Ultradian rhythm of plasma norepinephrine in rats.

This study examined the possibility that laboratory rats exhibit ultradian plasma catecholamine rhythms. Four rats were prepared with chronic venous cannulas. After recovery, blood was sampled every 15 min for 8 h. Spectrum analysis revealed ultradian norepinephrine rhythms with spectral peaks around 80-90 min and around 40 min. However, no reliable ultradian rhythms were found for epinephrine. Ultradian norepinephrine rhythms appeared to be synchronized across three of the four rats, suggesting that the rhythm can be synchronized by some environmental stimulus. Thus, rats exhibit ultradian norepinephrine rhythms similar to those found in humans and in rhesus monkeys, indicating (a) that laboratory rats can be used to study the biological mechanisms of ultradian rhythms and (b) that ultradian oscillations of sympathetic function are a common property of mammalian systems.