Fig. 4.

Phosphorylation of CREB in rat pineal gland is a circadian event. a, The phosphorylation of CREB occurs under conditions of 12:12 hr light/dark (LD) shortly after the onset of the dark period (lights off at ZT12), as shown with immunocytochemical images from sections of rat pineal glands takenex vivo at the indicated time points. Scale bar, 50 μm. b, The onset of CREB phosphorylation persists under constant darkness (DD). For comparison of CREB with pCREB, SUMDENS values are normalized from immunoblot analyses (n = 3) so that peak values equal 100%. Theopen bar (11.5) represents a time point during subjective daytime; the closed bars represent time points at subjective night. c, A 30 min exposure of animals to light during subjective nighttime (ZT18) leads to the expected decline of AA-NAT protein levels (rightmost column) (see also Gastel et al., 1998); however, CREB, pCREB, and ICER levels remain unaltered. n.d., Not detectable.