Circadians

The components of the chronome are internally coordinated through feedsidewards, in a network of spontaneous, reactive and modulatory rhythms. The endogenicity of a chronome component was first demonstrated, statistically validated and quantified by objective numerical measures of the uncertainty of its characteristics for the case of circadians in the blinded mouse models (Figure 7/I). In studies on the effect of the lighting regimen as a synchronizer, the question as to the transducer arose. Do the eyes mediate the effect? To answer this question, two models were studied: the blinded C mouse and the mouse born anophthalmic. In the study on blinded mice, the controls were sham-operated and had consistently on the average high blood eosinophil counts in the middle of the daily light span and low counts during the dark span. By contrast, the blinded mice showed the same result in one study and the opposite effect a few weeks later. It was postulated that the rhythm of the blinded mice may have a circadian period slightly different from that of the 24-hour synchronized control mice. Since it was not practical to bleed a mouse every 4 hours over a long span, rectal temperature was measured around the clock, in some studies for the lifetime of the groups of mice investigated. This work led to the discovery of free-running rhythms with a period that differed invariably from 24 hours, and also differed among some of the mice. It can be seen from the average rectal temperature curves of two groups of mice (Figure 7/IA) that the daily peaks occurred, on the average, about every 24 hours in the sham-operated mice. These temperature peaks in the blinded mice occurred earlier and earlier each day. Accordingly, a plot of the circadian acrophases as a function of time post-operation (Figure 7/IB) shows a downward drift to earlier and earlier clock-hours for the blinded mice but not for the control mice. A histogram of the estimated circadian periods (Figure 7/IC) shows that the sham-operated animals have a 24-hour synchronized circadian rhythm (the periods cluster very tightly around 24 hours; light bars), whereas the mice that had a bilateral optic enucleation have a circadian period shorter than 24 hours (dark bars). A deviant circadian period notwithstanding (the free-running circadian period of inbred C mice being shorter and that of a woman isolated from society longer than 24 hours), the internal timing can be preserved, as shown for three variables in Figure 7/ID. These experiments on mice establish, on an inferential statistical basis, the phenomenon of free-running of several variables of a circadian system with some degree of maintained internal synchronization after removal of the eyes (transducers for the primary environmental synchronizer, the 12-hourly alternation of light and darkness). Similar studies of the free-running of human as well as murine and other systems with circadian and also with other frequencies constituted an indirect demonstration of the endogenicity (i.e., of the genetic basis) of the chronome, now amply validated by chemical mutagenesis and gene transfer.

Rhythms being a fundamental feature of life, found at all levels of organization, it is important to recognize their coordinating role. Apart from the spontaneous rhythms characterizing functions such as serum corticosterone or melatonin (Figure 7/IIA and B), reactive rhythms are found in response to a given stimulus applied under standardized controlled conditions of the laboratory: the adrenal response to ACTH is a case in point (broken line in Figure 7/IIA). Such response rhythms have been named [beta]-rhythms, the spontaneous rhythms being called a-rhythms, whether or not they are 24-hour or otherwise synchronized.

Much controversy can be resolved by studying the effect of the interaction by more than two variables at different rhythm stages; a third entity may modulate, in a predictable insofar as rhythmic fashion, the effect of one entity upon the second. Predictable sequences of attenuation, no-effect and amplification can then be found. A case in point is corticosterone production by bisected adrenals stimulated by ACTH 1-17 in the presence vs. absence of pineal homogenate (Figure 7/IIC). Such chronomodulation is also observed for the effect of ACTH 1-17 upon the metaphyseal bone DNA labelling in the rat (Figure 7/IID). Some of these multiple entity interactions involve more than one frequency; this is the case for the effect of the immunostimulator cefodizime (HR221) on corticosterone production by the adrenals stimulated by ACTH 1-17 (Figure 7/IIE). Chronomodulations involving one or several frequencies are known as [gamma]- or d-rhythms, respectively; they are part of feedsidewards, i.e., rhythmic sequences of attenuation, amplification and no-effect by a modulator upon the interaction of an actor and a reactor (Figure 7/III).

The above and a wealth of other evidence refutes the assumption of homeostasis as a regulation for a putative constancy, which presumes, at all times, a similar response to a given stimulus until time-unspecified feedbacks come into play, Table 1. The alternative is the recognition of the reality of chronobiologic dynamics by feedsidewards. Thus one does not deal with a posteriori rhythms, as the result of feedbacks, but with intermodulations among a priori genetically coded rhythms, the sine qua non of life. The characteristics of rhythms and trends and the coordination resulting from relations within chronomes serve as a definition of health. Chronome alterations provide harbingers of increased risk of developing disease. Chronome mapping can also be used for guiding the timing of treatment when needed.