DAY-NIGHT VARIATIONS OF ADENOSINE AND ITS METABOLIZING ENZYMES IN THEBRAIN CORTEX OF THE RAT - POSSIBLE PHYSIOLOGICAL SIGNIFICANCE FOR THEENERGETIC HOMEOSTASIS AND THE SLEEP-WAKE CYCLE

The role of adenosine as a metabolic regulator of physiological proces ses in the brain was studied by measuring its concentrations and the a ctivity of adenosine-metabolizing enzymes: 5'-nucleotidase, S-adenosyl homocysteine hydrolase, adenosine deaminase and adenosine kinase in th e cerebral cortex of the rat. Other purine compounds, such as, inosine , hypoxanthine and adenine nucleotides were also studied. The purines' pattern was bimodal with high levels of adenosine, inosine and hypoxa nthine during the light period reaching their peak at 12.00 h, 08.00 h and 16.00 h, respectively, and small increments during the night betw een 02.00 h and 04.00 h. The enzymatic activities showed, in general, an unimodal profile with low activity during the day and high activiti es at night. The adenine nucleotide profile showed a significant dimin ution between 12.00 h and 24.00 h. The high adenosine level during the day might be due to a diminution of adenine nucleotide and to the low activity of adenosine-metabolizing enzymes, suggesting an accumulatio n of the nucleoside. The night increase, although of less magnitude, i s simultaneous to high activity of adenosine-metabolizing enzymes and could be due to an increased formation of the nucleoside. The present data and the findings from other authors strongly suggest that adenosi ne in the brain cortex of the rat can participate at least in two phys iological processes: regulation of the sleep-wake cycle and replenishm ent of the adenine nucleotide pool.