– Adenosine is a neuromodulator in the adult central nervous system. Membrane-bound receptors for adenosine have been identified and cDNAs for A1, A2a, A2b, and A3 adenosine receptor subtypes have been cloned recently.Expression of A2a adenosine receptor mRNA in cranial ganglia, carotid body, and intermediate lobe of the pituitary gland similarly suggests novel sites of adenosine action during development and in the adult.