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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. 0 1987 Society for Experimental Biology and Medicine. In the rat, the administration of either nor- epinephrine (NE) (especially when preceded by the injection of a NE uptake inhibitor) or isoproterenol, a @-receptor agonist, during the photophase of the 1ight:dark (LD) cycle readily stimulates production of melatonin within the pineal gland (1, 2). On the other hand, the daytime administration of these compounds to the Syrian hamster has no stimulatory ac- tion on pineal melatonin (3-5). It was recently discovered, however, that during the dark phase of the LD cycle the Syrian hamster pi- neal will respond to the catecholamine or its agonist. This was proven using two experi- mental paradigms. First, if isoproterenol is given prior to acute light exposure at night the normal light-associated reduction in pineal melatonin levels is delayed (6). Second, if ei- ther NE (preceded by an uptake inhibitor) or isoproterenol is injected into Syrian hamsters with light-suppressed melatonin levels at night, melatonin production within the pineal gland is increased (7-9). In both of these experi- mental paradigms the hamsters were exposed to 6.5-8 hr of darkness before a positive re- sponse to either NE or its agonists could be evoked. The purpose of the first study in this series was to determine whether this period of dark ~ ~~~

The presence of type II 5′-deiodinase activity in the Syrian hamster Harderian gland was investigated. This enzyme exhibited an increase of its activity after animals entered the normal dark phase, with maximal activity occurring at 04.00 hr (8 hr after lights off). The nocturnal increase was prevented by maintaining the animals in light during the night. Isoproterenol subcutaneously injected every 2 hr (1.0 mg/kg body wt) from 20.00 hr through 0.400 hr to animals exposed to light during the normal dark period mimicked the effect of darkness, i.e., with this treatment an increase in 5′-deiodinase activity with maximal peak values at 02.00 hr was observed. The results show that 5′-deiodinase activity in the Syrian hamster Harderian gland exhibits a nyctohemeral profile dependent on β-adrenergic activation of the gland.