Is the discriminative stimulus produced by phencyclidine due to an interaction with N-methyl-D-aspartate receptors?

Discriminative Stimulus Effects of Abused Inhalants

Inhalants are a loosely organized category of abused compounds defined entirely by their common route of administration. Inhalants include volatile solvents, fuels, volatile anesthetics, gasses, and liquefied refrigerants, among others. They are ubiquitous in modern society as ingredients in a wide variety of household, commercial, and medical products. Persons of all ages abuse inhalants but the highest prevalence of abuse is in younger adolescents. Although inhalants have been shown to act upon a host of neurotransmitter receptors, the stimulus effects of the few inhalants which have been trained or tested in drug discrimination procedures suggest that their discriminative stimulus properties are mediated by a few key neurotransmitter receptor systems. Abused volatile solvent inhalants have stimulus effects that are similar to a select group of GABAA positive modulators comprised of benzodiazepines and barbiturates. In contrast the stimulus effects of nitrous oxide gas appear to be at least partially mediated by uncompetitive antagonism of NMDA receptors. Finally, volatile anesthetic inhalants have stimulus effects in common with both GABAA positive modulators as well as competitive NMDA antagonists. In addition to a review of the pharmacology underlying the stimulus effects of inhalants, the chapter also discusses the scientific value of utilizing drug discrimination as a means of functionally grouping inhalants according to their abuse-related pharmacological properties.

Excitotoxicity and stroke: identifying novel targets for neuroprotection

Excitotoxicity, the specific type of neurotoxicity mediated by glutamate, may be the missing link between ischemia and neuronal death, and intervening the mechanistic steps that lead to excitotoxicity can prevent stroke damage. Interest in excitotoxicity began fifty years ago when monosodium glutamate was found to be neurotoxic. Evidence soon demonstrated that glutamate is not only the primary excitatory neurotransmitter in the adult brain, but also a critical transmitter for signaling neurons to degenerate following stroke. The finding led to a number of clinical trials that tested inhibitors of excitotoxicity in stroke patients. Glutamate exerts its function in large by activating the calcium-permeable ionotropic NMDA receptor (NMDAR), and different subpopulations of the NMDAR may generate different functional outputs, depending on the signaling proteins directly bound or indirectly coupled to its large cytoplasmic tail. Synaptic activity activates the GluN2A subunit-containing NMDAR, leading to activation of the pro-survival signaling proteins Akt, ERK, and CREB. During a brief episode of ischemia, the extracellular glutamate concentration rises abruptly, and stimulation of the GluN2B-containing NMDAR in the extrasynaptic sites triggers excitotoxic neuronal death via PTEN, cdk5, and DAPK1, which are directly bound to the NMDAR, nNOS, which is indirectly coupled to the NMDAR via PSD95, and calpain, p25, STEP, p38, JNK, and SREBP1, which are further downstream. This review aims to provide a comprehensive summary of the literature on excitotoxicity and our perspectives on how the new generation of excitotoxicity inhibitors may succeed despite the failure of the previous generation of drugs.

Sigma-1 receptor function is critical for both the discriminative stimulus and aversive effects of the kappa-opioid receptor agonist U-50488H

The present study was undertaken to identify possible similarities between the effects of kappa-opioid receptor agonist, N-methyl-D-aspartate-receptor antagonist, and sigma receptor agonist on the discriminative stimulus effects of U-50488H, and the possible involvement of sigma receptors in the discriminative stimulus and aversive effects of U-50488H. The kappa-opioid receptor agonist U-50488H produced significant place aversion as measured by the conditioned place preference procedure, and this effect was completely abolished by treatment with the putative sigma-1 receptor antagonist NE-100. In addition, phencyclidine (+)-SKF-10047 and (+)-pentazocine, which are sigma receptor agonists, generalized to the discriminative stimulus effects of U-50488H in rats that had been trained to discriminate between U-50488H (3.0 mg/kg) and saline. Furthermore, NE-100 significantly attenuated the discriminative stimulus effects of U-50488H and the U-50488H-like discriminative stimulus effects of phencyclidine. These results suggest that the sigma-1 receptor is responsible for both the discriminative stimulus effects and aversive effects of U-50488H.

Role of training dose in drug discrimination: a review

Drug discrimination has been an important technique in behavioural pharmacology for at least 40 years. The characteristics of drug-produced discriminative stimuli are influenced by behavioural and pharmacological variables, including the doses used to establish discriminations. This review covers studies on the effects of varying the training dose of a drug in a search for general principles that are applicable across different drug classes and methodological approaches. With respect to quantitative changes, relationships between training dose and the rate of acquisition or magnitude of stimulus control were found for most drug classes. Acquisition accelerated with dose up to a point beyond which drug-induced impairments of performance had a deleterious impact. Sensitivity to the training drug as measured by ED(50) values typically increased when the training dose was reduced. Qualitative changes were more complex and appeared to fall into three categories: (a) changes in profiles of generalization between partial and full agonists; (b) reduced specificity of some discriminations at small training doses; and (c) changes in the relative salience of actions mediated through different neurotransmitter systems or from central and peripheral sites. Three-lever discrimination procedures incorporating 'drug versus drug' or 'dose versus dose' contingencies enabled detection of more subtle differences than the simple 'drug versus no drug' approach when applied to the opioid, hallucinogen and barbiturate classes of drugs. These conclusions have implications for the interpretation of data from studies that use either within-subject or between-subject designs for studying the discriminative stimulus effects of drugs.

Generalization of NMDA-receptor antagonists to the discriminative stimulus effects of kappa-opioid receptor agonists U-50,488H, but not TRK-820 in rats

Generalizations of NMDA-receptor antagonists to the discriminative stimulus effects of kappa-opioid receptor agonists in rats were examined. Phencyclidine, MK-801, and ketamine, non-competitive NMDA-receptor antagonists, generalized to the discriminative stimulus effects of U-50,488H, but not those of TRK-820, whereas (+/-)-3-(2-carbaxypiperazine-4-yl) propyl-1-phosphonic acid (CPP), a competitive NMDA-receptor antagonist, and ifenprodil, an NR1/NR2B NMDA-receptor antagonist, did not, suggesting that non-competitive NMDA-receptor antagonists possess U-50,488H-like discriminative stimulus effects in rats. Since U-50,488H and phencyclidine both induce aversive effects, our findings indicate that the cue of the discriminative stimulus effects of U-50,488H and non-competitive NMDA-receptor antagonists may be associated with their aversive effects.

Evaluation of the phencyclidine-like discriminative stimulus effects of novel NMDA channel blockers in rats

RATIONALE

Because of their potential therapeutic effects, N-methyl-D-aspartate (NMDA) receptor antagonists have been investigated for clinical use. Unfortunately, many channel-blocking antagonists have been associated with the production of side effects, including motor impairment and phencyclidine (PCP)-like subjective effects.

OBJECTIVE

This study investigated the relationship between NMDA receptor channel blockade and production of PCP-like side effects by evaluating a variety of NMDA channel blockers with different binding characteristics for the production of PCP-like discriminative stimulus effects.

METHODS

The NMDA channel blockers were tested in rats trained to discriminate 2 mg/kg PCP, i.p., from saline using a standard two-lever drug discrimination procedure with responding under a fixed ratio (FR) 32 schedule of food reinforcement.

RESULTS

The high-affinity channel blockers PD 138289, PD 137889 and FR 115427, produced full, dose-dependent substitution for PCP. Of the moderate-affinity channel blockers, MRZ 2/579 fully substituted for PCP while 1-(4-methoxyphenyl)-1,2,3,4-tetrahydroisoquinoline, 8-(2-methoxyphenyl)-1,2,3,4-tetrahydroisoquinoline and alaproclate produced partial substitution. Drugs with the lowest affinity for the channel site and/or higher affinity for non-NMDA CNS sites, antazoline, idazoxan, 1-phenyl-1,2,3,4-tetrahydroisoquinoline, alpha-benzyl- N-methylphenethylamine and orphenadrine, failed to substitute for PCP.

CONCLUSIONS

The results demonstrate that the cellular actions of the individual channel-blocking NMDA antagonists, in particular affinity for the channel site and NMDA receptor specificity, are important determinants of their discriminative stimulus effects. While higher affinity channel blockers show a correlation between affinity and PCP-like discriminative stimulus effects, behavioral disruption through action at non-NMDA receptors probably prevents achieving sufficient concentrations of the lower affinity compounds at NMDA receptors to produce PCP-like discriminative stimulus effects.

Role of the NMDA receptor NR2B subunit in the discriminative stimulus effects of ketamine

The noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, ketamine, is a dissociative anesthetic with antihyperalgesic properties. However, its clinical use is compromised by psychotomimetic side-effects. As ketamine and other noncompetitive NMDA antagonists, such as phencyclidine and dizocilpine, are not selective for the NR2A-2D subunits of the NMDA receptor, it is unclear which of these subunits is responsible for the psychotomimetic side-effects. This study investigated the role of the NR2B subunit in the ketamine drug discrimination model, a possible correlate for such side-effects. In a first experiment aimed at assessing general potency and time dependency, ketamine, dizocilpine, phencyclidine and the NR2B-selective antagonists ifenprodil and Ro 25-6981, dose-dependently suppressed fixed ratio 10 food-reinforced responding in rats, with peak efficacy obtained around 15-40 min. In rats trained to discriminate ketamine from vehicle in a two-lever fixed ratio 10 food-reinforced procedure, ketamine, dizocilpine, phencyclidine and Ro 25-6981 induced complete generalization (>80%); whereas ifenprodil induced partial generalization (33%). These findings suggest that the NR2B subunit is involved in the discriminative stimulus effects of noncompetitive NMDA antagonists, and that selective NR2B antagonists may also induce psychotomimetic side-effects.

Involvement of dopaminergic system in the nucleus accumbens in the discriminative stimulus effects of phencyclidine

The effects of microinjection of phencyclidine (PCP) and dizocilpine, non-competitive NMDA receptor antagonists, and dopamine into the nucleus accumbens 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. Microinjection of PCP (2-40 microg) and dizocilpine (2-12 microg) into the bilateral nucleus accumbens produced a dose-dependent increase in PCP-appropriate responding and fully substituted for systemically administered PCP, whereas microinjection of dopamine (1-4 microg) did not produce PCP-like discriminative stimulus effects. The performance of PCP discrimination was assessed after bilateral destruction of the dopaminergic nerve neurons in the nucleus accumbens with dopaminergic neurotoxin, 6-hydroxydopamine (6-OHDA, 4 microg/1 microl/side). The destruction of dopaminergic nerve neurons in the nucleus accumbens failed to prevent the performance of PCP discrimination. There was no difference in the average percentages of PCP-appropriate responding between vehicle and 6-OHDA-treated rats in the dose-response tests. These results suggest that the dopaminergic system in the nucleus accumbens does not play a critical role in the discriminative stimulus effects of PCP.

NMDA receptor antagonists as analgesics: focus on the NR2B subtype

Ifenprodil and a group of related compounds are selective antagonists of NR2B-containing NMDA receptors. These compounds are antinociceptive in a variety of preclinical pain models and have a much lower side-effect profile compared with other NMDA receptor antagonists. It remains unclear whether the improved safety of these compounds is due to their subtype selectivity or to a unique mode of inhibition of the receptor. Human trials have so far confirmed the good tolerability of these subtype-selective NMDA receptor antagonists; however, whether they are as effective as other NMDA receptor antagonists in pain patients remains to be demonstrated.

A putative sigma1 receptor antagonist NE-100 attenuates the discriminative stimulus effects of ketamine in rats

Ketamine, one of the dissociative anaesthetic agents, has been shown to produce psychotomimetic effects. It has been well documented that activation of sigma receptors is responsible for the pathogenesis of some psychiatric disorders. In the present study, the effects of NE-100, a putative sigma(1) receptor antagonist, was investigated in rats trained to discriminate between ketamine (5 mg/kg, i.p.) from saline under a fixed-ratio 10 food-reinforced procedure. Here we report for the first time that NE-100 (1 mg/kg) produced a shift to the right in the dose-response curve for ketamine's discriminative stimulus effects. These results suggest that the sigma(1) receptor is, at least in part, involved in the discriminative stimulus effects of ketamine.

Supraspinal vs spinal sites of the antinociceptive action of the subtype-selective NMDA antagonist ifenprodil

The N-methyl-D-aspartate (NMDA) antagonist ifenprodil and several structurally related compounds are highly selective for the NR2B-containing receptor subtype. This selectivity could provide an explanation for the reported difference of the analgesic and side-effect profile of ifenprodil-like compounds from other NMDA antagonists. In this work, we have queried if the ifenprodil-induced antinociception can be attributed to the block of NMDA receptors in the spinal cord. Ifenprodil and some other NMDA antagonists (MK-801, memantine) were tested in a model of inflammatory pain (Randall-Selitto) in rats. The in vivo NMDA antagonism was assessed in anaesthetised rats on responses of spinal dorsal horn (DH) neurones to iontophoretic NMDA and in the model of single motor unit (SMU) wind-up. Ifenprodil, MK-801 and memantine dose-dependently increased nociceptive thresholds in the Randall-Selitto model. Antinociceptive doses of the channel blockers selectively antagonised NMDA responses of DH neurones and inhibited wind-up. In contrast, antinociceptive doses of ifenprodil did not show any NMDA antagonism in electrophysiological tests. Although ifenprodil did not inhibit the SMU responses to noxious stimuli in spinalised rats, it markedly and dose-dependently inhibited nociceptive SMU responses in sham-spinalised rats. These results argue against the spinal cord being the principal site of antinociceptive action of ifenprodil; supraspinal structures seem to be involved in this effect.

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.

N-methyl-D-aspartate antagonists and drug discrimination

Excitatory amino acids (EAA), such as glutamate, are thought to be involved in various disorders (e.g., ischemic brain damage, epilepsy, Parkinson's disease), and EAA antagonists have been suggested as potential treatments for these disorders. Phencyclidine (PCP), with produces psychotomimetic effects in humans, has antagonist properties at the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors that have been suggested to underlie some of its actions. This suggestion, and concern about possible psychotomimetic activity, has stimulated research aimed at examining to what extent the behavioral profile of other NMDA antagonists resembles that of PCP. Drug discrimination (DD) is prominent among the procedures used to carry out such comparisons. The results of clinical studies with NMDA antagonists provide feedback about the predictive validity of the DD procedures used to characterize their preclinical behavioral profile. Further, DD is used also to examine the ability of compounds to attenuate the discriminative stimulus (DS) effects of PCP-type drugs, and results of such studies have been suggested to provide evidence of antipsychotic potential. Finally, although many instances of intermediate responding in DD can be explained by low efficacy at the receptors that mediate the DS effects of the training drug, certain outcomes produced by PCP-type drugs do not offer valid measures of efficacy, and require more detailed behavioral analyzes.

An allosteric interaction between the NMDA receptor polyamine and ifenprodil sites in rat cultured cortical neurones

1. The atypical NR2B subunit-selective NMDA receptor antagonist ifenprodil was originally believed to act as a competitive antagonist at the polyamine binding site of the NMDA receptor. However, a number of studies have suggested that ifenprodil might bind to a distinct site. 2. Using whole-cell voltage clamp recordings, we have studied the interaction of spermine with both ifenprodil and the related NR2B selective antagonist Ro 8-4304 at the NMDA receptor in rat cultured cortical neurones in the presence of saturating concentrations of glycine. 3. Ifenprodil and Ro 8-4304 inhibited steady-state currents evoked by 100 microM NMDA in the absence of spermine with IC50 values of 0.3 and 0.6 microM, respectively. In the presence of 1 and 3 mM spermine, IC50 values for ifenprodil were 1.4 and 1.8 microM and for Ro 8-4304 they were 3. 0 and 7.5 microM, respectively. 4. In the presence of spermine, the on-time constant of receptor blockade by both antagonists was significantly slower than control and the off-time constant of recovery from receptor blockade following removal of Ro 8-4304 was significantly faster. 5. Fast application of spermine during an NMDA steady-state current in the continuous presence of a subsaturating concentration of either antagonist resulted in a biphasic increase in the current, consistent with a fast increase upon spermine binding and a slow increase resultant from dissociation of antagonist due to spermine binding-induced allosteric reduction in receptor antagonist affinity. In agreement with this, at higher, saturating concentrations of antagonist, the slow increase in current amplitude was markedly reduced or absent. 6. These observations are consistent with a non-competitive, allosteric interaction between spermine and the antagonists, such that spermine binding to the NMDA receptor results in a reduction in receptor affinity for the antagonists and vice versa. 7. The effects of Mg2+ on the NMDA-evoked currents and its interaction with ifenprodil were similar to those of spermine, supporting the suggestion that Mg2+ might be the physiological ligand acting at the spermine site mediating glycine-independent stimulation.

Phencyclidine-like discriminative stimulus effects of polyamine modulators of N-methyl-D-aspartate receptor activity in rats

Excessive N-methyl-D-aspartate (NMDA) receptor activation has been implicated in many acute and chronic neuropathologies. NMDA antagonists might prove to be useful treatments, unfortunately, some can produce phencyclidine (PCP)-like side effects. The polyamine-site modulators, spermine (SPM) and spermidine (SPD), produce dose related biphasic modulation of NMDA channel currents while another polyamine, arcaine (ARC), produces only negative modulatory effects. The PCP-like effects of these compounds were tested in rats trained to discriminate PCP from saline in a standard two-lever drug discrimination paradigm under a fixed ratio schedule of food reinforcement. SPM, SPD and ARC occasioned little, if any, responding on the PCP-associated lever, even at response rate suppressing doses. The results provide further evidence that differences exist between the discriminative stimulus effects produced by drugs active at different sites on the NMDA receptor and suggest that the polyamine modulatory site should be a good target for development of NMDA antagonist medications with a reduced propensity for PCP-like acute behavioral effects.

State-dependent NMDA receptor antagonism by Ro 8-4304, a novel NR2B selective, non-competitive, voltage-independent antagonist

1. Subunit-selective blockade of N-methyl-D-aspartate (NMDA) receptors provides a potentially attractive strategy for neuroprotection in the absence of undesirable side effects. Here, we describe a novel NR2B-selective NMDA antagonist, 4-¿3-[4-(4-fluoro-phenyl)-3,6-dihydro-2H-pyridin-1-yl]-2-hydroxy-propoxy ¿-benzamide (Ro 8-4304), which exhibits >100 fold higher affinity for recombinant NR1(001)/NR2B than NR1(001)/NR2A receptors. 2. Ro 8-4304 is a voltage-independent, non-competitive antagonist of NMDA receptors in rat cultured cortical neurones and exhibits a state-dependent mode of action similar to that described for ifenprodil. 3. The apparent affinity of Ro 8-4304 for the NMDA receptor increased in an NMDA concentration-dependent manner so that Ro 8-4304 inhibited 10 and 100 microM NMDA responses with IC50s of 2.3 and 0.36 microM, respectively. Currents elicited by 1 microM NMDA were slightly potentiated in the presence of 10 microM Ro 8-4304, and Ro 8-4304 binding slowed the rate of glutamate dissociation from NMDA receptors. 4. These results were predicted by a reaction scheme in which Ro 8-4304 exhibits a 14 and 23 fold higher affinity for the activated and desensitized states of the NMDA receptor, respectively, relative to the agonist-unbound resting state. Additionally, Ro 8-4304 binding resulted in a 3 4 fold increase in receptor affinity for glutamate site agonists. 5. Surprisingly, whilst exhibiting a similar affinity for NR2B-containing NMDA receptors as ifenprodil, Ro 8-4304 exhibited markedly faster kinetics of binding and unbinding to the NMDA receptor. This spectrum of kinetic behaviour reveals a further important feature of this emerging class of NR2B-selective compounds.

The NMDA/glycine receptor antagonist, L-701,324, produces discriminative stimuli similar to those of ethanol

The ethanol-like discriminative stimulus properties of a novel NMDA glycine receptor antagonist, L-701,324 ((7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-2-(1H)-quinolone), a polyamine receptor antagonist, eliprodil, and a non-competitive NMDA receptor antagonist, MK-801 (dizocilpine), were examined in rats trained to discriminate ethanol from vehicle in a two-lever discrimination procedure. In rats trained to discriminate ethanol from vehicle, L-701,324 and MK-801 substituted for ethanol in a dose-dependent fashion with a complete substitution noted following administration of 7.5 mg/kg L-701,324 and 0.2 mg/kg MK-801, respectively. Full substitution for ethanol was achieved with no alteration in the rate of responding. In contrast, administration of eliprodil (in doses up to 5 mg/kg) showed only a partial, but not dose-dependent, substitution for ethanol. These findings indicate that a reduction of NMDA receptor activity, produced either via a blockade of non-competitive NMDA recognition sites or of NMDA/glycine-sensitive regulatory sites, had discriminative stimulus properties that are similar to those produced by ethanol. Furthermore, the observation that the NMDA/glycine receptor antagonist, L-701,324, was a more effective substitute for ethanol than was the polyamine antagonist, eliprodil, suggests that several NMDA receptor subunits, and thus not only NMDAR2B receptor subunits, are of importance for the discriminative stimulus effects of ethanol.

A novel mechanism of activity-dependent NMDA receptor antagonism describes the effect of ifenprodil in rat cultured cortical neurones

1. Ifenprodil is a selective, atypical non-competitive antagonist of NMDA receptors that contain the NR2B subunit with an undefined mechanism of action. Ifenprodil is neuroprotective in in vivo models of cerebral ischaemia but lacks many of the undesirable side-effects associated with NMDA antagonist. 2. Using whole-cell voltage-clamp recordings, we have studied the mechanism of inhibition of NMDA-evoked currents by ifenprodil in rat cultured cortical neurones in the presence of saturating concentrations of glycine. 3. Ifenprodil antagonized NMDA receptors in an activity-dependent manner, whilst also increasing the receptor affinity for glutamate recognition-site agonists. Ifenprodil inhibition curves against 10 and 100 microM NMDA-evoked currents yielded IC50 values of 0.88 and 0.17 microM, respectively. Thus, the apparent affinity of ifenprodil for the NMDA receptor is increased in an NMDA concentration-dependent manner. 4. Currents evoked by 0.3 and 1 microM NMDA were potentiated to approximately 200% of control levels in the presence of 3 microM ifenprodil. Thus, with increasing concentration of NMDA the effect of ifenprodil on NMDA-evoked currents changed from one of potentiation to one of increasing inhibition. 5. These results are predicted by a reaction scheme in which ifenprodil exhibits a 39- and 50-fold higher affinity for the agonist-bound activated and desensitized states of the NMDA receptor, respectively, relative to the resting, agonist-unbound state. Furthermore, ifenprodil binding to the NMDA receptor results in a 6-fold higher affinity for glutamate site agonists. 6. This represents a novel mechanism of NMDA receptor antagonism that, together with the subunit selectivity, probably contributes to the attractive neuropharmacological profile of this and related compounds.

Inhibition of morphine tolerance by NMDA receptor antagonists in the formalin test

5-Nitro-6,7-dimethyl-1,4-dihydro-2,3-quinoxalinedione (ACEA-1328) was characterized in vitro for antagonism of excitatory amino acid receptors, and subsequently tested in vivo and compared with MK-801 for phencyclidine (PCP)-like motor stimulation, antinociception, and effects on morphine tolerance in mice. Assayed on rat cerebral cortical glutamate receptors expressed in Xenopus oocytes ACEA-1328 showed potent (Kb approximately 40 nM) antagonism at NMDA receptor/glycine sites and moderate (Kb approximately 3 microM) antagonism at non-NMDA receptors. In both cases inhibition was predominantly competitive. ACEA-1328 was weak, or inactive, at NMDA receptor glutamate recognition sites, metabotropic receptors and opioid binding sites. In the formalin and rotarod tests ACEA-1328 and MK-801 produced both antinociception and disturbances of motor coordination. MK-801 caused a PCP-like motor stimulatory effect, whereas ACEA-1328 was devoid of such an effect. In tolerance studies, ACEA-1328 and MK-801 each blocked morphine tolerance in the formalin test, the effect of ACEA-1328 was dose-dependent. Our data contribute to a growing body of evidence which suggests that activation of NMDA receptors is critical for the development of opioid tolerance, and that antagonism at NMDA receptor/glycine sites may have potential as a means of diminishing tolerance with no PCP-like motor stimulatory side effects.

Dizocilpine prevents the development of tolerance to ethanol-induced error on a circular maze test

Dizocilpine [(+)MK-801] and ketamine, in doses that disrupt learning and memory, also prevent the development of tolerance to the motor impairing effects of ethanol (EtOH). However, dizocilpine itself affects motor behavior. In order to separate the possible influence of these two effects on the development of tolerance to EtOH, food-reinforced performance on a circular maze test was used in two different experiments. EtOH alone (1.2 g/kg) tended to increase the error score and reduce number of runs per trial, running speed, and total distance run, but on chronic administration of EtOH, tolerance developed progressively to all these effects. Dizocilpine also increased the error score, but had a biphasic effect on measures of running: low and intermediate doses (0.009 and 0.075 mg/kg, IP) increased running distance, whereas a high dose (0.15 mg/kg) decreased running speed and distance. When combined with EtOH, dizocilpine tended to overcome the effect of EtOH on running activity, but not on error score. Chronically, dizocilpine (0.075 and 0.15 mg/kg) prevented the development of tolerance to the effect of EtOH on error score, even though the lower dose of dizocilpine permitted tolerance to the effects of EtOH on running. These results suggest that NMDA receptor antagonists selectively inhibit tolerance to cognitive effects of ethanol even when the antagonists do not affect motor performance.

Dizocilpine-like discriminative stimulus effects of low-affinity uncompetitive NMDA antagonists

The dizocilpine-like discriminative stimulus effects of a variety of channel blocking (uncompetitive) N-methyl-D-aspartate (NMDA) receptor antagonists were examined in rats trained to discriminate dizocilpine (0.17 mg/kg, i.p) from saline in a two-lever operant procedure. The dissociative anesthetic-type NMDA antagonists dizocilpine (ED50 0.05 mg/kg), phencyclidine (ED50 3.4 mg/kg) and ketamine (ED50 14 mg/kg) showed complete substitution without producing significant decreases in response rates, whereas dexoxadrol (ED50 4.3 mg/kg) also produced complete substitution with a concomitant decrease (35%) in response rate. Similarly, the low-affinity antagonist memantine resulted in complete substitution (ED50 9.7 mg/kg) at doses that significantly reduced (68%) the response rate. All other low-affinity antagonists resulted in either partial or no substitution for the discriminative stimulus effects of dizocilpine at doses that significantly decreased average response rates. These include (ED50 values in parentheses) remacemide (29 mg/kg), the remacemide metabolite 1,2-diphenyl-2-propylamine (ARL 12495) (14 mg/kg), phencylcyclopentylamine (25 mg/kg), dextromethorphan (46 mg/kg), (+/-)-5-aminocarbonyl-10,11-dihydro -5H-dibenzo-[a,d]cyclohepten-5,10-imine (ADCI; no substitution) and levoxadrol (no substitution). We conclude that low-affinity uncompetitive NMDA antagonists have discriminative stimulus properties distinct from dissociative anesthetic-type uncompetitive NMDA antagonists. The lowest-affinity antagonists show virtually no substitution for dizocilpine, whereas the relatively more potent low-affinity antagonists (such as memantine) exhibit greater substitution, but complete substitution is obtained only at rate-reducing doses.

Competitive and uncompetitive N-methyl-D-aspartate antagonist discriminations in pigeons: CGS 19755 and phencyclidine

The purpose of the present studies was to examine representative uncompetitive and competitive NMDA antagonists, as well as the glycine/NMDA antagonist, HA 966, in pigeons trained to discriminate either PCP or CGS 19755 from saline. Separate groups of pigeons were trained to discriminate either the uncompetitive, phencyclidine (PCP; 0.32 and 1.0 mg/kg, IM), or the competitive, CGS 19755 (cis-4-phosphonomethyl-2-piperidine-carboxylic acid; 1.8 mg/kg, IM), NMDA antagonists from saline. Uncompetitive and competitive NMDA antagonists were examined in generalization studies, as were the racemate and the (+) and (-) stereoisomers of HA 966 (3-amino-1-hydroxypyrrolid-2-one). Dizocilpine (MK 801) was fully generalized to PCP but not to CGS 19755. All competitive NMDA antagonists tested were fully generalized to CGS 19755, but not to PCP. The competitive antagonists, however, produced > 50% PCP-appropriate responding. The (+) isomer of HA 966 was fully generalized by three of four pigeons discriminating PCP (1.0 mg/kg) or CGS 19755, whereas the racemate and the (-) isomer produced < 40% drug-appropriate responding in either group. Neither NMDA, morphine, nor pentobarbital produced > 10% drug-appropriate responding in either discrimination group. The competitive antagonists tended to produce peak drug-appropriate responding at times greater than 60 min after administration, whereas uncompetitive antagonists produced peak drug-appropriate responding at earlier times. HA 966 also had a relatively slow onset of action as compared to PCP. These results suggest that antagonists acting at different modulatory sites of the NMDA receptor complex produce similar, but not identical, discriminative stimuli.

Pharmacological heterogeneity of NMDA receptors in cerebellar granule cells in immature rat slices. A microfluorimetric study with the [Ca2+]i sensitive dye Indo-1

We have developed a technique for monitoring the internal calcium concentration--[Ca2+]i--in a single selected cell in cerebellar slices of 8-day-old rats. In post-migratory granule cells NMDA induced a rapid, reversible and concentration-dependent increase in [Ca2+]i (+22% at 10 microM and +210% at 100 microM). This effect was dependent on the presence of Ca2+ in the extracellular medium. When Mg2+ was absent from the perfusion buffer the sensitivity to NMDA was greatly increased (+108% at 10 microM). The NMDA response was not affected by glycine site agonists or by tetrodotoxin, a sodium channel blocker. In the absence of magnesium, the NMDA (10 microM)-induced increase in [Ca2+]i was prevented in a monophasic manner by the recognition site antagonist 2-amino-phosphonovalerate (2-APV; IC50 = 13 microM), and in a biphasic manner by the glycine site antagonist 7-chlorokynurenate (IC50s = 25 nM and 5.9 microM) and by the channel blocker dizocilpine (IC50s = 5 nM and 3 microM). In contrast, this NMDA response was only partially antagonized by the polyamine site antagonists ifenprodil and eliprodil (maximal inhibition approximately 50% at concentrations > or = 10 microM, IC50 1 and 2 microM, respectively). These results demonstrate the presence in granule cells from immature rat cerebellum of at least two NMDA receptor populations which can be differentiated by their sensitivity to inhibitors acting upon the different sites of the NMDA receptor complex.

Tolerance to and dependence on MK-801 (dizocilpine) in rats

Rats were trained to respond under a fixed-ratio 30 schedule for food presentation during four daily 0.5-h sessions occurring every 6 h. After stable baseline response was established, osmotic minipumps were implanted that infused vehicle or (+)-5 methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine hydrogen maleate (dizocilpine; MK-801), SC. Behavioral sessions continued to be conducted daily. After 10 days the infusion pumps were removed. Vehicle and 0.10 mg/kg per day MK-801 did not affect behavior during infusions or after cessation of dosing. Dosing with 0.32 and 0.56 mg/kg per day initially suppressed responding, but tolerance developed to these effects. After the infusions were stopped, a dose-dependent disruption of operant behavior occurred. Response rates for the 0.32 and 0.56 mg/kg per day infusion groups were suppressed to 41 and 27% of preinfusion control response rates, respectively, the day after dosing stopped; however, no physical signs of abstinence were observed. Response rates recovered toward control over the next 2-4 days. In a separate experiment, the suppression of response produced by abstinence from 0.32 mg/kg per day of MK-801 (SC) for 10.5 days was reversed by readministration of MK-801 (IP). These results demonstrate that MK-801 produces dependence, as evidenced by the emergence of a behavioral abstinence syndrome after cessation of dosing.

The pharmacology of SDZ EAA 494, a competitive NMDA antagonist

SDZ EAA 494 (D-CPPene) was characterized as a competitive NMDA antagonist, having a pA2 value against NMDA depolarizations in frog spinal cord and rat neocortex of 6.7-6.8 and a pKi of 7.5 in a [3H]CGP39653 binding assay, with no action on other receptors or amine reuptake. The compound was orally active in rodent maximal electroshock models with an ED50 of around 16 mg/kg, was protective in rats even 24 hours after oral application and had an oral therapeutic index of around 8. Muscle relaxation, ataxia, flattened body posture and reduced acquisition of a passive avoidance task, suggesting potential effects on memory formation, occurred at supra-anticonvulsant doses in rodents, with PCP-like stimulatory effects produced only by high i.p. doses or constant i.v. infusions. This favourable profile is discussed in relation to the negative outcome of a recent trial of the compound in patients with intractable epilepsy. The conclusion is drawn that standard models for screening new anticonvulsants are inappropriate to seeking drugs active in patients with a protracted convulsive history. The anti-ischaemic action of SDZ EAA 494 encourages further testing in brain trauma, in which the anticonvulsant action of the compound may be an added benefit.

Evaluation of the reinforcing effects of eliprodil in rhesus monkeys and its discriminative stimulus effects in rats

Two studies examined the abuse potential of the N-methyl-D-aspartate (NMDA) non-competitive antagonist eliprodil [(+/-)-alpha-(4-chlorophenyl)-4-[(fluorophenyl)methyl]-1- piperidineethanol] by evaluating its reinforcing effects in rhesus monkeys and its phencyclidine (PCP)-like discriminative stimulus effects in rats. The monkeys were trained to self-administer PCP i.v. under a fixed ratio 10 schedule of reinforcement. After the monkeys were trained, saline, vehicle and various doses of eliprodil were substituted for PCP. The rats were trained to discriminate 3 micrograms/kg PCP from saline using a standard two-lever discrimination procedure with correct-lever responding reinforced under a fixed ratio 10 schedule of food reinforcement. After acquiring the discrimination, the rats were tested with various doses of PCP, dizocilpine and eliprodil. The self-administration study showed that eliprodil did not have reinforcing effects, since it maintained injection rates comparable to the negative controls, saline and vehicle. In the discrimination study it was found that the higher doses of PCP and dizocilpine resulted in 100% PCP-associated lever responding, whereas eliprodil occasioned no responding on the PCP-associated lever. The results from these studies suggest that eliprodil has a low potential for abuse in humans as well as providing further evidence that eliprodil produces a profile of behavioral effects unlike the PCP-site selective NMDA antagonists.

Glutamate antagonists have different effects on spontaneous locomotor activity in rats

Locomotor activity, ataxia, and stereotypy were assessed in the open field after administration of NMDA and AMPA antagonists acting by different mechanisms. The interaction with glutamatergic receptors was confirmed in the binding assay. (+)MK-801 and phencyclidine (PCP) produced similar changes in horizontal activity, i.e., a strong increase from the beginning of the test. Ketamine, and to a lesser extent, memantine, enhanced horizontal activity at the later observation periods only. Amantadine and NBQX produced a slight inhibition, while GYKI-52466, d-cycloserine, (+R)-HA-966, CGP-37849, and dextromethorphan were ineffective. Vertical activity (rearings) were inhibited by most agents except GYKI-52466 and gly-B partial agonists. At higher doses ataxia was seen after: MK-801, PCP, ketamine, memantine, amantadine, CGP-37849, dextromethorphan, and GYKI-52466. Hence, the inhibition of NMDA and AMPA receptors by agents acting at different recognition sites produces qualitatively different behavioral consequences.

Sensitive and rapid behavioral differentiation of N-methyl-D-aspartate receptor antagonists

Behavioral effects of PCP-type noncompetitive antagonists of N-methyl-D-aspartate (NMDA) receptors overlap with those of a host of other centrally acting compounds. In the present experiment, locomotor activity and performance on an inverted screen test in untrained mice were used to differentiate PCP-type non-competitive NMDA antagonists from other drug classes. These uncompetitive NMDA antagonists [PCP, dizocilpine, (-)-MK-801, TCP, (+)-SKF 10,047, dextrorphan, ketamine] produced dose-related increases in locomotor activity and the percentage of mice falling off an inverted, elevated wire mesh screen. Both effects demonstrated stereoselectivity, occurred at comparable dose levels, and were within the range of doses producing other biological effects (e.g., anticonvulsant). The potencies of these drugs for producing behavioral effects were positively correlated with affinities for PCP ([3H]MK-801) but not sigma([3H]SKF 10,047) receptors. Although muscarinic antagonists (benactyzine, atropine) produced effects in the same direction, locomotor stimulation was small and occurred at lower doses than those inducing screen failures. Competitive NMDA antagonists (LY 274614, LY 233536, CPP, NPC 12626), sigma receptor ligands (DTG, dextromethorphan), postsynaptic dopamine agonists (quinpirole, SKF 38393) and antagonists (haloperidol, SCH 39166), and some depressant compounds (morphine, diazepam) increased failures on the screen test but decreased locomotor activity. Ligands of the polyamine regulatory site of the NMDA receptor (ifenprodil, SL 82.0715-10) and the AMPA receptor antagonist NBQX decreased locomotor activity without increasing screen failures. An antagonist of the strychnine-insensitive glycine receptor (7-chlorokynurenic acid) did not affect performance on either test. Psychomotor stimulants (cocaine and methamphetamine) stimulated locomotor activity without affecting screen performance. The only false positives occurred with barbiturates (pentobarbital, phenobarbital). Nonetheless, the present procedure demonstrates excellent sensitivity and power for rapid discrimination of uncompetitive NMDA antagonists.

NMDA receptors with different sensitivities to magnesium and ifenprodil control the release of [14C]acetylcholine and [3H]spermidine from rat striatal slices in vitro

KCl (20-100 mM) and N-methyl-D-aspartate (NMDA, 100-1,000 microM) produce concomitant concentration-dependent increases in the release of previously captured [14C]acetylcholine and [3H]spermidine from rat striatal slices in vitro. The effects of NMDA (300 microM) on striatal [14C]acetylcholine and [3H]spermidine release were blocked with equal potencies by the competitive NMDA antagonist CGP 37849, the glycine site antagonist L-689,560, and the NMDA channel blocker dizocilpine. In contrast, although NMDA-evoked [14C]acetylcholine release was antagonized by ifenprodil (IC50 = 5.3 microM) and MgCl2 (IC50 = 200 microM), neither compound antagonized the NMDA-evoked release of [3H]spermidine at concentrations up to 100 microM (ifenprodil) or 1 mM (MgCl2). Distinct NMDA receptor subtypes with different sensitivities to magnesium and ifenprodil therefore exist in the rat striatum.

An investigation into the discriminative stimulus and reinforcing properties of the CCKB-receptor antagonist, L-365,260 in rats

The discriminative stimulus properties of the selective CCKB-receptor antagonist, L-365,260 were evaluated in rats trained to discriminate diazepam (2 mg/kg) or morphine (5 mg/kg) from vehicle, using a two-lever food reinforced technique. In the diazepam drug discrimination, the benzodiazepine-receptor agonist FG8205 (0.063-2 mg/kg) produced dose-related drug associated responding, whereas L-365,260 (0.125-4 mg/kg) treated animals showed vehicle appropriate behaviour. In rats trained to discriminate morphine from saline, L-365,260 (0.063-4 mg/kg) produced saline lever responding. When a dose of 1 mg/kg L-365,260 was administered in combination with morphine, the dose response curve for drug lever responding was not significantly affected. This was in contrast to the effect produced by the opiate antagonist naloxone (0.3 mg/kg) which shifted the dose-response curve to the right. Another group of rats underwent training to discriminate a dose of 6 mg/kg L-365,260 from vehicle. None of the animals learned the discrimination within 50 daily training sessions. In addition, unlike morphine (3 mg/kg), or changing the training dose of cocaine, intravenous administration of L-365,260 (0.3-10 mg/kg) did not modify lever pressing or the number of injections received by rats trained to self administer cocaine (0.25 mg/injection). L-365,260 (0.1-3 mg/kg) produced a dose-related inhibition of pentagastrin-stimulated gastric acid secretion in vivo. When administered dissolved in a mixture of ethanol/propylene glycol/saline, the ID50 was 0.83 mg/kg, and when suspended in an ethanol/carboxymethylcellulose vehicle, it was 0.7 mg/kg. It was concluded: 1) that L-365,260 does not produce discriminative stimuli similar to either diazepam or morphine; 2) that the potentiation of morphine-induced behaviour by L-365,260 does not extend to the discriminative stimulus properties of morphine; 3) that L-365,260 itself does not produce readily discriminable interoceptive stimuli in rats; and 4) that L-365,260 does not substitute for the reinforcing drug cocaine.

Common and uncommon behavioural effects of antagonists for different modulatory sites in the NMDA receptor/channel complex

The behavioural significance of the N-methyl-D-aspartate (NMDA) receptor/channel complex was investigated in rats, using different types of antagonists including D-2-amino-phosphonovaleric acid, (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid, 2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid, 7-Cl kynurenate, ifenprodil, phencyclidine (PCP), MK-801, ketamine, (+/-)-N-allylnormetazocine, dextromethorphan, ZnCl2, and MgCl2. All antagonists produced an impairment of learning and muscle relaxation when administered i.c.v. Frequent circling was characteristically produced by ligands for PCP binding sites and was unaffected by concomitant administration of a competitive NMDA receptor antagonist. These results suggest that the NMDA receptor/channel plays an important role in regulating learning/memory processes and muscle tone, whereas PCP binding sites unassociated with the NMDA channel are involved in the production of characteristic circling behaviour.

Effects of NMDA receptor antagonists in squirrel monkeys trained to discriminate the competitive NMDA receptor antagonist NPC 12626 from saline

Because excitatory amino acids have been implicated in several physiological phenomena, antagonists of excitatory amino acid function may have significant therapeutic potential as anticonvulsants, neuroprotectants and anxiolytics. Drug discrimination procedures in animals have proven useful to compare and contrast the behavioral effects of site-selective NMDA antagonists. In the only previous study using a competitive NMDA antagonist as a training drug, rats were trained to discriminate NPC 12626 (2-amino-4,5-(1,2-cyclohexyl)-7-phosphonohepatanoic acid) from nondrug. The major goal of the present study was to establish and characterize a nonhuman primate model of NPC 12626 discrimination. Adult male squirrel monkeys were trained to discriminate NPC 12626 from saline under a two-lever fixed ratio-30 schedule of food reinforcement. The monkeys required between 80 and 120 training sessions to acquire this discrimination after the training dose had been raised from 3 to 20 mg/kg i.m. The competitive NMDA antagonists CGP 37849 (D,L-(E)-2-amino-4-methyl-5- phosphono-3-pentanoic acid) and CPPene (D-3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid) substituted completely for NPC 12626, while the potent noncompetitive NMDA antagonist, dizocilpine (MK-801), did not. These results reflect a profile of discriminative stimulus effects which support that observed in rats and establish a primate model for use in further study of the behavioral effects of the competitive NMDA antagonists.

Contrasting effects of the competitive NMDA antagonist CPP and the non-competitive NMDA antagonist MK 801 on performance of an operant delayed matching to position task in rats

The effects of the competitive NMDA antagonist CPP and the non-competitive NMDA antagonist MK 801 (dizolcipine) on short term working memory in the rat were investigated. The behavioural paradigm used was discrete trial, operant delayed matching to position, as originally described by Dunnett (1985), with delays of 0, 5, 15 and 30 s. These delays generated an orderly "forgetting" curve in control rats, with matching accuracy decreasing from approximately 100% at 0-s delay to approximately 75% at 30-s delay. Intraperitoneal (IP) administration of CPP (10 mg/kg) produced a marked delay dependent impairment in performance, suggesting a specific effect on short term working memory. This effect was accompanied by a minor decrease in the speed of responding, and a slight increase in the number of missed trials. Lower doses of CPP had no significant effects on either matching accuracy or sedation. In contrast, IP administration of MK 801 (0.1 and 0.2 mg/kg) caused a marked delay independent impairment in the accuracy of delayed matching performance, suggesting a non-specific disruption of performance. A lower dose (0.05 mg/kg) of MK 801 had no significant effect on matching accuracy. The two lower doses of MK 801 increased the number of nose pokes made during the delays and tended to increase the speed of responding, suggesting a stimulant-like action. The highest dose of MK 801 had the opposite effects and also decreased the number of trials completed.(ABSTRACT TRUNCATED AT 250 WORDS)

Drug discrimination analysis of ethanol as an N-methyl-D-aspartate receptor antagonist

Ethanol has been shown to antagonize N-methyl-D-aspartate (NMDA) receptor-mediated neurotransmission in a number of in vitro systems. Drug discrimination procedures in rats were used to evaluate ethanol as an antagonist of NMDA discrimination and for its ability to produce discriminative stimulus effects similar to those of competitive and noncompetitive NMDA antagonists. Ethanol (300-1500 mg/kg i.p.) failed to antagonize the stimulus effects of 30 mg/kg NMDA, nor did it substitute fully for either the competitive antagonist NPC 12626 nor the noncompetitive antagonist phencyclidine (PCP). A maximum average of 55.4% PCP-lever responding provided evidence for partial substitution in this model. The effects of ethanol on NMDA discrimination are distinct from those previously reported for competitive NMDA antagonists but similar to those of noncompetitive antagonists. On the other hand, ethanol can be distinguished from both competitive and PCP-like noncompetitive NMDA antagonists using drug discrimination procedures.

SL 82.0715, an NMDA antagonist acting at the polyamine site, does not induce neurotoxic effects on rat cortical neurons

In the present study, we have examined by light and electron microscopy whether SL 82.0715, a polyamine site-directed N-methyl-D-aspartate (NMDA) antagonist, causes pathological changes in cerebrocortical neurons similar to those observed with NMDA receptor channel blockers in the rat brain. Dizocilpine (1, 2 and 5 mg.kg-1, s.c.) induced a dose-dependent vacuolization of the neuronal cytoplasm in specific neurons of the retrosplenial and posterior cingulate cortices (layers III and IV) even at the lowest dose studied, at 6 h post-injection. In contrast, SL 82.0715 (10 and 30 mg.kg-1 i.p., 6 h post-injection) did not induce such morphological alterations. These results indicate that NMDA receptor blockade is not necessarily associated with alterations of cortical neuronal morphology.

Effects of competitive N-methyl-D-aspartate antagonists on midbrain dopamine neurons: an electrophysiological and behavioral comparison to phencyclidine

Electrophysiological and behavioral methods were used to evaluate and compare the effects of the competitive N-methyl-D-aspartate (NMDA) receptor blocker, NPC 12626, with the non-competitive NMDA antagonist, phencyclidine (PCP), on the activity of mesolimbic dopamine neurons. NPC 12626 (50 mg/kg, i.p.) produced a degree of locomotor hyperactivity comparable to that seen with PCP (5 mg/kg). However, 6-hydroxydopamine lesions of the nucleus accumbens blocked the PCP-induced hyperactivity but not the behavioral activation evoked by NPC 12626. Single-unit extracellular recordings from ventral tegmental A10 dopamine neurons also found marked differences between the competitive and non-competitive NMDA antagonists. Intravenous injections of NPC 12626 and CGS 19755 in doses up to 60 mg/kg failed to change A10 activity. This was in contrast to the striking bimodal dose-dependent increase-decrease in firing rate elicited by PCP. The absence of an effect of NPC 12626 on A10 neurons was not evidently related to a lack of access to central sites since NPC pretreatment (40 mg/kg, i.v.) completely antagonized the neurotoxicity caused by intrastriatal injection of quinolinic acid, an NMDA agonist, but not that caused by the non-NMDA compound, kainic acid. Thus, competitive NMDA antagonists do not share PCP's properties of activating mesolimbic dopaminergic systems, and as such they may be devoid of the potent psychotomimetic effects or the abuse liability associated with non-competitive NMDA receptor blockers such as PCP.

[3H]1-[2-(2-thienyl)cyclohexyl]piperidine labels two high-affinity binding sites in human cortex: further evidence for phencyclidine binding sites associated with the biogenic amine reuptake complex

Previous work demonstrated two high-affinity PCP binding sites in guinea pig brain labeled by [3H]TCP (1-(1-[2-thienyl]cyclohexyl)piperidine): site 1 (N-methyl-D-aspartate [NMDA]-associated) and site 2 (dopamine-reuptake complex associated). The present study examined brain membranes prepared from various species, including human, for the presence of site 2, defined as binding in the presence of (+)-5-methyl-10,11-dihydro-5H-dibenzo [a, d]cyclohepten-5,10-imine maleate ((+)-MK801) minus binding in the presence of 10 microM TCP (nonspecific binding). Studies were conducted in absence of sodium which was found to be inhibitory to [3H]TCP binding. The results demonstrated detectable levels of site 2 in brain membranes of guinea pig, rabbit, pig, mouse, sheep, and human but not in the rat or chicken. Using human cortical membranes, site 2 was the predominant binding site. Detailed studies conducted with human cortical tissue showed that high-affinity dopamine (1-[2- [bis(4-fluorophenyl)-methoxy]ethyl]-4-(3-phenylpropyl)piperazine (GBR12909)], [1,2]benzo(b)thiophenylcyclo-hexylpiperidine (BTCP), and serotonin (fluoxetine) uptake inhibitors produced a wash-resistant inhibition of [3H]TCP binding to site 2, but not site 1. Preincubation of guinea pig brain membranes with BTCP was shown to produce an increase in the dissociation rate of [3H]TCP from PCP site 2. Structure activity studies with various uptake inhibitors showed that GBR12909, benztropine, fluoxetine, and BTCP have higher affinity for site 2 than for site 1. (+)-MK801, ketamine, and tiletamine were very selective for site 1, whereas dexoxadrol and TCP were moderately selective for site 1. These results suggest that human cortex possesses high-affinity PCP binding sites associated with biogenic reuptake binding sites, and that guinea pig brain, but not rat brain, may be an appropriate animal model for studying PCP site 2 in human brain.

Pharmacological specificity of the phencyclidine discriminative stimulus in rats

The discriminative stimulus effects of phencyclidine (PCP), pentobarbital and the competitive N-methyl-D-aspartate antagonist 3-([+/-]-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) were examined in rats trained to discriminate PCP from saline under a 2-lever, food-maintained operant schedule. Dose-response curves were obtained for all three drugs at a PCP training dose of 1.25 mg/kg; subsequently, rats were retrained to discriminate either 0.56 or 3.0 mg/kg PCP. The dose-response to PCP was not substantially changed by raising or lowering the training dose. However, doses of pentobarbital and CPP produced augmented levels of substitution when the training dose was lowered and decreased substitution when it was raised. The changes in PCP training dose were, therefore, effective in either diminishing or amplifying the pharmacological specificity of the PCP stimulus. Under conditions where specificity was high (high training dose), neither pentobarbital (0.1-17 mg/kg) nor CPP (1-17 mg/kg) produced appreciable PCP-like stimulus effects, supporting evidence that competitive NMDA antagonists may be no more PCP-like than are barbiturates. These data provide additional evidence for differences in the behavioral effects of noncompetitive and competitive NMDA antagonists.

NMDA receptor antagonists suppress behaviors but not norepinephrine turnover or locus coeruleus unit activity induced by opiate withdrawal

Pretreatment with the non-competitive NMDA (N-methyl-D-aspartate) antagonist MK801 (0.5, 1.0 mg/kg, s.c.) suppressed the behavioral signs of withdrawal in morphine-dependent rats. However, the same doses of MK801 that suppressed morphine withdrawal also simultaneously produced phencyclidine (PCP)-like behaviors. Pretreatment with the competitive NMDA antagonist LY274614 (25, 50, 100 mg/kg i.p.) also suppressed the behavioral signs of withdrawal in morphine-dependent rats but did not produce PCP-like behavioral effects. Single unit recordings were made from noradrenergic neurons in the locus coeruleus (LC) and, at doses that suppressed morphine withdrawal behaviors, neither MK801 nor LY274614 blocked the withdrawal-induced activation of LC neurons. Biochemical analysis indicated that, at the same behaviorally relevant doses, neither MK801 nor LY274614 blocked the withdrawal-induced increase in norepinephrine turnover in the hippocampus, cerebral cortex, or hypothalamus. These results indicate that NMDA antagonists attenuate the behavioral signs of morphine withdrawal without blocking the withdrawal-induced increase in norepinephrine turnover or the withdrawal-induced increase in LC unit activity. In addition, non-competitive NMDA antagonists, like MK801, may not be useful to alleviate opiate withdrawal symptoms in man because of their PCP-like side effects. However, competitive NMDA antagonists, like LY274614, could be of great benefit for alleviating opiate withdrawal symptoms in man.

Effects of NMDA receptor antagonists and sigma ligands on the acquisition of conditioned fear in mice

Recent studies have shown that several compounds known to act as competitive or non-competitive antagonists of NMDA receptors can disrupt learning in rodents. The present study was carried out to investigate the effects of a range of NMDA antagonists, acting at several sites in the NMDA receptor complex, on the acquisition of learned fear in mice. Dose-related disruptions of learning were produced by the non-competitive antagonists phencyclidine, dizocilpine, dextromethorphan and (+) and (-)N-allylnormetazocine. The (+) enantiomer of N-allylnormetazocine was approximately twice as potent as the (-) enantiomer. The competitive NMDA receptor antagonist, CGS 19755, also blocked the acquisition of learned fear as did the non-specific glutamate antagonist riluzole. In contrast, the anti-ischaemic drugs ifenprodil and SL 82.0715, which probably act as NMDA antagonists through an effect on the polyamine site, had no effect on learning up to doses which substantially reduced locomotion. The sigma receptor ligand DTG was also inactive. These results confirm that both competitive and non-competitive NMDA antagonists disrupt learning but indicate that the extent to which such an effect is observed may depend on the site at which the compounds act within the receptor complex. Activity at sigma receptors is unrelated to the effect on learning.

A rapid method for evaluating the behavioral effects of phencyclidine-like dissociative anesthetics in mice

A simple and rapid method for detecting the behavioral effects of phencyclidine and related dissociative anesthetics is described. Dissociative anesthetics such as phencyclidine (PCP) and dizolcipine, which bind with high affinities at N-methyl-D-aspartate (NMDA) coupled cation channels ("PCP receptors"), produced a dose-related increase in the percentage of mice that fell from a 1.5 cm deep circular arena mounted on a 60 cm platform. A similar behavior was not manifest by other classes of compounds examined including competitive NMDA antagonists, an antagonist at strychnine-insensitive glycine receptors, and sigma-receptor ligands with moderate to low affinities for PCP receptors. Pretreatment of mice with glycine reduced in a dose-dependent manner the percentage of falls elicited by a maximally effective dose of dizolcipine. This simple procedure may prove useful for both the rapid detection of dissociative anesthetics and evaluation of putative PCP antagonists.

The behavioral pharmacology of NMDA receptor antagonists

There is considerable interest in the development of NMDA antagonists as potential therapeutic agents in the treatment of convulsant, neurodegenerative and anxiety disorders. Because the clinical use of phencyclidine (PCP) has been precluded by its psychotomimetic effects and abuse potential, there has been concern that other NMDA antagonists including those acting competitively might produce similar untoward effects. However, the studies in animals, reviewed here by Joyce Willetts, Robert Balster and David Leander, suggest that while there are certain similarities in the behavioral effects of PCP-like and competitive antagonists, there are also differences. These differences have implications for the development of NMDA antagonists with less likelihood for producing PCP-like side-effects.

The discriminative stimulus properties of (+)-HA-966, an antagonist at the glycine/N-methyl-D-aspartate receptor

Using a two-lever operant drug discrimination paradigm, rats have been trained to discriminate between the administration of saline and R-(+)-HA-966 (R-(+)-3-amino-1-hydroxypyrrolid-2-one, 30 mg/kg i.p.) an antagonist at the glycine modulatory site on the N-methyl-D-aspartate (NMDA) receptor/ion channel complex. Drug-appropriate responding was not induced in stimulus generalisation experiments when the non-competitive NMDA receptor antagonist, phencyclidine (PCP, 1-8 mg/kg i.p.) was substituted for (+)-HA-966. Similarly, (+)-HA-966 (6-50 mg/kg i.p.) did not induce drug-appropriate responding in animals trained to discriminate PCP (3 mg/kg i.p.) from saline. The results suggest that the behavioural profile of compounds attenuating the actions of NMDA via blockade of the glycine modulatory site may be substantially different from those acting at the ion channel of the NMDA receptor complex.

Ethological analysis of the effects of MK-801 upon aggressive male mice: similarity to chlordiazepoxide

Previous studies have demonstrated benzodiazepine-like effects of competitive and noncompetitive antagonists of the N-methyl-D-aspartate (NMDA) type of excitatory amino acid receptor. The present experiment compared the effects of the benzodiazepine chlordiazepoxide and the NMDA noncompetitive antagonist MK-801 upon the behavior of aggressive male mice in dyadic interactions using ethological analysis. OF-1 male mice housed with females were administered either chlordiazepoxide (Vehicle, 3.0, 10.0 and 30.0 mg/kg IP) or MK-801 (Vehicle, 0.1, 0.3 and 0.3 mg/kg, IP) in a randomised order thirty minutes prior to pairing with unfamiliar male opponents in an unfamiliar environment. It was found that both compounds tended to increase aggressiveness and social behavior and reduce ambivalent activity consistent with approach-avoidance conflict. The increases in aggressiveness and decreases in ambivalent activity were induced by MK-801 at doses lower than those resulting in gross motor effects. These data confirm that noncompetitive antagonists of NMDA may generate a benzodiazepine-like behavioral profile.