Fig. 3.
Titration of adenosine A2A receptor constitutive activity by changes in receptor expression. Response of the adenosine A2A(1-316) receptor in functional assays is system-dependent. (A) In the absence of doxycycline, there is a good “window” to see agonist (NECA; filled circle) responses, although the system cannot detect the difference in inverse agonist (SCH58261; filled square) and neutral antagonist (istradefylline; filled triangle) responses. (B) At high levels of receptor expression (10 ng/ml doxycycline), constitutive activity levels have reached the system maximal response level; therefore, no further increase in response is seen after challenge by NECA (filled circle), although there is a clear inverse agonist response by SCH58261 (filled square). (C) At 3 ng/ml doxycycline, receptor expression levels are such to differentiate between agonist (NECA; filled circle) and inverse agonist (SCH58261; filled square) responses. Data plotted as mean ± S.E.M. of n = 3.