Major role of suckling stimulation for inhibition of estrous behaviors in lactating rabbits: acute and chronic effects

Lactation in rabbits induces anestrus: sexual receptivity and scent-marking (chinning) are reduced despite the brevity of suckling (one daily nursing bout, lasting around 3 min). The mechanisms underlying this effect are unknown but, as chinning, lordosis, and ambulation in an open field are immediately inhibited by the peripheral stimulation received during mating we hypothesized that, across lactation, suckling stimulation would provoke a similar effect. To test this possibility we provided litters of 1, 3, 5, or 10 pups across lactation days 1-15 and quantified chinning and ambulation frequencies, the lordosis quotient, and milk output. Baseline chinning frequency, determined before the daily nursing bout, was low across lactation days 1-15 in does nursing 3, 5 or 10 pups but it increased steadily across days 1-10 in rabbits suckling one pup. Yet, a single young was sufficient to abolish chinning for about 1h, after which this behavior rose again. Suckling litters of all sizes reduced (but did not abolish) ambulation frequency, both chronically (baseline levels declined across days 1-5) and acutely. Sexual receptivity was significantly reduced on lactation day 15 only in does that had nursed 10 pups. Large litter size promoted a larger milk output and a normal duration of nursing episodes. Results support a major role of suckling stimulation for the suppression of estrous behaviors and ambulation through as yet unidentified mechanisms.     

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.     

Kin Recognition and Maternal Care under Restricted Feeding in House Mice (Mus domesticus)

This study tests the hypothesis that female house mice (F1 generation of wild caught Mus domesticus) should preferentially invest in own offspring if confronted with young of different degrees of relatedness. The maternal behaviour of females with litters of 4 own and 4 unrelated alien young (cross-fostered at day 1 of lactation) was analysed during a lactation period of 22 days both under ad libitum and under restricted feeding (food was restricted by 20%). Cross-fostering and restricted feeding had no effect on the amount of time spent nursing until weaning. Under both feeding conditions the females did not differ in their maternal behaviour towards own and alien young: there were no significant differences either in the amount of time spent nursing own versus alien pups or in the time spent licking own versus alien young. Weight gain of own and alien = wild littermates did not differ significantly in mixed litters and was similar both under ad libitum and under restricted feeding. Such indiscriminate behaviour might be adaptive if female house mice prefer to communally nest with a relative and thus improve their inclusive fitness by investing in own and related offspring in a communal nest. Under moderate restricted feeding females could not wean the entire litter but reduced litter size by cannibalizing on average 2.7 pups (75% of the pups were killed when they were 4–8 days old). Females with cross-fostered litters killed as many own as alien young. This suggests that females cannot discriminate between own and unrelated young if cross-fostering takes place at day 1 of lactation. Besides testing kin recognition abilities, the experiments also allow analysis of the weaning strategy of females under food shortage. Under restricted feeding, body weight of the females was significantly lower during middle lactation than under ad libitum feeding. Weaning weight of young in reduced litters under food restriction (9–10 g) did not differ significantly from weaning weight of young in litters of 7–10 young, but was lower than that of young in similar sized litters (litter size 6), under ad libitum feeding. The maternal behaviour of cannibalizing some young under food shortage can be interpreted as a weaning strategy which results in the largest number of offspring that can be raised to a minimal weaning weight of 9–10 g. Such a weaning strategy might represent a favourable trade-off between number and size of young produced.     

Causes and predictors of nest mortality in a European rabbit population

We conducted a study on nest mortality of an individually marked population of European rabbits (Oryctolagus cuniculus L., 1758) living in a field enclosure. Over 4 years, we determined maternities and quantified pup mortality during the nest period of 703 pups born in subterranean breeding burrows. Overall, pup mortality occurred in 42.7% of the litters, whereas 32.4% of all born pups were affected. Mortality was highest during the first few postnatal days. In about 50% of the cases, we managed to quantify different causes of mortality such as malnutrition, flooding, cooling of the pups, infanticide, or predation. The pups’ body mass on postnatal day 1, the thermal environment and the number of litter mates were the most important predictors of nest mortality. Litter mortality risk decreased with increasing soil temperature around the subterranean nests. A comparatively higher average pup body mass lowered the nest mortality risk of a litter, whereas this effect was more pronounced when soil temperatures were low. Furthermore, mortality was lowest in medium-sized litters, most probably due to the balance between the thermal benefits of huddling with litter siblings and the costs of having them due to the lower share of milk obtained by the individual pups in larger litters. In addition, nest mortality depended on characteristics of the mother; mortality was increased in litters of low-ranking females and of mothers with lower body mass. In conclusion, our study highlights multiple causes and the effects of different environmental and social factors on nest mortality of this small mammal.     

Neuroendocrinology of maternal behavior in the rabbit

Rabbit maternal behavior consists of building an underground nest of straw and body hair during late pregnancy and displaying, with circadian periodicity, a single 3-min nursing bout/day across lactation. Estrogen, androgen, progesterone, and prolactin regulate specific aspects of nest-building and promote the onset of maternal responsiveness. However, the maintenance of this behavior relies on stimuli from the litter: by preventing mother/young contact at parturition or during early lactation maternal responsiveness is altered or abolished. The brain areas controlling the expression of nest-building and nursing were investigated by implanting estradiol, locating the distribution of estrogen and prolactin receptors, quantifying the expression of immediate-early genes, and lesioning structures of the olfactory system. These studies revealed that: (a) estrogen receptor-alpha, alpha, present in the preoptic region, may mediate the stimulation of nest-building by estradiol; (b) prolactin binding sites, located mainly in periventricular structures, are more abundant in late pregnancy and early lactation; (c) the number of FOS-immunoreactive neurons increases in the lateral septum, but not in the mediobasal hypothalamus, following nursing; (d) the accessory olfactory bulb tonically inhibits the expression of maternal behavior because its removal promotes maternal responsiveness in virgins, which are otherwise unresponsive to daily pup exposure. In summary, rabbits rely on the same hormonal and extrahormonal factors that stimulate maternal behavior in other mammals, yet the way in which such factors promote elaborate nest-building and the unfailing display of circadian nursing is unique to rabbits and warrants future investigation.     

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. 24 h periodicity, and lasts around 3 min. 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.     

Pup Shoving by Adult Naked Mole-Rats

Adult naked mole‐rats (Heterocephalus glaber) characteristically perform an unusual behavior toward young: they shove small pups frequently and vigorously around the nest. We studied 15 litters in five captive colonies to quantify which adults shove pups, changes in shoving frequencies as pups develop, how external disturbances affect pup‐shoving frequencies, and behavior of juveniles that were not shoved as pups. In all litters and colonies the breeding female shoved pups significantly more often than any other individual. Breeding females also shoved adult colony mates, but at far lower rates than they shoved pups. Breeding males shoved pups about half as often as did breeding females. Together, the parents shoved pups ten times more often than did nonbreeders. Frequencies of pup shoving peaked when pups were 3–4 wk old, roughly coincident with weaning. When colonies were disturbed experimentally, frequencies of pup shoving increased dramatically, whereas rates at which nonbreeding adults were shoved decreased sharply. We separated four newly‐weaned litters and raised half the pups apart from their colony. When these litters were reunited 4–9 wk later, the unshoved (experimental) pups were the same size as the frequently shoved (control) pups, but the unshoved pups were significantly less likely to flee from a disturbance. Shoving of small pups encourages them to flee from danger, and also may enforce weaning.     

Circadian thermoregulation in suckling rabbit pups

The rabbit pup is well suited to track the age-dependent development of periodic thermoregulation during the suckling period. Since the litters are regularly nursed once per day for a total of 3 to 4 min, an exogenous, metabolic, nonphotic periodic variable is supposed to have an impact on the 24-h rhythm of body temperature. The authors monitored the course of core body temperature during the suckling period of 20 pups by means of a transmitter implanted intraperitoneally on day 3 postpartum. The 24-h mean rose from an average of 37.8+/-0.3 degrees C on day 4 of life to 39.5+/-0.2 degrees C at weaning on day 27, for 2 out of 20 pups, and day 28, for 18 out of 20 pups. In constant dim illumination, the pups exhibited a 24-h rhythm even on postnatal day 4, which consolidated around days 5 to 7. The rhythm consisted of a significant anticipatory rise of 0.4 to 0.6 degrees C above the respective 24-h mean commencing 2.5 to 3.5 h prior to nursing. Milk intake was followed by a further increase of temperature for an additional 0.3 to 0.6 degrees C. Then the temperature dropped for 1.2 to 1.5 degrees C within 1 to 3 h and returned to average 3 to 5 h later. During a 48-h fast, the rhythm continued to exist, though in a modified shape: the anticipatory component persisted almost unchanged; a further elevation of temperature, however, did not occur. Thus, the anticipatory component apparently is generated endogenously and the second surge represents an exogenous suckling-induced, thermogenic peak. When maternal nursing was advanced for 15 min/day for a total of 5 h, the temperature rhythm of the pups followed the shift of the zeitgeber in parallel. These data confirm the assumption that a circadian rhythm exists during the first postnatal days of the rabbit and that this rhythm is entrained by the 24-h nursing rhythm. The authors suggest that the biological significance of a feeding entrainable oscillator (FEO) in the rabbit might be to activate the pups prior to the periodic nursing visit of the rabbit doe. Thus, the pups are prepared to quantitatively use the one and only short nursing episode per day for maximal milk ingestion.     

Mimicking Natural Nursing Conditions Promotes Early Pup Survival in Domestic Rabbits

In the wild rabbit, Oryctolagus cuniculus, mother–young relationships are based on restricted, once-per-day nursing interactions. Correspondingly, pups have evolved an efficient strategy of energy saving. Here we investigate under breeding conditions, whether matching or not, the once-daily nursing visit by the rabbit females has an effect on pup survival and growth. Two nursing regimen were applied to 89 primiparous (P) and to 78 multiparous (M) does: (a) one that matched the once daily nursing pattern (closed nest-box during the whole day except for a few minutes devoted to nursing) and (b) one that did not match it (24 h free nest access). In P females, the controlled nest access resulted in lower mortality between birth and weaning (8.1%) as compared to the free nest-access (18%). This effect was recorded from postnatal d 3–4 onwards. Both treatments induced different death causes (starvation (63%) in controlled-access regimen, and wounds and nest-soiling (29%) in free-access regimen). While both experimental nest-access regimens differentially affected pup survival in P or M females, they were without influence on pup growth rate in does of either parity. It is concluded that repeated nest visits by the female increase risks of injury to pups, and of out-of-time pup activation or sucking, and that, more generally, it plays against the ethophysiologigal strategy of biomass conservation evolved by rabbit newborns. The fact that the nest-access regimen no longer affected pup survival from the second parity suggests that the behaviour of multiparous does more adequately models the offspring demands.     

Patterns of maternal behavior of rats genetically selected for opposite coping styles

Maternal behavior of Koltushi High- and Low-avoidance (KHA and KLA) rats strains was assessed over the prewealing period (days 6-21). Ten litters of each strain were observed during the light phase of the light/dark cycle. In a series of experiments, rat pups were taken from the maternal nest and placed into the opposite corner of the cage. The following parameters of the maternal behavior were recorded: the latency of the first contact with the pups, pup licking, latency of carrying/retrieval of the first pup back to the nest, time of returning to the nest of the whole litter, and mother's spontaneous behavior (grooming and locomotion time) over the course of 10 min of observation. KLA mothers stayed with their pups and took care of them more frequently than KHA mothers during the light phase of the circadian cycle. Time of self-grooming was significantly higher in KHA rats. The highest levels of self-grooming of mothers was registered on the first day of testing. The latency of the first coming to pups after their removal from the nest was lower in KHA rats, but they needed more time to returned all pups to the nest. The experimental evidence suggests that the KHA but not KLA rats with the active coping style and higher stress reactivity display disorders in maternal behavior in a novel situation.     

The role of the young in the control of the hormonal events during lactation and behavioral weaning in the golden hamster

This study was designed to determine the relationship between the behavioral and physiological changes occurring in the hamster over the course of lactation. Experimental (E) females were given litters of six 3-day-old pups on Day 13 of lactation and allowed to raise them. Control (C) females raised their own litters of six pups. Groups of E and C females were decapitated at 1000 and 1700 hr when their pups were either 8, 18, or 28 days old. Nursing was observed for 1 hr on the 3 days prior to autopsy. Nursing behavior disappeared by Day 28 in C females. E females exhibited nursing behavior at levels equal to those observed in C females until E pups were 28 days old and 38 days had elapsed since parturition. Despite the fact that E mothers continued to nurse pups on day 18 postpartum when pups were 8 days old, E pups showed lower growth rates than did C pups. Prolactin (PRL) levels remained elevated when E pups were 8 days old even though E pups did not grow normally. PRL levels decreased over time in both C and E females and reached baseline by Day 28 although nursing behavior was still elevated in E females. Thus, nursing behavior did not stimulate PRL release during late lactation. Estradiol (E₂) levels in C females remained at baseline until Day 28 when levels increased. LH, FSH, and Progesterone (P) levels in C females showed a dramatic diurnal pattern which disappeared by Day 28, when levels dropped. Levels of E₂, P, LH, and FSH in E females closely paralleled those of C females only when groups were compared with respect to pup age. Thus, behavioral weaning and the return to estrous cyclicity appear to be dependent upon the age of the pups rather than the time elapsed since parturition. However, milk production and PRL release appear to be more closely tied to the number of days postpartum and can be dissociated from the amount of nursing behavior observed.     

Metabolic correlates of the circadian pattern of suckling-associated arousal in young rabbits

Young rabbits (Oryctolagus cuniculus) are only nursed for 3–5 min every 24 h. They show a circadian increase in activity in anticipation of this, which is entrained by suckling. Our aim was to determine whether serum and liver metabolites show diurnal fluctuations which could act to regulate this circadian pattern. Stomach weight, liver glycogen and serum metabolites were measured every 3 h in 7- to 8-day-old pups when normally nursed (up to 24 h after suckling) and fasted (up to 48 h after suckling). The results suggest: Accepted: 9 October 1999     

Circadian temperature rhythms in rabbit pups and in their does.

Circadian rhythms of mammals are generated endogenously, the master oscillator system residing in the suprachiasmatic nuclei (SCN). Previous experiments have indicated that the rabbit has a feeding entrainable circadian oscillator (FEO) which is supposed to be of greatest importance during the early infancy of the rabbit. Here we report the course of telemetrically monitored core body temperature of rabbit pups and of their does. Temperature increased from 37.6+/-0.3 degrees C on day 2 to 39.5+/-0.1 degrees C on day 28 of life. The pups showed a 24 h temperature rhythm even during their first days of life. Temperature increased 2 1/2-3 h prior to nursing for 0.4-0.8 degrees C and rose for an additional 0.4-0.6 degrees C immediately after milk ingestion. The anticipatory, but not the postprandial component persisted when nursing was skipped twice. The persistence of a rhythm in the absence of any entraining agent is crucial for its endogenous generation. In the doe, the core body temperature gradually decreased during the last 2/3 of pregnancy. During parturition it steeply rose for 1.5-1.7 degrees C and attained a plateau of 39.7+/-0.2 degrees C during lactation. The circadian rhythm persisted during the whole course of pregnancy and lactation. Thus, in the rabbit an endogenous, feeding entrainable circadian oscillator appears to operate from the first days of life. It is of functional significance in that it alerts the pup in time so that it is able to utilize the singular short presence of the doe for maximal milk intake.     

Interaction of late pregnancy and lactation in rats.

The effect of pregnancy on lactation was studied during the third week of lactational pregnancy in postpartum pregnant rats with a delay in implantation of only 1 day (1d-LP rats). In an experimental design in which the suckling litter was prevented from consuming solid food, lactational performance was estimated by weighing the ten-pup suckling litters on days 16-21 of lactation or by measuring maternal weight loss after a nursing spell on day 21. In 1d-LP rats, food consumption as well as lactational performance was lower than it was in nonpregnant lactating rats (L rats) and pregnant-lactating rats with a normal long delay of implantation of at least 6 days (LP rats). The time spent by the pups sucking at the nipples was not different among the three groups, but the number of milk ejections was diminished in 1d-LP dams. Restriction of daily food supply during days 16 to 21 of lactation diminished lactational performance more strongly in 1d-LP rats than it did in L rats; 1d-LP rats conserved protein stores and mobilized fewer minerals than did L rats. The weight and composition of the litter in vitro were not affected by the food restriction. In pregnant-lactating rats (LP and 1d-LP rats), the number of early resorptions was increased in comparison with pregnant rats, showing that lactation can affect the earlier stages of pregnancy. It was concluded that late pregnancy does not affect nursing behaviour, but suppresses lactation by restricting maternal food intake and mobilization of maternal stores. Measurements in serum indicate a causative role for oestradiol, but not for leptin.     

Lactating Females Do Not Discriminate Between Their Own Young and Unrelated Pups in the Communally Breeding Rodent, Octodon degus

Females in numerous rodent species engage in communal nesting and breeding, in which they share one or more nests to rear their young. A potential cost of communal nesting and breeding is that mothers divert resources to unrelated offspring. One way mothers could avoid this cost is to recognize and favour their own young over unrelated offspring when allocating maternal effort. We assessed whether female degus (Octodon degus), a communally nesting and breeding caviomorph rodent, discriminate between their own and unrelated offspring during lactation. Female degus previously have been shown to distinguish between their own and unrelated pups when exposed to odours from both. We measured pup discrimination based on differences in the retrieval behaviour of females that were in early or intermediate lactation directed towards their own and unrelated offspring; offspring presented were of similar or different age. Before any event of pup retrieval, lactating females spent similar amounts of time and interacted to a similar extent with their own and unrelated pups. During pup retrieval, all lactating females transported both pups to the nest. Neither relatedness to pups, nor pup‐age differences, influenced the order in which pups were retrieved to the nest. Dams waited similar amounts of time before retrieving the first pup when the first transported young was their own or unrelated. Likewise, females waited similar amounts of time before retrieving the second pup when the pup transported first was their own or unrelated. The time between first and second pup transport events was longer when dams were in early when compared with intermediate lactation, but only when pups were of similar age. All experimental subjects nursed unrelated pups after they were retrieved. Collectively, our results do not support the hypothesis that communally breeding female degus use their recognition ability to discriminate against unrelated offspring in favour of their own young.     

Effects of the intensity of the suckling stimulus and ovarian steroids on pituitary gonadotropin-releasing hormone receptors during lactation

These studies examined whether the decrease in pituitary responsiveness to gonadotropin-releasing hormone (GnRH) observed during lactation in the rat results from a change in pituitary GnRH receptors. GnRH binding capacity was determined by saturation analysis using D-Ala6 as both ligand and tracer. During the estrous cycle, the number of GnRH binding sites increased from 199 +/- 38 fmol/mg protein on estrus to 527 +/- 31 fmol/mg protein on the morning of proestrus, whereas there was no change in receptor affinity (Ka, 6-10 X 10(9) M-1), During lactation, females nursing 8 pups on Days 5 or 10 postpartum had 50% fewer GnRH receptors (109-120 fmol/mg protein) than observed during estrus or diestrus 1 (199-242 fmol/mg protein) although receptor affinity was similar among all the groups. No deficits in pituitary GnRH receptors were observed in females nursing 2 pups on Day 10 postpartum. Removal of the 8-pup suckling stimulus for 24 or 48 h resulted in a dramatic increase in GnRH receptor capacity by 24 h from 120 +/- 16 to 355 +/- 39 fmol/mg protein. The rise in GnRH receptors after pup removal was accompanied by an increase in serum luteinizing hormone (LH) and estradiol concentrations. To assess the role of ovarian steroids in determining GnRH receptor capacity during lactation, females were ovariectomized (OVX) on Day 2 postpartum. Suckling of a large litter (8 pups) completely blocked the postcastration rise in serum LH and in pituitary GnRH receptors on Day 10 postpartum (OVX+ 8, 77 +/- 12 fmol/mg protein; OVX+ 0, 442 +/- 38 fmol/mg protein).(ABSTRACT TRUNCATED AT 250 WORDS)     

The response of plasma prolactin to suckling during normal and prolonged lactation in the rat

In dams which had been kept isolated from pups for 8-10 h, the magnitude of the suckling-induced prolactin rise in the plasma was studied in relation to intensity of suckling stimulus and lactational age of the mother. At midlactation the response of prolactin evoked by suckling was enhanced as litter size increased. Suckling of 2 pups induced a greater prolactin rise in dams adjusted to 2 pups than in dams adjusted to 8 pups. Suckling of 8 pups caused a greater prolactin rise in dams which had been adjusted to an 8-pup litter, than in rats with a 2-pup litter. At late and prolonged lactation the rise of prolactin in the plasma induced by the suckling stimulus of 8 pups was significantly lower than at midlactation. Injection of perphenazine after a period of suckling induced a moderate increase of plasma prolactin in dams at midlactation, and a similar increase in dams at late lactation and at day 42 of lactation. It is concluded that in the first half of lactation the number of pups, i.e. the intensity of the suckling stimulus, is an important factor in determining the magnitude of the prolactin response to suckling. The lower response of plasma prolactin to suckling in late lactation is neither caused by a decrease in suckling stimulus from the pups nor by an increase in prolactin clearance; it is probably due to a gradual reduction in prolactin synthesizing and releasing capacity of the pituitary, brought on by a desensitization of the neural or neuroendocrine system to suckling stimuli as lactation proceeds.     

Nipple-search performance by rabbit pups: Changes with age and time of day

Developmental changes in the nipple-search behaviour of rabbit pups (Oryctolagus cuniculus) were investigated from day 1 to day 13. The performance of four litters was tested 1 h before and 6 h after the once-daily nursing, and of two litters 6 h before and 1 h after nursing on lactating does restrained on their back. During the first 5 days, the median times taken by pups to attach to nipples decreased from 11.8 s to 3.2 s. This improvement was mainly due to a decrease in the latency to search and suggested an improvement in the ability to react to the nipple-search pheromone. Except for the first day, pups located nipples faster before than after nursing, and were best 1 h before. After the first week, latencies to search remained minimal at this time, but increased again at the other times, and were longest on day 13 at 1 h after nursing. Performance changes with time of day corresponded to daily changes in activity, and, after the first week, suggested a further development of underlying motivational mechanisms.     

Thermal versus tactile stimuli and audible vocalization in rat pups

The influences of thermal and tactile stimuli on the audible vocalization in pup rats,Rattus norvegicus were investigated. Mother rats and their litters were exposed to ambient temperature (AT) depression by 5°C for 2 hours every other day from 3 to 11 days postpartum. Nearly all audible calls emitted by pups were recorded during tactile contact with the mother. AT depression alone neither prolonged the total duration of pup-licking by mother nor intensified audible vocalization. It was found, however, that the frequency (No./h.) of nest exiting by the dam doubled during the cooling and that the sum of audible peeps emitted by cooled pups in 4 cooling days was higher than sum of peeps emitted by the control pups. This difference is apparently related to an increase in the mother's motor activity, especially nest-building activity, during cooling. Analysis of circumstances accompanying peep emission supports the hypothesis that the tactile stimuli delivered by mother play the predominant role in causing audible vocalization in pup rats. This investigation was supported by Project CPBP-04.01. of the Polish Academy of Science     

Endocrine profiles during doe-litter separation and the subsequent pregnancy in rabbits

This study was carried out to determine the effects of a transient doe-litter separation on plasma prolactin, LH, FSH, estradiol-17β and progesterone concentrations before artificial insemination and during the subsequent pregnancy. Control does (n=12) had free access to nursing, whereas separated does (n=12) were kept away from their litters for 48 hours before artificial insemination. Both groups were inseminated on day 11 after parturition. Teat stimulation by suckling caused a high increase in prolactin concentrations in separated does (p<0.0001). Basal prolactin concentrations were observed in both groups on days 8 and 18 of pregnancy. No effect of the treatment was detected on LH and FSH concentrations during the sampling period. A rise of estradiol-17β concentrations was observed 48 hours after doe-litter separation, compared to control does and to previous values (p<0.003). Both groups showed low progesterone concentrations before artificial insemination. Pregnant rabbits in both groups showed increased progesterone concentrations on days 8 and 18 of pregnancy. Lower estradiol-17β concentrations were observed in control does on day 18 of pregnancy compared with separated rabbits (p<0.003). The results suggest that a transient separation of nursing does from their litters before artificial insemination may promote high follicular steroidogenesis activity leading to increased estradiol-l7β concentrations. This hormonal change could be a result of several stimulatory actions probably triggered by the absence of suckling episodes and may affect the luteotrophic function during the subsequent pregnancy.