Isomers of cyclazocine as excitatory amino acid antagonists

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.

Effect of chronic administration of NMDA antagonists on synaptic development

The current research assessed the role of the N-methyl-D-aspartate (NMDA) receptor in developmental synaptic plasticity. This was accomplished by quantitative analysis of synaptic number and morphology following pharmacological manipulation of NMDA receptor activity using either the competitive antagonist 2-amino-5-phosphonovaleric acid (APV) or the noncompetitive antagonist phencyclidine (PCP). In the first group, 15-day-old male Long-Evans rats were implanted with osmotic minipumps, which administered 50 mM APV or vehicle at a rate of 0.5 microliter per h into the subjects' occipital cortex for 14 days. At age 30 days (P30), the rats were sacrificed and their occipital neocortices were examined. A second group of rats was given subcutaneous injections of 10 mg/kg PCP or vehicle once daily beginning on P5 for a period of 15 days, and was sacrificed on P20. To determine the effects following withdrawal from long-term NMDA antagonism, a third group of animals was given the same PCP injection routine until P20, but was sacrificed on P21, P26, P36, and P56. Developmental administration of APV was associated with a decreased molecular layer depth and estimated total number of synapses. Similarly, PCP induced a reduction in brain weight, molecular layer depth, and estimated total number of synapses. Withdrawal from NMDA antagonism was initially associated with similar results, i.e., reduced brain weight, cortex depth, synaptic density, and estimated total number of synapses, along with an increase in synaptic length. By P36, however, there was a transitory rebound associated with increased molecular layer depth and estimated total number of synapses. These results support the suggestion that NMDA receptor activation is integral to naturally occurring developmental synaptogenesis, and underscore the importance of NMDA receptor involvement in the process of synaptic plasticity.

Spinal paralysis and catalepsy induced by intrathecal injection of opioid agonists

The intrathecal administration of high (1.05 mumol) doses of D-Ala2-Met5-enkephalinamide (DAMA), D-Ala2-Leu5-enkephalinamide (DADLE), Try-D-Thr-Gly-Phe-Leu-Thr, MR2034-TA, dextrorphan tartrate, U50,488H, levorphanol tartrate, methadone hydrochloride, and 1-methyl-4-phenyl-4-propionoxypiperidine induced spinal hypokinesia. The first 5 of these compounds caused spinal paralysis, whereas the other compounds and lower doses of the first 4 induced waxy catalepsy that was restricted to the hindquarters of rats. The paralysis induced by DAMA was not reversible by IT injections of 50 micrograms naltrexone, indicating, together with the paralytic effects of dextrorphan, that traditional opiate receptors are not involved in this behavioral effect. The spinal catalepsy induced by 0.26 mumol of DAMA was prevented by IT pretreatment with 10 micrograms of naltrexone. In view of this finding and the observation that spinal catalepsy can be induced by agonists of all opiate receptor classes, it seems likely that spinal catalepsy is produced by activation of specific opiate receptors, although the subtype remains to be established.

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.

Opioids and non-opioid enantiomers selectively attenuate N-methyl-D-aspartate neurotoxicity on cortical neurons

Addition of 1 microM-1 mM methadone to the bathing medium produced a concentration-dependent reduction in the neurotoxicity of exogenously applied N-methyl-D-aspartate (NMDA) in murine cortical cell culture (EC50 about 100 microM); the reduction persisted at intense NMDA exposure, consistent with non-competitive inhibition. Methadone also protected against exposure to quinolinate but not quisqualate or kainate. Concentrations (100 microM-3 mM) of several other opioids - morphine, fentanyl, codeine, meperidine, dextropropoxyphene, and naltrexone - were additionally found to produce concentration-dependent reductions in NMDA neurotoxicity. This novel neuron-protective effect of opioids was not mediated by conventional opioid receptors: the non-opioid enantiomer of methadone and morphine exhibited a potency equal to or greater than that of the opioid enantiomer, and 1 mM naloxone did not act as an antagonist. The possibility that opioids, or especially non-opioid enantiomers of opioids, might provide a useful therapeutic approach in diseases states involving NMDA receptor-mediated neurotoxicity, warrants further study.

Tetrahydroaminoacridine selectively attenuates NMDA receptor-mediated neurotoxicity

Addition of the acetylcholinesterase inhibitor 1,2,3,4-tetra-9-hydroaminoacridine (THA) at 1-3 mM markedly reduced the neuronal cell loss that otherwise followed brief exposure of murine cortical cell cultures to 500 microM N-methyl-D-aspartate (NMDA). This novel antagonism was selective for NMDA receptor-mediated toxicity, as it extended to glutamate toxicity but not to quisqualate toxicity, and was THA concentration-dependent between 100 microM and 3 mM, with IC50 approximately 500 microM. The antagonism was probably not due to enhancement of endogenous cholinergic action, as it was not mimicked by acetylcholine, carbachol, or bethanechol; rather, it likely reflected a recently described interaction of THA with the phencyclidine receptor. Exploration of structural specificity revealed some partial neuron-protection with high concentrations of other cholinesterase inhibitors--physostigmine, neostigmine, and edrophonium, but not the structurally related potassium channel blocker, 4-aminopyridine. Further examination of correlations between THA-like structure, and neuron-protective activity, may provide useful insights in the development of new antagonists of NMDA receptor-mediated neurotoxicity.

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.