Abstract
UNDER 1 normal lighting conditions the rat pineal gland exhibits a diurnal variation in its content of the amines serotonin 1 and norepinephrine 2 and the enzyme hydroxyindole-O-methyl transferase (HIOMT). 3 Each of these rhythms is affected by changes in environmental lighting. The norepinephrine and HIOMT rhythms appear to be predominantly under photic control and are suppressed in animals kept in continuous light or darkness and in blinded animals. 3,4 The serotonin rhythm, in contrast, is eliminated when animals are placed in constant light but persists in animals deprived of visual stimulation. 1 Each of the rhythms is maintained by neural information transmitted to the pineal from the central nervous system by the cervical sympathetics. Denervation of the pineal by superior cervical ganglionectomy abolishes all of the rhythms. 2-4 In addition, destroying these nerves in the female rat suppresses the vaginal estrus response to continuous environmental illumination, a response
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