Possible Involvement of Pertussis Toxin‐Sensitive G Proteins and D 2 Dopamine Receptors in the A 1 Adenosine Receptor‐Adenylate Cyclase System in Rat Cerebral Cortex

To identify the involvement of dopamine receptors in the transmembrane signaling of the adenosine receptor‐G protein‐adenylate cyclase system in the CNS, we examined the effects of pertussis toxin (islet‐activating protein, IAP) and apomorphine on A 1 adenosine agonist (‐) N 6 ‐R ‐[ 3 H]phenylisopropyladenosine ([ 3 H]PIA) and antagonist [ 3 H]xanthine amine congener ([ 3 H]XAC) binding activity and adenylate cyclase activity in cerebral cortex membranes of the rat brain. Specific binding to a single class of sites for [ 3 H]XAC with a dissociation constant ( K D ) of 6.0 ± 1.3 n M was observed. The number of maximal binding sites ( B max ) was 1.21 ± 0.13 pmol/mg protein. Studies of the inhibition of [ 3 H]XAC binding by PIA revealed the presence of two classes of PIA binding states, a high‐affinity state ( K D = 2.30 ±1.16 n M ) and a low‐affinity state ( K D = 1,220 ± 230 n M ). Guanosine 5′‐(3‐ O ‐thio)triphosphate or IAP treatment reduced the number of the high‐affinity state binding sites without altering the K D for PIA. Apomorphine (100 μ M ) increased the K D value 10‐fold and decreased B max by ∼20% for [ 3 H]PIA. The effect of apomorphine on the K D value increase was irreversible and due to a conversion from high‐affinity to low‐affinity states for PIA. The effect was dose dependent and was mediated via D 2 dopamine receptors, since the D 2 antagonist sulpiride blocked the phenomenon. The inhibitory effect of PIA on adenylate cyclase activity was abolished by apomorphine treatment. There was no effect of apomorphine on displacement of [ 3 H]quinuclidinyl benzilate (muscarinic ligand) binding by carbachol. These data suggest that A 1 adenosine receptor binding and function are selectively modified by D 2 dopaminergic agents.