Photic and neural control of the 24-hour norepinephrine rhythm in the rat pineal gland.

The norepinephrine content of the rat pineal gland varies 3-fold during each 24-hr day. Norepinephrine levels are highest at the end of the dark period and fall during the light period. This rhythmic variation in pineal norepinephrine is abolished when animals are blinded or are kept in continuous light or darkness. It appears to be generated by nerve impulses which are initiated by photic stimulation of the retina. These impulses are carried to the brain by the inferior accessory optic tract and reach the pineal by a pathway which includes the preganglionic sympathetic trunk to the superior cervical ganglion.