Nocturnal increase in the sensitivity of the Syrian hamster pineal gland to isoproterenol is darkness dependent.

The sensitivity of the Syrian hamster pineal gland to stimulation by isoproterenol is greatly increased in the latter half of the daily dark phase. This increased sensitivity requires a period of dark exposure for up to 6.5 hr. Also, if dark-maintained hamsters are exposed to light in the latter half of the night pineal melatonin levels drop precipitously but can be restimulated by isoproterenol administration. As the interval of light exposure continues, however, the pineal sensitivity to isoproterenol decreases.