In the Syrian hamster, pineal melatonin levels exhibit a 15-fold rise during the dark phase of the light: dark cycle. This rise is believed to be mediated by the release of norepinephrine from the postganglionic sympathetic fibers which terminate within the pineal. In order to determine the nature of the adrenergic receptor involved in the norepinephrine mediated nocturnal increase in melatonin, male hamsters were treated with either α- or β-adrenergic blockers just prior to lights out. Subsequently, radioimmunoassayable levels of melatonin were measured at 7, 8 and 9 hours (0300, 0400 and 0500 h, respectively) into the dark period. Propranolol (20 mg/kg) completely suppressed the nocturnal rise of melatonin while phentolamine (10 mg/kg) had no effect upon the increase. The minimum amount of propranolol necessary to block the nighttime rise of melatonin was determined to lie between 1 mg/kg and 10 mg/kg. Phenoxybenzamine (20 mg/kg) exhibited a slight, although statistically significant, blockade of the nocturnal melatonin rise.
ASJC Scopus subject areas
- Biochemistry, Genetics and Molecular Biology(all)
- Pharmacology, Toxicology and Pharmaceutics(all)