Circadian rhythm of serotonin receptor in rat brain.

Bmax and Kd for the serotonin receptors (5-HT-1) as well as the ratios of 5-HT-1A and 5-HT-1B receptors were assessed at 3-hour intervals over a 24-hour period in the cortex of rats that were housed under a 12-hour lighting cycle, with the light turned on at 18:00. The circadian rhythm of the Bmax for the high- and low-affinity sites in 5-HT-1 receptor became evident. The peak of the Bmax for the high- and low-affinity sites occurred between 21:00 and 00:00. No circadian rhythm was observed for Kd at each site for the 5-HT-1 receptors. The ratios of Bmax for the high- and low-affinity sites of the 5-HT-1 receptors were constant at 8.6 +/- 1.4% and 91.4 +/- 1.4% respectively over the test period. The ratios of 5-HT-1A and 5-HT-1B receptors were constant at 36.8 +/- 1.3% and 63.2 +/- 1.2% respectively over the test period. No circadian rhythm was observed for Kd. These results suggest that the Bmax for the 5-HT-1 receptors may have the same circadian rhythm as high- and low-affinity sites and the Bmax for the 5-HT-1A and 5-HT-1B receptors may also have circadian rhythm.