In vivo responses of the pineal gland of the Syrian hamster to isoproterenol or norepinephrine.

The influence of isoproterenol, a norepinephrine (NE) agonist, or NE itself on pineal melatonin levels in adult male Syrian hamsters was examined in a series of studies. Isoproterenol (3 mg/kg) failed to stimulate pineal melatonin production in hamsters kept under light:dark cycles of 14:10 or in hamsters that had been superior cervical ganglionectomized for 1 week. When the drug was given to animals kept under continual light for 7 days it caused a low-grade rise in pineal melatonin, however, this was not confirmed in a second experiment. When the dosage of isoproterenol was increased to 10 mg/kg and 2 injections of the drug were given (at 2 h intervals) pineal melatonin levels also did not exhibit a significant increase. Finally, the administration of NE at a dosage of either 2.5 or 5 mg/kg also failed to augment melatonin levels in the hamster pineal gland.