Inhibition of adenosine uptake into rat brain synaptosomes by the benzodiazepines

Adjusting the brakes to adjust neuronal activity: Adenosinergic modulation of GABAergic transmission

About 50 years elapsed from the publication of the first full paper on the neuromodulatory action of adenosine at a 'simple' synapse model, the neuromuscular junction (Ginsborg and Hirst, 1972). In that study adenosine was used as a tool to increase cyclic AMP and for the great surprise, it decreased rather than increased neurotransmitter release, and for a further surprise, its action was prevented by theophylline, at the time only known as inhibitor of phosphodiesterases. These intriguing observations opened the curiosity for immediate studies relating the action of adenine nucleotides, known to be released together with neurotransmitters, to that of adenosine (Ribeiro and Walker, 1973, 1975). Our understanding on the ways adenosine uses to modulate synapses, circuits, and brain activity, vastly expanded since then. However, except for A_2A receptors, whose actions upon GABAergic neurons of the striatum are well known, most of the attention given to the neuromodulatory action of adenosine has been focusing upon excitatory synapses. Evidence is growing that GABAergic transmission is also a target for adenosinergic neuromodulation through A₁ and A_2A receptors. Some o these actions have specific time windows during brain development, and others are selective for specific GABAergic neurons. Both tonic and phasic GABAergic transmission can be affected, and either neurons or astrocytes can be targeted. In some cases, those effects result from a concerted action with other neuromodulators. Implications of these actions in the control of neuronal function/dysfunction will be the focus of this review.

Benzodiazepines alter nucleotide and nucleoside hydrolysis in zebrafish (Danio rerio) brain

Anxiety is characterized by unpleasant bodily sensations, such as pounding heart and intense fear. The therapy involves the administration of benzodiazepine drugs. Purinergic signaling participates in the induction of several behavioral patterns and their actions are inactivated by ectonucleotidases and adenosine deaminase (ADA). Since there is evidence about the involvement of purinergic system in the actions mediated by benzodiazepines, we evaluated the effects in vitro and in vivo of administration of diazepam and midazolam on nucleoside triphosphate diphosphohydrolases, ecto-5'-nucleotidase, and ADA activities in zebrafish brain, followed by the analysis of gene expression pattern of these enzymes and adenosine receptors (A1, A2a1, A2a2, A2b). The in vitro studies demonstrated that diazepam decreased ATP (66 % for 500 µM) and ADP hydrolysis (40-54 % for 10-500 µM, respectively). Midazolam decreased ATP (16-71 % for 10-500 µM, respectively) and ADP (48-73.5 % for 250-500 µM, respectively) hydrolysis as well as the ecto-ADA activity (26-27.5 % for 10-500 µM, respectively). AMP hydrolysis was decreased in animals treated with of 0.5 and 1 mg/L midazolam (32 and 36 %, respectively). Diazepam and midazolam decreased the ecto-ADA activity at 1.25 mg/L and 1 mg/L (31 and 33 %, respectively), but only 0.1 mg/L midazolam induced an increase (40 %) in cytosolic ADA. The gene expression analysis demonstrated changes on ecto-5'-nucleotidase, A1, A2a1, A2a2, and A2b mRNA transcript levels after acute treatment with benzodiazepines. These findings demonstrated that benzodiazepine exposure induces a modulation of extracellular nucleotide and nucleoside metabolism, suggesting the purinergic signaling may be, at least in part, related to benzodiazepine effects.

Adenosine receptor agonists attenuate the development of diazepam withdrawal-induced sensitization in mice

In the present study, the effects of adenosine agonists on the development of sensitization to withdrawal signs precipitated after sporadic treatment with diazepam, in mice, were investigated. To obtain the sensitization, the animals were divided into groups: continuously and sporadically treated with diazepam (15.0 mg/kg, s.c.). The adenosine receptor agonists (CPA, CGS 21,680 and NECA) were administered in sporadically diazepam treated mice during two diazepam-free periods. Concomitant administration of pentetrazole (55.0 mg/kg, s.c.) with flumazenil (5.0 mg/kg, i.p.) after the last injection of diazepam or vehicle, induced the withdrawal signs, such as clonic seizures, tonic convulsion and death episodes. The major finding of our experiments is attenuation of withdrawal signs in sensitized mice, inducing by all adenosine agonists. Only higher dose of CPA produced significantly decreased the number of withdrawal incidents, while both used doses of CGS 21,680 and NECA produced more clear effects. These results support the hypothesis that adenosinergic system is involved in the mechanisms of sensitization to the benzodiazepine withdrawal signs, and adenosine A(2A) receptors play more important role in that process.

Effect of the adenosine A2a receptor antagonist 3,7-dimethyl-propargylxanthine on anxiety-like and depression-like behavior and alcohol consumption in Wistar Rats

BACKGROUND

It has been suggested that the reinforcing properties of ethanol are in part mediated via an A2 activation of cAMP/PKA signaling in the nucleus accumbens, predicting that administration of an A2a antagonist might reduce ethanol reward and consumption. We therefore examined the effect of the adenosine A2a receptor antagonist 3,7-dimethylpropargylxanthine (DMPX, 3, and 10 mg/kg intraperitoneal) on alcohol reinforcement, anxiety-related, depression, and rewarding behaviors in nonselected Wistar rats.

METHODS

Operant ethanol self-administration was used for examining alcohol intake, elevated plus-maze and Vogel conflict test for anxiety-related behavior, Porsolt swim test for depression-like behavior, and conditioned place preference for examination of the rewarding properties of the drug.

RESULTS

3,7-Dimethylpropargylxanthine decreased lever-pressing for ethanol in a dose-dependent manner. When analyzed as percentage of pretreatment baseline, maximum suppression was approximately 60% (39+/-7.5 vs 98+/-12%, mean+/-SEM, p=0.017). This effect was behaviorally specific, as no effect was found on the water lever. In agreement with previously published data, stimulation of locomotion was found (beam-breaks: 3590+/-540 vs 2475+/-240, 10 mg/kg vs saline, p=0.048). No anxiety-modulating effects were seen in either the elevated plus-maze or the Vogel conflict test. 3,7-Dimethylpropargylxanthine was not found to have intrinsic rewarding properties in the conditioned place preference model.

CONCLUSIONS

In summary, DMPX produced a robust and behaviorally selective reduction of ethanol reinforcement, while anxiety-modulating effects were less consistent. These results bring further support to a role for adenosine in the regulation of ethanol consumption and possibly alcohol addiction/abuse, and the A2a receptor as a potential target for the treatment of alcoholism and alcohol abuse.

Influence of adenosine receptor agonists on benzodiazepine withdrawal signs in mice

The involvement of adenosine receptor agonists in benzodiazepine withdrawal signs was evaluated as the seizure susceptibility of mice. The concomitant administration of subthreshold dose of pentetrazole (55.0 or 60.0 mg/kg, s.c.) with flumazenil (10.0 mg/kg, i.p.) in mice chronically treated with temazepam or diazepam induced the appearance of withdrawal signs: clonic seizures, tonic convulsions and death episodes. The administration of the selective A1 (CPA-N6-cyclopentyladenosine), A2A (CGS 21680-2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride) and the non-selective A1/A2A (NECA-5'-N-ethylcarboxamidoadenosine) adenosine receptor agonists (i.p.) evoked the significant attenuation of benzodiazepine withdrawal signs, and these effects were more expressed in temazepam- than in diazepam-dependent mice. CPA has shown the most apparent and dose-dependent attenuating effect. The results confirm that adenosine A1 and A2A receptors are involved in benzodiazepine withdrawal signs, and adenosine A1 receptor plays a predominant role in this phenomenon.

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.

Hydrogen bonding interaction of diphenylbarbituric acid and 9-ethyladenine. Crystal structure of a 1:1 complex

There are several different types of adenosine receptors found in the brain and molecules capable of interacting with the adenine ring system may have an effect on brain activity. Phenobarbital and diphenylhydantoin, two commonly used antiepileptic drugs have been shown to form hydrogen bonded complexes with adenine derivatives. Although 5,5-diphenylbarbituric acid has anticonvulsant properties equivalent to phenobarbital, neither its structure nor possible interactions with adenine have been investigated. 5,5-Diphenylbarbituric acid and 9-ethyladenine were, therefore, cocrystallized and the three-dimensional 1:1 hydrogen bonded complex was determined by X-ray diffraction. The crystals are monoclinic, C2/c, a = 28.457(9), b = 10.633(3), c = 13.995(4) Å, β = 100.20(2)°, Z = 8, R = 0.067 for 1680 reflections with I > 3σ(I). The molecules propagate along the b crystallographic axis in an alternating fashion in which adjacent adenine and barbiturate molecules interact through a pair of hydrogen bonds. Two-fold symmetry related barbiturate carbonyl groups in neighboring planes of hydrogen bonded ribbons are involved in dipole–dipole attractive interactions. This hydrogen bonding pattern is compared to patterns found in other barbiturate–adenine complexes.Key words: hydrogen bonding, crystal structure, barbiturates, adenine, anticonvulsants.

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.

Profile of anticonvulsant activity and minimal toxicity of methyl 4-[(p-chlorophenyl)amino]-6-methyl-2-oxo-cyclohex-3-en-1-oate and some prototype antiepileptic drugs in mice and rats

The anticonvulsant and toxic properties of methyl 4-[(p-chlorophenyl)amino]-6-methyl-2-oxocyclohex-3-en-1-oate (ADD 196022), were compared with those of phenytoin (PHT), carbamazepine (CBZ), and valproate (VPA). These compounds were evaluated in mice and rats using well-standardized anticonvulsant testing procedures. Results indicate that ADD 196022 is a very potent anticonvulsant in the maximal electroshock seizure (MES) model. The compound was effective in nontoxic doses after intraperitoneal (i.p.) administration in mice and oral administration in rats. In mice, i.p. administration of ADD 196022 resulted in an ED50 value of 26.2 mg/kg as compared with a value of 6.48 mg/kg for PHT in the same assay. ADD 196022 was more potent that PHT in the oral rat model, having an ED50 value of 5.79 mg/kg as compared to 23.2 mg/kg for PHT. ADD 196022 was ineffective in nontoxic doses against all other seizure models evaluated and thus has a pharmacologic profile similar to that of PHT.

Characterization of the anticonvulsant properties of F-721

The anticonvulsant properties of F-721 (3-diethylamino-2,2-dimethylpropyl-5-[p-trifluoromethylphenyl]-2-f uroate hydrochloride) were investigated in a battery of in vivo and in vitro anticonvulsant model systems. After intraperitoneal (ip) administration in mice, F-721 was effective in nontoxic doses against maximal electroshock (MES), subcutaneous picrotoxin clonic, intracerebroventricular (icv) N-methyl-D-aspartate (NMDA) tonic, icv NMDA clonic and icv quisqualic acid tonic seizures (ED50s: 11.1, 28.4, 1.76, 3.4, and 4.4 mg/kg, respectively). F-721 exhibited only partial activity against clonic seizures induced in the subcutaneous Metrazol and subcutaneous bicuculline test in mice and was inactive in this species against tonic seizures induced in the subcutaneous strychnine test. F-721 was effective against MES seizures following oral administration to mice (ED50: 31.3 mg/kg) and only partially effective by this route against clonic seizures induced by subcutaneous Metrazol. In rats, F-721 was a potent anticonvulsant in the maximal electroshock model following oral administration (ED50: 9.9 mg/kg). F-721 was also effective against corneal-kindled and amygdaloid-kindled seizures in rats. F-721 suppressed stage 5 seizures in corneal-kindled rats with an ED50 of 15 mg/kg, ip. In addition, it also decreased the afterdischarge duration and behavioral seizure stage in amygdaloid-kindled rats at doses that did not cause sedation or ataxia. At 40 mg/kg, F-721 reduced afterdischarge duration by 83.2% and reduced the seizure severity score to 1.7. The ED50 for 50% reduction of afterdischarge duration was 16.3 mg/kg, ip. In cultured mouse spinal cord neurons, F-721 suppressed sustained repetitive firing in response to a depolarizing current with a median inhibitory concentration (IC50) of 1.9 microM. F-721 had no effect on adenosine uptake, gamma-aminobutyric acid or NMDA receptor binding. Comparative data from previous studies with clinically established antiepileptic agents reveal that F-721's profile of activity most closely resembles that of phenytoin and carbamazepine. However, F-721 was notably more efficacious in suppressing amygdaloid-kindled seizures in rats and was a more potent antagonist of icv NMDA clonic seizures. Our studies indicate that F-721 is a potent, orally active anticonvulsant with a favorable margin of safety. The profile of anticonvulsant activity of F-721 suggests potential utility in the management of generalized tonic-clonic, simple and complex partial seizures.

Strategies for identifying and developing new anticonvulsant drugs

The identification of new anticonvulsant drugs depends on the use of different animal models of epilepsy. The models should be mechanism-independent, able to screen a large number of compounds, at limited cost and technical expertise. Primary screening models include genetic or reflex models of epilepsy and electrically and chemically induced seizures. Once active compounds have been identified, more advanced mechanistic and seizure-specific models are needed to refine the choice of a lead compound. These can be either in vivo or in vitro models. Models known to interact with specific receptors or the production of the putative neurotransmitters of neural excitability or inhibition are valuable in assessing possible mechanisms of action. In vitro models have evolved as important tools in correlating changes in electrical phenomena and therapeutic spectrum. The use of the hippocampal slice and the cultured neuron permits classification of anticonvulsant activity based on cellular actions of the drug. Interactions by the experimental drugs with specific subcellular fractions of the central nervous system augment information on possible mechanisms of action. The final choice of compounds for development requires synthesizing and comparing all of the pharmacodynamic information with the pharmacokinetic and toxicologic data. In the final analysis, no single animal model of epilepsy known today can assure the development of better drugs for all treatment of the epilepsies.

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.

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.

Preclinical profile of remacemide: a novel anticonvulsant effective against maximal electroshock seizures in mice

Anticonvulsant tests in mice revealed specific, potent actions of remacemide for protection of mice against maximal electroshock seizures (MES). Comparisons of oral efficacy to reference compounds yielded the following ED50 values (expressed as mg/kg): remacemide = 33, phenytoin = 11, phenobarbital = 20, carbamazepine = 13 and valproate = 631. The duration for protection by remacemide was longer than carbamazepine or valproate, but shorter than phenytoin or phenobarbital. In neural impairment tests (inverted screen or rotorod) to determine the oral toxic dose 50 (TD50) the following therapeutic indices (TD50/ED50) were obtained: (1) inverted screen--remacemide = 17.6, phenytoin = 57.4, phenobarbital = 5.1, carbamazepine = 10.2, and valproate = greater than 3; and (2) rotorod--remacemide = 5.6, phenytoin = 9.6, phenobarbital 4.8, and valproate = 1.9. Remacemide was devoid of sedative actions and possessed a favorable 28.1 margin of safety value (median estimated lethal dose/ED50 for MES). An intermediate potency against either audiogenic- or N-methyl-D-aspartate-induced seizures was exhibited by remacemide. Tolerance to MES was not apparent after 5 days of oral daily dosing of remacemide. Remacemide was inactive in vitro against gamma-aminobutyrate or benzodiazepine receptors and adenosine uptake mechanisms. Therapeutic utility for generalized tonic/clonic seizures is predicted for remacemide.

Adenosine transport by rat and guinea pig synaptosomes: basis for differential sensitivity to transport inhibitors

Adenosine transport by rat and guinea pig synaptosomes was studied to establish the basis for the marked differences in the potency of some transport inhibitors in these species. An analysis of transport kinetics in the presence and absence of nitrobenzylthioinosine (NBTI) using synaptosomes derived from several areas of rat and guinea pig brain indicated that at least three systems contributed to adenosine uptake, the Km values of which were approximately 0.4, 3, and 15 microM in both species. In both species, the system with the Km of 3 microM was potently (IC50 of approximately 0.3 nM) and selectively inhibited by NBTI. This NBTI-sensitive system accounted for a greater proportion of the total uptake in the guinea pig than in the rat and was inhibited by dipyridamole, mioflazine, and related compounds more potently in the guinea pig. Preliminary experiments with other species indicate that adenosine transport in the mouse is similar to that in the rat, whereas in the dog and rabbit, it is more like that in the guinea pig. In the rat, none of the systems appeared to require Na+, but the two systems possessing the higher affinities for adenosine were inhibited by veratridine- and K(+)-induced depolarization. The transport systems were active over a broad pH range, with maximal activity between pH 6.5 and 7.0. Our results are consistent with the possibility that adenosine transport systems may be differentiated into uptake and release systems.

Benzodiazepines and their antagonists interfere with opioid-dependent stress-induced analgesia

Timing or intensity of shocks significantly modify the characteristics of the analgesia induced by footshock, and conditioning to footshock induces analgesia, independently from the time and shock parameters used for conditioning. However, whatever the parameters of shock, and the presence of conditioning or not, the stress has to be inescapable in order to produce an increase in pain thresholds. This observation suggests that anxiety plays a major role in the development of stress-induced analgesia. In order to test this hypothesis we investigated the effects of the benzodiazepine agonists diazepam and clonazepam, the antagonists RO 15-1788, CGS 8216, CGS 9896, and the inverse agonists FG 7142 and FG 7041 on the development and maintenance of stress-induced analgesia. Benzodiazepine receptor agonists decreased the analgesic effect of inescapable footshock, benzodiazepine receptor antagonists increased the footshock induced analgesia, whereas inverse agonists did not modify the analgesia induced by the shock. All the benzodiazepine receptor ligands blocked the antagonism of the footshock analgesia induced by naloxone.

Antagonism of the anti-conflict effects of phenobarbital, but not diazepam, by the A-1 adenosine agonist l-PIA

The present study examined the effects of the anxiolytics diazepam and phenobarbital, the A-1 adenosine agonist N6-R-phenylisopropyladenosine (l-PIA), and the A-2 adenosine agonist 5'-N-ethylcarboxamidoadenosine (NECA) on conflict behavior. Water-restricted rats were trained to drink from a tube that was electrified (0.5 mA intensity) on a FI-29s schedule, electrification being signaled by a tone. After 3 weeks of daily 10-min sessions, the animals accepted a stable number of shocks (punished responding) and consumed a consistent volume of water (unpunished responding) per session. Different doses of l-PIA and NECA were then tested separately at weekly intervals. In addition, the effects of diazepam and phenobarbital were determined in animals pretreated with saline, l-PIA, or NECA. Neither l-PIA (15-250 nmole/kg) nor NECA (2.5-20 nmole/kg) produced a significant anti-conflict effect when administered alone. Diazepam (1.25-10 mg/kg) or phenobarbital (10-40 mg/kg) administration to saline-pretreated rats resulted in a dose-dependent increase in punished responding (shocks received) with minimal effects on unpunished responding (water intake). Neither l-PIA nor NECA pretreatment reliably altered the effects of diazepam on conflict behavior. Pretreatment with l-PIA, but not NECA, significantly reduced the anti-conflict effects of phenobarbital on conflict behavior. These data suggest that phenobarbital, but not diazepam, anti-conflict responses may involve interactions with A-1 adenosine receptors.

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.

The effect of benzodiazepines on the 5-HT agonist-induced head-twitch response in mice

The effects of four benzodiazepines (diazepam, clonazepam, oxazepam and clobazam) were studied on the head-twitch response induced in mice by several 5-HT receptor agonists. All the benzodiazepines tested potentiated the effects of the directly acting agonists 5-methoxy-N,N-dimethyltryptamine (5-MeODMT), quipazine and mescaline, without themselves inducing head-twitches. In contrast, none of them potentiated head-twitches induced by the indirectly acting agonist 5-hydroxytryptophan (5-HTP; with carbidopa 25 mg/kg), and in some experiments a clear inhibition was seen. The clonazepam (10 mg/kg) potentiation of 5-MeODMT-induced head-twitches was not antagonised by flumazenil, (+)-bicuculline, or by pretreatment with p-chlorophenylalanine. Neither was it mimicked by muscimol, which inhibited head-twitches. These results indicate that the observed potentiation is not mediated by benzodiazepine receptors and that it occurs postsynaptically to the initiating 5-HT receptors. The inability of the benzodiapines to potentiate 5-HTP-induced head-twitches probably reflects a reduction in 5-HT neuronal activity mediated by benzodiazepine receptors, as co-administration of flumazenil and clonazepam potentiated the effects of 5-HTP whereas each compound alone had no effect.

Potentiation of adenosine-evoked depression of rat cerebral cortical neurons by triazolam

The triazolobenzodiazepine triazolam, applied iontophoretically onto rat cerebral cortical neurons, potentiated the magnitude and duration of adenosine-elicited depressions of spontaneous activity. Triazolam did not enhance the depressions evoked by adenosine 5'-N-ethylcarboxamide, an uptake resistant analog of adenosine, suggesting that potentiation of adenosine resulted from an inhibition of adenosine uptake. With larger application currents, triazolam depressed the firing of cortical neurons. This action was blocked by the adenosine antagonist caffeine (20 mg/kg, i.v.) implying that the depression resulted from an accumulation of endogenously released adenosine.

Nucleoside transport in rat cerebral-cortical synaptosomes. Evidence for two types of nucleoside transporters

The transport of [U-14C]uridine was investigated in rat cerebral-cortical synaptosomes using an inhibitor-stop filtration method. Under these conditions the rapid efflux of uridine from the synaptosomes is prevented and uridine is not significantly metabolized in the synaptosome during the first 1 min of uptake. The dose-response curve for the inhibition of uridine transport by nitrobenzylthioinosine (NBMPR) was biphasic: approx. 40% of the transport activity was inhibited with an IC50 (concentration causing half-maximal inhibition) value of 0.5 nM, but the remaining activity was insensitive to concentrations as high as 1 microM. Similar biphasic dose-response curves were observed for dilazep inhibition, but both transport components were equally sensitive to dipyridamole inhibition. Uridine influx by both components was saturable (Km 300 +/- 51 and 214 +/- 23 microM, and Vmax. 12 +/- 3 and 16 +/- 3 pmol/s per mg of protein, for NBMPR-sensitive and NBMPR-insensitive components respectively), and inhibited by other nucleosides such as 2-chloroadenosine, adenosine, inosine, thymidine and guanosine with similar IC50 values for the two components. Inhibition of uridine transport by NBMPR was associated with high-affinity binding of NBMPR to the synaptosome membrane (Kd 58 +/- 15 pM). Binding of NBMPR to these sites was competitively blocked by uridine and adenosine and inhibited by dilazep and dipyridamole, with Ki values similar to those measured for inhibiting NBMPR-sensitive uridine influx. These results demonstrate that there are two components of nucleoside transport in our rat synaptosomal preparation that differ in their sensitivity to inhibition by NBMPR. Thus conclusions regarding nucleoside transport in rat brain based only on NBMPR-binding activity must be viewed with caution.

Adenosine uptake site heterogeneity in the mammalian CNS? Uptake inhibitors as probes and potential neuropharmaceuticals

Inhibitors of adenosine uptake or transport have been used clinically for some time in certain cardiovascular diseases. More recently, some of them have also been investigated for possible clinical use in combination with antimetabolites based on the observed heterogeneity of nucleoside transport in mammalian tumor cells. Such a heterogeneity of adenosine uptake and uptake sites has now also been suggested in the mammalian CNS. The aim of this article is, therefore, to review the present status of our knowledge of adenosine uptake in the mammalian CNS, compare it with our far more advanced knowledge of nucleoside transport in other mammalian cells and suggest direction of future research. The possible implications for the development of adenosine uptake inhibitors as adenosinergic neuropharmaceuticals will be discussed based on our knowledge of the physiological function of adenosine in the CNS.

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)

The role of adenosine in the central actions of the benzodiazepines

1. Evidence is presented which indicates that the central actions of the benzodiazepines cannot be fully accounted for by assuming an action only at the GABAA-Cl- channel supramolecular complex. 2. The hypothesis is presented, together with supporting evidence, that inhibition of adenosine uptake can account for many of the actions of the benzodiazepines. 3. New findings showing that Ro 15-1788 and Ro 5-4864 have both potentiative and antagonistic interactions with adenosine are discussed. 4. The proconvulsant beta-carbolines are shown to be adenosine antagonists. 5. The concept that benzodiazepine action may involve several mechanisms is presented.

Adenosine potentiation and antagonism may account for the diverse behavioral actions of Ro 5-4864

The actions of the p-chloroderivative of diazepam, Ro 5-4864 on the spontaneous discharges of rat cerebral cortical neurons and its interactions with depressions evoked by adenosine and adenosine 5'-N-ethylcarboxamide (NECA) were observed. Iontophoretically applied Ro 5-4864 had variable actions on cortical neuronal activity, exciting some neurons and depressing others. During the period of applications. Ro 5-4864 antagonized the depressant effects of NECA, whilst having less of an action on adenosine depressions. Following Ro 5-4864 application adenosine, but not NECA, depressions were potentiated in amplitude and duration for periods of up to 20 min. It is suggested that Ro 5-4864 has both antagonistic and potentiative interactions with adenosine.

Effects of drugs on the initiation and maintenance of status epilepticus induced by administration of pilocarpine to lithium-pretreated rats

The ability of various drugs to prevent the onset of status epilepticus induced by administration of the muscarinic agonist, pilocarpine, to lithium-pretreated rats was determined. Motor limbic seizures and status epilepticus occurred in 100% of rats administered pilocarpine (30 mg/kg, s.c.) 20 h after pretreatment with lithium (3 meq/kg, i.p.). The latency to spike activity and to status epilepticus was 20 +/- 1 min and 24 +/- 1 min, respectively. Atropine, diazepam, phenytoin, carbamazepine, phenobarbital, paraldehyde, and L-phenylisopropyladenosine (L-PIA) prevented all phases of seizure activity induced by lithium/pilocarpine treatment. The initiation of status epilepticus was significantly prolonged by pretreatment with sodium valproate. These findings indicate that the seizures induced by administration of lithium and pilocarpine accurately model generalized tonic-clonic epilepsy. The anticonvulsant activity of L-PIA was prevented by prior treatment with the adenosine antagonist, theophylline. The latency to spike and seizure activity was decreased by theophylline, indicating that endogenous adenosine may have a tonic inhibitory influence on cholinergic neurons. Atropine, diazepam, phenobarbital, phenytoin, sodium valproate, L-PIA, and carbamazepine did not interrupt seizure activity when administered 60 min after pilocarpine (approximately 35 min after initiation of status epilepticus). When rats were administered paraldehyde at this time, status epilepticus was rapidly terminated and all rats survived. Thus, status epilepticus induced by lithium and pilocarpine provides a seizure model that is not responsive to conventional anticonvulsants.

Ro 15-1788 both antagonizes and potentiates adenosine-evoked depression of cerebral cortical neurons

The effects of Ro 15-1788, a benzodiazepine antagonist with some agonist properties, were studied on adenosine and adenosine 5'-N-ethyl-carboxamide (NECA)-evoked depressions of rat cerebral cortical neuronal activity. Iontophoretically applied Ro 15-1788 had both antagonistic and potentiative interactions with adenosine. Reductions in the magnitude of adenosine-evoked depressions of firing were evident during the period of Ro 15-1788 application, with a long-lasting potentiative effect becoming apparent upon termination of the Ro 15-1788 application. Depressions of cell firing evoked by NECA, an uptake-resistant analog of adenosine, were antagonized by Ro 15-1788, with no subsequent potentiation. Larger applications of Ro 15-1788 had a depressant action on neuronal firing, which was antagonized by caffeine (20 mg/kg), an adenosine receptor blocker. These results indicate that Ro 15-1788 may be an antagonist at the adenosine receptor as well as a potentiator of the adenosine response. The prolongation of the adenosine depression is likely to be the result of a persistent inhibition of adenosine uptake by Ro 15-1788. These diverse effects on the adenosine response may account for some of the complex behavioral actions of Ro 15-1788.

Effects of diazepam on circadian phase advances and delays

Phase advances of hamster locomotor rhythms, which normally can be induced by light pulses in the late subjective night, were blocked in a dose-dependent manner by the benzodiazepine, diazepam. Light-induced phase delays were unaffected at doses that significantly blocked phase advances. Diazepam caused small phase delays of the free-running rhythm when given without a light pulse at either phase advance or phase delay time points. These results are discussed with regard to the possibility that different neurochemical mechanisms are required to process light-induced phase advances and delays and that GABA neurotransmission may be involved in the modulation of light input to the clock.

Photoaffinity labelling of benzodiazepine receptors: lack of effect on ligand binding to the nucleoside transport system

This study was undertaken to investigate the possibility of an allosteric interaction between benzodiazepine receptors and the CNS nucleoside transport system. Irreversible (photoaffinity) labelling of the benzodiazepine receptors in guinea pig cortical membranes resulted in a marked reduction in the binding (Bmax) of both [3H]flunitrazepam (71%) and [3H]ethyl-beta-carboline-3-carboxylate (22%) to the benzodiazepine receptors but had no effect on the binding of [3H]nitrobenzylthioinosine to the nucleoside transport system. Furthermore, although photoaffinity labelling resulted in a significant decrease in the affinities of flunitrazepam (approximately equal to 16-fold) and dipyridamole (approximately equal to sevenfold) for the [3H]Ro 15-1788 binding site of the benzodiazepine receptor complex, the affinities of these compounds for the nucleoside transport system were unaltered. These results suggest that the CNS nucleoside transport system and the benzodiazepine receptor complex are distinct, noninteractive ligand recognition sites.

Effects of carbamazepine on noradrenergic mechanisms in affectively ill patients

Noradrenergic mechanisms have been postulated to account for the anticonvulsant and psychotropic effects of carbamazepine. In order to assess this possibility in man, cerebrospinal fluid (CSF) was obtained from affectively ill patients before and during treatment with carbamazepine (average duration 29 days) at doses averaging 860 mg/day, achieving blood levels of 8.86 micrograms/ml. Neither plasma nor CSF norepinephrine (NE) nor CSF 3-methoxy-4-hydroxy-phenylglycol (MHPG) was significantly altered by carbamazepine. Baseline medication-free values in 21 depressed patients were not predictive of the degree of subsequent clinical antidepressant response. CSF NE decreased in four manic patients treated with carbamazepine. The many effects of carbamazepine on noradrenergic mechanisms in animals are discussed in relationship to these first studies of carbamazepine in man.

Effect of aminophylline on muscle relaxant action of diazepam and phenobarbitone in genetically spastic rats: further evidence for a purinergic mechanism in the action of diazepam

The effect of aminophylline on the muscle relaxant action of both diazepam and phenobarbitone was studied in genetically spastic rats of the Han-Wistar strain which exhibit spontaneous tonic activity in the electromyogram of the gastrocnemius-soleus muscle. Both diazepam (0.8 and 4.0 mg/kg i.p.) and phenobarbitone (20 and 30 mg/kg i.p.) reduced the spontaneous activity measured in the electromyogram in a dose-related manner. Aminophylline (50 mg/kg i.p.), a methylxanthine with potent antagonistic activity of adenosine-mediated inhibition, partially reversed the muscle relaxant action of diazepam (4 mg/kg) but not that produced by phenobarbitone. The muscle relaxant effect of phenobarbitone (30 mg/kg) was antagonised by beta-carboline-3-carboxylic acid methylester (beta-CCM), 2 mg/kg i.p. The reversal of the muscle relaxant effect of phenobarbitone produced by beta-CCM was abolished by CGS 8216 (2-phenylpyrazolo-(4,3c)quinolin-3(5H)-one), 5 mg/kg i.p. Aminophylline altered neither the muscle relaxant effect of a low dose of diazepam (0.8 mg/kg) nor the reversal of the muscle relaxant effect of phenobarbitone produced by beta-CCM. These findings indicate that the interaction between diazepam and aminophylline does not involve competition for the benzodiazepine receptor and add further support to the suggestion that purinergic mechanisms may be engaged in the muscle relaxant action of diazepam.

Involvement of adenosine receptor activities in aggressive responses produced by clonidine in mice

A behavioral study was made of the mechanisms underlying the aggressive behavior induced by high doses of clonidine in mice. The frequency of clonidine-induced aggressive responses such as attacking and biting was increased dose-dependently from 10 to 50 mg/kg. Aggressive behavior induced by clonidine at doses of 10-30 mg/kg was potentiated under conditions of isolation and food deprivation for 24 h. Clonidine (30 mg/kg)-induced aggressive behavior was attenuated by adenosine (10 mg/kg IP) or dipyridamole (10 mg/kg IP), but markedly antagonized by combined pretreatment with both drugs. The behavior was strongly reduced by potent adenosine analogs, such as N6-cyclohexyl adenosine (CHA, 0.1 and 0.2 mg/kg IP) and N6-(L-phenyl isopropyl) adenosine (L-PIA, 0.2 mg/kg IP), but conversely was potentiated by phentolamine (10 mg/kg IP) or theophylline (10 mg/kg IP). Diazepam (2.5 mg/kg IP) and Ro15-1788 (2.5 mg/kg IP), a benzodiazepine receptor antagonist, also blocked the aggressive behavior. The inhibition by CHA (0.2 mg/kg IP) or diazepam (2.5 mg/kg) of clonidine-induced aggression was not antagonized by additional pretreatment with bicuculline (2 mg/kg IP). The aggressive response to apomorphine (8 mg/kg IP) was not affected by those drugs which inhibited the response to clonidine. The results suggest that the aggressive behavior evoked by high doses of clonidine, but not that by apomorphine, involves a blockade of adenosine receptors.

Adenosine's role in the central actions of the benzodiazepines

The hypothesis that inhibition of adenosine uptake may play an important role in the central actions of the benzodiazepines is presented. The evidence supporting this hypothesis is discussed. Brain concentrations of the benzodiazepines are adequate for inhibition of adenosine uptake. Benzodiazepines, such as RO15-1788 and RO5-4864, which do not enhance gamma-aminobutyric acid binding, may exert some of their central effects by inhibiting the uptake of adenosine.

Calmodulin antagonists inhibit adenosine uptake by rat brain cortical synaptosomes

The purpose of these experiments was to determine if the adenosine uptake process in brain synaptosomes is regulated by calmodulin. Several calmodulin antagonists including trifluoperazine, W-7 and R24571 were tested for their ability to inhibit adenosine uptake by rat brain cortical synaptosomes. The results indicate that these agents inhibit adenosine uptake in a competitive manner. Their potencies as inhibitors of uptake were in good agreement with those reported for their inhibition of identified calmodulin regulated reactions. It is therefore concluded that the adenosine uptake process in rat brain synaptosomes is regulated by calmodulin or a calmodulin-like protein.

Benzodiazepine inhibition of adenosine uptake is not prevented by benzodiazepine antagonists

Uptake of [3H]adenosine into rat cerebral cortex synaptosomes was studied. Hexobendine (10(-5) M) and the benzodiazepine agonists diazepam (10(-5) M) and flurazepam (10(-4) M) significantly inhibited this uptake, but only if the compounds were pre-incubated for 10 min in the case of the benzodiazepines. The benzodiazepine antagonists Ro15-1788 (10(-5) M) and CGS 8216 (10(-5) M) failed to reverse the action of benzodiazepine agonists or hexobendine on [3H]adenosine uptake. The results add weight to the view that inhibition of adenosine uptake processes by benzodiazepines do not contribute to their behavioural effects.

Diazepam inhibition of adenosine uptake in the CNS: lack of effect on adenosine kinase

1. When tested at concentrations resulting in significant inhibitions of adenosine uptake in brain slices, diazepam and several other benzodiazepines were without effect on brain adenosine kinase. 2. This suggests that the inhibitory effect of the benzodiazepines on adenosine accumulation is at the membrane transport level and not by an inhibition of the intracellular phosphorylation step which facilitates adenosine uptake.

Inhibition of adenosine uptake into rat brain synaptosomes by prostaglandins, benzodiazepines and other centrally active compounds

A number of compounds have been tested for their abilities to inhibit the rapid uptake of adenosine by rat cerebral cortical synaptosomes. Prostaglandins PGI2, PGA2, and PGE1 and PGE2 were potent inhibitors of adenosine uptake with IC20 values in the 10(-7) M-10(-6) M range. PGA1, PGD2 and PGF2 alpha also inhibited uptake but were less active. The benzodiazepine antagonist Ro 15-1788 inhibited adenosine uptake and failed to antagonize the effects of diazepam. Another antagonist, ethyl-beta-carboline-3-carboxylate, was a weak inhibitor of adenosine uptake. Ro 5-4864, the so-called peripheral benzodiazepine ligand, inhibited adenosine uptake. Hydroxyzine and tracazolate, two anxiolytic agents, inhibited uptake as did flunarizine, a coronary vasodilator. Two calmodulin antagonists, W7 and R 24571, were effective inhibitors of adenosine uptake. Their IC50 values were comparable to those at which they have been demonstrated to inhibit calmodulin-mediated reactions in other systems. These observations suggest that adenosine uptake may be a calmodulin-regulated process.

Cell-membrane receptors for purines. Review

Purines are involved in many aspects of cell chemistry - intermediary metabolism, nucleic acid synthesis, and the supply of high-energy phosphates to various active transport systems. In addition, however, there appear to be specific receptor molecules located within the plasma membrane of some cells, which mediate changes of cell function in response to purines present in the extracellular fluid. It is the purpose of this review to summarize the kind of functions subserved by those receptors as well as the basic structural requirements for their activation.

Nucleoside transport in rat cerebral cortical synaptosomal membrane: a high affinity probe study

1. The nucleoside transport system in rat cerebral cortical synaptosomes was investigated using [H3]p-nitrobenzylthioinosine (NBMPR) as a high affinity probe. 2. There are high affinity and low affinity binding sites for NBMPR on rat synaptosomal membranes. The high affinity sites showed a KD value of 0.05 nM and a Bmax value of 113 fmol/mg protein. 3. Biochemical characterization of the high affinity [H3]NBMPR binding sites indicated that they probably correspond to nucleoside transport sites. 4. Several known adenosine uptake inhibitors including clonazepam were tested for their interaction with this high affinity binding site. 5. The results suggest that hexobendine and papaverine inhibit adenosine uptake by occupying the [H3]NBMPR high affinity binding sites. 6. Clonazepam and dipyridamole appear to inhibit adenosine uptake in rat cerebral cortical synaptosomes via an interaction at a different site.