The concentration ol cyclic AMP in the pineal glands of rats exposed to constant light for 7 days and then placed in darkness for up to 3 hr was no different from that measured in the pineal glands of rats kept in constant light. However, if the norepinephrine re-uptake inhibitor desmethyli-mipramine (DMI) was injected into rats 1 hr before placing them in the dark, then exposure to darkness caused a rapid, sustained increase in pineal gland concentrations of cyclic AMP. While bilateral adrenal demedullation did not prevent the increase in pineal cyclic AMP in rats treated with DMI and exposed to darkness, bilateral superior cervical ganglionectomy abolished completely the darkness-induced rise in cyclic AMP in DMI-trcated rats. Furthermore, prior treatment of rats with the β-adrenergic antagonist ( ± )-propranolol prevented the darkness-induced increase in cyclic AMP in rats treated with the inhibitor of norepinephrine re-uptake. It appears that increases in sympathetic activity to the pineal gland, induced by darkness, can raise the concentration of cyclic AMP in the gland but only when nore-pinephrine re-uptake is prevented.
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