigma/PCP, sigma and kappa-opioid ligands were potent inhibitors of hypoxia/hypoglycemia-induced neurotoxicity, although their neuroprotective potency did not correlate with their affinity for either the sigma or PCP binding sites. In conclusion, the ability of sigma and kappa-opioid ligands to attenuate hypoxia/hypoglycemia, but not NMDA or kainate-induced toxicity, suggests that these drugs exert their neuroprotective role by a predominantly presynaptic mechanism possibly by inhibiting ischemic-mediated glutamate release.

Quantitative autoradiographic localization of [3H]3-OH-PCP (1-(1(3-hydroxyphenyl)cyclohexyl)piperidine) binding sites in rat brain.

Binding of a novel radioligand, [3H]3-OH-PCP (1(1(3-hydroxyphenyl) cyclohexyl)piperidine), to N-methyl-D-aspartate (NMDA) receptor-coupled and -uncoupled PCP sites was investigated in the rat brain. The highest densities of [3H]3-OH-PCP binding were observed in the hippocampal formation, notably in the stratum radiatum and oriens of CA1 region, and dentate gyrus. There were relatively high levels of binding in the olfactory system, superficial layer of cortices, the amygdala and the thalamus. In contrast, lower levels of binding were found in the globus pallidus, cerebellum, and brain stem, except for the superior colliculus. These findings demonstrate that [3H]3-OH-PCP binds to discrete regions within the rat brain. Its distribution is consistent with autoradiographic localization of [3H]TCP and [3H]MK-801 binding sites in the rat brain, suggesting that [3H]3-OH-PCP binds to NMDA/PCP ion-channel complexes in preference to sigma sites.

Orphenadrine is an uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist: binding and patch clamp studies.

Orphenadrine has been used as an antiparkinsonian, antispastic and analgesic drug for many years. Here we show that orphenadrine inhibits [3H]MK-801 binding to the phencyclidine (PCP) binding site of the N-methyl-D-aspartate (NMDA)-receptor in homogenates of postmortem human frontal cortex with a Ki-value of 6.0 +/- 0.7 microM. The NMDA receptor antagonistic effects of orphenadrine were assessed using concentration- and patch-clamp techniques on cultured superior colliculus neurones. Orphenadrine blocked open NMDA receptor channels with fast kinetics and in a strongly voltage-dependent manner. The IC50-value against steady state currents at -70 mV was 16.2 +/- 1.6 microM (n = 6). Orphenadrine exhibited relatively fast, concentration-dependent open channel blocking kinetics (Kon 0.013 +/- 0.002 10(6) M-1S-1) whereas the offset rate was concentration-independent (Koff 0.230 +/- 0.004 S-1). Calculation of the ratio Koff/Kon revealed an apparent Kd-value of 17.2 microM which is nearly identical to the IC50 calculated at equilibrium.

Changes in muscarinic (M1 and M2 subtypes) and phencyclidine receptor density in the rat brain following trimethyltin intoxication.

The main purpose of this study was to determine whether one or both of the muscarinic receptor subtypes (M1 and M2) contributed to the total cholinergic receptor loss found in trimethyltin (TMT) treated rats and to assess the effect of TMT on phencyclidine (PCP) receptor density in several regions of the rat brain. The distribution and changes in muscarinic (M1 and M2) receptor and PCP receptor sites were analysed by means of quantitative autoradiography using [3H]quinuclidinyl benzilate (QNB) and [3H] N-(1-(2-thienyl) cyclohexyl) 3,4-piperidine (TCP) respectively. The results demonstrate a TMT induced decrease in [3H]QNB binding in a large number of brain regions particularly the hippocampal formation, for both M1 and M2 muscarinic receptor subtypes. There is also a decrease in [3H]TCP binding in several brain regions. The effects of TMT on PCP receptors suggest that TMT induced damage is not restricted to the cholinergic system and that N-methyl-D-aspartate (NMDA) receptors are also affected.