Maternal behavior in male rats: critical times for the suppressive action of androgens

The parental brain and behavior: A target for endocrine disruption

During pregnancy, the sequential release of progesterone, 17β-estradiol, prolactin, oxytocin and placental lactogens reorganize the female brain. Brain structures such as the medial preoptic area, the bed nucleus of the stria terminalis and the motivation network including the ventral tegmental area and the nucleus accumbens are reorganized by this specific hormonal schedule such that the future mother will be ready to provide appropriate care for her offspring right at parturition. Any disruption to this hormone pattern, notably by exposures to endocrine disrupting chemicals (EDC), is therefore likely to affect the maternal brain and result in maladaptive maternal behavior. Development effects of EDCs have been the focus of intense study, but relatively little is known about how the maternal brain and behavior are affected by EDCs. We encourage further research to better understand how the physiological hormone sequence prepares the mother's brain and how EDC exposure could disturb this reorganization.

Fathering in rodents: Neurobiological substrates and consequences for offspring

This article is part of a Special Issue "Parental Care". Paternal care, though rare among mammals, is routinely displayed by several species of rodents. Here we review the neuroanatomical and hormonal bases of paternal behavior, as well as the behavioral and neuroendocrine consequences of paternal behavior for offspring. Fathering behavior is subserved by many of the same neural substrates which are also involved in maternal behavior (for example, the medial preoptic area of the hypothalamus). While gonadal hormones such as testosterone, estrogen, and progesterone, as well as hypothalamic neuropeptides such as oxytocin and vasopressin, and the pituitary hormone prolactin, are implicated in the activation of paternal behavior, there are significant gaps in our knowledge of their actions, as well as pronounced differences between species. Removal of the father in biparental species has long-lasting effects on behavior, as well as on these same neuroendocrine systems, in offspring. Finally, individual differences in paternal behavior can have similarly long-lasting, if more subtle, effects on offspring behavior. Future studies should examine similar outcome measures in multiple species, including both biparental species and closely related uniparental species. Careful phylogenetic analyses of the neuroendocrine systems presumably important to male parenting, as well as their patterns of gene expression, will also be important in establishing the next generation of hypotheses regarding the regulation of male parenting behavior.

Maternal care interacts with prenatal stress in altering sexual dimorphism in male rats

The present study analyzes the interaction between prenatal stress and mother's behavior on brain, hormonal, and behavioral development of male offspring in rats. It extends to males our previous findings, in females, that maternal care can alter behavioral dimorphism that becomes evident in the neonates when they mature. Experiment 1 compares the maternal behavior of foster mothers toward cross-fostered pups versus mothers rearing their own litters. Experiment 2 ascertains the induced "maternal" behavior of the male pups, derived from Experiment 1 when they reached maturity. The most striking effect was that the males non-exposed to the stress as fetuses and raised by stressed foster mothers showed the highest levels of "maternal" behavior of all the groups (i.e., induction of maternal behavior and retrieving behavior), not differing from the control, unstressed, female groups. Furthermore, those males showed significantly fewer olfactory bulb mitral cells than the control males that were non-stressed as fetuses and raised by their own non-stressed mothers. They also presented the lowest levels of plasma testosterone of all the male groups. The present findings provide evidence that prenatal environmental stress can "demasculinize" the behavior, brain anatomy and hormone secretion in the male fetuses expressed when they reach maturity. Moreover, the nature of the maternal care received by neonates can affect the behavior and physiology that they express at maturity.

Parental responsiveness is feminized after neonatal castration in virgin male prairie voles, but is not masculinized by perinatal testosterone in virgin females

We previously found a large sex difference in the parental responsiveness of adult virgin prairie voles (Microtus ochrogaster) such that most males are spontaneously parental, whereas most females are not. Because this sex difference is independent of the gonadal hormones normally circulating in adult virgin voles, the present study examined whether perinatal hormones influence the development of this sex difference. Males were treated prenatally (via their pregnant dam) with both the androgen receptor blocker flutamide (5 mg/day/dam) and the aromatase inhibitor ATD (1 mg/day/dam), or oil, for the last 2 weeks of gestation. Half of the subjects from each group were castrated on the day of birth and the other half received a sham surgery. As adults, intact males were castrated and all males received a silastic capsule filled with testosterone. Prenatal treatment with flutamide and ATD had no effect on males' behavior toward pups, but neonatal castration significantly reduced the percentage of males acting parentally. In a second experiment, females were exposed to testosterone propionate (TP; 50 microg/day/dam) or oil via their dam during the last 2 weeks of gestation. For the first neonatal week, half of the females from each group were injected with TP (1 mg/day) and the other half oil. As adults, females were ovariectomized and half from each group received a testosterone-filled capsule and the other half received an empty capsule. None of the perinatal TP treatments increased females' parental responsiveness, although females from all groups that received testosterone capsules as adults were highly parental. Therefore, although postnatal testicular hormones are necessary for high parental responsiveness in males, the behavior of females is not influenced by perinatal exposure to testosterone.

Influence of gonadal hormones on the development of parental behavior in adult virgin prairie voles (Microtus ochrogaster)

Prairie voles (Microtus ochrogaster) are a socially monogamous species and both sexes are parental after the birth of pups. In contrast, sexually inexperienced adult prairie voles differ in their behavior towards pups such that virgin males are paternal whereas virgin females are often infanticidal. To test whether there exists a discrete perinatal 'sensitive period' during which gonadal hormones influence this behavior, and to distinguish between the relative contributions of estrogenic and androgenic mechanisms to this influence, prairie voles were exposed to testosterone propionate (TP), the anti-androgen flutamide, or the aromatase inhibitor 1,4,6-androstatriene-3,17-doine (ATD) either prenatally via their pregnant dam for the last 15-19 days of the 22-day gestational period or postnatally on days 1-7. None of the treatments altered the high paternal responsiveness of males or the high infanticide rate in females when compared with controls. Females exposed prenatally to ATD, however, had levels of parental behavior that were significantly higher than the lowest levels observed in prenatally TP-treated females. These results suggest that sex differences in the parental behavior of adult virgin prairie voles are not generated exclusively by androgenic or estrogenic mechanisms during a restricted prenatal or early postnatal 'sensitive period' and that the parental behavior of virgin females may be more susceptible to any influence of gonadal hormones during development than males.

Sex differences in the parental behavior of rodents

The reproductive strategy of many mammalian species that give birth to altricial young involves intense and prolonged care of their offspring. In most cases, the mother provides all nurturance, but in some cases fathers, older siblings, or unrelated conspecifics participate in parental care. The display of these behaviors by animals other than mothers is affected by numerous factors, including their sex. We herein review the literature on similarities and/or differences between male and female laboratory rodents (rats, mice, voles, gerbils, and hamsters) in their parental responsiveness and discuss how the parental behavior of males and females is influenced by hormones, developmental processes, and prior social experiences. Understanding the mechanisms that generate sex differences in the parental responsiveness of rodents may indicate how similar sex differences in parental care are generated in other mammals.

Sex differences in the parental behaviour of adult virgin prairie voles: independence from gonadal hormones and vasopressin

Sexually and parentally experienced prairie voles display robust biparental care of pups that is similar between the sexes. Little is known, however, about possible sex differences in the parental behaviours of sexually inexperienced prairie voles. Parental behaviour of adult virgin male and female prairie voles was examined in sham-operated and gonadectomized subjects treated with vehicle or oestradiol. Since arginine-vasopressin (AVP) has been suggested to stimulate parental behaviour in sexually inexperienced males, neural AVP immunoreactivity (AVP IR) was quantified. Most sham-operated and castrated males displayed high levels of parental behaviour (9/9 controls, 6/9 castrates) during a 15-min exposure to pups 4 weeks after surgery, and few behavioural differences were seen between groups. Conversely, almost all gonadally intact (8/9) and gonadectomized (8/9) females attacked pups. Implantation of a 0.1-mg pellet of oestradiol immediately after gonadectomy had little effect on males (9/9 parental), whereas most (5/9) oestradiol-treated females acted maternally. AVP-immunoreactive (AVP-ir) fibre density in the lateral septum (LS) and lateral habenula (LHb), expressed by the number of pixels that covered AVP-ir fibres during computerized optical density analysis, was greater in males than females, was non-significantly reduced in castrated males, and doubled in the LS of oestradiol-treated females. In a second experiment, males tested 8 weeks after similar manipulations remained highly parental though castrated males had almost no AVP-ir fibres in the LS and LHb. Levels of AVP IR in males treated with oestradiol were similar to those observed in intact males. A dramatic sex difference therefore exists in the parental behaviour of adult sexually naive prairie voles which cannot be explained by sex differences in gonadal hormones. Because both castrated and intact males were highly parental, even though castrates had virtually no AVP-ir in the LS or LHb, AVP does not appear to be crucial for their responsiveness toward pups.

Early postnatal diazepam exposure facilitates maternal behavior in virgin female rats

Virgin female rats do not display maternal behavior if they are not exposed to the pups during several days. This exposure is called induction. In this work we have studied the effects of early postnatal (PO-P16) diazepam (DZ) administration (1 and 2.5 mg/kg, SC) on the display of maternal behavior of virgin female rats when adults. Although we did not find statistically significant differences between P0-P16 DZ treated and control females with respect to the latency of retrieval, P0-P16 DZ administration resulted in a statistically significant increase of the percentage of female rats that became maternal, showing retrieval behavior. This early postnatal treatment with DZ also increased other variables that are currently measured in maternal behavior tests, such as: time of physical contacts, grooming, crouching, and nest building quality. No statistically significant differences were found in the body weight of treated versus control animals during development, nor during adulthood. Our results provide further evidence that the GABAA-BDZ-Cl- receptor complex is implicated in the development of maternal behavior in female rats.

Developmental psychobiological influences on rodent parental behavior

The present paper focuses on the development, broadly defined, of parental responsiveness. Both sexes can respond to young with parental behavior, in the absence of elaborate experimental manipulations; yet, through the combined natural forces of ontogeny and maturation, organizational influences of hormones, and adult modifications of neurochemical sensitivity, these animals can be rendered more or less sensitive to young. We will explore the ways in which such developmental alterations determine whether or not an animal is responsive to neonates.

Pubertal decline in maternal responsiveness in Long-Evans rats: maturational influences

Latency to onset of maternal behavior increases progressively from the weanling to the peripubertal period in female and male Long-Evans rats. Gonadectomy or hypophysectomy on day 21 had no influence on the pubertal decline in maternal responsiveness. Apparently, inhibitory neural pathways mature which are independent of pituitary hormonal influences.

Reproductive success, postpartum maternal behavior, and masculine sexual behavior of neonatally androgenized female hamsters

Sex differences in maternal behavior induced by pup stimulation (sensitization) have been reported for rats and hamsters and may be affected by the presence or absence of perinatal androgen treatment. Postpartum maternal behavior and litter survival in golden hamsters treated with testosterone propionate (TP) as neonates were studied. A high dose of TP (300 micrograms)1 eliminated feminine reproductive capacity when given on Day 2 or 4 postpartum and had no discernible effect on Day 12. Treatment on Days 6, 8, or 10 resulted in treatment day-dependent deficiencies in reproductive success which fell short of sterility in most females. These deficiencies included low birth weight, weight gain, and higher litter losses than controls. However, the maternal behavior of TP dams, as measured by retrieval and crouching, appeared to be normal. The disparity between delivery and successful rearing of normal-weight young may include uterine incompetence, lactation deficiency, and hypercannibalism. Behavioral masculinization was a more sensitive index of neonatal androgen action than any aspect of defeminization, but the two phenomena were dissociated in individuals.