Benz(F)isoquinolines as excitatory amino acid antagonists: An indication of their mechanism of action?

Glutamate in schizophrenia: Neurodevelopmental perspectives and drug development

Research into the neurobiological processes that may lead to the onset of schizophrenia places growing emphasis on the glutamatergic system and brain development. Preclinical studies have shown that neurodevelopmental, genetic, and environmental factors contribute to glutamatergic dysfunction and schizophrenia-related phenotypes. Clinical research has suggested that altered brain glutamate levels may be present before the onset of psychosis and relate to outcome in those at clinical high risk. After psychosis onset, glutamate dysfunction may also relate to the degree of antipsychotic response and clinical outcome. These findings support ongoing efforts to develop pharmacological interventions that target the glutamate system and could suggest that glutamatergic compounds may be more effective in specific patient subgroups or illness stages. In this review, we consider the updated glutamate hypothesis of schizophrenia, from a neurodevelopmental perspective, by reviewing recent preclinical and clinical evidence, and discuss the potential implications for novel therapeutics.

Ketamine and phencyclidine: the good, the bad and the unexpected

The history of ketamine and phencyclidine from their development as potential clinical anaesthetics through drugs of abuse and animal models of schizophrenia to potential rapidly acting antidepressants is reviewed. The discovery in 1983 of the NMDA receptor antagonist property of ketamine and phencyclidine was a key step to understanding their pharmacology, including their psychotomimetic effects in man. This review describes the historical context and the course of that discovery and its expansion into other hallucinatory drugs. The relevance of these findings to modern hypotheses of schizophrenia and the implications for drug discovery are reviewed. The findings of the rapidly acting antidepressant effects of ketamine in man are discussed in relation to other glutamatergic mechanisms.

New advances in NMDA receptor pharmacology

N-Methyl-D-aspartate (NMDA) receptors are tetrameric ion channels containing two of four possible GluN2 subunits. These receptors have been implicated for decades in neurological diseases such as stroke, traumatic brain injury, dementia and schizophrenia. The GluN2 subunits substantially contribute to functional diversity of NMDA receptors and are distinctly expressed during development and among brain regions. Thus, subunit-selective antagonists and modulators that differentially target the GluN2 subunit might provide an opportunity to pharmacologically modify the function of select groups of neurons for therapeutic gain. A flurry of clinical, functional and chemical studies have together reinvigorated efforts to identify subunit-selective modulators of NMDA receptor function, resulting in a handful of new compounds that appear to act at novel sites. Here, we review the properties of new emerging classes of subunit-selective NMDA receptor modulators, which we predict will mark the beginning of a productive period of progress for NMDA receptor pharmacology.

Comparative antipsychotic profiles of neurotensin and a related systemically active peptide agonist

Several lines of evidence have shown that neurotensin can modulate dopamine neurotransmission. It has been suggested that neurotensin has potential antipsychotic activity because it reduces dopaminergic activity preferentially in the nucleus accumbens. In the present study, the effects of neurotensin and NT1 (N alpha Me-Arg-Lys-Pro-Trp-TIe-Leu or Eisai hexapeptide), a metabolically stable and systemically active neurotensin agonist, were examined in several models of antipsychotic activity and side effect liability in mice; analgesic and hypothermic effects of both compounds also were determined. Up to high doses, neurotensin (5.0 and 10.0 micrograms, i.c.v.) and NT1 (10.0 and 20.0 mg/kg, i.p.) did not produce catalepsy. A much lower dose of neurotensin (0.03 microgram, i.c.v.) significantly reduced amphetamine- and phencyclidine-stimulated locomotor activity; NT1 also diminished amphetamine- and phencyclidine-stimulated locomotion with ED50 values of 0.3 and 0.4 mg/kg, i.p., respectively. Neurotensin (0.01-0.3 microgram, i.c.v.) and NT1 (0.1-1.0 mg/kg, s.c.) also produced dose-dependent analgesia in the paw pressure test and decreased body temperature; these effects were insensitive to pretreatment with naloxone (10.0 mg/kg, i.p.). Together, the results support the hypothesis that neurotensin agonists have antipsychotic and analgesic activity. Moreover, the data suggest that such compounds may not produce extrapyramidal side effects.

Dopamine receptor antagonists block amphetamine and phencyclidine-induced motor stimulation in rats

d-Amphetamine (DEX) and phencyclidine (PCP) increased motor activity in rats as measured in automated activity cages. Analysis of the stimulation indicated that both drugs increased horizontal activity (total activity), locomotion, and peripheral activity. However, DEX increased while PCP decreased the incidence of rearing. The ability of different drugs to antagonise DEX- and PCP-induced increases in total activity (called stimulation) was measured. Dopamine (DA) D1 receptor antagonists (SCH23390, NNC-01-0112) were 7-8 times more potent in blocking DEX than PCP. DA D2 receptor antagonists (raclopride, remoxipride, haloperidol) were only 1-2 times more potent against DEX-induced stimulation. Nonselective DA receptor antagonists were also tested. Chlorpromazine was more potent against DEX than against PCP. Buspirone and sertindole were slightly more potent in blocking PCP than DEX. Ritanserin (5-HT2 receptor antagonist) was inactive against both stimulants. 8-OH-DPAT (5-HT1A receptor agonist) potentiated the stimulant effects of DEX and PCP. Prazosin (alpha 1-adrenergic receptor antagonist) partially blocked both DEX and PCP. Most drugs tested depressed spontaneous motor activity. Remoxipride and sertindole, however, caused very little depression even at doses several times higher than those needed to block DEX or PCP. The data show clear pharmacological differences between DEX- and PCP-induced stimulation.

Anticonvulsant actions of phencyclidine receptor ligands: correlation with N-methylaspartate antagonism in vivo

1. Drugs with phencyclidine (PCP)-like activity in behavioural discrimination and [3H]PCP binding studies share anticonvulsant properties. 2. We have compared the rank order potency of a series of PCP-like compounds as N-methylaspartate (NMA) antagonists, determined from previously published studies from our laboratory, with their rank order anticonvulsant potencies as determined by two independent research groups in three different in vivo models of experimentally-induced epilepsy. 3. Rank order potency for NMA antagonism correlated well with rank order anticonvulsant potency. Furthermore, the systemic doses required for an effective blockade of NMA-evoked excitations were, in most cases, similar to those which produced anticonvulsant activity. 4. The results suggest that functional NMA antagonism may underlie the shared anticonvulsant properties of structurally dissimilar compounds with PCP-like activity.

Anticonvulsant effects of phencyclidine-like drugs: relation to N-methyl-D-aspartic acid antagonism

Various compounds that have been identified in the literature as binding to the [3H]phencyclidine receptor site and as producing behavioral effects similar to phencyclidine (phencyclidine-like) protected mice from maximal electric shock-induced tonic-extensor seizures. These anticonvulsant effects appear to be due to blockade of the N-methyl-D-aspartic acid receptor, as recently reported for phencyclidine-like compounds. Phencyclidine-like compounds produced their anticonvulsant effects at doses that were also neurologically impairing.

N-methyl-D-aspartic acid-induced lethality in mice: selective antagonism by phencyclidine-like drugs

N-Methyl-D-aspartic acid (NMDA) produced a dose-related increase in lethality in mice, with 200 mg/kg (i.p.) effecting 100% lethality. Upon daily dosing, acutely sublethal doses of NMDA produced deaths. This NMDA-induced lethality was stereoselective; N-methyl-L-aspartic acid had no effects at doses as high as 400 mg/kg. Moderate doses of phencyclidine (PCP) and drugs having PCP-like behavioral effects blocked the NMDA-induced lethality. Other classes of psychoactive drugs, including opioids, anticonvulsants and antipsychotics, were ineffective in preventing NMDA-induced lethality. The potency of PCP-like drugs to block the NMDA-induced lethality correlates highly with the dose necessary to produce PCP-like catalepsy and PCP-like discrimination in pigeons. These data support the hypothesis that PCP-like drugs produce many of their effects by impairing the normal functioning of the NMDA-defined excitatory neurotransmitter receptor in the central nervous system.

Anticonvulsant effects of dextrorphan in rats: possible involvement in dextromethorphan-induced seizure protection

The major metabolite of the non-opioid anticonvulsant/antitussive dextromethorphan is dextrorphan. In the present study, the effects of dextrorphan were determined in an experimental model of seizure activity (maximal electroshock convulsions) (MES). Subcutaneous administration of dextrorphan produced dose-related blockade of tonic hindlimb extension (THE) and a decrease in the duration of tonic forelimb extension (TFE). The anticonvulsant effect of dextrorphan was linear and maximally efficacious. Compared to the prototypical anticonvulsant drug diphenylhydantoin, dextrorphan was 2.5 times more potent (ED50's = 30 mumol/kg and 12 mumol/kg, respectively). Pretreatment with naloxone failed to antagonize dextrorphan-induced blockade of THE. Moreover, pretreatment with dextrophan failed to significantly enhance the anticonvulsant potency of diphenylhydantoin. It is likely that the anticonvulsant effects of dextrorphan are related to its actions at the phencyclidine/N-methyl-D-aspartate receptor complex, whereas the anticonvulsant effects of dextromethorphan have been attributed to binding to a specific dextromethorphan site in the brain. Therefore, we suggest that while metabolism to dextrorphan could possibly contribute to the anticonvulsant effects of dextromethorphan, it is probably through an unrelated receptor mechanism.

Phencyclidine. Physiological actions, interactions with excitatory amino acids and endogenous ligands

Phenycyclidine (PCP) produces many profound effects in the central nervous system. PCP has numerous behavioral and neurochemical effects such as inhibiting the uptake and facilitating the release of dopamine, serotonin, and norepinephrine. PCP also interacts with sigma, mu opioid, muscarinic, and nicotinic receptors. However, the psychotomimetic effects induced by PCP are believed to be mediated by specific PCP receptors, where PCP binds with greater potency than sigma compounds. Electrophysiological, behavioral, and neuro-chemical evidence strongly suggests that at least some of the many PCP actions result from antagonism of excitatory amino acid-induced responses via PCP receptors. The recent isolation and partial characterization of the alpha and beta endopsychosins and the identification of other endogenous ligands for the PCP and sigma receptors, is another promising area of research in the elucidation of the physiological role of an endogenous PCP and sigma system.

Phencyclidine-like behavioral effects in pigeons induced by systemic administration of the excitatory amino acid antagonist, 2-amino-5-phosphonovalerate

A selective N-methyl-D-aspartate antagonist, DL-2-amino-5-phosphonovalerate, was found to produce PCP-like catalepsy, discriminative stimulus effects, and stereotyped operant responding in pigeons when administered intramuscularly. These results support the hypothesis that the behavioral effects of PCP-like drugs result at least in part from reduced neurotransmission at excitatory amino acid synapses utilizing N-methyl-D-aspartate preferring receptors.

Phencyclidine-like catalepsy induced by the excitatory amino acid antagonist DL-2-amino-5-phosphonovalerate

This study presents experimental evidence for the mediation of a behavioral effect of phencyclidine-like drugs by inhibition of neurotransmission at excitatory synapses utilizing N-methyl-aspartate (NMA) receptors by showing that DL-2-amino-5-phosphonovalerate, a selective NMA antagonist, produces phencyclidine-like catalepsy in pigeons. This finding suggests the possibility that other behavioral actions of phencyclidine-like substances may be mediated in a similar fashion.

2-Methyl-3,3-diphenyl-3-propanolamine (2-MDP) selectively antagonises N-methyl-aspartate (NMA)

Using electrophoretic application to rat central neurones in vivo, and bath application to frog spinal cord in vitro, 2-methyl-3,3-diphenyl-3-propanolamine was found to be a selective antagonist of N-methyl-DL-aspartate, but not of quisqualate or kainate. In this respect the (-) isomer proved to be about three times more potent than the (+) in both preparations.

Differential effects of dextrorphan and levorphanol on the excitation of rat spinal neurons by amino acids

The effects of the stereoisomers dextrorphan and levorphanol on the excitation of spinal neurons by electrophoretically administered excitatory amino acids were studied in pentobarbitone-anaesthetised rats. Both isomers reduced responses to N-methyl-DL-aspartate (NMA), dextrorphan being both more selective and more potent than levorphanol in this respect. This observation supports the proposal that the NMA-blocking activity of a variety of drugs with psychotomimetic properties is subserved by actions at phencyclidine (PCP)/sigma opiate receptors.