The influence of different light irradiances on pineal N-acetyltransferase activity and melatonin levels in the cotton rat, Sigmodon hispidus

Wild-captured cotton rats (Sigmodon hispidus) trapped and tested in September and October exhibited a rapid reduction in pineal N-acetyltransferase (NAT) activity and melatonin levels after exposure to a light irradiance of 300 ωW/cm² during the dark period. The half-time for the depression of both NAT and melatonin was on the order of 2 min. The exposure of cotton rats during darkness to much lower irradiances of light, i.e., 5.0, 0.04, 0.03 or 0.01 W/cm², for 32 min also greatly diminished pineal NAT activity and radioimmunoassayable melatonin levels; however, a light irradiance of 0.005 ωW/cm² failed to significantly depress either the acetylating enzyme or the melatonin content of the pineal gland. The results show that the pineal gland of the wild-captured cotton rat, as judged by NAT activity and melatonin levels, is inhibited even by very low irradiances of light.