Adenosinergic system is involved in development of diazepam tolerance in mice

In the present study the effect of adenosinergic system on the development of diazepam tolerance to motor disturbances in mice was investigated. Diazepam tolerance was obtained by administration of diazepam at a dose of 5.0 mg/kg, s.c. for ten consecutive days. On the 1st and the 10th day of the experiment motor impairments were measured in two behavioural tests: rota-rod and chimney test. We showed that acute diazepam injection produced significant motor impairments in mice and that effect was decreased by repeated diazepam treatment, confirming the development of tolerance to the motor impairing effect of diazepam. We demonstrated that adenosine A₁ and/or A_2A receptor agonists: CPA (0.025 and 0.05 mg/kg, i.p.), CGS 21680 (0.1 and 0.2 mg/kg, i.p.), NECA (0.005 and 0.01 mg/kg, i.p.) pretreatment with diazepam were able to attenuate the development of diazepam tolerance and adenosine receptor antagonists: DPCPX (1.0 and 3.0 mg/kg, i.p.), DMPX (3.0 and 6.0 mg/kg, i.p.) and caffeine (10.0 and 20.0 mg/kg, i.p.) induced the opposite effect. The most apparent effects were obtained by non-selective agonist (NECA) and antagonist (caffeine) of adenosine receptors. We conclude that adenosinergic system plays an important role in mechanisms underlying the development of benzodiazepine tolerance.     

Antagonistic interaction between adenosine A2A and dopamine D2 receptors modulates the social recognition memory in reserpine-treated rats

Increasing evidence suggests that antagonistic interactions between specific subtypes of adenosine and dopamine receptors in the basal ganglia are involved in the control of motor activity. However, there are few studies investigating this interaction in other brain regions and its role in additional functions. In the present study, we evaluated whether reserpine-treated rats (1.0 mg/kg, i.p.) exhibit altered social recognition memory abilities. The effects of acute administration of the dopamine receptor agonists 7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3 benzazepine (SKF 38393, dopamine D(1) receptor agonist) and quinpirole (dopamine D(2) receptor agonist), together with the adenosine receptor antagonists caffeine (non-selective), 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, adenosine A(1) receptor antagonist) and 4-(2-[7-amino-2-{2-furyl}{1,2,4}triazolo-{2,3-a}{1,3,5}triazin-5-yl-amino]ethyl)phenol (ZM241385, adenosine A(2A) receptor antagonist), were also investigated. Twenty-four hours after treatment, reserpine-treated rats exhibited a significant disruption in the ability to recognize a juvenile rat after a short period of time. These animals did not show any motor deficit. The social recognition disruption induced by reserpine was reversed by acute treatment with quinpirole (0.05-0.1 mg/kg, i.p.), caffeine (10.0-30.0 mg/kg, i.p.) or ZM241385 (0.5-1.0 mg/kg, i.p.), but not with SKF 38393 (0.5-3.0 mg/kg, i.p.) or DPCPX (0.5-3.0 mg/kg, i.p.). Moreover, a synergistic response was observed following the co-administration of 'non-effective' doses of ZM241385 (0.1 mg/kg, i.p.) and quinpirole (0.01 mg/kg, i.p.). These results reinforce and extend the notion of antagonistic interactions between adenosine and dopamine receptors, and demonstrate, for the first time, that the blockade of adenosine A(2A) receptors and the activation of dopamine D(2) receptors can reverse the social recognition deficits induced by reserpine in rats.     

Adenosine A1 receptors modulate the anxiolytic-like effect of ethanol in the elevated plus-maze in mice

The anxiolytic property of ethanol is generally accepted to be an important motivational factor for its consumption and the development of alcohol dependence. Recent studies suggest that adenosine receptors mediate important actions of ethanol, such as motor incoordination and hypnotic effects. In addition, several lines of evidence support the involvement of adenosine in anxiety. The aim of the present study was to evaluate the role of adenosine receptors in the anxiolytic-like effect of ethanol in mice. The effects of acute administration of the adenosine receptor antagonists caffeine (nonselective), 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, adenosine A1 receptor antagonist) and 4-(2-[7-amino-2-[2-furyl][1,2,4]triazolo-[2,3-a][1,3,5]triazin-5-yl-amino]ethyl)phenol (ZM241385, adenosine A(2A) receptor antagonist), together with the adenosine A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA), and their interaction with ethanol in the elevated plus-maze test in mice were studied. The highest doses of caffeine (30.0 mg/kg, i.p.) and DPCPX (6.0 mg/kg, i.p.) produced an anxiogenic-like effect, while CCPA administration (0.25 mg/kg, i.p.) showed an anxiolytic-like activity. The prior administration of "non-anxiogenic" doses of caffeine (10.0 mg/kg, i.p.) and DPCPX (3.0 mg/kg, i.p.), but not ZM241385 (1.0 mg/kg, i.p.), significantly reduced the anxiolytic-like effect of ethanol (1.2 g/kg, i.p.). Moreover, anxiolytic-like response was observed by the co-administration of "non-anxiolytic" doses of CCPA (0.125 mg/kg) and ethanol (0.6 g/kg). These results reinforce the involvement of adenosine in anxiety and suggest that the activation of adenosine A1 receptors, but not adenosine A(2A) receptors, mediate the anxiolytic-like effect induced by ethanol in mice.     

Blockade of adenosine A2A receptor counteracts neuropeptide-S-induced hyperlocomotion in mice

Neuropeptide S (NPS) is the endogenous ligand of a G-protein-coupled receptor named as NPSR. Behavioral effects have been recently attributed to NPS, i.e. hyperlocomotion, anxiolysis, and wakefulness. However, little is known about the mechanisms by which NPS evokes such biological actions. The present study aimed to investigate the role played by the adenosine A_2A and A₁ receptors in hyperlocomotion induced by NPS. Spontaneous locomotion was assessed in an activity cage for 30 min in mice acutely treated with caffeine (a nonselective adenosine receptor antagonist), ZM241385 (a selective A_2A receptor antagonist), or CPT (a selective A₁ receptor antagonist) before NPS challenge (0.1 nmol, i.c.v.), which induce hyperlocomotion in mice. The pretreatment with caffeine (3 mg/kg, i.p.), in an inactive dose per se, prevented the increase in locomotion evoked by NPS. The co-administration of NPS (0.1 nmol, i.c.v.) and ZM241385 (0.1 pmol, i.c.v.) counteracted hyperlocomotion evoked by NPS. The co-administration of NPS and CPT (0.1 pmol, i.c.v.) slightly facilitated the increase in locomotion evoked by NPS alone. In summary, the pharmacological blockade of A_2A receptors significantly attenuated the stimulatory effects of NPS. By contrast, the antagonism of A₁ receptors facilitated NPS-induced hyperlocomotion in mice, but we cannot rule out a merely additive effect of two stimulatory systems in the brain. Altogether, this is the first evidence of a putative role played by A_2A and A₁ receptors in modulating hyperlocomotion induced by NPS.     

The adenosine receptor antagonist theophylline induces a monoamine-dependent increase of the anticataleptic effects of NMDA receptor antagonists

Previous work revealed that adenosine antagonists as theophylline reversed neuroleptic-induced catalepsy and potentiated anticataleptic effects of dopamine agonists reflecting specific adenosine-dopamine receptor interactions in the central nervous system. We tested whether similar functional interactions exist between adenosine receptors and glutamate receptors of the N-methyl-D-asparte (NMDA) subtype. The present study demonstrates that the anticataleptic effects of the competitive NMDA receptor antagonist CGP37849 and the non-competitive NMDA receptor antagonist dizocilpine can be potentiated by coadministration of a threshold dose of the adenosine receptor antagonist theophylline (2.5 mg/kg, i.p.) in haloperidol (0.5 mg/kg, i.p.)-pretreated rats. This potentiation was elicited only with higher doses of CGP37849 (4 and 8 mg/kg, i.p.) or dizocilpine (0.16 mg/kg, i.p.) in haloperidol (0.5 mg/kg, i.p.), but not in reserpine (5 mg/kg, i.p.) plus -methyl-ptyrosine (100 mg/kg, i.p.)-pretreated animals. Therefore, these synergistic interactions seem to be brought about by indirect monoamine-dependent mechanisms rather than direct functional interrelationships between NMDA and adenosine A_2a receptors.     

The adenosine A<Subscript>2A</Subscript> antagonist MSX-3 reverses the effects of the dopamine antagonist haloperidol on effort-related decision making in a T-maze cost/benefit procedure

Rationale  Mesolimbic dopamine (DA) is a critical component of the brain circuitry regulating behavioral activation and effort-related processes. Research involving choice tasks has shown that rats with impaired DA transmission reallocate their instrumental behavior away from food-reinforced tasks with high response requirements and instead select less effortful food-seeking behaviors. Objective  Previous work showed that adenosine A_2A antagonism can reverse the effects of the DA antagonist haloperidol in an operant task that assesses effort-related choice. The present work used a T-maze choice procedure to assess the effects of adenosine A_2A and A₁ antagonism. Materials and methods  With this task, the two arms of the maze have different reinforcement densities (four vs. two food pellets), and a vertical 44 cm barrier is positioned in the arm with the higher density, presenting the animal with an effort-related challenge. Untreated rats strongly prefer the arm with the high density of food reward and climb the barrier in order to obtain the food. Results  Haloperidol produced a dose-related (0.05–0.15 mg/kg i.p.) reduction in the number of trials in which the rats chose the high-barrier arm. Co-administration of the adenosine A_2A receptor antagonist MSX-3 (0.75, 1.5, and 3.0 mg/kg i.p.), but not the A₁ antagonist 8-cyclopentyl-1,3-dipropylxanthine (0.75, 1.5, and 3.0 mg/kg i.p.), reversed the effects of haloperidol on effort-related choice and latency. Conclusions  Adenosine A_2A and D2 receptors interact to regulate effort-related decision making, which may have implications for the treatment of psychiatric symptoms such as psychomotor slowing or anergia that can be observed in depression, parkinsonism, and other disorders.     

A1 AND A2 adenosine receptors mediate opposite effects on nsaid-induced gastric ulcers in the rat

Some adenosine modulators can modify stress- and ethanol-induced ulcers. However, little is known about their effects on NSAID-induced gastric ulcers. Our studies show that cyclopentyladenosine (CPA), an A₁ selective receptor agonist, produced exacerbation of the NSAID-induced gastric lesion (100% at 10 mg/kg p.o.). CPA administered alone also demonstrated ulcerogenic capacity. PD-125944 (DPMA) and CGS 21680, A_2a selective agonists, protected gastric mucosa from NSAID-induced damage with ED₅₀ of 4.4 and 1.2 mg/kg p.o. respectively. The effect of DPMA was reversed by 8-(3-chlorostyryl) caffeine (CSC), an A_2A antagonist, with an ID₅₀ of 9 mg/kg i.p. The adenosine uptake blocker S-(4-nitrobenzyl)-6-thioinosine (NBTT; 1 mg/kg i.p.) reversed the damage with maximum inhibition of 50-60%. This response was reversed by CSC (10 mg/kg i.p.) but not by 8-cyclopentyl-l,3-dipropylxanthine (5 mg/kg i.p.), an A₁ receptor antagonist. These data suggest that selective activation of A₂ adenosine receptors completely prevents indomethacin-induced gastric ulcers. However, selective stimulation of A₁ receptors causes both direct gastric damage and enhances that induced by NSAIDs. Therefore, an increase in endogenous adenosine produces partial inhibition of the injury, suggesting a predominant role for A₂ receptors in the regulation of gastric mucosa integrity.     

Pharmacodynamics of ZM 241385, a Potent A2a Adenosine Receptor Antagonist,after Enteric Administration in Rat,Cat and Dog

4-(2-[7-Amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385) is currently the most selective for the A_2a adenosine receptor antagonist. This paper describes the in-vivo activity of ZM 241385 after administration by both oral and intraduodenal routes. In conscious spontaneously hypertensive rats, ZM 241385 (1–10 mg kg^?1) selectively attenuated the mean arterial blood pressure response produced by exogenous adenosine (1 mg kg^?1 min^?1, i.v.) by up to 45% after oral administration. Activity of ZM 241385 was maintained for at least 6 h after 3 and 10 mg kg^?1 (p.o.). In conscious normotensive cats, ZM 241385 attenuated the blood pressure responses to adenosine (0.6–10 mg kg^?1, i.v.) by 94% after 10 mg kg^?1 (p.o.) and by up to 74% after 0.3 mg kg^?1 (i.v.). Duration of action of ZM 241385 up to 12 h (36% inhibition) was observed after 3 mg kg^?1 (p.o.). In anaesthetized dogs and cats, ZM 241385, after intraduodenal administration (1–10 mg kg^?1), produced a rapid (dose ratio 100-fold 15 min after administration of 10 mg kg^?1 in the cat) and prolonged (dose ratio of 14 at 6 h after administration of 10 mg kg^?1) attenuation of the vasodilatation responses to adenosine receptor stimulation. When administered by this route ZM 241385 was six times more potent than theophylline in the cat and at least twice as potent as theophylline in the dog. In conclusion, ZM 241385 is a potent, selective A_2a adenosine receptor antagonist which is orally active, with a good duration of action by the enteric route in cat, rat and dog. It could therefore be used to evaluate the role of adenosine A_2a receptors in the action of adenosine in-vivo.     

Interactive role of adenosine and dopamine in the opiate withdrawal syndrome

Adenosine reduces opioid withdrawal symptoms by activating A₁ adenosine receptors, probably by inhibiting excitatory amino acid release. Since blockade of A_2A adenosine receptors seems to enhance dopaminergic striatopallidal transmission, we evaluated the role of the purinergic system in the opiate withdrawal syndrome by using two A₁ receptor agonists [N⁶-cyclohexyladenosine, CHA and 2-chloro-N⁶-cyclopentyladenosine, CCPA], and two A_2A receptor antagonists (SCH 58261 and 8-(3-chlorostyryl)caffeine, CSC).Male adult rats received increasing doses of morphine sulphate suspended in 5 ml/kg of a sustained release preparation (40–100 mg/kg s.c.) daily for 4 days and 20 h after the last administration, the withdrawal syndrome was evoked by naloxone (5 mg/kg i.p.). Animals were observed for 30 min for signs of opiate withdrawal. Other groups of rats were implanted with concentric probes for microdialysis and dopamine levels were measured in the nucleus accumbens. CHA and CCPA (0.05, 0.1 or 0.5 mg/kg i.p.) significantly reduced "wet-dog" shakes, diarrhoea, teeth chattering, jumping and writhing. SCH 58261 and CSC (0.1, 0.5 or 1 mg/kg i.p.), given 10 min before naloxone, also reduced signs of opiate withdrawal. CHA plus SCH 58261 and CCPA plus CSC greatly enhanced the reduction of withdrawal signs observed with CHA and CCPA or CSC and SCH 58261 alone. In vivo microdialysis showed that naloxone significantly decreased DA release; this effect was prevented by pretreatment with systemic SCH 58261 and CSC, but not with CHA and CCPA. Our results demonstrate that A₁ and A_2A adenosine receptors mediate the effect induced by adenosine in opiate withdrawal syndrome and suggest that adenosine A₁ agonists and adenosine A_2A antagonists may be beneficial in the treatment of this syndrome.L. Stella and V. de Novellis share first authorship     

SCH 58261 and ZM 241385 differentially prevent the motor effects of CGS 21680 in mice: evidence for a functional 'atypical' adenosine A(2A) receptor

The acute motor effects elicited by drugs acting upon adenosine A(2A) receptors, namely the highly selective agonist CGS 21680 or the antagonists SCH 58261 and ZM 241385, were investigated in mice. CGS 21680 dose-dependently (0.1-2.5 mg/kg i.p.) decreased horizontal and vertical motor activities. The depressant effect of CGS 21680 (0. 5 mg/kg i.p.) was maintained in mice pretreated by the adenosine receptor antagonist 8-(p-sulfophenyl)-theophylline (10-30 mg/kg i.p. ), which poorly penetrates the blood-brain barrier, but was completely lost in adenosine A(2A) receptor knockout mice. Thus, the adenosine A(2A) receptor is critically involved in motor activity. SCH 58261 (1-10 mg/kg i.p.) increased locomotion and rearing with a quick onset, but for a shorter period in mice habituated to the environment than in mice unfamiliar to it. ZM 241385 (7.5-60 mg/kg i. p.) stimulated horizontal and vertical activities with a slow onset at the two highest tested doses, similarly in naive and in habituated mice. The increase in locomotion elicited by ZM 241385 (15-30 mg/kg i.p. and 10-20 nM i.c.v.) was retained in mice treated by CGS 21680 (0.5 mg/kg i.p.) but that elicited by SCH 58261 (1-3-10 mg/kg i.p. and 10-20 nM i.c.v.) partially subsided. In conclusion, both 'striatal-like'/'SCH 58261-sensitive' adenosine A(2A) receptors and 'ZM 241385-sensitive'/'atypical' CGS 21680 binding sites may mediate CGS 21680-induced motor effects. Moreover, our results suggest that 'atypical' CGS 21680 binding sites could be adenosine A(2A) receptors with a peculiar pharmacological profile.     

The anxiogenic-like effect of caffeine in two experimental procedures measuring anxiety in the mouse is not shared by selective A2A adenosine receptor antagonists

Rationale: The elevated plus-maze and the light/dark box are two established anxiety tests in rodents, which are useful to screen putative anxiogenic effects of drugs. Objective: Caffeine is well known to promote anxious behaviour in humans and animal models, but the precise site of action of the drug is still a matter of debate. The present study investigated whether the anxiogenic effects of caffeine observed in mice depend on the blockade of A_2A receptor. First, the effects induced by the non-selective drug caffeine were compared with those elicited by two selective A_2A receptor antagonists over a wide range of doses in the same experimental conditions. The effects of A_2A or A₁ adenosine receptor agonists and of a selective A₁ adenosine receptor antagonist were also investigated. Second, wild-type and A_2A receptor knockout mice offered another approach to delineate the role played by A_2A receptor in caffeine’s anxiogenic effects. Methods: Mice were exposed to the elevated plus-maze or to the light/dark box for 5 min after acute or chronic administration of tested drugs. Results: Caffeine acutely administered (50 or 100 mg/kg IP) induced anxiety-like effects in both procedures. Its chronic administration (50 mg/kg IP twice daily) for 1 week or consumption in the drinking water (0.3 g/l) for 8 days or 2 months were also anxiogenic in the plus-maze test. The A_2A receptor antagonists ZM241385 (up to 60 mg/kg IP) and SCH58261 (up to 10 mg/kg IP) were devoid of acute effects in both tests. One week administration of ZM241385 (30 mg/kg IP) or SCH58261 (3 mg/kg IP) had no effects in the plus-maze test. An antagonist (DPCPX) and an agonist (CPA) at A₁ receptors had no acute effects on anxiety-related indices, whereas an A_2A receptor agonist (CGS 21680) displayed non-specific motor effects in the plus-maze test. Acute administration of caffeine (50 mg/kg IP) induced no clear-cut anxiety-like effects in the plus-maze test in A_2A receptor knockout mice that exhibited higher basal anxiety levels than wild-type mice. Chronic administration (50 mg/kg IP twice daily) for 1 week elicited less anxiety-like behaviour in A_2A receptor knockout than in wild-type mice. Conclusions: Adaptative mechanisms following mutation in A_2A receptors or their long-term blockade after chronic ingestion of caffeine may be responsible for increase proneness to anxiety. However, the short-term anxiety-like effect of caffeine in mice might not be related solely to the blockade of adenosine A_2A receptors, since it is not shared by A_2A selective antagonists. Received: 21 March 1999 / Final version: 24 June 1999     

Effects of SR141716 and WIN 55,212-2 on tolerance to ethanol in rats using the acute and rapid procedures

Rationale Our previous findings have shown rapid cross-tolerance between ethanol and Δ⁹-tetrahydrocannabinol and that intraperitoneal (i.p.) injection of cannabinoid receptor type 1 (CB1R) antagonist SR141716 (SR) does not interfere with tolerance to either of these drugs in mice. Objectives This study investigates the effects of SR, alone or in combination with the CB receptor agonist WIN 55,212-2 (WIN), on the development of acute and rapid tolerance to the incoordinating effect of ethanol in rats. Materials and methods Male Wistar rats received SR, through i.p. (0.5–2.0 mg/kg) or intracerebroventricular (i.c.v.) injections (0.5–4.0 μg), alone or together with WIN (1.0 μg, i.c.v.), in combination with ethanol (2.7 g/kg, i.p.). Another group received WIN (1.0 μg, i.c.v.) in combination with ethanol (2.3 g/kg), and the rats were tested for motor coordination. Rapid tolerance was assessed 24 h later by administering ethanol to all animals and retesting them under the same dose regimen. Acute tolerance was evaluated for 75 min after ethanol (3.0 g/kg, i.p.) in animals treated with SR or WIN (i.c.v.). Results The reduced motor impairment on day 2 (i.e., rapid tolerance) was blocked by SR (i.p. and i.c.v.). WIN (1.0 μg, i.c.v.) facilitated rapid tolerance and also prevented the blockade of rapid tolerance by SR (1.0 μg, i.c.v.). In the acute tolerance procedure, SR did not affect the motor incoordination induced by ethanol. Conclusions The results suggest that the endocannabinoid system may contribute to the development of rapid tolerance to ethanol.     

Defaecation, intestinal fluid accumulation and motility in rodents: implications of cannabinoid CB1 receptors

We have studied the effect of SR141716A (0.1–5 mg/kg, i.p.), a cannabinoid CB₁ receptor antagonist, and WIN (0.1–5 mg/kg, i.p.), a cannabinoid receptor agonist, on acute defaecation and gastrointestinal transit in mice and on intraluminal fluid accumulation in the rat small intestine. SR141716A increased while WIN 55,212-2 decreased defaecation, gastrointestinal transit and fluid accumulation. A per se non-effective dose of SR141716A (0.3 mg/kg) counteracted the inhibitory effect of WIN 55,212-2 (1 mg/kg) on gastrointestinal functions studied. The effect of SR141716 on both intestinal fluid accumulation in rats and gastrointestinal transit in mice was inhibited by atropine (1 mg/kg, i.p.), but not by hexamethonium (1 mg/kg, s.c.), SR140333 (20 μg/kg, i.p.) or SR48968 (20 μg/kg, i.p.), antagonists of NK₁ and NK₂ receptors, respectively. These results suggest that intestinal fluid accumulation and motility are inhibited by endogenous cannabinoid(s) acting at the cannabinoid CB₁ receptors. This effect may be mediated by mechanisms involving muscarinic cholinoceptors. Received: 13 July 1998 / Accepted: 25 September 1998     

Involvement of diazepam-insensitive benzodiazepine receptors in the suppression of DOI-induced head-twitch responses in diabetic mice

Rationale We previously reported that the head-twitch responses induced by the 5-HT₂ receptor agonist (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) (DOI-HTRs) were decreased in streptozotocin-induced diabetic mice.Objectives We examined the involvement of γ-aminobutyric acid (GABA)/benzodiazepine system on the suppression of DOI-HTRs in diabetic mice.Results The benzodiazepine receptor antagonist flumazenil (0.1–1 mg/kg, i.v.) dose-dependently and significantly increased DOI-HTRs in diabetic mice to the same levels as in nondiabetic mice. However, flumazenil (0.1–1 mg/kg, i.v.) did not affect DOI-HTRs in nondiabetic mice. The benzodiazepine receptor agonist diazepam (0.1–1 mg/kg, i.p.) had no effect on DOI-HTRs in either nondiabetic or diabetic mice. The GABA_A receptor antagonist bicuculline (0.1–1 mg/kg, i.p.) and the benzodiazepine receptor partial inverse agonist Ro 15-4513 (0.1–1 mg/kg, i.v.) dose-dependently and significantly suppressed DOI-HTRs in nondiabetic mice to the same levels as in diabetic mice. Ro 15-4513-induced reduction of DOI-HTRs in nondiabetic mice was completely antagonized by flumazenil (1 mg/kg, i.v.), but not diazepam (0.3 mg/kg, i.p.).Conclusions We suggest that the abnormal diazepam-insensitive benzodiazepine receptor function partly underlies the suppression of DOI-HTRs in diabetic mice.     

Striatal adenosine A(2A) receptor blockade increases extracellular dopamine release following l-DOPA administration in intact and dopamine-denervated rats

The influence of the selective adenosine A(2A) receptor antagonist ZM 241385 on exogenous l-DOPA-derived dopamine (DA) release in intact and dopamine-denervated rats was studied using an in vivo microdialysis in freely moving animals. Local infusion of l-DOPA (2.5 microM) produced a marked increase in striatal extracellular DA level in intact and malonate-lesioned rats. Intrastriatal perfusion of ZM 241385 (50-100 microM) had no effect on basal extracellular DA level, but enhanced dose-dependently the l-DOPA-induced DA release in intact and malonate-lesioned animals. A non-selective adenosine A(2A) receptor antagonist DMPX (100 microM), similarly to ZM 241385, accelerated conversion of l-DOPA in intact and malonate-denervated rats. This effect was not produced by the adenosine A(1) receptor antagonist, CPX (10-50 microM). However, ZM 241385 did not affect the l-DOPA-induced DA release in rats pretreated with reserpine (5 mg/kg i.p.) and alpha-methyl-p-tyrosine (AMPT, 300 mg/kg i.p.). Obtained results indicate that blockade of striatal adenosine A(2A) receptors increases the l-DOPA-derived DA release possibly by indirect mechanism exerted on DA terminals, an effect dependent on striatal tyrosine hydroxylase activity. Selective antagonists of adenosine A(2A) receptors may exert a beneficial effect at early stages of Parkinson's disease by enhancing the therapeutic efficacy of l-DOPA applied exogenously.     

Serotonergic modulation of extrapyramidal motor disorders in mice and rats: Role of striatal 5-HT3 and 5-HT6 receptors

Previous studies showed that 5-HT_1A and 5-HT₂ receptors play an important role in controlling the extrapyramidal motor disorders. However, the functions of other 5-HT receptor subtypes remain elusive. To elucidate the role of 5-HT receptors, specifically of 5-HT₃～5-HT₇ subtypes, in modifying antipsychotic- induced extrapyramidal side effects (EPS), we studied the effects of the 5-HT stimulant 5-hydroxytryptophan (5-HTP) and various 5-HT receptor antagonists on haloperidol (HAL)-induced bradykinesia and catalepsy in mice and rats. Pretreatment of mice with 5-HTP (25–100 mg/kg, i.p.) dose-dependently enhanced HAL (0.3 mg/kg, i.p.)-induced bradykinesia and catalepsy. The potentiation of HAL-induced EPS by 5-HTP (50 mg/kg, i.p.) was significantly inhibited by ritanserin (5-HT₂ antagonist, 0.3-3 mg/kg, i.p.), ondansetron (5-HT₃ antagonist, 0.1–1 mg/kg, i.p.), or SB-258585 (5-HT₆ antagonist, 1–10 mg/kg, i.p.) in a dose-dependent manner. However, neither WAY-100135 (5-HT_1A antagonist, 1–10 mg/kg, i.p.), GR-125487 (5-HT₄ antagonist, 1–10 mg/kg, i.p.), SB-699551 (5-HT_5A antagonist, 1–10 mg/kg, i.p.) nor SB-269970 (5-HT₇ antagonist, 1–10 mg/kg, i.p.) reduced the 5-HTP and HAL-induced bradykinesia or catalepsy. In addition, both ondansetron (0.1–1 mg/kg, i.p.) and SB-258585 (3 and 10 mg/kg, i.p.) also alleviated bradykinesia and catalepsy induced by HAL (0.5 mg/kg, i.p.) alone in mice. Furthermore, bilateral microinjection of ondansetron (5 μg (13.7 nmol) per side) or SB-258585 (5 μg (8.92 nmol) per side) into the dorsolateral striatum (dlST) attenuated haloperidol-induced catalepsy in rats. These results suggest that serotonergic stimulation augments extrapyramidal motor disorders by activating the striatal 5-HT₃ and 5-HT₆ receptors and the antagonism of these receptors effectively alleviates antipsychotic-induced EPS.     

Actions of adenosine A2A receptor antagonist KW-6002 on drug-induced catalepsy and hypokinesia caused by reserpine or MPTP

Rationale: Current treatment of Parkinson’s disease (PD) is based on dopamine replacement therapy, but this leads to long term complications, including dyskinesia. Adenosine A_2A receptors are particularly abundant in the striatum and would be a target for an alternative approach to the treatment of PD. Objectives: The purpose of this study is to examine the efficacy and potency of the novel selective adenosine A_2A receptor antagonist (E)-1,3-diethyl-8-(3,4-dimethoxystyryl)-7-methyl-3,7-dhydro-1H-purine-2,6-dione (KW-6002) in ameliorating the motor deficits in various mouse models of Parkinson’s disease. Methods: We evaluated the efficacy and potency of KW-6002 and other reference compounds in the selective adenosine A_2A receptor agonist 2-[p-(2-carboxyethyl)phenethylamino]-5’-N-ethylcarboxamidoadenosine (CGS 21680)-, haloperidol- or reserpine-induced catalepsy models. The effect of KW-6002 on reserpine or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride(MPTP)-induced hypolocomotion was also examined. Results: The ED₅₀s of KW-6002 in the reversal of CGS21680-induced and reserpine-induced catalepsy were 0.05 mg/kg, PO and 0.26 mg/kg, PO, respectively. Compared to the ED₅₀ of other adenosine antagonists and dopamine agonist drugs, KW-6002 is over 10 times as potent in these models. KW-6002 also ameliorated the hypolocomotion (minimum effective dose; 0.16 mg/kg) induced by nigral dopaminergic dysfunction with MPTP or reserpine treatment. Combined administrations of subthreshold doses of KW-6002 and l-dopa (50 mg/kg, PO) exerted prominent effects on haloperidol-induced and reserpine-induced catalepsy, suggesting that there may be a synergism between the adenosine A_2A receptor antagonist KW-6002 and dopaminergic agents. Conclusions: To our knowledge, KW-6002 is the most potent and orally active adenosine A_2A receptor antagonist in experimental models of Parkinson’s disease, and may offer a new therapeutic approach to the treatment of Parkinson’s disease. Received: 22 March 1999 / Final version: 25 May 1999     

Yohimbine produces antinociception in the formalin test in rats: involvement of serotonin1A receptors

Rationale: Previous studies have suggested that the α₂-adrenergic receptor antagonist yohimbine produced antinociceptive effects in the formalin test in rats. However, yohimbine is also an agonist at serotonin (5-HT)_1A receptors, suggesting the possibility that the antinociceptive effects of yohimbine might be mediated via these receptors. Objective: The purpose of the present studies was to evaluate the potential role of 5-HT_1A receptors in mediating the antinociceptive effects of yohimbine. Methods: The antinociceptive effects of yohimbine were evaluated using the formalin test in rats. Results: Yohimbine (2.5–10 mg/kg s.c.) produced dose-related antinociception during both phase I and phase II of the formalin test, and was approximately equipotent and equiefficacious to morphine. The selective 5-HT_1A receptor antagonist WAY 100,635 (0.03–3.0 mg/kg s.c.) produced a partial reversal of yohimbine. In comparison, the selective 5-HT_1A receptor agonist (±)8-hydroxy- dipropylaminotetralin HBr (8OH-DPAT; 1.0 mg/kg s.c.) also produced a dose-related antinociception in the formalin test, although 8OH-DPAT was completely reversed by WAY 100,635 (3.0 mg/kg s.c.). The antinociceptive effects of yohimbine were not antagonized by the 5-HT_1B/1D antagonist GR 127935 (1.0 mg/kg and 3.0 mg/kg s.c.), the 5-HT₂ antagonist LY53857 (1.0 mg/kg s.c.), or the 5-HT₃ antagonist zatosetron (3.0 mg/kg s.c.). Conclusions: The present results demonstrate that yohimbine produces a dose-related antinociception in the formalin test in rats which is mediated in part by the agonistic actions at 5-HT_1A receptors. Received: 10 September 1999 / Final version: 5 November 1999     

An endocannabinoid signaling system modulates anxiety-like behavior in male Syrian hamsters

Rationale  An endocannabinoid signaling system has not been identified in hamsters. Objective  We examined the existence of an endocannabinoid signaling system in Syrian hamsters using neuroanatomical, biochemical, and behavioral pharmacological approaches. Materials and methods  The distribution of cannabinoid receptors was mapped, and membrane fatty-acid amide hydrolase (FAAH) activity and levels of fatty-acid amides were measured in hamster brain. The impact of cannabinoid CB₁ receptor blockade and inhibition of FAAH was evaluated in the elevated plus maze, rota-rod test, and models of unconditioned and conditioned social defeat. Results  A characteristic heterogeneous distribution of cannabinoid receptors was detected in hamster brain using [³H]CP55,940 binding and autoradiography. The FAAH inhibitor URB597 inhibited FAAH activity (IC₅₀ = 12.8 nM) and elevated levels of fatty-acid amides (N-palmitoyl ethanolamine and N-oleoyl ethanolamine) in hamster brain. Anandamide levels were not reliably altered. The cannabinoid agonist WIN55,212-2 (1– 10 mg/kg i.p.) induced CB₁-mediated motor ataxia. Blockade of CB₁ with rimonabant (5 mg/kg i.p.) induced anxiogenic-like behavior in the elevated plus maze. URB597 (0.1–0.3 mg/kg i.p.) induced CB₁-mediated anxiolytic-like effects in the elevated plus maze, similar to the benzodiazepine diazepam (2 mg/kg i.p.). Diazepam (2–6 mg/kg i.p.) suppressed the expression, but not the acquisition, of conditioned defeat. By contrast, neither URB597 (0.3–3.0 mg/kg i.p.) nor rimonabant (5 mg/kg i.p.) altered unconditioned or conditioned social defeat or rota-rod performance. Conclusions  Endocannabinoids engage functional CB₁ receptors in hamster brain to suppress anxiety-like behavior and undergo enzymatic hydrolysis catalyzed by FAAH. Our results further suggest that neither unconditioned nor conditioned social defeat in the Syrian hamster is dependent upon cannabinoid CB₁ receptor activation.