Benzodiazepine interaction with adenosine systems explains some anomalies in GABA hypothesis

Synthesis of 11-aryl-5H-imidazo[2,1-c][1,4]benzodiazepines and their benzodiazepine and A1 adenosine binding activity

In the context of a research program aimed at elucidating the properties of the 5H-imidazo[2,1-c][1.4]benzodiazepine system, a series of 11-aryl-5H-imidazo[2,1-c][1,4]benzodiazepines (3a-i) and their 10,11-dihydro-derivatives (4a-i) has been synthesized. The synthetic strategy includes the preparation of the aryl-[1-(2-nitrobenzyl)-1H-imidazol-2-yl]methanones (5a-i) followed by their reduction and subsequent cyclization. Affinities of compounds 3a-i and 4a-i for central benzodiazepine as well as for adenosine A1-receptors were determined by radioligand binding assays. Among the unsaturated analogues, the highest activity at both receptors is displayed by 1H-(2-thienyl) derivative 3e. The hydrogenated analogues 4a-i do not exhibit considerable binding affinity either for central benzodiazepine or for adenosine A1-receptors.

Effects of diazepam on extracellular brain neurotransmitters in pilocarpine-induced seizures in rats

The present study was undertaken to gain insights into the mechanism of action of diazepam in focally-evoked pilocarpine-induced seizures by concomitantly assessing the changes produced in the extracellular levels of glutamate, GABA (gamma-aminobutyric acid) and dopamine. In vivo microdialysis, coupled to continuous monitoring of electrocorticographic (ECoG) recordings, was performed in freely moving rats. Intrahippocampal perfusion with 10 mM pilocarpine (40 min, 2 microl/min) produced limbic seizures. A single dose of intraperitoneal diazepam (5 mg/kg) was administered 2 h after pilocarpine perfusion was started. Dialysates were sampled both from hippocampus and cerebellum and analysed by microbore liquid chromatography. Diazepam produced instant inhibition of behavioural and ECoG seizure activity. Pilocarpine-induced increases in the extracellular levels of glutamate and dopamine in hippocampus were promptly reduced by diazepam. No concurrent alterations in pilocarpine-induced increases in the extracellular levels of GABA in either hippocampus or cerebellum were seen. Pilocarpine enhanced cerebellar glutamate levels only transiently and levels returned to baseline before diazepam administration. No further changes in cerebellar glutamate levels were observed with diazepam. Our findings suggest that the anti-convulsant action of diazepam against pilocarpine-induced seizures is associated with a prompt attenuation of extracellular hippocampal glutamate overflow without concurrent alteration of pilocarpine-induced increases in endogenous GABA levels. Diazepam also significantly decreased pilocarpine-induced increases in extracellular dopamine levels within the hippocampus. No immediate alterations of the basal levels of the neurotransmitters monitored were observed with diazepam.

Midazolam inhibits long-term potentiation through modulation of GABAA receptors

Benzodiazepine drugs (BZ) are used for anxiety, insomnia, and seizures. They worsen memory, especially in large doses, but the mechanism of this action is uncertain. In micromolar concentrations, benzodiazepines have been shown to reduce long-term potentiation (LTP), which could be a cellular basis for their amnesic action. We have found that the LTP-inhibiting effects of BZ occur in the nanomolar concentrations attained in humans, and that this effect occurs through modulation of GABAA receptor function. We recorded extracellular synaptic input/output (I/O) curves for population spikes (PS) and EPSPs in rat hippocampal slices before and after induction of LTP. LTP increased maximal PS and EPSPs and shifted I/O curves for PS and EPSPs to the left, reflecting increased synaptic responsiveness after LTP. Curves relating EPSPs to PS were also shifted, so that after LTP larger PS were elicited for the same size EPSP (E-S potentiation). Midazolam (0.5 microM) markedly inhibited the left-shift in PS I/O curves due to E-S potentiation but did not significantly affect other parameters. 8-Phenyltheophylline (10 microM), an adenosine receptor antagonist, did not prevent midazolam inhibition of LTP. Bicuculline, a GABAA receptor antagonist, caused a dose-dependent antagonism of midazolam's LTP inhibition. Our results suggest that benzodiazepines reduce LTP primarily through reduction of E-S potentiation, and that this effect occurs through modulation of GABAA receptor function. This could in part account for the ability of benzodiazepines to disturb new memory formation.

Enhancement by benzodiazepines of the inhibitory effect of adenosine on skeletal neuromuscular transmission

1. Interactions of benzodiazepines with adenosine on the neuromuscular transmission were studied in mouse diaphragm preparations. 2. In tubocurarine (0.6-0.8 microM)-partially paralyzed preparations, diazepam (35 microM) and Ro 5-4864 (3-30 microM), a peripheral type benzodiazepine receptor agonist, potentiated the inhibitory effect of adenosine on indirect twitch responses. 3. The central type receptor agonist, clonazepam did not affect the inhibitory effect of adenosine. 4. The peripheral benzodiazepine receptor antagonist, PK11195 (1-10 microM) attenuated the adenosine inhibition and antagonized the potentiation by Ro 5-4864. 5. Ro 5-4864 failed to enhance further the inhibitory effect of adenosine in the presence of dipyridamole, an adenosine uptake inhibitor that also potentiated adenosine inhibition. 6. Neither Ro 5-4864 nor PK 11195 affected the inhibition produced by a stable adenosine analogue, 2-chloroadenosine, which is not a substrate for the adenosine uptake system. 7. Ro 5-4864 did not affect endplate potentials (e.p.ps) in the absence of adenosine, but reduced the amplitude of e.p.ps in the presence of adenosine without affecting miniature e.p.ps. 8. It is suggested that benzodiazepines potentiate the adenosine-effected presynaptic inhibition of neuromuscular transmission by an inhibition of adenosine uptake through activation of peripheral type benzodiazepine receptors.

Opposite effects of midazolam and beta-carboline-3-carboxylate ethyl ester on the release of dopamine from rat nucleus accumbens measured by in vivo microdialysis

This report describes the effects of midazolam and beta-carboline-3-carboxylate ethyl ester (beta-CCE) on extracellular concentrations of dopamine in the nucleus accumbens of freely moving rats measured by in vivo microdialysis. The two compounds had opposite effects, midazolam (0.075 and 0.15 mg/kg i.v.) dose dependently decreasing, and beta-CCE (3 and 10 mg/kg i.p.) dose dependently increasing, dialysate concentrations of dopamine. Flumazenil (6 micrograms/kg i.v.) did not affect the efflux of dopamine but it prevented the effects of both midazolam and beta-CCE on dopamine efflux. N6-Cyclohexyladenosine (0.1, and 1 mg/kg i.p.), a selective adenosine A1 agonist, dose dependently increased the efflux of dopamine. This effect was blocked by 8-cyclopentyl-1,3-dipropylxanthine (25 mg/kg i.p.), a selective adenosine A1 receptor antagonist, a dose which given alone did not affect dopamine efflux; responses to midazolam were not affected. 3,7-Dimethyl-1-propargylxanthine (1 and 3 mg/kg i.p.), a selective adenosine A2 receptor antagonist, did not mimic the effects of beta-CCE. The results suggest that midazolam and beta-CCE modulate dopamine release in the nucleus accumbens by an action at the benzodiazepine binding site associated with the GABAA receptor complex.

Perceptual masking of the chlordiazepoxide discriminative cue by both caffeine and buspirone

Twelve male Sprague-Dawley rats were trained to discriminate between the interoceptive stimulus attributes of 5 mg/kg chlordiazepoxide (CDP) and saline in a two-lever operant task under a fixed-ratio 10 (FR-10) schedule of food reinforcement. Caffeine, buspirone, and Ro 15-1788 failed to engender complete generalization when tested in combination with saline. In drug interaction test sessions caffeine (56 mg/kg) blocked the discriminative stimulus properties of the training dose of CDP and shifted the CDP discriminative dose-response function to the right. This rightward shift in CDP discriminative function was paralleled by a concomitant downward shift in the rate-of-responding dose-response function. Drug interaction test sessions conducted with 3.2 mg/kg of buspirone in combination with various doses of CDP engendered a downward shift in both the discriminative and rate-of-responding dose-response functions. Because 3.2 mg/kg buspirone in combination with the training dose of CDP resulted in complete response rate suppression, additional combination tests were conducted with 3 mg/kg CDP, a dose which reliably engendered > 90% CDP-appropriate responding, and various doses of buspirone. Similar to the CDP-caffeine interactions, buspirone blocked the cueing properties of 3 mg/kg CDP with a parallel reduction in response rates. Interaction test sessions conducted with Ro 15-1788 and CDP resulted in rightward shifts in both the discriminative and rate functions of CDP. We suggest that the interactions between CDP and both caffeine and buspirone resulted from the perceptual masking of the interoceptive (subjective) effects of CDP, whereas the interaction between Ro 15-1788 and CDP reflect pharmacological antagonism.

Contrasting effects of adenosine A1 and A2 receptor ligands in different chemoconvulsive rodent models

The pro- and anticonvulsive properties of selective adenosine A1 and A2 receptor agonists and antagonists were investigated in mice using seizure models involving a specific blockade of adenosine A1 and A2 receptors, modulation of the gamma-aminobutyric acid/benzodiazepine receptor complex or activation with the excitatory amino acid glutamate. The selective adenosine A1 receptor agonists N-cyclopentyladenosine (CPA) and R-N-(phenylisopropyl) adenosine (R-PIA) in doses of 1 and 10 mg/kg i.p. potentiated seizures induced by the selective adenosine A1 receptor antagonist 8-[4-[[[[(2-aminoethyl)amino]carbonyl]methyl]oxy]-phenyl]- 1,3-dipropylxanthine (XAC). Likewise, the selective adenosine A2 receptor agonists N-[(2-methylphenyl)methyl]adenosine (metrifudil) and N-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl]adenosine (DPMA), in doses of 30 and 100 mg/kg i.p., respectively, potentiated seizures induced by the selective adenosine A2 receptor antagonist 3,7-dimethyl-1-propargylxanthine (DMPX). In contrast, the adenosine A1 and A2 receptor agonists both antagonized seizures induced by methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM--an inverse agonist at benzodiazepine receptors) and the adenosine A1 receptor agonists also protected against seizures induced by glutamate. Paradoxically, the selective adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT) antagonized DMCM- and pentylenetetrazole-induced seizures. Thus, it appears that adenosine A1 and A2 receptor agonists can be both pro- and anticonvulsive depending on the mechanism of action of the chemoconvulsant used in the seizure model. The findings with CPT suggest that other types of adenosine analogues than agonists may possess anticonvulsive properties.

Effects of benzodiazepine administration on A1 adenosine receptor binding in-vivo and ex-vivo

The adenosine receptor has been implicated in the central mechanism of action of benzodiazepines. The specific binding of an A1-selective adenosine antagonist radioligand, [3H]8-cyclopentyl-1,3-dipropylxanthine, was measured in-vivo in mice treated with alprazolam (2 mg kg-1, i.p.), lorazepam (2 mg kg-1, i.p.) and vehicle. Binding studies were performed in-vivo and ex-vivo in mice receiving continuous infusion of alprazolam (2 mg kg-1 day-1), lorazepam (2 mg kg-1 day-1) and vehicle by mini-osmotic pumps for 6 days. Continuous infusion of alprazolam and lorazepam significantly decreased specific binding by 34 and 53%, respectively, compared with vehicle treatment (P less than 0.01). Single doses of alprazolam and lorazepam induced a similar trend in specific binding in-vivo (P = 0.07). There were no alterations in A1-receptor density (Bmax) or affinity (Kd) in cortex, hippocampus or brainstem in ex-vivo studies. Benzodiazepine treatment may diminish A1- receptor binding in-vivo by inhibiting adenosine uptake or by direct occupancy of the A1 adenosine receptor recognition site.

Functional activity of the adenosine binding enhancer, PD 81,723, in the in vitro hippocampal slice

The adenosine receptor binding enhancer, PD 81,723, enhances the inhibitory effects of exogenously applied adenosine in a dose-dependent manner in hippocampal brain slices. Extracellular recordings were obtained from the CA1 cell layer while electrically stimulating the stratum radiatum. Application of 1, 10 or 32 microM PD 81,723 in the presence of adenosine resulted in a dose-dependent reduction in the amplitude of the population spike which could be partially reversed by theophylline. In addition, hippocampal slices exposed to adenosine showed greater paired-pulse facilitation compared to control and this facilitation was significantly enhanced by the presence of PD 81,723. PD 81,723 had no effect when administered alone, but required the presence of adenosine. These results demonstrate that in addition to enhancing adenosine receptor binding, PD 81,723 also enhances the functional activity of adenosine in the hippocampal slice.

Influence of diazepam, alpidem, zolpidem and zopiclone, on the response to adenosine of the guinea pig isolated trachea

It has been reported that dipyridamole and some benzodiazepines potentiate the responses to adenosine in peripheral organs and in particular in the guinea pig isolated atria or trachea by inhibition of adenosine uptake and/or metabolism. In this study, we have examined the sensitization of guinea pig isolated trachea to relaxant responses to adenosine produced by dipyridamole, diazepam and 3 compounds chemically unrelated to benzodiazepines but which display selective agonistic activity towards the central (zolpidem and zopiclone) or peripheral (alpidem) type benzodiazpine receptors. In preparations under spontaneous tone and in the absence of adenosine, dipyridamole (10(-5) M) and diazepam (10(-5)-10(-4) M), alpidem (3 x 10(-6) M-10(-5) M) and zopiclone (10(-6)-10(-4) M) induced a relaxation of the airway smooth muscle. In addition, dazepam (10(-4) M) attenuated the phasic response to histamine (10(-5) M). Dipyridamole (10(-5) M) and diazepam (10(-4) M) respectively produced a 56.2 and 32.4-fold potentiation of adenosine relaxant effects. Alpidem (10(-6)-10(-5) M), zolpidem (10(-6)-10(-4) M) and zopiclone (10(-6)-10(-4) M) were without any significant effect on the adenosine concentration-response curves. Moreover, alpidem, zolpidem, and zopiclone did not modify the 2-chloroadenosine dose-response curves nor the diazepam induced sensitization of adenosine-induced relaxation. In conclusion, adenosine sensitization of the guinea pig isolated trachea caused by diazepam might involve a peripheral benzodiazpine receptor subtype coupled to a nucleoside transporter system which is different from those recognized by compounds derived from the imidazopyridine series.

Physiological and pharmacological properties of adenosine: therapeutic implications

Adenosine is a nucleoside which has been shown to participate in the regulation of physiological activity in a variety of mammalian tissues, and has been recognized as a homeostatic neuromodulator. It exerts its actions via membrane-bound receptors which have been characterized using biochemical, electrophysiological and radioligand binding techniques. Adenosine has been implicated in the pharmacological actions of several classes of drugs. A number of studies strongly suggest that the nucleoside may regulate cellular activity in many pathological disorders and, in that respect, adenosine derivatives appear as promising candidates for the development of new therapeutic compounds, such as anticonvulsant, anti-ischemic, analgesic and neuroprotective agents.

Adenosine analogs attenuate tolerance-dependence on alprazolam

1. Tolerance to and physical dependence on alprazolam were induced in mice by administering two doses of a slow release preparation. 2. Physical dependence was evaluated by the abstinence syndrome induced by flumazenil. Tolerance was studied by measuring the motor incoordination induced by a test dose of alprazolam. 3. The intensity of tolerance was decreased by the administration of L-phenylisopropyl adenosine (L-PIA), cyclopentyl adenosine (CPA), cyclohexyl adenosine (CHA), N-ethylcarboxamide adenosine (NECA), 8-phenyltheophylline (8-PTP) and theophylline (TP). 4. The intensity of the abstinence syndrome induced by flumazenil was attenuated by L-PIA, CPA NECA, TP and 8-PTP. 5. The results suggest that benzodiazepines may exert, at least in part, their effects by involving adenosine in the central nervous system.

Possible involvement of pertussis toxin-sensitive G proteins and D2 dopamine receptors in the A1 adenosine receptor-adenylate cyclase system in rat cerebral cortex

To identify the involvement of dopamine receptors in the transmembrane signaling of the adenosine receptor-G protein-adenylate cyclase system in the CNS, we examined the effects of pertussis toxin (islet-activating protein, IAP) and apomorphine on A1 adenosine agonist (-)N6-R-[3H]phenylisopropyladenosine ([3H]PIA) and antagonist [3H]xanthine amine congener ([3H]XAC) binding activity and adenylate cyclase activity in cerebral cortex membranes of the rat brain. Specific binding to a single class of sites for [3H]XAC with a dissociation constant (KD) of 6.0 +/- 1.3 nM was observed. The number of maximal binding sites (Bmax) was 1.21 +/- 0.13 pmol/mg protein. Studies of the inhibition of [3H]XAC binding by PIA revealed the presence of two classes of PIA binding states, a high-affinity state (KD = 2.30 +/- 1.16 nM) and a low-affinity state (KD = 1.220 +/- 230 nM). Guanosine 5'-(3-O-thio)triphosphate or IAP treatment reduced the number of the high-affinity state binding sites without altering the KD for PIA. Apomorphine (100 microM) increased the KD value 10-fold and decreased Bmax by approximately 20% for [3H]PIA. The effect of apomorphine on the KD value increase was irreversible and due to a conversion from high-affinity to low-affinity states for PIA. The effect was dose dependent and was mediated via D2 dopamine receptors, since the D2 antagonist sulpiride blocked the phenomenon. The inhibitory effect of PIA on adenylate cyclase activity was abolished by apomorphine treatment. There was no effect of apomorphine on displacement of [3H]quinuclidinyl benzilate (muscarinic ligand) binding by carbachol. These data suggest that A1 adenosine receptor binding and function are selectively modified by D2 dopaminergic agents.

Relaxant effect of benzodiazepines on uterine rings isolated from estrogen-treated rats

K(+)-contracted rat uterine rings were relaxed in a concentration-dependent manner by the benzodiazepines Ro 5-4864, diazepam and clonazepam, as well as by the putative peripheral benzodiazepine antagonist PK 11195. The relaxation induced by diazepam was not counteracted by the central antagonist Ro 15-1788 (10 microM), and the relaxant effects of Ro 5-4864 and of diazepam were not prevented by either the GABAA antagonist bicuculline (10 microM) or the beta-adrenoceptor antagonist propranolol (1 microM). The mechanism underlying the relaxant effects of benzodiazepines on K(+)-contracted uterine rings is still under study.

Review: Normal and abnormal central nervous system GABA metabolism in childhood

The metabolism and function of central nervous system GABA is briefly reviewed. Hereditary disorders of the GABA metabolism presenting in childhood are discussed with particular emphasis on the recently identified succinic semialdehyde dehydrogenase deficiency and GABA-transaminase deficiency, and on diseases associated with low CSF GABA which await further unravelling. Low CSF GABA concentrations are not always associated with convulsions. A separate section is devoted to the CSF as a tool in the diagnosis of these disorders. Finally, we present a few diagnostic and therapeutic guidelines.

Therapeutic and hypothermic properties of diazepam altered by a diazepam-chlorpromazine association

Rats were injected (IP) with diazepam (2.5 mg/kg) and chlorpromazine (10.0 mg/kg) with a 30-min interval between the two injections. After 10-12 repeated drug pairings of this type, the thermic, muscle relaxant, and anxiolytic responses of the animals to diazepam alone were tested. These tests revealed: 1) an enhanced hypothermia (rectal temperature), 2) an attenuated muscle relaxant effect (inclined plane test), and 3) a potentiated anxiolytic effect (plus-maze test). Although various interdrug associations have previously been demonstrated using other measures of conditioning, this is the first instance in which changes in the therapeutic effects of a drug (in this case, muscle relaxation and anxiety reduction) have been obtained with this procedure.

Diminution by benzodiazepines of the chronotropic responses to noradrenaline in rat isolated atria

The effects of various benzodiazepines on chronotropic responses were assayed in spontaneously beating rat isolated atria. The increases in atrial rate obtained from concentration-response curves to noradrenaline were reduced dose dependently by both the peripheral agonist, Ro 5-4864 5 and 10 microM, and the mixed agonist, diazepam 5, 10 and 50 microM, but not by the central benzodiazepine agonist, clonazepam 10 and 30 microM. The inhibitory effects of the benzodiazepines on the atrial responses to noradrenaline were not counteracted by either the peripheral benzodiazepine antagonist, PK 11195 10 microM, or the central benzodiazepine antagonist, Ro 15-1788 10 and 100 microM. Both 10 microM Ro 5-4864 and 10 microM diazepam also reduced the increases in atrial rate produced by either the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, or the adenylate cyclase activator, forskolin. On the contrary, diazepam and Ro 5-4864 did not modify the chronotropic responses of the atria either to direct exposure to CaCl2 or to the calcium agonist, BAY K 8644. The increases in the intracellular levels of cAMP induced by noradrenaline were not modified by Ro 5-4864 and were even increased by 50% in the presence of diazepam. It is concluded that benzodiazepines probably reduce the chronotropic responses to noradrenaline in rat isolated atria through the interaction with the cAMP-linked chain of events that follows the activation of beta-adrenoceptors.

Electrophysiology of benzodiazepine receptor ligands: multiple mechanisms and sites of action

Electrophysiology of BZR ligands has been reviewed from different points of view. A great effort was made to critically discuss the arguments for and against the temporarily leading hypothesis of the mechanism of action of BZR ligands, the GABA hypothesis. As has been discussed at length in the present article, an impressive body of electrophysiological and biochemical evidence suggests an enhancement of GABAergic inhibition in CNS as a mechanism of action of BZR agonists. Biochemical data even indicate a physical coupling between GABA recognition sites and BZR which, together with the effector site build-up by Cl- channels, form a supramolecular GABAA/BZR complex. By binding to a specific site on this complex, BZR agonists allosterically increase and BZR inverse agonists decrease the gating of GABA-linked Cl- channels, whereas BZR antagonists bind to the same site without an appreciable intrinsic activity and block the binding and action of both agonists as well as inverse agonists. While this model is supported by many electrophysiological experiments performed with BZR ligands in higher nanomolar and lower micromolar concentrations, it does not explain much controversial data from animal behavior and, more importantly, is not in line with electrophysiological effects obtained with low nanomolar BZ concentrations. The latter actions of BZR ligands in brain slices occur within a concentration range compatible with concentrations of BZ observed in CSF fluid, which would be expected to be found in the biophase (receptor level) during anxiolytic therapy in man. Enhanced K+ conductance seems to be a suitable candidate for this effect of BZR ligands. This direct action on neuronal membrane properties may underlie the many electrophysiological observations with extremely low systemic doses of BZR ligands in vivo which demonstrated a depressant effect on spontaneous neuronal firing in various CNS regions. Skeletomuscular spasticity and epilepsy are two neurological disorders, where both the enhanced GABAergic inhibition and increased K+ conductance may contribute to the therapeutic effect of BZR agonists, since electrophysiological and behavioral studies strongly support GABA-dependent as well as GABA-independent action of BZR ligands elicited by low to intermediate doses of BZ necessary to evoke anticonvulsant and muscle relaxant effects. Somewhat higher doses of BZR ligands, inducing sedation and sleep, lead perhaps to the only pharmacologically relevant CNS concentrations (ca. 1 microM) which might be due entirely to increased GABAergic inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)