Histochemical studies on the distribution of hexokinase and several enzymes related to catecholamine production in the locus coeruleus of the squirrel monkey

Abstract

Summary Detailed histochemical studies have been conducted on the distribution of hexokinase (HK), monoamine oxidase (MAO), L-gulonolactone oxidase (GO) and NAD-linked xylitol dehydrogenase (XY) in every component of the locus coeruleus (LC) of the healthy juvenile male squirrel monkey (Saimiri sciureus). In addition, the reactions of MAO and GO have been tested in the LC of the healthy adult male rat of the Wistar strain. In contrast with many other species of mammals whose LC is equally composed of both medium-sized neurons and small neurons, the LC of the squirrel monkey consists almost exclusively of medium-sized neurons. The LC was clearly discerned from the surrounding brain tissue by the HK reaction, because the neurons with relatively weak activity were embedded in the neuropil with moderate activity and surrounded by the glial cells with moderate to strong activity. The neurons of C4 revealed varying degrees of positive reactions. Most of its neurons exhibited moderate activity suggesting that these neurons may receive energy supplies indirectly from the glial cells; whereas a few neurons on its surface showed strong activity hinting that they may receive energy supplies directly from glucose in the circulating blood. The presence of both types of energy supplies in the LC may facilitate keeping a glycemie level constant in the respiratory center of the monkey. The perikarya of neurons were moderately to strongly positive while the blood capillaries were more strongly positive and some glial cells were strongly positive with regard to the MAO reaction. This gave the LC a marked coloration as a whole. It is suggested that so far as the squirrel monkey is concerned, active monoamine metabolism in an individual neuron of the LC may undergo phasic changes depending on the functional state of that cell. In LC, MAO may be concerned with inactivation of catecholamines rather than the general metabolism of an autonomic nucleus. Most of the neurons revealed varying degrees of positive activity for the GO test while some neurons showed much more positive granules than the others for the XY test. These results strongly suggest that there is a high possibility that at least some neurons of the LC may have the ability to synthesize vitamin C. This ability may be closely related to the catecholamine production, which is involved in the process of formation of neuromelanin, in the LC. In contrast to the squirrel monkey, the LC of the rat was devoid of GO and XY, although its liver showed strong GO activity. It is suggested that the LC is independent of the liver in regard to the ability to synthesize vitamin C in some species of mammals.