SPECIES-DIFFERENCES IN BRAIN ADENOSINE A(1) RECEPTOR PHARMACOLOGY REVEALED BY USE OF XANTHINE AND PYRAZOLOPYRIDINE BASED ANTAGONISTS

1 The pharmacological profile of adenosine A(1) receptors in human, gu inea-pig, rat and mouse brain membranes was characterized in a radioli gand binding assay by use of the receptor selective antagonist, [H-3]- 8-cyclopentyl-1,3-dipropylxanthine ([H-3]-DPCPX). 2 The affinity of [H -3]-DPCPX binding sites in rat cortical and hippocampal membranes was similar. Binding site affinity was higher in rat cortical membranes th an in membranes prepared from guinea-pig cortex and hippocampus, mouse cortex and human cortex. pK(D), values (M) were 9.55, 9.44, 8.85, 8.9 4, 8.67, 9.39 and 8.67, respectively. The binding site density (B-max) was lower in rat cortical membranes than in guinea-pig or human corti cal membranes. 3 The rank order of potency of seven adenosine receptor agonists was identical in each species. With the exception of 5'-N-et hylcarboxamidoadenosine (NECA), agonist affinity was 3.5-26.2 fold hig her in rat cortical membranes than in human and guinea-pig brain membr anes; affinity in rat and mouse brain membranes was similar. While NEC A exhibited 9.3 fold higher affinity in rat compared to human cortical membranes, affinity in other species was comparable. The stable GTP a nalogue. Gpp(NH)p (100 mu M) reduced 2-chloro-N-6-cyclopentyladenosine (CCPA) affinity 7-13.9 fold, whereas the affinity of DPCPX was unaffe cted. 4 The affinity of six xanthine-based adenosine receptor antagoni sts was 2.2-15.9 fold higher in rat cortical membranes compared with h uman or guinea-pig membranes. The rank order of potency was species-in dependent In contrast, three pyrazolopyridine derivatives, (R)-1-[(E)- 3-(2-phenylpyrazolo[1,5-alpyridin-3-yl) acryloyl]-2-piperidine ethanol (FK453), (R)-1-[(E)-3-(2-phenylpyrazolo[1,5-alpyridin-3-yl) acryloyl] -piperidin-2-yl acetic acid (FK352) and razolo[1,5-alpyridin-3-yl)-1(6 H)-pyridazinebutyric acid (FK838) exhibited similar affinity in human, guinea-pig, rat and mouse brain membranes. pK(t) values (M) for [H-3] -DPCPX binding sites in human cortical membranes were 9.31, 7.52 and 7 .92, respectively. 5 Drug affinity-for adenosine A(2A) receptors was d etermined in a hyl)phenethylamino]-5'-N-ethylcarboxamidoadenosine ([H- 3]-CGS 21680) binding assay in rat striatal membranes. The pyrazolopyr idine derivatives, FK453, FK838 and FK352 exhibited pK(t) values (M) o f 5.90, 5.92 and 4.31, respectively, compared with pK(t) values of 9.3 1, 8.18 and 7.57 determined in the [H-3]-DPCPX binding assay in rat co rtical membranes. These novel pyrazolopyridine derivatives therefore r epresent high affinity, adenosine A(1) receptor selective drugs that, in contrast to xanthine based antagonists, exhibit similar affinity fo r [H-3]-DPCPX binding sites in human, rat, mouse and guinea-pig brain membranes.