Adenosine A(1) receptor: analysis of the potential therapeutic effects obtained by its activation in the central...

Adenosine modulates several physiological functions in the CNS and in peripheral tissues via membrane receptors which have been classified into four adenosine subtypes. A(1) activation produces neuronal depression: this inhibition allows A(1) agonists to produce ischemic tolerance and protection in neuronal tissue. In order to selectively reproduce these effects, several A(1) selective ligands have been synthesised and evaluated to understand how they interact with the adenosine A(1) receptor. The investigation methods include SAR studies using native and chemically modified A(1) receptors, molecular cloning of native and mutant adenosine A(1) receptors, molecular modeling and thermodynamic analysis of drug-receptor interaction. Despite the great quantity of information available on the adenosine A(1) receptor, no A(1) agonist has so far entered in clinical use against brain diseases in view of the side effects; moreover selective A(1) agonists appear to be poorly adsorbed into the brain and can be quickly degraded in vivo or in the whole blood. In an attempt to overcome these problems studies have been undertaken dealing with the use of partial agonists to inhibit side-effects and the employment of prodrugs to increase stability and diffusion through lipid barriers of A(1) ligands. Other attempts involve either the use of A(1) receptor enhancers as modulators able to locally enhance the action of endogenously produced adenosine, or the encapsulation of A(1)agonists in drug delivery systems targeted to the brain. In this review, these approaches will be described together with the effects of adenosine A(1) receptor ligands and their binding mechanisms on the central nervous system.