Studies on pineal melatonin levels in a diurnal species, the Eastern chipmunk (Tamias striatus): Effects of light at night, propranolol administration or superior cervical ganglionectomy

Five experiments were carried out on the control of melatonin levels in the pineal gland of a diurnal species, the Eastern chipmunk (Tamias striatus). We confirmed that the exposure of chipmunks to fluorescent white light of 3,981-4,304 lux during the normal dark period does not prevent the rise in pineal melatonin levels normally associated with darkness. Also, the administration of propranolol (20mg/kg) at 8 p.m. did not block the rise in pineal melatonin in animals exposed to either dark or light at night. Similarly, if chipmunks received propranolol 4 hours into the dark phase, pineal melatonin levels were not depressed 2 hours later. When animals were superior cervical ganglionectomized, however, the pineal content of melatonin remained low regardless of whether the animals were exposed to darkness or light at night. The exposure of chipmunks acutely to light at midnight (4 hours after darkness onset) had only a slight depressive effect on pineal melatonin 30 min later; by comparison, when chipmunks were acutely exposed to light at 3 a.m. (7 hours after darkness onset) daytime pineal melatonin levels were reached within 15 min after light onset. These findings in a diurnal species, the Eastern chipmunk, differ markedly when compared to previously reported observations on nocturnal laboratory rodents.