Ralph E. Mistlberger

Circadian food-anticipatory activity: Formal models and physiological mechanisms

Rats and other species exhibit food-anticipatory activity (FAA) to daily mealtime under circadian (24 h) food access schedules. A critical review of several explanatory models indicates that hourglass clocks and associative learning processes are inadequate to explain many properties of FAA in intact and suprachiasmatic nuclei ablated rodents. A computational learning model, involving circadian clock consultation and phase memory, accounts for some but not all of these properties. An entrainment model, invoking separate, compound food- and light-entrainable oscillators, provides a more complete account of FAA. However, FAA may be simulated best by a model that combines oscillator entrainment with clock consultation and memory for circadian phase. Species as diverse as bees, birds, and mammals appear to share many features of FAA in common; differences may be explained in terms of oscillator organization and the ability to represent multiple circadian phases memorially. Physiological mechanisms of FAA are largely unknown; strategies for localization of entrainment pathways and oscillators, and a modest data base, are reviewed.

Social influences on mammalian circadian rhythms: animal and human studies

While light is considered the dominant stimulus for entraining (synchronizing) mammalian circadian rhythms to local environmental time, social stimuli are also widely cited as‘zeitgebers’(time‐cues). This review critically assesses the evidence for social influences on mammalian circadian rhythms, and possible mechanisms of action. Social stimuli may affect circadian behavioural programmes by regulating the phase and period of circadian clocks (i.e. a zeitgeber action, either direct or by conditioning to photic zeitgebers), by influencing daily patterns of light exposure or modulating light input to the clock, or by associative learning processes that utilize circadian time as a discriminative or conditioned stimulus. There is good evidence that social stimuli can act as zeitgebers. In several species maternal signals are the primary zeitgeber in utero and prior to weaning. Adults of some species can also be phase shifted or entrained by single or periodic social interactions, but these effects are often weak, and appear to be mediated by social stimulation of arousal. There is no strong evidence yet for sensory‐specific nonphotic inputs to the clock. The circadian phase‐dependence of clock resetting to social stimuli or arousal (the‘nonphotic’phase response curve, PRC), where known, is distinct from that to light and similar in diurnal and nocturnal animals. There is some evidence that induction of arousal can modulate light input to the clock, but no studies yet of whether social stimuli can shift the clock by conditioning to photic cues, or be incorporated into the circadian programme by associative learning. In humans, social zeitgebers appear weak by comparison with light. In temporal isolation or under weak light‐dark cycles, humans may ignore social cues and free‐run independently, although cases of mutual synchrony among two or more group‐housed individuals have been reported. Social cues may affect circadian timing by controlling sleep‐wake states, but the phase of entrainment observed to fixed sleep‐wake schedules in dim light is consistent with photic mediation (scheduled variations in behavioural state necessarily create daily light‐dark cycles unless subjects are housed in constant dark or have no eyes). By contrast, discrete exercise sessions can induce phase shifts consistent with the nonphotic PRC observed in animal studies. The best evidence for social entrainment in humans is from a few totally blind subjects who synchronize to the 24 h day, or to near‐24 h sleep‐wake schedules under laboratory conditions. However, the critical entraining stimuli have not yet been identified, and there are no reported cases yet of social entrainment in bilaterally enucleated blind subjects. The role of social zeitgebers in mammalian behavioural ecology, their mechanisms of action, and their utility for manipulating circadian rhythms in humans, remains to be more fully elaborated.

Adult hippocampal neurogenesis and voluntary running activity: circadian and dose-dependent effects.

Running activity increases cell proliferation and neurogenesis in the dentate gyrus of adult mice. The present experiment was designed to investigate whether the effect of activity on adult neurogenesis is dependent on the time of day (circadian phase) and the amount of activity. Mice received restricted access to a running wheel (0, 1, or 3 hr) at one of three times of day: the middle of the light phase (i.e., when mice are normally inactive), dark onset (i.e., when mice begin their nocturnal activity), and the middle of the dark period (i.e., when mice are in the middle of their active period). Cell proliferation and net neurogenesis were assessed after incorporation of the thymidine analog bromodeoxyuridine (BrdU) and immunohistochemical detection of BrdU and neuronal markers. Running activity significantly increased cell proliferation, cell survival, and total number of new neurons only in animals with 3 hr of wheel access during the middle of the dark period. Although activity was positively correlated with increased neurogenesis at all time points, the effects were not statistically significant in animals with wheel access at the beginning of the dark period or during the middle of the light period. These data suggest that the influence of exercise on cell proliferation and neurogenesis is modulated by both circadian phase and the amount of daily exercise, thus providing new insight into the complex relationship between physiological and behavioral factors that can mediate adult neuroplasticity.

Neurobiology of food anticipatory circadian rhythms

Circadian rhythms in mammals can be entrained by daily schedules of light or food availability. A master light-entrainable circadian pacemaker located in the suprachiasmatic nucleus (SCN) is comprised of a population of cell autonomous, transcriptionally based circadian oscillators with defined retinal inputs, circadian clock genes and neural outputs. By contrast, the neurobiology of food-entrainable circadian rhythmicity remains poorly understood at the systems and cellular levels. Induction of food-anticipatory activity rhythms by daily feeding schedules does not require the SCN, but these rhythms do exhibit defining properties of circadian clock control. Clock gene rhythms expressed in other brain regions and in peripheral organs are preferentially reset by mealtime, but lesions of specific hypothalamic, corticolimbic and brainstem structures do not eliminate all food anticipatory rhythms, suggesting control by a distributed, decentralized system of oscillators, or the existence of a critical oscillator at an unknown location. The melanocortin system and dorsomedial hypothalamus may play modulatory roles setting the level of anticipatory activity. The metabolic hormones ghrelin and leptin are not required to induce behavioral food anticipatory rhythms, but may also participate in gain setting. Clock gene mutations that disrupt light-entrainable rhythms generally do not eliminate food anticipatory rhythms, suggesting a novel timing mechanism. Recent evidence for non-transcriptional and network based circadian rhythmicity provides precedence, but any such mechanisms are likely to interact closely with known circadian clock genes, and some important double and triple clock gene knockouts remain to be phenotyped for food entrainment. Given the dominant role of food as an entraining stimulus for metabolic rhythms, the timing of daily food intake and the fidelity of food entrainment mechanisms are likely to have clinical relevance.

New neurons in the adult brain: The role of sleep and consequences of sleep loss

Research over the last few decades has firmly established that new neurons are generated in selected areas of the adult mammalian brain, particularly the dentate gyrus of the hippocampal formation and the subventricular zone of the lateral ventricles. The function of adult-born neurons is still a matter of debate. In the case of the hippocampus, integration of new cells in to the existing neuronal circuitry may be involved in memory processes and the regulation of emotionality. In recent years, various studies have examined how the production of new cells and their development into neurons is affected by sleep and sleep loss. While disruption of sleep for a period shorter than one day appears to have little effect on the basal rate of cell proliferation, prolonged restriction or disruption of sleep may have cumulative effects leading to a major decrease in hippocampal cell proliferation, cell survival and neurogenesis. Importantly, while short sleep deprivation may not affect the basal rate of cell proliferation, one study in rats shows that even mild sleep restriction may interfere with the increase in neurogenesis that normally occurs with hippocampus-dependent learning. Since sleep deprivation also disturbs memory formation, these data suggest that promoting survival, maturation and integration of new cells may be an unexplored mechanism by which sleep supports learning and memory processes. Most methods of sleep deprivation that have been employed affect both non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. Available data favor the hypothesis that decreases in cell proliferation are related to a reduction in REM sleep, whereas decreases in the number of cells that subsequently develop into adult neurons may be related to reductions in both NREM and REM sleep. The mechanisms by which sleep loss affects different aspects of adult neurogenesis are unknown. It has been proposed that adverse effects of sleep disruption may be mediated by stress and glucocorticoids. However, a number of studies clearly show that prolonged sleep loss can inhibit hippocampal neurogenesis independent of adrenal stress hormones. In conclusion, while modest sleep restriction may interfere with the enhancement of neurogenesis associated with learning processes, prolonged sleep disruption may even affect the basal rates of cell proliferation and neurogenesis. These effects of sleep loss may endanger hippocampal integrity, thereby leading to cognitive dysfunction and contributing to the development of mood disorders.

Suprachiasmatic nuclei lesions eliminate circadian temperature and sleep rhythms in the rat

We conducted a long-term study of the circadian rhythms of temperature and sleep in the rat after lesions of the suprachiasmatic nuclei (SCN). Brain temperature was measured with thermistors and sleep-wake was scored on the basis of continuously recorded EEG using a computerized system. Rats with complete SCN lesions did not exhibit circadian rhythms in constant dim illumination. Rats with partial SCN lesions generated weak and variable free-running rhythms, and when exposed to a light-dark cycle, some showed a reduced amplitude and altered waveform relative to normal rats. A few rats with partial SCN lesions showed a recovery of function. There was little difference between the circadian rhythms in temperature and waking, and these measures responded similarly to all lesions. Thus, no support was found for the notions that anatomically distinct oscillators control the circadian rhythms of temperature and activity, or that an oscillator outside of the SCN controls the circadian rhythm of temperature.

Food-anticipatory circadian rhythms: concepts and methods

Rats, mice and other species can behaviorally anticipate a predictable daily mealtime by entrainment of circadian oscillators (food‐entrainable oscillators) distinct from those (light‐entrainable oscillators) that regulate light‐dark entrained rhythms of behavior and physiology. Neurobiological analysis of food‐anticipatory rhythms has progressed slowly but is gaining pace. Food‐anticipatory rhythms have proven to be surprisingly robust to many neural and circadian clock gene perturbations. A few neural ablation sites or gene mutations have been associated with loss or marked attenuation of anticipatory rhythms, but in each case there are apparently conflicting reports. Attenuation of food‐anticipatory rhythms following neural or genetic perturbations could result from actions upstream or downstream from the clock mechanism, and could be limited to certain behavioral endpoints or recording conditions. Failure to observe attenuation could reflect compensation by alternate timing mechanisms that do not involve food‐entrainable oscillators. To facilitate progress in neurobiological analysis of food‐anticipatory rhythms, criteria for distinguishing among formally distinct mechanisms by which animals might anticipate a daily meal are reviewed, and procedural variables that can affect the expression of food‐anticipatory rhythms in neurobiologically intact or compromised animals are identified.

Nonphotic Entrainment in Humans

Although light is accepted as the dominant zeitgeber for entrainment of the human circadian system, there is evidence that nonphotic stimuli may play a role. This review critically assesses the current evidence in support of nonphotic entrainment in humans. Studies involving manipulations of sleep-wake schedules, exercise, mealtimes, and social stimuli are re-examined, bearing in mind the fact that the human circadian clock is sensitive to very dim light and has a free-running period very close to 24 h. Because of light confounds, the study of totally blind subjects with free-running circadian rhythms represents the ideal model to investigate the effects of nonphotic stimuli on circadian phase and period. Strong support for nonphotic entrainment in humans has already come from the study of a few blind subjects with entrained circadian rhythms. However, in these studies the nonphotic stimulus(i) responsible was not identified. The effect of appropriately timed exercise or exogenous melatonin represents the best proof to date of an effect of nonphotic stimuli on human circadian timing. Phase-response curves for both exercise and melatonin have been constructed. Given the powerful effect of feeding as a circadian zeitgeber in various nonhuman species, studies of meal timing are recommended. In conclusion, the available evidence indicates that it remains worthwhile to continue to study nonphotic effects on human circadian timing to identify treatment strategies for shift workers and transmeridian travelers as well as for the blind and possibly the elderly.

Palatable daily meals entrain anticipatory activity rhythms in free-feeding rats: Dependence on meal size and nutrient content

Circadian wheel-running rhythms were monitored continuously in 3 groups of female Sprague-Dawley rats under different palatable food availability schedules. All rats had free access to standard rat chow and water throughout the study. In addition, Group 1 rats received a palatable nutrient-rich mash for 2 hr each day for 28 days, beginning 3 hr after light onset of a 12:12 LD cycle. Group 2 rats received the same mash but were limited to 4 g daily. Group 3 rats received a palatable non-nutritive mash. Ten of 13 Group 1 rats, 2 of 13 Group 2 rats, and 0 of 13 Group 3 rats showed anticipatory running prior to the daily palatable meal. Palatable mash intake was generally lower among Group 3 rats than among Group 1 rats. However, several Group 3 rats consumed non-nutritive mash in amounts which equalled or exceeded the nutritive mash intake of Group 1 rats showing anticipatory running. The results indicate that temporally limited daily access to a palatable food can entrain anticipatory wheel-running in rats that are not food-deprived. They also indicate that entrainment to periodic food availability depends on stimuli associated with the concentrated intake of nutrients rather than on the absolute size or palatability of a meal.

Entrainment and phase shifting of circadian rhythms in mice by forced treadmill running

Daily schedules of spontaneous, drug-, or novelty-induced running can entrain circadian rhythms in rodents. Forced running, by contrast, has been reported to have weak or no effects, although a thorough comparative study in a single species is lacking. To fill this gap, drinking or activity rhythms were monitored in C57 mice subjected to daily, 3-h bouts of forced treadmill running or to 3-h daily access to home cage running wheels. Entrainment to treadmill running was observed in 17/27 mice, and to restricted wheel access in 11/20 mice. Entrainment was affected by availability of a home cage wheel (e.g., 14/16 mice with no wheel entrained to treadmill running). Phase angle of entrainment was related to prior circadian period (tau), and tau following entrainment exhibited aftereffects. No mice entrained to a 3-h daily schedule of water access, suggesting that entrainment to scheduled running was not related to water or associated food intake. Phase shifts in response to single 3-h bouts of treadmill running or wheel access were small and not reliably induced. The entrainment paradigm is thus recommended for further study of behavioral effects on the mouse circadian system; forced running, in particular, offers several methodological advantages. The results do not support prior suggestions that forced and voluntary activity differ in value as nonphotic zeitgebers.