Abstract
When the lights were extinguished, the plasma prolactin (PRL) concentration rose rapidly in 6 non-lactating female Saanen goats which were accommodated with 14 hours of lighting per day, 350 lux at the head of a standing goat (i, e. 14L, the light is extinguished or turned on by gradually reducing or increasing the light intensity, room temperature 20±2°C) for 3 months. In Experiment I, when the illumination was shortened to 8 hours/day (8L) for the 3 goats, the time for the plasma PRL concentration to increase after lights-off on that day was delayed 4-5 hours compared to that on the control day. When the lights-off time was returned to 14L on the next day, the time for the plasma PRL concentration to increase also returned to the same time as the control day. In Experiment II, when the illumination was shortened to 10 hours/day (10L) and was continued for 28 days for the other 3 goats, the time for the plasma PRL concentration to increase after lights-off on the first day was delayed 2-3 hours vs. that on the control day. On the 7th day, however, the time for the plasma PRL concentration to increase after lights-off was almost the same as that on the control day. On the other hand, both the level of plasma PRL during the light period and the peak in plasma PRL concentration after lights-off in 2 goats declined markedly on the 21st day at 10L. When the light period was returned to 14L on 29th day, the plasma PRL concentrations in these goats during the light period remained low but those after lights-off rose. The plasma PRL concentrations in the remaining goat during both periods (the light and dark) on the 29th day remained as low as the levels on the 28th day.
