Among its many functions, adenosine is a neurotransmitter which acts at the adenosine receptor. By creating knockout mice for one subgroup of adenosine receptors (A2a), the normal functions of the receptor can be studied. The knockout mice show reduced sensitivity to pain, increased anxiety and similar responses to those shown by wild-type mice to high levels of caffeine. This has led to the conclusion that caffeine normally exerts its responses by blocking the action of the A2areceptor.