Effect of acute light exposure upon melatonin content, NAT activity, and nuclear volume in the gerbil pineal complex.

This report describes the response of both the superficial and deep components of the gerbil pineal gland to a 30-min pulse of fluorescent light at the time of the nocturnal peak in pineal activity. When gerbils were maintained in a 14 h light:10 h dark photoperiod, mean melatonin concentrations in the superficial pineal gland ranged between 72 and 108 pg/gland during the day and first 4 h of darkness, rose to a peak of 399 pg/gland 7 h after lights out, then returned to basal levels at the expected time of light onset. Pineal melatonin contents were reduced to daytime values within 30 min after exposure of gerbils to fluorescent light at the time of the nocturnal peak. These changes in superficial pineal melatonin content reflected the circadian pattern in serotonin N-acetyltransferase (NAT) activity, as would be expected if NAT controls melatonin synthesis. In addition to the reductions in NAT and melatonin content induced by nocturnal photic stimulation, there was a 30% reduction in mean pinealocyte nuclear volume. A nocturnal elevation in melatonin content but not in NAT activity occurred in the deep pineal gland. Unlike the superficial pineal gland, the deep pineal gland did not demonstrate a response in melatonin content, NAT activity, or pinealocyte nuclear volumes upon exposure of gerbils to light at the time of the nocturnal peak in superficial pineal activity.