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VOL. 69 (4) ADENOSINE RECEPTORS

on early findings showing that expression of immediate early genes could
be used to pinpoint changes in neuronal activity and/or signal
transduction (Sheng and Greenberg; 1990). We observed that stimulatory
doses of caffeine and selective A2A receptor antagonists caused a decrease
in the expression of IEGs, known to be regulated by the cAMP/CREB
cascade, in striatopallidal neurons (Svenningsson, Nomikos and
Fredholm; 1995, Svenningsson, Nomikos, Ongini and Fredholm; 1997).
These and subsequent studies (Pinna, Wardas, Cozzolino and Morelli;
1999, Chen, Moratalla, Impagnatiello, Grandy, Cuellar, Rubinstein,
Beilstein, Hackett, Fink, Low, Ongini and Schwarzschild; 2001) provide
strong evidence that adenosine, via A2A receptors, exert a robust tonic
activation of the cAMP/CREB/IEG cascade in striatopallidal neurons.
Moreover, this result also provided evidence that multiple D2 receptor-
mediated effects by dopamine can be attributed to an antagonism of this
adenosine-mediated activation of striatopallidal neurons.

In order to increase our understanding of the interactions of adenosine and
dopamine at the signal transduction level, we proposed a collaborative
project with Paul Greengard to study the effects of adenosine A2A
selective compounds and caffeine on the phosphorylation of dopamine
and cAMP phosphoprotein of 32 kDa (DARPP-32). DARPP-32 is highly
enriched in all striatal GABAergic medium-sized projection neurons and
is an important mediator of dopaminergic signaling (Greengard; 2001). Its
function is determined by its relative phosphorylation state at several
different threonine/serine residues, of which the most studied is a PKA-
site Thr34. When this residue is phosphorylated it converts DARPP-32
into an inhibitor of protein phosphatase-1, which, in turn, regulates the
activity of multiple transcription factors, including CREB, ion channels
and ionotropic receptors (Fig 2). In initial studies conducted in brain
slices prepared from striatum, it was found that CGS 21680 potently
increases phosphorylation at Thr34 (Svenningsson, Lindskog, Rognoni,
Fredholm, Greengard and Fisone; 1998). This effect was additive to that
of SKF81297, a selective D1 agonist, and could be counteracted by
quinpirole, a selective D2 agonist (Lindskog, Svenningsson, Fredholm,
Greengard and Fisone; 1999). This result identified adenosine, via A2A
receptors, as a key regulator of the phosphorylation state of DARPP-32 in

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