A degree of ambiguity and uncertainty exists concerning the distribution of mRNAs encoding the four cloned adenosine receptors. In order to consolidate and extend current understanding in this area, the expression of the adenosine receptors has been examined in the rat by use of in situ hybridisation and the reverse transcription‐polymerase chain reaction (RT‐PCR).

In accordance with earlier studies, in situ hybridisation revealed that the adenosine A₁ receptor was widely expressed in the brain, whereas A_2A receptor mRNA was restricted to the striatum, nucleus accumbens and olfactory tubercle. In addition, A₁ receptor mRNA was detected in large striatal cholinergic interneurones, 26% of these neurones were also found to express the A_2A receptor gene. Central levels of mRNAs encoding adenosine A_2B and A₃ receptors were, however, below the detection limits of in situ hybridisation.

The more sensitive technique of RT‐PCR was then employed to investigate the distribution of adenosine receptor mRNAs in the central nervous system (CNS) and a wide range of peripheral tissues. As a result, many novel sites of adenosine receptor gene expression were identified. A₁ receptor expression has now been found in the heart, aorta, liver, kidney, eye and bladder. These observations are largely consistent with previous functional data. A_2A receptor mRNA was detected in all brain regions tested, demonstrating that expression of this receptor is not restricted to the basal ganglia. In the periphery A_2A receptor mRNA was also found to be more widely distributed than generally recognised. The ubiquitous distribution of the A_2B receptor is shown for the first time, A_2B mRNA was detected at various levels in all rat tissues studied. Expression of the gene encoding the adenosine A₃ receptor was also found to be widespread in the rat, message detected throughout the CNS and in many peripheral tissues. This pattern of expression is similar to that observed in man and sheep, which had previously been perceived to possess distinct patterns of A₃ receptor gene expression in comparison to the rat.

In summary, this work has comprehensively studied the expression of all the cloned adenosine receptors in the rat, and in so doing, resolves some of the uncertainty over where these receptors might act to control physiological processes mediated by adenosine.