Adenosine deaminase and histidine decarboxylase coexist in certain neurons of the rat brain

The enzymes adenosine deaminase (ADA) and histidine decarboxylase (HDC) were immunocytochemically detected in rat brain. The gross distributions of ADA- and HDC-immunoreactive neurons in the basal hypothalamus were very similar. The superficial layers of the superior colliculus showed only ADA-containing neurons. Using adjacent thin-sections of basal hypothalamus, stained alternately for ADA and HDC immunoreactivity, it was possible to show the two labels localized within the same neurons. These observations imply a relationship between two neurochemically distinct putative neurotransmitter/modulator systems, that of histamine and adenosine.