Inhibition of the Chick Pineal Rhythm in Rate of Thymidine Incorporation by Cyclic AMP

Chick pineal glands in organ culture showed a circadian rhythm in the rate of thymidine incorporation. Thymidine incorporation was very markedly inhibited when 3-isobutyl-l-methylxanthine (IBMX) was continuously present. When IBMX was added to cultures in control medium during the photoperiod of the second day in culture, the extent of inhibition of incorporation during that photoperiod increased with the increase in length of the photoperiod remaining. Incorporation did not resume at the start of a second photoperiod if IBMX was added within the first 10 h of the first photoperiod. Corresponding results were obtained with glands continuously cultured in constant darkness. Similar results were also obtained using glands treated with 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Ro 20-1724), 7 beta-acetoxy-8,13-epoxy-1 alpha, 6 beta, 9 alpha-trihydroxy-1abd-14-ene-11-one (forskolin), or 8-bromo-cyclic AMP, but not with 8-bromo-cyclic GMP. When glands cultured with IBMX were transferred to control medium, incorporation remained inhibited until the start of the next photoperiod. We conclude that the increase in the rate of thymidine incorporation at the start of each new photoperiod is dependent on a "switch" process that is inhibited by elevated concentrations of cyclic AMP.