Regulation of the cycle in chick pineal serotonin N-acetyltransferase activity in vitro by light.

The diurnal cycle in vivo in the level of serotonin N-acetyltransferase (NAT) activity found in the pineal gland of chicks kept under diurnal lighting was reproduced in vitro with glands incubated in organ culture under the same diurnal lighting conditions. We have examined the effects of varying the lighting conditions of culture upon development of NAT activity in vitro with pineal glands from birds which were killed at different times during the photoperiod in vivo. The ability of chick pineal glands to develop increased NAT activity in culture during the dark period of a diurnal cycle of illumination was determined primarily by the proportion of the photoperiod in vivo elapsed at the time of sacrifice of the birds. The time of development of maximal NAT activity in cultured chick pineal glands could be advanced or delayed by corresponding changes in time of start of the dark period. However, the 'settings' of the pineal 'biological clock' remained unchanged. Glands from birds sacrificed in the final 30 min of the photoperiod in vivo developed increased NAT activity without lag when cultured in the light. Similarly, pineal glands cultured under continuous illumination developed an increased photostable NAT activity beginning at the time of the "subjective" end of the photoperiod in vitro. Ability to develop additional NAT activity in the dark was low rapidly at the "subjective" time in vitro of start of the next photoperoid. The maximal NAT activity developed in the dark consisted of both photolabile and photostable components.