A Functional Analysis of Circadian Pacemakers in Nocturnal Rodents. III. Heavy Water and Constant Light: Homeostasis of Frequency?

1. In a preceding paper differences in the lability of the freerunning circadian period (τ) in constant darkness (DD) were described among four species of rodents. This lability (i) is strongly correlated with the responses of τ to (ii) D2O-administration and to (iii) constant light (LL) of various intensities. The question is raised whether these are three reflections of the action of the same mechanism conserving circadian frequency. 2. A number of qualitative differences exist in the responses to D2O and LL: (i) D2O always decelerates, while LL may decelerate (as in nocturnal rodents) or accelerate circadian rhythms. (ii) D2O does not affect the pattern of activity or cause aperiodicity or "splitting" as sometimes observed in LL. (iii) The magnitude of the response to D2O is independent of τ in DD (Mus musculus); the response to LL is negatively correlated with τ in DD (Peromyscus maniculatus). (iv) The response to D2O appears subject only to the time constants of the processes of deuteriation and dedeuteriation of body tissues; the response to LL involves long time constants, gradual approach to equilibrium frequency, and "after-effects" upon return to DD. 3. Phase response curves for 15' light pulses are virtually identical in mice drinking D2O (25 %) and in mice drinking tap water, although their τ's differ by as much as 1.8 h. This is seen as evidence that D2O-action is not restricted to a specific phase of the circadian cycle. 4. Serum concentrations of D2O, 12 days after onset of deuteriation of the drinking water are 8.6% in hamsters and 13.9% in C57 mice. The difference accounts for the difference in pacemaker response (change in τ by 20% D2O: 3.8% and 6.6%, respectively). Thus the response to D2O is related to the characteristics of water metabolism, and species differences do not reflect differences in the homeostatic mechanism conserving frequency. 5. Concerning the action of constant light, no firm conclusion can be made. The long time constants in the response to LL suggest that τ is homeostatically protected in the face of alterations in LL-intensity. 6. On the other hand, the strong correlation both among and within species between LL-response and shape of the phase response curve (PRC; linearly transformed in the analysis to a "velocity response curve", VRC) suggests that the change of τ with LL is best explained as an artifact caused by the daily curve of light sensitivity, which itself is necessary for entrainment. PRC-shape and the lability of homeostatic conservation of frequency are believed to be functionally interrelated. 7. "Aschoff's Rule", concerning the differences in response of τ to LL between nocturnal and diurnal animals, is given new support by a literature survey of pertinent data. It is again most readily understood as an artifact reflecting different light response curves involved in different strategies of entrainment in nocturnal and diurnal animals.