Piperazine imidazo[1,5-a]quinoxaline ureas as high-affinity GABAA ligands of dual functionality.

A series of imidazo[1,5-a]quinoxaline piperazine ureas appended with a tert-butyl ester side chain at the 3-position was developed. Analogues within this series have high affinity for the gamma-aminobutyric acid A (GABAA)/benzodiazepine receptor complex with efficacies ranging from inverse agonists to full agonists. Many analogues were found to be partial agonists as indicated by [35S]TBPS and Cl- current ratios. Uniquely, a number of these analogues were found to have a bell-shaped dose-response profile in the alpha1 beta2 gamma2 subtype as determined by whole cell patch-clamp technique, where in vitro efficacy was found to decrease with increasing drug concentration. Many of the compounds from this series were effective in antagonizing metrazole-induced seizures, consistent with anticonvulsant and possibly anxiolytic activity. Additionally, several analogues were also effective in lowering cGMP levels (to control values) after applied stress, also consistent with anxiolytic-like properties. The most effective compounds in these screens were also active in animal models of anxiety such as the Vogel and Geller assays. The use of the piperazine substituent allowed for excellent drug levels and a long duration of action in the central nervous system for many of the quinoxalines, as determined by ex vivo assay. Pharmacokinetic analysis of several compounds indicated excellent oral bioavailability and a reasonable half-life in rats. From this series emerged two partial agonists (55, 91) which had good activity in anxiolytic models, acceptable pharmacokinetics, and minimal benzodiazepine-type side effects.

Ibuprofen: effect on inducible nitric oxide synthase.

Treatment with ibuprofen and other non-steroidal anti-inflammatory drugs (NSAIDS) has been reported to decrease the incidence as well as slow down the progression of Alzheimer's disease. Understanding the mechanism of this therapeutic effect would provide a target for development of drugs which may be devoid of side effects observed with NSAIDs. In addition to inhibiting cyclooxygenase (COX), the NSAIDs have recently been shown to decrease inducible nitric oxide synthase (iNOS) activity. Ibuprofen and other NSAIDs had no direct effect on catalytic activity of iNOS, but decreased levels of iNOS mRNA. The mechanism of action of ibuprofen on reduction of iNOS activity has been further investigated in the present study using rat primary cerebellar glial cell cultures. In addition, the effect of ibuprofen on COX mRNA expression and prostaglandin formation was also studied. Glial cells treated with E. coli lipopolysaccharide (LPS) and interferony (INFgamma) for 16 h expressed iNOS and COX. Ibuprofen did not directly inhibit iNOS activity. However, when ibuprofen was incubated at the same time with LPS and INFgamma for 16 h, enzyme activity was reduced, with an IC50 of 0.76 mM. Ibuprofen concentration-dependently decreased iNOS mRNA levels, with an IC50 > 2 mM. Thus, there was no correlation between decrease in iNOS activity and reduction in iNOS mRNA levels. Ibuprofen decreased iNOS protein levels, as determined by Western blot, with an IC50 of 0.89 mM. The data suggest that the reduction in iNOS activity by ibuprofen is due to inhibition of post-transcriptional processing of this enzyme. Ibuprofen had no effect on constitutive COX (COX-1) or inducible COX (COX-2) mRNA expression. However, ibuprofen inhibited PGE2 formation with an IC50 of 0.86 mM. The anti-inflammatory actions of ibuprofen have been related to inhibition of COX and, subsequently, reducing prostaglandin formation. Since the potency of ibuprofen for inhibition of PGE2 formation and reduction in iNOS activity are similar, it is suggested that, at therapeutically effective doses, a decrease in iNOS activity may also occur in vivo. Therefore, reduction in iNOS protein levels in the brain may have a role in preserving the integrity of neurons in individuals susceptible to Alzheimer's disease.

The novel anxiolytic U-101017: in vitro and ex vivo binding profile and effect on cerebellar cGMP.

The binding affinities (Ki) of U-101017 and diazepam for the GABA(A) receptor in rat cortical membranes were determined using [3H]flunitrazepam ([3H]FNZ) as the ligand. The inhibition constants of U-101017 and diazepam were 3.78 nM and 6.36 nM, respectively. Brain uptake of U-101017 was studied by the ex vivo [3H]FNZ binding assay. A significant ex vivo inhibition of [3H]FNZ binding was observed 10 min after oral administration of U-101017, and the effect lasted for at least 240 min (the last time point of investigation). The potential anxiolytic activity of U-101017 and diazepam was investigated in nonstressed and stressed (electric foot shock) mice by quantitative estimation of cerebellar cyclic 3',5'-guanosine monophosphate (cGMP). Both U-101017 and diazepam dose-dependently decreased cGMP and attenuated stress-induced elevations in cGMP. These effects were antagonized by the GABA(A) receptor antagonist flumazenil. U-101017 was about two orders of magnitude more potent in stressed animals than in controls. The results of our investigation indicate that the anxiolytic-like activity of U-101017 is mediated via GABA(A) receptors.

U-95666E: a potential anti-parkinsonian drug with anxiolytic activity.

1. U-95666E, a D2 selective dopamine agonist, was investigated for its effect on rat striatal acetylcholine (ACh) concentration and the results were compared with those obtained with pergolide, pramipexole and bromocriptine under similar conditions. 2. U-95666E, pergolide, pramipexole and bromocriptine dose-dependently increased striatal ACh concentration both in the non-reserpinized and reserpinized rats. 3. Intrinsic activity of U-95666E was similar to pergolide and pramipexole in non-reserpinized rats, but significantly lower in reserpinized rats. 4. The sensitivity of these dopamine agonists for increasing ACh levels in the denervated as compared to innervated striatum were significantly (p < 0.01) higher. 5. U-95666E also has anxiolytic activity in mice. 6. In conclusion, U-95666E may have potential for the treatment of Parkinson's Disease and associated anxiety.

Anxiolytic-like effects of PNU-101017, a partial agonist at the benzodiazepine receptor.

PNU-101017 is a chemically novel ligand at the benzodiazepine recognition site of cloned GABAA receptors. It was reported to potentiate GABA-mediated chloride current in cultured cells with a moderate intrinsic activity and a biphasic dose-response relationship. In this study, we confirmed that PNU-101017 has a partial agonist-like effect in the antagonism of metrazole-induced seizures in mice. It produced no sedation or ataxia, but did antagonize diazepam-induced motor deficit of mice in the rotarod test. PNU-101017 was weakly active in anti-conflict anxiolytic tests, but attenuated the plasma corticosteroid response to mild stress in rats. It also antagonized stress-induced elevation of cerebellar cGMP levels in mice. Like chlordiazepoxide, it increased drinking of saline solution in thirsty rats. PNU-101017 did not potentiate the CNS-depressant effects of ethanol, and produced no evidence of physical dependence when administered repeatedly. Agonists with low intrinsic activity at the benzodiazepine receptor, such as PNU-101017, should be further explored for therapeutic uses.

Neuroprotective effects of the GABA(A) receptor partial agonist U-101017 in 3-acetylpyridine-treated rats.

The neuroprotective effects of U-101017, [7-chloro-5-[cis-3,5-dimethylpiperazine)carbonyl]-imidazole[1,5a]quinoli ne-3-carboxylate], a GABA(A) receptor partial agonist, were investigated in 3-acetylpyridine (3-AP) treated Wistar rats. A significant (P < 0.01) reduction in both cGMP and ATP in the cerebellum was observed at 96 h after treatment with 3-AP (500 micromol/kg i.p.). Oral administration of U-101017 before and after treatment with 3-AP significantly attenuated 3-AP-induced decreases in cGMP and ATP, and this effect was dose related. Consistent with the neurochemical effect, U-101017 prevented 3-AP-induced loss of motor coordination. Treatment with U-101017 partially, but significantly (P < 0.01) prevented the loss of inferior olivary neurons. U-101017 had no significant effect on body temperature. Thus, hypothermia was not involved in neuroprotective effects of U-101017. Co-administration of flumazenil with each treatment of U-101017 blocked the neuroprotective effect of U-101017, indicating that it mediated neuroprotection via the benzodiazepine binding sites on the GABA(A) receptor complex. Delayed administration of U-101017 at various time intervals after treatment with 3-AP demonstrated a significant neuroprotective effect even at 8 h, suggesting that this drug has a wide therapeutic window.

Neuroprotective effects of the dopamine agonists pramipexole and bromocriptine in 3-acetylpyridine-treated rats.

The neuroprotective effects of pramipexole, a dopamine agonist, were investigated in 3-acetylpyridine (3-AP)-treated Wistar rats. Bromocriptine was used as a reference compound to compare the results obtained with pramipexole. A significant reduction (P < 0.01) in cerebellar cGMP and ATP was observed 96 h after treatment with 3-AP (500 micromol/kg, i.p.). Both pramipexole and bromocriptine significantly attenuated 3-AP-induced reduction in cerebellar cGMP and ATP. Consistent with the neurochemical effect, both pramipexole and bromocriptine prevented 3-AP-induced loss of motor coordination. 3-Acetylpyridine produced a significant (P < 0.01) loss of neurons in the inferior olivary nucleus. Treatment with pramipexole and bromocriptine partially, but significantly (P < 0.01), prevented the loss of inferior olivary neurons. There was no reduction in the temperature of the animals, indicating that hypothermia was not responsible for neuroprotection.

D1 dopamine receptor activity of anti-parkinsonian drugs.

Clinical and preclinical investigations suggest that stimulation of D1 dopamine receptors may be responsible for dyskinesias induced by dopamine agonist treatment of Parkinson's Disease (PD), and that these dyskinesias may be decreased by treatment with a D1 antagonist (clozapine). Therefore, the effects of dopamine agonists and antagonists have been investigated in a primary cerebellar granule cell model of cAMP formation that seems to be highly responsive to the D1 receptors. SKF 38393, lisuride, apomorphine, pergolide, dopamine, bromocriptine and 7-OH-DPAT showed concentration-dependent increases in cAMP formation, with EC50s (in microM) of 0.013, 0.053, 0.25, 1.04, 2.18, 50.9 and 54.4, respectively. SKF 38393, apomorphine, dopamine and pergolide had similar intrinsic activity (100%), while the intrinsic activities of 7-OH-DPAT, bromocriptine and lisuride were 28.0%, 20.7% and 17.2%, respectively. SCH 23390, a selective D1 dopamine receptor antagonist, blocked an increase in cAMP formation produced by EC50 concentrations of all of the dopamine agonists investigated in this study. Clozapine concentration-dependently blocked pergolide-induced increases in cAMP and was approximately 1700-fold less potent than SCH 23390 (IC50: 0.97 microM and 0.56 nM, respectively). U-95666A (1-1000 microM), selective for the D2 receptors, showed no significant effect on cAMP, while pramipexole (0.1-100 microM), a D3 preferring agonist, did not elevate cAMP. These data suggest that primary cerebellar granule cell cultures are an excellent model for measuring D1 dopamine receptor-mediated changes in cellular cAMP. The results are discussed with reference to the relationship between the D1 receptor-stimulated increase in cAMP formation and the induction of dyskinesia in humans by these anti-parkinsonian drugs.

Pharmacology of U-91356A, an agonist for the dopamine D2 receptor subtype.

U-91356A [(R)-5,6-dihydro-5-(propylamino)4H-imidazo[4,5,1-ij]quinolin -2-(1H)-one, monohydrochloride], bound with highest affinity to the dopamine D2 receptor subtype, although it also bound with somewhat lower affinities to the dopamine D3 and D4, as well as the 5-HT1A receptor subtypes. In addition to depressing dopamine synthesis and turnover, injection of U-91356A increased striatal acetylcholine concentrations. U-91356A also depressed firing rates of dopamine neurons. In mice, this compound stimulated cage climbing and locomotor activity in reserpinized animals; it also antagonized D-amphetamine-stimulated locomotor activity. It produced contralateral turning in rats with unilateral lesions of the substantia nigra. These data are consistent with roles for the dopamine D2 receptor subtype as a dopamine autoreceptor and as a stimulatory, postsynaptic dopamine receptor.

Pharmacological characterization of U-101387, a dopamine D4 receptor selective antagonist.

Dopamine D2-like receptors play an important role in the pharmacotherapy of psychotic disorders. Molecular and cellular techniques have identified distinct gene products (D2-long, D2-short, D3 and D4) displaying the D2 receptor pharmacology. However, the contribution of each subtype in antipsychotic effects of or their physiological role remain unclear. Here we describe the pharmacological effects of a selective D4 antagonist, U-101387. U-101387 displayed moderately high affinity (Ki = 10 nM) and selectivity for the dopamine D4.2 receptor expressed in clonal cell lines. It lacked measurable affinity for not only other dopamine receptors but also noradrenalin, serotonin and histamine receptor families (Ki > 2000 nM). It fully and dose-dependently antagonized quinpirole-induced cAMP inhibition (without producing any effect by itself) in stably transfected cells. U-101387 also displayed excellent oral bioavailability, brain penetration and other pharmacokinetic characteristics. Unlike classical neuroleptics (e.g., haloperidol), U-101387 neither blocked acute behavioral effects of amphetamine or apomorphine nor did it alter spontaneous locomotion by itself. Additionally, U-101387 was without effect in behavioral and biochemical tests predictive of extrapyramidal and neuroendocrine side effects. Consistent with the lack of autoreceptor function of D4, acute administration of U-101387 failed to alter dopamine neuronal firing by itself or reverse the inhibition produced by dopamine agonists and to affect monoamine turnover in areas innervated by the mesencephalic or hypothalamic dopamine neurons. However, U-101387 potently induced c-fos mRNA expression in the infralimbic/ventral prelimbic cortex to a level similar to that produced by the atypical antipsychotic, clozapine. This is consistent with the predominantly cortical distribution of the D4 receptor. Taken together, these results demonstrate that the D4-selective antagonist, U-101387, produces effects that are distinct from those of the nonselective D2 antagonists as well as D3-preferring agents. U-101387 offers a unique tool to understand the role of dopamine D4 receptors in diseases involving central dopamine systems.

High-affinity alpha-aminobutyric acid A/benzodiazepine ligands: synthesis and structure-activity relationship studies of a new series of tetracyclic imidazoquinoxalines.

A series of tetracyclic imidazoquinoxaline analogs was developed which constrain the carbonyl group of the partial agonist 3-(5-cyclopropyl-1,2,4-oxadiazol-3-yl)-5-[(dimethylamino)carbonyl] - 4,5-dihydroimidazo[1,5-alpha]quinoxaline (2, U-91571) away from the benzene ring. These analogs orient the carbonyl group in the opposite direction of the previously reported full agonist 1-(5- cyclopropyl-1,2,4-oxadiazol-3-yl)-12,12a-dihydroimidazo[1,5- alpha]pyrrolo [2,1-c]quinoxalin-10(11H)-one (3, U-89267). A number of approaches were utilized to form the "bottom" ring of this tetracyclic ring system including intramolecular cyclizations promoted by Lewis acids or base, as well as metal-carbenoid conditions. The size and substitution pattern of the additional ring was widely varied. Analogs within this series had high affinity for the benzodiazepine receptor on the alpha-aminobutyric acid A chloride ion channel complex. From TBPS shift and Cl- current assays, the in vitro efficacy of compounds within this class ranged from antagonists to partial agonists with only 18a identified as a full agonist. Additionally, several analogs were quite potent at antagonizing metrazole-induced seizures indicating possible anticonvulsant or anxiolytic activity. Unlike 3, analogs in this series did not have high affinity for the diazepam insensitive alpha 6 beta 2 delta 2 subtype. These results suggest that either constraining the carbonyl group away from the benzene ring or the greater planarity that results from the additional cyclic structure provides analogs with partial agonist properties and prevents effective interaction with the alpha 6 beta 2 delta 2 subtype.

3-Phenyl-substituted imidazo[1,5-alpha]quinoxalin-4-ones and imidazo[1,5-alpha]quinoxaline ureas that have high affinity at the GABAA/benzodiazepine receptor complex.

A series of imidazo[1,5-alpha]quinoxalin-4-ones and imidazo[1,5-alpha]quinoxaline ureas containing substituted phenyl groups at the 3-position was developed. Compounds within the imidazo[1,5-alpha]quinoxaline urea series had high affinity for the GABAA/benzodiazepine receptor complex with varying in vitro efficacy, although most analogs were partial agonists as indicated by [35S]TBPS and Cl- current ratios. Interestingly, a subseries of piperazine ureas was identified which had biphasic efficacy, becoming more antagonistic with increasing concentration. Analogs within the imidazo[1,5-alpha]quinoxalin-4-one series had substantially decreased binding affinity as compared to the quinoxaline urea series. These compounds ranged from antagonists to full agonists by in vitro analysis, with several derivatives having roughly 4-fold greater intrinsic activity than diazepam as indicated by Cl- current measurement. Numerous compounds from both series were effective in antagonizing metrazole-induced seizures, consistent with anti-convulsant properties and possible anxiolytic activity. Most of the quinoxaline ureas and quinoxalin-4-ones were active in an acute electroshock physical dependence side effect assay in mice precluding further development.

D2 and D1 dopaminergic activity of 7-OH-DPAT.

The D1 and D2 dopamine receptor agonist properties of 7-hydroxy-2-(N,N-di-n-propylamino) tetraline (7-OH-DPAT) was determined by investigating the effect of this compound on rat striatal acetylcholine (ACh) concentration and increase in cAMP formation in primary cerebellar granule cell cultures. 7-OH-DPAT at low doses (0.01 to 0.1 mumol/ kg) had no significant effect, and at high doses (0.3 to 30 mumol/kg) significantly (P < 0.01) increased striatal ACh levels. Likewise, quinpirole was found to significantly elevate ACh content. Pretreatment with haloperidol, a non-selective antagonist of the D2 family of receptors, significantly (P < 0.01) blocked 7-OH-DPAT- and quinpirole-induced increases in ACh. U-99194A, a D3 selective dopamine antagonist, had no significant effect on 7-OH-DPAT-induced increases in striatal ACh. However, raclopride, a D2 selective dopamine antagonist, completely blocked 7-OH-DPAT-induced elevations in ACh. 7-OH-DPAT in the mumolar range increased cAMP formation in granule cell cultures, and this effect was antagonized by SCH 23390, a D1 selective dopamine antagonist. The neurochemical study indicates that, at high doses, 7-OH-DPAT has both D1 and D2 agonist activities.

Neuroprotective effects of the pyrrolopyrimidine U-104067F in 3-acetylpyridine-treated rats.

The neuroprotective effects of U-104067F[(9-(2-morpholinyl)ethyl) 2,4-di-l-pyrrolidinyl-9H-pyrimidino (4,5-b)(4,5indole monohydrochloride hydrate], a pyrrolopyrimidine antioxidant, were investigated in 3-acetylpyridine (3-AP)-treated Wistar rats. A significant (P < 0.01) reduction in cerebellar cGMP and ATP was observed at 96 h after treatment with 3-AP (500 micromol/kg ip). Oral administration of U-104067F significantly attenuated 3-AP-induced reductions in cGMP and ATP, and this effect was dose related. Consistent with the neurochemical effect, U-104067F prevented 3-AP-induced loss of motor coordination. 3-Acetylpyridine produced a significant (P < 0.01) loss of neurons in the inferior olivary nucleus. Treatment with U-104067F partially, but significantly (P < 0.01), prevented the loss of inferior olivary neurons. The physiological and neurochemical parameters were maintained within normal limits by U-104067F in 3-AP-treated rats despite only partial preservation of inferior olivary neurons.

High-affinity partial agonist imidazo[1,5-a]quinoxaline amides, carbamates, and ureas at the gamma-aminobutyric acid A/benzodiazepine receptor complex.

A series of imidazo[1,5-a]quinoxaline amides, carbamates, and ureas which have high affinity for the gamma-aminobutyric acid A/benzodiazepine receptor complex was developed. Compounds within this class have varying efficacies ranging from antagonists to full agonists. However, most analogs were found to be partial agonists as indicated by [35S]TBPS and Cl- current ratios. Many of these compounds were also effective in antagonizing metrazole-induced seizures in accordance with anticonvulsant and possible anxiolytic activity. Selected quinoxalines displayed limited benzodiazepine-type side effects such as ethanol potentiation and physical dependence in animal models. Dimethylamino urea 41 emerged as the most interesting analog, having a partial agonist profile in vitro while possessing useful activity in animal models of anxiety such as the Vogel and Geller assays. In accordance with its partial agonist profile, 41 was devoid of typical benzodiazepine side effects.

U-19451A: a selective inducible nitric oxide synthase inhibitor.

Drugs with high selectivity for iNOS inhibition may be useful for treatment of neurodegenerative disorders, chronic inflammatory diseases, and septic shock. Therefore, U-19451A (2-benzyl-2-thio-pseudourea hydrochloride), a potential NOS inhibitor, has been investigated for its selectivity for iNOS using tissues, primary cerebellar granule cell cultures and glial cell cultures. Lungs isolated from rats treated with intravenous injection of E coli lipopolysaccharide and glial cell cultures treated with the same bacterial toxin plus gamma-interferon were used for iNOS activity. Rat cerebellum and primary cerebellar granule cell cultures were utilized for neuronal NOS (nNOS) activity. S-methylthiourea (SMT) and L-nitroarginine methyl ester (L-NAME), selective iNOS and nNOS inhibitors, respectively, were chosen as standards. Both U-19451A and SMT were 4-times more selective for iNOS as compared to nNOS in tissues. U-19451A was more selective than SMT for iNOS inhibition using cultures. L-NAME was 16-31 times more selective for inhibiting nNOS activity. Based on the selectivity of U-19451A for iNOS inhibition, this drug would be expected to be effective in the treatment of diseases with inflammatory pathology without producing side effects associated with nNOS inhibition.

Comparative dopaminergic and muscarinic antagonist activity of clozapine and haloperidol.

Clozapine is an atypical antipsychotic drug, and its dopamine and muscarinic antagonist activity has been compared with haloperidol in rodents. Elevation in rat striatal acetylcholine (ACh) and mice cerebellar cGMP has been used as an agonist response for oxotremorine and quinpirole. Pretreatment with clozapine significantly blocked oxotremorine-induced elevation in striatal ACh (p<0.01) and cerebellar cGMP(p<0.05). At the same doses, clozapine had no significant effect on quinpirole-induced increases in ACh and cGMP levels. Pretreatment with haloperidol significantly antagonized quinpirole-induced elevation in striatal ACh (p<0.01) and cerebellar cGMP(p<0.05), and haloperidol had no significant effect on oxotremorine-induced agonist responses. Thus, clozapine is antimuscarinic at a dose level that lacks dopamine antagonist properties, whereas haloperidol is a dopamine antagonist and lacks antimuscarinic activity. The atypical neuroleptic profile of clozapine may be due to its high antimuscarinic and low antidopaminergic activity.

Motor effects of the partial dopamine agonist (-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine (preclamol) in Parkinson's disease.

The motor effects of the partial dopamine agonist (-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine [(-)-3-PPP, preclamol] were evaluated in nine patients with Parkinson's disease using a double-blind, placebo-controlled design. (-)-3-PPP monotherapy had an antiparkinsonian effect in five of nine patients at a mean dose of 37 +/- 10 mg intramuscularly. The co-administration of (-)-3-PPP and a mildly dyskinetic dose of levodopa, infused intravenously at steady-state, resulted in complete suppression of dyskinesias and reemergence of parkinsonian signs in two of seven patients. These dopamine antagonist effects of (-)-3-PPP occurred at relatively low (2.5 and 5 mg) doses. Our results suggest that partial dopamine agonists can exert agonist or antagonist activity in parkinsonian patients depending on concurrent dopaminergic tone. Although this dual action of (-)-3-PPP and other partial agonists could be therapeutically important on theoretical grounds, the small number of patients manifesting a clinically significant response and the frequently inconsistent effects could indicate that this class of agents may have relatively limited clinical utility.

Structure activity relationships of peroxynitrite scavengers an approach to nitric oxide neurotoxicity.

Nitric oxide (NO) is made by NO synthase during the conversion of arginine to citrulline. Researchers have found that they can block the actions of excitotoxins by inhibiting NO synthase. Released from excitable cells during trauma, NO may react with superoxide to form peroxynitrite. Once formed, peroxynitrite and its products can then react with proteins, lipids and nucleic acids resulting in cell injury and death. The present study was undertaken to investigate analogs of cysteine as scavengers of peroxynitrite. Peroxynitrite scavengers were assayed by Attoflo, an automated radioimmunoassay. Briefly, peroxynitrite, in a dose-dependent manner (0.1 to 10 mM), inhibited the binding of I125 cAMP to a polyclonal antibody used in the assay of cAMP. Drugs were tested for blockade of the inhibition (90%) caused by peroxynitrite at 10 mM. Cysteine blocked the inhibition of ligand/antibody binding in a dose-dependent manner (EC50 = 3 mM). Cysteine, cysteine esters, penicillamine, penicillamine esters and cysteamine were the most effective peroxynitrite scavengers. Analogs of cysteine may thereby protect cells from nitric oxide toxicity.

Antagonist, partial agonist, and full agonist imidazo[1,5-a]quinoxaline amides and carbamates acting through the GABAA/benzodiazepine receptor.

(4RS)-1-(5-Cyclopropyl-1,2,4-oxadiazol-3-yl)-12,12a-dihyd roimidazo[1,5- a]pyrrolo[2,1-c]quinoxalin-10(11H)-one (1a), 5-benzoyl-3-(5-cyclopropyl-1,2,4-oxadiazol-3-yl)-4,5- dihydroimidazo[1,5-a]quinoxaline (13b), and tert-butyl (4S)-12,12a-dihydroimidazo[1,5-a]pyrrolo[2,1- c]quinoxaline-1-carboxylate (1e), as well as other imidazo[1,5-a]quinoxaline amides and carbamates, represent a new series of compounds which bind with high affinity to the GABAA/benzodiazepine receptor. These compounds exhibit a wide range of intrinsic efficacies as measured by [35S]TBPS binding ratios. The synthesis of 1a begins with the addition of DL-glutamic acid to 1-fluoro-2-nitrobenzene, followed by reduction of the nitro group and subsequent ring closure to form 3-(carbethoxymethyl)-1,2,3,4-tetrahydroquinoxalin-2-one, followed by a second ring closure to afford (4RS)-1,5-dioxo-1,2,3,4,5,6-hexahydropyrrolo[1,2-a]quinoxali ne as the key intermediate. Appendage of a substituted imidazo ring via the anion of 5-cyclopropyl-1,2,4-oxadiazol-3-yl gives 1a. The (-)- and (+)-isomers of 1a were prepared from 1-fluoro-2-nitrobenzene and L- and D-glutamic acid, respectively. 1a and its enantiomers demonstrated affinity for the [3H]flunitrazepam binding site with Ki's of 0.87, 0.62, and 0.65 nM, respectively.