A novel pharmacological activity of caffeine in the cholinergic system

Cholinergic deficit is regarded as an important factor responsible for Alzheimer's disease (AD) symptoms. Acetylcholinesterase (AChE) and nicotinic receptor (AChR) are two molecular targets for the treatment of this disease. We found here that methanolic extracts of Camellia sinensis exhibited anticholinesterase activity and induced AChR conformational changes. From bioguided fractionation we confirmed that caffeine was the active compound exerting such effects. It is well-known that caffeine acts as an inhibitor of AChE and here we explored the effect of caffeine on the AChR by combining single channel recordings and fluorescent measurements. From single channel recordings we observed that caffeine activated both muscle and α7 AChRs at low concentrations, and behaved as an open channel blocker which was evident at high concentrations. Fluorescent measurements were performed with the conformational sensitive probe crystal violet (CrV) and AChR rich membranes from Torpedo californica. Caffeine induced changes in the K_D value of CrV in a concentration-dependent manner taking the AChR closer to a desentisized state. In the presence of α-bungarotoxin, an AChR competitive antagonist, high concentrations of caffeine increased the K_D value of CrV, compatible with a competition with CrV molecules for the luminal channel. Our electrophysiological and fluorescent experiments show that caffeine has a dual effect on nicotinic receptors, behaving as an agonist and an ion channel blocker, probably through distinct AChR sites with quite different affinities. Thus, caffeine or its derivatives can be considered for the design of promising multitarget-directed drugs for AD treatment by modulation of different targets in the cholinergic pathway.