Adenosine deaminase and purinergic neuroregulation

During the early 1970s purines and their associated enzymes were largely believed to be ubiquitous and evenly distributed throughout neural tissues. There is now firm immunohistochemical and neurochemical evidence that this is not the case for the catabolic enzyme adenosine deaminase (ADA) nor is it true for adenosine transport sites labelled by the ligand [(3)H]nitrobenzylthioinosine. ADA-immunohistochemistry, in fact, reveals a widespread and heterogeneous distribution of neuronal cell and fibre types which is remarkably reminiscent of distribution maps of neurotransmitter synthetic enzymes or neuropeptides. What we now know about the neuroanatomy of ADA-containing systems should facilitate tests of the hypothesis that ADA acts on some as yet uncharacterized pool of adenosine and help in answering the more specific question of how this enzyme relates to the role of adenosine as a neuroregulatory substance in discrete neural systems.