Chronic Caffeine Ingestion Sensitizes the A1 Adenosine

Caffeine consumption causes significant physiologic effects due to its antagonism of adenosine receptors. The Al adenosine receptor is coupled in an inhibitory manner to adenylate cyclase. To study the effects of chronic caffeine ingestion, rats were provided with 0.1% caffeine drinking solution for 28 d. The Al adenosine receptor agonist radioligand [3Hjphenylisopropyladenosine identifies two affinity states in control rat cerebral cortex membranes with a high affinity dissociation constant (KH) of 0.40±0.08 nM and low affinity dissociation constant (KL) of 13.7±3.9 nM, with 33% of the receptors in the high affinity state. In membranes from caffeine-treated animals, all of the Al receptors are shifted to the high affinity state with a dissociation constant (KD) of 0.59±0.06 nM. Guanylyl-imidodiphosphate (10-4 M) decreases binding by 43% in control membrane, with no change in KH or KL, while membrane binding in caffeinetreated animals decreases by 45% with a threefold shift in KD to 1.5±03 nM. Concomitant with the enhanced high affinity Al receptor state and increased sensitivity to guanine nucleotides, membranes from treated animals show a 35% enhancement in (-)-N'-(R-phenylisopropyl)adenosine-mediated inhibition of adenylate cyclase compared with controls (P < 0.03). Photoaffinity crosslinking the receptors with I'25I1N'-243-iodo4-aminophenyl)ethyladenosine reveals that Al receptors from both groups migrate as M, 38,000 proteins. fi-adrenergic receptor binding with 1125Iliodocyanopindolol shows a decrease in the number of Pl-receptors from 233±7 fmol/mg protein in control membranes to 190±10 fmol/mg protein in treated membranes (P = 0.01). These data indicate that the adenosine receptor antagonist, caffeine, induces a compensatory sensitization of the Al receptor-adenylate cyclase system and downregulation of jBadrenergic receptors, and provides a molecular mechanism for the caffeine withdrawal syndrome.