Small-Animal PET Study of Adenosine A₁ Receptors in Rat Brain: Blocking Receptors and Raising Extracellular Adenosine

Abstract

Activation of adenosine A 1 receptors (A 1 R) in the brain causes sedation, reduces anxiety, inhibits seizures, and promotes neuroprotection. Cerebral A 1 R can be visualized using 8-dicyclopropylmethyl-1-11 C-methyl-3-propyl-xanthine ( 11 C-MPDX) and PET. This study aims to test whether 11 C-MPDX can be used for quantitative studies of cerebral A 1 R in rodents. Methods: 11 C-MPDX was injected (intravenously) into isoflurane-anesthetized male Wistar rats (300 g). A dynamic scan of the central nervous system was obtained, using a small-animal PET camera. A cannula in a femoral artery was used for blood sampling. Three groups of animals were studied: group 1, controls (saline-treated); group 2, animals pretreated with the A 1 R antagonist 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX, 1 mg, intraperitoneally); and group 3, animals pretreated (intraperitoneally) with a 20% solution of ethanol in saline (2 mL) plus the adenosine kinase inhibitor 4-amino-5-(3-bromophenyl)-7-(6-morpholino-pyridin-3-yl)pyrido [2,3-d] pyrimidine dihydrochloride (ABT-702) (1 mg). DPCPX is known to occupy cerebral A 1 R, whereas ethanol and ABT-702 increase extracellular adenosine. Results: In groups 1 and 3, the brain was clearly visualized. High uptake of 11 C-MPDX was noted in striatum, hippocampus, and cerebellum. In group 2, tracer uptake was strongly suppressed and regional differences were abolished. The treatment of group 3 resulted in an unexpected 40%-45% increase of the cerebral uptake of radioactivity as indicated by increases of PET standardized uptake value, distribution volume from Logan plot, nondisplaceable binding potential from 2-tissue-compartment model fit, and standardized uptake value from a biodistribution study performed after the PET scan. The partition coefficient of the tracer (K 1 /k 2 from the model fit) was not altered under the study conditions. Conclusion: 11 C-MPDX shows a regional distribution in rat brain consistent with binding to A 1 R. Tracer binding is blocked by the selective A 1 R antagonist DPCPX. Pretreatment of animals with ethanol and adenosine kinase inhibitor increases 11 C-MPDX uptake. This increase may reflect an increased availability of A 1 R after acute exposure to ethanol.

Activation of Adenosine1 Receptors Induces Antidepressant-Like, Anti-Impulsive Effects on Differential Reinforcement of Low-Rate 72-s Behavior in Rats

“…Unfortunately, the 0.3 mg/kg DPCPX dose completely suppressed the antidepressant-like effect of CHA. A more complete understanding of CHA and DPCPX pharmacological potency that was available after the jpet.aspetjournals.org present studies (Collins et al, 2010;Paul et al, 2011) were conducted suggests that the 0.3 mg/kg DPCPX dose results in maximal effects at blocking adenosine A 1 receptors. Furthermore, similar antidepressant-like effects of CHA alone were observed in this experiment both times the agonist was administered.…”

“…First, complete displacement of the binding for a different adenosine A 1 receptor radiotracer was obtained with a 1 mg/kg i.p. dose of DPCPX (Paul et al, 2011). Second, CHA and DPCPX (each 0.03-0.30 mg/kg doses) altered locomotor activity through a specific interaction with brain adenosine A 1 receptors (Marston et al, 1998).…”

Use of 11C-MPDX and PET to Study Adenosine A1 Receptor Occupancy by Nonradioactive Agonists and Antagonists

“…In a previous study, we reported that 11 C-MPDX and small-animal PET can be used to quantify regional A 1 R densities in the rodent brain (7). PET offers unique opportunities for measuring the fraction of receptor populations occupied by nonradioactive drugs in the living brain and relating levels of occupancy to the magnitude of the therapeutic effect or to unwanted side effects.…”

Adenosine A1 Receptors in the Central Nervous System: Their Functions in Health and Disease, and Possible Elucidation by PET Imaging

“…Only a single PET study has examined changes of A 1 R expression after acute exposure of animals to ethanol [126]. In that study, Wistar rats were treated with a combination of ethanol and the AK inhibitor ABT-702, and scanned with the A 1 R ligand [ 11 C]MPDX.…”

“…A microPET study from our own group has indicated that A 1 R in rat brain are almost completely occupied after administration of DPCPX (3 mg/kg, i.p. ), 20 min before injection of 11 C-MPDX [126].…”