Carbonic anhydrase activity in first-order sensory neurons of the rat.

Abstract

The localization of carbonic anhydrase within nervous tissues is to some extent controversial and, despite the general agreement that enzyme activity is present in choroid plexus and neuroglia and absent from neuronal perikarya, it is still uncertain whether all glial cells (astrocytes, oligodendrocytes, satellite cells and Schwann cells) are reactive and all neurons nonreactive. An unusually selective neuronal localization of enzymatic activity was observed by light microscopy: some of the large and medium-sized neurons of the rat''s spinal, nodose and trigeminal ganglia reacted positively for carbonic anhydrase, whereas other peripheral and central neurons did not. The perikarya and the central and peripheral processes of the reactive cells were clearly visualized by the histochemical reaction. The central processes were easily traced from their origin in the spinal ganglion into the dorsal columns of the spinal cord and to their terminations in the gray matter of the spinal cord and the gracile and cuneate nuclei of the medulla. The peripheral processes of these cells in the lumber spinal ganglia were traced into the sciatic nerve and its muscular and cutaneous branches. The histochemical reaction appeared to be specific for demonstrating carbonic anhydrase activity because it was abolished by raising the pH, omitting the substrate, or adding the specific inhibitor, acetazolamide and because immunocytochemical staining with antibodies to purified erythrocyte carbonic anhydrase gave a positive reaction with the same classes of neurons that reacted enzymatically. The unusual chemical specificity of these neurons suggests that they possess some unique metabolism; whether this is related to synthesis of a specific transmitter, to high glycolytic metabolism, to the maintenance of a high intracellular concentration of chloride ion, or to some other metabolic attribute will require analysis by quantitative biochemical methods.