The medial preoptic area is necessary for motivated choice of pup- over cocaine-associated environments by early postpartum rats

Converging evidence suggests that the motivation to seek cocaine during the postpartum period is significantly impacted by the competing incentives of offspring, a stimulus unique to this life stage. In the present study, the functional role of the medial preoptic area (mPOA), a critical site involved in maternal responsiveness, on processing incentive value of pup-associated cues and influencing response allocation for pup- over cocaine-associated environments was investigated using a concurrent pup/cocaine choice conditioned place preference (CPP) paradigm. Early postpartum females with bilateral guide cannulae aimed into the mPOA or into anatomical control sites were conditioned, from postpartum days (PPD) 4 to 7, to associate different uniquely featured environments with pups or cocaine. CPP was tested on PPD8 following intra-mPOA infusions of either 2% bupivacaine or saline vehicle. In two additional experiments, the effects of intra-mPOA infusions of bupivacaine on expression of conditioned responding induced by environments associated with either pups or cocaine were examined separately. Transient inactivation of the mPOA selectively blocked the conditioned preferences for pup-associated environments, significantly contrasting the robust pup-CPP found in non-surgical and intra-mPOA vehicle-treated females. In contrast, mPOA inactivation failed to alter cocaine-CPP in postpartum females. When given a choice between environments associated with pups or cocaine, transient functional inactivation of the mPOA altered choice behavior, biasing the preference of females toward cocaine-associated environments, such that almost all preferred cocaine- and none the pup-associated option. The anatomical specificity was revealed when inactivation of adjacent regions to the mPOA did not affect CPP responses for pups. The findings support a critical role for the mPOA in mediating pup-seeking behavior, and further suggest that the competing properties of pups over alternative incentives, including drugs of abuse, rely on mPOA integrity to provide relevant pup-related information to the circuitry underlying the choice behavior between pups and alternative stimuli.     

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.     

Failure of pituitary transplants to facilitate the onset of maternal behavior in ovariectomized virgin rats

Ovariectomized virgin female rats received two pituitary glands grafted under a kidney capsule in an attempt to assess the effect of chronically increased plasma levels of prolactin on the onset of maternal behavior. When exposed to fresh foster pups on 6 consecutive days, only 5 out of 12 of these females and 2 out of 12 females in an ovariectomized control group displayed maternal behavior, whereas all 4 rats in an additional control group of lactating mothers were maternal throughout these tests. Ovariectomized animals which failed to become maternal then received subcutaneous injections of either 10 μg estradiol benzoate (EB) or oil on two consecutive days, and 1 day thereafter foster pups were again presented to these females for 18 hr. Significantly more EB-treated females displayed maternal behavior, regardless of whether or not they bore pituitary grafts, suggesting that estradiol and not prolactin may be essential for the rapid onset of maternal behavior in parturient rats.     

Experience with pups sustains maternal responding in postpartum rats

In these studies, we investigated the sustaining of postpartum maternal responsiveness through pup experience. In the first study, females were tested for maternal behavior at various times after pregnancy termination by Caesarean section (CS) at term. The females remained rapidly responsive to young for 7 days after pregnancy termination, but were no more responsive than virgins after 10 days. In the second study, females were allowed varying times of complete access to pups, beginning 24 hours after CS, and were tested for maternal behavior 10 days later. Females who retrieved and cared for pups for 30 minutes during exposure, remained rapidly maternal 10 days after CS; those who were allowed only 15 minutes of caring for pups did not. In the third study, females received 24 hours of exposure to distal sensory stimuli from pups, beginning 24 hours after CS. The proportion of females who became maternal within 24 hours of the beginning of maternal testing, 10 days after CS, was greater than that among females who received no exposure to pups.     

Changes in responsiveness to newborn pups in pregnant,nulliparous golden hamsters

Virgin female hamsters were mated and tested once daily for maternal retrieving behavior beginning on days 0, 5, 9, 13, 15, of the 16 day gestation period to determine if responsiveness toward newborn pups changes as pregnancy proceeds. Upon initial exposure to 3 newborn pups, only a small percentage of early-to-mid-pregnant females exhibited maternal retrieving behavior spontaneously. In contrast, over half of the 15 day pregnant females displayed retrieving during the first test. Despite the high frequency of initial pup-directed aggression and cannibalism, maternal retrieval was induced in the majority of the females in all groups by repeated daily exposure to 3 newborn pups. However, no significant differences were observed in the number of pup exposure periods required to induce maternal retrieving in 0, 5, and 9 day pregnant females. It is concluded that the high level of maternal responsiveness observed in the parturient hamster develops somewhat abruptly during late pregnancy. In this respect, the pattern observed in the hamster differs from the more gradual increase in maternal responsiveness reported in mid-to-late-pregnant mice and rats.     

Effects of mother-litter separation on later maternal responsiveness in the hamster

Primiparous hamsters were permitted either 1,24, or 48 hr of contact with their own litters beginning at parturition followed by varying periods of separation from pups. A different subgroup in each of these 3 conditions was tested for maternal behavior each day starting 24 hr after litter removal. Testing for the display of retrieving and the adoption of the nursing posture using 1–2 day old foster pups continued for each condition until 2 subgroups failed to behave maternally on 2 consecutive days. Nulliparous females were similarly exposed to foster pups and tested at selected intervals; regardless of initial exposure, only 4 of 24 nulliparous animals ultimately displayed maternal behavior. Different primiparous subgroups given 1 hr of exposure were maternal on Days 2–4 postpartum but not on Days 5 or 6 while females in the 24 hr group were maternal on Days 3–5 but not on Days 7 or 8. Groups permitted 48 hr of litter stimulation were maternal through Day 15 postpartum demonstrating that the initial 24–48 hr of pup contact is essential for the development of a long-term mother-young bond.     

Sensory regulation of maternal behavior in rats: Effects of pup age

Maternal behavior (retrieving, crouching, and licking) was induced in Sprague-Dawley virgin female rats by constant exposure to pups aged 1-2, 3-4, 5-6, 7-8, 9-10, 11-12, 13-14, or 15-16 days. The incidence of spontaneous components of maternal behavior, notably retrieving, was geater towards pups 1-8 days of age than towards older pups, whereas the occurrence of cannibalism did not differ as a function of pup age. With pups 1-2 through 13-14 days, the latency to onset of full maternal behavior was shortest with 1-2-day-old pups (2-day median) and longest with 13-14-day-old pups (7-day median). Females exposed to pups aged 3-4 through 11-12 days did not differ significantly in their latencies, the medians of which ranged from 4.0 to 5.5 days. Only 1 female out of 8 exposed to pups aged 15-16 days became fully maternal, but 5 more displayed components of maternal responsiveness. The optimal nature of neonates and the general attractiveness of a wider range of pup ages as stimuli for the elicitation of maternal behavior in rats, as well as comparisons to mice and hamsters, were discussed.     

Disruption of maternal behavior in rats with lesions of the septal area

Rats given septal lesions prior to mating did not build nests or nurse their pups. During retrieving tests they carried pups about the cage repeatedly and dropped them in scattered locations. Virgin rats induced to be maternal by housing with foster pups showed similar, but less marked, behavioral deficits. Rats administered septal lesions after parturition became hyperresponsive, ceased all maternal behaviors, and cannibalized their pups. The results indicated that the aberrant maternal behaviors shown by rats with septal lesions did not result from alterations in hormonal status. It was concluded that the lesions disrupted the typical pattern or sequence of maternal activities and that the deficits were probably related to an enhancement of response perseveration.     

Neonatal gonadal hormones: Effect on maternal and sexual behavior in the female rat

It has been found that significant differences exist between male and female rats of the Long-Evans strain on maternal behavior when the animals were presented with rat pups for seven consecutive days. The presence of ovarian or testicular products at the time of maternal testing did not have a facilitating or suppressing effect on maternal behavior in the Long-Evans rat. Females given 100 μg or 1 mg of testosterone propionate (TP) four days after birth and gonadectomized at 25 days of age behaved like control females (oil injected four days after birth and gonadectomized at 25 days of age) on measures of maternal behavior, but showed significantly lower sexual receptivity when primed with estrogen and progesterone. Thus the female sexual behavior system was suppressed by neonatal TP, but the maternal mediating system was not suppressed by the same treatment. It is concluded that the critical periods of differentiation may be different for sex and maternal behavior.     

Morphine treatment and reproductive condition alter olfactory preferences for pup and adult male odors in female rats

Administration of morphine sulfate (MS) to pregnancy-terminated and postpartum lactating female rats disrupts both maternal behavior and postpartum aggression. Since the display of these behaviors may be heavily dependent on olfactory cues provided by the stimulus animals (rat pups and adult male rats, respectively), we examined whether MS was affecting the perception of the olfactory stimuli, and whether olfactory perception was modified by reproductive condition. In Experiment 1, lactating rats had their pups removed and were injected with MS (5.0 mg/kg, sc.) or saline. 60 min later they were placed into a two-choice apparatus, one side of which contained bedding soiled by neonates and the other clean bedding. Time spent on each side was recorded for a total of 5 min (300 s; chance = 150 s). Saline-treated mothers spent significantly more time on the pup-odor side, whereas MS-treated females spent significantly less. In Experiment 2, lactating females were treated with MS or saline and exposed to male odors (soiled bedding). MS significantly increased time spent on the side with male odors; when treated with saline, time spent was significantly reduced. Thus, in lactating rats. MS creates an aversion for pup odors while reducing the female's normal aversion toward male odors. In Experiment 3, ovariectomized (ovx) virgin females expressed neither an aversion nor a preference for the odor of pups following saline administration. After MS treatment, however, the virgins showed a distinct preference for pup odors. When exposed to male odors in Experiment 4 ovx virgins showed a marked preference for male odors after MS treatment, and neither a preference nor an aversion after saline. Experiment 5 examined pup odor preferences in intact virgins, early (Day 7), middle (Day 14), late-pregnant (Day 21), and prepartum (Day 22) rats. The pup odor preferences of virgin, Day 7, Day 14, and Day 21 pregnant rats were not different and generally were at chance levels. Day 22 pregnant females exhibited a marked preference for pup odors compared to chance levels, as well as compared to the other four groups. These findings suggest that opiates and endogenous opioids may regulate olfactory preferences and that alterations in this system may underlie normal behavioral changes toward conspecifics prepartum as well as during lactation.     

Temporal parameters of responsiveness to maternal pheromone in Acomys cahirinus

In a series of two-choice preference tests, 15-and 20-day-old A. cahirinus pups were found to prefer chemical cues produced by a lactating female over such cues emanating from a nonlactating female. Twenty-five-day-old pups, however, showed reliable preferences for bedding material soiled by a nonlactating female when paired against either lactating female-soiled bedding or clean bedding material. Although 25-day-old pups are no longer responsive to chemical stimuli produced by lactating females (maternal pheromone), 2-day-old pups are attracted to such cues emanating from lactating females as late as 35 and 38 days postpartum. Therefore, there does not appear to be close temporal synchrony between the waning of responsiveness to maternal pheromone and the production thereof.     

Attenuation of maternal behavior in virgin CD-1 mice by methylphenidate hydrochloride

The administration of methylphenidate hydrochloride (MPH) to girls and women has increased in the last decade and the potential for mothers to receive this medication has also increased. Because substances that alter the dopaminergic systems can also disrupt maternal behavior, and MPH acts on dopaminergic neurons, we evaluated the influence of MPH on maternal behavior. The maternal induction paradigm allowed us to assess changes in spontaneous maternal behavior as a result of repeated exposure to MPH without exposing pups to the drug. Virgin female CD-1 mice received MPH (5 mg/kg) or saline daily, starting 3 days before pup exposure, and for the duration of the 10-day test period. Naïve groups of three pups were placed with the female each day and maternal behavior was assessed during 10-minute observation periods 1 h post-injection. MPH-treated females showed significantly less maternal behavior, including reduced pup licking and crouching over pups, compared to saline treated females. MPH-treated females also exhibited higher activity levels than saline treated females. Given the disruption in spontaneous maternal behavior of MPH-treated mice, further research examining the relationship between maternal behavior and MPH exposure is warranted.     

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.     

Parturition: Its role in the long term retention of maternal behavior in the rat

The role of parturition in the establishment of the long-term retention of maternal behavior in the rat was investigated. Primigravid rats caesarean sectioned on Day 22 of gestation that responded maternally immediately (<22 hr) following surgery were permitted to be maternal towards test pups for 48–70 hr at which time test pups were separated from the experimental animals. When reexposed to rat pups some 25 days later 50% of the caesarean sectioned animals responded maternally within 1 hr while the remaining 50% responded maternally within 5 test days. The level of maternal responsiveness of caesarean sectioned animals did not differ from that of primiparous animals given only 2 days of pup exposure postpartum and then tested for the retention of maternal behavior 25 days later. Forty-two percent of the primiparous animals responded maternally during the first test period 25 days following litter removal. Although a slight reduction in latency to respond maternally was evident in nulliparous animals upon reexposure to rat pups 25 days after an initial sensitization period, both caesarean sectioned and primiparous animals responded maternally significantly faster than did previously sensitized nulliparous females. The data demonstrate that the act of parturition is not essential for the establishment of the long-term retention of maternal behavior in the rat, and suggest that exposure to pups following termination of pregnancy may serve to intensify the process whereby the retention of maternal behavior is established.     

Effects of ethanol consumption on maternal behavior in the female rat

Female rats consuming 30% ethanol (v/v) for 30 days prior to and during gestation, and during the post-partum period were tested for maternal behavior with either their own pups or pups provided by normal foster mothers. When tested with their own pups, ethanol consuming females displayed significant deficits in maternal behavior. However, when a second group of ethanol consuming females were tested with normal pups, the females displayed maternal behavior comparable to that of control females. These data suggest that gestational ethanol consumption per se does not produce a deficit in the ability to display maternal behavior. Possibly, due to ethanol related changes in the pup's own characteristics, they became a less attractive stimulus in eliciting the display of maternal behaviors from their mothers.     

Long-term effects of pregnancy and parturition upon maternal responsiveness in the rat.

Latency to retrieve and group rat pups was measured in nonlactating primiparous and nulliparous rats. Primiparous females that raised litters to weaning and were tested for responsiveness to donor's pups 25 days postweaning exhibited an immediate onset of maternal responsiveness. The 21 day lactation period was found not be essential for the establishment of this level of responsiveness, since animals that experienced pregnancy and parturition, and interacted with their young for 1–$\text{1}\text{1}\text{2}$ hr during parturition also retrieved and grouped pups immediately when tested 25 days postpartum. The establishment of this behavioral response in the newly parturient animal was found to be independent of lactogenesis and the presence of the ovaries during parturition. Virgin females with or without previous pup exposure, animals caesarean sectioned on Day 22 of pregnancy, and primiparous females whose pups were removed immediately at birth did not exhibit this level of maternal responsiveness immediately when tested later in adulthood. The data suggest that an organization of maternal behavioral responsiveness occurs during parturition in the rat.     

The reproductive stage and experience of sexually receptive mothers alter their preference for pups or males

Female rats show postpartum estrus, a unique stage in their reproductive cycle in which they are able to display maternal and sexual responses at the same time. To assess the relative value of pups or males for sexually receptive mothers with different hormonal profiles and reproductive experiences, we employed a 3-point star maze with 3 choice compartments containing: pups, a sexually active male, or no stimulus (neutral). Cycling maternal and nonmaternal females in late proestrus, independently of their previous reproductive experience, strongly preferred the male to the pups, although most postpartum estrous dams did not exhibit preference for the male. The majority of the postpartum primiparous females did not prefer the litter's chamber either, but a previous reproductive experience strongly determined their preference for the pups. These results suggest that the hormonal changes of the proestrus, in contrast to those of the postpartum estrus, promote a strong preference for the male that is not diminished by the maternal condition. Conversely, the endocrine changes of the postpartum facilitate the effect of previous reproductive experience in strengthening the incentive value of the pups.     

Maternal pheromone: A demonstration of its existence in the mouse (Mus musculus)

A series of preference tests were conducted to determine whether the mouse produces a pup-attracting odor during lactation, similar to the case of the albino rat. Mouse pups preferred to be near the odor of lactating mice rather than the odor of virgin females, and the pups did not appear to discriminate between their own mothers and unfamiliar lactating females. Thus, the mouse, like the rat, appears to produce a maternal pheromone during lactation.     

Latency and duration of estrogen induction of maternal behavior in hysterectomized-ovariectomized virgin rats: Effects of pup stimulation

The latency for the onset of maternal behavior was measured in virgin rats which received either 100 μg/kg estradiol benzoate (EB) or Oil immediately following ovariectomy and hysterectomy. Different groups of EB and Oil-treated females were presented with 3–8 day old test pups at 0, 24, and 48 hr after their injections. Only the females given EB exhibited short-latency maternal behavior; latencies as short as 24 hr were observed in 46% of the animals which were presented with pups at 0 hr and within 48 hr, 92% of the group were maternal. The effects of EB on maternal responsiveness could be detected between 48 to 72 hr after injection: these effects were equal to those occurring over the first and second 24-hr periods.