High Licking/grooming Research Articles

Age-dependent differences on neurochemistry and behavior in rats raised with low and high levels of maternal care

In laboratory rats, naturally-occurring variations in maternal care have been used to study the neurobehavioral consequences of maternal nursing and to model the early-life adversity associated with many psychiatric disorders. This study aimed to determine the role of maternal care on behavior and monoamine concentrations at the prepubertal and young adulthood ages. We observed the licking/grooming (LG) behavior of Sprague-Dawley (SD) dams and assigned the litter to either low (LLG) or high (HLG) LG groups. Behavioral testing in the male offspring consisted of the open-field test, the elevated plus-maze, and the forced swimming test. Afterward, neurotransmitters contents were measured in the prefrontal cortex, the nucleus accumbens, the amygdala, and the hippocampus. We found that at the prepubertal stage, the effects of maternal care were only noticeable in the elevated plus-maze and the serotonin concentration in the nucleus accumbens. At adulthood, body weight and monoamines contents increased substantially in LLG rats. Specifically, they showed higher serotonin contents with a reduced turnover in almost all brain regions, followed by higher contents of norepinephrine and dopamine, especially in the nucleus accumbens. Changes in monoamines concentrations seem to be independent of the behavioral phenotype shaped by variations in maternal care, as behavioral effects were somewhat weak in both experiments. If higher monoamines contents in LLG rats represent an adaptive mechanism to deal with further adverse events, the behavioral paradigms used here were insufficiently challenging to bring out noticeable differences, at least in SD rats.

Maternal care and affective behavior in female offspring: Implication of the neurosteroid/GABAergic system

In female rats, the proestrus phase of the estrous cycle is associated with decreased levels of anxiety-like and depressive-like behavior relative to the metestrus phase. Progesterone likely modulate these behaviors, in part through the influence of its metabolite, allopregnanolone (THP) on hippocampal GABAAR subunit expression. As natural variations in maternal care have been found to influence both progesterone levels at proestrus and anxiety-like behavior in female offspring, we sought to investigate the importance of maternal care and the estrous cycle on affective behavior in female rats that had received Low or High levels of licking/grooming (LG) during early life. Subjects were tested for anxiety-like behavior in the elevated plus maze at proestrus or metestrus or for estrous cycle-dependent changes in depressive-like anhedonic behavior with a saccharin preference test. GABAAR subunit expression, and THP levels in the dorsal hippocampus and in plasma were also evaluated. Estrous cycle phase influenced saccharine preference and hippocampal THP level in both phenotypes. Low LG animals showed higher levels of hedonic behavior and anxiety-like behavior, irrespective of estrous cycle phase, as well as lower THP levels within the dorsal hippocampus when compared to High LG animals. Only High LG animals showed positive correlations between hippocampal THP levels and GABAAR subunit expression, suggesting a relative insensitivity to THP’s modulation of these receptor subunits in Low LG offspring. These findings suggest that natural variations in maternal care influence anxiety-like and hedonic behavior through the modulation of the neurosteroid/GABAergic system.

Natural variation in maternal care and cross-tissue patterns of oxytocin receptor gene methylation in rats

This article is part of a Special Issue “Parental Care”. Since the first report of maternal care effects on DNA methylation in rats, epigenetic modifications of the genome in response to life experience have become the subject of intense focus across many disciplines. Oxytocin receptor expression varies in response to early experience, and both oxytocin signaling and methylation status of the oxytocin receptor gene (Oxtr) in blood have been related to disordered social behavior. It is unknown whether Oxtr DNA methylation varies in response to early life experience, and whether currently employed peripheral measures of Oxtr methylation reflect variation in the brain. We examined the effects of early life rearing experience via natural variation in maternal licking and grooming during the first week of life on behavior, physiology, gene expression, and epigenetic regulation of Oxtr across blood and brain tissues (mononucleocytes, hippocampus, striatum, and hypothalamus). Rats reared by “high” licking-grooming (HL) and “low” licking-grooming (LL) rat dams exhibited differences across study outcomes: LL offspring were more active in behavioral arenas, exhibited lower body mass in adulthood, and showed reduced corticosterone responsivity to a stressor. Oxtr DNA methylation was significantly lower at multiple CpGs in the blood of LL versus HL males, but no differences were found in the brain. Across groups, Oxtr transcript levels in the hypothalamus were associated with reduced corticosterone secretion in response to stress, congruent with the role of oxytocin signaling in this region. Methylation of specific CpGs at a high or low level was consistent across tissues, especially within the brain. However, individual variation in DNA methylation relative to these global patterns was not consistent across tissues. These results suggest that blood Oxtr DNA methylation may reflect early experience of maternal care, and that Oxtr methylation across tissues is highly concordant for specific CpGs, but that inferences across tissues are not supported for individual variation in Oxtr methylation.

Postnatal maternal care predicts divergent weaning strategies and the development of social behavior.

Maternal care experienced during postnatal development predicts long-term neurobiological and behavioral outcomes. However, the cascade of behavioral changes that emerge in response to maternal care has not been elucidated. In the current study, we examine naturally occurring variation in postnatal licking/grooming (LG) in C57BL/6J mice to determine its impact on preweaning maternal and pup behavior, the weaning process, the pace of developmental change, the emergence of social behavior, and indices of anxiety-like behavior in adulthood. Our analyses indicate that lower postnatal LG is associated with truncated and more infrequent maternal behavior during the preweaning period. Moreover, compared to High LG dams, Low LG dams are observed to actively wean their offspring sooner and have offspring that play more frequently. The heightened pace of developmental change observed in offspring of Low LG dams suggests a more rapid transition to behavioral and nutritional independence, which could have implications for future reproductive strategies.

Neonatal overexpression of estrogen receptor-α alters midbrain dopamine neuron development and reverses the effects of low maternal care in female offspring.

Maternal behavior is dependent on estrogen receptor-alpha (ERα; Esr1) and oxytocin receptor (OTR) signaling in the medial preoptic area (MPOA) of the hypothalamus, as well as dopamine signaling from the ventral tegmental area (VTA) to forebrain regions. Previous studies in rats indicate that low levels of maternal care, particularly licking/grooming (LG), lead to reduced levels of MPOA ERα and VTA dopamine neurons in female offspring and predict lower levels of postpartum maternal behavior by these offspring. The aim of this study was to determine the functional impact on maternal behavior of neonatal manipulation of ERα in females that had experienced low versus high levels of postnatal maternal LG. Adenovirus expressing ESR1 was targeted to the MPOA in female pups from low and high LG litters on postnatal day 2-3. Overexpression of ESR1 in low LG offspring elevated the level of ERα-immunoreactive cells in the MPOA and of tyrosine hydroxylase cells in the VTA to that observed in high LG females. Amongst juvenile female low LG offspring, ESR1 overexpression also decreased the latency to engage in maternal behavior toward donor pups. These results show that virally mediated expression of ESR1 in the neonatal rat hypothalamus results in lasting changes in ESR1 expression through the juvenile period, and can "rescue" hormone receptor levels and behavior of offspring reared by low LG dams, potentially mediated by downstream alterations within reward circuitry. Thus, the transmission of maternal behavior from one generation to the next can be augmented by neonatal ERα in the MPOA.

Developmental Timing of the Effects of Maternal Care on Gene Expression and Epigenetic Regulation of Hormone Receptor Levels in Female Rats

Maternal care experienced during postnatal development has enduring effects on neuroendocrine function and behavior. Previous studies in rats have illustrated the effect of maternal licking/grooming (LG) on hormone receptors and maternal behavior of adult female offspring associated with altered DNA methylation. However, the developmental timing of these effects, which provide insight into the cellular and molecular pathways through which early experience alters later behavior, had not been explored. Here, we demonstrate the developmental emergence of these outcomes and use cross-fostering to identify sensitive periods for these effects. Estrogen receptor (ER)α and ERβ mRNA levels within the medial preoptic area (MPOA) of the hypothalamus were increased by postnatal day (PN)21 in female offspring of high LG dams; LG-associated increases in oxytocin receptor mRNA levels were observed beyond the weaning period. Quantification of ERα-immunoreactivity indicated a high degree of neuroanatomical specificity of LG effects within the MPOA that were observed by PN6. Reduced DNA methylation and histone 3 lysine 9 tri-methylation and increased histone 3 lysine 4 tri-methylation at the ERα gene promoter (Esr1) were detected at PN21 in high LG female offspring. Latency to engage in maternal behavior toward donor pups was significantly shorter among high LG females. Cross-fostering revealed that maternal sensitization and MPOA ERα levels are sensitive to maternal care experienced before but not after PN10. Differential windows of plasticity were identified for ERβ and oxytocin receptor mRNA levels. These studies contribute significantly to our understanding of the molecular, neurobiological, and behavioral pathways through which variation in maternal behavior is transmitted from one generation to the next.

Perinatal Testosterone Exposure and Maternal Care Effects on the Female Rat's Development and Sexual Behaviour

Natural variations in maternal care have profound influences on offspring behaviour, brain activity and hormone release. Measuring the amount of time that a rat dam spends licking/grooming (LG) her pups during their first week of life allows for characterisation of distinctive Low, Mid and High LG phenotypes. We have previously found that female offspring of High LG mothers are less sexually receptive, less motivated to mate and show a later onset of puberty relative to Low LG offspring. Given that High LG females are exposed to greater levels of testosterone in utero, we hypothesise that differences in sexual behaviour between High and Low LG female offspring are driven in part by differences in prenatal hormone exposure. To test this hypothesis, pregnant dams pre-characterised as Low, Mid, or High LG mothers were implanted with testosterone or placebo on gestational day (GD) 16. Offspring body weight and anogenital index were assessed at GD 21 and in adulthood. Age of vaginal opening and oestrous cyclicity were assessed to determine the timing of pubertal onset. Testosterone exposure removed the difference between LG phenotypes in pubertal onset by delaying vaginal opening and the appearance of first pro-oestrus. In adulthood, sexual behaviour in a paced mating chamber after sham surgery or ovariectomy with steroid-replacement was examined. Our findings show that Low, Mid and High LG female offspring are differentially affected by perinatal testosterone exposure, and that this exposure removes the precocial pubertal onset of Low LG offspring and increases the sexual proceptivity and receptivity of High LG offspring. These results suggest that maternal programming of the female reproductive system may be mediated, in part, through differences in perinatal testosterone exposure, instead of solely through maternal behaviour.

Maternal behavior as an early modulator of neurobehavioral offspring responses by Sprague-Dawley rats

Maternal care plays an important role as an early modeler of neurodevelopment and brain function, and its effects remain until adulthood. Such modeling or programming has shown to influence the stress response and represents a key susceptibility factor in the development of mood disorders. In order to characterize such process which is still not clear, male offspring were classified in animals with low, medium and high licking/grooming (LG) according to the maternal behavior. Juvenile animals were subjected to the open field test (OFT) and the forced swimming test (FST), and offspring of low and high LG mothers were compared. Seven days after the FST, neurochemical and gene expression analyses were carried out in order to identify possible changes on relevant targets. Maternal care did determine locomotor behaviors in the OFT, supporting an anxiogenic effect of low maternal investment. This effect seems to be associated with the serotonergic systems in both nucleus accumbens (NAc) and hippocampus (HPC), since offspring of low LG mothers showed decreased 5-HT neurotransmission in those brain regions compared with animals of high LG mothers. Furthermore, TrkB expression was higher in offspring of high LG compared to the group of low LG mothers, supporting its influence as a mechanistic intermediate of such effect, at least in the NAc. Taken together, these findings strongly support the influence of differential maternal care on the neurodevelopment and responsivity of juvenile rats.

Variations in maternal care associated with differences in female rat reproductive behavior in a group‐mating environment

Maternal care influences the development of sexual behavior in pair mating rats, under laboratory conditions. This study examined the effect of variations in maternal care in a group-mating condition. Groups of two low and two high licking/grooming (LG) female offspring mated with two males in a large pacing chamber for 36 hr. Sexual behaviors were scored for the first 15 vaginal-cervical stimulations (VCS) and the entire 36 hr. Low LG females spent more time mating, required more time to receive an intromission after entering the male compartment. They also received more ejaculations compared to high LG females during the first 15 VCS. This difference disappeared as mating continued. Males were more responsive to high than low females. No pregnancy rate difference was seen between the two female phenotypes, demonstrating that variation in maternal care received results in two mating strategies that are both reproductively successful under group-mating conditions.

Maternal Care Influences Hippocampal N-Methyl-D-Aspartate Receptor Function and Dynamic Regulation by Corticosterone in Adulthood

Variations in maternal care in the rat associate with robust differences in hippocampal development and synaptic plasticity in the offspring. Maternal care also influences pituitary-adrenal stress responses and corticosterone (CORT) regulation of hippocampal plasticity. N-methyl-D-aspartate receptors (NMDAR) regulate synaptic plasticity, and NMDAR function is modulated by stress and CORT. We hypothesized that altered NMDAR function underlies the interaction of maternal and stress effects on hippocampal synaptic plasticity. We used electrophysiology and western blot to examine NMDAR synaptic function/expression and NMDAR-dependent long-term potentiation (LTP) in adult offspring of mothers that varied in the frequency of pup licking/grooming (LG) (i.e., High or Low LG). Basal NMDAR synaptic function was enhanced in the hippocampal dentate gyrus (DG) of adult Low LG offspring. Synaptic expression of NMDAR but not α-amino-3-hydroxy-methyl-4-isoxazole propionic acid receptors was also increased. Stress level CORT (100 nmol/L) rapidly (< 20 min) and robustly increased NMDAR function in High LG offspring, eliminating the maternal effect. Corticosterone did not affect NMDAR function in Low LG offspring. Bovine serum albumin-conjugated CORT reproduced the CORT effect in High LG offspring, implicating a membrane-bound corticosteroid receptor. NMDAR hyperfunction might impair synaptic plasticity. Partial NMDAR antagonism by low concentration DL-2-Amino-5-phosphonopentanoic acid rescued a basal LTP deficit in Low LG offspring and inhibited LTP in High LG offspring. Low LG offspring exhibit basally elevated NMDAR function coupled with insensitivity to CORT modulation indicative of a chronic alteration of NMDAR function. Elevated NMDAR function in the hippocampus might underlie impaired LTP in Low LG offspring.

Oxytocin-Dopamine Interactions Mediate Variations in Maternal Behavior in the Rat

Variations in maternal behavior among lactating rats associate with differences in estrogen-oxytocin interactions in the medial preoptic area (mPOA) and in dopamine levels in the nucleus accumbens (nAcc). Thus, stable, individual differences in pup licking/grooming (LG) are abolished by oxytocin receptor blockade or treatments that eliminate differences in the nAcc dopamine signal. We provide novel evidence for a direct effect of oxytocin at the level of the ventral tegmental area (VTA) in the regulation of nAcc dopamine levels. Mothers that exhibit consistently increased pup LG (i.e. high LG mothers) by comparison with low LG mothers show increased oxytocin expression in the mPOA and the paraventricular nucleus of the hypothalamus and increased projections of oxytocin-positive cells from both mPOA and paraventricular nucleus of the hypothalamus to the VTA. Direct infusion of oxytocin into the VTA increased the dopamine signal in the nAcc. Finally, high compared with low LG mothers show greater increases in dopamine signal in the nAcc during bouts of pup LG, and this difference is abolished with infusions of an oxytocin receptor antagonist directly into the VTA. These studies reveal a direct effect of oxytocin on dopamine release within the mesocorticolimbic dopamine system and are consistent with previous reports of oxytocin-dopamine interactions in the establishment and maintenance of social bonds.

Maternal Programming of Sexual Behavior and Hypothalamic-Pituitary-Gonadal Function in the Female Rat

Variations in parental care predict the age of puberty, sexual activity in adolescence and the age at first pregnancy in humans. These findings parallel descriptions of maternal effects on phenotypic variation in reproductive function in other species. Despite the prevalence of such reports, little is known about potential biological mechanisms and this especially true for effects on female reproductive development. We examined the hypothesis that parental care might alter hypothalamic-pituitary-ovarian function and thus reproductive function in the female offspring of rat mothers that vary pup licking/grooming (LG) over the first week postpartum. As adults, the female offspring of Low LG mothers showed 1) increased sexual receptivity; 2) increased plasma levels of luteinizing hormone (LH) and progesterone at proestrus; 3) an increased positive-feedback effect of estradiol on both plasma LH levels and gonadotropin releasing-hormone (GnRH) expression in the medial preoptic region; and 4) increased estrogen receptor α (ERα) expression in the anterioventral paraventricular nucleus, a system that regulates GnRH. The results of a cross-fostering study provide evidence for a direct effect of postnatal maternal care as well as a possible prenatal influence. Indeed, we found evidence for increased fetal testosterone levels at embryonic day 20 in the female fetuses of High compared to Low LG mothers. Finally, the female offspring of Low LG mothers showed accelerated puberty compared to those of High LG mothers. These data suggest maternal effects in the rat on the development of neuroendocrine systems that regulate female sexual behaviour. Together with studies revealing a maternal effect on the maternal behavior of the female offspring, these findings suggest that maternal care can program alternative reproductive phenotypes in the rat through regionally-specific effects on ERα expression.

Transgenerational effects of social environment on variations in maternal care and behavioral response to novelty.

Cross-fostering studies in the rat have illustrated the importance of the postnatal environment in mediating the transmission of maternal licking/grooming (LG) from mother to offspring. The authors addressed the question of how postweaning social conditions can alter the patterns of maternal behavior. Juvenile female offspring of high LG and low LG mothers were placed in either standard, enriched, or impoverished postweaning environments for 50 consecutive days and then mated and observed with their own litters. Analysis of LG behavior indicated that the effect of postweaning environment was dependent on the level of postnatal mother-infant interaction. Postweaning isolation reduced exploratory behavior, maternal LG, and oxytocin receptor binding in the offspring of high LG mothers, whereas social enrichment enhanced exploration, LG behavior, and oxytocin receptor binding of low LG offspring. These effects were also transmitted to the next generation of offspring. Thus, maternal LG and the neural mechanisms that regulate this behavior exhibited a high degree of plasticity in response to changes in environment both within and beyond the postnatal period, with implications for the transmission of behavioral response to novelty and maternal care across generations.

Variations of maternal care alter offspring levels of behavioural defensiveness in adulthood: Evidence for a threshold model

Natural variations of maternal care in the rat influence the development of neuronal systems that regulate defensive responses to threat. Thus, as adults, rats that received higher levels of maternal licking/grooming (LG) in infancy display dramatic reductions in burying in the shock-probe test, relative to offspring of low LG mothers. We sought to replicate that finding and determine whether maternal care similarly influences offspring responses to social threat, using the resident-intruder test. We also examined whether maternal LG influences offspring behaviour along a continuum by comparing defensive responses of offspring of mid LG mothers to those of offspring of high LG and low LG mothers. A final goal was to assess whether the reductions in adult offspring reactivity to threat that typically follow corticosterone (CORT) administration to dams across lactation are mediated through CORT-induced changes in maternal care. Adult offspring of high LG mothers spent less time burying the shock-probe, relative to offspring of mid and low LG mothers, whereas offspring of CORT-treated mothers did not differ from any group. Similarly, offspring of high LG (but not CORT-treated) mothers displayed fewer defensive responses in the resident-intruder test. Thus, only natural variations of maternal care were associated with individual differences in offspring reactivity to threat. Furthermore, because offspring of mid and low LG mothers displayed equivalent levels of defensive responding in both tests, it appears that a critical threshold of maternal LG is necessary to alter the developmental trajectory of neural systems mediating defensive behaviours.

Maternal Care Associated with Methylation of the Estrogen Receptor-α1b Promoter and Estrogen Receptor-α Expression in the Medial Preoptic Area of Female Offspring

Variations in maternal behavior are associated with differences in estrogen receptor (ER)-alpha expression in the medial preoptic area (MPOA) and are transmitted across generations such that, as adults, the female offspring of mothers that exhibit increased pup licking/grooming (LG) over the first week postpartum (i.e. high LG mothers) show increased ERalpha expression in the MPOA and are themselves high LG mothers. In the present studies, cross-fostering confirmed an association between maternal care and ERalpha expression in the MPOA; the biological offspring of low LG mothers fostered at birth to high LG dams show increased ERalpha expression in the MPOA. Cross-fostering the biological offspring of high LG mothers to low LG dams produces the opposite effect. We examined whether the maternal programing of ERalpha expression is associated with differences in methylation of the relevant ERalpha promoter. Levels of cytosine methylation across the ERalpha1b promoter were significantly elevated in the adult offspring of low, compared with high, LG mothers. Differentially methylated regions included a signal transducer and activator of transcription (Stat)5 binding site and the results of chromatin immunoprecipitation assays revealed decreased Stat5b binding to the ERalpha1b promoter in the adult offspring of low, compared with high, LG mothers. Finally, we found increased Stat5b levels in the MPOA of neonates reared by high, compared with low, LG mothers. These findings suggest that maternal care is associated with cytosine methylation of the ERalpha1b promoter, providing a potential mechanism for the programming of individual differences in ERalpha expression and maternal behavior in the female offspring.

Stress During Gestation Alters Postpartum Maternal Care and the Development of the Offspring in a Rodent Model

Epidemiological studies suggest that environmental adversity can alter parental care and thus influence child development. We addressed the question of whether stressors can directly affect parental behavior using a rodent model of stable, individual differences in maternal behavior. Lactating rat mothers were characterized as high or low in pup-directed licking/grooming (LG) behavior, rebred, and subjected to 7 days of intermittent stress or control conditions during gestation. Female rats were mated a third time without any subsequent intervention. Maternal behavior, oxytocin receptor (OTR) binding, and offspring behavior were examined. Stress reduced OTR levels and pup LG of high LG mothers to levels comparable with those of low LG mothers. The adult offspring of the gestational stress/high LG mothers resembled those of low LG mothers on behavioral measures of anxiety and maternal behavior, as well as OTR levels. The results of the third mating revealed an enduring effect of gestational stress on both mother and offspring maternal LG. These findings suggest that stress can directly alter maternal care through the neuroendocrine systems that normally regulate this behavior. Thus, the effects of environmental adversity can be transmitted across generations through a nongenomic mechanism involving maternal care.

Variations of maternal care differentially influence ‘fear’ reactivity and regional patterns of cFos immunoreactivity in response to the shock-probe burying test

Natural variations in maternal care in the rat influence the development of neuronal systems that regulate endocrine and behavioral responses to stress. Thus, as adults, rats that received higher levels of maternal licking/grooming (LG) in infancy are less ‘fearful’ in response to novelty, compared with adult offspring of Low LG mothers. The present study examined the influence of maternal care on behavioral and neuronal responses to a more specific, localizable form of threat using an electrified probe in the shock-probe burying test. Even under these conditions, adult offspring of High LG mothers displayed lower levels of fear reactivity (i.e. less shock-induced freezing and probe burying) throughout the test than did offspring of Low LG mothers. These differences in fearfulness were associated with differential patterns of cFos immunoreactivity (cFos-IR), 120 min following test exposure. Relative to control rats exposed to a non-electrified probe, cFos-IR was increased in the offspring of High LG mothers exposed to an electrified probe in the dentate gyrus, ventral subiculum, lateral and medial septum, nucleus accumbens and the dorsal periaqueductal gray. Shock-exposed offspring of Low LG dams displayed a very different pattern of neuronal activation characterized by both increases (area CA1of the ventral hippocampus and the inferior colliculus) and decreases (paraventricular nucleus of the hypothalamus and the ventrolateral periaqueductal gray) in cFos-IR compared with the no-shock controls. Together these results suggest that maternal care serves to ‘program’ neuronal circuits that modulate fear-related responding in the rat resulting in qualitatively different neuronal responses to stress.

Natural variations in maternal care are associated with estrogen receptor alpha expression and estrogen sensitivity in the medial preoptic area.

Lactating rats exhibit stable individual differences in pup licking/grooming (LG) over the first week postpartum. Such naturally occurring variations in maternal behavior are associated with differences in estrogen-inducible oxytocin receptors in the medial preoptic area (MPOA) of the hypothalamus. We compared levels of ER alpha and ER beta mRNA in the MPOA of lactating High or Low LG mothers as well as in their nonlactating, female offspring, which inherit the maternal phenotype of their mothers. Among lactating females, High LG females exhibited significantly elevated levels of ER alpha mRNA compared with Low LG females. Likewise, the adult, virgin female offspring of High LG mothers showed higher levels of ER alpha mRNA in the MPOA compared with those of Low LG mothers. There were no group differences in levels of ER beta mRNA. Differences in ER alpha protein expression in the MPOA were confirmed using Western blot analysis. To further characterize the effects of estrogen in the MPOA, cFos immunoreactivity was compared in ovariectomized, adult offspring of High and Low LG dams treated with estradiol or oil. Increased cFos activity in the anterior ventral nucleus of the MPOA was observed in estradiol-treated High LG, but not Low LG females. These findings suggest that natural variations in maternal care are associated with differences in ER alpha expression in the MPOA and that such differences are transmitted from the mother to her female offspring.