Circadian rhythm of cortical and striatal adenosine receptors

In order to investigate diurnal variations in binding parameters of A1 and A2 adenosine receptors, Kd and Bmax were calculated in mice that had been housed under controlled light-dark cycles for 4 weeks (light on from 7.00 to 19.00 h). A1 cortical receptors were labelled by N6-cyclohexyl-[3H]adenosine, and A1 and A2 striatal receptors by 5'-N-ethylcarboxamido[8-3H]adenosine. Significant differences were found for Bmax values measured at 3-h intervals across a 24-h period. Cyclic variations of the number of binding sites were shown, with a minimum number of A1 and A2 receptors during the light period and a maximum during the dark period. The amplitude for cortical A1 receptors, between 03.00 and 18.00 h, was 39%, while for striatal A1 receptors, between 03.00 and 15.00 h, was 92%. The amplitude for the A2 striatal receptors, between 03.00 and 15.00 h, was 147%. No substantial rhythm was found in the Kd values. These differences in the amplitude could suggest a different physiological modulation of the two adenosine receptor subtype moiety which could reflect a physiologically-relevant mechanism by which adenosine exerts its modulatory role in the central nervous system.