Phase Reference Point Research Articles

Phase-response and Aschoff illuminance curves for locomotor activity rhythm of the rat.

A phase-response curve (PRC) for the circadian rhythm of locomotor activity was constructed for female Sprague-Dawley-derived rats kept in continuous darkness (DD) except when given a 1-h light pulse (150 lx) once each 2 wk. By use of the circadian onset of wheel running as the phase-reference point, the free-running period (tau) in DD was 24.09 h. Maximum phase delays and phase advances occurred in response to light pulses given during the first 5 and last 6 h of activity, respectively. The delay-to-advance ratio (D/A) of the PRC was 1.5. In a separate group of rats exposed to continuous light, tau increased by 1.45 h as illuminance was increased in log steps from 0.1 to 10 lx, thus demonstrating the Aschoff effect in rats. This increase in tau was large, considering the relatively low D/A of the PRC, suggesting that factors in addition to the D/A contribute to the Aschoff effect.

Cell Division Cycles and Circadian Clocks

The cell division rhythm in Euglena gracilis Klebs (Z strain) freeruns with a circadian period (30.2 +/- 1.8 hours for 156 monitored oscillations) in aerated, magnetically stirred, 8-liter, axenic batch cultures grown photoautotrophically at 25 degrees C in LD: 3,3, (7,500 lux, cool-white fluorescent) 6-hour light cycles from the moment of inoculation. Cell number was measured at 2-hour intervals with an automatic fraction collector and Coulter Electronic Particle Counter. At different circadian times throughout the 30-hour division cycle, 3-hour light perturbations were imposed on free-running cell populations by giving light during one of the intervals when dark would have fallen in the LD: 3,3 regimen. Using the onset of division as the phase reference point, the net steady-state phase advance or delay (+/-Deltaphi) of the rhythm was determined after transients, if any, had subsided (usually in one or two days) relative to an unperturbed control culture. Both +Deltaphi and -Deltaphi were found, with maximum values of approximately +/-11 to 12 hours being obtained at circadian time (CT) 20 to 22 (the ;breakpoint'); little, if any phase shift occurred if the light signal was given between CT 6 and CT 12. The phase-resetting curve obtained by plotting new phase (phi') versus old phase (phi) was of the type 0 (;strong') variety. Light perturbations, no matter when imposed, engendered new phases which mapped to a relatively restricted portion (CT 6 to CT 13) of the circadian cycle.These data provide the first detailed phase-response curve for a circadian mitotic clock. The findings, therefore, not only further support the hypothesis that a circadian oscillator (perhaps exhibiting limit cycle behavior) can modulate cell division in eukaryotic cells, but also provide a useful basis for the dissection of the nature and extent of the coupling between cell division and circadian cycles.

Light-dark entrainment of proestrous LH surges and circadian locomotor activity in female hamsters

The temporal relationships of the proestrous LH surge and the circadian locomotor activity rhythm were compared in hamsters entrained to four different 24-hr light-dark (LD) cycles. Animals were housed in cages equipped with running wheels to obtain continuous activity records. Stable entrainment of locomotor activity was complete within 3 weeks of exposure to each photoperiod at which time hamsters were randomly assigned to hourly sample groups. Serum was obtained by cardiac puncture under light ether anesthesia on the day of proestrous and was assayed by RIA for LH. A computer-based least-squares sine wave-fitting technique determined a single objective phase reference point for the time of the hormone maximum. In each photoperiod, precise temporal relationships were maintained between the LH surge and activity onset, whereas the phase relationship between the LH surge and the LD cycle was more variable. These data indicate that the environmental LD cycle entrains the circadian timing system which, in turn, provides temporal information to the rhythms of proestrous gonadotropin and locomotor activity.

Comparison of synchronization of primate circadian rhythms by light and food

Several circadian rhythms in squirrel monkeys (Saimiri sciureus) entrained by two different agents were studied to compare their mode of coupling with the environmental zeitgebers. Synchronization was accomplished either by light-dark cycles consisting of 12 h of 600 lx followed by 12 h of less than 1 lx (LD 12:12), or by eat-fast cycles in which the animals could eat for 3 h and then had to fast for the remaining 21 h each day (EF 3:21). The rhythms of drinking, colonic temperature, and urinary potassium and water excretion were measured in chair-acclimatized monkeys. The drinking and urinary rhythms were more reproducible (smaller mean variance) and more stable (smaller standard deviation of the timing of a phase reference point) in EF than in LD cycles, whereas the temperature rhythm was more tightly controlled by LD cycles than by EF cycles. In constant light an 8-h phase delay in the EF cycle caused the drinking and urinary rhythms to resynchronize to the EF cycle within one day, while the temperature rhythm required about 6 days to resynchronize. In contrast, previously published data for a similar phase delay in the LD cycle with food available ad libitum show that the drinking and temperature rhythms resynchronized more rapidly than the urinary rhythms. These results indicate that separate mechanisms are involved in transducing temporal cues from LD and EF cycles in the circadian timekeeping system of these nonhuman primates.

Circadian rhythms of rat locomotor activity after lesions of the midbrain raphe nuclei

Electrolytic lesions that damaged the midbrain raphe nuclei of albino rats increased the proportion of daily wheel-running behavior that occurred during the light phase of a LD 12∶:12 illumination cycle. In constant dim illumination, rats with midbrain raphe lesions generated free-running circadian locomotor rhythms with periods comparable to those of control animals. This suggests that the serotonergic projection from the midbrain raphe nuclei to the suprachiasmatic nuclei (SCN) of the hypothalamus is not critical to the generation and/or integration of circadian periodicity. Because of the disruptive effects of the lesions on activity, including the loss of a well-defined phase reference point, it was not possible to exclude more subtle effects of the raphe system on various circadian parameters. Nevertheless, the data are consistent with the hypothesis that the SCN are part of an autonomous circadian oscillatory system that does not depend on afferent input from the raphe nuclei.

Field Range and Resolution in Holography*

This article describes a model for predicting the effects of finite hologram-emulsion resolving power on the range of field recorded in the hologram and the resolution in the reconstructed image. The object must be located within a prescribed volume about the phase reference point to attain a certain required resolution in the reconstructed image. When the object is moved from this volume, the resolution in the reconstructed image deteriorates. A hologram and its reconstruction synthesized by numerical evaluation of the Fresnel–Kirchhoff integral supports this model. Pictures taken in various image planes of a thick object are shown. The limited range of field is not a problem in microscopy because of the small size of the specimen, so that complete three-dimensional records of a microscope specimen can be made in a single microhologram.