Mesotelencephalic dopamine system and reproductive behavior in the female rat: effects of ventral tegmental 6-hydroxydopamine lesions on maternal and sexual responsiveness

The effects of lesions to the mesolimbic dopamine system on maternal and sexual behaviors in the female rat was assessed. Rat dams that were given ventral tegmental area microinfusions of 6-hydroxydopamine (6-OHDA) during lactation showed a persistent deficit in pup retrieval but were not impaired with respect to nursing, nest building, or maternal aggression. In addition, 6-OHDA-lesioned females failed to respond to amphetamine by showing locomotor hyperactivity. Administration of the dopamine blocker raclopride to neurologically intact dams also inhibited pup retrieval but had no effect on nursing. Females given 6-OHDA during pregnancy appeared completely unresponsive to pups, whereas no maternal deficits were seen in females that received 6-OHDA 8 weeks before parturition. Proceptive (hopping and darting) and receptive (lordosis) components of sexual behavior, assessed after ovariectomy and exogenous steroid hormone treatment, were not affected by mesolimbic 6-OHDA lesions.     

Effects of high fat,protein supplemented diets on maternal behavior in rats

Female albino rats were maintained on either chow, a 25% fat, protein supplemented diet or a 45% fat, protein supplemented diet during breeding, gestation and lactation. Materanal behavior was assessed using observational techniques. Maternal bodyweight of the females on the high fat diets was not different from females on chow. Reproductive success, as indicated by pregnancy and pup survival, was reduced in the females on the 45% fat diet. However, unlike offspring of obese dams, pup growth was enhanced in most of the offspring of the dams on the high fat, protein supplemented diets. The dams on these diets showed more postural nursing, less non-postural nursing, more pup grooming and were observed more frequently with a majority of their pups. These dietary differences in maternal behavior did not appear to be necessarily due to differences in the body weight of the pups.     

Effects of social stress and intrauterine position on sexual phenotype in wild-type house mice (Mus musculus)

Wild-type house mice were used to test the effect of intrauterine position on anogenital distance (AGD) and to verify whether crowding stress would masculinize female pups, developing at all intrauterine positions, as has been demonstrated in CF-1 mice stressed by restraint, heat, and light. Stress of crowding was documented by comparing aggressive behavior, litter birth weights, and plasma corticosterone levels among females in different densities. AGDs were recorded from pups born to females housed from day 12 to 19 of gestation either individually with their mate (nonstressed) or in one of two group-housed densities. Female pups from nonstressed dams positioned between two males in utero (2M females) had longer AGDs than females positioned between two females (0M females). AGDs of males from nonstressed dams did not differ on the basis of intrauterine position. Group-housed pregnant females in the higher of two densities had female pups with longer AGDs than female pups of other dams. However, variance in female pup AGD was no different among dams in different densities. These results extend to the wild house mouse previous findings in albino mice that intrauterine position influences sexual phenotype. In addition, social stress can induce masculinization of female pups in wild mice as physical stress has been shown to do in albinos. This suggests that intrauterine position effects and their modification by crowding stress can potentially influence the dynamics of wild house mouse populations.     

Prenatal stress eliminates differential maternal attention to male offspring in Norway rats

Maternal licking behavior was observed in 20 Long-Evans rat dams on two consecutive days. Stimulus pups were male and female foster pups from dams that were either housed with 5 adult males during the last trimester of pregnancy (stressed) or housed alone (unstressed). Unstressed male pups received significantly more maternal licking than their female siblings, but prenatally stressed males and females received similar levels of maternal licking, comparable to that directed to unstressed females. In a second study, urine collected from prenatally stressed male pups elicited significantly less investigation from dams in a choice test than urine from age-matched unstressed males. It is concluded that the chemosignals which stimulate dams normally to provide more maternal attention to male than female neonates are deficient in prenatally stressed males. The results raise the possibility that differential maternal care may mediate some effects of prenatal stress on behavioral development in males.     

Prenatal stress affects mother-infant interaction in Norway rats

Pregnant Long-Evans rats were stressed by crowding, and subsequent mother-infant interactions were described after cross-fostering. Prenatally stressed pups elicited less maternal licking from unstressed foster dams than controls, and previously stressed dams licked unstressed foster pups less than controls. No other differences in mother-infant interactions were detected. Adult offspring reared by foster dams that were stressed during pregnancy were more active in an open field than controls, but prenatally stressed and unstressed animals reared by control dams did not differ. Thus, stress during pregnancy can alter the maternal behavior of stressed dams, and the differential maternal stimulation can affect adult open-field behavior. Because prenatally stressed pups elicit different maternal care, cross-fostering does not eliminate the possibility that maternal stimulation may mediate some prenatal stress effects.     

Opioid receptor antagonism during early lactation results in the increased duration of nursing bouts

High levels of mu opioid receptor activation during the postpartum period result in the disruption of ongoing maternal behavior. The role of physiological levels of endogenous opioids on the mediation of maternal behavior in postpartum females, however, has not been closely examined. The purpose of the present experiments was to examine the function of endogenous opioids during early and mid-lactation by treating postpartum females with the opioid antagonist naloxone and monitoring their behavioral interactions with pups. Although this treatment did not lead to any qualitative differences in the maternal behaviors measured (pup retrieval and grooming, nest building, grouping of pups, or crouching over pups), there was a quantitative difference in the amount of time the females spent with pups on the nest and actively nursing pups. Naloxone, given either systemically or centrally (intracerebroventricularly), resulted in prolonged nursing and nesting bouts. This effect, however, was only observed during the early lactation time point (postpartum days 5-7). Females tested later in lactation (postpartum days 10-12 or 12-14) did not display the increased nursing or nesting bouts in response to the antagonist. These data indicate that central opioids play a role in the duration of nursing bouts during early lactation.     

Oxytocin antagonism alters rat dams' oral grooming and upright posturing over pups

Studies involving intracerebral administration of antiserum or antagonists have demonstrated that central oxytocin (OT) plays a prominent role in initiating but not maintaining postpartum maternal behavior in rats. There has been little investigation, however, of OT's influence on the levels of maternal behavior exhibited during the maintenance phase. We measured rat dam behavior during the 105-min observation periods preceding and beginning 2 h after intracerebroventricular infusion of the selective OT antagonist (OTA) (1 microg), or normal saline (NS) vehicle (5 microl) on postpartum days 2-3 and 6-7. Compared to NS, OTA significantly decreased pup licking as a proportion of dams' total oral grooming, increased self-grooming, decreased the frequency of elevated upright posture over pups and increased the frequency of lying prone on pups. Quiescent, kyphotic nursing was also significantly lower in OTA-treated dams. Other components of maternal behavior were not significantly affected by OTA or NS treatment. These findings suggest that central OT may shift the focus of the dams' oral grooming from self to pups and may also facilitate elevation of dams' upright posture over pups. Acute stress responses, maternal behavior and central OT receptor binding in adult rats have been linked to the amount of maternal licking and arched back, upright nursing received during infancy. OT activity in dams' brains may influence these developmental outcomes in their offspring by selectively regulating their pup licking and crouching posture.     

Litter deficits of socially stressed and low status hamster dams

The effects of maternal social stress, induced by pairing pregnant females, and social status, measured by agonistic behavior within pairs, on pup development was investigated in hamsters. Using litters from untreated first pregnancies as a control, 40 dams were housed either alone or paired with another dam during their second pregnancy. Within a pair, the dam with more aggressive and fewer defensive responses than the other was considered dominant. Compared to their first unpaired litters, second litters of paired dams were smaller in number. Comparisons among second litters showed that paired dams, especially submissive dams, had fewer pups than unpaired dams. It was concluded that both maternal social stress and low status increase intrauterine mortality.     

Long-term effects of prenatal morphine exposure on maternal behaviors differ from the effects of direct chronic morphine treatment

Previous studies have demonstrated that chronic morphine treatment of pregnant rats alters maternal behavior. Other studies have shown long-term effects of prenatal exposure to morphine, including changes in reproductive behavior in adult females. The present study investigated the effects of prenatal morphine exposure on a variety of maternal behaviors such as nursing, maternal activities, nonmaternal activities, and pup retrieval. Prenatal morphine exposure increased active and decreased passive nursing. There were no differences in maternal activities such as presence in the nest, contact with pups, grooming of pups, and/or manipulation of nest shavings. In the retrieval test, prenatally morphine-exposed mothers were faster in carrying the first pup, retrieving the first pup back to the nest, and returning all pups to the nest than prenatally saline-exposed mothers. Maternal and nonmaternal activities also were affected by the light: dark cycle. All saline- and morphine-exposed mothers nursed more, were more often in the nest, and more often in contact with greater than half of their litter during the light than the dark sessions. On the other hand, nonmaternal activities increased during the dark sessions: Mothers cared for themselves (groomed, ate) more and displayed more rearing and sniffing. Mothers spent more time resting with their eyes closed during the light sessions regardless of prenatal drug exposure. The present study demonstrated that the effect of morphine on maternal behavior is different in adult exposed and prenatally exposed mothers. While direct morphine treatment impaired maternal behaviors, prenatal morphine exposure has the opposite effect.     

Examining maternal influence on OLETF rats' early overweight: Insights from a cross‐fostering study

Obese female Otsuka Long Evans Tokushima Fatty (OLETF) rats display increased nursing time and frequency compared to lean LETO controls, suggesting a maternal contribution to pup preobesity. In previous studies, OLETF pups presented high adiposity, showed greater suckling efficiency, initiative and weight gain from nursing than controls throughout lactation. To further elucidate maternal–infant interactions contributing to pup preobesity, we cross‐fostered pups a day after birth and examined maternal behavior. Nursing frequency decreased in OLETF dams raising LETO pups (OdLp) in the third postnatal week, while LETO dams raising OLETF pups showed no significant changes. Fat % was greater in the milk of OLETF versus LETO dams. OdLp pups showed long‐term body weight (BW) increase, suggesting that maternal environment can induce BW increases even in the absence of a genetic tendency. Additionally, interaction between OLETF dams and pups produces high nursing frequency, exposing the pups to abundant high‐fat milk, thus strengthening their preobese phenotype. © 2009 Wiley Periodicals, Inc. Dev. Psychobiol 51: 358–366, 2009.     

Prenatal stress produces sex differences in nest odor preference

Prenatal stress (PS) and early postnatal environment may alter maternal care. Infant rats learn to identify their mother through the association between maternal care and familiar odors. Female Wistar rats were exposed to restraint stress for 30 min, 4 sessions per day, in the last 7 days of pregnancy. At birth, pups were cross-fostered and assigned to the following groups: prenatal non-stressed mothers raising non-stressed pups (NS:NS), prenatal stressed mothers raising non-stressed pups (S:NS), prenatal non-stressed mothers raising stressed pups (NS:S), prenatal stressed mothers raising stressed pups (S:S). Maternal behaviors were assessed during 6 postpartum days. On postnatal day (PND) 7, the behavior of male and female pups was analyzed in the odor preference test; and noradrenaline (NA) activity in olfactory bulb (OB) was measured. The results showed that restraint stress increased plasma levels of corticosterone on gestational day 15. After parturition, PS reduced maternal care, decreasing licking the pups and increasing frequency outside the nest. Female pups from the NS:S, S:NS, S:S groups and male pups from the S:S group showed no nest odor preference. Thus, at day 7, female pups that were submitted to perinatal interventions showed more impairment in the nest odor preference test than male pups. No changes were detected in the NA activity in the OB. In conclusion, repeated restraint stress during the last week of gestation reduces maternal care and reduces preference for a familiar odor in rat pups in a sex-specific manner.     

Correlation of maternal and pup NK-like activity and TNF responses against cytomegalovirus to pregnancy outcome in inbred guinea pigs

Immunologic causes for poor outcome of pregnancy complicated by primary cytomegalovirus (CMV) infection are only partially understood. Maternal and pup tumor necrosis factor (TNF) and natural killer cell (NK)-like activity associated with primary gestational CMV infection initiated in either the first or third trimester equivalent in the inbred guinea pig model were investigated. Poor pregnancy outcome defined as fetal resorptions, premature delivery, stillbirths, and intrauterine growth retardation occurred with infection at either gestational time. Induction of TNF and NK activity by CMV infection during pregnancy correlated with resorptions in early pregnancy infection and with premature labor in late pregnancy infection. Stillbirths occurred with CMV infection at either time. Regardless of the gestational time of CMV acquisition, poor outcome correlated with higher maternal NK and TNF responses during the first weeks after maternal virus acquisition. Furthermore, CMV infected dams with loss of >/= 50% of conceptus had higher TNF responses than infected dams with < 50% conceptus loss. Likewise, pups born in litters from CMV-infected dams with resorptions and/or premature labor also had enhanced NK activity and TNF response to CMV compared with pups born to dams not having resorptions or premature labor. TNF responses in the delivered pups of infected dams were higher than from pups of uninfected dams regardless of litter outcome, whereas pup NK responses were enhanced only in pups from litters of dams with premature labor or resorptions. Enhanced NK and TNF activity appear to be associated with premature delivery and other poor outcomes of pregnancy.     

Dysfunctional nurturing behavior in rat dams with limited access to nesting material: a clinically relevant model for early-life stress

BACKGROUND: Early-life emotional stress may be associated with affective and cognitive disorders later in life, yet satisfactory animal models for studying the underlying mechanisms are limited. Because maternal presence and behavior critically influence molecular and behavioral stress responses in offspring, we sought to create a model of dysfunctional, fragmented maternal nurturing behavior that would, in turn, provoke chronic early-life stress in the offspring. METHODS: Sprague-Dawley rat dams' nursing and nurturing behaviors were altered by limiting their ability to create satisfactory nests during postpartum days 2-9. Maternal behavior was observed throughout the diurnal cycle, and the frequency and duration of nurturing behaviors were scored. In addition, potential stress and anxiety of the dams were assessed using behavioral, molecular and hormonal measures. RESULTS: Both the quantity and the quality of dams' care of their pups were profoundly influenced by restriction of nesting materials in their cages: licking/grooming activities decreased and the frequency of leaving the pups increased, resulting in fragmented interactions between the dams and pups. The abnormal activity patterns of the dams were accompanied by increased anxiety-like behavior in the open field, but not in the elevated plus maze tests. Additionally, dams' plasma corticosterone levels and adrenal weights were augmented, suggesting chronic stress of these dams. By the end of the limited-nesting, stress-inducing period, hypothalamic corticotropin releasing hormone (CRH) mRNA expression was reduced in the limited-nesting dams, while arginine-vasopressin (AVP) mRNA levels were not significantly affected. CONCLUSION: Limiting dams' ability to construct a nest for their pups leads to an abnormal repertoire of nurturing behaviors, possibly as a result of chronic stress and mild anxiety of the dams. Because the fragmented and aberrant maternal behavior provoked chronic stress in the pups, the limited-nesting paradigm provides a useful tool for studying the mechanisms and consequences of such early-life stress experience in the offspring.     

The influence of natural variations in maternal care on play fighting in the rat

Naturally occurring variations in maternal care in the rat influence the sensitivity of offspring to stress in adulthood. The offspring of mothers that show lower levels of pup licking/grooming (i.e., low-LG mothers) demonstrate enhanced responses to stress and increased anxiety compared to those of high-LG mothers. Low-LG offspring are also more sensitive to the influence of environmental enrichment than high-LG offspring. This study examined play fighting in the juvenile offspring of high-LG and low-LG dams in a multiple-play partners housing environment. Male offspring from low-LG dams demonstrated a significantly higher frequency of pouncing, pinning and aggressive social grooming than did high-LG males and high-LG and low-LG females. Consistent with earlier reports, male pups engaged in more play fighting than did females and maternal care was associated with differences in play fighting but only in males. Lower levels of stimulation in the form of LG from the dam during perinatal development may thus increase sensitivity for the stimulating effects of play behavior in periadolescence, in part explaining the increased solicitation of play fighting through increased pouncing in the male offspring of the low-LG mothers. These findings identify a possible influence of variations in maternal care on play fighting and suggest that maternal care in the perinatal period influence social interactions during periadolescence.     

Prepartal chronic stress increases anxiety and decreases aggression in lactating female mice.

The effects of two types of prepartal chronic stress on maternal behavior, anxiety, and maternal aggression during lactation were assessed in female outbred Swiss mice. Two groups (n = 18 each) were subjected to 10 consecutive daily sessions of restraint stress (RS) or novel environment stress (NES) from Day 4 to Day 14 of pregnancy. A third group (n = 18) was left undisturbed during pregnancy (controls, C group). All females underwent a maternal behavior test (10-min observation after separation and reunion with the pups) on Postpartum Day (PD) 1, an anxiety test (a light-dark conflict test) on PD 6, and a maternal aggression test (5-min exposure to an unfamiliar adult male) on PD 7. NES and RS groups tended to show higher amounts of maternal care. Anxiety increased in the stressed females compared with controls, whereas the opposite was true for maternal aggression. In the RS group, anxiety was negatively correlated with maternal aggression. These results suggest that chronic stress may have produced neuronal and endocrine alterations in the dams, which may have led to increased avoidance of aversive stimuli. The results also support the hypothesis that, in lactating mice, anxiety is inversely related to the probability of displaying intense forms of aggression.     

The effects of protein malnutrition on the behavior of rats during the suckling period.

Observations of 2 groups of dams and their litters were made every 3 hr around the clock on Days 1-20 postpartum. The dams fed either an 8% or a 25% casein diet for 5 weeks before mating and during gestation and lactation. Dams on the low protein diet spent more time in the nest actively nursing their young than did high protein dams, and they exhibited no deficits in other maternal behaviors. Five of 11 kinds of behavior developed more slowly in the undernourished pups than in the well-nourished ones, but the developmental delays were no longer than a few days. Circadian fluctuations were apparent in all of the pups' activities during the 3rd postnatal week as well as in grooming and horizontal movement, 2 behaviors that were present earlier, by 6-10 days of age.     

Perinatal high-fat diet impairs pup retrieval and induces sex-specific changes in ultrasonic vocalization characteristics of rat pups

Rodent pups emit ultrasonic vocalizations (USVs) to solicit maternal behavior, promoting their survival. Conversely, maternal behaviors affect the expression of pup USVs. We previously demonstrated that a maternal diet high in saturated fat (HFD) alters maternal behavior and is associated with early maturation of pups and their stress physiology. Here, we assessed the developmental profiles of pup USVs using quantitative and qualitative measures on postnatal days (PND)7 and 13. Quantitative measures included call counts, duration, and frequency, while qualitative measures examined calls' sonographic structures. HFD offspring lacked the typical decrease in USV numbers with age observed among control offspring. They also had shorter calls at PND7 compared to control and HFD offspring at PND13. HFD female offspring showed a greater number of one-frequency-sweep calls, while male pups showed a greater number of two-frequency-sweep calls compared to control offspring. Concomitantly, HFD dams showed impaired pup retrieval on PND7. The data suggest that fewer USVs of shorter duration in HFD offspring may alter dam solicitation and thus impair maternal pup retrieval. This study highlights the impacts of perinatal HFD exposure on the dyadic reciprocal interaction between dam and pups, which may set the stage for long-lasting effects on offspring physiology and behavior.     

Development of cocaine sensitization before pregnancy affects subsequent maternal retrieval of pups and prefrontal cortical activity during nursing

Pups are a highly rewarding stimulus for early postpartum rats. Our previous work supports this notion by showing that suckling activates the mesocorticolimbic system in mothers. In the present study, we tested whether development of behavioral sensitization to cocaine before pregnancy affects the neural response to pups during the early postpartum days (PD). Virgin rats were repeatedly administered cocaine for 14 days (15 mg kg(-1)) and withdrawn from treatment during breeding and pregnancy. The neural response to suckling was measured at PD 4-8 using blood-oxygen-level-dependent (BOLD) MRI or microdialysis. Our results show that BOLD activation in the medial prefrontal cortex (PFC), septum and auditory cortex was curtailed in cocaine-sensitized dams. No differences between cocaine sensitized and saline control dams were observed in the nucleus accumbens, olfactory structures, or in 48 additional major brain regions that were analyzed. Baseline, but not pup-stimulated, dopamine (DA) levels in the medial PFC were lower in cocaine-sensitized dams than in controls. When tested for maternal behaviors, cocaine-sensitized dams showed significantly faster retrieval of pups without changes in other maternal behaviors such as grouping, crouching and defending the nest. Taken together, the present findings suggest that maternal motivation to retrieve pups was enhanced by repeated cocaine exposure and withdrawal, a result reminiscent of 'cross-sensitization' between the drug and a natural reward. Changes in retrieval behavior in cocaine-sensitized mothers might be associated with a hypo-responsive medial PFC.     

Social play with an unfamiliar group in weanling rats (Rattus norvegicus)

Early signs of aggressive behavior toward an intruder were examined in Sprague-Dawley rats. The interactions between groups of four 24-day-old pups (2 males, 2 females) and an unfamiliar male pup from another litter were observed. Some patterns of social play were asymmetric. Compared to the playmate from the group, the unfamiliar pup was most likely to be underneath the other rats, either during play fighting or by crawling under a playmate. The single pup was most frequently involved in the following behaviors: on top (One animal climbs on top of another animal placing its forepaws on the second animal.), grooming, or crawling under a playmate. The effects of relative birth weight, ambient temperature during the lactation period, relative body weight, and relative body temperature during the observation period were studied. Results show that pups born and raised in a warm environment interacted socially more than pups raised in a colder environment. Pups lighter than the mean body weight of playmates tended to use submissive behaviors more than relatively heavier pups. Relatively heavy pups tended to use play behaviors that might be considered more aggressive during the interaction. The results suggest that under appropriate conditions, early evidence for dominance, i.e., asymmetry can be revealed. © 1997 John Wiley & Sons, Inc. Dev Psychobiol 30: 165–176, 1997     

A cross-fostering analysis of the effects of PCB 77 on the maternal behavior of rats

Polychlorinated biphenyls (PCBs) are environmental contaminants known to cause multiple behavioral and developmental problems in humans and animals. In rats, gestational exposure to the PCB congener 3, 4, 3', 4'-tetrachlorobiphenyl (PCB 77) affects the brain and behavior of the offspring as well as the maternal behavior of the dams. Whether the behavior of dams is affected by direct effects of the contaminant or indirectly by actions of the PCB on the developing offspring is not known. We investigated this question using a cross-fostering paradigm in which pregnant rats were exposed to either oil vehicle or 2 mg/kg of PCB 77 on gestational days 6 through 18, and then raised pups that had been exposed to either oil or PCB 77 during gestation. Maternal behavior was observed on postnatal days 1, 2, 4 and 6. Some of the effects on maternal behavior, including an increase in the frequency of nursing bouts and in the amount of maternal autogrooming, can be ascribed to prenatal exposure of the litters to the PCB. Other behavioral effects, including an increase in time on the nest and in the amount of pup grooming as well as a reduction in high-crouch nursing, appear to be due to both direct effects of the PCB on the dams and effects mediated by changes in the offspring. Our results show that exposure to PCB 77 can have complex effects on the behavioral interactions between the dams and their litters with a potential impact on the development of the offspring.