Intracellular preoptic and striatal monoamines in pregnant and lactating rats: possible role in maternal behavior

Oxytocin interactions with central dopamine and serotonin systems regulate different components of motherhood

The role of oxytocin in maternal caregiving and other postpartum behaviours has been studied for more than five decades. How oxytocin interacts with other neurochemical systems to enact these behavioural changes, however, is only slowly being elucidated. The best-studied oxytocin-neurotransmitter interaction is with the mesolimbic dopamine system, and this interaction is essential for maternal motivation and active caregiving behaviours such as retrieval of pups. Considerably less attention has been dedicated to investigating how oxytocin interacts with central serotonin to influence postpartum behaviour. Recently, it has become clear that while oxytocin-dopamine interactions regulate the motivational and pup-approach aspects of maternal caregiving behaviours, oxytocin-serotonin interactions appear to regulate nearly all other aspects including postpartum nursing, aggression, anxiety-like behaviour and stress coping strategy. Collectively, oxytocin's interactions with central dopamine and serotonin systems are thus critical for the entire suite of behavioural adaptations exhibited in the postpartum period, and these sites of interaction are potential pharmacological targets for where oxytocin could help to ameliorate deficits in maternal caregiving and poor postpartum mental health. This article is part of the theme issue 'Interplays between oxytocin and other neuromodulators in shaping complex social behaviours'.

A Scientometric Approach to Review the Role of the Medial Preoptic Area (MPOA) in Parental Behavior

Research investigating the neural substrates underpinning parental behaviour has recently gained momentum. Particularly, the hypothalamic medial preoptic area (MPOA) has been identified as a crucial region for parenting. The current study conducted a scientometric analysis of publications from 1 January 1972 to 19 January 2021 using CiteSpace software to determine trends in the scientific literature exploring the relationship between MPOA and parental behaviour. In total, 677 scientific papers were analysed, producing a network of 1509 nodes and 5498 links. Four major clusters were identified: “C-Fos Expression”, “Lactating Rat”, “Medial Preoptic Area Interaction” and “Parental Behavior”. Their content suggests an initial trend in which the properties of the MPOA in response to parental behavior were studied, followed by a growing attention towards the presence of a brain network, including the reward circuits, regulating such behavior. Furthermore, while attention was initially directed uniquely to maternal behavior, it has recently been extended to the understanding of paternal behaviors as well. Finally, although the majority of the studies were conducted on rodents, recent publications broaden the implications of previous documents to human parental behavior, giving insight into the mechanisms underlying postpartum depression. Potential directions in future works were also discussed.

Perinatal depression: Heterogeneity of disease and in animal models

Perinatal depression (PND) can have either an antepartum or postpartum onset. Although the greatest risk factor for PND is previous depression history,de novoPND occurs with the majority of cases occurring in the postpartum. Timing of depression can impact etiology, prognosis, and response to treatment. Thus, it is crucial to study the impact of the heterogeneity of PND for better health outcomes. In this review, we outline the differences between antepartum and postpartum depression onset of PND. We discuss maternal physiological changes that differ between pregnancy and postpartum and how these may differentially impact depression susceptibility. We highlight changes in the maternal steroid and peptide hormone levels, immune signalling, serotonergic tone, metabolic factors, brain morphology, and the gut microbiome. Finally, we argue that studying the heterogeneity of PND in clinical and preclinical models can lead to improved knowledge of disease etiopathology and treatment outcomes.

Chronic social defeat stress: Impacts on ethanol-induced stimulation, corticosterone response, and brain monoamine levels

BACKGROUND

Chronic exposure to stress may dysregulate the hypothalamic-pituitary-adrenal axis and brain monoamine levels, contributing to the development of ethanol dependence. Exposure to chronic social defeat stress may impact ethanol-related effects, neural, and endocrine functions.

AIM

This study assessed ethanol-induced locomotor activity, corticosterone responses, and brain monoamine levels in Swiss albino mice 10 days post-exposure to chronic social defeat stress.

METHODS

During a period of 10 days, male Swiss mice were exposed to daily defeat episodes, followed by housing with an aggressive mouse for 24 h. Control mice were housed in pairs and rotated every 24 h. Ten days post-stress, locomotor behavior was recorded after a challenge with ethanol (2.2 g/kg; intraperitoneal) or saline. After the test, blood and brain samples were collected for determination of plasma corticosterone and brain monoamines across different brain areas through high-performance liquid chromatography.

RESULTS

Defeated mice failed to show a stimulant locomotor response to ethanol, while controls displayed the expected ethanol-induced stimulation. Ethanol increased plasma corticosterone levels, with lower corticosterone secretion in defeated mice. Brain monoamines were affected by social defeat and ethanol, varying in different brain regions. Social stress reduced levels of dopamine, noradrenaline, and serotonin in the hypothalamus. Defeated mice presented reduced serotonin and dopamine levels in the frontal cortex. In the striatum, ethanol treatment increased dopamine levels in controls, but failed to do so in defeated mice.

CONCLUSIONS

Our results suggest that chronic exposure to social defeat blunted ethanol-induced locomotor stimulation, and reduced ethanol-induced corticosterone secretion. Social stress promoted differential reductions in brain monoamine levels in the hypothalamus and frontal cortex and blunted ethanol-induced dopamine increases in the striatum.

Decreased mesolimbic dopaminergic signaling underlies the waning of maternal caregiving across the postpartum period in rats

RATIONALE

Mesolimbic dopamine (DA) signaling is essential for the high maternal caregiving characteristic of the early postpartum period, but little is known about dopamine's role in the expression of maternal caregiving thereafter.

OBJECTIVES

We tested the hypothesis that decreased mesolimbic dopaminergic signaling is particularly responsible for the natural decline in maternal caregiving that occurs as the postpartum period progresses.

METHODS

Sprague-Dawley (SD) mother rats received intraperitoneal injections of either vehicle, the DA D1 receptor agonist SKF38393, the DA D2 receptor agonist quinpirole, or both agonists twice daily from postpartum days 9 to 15. In a separate experiment involving Long-Evans (LE) rats, we examined whether DA D1 and D2 receptor mRNAs in the nucleus accumbens (NA) shell and ventral tegmental area (VTA), along with DA turnover in the VTA, decline across the postpartum period in parallel with the decreasing maternal behavior.

RESULTS

All drug treatments significantly maintained higher frequencies of active maternal behaviors (nesting, pup licking, retrieval) compared to vehicle. Furthermore, the majority of mothers treated with SKF38393 either alone or combined with quinpirole maintained full expression of maternal behavior during behavioral testing. D2 receptor mRNA levels were found to be lower in the late postpartum NA shell and VTA compared to early postpartum, but D1 receptor mRNA levels in the NA shell were higher in the late postpartum period. Furthermore, both late postpartum and recently parturient LE mothers had higher VTA DA turnover compared to nulliparae, suggesting changes in mesolimbic signal-to-noise ratio both at the end and beginning of motherhood.

CONCLUSIONS

Collectively, our results suggest that alterations in mesolimbic DA is part of the neural substrate responsible for dynamic maternal caregiving across the entire postpartum period.

Changes in brain peptides associated with reproduction and energy homeostasis: Putative roles of gonadotrophin-releasing hormone-II and tyrosine hydroxylase in determining reproductive performance in response to daily food availability times in diurnal zebra finches

Previous studies have demonstrated 'quality-quantity' trade-offs with daily food availability times in zebra finches. Compared with food access ad lib., zebra finch pairs with restricted food access for 4 hours in the morning produced poor quality offspring, whereas those with the same food access in the evening produced fewer but better quality offspring. The present study investigated whether food-time-dependent differential effects on reproductive performance involved brain peptides associated with reproduction and energy homeostasis in zebra finches. We measured peptide/protein expression of gonadotrophin-releasing hormone (GnRH)-I, GnRH-II, gonadotrophin-inhibitory hormone (GnIH), tyrosine hydroxylase (TH), neuropeptide Y (NPY), cocaine- and amphetamine regulated transcript (CART) and ZENK (a neuronal activation marker) by immunohistochemistry and mRNA expression of genes coding for the type 2 (DIO2) and type 3 (DIO3) deiodinase by a quantitative polymerase chain reaction in male and female zebra finches that were paired and kept under a 12:12 hour light/dark photocycle at 24 ± 2°C temperature for > 12 months with access to food ad lib., or for only 4 hours in the morning or evening. In both sexes, GnRH-I, DIO2 and DIO3 expression did not differ significantly between the three feeding conditions, although levels showed an overall food effect. However, in males, GnIH expression was significantly higher in evening-fed birds compared to ad lib. fed birds. Interestingly, GnRH-II and TH levels were significantly lower in restricted feeding compared to the ad lib. group and, importantly, GnRH-II and TH-immunoreactivity levels were negatively and positively correlated with egg laying latency and reproductive success (offspring/brood/pair), respectively. At the same time, we found no effect on the hypothalamic expression of orexigenic (NPY) and anorexigenic (CART) peptides, or ZENK protein (ie, the neuronal activity marker). These results suggest the involvement of reproductive neuropeptides, with putative roles for GnRH-II and TH, in the food-time-dependent effect on reproductive performance, albeit with subtle sex differences, in diurnal zebra finches, which possess the ability to reproduce year-round, in a manner similar to other continuously breeding vertebrates.

Depression-related disturbances in rat maternal behaviour are associated with altered monoamine levels within mesocorticolimbic structures

The ability of mothers to sensitively attune their maternal responses to the needs of their developing young is fundamental to a healthy mother-young relationship. The biological mechanisms that govern how mothers adjust caregiving to the dynamic changes in the demands of the young remain an open question. In the present study, we examined whether changes in monoamine levels, within discrete mesocorticolimbic structures involved in cognitive and motivational processes key to parenting, modulate this flexibility in caregiving across the postpartum period. The present study used a Wistar-Kyoto (WKY) animal model of depression and control Sprague-Dawley (SD) rats, which differ dramatically in their cognitive, motivational, and parenting performance. Levels of the monoamine neurotransmitters, dopamine, noradrenaline and serotonin, as well as their major metabolites, were measured within the medial prefrontal cortex, striatum, nucleus accumbens and medial preoptic area of SD and WKY mothers at early (postpartum day [PPD]7-8), late (PPD15-16) and weaning (PPD25) postpartum stages using high-performance liquid chromatography with electrochemical detection. Consistent with our prior work, we find that caregiving of SD mothers declined as the postpartum period progressed. Relative to nulliparous females, early postpartum mothers had lower intracellular concentrations of monoamines, as well as lower noradrenaline turnover, and an elevated serotonin turnover within most structures. Postpartum behavioural trajectories subsequently corresponded to a progressive increase in all three monoamine levels within multiple structures. Compared to SD mothers, WKY mothers were inconsistent and disorganised in caring for their offspring and exhibit profound deficits in maternal behaviour. Additionally, WKY mothers had generally lower levels of all three monoamines, as well as different patterns of change across the postpartum period, compared to SD mothers, suggesting dysfunctional central monoamine pathways in WKY mothers as they transition and experience motherhood. Taken together, the results of the present study suggest a role for monoamines at multiple mesocorticolimbic structures with repect to modulating caregiving behaviours attuned to the changing needs of the young.

Serotonin and motherhood: From molecules to mood

Emerging research points to a valuable role of the monoamine neurotransmitter, serotonin, in the display of maternal behaviors and reproduction-associated plasticity in the maternal brain. Serotonin is also implicated in the pathophysiology of numerous affective disorders and likely plays an important role in the pathophysiology of maternal mental illness. Therefore, the main goals of this review are to detail: (1) how the serotonin system of the female brain changes across pregnancy and postpartum; (2) the role of the central serotonergic system in maternal caregiving and maternal aggression; and (3) how the serotonin system and selective serotonin reuptake inhibitor medications (SSRIs) are involved in the treatment of maternal mental illness. Although there is much work to be done, studying the central serotonin system's multifaceted role in the maternal brain is vital to our understanding of the processes governing matrescence and the maintenance of motherhood.

The dynamic serotonin system of the maternal brain

Many pregnant and postpartum women worldwide suffer from high anxiety and/or depression, which can have detrimental effects on maternal and infant well-being. The first-line pharmacotherapies for prepartum and postpartum affective disorders continue to be the selective serotonin reuptake inhibitors (SSRIs), despite the lack of large well-controlled studies demonstrating their efficacy in reproducing women and the potential for fetal/neonatal exposure to the drugs. Prepartum or postpartum use of SSRIs or other drugs that modulate the brain's serotonin system is also troubling because very little is known about the typical, let alone the atypical, changes that occur in the female central serotonin system across reproduction. We do know from a handful of studies of women and female laboratory rodents that numerous aspects of the central serotonin system are naturally dynamic across reproduction and are also affected by pregnancy stress (a major predisposing factor for maternal psychopathology). Thus, it should not be assumed that the maternal central serotonin system being targeted by SSRIs is identical to non-parous females or males. More information about the normative and stress-derailed changes in the maternal central serotonin system is essential for understanding how serotonin is involved in the etiology of, and the best use of SSRIs for potentially treating, affective disorders in the pregnant and postpartum populations.

Serotonin-specific lesions of the dorsal raphe disrupt maternal aggression and caregiving in postpartum rats

The behavioral modifications associated with early motherhood, which include high aggression, caring for the young, and low anxiety, are all affected by acute pharmacological manipulation of serotonin signaling. However, the effects on all these behaviors of permanently disrupting serotonin signaling from one of its primary sources, the dorsal raphe nucleus (DR), have not been examined in detail. To address this, serotonin-specific lesions centered on the dorsomedial DR (DRdm; DR subregion strongly implicated in emotional behaviors) were induced at mid-pregnancy (day 15) or early postpartum (day 2) in rats using a saporin-conjugated neurotoxin targeting the serotonin transporter (Anti-SERT-SAP). Prepartum or postpartum Anti-SERT-SAP reduced DRdm serotonin immunoreactivity by ∼40-65%, and postpartum Anti-SERT-SAP also reduced it in the ventromedial and lateral wings of the DR, as well as in the median raphe. Serotonin-immunoreactive fibers were significantly reduced in the anterior hypothalamus, but not medial preoptic area, of lesioned dams. Pre- or postpartum lesions both greatly reduced maternal aggression, but while prepartum lesions did not affect later undisturbed maternal caregiving, the larger postpartum lesions prevented the postpartum decline in kyphotic nursing and reduced pup licking. Serotonin lesions did not affect pup retrieval, but the prepartum lesions temporarily increased maternal hovering over and licking the pups observed immediately after the disruptive retrieval tests. Dams' anxiety-like behaviors and litter weight gains were unaffected by the lesions. These findings suggest that DRdm serotonin projecting to the AH is particularly critical for maternal aggression, but that more widespread disruption of midbrain raphe serotonin is necessary to greatly impair maternal caregiving. Postpartum anxiety may rely more on other neurochemical systems or different midbrain serotonergic cell populations.

Motherhood and infant contact regulate neuroplasticity in the serotonergic midbrain dorsal raphe

The adult brain shows remarkable neuroplasticity in response to hormones and the socioemotional modifications that they influence. In females with reproductive and maternal experience, this neuroplasticity includes the birth and death of cells in several forebrain regions involved in maternal caregiving and postpartum affective state. Such plasticity in midbrain sites critical for these behavioral and emotional processes has never been examined, though. By visualizing bromodeoxyuridine (BrdU) to label mitotic cells, NeuroD for neuronal precursors, and TUNEL to identify dying cells, we found that the midbrain dorsal raphe nucleus (DR, the source of most ascending serotoninergic projections) exhibited significant neuroplasticity in response to motherhood. Specifically, BrdU analyses revealed that DR newborn cell survival (but not proliferation) was regulated by reproductive state, such that cells born early postpartum were less likely to survive 12 days to reach the late postpartum period compared to cells born during late pregnancy that survived 12 days to reach the early postpartum period. Many of the surviving cells in the DR were NeuN immunoreactive, suggesting a neuronal phenotype. Consistent with these findings, late postpartum rats had fewer NeuroD-immunoreactive DR cells than early postpartum rats. Maternal experience contributed to the late postpartum reduction in DR newborn cell survival because removing the litter at parturition increased cell survival as well as reduced cell death. Unlike cytogenesis in the maternal hippocampus, which is reduced by circulating glucocorticoids, DR newborn cell survival was unaffected by postpartum adrenalectomy. These effects of reproductive state and motherhood on DR plasticity were associated with concurrent changes in DR levels of serotonin's precursor, 5-HTP, and its metabolite, 5-HIAA. Our results demonstrate for the first time that cytogenesis occurs in the midbrain DR of any adult mammal, that DR plasticity is influenced by female reproductive state and maternal experience, and that this plasticity is accompanied by changes in DR serotonergic function. Because serotonin is critical for postpartum caregiving behaviors and maternal affective state, plasticity in the DR may contribute to the neurochemical changes necessary for successful motherhood.

Current Models and Future Directions for Understanding the Neural Circuitries of Maternal Behaviors in Rodents

Maternal behaviors in rodents include a number of subcomponents, such as nursing, nest building, licking and grooming of pups, pup retrieval, and maternal aggression. Because each behavior involves a unique motor pattern, a unique ensemble neural circuitry must underlie each behavior. To what extent there is overlap in terms of brain regions and specific neurons for each circuit is being actively investigated. This review will first examine overlapping and separate components of pup retrieval and maternal aggression circuitries while examining a central role for medial preoptic area (MPA) in both behaviors. With an emphasis on experimental approaches, the review will then highlight recent findings and propose future directions for understanding maternal behavior regulation. Finally, examples for why studying the neural basis of maternal behaviors can bring insights to other areas of neuroscience, such as feeding, addiction, and anxiety and aggression regulation will be provided.

Associations among within-litter differences in early mothering received and later emotional behaviors, mothering, and cortical tryptophan hydroxylase-2 expression in female laboratory rats

This article is part of a Special Issue "Parental Care". The effects of differential maternal care received on offspring phenotype in rodents has been extensively studied between litters, but the consequences of differential mothering within litters on offspring neurobehavioral development have been rarely examined. We here investigated how variability in maternal care received among female rat siblings (measured four times daily on postnatal days 4, 6, 8, and 10) relates to the siblings' later emotional and maternal behaviors. As previously reported, we found that some female pups received up to three times more maternal licking bouts compared to their sisters; this difference was positively correlated with the pups' body weights. The number of maternal licking bouts that females received was negatively correlated with their later neophobic behaviors in an open field during periadolescence, but positively correlated with their anxiety-related behavior in an elevated plus maze during adulthood. Licking received was also positively correlated with females' later likelihood to retrieve pups in a maternal sensitization paradigm. In addition, females' neophobia during adolescence and anxiety-related behavior during adulthood predicted some aspects of both postpartum and sensitized maternal responsiveness. Medial prefrontal cortex expression of tryptophan hydroxylase-2 (TPH2; enzyme necessary for serotonin synthesis) was negatively associated with early maternal licking received. Interestingly, cortical TPH2 was positively associated with the maternal responsiveness of sensitized virgins but negatively associated with it in postpartum females. These results indicate that within-litter differences in maternal care received is an often neglected, but important, contributor to individual differences in offspring socioemotional behaviors as well as to the cortical serotonin neurochemistry that may influence these behaviors.

Common and divergent psychobiological mechanisms underlying maternal behaviors in non-human and human mammals

Maternal interactions with young occupy most of the reproductive period for female mammals and are absolutely essential for offspring survival and development. The hormonal, sensory, reward-related, emotional, cognitive and neurobiological regulators of maternal caregiving behaviors have been well studied in numerous subprimate mammalian species, and some of the importance of this body of work is thought to be its relevance for understanding similar controls in humans. We here review many of the important biopsychological influences on maternal behaviors in the two best studied non-human animals, laboratory rats and sheep, and directly examine how the conceptual framework established by some of the major discoveries in these animal "models" do or do not hold for our understanding of human mothering. We also explore some of the limits for extrapolating from non-human animals to humans. We conclude that there are many similarities between non-human and human mothers in the biological and psychological factors influencing their early maternal behavior and that many of the differences are due to species-characteristic features related to the role of hormones, the relative importance of each sensory system, flexibility in what behaviors are exhibited, the presence or absence of language, and the complexity of cortical function influencing caregiving behaviors.

Dopamine and prolactin involvement in the maternal care of chicks in the native Thai hen (Gallus domesticus)

The dopaminergic (DAergic) system plays a pivotal role in incubation behavior via the regulation of prolactin (PRL) secretion in birds, however the role of the DA/PRL system in rearing behavior is poorly understood. The objective of this study was to investigate the relationship between the DA/PRL system and rearing behavior in a gallinaceous bird, the native Thai chicken. Incubating native Thai hens were divided into two groups. In the first group, hens were allowed to care for their chicks (rearing hens; R). In the second group, hens were deprived of their chicks immediately after hatching (non-rearing hens; NR). In both groups, blood samples and brain sections were collected at different time points after the chicks hatched (days 4, 7, 10, 14, 17, 21, 24, and 28; 6 hens/time point/group). In this study, tyrosine hydroxylase (TH) was used as a marker for DAergic neurons. The numbers of TH-immunoreactive (-ir) neurons in the nucleus intramedialis (nI) and in the nucleus mamillaris lateralis (ML), which regulate the vasoactive intestinal peptide (VIP)/PRL system, were determined in R and NR hens utilizing immunohistochemical techniques. Plasma PRL levels were determined by enzyme-linked immunosorbent assays. The results revealed that both the number of TH-ir neurons in the nI and the plasma PRL levels were significantly higher in the R hens compared with the NR hens during the first 14 days of chick rearing (P<0.05). However, there was no significant change in the DAergic activity in the ML in either the R or NR groups throughout the 28-day rearing periods. These results suggest that the DA/PRL system is involved in early rearing behavior. The additional decline in DAergic activity and plasma PRL levels during the disruption of rearing behavior further supports their involvement in rearing behavior in this equatorial precocial species.

Rat dams exposed repeatedly to a daily brief separation from the pups exhibit increased maternal behavior, decreased anxiety and altered levels of receptors for estrogens (ERα, ERβ), oxytocin and serotonin (5-HT1A) in their brain

In the present study we investigated the neurobiological mechanisms underlying expression of maternal behavior. Increased maternal behavior was experimentally induced by a brief 15-min separation between the mother and the pups during postnatal days 1 to 22. On postnatal days (PND) 12 and 22, we determined in experimental and control dams levels of anxiety in the elevated plus maze (EPM) as well as the levels of receptors for estrogens (ERα, ERβ), oxytocin (OTR) and serotonin (5-HT1AR) in areas of the limbic system (prefrontal cortex-PFC, hippocampus, lateral septum-SL, medial preoptic area-MPOA, shell of nucleus accumbens-nAc-Sh, central-CeA and basolateral-BLA amygdala), involved in the regulation of maternal behavior. Experimental dams, which showed increased maternal behavior towards their offspring, displayed reduced anxiety in the EPM on both PND12 and PND22. These behavioral differences could be attributed to neurochemical alterations in their brain: On both PND12 and PND22, experimental mothers had higher levels of ERα and OTRs in the PFC, hippocampus, CeA, SL, MPOA and nAc-Sh. The experimental manipulation-induced increase in ERβ levels was less widespread, being localized in PFC, the hippocampal CA2 area, MPOA and nAc-Sh. In addition, 5-HT1ARs were reduced in the PFC, hippocampus, CeA, MPOA and nAc-Sh of the experimental mothers. Our results show that the experience of the daily repeated brief separation from the pups results in increased brain ERs and OTRs, as well as decreased 5-HT1ARs in the dam's brain; these neurochemical changes could underlie the observed increase in maternal behavior and the reduction of anxiety.

Beyond a means of exposure: a new view of the mother in toxicology research

Toxicological studies generally view pregnant animals as a conduit through which gestational exposure of offspring to chemicals can be achieved, allowing for the study of developmental toxicity.

Melanin-concentrating hormone in the medial preoptic area reduces active components of maternal behavior in rats

Melanin-concentrating hormone (MCH) is an inhibitory neuropeptide mainly synthesized in neurons of the lateral hypothalamus and incerto-hypothalamic area of mammals that has been implicated in behavioral functions related to motivation. During lactation, this neuropeptide is also expressed in the medial preoptic area (mPOA), a key region of the maternal behavior circuitry. Notably, whereas MCH expression in the mPOA progressively increases during lactation, maternal behavior naturally declines, suggesting that elevated MCHergic activity in the mPOA inhibit maternal behavior in the late postpartum period. To explore this idea, we assessed the maternal behavior of early postpartum females following bilateral microinfusions of either MCH (50 and 100 ng/0.2 μl/side) or the same volume of vehicle into the mPOA. As expected, females receiving 100 ng MCH into the mPOA exhibited significant deficits in the active components of maternal behavior, including retrieving and nest building. In contrast, nursing, as well as other behaviors, including locomotor activity, exploration, and anxiety-like behavior, were not affected by intra-mPOA MCH infusion. The present results, together with previous findings showing elevated expression of this neuropeptide toward the end of the postpartum period, suggest that modulation of mPOA function by MCH may contribute to the weaning of maternal responsiveness characteristic of the late postpartum period.

Distal pup cues evoke dopamine responses in hormonally primed rats in the absence of pup experience or ongoing maternal behavior

During the early postpartum period or following estrogen/progesterone administration, pups elicit maternal behavior accompanied by a robust dopamine (DA) response in the nucleus accumbens (NAC) of female rats (Afonso et al., 2009). To determine whether DA responds to ostensibly "salient" stimuli in the absence of consummatory behaviors, we examined NAC shell DA responses during restricted (stimuli placed in a perforated box), and unrestricted access to pup and food stimuli. Microdialysis samples were collected from female rats that were either cycling and postpartum (Experiment 1), or after ovariectomy and treated with empty and hormone-filled capsules (Experiment 2). Relative to nonprimed controls, hormonally primed females had suppressed basal DA concentrations and facilitated pup-evoked DA responses, regardless of stimulus access condition. In contrast, food-evoked DA responses were unchanged by hormonal priming and were greater when females consumed food compared with distal (restricted) exposure to food. During pup and food restriction conditions, the lack of any "appetitive" behavioral differences, even in pup experienced postpartum females, was surprising. In Experiment 3, we confirmed that postpartum dams allocated time equivalently to restricted pup and food stimuli, even after pup deprivation. This was in sharp contrast to the effects of deprivation during the unrestricted access phase. Together, our data demonstrated that, in hormonally primed females, distal pup cues could evoke DA responses without prior stimulus experience, ongoing maternal (behavioral) responses, or clear evidence of robust pup saliency. The results suggest that NAC DA response reflects a state of responsiveness related to basal DA suppression in the hormonally primed female rat.

Accumbal dopamine function in postpartum rats that were raised without their mothers

Postpartum rats that had been previously raised in an artificial rearing (AR) apparatus, without their mothers or siblings during the preweaning period, show altered maternal responses towards their own offspring in adulthood. In mother-reared (MR) rats, nucleus accumbens (NAC) dopamine (DA) responses to pups evoke a robust sustained rise during the postpartum period and following treatment with estrogen/progesterone parturient-like hormones (Afonso et al., 2009). These MR females had siblings that received AR rearing with varying amounts of preweaning tactile stimulation (ARmin; ARmax). The present study examined NACshell DA responses to pup and food stimuli in these AR rats, and statistically compared them to their MR siblings. Microdialysis samples were collected from adult (90 days postnatal) AR females in different parity states (cycling vs. postpartum, Exp. 1), or after ovariectomy with different hormone treatments (sham vs. hormone, Exp. 2. After basal sample collection, pup and then food stimuli were individually presented to the females in the dialysis chamber. As with their MR siblings, basal DA concentrations were lower and pup-evoked DA responses greater in hormonally-primed AR females than in non-primed AR controls. Compared to their postpartum MR sisters (Exp. 1), AR rats had increased basal DA levels, reduced pup related DA elevations, and disrupted maternal behavior. The postpartum AR impairment in pup-evoked DA was reversed by additional pre-weaning tactile stimulation. Exogenous hormones (Exp. 2) eliminated AR impairments on pup-evoked DA responses. Although MR and AR siblings had comparable DA responses to food stimuli, upon reanalyzing MR data it was found that only postpartum dams had DA responses to pups greater than to food. These data suggest that that the hormonally induced suppression of basal DA levels may reflect saliency of pups which was greater in MR than in AR dams. Preweaning tactile stimulation could partially reverse these effects only in naturally cycling or parturient animals.

Dopamine and mesotocin neurotransmission during the transition from incubation to brooding in the turkey

We investigated the neuroendocrine changes involved in the transition from incubating eggs to brooding of the young in turkeys. Numbers of mesotocin (MT; the avian analog of mammalian oxytocin) immunoreactive (ir) neurons were higher in the nucleus paraventricularis magnocellularis (PVN) and nucleus supraopticus, pars ventralis (SOv) of late stage incubating hens compared to the layers. When incubating and laying hens were presented with poults, all incubating hens displayed brooding behavior. c-fos mRNA expression was found in several brain areas in brooding hens. The majority of c-fos mRNA expression by MT-ir neurons was observed in the PVN and SOv while the majority of c-fos mRNA expression in dopaminergic (DAergic) neurons was observed in the ventral part of the nucleus preopticus medialis (POM). Following intracerebroventricular injection of DA or oxytocin (OT) receptor antagonists, hens incubating eggs were introduced to poults. Over 80% of those injected with vehicle or the D1 DA receptor antagonist brooded poults, while over 80% of those receiving the D2 DA receptor antagonist or the OT receptor antagonist failed to brood the poults. The D2 DA/OT antagonist groups also displayed less c-fos mRNA in the dorsal part of POM and the medial part of the bed nucleus of the stria terminalis (BSTM) areas than did the D1 DA/vehicle groups. These data indicate that numerous brain areas are activated when incubating hens initially transition to poult brooding behavior. They also indicate that DAergic, through its D2 receptor, and MTergic systems may play a role in regulating brooding behaviors in birds.

Dopaminergic projections to the medial preoptic area of postpartum rats

Dopamine receptor activity in the rodent medial preoptic area (mPOA) is crucial for the display of maternal behaviors, as well as numerous other physiological and behavioral functions. However, the origin of dopaminergic input to the mPOA has not been identified through neuroanatomical tracing. To accomplish this, the retrograde tracer Fluorogold was iontophoretically applied to the mPOA of postpartum laboratory rats, and dual-label immunocytochemistry for Fluorogold and tyrosine hydroxylase later performed to identify dopaminergic cells of the forebrain and midbrain projecting to the mPOA. Results indicate that the number of dopaminergic cells projecting to the mPOA is moderate ( approximately 90 cells to one hemisphere), and that these cells have an unexpectedly wide distribution. Even so, more than half of the dual-labeled cells were found in either what has been considered extensions of the A10 dopamine group (particularly the ventrocaudal posterior hypothalamus and adjacent medial supramammillary nucleus), or in the A10 group of the ventral tegmental area. The rostral hypothalamus and surrounding region also contained numerous dual-labeled cells, with the greatest number found within the mPOA itself (including in the anteroventral preoptic area and preoptic periventricular nucleus). Notably, dual-labeled cells were rare in the zona incerta (A13), a site previously suggested to provide dopaminergic input to the mPOA. This study is the first to use anatomical tracing to detail the dopaminergic projections to the mPOA in the laboratory rat, and indicates that much of this projection originates more caudally than previously suggested.

Pregnant rats show enhanced spatial memory, decreased anxiety, and altered levels of monoaminergic neurotransmitters

Spatial memory, anxiety and central monoaminergic activities were measured in non-pregnant (NP) and pregnant females during two time periods of pregnancy: gestational days 7-9 (GD7, GD9) and gestation days 16-18 (GD16, GD18). Pregnant females discriminated between object locations on both test days on an object placement task, whereas NP females were unable to discriminate between locations. Pregnant females displayed decreased anxiety on the elevated plus maze on GD9 compared to NP females, followed by increased anxiety-like behavior on the elevated plus maze on GD18. Monoamine levels and activity (as indexed by turnover ratio) were measured in prefrontal cortex (PFC), CA1 and CA3 regions of the hippocampus (areas important for memory), and medial preoptic area (mPOA, an area important in display of maternal behaviors). In the PFC, NP females generally had higher monoamine levels and turnover ratios; however, norepinephrine (NE) turnover was higher in pregnant females at GD18. In the CA1 and CA3 regions of the hippocampus, monoamine levels and turnover ratios were generally higher during pregnancy, particularly on GD9. In the mPOA, pregnancy was associated with increases in NE activity, a previously unreported finding. The present study expands upon existing research indicating that pregnancy is beneficial to spatial memory and may decrease anxiety. Changes in monoamine levels and activity in specific brain regions indicate that the dopamine, norepinephrine and serotonin systems may contribute to the observed behavioral differences.

Exploratory, anti-anxiety, social, and sexual behaviors of rats in behavioral estrus is attenuated with inhibition of 3alpha,5alpha-THP formation in the midbrain ventral tegmental area

The progesterone (P(4)) metabolite and neurosteroid, 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP) acts in the midbrain ventral tegmental area (VTA) to modulate lordosis of female rats. 3alpha,5alpha-THP also mediates exploratory, affective, and social behaviors; whether actions of 3alpha,5alpha-THP in the VTA mediate these behaviors is of interest. To elucidate the role of the VTA in mediating exploratory, affective, and social behaviors, the present study examined effects of inhibiting 3alpha,5alpha-THP formation in the VTA. Rats received intra-VTA infusions of either PK11195 (400ng/mul, which inhibits de novo 3alpha,5alpha-THP production), indomethacin (10mug/mul, which blocks metabolism of P(4) to 3alpha,5alpha-THP), PK11195 and indomethacin together, or beta-cyclodextrin vehicle and tested on a battery of anxiety (open field and elevated plus maze), social (partner preference and social interaction), and sexual (paced mating) tasks. Compared to rats infused with vehicle to the VTA, rats infused with inhibitor(s) demonstrated significant reductions in central entries in the open field, time on open arms of an elevated plus maze, time spent interacting with a conspecific, initiation and intensity of lordosis, sexual solicitations, and midbrain 3alpha,5alpha-THP levels. These findings suggest that actions of 3alpha,5alpha-THP in the VTA are important for mediating aspects of exploration, anxiety, and social behavior related to mating.

Effects of multiparity on recognition memory, monoaminergic neurotransmitters, and brain-derived neurotrophic factor (BDNF)

Recognition memory and anxiety were examined in nulliparous (NP: 0 litters) and multiparous (MP: 5-6 litters) middle-aged female rats (12 months old) to assess possible enduring effects of multiparity at least 3 months after the last litter was weaned. MP females performed significantly better than NP females on the non-spatial memory task, object recognition, and the spatial memory task, object placement. Anxiety as measured on the elevated plus maze did not differ between groups. Monoaminergic activity and levels were measured in prefrontal cortex, CA1 hippocampus, CA3 hippocampus, and olfactory bulb (OB). NP and MP females differed in monoamine concentrations in the OB only, with MP females having significantly greater concentrations of dopamine and metabolite DOPAC, norepinephrine and metabolite MHPG, and the serotonin metabolite 5-HIAA, as compared to NP females. These results indicate a long-term change in OB neurochemistry as a result of multiparity. Brain-derived neurotrophic factor (BDNF) was also measured in hippocampus (CA1, CA3, dentate gyrus) and septum. MP females had higher BDNF levels in both CA1 and septum; as these regions are implicated in memory performance, elevated BDNF may underlie the observed memory task differences. Thus, MP females (experiencing multiple bouts of pregnancy, birth, and pup rearing during the first year of life) displayed enhanced memory task performance but equal anxiety responses, as compared to NP females. These results are consistent with previous studies showing long-term changes in behavioral function in MP, as compared to NP, rats and suggest that alterations in monoamines and a neurotrophin, BDNF, may contribute to the observed behavioral changes.

Activation of maternal behavior of albino rats after combined treatment with dopamine and opioid receptor antagonists in low doses

We studied the effect of D1/D2 antagonist haloperidol on maternal motivation in nursing albino rats. Haloperidol in a dose of 0.2 mg/kg significantly attenuated parental reactions and motor and exploratory activities. In a lower dose (0.1 mg/kg) the drug produced the same effect on maternal behavior (number of approaches to newborns) without reducing motor activity. The effect of low-dose haloperidol was different after naloxone treatment (0.2 mg/kg intranasally): the number of pup transfers increased significantly. The detected phenomenon indicates good prospects of combined treatment with agents modifying the cerebral dopaminergic and opioid systems as the method for correction of disorders in maternal behavior.

The effects of D1 or D2 dopamine receptor antagonism in the medial preoptic area, ventral pallidum, or nucleus accumbens on the maternal retrieval response and other aspects of maternal behavior in rats

The medial preoptic area (MPOA), ventral pallidum (VP), and nucleus accumbens (NA) receive dopaminergic afferents and are involved in maternal behavior. Experiments investigated whether dopamine (DA) receptor antagonism in NA disrupts maternal behavior, determined the type of DA receptor involved, and investigated the involvement of drug spread to VP or MPOA. Injection of SCH 23390 (D1 DA receptor antagonist) into NA of postpartum rats disrupted retrieving at dosage levels that were ineffective when injected into MPOA or VP. Motor impairment was not the cause of the deficit. Injection of eticlopride (D2 DA receptor antagonist) into NA or VP was without effect. Results emphasize the importance of DA action on D1 receptors in NA for retrieval behavior.

Dopaminergic mechanisms involved in prolactin release after mifepristone and naloxone treatment during late pregnancy in the rat

BACKGROUND/AIMS

During late pregnancy, the antiprogesterone mifepristone facilitates prolactin release. This effect is enhanced by administration of the opioid antagonist naloxone, suggesting an inhibitory-neuromodulatory role of the opioid system. Since hypothalamic dopamine (DA) is the main regulator of prolactin release, in this study we explored the role of DA on prolactin release induced by mifepristone and naloxone treatment.

METHODS/RESULTS

Rats on day 19 of pregnancy were used. Naloxone treatment did not modify the 3,4-dihydroxyphenylacetic acid/DA (DOPAC/DA) ratio or serum prolactin concentration in control rats. After mifepristone treatment, DA activity diminished significantly without modifying serum prolactin levels. Naloxone administration to antiprogesterone-treated rats did not change the DOPAC/DA ratio but increased serum prolactin. Tyrosine hydroxylase (TH) expression in medial basal hypothalamus (MBH) protein extracts was lowered by pretreatment with mifepristone, with no additional effect of naloxone. While mifepristone decreased the intensity of TH immunoreactivity in the arcuate and periventricular nuclei and in fibers of the median eminence, naloxone treatment had no further effect.

CONCLUSIONS

(1) A reduction of tuberoinfundibular dopaminergic (TIDA) neuron activity is suggested by the fall of the DOPAC/DA ratio and the low expression of MBH TH; (2) this reduction facilitates prolactin secretion by naloxone, indicating that progesterone stimulates DA neurons to maintain low serum prolactin; (3) naloxone action seems to depend on a previous decrease of DA tone induced by mifepristone, without involve a direct effect on neuronal DA activity, and (4) endogenous opioids may inhibit prolactin secretion through a non-dopaminergic neuronal system that regulates prolactin secretion in which as yet undetermined prolactin-releasing factors may participate.

Effects of novel antipsychotics, amisulpiride and aripiprazole, on maternal behavior in rats

RATIONALE

Rat maternal behavior, which entails complex motivational and social factors, is disrupted by the currently available typical and atypical antipsychotics. It is thought that this disruption reflects a side effect of antipsychotics, modeling the neuroleptic-induced negative or deficit state. Amisulpiride and aripiprazole are new atypical antipsychotics with mechanisms of action distinct from the current typical and atypical antipsychotics. The effects of these drugs on maternal behavior have not been explored.

OBJECTIVE

In the present study, we systematically examined the behavioral effects of amisulpiride and aripiprazole on maternal behavior in postpartum female rats.

METHODS

Various components of maternal behavior (pup retrieval, pup licking, nest building and pup nursing) were examined repeatedly over a period of 24 h after a single injection of three doses of amisulpiride (10, 30, and 100 mg/kg s.c.) and aripiprazole (3, 10, and 30 mg/kg).

RESULTS

Amisulpiride at the lower doses (10 and 30 mg/kg) enhanced pup licking, and only at the highest dose disrupted the active components of maternal behavior such as pup retrieval and nest building. Its effect was delayed in onset and prolonged as compared to other antipsychotics. Aripiprazole, even at the highest dose (30 mg/kg) did not impair pup retrieval or pup licking. However, it did disrupt nest building and led to enhanced pup nursing.

CONCLUSIONS

The unique effects of these two drugs may be due to their unique actions at the mesolimbic dopamine synapses. The sparing of the major components of maternal behavior by aripiprazole may be related to its partial agonist effects, whereas the enhancement of pup licking by amisulpiride may be related to its dose-dependent preferential effect on the presynaptic autoreceptors. The potential clinical implications of these findings are discussed.

Maternal behaviors: central integration or independent parallel circuits? Theoretical comment on Popeski and Woodside (2004)

N. Popeski and B. Woodside (2004) report that the injection of L-NAME into the 3rd ventricle, which would suppress the synthesis of nitric oxide, disrupts both maternal retrieval of pups and maternal aggression in postpartum rats. These findings are discussed around the question of whether a single central integrative site regulates retrieval and maternal aggression or whether independent neural mechanisms, each dependent upon nitric oxide, regulate the 2 behaviors. A case is made for each point of view. Future research utilizing direct injections of L-NAME into specific neural sites is needed to resolve this important question.

Dopamine D1 and D2 Receptor Antagonism in the Preoptic Area Produces Different Effects on Maternal Behavior in Lactating Rats

The preoptic area (POA) is critical for maternal behavior in rats but little is known about what neurotransmitters released here influence maternal responding. POA infusion of 10 microg (but not 2 microg) of the dopamine D1 receptor antagonist SCH-23390 greatly impaired retrieval and licking of pups but not other maternal or nonmaternal behaviors in lactating rats. In contrast, POA infusion of 10 microg (but not 2 microg) of the D2 receptor antagonist raclopride facilitated nursing but did not affect oral maternal behaviors. SCH-23390 in the medial hypothalamus tended to impair licking but not retrieval. Raclopride in the medial hypothalamus had no effects. Therefore, D1 and D2 receptor activity, particularly in the POA, is important for regulating different maternal behaviors.

The content of dopamine, serotonin, and their metabolites in the neural circuit that mediates maternal behavior in juvenile and adult rats

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.

Effects of typical and atypical antipsychotic drugs on maternal behavior in postpartum female rats

Understanding the effects of antipsychotic drugs (APDs) on social behaviors such as maternal behavior is valuable for understanding the complete spectrum of therapeutic and side-effects of antipsychotics. Although previous studies have suggested that typical antipsychotics impair maternal behavior, the effects of the atypical antipsychotics have not been systematically explored. The purpose of the present report was to examine the effects of typical (haloperidol, HAL) and several atypical (clozapine, CLZ; risperidone, RIS; quetiapine, QUE) antipsychotics on maternal behavior in female rats. Maternal behaviors were examined repeatedly over a period of 24 h after a single injection of a range of doses of HAL, CLZ, RIS or QUE on Day 6 postpartum. All antipsychotic drugs, typical or atypical, elicited a qualitatively similar disruptive effect on the active components of maternal behavior such as pup approach, pup retrieval and nest building at clinically relevant doses. However, HAL caused a prolonged disruption, whereas CLZ, RIS and QUE induced an early onset but shorter duration disruption. In addition, only the atypical antipsychotics showed some inhibitory effects on nursing behavior, possibly due to sedative side-effects shared by all atypical antipsychotics. The current generation of atypical antipsychotics shows a disruptive influence on maternal behavior similar to that of the typical antipsychotics. This effect may be intrinsic to antipsychotic activity or may be reflective of a side-effect. Since the latter is more likely, this may be an effect to avoid in the design of future antipsychotics.

Minireview: Neuronal steroid hormone receptors: they're not just for hormones anymore

The ovarian steroid hormones have numerous effects on the brain, many of which are mediated, at least in part, by interaction with intracellular steroid hormone receptors acting as regulators of transcription. These intracellular steroid hormone receptors have often been considered to be activated solely by cognate hormone. However, during the past decade, numerous studies have shown that the receptors can be activated by neurotransmitters and intracellular signaling systems, through a process that does not require hormone. Although most of these have been in vitro experiments, others have been in vivo. Evidence from a wide variety of tissues and cells suggests that steroid hormone receptors are transcription factors that can be activated by a wide variety of factors, only one of which is cognate hormone. Furthermore, ligand-independent activation of neural steroid hormone receptors, rather than being a pharmacological or in vitro curiosity, seems to be a process that occurs in the normal physiology of animals. Thinking of steroid hormone receptors only as ligand-activated proteins may constrain our thinking about the many factors that may activate the receptors and cause receptor-dependent changes in neural gene expression and neuroendocrine function.