Effects of A<Subscript>2A</Subscript> adenosine receptor blockade or stimulation on alcohol intake in alcohol-preferring rats

Rationale  

A_2A adenosine receptors (A_2AARs) have been proposed to be involved in drug addiction; however, preclinical studies about the effects of A_2AAR ligands on alcohol consumption have provided inconsistent results.     

Cytotoxic purine nucleoside analogues bind to A<Subscript>1</Subscript>, A<Subscript>2A</Subscript>, and A<Subscript>3</Subscript> adenosine receptors

Fludarabine, clofarabine, and cladribine are anticancer agents which are analogues of the purine nucleoside adenosine. These agents have been associated with cardiac and neurological toxicities. Because these agents are analogues of adenosine, they may act through adenosine receptors to elicit their toxic effects. The objective of this study was to evaluate the ability of cytotoxic nucleoside analogues to bind and activate adenosine receptor subtypes (A₁, A_2A, A_2B, and A₃). Radioligand binding studies utilizing Chinese hamster ovary cells, stably transfected with adenosine A₁, A_2A, or A₃ receptor subtype, were used to assess the binding affinities of these compounds, whereas adenylyl cyclase activity was used to assess the binding to A_2B receptors. Clofarabine and cladribine both bound to the A_2A receptor with a K _i of 17 and 15 μM, respectively. Clofarabine was the only adenosine analogue to bind to the A₃ receptor with a K _i of 10 μM, and none of these compounds bound to the A_2B receptor. Results show that clofarabine, cladribine, and fludarabine bind to the A₁ receptor. In addition, clofarabine, cladribine, and fludarabine were A₁ agonists (IC₅₀ 3.1, 30, and 30 μM, respectively). Neither pyrimidine nucleoside analogues gemcitabine nor cytarabine associated with any of the adenosine receptor subtypes (K _i > 100μM). This is the first report of an interaction between all adenosine receptor subtypes and chemotherapeutic nucleoside analogues commonly used in the treatment of cancer. Therefore, activation of these receptors may be at least one mechanism through which fludarabine-associated toxicity occurs.     

Comparative pharmacology of antipsychotics possessing combined dopamine D<Subscript>2</Subscript> and serotonin 5-HT<Subscript>1A</Subscript> receptor properties

Rationale  

There is increasing interest in antipsychotics intended to manage positive symptoms via D₂ receptor blockade and improve negative symptoms and cognitive deficits via 5-HT_1A activation. Such a strategy reduces side-effects such as the extrapyramidal syndrome (EPS), weight gain, and autonomic disturbance liability.     

Stimulant effects of adenosine antagonists on operant behavior: differential actions of selective A<Subscript>2A</Subscript> and A<Subscript>1</Subscript> antagonists

Rationale  

Adenosine A_2A antagonists can reverse many of the behavioral effects of dopamine antagonists, including actions on instrumental behavior. However, little is known about the effects of selective adenosine antagonists on operant behavior when these drugs are administered alone.     

Interactive effects of mGlu5 and 5-HT<Subscript>2A</Subscript> receptors on locomotor activity in mice

Rationale  

Metabotropic glutamate (mGlu) receptors have been suggested to play a role in neuropsychiatric disorders including schizophrenia, drug abuse, and depression. Because serotonergic hallucinogens increase glutamate release and mGlu receptors modulate the response to serotonin (5-HT)_2A activation, the interactions between serotonin 5-HT_2A receptors and mGlu receptors may prove to be important for our understanding of these diseases.     

Occupancy of dopamine D<Subscript>1</Subscript>, D<Subscript>2</Subscript> and serotonin<Subscript>2A</Subscript> receptors in schizophrenic patients treated with flupentixol in comparison with risperidone and haloperidol

Rationale Flupentixol (FLX) has been used as a neuroleptic for nearly 4 decades. In vitro data show comparable affinity to dopamine D₂, D₁ and 5-HT_2A receptors and recently, FLX showed to be not inferior to risperidone in schizophrenic patients with predominant negative symptomatology, which was implicated with flupentixol’s interaction with 5-HT_2A and/or D₁ receptors. Objectives To assess in vivo receptor occupancy (RO) in patients clinically treated with FLX (n = 13, 5.7 ± 1.4 mg/day) in comparison with risperidone (RIS, n = 11, 3.6 ± 1.3 mg/day) and haloperidol (HAL, n = 11, 8.5 ± 5.5 mg/day). Materials and methods Each patient underwent two PET scans with 3-N-[¹¹C]methylspiperone (target: frontal 5-HT_2A), [¹¹C]SCH23390 (striatal D₁) or [¹¹C]raclopride (striatal D₂). RO was calculated as the percentage reduction of specific binding in comparison with healthy controls. Results D₂-RO under FLX was between 50% and 70%, indicating an ED₅₀ of about 0.7 ng/ml serum. 5-HT_2A and D₁-RO was 20 ± 10% and 20 ± 5% (mean, SEM). Under HAL, D₁-RO was 14 ± 6% and under RIS not significantly different from zero. Conclusions We were able to demonstrate a moderate 5-HT_2A and D₁ occupancy under clinically relevant doses of flupentixol, albeit lower than expected from in vitro data and clearly below saturation. Therefore, if flupentixol’s efficacy on negative symptoms is based on its interaction with 5-HT_2A and/or D₁ receptors, it should be highly dependent on serum concentration and thus on dosage and metabolism. However, these data suggest that mechanisms other than D₁ or 5-HT_2A antagonism may contribute to flupentixol’s efficacy on negative symptoms.     

Behavioral economics of food reinforcement and the effects of prefeeding,extinction, and eticlopride in dopamine D<Subscript>2</Subscript> receptor mutant mice

Rationale  

Several studies have investigated the reinforcing effects of food in genetically engineered mice lacking dopamine D₂ receptors (DA D₂Rs); however, behavioral economic analyses quantifying reinforcement have not been conducted.     

In vivo electrophysiological and neurochemical effects of the selective 5-HT<Subscript>1A</Subscript> receptor agonist,F13640, at pre- and postsynaptic 5-HT<Subscript>1A</Subscript> receptors in the rat

Rationale  

F13640 (befiradol) is a novel 5-HT_1A receptor agonist with exceptional selectivity vs. other receptors and binding sites. It shows analgesic activity in animal models and is currently developed for human use.     

Behavioral sensitization to cocaine in rats: evidence for temporal differences in dopamine D<Subscript>3</Subscript> and D<Subscript>2</Subscript> receptor sensitivity

Rationale

Cocaine-induced changes in D₂ receptors have been implicated in the expression of sensitized behavioral responses and addiction-like behaviors; however, the influence of D₃ receptors is less clear.

Objectives

To characterize the effects of repeated cocaine administration on the sensitivity of rats to D₂- and D₃-mediated behaviors, as well as the binding properties of ventral striatal D₂-like and D₃ receptors.

Methods

Pramipexole was used to assess the sensitivity of rats to D₃/D₂ agonist-induced yawning, hypothermia, and locomotor activity, 24 h, 72 h, 10, 21, and 42 days after repeated cocaine or saline administration. The locomotor effects of cocaine (42 day) and the binding properties of ventral striatal D₂-like and D₃ receptors (24 h and 42 days) were also evaluated.

Results

Cocaine-treated rats displayed an enhanced locomotor response to cocaine, as well as a progressive and persistent leftward/upward shift of the ascending limb (72 h–42 day) and leftward shift of the descending limb (42 days) of the pramipexole-induced yawning dose–response curve. Cocaine treatment also decreased B _max and K _d for D₂-like receptors and increased D₃ receptor binding at 42 days. Cocaine treatment did not change pramipexole-induced hypothermia or locomotor activity or yawning induced by cholinergic or serotonergic agonists.

Conclusions

These studies suggest that temporal differences exist in the development of cocaine-induced sensitization of D₃ and D₂ receptors, with enhancements of D₃-mediated behavioral effects observed within 72 h and enhancements of D₂-mediated behavioral effects apparent 42 days after cocaine. These findings highlight the need to consider changes in D₃ receptor function when thinking about the behavioral plasticity that occurs during abstinence from cocaine use.

     

Effect of the adenosine A<Subscript>2A</Subscript> receptor antagonist MSX-3 on motivational disruptions of maternal behavior induced by dopamine antagonism in the early postpartum rat

Rationale  

Mesolimbic dopamine (DA), particularly in the nucleus accumbens, importantly regulates activational aspects of maternal responsiveness. DA antagonism and accumbens DA depletions interfere with early postpartum maternal motivation by selectively affecting most forms of active maternal behaviors, while leaving nursing behavior relatively intact. Considerable evidence indicates that there is a functional interaction between DA D2 and adenosine A_2A receptors in striatal areas, including the nucleus accumbens.     

Genistein,a potent inhibitor of secretory phospholipase A<Subscript>2</Subscript>: a new insight in down regulation of inflammation

Objectives  

Some of the legumes, spices and medicinal herbs rich in genistein are known for their anti-inflammatory properties. Anti-inflammatory property of these herbs is determined by subjecting secretory phospholipase A₂ (sPLA₂) inhibition, a key enzyme in the inflammatory reactions by genistein.     

Synthesis, 5-HT<Subscript>1A</Subscript> and 5-HT<Subscript>2A</Subscript> receptor affinity and QSAR study of 1-benzhydryl-piperazine derivatives with xanthine moiety at N4

Abstract  

Three novel 1-benzhydryl-piperazines with xanthine moiety at N4 were synthesized and tested for 5-HT_1A and 5-HT_2A receptor affinity. One of the compounds showed the highest affinity (58.6 nM) and selectivity (34 times) to 5-HT_2A receptor known for this class of compounds. A set of the three new and 31 previously synthesized 1-arylpiperazines with xanthine moiety at N4 was compiled and a QSAR study was performed in order to rationalize the further synthesis. It was found that the 5-HT_1A affinity depends on the shape of the molecules (ovality and number of circuits), the distribution of the electron density in the structures (partial charges at piperazine N1 and xanthine N1) and their charge transfer ability (HOMO energy). The 5-HT_2A affinity depends on the lipophilicity of the ligands and the distribution of the electron density in the structures (partial charges at piperazine N4 and xanthine O6). The QSAR results are in a good agreement with known pharmacophore models.     

A positron emission tomography (PET) study of cerebral dopamine D<Subscript>2</Subscript> and serotonine 5-HT<Subscript>2A</Subscript> receptor occupancy in patients treated with cyamemazine (Tercian)

Rationale Cyamemazine (Tercian) is an antipsychotic drug with anxiolytic properties. Recently, an in vitro study showed that cyamemazine possesses high affinity for serotonin 5-HT_2A receptors, which was fourfold higher than its affinity for dopamine D₂ receptors (Hameg et al. 2003).Objectives The aim of this study is to confirm these previous data in vivo in patients treated with clinically relevant doses of Tercian.Methods Eight patients received 37.5, 75, 150 or 300 mg/day of Tercian depending on their symptomatology. Dopamine D₂ and serotonin 5-HT_2A receptor occupancies (RO) were assessed at steady-state plasma levels of cyamemazine with positron emission tomography (PET), using [¹¹C]raclopride and [¹¹C]N-methyl-spiperone, respectively. The effective plasma level of the drug leading to 50% of receptor occupancy was estimated by fitting RO with plasma levels of cyamemazine at the time of the PET scan.Results Cyamemazine induced near saturation of 5-HT_2A receptors (RO=62.1–98.2%) in the frontal cortex even at low plasma levels of the drug. On the contrary, occupancy of striatal D₂ receptors increased with plasma levels, and no saturation was obtained even at high plasma levels (RO=25.2–74.9%). The effective plasma level of cyamemazine leading to 50% of D₂ receptor occupancy was fourfold higher than that for 5-HT_2A receptors. Accordingly, individual 5-HT_2A/D₂ RO ratios ranged from 1.26 to 2.68. No patients presented relevant increased prolactin levels, and only mild extrapyramidal side effects were noticed on Simpson and Angus Scale.Conclusion This in vivo binding study conducted in patients confirms previous in vitro findings indicating that cyamemazine has a higher affinity for serotonin 5-HT_2A receptors compared to dopamine D₂ receptors. In the dose range 37.5–300 mg, levels of dopamine D₂ occupancy remained below the level for motor side effects observed with typical antipsychotics and is likely to explain the low propensity of the drug to induce extrapyramidal side effects.     

The effects of systemic,intrastriatal, and intrapallidal injections of caffeine and systemic injections of A<Subscript>2A</Subscript> and A<Subscript>1</Subscript> antagonists on forepaw stepping in the unilateral 6-OHDA-lesioned rat

Rationale and objectives  Given that adenosine A_2A antagonists appear to be therapeutic in several animal models of Parkinson’s disease (PD), we examined the extent to which caffeine and selective A_2A and A₁ antagonists could enhance contralateral forepaw stepping in the unilateral 6-OHDA-lesioned rat. Materials and methods  Following unilateral injections of 12 μg 6-OHDA into the medial forebrain bundle (MFB), frequency of stepping with both front paws was counted separately as the paws were dragged anteriorally and laterally by a treadmill. Results  The MFB lesions decreased contralateral stepping by 74–83%, and 8 mg/kg 3,4-dihydroxy-l-phenylalanine (L-DOPA) increased contralateral stepping by 25–26%. Caffeine given systemically (15 mg/kg) or into the dorsal striatum or external globus pallidus (GP_E; 20–40 μg) increased contralateral forepaw stepping by 14%, 27%, and 26%, respectively, and enhanced the effect of 8 mg/kg L-DOPA on stepping. The selective A_2A antagonist SCH-58261 (2 mg/kg) also increased stepping by 13% and enhanced the therapeutic effect of L-DOPA, whereas the selective A_2A antagonist 8-cyclopentyltheophylline (3–7 mg/kg) and A₁ agonist N ⁶-cyclopentyladenosine (0.03–0.2 mg/kg) had no effect. None of these drugs appeared to produce dyskinesic effects. Conclusions  In this well-validated animal model of the akinesic effects of PD, caffeine and a selective A_2A, but not an A₁, antagonist were able to provide both monotherapeutic and adjunctive therapeutic effects. These data are consistent with the hypothesis that A_2A antagonists may be therapeutic in human PD patients and indicate that the dorsal striatum and GP_E are critical sites of therapeutic action. An erratum to this article can be found at     

A detailed behavioral analysis of the acute motor effects of caffeine in the rat: involvement of adenosine A<Subscript>1</Subscript> and A<Subscript>2A</Subscript> receptors

Rationale There is no consensus on the contribution of adenosine A₁ and A_2A receptor blockade to motor-activating effects of caffeine.Objective Our aim was to use a detailed and continuous observational method to compare the motor effects induced by caffeine with those induced by selective A₁ and A_2A receptor antagonists.Methods The behavioral repertoire induced by systemic administration of caffeine (3, 10, and 30 mg/kg), A₁ receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT; 1.2, 4.8 and 7.2 mg/kg), and A_2A receptor antagonist 3-(3-hydroxypropyl)-8-(m-methoxystyryl)-7-methyl-1-propargylxanthine phosphate disodium salt (MSX-3; 1, 3, and 10 mg/kg) was analyzed. The effects of pretreatment with the selective A₁ receptor agonist N ⁶-cyclopentyladenosine (CPA; 0.1 mg/g) and the selective A_2A receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5′-N-ethylcarboxyamidoadenosine (CGS 21680; 0.2 mg/kg) on the pattern of motor activation induced by caffeine, CPT, or MSX-3 were also examined.Results The pattern of behavioral activation induced by caffeine was better mimicked by CPT than by MSX-3. Coadministration of CPT and MSX-3 gave different results depending on the dose and the type of behavioral response. CPA was more effective at decreasing the activating effects of caffeine and CPT than those of CGS 21680. On the other hand, CGS 21680 was more effective at decreasing the activating effects of MSX-3 than those of caffeine or CPT. Factor analysis revealed a complex three-dimensional behavioral profile for caffeine that was similar to the profile for CPT and was different from the profile for MSX-3.Conclusions The results indicate a predominant role for A₁ receptors in the motor-activating effects of acutely administered caffeine.     

Adenosine A<Subscript>2A</Subscript> receptor deficient mice are partially resistant to limbic seizures

The neuromodulator adenosine, acting through activation of four defined metabotropic receptors called A₁, A_2A, A_2B and A_3, has been proposed as an endogenous anticonvulsant. Here, the consequences of deleting the adenosine A_2A receptor have been examined in different experimental models of epilepsy. A_2AR KO mice were not protected against seizures originating from brainstem structures, namely electroshock-induced seizures. The intensities of seizures induced by pentylenetetrazol or pilocarpine, as well as the percentages of convulsing mice, were significantly reduced in A_2A receptor knockout (A_2AR KO) animals. A_2AR KO mice exhibited reduced pentylenetetrazol-induced kindled seizures, demonstrating an important role of the A_2A receptor in the acquisition of kindling. These data suggest that adenosine stimulating A_2A receptors modulates excitatory neurotransmission and exacerbates limbic seizures. It is therefore suggested that adenosine A_2A receptor antagonists might offer protection from some epileptic syndromes.     

Pharmacological characterisation of the agonist radioligand binding site of 5-HT<Subscript>2A</Subscript>, 5-HT<Subscript>2B</Subscript> and 5-HT<Subscript>2C</Subscript> receptors

In the present study we compared the affinity of various drugs for the high affinity agonist-preferring binding site of human recombinant 5-HT_2A, 5-HT_2B and 5-HT_2C receptors stably expressed in monoclonal mammalian cell lines. To ensure that the agonist-preferring conformation of the receptor was preferentially labelled in competition binding experiments, saturation analysis was conducted using antagonist and agonist radiolabels at each receptor. Antagonist radiolabels ([³H]-ketanserin for 5-HT_2A receptor and [³H]-mesulergine for 5-HT_2B and 5-HT_2C receptor) bound to a larger population of receptors in each preparation than the corresponding agonist radiolabel ([¹²⁵I]-DOI for 5-HT_2A receptor binding and [³H]-5-HT for 5-HT_2B and 5-HT_2C receptor binding). Competition experiments were subsequently conducted against appropriate concentrations of the agonist radiolabels bound to the agonist-preferring subset of receptors in each preparation. These studies confirmed that there are a number of highly selective antagonists available to investigate 5-HT₂ receptor subtype function (for example, MDL 100907, RS-127445 and RS-102221 for 5-HT_2A, 5-HT_2B and 5-HT_2C receptors respectively). There remains, however, a lack of highly selective agonists. (–)DOI is potent and moderately selective for 5-HT_2A receptors, BW723C86 has poor selectivity for human 5-HT_2B receptors, while Org 37684 and VER-3323 display some selectivity for the 5-HT_2C receptor. We report for the first time in a single study, the selectivity of numerous serotonergic drugs for 5-HT₂ receptors from the same species, in mammalian cell lines and using, exclusively, agonist radiolabels. The results indicate the importance of defining the selectivity of pharmacological tools, which may have been over-estimated in the past, and highlights the need to find more selective agonists to investigate 5-HT₂ receptor pharmacology.     

Pharmacological and behavioral characterization of the 5-HT<Subscript>2A</Subscript> receptor in C57BL/6N mice

Rationale  

The serotonin (5-HT) 2A receptor is implicated in numerous psychiatric disorders, making it an important, clinically relevant target. Despite the availability of transgenic mouse lines, the native mouse 5-HT_2A receptor is not well-characterized.     

Non-uniform blockade of intrastriatal D<Subscript>2</Subscript>/D<Subscript>3</Subscript> receptors by risperidone and amisulpride

Rationale Atypical antipsychotic drugs have been shown to preferentially affect extrastriatal (mesolimbic) D₂/D₃ receptors over those within the striatum (nigrostriatal). The striatum does not contain exclusively nigrostriatal dopamine tracts, however. The caudate nucleus and ventral parts of the striatum primarily contain limbic and associative dopamine pathways more relevant to psychosis. Objectives We tested the hypothesis that two pharmacologically distinct atypical antipsychotic drugs, amisulpride and risperidone, would preferentially occupy of D₂/D₃ dopamine receptors in limbic and associative regions of the striatum. Methods Eight amisulpride-treated patients, six risperidone-treated patients and six age- and sex-matched healthy controls were recruited. Dynamic SPET studies were performed after bolus injection of [¹²³I]epidepride. Binding potential (BP) images were generated using a modified Logan method and aligned between subjects. Regions of interest (ROIs) were placed around head of caudate and putamen bilaterally on an average BP map derived from aligned control images. These ROIs were then applied user-independently to the BP maps for each subject to calculate BP for head of caudate and putamen. Mean occupancy of D₂/D₃ receptors in each ROI was determined by reference to the drug-free healthy volunteer group. Occupancy values for head of caudate and putamen were compared using paired Student’s t test. Results D₂/D₃ receptor occupancy was 42% in caudate and 31% in putamen for risperidone (t=5.9, df=11, p=0.0001) and 51% in caudate and 37% in putamen for amisulpride (t=11.1, df=15, p<0.0001). Conclusions Amisulpride and risperidone both show selective occupancy for limbic and associative D₂/D₃ receptors within the striatum.     

Effects of adenosine A<Subscript>2A</Subscript> receptor stimulation on cocaine-seeking behavior in rats

Rationale  

Dopamine (DA) receptor stimulation in the nucleus accumbens (NAc) plays an important role in regulating cocaine-seeking behavior. Adenosine receptors antagonize the effects of DA receptor stimulation on intracellular signaling, neuronal output, and behavior.