A team led by Björn Johansson of the Karolinska Institute in Sweden looked at mice that had had the gene coding for an adenosine receptor in their brains disabled. A high dose of caffeine has the same effect in people. The mice seemed healthy, with normal growth and heart rates, but tests showed they were unusually anxious and sensitive to pain (Proceedings of the National Academy of Sciences, vol 98, p 9407).
People who drink up to around six coffees a day should be all right, since this amount of caffeine blocks only half their adenosine receptors. But higher doses might cause similar symptoms.