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Immunohistochemical analysis of adenosine deaminase in rat brain revealed an extensive plexus of adenosine deaminase-containing neurons in the basal hypothalamus. These neurons converged on and were most numerous in three major centers, namely, the tuberal, caudal, and postmammillary caudal magnocellular nuclei. Most other brain regions were devoid of cells containing adenosine deaminase. Some adenosine deaminase-containing neurons were retrogradely labeled with the fluorescent dye fast blue when the dye was injected into the frontal cortex and striatum. Specific populations of neurons having high levels of adenosine deaminase may release adenosine as a neurotransmitter.

Adenosine deaminase (ADA) was detected immunohistochemically in neuronal cell bodies of dorsal root ganglia (DRG) of the rat. ADA-immunoreactivity was confined exclusively to small type B ganglion neurons in cervical, thoracic and lumbar sensory ganglia; large type A neurons in sensory ganglia were devoid of immunostaining for ADA. It was consistently found that only a small proportion of type B neurons in DRG contain immunohistochemically detectable ADA. It is suggested that the expression of high ADA levels is a distinguishing feature of a subpopulation of type B DRG neurons and, further, that ADA in these neurons may reflect their utilization of purines (adenosine or adenine nucleotides) as transmitters or cotransmitters.