Michael Terman

Food availability and daily biological rhythms

Restricted daily feeding schedules result in the partial or complete synchronization of a wide range of rhythmic biological functions in rodents. In some cases, exemplified by drinking behavior and liver tyrosine transaminase activity, this represents primarily a direct, exogenous influence of food intake. In others, synchronization is achieved by entrainment of a circadian time-keeping mechanism distinct from that which underlies free-running rhythms in these functions. This food-entrained mechanism is responsible for the timing of anticipatory increases in locomotor and lever-pressing activity immediatley prior to food delivery, and may also underlie similar anticipatory increases in body temperature, corticosterone secretion, and in the activities of some intestinal enzymes. It is suggested that such a mechanism may enable an animal to recognize and take advantage of the periodic recurrence of significant events in its biotic environment.

Feeding schedules and the circadian organization of behavior in the rat.

Feeding and drinking behavior of rats maintained in constant light were recorded before, during and after feeding schedules with periods lying within or outside the range of circadian entrainment. Regardless of period, all schedules immediately resulted in the partial or complete synchronization of drinking behavior, but failed to entrain the free-running circadian feeding and drinking rhythms. This indicated that drinking can be passively driven by periodic access to food. However, other results suggested that a separate circadian system was entrained by feeding schedules: First, the 24-h periodicity induced by 24-h feeding schedules often continued for several days after termination of the schedules. Second, the rats showed anticipatory activity under schedules with periods within, but not outside, the circadian range of entrainment. Third, lesions of the suprachiasmatic nuclei (SCN), which resulted in the immediate elimination of free-running rhythms, did not alter the rhythmic influences of the feeding schedules. These results provide evidence for the participation of two distinct circadian systems in the control of behavior in the rat. The two systems appear to have different entrainment characteristics and separate physiological substrates.

Circadian organization of food intake and meal patterns in the rat

Abstract The temporal distribution of food intake and of the two parameters, meal size and meal frequency, was determined in rats exposed to light-dark (LD) cycles and to constant illumination (LL). All three measures displayed characteristic multimodal circadian patterns that entrained to the LD cycle and free-ran in LL. Under both conditions, the circadian rhythm of meal size showed a small phase lag relative to the meal frequency rhythm. Lighting condition had no effect on total daily food intake, or on mean meal size and frequency. However, exposure to LL resulted in an attenuation of the rhythms, accompanied by an increase in postprandial correlations. The results indicate that circadian rhythms of food intake are attributable to circadian oscillations in both meal parameters, and suggest a competitive relationship between circadian rhythms and the metabolic controls underlying the rats meal pattern.

Splitting of circadian rhythms in the rat

SummaryPatterns of splitting of circadian rhythms into two or more components are described in rats. The patterns were always the same when two or three behaviors were recorded concurrently from the same animal (drinking, feeding, and electrical brain self-stimulation).Several characteristics of the split rhythms were similar to those described for hamster locomotor activity (Pittendrigh and Daan, 1976): 1. The period of the split components was shorter than that of the pre-split free-running rhythm; 2. in cases of splitting of rhythms into two components, synchronization occurred when the components reached a 180° phase-relation; and 3. refusion of the split components followed a reduction in light intensity.In one case, a complete lesion of the suprachias-matic nuclei was made in a rat showing split rhythms. The lesion abolished both of the split components, although one remained visible for about a week following the lesion.The results suggest control of the three behavioral rhythms by a common pacemaker which may consist of two coupled populations of oscillators, as described by Pittendrigh and Daan (1976) for circadian locomotor activity rhythms in nocturnal rodents.

Circadian rhythm of luminance detectability in the rat.

Abstract Rats lived in a visual detection test environment which allowed for around-the-clock measurement of the detectability of a 500 nm stimulus of low luminance. Daily trends in visual sensitivity were compared with a behavioral activity measure, the rate of trial initiations in the self-paced testing procedure. Both variables showed circadian rhythms with periods exceeding 24 hr in the absence of day-night cues. However, the two oscillations were phase-displaced, such that maximal visual sensitivity occurred earlier than did maximal behavioral activity, each day. The temporal organization of visual sensitivity may underlie well-known photic influences on circadian rhythms and photoperiodic behavior.

Circadian feeding and drinking rhythms in the rat under complete and skeleton photoperiods

Feeding and drinking rhythms were studied in rats maintained under 24-hr light-dark (LD) cycles with various photoperiods, under two-pulse (2P) and one-pulse (1P) skeleton photoperiods, and under constant dark (DD). Rhythmic waveforms were similar under complete LD cycles and corresponding skeleton photoperiods, indicating that these rhythms mainly reflect the entrainment of underlying circadian pacemakers. Little or no role was found for masking effects of light on circadian feeding and drinking waveforms. Entrainment was found to depend mainly on the timing of the dawn light signal, whether it was a 15-min light pulse or a dark-to-light transition initiating a complete photoperiod. Furthermore, the use of 1P schedules revealed that a dawn signal was sufficient for entrainment. These results closely match those obtained for motor activity measures in other nocturnal rodent species, and generally conform to the predictions of Pittendrighs nonparametric theory of entrainment. Furthermore, the close correspondence of the two rhythms during entrainment, phase-jumps, and free-running (DD) conditions indicates that they are controlled by common circadian pacemakers.

Circadian rhythm of brain self-stimulation behavior.

Under constant conditions of light, sound, temperature, and humidity, rats exhibited circadian rhythmicity in rate of bar-pressing with hypothalamic and septal reinforcing brain stimulation. Variations in reinforcer magnitude aflected absolute levels of operant response emission but not the frequency of the circadian rhythm. In long sessions, the time of peak responding deviated systematically from a strict 24-hour period. Such data show marked similarity to free-running rhythms of motor activity.

Concurrent variation of response bias and sensitivity in an operant-psychophysical test

The yes-no signal detection procedure was applied to a single-response operant paradigm in which rats discriminated between a standard auditory intensity and attenuated comparison values. The payoff matrix was symmetrical (with reinforcing brain stimulation for correct detections and brief time-out for errors), but signal probability and intensity differences were varied to generate a family of isobias and isosensitivity functions. The d’ parameter remained fairly constant across a wide range of bias levels. Isobias functions deviated from a strict matching strategy as discrimination difficulty increased, although an orderly relation was maintained between signal probability value and the degree and direction of response bias.

Effects of illumination level on the rat's rhythmicity of brain self-stimulation behavior

Rhythmic patterns in the rats brain self-stimulation behavior were analyzed across levels of illumination, including conditions of constant illumination (LL), constant darkness (DD), and light-dark cycles (LD 12:12). LD entrainment was achieved with light intensities ranging from 0.25 to 440 lux, and little or no change was found in the phase-angle difference between the dominant spectral peak and the light transitions. Under constant conditions, the circadian period (tau) increased in proportion to illumination level, with means ranging from 24.10 h (DD) to 25.90 h (LL 440 lux). tau increased linearly as a function of long I within the range of 0.25 to 30 lux, yielding a change of 0.28 h for a 10-fold increment in illumination level, a value which closely matches Aschoffs [3] preliminary estimate of delta tau/delta ILL for the rat. The circadian spectral component was influenced by several factors. (1) Re-entrainment protocol. Given a succession of LL conditions without entrainment segments in between, circadian rhythmicity was obscured at high illumination levels. (2) Duration of LL exposure. Even following an entrainment segment, long-term LL resulted in reduced power or loss of the circadian component. (3) LD vs LL. Spectral power was consistently higher under entrainment than under corresponding LL intensities, and there was a trend toward reduced power as a function of LL intensity. A wide range of ultradian spectral components was found across conditions. Under entrainment, most such components were harmonics of the circadian fundamental; under constant conditions, the frequency relationships were relatively variable.

Control of the rat's circadian self-stimulation rhythm by light-dark cycles ☆

Rats with hypothalamic and septal electrodes were maintained in continuous test environments where bar-press responses produced brief reinforcing electrical stimulations. Long-term trends in response emission were measured under continuous exposure to light, dark and 12 hr light-dark alternations. In addition, transient behavioral adjustment to sudden 180 degrees phase shifts in the light-dark schedule was studied. The ambient light condition was found to control the period and phase of the circadian rhythm of brain self-stimulation behavior, as quantified by Fourier analysis. The circadian period was greatest under constant light (up to 24.90 hr under dim illumination), and approximated 24.00 hr under constant dark. Successful nocturnal entrainment to 12 hr light-dark alternations was obtained, with the peak of the 24 hr Fourier fundamental occurring in the middle-to-late dark segments. Three to 11 days were required for re-entrainment to 180 degrees light-dark phase shifts, during which the behavioral oscillation period increased to values comparable to periods under constant light. The rate of re-entrainment appeared to be proportional to illumination intensity during light segments.