Why are there no adenosine agonists available?

[CrimpJiggler]

Various xanthine alkaloids such as caffeine, theophylline and theobromine produce stimulant effects because they are adenosine receptor antagonists. Would adenosine agonists not make good sedatives/tranquilisers?

 

[Carvacrol]

Because adenosine doesn't works like histamine?

Edit: Caffeine withdrawal syndrome is unpleasant, and it is caused by an excess of adenosine receptors. Adenosine causes vasodilatation, and too much causes headaches and nausea.
Painful headaches don't help sedation and tranquilisation. Therefore I think an adenosine agonist wouldn't be a good sedative/tranquiliser.

 

[Pythagorean]

Hospitals keep adenosine itself around as an agonist for treating tachychardia, to slow down the heart.

Adenosine acts as a break in biological systems, having inhibiting effects, but caffeine doesn't just stop this break, it also makes other neurotransmitters more active. For instance, it prevents breakdown of acetylcholine (ACh), so ACh sticks around longer, increasing its effect.