The A1 adenosine receptor antagonist 1,3, dipropyl-8-cyclopentylxanthine (DPCPX) displays adenosine agonist properties in the FRTL5 thyroid cell line

Potential antipsychotic action of the selective agonist of adenosine A1 receptors, 5'-Cl-5'-deoxy-ENBA, in amphetamine and MK-801 rat models

Background

Disturbances of dopaminergic and glutamatergic transmissions have been suggested to be involved in the pathomechanisms underlying psychotic symptoms of schizophrenia. In line with this concept, hyperlocomotion induced by the dopaminomimetic amphetamine and the uncompetitive antagonist of NMDA receptors MK-801 (dizocilpine) in rodents is a generally established model for screening of new potential antipsychotic drugs. Since recent studies have indicated that receptors for adenosine may be targets for antipsychotic therapy, the aim of the present study was to investigate an influence of 5′-Cl-5′-deoxy-ENBA, a potent and selective adenosine A₁ receptor agonist, on hyperlocomotion induced by amphetamine and MK-801.

Methods

Locomotor activity was measured by Force Plate Actimeters where four force transducers located below the corners of the floor of the cage tracked the animal position on a Cartesian plane at each time point.

Results

Hyperlocomotion induced by either amphetamine (1 mg/kg sc) or MK-801 (0.3 mg/kg ip) was inhibited by 5′-Cl-5′-deoxy-ENBA (0.1 mg/kg ip). The effect of 5′-Cl-5′-deoxy-ENBA on the amphetamine- and MK-801-induced hyperlocomotion was antagonized by the selective antagonist of adenosine A₁ receptor DPCPX at doses of 1 and 2 mg/kg ip, respectively.

Conclusion

The present study suggests that stimulation of adenosine A₁ receptors may produce antipsychotic effects.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

Thyrotropin regulates adenosine A(1) receptor expression in rat thyroid FRTL-5 cells

The effect of thyrotropin (TSH), on adenosine A(1) receptor expression in thyroid FRTL-5 cells was examined by [(3)H]-1, 3-dipropyl-,8-cyclopentyl xanthine ([(3)H]-DPCPX) binding on cells in suspension and on membrane preparation, and by in situ mRNA labelling. The estimated K(D) for intact cells was 0.19 nM and about 47,000 binding sites per cell were found in cells constantly grown in the presence of TSH. Three days deprivation of TSH decreased the number of [(3)H]-DPCPX binding sites without any significant effect of K(D). Reintroduction of TSH to the cells returned the higher level of A(1) receptors both in suspension binding studies on whole cells and on membrane preparations. In situ hybridization revealed that TSH evoked an increase in the number of cells densely labelled with a probe against A(1) receptor mRNA. The potency of the A(1) receptor agonist N(6)-cyclohexyladenosine (CHA) as an inhibitor of cyclic AMP formation induced by forskolin was increased in TSH-treated cells, with a shift in the IC(50) from 2.05 nM in TSH-deprived cells to 0.14 nM in TSH-treated cells. Since the activation of A(1) receptors inhibits TSH-mediated cyclic AMP signalling, our results suggest a regulatory feedback mechanism between signalling via adenosine A(1) receptors and TSH receptors.

Extracellular adenosine increases Na+/I- symporter gene expression in rat thyroid FRTL-5 cells

We studied the effect of extracellular adenosine on iodide (I-) transport in FRTL-5 thyroid cells. I- accumulation increases after a 48 h exposure to adenosine in a concentration-dependent manner, reaching a maximum of 7.9-fold basal levels at 72 h after the addition of 300 microM adenosine. Neither I- efflux nor intracellular cyclic adenosine monophosphate accumulation is affected by the exposure to adenosine. The stimulation of I- transport by adenosine is partly as a result of an increase in Na+/I- symporter (NIS) mRNA and protein levels. Northern blot analysis revealed that adenosine increases NIS mRNA levels at 24 h, reaching a maximum at 36 h. Western blot analysis demonstrated that adenosine increases NIS protein levels at 36 h, reaching a maximum at 72 h, in parallel with the kinetics of adenosine-induced I- transport. Adenosine increased the promoter activity of a full-length NIS promoter-luciferase chimera, suggesting that the effect of adenosine on NIS mRNA levels is transcriptional. The stimulatory effect of adenosine on NIS mRNA levels, is mimicked by N6-(L-2-phenylisopropyl) adenosine (PIA), an A1 adenosine receptor agonist, and inhibited by 1,3-dipropyl-8-cyclopentylxanthine, an A1 adenosine receptor antagonist, suggesting that the effect is mediated via the A1 adenosine receptor stimulation in FRTL-5 cells. Incubating cells with islet-activating protein inhibited the adenosine-induced NIS mRNA levels. In sum, extracellular adenosine increases NIS gene expression and stimulates I- transport via the A1 adenosine receptor-Gi/Go protein signal transduction pathway.

N6-isopentenyladenosine affects cAMP-dependent microfilament organization in FRTL-5 thyroid cells

N6-Isopentenyladenosine (i6A), an adenosine and mevalonate derivative, inhibits, like adenosine, TSH-induced cAMP increase and its related events (I- uptake and DNA synthesis) in FRTL-5 cells. This inhibition is dose-dependent and is measurable at 10(-8) M. However, unlike adenosine, i6A prevents TSH-promoted microfilament disassembly. The effect of i6A on cytoskeletal structure is antagonized by pertussis toxin and could be assigned to its N6 substitution since it can be mimicked by other synthetic N6-adenosine derivatives. It is suggested that a step beyond cAMP is involved, since i6A prevents also microfilament disassembly induced by 8-bromo-cAMP. This is the first demonstration that an adenosine derivative, which is also an end-product of the isoprenoid pathway, affects cAMP-dependent microfilament organization.

Adenosine inhibits DNA synthesis stimulated with TSH, insulin, and phorbol 12-myristate 13-acetate in rat thyroid FRTL-5 cells

Adenosine has been shown to modulate cell proliferation in FRTL-5 thyroid cells, although the mechanisms by which this interaction occurs is still unclear. In the present study we investigated the effects of adenosine on the 3H-thymidine incorporation, cell cycle kinetics, and expression of the transcription factor c-Fos in cells stimulated via three different mitogenic pathways, i.e., by thyroid stimulating hormone (TSH) [adenosine 3',5'-cyclic monophosphate(cAMP)], insulin (tyrosine kinase), or phorbol 12-myristate 13-acetate (protein kinase C). Addition of adenosine to cells grown in medium containing hormones and serum did not inhibit the incorporation of 3H-thymidine. If adenosine was added to hormone-deprived cells together with any of the tested mitogens, the stimulation of the 3H-thymidine incorporation was inhibited in a dose-dependent manner. The inhibition was significantly lower when the cells were preincubated with TSH or insulin for 48 h. Flow cytometric studies showed that adenosine evoked an inhibition of the cells in the G0/G1 phase. Submaximal doses of adenosine (10 nM-10 microM) were able to induce c-Fos expression in FRTL-5 cells. However, the mitogen-induced expression of c-Fos was not reduced by maximal dose of adenosine (100 microM). The effect of adenosine on DNA synthesis was not dependent on pertussis toxin-sensitive G-proteins. In addition, adenosine A1- or A2- receptor antagonists did not block the effect of adenosine. The effect of adenosine was abolished by treatment of the cells with adenosine deaminase, suggesting that the observed effect was not mediated by a metabolite of adenosine. The results suggest that adenosine is an effective blocker of mitogen-evoked DNA synthesis of FRTL-5 cells, provided that adenosine is administered simultaneously with the mitogen.

1,3-Dipropyl-8-cyclopentyl xanthine (DPCPX): a useful tool for pharmacologists and physiologists?

There is now ample evidence in the literature to demonstrate the selectivity of action of DPCPX for adenosine A1 vs other adenosine receptor types in tissues derived from a wide range of species. However, care has to be exercised to ensure that its physiochemical properties do not result in the production of quantitatively misleading data. In experiments using canine tissues the still limited data available in the literature clearly and consistently demonstrate that DPCPX has a lower affinity than expected in preparations which would be anticipated to contain A1 receptors. A range of in vitro experiments also demonstrate that DPCPX is not always a "neutral" or "silent" antagonist. The mechanism underlying these additional effects is unclear, but may result from an ability of the compound to disrupt the normal interaction of the A1 receptor with Gi, or may be indicative of a lack of specificity of action. The limited evidence available suggests that the compound retains its selectivity and specificity of action in vivo, and early work indicates that the compound is proving to be a useful tool with which to explore the potential of activation of adenosine A1 receptors as an important mechanism in physiological and pathophysiological processes.