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

Atypical antipsychotic profile of flunarizine in animal models

RATIONALE

Flunarizine is known as a calcium channel blocker commonly used in many countries to treat migraine and vertigo. Parkinsonism has been described as one of its side-effects in the elderly, which is in agreement with its recently characterized moderate D2 receptor antagonism.

OBJECTIVES

To perform a pre-clinical evaluation of flunarizine as a potential antipsychotic.

METHODS

We evaluated the action of orally administered flunarizine in mice against hyperlocomotion induced by amphetamine and dizocilpine (MK-801) as pharmacological models of schizophrenia, induction of catalepsy as a measure for extrapyramidal symptoms and impairment induced by dizocilpine on the delayed alternation task for working memory.

RESULTS

Flunarizine robustly inhibited hyperlocomotion induced by both amphetamine and dizocilpine at doses that do not reduce spontaneous locomotion (3-30 mg/kg). Mild catalepsy was observed at 30 mg/kg, being more pronounced at 50 mg/kg and 100 mg/kg. Flunarizine (30 mg/kg) improved dizocilpine-induced impairment on the delayed alternation test.

CONCLUSIONS

These results suggest a profile comparable to atypical antipsychotics. The low cost, good tolerability and long half-life (over 2 weeks) of flunarizine are possible advantages for its use as an atypical antipsychotic. These results warrant clinical trials with flunarizine for the treatment of schizophrenia.

Anticonvulsant and adverse effects of MK-801, LY 235959, and GYKI 52466 in combination with Ca2+ channel inhibitors in mice

This study was designed to investigate the influence of the calcium (Ca2+) channel inhibitors nicardipine, nifedipine, and flunarizine on the protective action of MK-801, LY 235959 [N-methyl-D-aspartate (NMDA) receptor antagonists], and GYKI 52466 (a non-NMDA receptor antagonist) against electroconvulsions in mice. Unlike nicardipine (15 mg/kg) or flunarizine (10 mg/kg) nifedipine (7.5 and 15 mg/kg) potentiated the protective potency of MK-801 (0.05 mg/kg), as reflected by significant elevation of the convulsive threshold (a CS50 value of the current strength in mA producing tonic hind limb extension in 50% of the animals). The protective activity of LY 235959 and GYKI 52466 was reflected by their ED50 values in mg/kg, at which the drugs were expected to protect 50% of mice against maximal electroshock-induced tonic extension of the hind limbs. Nicardipine (3.75 15 mg/kg), nifedipine (0.94-15 mg/kg), and flunarizine (2.5-10 mg/kg) in a dose-dependent manner markedly potentiated the antiseizure efficacy of LY 235959. Flunarizine (5 and 10 mg/kg) was the only Ca2+ channel inhibitor to enhance the protective action of GYKI 52466 against electroconvulsions. Except with MK-801 + flunarizine (motor performance) or GYKI 52466 + flunarizine (long-term memory), combination of NMDA or non-NMDA receptor antagonists with Ca2+ channel inhibitors produced an impairment of motor performance (evaluated in the chimney test) and long-term memory acquisition (measured in the passive avoidance task) as compared with vehicle treatment.

Influence of nicardipine, nimodipine and flunarizine on the anticonvulsant efficacy of antiepileptics against pentylenetetrazol in mice

Among three calcium channel inhibitors, only nicardipine (10-40 mg/kg) significantly inhibited clonic seizures induced by pentylenetetrazol administered at its CD97 (convulsive dose 97%) of 81 mg/kg, subcutaneously. Nimodipine and flunarizine (both up to 80 mg/kg) did not suppress pentylenetetrazol-induced clonic seizures per se. Co-administration of nicardipine (5 mg/kg) resulted in a significant enhancement of the protective potency of either ethosuximide (50 mg/kg) or valproate (100 mg/kg) against clonic seizures in this test. Similar effects were noted in case of combined treatment of nimodipine (20-40 mg/kg) with these antiepileptics. On the contrary, flunarizine (up to 20 mg/kg) did not modify the anticonvulsive action of these antiepileptic drugs. Moreover, none of the studied calcium channel inhibitors influenced the protective activity of clonazepam (0.01 mg/kg). The antiepileptic drugs, administered alone in above doses, were ineffective against pentylenetetrazol-induced clonic convulsions. In case of ethosuximide and valproate, the motor performance in the chimney test was worsened by co-administration of nimodipine (40 mg/kg). We found no pharmacokinetic interactions (at least in relation to the plasma levels of ethosuximide and valproate) that could explain the observed results. Thus, we conclude that a combination of some calcium channel inhibitors and antiepileptic drugs may provide more efficient protection against experimental seizures which may bear a potential clinical significance.

Influence of BAY k-8644, a calcium channel agonist, on the anticonvulsant activity of conventional anti-epileptics against electroconvulsions in mice

BAY k-8644, an agonist at the dihydropyridine binding site of the L-type voltage dependent calcium channel, at the dose of 5 mg/kg (s.c.) did not significantly affect the threshold for electroconvulsions, but impaired the protective efficacy of flunarizine (15 and 20 mg/kg, i.p.) in the electroconvulsive test. Interestingly, the calcium channel agonist (at 1 and 5 mg/kg) distinctly diminished the protection offered by conventional anti-epileptic drugs (carbamazepine, diphenylhydantoin and phenobarbital) against maximal electroshock-induced seizures in mice. A pharmacokinetic interaction does not seem to be involved in the effect of BAY k-8644, since total plasma levels of these anti-epileptics (measured by immunofluorescence) were not affected by the calcium channel agonist. The only anti-epileptic drug resistant to BAY k-8644 (up to 5 mg/kg) was valproate, whose ED50 (in mg/kg) was not changed in the presence of the calcium channel agonist. Further, BAY k-8644 (5 mg/kg) did not influence the flunarizine (a calcium channel blocker)-induced potentiation of the protective action of valproate against maximal electroshock-induced convulsions. The calcium channel agonist (5 mg/kg) reversed the flunarizine-induced augmentation of the anticonvulsive activity of carbamazepine. It may be concluded that carbamazepine, diphenylhydantoin and phenobarbital partially exert their anticonvulsive effects via blockade of calcium influx whilst valproate does not seem to. In this context, the flunarizine-induced potentiation of the anticonvulsive activity of valproate is probably independent of calcium channel blockade.

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.

Effects of antiepileptic drugs, calcium channel blockers and other compounds on seizures induced by activation of voltage-dependent L calcium channel in DBA/2 mice

1. The convulsant activity of the calcium voltage L-channel agonist Bay k 8644 was studied in genetically epilepsy prone DBA/2 mice. 2. Seizures were induced by intracerebroventricular injection of Bay k 8644. 3. These seizures were reversed by some calcium channel blockers such as dihydropyridines, some excitatory amino acid antagonists such as 2-amino-7-phosphonoeptanoate and CPPene, 2-chloro-adenosine, some anticonvulsant drugs such as magnesium valproate, diazepam and clonazepam and two kappa opioid agonists (U-50488H and U-54494A). 4. The remaining antiepileptic drugs (carbamazepine, phenytoin, phenobarbital and trimethadione) were ineffective in this respect. Other anticonvulsant compounds such as dizocilpine (MK 801), ketamine and drugs enhancing GABAergic transmission did not significantly affect the clonic phase of the seizures induced by Bay k 8644. 5. These results show that Bay k 8644 seizures are relatively resistant to some anticonvulsant compounds. The role of some neurotransmitters on seizures induced by Bay k 8644 is discussed.

Relative anticonvulsant effects of GABAmimetic and GABA modulatory agents

Anticonvulsant properties of compounds that enhance GABA-mediated inhibition through modulatory sites on the GABAA receptor [phenobarbital (PB), clonazepam (CZP), alpha-ethyl-alpha-methyl-gamma-thiobutyrolactone (alpha-EMTBL)] were compared with anticonvulsant effects of compounds believed to be antagonists at these modulatory sites (Ro15-1788 and alpha-isopropyl-alpha-methyl-gamma-butyrolactone gamma-IMGBL)] and to 4,5,6,7-tetrahydroisoxazolo-[4,5-c]-pyridin-3-ol (THIP, GABAA receptor agonist), (+/-) baclofen (GABAB receptor agonist), and gamma-vinyl GABA, a compound that increases endogenous GABA. The compounds were tested for their ability to block experimental seizures caused by maximal electroshock, pentylenetetrazol, picrotoxin, methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), bicuculline (BIC), aminophylline, strychnine, and t-butyl-bicyclophosphorothionate (TBPS) in mice. CZP blocked all but strychnine seizures. PB was also highly effective, blocking all but TBPS seizures. alpha-EMTBL, representing a new class of experimental anticonvulsant drugs, prevented all seizures except strychnine (STR)- and aminophylline-induced seizures. The antagonists are effective only against one convulsant stimulus. Ro15-1788 and alpha-IMGBL prevented only DMCM- and pentylenetetrazol (PTZ)-induced seizures, respectively. THIP and gamma-vinyl GABA both blocked only BIC and picrotoxin seizures. Baclofen had no anticonvulsant activity. These data demonstrate that compounds that increase neuronal inhibition by potentiating the action of GABA have a broader spectrum of anticonvulsant action than either antagonists or GABAmimetic agents or compounds that increase endogenous GABA.

Effects of some calcium antagonists upon the activity of common antiepileptic compounds on sound-induced seizures in DBA/2 mice

1. Flunarizine (2.65 mumol/kg, i.p.) and nimodipine (5.25 mumol/kg, i.p.) potentiated the anticonvulsant properties of phenytoin, phenobarbital and valproate against audiogenic seizures in DBA/2 mice. 2. Diltiazem (5.25 mumol/kg, i.p.) was able to potentiate the antiseizure activity of phenytoin but was not effective against the anticonvulsant action of phenobarbital and valproate. 3. Verapamil (5.25 mumol/kg, i.p.) was unable to potentiate the anticonvulsant properties of all antiepileptic drugs studied. 4. Bay K 8644 (1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluorophenyl)-pyridine- 5-carboxylic acid), a calcium agonist at a dose of 2.65 mumol/kg, i.p., induced a reduction of anticonvulsant potency of phenytoin, phenobarbital and valproate. 5. None of the calcium antagonists used significantly increased the plasma levels of antiepileptic compounds or significantly affected the body temperature changes induced by anticonvulsant drugs. 6. It may be concluded that some calcium antagonists enhance the anticonvulsant properties of some antiepileptic drugs against audiogenic seizures. A pharmacokinetic interaction does not seem responsible for these effects.

Anticonvulsant properties of some calcium antagonists on sound-induced seizures in genetically epilepsy prone rats

1. The anticonvulsant activity of calcium channel antagonists, was studied after intraperitoneal or oral administration in genetically epilepsy prone rats (GEPR). 2. Flunarizine, dihydropyridines and HA 1004, administered intraperitoneally, were the most potent compounds. Diltiazem, prenylamine, perhexiline, verapamil and methoxyverapamil, given intraperitoneally, were able to reduce the incidence of the tonic phase but were completely ineffective in preventing clonic and running phases of sound-induced seizures in GEPR. Similar anticonvulsant activity was observed when these compounds were administered orally. 3. After intracerebroventricular administration of some of the hydrosoluble calcium antagonists studied, the anticonvulsant effects were similar to those observed after systemic administration. 4. The systemic administration of Bay K 8644, a dihydropyridine analogue, having the ability to stimulate calcium entry into cells produced a dose-dependent increase in clonic and tonic convulsions and other epileptic phenomena, which were prevented by pretreatment with nimodipine or nitrendipine. 5. The possible role of purinergic, excitatory amino acid, GABA-benzodiapine mechanisms as well as the role of Ca2(+)-calmodulin and calcium channel binding sites on the anticonvulsant effects of some calcium antagonists are discussed.

Flunarizine does not delay the development of generalized seizures by amygdala kindling

Adult rats underwent amygdala kindling after the administration of vehicle, flunarizine 20 mg/kg/day, or flunarizine 40 mg/kg/day. Stimuli were delivered thrice daily at current intensities twice after-discharge threshold (ADT). Flunarizine did not alter initial or post-kindling ADT and did not affect the latency (number of stimuli) to the first stage 5 seizure. Apart from a tendency to increase the latency between the first and the 4th stage 5 seizures, flunarizine had little if any effect on amygdala kindling in this protocol.

Quinolinic acid-induced seizures, but not nerve cell death, are associated with extracellular Ca2+ decrease assessed in the hippocampus by brain dialysis

Seizures, neuronal damage and extracellular Ca2+ concentration were studied in rats unilaterally injected in the dorsal hippocampus with quinolinic acid, a brain metabolite with excitotoxic properties. In freely moving animals, in the first 2 h after the injection of a convulsant and neurotoxic dose (156 nmol), quinolinic acid induced a tetrodotoxin-insensitive decrease in the extracellular Ca2+ concentration (nadir 40%) in the injected area, as assessed by brain dialysis coupled to a fluorimetric method for Ca2+ detection. Blockade of quinolinic acid-induced decrements in Ca2+ by 15.6 nmol D-(-)2-amino-7-phosphonoheptanoic acid indicated that this effect was receptor-mediated. Dose-response relationships showed a close association between seizure activity (measured by EEG) and extracellular Ca2+ changes in the injected area. Changes in Ca2+ were apparent at the site of injection prior to the onset of focal seizures and they were not found in the homotypic structure where seizures were conducted. Drugs effective in blocking seizures (carbamazepine and flunarizine) prevented the fall in extracellular Ca2+, while drugs without anticonvulsant activity (ethosuximide and nifedipine) did not. Destruction of nerve cells by quinolinic acid was not prevented by treatment with carbamazepine and flunarizine. The results suggest that the fall in extracellular Ca2+ observed in the first 2 h after quinolinic acid, probably reflecting the ion influx into neurons, is involved in triggering focal seizures but is not related to the occurrence of nerve cell death.

Effect of various calcium channel blockers on three different models of limbic seizures in rats

Voltage-dependent calcium channel-blockers were studied for their ability to modulate limbic seizures induced in rats by injection of quinolinic acid and kainic acid into the hippocampus or by hippocampal kindling. Flunarizine, at 40 mg/kg (but not 20 mg/kg), reduced the total number of seizures and total time spent in seizures induced by quinolinic acid by 75%; at 60 mg/kg, both parameters were reduced more than 90%, while at 80 mg/kg seizures induced by kainic acid were not affected. Forty and 60 mg/kg of flunarizine protected hippocampal-kindled rats from fully developed convulsions (Stage 5). Nifedipine, at 20 and 40 mg/kg, was ineffective on seizures induced by both quinolinate and kainate. However, at 20 mg/kg, 57% of the kindled animals were protected from Stage 5 and total protection was achieved at 40 mg/kg. Verapamil, at 40 mg/kg, reduced by respectively, 88% and 78%, the total number of seizures and the total time spent in seizures induced by quinolinic acid, but had no effect on seizures induced by kainate and Stage 5 seizures. The results suggest that, while seizures induced by kainic acid were refractory to all voltage-dependent calcium channel blockers, binding sites affected by flunarizine and verapamil in the brain may selectively facilitate ictal activity induced by quinolinic acid. Binding sites for dihydropyridine might contribute to the increased hippocampal excitability in kindled animals. The role of calcium entry through voltage-dependent calcium channels in the occurrence of seizures in these models of limbic epilepsy is discussed.

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.

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.

Effects of bicuculline-induced seizures on benzodiazepine and adenosine receptors in developing rat brain

The effects of seizures induced by an acute administration of bicuculline have been investigated on the central benzodiazepine and adenosine receptors in developing rats and in adults. Generalized seizures rapidly increased the total number of both benzodiazepine binding sites and adenosine A1 receptors, without changes in receptor affinity (KD). It was concluded that such a phenomenon may facilitate the anticonvulsant action of benzodiazepine and adenosine via receptor binding and that it could be an adaptative process to protect subjects against recurrent seizures, especially in newborns.

Dissimilarities between benzodiazepine-binding sites and adenosine uptake sites in astrocytes and neurons in primary cultures

The question whether the benzodiazepine receptor site in astrocytes or in neurons might be identical to the adenosine uptake site was studied by determining pharmacological profiles, inhibition types, and the effects of benzodiazepine antagonists in primary cultures of either astrocytes or neurons. Fourteen different benzodiazepines and five different adenosine uptake inhibitors displaced [3H] diazepam and inhibited adenosine uptake in both astrocytes and neurons. However, the rank orders (determined as IC50 values) with which these two parameters were affected were profoundly different, indicating dissimilarities between these two sites. For several of the compounds a difference in inhibition type (competitive vs. noncompetitive) was observed between the benzodiazepine-binding site and the adenosine uptake site in astrocytes and/or neurons, which further corroborated the conclusion of a difference between the benzodiazepine-binding site and the adenosine uptake site. Finally, the neuronal benzodiazepine antagonists RO 15-1788 and CGS-8216 and the astrocytic benzodiazepine antagonist PK 11195, which reverse the action of benzodiazepines, were not able to reverse inhibition of adenosine uptake by diazepam but exerted an inhibitory effect of their own.

The xanthine derivative 1-(5'-oxohexyl)-3-methyl-7-propyl xanthine (HWA 285) enhances the actions of adenosine

The effect of two closely related xanthine derivatives, pentoxifylline and HWA 285, on cyclic AMP accumulation in rat hippocampal slices and on adenosine uptake in erythrocytes was examined. Pentoxifylline was a weak competitive antagonist of adenosine effects on cyclic AMP accumulation. HWA 285, by contrast, had a small stimulatory effect per se and also potentiated the effect of adenosine (10-30 microM). Neither pentoxifylline nor HWA 285 significantly affected the cyclic AMP accumulation induced by the stable adenosine analogue NECA or by alpha- or beta-adrenoceptor activation. HWA 285 was a much more potent inhibitor of adenosine uptake into human erythrocytes than pentoxifylline and other examined xanthines including thiocaffeine, 8-p-sulphophenyltheophylline, theophylline, caffeine and enprofylline. It is suggested that HWA 285 may potentiate, rather than antagonize, the effects of endogenous as well as exogenous adenosine, partly as a consequence of adenosine uptake inhibition.

The effects of lidoflazine and flunarizine on cerebral reactive hyperemia

Cerebral blood flow in the rat was monitored by a venous outflow technique with an extracorporeal circulation, which allows for the continuous recording of flow over periods of several hours. The bi-fluorophenyl-piperazine derivatives, lidoflazine and flunarizine, enhanced the reactive hyperemia elicited by a brief (30 s) anoxic challenge. They did not alter resting cerebral blood flow rates. Verapamil, a potent calcium slow channel blocker, decreased resting flow rates but did not alter the duration of the reactive hyperemia. As lidoflazine and flunarizine are potent inhibitors of adenosine uptake, whereas verapamil is not, the results are consistent with the hypothesis that adenosine plays a significant role in cerebral vascular autoregulation.

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.