Opioid analgesics as noncompetitive N-methyl-D-aspartate (NMDA) antagonists.

Much evidence points to the involvement of N-methyl-D-aspartate (NMDA) receptors in the development and maintainance of neuropathic pain. In neuropathic pain, there is generally involved a presumed opioid-insensitive component, which apparently can be blocked by NMDA receptor antagonists. However, in order to obtain complete analgesia, a combination of an NMDA receptor antagonist and an opioid receptor agonist is needed. Recent in vitro data have demonstrated that methadone, ketobemidone, and dextropropoxyphene, in addition to being opioid receptor agonists, also are weak noncompetitive NMDA receptor antagonists. Clinical anecdotes suggest that the NMDA receptor antagonism of these opioids may play a significant role in the pharmacological action of these compounds; however, no clinical studies have been conducted to support this issue. In the present commentary, we discuss evidence for the NMDA receptor antagonism of these compounds and its relevance for clinical pain treatment; an overview of structure-activity relationships for the relevant opioids as noncompetitive NMDA receptor antagonists also is given. It is concluded that although the finding that some opioids are weak noncompetitive NMDA receptor antagonists in vitro has created much attention among clinicians, no clinical studies have been conducted to evaluate the applicability of these compounds in the treatment of neuropathic pain conditions.

Dextropropoxyphene acts as a noncompetitive N-methyl-D-aspartate antagonist.

In order to elucidate whether opioid analgesics available on the Scandinavian market also act as noncompetitive N-methyl-D-aspartate (NMDA) antagonists, a series of commercially available opioids were screened for their affinity in [3H](RS)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cycloheptene-5,10-imine ([3H]MK-801) binding assays and potential inhibitory actions on responses to NMDA in the rat cortical wedge preparation. Of the screened compounds (codeine, dextropropoxyphene, etorphine, fentanyl, and morphine), only dextropropoxyphene, with an IC50 value in [3H]MK-801 binding of 5 microM, was found to be active. Further characterization of the interaction of dextropropoxyphene with the NMDA response in the rat cortical wedge preparation illustrated the noncompetitive NMDA antagonist activity of dextropropoxyphene. Analysis of the dextropropoxyphene inhibition curve of NMDA gave an IC50 value of 190 microM and a Hill slope of 0.8.

Conformational analysis and molecular modelling of a partial GABAA agonist and a glycine antagonist related to the GABAA agonist, THIP.

A series of 3-hydroxyisoxazoles (3-isoxazoles) substituted in the 5-position by piperidyl moieties (2-, 3-, and 4-PIOL) were studied by molecular modelling and computer graphics methods. Whereas 2-PIOL is pharmacologically inactive, 3-PIOL is a glycine antagonist and 4-PIOL a low-efficacy partial GABAA agonist. A conformational analysis of the isomeric PIOLs was performed on the basis of molecular mechanics calculations. The conformational analysis revealed a large degree of conformational freedom for all three compounds, especially of conformers having the 3-hydroxyisoxazole moiety in equatorial positions. By comparison of the PIOLs with the semi-rigid GABAA agonist THIP and the conformationally restricted glycine antagonist THAZ, the conformations relevant for GABAA and glycine receptor recognition were determined. For 2-PIOL a predicted active conformation is energetically favourable, but it exhibits considerable extra volume as compared to THIP, which may explain its lack of affinity for GABAA receptors. 3-PIOL is capable of adopting a conformation resembling that of THAZ. The GABAA receptor affinity of 4-PIOL may be explained by its ability to orientate the functional groups in space in a manner that allows a comparison with THIP. The low efficacy and affinity of 4-PIOL may reflect that not all atoms are directly involved in receptor binding. Certain steric requirements for binding to the GABAA receptor site have been identified and discussed.

Hydroxylated analogues of 5-aminovaleric acid as 4-aminobutyric acidB receptor antagonists: stereostructure-activity relationships.

The (R) and (S) forms of 5-amino-2-hydroxyvaleric acid (2-OH-DAVA) and 5-amino-4-hydroxyvaleric acid (4-OH-DAVA) were designed as structural hybrids of the 4-aminobutyric acidB (GABAB) agonist (R)-(-)-4-amino-3-hydroxybutyric acid [(R)-(-)-3-OH-GABA] and the GABAB antagonist 5-aminovaleric acid (DAVA). (S)-(-)-2-OH-DAVA and (R)-(-)-4-OH-DAVA showed a moderately potent affinity for GABAB receptor sites in rat brain and showed GABAB antagonist effects in a guinea pig ileum preparation. The respective enantiomers, (R)-(+)-2-OH-DAVA and (S)-(+)-4-OH-DAVA, were markedly weaker in both test systems. All four compounds were weak inhibitors of GABAA receptor binding in rat brain, and none of them significantly affected synaptosomal GABA uptake. Based on molecular modeling studies it has been demonstrated that low-energy conformations of (R)-(-)-3-OH-GABA, (S)-(-)-2-OH-DAVA, and (R)-(-)-4-OH-DAVA can be superimposed. These conformations may reflect the shapes adopted by these conformationally flexible compounds during their interaction with GABAB receptors. The present studies emphasize the similar, but distinct, constraints imposed on agonists and antagonists for GABAB receptors.

Synthesis and biological activity of a GABAA agonist which has no effect on benzodiazepine binding and of structurally related glycine antagonists.

3-Isoxazolols substituted in the 5-position by pyrrolidinyl or piperidyl (referred to, respectively, as PYOLs and PIOLs; see Figure 2 for structures) were designed and synthesized as analogues of the potent and specific GABAA agonist THIP. Activity in the series was markedly dependent upon positional isomerism in the structures. Isomers in which the pyrrolidine or piperidine rings were attached via their 2-positions (2-PYOL and 2-PIOL) had no effect on GABAA receptors in vivo or in vitro. An isomer wherein attachment was via the 4-position (4-PIOL) was a GABAA agonist, but it was unique in not affecting the binding of diazepam in vitro; its 'ring-opened' analogue, (RS)-5-(1-methyl-3-aminopropyl)-3-isoxazolol (11) did not bind significantly to GABAA receptor sites in vitro. In contrast, the 3-positional isomer (3-PIOL) antagonized the inhibitory action of glycine on cat spinal neurons. A similar effect was earlier demonstrated for 3-PYOL. However, in contrast to 3-PYOL, which is approximately equipotent as an antagonist of glycine and GABA, 3-PIOL only marginally reduced the inhibitory effect of GABA. The R and S forms of 3-PIOL, synthesized from the respective isomers of piperidine-3-carboxylic acid with known absolute stereochemistry, had pharmacological profiles indistinguishable from that of racemic 3-PIOL.

Comparative stereostructure-activity studies on GABAA and GABAB receptor sites and GABA uptake using rat brain membrane preparations.

The affinities of a number of analogues of gamma-aminobutyric acid (GABA) for GABAA and GABAB receptor sites and GABA uptake were studied using rat brain membrane preparations. Studies on the (S)-(+)- and (R)-(-)-isomers of baclofen, 3-hydroxy-4-aminobutyric acid (3-OH-GABA), and 4,5-dihydromuscimol (DHM) revealed different stereoselectivities of these synaptic mechanisms in vitro. Although (S)-3-OH-GABA and, in particular, (S)-DHM were more potent than the corresponding (R)-isomers as inhibitors of GABAA binding, the opposite stereoselectivity was demonstrated for the GABAB binding sites. Thus, (R)-3-OH-GABA and (R)-baclofen were more potent than the (S)-isomers as inhibitors of GABAB binding, (R)-baclofen being some five times more potent than (R)-3-OH-GABA. These two (R)-isomers actually have opposite orientation of the substituents on the GABA backbones, suggesting that the lipophilic substituent of (R)-baclofen interacts with a structural element of the GABAB receptor site different from that that binds the very polar hydroxy group of (R)-3-OH-GABA. The O-methylated analogue of 3-OH-GABA, 3-methoxy-4-aminobutyric acid (3-OCH3-GABA), did not interact significantly with GABAB sites. The homologues of GABA, trans-4-aminocrotonic acid (trans-ACA), muscimol, and 3-OH-GABA, that is, 5-aminovaleric acid (DAVA), trans-5-aminopent-2-enoic acid, homomuscimol, and 3-hydroxy-5-aminovaleric acid (3-OH-DAVA), respectively, were generally much weaker than the parent compounds, whereas 2-hydroxy-5-aminovaleric acid (2-OH-DAVA) showed a significantly higher affinity for GABAB sites than the corresponding GABA analogue.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue distribution, metabolism, anticonvulsant efficacy and effect on brain amino acid levels of the glia-selective gamma-aminobutyric acid transport inhibitor 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol in mice and chicks.

Using tritium-labelled 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol (THPO) its tissue distribution and metabolism were investigated in adult mice and 4-day-old chicks after systemic administration of the drug. It was found not to be significantly metabolized in the brain since metabolites of THPO corresponding to only approximately 8% of the parent compound could be detected 30 min after administration of the drug intramuscularly in mice. In the liver, however, THPO was found to be metabolized to a considerable extent. In chicks THPO metabolites were found in the brain but they accounted for less than 35% of the radioactivity. The brain concentration of THPO in mice and chicks corresponded to respectively 10 and 50% of the dose injected intramuscularly and the tissue level was essentially constant for at least 3 h after injection. Following systemic administration of THPO to mice and chicks the contents of aspartate, glutamate, glutamine, and gamma-aminobutyric acid (GABA) in whole brain and in synaptosomes was determined. It was found that only GABA contents were affected being increased in synaptosomes from mice and decreased in whole brain in chicks. Doses of THPO, which in chicks but not in mice led to brain levels that were sufficient to inhibit glial GABA uptake, were found to protect chicks but not mice against isonicotinic acid hydrazide-induced seizures. The findings are compatible with the notion that THPO exerts its anticonvulsant activity by inhibition of astrocytic GABA uptake.

Amplification by glycine of the anticonvulsant effect of THPO, a GABA uptake inhibitor.

THPO, a GABA uptake inhibitor, when given in doses of up to 4 mmol/kg (i.p.) to mice, had only a marginal protective effect against seizures induced 1 hr later by 3-mercaptopropionic acid (MPA). THPO (4 mmol/kg), when given in combination with 10 mmol/kg of glycine, protected 60% of the mice from MPA-induced convulsions. The combination of THPO and glycine delayed the onset of metrazol-induced clonic convulsions and protected 30% of the animals from seizures, although neither glycine or THPO alone had a significant anticonvulsant effect against metrazol induced seizures. In agreement with earlier findings, the results presented in this work seem to indicate that the synergistic anticonvulsant effects of glycine and GABAergic agents are independent of their mode of action: the effects of GABA agonists (muscimol) GABA-T inhibitors (vinylGABA), or an inhibitor of glial GABA uptake (THPO) are similarly amplified by glycine.

Glycine antagonists structurally related to muscimol, THIP, or isoguvacine.

Microelectrophoretic methods were used to study the effects on cat spinal neurones of a number of compounds structurally related to the gamma-aminobutyric acid (GABA) agonists muscimol, THIP, and isoguvacine. While N-methylmuscimol was an agonist at bicuculline methochloride-sensitive GABA receptors, somewhat weaker than GABA and THIP, neither N,N-dimethylmuscimol nor N-methyl-THIP interfered significantly with GABA receptors in vivo or binding sites in vitro. Both N,N-dimethylmuscimol and N-methyl-THIP, however, reversibly antagonized the depressant action of glycine. The seven-membered ring analogues of THIP, namely THIA (5,6,7,8-tetrahydro-4H-isoxazolo[5,4-c]azepin-3-ol), THAZ (5,6,7,8-tetrahydro-4H-isoxazolo[4,5-d]azepin-3-ol) and iso-THAZ (5,6,7,8-tetrahydro-4H-isoxazolo[3,4-d]azepin-3-ol), also blocked neuronal inhibition by glycine, iso-THAZ being the most potent compound. The conformationally mobile isomer of THAZ and iso-THAZ, 3-PYOL (5-(3-pyrrolidinyl)-3-isoxazolol), was a much less selective glycine antagonist, being also an antagonist of GABA, 3,4-TAZA (2,5,6,7-tetrahydro-1H-azepine-4-carboxylic acid) and 4,5-TAZA (2,3,6,7-tetrahydro-1H-azepine-4-carboxylic acid), which are amino acid analogues of THIA and THAZ, respectively, and ring homologues of isoguvacine, were also shown to be glycine antagonists. The mechanism of action of the present class of zwitterionic glycine antagonists is unknown. The compounds are much less potent than strychnine.

Urinary excretion of succinylacetone and delta-aminolevulinic acid in patients with hereditary tyrosinemia.

Succinylacetone was excreted in the urine from four patients, with hereditary tyrosinemia i.e., two patients with the severe infantile type with fatal outcome and two patients with less severe juvenile form. In the urine from two patients with neonatal transient tyrosinemia and from normal individuals succinylacetone was not detectable. The urinary excretion of delta-aminolevulinic acid was also increased in all patients with hereditary tyrosinemia compared to patients with neonatal transient tyrosinemia and to normal individuals. The results presented support the hypothesis of a deficiency of fumarylacetoacetase in hereditary tyrosinemia. Furthermore an analytical method for the quantitative determination of succinylacetone in urine using GC-MS is described.