Factors influencing maternal responsiveness in humans: usefulness of an animal model

Like mother, like daughter: evidence for non-genomic transmission of parental behavior and stress responsivity

Considerable evidence demonstrates that the quality of the early environment influences patterns of development that, in turn, determine the health and productivity of the individual throughout their life span. However, the processes through which early life influences health are not clearly understood. Through the activation of the hypothalamo-pituitary-adrenal (HPA) axis and corticotropin-releasing hormone (CRH) pathways, prolonged or exaggerated responses to stress have profound effects on physiological and cognitive functions. Early maternal separation or handling of neonatal rats can program widespread and lifelong changes in various transmitter systems that regulate the HPA and CRH systems. Our studies show that a high level of maternal licking/grooming, and arched-back nursing correlates with reduced CRH mRNA expression and enhanced glucocorticoid negative feedback, and lower stress responses in the adult. This behavior is stably transmitted between generations and cross-fostering studies show that the offspring inherit the behavior from the nursing mother and not the biological mother. Such intergenerational transmission of maternal behavior is seen in rodents, primates and humans, and may underlie adaptive changes in the HPA axis. The neural basis of this inheritance pattern appears to reside in the central oxytocin system which determines features of maternal behavior. Through these various adaptive neural mechanisms the environmental demand on the mother is reflected in the quality of maternal care to her offspring. This, in turn, programs stress reactivity and maternal behavior patterns of the offspring. This not only determines certain health outcomes but also establishes the relationships between mother and offspring in the next generation. These findings suggest that for neurobiologists, the function of the family is an important level of analysis and the critical question is that of how environmental events regulate neural systems that mediate the expression of parental care.

Maternal care, gene expression, and the transmission of individual differences in stress reactivity across generations

Naturally occurring variations in maternal care alter the expression of genes that regulate behavioral and endocrine responses to stress, as well as hippocampal synaptic development. These effects form the basis for the development of stable, individual differences in stress reactivity and certain forms of cognition. Maternal care also influences the maternal behavior of female offspring, an effect that appears to be related to oxytocin receptor gene expression, and which forms the basis for the intergenerational transmission of individual differences in stress reactivity. Patterns of maternal care that increase stress reactivity in offspring are enhanced by stressors imposed on the mother. These findings provide evidence for the importance of parental care as a mediator of the effects of environmental adversity on neural development.

Variations in maternal behaviour are associated with differences in oxytocin receptor levels in the rat

Female Long-Evans rats exhibit stable individual differences in maternal behaviours such as pup licking/grooming and arched-back nursing posture (LG-ABN). These variations in maternal behaviour are accompanied by differences in lactation-induced increases in oxytocin receptor levels in brain regions known to mediate the expression of maternal care in this species (i.e. the bed nucleus of the stria terminalis, the medial preoptic area and the lateral septum). Oxytocin receptor levels in the central nucleus of the amygdala were significantly higher in high compared to low LG-ABN females regardless of reproductive status. These findings suggest that individual differences in maternal behaviour may be directly related to variations in oxytocin receptor expression.

Nongenomic transmission across generations of maternal behavior and stress responses in the rat

In the rat, variations in maternal care appear to influence the development of behavioral and endocrine responses to stress in the offspring. The results of cross-fostering studies reported here provide evidence for (i) a causal relationship between maternal behavior and stress reactivity in the offspring and (ii) the transmission of such individual differences in maternal behavior from one generation of females to the next. Moreover, an environmental manipulation imposed during early development that alters maternal behavior can then affect the pattern of transmission in subsequent generations. Taken together, these findings indicate that variations in maternal care can serve as the basis for a nongenomic behavioral transmission of individual differences in stress reactivity across generations.

The role of corticotropin-releasing factor--norepinephrine systems in mediating the effects of early experience on the development of behavioral and endocrine responses to stress

Naturally occurring variations in maternal care in early postnatal life are associated with the development of individual differences in behavioral and hypothalamic-pituitary-adrenal responses to stress in the rat. These effects appear to be mediated by the influence of maternal licking and grooming on the development of central corticotropin-releasing factor (CRF) systems, which regulate the expression of behavioral, endocrine, and autonomic responses to stress through activation of forebrain noradrenergic systems. These findings provide a neurobiologic basis for the observed relationship between early life events and health in adulthood. In more recent studies, we explored the behavioral transmission of individual differences in stress reactivity, and thus, vulnerability to stress-induced illness, across generations.

The biology of human parenting: insights from nonhuman primates

Primate and human parenting have often been viewed as completely emancipated from neuroendocrine influences and primarily dependent on experience, social and cognitive processes. A review of recent findings of primate research on the neurobiological regulation of parental responsiveness, the causes of variability in parenting styles, and the determinants of infant abuse suggests that primate parenting is more sensitive to neuroendocrine mechanisms than previously thought. The findings of primate research can have important implications for human research and encourage the investigation of biological influences on human parenting.

Maternal care and the development of stress responses

Studies dating from the 1950s have documented the impact of early life events on the development of behavioral and endocrine responses to stress. Recent findings suggest that these effects are mediated through changes in mother-offspring interactions and have identified central corticotropin-releasing factor systems as a critical target for the effects of variations in maternal care.

Cortisol, hedonics, and maternal responsiveness in human mothers

New mothers are more attracted to the body odor of newborn infants than are nonmothers. In this study we investigated the relation of postpartum hormones and of prior experience with infants to this enhanced maternal attraction to infant odors. New mothers were asked to complete a hedonics task, using a pleasantness scale to provide an attraction score to different odorants presented on a cotton substrate in a 1-pt Baskin-Robbins container. Mothers were "blind" to the contents of the container. Participants also completed an extensive set of 100-item likert scales concerning their attitudes toward infants, care taking, own maternal adequacy, and other interpersonal relations. Mothers were videotaped interacting with their infants and provided salivary samples prior to the interaction. Salivary samples were assayed by radioimmunoassay (RIA) for salivary concentrations of cortisol, progesterone, and testosterone. Results show that first-time mothers with higher cortisol concentrations were more attracted to their own infant's body odor. Mothers with higher cortisol levels were also better able to recognize their own infants' odors. While cortisol was not related to attitudinal measures of maternal responsiveness, mothers with more prior experience interacting with infants exhibited both more attraction to infant odors and more positive maternal attitudes. Together, prior maternal experience and postpartum cortisol explain a significant proportion of the variance in mothers' attraction to newborn infant odors. These relations are discussed in terms of the variety of "meanings" cortisol could have during the postpartum period.

Offspring-induced nurturance: animal-human parallels

Offspring provide mothers with stimuli that impel their own nurturance. In rats, distal sensory stimuli from pups--sight, sound, odor--contribute to contact-seeking, whereas tactile stimuli from pups to dam's snout and ventrum elicit essential maternal behavioral reflexes involved in retrieval, licking, and the quiescent, upright nursing posture (kyphosis). Brain sites involved with maternal behavior--assessed by lesions, immunocytochemical visualization of gene activity, and neurophysiological mapping--include the midbrain central gray, medial preoptic nucleus, limbic system, and somatosensory cortex; these may change with experience. Human mothers inadvertently learn to identify their own baby rapidly after birth and can do so via a single sensory modality. Subsequently maternal responsiveness and gratification are impaired by inappropriate, insufficient, or nonreciprocal interactions such as occurs when the baby cries excessively, is blind, deaf, or autistic. Thus, maternal behavior characterized by elicited responses and emotional reactions to stimuli from offspring may be evolutionarily conserved.

Hormonal and experiential correlates of maternal responsiveness during pregnancy and the puerperium in human mothers

Two studies were undertaken (1) to determine whether human mothers undergo a change in maternal responsiveness during pregnancy before the birth of the baby, as shown for other mammalian species, and (2) to establish whether a relation exists between changes in maternal feelings and attitudes and changes in hormones. In both studies prospective first-time mothers completed an extensive set of questionnaires, covering a broad range of issues, including a set of 76- to 100-item likert scales concerning attitudes toward infants, childbirth, pregnancy, caretaking, and other interpersonal relationships. In the first cross-sectional study, mothers completed the questionnaires at one of seven time points, ranging from prior to pregnancy to 3 months postpartum. In the longitudinal study, questionnaires were completed repeatedly throughout this same time period. In addition, blood was taken at these same time points and assayed by RIA for plasma concentrations of the steroids, estradiol, progesterone, testosterone, and cortisol. The primary findings are (1) feelings of nurturance grow during pregnancy and from pregnancy to postpartum; in the cross-sectional study, for most of the factors relating to infants or mothering, pregnancy and postpartum responses were more positive than prepregnancy responses; in the longitudinal study, many of these factors also showed elevations across pregnancy itself, as well as further elevations with the birth of the infant. (2) Pregnancy hormones were not related to the growth of attachment to the infant across pregnancy. (3) However, the pattern of change in the ratio of estradiol to progesterone from early to late pregnancy was related to postpartum attachment feelings. (4) Finally, hormonal correlates of attachment feelings may reflect effects both on feelings of nurturance directly and, indirectly, on mothers' feelings of well-being.

Psychobiology of maternal behavior: contribution to the clinical understanding of maternal behavior among humans

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.

Neuropsychology of maternal behavior in the rat: c-fos expression during mother-litter interactions

This series of studies used the pattern of nuclear Fos-like immunoreactivity (Fos-lir) to map the functional pathways in the brain that mediate the onset and retention of maternal behavior. In the first two experiments, parturient rat dams were exposed to either pups or to other stimuli on Day 1 postpartum. Dams interacting with pups were either intact or sustained ventral somatosensory, olfactory, or combined desensitizations. Results showed that 1) all intact pup-interacting dams showed elevated levels of Fos-lir in the medial preoptic area (MPOA) and the medial and cortical amygdala as compared to control groups, and 2) olfactory and ventral somatosensory desensitization, either alone or in combination, did not decrease Fos-lir in the MPOA. However, olfactory desensitizations did decrease Fos-lir in the medial amygdala and the combined desensitizations significantly reduced Fos-lir in both the basolateral and central amygdala. In the third study, dams were either exposed to pups or to other stimuli and were subsequently reexposed to pups or to pup cues. Regardless of prior maternal experience, females who were able to interact with pups upon reexposure showed increased Fos-lir in the MPOA, the basolateral and central nuclei of the amygdala, and the nucleus accumbens when compared to females which did not interact with pups. Taken together, these studies suggest that the neuroanatomy of maternal behavior is a complex one, involving multiple systems that interconnect with the MPOA and that mediate the many behavioral processes activated when an animal responds maternally.

Postpartum factors related to mother's attraction to newborn infant odors

Hedonic responses to a variety of infant (general body, urine, and feces) and noninfant (lotion, cheese, and spice) odorants were compared in four groups of subjects: new mothers, mothers a 1-month postpartum, and female and male nonparents. Using standard scaling procedures, subjects rated each of the odorants twice on a scale from extremely unpleasant (-20.5) to extremely pleasant (+20.5). In addition, all subjects completed a set of attitude questionnaires, and mothers also answered a childbirth questionnaire and were observed while feeding their infants.

The effects of a social support group on depression, maternal attitudes and behavior in new mothers

The present study examined the effects of a postpartum Social Support Group intervention on mood state, attitudes and behavior of new mothers. Intervention conditions consisted of an eight session Social Support intervention (n = 44), a no intervention condition (n = 83) and a Group-by-Mail intervention (n = 15). Attitudinal and behavioral assessments were made before and after the interventions at 6 and 20 weeks postpartum, respectively. The primary results indicate that regardless of intervention condition mothers undergo an improvement in mood from 2 weeks to 5 months postpartum. Although the Social Support intervention did not alleviate maternal depression and, in fact, may be detrimental to depressed mothers' self-confidence, it did increase mothers' proximal attention to their infants.

Postpartum psychiatric disorders

The literature on postpartum psychiatric disorders (PPD) is reviewed with particular reference to recent advances in the field. Despite the prevailing tendency to consider PPD as a manifestation of other pre-existing psychiatric disorders (i.e. affective, schizophreniform, organic, neurotic, etc.) a growing number of studies point to the uniqueness and specificity of postpartum symptomatology. Theories of etiology, psychosocial as well as biological, are still wide-ranging and unconvicing. Attempts to predict PPD and/or to identify mothers-to-be at risk are based largely on retrospective studies or else are hindered by the use of nonspecific instruments. Society's implicit belief in the "joys of motherhood" may have further contributed to the obscurity surrounding these frequently occurring disorders. Interventions follow conventional methods of psycho- and pharmacotherapy but the multidisciplinary treatment approach especially for women with acute PPD who require hospital admission is gaining wider acceptance. The inpatient psychiatric mother-baby intensive care unit model is part of this approach.