Circadian variation of [3H]N6-(L-phenylisopropyl)adenosine binding in rat brain.

Since considerable recent experimental evidence suggests a role for the purine nucleoside adenosine in the regulation of mammalian sleep, circadian variations of adenosine receptor binding were examined in whole rat brain using [3H]N6-(L-phenylisopropyl)adenosine ([3H]L-PIA). These results demonstrate a significant circadian variation in the number of [3H]L-PIA binding sites (Bmax) with a maximum 3 h after the beginning of the dark phase of a 12 h light/12 h dark cycle, and a minimum 8 h later (P less than 0.025). The dissociation constant (Kd) of [3H]L-PIA at adenosine receptors did not exhibit any statistically significant circadian variation. These data indicate a daily rhythm in the number of adenosine receptors without a change in Kd and may support the hypothesized involvement of adenosine in the regulation of sleep in rats.