A colony of adult rats was maintained on a standardized regimen with artificial photoperiods extending from 0600 to 1800 alternating with 12 hours of darkness. Every hour during a 24-hour period separate subgroups of rats were injected with 35 mg/kg of pentobarbital sodium, and the duration of time each animal remained under anesthesia was recorded.
The rats remained anesthetized for longer periods of time between the hours of 1600 and 2200 than at other times of the day; this indicates that the response to the drug is dependent on he circadian phase system of the animal. When the results were plotted along the 24-hour time scale, the difference between the crest and trough of the curve was about 100%.
A second colony of rats was maintained in a similar fashion except that the animals were subjected to continuous illumination. The rhythm of their susceptibility to pentobarbital sodium persisted for at least four weeks in a form highly modified from the animals synchronized to a light-dark cycle.
In a third colony of animals, the rhythm still was present four months after they had been blinded; but it was modified and different from either the light-dark synchronized animals or from rats subjected to continuous illumination.
The importance of recognizing the time structure of the living organism is discussed.