THE HISTOCHEMICAL IDENTIFICATION OF ACETYLCHOLINESTERASE IN CHOLINERGIC, ADRENERGIC AND SENSORY NEURONS

¹ Department of Physiology and Pharmacology, Graduate School of Medicine, University of Pennsylvania, Philadelphia, Pa.

A previously described histochemical method for the localization of specific or aeetylcholinesterase (AChE) and non-specific cholinesterase (ChE) has been modified to increase its specificity and sensitivity for studies of tissues of the cat, rabbit and rhesus monkey. The concentrations of B.W. 284C51, DFP or Nu 683, and eserine required for the selective complete inhibition of one or both types of enzyme were determined manometrically, using brain homogenates of each of the three species amid employing the specific substrates methacholine (MeCh) and butyrylcholine (BuCh), the histochemical substrates acetylthiocholine (AThCh) and butyrylthiocholine (BuThCh), and acetylcholine (ACh). Determinations were conducted both under optimal conditions for enzymatic activity, and in the presence of the same concentrations of Na₂SO₄ and at the same pH as in the histoehemical incubation media. Acetylthiocholine and BuThCh were found to be hydrolyzed by additional enzymes which are resistant to the selective inhibitors of AChE and non-specific ChE in the brains of all three species. By the addition of the inhibitors, alone and in combination, to the incubation media, it was possible to ascertain the specificity of staining for AChE and non-specific ChE when the selective inhibitors are absent. Sensitivity was increased by increasing the period of incubation to two hours.

As reported previously, high concentrations of AChE were localized in the cholinergic neurons of all three species. However, it was also found that lower, variable concentrations are present in all the adrenergic and sensory neurons examined. Non-specific ChE activity was found in the glial cells at all sites, and in the neurons and interstitial cells of Auerbach's plexus. The inhibitor-resistant esterases were apparently of low activity in peripheral ganglia, and were confined largely to the glial cells.

Implications of the present findings concerning the processes of transmission and conduction are discussed. On the basis of these results and those of other investigators, it is suggested that the terms cholinergic and adrenergic may refer to the predominant but not necessarily the exclusive transmitting agents of the respective nerve fibers.