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B. B. FREDHOLM ANAL. REAL ACAD. NAL. FARM.

Shimada, Ogi, Nariai, Tanaka, Endo, Suzuki and Senda; 2000, Hirani,
Gillies, Karasawa, Shimada, Kase, Opacka-Juffry, Osman, Luthra, Hume
and Brooks; 2001). However, these PET ligands do not appear ideal since
non-specific, extrastriatal binding is high.

Biochemical studies have demonstrated low levels of adenosine
A2B receptors on most neurons and glia cells and in situ hybrization
studies specifically demonstrated the presence of adenosine A2B receptor
mRNA in the hypophyseal pars tuberalis (Stehle et al.; 1992). The levels
of A3 receptors in the brain are also low, but there appear to be species
differences, with the levels being higher sheep and humans than in
rodents (Salvatore, Jacobson, Taylor, Linden and Johnson; 1993).

Signaling via Adenosine Receptors
The adenosine A1 and A2 receptors were initially subdivided on the basis
of their ability to inhibit and stimulate adenylyl cyclase, respectively
(Londos, Cooper and Wolff; 1980). Indeed, A1 and A2 receptors are
coupled to members of the Gi group and Gs group of G proteins,
respectively (Table 1).

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