Diversity and development of circadian rhythms in the European rabbit

Thermoregulation in periparturient rabbit does and their neonatal kits with different litter sizes during West African winter

The study evaluated the changes in body temperatures and hematological parameters in periparturient rabbit does and the dynamics of body surface temperature in neonatal rabbit kits during West African winter. The variables were measured in 21 rabbits, comprising periparturient (n = 12) and dry (n = 9) does, and after kindling, the kits were grouped based on litter size, into small (4 kits per litter; n = 24) and large (5-7 kits per litter; n = 34) litter. Results revealed that all body temperatures of the does were lower (p < 0.05) during gestation compared with lactation period or dry does in all hours of the day, and the PCV was significantly higher in lactating than dry does. At birth, small litter kits had higher (p < 0.05) body weight and morning body surface temperature than the large litter kits. The large litter kits showed distinct (p < 0.05) diurnal variation in body surface temperature from birth, while diurnal variation in the small litter kits was absent (P > 0.05) in the first three days after birth. It was concluded that pregnant rabbits and large litter kits may have poor thermoregulation during West African winter, and thus, the need for special protective housing to improve productivity.

Circadian rhythms in colonic function

A rhythmic expression of clock genes occurs within the cells of multiple organs and tissues throughout the body, termed "peripheral clocks." Peripheral clocks are subject to entrainment by a multitude of factors, many of which are directly or indirectly controlled by the light-entrainable clock located in the suprachiasmatic nucleus of the hypothalamus. Peripheral clocks occur in the gastrointestinal tract, notably the epithelia whose functions include regulation of absorption, permeability, and secretion of hormones; and in the myenteric plexus, which is the intrinsic neural network principally responsible for the coordination of muscular activity in the gut. This review focuses on the physiological circadian variation of major colonic functions and their entraining mechanisms, including colonic motility, absorption, hormone secretion, permeability, and pain signalling. Pathophysiological states such as irritable bowel syndrome and ulcerative colitis and their interactions with circadian rhythmicity are also described. Finally, the classic circadian hormone melatonin is discussed, which is expressed in the gut in greater quantities than the pineal gland, and whose exogenous use has been of therapeutic interest in treating colonic pathophysiological states, including those exacerbated by chronic circadian disruption.

What Are the Keys to the Adaptive Success of European Wild Rabbit (Oryctolagus cuniculus) in the Iberian Peninsula?

The European wild rabbit (Oryctolagus cuniculus) plays an important ecological role in the ecosystems of the Iberian Peninsula. Recently, rabbit populations have drastically reduced, so the species is now considered endangered. However, in some places, this animal is considered a pest. This is the conservation paradox of the 21st century: the wild rabbit is both an invasive alien and an endangered native species. The authors of this review aimed to understand the keys to the adaptive success of European rabbits, addressing all aspects of their biology in order to provide the keys to the ecological management of this species. Aspects including nutrition, genetics, immunity interactions with the environment, behaviour, and conflict with human activities were reviewed. Ultimately, rabbits are resilient and adaptable. The main adaptations that explain the rabbit's adaptive success are its nutrition (wide adaptation to food and good nutritional use of caecotrophy), immune system (powerful and developed), and other aspects related to genetics and behaviour. Rabbits' relationship with humans has led them to colonise other places where they have become pests. Despite these adaptations, populations in native places have been drastically reduced in recent years. Since it serves as a bastion of the Mediterranean ecosystem, a specific conservation program for this species must be carried out. Therefore, a study of the rabbit's response to diseases and nutrition (especially protein), as well as the interaction between them, is of special interest.

Regulation of the rabbit's once-daily pattern of nursing: a circadian or hourglass-dependent process?

The European rabbit Oryctolagus cuniculus has an unusual pattern of nursing behavior. After giving birth in a nursery burrow (or laboratory nest box), the mother immediately leaves the young and only returns to nurse for a few minutes once approximately every 24 h. It has been assumed this schedule, like a variety of other functions in the rabbit, is under circadian control. This assumption has been largely based on findings from mothers only permitted restricted access to their young once every 24 h. However, in nature and in the laboratory, mothers with free access to young show nursing visits with a periodicity shorter than 24 h, that does not correspond to other behavioral and physiological rhythms entrained to the prevailing 24 h light/dark (LD) cycle. To investigate how this unusual, apparently non-circadian pattern might be regulated, we conducted two experiments using female Dutch-belted rabbits housed individually in cages designed to automatically register feeding activity and nest box visits. In Experiment 1 we recorded the behavior of 17 mothers with free access to their young under five different LD cycles with long photo and short scotoperiods, spanning the limits of entrainment of the rabbit's circadian system. Whereas feeding rhythms were entrained by LD cycles within the rabbit's circadian range of entrainment, nursing visits showed a consistently shorter periodicity regardless of the LD regimen, largely independent of the circadian system. In Experiment 2 we tested further 12 mothers under more conventional LD 16:8 cycles but "trained" by having access to the nest box restricted to 1 h at the same time each day for the first 7 d of nursing. Mothers were then allowed free access either when their young were left in the box (n = 6), or when the litter had been permanently removed (n = 6). Mothers with pups still present returned to nurse them on the following days according to a similarly advancing pattern to the mothers of Experiment 1 despite the previous 7 d of "training" to an experimentally enforced 24 h nursing schedule as commonly used in previous studies of rabbit maternal behavior. Mothers whose pups had been removed entered the box repeatedly several times on the first day of unrestricted access, but on subsequent days did so only rarely, and at times of day apparently unrelated to the previously scheduled access. We conclude that the pattern of the rabbit's once-daily nursing visits has a periodicity largely independent of the circadian system, and that this is reset at each nursing. When nursing fails to occur nest box visits cease abruptly, with mothers making few or no subsequent visits. Together, these findings suggest that the rabbit's once-daily pattern of nursing is regulated by an hourglass-type process with a period less than 24 h that is reset at each nursing, rather than by a circadian oscillator. Such a mechanism might be particularly adaptive for rhythms of short duration that should end abruptly with a sudden change in context such as death or weaning of the young.

Misadjustment of diurnal expression of core temperature and locomotor activity in lactating rabbits associated with maternal over-nutrition before and during pregnancy

Metabolic parameters ranging from circulating nutrient levels and substrate utilization to energy expenditure and thermogenesis are temporally modulated by the circadian timing system. During critical embryonic developmental periods, maternal over-nutrition could alter key elements in different tissues associated with the generation of circadian rhythmicity, compromising normal rhythmicity development. To address this issue, we determine whether maternal over-nutrition leads to alterations in the development of circadian rhythmicity at physiological and behavioral levels in the offspring. For this, female rabbits were fed a standard diet (SD) or high-fat and carbohydrate diet (HFCD) before mating and during gestation. Core body temperature and gross locomotor activity were continuously recorded in newborn rabbits, daily measurements of body weight and the amount of milk ingested was carried out. At the end of lactation, tissue samples, including brown adipose tissue (BAT) and white adipose tissue (WAT), were obtained for determining the expression of uncoupling protein-1 (UCP1) and cell death-inducing DNA fragmentation factor-like effector A (CIDEA) genes. HFCD pups exhibited conspicuous differences in the development of the daily rhythm of temperature and locomotor activity compared to the SD pups, including a significant increase in the daily mean core temperature, changes in the time when temperature or activity remains above the average, shifts in the acrophase, decrease in the duration and intensity of the anticipatory rise previous to nursing, and changes in frequency of the rhythms. HFCD pups exhibited a significant increase in BAT thermogenesis markers, and a decrease of these markers in WAT, indicating more heat generation by brown adipocytes and alterations in the browning process. These results indicate that maternal over-nutrition alters offspring homeostatic and chronostatic regulation at the physiological and behavioral levels. Further studies are needed to determine whether these alterations are associated with the changes in the organization of the circadian system of the progeny.

The Clock Keeps Ticking: Circadian Rhythms of Free-Ranging Polar Bears

Life in the Arctic presents organisms with multiple challenges, including extreme photic conditions, cold temperatures, and annual loss and daily movement of sea ice. Polar bears ( Ursus maritimus) evolved under these unique conditions, where they rely on ice to hunt their main prey, seals. However, very little is known about the dynamics of their daily and seasonal activity patterns. For many organisms, activity is synchronized (entrained) to the earth’s day/night cycle, in part via an endogenous (circadian) timekeeping mechanism. The present study used collar-mounted accelerometer and global positioning system data from 122 female polar bears in the Chukchi and Southern Beaufort Seas collected over an 8-year period to characterize activity patterns over the calendar year and to determine if circadian rhythms are expressed under the constant conditions found in the Arctic. We reveal that the majority of polar bears (80%) exhibited rhythmic activity for the duration of their recordings. Collectively within the rhythmic bear cohort, circadian rhythms were detected during periods of constant daylight (June-August; 24.40 ± 1.39 h, mean ± SD) and constant darkness (23.89 ± 1.72 h). Exclusive of denning periods (November-April), the time of peak activity remained relatively stable (acrophases: ~1200-1400 h) for most of the year, suggesting either entrainment or masking. However, activity patterns shifted during the spring feeding and seal pupping season, as evidenced by an acrophase inversion to ~2400 h in April, followed by highly variable timing of activity across bears in May. Intriguingly, despite the dynamic environmental photoperiodic conditions, unpredictable daily timing of prey availability, and high between-animal variability, the average duration of activity (alpha) remained stable (11.2 ± 2.9 h) for most of the year. Together, these results reveal a high degree of behavioral plasticity in polar bears while also retaining circadian rhythmicity. Whether this degree of plasticity will benefit polar bears faced with a loss of sea ice remains to be determined.

Aggressiveness, Mating Behaviour and Lifespan of Group Housed Rabbit Does

Simple Summary

Aggressiveness is a well-known trait in different animal species, including the European wild and domesticated rabbit. It is one of the main problems in group housing of rabbit does. The aim of the experiment was to investigate the frequency of aggressiveness and mating behaviour as well as the lifespan of does depending on their rank order. Dominance hierarchy developed during aggressive behaviour, i.e., fighting. In most cases the primary position in the rank order was clear. Mating activity was very high after assembling the groups. In addition to male–female mating, some female-female and female-male mounting was also observed, which could lead to pseudo-pregnancy. A second small peak of sexual activity was observed at the end of the hypothetical pseudo-pregnancy. Based on the results it can be concluded that aggressive behaviour is frequent in group housing systems which is contrary to animal welfare. Natural mating is not effective in group-housing systems.

Abstract

Aggressiveness is one of the main problems in group housing of rabbit does. The aim of the experiment was to investigate the frequency of aggressiveness and mating behaviour as well as the lifespan of does depending on group composition. The female and male rabbits were housed in one of the 7.7 m² pens (four females and one male per pen). Based on the ages of female rabbits two homogenous groups (HOM) were formed containing four 17-week-old females and two heterogeneous groups (HET) containing three 17-week-old and one 1-year-old female. Twenty-four-hour video recordings were taken during the first month after assembling the groups, and the aggressive actions (fights) and matings were counted. The lifespan was examined over a 200-day experimental period. On the day after assembling the groups the number of fights among does was high in HET group. The same aggressive behaviour only started a week later in HOM group, and some fights between females and the male were also observed. The daily peaks of aggressiveness were in the morning (after the light on) and in the evening (before and after the lights off). The primary position of females in the hierarchy was clear but sometimes no differences were detected among the subordinate females. The mortality of does was connected with their rank order. The number of matings was very high on the day of assembling the groups and a second small peak was observed at the end of the hypothetical pseudo-pregnancy. In addition to mating between male and females, female–female and female–male mounting was also observed. Despite of the small sample size it seems that aggressive behaviour is frequent in group housing systems, which is contrary to animal welfare. Natural mating is not effective in group-housing system.

Housing Rabbit Does in a Combi System with Removable Walls: Effect on Behaviour and Reproductive Performance

We evaluated the effects of two types of colony cages, in which rabbit does were always in a group (C1), and where they were in combi cages furnished with removable internal walls to allow both individual and grouphousing (C2), in addition to the control group (C: conventional individual cage), on welfare, reproductive performance, and global efficiency. Forty-eight New Zealand White nulliparous rabbit does underwent artificially insemination (AI) and were divided into three groups, and reared in the different systems for about 1 year. The reproductive rhythm provides AIs at weaning (30d). In the C1 system, does were continuously grouped, while in C2, walls were inserted four days before kindling and removed 1week after it (60% of the timesheet in group). Reproductive traits and behaviour were evaluated during the entire year. The behavioural observations were performed around days 7, 36, and 44, corresponding to the inclusion of the does in the maternal cages, the insertion of walls four days before kindling, and the removal of the walls 1week after parturition in the C2 group, respectively. The percentages of does with severe skin injuries and the distribution of the injuries on different parts of body were also registered. Does reared in conventional cages showed the greatest presence of stereotype behaviours, while the C1 group showed the highest (p < 0.05) incidence of aggressiveness after regrouping (attack, dominance features, and lower allo-grooming) in comparison to the C2 group (17% and 22%, in C2 and C1 does, respectively).Individually caged does achieved the best productive performance (sexual receptivity, fertility, kindling rate, and number of kits born alive and at weaning). The C1 group showed the lowest performance (p < 0.05), whereas C2 showed an intermediate one. Does housed in the combi cage (C2) had higher (p < 0.05) receptivity and fertility rates and higher numbers of kits born alive and at weaning (79.2% and 76.2%; 7.95 and 7.20, respectively) than the C1 group, but lower values (p < 0.05) than does that were individually housed.

Maternal behaviour in domestic dogs

Mammalian parental care, in most of the cases, is given by the female, who provides food, warmth, and protection. In domestic dogs, maternal behaviour shown by the dam mainly consists of contact, nursing, grooming/licking, play, punishment, thermoregulation, and motion. Peer-reviewed literature published between 1952 and 2018 was retrieved from CAB Abstracts, PubMed, ISI Web of Knowledge, Scopus and book chapters. Keywords for this search included the following terms: behaviour, bonding, altricial, precocial, offspring, maternal, whelping, nursing, domestic dogs, female dog, aggression, puppies, anogenital licking. In this review, we reported and discussed scientific information about maternal behaviour in domestic bitches, comparing altricial vs precocial species; the importance of the bonding, grooming/licking and nursing, and their impacts on puppies' behaviour; altered maternal behaviours such as aggression, cannibalism, rejection, and also the relation between hormones and maternal care behaviours. We concluded that the level of interactions between the dam and the puppies influences the physiological, cognitive and behavioural development of the litter, and the main hormones in the bitch for inducing maternal care behaviours are estradiol, oxytocin, prolactin and progesterone.

Temporal variations of nucleosides and nucleotides in rabbit milk

Nucleotides and nucleosides have a preeminent role in physiological and biochemical processes for newborns, the major source of these during early development is the breast milk. Different biomolecules exhibit daily fluctuations in maternal milk that could transfer temporal information that synchronize newborn circadian system. As a first approach, we characterized the diurnal profile of nucleotides and nucleosides contained in maternal milk of rabbits during the first week of lactation. It is possible that some nucleosides, such as adenosine, play a relevant role in setting up the emerging circadian rhythmicity, whereas uridine and guanosine could participate in the maintenance of rhythmicity.

Participation of the Olfactory Bulb in Circadian Organization during Early Postnatal Life in Rabbits

Experimental evidence indicates that during pre-visual stages of development in mammals, circadian regulation is still not under the control of the light-entrainable hypothalamic pacemaker, raising the possibility that the circadian rhythmicity that occurs during postnatal development is under the control of peripheral oscillators, such as the main olfactory bulb (MOB). We evaluated the outcome of olfactory bulbectomy on the temporal pattern of core body temperature and gross locomotor activity in newborn rabbits. From postnatal day 1 (P1), pups were randomly assigned to one of the following conditions: intact pups (INT), intact pups fed by enteral gavage (INT+ENT), sham operated pups (SHAM), pups with unilateral lesions of the olfactory bulb (OBx-UNI), and pups with bilateral lesions of the olfactory bulb (OBx-BI). At the beginning of the experiment, from P1-8, the animals in all groups were fed at 11:00, from P9-13 the feeding schedule was delayed 6 h (17:00), and finally, from P14-15 the animals were subjected to fasting conditions. The rabbit pups of the INT, INT+ENT, SHAM and OBx-UNI groups exhibited a clear circadian rhythmicity in body temperature and locomotor activity, with a conspicuous anticipatory rise hours prior to the nursing or feeding schedule, which persisted even during fasting conditions. In addition, phase delays in the nursing or feeding schedule induced a clear phase shift in both parameters. In contrast, the OBx-BI group exhibited atypical rhythmicity in both parameters under entrained conditions that altered the anticipatory component, as well as deficient phase control of both rhythms. The present results demonstrate that the expression of circadian rhythmicity at behavioral and physiological levels during early stages of rabbit development largely depends on the integrity of the main olfactory bulb.

Mothers and offspring: The rabbit as a model system in the study of mammalian maternal behavior and sibling interactions

This article is part of a Special Issue "Parental Care". Jay Rosenblatt effectively promoted research on rabbit maternal behavior through his interaction with colleagues in Mexico. Here we review the activities of pregnant and lactating rabbits (Oryctolagus cuniculus), their neuro-hormonal regulation, and the synchronization of behavior between mother and kits. Changing concentrations of estradiol, progesterone, and prolactin throughout gestation regulate nest-building (digging, straw-carrying, fur-pulling) and prime the mother's brain to respond to the newborn. Nursing is the only mother-young contact throughout lactation. It happens once/day, inside the nest, with ca. 24h periodicity, and lasts around 3min. Periodicity and duration of nursing depend on a threshold of suckling as procedures reducing the amount of nipple stimulation interfere with the temporal aspects of nursing, though not with the doe's maternal motivation. Synchronization between mother and kits, critical for nursing, relies on: a) the production of pheromonal cues which guide the young to the mother's nipples for suckling; b) an endogenous circadian rhythm of anticipatory activity in the young, present since birth. Milk intake entrains the kits' locomotor behavior, corticosterone secretion, and the activity of several brain structures. Sibling interactions within the huddle, largely determined by body mass at birth, are important for: a) maintaining body temperature; b) ensuring normal neuromotor and social development. Suckling maintains nursing behavior past the period of abundant milk production but abrupt and efficient weaning occurs in concurrently pregnant-lactating does by unknown factors.

CONCLUSION

female rabbits have evolved a reproductive strategy largely dissociating maternal care from maternal presence, whose multifactorial regulation warrants future investigations.

Temporal modulation of the canonical clockwork in the suprachiasmatic nucleus and olfactory bulb by the mammary pheromone 2MB2 in pre-visual rabbits

During the early stages of development, the olfactory system plays a vital role in the survival of altricial mammals. One remarkable example is the Oryctolagus cuniculus, whose mother-young interaction greatly depends on the 2-methylbut-2-enal (2MB2) pheromone that triggers nipple search and grasping behaviors. Olfactory stimulation with 2MB2 regulates the expression of the core body temperature and locomotor activity rhythms in rabbit pups, indicating the modulation of the circadian system by this volatile cue. To address this issue, in the present study, we determined the effect of stimulation with pulses of 2MB2 on the molecular circadian clockwork in the suprachiasmatic nucleus (SCN) and in the main olfactory bulb (MOB). For this purpose, 7-day-old rabbits were stimulated with distilled water (CON), with ethyl isobutyrate (ETHYL) or with the pheromone (2MB2) at different times of the cycle, and 1h later, the expression of the activity marker C-FOS and of the clock proteins PER1, CRY1 and BMAL1 was evaluated in the SCN and in the three layers of the MOB. The clock proteins were abundantly expressed in both structures; nevertheless these showed diurnal rhythmicity only in the MOB, confirming that central pacemakers exhibit a heterochronical development of the molecular clockwork. C-FOS expression in the SCN and in the MOB was modulated by exposure to ETHYL and to 2MB2 only when these stimulants were presented at ZT00 and at ZT18. In contrast, the clock proteins were essentially modulated by 2MB2 at ZT00 and at ZT06 in both structures. In addition, the PER1 and CRY1 proteins exhibited differential responses to stimulation in the three layers of the MOB. For the first time, we report a modulatory and time-dependent effect of the mammary pheromone 2MB2 on the expression of the core clock proteins in the SCN and in the MOB in rabbits during pre-visual stages of development.

The influence of the time of day on midazolam pharmacokinetics and pharmacodynamics in rabbits

BACKGROUND

This study evaluates the time-of-day effect on midazolam and 1-OH midazolam pharmacokinetics, and on the sedative pharmacodynamic response in rabbits. Also, circadian fluctuations in rabbits' vital signs, such as the blood pressure, heart rate and body temperature were examined. The water intake was measured in order to confirm the presence of the animals' diurnal activity. The secondary aim involved the comparison of two methods of data analysis: a noncompartmental and a population modeling approach.

METHODS

Twelve rabbits were sedated with intravenous midazolam 0.35 mg/kg at four local times: 09.00, 14.00, 18.00 and 22.00 h. Each rabbit served as its own control by being given a single infusion at the four different times of the day on four separate occasions. The values of the monitored physiological parameters were recorded during the experiment and arterial blood samples were collected for midazolam assay. The pedal withdrawal reflex was used as the measurement of the sedation response. Two and one compartmental models were successfully used to describe midazolam and 1-OH midazolam pharmacokinetics. The categorical pharmacodynamic data were described with a logistic model.

RESULTS AND CONCLUSIONS

We did not find any time-of-day effects for the pharmacokinetic and pharmacodynamics parameters of midazolam. For 1-OH midazolam, statistically significant time-of-day differences in the apparent volume of distribution and clearance were noticed. They corresponded well with the rabbits' water intake. The noncompartmental and model-based parameters were essentially similar. However, more information can be obtained from the population model and this method should be preferred in chronopharmacokinetic and chronopharmacodynamic studies.

In search of a temporal niche: social interactions

Circadian rhythms can be entrained to periodic cues in the environment including the solar day, food resources, and temperature. Work on a variety of organisms has suggested that social interactions within and between species may also influence circadian rhythmicity, but conceptual and technical difficulties relating to animal models, housing environments, rhythm assays, and experimental design have complicated mechanistic investigations in the laboratory. We review these issues and introduce the gregarious Nile grass rat, Arvicanthis niloticus, as a suitable model for research on this problem. Understanding social influences on temporal organization at this supra-organismal, community level is of considerable translational value, as its implications range from conservation biology to human health.

Reducing variation in a rabbit vaccine safety study with particular emphasis on housing conditions and handling

This paper describes the results of a study of the effects of modified housing conditions, conditioning and habituation on humans using a rabbit model for monitoring whole-cell pertussis vaccine (pWCV)-induced adverse effects. The study has been performed with reference to previous vaccine safety studies of pWCV in rabbits in which results were difficult to interpret due to the large variation in experimental outcome, especially in the key parameter deep-body temperature (T(b)). Certain stressful laboratory conditions, as well as procedures involving humans, e.g. blood sampling, inoculation and cage-cleaning, were hypothesized to cause this large variation. The results of this study show that under modified housing conditions rabbits have normal circadian body temperatures. This allowed discrimination of pWCV-induced adverse effects in which handled rabbits tended to show a dose-related increase in temperature after inoculation with little variance, whereas non-handled rabbits did not. Effects of experimental and routine procedures on body temperature were significantly reduced under modified conditions and were within the normal T(b) range. Handled animals reacted less strongly and with less variance to experimental procedures, such as blood sampling, injection and cage-cleaning, than non-handled rabbits. Overall, handling had a positive effect on the behaviour of the animals. Data show that the housing modifications have provided a more robust model for monitoring pWCV adverse effects. Furthermore, conditioning and habituation of rabbits to humans reduce the variation in experimental outcome, which might allow for a reduction in the number of animals used. In addition, this also reduces distress and thus contributes to refining this animal model.

Why do heavy littermates grow better than lighter ones? A study in wild and domestic European rabbits

Birth mass can vary considerably among mammalian littermates. Heavier pups often show higher growth rates than their lighter siblings, which might positively affect fitness-relevant parameters during later life. Such a correlation between birth mass and pre-weaning growth within litters was confirmed by our study of wild-type and domestic European rabbits (Oryctolagus cuniculus) living in a semi-natural environment and under laboratory housing conditions, respectively. Our study indicates that at least two main mechanisms account for this relationship in our study species: heavier pups had a higher milk intake and also showed a more efficient conversion of milk into body mass. Furthermore, our study suggests that the better milk conversion by heavy pups was driven by three synergistic mechanisms: heavier pups had comparatively more huddling partners in the nest, they did not need to perform large amounts of proactive behavior in order to reach and remain in a central position within the litter huddle, and they could maintain a comparatively higher body temperature most probably due to their more favorable surface area to volume ratio. In conclusion, our study of European rabbits provides strong evidence that both under natural conditions and in the laboratory, within-litter differences in birth mass are maintained and may even increase during pups' early postnatal development.

Neuroendocrine-immune correlates of circadian physiology: studies in experimental models of arthritis, ethanol feeding, aging, social isolation, and calorie restriction

Virtually all neuroendocrine and immunological variables investigated in animals and humans display biological periodicity. Circadian rhythmicity is revealed for every hormone in circulation as well as for circulating immune cells, lymphocyte metabolism and transformability, cytokines, receptors, and adhesion molecules. Clock genes, notably the three Period (Per1/Per2/Per3) genes and two Cryptochrome (Cry1/Cry2) genes, are present in immune and endocrine cells and are expressed in a circadian manner in human cells. This review discusses the circadian disruption of hormone release and immune-related mechanisms in several animal models in which circulating cytokines are modified including rat adjuvant arthritis, social isolation in rats and rabbits and alcoholism, the aging process and calorie restriction in rats. In every case the experimental manipulation used perturbed the temporal organization by affecting the shape and amplitude of a rhythm or by modifying the intrinsic oscillatory mechanism itself.

Diurnal pattern of clock gene expression in the hypothalamus of the newborn rabbit

In the European rabbit (Oryctolagus cuniculus) nursing acts as a strong non-photic synchronizer of circadian rhythmicity in the newborn young. Rabbits only nurse for a few minutes once every 24 h and previous studies have shown that the pups, blind at birth, display endogenous circadian rhythms in behavior and physiology entrained by this regular daily event. As a further step toward understanding the neural organization of the rabbit's early circadian system, we investigated the expression of clock genes in the suprachiasmatic nucleus of the hypothalamus (SCN; the principal circadian pacemaker in adult mammals) across the pups' 24-h day. We used 43 pups from seven litters maintained in constant darkness and entrained non-photically by nursing at the same time each day until P7. After nursing on day 7, pups were killed in the dark at 3-h intervals so as to obtain eight groups (n=5-6 pups/group) distributed evenly across the 24 h before the next scheduled nursing. Profiles in the expression of the clock genes Per1, Per2, Cry1 and Bmal1 were determined using in situ hybridization in brain sections through the hypothalamus at the level of the SCN. We report for the first time: 1) that Per1, Per2, Cry1 and Bmal1 are all expressed in the SCN of the newborn rabbit, 2) that the expression of Per1, Per2 and Bmal1 but not Cry1 shows diurnal rhythmicity similar to that in adult mammals, and 3) that the expression of Per1, Per2 and Bmal1 is consistent with the strong entraining effect of nursing found in previous studies. Unexpectedly, and contrasting somewhat to the pattern in the SCN, we also found diurnal rhythmicity in the expression of Cry1 and Bmal1 but not of Per1 in the anterior ventromedial hypothalamic nucleus. Overall, our findings suggest that the SCN is a functional part of the newborn rabbit's circadian system and that it can be entrained by non-photic cues associated with the mother's daily nursing visit.

Rabbit pup response to the mammary pheromone: from automatism to prandial control

Rabbit pups display nipple-search behavior in response to maternal odor cues such as the Mammary Pheromone (MP). Here, we assessed in 3 steps whether the activity of the MP is influenced by prandial and/or rhythmic factors during early development. To address this point, the activity of the MP was tested in 690 pups varying in age (day 0, 2, 5 or 10), prandial state (sucked or un-nursed) and period of their 24-h cycle (-6 to +12 h after the daily sucking). In Experiment 1, the responsiveness of d0 pups was high and stable during the whole day, whether they are nursed or deprived of sucking. The MP activity was thus unaffected by colostrum ingestion. Experiment 2 revealed that some differences appear the next days: whereas the responsiveness of d2 pups was also very high during the 24-h cycle (>94%), it decreased in d5-d10 pups immediately after sucking (<48%), with a rapid recuperation in d5 pups. Experiment 3 showed that this postprandial drop in response to the MP disappeared in d5-d10 pups that missed the daily nursing, suggesting that it resulted from post-ingestive/post-absorptive events rather than from rhythm activity-related factors. Taken together, the results show a progressive change in the circadian dynamics of the MP-induced orocephalic response as a function of age modulated by prandial state. This indicates a transition from an automatic response to the MP to a response regulated by physiological factors associated with satiation.

Circadian rhythms of prolactin secretion in neonatal female rabbits after acute separation from their mothers

Newborn rabbits (Oryctolagus cuniculus) are only nursed for 3-5 min every 24 h and show a circadian increase in activity in anticipation of nursing. The objective of this study was to determine, in neonatal female rabbits after acute separation from the doe for 48 h, the changes in 24-h rhythms of plasma prolactin and median eminence and anterior pituitary concentration of dopamine (DA) and serotonin (5HT). In addition, median eminence concentration of the excitatory amino acid transmitters glutamate (GLU) and aspartate (ASP) and of the inhibitory amino acid transmitters gamma-aminobutyric acid (GABA) and taurine (TAU) was measured. A significant 21% increase of circulating prolactin occurred in isolated pups. In controls pups, plasma prolactin levels showed two peaks, during the first half of the light phase and at the beginning of the scotophase, respectively. In the isolated pups, a phase advance of about 4 h occurred for the two prolactin peaks. Hemicircadian changes of median eminence DA were found in controls, whereas a single daily peak (at 17:00 h) was found in the separated pups. Plasma prolactin and median eminence DA correlated significantly and inversely in the control group only. Pituitary DA content exhibited a single peak in controls and a hemicircadian pattern in isolated pups. Plasma prolactin and pituitary DA correlated significantly in isolated pups only 00000. Pup isolation decreased median eminence 5HT levels, augmented pituitary 5HT levels and disrupted their 24 h rhythmicity. Circulating prolactin correlated inversely with median eminence 5HT and directly with adenohypophysial 5HT only in controls. Isolation of pups generally modified the 24 h pattern of median eminence excitatory and inhibitory amino acid content by causing a prominent decrease at the beginning of the light phase. The results indicate that circadian rhythmicity of prolactin secretory mechanisms in female rabbit pups is significantly affected by pup's isolation from the doe.

Twenty-four hour rhythm of plasma prolactin in female rabbit pups

Lactation in the rabbit is a nocturnal activity, extremely short and regular, that can be a strong synchronizer for the development of circadian rhythmicity in the pups. In the present study, 24-h rhythmicity of plasma prolactin and median eminence and anterior pituitary content of dopamine (DA), serotonin (5HT), gamma-aminobutyric acid (GABA) and taurine were examined in 11 days old female pups kept under 16 h light:8 h dark photoperiods (lights on at 08:00 h). Groups of six to seven female rabbit pups were killed by decapitation at six different time points throughout a 24-h cycle, starting at 09:00 h. Plasma prolactin levels changed significantly throughout the day, showing two peaks, one at first half of rest span (at 13:00 h) and another one at the beginning of the scotophase (at 01:00 h), just preceding doe visit. Median eminence DA content changed in a bimodal way as a function of time of day, displaying two maxima, at the beginning of the rest span and of the activity phase. Median eminence DA and plasma prolactin correlated significantly in an inverse way. Two maxima in median eminence 5HT levels were found, about 4 h in advance to the prolactin peaks. Circulating prolactin correlated inversely with median eminence 5HT content and directly with adenohypophysial 5HT content. Median eminence GABA content reached its maximum at the beginning of the scotophase and correlated significantly with plasma prolactin concentration. A positive correlation between plasma prolactin and adenohypophysial taurine content was observed. These results show that the circadian rhythmicity in prolactin secretory mechanisms in female rabbit pups develops during the early neonatal life.

Circadian variation and intermuscular correlation of rabbit jaw muscle activity

The activation of jaw muscles varies with different tasks and must be coordinated to ensure proper function of the masticatory system. The activation patterns might differ in various muscles or over the time course. In order to evaluate the activation patterns and the intermuscular correlation during normal daily activity the electromyograms (EMG) of the superficial and deep masseter, medial pterygoid and digastric muscles were continuously recorded in rabbits and related to activity levels. Muscle use was assessed as the relative time per hour (duty time) during which predefined levels of the peak-EMG of the day were exceeded. Pearson's correlation of duty times was calculated for 6 muscle pairs at various activity levels. The duty times of the muscles differed significantly at levels exceeding 50% of the peak-EMG. The animals exhibited apparent intraday variations of duty times revealing a circadian covariant pattern of muscle use. These variations, however, were different in each individual animal. The activation of pairs of jaw-closing muscles was more highly correlated than that of pairs consisting of a jaw-closing and a jaw-opening muscle. The mutual dependence of hourly muscle activity among jaw-closers and among jaw-closers and jaw-openers varied with the activity level suggesting that those muscle groups might be independently controlled during non-powerful and powerful motor behaviors.

Effect of rabbit doe-litter separation on 24-hour changes of luteinizing hormone, follicle stimulating hormone and prolactin release in female and male suckling pups

BACKGROUND

The daily pattern of nursing of the rabbit pup by the doe is the most important event in the day for the newborn and is neatly anticipated by them. Such anticipation presumably needs a close correlation with changes in hormones that will allow the pups to develop an appropriate behavior. Although a number of circadian functions have been examined in newborn rabbits, there is no information on 24-h pattern of gonadotropin release or on possible sex-related differences in gonadotropin or prolactin (PRL) release of pups. This study examined the 24-h changes of plasma luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) in 11 days old suckling female and male rabbits left with the mother or after short-term (i.e., 48 h) doe-litter separation.

METHODS

Animals were kept under controlled light-dark cycles (16 h-8 h; lights on at 08:00 h). On day 9 post partum, groups of 6-7 female or male rabbit pups were separated from their mothers starting at 6 different time intervals in the 24 h cycle. Pups were killed 48 h after separation. At each time interval groups of male or female pups that stayed with the mother were killed as controls. Plasma, LH, FSH and PRL levels were measured by specific radioimmunoassays.

RESULTS

In pups kept with their mother plasma FSH and LH maxima occurred at the first and second part of the light phase (at 13:00 and 17:00-21:00 h, respectively) (females) or as two peaks for each of the hormones (at 13:00 and 01:00 h) (males). PRL release was similar in female and male rabbit pups kept with their mother, showing a 24-h pattern with two peaks, at 13:00 and 01:00 h, respectively. Mean 24-h values of gonadotropins and PRL did not differ between sexes. Isolation of pups for 48 h augmented circulating gonadotropin and PRL levels and distorted hormone 24-h pattern to a similar extent in both sexes.

CONCLUSION

Significant sex differences in 24-h changes in LH and FSH, but not in PRL, release occurred in rabbit pups kept with the doe. Separation of newborn pups from their mother augmented circulating gonadotropin and PRL levels and disrupted 24-h rhythmicity of gonadotropin and PRL release similarly in both sexes. The effect of pups' isolation can be attributed either to a modification of the circadian pacemaker or to a masking effect on some of its output overt rhythms.

Effect of litter separation on 24-hour rhythmicity of plasma prolactin, follicle-stimulating hormone and luteinizing hormone levels in lactating rabbit does

BACKGROUND

This work describes the effect of a 48-h litter separation on 24-h patterns of plasma prolactin, FSH and LH concentration in female lactating rabbits kept under a 16:8 light-dark photoperiod (lights on at 0800 h).

METHODS

Groups of 6-7 female lactating rabbits maintained with their litters or separated from them for 48 h were killed by decapitation on day 11 post-partum, at 6 different time points throughout a 24-h cycle, starting at 0900 h. Plasma levels of prolactin, FSH and LH were measured by specific double antibody radio-immunoassays.

RESULTS

Plasma level of prolactin in control and separated does changed in a similar way throughout the day, showing two maxima, at 0500-0900 h and at 1700-2100 h, respectively. Litter separation significantly augmented plasma FSH and LH and disrupted their 24-h rhythmicity.

CONCLUSION

Since previous studies had shown that litter separation for short periods of time augmented sexual receptivity and fertility of the doe, the changes in FSH and LH reported may influence the massive release of gonadotropin releasing hormone, LH and FSH triggered by mating or artificial insemination in litter-separated mothers.