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Circadian Systems: General Perspective

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Circadian Systems: General Perspective 5 Circadian Systems: General Perspective i COLIN S. PITTENDRIGH INNATE TEMPORAL PROGRAMS: BIOLOGICAL CLOCKS MEASURING ENVIRONMENTAL TIME The physical environment of life is characterized by several major periodicities that derive from the motions of the earth and the moon relative to the sun. From its origin some billions of years ago, life has had to cope with pronounced daily and annual cycles of light and temperature. Tidal cycles challenged life as soon as the edge of the sea was invaded; and on land, humidity and other daily cycles were added to the older challenges of light and temperature. These physical periodicities clearly raise challenges-caricatured by the hos- tility of deserts by day and of high latitudes in winter--- that natural selection has had to cope with; on the other hand, the unique stability of these cycles based on celestial mechan- ics presents a clear opportunity for selection: their predictability makes anticipatory pro- gramming a viable strategy. The result has been widespread occurrence in eukaryotic sys- tems of innate temporal programs for metabolism and behavior that are most appropriately undertaken during a restricted fraction of the external cycle of physical change. Feeding behavior in nocturnal rodents is programmed into early hours of the night; the behavior persists at thatphase--recurring at intervals close to 24 hr--in animals retained in cueless constant darkness; and mobilization of the enzymes necessary for digestion by the intestine and subsequent metabolic processing in the liver is appropriately programmed to that same (or slightly earlier) time. In plants, photosynthesis can, of course, occur only by day, but that obvious periodicity is not entirely forced exogenously; green cells kept in continuous illumination for weeks continue a circadian periodicity of O2 evolution reflecting a pro- Colin S. Pittendrigh Hopkins Marine Station, Stanford University, Pacific Grove, California 93950. 58 grammed shutdown of the photosystems during the cell’s “subjective night” (e.g., Schweiger, Wallraf, and Schweiger, 1964a,b). COLIN S. Pittendrigh Some intertidal isopods (e.g., Excirolana; Enright, 1976) feed only during high tides and retreat into the sand when the sea recedes; the intertidal amphipod Orchestoidea feeds only on the exposed beach at low tide and retreats below at high water (Benson and Lewis, 1976; McGinnis, 1972; Page, Block, and Pittendrigh, 1980). In both cases, the predicta- bility of the tidal cycle has been exploited by selection: the tidal periodicity of activity and retreat is programmed and persists in a tideless chamber in constant temperature and illu- mination. In Orchestoideu, which feeds on the exposed sand at low tide, activity is further restricted to the hours of darkness at night. Here, then, two periodic programs govern the timing of activity. Intertidal insects (Neumann, 1976) that undergo metamorphosis in the sea must limit the time of eclosion to low tide, when the pupae are no longer submerged and the freshly emerged adult can fly away. In nature, eclosion does indeed occur only during those nights (every two weeks) that coincide with the lowest neap tides, and in a cueless laboratory environment, it is found that the time of eclosion is again controlled by innate temporal programming; one component of the program has a circadian frequency, other semilunar. The predictability of even the annual cycles has provided natural selection with the option of innate programming as a viable strategy to ensure seasonally appropriate responses including the anticipation of oncoming opportunities or challenges. The repro- ductive cycles of several mammals and birds are now known to persist as circannual rhythms in laboratory environments that lack seasonal cues. As Gwinner discusses later (see Chapter 21) the whole gamut of seasonal change in some warblers-including molts, the duration of migratory restlessness, and even change in orientation to navigational cues-is timed by such an innate program. In all cases-daily, tidal, lunar, and annual-the temporal program is based on undamped biological oscillators whose period (frequency) is an evolved approximation (“circa-“) to the environmental cycle it copes with and exploits. In all cases where it has been examined (circadian, circatidal, circalunar), the period of the oscillator involved has the remarkable property of temperature compensation: it is stable over a wide range of temperatures (Pittendrigh, 1954, 1974; Sweeney and Hastings, 1960). The self-sustaining (undamped) feature of the oscillator driving the program is dem- onstrated by the persistence for years of the circadian rhythm of activity in a rodent. The dependence of the oscillator’s motion on metabolic energy is clearly shown by its abrupt arrest with the onset of anoxia in Drosophila and its equally prompt resumption even 30 hr later with the return of oxygen and aerobic respiration (Pittendrigh, 1974). The innate- ness (genetic control) of the oscillation is attested to in many ways, including its suscepti- bility to change of frequency (period) by laboratory selection (Pittendrigh, 1980b) or chem- ical mutagenesis (Konopka and Benzer, 1971). CLOCKLIKE PROPERTIES OF THE “CIRCA-“OSCILLATORS The pacemakers driving these temporal programs are a distinct subset of biological oscillators characterized by several features that relate to their clocklike function. They are biological clocks for the measurement of environmental time in two conceptually distinct ways: (1) in providing for a proper phasing of the program to the cycle of environmental 59 change, they, in effect, recognize local time; and (2) in assuring an appropriately stable CIRCADIAN SYSTEMS: temporal sequence in the program’s successive events, they, in effect, measure the lapse of GENERAL (sidereal) time. PERSPECTIVE RECOGNITION OF LOCAL TIME: PACEMAKER ENTRAINMENT The utility of a program intended to match a cycle of environmental change is clearly contingent on its proper phasing to that cycle. This functional prerequisite provides some general explanation of why natural selection has generated programs whose temporal framework is a self-sustaining oscillator rather than an hourglass. Like all such oscillators, the pacemakers driving circadian, circatidal, circalunar, and circannual programs can be entrained by (or can lock onto) an external periodicity whose frequency is reasonably close. In the biological cases that concern us here, the external (environmental) cycle is called a zeitgeber or entraining agent. In the circadian case, the 24 hr cycle of light and darkness (LD) is a virtually universal zeitgeber. When entrained by an LD cycle (with period T), the pacemaker’s period (T) is changed from T to r* =T. A mouse that displays a circadian rhythm with r = 23.2 hr in constant darkness assumes a 24-hr period when exposed to LD 12:12 (t =24). And in the entrained steady state a specific phase-relation ($J) is established between the pacemaker of the mouse’s program and the external light cycle; it is by virtue of its stable entrainment to the LD cycle that the pacemaker can be entrusted with timing, for example, the synthesis of digestive enzymes in anticipation of feeding in the early hours of the night. An hourglass timer might, in principle, provide the time reference for such a program, but only if initiation of its discharge could be phased-every day-to some well-defined external marker of local time. An oscillator has several advantages in this context, includ- ing, especially, its provision for complete isolation from a zeitgeber for several cycles: as a freerunning oscillator, it stores information on local time (phase) acquired by prior entrain- ment (bUNNING, 1960). That information is reliable, of course, only if the pacemaker’s period (7) is close approximation to t= 24 hr. In the few cases (e.g., Sarcophaga Saun- ders, 1973) where such isolation from zeitgebers for several days is known to occur, T is indistinguishably close to 24 hr. In the majority of cases, ? is in fact only a close approximation (hence circa) to the period (T) of the relevant environmental cycles: Why.? Why has selection not added the complete precision of r = t to the other functional distinctions of “circa-“pacemakers? It could be argued that given the period control (T + r* = t) inherent in entrainment, selection has only demanded that r be close enough to t to yield to entrainment, but that leaves unexplained a clear statistical trend in ‘? values. In day-active species 7 is generally >24 hr, and <24 hr in night-active species (Aschoff, 1960, 1979). It has been argued (Pittendrigh and Daan, 1976a) that any discrepancy of ? from 24 hr ( = t) contributes to the day-to-day stabilization of the program’s phase (JI) relative to local time. The phase relation $ of any oscillator to its zeitgeber is always sensitive to the ratio p = r/T and especially when p = 1.0. There is, then, some adaptive advantage to the very circa nature of r in contributing to the reliable recognition of local time. Further, it is seen in a later chapter (see Chapter 7) that the sign of r’s difference from T has itself some functional meaning: it contributes to maintaining a particular phase relation between program and 60 light cycle even as photoperiod (or day length) changes throughout the season. In general, the circa nature of the period of those biological oscillators that measure environmental COLIN S. PITTENDRIGH time is more than a tolerable approximation to the period of the cycles they match; it has clear utility in stabilizing $ and hence the reliable recognition of local time. MEASUREMENT OF THE LAPSE OF TIME: HOMEOSTASIS OF PACEMAKER PERIOD AND ANGULAR VELOCITY The period (7) of all those pacemakers that function as clocks measuring environmen- tal time is remarkably stable, even over a wide range of temperatures and other environ- mental variables (Pittendrigh, 1954, 1974; Pittendrigh and Caldarola, 1973). This general homeostasis of r, of course, contributes to a stable phase relation between program and light cycle because # is sensitive to variation in T/ T and hence in r.
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