| Popis: |
Purines and pyrimidines present an interesting paradox in the nervous system. Molecules such as adenosine, ATP, and UTP play an essential role in cellular metabolism, and are found in every cell in the brain. Nevertheless, it is now well-established that both of these molecules play an important secondary role as extracellular signaling molecules, and that there are specific receptors for both adenosine and ATP (and in some cases for other nucleotides such as ADP and UTP) on neurons as well as glial cells. The widespread distribution of ligands and receptors, the relatively recent cloning of multiple subtypes of nucleoside and nucleotide receptors, and their diverse effects throughout the nervous system, have provided the stimuli to accelerate research in this area. There are many lines of circumstantial evidence to suggest that the two major roles of purines may be linked, i.e., that activation of at least some purinergic receptors may be a way of communicating to other cells information about metabolic state. However, while there is a great deal known about purinergic signaling, some of the more important functional questions remain unresolved. For example, there is abundant evidence that some adenosine receptors in the brain are tonically activated by the basal concentrations of adenosine in the extracellular space. However, the significance of this inhibition is unclear and, in many cases, blockade of this tonic inhibitory influence (e.g., with caffeine) seems to have beneficial rather than deleterious effects on the overall function of the nervous system. |
