Responsiveness of Pineal N‐Acetyltransferase and Melatonin in the Cotton Rat Exposed to Either Artificial or Natural Light at Night

In three separate experiments, the effect of acute exposure to either artificial or natural light during darkness of pineal N‐acetyltransferase (NAT) activity and melatonin content was studied in the cotton rat (Sigmodon hispidus). The exposure of animals to an artificial‐light irradiance of 160,000 μW/cm² during darkness for either 1 s, 5 s, or 30 min was followed by a precipitous decline in pineal NAT activity and melatonin content when measured at either 15 or 30 min after light onset. When cotton rats were acutely exposed to light at night for 5 s, irradiances of either 3.2, 32, 320, and 3,200 did not suppress either pineal NAT or melatonin 30 min later; however, if the 5‐s exposure had an irradiance of either 32,000 or 160,000 μW/cm², the pineal enzyme activity and indole content were depressed. Moonlight, which had a maximal irradiance of 0.32 μW/cm², was unable to suppress pineal NAT activity and melatonin content even when the animals were exposed to the moonlight for 30 min. The treatment of cotton rats with either norepinephrine or its agonist, isoproterenol, before their exposure to light at night retarded slightly the suppressive effect of light on the pineal constituents measured. Also, these drug treatments suppressed the pre‐exposure levels of both NAT activity and melatonin content in the cotton rat pineal gland.