Maternal Virgins Research Articles

Oestrogen-Independent, Experience-Induced Maternal Behaviour in Female Mice

Nulliparous female mice that have not experienced mating, pregnancy or parturition show near immediate spontaneous maternal behaviour when presented with foster pups. The fact that virgin mice display spontaneous maternal behaviour indicates that the hormonal events of pregnancy and parturition are not necessary to produce a rapid onset of maternal behaviour in mice. However, it is not known how similar maternal behaviour is between virgin and lactating mice. In the present study, we show that naturally postpartum females are faster to retrieve pups and spend more time crouching over pups than spontaneously maternal virgin females, and that these differences diminish with increased maternal experience. Moreover, 4 days of experience with pups induced pup retrieval on a novel T-maze. Furthermore, the effects of experience on subsequent maternal responsiveness are not dependent on gonadal hormones because ovariectomised females with 4 days of pup experience show pup retrieval on a novel T-maze similar to that of postpartum mice. Four days of maternal experience also induced T-maze pup retrieval in ovariectomised aromatase knockout female mice that was not significantly different from the maternal responsiveness of ovariectomised wild-type littermates. These data suggest that maternal experience can induce maternal behaviour in females that have never been exposed to oestradiol at any time in development or adulthood. Finally, ovariectomised pup-experienced females continue to retrieve pups on a novel T-maze 1 month after the initial experience, suggesting that, even in the absence of oestradiol, maternal experience produces long-lasting modifications in maternal responsiveness.

Effects of maternal behavior induction and pup exposure on neurogenesis in adult, virgin female rats

The states of pregnancy and lactation bring about a range of physiological and behavioral changes in the adult mammal that prepare the mother to care for her young. Cell proliferation increases in the subventricular zone (SVZ) of the female rodent brain during both pregnancy and lactation when compared to that in cycling, diestrous females. In the present study, the effects of maternal behavior induction and pup exposure on neurogenesis in nulliparous rats were examined in order to determine whether maternal behavior itself, independent of pregnancy and lactation, might affect neurogenesis. Adult, nulliparous, Sprague-Dawley, female rats were exposed daily to foster young in order to induce maternal behavior. Following the induction of maternal behavior each maternal subject plus females that were exposed to pups for a comparable number of test days, but did not display maternal behavior, and subjects that had received no pup exposure were injected with bromodeoxyuridine (BrdU, 90 mg/kg, i.v.). Brain sections were double-labeled for BrdU and the neural marker, NeuN, to examine the proliferating cell population. Increases in the number of double-labeled cells were found in the maternal virgin brain when compared with the number of double-labeled cells present in non-maternal, pup-exposed nulliparous rats and in females not exposed to young. No changes were evident in the dentate gyrus of the hippocampus as a function of maternal behavior. These data indicate that in nulliparous female rats maternal behavior itself is associated with the stimulation of neurogenesis in the SVZ.

Maternal condition reduces fear behaviors but not the endocrine response to an emotional threat in virgin female rats

Lactating dams and maternal virgin females are less fearful in behavioral tests compared with non-maternal animals, suggesting that maternal condition per se reduces the negative value of threatening stimuli. In addition, lactating females exhibit a diminished hypothalamic–pituitary–adrenal response to potential environmental threats. Can the maternal condition, independently of the endocrine profile of lactation, promote a reduction in the behavioral as well as in the endocrine response to an emotional stressor? To answer this question, anxiety-related and fear behaviors as well as the levels of corticosterone were evaluated in response to a bright-lit open field-loud noise model in maternal and non-maternal non-ovariectomized virgin females and lactating dams in the presence of the pups. Maternal animals, both lactating and virgin, presented an increased exploration of the bright-lit open field and a significant reduction of fear behaviors, indicated by the decreased flight and immobility responses to the subsequent activation of a loud noise, in comparison to non-maternal virgins. Interestingly, maternal virgin females, as non-maternal rats, showed high corticosterone plasma levels, in contrast to the lower endocrine response exhibited by lactating dams when confronted to this threat. Present results suggest that maternal condition allows females to take risks when caring for their young, a behavioral strategy that is independent of the reduced hypothalamic–pituitary–adrenal axis response characteristic of lactation. This evidence points towards a clear dissociation in the mechanisms regulating behavioral and endocrine responses to emotional stressors during motherhood.

Reproductive experience and activation of maternal memory.

The maternal and neurobiological responses of biological mothers and pup-induced maternal virgin rats were compared 55 and 80 days after an initial 2-day maternal experience. When tested for home cage responsiveness after prolonged isolation from young, the biological, primiparous rats displayed shorter maternal latencies. Primiparous females tested in the presence of pups on the elevated plus-maze displayed increased exploration of the open arms and increased c-Fos expression in the cortical nucleus of the amygdala. Pup exposure and parity also enhanced activation of the nucleus accumbens shell and medial nucleus of the amygdala, respectively. Therefore, although both nulliparous and primiparous rats retain a maternal memory for a prolonged time, the memory and neurochemical response appear stronger in primiparous mothers.

Comparison of the “Nursing” and Other Parental Behaviors of Nulliparous and Lactating Female Rats

Virgin female rats display maternal behaviors after continuous exposure to pups (sensitization) that are in some respects similar to those of postpartum females. We herein provide a detailed comparison of the “nursing” and other parental behaviors of maternally sensitized virgin females and postpartum lactating dams. Ovariectomized and intact virgin females were exposed to pups until displaying maternal behavior. On the females' fourth day of maternal responsiveness, the pups were removed for 3 h and then returned, and subject–litter interactions were observed for 45 min. Behavior of maternal virgins was compared with that of lactating dams observed on day 4 postpartum interacting with either suckling pups or pups unable to suckle due to perioral anesthesia. Ovariectomy had no effect on behavior of virgins. Retrieval and licking of pups were deficient in virgins compared with lactating dams. Suckled dams showed prolonged kyphosis (upright crouched nursing), whereas nonsuckled dams displayed little kyphosis but rather were often in a hunched position over pups. Some aspects of quiescent “nursing” behaviors of virgins were surprisingly similar to those of suckled dams, including the latency to and duration of quiescence. Nonsuckling pup stimulation elicited more kyphosis in virgins than in lactating dams, which was still much less than in suckled dams. Virgins also “nursed” pups in hunched and prone postures. Differences between sensitized and postpartum females in their maternal behaviors likely reflect differences in motivation as well as sensory inputs they receive from pups. In particular, sensory regulation of “nursing” behaviors is influenced by reproductive state because nonsuckling pups elicit different postural responses in sensitized and lactating mothers.

Mechanisms involved in the control of punished responding in mother rats

In a previous study we found that mother rats show more drinking responses than virgins in the punished drinking paradigm, an animal model for anxiety. The present investigation was carried out to determine the possible mechanisms underlying this naturally occurring anticonflict effect. In Experiment 1, we investigated whether the induction of maternal behavior in virgin females (by long-term estrogen and progestin treatment in combination with pup exposure) enhances punished drinking. However, no release from shock-induced (0.25 mA) suppression of drinking was observed in maternally responsive virgins deprived of water for 24 hr. Unlike natural mothers, then, no anticonflict effect is seen in maternal virgins. A considerable body of evidence suggests that facilitation of γ-aminobutyric acid (GABA) activity in the brain increases punished responding in rats. In the second experiment, therefore, lactating females were injected with pentylenetetrazol, a GABA antagonist, before being monitored for punished drinking. The drug attenuated the enhanced acceptance of shock in mothers, the effect being observed at a dose level that did not reliably affect unpunished responding. Experiment 3 addressed the possible influence of ovarian and adrenal hormones on punished drinking in lactating females. No significant behavioral effects were observed in mothers subjected to adrenalectomy or ovariectomy 4 days before testing.

Maternal licking by virgin and lactating rats: Water transfer from pups

Rat dams provide water to their young via milk. Dams reclaim much of this water by licking the pups' anogenital areas, stimulating reflexive urination and consuming the pups' urine. Sensitized virgin rats, induced to act maternally do not provide water to pups, but they nevertheless lick them. To determine whether bidirectional transfer of water between the rat mother and her litter mediates maternal licking, water transfer from pups to sensitized virgins was compared with that to lactating dams. We used time-lapse video recordings to measure anogenital licking of pups. Sensitized virgins and lactating dams spent equivalent amounts of time licking the anogenital regions of test litters. We quantified the amount of water transferred from offspring to both virgins and dams by injecting pups with tritiated water and measuring the radioactive label in maternal plasma after interaction with a litter of 5-day-olds. Dams obtained more than twice as much urine from the litter in 4 hr than did the maternal virgins. Differences in the amount of water obtained from pups were due to differences in urine availability caused by the receipt of milk from the dams. When the dams' nipples were ligated, so that their pups received no milk, ligated dams and virgins consumed equivalent amounts of pup urine. Maternal licking and urine consumption are not dependent solely upon the bidirectional exchange of water between the dam and her offspring.

Two modes of prolactin secretion induced by young in maternally behaving virgin rats.

Virgin rats exposed for several days to foster young develop a typical pattern of maternal behavior. After the establishment of maternal behavior, the majority of virgins fail to maintain regular estrous cycles in the presence of young and enter a state of prolonged diestrus. The present study investigated whether maternal virgin rats are capable of secreting PRL in two modes which are known to be controlled differentially: 1) secretion in a circadian pattern, which is regulated autonomously, and 2) acute release, which is governed by stimulation from young. To check for circadian secretion of PRL in maternal virgins, we measured daytime (1200 h) and nigh time (0230 h) concentrations of PRL during pup-induced diestrus and throughout the estrous cycle. In maternal virgins which entered prolonged diestrus through exposure to young, PRL was secreted autonomously in a rhythmic pattern of nightly surges. In this respect, the pupinduced diestrus is similar to pseudopregnancy. In contrast, maternally behaving vi...

Induction by young of prolonged dioestrus in virgin rats behaving maternally.

The present investigation examined, by study of daily vaginal smears, whether the interaction of virgin female rats with young affected the oestrous cycle. Virgins behaving non-maternally or maternally were continuously exposed to young for extended periods. (1) All non-maternal virgins continued to cycle while 87% of maternal virgins ceased to cycle when exposed to young, and entered a series of prolonged dioestrous phases, each lasting about 14 days. Likewise, all maternal parous non-lactating controls exposed to young entered prolonged dioestrus. (2) The dioestrous phase induced by young in virgins was characterized by the decidual response. (3) Young aged 4-9 days were as effective as 10-15-day-old young for inducing and maintaining the dioestrus. (4) Confinement of young in a wire-mesh basket showed that body contact with young was essential to establish dioestrus. (5) Body contact with young was sufficient to induce dioestrus in maternal virgins with excised nipples. (6) Once established, the dioestrus could be maintained for about 12 days after removal of young as early as Day 4 of dioestrus in virgin and parous non-lactating females, but removal of young on Day 6 from post-partum lactating females resulted in a rapid resumption of cyclicity. (7) Prolonged suckling during a series of dioestrous phases induced lactation in 61% of maternal virgins, as well as in all parous control females.

Prolactin release in lactating, primiparous and multiparous thelectomized and maternal virgin rats exposed to pup stimuli.

Serum prolactin was measured by radioimmunoassay in primiparous lactating (L), primiparous thelectomized (nipple removed; PT), multiparous thelectomized (with 1 prior normal lactation experience; MT), virgins (MV) induced to display maternal behavior (retrieving, pup licking and adopting the lactating position) and in nonmaternal virgin females (NMV) not previously exposed to pups. On postpartum Day 7 or on 1 diestrous day between Days 6–10 after the onset of maternal care in the MV group, pups were removed at 0900 h and were either returned or not returned at 1500 h. Rats were decapitated at 1530 h. Prolactin was significantly higher when pups were returned to the L and MT groups and the MV group that had been maternal for 8–10 days and, among these groups, prolactin levels were: L>MT = MV (Days 8–10). Thus, prolactin is released in response to pup stimuli in the absence of suckling and is influenced by a previous lactation experience and by the concurrent duration of the maternal behavior state in virgin rats.

Effects of ergocryptine on postpartum maternal behavior, ovarian cyclicity, and food intake in rats

It is known that postpartum mothers are much more likely than maternal virgins to retrieve pups from a T-maze extension of their home cage. By administering ergocryptine (Ergo), a prolactin inhibitor, to thelectomized (nipple-removed) females, it was demonstrated that prolactin release in the postpartum rat in response to nonsuckling pup stimulation is not responsible for this difference. Indirect indexes of prolactin inhibition included earlier onset of estrus and decreased mammary gland weight in Ergo-treated mothers. Ergo was not found to affect food intake.

Selective retrieving by maternal and nonmaternal female rats.

An earler study reported that nonpregnant females do not selectively retrieve a pup over a small plastic toy, while lactating mothers retrieve the pup more often than the toy. The present study was undertaken to see whether nonpregnant females made maternal by exposure to pups would act more like nonmaternal females than lactating mothers. The behavior toward a pup and a toy was studied in 1 group of nonmaternal virgins, 2 groups of maternal virgins, and 1 group of lactating mothers, during 7 daily 5-min. tests. Measures of the frequency and duration of retrieving-related behavior and analysis of behavioral sequences in relation to the pup and toy showed that pup-induced maternal virgins more closely resemble the lactating mothers than nonmaternal virgins.

Spontaneous and testosterone-induced pup killing in female Rockland-Swiss mice: the efect of lactation and the presence of young.

AbstractCannibalism of pups by adult, virgin, Rockland‐Swiss mice did not occur during the lactation period following the subsequent pregnancy of these animals. However, upon retesting 2 weeks after the weaning of their young, 5 of 13 again killed pups. Testosterone propionate was ineffective in eliciting pup killing when administered during the lactation period. The continuation of androgen treatment for 1 week after weaning did lead to the cannibalism of 1‐day old foster young. Finally, testosterone administration did not induce pup killing in spontaneously maternal virgin female mice when pups were constantly present.

Aspects of nonhormonal maternal behavior in the rat

A new cross-transfusion procedure using single chronically implanted heart catheters was developed to permit animals to move freely during transfusion. Support for the concept of a nonhormonal basis of maternal behavior was found in the inability of blood, cross-transfused from pup-induced maternal virgins and spontaneous retrieving females, to shorten latencies for retrieving in recipient virgins. Spontaneous retrieving was not affected by the hormonal changes of pregnancy. On the other hand, the rate of induction of maternal behavior (i.e., retrieving) was shown to depend upon the size of the cage in which virgins were confined with pups: latencies were directly proportionate to cage area and opportunity for contact with pups.