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To investigate the participation of prolactin in nest-building and maternal behaviour in rabbits, we administered (from pregnancy day 26 to parturition) rabbit prolactin (rbPRL; or vehicle) intracerebroventricularly (i.c.v.) to primiparous animals injected with bromocriptine subcutaneously (s.c.). Control females (given vehicle s.c. and i.c.v.) built a maternal nest (of straw and body hair) in 77% of cases. This proportion decreased to 19% in the bromocriptine-only group (P < 0.05) and increased to 93% in the group given bromocriptine plus rbPRL (P > 0.05). Maternal behaviour (i.e. the adoption of a crouching posture over the litter inside the nest box) was expressed by 77% of control rabbits, 19% of bromocriptine-only animals (P < 0.05) and 57% of females given bromocriptine plus rbPRL (P > 0.05). Values of nonmaternal activities (i.e. scent-marking, ambulation in an open field) were similar among the three studied groups. These results suggest that prolactin, acting in late pregnancy, plays a major role in the stimulation of nest-building and maternal behaviour in rabbits.

Continuous exposure of non-parturient rats to pups can induce maternal behavior similar in most aspects to that found in the postpartum rat. Surprisingly, young juvenile rats (20-24 days of age) only require 1-3 days of exposure to pups, while adults require 4-8 days before maternal behavior emerges. Dopamine (DA) and possibly serotonin (5-HT) may mediate the expression of adult maternal behavior. We hypothesize that postnatal changes in DA and 5-HT within the neural circuit that supports maternal behavior including the medial preoptic area (MPOA), medial and cortical amygdala (MCA), and nucleus accumbens (NAC), may underlie these differences in responsiveness across juveniles and adults. We measured DA, 5-HT, and their metabolites in postmortem samples of these regions in maternal and non-maternal juvenile and adult females. The only difference found across behavioral groups was that the MPOA of adults induced into maternal behavior by pup exposure had more DA than did that of isolated adult females or maternal juveniles. However, when adults versus juveniles were compared, the content of DA and 3,4-dihydroxyphenylacetic (DOPAC) was higher in the adult than in the juvenile NAC and MCA; the content of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in these structures did not vary across the age groups. In contrast, higher levels of 5-HT and 5-HIAA were found in the MPOA in juveniles compared to adults. We propose that these region-specific age differences in DA and 5HT may underlie differences in juvenile-adult responses to pups.

This set of experiments investigated the appetitive or motivational processes underlying the performance of maternal behavior. The place preference paradigm was adapted to simultaneously investigate the reinforcing properties of cocaine and pups for maternal, lactating dams. These modifications allowed the authors to assess which stimulus, either a 10 mg/kg s.c. injection of cocaine or 3 pups, had the strongest reinforcing value. At Postpartum Days 10 and 16, the dams preferred the cocaine cue-associated chamber, whereas the dams tested at Postpartum Day 8 preferred the pup cue-associated chamber. Overall, the data revealed an interaction between the postpartum period at testing and the exhibited preference for cocaine or pups. Further testing will investigate the neural circuitry underlying the appetitive processes of each stimulus.

We investigated the role of prolactin and suckling stimulation in the expression of maternal behaviour of primiparous rabbits. Bromocriptine (1 mg/kg/day), given to intact mothers across postpartum days 1-5, decreased serum concentrations of prolactin to undetectable levels, reduced crouching, and increased time inside the nest. Failure of maternal nest-building, provoked by bromocriptine injections from pregnancy day 26 to parturition or to postpartum day 5, correlated with a stronger reduction in crouching and an increased time inside the nest, measures of disturbed maternal behaviour, on postpartum days 3 and 5. Preventing suckling by thelectomy did not prevent prolactin release but reduced crouching incidence and increased the time spent inside the nest on postpartum days 3 and 5. Bromocriptine, injected in thelectomized mothers across postpartum days 1-5, further reduced the incidence of crouching and increased the time spent inside the nest on postpartum days 3 and 5. We conclude that prolactin acting prepartum facilitates maternal behaviour initiation in rabbits and, together with pup stimulation, maintains this behaviour across lactation.

Injection of the serotonergic agonist, 8-hydroxy-2-(di-n-propylamino-tetralin (8-OH-DPAT) (0.5 mg/kg ip) produced a clear anxiolytic-like effect (as measured in the burying behavior test), after parturition, which remains until day 6 of lactation. Thereafter 8-OH-DPAT completely lacked action. In order to analyze whether lactation prevented the action of 8-OH-DPAT, dams were separated from their pups for five consecutive days. The blockade of the anxiolytic effect of 8-OH-DPAT does not disappear by isolation of the mothers from their offspring or from neighboring pups. Finally, to investigate the possible role of maternal behavior in the blockade of the anxiolytic effect of 8-OH-DPAT a third experiment was made in which ovariectomized females were rendered maternal by the sensitization procedure. These females respond normally to the antianxiety actions of 8-OH-DPAT. Results suggest that a long-term clue triggered by lactation, possibly related to prolactin secretion, interferes with the anxiolytic effect of 8-OH-DPAT.

On the basis of evidence that 14C-2-deoxyglucose (2-DG) autoradiography indicates activity at axonal terminals, whereas c-fos immunocytochemistry indicates activity of neuronal cell bodies, we combined these techniques in adjacent histological brain sections to assess excitatory and disinhibitory synaptic relations in selected sites in female rats in which maternal behavior was elicited by natural parturition, sensitization (7- to 10-day cohabitation with foster pups), or hysterectomy. All individuals in these three groups expressed maternal behavior immediately before 2-DG injection. Controls were non-maternal virgins. Parturient and Hysterectomized groups: elevation (compared with controls) in both 2-DG and c-fos activity in medial preoptic area (MPOA) indicated an increase in its input and output activity, i.e., an excitatory interaction; the MPOA was previously shown to be critical for maternal behavior. Sensitized group: a decrease in 2-DG activity of vomeronasal nuclei (bed nucleus of the accessory olfactory tract, BAOT, and medial amygdala, ME, replicating our previous study) and an elevation in c-fos activity, jointly indicate disinhibition of these nuclei, that were previously shown to modulate pup-chemostimulation-induced sensitization. All other sites showed evidence of excitatory input-output relationships (i.e., joint increase in both 2-DG and c-fos activity), e.g., bed nucleus of the stria terminalis (BNST), lateral habenula (LHAB), central gray (CG), thalamus (THAL), septum (SEPT), and ventral tegmental area (VTA). The present study demonstrates the feasibility of measuring 2-DG and c-fos activity jointly in adjacent sections of the same brain, thereby providing evidence to distinguish between localized excitation and disinhibition.

To determine whether the neurons of the medial preoptic area (MPOA) are necessary for pup-induced maternal behavior (MB) in juvenile and adult rats, subjects received bilateral injections of the neurotoxin N-Methyl-D-Aspartic acid into the MPOA. Controls were intact or were sham treated by surgical placement of the syringe barrel. The rats were then induced into MB by constant pup exposure. Starting at 27 (juvenile) or 60 (adult) days of age, rats were tested for MB for 12 consecutive days. After histological analysis, rats were categorized as having either large or small lesions of the MPOA. In juveniles, large lesions of the MPOA blocked retrieval and impaired nest-building, but crouching behavior was unaffected; small lesions had no effect on MB. In contrast, in adults, large or small lesions severely impaired all components of MB. The results suggest that in juvenile rats, the role of the MPOA neurons in MB is only partially established, whereas by 60 days of age, the unsubstitutable role of the MPOA in the neural circuit that mediates MB is fully established.

Juvenile rats can exhibit maternal behavior after being exposed continuously to rat pups, a process called sensitization. Maternal behavior in juveniles is robust and is similar to adult maternal behavior (Mayer and Rosenblatt [1979] Dev. Psychobiol. 12:407-424; Gray and Chesley [684] J. Comp. Psychol. 98:91-99). In this study, immunocytochemical detection of the protein products of two immediate-early genes, c-fos and fosB, was used as a tool to identify forebrain neuronal populations involved in the maternal behavior of 27-day-old juvenile rats compared with 60-day-old adults. To sensitize them, rats were exposed continuously to foster pups. Once they were maternal, they were isolated from pups overnight, reexposed to pups for 2 hours, and then killed. Nonmaternal control animals also were isolated overnight and were either reexposed to pups for 2 hours or kept isolated from pups before killing. The lateral habenula (LH) was the only area in which both maternal juveniles and maternal adults had more c-Fos-immunoreactive (-Ir) neurons compared with controls. In maternal adults, the number of neurons that expressed c-Fos and FosB immunoreactivity increased in the medial preoptic area (MPO) and the ventral bed nucleus of the stria terminalis (BSTv), whereas the dorsal bed nucleus of the stria terminalis (BSTd) and the medial and cortical nuclei of the amygdala (MEA and COA, respectively) had increases only in the number of neurons that expressed c-Fos immunoreactivity. In contrast, juveniles, whether or not they were maternal, had the same number of c-Fos-IR and FosB-Ir neurons in all these areas. The adult-like increase in the number of c-Fos-Ir neurons found in maternal juveniles suggests that the juvenile LH participates in the neural circuit that supports maternal behavior in an adult-like manner. The lack of c-fos or fosB induction in the MPO, BSTv, BSTd, COA, or MEA of maternal juveniles compared with maternal adults may reflect the immaturity of these brain regions in juvenile rats. Exactly what this immaturity consists of and when the responses of these regions become adult-like remain to be determined., (Copyright 2000 Wiley-Liss, Inc.)

Cocaine was microinfused bilaterally (50 microg/0.5 microl/side) into the medial preoptic area (MPOA) or nucleus accumbens (NA), 2 regions within the rat brain neural circuit known to mediate maternal behavior (MB). Additionally, 2 sites not involved in this neural circuit, the dorsal striatum and dorsal medial hippocampus, were used as control sites. Microinfusion of cocaine into the MPOA or NA impaired MB, whereas infusion into the control sites did not. MB impairment was not temporally coincident with the increased locomotor activity, also documented after cocaine infusion into the MPOA or NA, arguing strongly that impaired MB is a direct, specific effect of cocaine in these areas, not a derivative of increased motor activity. This is the first demonstration that cocaine action on single central nervous system (CNS) sites can impair MB to the same extent as systemic injections. Thus, cocaine's simultaneous effect on multiple CNS sites is not required for MB impairment.

We demonstrated previously that the lateral habenula (Lhb) mediates maternal behavior. Our present goal was to identify the first and second order afferent connections of the Lhb, particularly those relevant for maternal behavior. Using pseudorabies virus (PRV) as a retrograde transneuronal tracer and the retrograde tracer Fluoro-Gold, we identified first order Lhb afferent projections from the lateral preoptic area, bed nucleus of the stria terminalis, nucleus accumbens and ventral tegmental area, each important for the display of maternal behavior. Maternally relevant second order neurons originated from the medial preoptic area and amygdala. Additional regions with first and second order neurons afferent to the Lhb were also identified.

The natural onset of maternal behavior in the rat is hormonally mediated. Estrogen, progesterone, and prolactin administered to ovariectomized females in amounts and sequences that produce circulating levels similar to those found during pregnancy stimulate the onset of maternal behavior. In fact, maternal behavior can be stimulated by estrogen alone, administered either peripherally or by implant in the central nervous system. The lateral habenula (Lhb), which is a necessary component in the neural circuit that supports maternal behavior, contains a subset of neurons with estrogen receptors. The present study investigated whether estradiol implants directly in the Lhb are sufficient to stimulate maternal behavior. Female rats, hysterectomized and ovariectomized on day 16 of pregnancy, received estrogen implants in the Lhb or, as a positive control, in the medial preoptic area (MPOA). An additional control group received cholesterol implants in the Lhb. All females were tested for pup retrieval, nest building, crouching behavior, locomotor activity, and carrying behavior. Estradiol implants into the Lhb did not stimulate the onset of maternal behavior. Females with estrogen implants in the Lhb scored significantly lower in pup retrieval and crouching behavior compared to females with implants in the MPOA and were not significantly different from females with cholesterol implants in the Lhb. There were also no significant differences in overall activity or carrying behavior among the groups., (Copyright 1999 Academic Press.)

Our research has demonstrated that the lateral habenular nucleus (Lhb) is necessary for the hormonal onset but not the postpartum maintenance of maternal behavior in the rat (K. P. Corodimas, J. S. Rosenblatt, & J. I. Morrell, 1992; K. P. Corodimas, J. S. Rosenblatt, M. E. Canfield, & J. I. Morell, 1993; T. Matthews-Felton, K. P. Corodimas, J. S. Rosenblatt, & J. I. Morell, 1995). To test the role of the Lhb in the nonhormonal onset of maternal behavior, we used the sensitization model in which the continual exposure of females to pups induces maternal behavior. Ovariectomized females received bilateral cytotoxic lesions of neurons of either the Lhb or the dorsal medial cingulate cortex-hippocampus, or they were unoperated. Maternal behavior, activity, and oromotor carrying capability were tested. Complete lesions of the neurons of the Lhb induced significant deficits in pup retrieval and nest building. Sniffing, licking, and crouching behaviors were unaltered. Activity and carrying ability were normal. These results indicate a role for the Lhb that extends to the nonhormonally dependent onset of maternal behavior, but they also indicate a more limited role than in the mediation of the hormonal onset of the behavior.

The aim of the study was to determine whether there is an increase in responsiveness to estrogen stimulation of maternal behavior and lordosis responsiveness during pregnancy. Using separate groups of pregnancy-terminated females, we measured the initial maternal responsiveness of hysterectomized-ovariectomized (HO) females and their responsiveness to estrogen stimulation. Maternal behavior latencies were studied in females HO on the 8th, 10th, 13th, 16th, or 19th day of pregnancy (8HO-19HO) and in nonpregnant HO (NPHO) females. Groups were injected sc with estradiol benzoate (EB) in doses ranging from 0 to 200 microgram(s)/kg and tested for maternal behavior (retrieving, crouching, and licking pups). In addition, we investigated whether there is an increase during pregnancy (following HO) in lordosis responsiveness to estrogen stimulation. Lordosis behavior was studied in pregnant HO females (days 8, 16, and 22) and NPHO females given 0 to 200 microgram(s)/kg EB. There was an increase in maternal responsiveness in oil-treated HO females starting around midpregnancy. From early pregnancy on there was also an increase in maternal responsiveness to 20 microgram(s)/kg EB. In late pregnant females (16HO) there was a further increase with 50 microgram(s)/kg EB. There was no increase in lordosis responsiveness to EB stimulation during pregnancy; pregnant and nonpregnant HO females had the same EB threshold for stimulating lordosis behavior. The results of both studies were related to increases during the latter half of pregnancy in nuclear estrogen receptor concentrations in the MPOA, an area that mediates estrogen stimulation of maternal behavior, and the absence of such increases during pregnancy in the VMH, an area that mediates estrogen stimulation of lordosis behavior.

We prevented mother/pup contact at parturition or across early or midlactation to investigate the importance of such interaction for maintaining material behavior in rabbits. When pup contact was prevented across lactation Days 1-7 or 11-17 (by anesthetizing multiparous mothers during the oxytocin-induced milk letdown; Experiment 1), nursing incidence was reduced to 40% and 83%, respectively, on the day following anesthesia withdrawal. Both groups also showed a decreased milk output, long latencies to initiate nursing, and several entrances into the nest box not associated with nursing. In Experiment 2 we prevented mother/litter contact at parturition to determine the specific role of pup contact at this time. We found a reduction in the incidence of nursing on postpartum Day 1 from 80% (in control primiparous mothers) to 33%. By contrast, 100% of both deprived and control multiparous mothers displayed nursing on Day 1. These mothers also showed the unusual behaviors found in Experiment 1 and an extemporaneous nest-building. We conclude that: (a) mother/young contact at parturition is crucial for establishing maternal responsiveness in primiparous does, (b) the experience acquired by raising a previous litter allows the retention of maternal responsiveness despite a lack of pup contact at parturition, (c) maternal responsiveness is maintained across early lactation by daily interaction with pups, and (d) interaction with pups across midlactation allows the finely tuned display of maternal behavior.

The present study investigated whether the medial preoptic area (MPOA) mediates estrogen stimulation of maternal behavior in the male as it does in the female. Previous studies have shown that lesions of the medial preoptic area prevent sensitization of maternal behavior in male rats and that in gonadectomized, hormonally primed males, systemically administered estradiol benzoate stimulates short-latency maternal behavior. These findings are similar to those found in females. In the present study adult males were gonadectomized and hormonally primed with subcutaneously implanted capsules of estradiol (Days 1-16) and progesterone (Days 3-15) and then were stereotaxically implanted bilaterally in the MPOA with implants containing 10% estradiol. Tests with young pups were started 48 h later and continued for 10 days (11 tests). Control groups were implanted in the MPOA with cholesterol or were injected subcutaneously with estradiol benzoate (100 microg/kg). Estradiol implanted males had shorter latencies for maternal behavior (retrieving, crouching, licking pups) than cholesterol implanted males, but their latencies were slightly longer than those of estradiol benzoate injected males. The medial preoptic area, therefore, mediates estrogen stimulation of maternal behavior in males as it does in females.

Estradiol, progesterone, and prolactin regulate digging, carrying straw, and plucking hair for maternal nest-building in rabbits. To explore whether external factors also modulate this process, we assessed whether shaved pregnant rabbits with straw nests would collect their own, male, or synthetic hair for nest-building. Pregnant (but not estrous) does collected and used all hair types, indicating that hair-plucking can be bypassed and a nest constructed with "alternative" hair. Unshaved pregnant does with straw nests also collected synthetic hair, indicating that this behavior is not triggered by the absence of maternal hair. Yet, if hair-plucking/nest-building had occurred, hair-collecting was prevented, suggesting that an internally triggered "drive" was turned off by perceiving a built nest. When given only straw or hair, shaved pregnant does collected and used the material provided, indicating that nest-building is internally driven and accomplished by using any available elements. When given both materials, increasingly more shaved does built straw nests across prepartum days 7 to 2. Straw-carrying declined thereafter, suggesting that perceiving a straw nest limits the collection of such material. Hair-collecting was postponed until prepartum day 1 to postpartum day 2, indicating that: (a) mothers distinguish between straw and hair and (b) hormonal factors regulate the sequential selection of straw and then hair and when the change from straw to hair occurs. Maternal behavior was normal at parturition and for the next 4 days in a similar proportion of does among all experimental groups. We conclude that hormones and external factors regulate nest-building by acting as "on/off/on" signals.

Maternal nest-building in rabbits, expressed across the last third of pregnancy, consists of: digging a burrow, collecting straw and shaping it into a nest inside the burrow, plucking body hair and lining the straw nest with it. The sequential expression of these activities is correlated with specific changes in the plasma concentration of estradiol, progesterone (P), and prolactin (PRL). To further substantiate the participation of these hormones in the control of maternal nest-building we explored in ovariectomized (ovx) New Zealand white rabbits the capacity of several combinations of such hormones to stimulate digging, straw-carrying, and hair-pulling. Does given estradiol benzoate (EB; 5 micrograms/day from days 3 to 21) plus P (2 or 10 mg/day from days 4 to 16) dug into a substrate from the fourth day of the P treatment until withdrawal of this hormone. The intensity of this effect was greater in the group treated with the high dose of P. Straw-carrying and hair-pulling occurred after P withdrawal in a dose-response way. Food intake, which declines in pregnant females shortly before parturition, decreased to the same extent in both groups of ovx EB-treated does after P withdrawal. A significant increase in PRL plasma levels was observed on day 9 in does given EB plus 2 mg P/day and at two days following P withdrawal in does given EB plus 10 mg P/day. When such ovx EB/P-treated does were given bromocriptine to block PRL release (1 or 3 mg/Kg/day, from days 11 to 21) the expression of digging was unmodified. By contrast, bromocriptine abolished the display of straw-carrying and hair-pulling, and also prevented the decline in food intake normally following P withdrawal. The addition of ovine PRL to ovx EB/P-treated does given bromocriptine reduced the expression of digging, did not restore straw-carrying or hair-pulling, and provoked a sharp decline in food intake. The possible mechanisms of interaction between PRL and steroid hormones for the regulation of specific aspects of the pregnant doe's physiology and behavior are discussed.

Male rats exhibit maternal behavior prepuberally and in adulthood, but the neural mechanisms and the ability of males to respond to hormones that stimulate maternal aggression (following arousal of maternal behavior) in females have not been studies. In Experiment 1, males were exposed to pups to stimulate maternal behavior (sensitization) after either radiofrequency lesions of the MPOA or sham lesions with nonactivated electrodes that penetrated the MPOA. Nonsurgical males served as a CONTROL group. The LESION male group showed severe deficits in all components of maternal behavior compared to the latter two groups that showed no behavioral deficits. Females in the LESION group and those given SHAM 1 lesions (produced by electrodes without current introduced into the MPOA) also showed severe deficits in maternal behavior compared to SHAM 2 females (electrode lowered to above the MPOA without current) and nonsurgical CONTROL females. In Experiment 2, prolonged estradiol (E2), progesterone (P) treatment followed by an injection of either 20 micrograms or 100 micrograms/ kg estradiol benzoate (EB) or oil in castrated males was effective in stimulating short-latency maternal behavior, mainly in the 100 micrograms/kg EB group. Males of this group also showed a high level of maternal aggression that was inversely correlated with their latencies for maternal behavior. All groups showed maternal aggression when maternal behavior was established. The results indicate the MPOA mediates maternal behavior in males as it does in females; maternal aggression in males accompanies the stimulation of maternal behavior and may be stimulated by the same hormones.

This paradigm distinguished between two hypotheses not previously directly addressed. Do repeated exposures to cocaine at critical times during pregnancy, when the neural mechanisms that support maternal behavior are being read, alter some fundamental neural underpinning of maternal behavior in rats? Alternatively, does cocaine alter maternal behavior only when circulating? During the 4 hr after cocaine injection (20 or 40 mg/kg), there were significant deficits in maternal behavior. In contrast, 16 hr after cocaine injection, drug-injected females, in which plasma cocaine had fallen to nondetectable levels, showed the normal maternal behavior of saline-injected controls. This pattern of impaired maternal behavior after cocaine injection, followed by normal behavior as blood levels returned to zero, was replicated over 8 days. It was concluded that cocaine impairs maternal behavior only when circulating and does not have a residual effect in the transiently drug-free, chronically drug-treated dam.

In 16-day pregnant, hysterectomized-ovariectomized and estradiol benzoate-treated rats, cytotoxic lesions of the lateral habenula (Lhb) produced severe deficits in maternal behavior (K.P. Corodimas, J.S. Rosenblatt, M.E. Canfield, & J.I. Morrell, 1993). To determine if deficits could be found in parturient rats, females were bilaterally injected with kainic acid (KA) to produce cytotoxic lesions of the Lhb. Controls either received bilateral KA-induced lesions to the hippocampus or were unoperated. All females maintained their pregnancies, underwent parturition, and were tested for maternal behavior (pup retrieval, nursing, and nestbuilding), general activity, and oromotor carrying. Females with lesions of the Lhb had severe disruptions of all components of maternal behavior and postpartum estrus. These results extend previous findings that the Lhb is involved in the hormonal onset of maternal behavior and also demonstrate that the Lhb supports the display of postpartum estrus.

Levels of [14C]2-deoxyglucose (2-DG), measured autoradiographically, in the medial preoptic area (MPOA), were higher during natural parturition with concurrent maternal behavior than in non-pregnant non-maternal controls, whereas levels in the vomeronasal system were lower in virgin rats made maternal by cohabitation with young than in control and parturient rats. Previous studies have shown that lesions of MPOA disrupt maternal behavior, whereas lesions of vomeronasal structures stimulate it, and that an increase in 2-DG levels is indicative of an increase in firing activity in neuron terminals. Consequently, the present findings suggest that maternal behavior can be induced by: (a) an increase in parturition-generated sensory stimulatory input to the MPOA in response to mechanostimulation of the birth canal, and (b) a separate chemosensory vomeronasal pathway whose activity is reduced cohabitation with young, thereby disinhibiting maternal behavior.

Psychobiological studies of maternal behavior among non-primate mammals have arrived at several general conclusions that may aid in our understanding of human maternal behavior and in the clinical treatment of parenting disorders. The hormonal basis of maternal behavior arises during pregnancy and consists of a hormonal priming phase, extending over most of pregnancy, and a hormonal triggering phase at the end of pregnancy. Hormonal priming in several species depends upon estrogen, progesterone and prolactin, and hormonal triggering depends upon a decline in progesterone and an increase in estrogen, prolactin and oxytocin in different species. The onset of maternal behavior is hormonally based but postpartum maternal behavior in several species is based upon the stimulation that the mother receives from her young (i.e. is non-hormonally based). There is a transition period between these two phases soon after parturition during which contact with the young plays a crucial role in the maintenance of maternal behavior. The onset of maternal behavior may be viewed as the resolution of conflict between approach and withdrawal responses of females to their offspring.

Several parameters associated with maternal behavior were quantified under laboratory conditions in New Zealand white rabbits. Digging behavior appeared earliest (8-6 days prepartum), its decline preceding the onset of straw carrying (3-1 days prepartum). Hair pulling consummated the construction of the maternal nest. Food intake significantly decreased on days 2 and 1 prepartum. On parturition day, all females spent 300-500 s with the litter while, for the rest of lactation, nursing bouts lasted 199 +/- 7 s. Milk yield increased linearly up to lactation day 19, declining thereafter. Pup weight increased linearly throughout lactation despite the decline in milk yield. Plasma estradiol (E) levels did not significantly vary across pregnancy: 60 +/- 2 pg/ml (days 10-25) and 75 +/- 6 pg/ml (day 30). The testosterone (T) levels at these times were: 200 +/- 10 and 308 +/- 0.03 pg/ml, respectively. Testosterone significantly declined from pregnancy day 30 to lactation day 1 (202 +/- 0.02 pg/ml). Progesterone (P) levels significantly declined from pregnancy day 20 (9 +/- 1 ng/ml) onwards. Progesterone levels were negligible across lactation. Thus, mother rabbits display a sequence of motor patterns and somatic events correlated with changes in plasma levels of T and P against a background of E.

Estrogen stimulation of maternal behavior during pregnancy in the rat has been studied at several levels of analysis. These include (a) changes in maternal responsiveness during pregnancy; (b) hormonal stimulation of maternal behavior; and (c) correlation between nuclear binding of estradiol in the medial preoptic area and the stimulation of maternal behavior (i.e., in pregnancy-terminated, ovariectomized females treated with estradiol benzoate). These studies have given rise to the concepts of hormonal priming and triggering of maternal behavior during pregnancy and at parturition. More recently, using in situ hybridization, ER mRNA was measured during pregnancy (also diestrus and postpartum) in brain regions in which binding previously had been studied, to investigate further the regulation of hormonal priming. Steady state levels of ER mRNA per cell and cell densities of ER mRNA produced a measure of total ER mRNA per brain region which was then compared to nuclear estrogen receptor binding. The relation between binding and ER mRNA is presented for one of the brain regions, the rostral medial preoptic nucleus. The results indicate that ER transcription is regulated during pregnancy, but regulation is specific to each brain region and there is no simple relation between ER mRNA and nuclear estrogen receptor binding.

Expanding on research showing that radio-frequency-induced lesions of the habenular complex disrupt the hormonal onset of maternal behavior in rats, we explored the importance of neurons in the lateral (Lhb) subdivision of the habenular complex for the onset of maternal behavior. On Day 12 of pregnancy, bilateral cytotoxic lesions were produced in the Lhb with kainic acid or, as a control, just dorsal in the medial hippocampus. A 3rd group had radio-frequency-induced Lhb and medial (Mhb) lesions. On Day 16 of pregnancy, Ss were hysterectomized-ovariectomized, given estradiol, and tested 48 hr later for 10 days. Neuroanatomical tracing with fluoro-gold was then used to directly quantify the extent and location of Lhb neuron loss and to verify that Mhb neurons were intact. Cytotoxic lesions of the entire Lhb prevented the onset of all components of maternal behavior compared with controls. Results show that neurons in the Lhb, not the Mhb, are important for onset of maternal behavior in rats.

We examined the effects in postpartum female rats (Rattus norvegicus) of olfactory deafferentation by intranasal ZnSO4 on maternal aggression and retention of aggression 24 hr after pups are removed. When females were tested with pups, effects of deafferentation depended on postpartum interval; anosmia failed to reduce aggression on Lactation Day 3 (L3), but anosmia was associated with moderate reductions on L8-L12. We propose that postpartum aggression is stimulated initially by hormonal factors associated with late pregnancy but later is supported by exteroceptive stimulation. After removal of pups, although aggression scores were lower across treatment conditions, anosmic females had shorter latencies to attack and tended to elicit more submissive behavior from intruders. Data suggest that odor cues from conspecifics normally inhibit aggression by females, but this inhibition is reduced during lactation.

A modified method of applying ZnSO4 to the olfactory mucosa is described. Treated rats experienced severe nasal congestion that cleared within 24 h; more persistent morbidity did not occur. Nonpregnant females observed with male intruders 24 h following ZnSO4 showed no alterations in behavior other than a reduction in anogenital sniffing, indicating that they were not hypoactive or irritable. In other experiments, lactating females were observed in a hole-board apparatus; 2 days posttreatment anosmia was confirmed in 80% of bilaterally ZnSO4-treated females by the absence of preference for pup odors. After bilateral but not unilateral ZnSO4 treatment, initially activity scores and nose pokes were equivalent in all groups, but later they both were lower than in controls, probably due to a more rapid habituation to the novel apparatus. We conclude that intranasal ZnSO4 by small-volume spray is a useful experimental tool.

Two experiments explored the effects on subsequent maternal aggression of interspersing 5-24 days between pregnancy termination and pup exposure to elicit maternal behavior. Pregnancy-terminated females displaying maternal behavior were compared to nonpregnant females also stimulated by pups to behave maternally and to pregnancy-terminated groups that were not exposed to pups. Pregnancies were terminated by hysterectomy-ovariectomy on Gestation Day 19 (G19), with pup exposure commencing after 6 days (Expt. 1), and by Cesarean section + ovariectomy (OVX), after delays of 5, 10, or 24 days (Expt. 2). In each study pregnancy-terminated, maternal groups were more aggressive than both nonpregnant, maternal groups and pregnancy-terminated, nonmaternal groups regardless of the imposed delays. In Expt. 2, postsurgical interval also was a significant factor, suggesting that surgical stress may elevate maternal aggression for several days. Prolonged treatment of nonpregnant OVX females (Expt. 3) with estrogen and progesterone elevated maternal aggression when pup exposure commenced 2 but not 7 days after termination of hormone treatment (Expt. 3). Therefore the physiological changes initiated during pregnancy that support maternal aggression persist for at least 4 weeks and may involve factors in addition to ovarian hormones that stimulate maternal behavior and maternal aggression. An additional unexpected finding was that nonpregnant (OVX) females had significantly higher Aggression scores after initiating maternal behavior.

Bilateral peripeduncular (PPN) lesions made on the seventh postpartum day (L7) with either radiofrequency (RF) current or N-methyl-d,l-aspartic acid (NMDA)/phosphate buffered saline (PBS) reduced maternal aggression (MA) and partially inhibited lactation without producing significant deficits in other items of maternal behavior (MB). RF-PPN lesions did not interfere with prolactin secretion, which suggests that there was deficient oxytocinergic activity. The deficit in MA was not due to interruption of afferent suckling input to the PPN: either thelectomizing females (day L6) or producing bilateral knifecuts in the mesencephalon (placed caudal to the level of the PPN; Day L7) had no effect on MA, but both procedures impaired lactation. Deficits in MA produced by RF-PPN lesions developed gradually between Days L4 and L7; lesions made either prepartum or on Day L1 did not impair MA or MB. Deficits in lactation first appeared after RF-PPN lesions on Day L1.

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.

Previously, we showed that changes in nuclear estrogen receptor (NER) concentrations within large dissections of the preoptic area and hypothalamus of pregnant rats might underlie changes in both maternal and sexual behaviors. To more precisely localize these biochemical changes, NER concentrations were measured in microdissected nuclei within the preoptic area and hypothalamus of pregnant rats on days 8, 16 and 22. Results indicated that NER levels changed significantly throughout pregnancy and that the magnitude and direction of change was site specific. Although a different pattern of change was found in each nucleus, NER levels in most preoptic area nuclei were significantly higher by the 16th day of pregnancy compared to those on day 8, while receptor levels in most hypothalamic nuclei did not increase until day 22 of pregnancy. The higher NER levels found in preoptic area nuclei of female rats as pregnancy advances can be correlated with growing maternal responsiveness during pregnancy, while elevated concentrations of NER in hypothalamic nuclei at the end of pregnancy may indicate preparation for postpartum estrus.

A 16-day treatment of nonpregnant, ovariectomized rats using 5-mm Silastic implants of estradiol (E), daily injections of 4 mg of progesterone (P), and terminal injections of 5 micrograms/kg of estradiol benzoate (EB) to provide a pregnancy-like pattern of hormone exposure, stimulates (a) home-cage aggression toward unfamiliar intruder rats, (b) short-latency maternal behavior when the females are exposed continuously to pups, and (c) maternal aggression after maternal care has been initiated. Preliminary experiments examined the persistence of stimulation of aggression by the 16-day treatment in the absence of exposure to pups eliciting maternal care, and whether an abbreviated, 1-week treatment stimulates aggression equally. Subsequent experiments examined the importance of the elements of the treatment (E implants, P injections, EB injection), and whether prolonging exposure to P or E would alter its behavioral effects. The full 16-day E/P/EB treatment stimulated higher levels of home-cage and maternal aggression, and shorter maternal behavior latencies than abbreviated and partial treatments. E in combination with P or EB significantly raised home-cage aggression, whereas P alone was without effect. Administering P for 2 additional days attenuated reductions in maternal behavior latencies by E/P/EB, but did not reduce home-cage or maternal aggressiveness. Continuous exposure to E throughout testing did not affect any dependent variable. Comparing these findings to earlier data and reports suggests that hormone exposure for 2 weeks or more, and provision of P levels approaching those of pregnancy are important to the effects of the E/P/EB treatment on aggression.

Lactating rats were used in experiments to examine the role of the dopamine system in the regulation of ongoing maternal behavior. Administration of haloperidol disrupted retrieval behavior and nest building in a dose-dependent manner but not nursing behavior and pup licking. Feeding behavior including carrying of food pellets to the nest and general locomotor ability were not disrupted by haloperidol. Apomorphine effectively reversed the inhibition caused by dopaminergic blockade. These results indicate a role for dopamine in the motivational regulation of retrieval and nest building during the postpartum phase of maternal behavior.

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.

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.

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.

Maternal behaviour in the rat consists of four principal components: nursing or crouching over the young, retrieving pups to the nest, body and genital licking of pups, and nest-building. Normally the onset of maternal behaviour occurs at parturition but studies reveal that the true onset is somewhat earlier, around 24 hours pre partum. The onset of maternal behaviour is hormonally determined and it is most likely that the ovarian hormone oestradiol plays a major role under the specific conditions that exist pre partum. The onset of maternal behaviour requires the resolution of tendencies of fear-avoidance of pups, based on their olfactory characteristics, and attraction to them and response to the pup stimuli that elicit maternal behaviour. In most animals this resolution occurs almost immediately but in some animals several hours of contact with the pups is required for maternal behaviour to be firmly established. After parturition, maternal behaviour appears to be regulated chiefly by stimuli from the pups, and hormones do not play a role apart from their role in lactogenesis. Size of litter, age of pups, amount of externally induced stress and other factors can affect the mother-young interaction: there is a period of special vulnerability to disruption of the mother-young interrelationship shortly after parturition which corresponds, we believe, to the period of transition from hormonal to non-hormonal regulation of maternal behaviour. Weaning and the decline of maternal behaviour is a specific phase of maternal care. Maternal behaviours gradually decline as avoidance behaviours increase.

The present series of experiments examined whether the medial preoptic area (MPOA) is involved in the onset of maternal behavior in the rat. Previously, the MPOA had been shown to be important in the maintenance of maternal behavior in the lactating rat. The first experiment investigated whether estradiol benzoate (EB) acts on the MPOA to facilitate the onset of maternal behavior in the 16-day pregnant, hysterectomized, and ovariectomized female rat. Such rats when given EB implants in the MPOA had significantly shorter latencies for the onset of maternal behavior than had females implanted with cholesterol in the MPOA or with EB in the ventromedial hypothalamus, in mammillary bodies, or under the skin. A second experiment showed that estrogen-induced prolactin release was not involved in this facilitation. A third experiment indicated that MPOA lesions disrupt the onset of maternal behavior that is induced by pup stimulation in virgin females. It was concluded that the MPOA is involved not only in the maintenance of maternal behavior but in the hormonally mediated onset of maternal behavior and the onset of maternal behavior induced in virgin females by pup stimulation.

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.

This study examined the effects of a 24 hr mother-litter separation on maternal aggression and pup mortality in hamsters. There were four lactating groups tested for aggression in their home cages on days 5 and 15 postpartum: a group that was separated from their litters for 24 hr, a group that was not separated from their litters and two 24 hr separation groups in which the litters were returned 30 and 120 min prior to the aggression test. Nonreceptive, estrous cycling animals were used as controls (tested twice, 10 days apart) and as intruders. Measures of aggression recorded during the 10 min tests included the number of attacks, fights, chases and intruder retreats. Pup mortality was examined in the 4 lactating groups and in two additional non-tested groups, one of which was separated from their litters for 24 hr. Lactating hamsters initiated significantly higher levels of aggression than control animals. Mother-litter separation (24 hr) significantly decreased levels of aggression and these levels were restored when litters were replaced 30 and 120 min prior to aggression tests. There were few differences in levels of aggression between days 5 and 15 of lactation. Pup mortality was significantly greater between days 5-6 and 15-16 of lactation among those groups that were separated from their litters compared to those groups that were not separated. Significantly higher mortality levels were found between days 7 and 15 of lactation among the 2 groups in which pups were returned prior to the aggression test compared to all other groups.

Sensitized virgins and postpartum lactating mothers, both exhibiting maternal behavior, were given donor litters that increased in age by 1 day, for 28 days, starting at the onset of maternal behavior. Each day females were tested for maternal behavior with pups 4-8 days old: Maternal care (i.e., nursing/crouching, retrieving, nest building and licking) and maternal withdrawal, rejection, and prevention of nursing were recorded. After the ninth day, females were also tested with the progressively older pups from 10 to 28 days of age with which they were living. Virgins and lactating mothers showed generally similar patterns of maternal care although some differences were found, and they declined in maternal behavior toward the older pups in a similar manner. Maternal behavior did not decline in tests with younger pups. The results are interpreted as supporting the hypothesis that the decline as well as the maintenance of maternal behavior postpartum is nonhormonally mediated.

Terminating the pregnancies of female rats on day 16 (D 16) by hysterectomy and ovariectomy (HO) and administering estradiol benzoate (EB) systemically or centrally into the medial preoptic area (MPOA) stimulates the onset of maternal behavior 48 h later. Since estrogen's effects are mediated through its intracellular receptors, the present studies measured nuclear and cytosol estrogen receptor concentrations in the POA and hypothalamus (HYP) of female rats during the 48 h when estrogen is active in stimulating maternal behavior. A low dose of EB (5 micrograms/kg) was used which was effective in stimulating maternal behavior in D16HO females but not in nonpregnant HO females of two types. In D16 animals there was a significantly higher concentration of nuclear receptors in the POA than in the HYP at time 0 (before HO and EB) but thereafter levels were similar in the two areas: nuclear receptor levels rose between 6 and 24 h after which they declined. In nonpregnant females, nuclear receptor levels at time 0 were low in both brain regions, rose significantly between 0 and 24 h, and significantly declined between 24 and 48 h similar to the D16 females. There were minimal differences in cytosol receptor levels among all groups at any time. Results from additional groups, nonpregnant HO females given 100 micrograms/kg EB and D16H animals (females whose pregnancies were terminated by hysterectomy only and their ovaries left intact), both of which exhibit maternal behavior at 48 h, support the idea that a high level of nuclear estrogen receptors in the POA followed by a sustained high level is necessary for estrogen stimulation of maternal behavior.

This article reviews the current state of our knowledge about the hormonal basis of maternal behavior in the rat. Considered are the ovarian hormones estrogen and progesterone, the pituitary hormones beta-endorphin and prolactin, and the hormone oxytocin, secreted by several hypothalamic nuclei and associated brain regions. The hormones of pregnancy, estrogen and progesterone, prime the female to respond to a terminal rise in estrogen that stimulates a high level of maternal responsiveness even before parturition begins. Studies on the role of prolactin, using hypophysectomy, prolactin release blockers and anterior pituitary and prolactin replacement, indicate that prolactin is required for the ovarian hormones to be effective in stimulating maternal behavior. During the latter half of pregnancy, placental lactogen may displace prolactin in this role. Although prolactin serves as a chronic stimulus for maternal behavior, it also may act over a short period. Oxytocin stimulates maternal behavior in a specific strain of rat, but not in other strains, and only when administered introcerebroventricularly (ICV) in estrogen-primed females. The decline in the high brain levels of beta-endorphin around parturition has been proposed as a requirement for the onset of maternal behavior; morphine blocks the onset of maternal behavior and disrupts ongoing maternal behavior and maternal aggression in lactating females. However, blocking beta-endorphin action at parturition interferes with pup cleaning and eating of the placenta as well.

This review covers a description of the maternal behavior cycle in the hamster and relationships between the endocrine system and prepartum changes in activity, aggression, sexual receptivity, and maternal care. Postpartum maternal behavior consists of the mother's "normal" cannibalism of some portion of her litter, the formation of a bond between mother and her young, the development of behavioral synchrony between the mother and her pups, and maternal aggression toward conspecifics. The maternal responses of virgin female, male, and juvenile animals are described and methodological considerations in the testing of both virgin and parturient animals are presented. Finally, a theoretical framework for considering the regulation of maternal care in the hamster is presented. Two main phases of regulation are proposed; an onset phase that that originates prepartum and depends on the physiological conditions of pregnancy and a maintenance phase which provides for the continuation of maternal care during lactation and is based on both endogenous maternal factors and external litter-related stimuli. A transition period beginning at parturition facilitates the shift between the onset and maintenance phases.