Comparison of the Effects of β‐Adrenergic Agents on Pineal Serotonin N‐Acetyltransferase Activity and Melatonin Content in Two Species of Hamsters

The nighttime rise in pineal melatonin levels can be blocked by administration of the β‐adrenergic receptor antagonist, propranolol, in both Syrian hamsters and rats. Although the administration of β‐adrenergic receptor agonists such as norepinephrine or isoproterenol stimulates pineal melatonin production in the rat, these drugs are without apparent effect on indole production in the Syrian hamster. To determine whether this lack of stimulatory effect in the Syrian hamster is characteristic of this species, a comparison of the effects of norepinephrine and isoproterenol on pineal serotonin N‐acetyltransferase activity and melatonin content was conducted. In contrast to their lack of effect in the Syrian hamster, norepinephrine and isoproterenol stimulated pineal serotonin N‐acetyltransferase activity and melatonin content in the Djungarian hamster. Hourly injection of norepinephrine during a continuation of light into the normal dark period stimulated increases in the activity of serotonin N‐acetyltransferase and melatonin content in the Djungarian hamster but was without effect on these pineal parameters in the Syrian hamster.