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.     

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.     

Sexual behavior of male and female septal lesioned rats

Gonadectomized male and female rats were given septal lesions (SL) or sham surgery at approximately 60 days of age. After 3 weeks lordosis behavior tests were initiated. Females were tested after daily injections of 2 μg estradiol benzoate (EB) for 3 days, while males were tested after EB only (2 μg×3 days), and after EB plus progesterone (Prog). The mean lordosis quotients (LQ) of septal lesioned female rats were significantly higher than those of sham operated controls. No increase in lordosis responding was seen in male rats with either EB alone or EB+Prog. Following an additional 3 week interval without steroid treatment masculine behavior tests began. All animals received a pretest and were tested again on Day 4, 7, 11 and 15 daily tesosterone propionate (150 μg/day) treatment. No alterations in masculine sexual behavior (relative to that of controls) were found in either male or female septal lesioned rats. It is concluded that the increased hormone sensitivity is specific for lordosis behavior, at least when the SL are given in adulthood.     

Cholecystokinin octapeptide increases passive avoidance latencies in rats

Intraperitoneal injections of either 0.9% NaCl or cholecystokinin octapeptide (CCK-8) were paired with electrical footshock using a standard passive avoidance conditioning procedure for rats. Passive avoidance behavior was measured either 24 or 48 hr following the conditioning. No CCK-8 effect upon passive avoidance behavior was observed at the 24 hr test, but CCK-8 (100 micrograms/kg and 200 micrograms/kg) produced longer passive avoidance latencies at the 48 hr test. Control rats showed a decrease in passive avoidance latencies from 24 hr to 48 hr, while the CCK-8 rats did not show such a trend.     

The habenular complex mediates hormonal stimulation of maternal behavior in rats.

The role of the habenular complex (Hbc) in the hormonal onset and nonhormonal maintenance of maternal behavior in rats was examined. In Experiment 1, bilateral lesions were produced in the Hbc on Gestational Day (GD) 12. On GD 16, animals were hysterectomized-ovariectomized and given estradiol benzoate (EB); they were then tested for maternal behavior 48 hr later. Hbc lesions delayed the appearance of all components of maternal behavior for several days. In Experiment 2, large Hbc lesions that were produced on Postpartum Day 4 caused only 1- or 2-day deficits in maternal behavior. These data suggest that the Hbc mediates the hormonal onset of maternal behavior. During the postpartum period, however, the importance of the Hbc for maternal behavior diminishes as the hormones of pregnancy become less important.     

Bulbectomy and sensitivity to estrogen: Anatomical and functional specificity

In order to clarify the influence of the olfactory system on female sexual behavior, ovariectomized rats were given sham operations (SHAM), total bilateral olfactory bulbectomy (TBULBX), partial bulbectomy (PBULBX), anterior olfactory nucleus lesions (AON) or accessory olfactory bulb lesions (AOB), and tested for lordosis behavior. Only TBULBX resulted in increased sensitivity to estradiol benzoate (EB) in that lordosis quotients (LQ) were increased and rejection behavior decreased following administration of 2, 4 or 8 μg EB/kg/day for 3 days. Only TBULBX group rats were anosmic on 2 postoperative tests. TBULBX group rats showed very mild hyperresponsiveness on an emotionality test. Effects of TBULBX on LQ are not due to general sensory hyperresponsiveness or EB-induced hyperresponsiveness since no differences in the quality of lordosis occurred, and no differences occurred in latency to paw-lift on hot plate tests with or without EB. Heightened EB sensitivity in the TBULBX group is not due to adrenal steroids since following adrenalectomy and 8 μg EB/kg treatment, TBULBX group LQ scores were still elevated relative to those of SHAM controls. The LQ scores of PBULBX group rats were intermediate to those of SHAM and TBULBX group rats. Bulbectomy-induced alterations in sensitivity to EB as measured by the LQ do not appear to be due to alterations in “arousal” mechanisms in general. While deficits in olfactory perception might exacerbate the effect, it is unlikely that anosmia per se is sufficient to induce major alterations in the degree of sexual receptivity following EB. The magnitude of behavioral effects of bulbectomy on EB sensitivity may be related, to some extent, to the amount of bulb tissue removed. It is possible that bulbectomy may enhance behavioral sensitivity to EB by disrupting biochemical responses to EB in limbic system structures which normally exert an inhibitory influence over sexual receptivity.     

Estrogen and progesterone interaction in the regulation of scent marking in the female Mongolian gerbil (Meriones unguiculatus)

Ovariectomized female gerbils were treated with either 0, 20, 40, 80 or 160 μg estradiol benzoate (EB) followed in 36 hr by either 0, 250, 500 or 1000 μg progesterone. The animals were tested for ventral scent marking nine hr after progesterone treatment. Treatment with 20, 40 or 80 μg EB resulted in a significant increase in marking in the ovariectomized female for each dose of progesterone used. One hundred and sixty μg EB was not effective while 80 μg was the most effective dose at each dose of progesterone. There were no differences among the 80 μg EB groups in mean marking frequency as long as progesterone was present. Neither estrogen alone nor progesterone alone was effective in stimulating a significant level of marking. An additional sample of ovariectomized females were maintained for four weeks on either 2.5, 5, 10, 20 or 40 μg EB and tested for marking each week 45 hr after EB administration. On the fifth week 250 μg progesterone was given each week 36 hr following EB with behavioral testing nine hr later. Such a regime was followed for four weeks. Marking remained at zero levels on weeks one through four. EB + progesterone on weeks five through eight, however, resulted in a significant increase in marking in animals receiving 10 μg EB (mean = 9.0 ± 3.5), 20 μg EB (mean = 6.6 ± 2.5) and 40 μg EB (mean = 9.2 ± 4.2). It was concluded that both estrogen and progesterone were essential for the support of marking in this sample of low-marking females and that since there were no reliable differences between groups receiving different doses of progesterone, estrogen was the primary stimulus controlling marking behavior, with progesterone perhaps playing a modifying role.     

Facilitation of progesterone induced lordosis behavior by phosphodiesterase inhibitors in estrogen primed rats

The effect of 1-methyl,3-isobutylxanthine (MIX) and theophylline (TPH), two phosphodiesterase inhibitors, on the behavioral response induced by low doses of progesterone (P) was studied in ovariectomized estrogen primed Wistar rats. Administration of 50, 100 or 200 μg of P 44 hr after 2 μg of estradiol benzoate (EB) induced lordosis in 25%, 69% and 71% respectively of the rats used. Maximal mean lordosis quotients (LQ) obtained in the control groups were as follows: (50 μg P=7; 100 μg P=32; 200 μg P=41). Maximal mean LQ's were seen in control groups 4 hr after P. Lordosis behavior disappeared in nearly all control rats within 36 hr of P injection. Repeated injections of 4 mg MIX or 10 mg TPH (40 and 44 hr after EB and thereafter every 8 hr up to 96 hr) significantly enhanced and prolonged the response to P. Maximal mean LQ's of experimental groups were as follows: 50 μg P + MIX=45; 50 μg P + TPH=37; 100 μg P + MIX=46; 100 μg P + TPH=68; 200 μg P + MIX=91; 200 μg P + TPH=93. Lordosis was still displayed by more than 50% of the rats treated with MIX or TPH 36 hr after P. Neither MIX nor TPH alone elicited significant lordosis behavior when given to estrogen primed rats. The results suggest that the induction of lordosis behavior by P is mediated through a rise in cyclic nucleotide levels in neurons related to the expression of sexual behavior.     

Bilateral lesions of the bed nucleus of the accessory olfactory tract facilitate maternal behavior in virgin female rats

In this study the effects of electrolytic lesions of the bed nucleus of the accessory olfactory tract (BAOT) on the induction of maternal behavior in virgin female Wistar rats was investigated. Results demonstrate a functional role of the BOAT in the neural control of maternal behavior in virgin female rats since bilaterally BOAT lesioned (BL) animals showed significantly shorter sensitization latencies (BL = 6 days) than sham lesioned (SH = 12 days) and intact control (C = 12 days) female rats. At the same time, statistically significant differences were observed in retrieval latency between BL (5.5 days) and C (10 days) groups, but not in the SH group (8 days). In physical contact frequency, the BL group reached higher scores than SH or C group. However, bilateral BAOT lesions failed to affect other maternal patterns such as nest building quality. Thus these results indicate that the BOAT, a vomeronasal system structure, is involved in the control of maternal behavior and that BL electrolytic lesions facilitate the onset of this behavior in virgin female Wistar rats.     

Effects of adrenalectomy on maternal behavior in pregnancy-terminated rats

Latencies to the onset of maternal behavior were measured in 16 day pregnancy-terminated rats. Three conditions were tested and included hysterectomy-ovariectomy (HO) with testing beginning 24 hr after surgery and HO plus a concurrent subcutaneous injection of 5 μg/kg estradiol benzoate or hysterectomy only (H) with testing starting 48 hr post-operatively. Within each condition, one group was also adrenalectomized while another was sham-operated at the same time as HO or H. There were no differences in the distribution of latencies between the adrenalectomized and sham-operated controls in any of the conditions although a significantly greater percentage of the sham adrenalectomized H animals were maternal by Day 2 of testing. It was concluded that adrenal secretions are not essential for the onset of maternal behavior in pregnancy-terminated animals while they may add a facilitatory effect following H only.     

Maternal behavior induced in male rats by bilateral lesions of the bed nucleus of the accessory olfactory tract.

In the present study, we investigate the effect of bilateral electrolytic lesions of the bed nucleus of the accessory olfactory tract (BAOT) in male Wistar rats that did not have care-pups experience, using a test of induced maternal behavior. Consistent with our previous findings in virgin female rats (10), there was a significantly shorter sensitization (3 days) and retrieval (2 days) latencies in the BAOT-lesioned group than in the sham-lesioned and intact-control male groups (12 days for both). Based on these findings, we propose that BAOT, a sexually dimorphic nucleus of the vomeronasal system, exerts an inhibitory modulation in the expression of parental behavior in male and female virgin rats. It may do so by maintaining an olfactory-based tonic inhibition of maternal behavior, thereby resulting in the adults' tonic avoidance of the pups until this inhibition is abolished by lesion, or reduced or overridden by appropriate hormonal and/or sensory influences.     

The effect of hysterectomy on hormone-induced lordosis behavior in hamsters

Ovariectomized-hysterectomized (OH) and ovariectomized-sham hysterectomized (OSH) hamsters were tested for lordosis behavior following treatments including either 1, 5, or 10 micrograms estradiol benzoate (EB) in combination with 0.1, 0.25, and 0.5 mg progesterone. Few animals responded at the 1 microgram dose of EB and there were no differences in latency to the first display of lordosis or in the total lordosis duration among responding animals in the 5 and 10 micrograms EB groups. However, there was significantly more positive tests in the OSH group injected with 5 micrograms EB than in the OH group and this difference approached statistical significance in the 10 micrograms EB groups. The results are compared to similar studies in rats and possible mechanisms for the effects of hysterectomy are discussed.     

Induction of maternal behavior in adult female rats following chronic morphine exposure during puberty

The peripubertal period in the female rat is the time when the stimulatory effects of opioids on prolactin (PRL) secretion develop. In the adult rat, the administration of chronic high-dose morphine has been shown to attenuate the ability of opiates to stimulate PRL secretion. One function of PRL in adult virgin rats is the induction of maternal behavior. The present study examined whether chronic high-dose morphine exposure during the peripubertal period alters PRL-mediated induction of maternal behavior in adult female rats. Two groups of juvenile female rats were administered increasing doses of morphine or vehicle (s.c.) from age 30 to 50 days. As adults, these females either remained intact, or were ovariectomized and treated with a PRL-dependent, steroid hormone regimen that stimulates a rapid onset of maternal behavior. All females were then exposed daily to rat foster pups to determine whether peripubertal morphine exposure affected their latencies to induce maternal behavior. Morphine treatment resulted in a delay in vaginal opening and a temporary reduction in the rate of weight gain; however, the rate of onset of maternal behavior was unaffected by peripubertal morphine treatment. Thus, chronic morphine exposure in the pubertal female did not impact the expression of pup-induced maternal care.     

Cholecystokinin stimulates and inhibits lordosis behavior in female rats

Recently, IP CCK-8 has been shown to inhibit lordosis in sexually experienced, estradiol benzoate (EB) and progesterone (P) primed rats. However, receptivity is influenced by prior sexual experience and/or exposure to sex steroids, as well as the steroid dosage administered before testing. Thus, we examined the effect of CCK-8 (3 micrograms/kg; IP) on lordosis in rats with different degrees of receptivity. Three weeks after ovariectomy, females were treated with EB followed 48 hr later with P, or with EB alone. CCK-8 significantly facilitated lordosis in rats given 5 micrograms EB. Following a 5 week nonexperimental period, animals were more receptive and CCK-8 significantly inhibited lordosis in the 5 or 10 micrograms EB groups. In a separate experiment, rats were ovariectomized, adrenalectomized, and treated with EB alone. As in the first experiment, CCK-8 facilitated and inhibited lordosis. CCK-8's effects were highly dependent on the female's receptivity, facilitating lordosis when receptivity was low and inhibiting lordosis when receptivity was high (but not maximal). In conclusion, IP CCK-8 modulates lordosis behavior independent of P, but its effects depend on the female's degree of receptivity.     

Environmental enrichment delays pup-induced maternal behavior in rats

Adult, virgin rats do not spontaneously display maternal behavior when exposed to foster pups. However, continuous daily exposure of the female to foster pups for about 5-7 days can induce a set of maternal behaviors similar to those shown by postpartum dams. Induction latencies depend upon a number of factors, including the stress and anxiety levels of the female. The goal of this study was to attempt to mitigate the likely stressfulness of being singly housed during testing by enriching the rat's home cage environment and to determine if the concomitant environmental change would alter the latency to express maternal behavior. In addition, the effect of varying the number of test pups used for testing was examined. Two groups of virgin Sprague-Dawley rats were first tested on the elevated plus maze after 1 week of exposure to either control (standard housing) or enriched conditions. One week later, maternal behavior testing began using one or three pups. Upon completion of maternal behavior testing, plasma corticosterone concentrations were determined following a mild stressor. The data indicate that enrichment tends to increase anxiety-like behaviors in the elevated plus maze. In addition, enrichment delayed the onset of maternal behavior irrespective of the number of test pups. There were no effects of environmental enrichment on plasma corticosterone levels following exposure to a stressor. These results indicate that what is considered a modestly enriched environment delays the expression of pup-oriented responses and does not apparently reduce stress or improve performance on all behavioral tasks.     

Maternal behavior in the mouse: effect of estrogen and progesterone

The administration of 100 μg and 50 μg of estradiol benzoate for 5 consecutive days to adult, nulliparous. Rockland-Swiss mice that previously exhibited maternal behavior resulted in a loss of such behavior. The effect of estradiol was temporary since normal maternal activities were exhibited when the animals were retested two weeks after the termination of hormone treatment. Treatment with 5 μg and 0.5 μg estradiol benzoate did not significantly affect maternal behavior. Progesterone treatment at dosages of 1000 μg and 500 μg was without affect.     

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.     

Obese mice and the satiety effects of cholecystokinin, bombesin and pancreatic polypeptide

Cholecystokinin (CCK-8), bombesin (BBS) and pancreatic polypeptide (PP) are gastrointestinal hormones which are released during a meal and which decrease food intake when administered exogenously. The satiety effects of CCK-8, BBS and PP were measured in weanling and adult Bar Harbor obese and lean mice. Weanling mice fasted 5.5 hr were less sensitive to both CCK-8 and BBS at 5–6 weeks of age, when body weights of the obese were 20% greater than lean. The obese were equally sensitive as lean mice to CCK-8 and BBS at 7–8 weeks of age when the obese were 50% heavier. After a 3.0 hr fast adult obese mice, weighing 100% more than lean mice, were less sensitive to satiety effects of lower doses of CCK-8 (1.0 μg/kg) and PP (8 μg/kg) but equally sensitive to higher doses of CCK-8 (2.0 μg/kg), PP (16 μg/kg) and all doses of BBS (2.0, 4.0 and 8.0 μg/kg). After a 6-hr fast, 16 μg/kg PP did not affect food intake in obese or lean mice, whereas 32 μg/kg PP decreased food intake more in obese than lean mice. Thus both weanling and adult obese mice, as obese rats, are less sensitive to the putative satiety agent CCK-8.     

Maternal memory in adult, nulliparous rats: effects of testing interval on the retention of maternal behavior

The retention of maternal behavior (i.e., maternal memory) was measured in adult, nulliparous rats induced to respond maternally by continuous exposure to foster pups. Specifically, the effects of the interval duration between the initial induction and the reinduction of maternal behavior were determined. Intact virgin rats were first exposed to foster young to induce maternal behavior. During the initial induction phase, females were required to be fully maternal on 2 consecutive test days. Animals were then assigned to one of three interval groups (10, 20, or 40 days). After being isolated from rat pups for these designated periods, females in each group were tested again for their latencies to induce maternal behavior. Whereas the initial median latencies to display full maternal behavior ranged from 4.5 to 5 days for each group, upon retesting, median latencies for each group declined to 1 to 4 days. The greatest reduction in latency was present in the 10-day group (80%), and the smallest reduction was detected in the 40-day group (20%). A significant negative linear correlation was found between test interval and percentage reduction in behavioral latency. Based upon this relationship and under these test conditions, "maternal memory" in the adult, nulliparous rat would be expected to be nondetectable after about an interval of 50 days between tests. The pattern of maternal memory acquisition and loss appears similar to that reported in parous animals. The present study highlights similarities and possible differences underlying the establishment of the retention of maternal behavior (i.e., maternal memory).     

The connections of the medial preoptic region and maternal behavior in the rat

Female rats which are hysterectomized and ovariectomized on Day 16 of pregnancy and injected with estrogen show a short latency to onset of maternal behavior when presented with test pups 48 hrs later. In the present experiment, female rats were treated similarly except that on Day 16 of pregnancy they received knife cuts which severed either the lateral, anterior, dorsal, or posterior connections of the medial preoptic area (MPOA), or sham knife cuts. Severing the lateral connections of the MPOA severely disrupted materal behavior, while severing the dorsal or posterior connections of the MPOA produced either minor deficits or no deficits. Severing the anterior connections of the MPOA did produce large deficits in maternal behavior, but this was associated with hypoactivity and loss of body weight. Therefore, the maternal behavior deficits observed in the anterior cut group may have been a secondary effect of the knife cut. The results emphasize the importance of the lateral connections of the MPOA for maternal behavior.