ALBERT

Description: The existence of a biological rhythm in the response of animals to noxious stimuli and drugs is well known. However, the mechanism of this response is not well understood. This study was undertaken to describe the existence of a diurnal rhythm in the hypothalamic-pituitary-adrenocortical system before and after stress in female rats kept in controlled environmental conditions in 12L:12D, 24L:OD, or OL:24D. Plasma ACTH and plasma corticosterone concentrations were compared in unstressed animals. The time pattern in the response to stress was determined at four hourly intervals during a 24 hr period in which plasma ACTH and plasma corticosterone were measured at different time intervals. The stress response varied considerably with time of day in both magnitude and duration. The adrenals of rats exposed to constant light for 45 days atrophied, whereas the adrenals of animals kept in constant dark for the same period did not differ significantly from those of controls kept in 12L:12D. The increase in plasma ACTH in response to stress was greater both in the animals maintained in constant light and in constant dark than in the 12L:12D controls. Homeostatic mechanisms involved in these changes are discussed.

Description: Light is considered by many investigators to be the primary Zeitgeber for most physiologic rhythms. In order to study the effects on biorhythms of changing photoperiods and to provide information on the nature of the wave forms and the mechanisms of entrainment, unrestrained male monkeys (Cebus albifrons, Macaca nemestrina) were maintained in a sound-proofed environmental chamber. The Cebus was initially maintained on a 12L:12D schedule; it was subjected to a 180 degrees phase shift for 14 days, then returned to the original photoperiod. In two experiments (24 days; 27 days each) the same monkey was again maintained on a 12L:12D schedule which was gradually altered to a constant light environment. Deep body temperature (DBT) data were obtained with miniature radiotransmitters. Locomotor activity (LMA) was measured by strain gauges. Under the 12L:12D regimens the Macaca DBT cycles were uniform as to phase and wave form for over 60 weeks. These wave forms were analyzed by the use of periodogram and correlogram analyses and by fitting to the Volterra Integro-Differential Equation. Phase angle relationships between Zeitgeber and physiologic parameters were characterized. After the photoperiod phase shift the DBT cycle rephased in about 9 days. During the rephasing process the wave form changed. The shapes of the wave forms of DBT and activity were maintained with increasing light until an 18L:6D photoperiod was reached. The rhythms were entrained to the onset of darkness rather than lights on. Major and minor periods of LMA were detected. Hysteresis diagrams showed that DBT led the onset of major LA by 6 hr and the end of major activity by 2 hr.