Effects of changes in environmental lighting upon levels of hydroxyindole O-methyltransferase activity in the developing-chick gland.

The level of hydroxyindole O-methyltransferase (HIOMT) activity in the pienal gland of developing chicks raised under constant illumination rose more rapidly and to higher values than in the gland of birds maintained in constant darkness. Rates of net increase in activity, and levels of activity attained, for birds raised under a diurnal cycle of illumination were intermediate between those maintained in constant light or darkness. Under each of the lighting conditions, the course of increase in enzymic activity was markedly affected by variations in an unidentified factor, the source of which appeared to be the hatching eggs. Birds transferred from constant light to the dark showed either an arrest of increase in enzyme activity or a loss of activity until the levels equalled that observed for chicks of the same age raised in constant darkness. Chicks transferred from constant darkness to constant illumination showed marked increases in levels of enzyme activity at rates comparable with the maximal values observed with birds maintained under constant illumination, regardless of age and without delay. No diurnal cycle in level of HIOMT activity was observed in the pineals of 15-day birds.