Circadian and photic-induced expression of Fos protein in the suprachiasmatic nucleus of the rabbit

Maternal responsiveness to suckling is modulated by time post-nursing: A behavioural and c-Fos/oxytocin immunocytochemistry study in rabbits

Doe rabbits nurse once/day for approximately 3 minutes, with circadian periodicity, inside the nest box. The amount of suckling received at each bout regulates this behaviour because reducing the litter size to four kits or less disrupts nursing circadian periodicity and increases suckling bout duration. Additionally, the likelihood that does will nurse kits at a given time of day depends on the time elapsed since the last suckling episode and the litter size nursed then. We hypothesised that the time elapsed since the last nursing would impact the number of c-Fos immunoreactive (IR) cells observed after suckling five kits and also that observed before the next nursing ("no kits"). Suckling significantly increased, relative to "no kits", the number of c-Fos-IR cells in the medial preoptic area and lateral septum but not in the bed nucleus of the stria terminalis (BNST), suprachiasmatic nucleus or ventromedial hypothalamus in does nursing at 18 or 24 hours after the previous bout. No effects of suckling were observed in mothers nursing at 6 hours. Does given kits at 3 hours post last suckling refused to nurse but, in the remaining three groups, all does nursed normally. At "no kits", more c-Fos-IR cells were seen (in all regions except the BNST) in does given kits at 24 hours after the last nursing and killed 1 hour later (ie, 4 hours after lights on) than in those killed earlier. The percentage of oxytocinergic (OT) cells co-expressing c-Fos was not modified by nursing in the paraventricular or supraoptic nuclei but, in the latter, the largest number of total OT-IR cells occurred at 18 and 24 hours post-last nursing. In conclusion, the responsiveness of particular forebrain regions involved in regulating circadian rhythms, lactation, and maternal behaviour is modulated by suckling and time of day.

Behavioral, neuroendocrine and physiological indicators of the circadian biology of male and female rabbits

Adult rabbits show robust circadian rhythms of: nursing, food and water intake, hard faeces excretion, locomotion, body temperature, blood and intraocular pressure, corticosteroid secretion, and sleep. Control of several circadian rhythms involves a light-entrained circadian clock and a food-entrained oscillator. Nursing periodicity, however, relies on a suckling stimulation threshold. Brain structures regulating this activity include the paraventricular nucleus and preoptic area, as determined by lesions and quantification of cFOS- and PER1 clock gene-immunoreactive proteins. Melatonin synthesis in the rabbit pineal gland shows a diurnal rhythm, with highest values at night and lowest ones during the day. In kits the main zeitgeber is milk intake, which synchronizes locomotor activity, body temperature, and corticosterone secretion. Brain regions involved in these effects include the median preoptic nucleus and several olfactory structures. As models for particular human illnesses rabbits have been valuable for studying glaucoma and cardiovascular disease. Circadian variations in intraocular pressure (main risk factor for glaucoma) have been found, with highest values at night, which depend on sympathetic innervation. Rabbits fed a high fat diet develop cholesterol plaques and high blood pressure, as do humans, and such increased fat intake directly modulates cardiovascular homeostasis and circadian patterns, independently of white adipose tissue accumulation. Rabbits have also been useful to investigate the characteristics of sleep across the day and its modulation by infections, cytokines and other endogenous humoral factors. Rabbit circadian biology warrants deeper investigation of the role of the suprachiasmatic nucleus in regulating most behavioral and physiological rhythms described above.

Biostimulation and nursing modify mating-induced c-FOS immunoreactivity in the female rabbit forebrain

Mating in rabbits lasts only 3-5s but profoundly changes the female׳s physiology and behavior (e.g., inhibition of scent-marking and ambulation, changes in EEG, and release of GnRH). The behavioral responsiveness to copulation is reduced in lactating rabbits, relative to estrous does, but is enhanced after suppressing a single nursing bout ("biostimulation"). Little is known about the mechanisms mediating the differential responsiveness to mating among estrous, lactating, and biostimulated rabbits. To begin addressing this issue we quantified the number of c-FOS-immunoreactive (IR) cells in the preoptic area (POA), dorsomedial hypothalamus (DMH), ventromedial hypothalamus (VMH), infundibular nucleus (INF), paraventricular nucleus (PVN), supraoptic nucleus (SON), and lateral septum (LS) in mated and unmated does from the above three reproductive conditions. Mating increased c-FOS-IR cells in the POA and PVN relative to unmated estrous does. Biostimulation increased c-FOS-IR cells in the PVN, relative to lactating does, regardless of mating. Lactation reduced the responsiveness of the LS and INF to copulation but increased it in the DMH. No differences were found in the VMH.

CONCLUSIONS

a) copulation activates forebrain nuclei that regulate scent-marking (POA), ovulation (INF), and post-coital oxytocin release (PVN); b) lactation and suppression of one nursing bout modulate the magnitude of such changes.

Development of retinal projections and response to photic input in the suprachiasmatic nucleus of New Zealand White Rabbits

In rabbit pups, nursing by the mother is the prevailing entraining signal for their circadian rhythms during at least the first two weeks of life. Therefore, they are considered a natural model of food anticipatory activity. However, the photic entrainment of the circadian system in rabbit pups during this developmental stage is not well understood. The present study examined the retinal projections to the suprachiasmatic nucleus (SCN) and the functional responses of the SCN to light exposure. Using the anterograde tracer cholera toxin-B, we examined the retinal projections to the SCN at postnatal days (PD) 1, 9, 19 and in adult animals. The results revealed that the retinal projections were present at PD1 with a bilateral symmetry, and with a contralateral tendency at PD19 and adults. We also explored the response of the SCN to a light pulse by assessing the induction of FOS protein, a marker of neuronal activation, at PD1, 12, 19 and in adults. Light-induced FOS was observed during day and night at PD1, but mainly during night at PD12, 19 and adults. We conclude that in the SCN there is a "gating" mechanism to FOS induction by light that develops several days after birth, as in other mammals, and in the rabbit is already present at PD12. Moreover, in contrast to other altricial mammals, the circadian visual system, although not essential for entraining the rhythm during first two weeks of life, is present and functional in rabbit pups from birth.

The rabbit pup, a natural model of nursing-anticipatory activity

Mother rabbits nurse their young once a day with circadian periodicity. Nursing bouts are brief (ca. 3 min) and occur inside the maternal burrow. Despite this limited contact mother rabbits and their pups are tuned to each other to ensure that the capacities of each party are used efficiently to ensure the weaning of a healthy litter. In this review we present behavioral, metabolic and hormonal correlates of this phenomenon in mother rabbits and their pups. Research is revealing that the circadian rhythm of locomotion shifts in parallel to the timing of nursing in both parties. In pups corticosterone has a circadian rhythm with highest levels at the time of nursing. Other metabolic and hormonal parameters follow an exogenous or endogenous rhythm which is affected by the time of nursing. In the brain, clock genes and their proteins (e.g. Per1) are differentially expressed in specific brain regions (e.g. suprachiasmatic nucleus, paraventricular nucleus) in relation to providing or ingesting milk in mothers and young, respectively. These findings suggest that circadian activities are modulated, in the mothers, by suckling stimulation and, in the young, by the ingestion of milk and/or the perception of the mammary pheromone. In conclusion, the rabbit pup is an extraordinary model for studying the entraining by a single daily food pulse with minimal manipulations. The mother offers the possibility of studying nursing as a non-photic synchronizer, also with minimal manipulation, as suckling stimulation from the litter occurs only once daily.

Nature's food anticipatory experiment: entrainment of locomotor behavior, suprachiasmatic and dorsomedial hypothalamic nuclei by suckling in rabbit pups

In nature and under laboratory conditions, dams nurse rabbit pups once daily for a duration of fewer than 5 min. The present study explored neural mechanisms mediating the timing of nursing in this natural model of food anticipatory activity, focussing on the suprachiasmatic nucleus (SCN), the locus of the master circadian clock and on the dorsomedial hypothalamic nucleus (DMH), a region implicated in timing of food-entrained behavior. Rabbit pups are born in the dark, with eyelids closed. Nursing visits to the litters also occurs during the dark phase. To explore the effect of the timing of feeding, pups were maintained in constant darkness, while females housed in a light-dark cycle were permitted to nurse their pups either during the night (night-fed group) or day (day-fed group). All pups exhibited anticipatory locomotor activity before daily nursing. In the SCN, PER1 and FOS peaked during the night in both groups, with a longer duration of elevated protein expression in the night-fed group. In contrast, DMH peak PER1 expression occurred 8 h after pups were fed, corresponding to the shift in timing of nursing. Comparison of nursed and 48 h fasted pups indicates that the timing of PER1 expression was similar in the SCN and DMH, with fewer PER1-positive cells in the latter group. The results indicate that rabbit pups show food anticipatory activity, and that timing of nursing differentially affects PER1 expression in the SCN and DMH.