Olfactory bulbectomy impedes social but not photic reentrainment of circadian rhythms in female Octodon degus

Social information as an entrainment cue for the circadian clock

Animals adapt to the daily changes in their environmental conditions by means of genetically encoded circadian clocks. These clocks, found throughout the tree of life, regulate diverse biological functions, and allow periodical changes in physiology and behaviour. The molecular underpinnings of these clocks have been extensively studied across taxa, revealing a brain-based system that coordinates rhythmic activities through neuronal networks and signalling pathways. Entrainment, the alignment of internal rhythms with external cues or zeitgebers, is crucial for the adaptive value of these internal clocks. While the solar light-dark cycle is a primary zeitgeber for most animals, other relevant cues such as temperature, meal timing, predators, anxiety, fear, physical activity, and social interactions also play roles in entraining circadian clocks. The search of a detailed description of the circadian clocks is a goal for neurobiology and an area of growing societal interests. Moreover, as disruptions in circadian rhythms are implicated in various diseases, understanding the entrainment pathways contributes to developing interventions for improved wellbeing and health outcomes. This review focuses on socially relevant cues, examining their impact on animal physiology and behaviour, and explores the sensory pathways transmitting information to the central clock.

The presence/absence of conspecifics modulates the circadian locomotor activity and body mass in spotted munia (Lonchura punctulata)

Biological clocks regulate the behavior and physiology of animals by tracking the local time using diverse time cues. Social cues are relevant in studying the behavior of gregarious animals, but these cues have not been widely studied in birds. Temporal information for circadian timekeeping is socially communicated through visual, physical, olfactory, and auditory means. We examined the efficacy of pulsatile social interactions on locomotor activity and its associated characteristics such as distribution profile of rest and activity, total counts, activity duration, phase shift in activity onset, and circadian periodicity in spotted munia. Besides, we analyzed the effect of such social interactions on their body mass. Spotted munia exhibited phase shift in the onset of activity when subjected to social isolation, but these cues could not affect their circadian periodicity. In Pair as well as in Group, social isolation led to increased activity and activity duration, and decreased body mass in guests relative to the host bird. Our results suggest that the circadian rhythm of locomotor activity in spotted munia is quite sensitive to socialization and isolation, and isolation is detrimental for the birds. Consistent with these observations, the decline in body mass revealed the physiological consequences of social isolation on spotted munia.

Social synchronization of circadian rhythms with a focus on honeybees

Many animals benefit from synchronizing their daily activities with conspecifics. In this hybrid paper, we first review recent literature supporting and extending earlier evidence for a lack of clear relationship between the level of sociality and social entrainment of circadian rhythms. Social entrainment is specifically potent in social animals that live in constant environments in which some or all individuals do not experience the ambient day-night cycles. We next focus on highly social honeybees in which there is good evidence that social cues entrain the circadian clocks of nest bees and can override the influence of conflicting light-dark cycles. The current understanding of social synchronization in honeybees is consistent with self-organization models in which surrogates of forager activity, such as substrate-borne vibrations and colony volatiles, entrain the circadian clocks of bees dwelling in the dark cavity of the nest. Finally, we present original findings showing that social synchronization is effective even in an array of individually caged callow bees placed on the same substrate and is improved for bees in connected cages. These findings reveal remarkable sensitivity to social time-giving cues and show that bees with attenuated rhythms (weak oscillators) can nevertheless be socially synchronized to a common phase of activity. This article is part of the theme issue 'Synchrony and rhythm interaction: from the brain to behavioural ecology'.

Context and novelty increase strength of auditory cues as a weak circadian zeitgeber in songbirds

Light is the best-studied external cue (zeitgeber) for the entrainment of circadian rhythms. Non-photic entrainment is also possible; some organisms can entrain to rhythmic temperatures, drug administration, feeding, water turbulence, exercise and social cues. One such social cue that has the capacity to act as a weak zeitgeber to songbirds is the rhythmic presentation of conspecific vocalization. To better characterize this phenomenon, we performed several trials in which male and female zebra finches were maintained in constant dim light and allowed to free-run for 1 week before being presented with different audio cues of various lengths of playback and audio design every day at the same time of day for 15-31 days. Live audio monitoring from a nearby colony housed in light: dark (LD) conditions proved the strongest zeitgeber we tested, suggesting the phenomenon is enhanced with dynamic, context-appropriate vocalizations. Live colony playback was more efficacious than was a 2 h or 4 h presentation of the same, single zebra finch song but not a 1 h presentation, suggesting that habituation may have occurred in some of these experiments. The monitoring of the colony was also not statistically different from a 4 h playback of that same song, reversed, suggesting that social context is not required. It was, however, more effective than a 4 h presentation of synthesized, pseudorandom tones. When birds entrained to the period of the zeitgebers, their expressed period closely matched 24 h with phases closely matched to the onset of the zeitgeber. Masking was not evident in contrast to masking observed following transfer from constant dim light to LD and vice versa.This series of experiments could prove a means of quantifying the capacity for reciprocal social interaction, a state which can be dynamic in songbirds, as well as the integration between sociality and the circadian clock.

Gender difference in circadian clock responses for social interaction with conspecific of the opposite-sex

Social cues are potent non-photic modulators of the circadian clock and play a vital role in resetting the endogenous clock. Several lines of evidence strongly suggest a functional link between olfactory cues and the circadian clock. However, there is a paucity of information on the effects of social interaction with the conspecifics of the opposite sex on the circadian clock. Hence, we studied the effect of social cues of sexually mature naïve opposite sex of the conspecific on the phase resetting of the circadian clock at various circadian times (CT) and molecular changes at the suprachiasmatic nuclei (SCN) and odor responsive structure in the brain of mice. Sexually naïve adult male and female free-running mice (designated as 'runners') were exposed to the conspecifics of the opposite-sex ('strangers') for 30 min at CT3, CT9, CT15, and CT21. Both male and female 'runners' exhibited a phase advance at CT3, delay at CT21, and no response at CT9. However, at CT15 only the male 'runners' exhibited phase advance but not the female 'runners'. Control mice did not elicit any significant phase shifts at all CTs. Social interactions with conspecifics of the opposite-sex up-regulated c-fos/C-FOS, omp in the olfactory bulb, per-1/PER-1 in the SCN, C-FOS, and PER-1 in the piriform cortex of both male and female runners at CT3. However, at CT15 up-regulation of variables only occurred in male but not in female runners. Together, the present investigation has shown the gender difference in circadian clock responses for social cues with conspecific of the opposite-sex in mice.

The Biological Clock in Gray Mouse Lemur: Adaptive, Evolutionary and Aging Considerations in an Emerging Non-human Primate Model

Circadian rhythms, which measure time on a scale of 24 h, are genetically generated by the circadian clock, which plays a crucial role in the regulation of almost every physiological and metabolic process in most organisms. This review gathers all the available information about the circadian clock in a small Malagasy primate, the gray mouse lemur (Microcebus murinus), and reports 30 years data from the historical colony at Brunoy (France). Although the mouse lemur has long been seen as a "primitive" species, its clock displays high phenotypic plasticity, allowing perfect adaptation of its biological rhythms to environmental challenges (seasonality, food availability). The alterations of the circadian timing system in M. murinus during aging show many similarities with those in human aging. Comparisons are drawn with other mammalian species (more specifically, with rodents, other non-human primates and humans) to demonstrate that the gray mouse lemur is a good complementary and alternative model for studying the circadian clock and, more broadly, brain aging and pathologies.

A new device for monitoring individual activity rhythms of honey bees reveals critical effects of the social environment on behavior

Chronobiological studies of individual activity rhythms in social insects can be constrained by the artificial isolation of individuals from their social context. We present a new experimental set-up that simultaneously measures the temperature rhythm in a queen-less but brood raising mini colony and the walking activity rhythms of singly kept honey bees that have indirect social contact with it. Our approach enables monitoring of individual bees in the social context of a mini colony under controlled laboratory conditions. In a pilot experiment, we show that social contact with the mini colony improves the survival of monitored young individuals and affects locomotor activity patterns of young and old bees. When exposed to conflicting Zeitgebers consisting of a light–dark (LD) cycle that is phase-delayed with respect to the mini colony rhythm, rhythms of young and old bees are socially synchronized with the mini colony rhythm, whereas isolated bees synchronize to the LD cycle. We conclude that the social environment is a stronger Zeitgeber than the LD cycle and that our new experimental set-up is well suited for studying the mechanisms of social entrainment in honey bees.

Olfactory Bulbectomy Modifies Photic Entrainment and Circadian Rhythms of Body Temperature and Locomotor Activity in a Nocturnal Primate

Studies on rodents have emphasized that removal of the olfactory bulbs modulates circadian rhythmicity. Using telemetric recordings of both body temperature (Tb) and locomotor activity (LA) in a male nocturnal primate, the gray mouse lemur, the authors investigated the effects of olfactory bulbectomy on (1) the circadian periods of Tb and LA in constant dim light condition, and (2) photic reentrainment rates of circadian rhythms following 6-h phase shifts of entrained light-dark cycle (LD 12:12). Under free-running condition, bulb-ectomized males had significantly shorter circadian periods of Tb and LA rhythms than those of control males. However, the profiles of Tb rhythms, characterized by a phase of hypothermia at the beginning of the subjective day, and Tb parameters were not modified by olfactory bulbectomy. Under a light-dark cycle, olfactory bulbectomy significantly modified the expression of daily hypothermia, especially by an increase in the latency to reach minimal daily Tb, suggesting a delayed response to induction of daily hypothermia by light onset. Re-entrainment rates following both a 6-h phase advance and a 6-h phase delay of entrained LD were also delayed in bulbectomized males. Olfactory bulbectomy led to significant fragmentation of locomotor activity and increased locomotor activity levels during the resting period. The shortening of circadian periods in bulbectomized males could partly explain the delayed responses to photic stimuli since in control males, the longer the circadian period, the better the response to light entrainment. This experiment shows for the 1st time that olfactory bulbs can markedly modify the circadian system in a primate.

Morphology and Topography of the Celiac Plexus in Degu (Octodon Degus)

Here, we investigate the morphology and topography of the celiac plexus components in degu (Octodon degus). The study was performed using six adult individuals of both sexes. Macromorphological observations were performed using a derivative of the thiocholine method specially adapted for this study type (Gienc, 1977). The classical H&E technique was used for analysis of the cytoarchitectonic of the ganglion, and the AChE (Karnovsky and Roots, 1964) and SPG (De la Torre, 1980) techniques to observe cholinergic and adrenergic activity. The celiac plexus of degu is located on the ventral and lateral surface of the abdominal aorta, at the level where the celiac artery separates from the aorta. This structure consists of two large and two smaller aggregations of neurocytes connected with postganglionic fibers. Histochemical investigations have demonstrated the mainly cholinergic characteristic of the intraganglionic and postganglionic fibers of the celiac plexus, while the adrenergic fibers accompanied only the blood vessels and neurocytes revealed differentiation of adrenergic activity. Histological analysis revealed that neurocytes occupied about half of the cross-section area, with the nerve fibers, connective tissue, and blood vessels forming the remaining part. Ganglionic cells were oval, and usually contained a single nucleus, although two nuclei were sometimes observed.

Temporal behaviour profiles of Mus musculus in nature are affected by population activity

Animals have circadian clocks that govern their activity pattern, resulting in 24h rhythms in physiology and behaviour. Under laboratory conditions, light is the major external signal that affects temporal patterns in behaviour, and Mus musculus is strictly nocturnal in its behaviour. In the present study we questioned whether under natural conditions, environmental factors other than light affect the temporal profile of mice. In order to test this, we investigated the activity patterns of free-ranging M. musculus in a natural habitat, using sensors and a camera integrated into a recording unit that the mice could freely enter and leave. Our data show that mice have seasonal fluctuations in activity duration (6.7±0.82 h in summer, 11.3±1.80 h in winter). Furthermore, although primarily nocturnal, wild mice also exhibit daytime activity from spring until late autumn. A multivariate analysis revealed that the major factor correlating with increased daytime activity was population activity, defined as the number of visits to the recording site. Day length had a small but significant effect. Further analysis revealed that the relative population activity (compared to the past couple of days) is a better predictor of daytime activity than absolute population activity. Light intensity and temperature did not have a significant effect on daytime activity. The amount of variance explained by external factors is 51.9%, leaving surprisingly little unexplained variance that might be attributed to the internal clock. Our data further indicate that mice determine population activity by comparing a given night with the preceding 2-7 nights, a time frame suggesting a role for olfactory cues. We conclude that relative population activity is a major factor controlling the temporal activity patterns of M. musculus in an unrestricted natural population.

Timekeeping through social contacts: social synchronization of circadian locomotor activity rhythm in the carpenter ant Camponotus paria

In ant colonies a large proportion of individuals remain inside nests for most of their lives and come out only when necessary. It is not clear how, in a nest of several thousand individuals, information about local time is communicated among members of the colony. Central to this seem to be circadian clocks, which have an intrinsic ability to keep track of local time by entraining to environmental light-dark, temperature, and social cycles. Here, the authors report the results of their study aimed at understanding the role of cyclic social interactions in circadian timekeeping of a day-active species of carpenter ant Camponotus paria. The authors found that daily social interactions with visitors (worker ants) was able to synchronize the circadian locomotor activity rhythm of host worker ants and queens, in one-on-one (pair-wise) and multi-individual (group-wise) interactions. Interestingly, the outcome of cyclic social interactions was context specific; when visitor workers socially interacted with host workers one-on-one, host workers considered the time of interaction as subjective day, but when visitor workers interacted with a group of workers and queens, the hosts considered the time of interaction as subjective night. These results can be taken to suggest that members of the ant species C. paria keep track of local time by socially interacting with workers (foragers) who shuttle in and out of the colony in search of food. (Author correspondence: vsharma@jncasr.ac.in ).

Social synchronization of circadian locomotor activity rhythm in the fruit fly Drosophila melanogaster

Circadian clocks regulate the physiology and behaviour of organisms across a wide range of taxa. To keep track of local time, these clocks use a variety of time cues such as light-dark, temperature, food availability and social interaction cycles. This study assessed the role of social cues in modulating circadian clocks of the fruit fly Drosophila melanogaster. Using pair-wise interactions, we first estimated the percentage contribution of each interacting partner on the cumulative rhythmic behaviour of the pairs. Subsequently, we studied the effects of multi-individual (group-wise) interactions on the rhythmic behaviour of the group by estimating phase synchrony between individuals of different strains (having different circadian periods) maintained in both homogeneous and heterogeneous groups. Although it is known that social interactions improve synchrony between interacting individuals, we asked whether such interactions are able to synchronize the circadian rhythms of highly phase-desynchronized flies. We found that, although interactions between fly strains possessing different circadian periods failed to produce synchrony, social interactions among phase-desynchronized flies did enhance the phase synchrony of the interacting individuals. Differently phased individuals living in social groups displayed significantly greater phase synchrony than those living solitarily. Social synchronization is olfaction mediated as group-wise interactions among phase-desynchronized flies possessing compromised olfactory ability (Or83b(0)) did not improve phase synchrony. These results suggest that social cues synchronize the circadian clocks of Drosophila provided that the interacting individuals have similar clock periods.

Social influences on circadian rhythms and sleep in insects

The diverse social lifestyle and the small and accessible nervous system of insects make them valuable for research on the adaptive value and the organization principles of circadian rhythms and sleep. We focus on two complementary model insects, the fruit fly Drosophila melanogaster, which is amenable to extensive transgenic manipulations, and the honey bee Apis mellifera, which has rich and well-studied social behaviors. Social entrainment of activity rhythms (social synchronization) has been studied in many animals. Social time givers appear to be specifically important in dark cavity-dwelling social animals, but here there are no other clear relationships between the degree of sociality and the effectiveness of social entrainment. The olfactory system is important for social entrainment in insects. Little is known, however, about the molecular and neuronal pathways linking olfactory neurons to the central clock. In the honey bee, the expression, phase, and development of circadian rhythms are socially regulated, apparently by different signals. Peripheral clocks regulating pheromone synthesis and the olfactory system have been implicated in social influences on circadian rhythms in the fruit fly. An enriched social environment increases the total amount of sleep in both fruit flies and honey bees. In fruit flies, these changes have been linked to molecular and neuronal processes involved in learning, memory, and synaptic plasticity. The studies on insects suggest that social influences on the clock are richer than previously appreciated and have led to important breakthroughs in our understanding of the mechanisms underlying social influences on sleep and circadian rhythms.

Cyclic presence and absence of conspecifics alters circadian clock phase but does not entrain the locomotor activity rhythm of the fruit fly Drosophila melanogaster

Circadian clocks use a wide range of environmental cues, including cycles of light, temperature, food, and social interactions, to fine-tune rhythms in behavior and physiology. Although social cues have been shown to influence circadian clocks of a variety of organisms including the fruit fly Drosophila melanogaster, their mechanism of action is still unclear. Here, the authors report the results of their study aimed at investigating if daily cycles of presence and absence (PA) of conspecific male visitors are able to entrain the circadian locomotor activity rhythm of male hosts living under constant darkness (DD). The results suggest that PA cycles may not be able to entrain circadian locomotor activity rhythms of Drosophila. The outcome does not change when male hosts are presented with female visitors, suggesting that PA cycles of either sex may not be effective in bringing about stable entrainment of circadian clocks in D. melanogaster. However, in hosts whose clock phase has already been set by light/dark (LD) cycles, daily PA cycles of visitors can cause measurable change in the phase of subsequent free-running rhythms, provided that their circadian clocks are labile. Thus, the findings of this study suggest that D. melanogaster males may not be using cyclic social cues as their primary zeitgeber (time cue) for entrainment of circadian clocks, although social cues are capable of altering the phase of their circadian rhythms.

The Social Clock of the Honeybee

The honeybee has long been an important model for studying the interplay between the circadian clock and complex behaviors. This article reviews studies further implicating the circadian clock in complex social behaviors in bees. The article starts by introducing honeybee social behavior and sociality and then briefly summarizes current findings on the molecular biology and neuroanatomy of the circadian system of honeybees that point to molecular similarities to the mammalian clockwork rather than to that of Drosophila. Foraging is a social behavior in honeybees that relies on the circadian clock for timing visits to flowers, time-compensated sun-compass navigation, and dance communication used by foragers to recruit nestmates to rewarding flower patches. The circadian clock is also important for the social organization of honeybee societies. Social factors influence the ontogeny of circadian rhythms and are important for social synchronization of worker activities. Both queen and worker bees switch between activities with and without circadian rhythms. In workers this remarkable plasticity is associated with the division of labor; nurse bees care for the brood around the clock with similar levels of clock gene expression throughout the day, whereas foragers have strong behavioral circadian rhythms with oscillating brain clock gene levels. This plasticity in circadian rhythms is regulated by direct contact with the brood and is context-specific in that nurse bees that are removed from the hive exhibit activity with strong behavioral and molecular rhythms. These studies on the sociochronobiology of honeybees and comparative studies with other social insects suggest that the evolution of sociality has influenced the characteristics of the circadian system in honeybees.

Olfactory Discrimination and Transient Mood Change in Young Men and Women: Variation by Season, Mood State, and Time of Day

Odor performance varies by clinical state and gender, though little is known about its variation by season or time of day. Many odors, including lavender, induce transient mood changes. This study explored discrimination differences between various lavender oil blends and their effects on transient mood in the morning and evening in depressed and nondepressed adults. We also determined seasonal influences on these measures. A total of 169 subjects, 98 women and 71 men (mean age +/- SD, 19.3 +/- 1.6 y) participated, with different subjects studied at different times of the year. The Beck Depression Inventory (BDI) classified subjects as depressed (score > or =10; N= 57) or nondepressed (score <10; N= 112). In the discrimination test, subjects compared pairs of two different lavender oil blends or a control. Transient change in mood was assessed by the Profile of Mood States (POMS) after each trial of five lavender blends interspersed by three control odors. Tests were conducted in the morning (08:00-10:00 h) and evening (18:00-20:00h). In all subjects, discrimination was significantly better for some odor pairs than for others, thus demonstrating test specificity. Discrimination was better overall in the fall than winter/spring and better in depressed than nondepressed subjects for specific odor pairs. No significant gender or time-of-day differences in discriminability were detected. There were, however, significant group differences in transient mood profiles. Current depressed state affected mood response, with lavender increasing anger in depressed subjects only. In addition, depressed subjects and men, whether or not depressed, exhibited diurnal mood variation, with better mood in the evening; the former group also showed more evening energy. All subjects were more confused in the morning than evening. Season also affected transient mood; winter/spring-tested subjects reported more vigor than fall-tested subjects. In addition, summer-tested subjects showed more tension in the morning, whereas fall-tested subjects showed the opposite pattern in the evening. In all subjects, lavender increased fatigue, tension, confusion, and total mood disturbance, and it decreased vigor. The study showed that both chronobiological (seasonal and time-of-day) and clinical factors modify discrimination and mood response to odors. Brief lavender odor presentation may serve as a nonphotic method for altering mood in young depressed and nondepressed adults particularly during the fall, a time of heightened discriminability.

A circadian clock in the olfactory bulb controls olfactory responsivity

Recently, it has been shown that multiple mammalian cell types express daily rhythms in vitro. Although the suprachiasmatic nucleus (SCN) of the hypothalamus is known to regulate a wide range of circadian behaviors, the role for intrinsic rhythmicity in other tissues is unknown. We tested whether the main olfactory bulb (OB) of mice mediates daily changes in olfaction. We found circadian rhythms in cedar oil-induced c-Fos, a protein marker of cellular excitation, in the mitral and granular layers of the OB and in the piriform cortex (PC). These oscillations persisted in constant darkness with a fourfold change in amplitude and a peak approximately 4 h after the onset of daily locomotor activity. Electrolytic lesions of the SCN abolished circadian locomotor rhythms, but not odor-induced c-Fos rhythms in the OB or PC. Furthermore, removal of the OB abolished spontaneous circadian cycling of c-Fos in the PC, shortened the free-running period of locomotor rhythms, and accelerated re-entrainment after a 6 h advance and slowed re-entrainment after a 6 h delay in the light schedule. OB ablation or odorant altered the amplitude of c-Fos rhythms in the SCN and ablation of one OB abolished c-Fos rhythms in the ipsilateral PC, but not in the contralateral OB and PC. We conclude that the OB comprises a master circadian pacemaker, which enhances olfactory responsivity each night, drives rhythms in the PC, and interacts with the SCN to coordinate other daily behaviors.

Octodon degus: a diurnal, social, and long-lived rodent

Octodon degus is a moderate-sized, precocious, but slowly maturing, hystricomorph rodent from central Chile. We have used this species to study a variety of questions about circadian rhythms in a diurnal mammal that readily adapts to most laboratory settings. In collaboration with others, we have found that a number of fundamental features of circadian function differ in this diurnal rodent compared with nocturnal rodents, specifically rats or hamsters. We have also discovered that many aspects of the circadian system are sexually dimorphic in this species. However, the sexual dimorphisms develop in the presence of pubertal hormones, and the sex differences do not appear until after gonadal puberty is complete. The developmental timing of the sex differences is much later than in the previously studied altricial, rapidly developing rat, mouse, or hamster. This developmental timing of circadian function is reminiscent of that reported for adolescent humans. In addition, we have developed a model that demonstrates how nonphotic stimuli, specifically conspecific odors, can interact with the circadian system to hasten recovery from a phase-shift of the light:dark cycle (jet lag). Interestingly, the production of the odor-based social signal and sensitivity to it are modulated by adult gonadal hormones. Data from degu circadian studies have led us to conclude that treatment of some circadian disorders in humans will likely need to be both age and gender specific. Degus will continue to be valuable research animals for resolving other questions regarding reproduction, diabetes, and cataract development.

The suprachiasmatic nucleus entrains, but does not sustain, circadian rhythmicity in the olfactory bulb

The suprachiasmatic nucleus (SCN) of the hypothalamus has been termed the master circadian pacemaker of mammals. Recent discoveries of damped circadian oscillators in other tissues have led to the hypothesis that the SCN synchronizes and sustains daily rhythms in these tissues. We studied the effects of constant lighting (LL) and of SCN lesions on behavioral rhythmicity and Period 1 (Per1) gene activity in the SCN and olfactory bulb (OB). We found that LL had similar effects on cyclic locomotor and feeding behaviors and Per1 expression in the SCN but had no effect on rhythmic Period 1 expression in the OB. LL lengthened the period of locomotor and SCN rhythms by approximately 1.6 hr. After 2 weeks in LL, nearly 35% of rats lost behavioral rhythmicity. Of these, 90% showed no rhythm in Per1-driven expression in their SCN. Returning the animals to constant darkness rapidly restored their daily cycles of running wheel activity and gene expression in the SCN. In contrast, the OB remained rhythmic with no significant change in period, even when cultured from animals that had been behaviorally arrhythmic for 1 month. Similarly, we found that lesions of the SCN abolished circadian rhythms in behavior but not in the OB. Together, these results suggest that LL causes the SCN to lose circadian rhythmicity and its ability to coordinate daily locomotor and feeding rhythms. The SCN, however, is not required to sustain all rhythms because the OB continues to oscillate in vivo when the SCN is arrhythmic or ablated.

Birds of a feather clock together – sometimes: social synchronization of circadian rhythms

Biological systems use internal circadian clocks to efficiently organize physiological and behavioral activity within the 24-hour time domain. In the absence of time cues, circadian periods vary slightly from 24 hours, but in nature, ambient light serves as the most salient synchronizer for these rhythms, fine-tuning them to exactly 24 hours each day. For some species, social cues can serve to synchronize circadian rhythms in the absence of other time cues or to amplify ambiguous light cues. This has been demonstrated to various degrees in fruit flies, degus, birds, fish, bats, beavers and humans; however, studies in rats and hamsters have shown that social cues are less salient time cues for these species. Social influences on circadian timing might function to tightly organize the social group, thereby decreasing the chances of predation and increasing the likelihood of mating.

Olfactory cues accelerate reentrainment following phase shifts and entrain free-running rhythms in female Octodon degus (Rodentia)

Social interactions between conspecifics is a type of nonphotic zeitgeber common to several species. In the diurnal rodent Octodon degus, social interactions enhance reentrainment after phase shifts and can act as a weak zeitgeber. Olfactory stimuli appear necessary for these effects since bulbectomy eliminates socially enhanced reentrainment. In Experiment 1, the authors examined whether stimulation of the main olfactory system was sufficient to enhance reentrainment after 6-h phase advances and delays in the adult female O. degus. When test animals received conspecific odor cues during reentrainment, they entrained 39% faster after phase advances (p < 0.05) and 33% faster after phase delays (p < 0.001) than when they did not receive odor cues. Thus, olfactory cues from distant female donors were sufficient to enhance rates of entrainment in female O. degus and provided results equivalent to earlier studies with donors and shifters housed in the cages together. In Experiment 2, the authors examined whether discrete 3-h and 1-h daily pulses of airborne odors from a group of 5 entrained female degus would be sufficient to produce entrainment of wheel-running activity in adult female conspecifics. During the period of exposure to 3-h pulses, 50% (4/8) of the subjects temporarily entrained to a 24-h cycle, while 12.5% (1/8) of the subjects fully entrained. Exposure to 1-h pulses allowed 37.5% (3/8) of the subjects to temporarily entrain and 12.5% (1/8) of the subjects to fully entrain. Duration of entrained episodes was positively correlated with psi, daily onset of activity with respect to the timing of odor exposure (Pearson r = 0.731; p < 0.05), such that animals with the entraining odor pulse beginning during subjective day (psi = 7.8 h, CT 7.8 +/- 1.4) had longer periods of entrainment (22.2 +/- 5.6 days) than animals with the entraining pulse occurring during subjective night (psi = -4.6 h; CT 19.4 +/- 0.9; 5.6 +/- 0.9 days; p < 0.001). In addition, for each animal, the combined duration of all episodes of 24-h entrainment correlated with increased period length (tau) of free-running rhythms (Pearson r = 0.733; p < 0.05). Thus, daily discrete pulses of odors with durations of either 1 or 3 h from female conspecifics were sufficient to produce both temporary and full entrainment to a 24-h cycle in the majority of female O. degus, and the likelihood of long periods of entrainment correlated with long taus and coordination of the odor pulse with mid subjective day.

Extraocular phototransduction and circadian timing systems in vertebrates

It is widely accepted that, for organisms with eyes, the daily regulation of circadian rhythms is made possible by light transduction through those organs. Yet, it has been demonstrated repeatedly in recent years that ocular light receptors that mediate vision, at least in mammals, are not the same photoreceptors involved in circadian regulation. Moreover, it has been recognized for many years that circadian regulation can occur in organisms without eyes. In fact, extraocular circadian phototransduction (EOCP) appears to be a phylogenetic rule for the vast majority of species. EOCP has been reported in every nonmammalian species studied to date. In mammals, however, the story is very different. This paper presents findings from studies that have examined specifically the capacity for EOCP in vertebrate species. In addition, the literature addressing noncircadian aspects of extraocular phototransduction is briefly discussed. Finally, possible mechanisms underlying EOCP are discussed, as are some of the implications of the presence, or absence, of EOCP across phylogeny.

Testicular hormones modulate circadian rhythms of the diurnal rodent, Octodon degus

Sex differences have been identified in a variety of circadian rhythms, including free-running rhythms, light-induced phase shifts, sleep patterns, hormonal fluctuations, and rates of reentrainment. In the precocial, diurnal rodent Octodon degus, sex differences have been found in length of free-running rhythm (tau), phase response curves, rates of reentrainment, and in the use of social cues to facilitate reentrainment. Although gonadal hormones primarily organize circadian rhythms during early development, adult gonadal hormones have activational properties on various aspects of circadian rhythms in a number of species examined. Gonadectomy of adult female O. degus did not influence tau, phase angle of entrainment, or activity patterns in previous experiments. The present experiment examined the role of gonadal hormones in adult male degus' circadian wheel-running rhythms. We predicted that male gonadal hormones would have an activational effect on some aspects of circadian rhythms, particularly those in which we see sex differences. Phase angles of entrainment, tau, length of the active period (alpha), maximum and mean activity levels, and activity amplitude were examined for intact and castrated males housed in LD 12:12. Responses to light pulses while housed in constant darkness (DD) were also compared. Castration had no significant effect on tau or light-induced phase shifts. However, castration significantly increased phase angle of entrainment and decreased activity levels. The data indicate that adult gonadal steroids are not responsible for the sex differences in endogenous circadian mechanisms of O. degus (tau, PRC), although they influence activity level and phase angle of entrainment. This is most likely due to masking properties of testosterone, similar to the activity-increasing effects of estrogen during estrus in O. degus females.

Effects of intergeniculate leaflet lesions on circadian rhythms in Octodon degus

The intergeniculate leaflet (IGL) modulates photic and nonphotic entrainment of circadian rhythms in nocturnal species, but nothing is known about its role in diurnal species. We investigated the significance of the IGL for circadian rhythm function in the diurnal rodent, Octodon degus, by determining the effects of bilateral electrolytic IGL lesions (IGL(X)) on: (i) photic entrainment; (ii) reentrainment rates to photic cues following a 6-h phase advance of the light-dark (LD) cycle; (iii) reentrainment rates to nonphotic social and photic cues following a 6-h phase advance of the LD cycle; and (iv) the circadian period (tau) of the activity rhythm in constant darkness (DD). IGL(X) significantly lengthened the duration (alpha) of the entrained activity rhythm and produced a significantly earlier phase of activity onset under entrained (LD 12:12) conditions, but did not change phase of activity offset, rhythm amplitude or mean daily activity levels. IGL(X) failed to modify tau of free-running activity rhythms in DD or alter reentrainment rates of circadian rhythms to nonphotic social and photic cues or photic cues alone. Thus, the IGL modulates two parameters of photic entrainment, but is not necessary for reentrainment to either nonphotic social or photic cues. Our results contribute to the growing comparative database on the neural mechanisms controlling circadian rhythms and indicate that the role of the IGL varies across species with no apparent relationship between diurnality-nocturnality and circadian function.

In rats, odor-induced Fos in the olfactory pathways depends on the phase of the circadian clock

We used immunostaining for Fos to study the effect of circadian clock phase on odor-induced neuronal activation in the olfactory system in rats. Brief presentation of cedar odor to rats housed in constant darkness stimulated Fos expression in the main olfactory bulb, anterior olfactory nucleus, piriform cortex, and several other odor-responsive structures, both in the subjective day and subjective night phases of the cycle. Fos expression in response to odor, but not basal expression, was greatly enhanced in the subjective night in all structures examined. These findings are consistent with the idea that odor-induced neuronal activation in the olfactory pathways is modulated by the phase of the circadian clock.

Suprachiasmatic nucleus and intergeniculate leaflet in the diurnal rodent Octodon degus: retinal projections and immunocytochemical characterization

The neural connections and neurotransmitter content of the suprachiasmatic nucleus and intergeniculate leaflet have been characterized thoroughly in only a few mammalian species, primarily nocturnal rodents. Few data are available about the neural circadian timing system in diurnal mammals, particularly those for which the formal characteristics of circadian rhythms have been investigated. This paper describes the circadian timing system in the diurnal rodent Octodon degus, a species that manifests robust circadian responses to photic and non-photic (social) zeitgebers. Specifically, this report details: (i) the distribution of six neurotransmitters commonly found in the suprachiasmatic nucleus and intergeniculate leaflet; (ii) the retinohypothalamic tract; (iii) the geniculohypothalamic tract; and (iv) retinogeniculate projections in O. degus. Using immunocytochemistry, neuropeptide Y-immunoreactive, serotonin-immunoreactive and [Met]enkephalin-immunoreactive fibers and terminals were detected in and around the suprachiasmatic nucleus; vasopressin-immunoreactive cell bodies were found in the dorsomedial and ventral suprachiasmatic nucleus; vasoactive intestinal polypeptide-immunoreactive cell bodies were located in the ventral suprachiasmatic nucleus; [Met]enkephalin-immunoreactive cells were located sparsely throughout the suprachiasmatic nucleus; and substance P-immunoreactive fibers and terminals were detected in the rostral suprachiasmatic nucleus and surrounding the nucleus throughout its rostrocaudal dimension. Neuropeptide Y-immunoreactive and [Met]enkephalin-immunoreactive cells were identified in the intergeniculate leaflet and ventral lateral geniculate nucleus, as were neuropeptide Y-immunoreactive, [Met]enkephalin-immunoreactive, serotonin-immunoreactive and substance P-immunoreactive fibers and terminals. The retinohypothalamic tract innervated both suprachiasmatic nuclei equally; in contrast, retinal innervation to the lateral geniculate nucleus, including the intergeniculate leaflet, was almost exclusively contralateral. Bilateral electrolytic lesions that destroyed the intergeniculate leaflet depleted the suprachiasmatic nucleus of virtually all neuropeptide Y- and [Met]enkephalin-stained fibers and terminals, whereas unilateral lesions reduced fiber and terminal staining by approximately half. Thus, [Met]enkephalin-immunoreactive and neuropeptide Y-immunoreactive cells project equally and bilaterally from the intergeniculate leaflet to the suprachiasmatic nucleus via the geniculohypothalamic tract in degus. This is the first report examining the neural circadian system in a diurnal rodent for which formal circadian properties have been described. The data indicate that the neural organization of the circadian timing system in degus resembles that of the most commonly studied nocturnal rodents, golden hamsters and rats. Armed with such data, one can ascertain differences in the functional organization of the circadian system between diurnal and nocturnal mammals.

Olfactory stimulation enhances light-induced phase shifts in free-running activity rhythms and Fos expression in the suprachiasmatic nucleus

There is evidence to suggest that the olfactory and circadian systems are linked, functionally, and that olfactory stimuli can modulate circadian rhythms in mammals. Furthermore, olfactory bulb removal can alter free-running rhythms in animals housed in constant darkness and can attenuate the effect of social stimuli on photic entrainment of circadian rhythms. The mechanisms through which olfactory stimuli influence circadian rhythms are not known. One possibility is that olfactory stimuli influence circadian rhythms by modulating the activity of the circadian clock located in the hypothalamic suprachiasmatic nucleus. To study this, we assessed the effect of olfactory stimulation on free-running rhythms and on photic resetting of the circadian clock in rats using phase shifts in wheel-running rhythms and expression of the transcription factor Fos in the suprachiasmatic nucleus. We found that brief exposure to an olfactory stimulus, cedar wood essence, in the subjective day or subjective night had no effect on either free-running rhythms or Fos expression in the suprachiasmatic nucleus, but that when presented in combination with light, the odor dramatically enhanced light-induced phase shifts and Fos expression in the suprachiasmatic nucleus. Olfactory stimulation alone induced Fos expression in several structures that innervate the suprachiasmatic nucleus, pointing to ways by which stimulus information transmitted in the olfactory pathways could gain access to the suprachiasmatic nucleus to modulate photic resetting. These findings, showing that clock resetting by light can be facilitated by olfactory stimulation, point to a mechanism by which olfactory cues can modulate entrainment of circadian rhythms.

Social contact synchronizes free-running activity rhythms of diurnal palm squirrels

Social contact with conspecifics entrains rhythms of a number of species, although convincing demonstrations of the phenomenon in diurnal mammals are limited. The present study examined the question of whether social contact mutually synchronizes free-running locomotor activity rhythms of the diurnal Indian palm squirrel, Funambulus pennanti. Twelve male squirrels were housed individually, without visual contact, in two separate laboratories (six in each laboratory). The squirrels were initially held under opposing light-dark (LD) schedules (with an 11 h phase difference), and were then placed under constant bright light (LL). Squirrels from separate laboratories were paired together, and each pair was placed into a fresh cage on the day of the pairing. After 48 days of social contact, the squirrel pairs were separated, and returned to their original positions in the two laboratories in fresh cages. Free-running phase and period were assessed prior to and after the social contact for each squirrel. The phase difference in the free-running rhythms of pairs of squirrels was significantly decreased following social contact. Actogram records revealed strong evidence of social synchronization of free-running rhythms in four of the six pairs. For the remaining two pairs, the data were ambiguous. This study confirmed the findings in other species, that social cues are a potent zeitgeber for F. pennanti.

Removal of the olfactory bulbs delays photic reentrainment of circadian activity rhythms and modifies the reproductive axis in male Octodon degus

The diurnal rodent, Octodon degus, exhibits robust sex differences in several circadian measures, including circadian period (tau) and reentrainment rates to photic and nonphotic (social) zeitgebers. The neural substrates underlying such physiological differences remain unknown. In female degus, olfactory bulbectomies (BX) inhibit socially-facilitated reentrainment, but do not alter photic reentrainment, entrained measures, or tau in constant darkness (DD). This experiment investigated the effects of BX in male degus on (i) photic reentrainment rates of circadian rhythms following a 6-h phase advance of the light-dark (LD) cycle; (ii) photic entrainment; (iii) tau of free-running activity rhythms in DD; and (iv) body weight, paired testis weight, and the reproductive hormones, testosterone, androstenedione and follicle stimulating hormone (FSH). BX significantly delayed photic reentrainment rates. They did not, however, modify tau, the phase of activity onset or offset, amplitude or duration (alpha) of the activity rhythm, mean daily locomotor activity levels, or body weight. FSH, testosterone and androstenedione were unaffected by BX, whereas paired testis weights were significantly greater in BX degus compared with shams. Thus, the olfactory bulbs influence photic reentrainment of circadian rhythms and modestly affect the reproductive axis in male degus. Our results suggest that the olfactory bulbs may be a neural source of observed sex differences in photic reentrainment in degus, and highlight interspecies variation in the olfactory bulbs' effects on entrained and free-running circadian rhythms and on reproduction.