Circadian variations of adrenergic receptors in the mammalian pineal gland: A review

Pineal adrenergic receptor numbers show circadian variations in both rat and Syrian hamster. In the rat pineal Β-adrenergic receptor density reaches peak values either late in the light phase or at middark; the differences in the circadian phase seem related to the light:dark cycle to which the animals are exposed. No circadian rhythm of pineal α-adrenergic receptors is documented in intact rats. In the Syrian hamster pineal Β-adrenergic receptor density is high throughout the light phase and drops to minimal values at the time of the nocturnal peak of melatonin production. The circadian rhythm of pineal α-adrenergic receptor numbers runs parallel to the Β-adrenergic receptor variation, but is less pronounced. In the rat, pineal melatonin production is rapidly induced by Β-adrenergic agonists at any time during a 24-hour period, even when the pinealocyte Β-adrenergic receptor number is lowest (early in the light phase). In contrast, the Syrian hamster pineal seems most responsive to Β-adrenergic agonists in the late night while being less responsive during the day when Β-adrenergic receptor density is high. Interestingly, the human pineal gland is also not especially responsive to adrenergic stimulation during the light phase, possibly making the Syrian hamster pineal a better model than the rat pineal for determining neural/pineal interactions in humans. Comparison of the circadian variations in pineal adrenergic receptors leads to the conclusion that the functional differences between rat and hamster pineal are probably not explicable in terms of the adrenergic receptors, but are caused most likely by (a) intracellular mechanism(s) beyond the adrenergic receptors.