The effect of the dioxolanes on amino acid induced excitation in the mammalian spinal cord

Hydroxynorketamine Blocks N-Methyl-d-Aspartate Receptor Currents by Binding to Closed Receptors

Ketamine, a dissociative anesthetic, is experiencing a clinical resurgence as a fast-acting antidepressant. In the central nervous system, ketamine acts primarily by blocking NMDA receptor currents. Although it is generally safe in a clinical setting, it can be addictive, and several of its derivatives are being investigated as preferable alternatives. 2R,6R-Hydroxynorketamine (HNK), a ketamine metabolite, reproduces some of the therapeutic effects of ketamine and appears to lack abuse liability. Here, we report a systematic investigation of the effects of HNK on macroscopic responses elicited from recombinant NMDA receptors expressed in human embryonic kidney 293 cells. We found that, like ketamine, HNK reduced NMDA receptor currents in a dose-, pH-, and voltage-dependent manner. Relative to ketamine, it had 100-fold-lower potency (46 µM at pH 7.2), 10-fold-slower inhibition onset, slower apparent dissociation rate, weaker voltage dependence, and complete competition by magnesium. Notably, HNK inhibition was fully effective when applied to resting receptors. These results revealed unexpected properties of hydroxynorketamine that warrant its further investigation as a possible therapeutic in pathologies associated with NMDA receptor dysfunction. SIGNIFICANCE STATEMENT: NMDA receptors are excitatory ion channels with fundamental roles in synaptic transmission and plasticity, and their dysfunction associates with severe neuropsychiatric disorders. 2R,6R-Hydroxynorketamine, a metabolite of ketamine, mimics some of the neuroactive properties of ketamine and may lack its abuse liability. Results show that 2R,6R-hydroxynorketamine blocks NMDA receptor currents with low affinity and weak voltage dependence and is effective when applied to resting receptors. These properties highlight its effectiveness to a subset of NMDA receptor responses and recommend it for further investigation.

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.

Affinity of 1-aryl-1,2,3,4-tetrahydroisoquinoline derivatives to the ion channel binding site of the NMDA receptor complex

A series of 1-aryl-1,2,3,4-tetrahydroisoquinoline and 8-methyl-1-aryl-1,2,3,4-tetrahydroisoquinoline derivatives was evaluated for affinity to the PCP binding site of the NMDA receptor complex. The (S)-configured tetrahydroisoquinoline derivative (S)-4 e x HCl bearing a 2-methylphenyl substituent in position 1 of the heterocyclic ring system and a methyl group in position 8 was found to exhibit the highest affinity among the derivatives with a K(i)-value of 0.0374 microM. In addition, this compound shows a remarkable enantioselectivity of binding by being almost 90 times more potent than the corresponding (R)-enantiomer (R)-4 e x HCl. Additionally, a convenient and efficient synthetic approach to racemic 1-aryl-1,2,3,4-tetrahydroisoquinoline derivatives is described.

Chemical dissociation of human awareness: focus on non-competitive NMDA receptor antagonists

Since the mid-1950s the pharmaceutical industry has developed a number of chemicals, including phencyclidine, ketamine and related arylcyclohexylamines (PCE and TCP), dizocilpine (MK-801), N-allylnormetazocine [ NANM, (±)SKF-10,047], etoxadrol, dioxadrol and its enantiomers dexoxadrol and levoxadrol, which produce a constellation of unusual behavioral effects in animals and man. The compounds best studied in humans are phencyclidine and ketamine. They produce a remarkable dose-dependent dissociation of awareness. All of these substances are now known to be non-competitive antagonists of NMDA receptors of glutamic acid. They act in the NMDA receptor ion channel. One can conclude, on the basis of the effects observed with these agents, that glutamic acid and related excitatory amino acids are extremely important in the maintenance of human awareness.

(+)-3-[123I]Iodo-MK-801: synthesis and characterization of binding to the N-methyl-D-aspartate receptor complex

Synthetic methods have been established for preparing high specific activity (+)-3-[123I]Iodo-MK-801 in high radiochemical yield. The binding of the radiotracer to rat cortical membranes has been examined to assess its potential use as an in vivo imaging agent for the N-methyl-D-aspartate (NMDA) receptor-ion channel complex. Under the conditions of the assay, specific (+)-3-[123I]Iodo-MK-801 binding to membrane homogenates represented greater than 95% of the total binding. Several structurally distinct, noncompetitive NMDA receptor antagonists inhibited binding with potencies in accordance with their reported inhibitory activity at the receptor complex. The concentration of (+/-)-3-Iodo-MK-801 required to inhibit 50% of (+)-3-[123I]Iodo-MK-801 binding (IC50) was 3.4 nM when using a low ionic strength assay buffer and 5.5 nM in a physiological buffer. In a thoroughly washed membrane preparation, (+)-3-[123I]Iodo-MK-801 binding was enhanced by L-glutamate and glycine at concentrations known to activate the NMDA receptor. The results indicate that (+)-3-[123I]Iodo-MK-801 specifically labels the NMDA receptor complex in rat brain membranes and the retention of high affinity under near physiological assay conditions suggests that it may be useful as a SPECT imaging agent for the receptor in vivo.

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.

Conditioned taste aversions induced by phencyclidine and other antagonists of N-methyl-D-aspartate

Taste aversions can be conditioned in rats by a variety of psychoactive drugs, including those with reinforcing properties. Previous research, however, has not established clearly whether phencyclidine and related drugs are active in such procedures. The present study was carried out to investigate whether phencyclidine would induce a conditioned taste aversion and whether several other compounds (MK-801, the stereoisomers of NANM and ifenprodil) which, like phencyclidine, are known to antagonise the actions of N-methyl-D-aspartate (NMDA), would produce similar effects. When rats received injections of these compounds, after consuming a novel solution of saccharin, their subsequent consumption of the same solution decreased. The smallest doses of the different drugs which induced clear taste aversions were: phencyclidine 3 mg/kg, MK-801 0.3 mg/kg, (+)-NANM 10 mg/kg, (-)-NANM 3 mg/kg and ifenprodil 10 mg/kg. Thus, all the drugs were active. However, as neither the potencies nor the efficacies of the different compounds in inducing taste aversions correlated with their other behavioural effects or with their relative potencies in antagonising the effects of NMDA or in displacing phencyclidine from its binding sites, the mechanisms involved are unclear.

Phencyclidine receptors and N-methyl-D-aspartate antagonism: electrophysiologic data correlates with known behaviours

Using cortical wedges and isolated frog spinal cords, the potency of a series of psychoactive phencyclidine (PCP) and sigma receptor ligands as antagonists of N-methyl-D-aspartate (NMDA) has been compared with their potency in neurochemical and behavioural studies. Phencyclidine receptor, but not sigma or kappa, ligands were selective antagonists of NMDA on both preparations. Combination studies suggested that dissociative anaesthetics and sigma benzomorphans act at the same site. The relative potencies of the drugs as NMDA antagonists correlated well with their potency in PCP receptor binding studies in vitro and in PCP discrimination studies in vivo.

Phencyclidine selectively blocks the sustained voltage-dependent potassium conductance in PC12 cells

We investigated the effects of phencyclidine (PCP), a psychotomimetic dissociative anesthetic, and several related drugs on voltage-dependent K+ currents in PC12 cells, a neuron-like clonal cell line derived from a rat pheochromocytoma. Whole-cell voltage clamp recordings demonstrated two kinetically distinct voltage-dependent outward (K+) current components in these cells: a rapidly activating and inactivating component, IA, that was selectively eliminated by 4-aminopyridine (2 mM) and a slowly activating, minimally inactivating (sustained) component, IK, that was specifically blocked by tetraethylammonium (20 mM). PCP (1-100 microM) produced a dose-dependent blockade of both IK and IA, however, at low doses the drug selectively reduced IK with little effect on IA; the IC50s for blockade of IK and IA were 4 and 25 microM, respectively. The blockade of IK was voltage-dependent so that the degree of block decreased with increasing depolarization, indicating that the blocking mechanism is likely one in which the positively charged PCP molecule is drawn into the channel pore. Several PCP related drugs also suppressed IK. Thienyl-PCP (TCP), a drug that is behaviorally more potent than PCP, partially blocked IK at low doses (31% at 1 microM), but even at high doses (25 microM) the degree of block was never as great as that produced by PCP. The optically active PCP congeners (+)-PCMP (1-(1-phenylcyclohexyl)-3-methyl-piperidine) and dexoxadrol were also potent blockers of IK. However, in contrast to the stereospecificity these compounds demonstrate in binding to high-affinity PCP receptors and in eliciting PCP-like behavioral responses, their enantiomers (-)-PCMP and levoxadrol showed similar potencies as the parent compounds in blocking IK. These results demonstrate that PCP and related drugs are powerful, selective blockers of IK in PC12 cells. The structure-activity studies indicate that this effect occurs at a site that is pharmacologically distinct from the behaviorally relevant PCP receptor. Blockade of K+ channels is unlikely to be responsible for the psychotomimetic or anti-convulsant properties of PCP, but could account for the convulsant potential of the drug.

[3H]D-2-amino-5-phosphonopentanoate as a ligand for N-methyl-D-aspartate receptors in the mammalian central nervous system

Tritiated D-2-amino-5-phosphonopentanoate has been prepared and evaluated as a radioligand for investigating N-methyl-D-aspartate receptors in rat brain membranes. A radioactive impurity, which was more acidic than 2-amino-5-phosphonopentanoate, interfered with the binding assay for [3H]D-2-amino-5-phosphonopentanoate in preliminary experiments and developed progressively with time of storage of the ligand, was isolated by ion-exchange purification and its binding site characterized. Binding of the 3H-impurity was increased in the presence of calcium ions, with a maximum effect at a concentration of 1-3 mM, but not by sodium, potassium or magnesium ions. It was inhibited by omega-phosphonate analogues of D-2-amino-5-phosphonopentanoate and by inorganic phosphate but not by L-glutamate or any other omega-carboxylates, omega-sulphinates or omega-sulphonates tested. The site of binding for the 3H-impurity was not identified, but from its pharmacological profile it appears to be unrelated to any excitatory amino acid receptor so far described. Binding of purified [3H]D-2-amino-5-phosphonopentanoate to rat cerebral cortical membranes was saturable (KD, 0.53 microM; Bmax, 4.3 pmol/mg protein), was maximal at pH 7.3, but was not particularly temperature sensitive. Dissociation of the receptor-ligand complex was very rapid. Magnesium ions had an inhibitory effect on the binding of [3H]D-2-amino-5-phosphonopentanoate, but the mechanism of this action was not clear. For a wide range of competitive excitatory amino acid antagonists with different potencies and receptor specificities there was a direct relationship between their Ki values as inhibitors of [3H]D-2-amino-5-phosphonopentanoate binding and their KD values for antagonism of N-methyl-D-aspartate induced depolarizations. Thus, [3H]D-2-amino-5-phosphonopentanoate binds to electrophysiological N-methyl-D-aspartate receptors. Among endogenous agonists, L-glutamate had the highest affinity (Ki 0.9 microM) for the [3H]D-2-amino-5-phosphonopentanoate binding site; L-homocysteate and S-sulpho-L-cysteine also had high affinity. However, quinolinate and N-acetylaspartylglutamate had relatively low affinity. It is considered that L-glutamate is the most likely substance to be the transmitter activating N-methyl-D-aspartate receptors physiologically. A study of the regional distribution of [3H]D-2-amino-5-phosphonopentanoate binding sites showed the hippocampus and cerebral cortex to have the highest density of these sites, while the cerebellum and spinal cord had the lowest.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of N-methyl-D-aspartate evoked sodium flux by MK-801

The inhibition of N-methyl-D-aspartate (NMDA) stimulated 22Na+ efflux from rat hippocampal slices was studied using competitive and non-competitive receptor antagonists. There was a good correlation between the abilities of the competitive antagonists to block NMDA evoked 22Na+ efflux and their potencies as inhibitors of L-[3H]glutamate binding. The recently reported novel NMDA receptor antagonist, (+)-5-methyl-16,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) was shown to non-competitively inhibit NMDA stimulated 22Na+ efflux with an IC50 value of 0.4 microM. Relatively high (10 microM) concentrations of MK-801 had no effects on quisqualic acid, alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA), or kainic acid stimulated efflux. However, MK-801 was able to block 22Na+ efflux induced by ibotenic acid and L-homocysteic acid, amino acids that act as NMDA receptor agonists. MK-801, (-)-MK-801, and non-competitive NMDA receptor antagonists of the arylcyclohexylamine and dioxolane classes inhibited NMDA stimulated 22Na+ efflux with potencies that reflected their abilities to compete for [3H]MK-801 binding sites in rat cortical membranes. These results indicate the utility of the 22Na+ efflux assay in studying the properties of NMDA receptors and confirm the nature and selectivity of the inhibition of NMDA receptor linked ion channel activation by MK-801.

[3H]MK-801 labels a site on the N-methyl-D-aspartate receptor channel complex in rat brain membranes

The potent noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist [3H]MK-801 bound with nanomolar affinity to rat brain membranes in a reversible, saturable, and stereospecific manner. The affinity of [3H]MK-801 was considerably higher in 5 mM Tris-HCl (pH 7.4) than in previous studies using Krebs-Henseleit buffer. [3H]MK-801 labels a homogeneous population of sites in rat cerebral cortical membranes with KD of 6.3 nM and Bmax of 2.37 pmol/mg of protein. This binding was unevenly distributed among brain regions, with hippocampus greater than cortex greater than olfactory bulb = striatum greater than medulla-pons, and the cerebellum failing to show significant binding. Detailed pharmacological characterization indicated [3H]MK-801 binding to a site which was competitively and potently inhibited by known noncompetitive NMDA receptor antagonists, such as phencyclidine, thienylcyclohexylpiperidine (TCP), ketamine, N-allylnormetazocine (SKF 10,047), cyclazocine, and etoxadrol, a specificity similar to sites labelled by [3H]TCP. These sites were distinct from the high-affinity sites labelled by the sigma receptor ligand (+)-[3H]SKF 10,047. [3H]MK-801 binding was allosterically modulated by the endogenous NMDA receptor antagonist Mg2+ and by other active divalent cations. These data suggest that [3H]MK-801 labels a high-affinity site on the NMDA receptor channel complex, distinct from the NMDA recognition site, which is responsible for the blocking action of MK-801 and other noncompetitive NMDA receptor antagonists.

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.

An endogenous ligand of the brain sigma/PCP receptor antagonizes NMDA-induced neurotransmitter release

The present study provides evidence for the presence of an endogenous ligand for the phencyclidine (PCP) receptor of mammalian brain. Partially purified bovine hippocampal extracts potently and dose dependently inhibit binding to PCP receptors of [3H]N-(1-[2-thienyl]-cyclohexyl)piperidine (TCP), a highly potent and specific ligand of PCP receptors. In addition to demonstrating PCP-like binding properties, the partially purified extract mimics biological actions of PCP upon neurotransmitter release. HPLC fractions active in the [3]TCP binding assay, by contrast to fractions inactive in the binding assay, potently elicited stimulation of spontaneous acetylcholine and dopamine efflux and inhibited NMDA-stimulated release of acetylcholine and dopamine. The transmitter release assay provides validation of a PCP-like physiological activity exerted by bovine hippocampal extracts partially purified by HPLC.

Effects of phencyclidine, SKF 10,047 and related psychotomimetic agents on N-methyl-D-aspartate receptor mediated synaptic responses in rat hippocampal slices

The effects of representative drugs from three classes of psychotomimetic compounds (arylcyclohexylamines, benzomorphan opioids and dioxolanes) have been examined on synaptic transmission at an identified monosynaptic pathway in rat hippocampal slices. The compounds tested were phencyclidine (PCP) and ketamine, the racemate and isomers of SKF 10,047 (N-allylnormetazocine), and the isomers of dioxadrol (dexoxadrol and levoxadrol). In the absence of added magnesium ions (Mg) in the perfusion medium low frequency stimulation of the Schaffer collateral-commissural pathway evoked a burst of population spikes in the CA1 cell body region. The secondary components of this response could be abolished by the selective N-methyl-D-aspartate (NMDA) antagonist D-2-amino-5-phosphonovalerate (APV). PCP (1 microM) or ketamine (10 microM) selectively blocked the secondary components of the synaptic response. The effect of PCP was neither mimicked nor prevented by hexamethonium and atropine, phentolamine and propranolol, or clonidine and was therefore unlikely to involve cholinergic or adrenergic neurotransmitter systems. The sigma opiate, (+/-)-SKF 10,047 (10 microM) also abolished selectively the secondary components of the synaptic response. There was no apparent difference between the potency of the stereoisomers of this compound. The action of (+/-)-SKF 10,047 was not affected by either naloxone or haloperidol, indicating that this effect did not involve opioid receptors or the haloperidol-sensitive sigma site. Dexoxadrol (10 microM), but not levoxadrol (10 microM), also selectively blocked the secondary components of the synaptic response. It is concluded that these psychotomimetic agents can block an NMDA receptor-mediated component of synaptic transmission in the hippocampus and that this effect is mediated by a specific PCP/sigma site.

Phencyclidine (PCP) receptors: autoradiographic localization in brain with the selective ligand, [3H]TCP

Receptor binding sites for the phencyclidine (PCP) analogue, [3H]TCP, have been localized in the rat and guinea pig central nervous systems by in vitro autoradiography. Quantitation of [3H]TCP binding site densities in rat brain reveals highest levels in the forebrain, in particular the strata oriens and radiatum of the hippocampus, the molecular layer of the dentate gyrus and superficial layers of the cerebral cortex. Moderate levels of binding occur in the amygdala, thalamus, anterior olfactory nucleus, external plexiform layer of the olfactory bulb, olfactory tubercle, geniculate nuclei and deep layers of the cortex. Low levels of binding occur throughout most of the septum, diagonal band, hypothalamus, pons-medulla and cerebellum. Spinal cord grey matter also has low levels of binding. Excitotoxin lesions of the hippocampal formation, which destroy the pyramidal and granule cells, reduce the binding of [3H]TCP to strata radiatum and oriens and the molecular layer of the dentate gyrus by 60% suggesting that [3H]TCP labels intrinsic neurons in these regions. Residual binding is probably on afferent terminals. Ibotenic acid lesions of the caudate-putamen reduce [3H]TCP binding by 70%, indicating that binding sites are localized on intrinsic striatal neurons. 6-Hydroxydopamine lesions do not alter [3H]TCP binding levels in the caudate, suggesting the absence of binding sites on dopaminergic terminals in the caudate.

Autoradiographic distribution of [3H]dexoxadrol (a phencyclidine-related ligand) binding sites in rat and human brain

[3H]dexoxadrol, a dissociative anesthetic, binds with high affinity to specific sites in rat brain (membrane binding and light microscopic autoradiography). Various phencyclidine (PCP) analogues compete for [3H]dexoxadrol sites in a slightly different manner than against [3H]PCP binding sites. As for [3H]PCP binding, [3H]dexoxadrol binding sites are highly concentrated in brain regions such as the cortex and the hippocampus. However, other areas such as the hypothalamus are enriched only in [3H]dexoxadrol binding sites. This suggests that [3H]dexoxadrol binds to PCP-related sites in certain brain regions but not in others. In the human forebrain, [3H]dexoxadrol binding sites are distributed as in the rat brain and mainly found in the caudate, putamen and cortex.

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

Using the technique of microelectrophoresis on cat and rat spinal neurones, the bridged benz(f)isoquinoline, LY154045, like ketamine and dextrorphan, was found to be a selective antagonist of N-methylaspartate, an amino acid used for characterizing excitatory amino acid synaptic receptors. The unbridged analogue, LY154005, was inactive as an amino acid antagonist. This result correlates well with the ability of LY154045, but not LY154005, to displace phencyclidine from CNS tissue and to mimic phencyclidine in behavioural tests. The potential role of N-methylaspartate antagonism in the aetiology of some of the behavioural effects of LY154045, phencyclidine and related drugs is considered.