Mono N-aryl ethylenediamine and piperazine derivatives are GABAA receptor blockers: implications for psychiatry

GABAA receptor-mediated seizure liabilities: a mixed-methods screening approach

GABAA receptors, members of the pentameric ligand-gated ion channel superfamily, are widely expressed in the central nervous system and mediate a broad range of pharmaco-toxicological effects including bidirectional changes to seizure threshold. Thus, detection of GABAA receptor-mediated seizure liabilities is a big, partly unmet need in early preclinical drug development. This is in part due to the plethora of allosteric binding sites that are present on different subtypes of GABAA receptors and the critical lack of screening methods that detect interactions with any of these sites. To improve in silico screening methods, we assembled an inventory of allosteric binding sites based on structural data. Pharmacophore models representing several of the binding sites were constructed. These models from the NeuroDeRisk IL Profiler were used for in silico screening of a compiled collection of drugs with known GABAA receptor interactions to generate testable hypotheses. Amoxapine was one of the hits identified and subjected to an array of in vitro assays to examine molecular and cellular effects on neuronal excitability and in vivo locomotor pattern changes in zebrafish larvae. An additional level of analysis for our compound collection is provided by pharmacovigilance alerts using FAERS data. Inspired by the Adverse Outcome Pathway framework, we postulate several candidate pathways leading from specific binding sites to acute seizure induction. The whole workflow can be utilized for any compound collection and should inform about GABAA receptor-mediated seizure risks more comprehensively compared to standard displacement screens, as it rests chiefly on functional data.

Fifty years of experience with loxapine for the rapid non-coercive tranquilization of acute behavioral disturbances in schizophrenia patients, and beyond

INTRODUCTION

Acute behavioral disturbances in psychosis, including agitation, comprise a heterogeneous group of manifestations varying in intensity and duration they last for. They require rapid, non-coercive treatments ranging from verbal de-escalation to the calming effect of pharmacological agents. The treatment goals are reduction of patient suffering and prevention of disease deterioration. Stabilizing rather than sedating is preferred to ensure improved compliance and a stronger therapeutic alliance. Furthermore, animal pharmacology and clinical studies on agitation reveal the robust calming and anxiolytic properties of loxapine.

AREAS COVERED

This review covers the pharmacological and clinical history of loxapine along with research developments. It emphasizes the advantages of its multiple formulations ranging from injectable forms and tablets to orally inhaled forms to attain rapid and fine-tuned tranquilization.

EXPERT OPINION

Rapid tranquillization is achieved within 2-6 hours using liquid orally-consumed loxapine, and within an hour or less with its IM or orally inhaled forms. Loxapine has been adopted in the management of a wide range of acute disturbances, such as agitation in psychosis. In the context of personalized medicine, key cellular and molecular elements of the schizophrenia phenotype were recently shown to be improved with loxapine.

Tricyclic antipsychotics and antidepressants can inhibit α5-containing GABAA receptors by two distinct mechanisms

Background and Purpose

Many psychotherapeutic drugs, including clozapine, display polypharmacology and act on GABA_A receptors. Patients with schizophrenia show alterations in function, structure and molecular composition of the hippocampus, and a recent study demonstrated aberrant levels of hippocampal α5 subunit‐containing GABA_A receptors. The purpose of this study is to investigate the effects of tricyclic compounds on α5 subunit‐containing receptor subtypes.

Experimental Approach

Functional studies of effects by seven antipsychotic and antidepressant medications were performed in several GABA_A receptor subtypes by two‐electrode voltage‐clamp electrophysiology using Xenopus laevis oocytes. Computational structural analysis was employed to design mutated constructs of the α5 subunit, probing a novel binding site. Radioligand displacement data complemented the functional and mutational findings.

Key Results

The antipsychotic drugs clozapine and chlorpromazine exerted functional inhibition on multiple GABA_A receptor subtypes, including those containing α5‐subunits. Based on a chlorpromazine binding site observed in a GABA‐gated bacterial homologue, we identified a novel site in α5 GABA_A receptor subunits and demonstrate differential usage of this and the orthosteric sites by these ligands.

Conclusion and Implications

Despite high molecular and functional similarities among the tested ligands, they reduce GABA currents by differential usage of allosteric and orthosteric sites. The chlorpromazine site we describe here is a new potential target for optimizing antipsychotic medications with beneficial polypharmacology. Further studies in defined subtypes are needed to substantiate mechanistic links between the therapeutic effects of clozapine and its action on certain GABA_A receptor subtypes.

Intentional Recreational Abuse of Quetiapine Compared to Other Second-generation Antipsychotics

INTRODUCTION

Case reports and poison center data have demonstrated that the second-generation antipsychotic quetiapine is being obtained and used for recreational abuse. The purpose of this study was to describe the relative rates of single-substance abuse for different atypical antipsychotics and compare their demographic and clinical features.

METHODS

We conducted a 10-year retrospective analysis of the National Poison Data System (NPDS) database (2003 - 2013). Trained nurses and pharmacists with specialty training in toxicology prospectively collect all NPDS data at poison control centers around the United States. We queried the NPDS for all cases of single-substance second-generation antipsychotic exposures coded as "intentional abuse." The data provided by the NPDS regarding rates and clinical features of quetiapine abuse and the abuse of all other second-generation antipsychotics were compared and described descriptively.

RESULTS

During the study period, 2,118 cases of quetiapine abuse and 1,379 cases of other second-generation antipsychotic abuse were identified. Quetiapine abuse was more common than the abuse of other second-generation antipsychotics, compromising 60.6% of all abuse cases during the study period. After quetiapine, the next most frequently abused medications were risperidone (530 cases, 15.2%) and olanzapine (246 cases, 7.0%). For all second-generation antipsychotics including quetiapine, central nervous system clinical effects were most common, including drowsiness, confusion, and agitation. Other serious clinical effects observed with second-generation antipsychotic abuse included hypotension, respiratory depression, and seizures.

CONCLUSION

Quetiapine abuse is relatively common, and is abused far more often than any other second-generation antipsychotic. Emergency physicians should be aware of the clinical effects that may occur after second-generation antipsychotic abuse.

Investigation on the inclusion interaction of 4-sulfonatocalix[n]arenes with 1-(4-nitrophenyl)piperazine

The inclusion behaviors of 4-Sulfonatocalix[n]arenes (SCXn) (n=4, 6, 8) with 1-(4-nitrophenyl)piperazine (NPP) were investigated by UV spectroscopy and fluorescence spectroscopy at different pH values (pH=3.05, 6.50, 8.40). The UV absorption and fluorescence intensity of NPP remarkably increased in presence of SCXn revealing formation of the inclusion complexes between NPP and SCXn. Moreover, the formation constants (K) of inclusion complexes were also determined by the non-linear fitting method, and the obtained data showed that the formation constants decreasedgradually with the increasing of the pH value. When the pH value was 3.05, the formation constant of NPP with SCX8 reached a maximum of 1.7×10(7) L mol(-1). The stoichiometric ratio was verified to be 1:1 by the continuous variation method. Meanwhile FT-IR and DSC analysis also indicated that NPP could form the inclusion complex with SCXn. In order to explore the inclusion mechanism of NPP with SCXn, 1H NMR and molecular modeling studies were carried out and experimental results showed that the part of benzene ring of NPP penetrated into the hydrophobic cavity of SCXn.

pH-dependent inhibition of native GABA(A) receptors by HEPES

BACKGROUND AND PURPOSE

Artificial buffers such as HEPES are extensively used to control extracellular pH (pH(e) ) to investigate the effect of H(+) ions on GABA(A) receptor function.

EXPERIMENTAL APPROACH

In neurones cultured from spinal cord dorsal horn (DH), dorsal root ganglia (DRG) and cerebellar granule cells (GC) of neonatal rats, we studied the effect of pH(e) on currents induced by GABA(A) receptor agonists, controlling pH(e) with HCO(3) (-) or different concentrations of HEPES.

KEY RESULTS

Changing HEPES concentration from 1 to 20 mM at constant pH(e) strongly inhibited the currents induced by submaximal GABA applications, but not those induced by glycine or glutamate, on DH, DRG or GC neurones, increasing twofold the EC(50) for GABA in DH neurones and GC. Submaximal GABA(A) receptor-mediated currents were also inhibited by piperazine-N,N'-bis(2-ethanesulfonic acid) (PIPES), 3-(N-morpholino)propanesulfonic acid, tris(hydroxymethyl)aminomethane or imidazole. PIPES and HEPES, both piperazine derivatives, similarly inhibited GABA(A) receptors, whereas the other buffers had weaker effects and 2-(N-morpholino)ethanesulfonic acid had no effect. HEPES-induced inhibition of submaximal GABA(A) receptor-mediated currents was unaffected by diethylpyrocarbonate, a histidine-modifying reagent. HEPES-induced inhibition of GABA(A) receptors was independent of membrane potential, HCO(3) (-) and intracellular Cl(-) concentration and was not modified by flumazenil, which blocks the benzodiazepine binding site. However, it strongly depended on pH(e) .

CONCLUSIONS AND IMPLICATIONS

Inhibition of GABA(A) receptors by HEPES depended on pH(e) , leading to an apparent H(+) -induced inhibition of DH GABA(A) receptors, unrelated to the pH sensitivity of these receptors in both low and physiological buffering conditions, suggesting that protonated HEPES caused this inhibition.

The limited utility of electrocardiography variables used to predict arrhythmia in psychotropic drug overdose

OBJECTIVE

The aim of the present study was to examine the relationship between serious arrhythmias in patients with psychotropic drug overdose and electrocardiography (ECG) findings that have been suggested previously to predict this complication.

METHODS

Thirty-nine patients with serious arrhythmias (ventricular tachycardia, supraventricular tachycardia or cardiac arrest) after tricyclic antidepressant overdose or thioridazine overdose were compared with 117 controls with clinically significant overdose matched to each case for the drug ingested. These patients with psychotropic drug overdose had presented for treatment to the Department of Clinical Toxicology, Newcastle and to the Princess Alexandra Hospital, Brisbane. The heart rate, the QRS width, the QTc and QT intervals, the QT dispersion, and the R wave and R/S ratios in aVR on the initial ECGs were compared in cases and controls.

RESULTS

The cases had taken dothiepin (16 patients), doxepin (six patients), thioridazine (five patients), amitriptyline (five patients), nortriptyline (three patients), imipramine (one patient) and a combination of dothiepin and thioridazine (three patients). In 20 of the 39 patients with arrhythmias, the arrhythmia had been a presumed ventricular tachycardia. Of the other 19 patients, 15 patients had a supraventricular tachycardia, two patients had cardiac arrests (one asystole, one without ECG monitoring) and two patients had insufficient data recorded to make classification of the arrhythmias possible. The QRS was >/= 100 ms in 82% of cases but also in 76% of controls. QRS >/= 160 ms had a sensitivity of only 13% and occurred in 2% of controls. QRS > 120 ms, QTc > 500 and the R/S ratio in aVR appeared to have a stronger association with the occurrence of arrhythmia: QRS > 120 ms (odds ratio [OR], 3.56; 95% confidence interval [CI], 1.46-8.68), QTc > 500 (OR, 3.07; 95% CI, 1.33-7.07), and R/S ratio in aVR > 0.7 (OR, 16; 95% CI, 3.47-74). Excluding thioridazine overdoses and performing the analysis for tricyclic antidepressant overdoses alone gave increased odds ratios for QRS > 120 ms (OR, 4.83; 95% CI, 1.73-13.5) and QTc > 500 (OR, 4.5; 95% CI, 1.56-13) but had little effect on that for the R/S ratio in aVR > 0.7 (OR, 14.5; 95% CI, 3.10-68).

CONCLUSION

ECG measurements were generally weakly related to the occurrence of arrhythmia and should not be used as the sole criteria for risk assessment in tricyclic antidepressant overdose. The frequently recommended practice of using either QRS >/= 100 ms or QRS >/= 160 ms to predict arrhythmias is not supported by our study. R/S ratio in aVR > 0.7 was most strongly related to arrhythmia but had estimated positive and negative predictive values of only 41% and 95%, respectively. The use of these specific predictors in other drug overdoses is not recommended without specific studies.

Clozapine's antipsychotic effects do not depend on blockade of 5-HT3 receptors

Sixteen known 5-HT3 receptor blockers, including clozapine, fully or partially reverse the inhibitory effect of 1 microM GABA on [35S]TBPS binding, indicating that they are also GABA(A) antagonists, some of them selective for subsets of GABA(A) receptors. The 5-HT3 receptor blocker, ondansetron, has been reported to produce some antipsychotic and anxiolytic effects. However, no antipsychotic effects have been reported for a large number of highly potent 5-HT3 receptor blockers. Like clozapine, ondansetron partially reverses the inhibitory effect of GABA on [35S]TBPS binding. Additivity experiments suggest that ten 5-HT3 receptor blockers tested at low concentrations preferentially block subtypes of GABA(A) receptors that are among those blocked by clozapine. Wiley and Porter (29) reported that MDL-72222, the most potent GABA(A) antagonist described here, partially generalizes (71%) with clozapine in rats trained to discriminate an interoceptive clozapine stimulus, but only at a dose that severely decreases responding. Tropisetron (ICS-205,930) exhibits both GABA-positive and GABA-negative effects. R-(+)-zacopride is 6-fold more potent than S-(-)-zacopride as a GABA(A) antagonist. We conclude that the observed antipsychotic and, possibly, anxiolytic effects of some 5-HT3 receptor blockers are due to selective antagonism of certain GABA(A) receptors, and not to blockade of 5-HT3 receptors. We speculate that the anxiolytic and sedative effects of clozapine and several other antipsychotic drugs may be due to selective blockade of alpha1beta2gamma2 GABA(A) receptors which are preferentially located on certain types of GABAergic interneurons (probably parvalbumin positive). Blockade of these receptors will increase the inhibitory output of these interneurons. So far, no highly potent GABA(A) antagonists with clozapine-like selectivity have been identified. Such compounds may exhibit improved clozapine-like antipsychotic activity.

Antagonistic action of pitrazepin on human and rat GABA(A) receptors

Pitrazepin, 3-(piperazinyl-1)-9H-dibenz(c,f) triazolo(4,5-a)azepin is a piperazine antagonist of GABA in a variety of electrophysiological and in vitro binding studies involving GABA and glycine receptors. In the present study we have investigated the effects of pitrazepin, and the GABA(A) antagonist bicuculline, on membrane currents elicited by GABA in Xenopus oocytes injected with rat cerebral cortex mRNA or cDNAs encoding alpha1beta2 or alpha1beta2gamma2s human GABA(A) receptor subunits. The three types of GABA(A) receptors expressed were reversibly antagonized by bicuculline and pitrazepin in a concentration-dependent manner. GABA dose-current response curves for the three types of receptors were shifted to the right, in a parallel manner, by increasing concentrations of pitrazepin. Schild analyses gave pA2 values of 6.42+/-0.62, n = 4, 6.41+/-1.2, n = 5 and 6.21+/-1.24, n = 6, in oocytes expressing rat cerebral cortex, alpha1beta2 or alpha1beta2gamma2s human GABA(A) receptors respectively (values are given as means +/- s.e. mean), and the Hill coefficients were all close to unity. All this is consistent with the notion that pitrazepin acts as a competitive antagonist of these GABA(A) receptors; and that their antagonism by pitrazepin is not strongly dependent on the subunit composition of the receptors here studied. Since pitrazepin has been reported to act also at the benzodiazepine binding site, we studied the effect of the benzodiazepine antagonist Ro 15-1788 (flumazenil) on the inhibition of alpha1beta2gamma2s receptors by pitrazepin. Co-application of Ro 15-1788 did not alter the inhibiting effect of pitrazepin. Moreover, pitrazepin did not antagonize the potentiation of GABA-currents by flunitrazepam. All this suggests that pitrazepin does not affect the GABA receptor-chloride channel by interacting with the benzodiazepine receptor site.

Acute and chronic administration of clozapine produces greater proconvulsant actions than haloperidol on focal hippocampal seizures in freely moving rats

In this study, we assessed the effects of the acute (a single injection) and repeated (once daily injections for 21 days) administration of the atypical antipsychotic drug clozapine (1.5, 5, or 15 mg/kg i.p.) and the typical antipsychotic drug haloperidol (0.15, 0.5, and 1.5 mg/kg, i.p.) on hippocampal partial seizures generated by low-frequency electrical stimulation in male Wistar rats. The seizure threshold and severity were determined by measuring the pulse number threshold (PNT) and the primary afterdischarge duration (ADD), respectively. A single injection of either 5 or 15 mg/kg of clozapine significantly decreased the PNT and significantly increased the primary ADD, indicating a proconvulsant action. The repeated administration of clozapine (1.5, 5, or 15 mg/kg, i.p.) produced dose-dependent, proconvulsant effects by significantly decreasing the PNT and by significantly increasing the primary ADD. In contrast to clozapine, the acute administration of haloperidol did not significantly alter the PNT or the primary ADD. The repeated administration of haloperidol (0.5 and 1.5 mg/kg, i.p.), unlike clozapine, significantly decreased the primary ADD, but did not alter the PNT. Overall, clozapine produces a greater proconvulsant action than haloperidol in an animal model of hippocampal seizures.

Can the fatal toxicity of antidepressant drugs be predicted with pharmacological and toxicological data?

Antidepressant drugs are among the most common drugs involved in fatal poisoning and large variations between antidepressant drugs have been noted. Despite the fact that a large number of studies have calculated a fatal toxicity index (FTI) for antidepressants, no serious attempts have been made to compare the differences in fatal toxicity against known pharmacological and toxicological differences in receptor affinity. It is potentially from such data that screening of drugs during their pre-clinical development can be facilitated. We examined correlations between the FTI and noradrenaline (norepinephrine)/serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibition selectivity, the dose that is lethal to 50% of animals (LD50), lipid solubility, and antagonist activity at cholinergic, histaminergic, alpha-adrenergic and gamma-aminobutyric acid (GABA)A receptors or sodium and potassium channel blocking effects. We obtained data on the number of fatal poisonings between 1983 and 1992 in England and Wales caused by a single antidepressant drug from the Department of Health in the UK. This number was divided by the number of prescriptions in England for these drugs over this time to derive a FTI of deaths per million prescriptions. The highest FTIs were for amoxapine, viloxazine, desipramine and dothiepin. Lofepramine, paroxetine and fluoxetine had very low FTIs. Using Poisson regression, there was a significant positive relationship between the FTI of antidepressant drugs and their lethal toxicity in animals, and measures of their cardiac effects. The relative noradrenaline/serotonin reuptake inhibition, lipid solubility and their potency at histamine H1, muscarinic and alpha 1-adrenergic receptors had no substantial association with the FTI. Limited data suggest that some cardiac effects and potency as a GABAA antagonist may be important predictors of significant toxicity. Further data using standardised bio-assays are needed to compare the direct cardiac effects of antidepressants. Thus, the best current pre-clinical indicator of fatal toxicity in humans is the LD50 in animal studies. Clearly, there are humane and practical reasons for developing a better pre-clinical indicator of toxicity in overdose for this rapidly expanding group of drugs.

Differential effects of acute administration of clozapine or haloperidol on local cerebral glucose utilization in the rat

We employed the [14C]2-deoxyglucose method in order to map local brain metabolic activity of rats administered 1, 5, or 20 mg/kg of clozapine, or 0.5 mg/kg of haloperidol, as compared to saline. Clozapine produced a dose-dependent reduction of glucose utilization. At the dose of 1 mg/kg, the effects were limited to limbic areas. An additional number of structures were significantly affected following administration of 5 mg/kg (the whole hippocampal formation and septal area, and cortical limbic areas). The dose of 20 mg/kg markedly reduced glucose utilization in most of the areas examined. Haloperidol (0.5 mg/kg) reduced glucose utilization of the orbital cortex, hippocampal formation and septal area, globus pallidus, amygdala, ventral thalamus, and substantia nigra reticulata. The results show that acute administration of clozapine or haloperidol are associated with different distribution patterns of altered cerebral energy metabolism. Clozapine differently from haloperidol, reduces energy metabolism of the nucleus accumbens and other limbic areas. Haloperidol, but not clozapine (1 or 5 mg/kg), affects the substantia nigra reticulata.

Clozapine and some other antipsychotic drugs may preferentially block the same subset of GABA(A) receptors

Selective blockade of a subset of GABA(A) receptors may be involved in the antipsychotic effects of Clozapine and several other antipsychotic drugs. Seven antipsychotic drugs, and 11 drugs classified as antidepressants that only partially reverse the inhibitory effect of 1 microM GABA on [35S]TBPS binding, do not yield additive reversal when tested pairwise with Clozapine, which also only partially reverses the inhibitory effect of GABA. This suggests that all of these antipsychotic/antidepressant drugs may block a common subset of GABA(A) receptors. DMCM and Ro 5-4864 are also partial reversers of GABA's inhibitory effect, but they yield additive reversals when tested pairwise with the antipsychotic/antidepressant drugs, and also with each other, suggesting that DMCM, Ro 5-4864, and the antipsychotic drugs define three heterogeneous subsets of GABA(A) receptors, with variable overlap, depending on the drug. Several potent ligands for benzodiazepine binding sites can block the GABA inhibitory effects of DMCM and Ro 5-4864, but with different patterns: the ligands generally blocked DMCM less potently, but more completely than Ro 5-4864. Ro 5-4864 was not blocked by Flumazenil or CGS-8216, ligands that potently blocked DMCM. Nine additional antipsychotic/antidepressant drugs, as well as Clozapine, and 7 "classical" GABA(A) receptor blockers, all of which reversed GABA nearly completely, when tested at lower concentrations that only reverse approximately 20-35%, yielded almost complete additivity when tested pairwise with DMCM or Ro 54864. Another convulsant benzodiazepine, KW-1937, a positional isomer of Brotizolam, fully reverses the inhibitory effect of 1 microM GABA. At a lower concentration yielding about 50% reversal, KW-1937 is completely additive with DMCM, but entirely nonadditive with Ro 5-4864. The 50% reversal obtained with KW-1937 was potently blocked by Triazolam, but with a plateau similar to that obtained with Ro 5-4864. The results with KW- 1937 suggest that its 50% reversal largely corresponds to the reversal obtained with Ro 5-4864, and that virtually all of the [35S]TBPS binding sites inhibited by 1 microM GABA are coupled to benzodiazepine binding sites. The fraction of GABA(A) receptors preferentially blocked by all the antipsychotic/antidepressant drugs, roughly 25% of the [35S]TBPS binding sites inhibited with 1 microM GABA, are sensitive to KW-1937, but not to DMCM or to Ro 5-4864.

Inhibition of GABAA receptor chloride channel by quinolones and norfloxacin-biphenylacetic acid hybrid compounds

Receptor binding studies have shown that the combination of some new quinolone antibacterial agents with 4-biphenylacetic acid (BPAA), a metabolite of fenbufen, inhibits GABAA receptors. In order to elucidate further the mechanism of these drug interactions, the effect of quinolone antibacterial agents on muscimol-stimulated 36Cl- uptake in rat cerebral cortical synaptoneurosomes was investigated in the absence or presence of BPAA. In the absence of BPAA, quinolones such as norfloxacin (NFLX) and enoxacin attenuated muscimol-stimulated 36Cl- uptake at 10 microM and above. In combination with 10 microM BPAA, the inhibitory effect of these drugs was potentiated and there was a parallel shift of the inhibition curves to the left for these drugs. BPAA alone (1 and 10 microM) did not affect basal or muscimol-stimulated 36Cl- uptake. Hybrid molecules of NFLX and BPAA were synthesized and their inhibitory potency was also investigated. Inhibition curves of muscimol-stimulated 36Cl- uptake revealed that a hybrid with a -CONH(CH2)3- chain between NFLX and BPAA (flexible structure) (1 nM-20 microM) inhibited muscimol-stimulated 36Cl- uptake more potently than did the combination of NFLX (10 nm-100 microM) and 10 microM BPAA. In contrast, another hybrid linked by -CONH-(stretched structure) exhibited a weak inhibitory effect at 10 microM. These results suggest that quinolones in combination with BPAA bind to GABAA receptors, thus inhibiting Cl- channel activity, and that the inhibitory potency of quinolones may be enhanced by an intermolecular interaction with BPAA.

GABAA receptor pharmacology

gamma-Aminobutyric acid (GABA)A receptors for the inhibitory neurotransmitter GABA are likely to be found on most, if not all, neurons in the brain and spinal cord. They appear to be the most complicated of the superfamily of ligand-gated ion channels in terms of the large number of receptor subtypes and also the variety of ligands that interact with specific sites on the receptors. There appear to be at least 11 distinct sites on GABAA receptors for these ligands.

Possible intermolecular interaction between quinolones and biphenylacetic acid inhibits gamma-aminobutyric acid receptor sites

The combination of some new quinolone antibacterial agents with 4-biphenylacetic acid (BPAA), a metabolite of fenbufen, is known to specifically induce functional blockade of the gamma-aminobutyric acid (GABA) receptors. The mechanisms of these drug interactions were further examined. Scatchard analysis of [3H]muscimol binding to rat brain plasma membranes in the presence of enoxacin and BPAA revealed that a significant decrease in the number of muscimol binding sites was produced without affecting the affinity of binding to the receptors. In the presence of norfloxacin, BPAA inhibited muscimol binding the most potently of the six BPAA-related compounds tested. Fenbufen and 9,10-dihydro-gamma-oxo-2-phenanthrenebutyric acid also inhibited the binding, and 4-biphenylcarboxylic acid and methyl 4-biphenylacetate inhibited it slightly, but 3-benzoylpropionic acid exhibited no competitive inhibition. Accordingly, hybrid molecules of norfloxacin and BPAA were synthesized for stereochemical analysis of these drug interactions. A hybrid with a -CONH(CH2)3- chain between norfloxacin and BPAA (flexible structure) inhibited muscimol binding, and intracisternal injection of this hybrid caused clonic convulsions in mice more potently than the combination of norfloxacin and BPAA did. In contrast, a hybrid linked by -CONH- (stretched structure) showed almost no such inhibitory effect. 1H NMR analysis indicated the presence of intramolecular attraction at the quinoline ring of the hybrid exhibiting the antagonistic activity. These results suggest the possibility that quinolones and BPAA interact with the GABA receptor at nearby sites and that the binding affinity of quinolones to the GABA receptors is largely enhanced by the intermolecular interaction with BPAA.