Effects of prolonged estrogen-progesterone treatment and hypophysectomy on the stimulation of short-latency maternal behavior and aggression in female rats

Two experiments were undertaken to examine the stimulation of home-cage and/or maternal aggressiveness by a hormonal treatment stimulating short-latency maternal behavior. Nonpregnant ovariectomized rats were treated with a 16-day regimen providing pregnancy levels of estrogen (E, 5-mm Silastic capsule) and progesterone (P, daily injection of 4 mg) followed by E and P withdrawal, with or without a terminal injection of estradiol benzoate (EB, 5 micrograms/kg). In Experiment 1, hormonally treated and control females were exposed continuously to pups and tested for aggression toward male intruders on the fifth day of pup exposure. Females receiving E/P/Oil and E/P/EB were highly aggressive whether or not they had yet shown maternal behavior, whereas vehicle-treated females were nonaggressive. In Experiment 2, hypophysectomized (HYPX) and Sham-HYPX females received either E/P/EB or a control treatment and were tested with male intruders (a) immediately preceding and (b) on the fifth day of continuous pup exposure. HYPX and Sham-HYPX females treated with E/P/EB were almost equally aggressive both preceding and following pup exposure (during which they initiated maternal care), whereas HYPX and Sham-HYPX vehicle-treated females were nonaggressive at both tests. In contrast, maternal behavior latencies were reduced by E/P/EB only among Sham-HYPX females. The results establish that an E/P/EB-treatment which elicits short-latency maternal responses also increases aggressiveness toward intruders. Pituitary products, although involved in the mediation of maternal responsiveness, do not contribute significantly to the stimulation of female aggressiveness by ovarian hormones.     

Duration of estrogen stimulation and progesterone inhibition of maternal behavior in pregnancy-terminated rats

The duration of the effectiveness of estradiol benzoate (EB) on the latency to the onset of maternal behavior was measured in 16-day pregnant rats that were hysterectomized-ovariectomized (HO). Eight groups of HO animals were treated with either a single SC injection of 5 μg/kg of EB or oil at surgery and were initially presented with foster pups at either 24, 48, 72, or 96 hr postoperatively. Compared to their respective controls, EB-treated animals showed singificantly shorter latencies when testing began at 48 and 72 hr but not 24 or 96 hr. In the second experiment, 16-day HO rats were treated with 5 μg/kg of EB at surgery and either oil or 0.5 mg of progesterone at 0, 24, or 44 hr postoperatively. Additional groups received either progesterone or oil at surgery (instead of EB) and a second injection of oil 44 hr later. Testing began 48 hr following surgery for all groups, and the results showed that only the groups injected with EB alone or EB plus progesterone at 44 hr displayed short-latency maternal behavior. It was concluded that a significant reduction in the latency to the onset of maternal behavior can be obtained between 24 and 72 hr after EB treatment and that progesterone when injected concurrently or 24 hr later can inhibit the effectiveness of EB.     

Progesterone inhibition of maternal behavior in the rat

The present series of experiments investigated the role of progesterone in inhibiting the onset of maternal behavior in the rat. Female rats hysterectomized and ovariectomized on Day 16 of pregnancy and injected subcutaneously with 20 μg/kg of estradiol benzoate (EB) show a short latency to onset of maternal behavior when presented with test pups 48 hr later. A subcutaneous injection of either 1 or 5 mg of progesterone on Day 16 of pregnancy and again 24 hr later inhibited this EB-induced short-latency onset of maternal behavior. The central neural site at which progesterone might act to produce this inhibitory effect was explored. Famale rats, hysterectomized and ovariectomized on Day 16 of pregnancy and injected subcutaneously with EB, received implants of crystalline progesterone on Day 16 of pregnancy into either the medial preoptic area, ventromedial hypothalamus, midbrain tegmentum, dorsal raphe nucleus, or median raphe nucleus. No inhibitory effects were found and all females showed a short-latency onset of maternal behavior. Several possible explanations for this lack of inhibitory effect of intracerebral implantation of progesterone are discussed.     

Cytosol and nuclear estrogen receptor binding in the preoptic area and hypothalamus of female rats during pregnancy and ovariectomized, nulliparous rats after steroid priming: correlation with maternal behavior

In a previous study, high nuclear estrogen receptor concentrations in the preoptic area (POA) were found on Day 16 of pregnancy to prime females to respond to a subsequent low dose of estradiol benzoate (EB) after hysterectomy-ovariectomy by exhibiting maternal behavior in 48 hr. Receptor concentrations in the POA were found to be higher than those in the hypothalamus (HYP). The present study investigated when nuclear estrogen receptors increase during pregnancy in POA and when the difference in receptor concentrations between POA and HYP occurs. An attempt was made to reproduce these pregnancy changes with a 16-day treatment of estrogen and progesterone in ovariectomized (OVX), nulliparous rats. In Experiment 1, we measured cytosol and nuclear estrogen receptor concentrations in the POA and HYP of female rats during pregnancy. Nuclear receptor concentrations in the POA increased beginning on Day 10, increased again on Day 16, and continued at this high level for the remainder of pregnancy. Nuclear estrogen receptor concentrations in the HYP remained at a lower level throughout most of pregnancy until Day 22 when they increased significantly. In Experiment 2, we tested the maternal behavior and measured estrogen receptor concentrations in OVX, steroid-primed, nulliparous rats after hysterectomy (H) and EB treatment. While 90% of estradiol (E) + progesterone (P)-primed females displayed short-latency maternal behavior 48 hr after H and EB treatment, 46% of E + vehicle (V)-treated controls were maternal. At 0 hr (prior to H and EB treatment), there was a significantly larger nuclear receptor accumulation in the POA but significantly attenuated receptor binding in the HYP. P treatment significantly affected cytosol and nuclear estrogen receptor dynamics. Differences in nuclear estrogen receptor concentrations were shown to be based on the number of available binding sites and not to changes in receptor affinity for estradiol.     

Forebrain implants of estradiol stimulate maternal nest-building in ovariectomized rabbits

In rabbits, estradiol and progesterone (P) stimulate digging a maternal burrow while P withdrawal promotes straw-carrying. To investigate where such hormones act to regulate those activities, ovariectomized rabbits were implanted with estradiol benzoate (EB; Experiment 1) in the nucleus accumbens (ACC), the principal nucleus of the medial preoptic area or the dorsal hippocampus. Implants were combined with s.c. P injections. In Experiment 2, P (in crystals or dissolved in oil) was implanted in the same regions as in Experiment 1, combined with s.c. injections of EB. Implants of EB into the ACC or MPOA-bed nucleus of the stria terminalis (BNST) stimulated significant digging across the period of P injections in 72% and 67% of females, respectively. Neither EB implants in the hippocampus nor cholesterol implants in the MPOA-BNST were effective in eliciting digging. P withdrawal provoked a rapid decline of digging in all animals; it also stimulated straw-carrying in 53% of females implanted with EB in the MPOA-BNST. P implants failed to stimulate digging in most females injected with EB. Removal of P crystals did not promote straw-carrying. Results support an action of estradiol on the ACC and MPOA-BNST to promote digging while only the MPOA-BNST is involved in stimulating straw-carrying. The failure of P implants to stimulate digging or straw-carrying in EB-treated females suggests that the stimulation of other or additional brain areas by P is necessary to fully activate maternal nest-building.     

Ovarian hormone-induced short-latency maternal behavior in ovariectomized virgin Long-Evans rats

Two ovarian hormone regimens reported to induce rapid-onset maternal behavior (MB) in maternally naive virgin, ovariectomized Sprague-Dawley (SD) albino rats (R. S. Bridges, 1984, Endocrinology 114, 930-940; A. L. Giordano, 1987, Doctoral Dissertation, Rutgers University, Newark, NJ) were assessed in Long-Evans (LE) hooded rats, a strain which tends to be less maternally responsive in various situations dissociated from parturition. The combination of sufficiently high and long-lasting treatments with estradiol (E, 10-mm Silastic capsule, sc, on Day 1) and progesterone (P, 3 x 30-mm Silastic capsules, Days 3-13) resulted in a mean MB onset latency (after pup presentation on Day 14) of 1.8 days. In contrast, no-hormone or P-only controls had latencies of about 5.5 days. However, the E + P combination was completely ineffective if the E capsule was withdrawn along with the P capsules, unlike the case for SD rats. Also in contrast to the albinos, E alone was ineffective, while E treatment following P withdrawal was only partially effective. The most efficacious regimen, which included a P treatment (injections of 4 mg/day, Days 3-12 or 3-15) known to maintain pregnancy in ovariectomized rats, resulted in mean latencies of less than or equal to 1 day; 39% overall displayed MB rapidly, i.e., retrieval within 15 min of exposure to pups and crouching by 3 hr, and 89% became maternal by the next day. With this regimen, neither duration of 4 mg/day P treatment (10 or 13 days) nor hysterectomy 2 days before testing affected MB latencies. Thus, the essential features of the previously reported ovarian hormone regimens for induction of short-latency MB are efficacious in LE rats, but the hormonal requirements in this strain seem to be more precise. Factors which might contribute to an even higher percentage maternal on the first day of pup exposure are considered.     

Effect of preoptic region implants of dilute estradiol on the maternal behavior of ovariectomized, nulliparous rats

Estrogen implanted directly into the medial preoptic region of pregnant Charles River Sprague-Dawley rats hysterectomized and ovariectomized on Day 16 of gestation mimics the effects of systemic estrogen treatment at this time by reducing the latency to respond to foster pups with maternal behavior (Numan, Rosenblatt, and Komisaruk, 1977). The present report describes the pup-directed responses of ovariectomized, nulliparous Zivic-Miller Sprague-Dawley rats that received bilateral medial preoptic implants of either cholesterol (n = 11) or estradiol diluted 1:10 with cholesterol (n = 11). Two days after treatment these animals were housed with three foster pups: their responsivity to pups and quality of nests built were then assessed, at first hourly and then daily. Rats receiving intracranial estradiol required significantly shorter exposures to pups than did cholesterol-treated animals before initiating carrying and grouping of 3 dispersed pups in a maternal nest during a 15-min test. On other measures, however, the groups did not differ (e.g., proportion grouping pups overnight, time required to complete retrieval of pups to the nest, time required to rebuild a disrupted nest). Animals treated with cholesterol and animals with estradiol implants did not differ in uterine weight at the time of sacrifice, suggesting that estrogen did not leak, even from this well-vascularized implant site, into the circulation. Thus, as in the pregnant animal, the facilitating effects of estrogen on maternal behavior can be mediated through the medial preoptic region; however, the effects were evident only when a test requiring retrieval of several pups within an arbitrarily short interval was given.     

Hormonal factors influence the onset of maternal aggression in laboratory rats

This experiment addressed the hypothesis that aggressiveness toward conspecifics is stimulated by hormonal factors known to mediate the onset of maternal care. Subjects included both pregnant and virgin females. Sixteen-day pregnant rats were hysterectomized (H), hysterectomized-ovariectomized and injected with estrogen (HO-EB), or subjected to sham procedures. Nonpregnant females were HO-EB or sham operated. The females were sensitized by continuous exposure to pups and were judged to have initiated maternal care when all pups were retrieved and grouped, Aggressiveness was observed during 5-min intruder tests using unfamiliar males, administered (a) 10 min prior to the introduction of test pups, (b) following the first 3 hr of pup exposure, and (c) after females had initiated maternal care. The results revealed that treatments known to reduce sensitization latencies also increased aggressiveness even prior to exposure to pups. Aggressiveness was displayed before sensitization only in groups having elevated estrogen levels. After initiating maternal behavior, pregnant and pregnancy-terminated females increased further in aggressiveness whereas nonpregnant females did not. Pregnancy-terminated, HO-Oil females became aggressive (only) after initiating maternal behavior, indicating that factors other than estrogen also influence the onset of maternal aggression.     

Hormonal regulation of maternal behavior in rats: stimulation following treatment with ectopic pituitary grafts plus progesterone

Changes in hormone secretions during pregnancy help to stimulate the onset of maternal behavior at parturition. To date, studies have demonstrated that estradiol (E2) appears to be a necessary component in the hormonal induction of maternal behavior in rats and other mammals. In the present study, we have reevaluated the contribution of E2, progesterone (P), and hormone-secreting pituitary grafts in the rapid induction of maternal behavior by measuring the behavioral effects of exposure to various combinations of P and prolactin-secreting ectopic pituitary grafts in the absence of estrogen. Adult hypophysectomized and nonhypophysectomized nulliparous rats were ovariectomized 2-3 days (Treatment Day 1) after their arrival in our laboratory. In Experiment #1, experimental, hypophysectomized rats were implanted s.c. with 6 P-filled Silastic capsules and given 2 anterior pituitary (AP) glands that were grafted beneath the kidney capsule on Treatment Day 1. Controls were given blank implants and were sham-grafted. P-filled and blank Silastic capsules were removed on Day 11, and behavioral testing was conducted once-a-day beginning on Day 12 for eleven days. Animals treated with P-plus-pituitary grafts displayed full maternal behavior significantly faster than did controls (median latencies of 3.0 and 7.5 days, respectively). In Experiment #2, nonhypophysectomized rats were assigned to one of three treatments. On Treatment Day 1, one group of rats received 6 P-filled Silastic implants and had 2 AP glands grafted under their renal capsules. A second group of animals received 6 P capsules and was sham-grafted, while controls were given blank implants and were sham-grafted.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mammary stimulation and maternal aggression in rodents: thelectomy fails to reduce pre- or postpartum aggression in rats

In mice, tactile stimulation of the nipples appears to be critical for the onset of postpartum maternal aggression. Surgical removal of the nipples (thelectomy) blocks aggression if performed prior to parturition. In rats, indirect evidence suggests a similar role for nipple stimulation in maternal aggression. Two experiments were undertaken to determine whether thelectomy prior to mating reduces pregnancy-induced and/or postpartum aggression in this species. In the first, thelectomized and sham-thelectomized females were subjected to home cage tests (pups, if any, present) with unfamiliar male intruders on Gestation Days 18 and 21 and Lactation Days 3 and 5. Additional groups of thelectomized females were tested one time only on either Lactation Day 5 or 12. Thelectomized and control females were equally aggressive; postpartum, nearly all females in both groups attacked. Experiment 2 used females that were hysterectomized-ovariectomized (HO) on Gestation Day 16. Such females are not aggressive prior to initiating maternal behavior, but become highly aggressive (over 80% attacking) after commencing maternal care. Females again were thelectomized or sham-thelectomized prior to mating. On Day 16 HO was performed, and 48 hr later continuous exposure to pups was begun. After the females had displayed maternal behavior for 1.5-2 days, intruder tests were conducted. All females attacked at least once, with no differences between treatment groups. Thus thelectomy does not reduce maternal aggression in the rat. This finding, however, does not preclude a role for tactile ventral stimulation in mediating maternal aggression.     

Postpartum,hormonal, and nonhormonal induction of maternal behavior in rats: Effects on T-maze retrieval of pups

It is known that the home-cage maternal behavior of rats which become maternal after daily pup exposure (sensitization) is almost indistinguishable from that of lactating mothers, but that sensitized and lactating rats can be distinguished by their pup-retrieval performance in a T-maze extension of the home cage. The present study explored this difference further. Postpartum mothers which could not suckle due to prior nipple removal (thelectomy) retrieved as well, if not better, than intact controls in the T-maze. Hormonal induction of maternal behavior (in ? 3 days) was carried out by hysterectomy-ovariectomy plus 100 μg/kg estradiol benzoate; the performance of these females was similar to that of the postpartum groups. In contrast, only a small percentage of the sensitized mothers retrieved in the T-maze, whether the latency to onset of their maternal behavior was long (4–10 days) or short (? 3 days). Thus, hormonal factors associated with pregnancy and/or parturition, but not suckling stimulation, may facilitate T-maze retrieval of pups. The possible ethological significance of the T-maze test as a measure of maternal responsiveness is discussed.     

Hormonal basis of hysterectomy-induced maternal behavior during pregnancy in the rat.

Hysterectomy during the last half of pregnancy (i.e., Day 10–19) induces a rapid onset of maternal behavior; ovariectomy in addition to hysterectomy, prevents this effect. Estradiol and progesterone were tested for their ability to restore short-latency maternal behavior in hysterectomized-ovariectomized (HO) females operated on the 10th, 13th, 16th and 19th days of pregnancy. A single injection of either 20 μg/kg or 100 μg/kg estradiol benzoate (EB) immediately following HO either alone or followed by 0.5 mg progesterone (P) 44 hr later restored short-latency maternal behavior similar to that observed following hysterectomy only. The lower dose of EB was found to be equally effective at all stages of pregnancy and P was unnecessary to induce maternal behavior. The effectiveness of EB in inducing maternal behavior was discussed in relation to the hormonal changes which follow hysterectomy during pregnancy and to those which are associated with the normal onset of maternal behavior around parturition.     

Mice: progesterone stimulates aggression in pregnancy-terminated females

Three experiments were conducted in order to assess the role of progesterone (P) in the aggressive behavior displayed by late pregnant Rockland-Swiss mice toward adult male intruders. In Experiment 1, hysterectomy on the 15th day of gestation reduced the aggressive behavior normally displayed by pregnant mice toward male intruders. In Experiment 2, Silastic implants of P stimulated aggression in hysterectomized mice but did not fully restore the behavior to the level of fighting normally observed in pregnant animals. In Experiment 3, aggressive behavior in P-treated hysterectomized animals was inhibited by simultaneous exposure to estradiol (E). Also, treatment with E alone did not stimulate aggression in hysterectomized mice. While pregnancy-induced aggression is promoted by P, other neuroendocrine factors may act in concert with the steroid to fully stimulate aggression in gravid mice.     

Embryo survival, and fetal and placental growth following elevation of maternal estradiol blood concentrations in the rat.

High doses of estrogens cause embryonic mortality, and fetal and placental growth retardation in rats. This study addresses the physiological relevance of such findings. Estradiol benzoate (EB), by s.c. injection, or estradiol-17beta (E2), delivered by a miniosmotic pump, raised maternal E2 concentrations from only slightly above control values to 5-fold. EB (1 microgram/day) over Days 6-13, 8-13, and 11-13, and continuous infusion of E2 (15 ng/h; Days 10-13) reduced fetal survival to 0%, 0%, 22%, and 75%, respectively. Single injections of EB showed that its lethal effect declined rapidly over Days 9 (44% survival) to 13 (90% survival). Embryos died within 48 h, but death was not due to luteal failure since progesterone levels were maintained and progesterone administered with EB did not reduce mortality. Administration of EB at 1 microgram/day (Days 14-21) or E2 at 40 ng/h (Days 13-16) retarded fetal and placental growth but did not affect survival. The rat embryo is highly sensitive to elevated maternal estradiol concentrations over much of gestation. The early lethal effect implies that endogenous E2 production is carefully regulated to maintain pregnancy; the latter growth-retarding effect suggests that E2 may have a role in the normal control of fetal growth.     

Growth hormone is secreted by ectopic pituitary grafts and stimulates maternal behavior in rats

The onset of maternal behavior at parturition in rats is hormonally regulated. Recently, we reported that treatment of behaviorally inexperienced, hypophysectomized (hypox), ovariectomized (ovx) rats with a sequential steroid treatment of progesterone (P) and estradiol (E2), and either ectopic anterior pituitary grafts or prolactin (PRL), stimulated maternal responsiveness toward foster young. That growth hormone (GH) has a number of PRL-like activities led us to ask whether the actions of PRL on maternal behavior were specific to PRL or might be shared by other PRL-like protein hormone, i.e., GH. In Experiment 1 we quantified plasma concentrations of GH and PRL by RIA in groups of hypox female rats that were ovariectomized and treated with a combination of ectopic pituitary grafts (Days 1-23) and Silastic capsules filled with P (Days 1-11) and E2 (Days 11-23). Blood samples were collected from Days 1 to 23 of treatment. Both plasma PRL and GH levels increased after grafting, initially rising 10- to 60-fold by Day 4 and gradually declining throughout the remainder of the 23-day sampling period. Throughout the 3-week period after grafting plasma GH levels were as high or higher than those of PRL. In Experiment 2 the behavioral effects of exogenously administered ovine (o)-GH were measured in groups of hypox, ovx rats that were treated with P and E2 as in Experiment 1. Experimental rats were injected twice daily with 0.25 mg oGH beginning on Day 1. Testing for maternal behavior toward foster young was conducted daily from Day 12 to Day 22. In steroid-treated rats, GH treatment stimulated a more rapid onset of maternal behavior (latencies of 3 vs greater than 10 days for vehicle-injected controls). These data indicate that GH, like PRL, is secreted by ectopic pituitary grafts and is capable of stimulating maternal behavior.     

Components of early maternal environment affecting the predisposition to catalepsy

Reciprocal pup substitution (cross-fostering) in cataleptic GC (designated so by the initials of words "genetic" and "catalepsy") and control Wistar females resulted in attenuation of cataleptic predisposition in GC rats fostered by Wistar foster-mothers. The latter demonstrate a more intense maternal care than GC females. There was a significant negative correlation between the frequency of mother staying in nest and the duration of pinch-induced catalepsy in pups fostered by her. In the home-cage retrieval test, the females of the strains compared showed a significant dependence of the latencies of approach to, and retrieval of, pups on their own and the pups' genotype.     

The effects of early rearing environment on the hormonal induction of maternal behavior in virgin rats

The present study investigated the effects of isolation rearing, through the artificial rearing paradigm (AR), on the hormonal induction of maternal behavior (MB) in female Sprague–Dawley rats. Between postnatal days (PND) 4 and 18, rat pups were raised either with their mothers (MR) or artificially, without their mothers (AR). As well, some of the AR pups were provided with additional maternal-like licking stimulation (AR-MAX) while the others were not given any additional stimulation (AR-MIN). At PND 60–100, AR (n = 28) and MR (n = 25) animals were ovariectomized (OVX). One week after the surgery, rats were either treated with a 2-week estrogen (E2) and progesterone (P) hormonal regimen (Bridges, R.S., 1984. A quantitative analysis of the roles of dosage, sequence, and duration of estradiol and progesterone exposure in the regulation of maternal behavior in the rat. Endocrinology 114, 930–940) or not treated with the hormone replacement. Maternal behavior testing with foster pups commenced 24 h following the removal of P treatment. Results demonstrated that MR animals showed increased pup licking and hover-crouching in comparison to AR animals and that hormonally primed groups became maternal more quickly than non-primed groups, regardless of the rearing history. There was also a significant interaction between the rearing condition (MR vs. AR) and hormonal treatment on the quality of maternal behavior exhibited. The highest level of licking and crouching was shown by the hormone-treated MR group. Mechanisms for these effects are discussed.     

Progesterone inhibition of estrogen-induced maternal behavior in hysterectomized-ovariectomized virgin rats.

Hysterectomized-ovariectomized virgin rats were tested for maternal behavior following treatment with 100 μg/kg EB immediately at surgery and either oil, 0.5 or 5.0 mg progesterone either 0, 24 or 44 hr following surgery. Stimulus pups were presented 48 hr postoperatively which is counted as Day 0 of testing. EB + oil-treated females displayed short-latency maternal behavior beginning on Day 0. The injection of 5.0 mg progesterone at 0, 24, or 44 hr significantly inhibited the onset of maternal care while the effect of the lower dose of progesterone depended upon the timing of its administration in relation to that of EB. At a dose of 0.5 mg, progesterone given 24 hr following EB, inhibited the appearance of maternal behavior but had no effect given at 44 hr, and resulted in only a partial delay when given at the same time as the EB. Possible mechanisms by which progesterone interfered with the display of maternal behavior were discussed.     

The estrogenic arousal of aggressive behavior in female mice

Experiments were conducted to determine the conditions under which estrogen would promote male-like aggressive behavior in female mice. The results of the first experiment showed that most females chronically exposed to testosterone propionate (TP) in adulthood fought, whereas females similarly treated with estradiol benzoate (EB) did not display aggression. Another experiment found that, when either TP or EB was administered on the day of birth, adult females displayed aggression in response to daily EB injections during adult life. Also, the potentiating effect of neonatal hormone exposure declined over the first 12 days postpartum, as 100% of the Day 0, 75% of the Day 6, and 0% of the Day 12 and 18 TP-treated females fought in response to daily injections of 40 μg of EB in adulthood. The final study showed that, under the test conditions employed, the failure of a chronic adult EB regimen to promote aggression was not due to a competing tendency to display female sexual behavior.