A test of the hypothesis that D2O affects circadian oscillations by diminishing the apparent temperature.

The period (tau) of a circadian pacemaker in the cockroach Leucophaea maderae is a nonmonotonic function of temperature. The slope of the curve (tau as a function of temperature) is negative at 20 degrees and positive at 30 degrees . When these insects are deuterated at 20 degrees and 30 degrees the period (tau) of the pacemaker lengthens in both cases, although there is a marked temperature dependence of D(2)O action. The increase in tau is nearly three times greater at 20 degrees than 30 degrees . This observation is a flat contradiction of a prediction made earlier that when D(2)O affects circadian pacemakers it does so by diminishing the apparent temperature of the cell. That prediction, however, involves an assumption that may well be unfounded. Unless D(2)O acts nonselectively on all the components in the system regulating tau, the prediction we sought to test is unfounded; and if D(2)O does not act nonselectively, the observed temperature dependence of D(2)O action is understandable in terms of simulating a lower temperature for those components it does affect.