Ultradian rhythm of plasma noradrenaline in rhesus monkeys

KLEITMAN has postulated that a 'basic rest-activity cycle' exists which is an extension of the rhythmically recurring patterns of sleep cycles in man ¹. Such ultradian rhythms, with a periodicity of approximately 90 min in non-human primates, have been associated with motor activity² and feeding behaviour³. Additionally, we have reported that plasma cortisol secretion in the rhesus monkey occurred in 90-min cycles which were synchronised for an entire group of monkeys over a 6-mth period, despite individual housing conditions of the animals⁴. Plasma noradrenaline (NA) is a reliable indicator of sympathetic nervous system activity⁵, in part because of its short half life of less than 2 min⁶. If Kleitman's hypothesis is correct, one might expect to see a basic 90-min rhythm for plasma NA, even though its short half life would favour the finding of a much faster rhythm. We have found a 90-min rhythm for plasma NA in the rhesus monkey, which was temporally synchronised for the experimental group as a whole. The presence of this rhythm suggests a tonic waxing and waning of sympathetic nervous system activity compatible with Kleitman's hypothesis.