Effect of Melatonin on the Intensity of Adenosine Production in the Rat Forebrain under Conditions of Acute Hypoxia and Varied Photoperiodicity

We studied the effect of injections of melatonin and modifications of the duration of illumination on the activity of 5-nucleotidase, an enzyme providing synthesis of adenosine, in the forebrain of juvenile male albino rats. The measurements were performed under conditions of acute hypobaric hypoxia. We found that, under conditions of natural illumination, neither isolated injections of melatonin nor acute hypoxia noticeably changed the activity of 5-nucleotidase. At the same time, acute hypoxia combined with melatonin injections increased the activity of this enzyme. A similar noticeable rise in the activity of 5-nucleotidase was observed after melatonin injections in normoxic animals kept in constant darkness, and in rats subjected to hypoxia without the above injections but under conditions of constant illumination. These data allow us to suppose that melatonin (whose level in the extracellular medium is a factor providing synchronization of endogenous temporal rhythms) stimulates 5-nucleotidase-mediated production of adenosine in brain neurons. Acute hypoxia promotes such an effect of melatonin.