Physiology and pathophysiology of central adenosine A1 and A2A receptors

Abstract: The aim of this thesis was to further investigate the individual roles of central adenosine A1 and A2A receptors in the physiological and pathophysiological effects of adenosine. In addition, the characteristics of different adenosine receptor ligands were studied. For these purposes pharmacological tools as well as mice lacking adenosine A1 and/or A2A receptors were used. Both adenosine A1 receptor knock-out and adenosine A1/A2A receptor double knock-out mice survived fetal life and developed no major defects. No significant change in baseline motor activity was seen in these knock-out mice. In situ hybridization and receptor autoradiography confirmed that the adenosine A1 receptor knock-out did not express any central adenosine A1 receptors. Adenosine A1 receptor heterozygotes expressed half the amount of the receptor. The same was due for the adenosine A2A receptor heterozygote regarding the adenosine A2A receptor expression. The expression of adenosine A2A receptors was not affected by absence of the adenosine A1 receptors. Rats that were sleep-deprived for 3 or 6 hours had altered levels of both adenosine A1 and A2A receptor mRNA. Adenosine A1 receptor mRNA was up-regulated specifically in the horizontal limb of the diagonal band in the basal forebrain. This up-regulation was not seen at the protein level. On the contrary, adenosine A2A receptor mRNA was down-regulated, and this was accompanied with a decrease in receptor binding after 3 but not 6 hours of sleep-deprivation. The alterations of the adenosine A2A receptor expression was observed only in the olfactory tubercle. The sleep pattern and the response to sleep deprivation was studied in mice lacking adenosine A1 receptors. These mice did unexpectedly not differ from their wild-type siblings in these respects. Adenosine A2A receptor knock-outs were more sensitive to hypoxia-ischemia than wild-type mice. They developed an aggravated brain damage, which resulted in altered behavior later in life. Adenosine A2A receptor knock-outs exposed to hypoxia-ischemia showed altered behavior in the open field test and performed less well in the rotarod test. Chronic administration of the locomotor activity stimulating adenosine A2A receptor antagonist SCH 58261 was given to rats. In contrast to previous studies with the non-selective adenosine antagonist caffeine, no tolerance developed to the motor stimulating effect after repeated injections of the drug. The locomotor effects of caffeine were analyzed in mice lacking adenosine A1 receptors. The dose-response curve for caffeine seemed shifted to the left, whereas response to high dose caffeine was unaltered in the knock-out. No locomotor stimulating effect was observed in adenosine A1/A2A receptor double knock-out mice in congruence with mice lacking only the adenosine A2A receptor. Finally, the selectivity of the adenosine receptor ligands DPCPX (A1 antagonist), SCH 58261(A2A antagonist), ZM 241385(A2A antagonist) and CGS 21680(A2A agonist) were tested by means of receptor binding in adenosine receptor knock-out brains. DPCPX did not bind to brains from adenosine A1 receptor knock-outs, whereas SCH 58261 and ZM 241385 did not bind in brains from adenosine A2A receptor knock-outs. CGS 21680 did however not bind in the striatum from adenosine A2A receptor knock-outs, but extra-striatal binding