Anatomical and cytochemical relationships of adenosine deaminase-containing primary afferent neurons in the rat

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The distribution of adenosine deaminase-containing neurons and fibers in the spinal cord and medulla was examined and the relationship of dorsal root ganglia neurons containing this enzyme to those containing somatostatin, substance P, fluoride-resistant acid phosphatase (FRAP) and 5′-nucleotidase was determined using immunohistochemical and histochemical methods. In the spinal cord adenosine deaminase-immunoreactive fibers and neurons were confined to layer I and Ho. A similar localization of these was observed in the spinal trigeminal nucleus. In adult animals treated neonatally with capsaicin adenosine deaminase-positive fibers were totally depleted in layer IIo but only partially depleted in layer I. Anatysis of lumbar sensory ganglia revealed that small type-B neurons immunoreactive for adenosine deaminase were also immunoreactive for somatostatin but not substance P. In addition, adenosine deaminase-positive neurons lacked histochemical reaction-product for FRAP and exhibited the lowest activity of 5′-nucleotidase. Examination of the neuronal populations containing the two phosphatase enzymes showed that a proportion of neurons exhibiting 5′-nucleotidase activity were devoid of FRAP activity. It is concluded that dorsal root ganglia neurons immunoreactive for adenosine deaminase and somatostatin constitute a single subpopulation of type-B ganglion cells separate from those containing substance P or FRAP. It appears that the lack of coexistence of adenosine deaminase with either FRAP or 5′-nucleotidase cannot be attributed simply to a coexistence of the two latter enzymes since some 5′-nucleotidase-positive neurons lacking FRAP were also devoid of adenosine deaminaseimmunoreactivity. Insofar as these three enzymes may contribute to the regulation of transmission processes in primary sensory neurons, our results indicate a minimal functional relationship between adenine nucleoside and nucleotide degrading enzymes in these neurons. In addition, FRAP appears to have some functional independence from 5′-nucleotidase.