The diurnal variation of norepinephrine in the rat parotid gland

The parasympathetic and sympathetic nervous systems are both involved in the control of secretion of the major salivary glands although the parasympathetic system is more dominant. Previous work by Sreebny and Johnson (1969) has demonstrated a diurnal variation in three secretory enzymes of the rat parotid gland. The possibility that there may also be a diurnal variation of the sympathetic neural transmitter, norepinephrine, was investigated. It was also anticipated a diurnal variation of norepinephrine in the rate parotid might show some correlation with the eating cycle. After the 24 hour eating pattern of the rat was determined, the parotid glands were removed form separate groups of experimental animals and assayed for norepinephrine content and tyrosine hydroxylase activity. The results obtained indicate that the levels of norepinephrine in the rate parotid did exhibit a diurnal variation with higher levels occurring during periods of light and lower levels occurring during periods of darkness. The activiaty of tyrosine hydroxylase, which is the rate limiting enzyme in the synthesis of norepinephrine from tyrosine, correlated with levels of norepinephrine throughout the 24-hour period. It was concluded that norepinephrine in the rat parotid demonstrates a diurnal variation that appears to parallel the activiaty of tyrosine hydroxylase. Also, norepinephrine and tyrosine hydroxylase activity appear to parallel the diurnal variation of three secretory enzymes as shown by Sreebny. These findings suggest that the sympathetic nervous systems may have a more functional role in the secretory control of the rat parotid gland than was previously considered.