Abstract
Adenosine is an important molecule in the central nervous system that modulates neurotransmitter release and induces neuroprotective effects during physiological and pathological conditions. Rapid adenosine release has been discovered using fast-scan cyclic voltammetry, which is a paradigm shift for the understanding of the time course of adenosine. However, there are still questions associated with the mechanism of rapid adenosine formation and its functions on a rapid time scale. Here, pharmacological tests as well as transgenic animals were used to investigate the formation and functions of rapid adenosine release.
In Chapter 2, two modes of rapid adenosine release: (1) spontaneous adenosine release, with no stimulation and (2) mechanically-stimulated adenosine release in the prefrontal cortex, hippocampus, and striatum were characterized. The frequency of spontaneous adenosine release and the concentration of mechanically-stimulated adenosine release differ among regions. The hippocampus had higher frequency of spontaneous release events and higher concentration of stimulated release, whereas the prefrontal cortex had less frequent spontaneous release and lower concentrations of mechanically-stimulated release. Rapid adenosine release differs in brain regions and therefore its neuromodulator effects vary, which is important for the future development of novel therapeutic treatments for different diseases. In Chapter 3, the mechanism of adenosine formation for the two modes of adenosine was studied in hippocampus. To understand whether rapid adenosine release was from extracellular breakdown of ATP, transgenic mice that lack of NTPDase1 (CD39), converting ATP or ADP to AMP or ecto-5′-nucleotidase (CD73), converting AMP to adenosine, were used. CD39 and CD73 knockout (KO) did not affect the concentration of spontaneous and mechanically-stimulated adenosine, indicating other pathways are involved in the rapid adenosine release. However, the frequency of spontaneous release events decreased in CD73KO and CD39KO mice, so these enzymes differentially regulate the frequency of spontaneous release. Thus, future pharmacological agents could target these enzymes and preferentially target just the spontaneous mechanism.
Chapters 4-5 described the function of spontaneous adenosine release. In Chapter 4, I developed a method to simultaneously detect rapid adenosine release and oxygen levels in vivo using a modified voltage waveform. Adenosine and oxygen events were correlated, as 34% of adenosine events were followed by an oxygen event. The frequency of rapid adenosine and oxygen release was regulated through A2A receptors but not A1 receptors. In Chapter 5, the correlated adenosine and oxygen events during ischemia and reperfusion (I/R) injury were further studied. Ischemia was induced with bilateral common carotid artery occlusion. The frequency of adenosine and oxygen events increased during I/R, indicating the local blood flow was transiently increased. However, blockade of A2A receptors eliminated the increase of correlated adenosine and oxygen events. These studies provide an understanding that adenosine transiently changes local blood flow even during ischemia, thus, adenosine may exert local neuroprotective effects during I/R injury.
Overall, this thesis highlights the possibility of rapid adenosine release as a therapeutic candidate for treatment of various diseases, including stroke. By understanding how and where rapid adenosine releases, strategies to regulate the frequency of spontaneous release or increase local concentration of adenosine by stimulation would be practical for future treatment of adenosine-related diseases.
