The role of adenosine in the central actions of the benzodiazepines

In vivo phenotypic validation of adenosine receptor-dependent activity of non-adenosine drugs

Some non-adenosinergic drugs are reported to also act through adenosine receptors (ARs). We used mouse hypothermia, which can be induced by agonism at any of the four ARs, as an in vivo screen for adenosinergic effects. An AR contribution was identified when a drug caused hypothermia in wild type mice that was diminished in mice lacking all four ARs (quadruple knockout, QKO). Alternatively, an adenosinergic effect was identified if a drug potentiated adenosine-induced hypothermia. Four drugs (dipyridamole, nimodipine, cilostazol, cyclosporin A) increased the hypothermia caused by adenosine. Dipyridamole and nimodipine probably achieved this by inhibition of adenosine clearance via ENT1. Two drugs (cannabidiol, canrenoate) did not cause hypothermia in wild type mice. Four other drugs (nifedipine, ranolazine, ketamine, ethanol) caused hypothermia, but the hypothermia was unchanged in QKO mice indicating non-adenosinergic mechanisms. Zinc chloride caused hypothermia and hypoactivity; the hypoactivity was blunted in the QKO mice. Interestingly, the antidepressant amitriptyline caused hypothermia in wild type mice that was amplified in the QKO mice. Thus, we have identified adenosine-related effects for some drugs, while other candidates do not affect adenosine signaling by this in vivo assay. The adenosine-modulating drugs could be considered for repurposing based on predicted effects on AR activation.

β-Caryophyllene, a CB2 receptor agonist produces multiple behavioral changes relevant to anxiety and depression in mice

Recent evidence suggests that the cannabinoid receptor subtype 2 (CB2) is implicated in anxiety and depression disorders, although few systematic studies in laboratory animals have been reported. The aim of the current experiments was to test the effects of the CB2 receptor potent-selective agonist β-caryophyllene (BCP) in animals subjected to models of anxiolytic- and antidepressant-like effects. Therefore effects of BCP (50mg/kg) on anxiety were assessed using the elevated plus maze (EPM), open field (OF), and marble burying test (MBT). However for depression, the novelty-suppressed feeding (NSF), tail suspension test (TST), and forced swim tests (FST) were used. Results indicated that adult mice receiving BCP showed amelioration of all the parameters observed in the EPM test. Also, BCP significantly increased the time spent in the center of the arena without altering the general motor activity in the OF test. This dose was also able to decrease the number of buried marbles and time spent digging in the MBT, suggesting an anti-compulsive-like effect. In addition, the systemic administration of BCP reduced immobility time in the TST and the FST. Finally, BCP treatment decreased feeding latency in the NSF test. Most importantly, pre-administration of the CB2 receptor antagonist AM630, fully abrogated the anxiolytic and the anti-depressant effects of BCP. Taken together, these preclinical results suggest that CB2 receptors may provide alternative therapeutic targets for the treatment of anxiety and depression. The possibility that BCP may ameliorate the symptoms of these mood disorders offers exciting prospects for future studies.

Actions of a novel water-soluble benzodiazepine-receptor agonist JM-1232(-) on synaptic transmission in adult rat spinal substantia gelatinosa neurons

Although the intrathecal administration of JM-1232(-) reportedly produces antinociception, this action has not yet been examined at the cellular level. We examined the action of JM-1232(-) on synaptic transmission in spinal substantia gelatinosa (SG) neurons which play an important role in regulating nociceptive transmission from the periphery. The whole-cell patch-clamp technique was applied to the SG neurons of adult rat spinal cord slices. Bath-applied JM-1232(-) prolonged the decay phase of GABA(A)-receptor mediated spontaneous inhibitory postsynaptic current (sIPSC) and increased its frequency without a change in amplitude. The former but not latter action was sensitive to a benzodiazepine-receptor antagonist flumazenil. JM-1232(-) also increased glycinergic sIPSC frequency with no change in amplitude and decay phase. On the other hand, glutamatergic spontaneous excitatory transmission was unaffected by JM-1232(-). These results indicate that JM-1232(-) enhances inhibitory transmission by (1) prolonging the decay phase of GABAergic sIPSC through benzodiazepine-receptor activation and by (2) increasing the spontaneous release of GABA and glycine from nerve terminals without its activation. This enhancement could contribute to at least a part of the antinociceptive effect of intrathecally-administered JM-1232(-).

Pharmacology of the beta-carboline FG-7,142, a partial inverse agonist at the benzodiazepine allosteric site of the GABA A receptor: neurochemical, neurophysiological, and behavioral effects

Given the well-established role of benzodiazepines in treating anxiety disorders, beta-carbolines, spanning a spectrum from full agonists to full inverse agonists at the benzodiazepine allosteric site for the GABA(A) receptor, can provide valuable insight into the neural mechanisms underlying anxiety-related physiology and behavior. FG-7,142 is a partial inverse agonist at the benzodiazepine allosteric site with its highest affinity for the alpha1 subunit-containing GABA(A) receptor, although it is not selective. FG-7,142 also has its highest efficacy for modulation of GABA-induced chloride flux mediated at the alpha1 subunit-containing GABA(A) receptor. FG-7,142 activates a recognized anxiety-related neural network and interacts with serotonergic, dopaminergic, cholinergic, and noradrenergic modulatory systems within that network. FG-7,142 has been shown to induce anxiety-related behavioral and physiological responses in a variety of experimental paradigms across numerous mammalian and non-mammalian species, including humans. FG-7,142 has proconflict actions across anxiety-related behavioral paradigms, modulates attentional processes, and increases cardioacceleratory sympathetic reactivity and neuroendocrine reactivity. Both acute and chronic FG-7,142 treatment are proconvulsive, upregulate cortical adrenoreceptors, decrease subsequent actions of GABA and beta-carboline agonists, and increase the effectiveness of subsequent GABA(A) receptor antagonists and beta-carboline inverse agonists. FG-7,142, as a partial inverse agonist, can help to elucidate individual components of full agonism of benzodiazepine binding sites and may serve to identify the specific GABA(A) receptor subtypes involved in specific behavioral and physiological responses.

QSAR modeling of the interaction of flavonoids with GABA(A) receptor

Experimentally assigned values to binding affinity constants of flavonoid ligands towards the benzodiazepine site of the GABA(A) receptor complex were compiled from several publications, and enabled to perform a predictive analysis based on Quantitative Structure-Activity Relationships (QSAR). The best linear model established on 78 molecular structures incorporated four molecular descriptors, selected from more than a thousand of geometrical, topological, quantum-mechanical and electronic types of descriptors and calculated by Dragon software. An application of this QSAR equation was performed by estimating the binding affinities for some newly synthesized flavonoids displaying 2-,7-substitutions in the benzopyrane backbone which still do not have experimentally measured potencies.

From cAMP to adenosine: an illuminating shift of focus

In a remarkable career, straddling five decades, John Phillis pursued with fierce determination and exceptional energy the main goal of his scientific life, to throw light on the chemical agents that control brain function. Starting in Australia, he settled in North America, first in Canada, then in the USA, where his long tenure at Wayne State brought his career to its culmination.

Adenosine A1 receptors and the anticonvulsant potential of drugs effective in the model of 3-nitropropionic acid-induced seizures in mice

The role of adenosine A1 receptors in the activity of drugs and substances protecting against seizures evoked by mitochondrial toxin, 3-nitropropionic acid (3-NPA) was studied in mice. Non-selective A1/A2 adenosine receptor antagonist, aminophylline and selective A1 adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) diminished the anticonvulsive effects of diazepam, phenobarbital, valproate and gabapentin. In contrast, A1/A2 adenosine receptor antagonist, 8-(p-sulfophenyl)theophylline (8pSPT) not penetrating via blood-brain barrier was ineffective. Aminophylline and DPCPX but not 8pSPT also reversed the protective action of A1/A2 adenosine receptor agonist, 2-chloroadenosine (2-CADO) and selective A1 adenosine receptor agonist, R-N6-phenylisopropyloadenosine (R-PIA), against 3-NPA-evoked convulsions. Obtained results suggest that the central adenosine A1 receptor stimulation may play a role in the anticonvulsive potential of diazepam, phenobarbital, valproate and gabapentin in a novel model of 3-NPA-evoked seizures. Moreover, concomitant application of aminophylline with these drugs may reduce their clinical antiepileptic efficacy, especially among patients suffering from seizures related to the disturbances of mitochondrial respiratory chain.

Molecular modeling and QSAR analysis of the interaction of flavone derivatives with the benzodiazepine binding site of the GABA(A) receptor complex

A large number of structurally different classes of ligands, many of them sharing the main characteristics of the benzodiazepine (BDZ) nucleus, are active in the modulation of anxiety, sedation, convulsion, myorelaxation, hypnotic and amnesic states in mammals. These compounds have high affinity for the benzodiazepine binding site (BDZ-bs) of the GABA(A) receptor complex. Since 1989 onwards our laboratories established that some natural flavonoids were ligands for the BDZ-bs which exhibit medium to high affinity in vitro and anxiolytic activity in vivo. Further research resulted in the production of synthetic flavonoid derivatives with increased biochemical and pharmacological activities. The currently accepted receptor/pharmacophore model of the BDZ-bs (Zhang, W.; Koeler, K. F.; Zhang, P.; Cook, J. M. Drug Des. Dev. 1995, 12, 193) accounts for the general requirements that should be met by this receptor for ligand recognition. In this paper we present a model pharmacophore which defines the characteristics for a ligand to be able to interact and bind to a flavone site, in the GABA(A) receptor. closely related to the BDZ-bs. A model of a flavone binding site has already been described (Dekermendjian, K.; Kahnberg, P.; Witt, M. R.; Sterner, O.; Nielsen, M.; Liljerfors, T. J. Med. Chem. 1999, 42, 4343). However, this alternative model is based only on graphic superposition techniques using as template a non-BDZ agonist. In this investigation all the natural and synthetic flavonoids found to be ligands for the BDZ-bs have been compared with the classical BDZ diazepam. A QSAR regression analysis of the parameters that describe the interaction demonstrates the relevance of the electronic effects for the ligand binding, and shows that they are associated with the negatively charged oxygen atom of the carbonyl group of the flavonoids and with the nature of the substituent in position 3'.

Naloxone potentiates anxiolytic-like actions of diazepam, pentobarbital and meprobamate but not those of Ro19-8022 in the rat

The elevated plus-maze test was used to determine if the opiate antagonist naloxone could potentiate the anxiolytic-like effects of the benzodiazepine diazepam, the barbiturate pentobarbital, the propanediol carbamate meprobamate and the partial benzodiazepine receptor agonist [R]-1-[(10-chloro-4-oxo-3-phenyl-4H-benzo[a]quinolizin-1-yl) carbonyl]-2-pyrrolidine-methanol (Ro19-8022) in the rat. A subeffective dose of each of these compounds was combined with naloxone, 10 mg/kg. Naloxone had no effect by itself, but potentiated all drugs except Ro19-8022. The proportion of entries on the open arm increased while the total number of arms entries was not modified. These results coincide with and extend data previously obtained in the mouse. One possible explanation for naloxone's effect could be that it blocks opioid inhibition of GABAergic (gamma-aminobutyric acid) neurons thereby enhancing the effects of benzodiazepines. Another possibility is that naloxone blocks opioid effects on adenosinergic systems.

Anxiolytic-like effects of meprobamate. Interactions with an opiate antagonist in Swiss and BALB/c mice

Naloxone has previously been shown to block the effects of benzodiazepines in the Swiss but not in the BALB/c strain. We have also reported that naloxone potentiates subeffective doses of benzodiazepines in Swiss mice. In the present studies we first determined whether naloxone could block anxiolytic-like effects of meprobamate in Swiss and BALB/c mice. Then we evaluated if subeffective doses of meprobamate could be potentiated in Swiss as well as in BALB/c mice. The elevated plus-maze test and the light/dark choice procedure were used. The lowest dose of meprobamate with anxiolytic-like effects was 60 mg/kg in the BALB/c mice. This dose was effective in both the plus-maze and in the light/dark choice procedure. In Swiss mice the same dose was effective in the plus-maze, whereas 120 mg/kg was required in the light/dark choice procedure. When an effective dose of meprobamate was combined with naloxone, 10 mg/kg, no blockade of anxiolytic-like effects was obtained in any strain in any procedure. To the contrary, when a subeffective dose of meprobamate was combined with naloxone, 10 mg/kg, an anxiolytic-like effect was obtained in both strains in both procedures. The present series of experiment shows that the ability of naloxone to block anxiolytic-like drug effects do not apply to meprobamate. However, the naloxone-induced potentiation of subeffective doses previously observed after treatment with benzodiazepines or buspirone was present also after treatment with meprobamate. Moreover, although blockade of anxiolytic-like drug effects with naloxone has not been observed in BALB/c mice, potentiation was as evident in that strain as in the Swiss. This suggests that the mechanisms behind naloxone's blockade of anxiolytic-like effects are independent from those behind its potentiation of such effects.

Involvement of the adenosine neuromodulatory system in the benzodiazepine-induced depression of excitatory synaptic transmissions in rat hippocampal neurons in vitro

We investigated whether adenosine neuromodulation is involved in a benzodiazepine (midazolam)-induced depression of excitatory synaptic transmissions in the CA1 and dentate gyrus (DG) regions in rat hippocampal slices. Field excitatory postsynaptic potentials (fEPSPs), evoked by electrical stimulation of the CA1-Schaffer collateral or the DG-perforant path, were recorded with extracellular microelectrodes from CA1-stratum radiatum or DG-stratum moleculare in oxygenated ACSF. The initial slope of the fEPSPs was analyzed for assessing the drug effects. Midazolam (1 microM) transiently depressed CA1- and DG-fEPSPs. The fEPSPs were depressed to approximately 75% of the control values, and then gradually recovered. The depression was not affected by bicuculline, a GABAA receptor antagonist, although it was completely antagonized by aminophylline, an adenosine receptor antagonist. Dipyridamole (5 microM), an adenosine uptake inhibitor, depressed the fEPSPs in a similar manner to midazolam. An adenosine deaminase inhibitor, EHNA, also transiently depressed the fEPSPs, but in a different manner. Exogenous adenosine persistently depressed the fEPSPs. The effects of the drugs were not significantly different in the CA1 and DG regions. The results suggest that midazolam (1 microM) depresses excitatory synaptic transmissions through the adenosine neuromodulatory system by inhibiting adenosine uptake in the CA1 and DG regions of the hippocampus.

Exogenous adenosine potentiates hypnosis induced by intravenous anaesthetics

We investigated the effect of adenosine on hypnosis induced by thiopentone, propofol and midazolam in mice. The onset and duration of hypnosis were determined by the loss of righting reflex. Adenosine and 2-chloroadenosine caused a significant shortening of onset of sleep-time and prolongation of duration of sleep-time in all groups (p < 0.05). Dipyridamole administration before combined intravenous anaesthetic-adenosine or intravenous anaesthetic-2-chloroadenosine administration produced similar effects to adenosine (p < 0.05). The adenosine antagonist theophylline, given before intravenous anaesthetic-adenosine or intravenous anaesthetic-2-chloroadenosine administration caused a significant delay in onset of sleep-time and shortening in the duration of sleep-time (p < 0.05). We conclude that central excitatory noradrenergic neurones play an important role in adenosine, 2-chloroadenosine and dipyridamole-induced hypnotic responses to intravenous anaesthetics and their inhibition by adenosine antagonists.

Overview--flavonoids: a new family of benzodiazepine receptor ligands

Benzodiazepines (BDZs) are the most widely prescribed class of psychoactive drugs in current therapeutic use, despite the important unwanted side-effects that they produce such as sedation, myorelaxation, ataxia, amnesia, ethanol and barbiturate potentiation and tolerance. Searching for safer BDZ-receptor (BDZ-R) ligands we have recently demonstrated the existence of a new family of ligands which have a flavonoid structure. First isolated from plants used as tranquilizers in folkloric medicine, some natural flavonoids have shown to possess a selective and relatively mild affinity for BDZ-Rs and a pharmacological profile compatible with a partial agonistic action. In a logical extension of this discovery various synthetic derivatives of those compounds, such as 6,3'-dinitroflavone were found to have a very potent anxiolytic effect not associated with myorelaxant, amnestic or sedative actions. This dinitro compound, in particular, exhibits a high affinity for the BDZ-Rs (Ki = 12-30 nM). Due to their selective pharmacological profile and low intrinsic efficacy at the BDZ-Rs, flavonoid derivatives, such as those described, could represent an improved therapeutic tool in the treatment of anxiety. In addition, several flavone derivatives may provide important leads for the development of potent and selective BDZ-Rs ligands.

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.

The scorpion envenoming syndrome

The pathophysiology of the scorpion envenoming syndrome is reviewed with emphasis on the body systems commonly affected. Concepts of the mechanisms underlying venom action, as can be explained by the recently discovered effects on ionic channels, are discussed. The results of clinical analysis of cases of scorpion sting victims and animal experiments with scorpion envenomation supporting these concepts are presented. The pharmacokinetic characteristics of scorpion venoms and their correlation to the magnitude of toxic effects are presented in relation to the potentials of therapeutic intervention. The pharmacological basis of the therapeutic usefulness and toxicities of the drugs commonly used in the treatment of scorpion envenoming is also projected. Finally, the results of a successful nation-wide clinical study with serotherapy of scorpion envenoming are presented and evaluated.

Anticonvulsant activities of 4-bromobenzaldehyde semicarbazone

Some of the properties of 4-bromobenzaldehyde semicarbazone (compound IV), a prototype molecule of a new class of anticonvulsants, aryl semicarbazones, are described. Compound IV demonstrated activity in the maximal electroshock (MES) and subcutaneous pentylenetetrazol (scPTZ) tests in mice, with low neurotoxicity. When given orally to rats, it displayed high potency in the MES test and very low neurotoxicity, resulting in a high protective index (PI). Compound IV displayed no proconvulsant properties, and development of rapid tolerance was not noted. When administered intraperitoneally (i.p.) at doses of 100, 300, or 600 mg/kg to peritoneally (i.p.) at doses of 100, 300, or 600 mg/kg to rats, compound IV had no effect on levels of gamma-aminobutyric acid (GABA) or on GABA-T activity in whole brain. When tested in vitro, compound IV had no effect on rat brain GABA-T at a drug concentration of 100 microM. Although the activities of certain drug-metabolizing enzymes were increased after oral administration of compound IV to rats, these effects were less prominent than those of phenytoin (PHT) and carbamazepine (CBZ). The principal mode of action of compound IV does not appear to be an interaction with the GABAA receptor complex, and other mechanisms, involving excitatory amino acid neurotransmission, will have to be considered in future investigations of the anticonvulsant activity of this compound.

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.

Modulation of the in vivo antibody response by a benzodiazepine inverse agonist (DMCM) administered centrally or peripherally

Exposure to stressors can result in changes in immune function. Although there is increasing information concerning the peripheral hormonal and neural mediators of stress-induced changes in immune function, there is little information concerning the central nervous system mechanisms that lead to the peripheral changes. The following experiments examined the possible involvement of the benzodiazepine-GABAA-chloride complex in modulation of the in vivo antibody response. Rats were given either peripheral or intracerebroventricular injections of methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), a drug that has been shown to act at the benzodiazepine-GABAA complex and produces a behavioral state similar to anxiety. Rats were then immunized with keyhole limpet hemocyanin (KLH) and serum levels of KLH-specific antibody were measured for 2 weeks after immunization. Both peripheral and central administration of DMCM modulated the in vivo antibody response. The dose-response relationship of DMCM and changes in antibody levels was nonmonotonic, with high doses resulting in an increase in serum antibody levels and moderate doses resulting in a decrease in serum antibody levels. A possible role of the benzodiazepine-GABAA system in stress-induced immunomodulation is discussed.

Adenosine modulates the antinociceptive action of benzodiazepines

1. In the present study, adenosine modulation on the antinociceptive action of benzodiazepines (BZD) using the writhing test in mice was evaluated. 2. The i.c.v. BZDs tested (diazepam, midazolam and lorazepam) induced a dose-dependent antinociceptive action, that was not antagonized either by naloxone or by aminophylline. 3. The i.p. administration of adenosine-related compounds also produced a dose-dependent reduction in the number of writhings in mice. These effects were not antagonized by i.p. injection of naloxone but were antagonized by i.p. aminophylline. 4. The antinociceptive effect of BZDs was significantly enhanced by the administration of adenosine compounds, but this increased response was not modified by naloxone or by aminophylline. 5. The present findings could be explained by the fact that BZDs and adenosine-related compounds may interact in an additive manner, since the effects of these drugs may be due to a common mechanism of action or a common pathway.

Nuclear benzodiazepine binding: possible interaction with thyroid hormone receptors

The biochemical and pharmacological properties of nuclear [3H]flunitrazepam in brain tissues were studied. Nuclear [3H]flunitrazepam binding is saturable for both central and peripheral binding sites. Inosine and hypoxanthine displace nuclear [3H]flunitrazepam binding with greater potency than the membrane [3H]flunitrazepam binding. Triiodothyronine (T3) increases the maximum number of binding sites (Bmax) of nuclear [3H]flunitrazepam binding in vitro while thyroxine (T4) does not have any effect. Diazepam reduces the affinity of nuclear 125I-T3 binding in vitro, while the Bmax is not affected significantly. Mild digestion of chromatin, using micrococcal nuclease, reveals that a major portion of nuclear [3H]flunitrazepam binding sites are located on chromatin. These data suggest a functional role for nuclear benzodiazepine binding and a possible modulatory effect of benzodiazepines on T3 binding with its nuclear receptors.

Potential use of DPCPX as probe for in vivo localization of brain A1 adenosine receptors

The suitability of (3H)DPCPX (8-cyclopentyl-1,3-dipropylxanthine), a xanthine derivative, as an vivo probe for labelling adenosine A1 receptors was studied in rats. [3H]DPCPX (nM) penetrated largely into the brain (0.8% of the injected dose per gram of brain tissue 5 min after injection). Brain concentrations stayed at a plateau level from 5 to 15 min after the injection. The distribution in the different brain regions was heterogeneous with the highest amount of [3H]DPCPX in cerebellum and hippocampus and the lowest concentrations in hypothalamus and brain stem. Displacement (45-70% of total radioactivity) was obtained by the injection of 250 nM of cold DPCPX or cyclopentylxanthine, an analog of DPCPX. The ex vivo autoradiographic distribution of [3H]DPCPX was similar to the in vitro autoradiographic distribution of tritiated A1 adenosine receptor ligand as [3H]CHA. These results suggest the potential use of DPCPX for further in vivo investigation of A1 adenosine receptors with techniques such as positron emission tomography.

Analgesic effect of benzodiazepines and flumazenil

1. In the present work the analgesic effect of benzodiazepines (BZD) and flumazenil (FLU) using the writhing test in mice was studied. 2. Intracerebroventricular administration of BZD exhibited a dose-dependent antinociceptive effect when compared to control value. 3. Intracerebroventricular administration of FLU induced a dose-dependent antinociceptive action that was not antagonized by naloxone (NX). 4. BZD administered as subcutaneous pellets produced an antinociceptive action in the writhing test, when compared to control mice, only at relative high doses and was partially antagonized by naloxone. 5. These findings could be explained assuming that NX and/or FLU have partial agonist properties in a common receptor which mediates the antinociceptive action. 6. The antinociceptive action of BZD could be related to an increased release of adenosine, which by itself has analgesic effects.

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.

Do adenosinergic substrates mediate methylxanthine effects upon reinforcement thresholds for electrical brain stimulation in the rat?

Caffeine and other methylxanthines elevate reinforcement threshold for electrical brain stimulation with an order of potency suggesting that the effect is mediated by antagonism of adenosine A2 receptors. The purpose of this study was to evaluate further the possible mechanism by which caffeine and other methylxanthines elevate reinforcement thresholds for ICSS. Drugs known to affect adenosinergic transmission in predictable ways, adenosine receptor agonists and antagonists and benzodiazepine agonists and inverse agonists, were tested to determine their effect upon reinforcement threshold. Both the selective A1 adenosine agonist, R(-)-PIA, and the non-selective A1/A2 adenosine agonist NECA failed to alter reinforcement thresholds, as did CGS 15943, a potent non-xanthine non-selective adenosine receptor antagonist. Chlordiazepoxide, a benzodiazepine agonist, lowered reinforcement thresholds and FG 7142, a benzodiazepine inverse agonist, elevated reinforcement thresholds, perhaps corresponding to their anxiolytic and anxiogenic subjective effects in humans. However, another benzodiazepine agonist, midazolam and another inverse agonist, beta-CCE, did not alter reinforcement thresholds. These results fail to support a general role for adenosinergic systems in the threshold-elevating effect of methylxanthines.

Gestational exposure to diazepam increases sensitivity to convulsants that act at the GABA/benzodiazepine receptor complex

Experiments examining seizure sensitivity were conducted on adult male offspring exposed to diazepam at 1.0 or 2.5 mg/kg per day in utero over gestational days 14-20. Threshold dosages to facial clonus, myoclonic jerk, clonic seizure, and extensor tonus were determined via i.v. infusion of bicuculline, methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), picrotoxin, pentylenetetrazol, caffeine and strychnine. Relative to uninjected and vehicle-exposed adult male offspring, prenatal diazepam administration reduced the threshold for bicuculline- and DMCM-induced facial clonus and myoclonic jerk by 40-50%. The threshold dosages to facial clonus, myoclonic jerk and clonic seizure from picrotoxin infusion were similarly reduced in animals exposed to diazepam in utero. In contrast, seizure thresholds to pentylenetetrazol, caffeine and strychnine were not affected by early developmental exposure to diazepam. In parallel biochemical studies, an increased sensitivity to the antagonistic effects of bicuculline methiodide on gamma-aminobutyrate (GABA)-stimulated chloride influx was observed in cortical synaptoneurosomes from adult male progeny of diazepam-treated dams. The results are interpreted to reflect a long-lasting alteration in the function of the GABA/benzodiazepine receptor complex by prenatal diazepam exposure that is manifest at the behavioral and neurochemical level in a pharmacologic specific manner.

Evidence for a role for dopamine in the diazepam locomotor stimulating effect

It is well known that benzodiazepines produce dependence in humans and locomotor stimulation in experimental animals. In this study the possible involvement of catecholamines in the diazepam-induced locomotor stimulation in mice were investigated. Diazepam was found to have a biphasic effect; increasing locomotor activity at a low dose (0.25 mg/kg), while decreasing it at higher doses (greater than 0.5 mg/kg). The locomotor stimulating effect of diazepam was effectively blocked by pretreatment with the benzodiazepine receptor antagonist flumazenil, as well as with the catecholamine synthesis inhibitor alpha-methyltryrosine and the dopamine receptor antagonists haloperidol, spiperone and SCH 23390. Taken together, these data indicate that the locomotor stimulating effect observed after low doses of diazepam is due to activation of brain dopaminergic systems involved in locomotor activity. The observations are discussed in relation to the hypothesis that dependence-producing drugs activate specific brain reward systems.

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.

Interaction of caffeine with the GABAA receptor complex: alterations in receptor function but not ligand binding

Behavioral and neurochemical evidence indicates interactions between caffeine and other adenosine receptor ligands and the gamma-aminobutyric acid (GABA)-benzodiazepine system. To assess the effects of caffeine on binding and function at the GABAA receptor, we studied the effects of behaviorally-active doses of caffeine on benzodiazepine and Cl- channel binding and on overall function of the GABAA receptor as measured by Cl- uptake. There was no effect of caffeine on benzodiazepine receptor binding in cortical synaptosomal membranes at concentrations of 1-100 microM. No effects on benzodiazepine binding were found ex vivo in mice treated with caffeine, 20 and 40 mg/kg. At the putative Cl- channel site labeled by t-butylbicyclophosphorothionate (TBPS), binding was unchanged in vitro after caffeine treatment (1 and 10 microM) in washed and unwashed membranes. However, in ex vivo studies caffeine (20 and 40 mg/kg) increased numbers of TBPS sites in unwashed but not washed membranes. Muscimol-stimulated Cl- uptake into cortical synaptoneurosomes was decreased in mice treated with caffeine, 20 and 40 mg/kg. Similar results were observed in in vitro preparations treated with 50 microM but not 100 microM caffeine. These results indicate that caffeine administration significantly alters the Cl- transport function of the GABAA receptor complex.

Antiepileptic drug development program: a cooperative effort of government and industry

The most important step in antiepileptic drug discovery is the choice of an appropriate animal model for the initial screening as well as for the more complex procedures that elucidate mechanisms of action. The currently available models fall short in their inability to identify all drugs for all types of seizures in a mechanism-independent manner. Nevertheless, spontaneous models of epilepsy are the most commonly used, and chemically or electrically induced seizures in rodents can also identify potential anticonvulsants. In the latter models, the intensity of the seizure stimulus is of paramount importance. The Antiepileptic Drug Development Program evaluates approximately 800 compounds each year, using two models for preliminary screening. One model assesses the ability of a compound to prevent seizure spread; the other weighs the ability to raise seizure threshold. In vivo tests, featuring amygdala- and corneal-kindled seizures, and in vitro assays, employing gamma-aminobutyric acid (GABA) receptors and synaptosomal uptake of adenosine, define drug-drug interactions and elucidate the pharmacological profiles of potential anticonvulsants.