The effects of paraventricular hypothalamic lesions on maternal behavior in rats

Neurobiological mechanisms underlying oxytocin-mediated parental behavior in rodents

Parental behavior is essential for mammalian offspring to survive. Because of this significance, elucidating the neurobiological mechanisms that facilitate parental behavior has received strong interest. Decades of studies utilizing pharmacology and molecular biology have revealed that in addition to its facilitatory effects on parturition and lactation, oxytocin (OT) promotes the expression of parental behavior in rodents. Recent studies have also described the modulation of sensory processing by OT and the interaction of the OT system with other brain regions associated with parental behavior. However, the precise neurobiological mechanisms underlying the facilitation of caregiving behaviors by OT remain unclear. In this Review, I summarize the findings from rats and mice with a view toward integrating past and recent progress. I then review recent advances in the understanding of the molecular, cellular, and circuit mechanisms of OT-mediated parental behavior. Based on these observations, I propose a hypothetical model that would explain the mechanisms underlying OT-mediated parental behavior. Finally, I conclude by discussing some major remaining questions and propose potential future research directions.

Is melanin-concentrating hormone in the medial preoptic area a signal for the decline of maternal care in late postpartum?

This manuscript proposes that melanin-concentrating hormone (MCH) in the medial preoptic area (MPOA) is an neurochemical signal evolved to trigger the declining process of maternal care. MCH in the MPOA appears only after parturition and is progressively increased with the progression of lactation, while maternal behavior declines progressively. Intra-MPOA injection of MCH decreases active maternal responses. MCH is also highly responsive to infant characteristics and maternal condition. Behavioral changes induced by MCH in late postpartum period are conducive to the decline of infant-directed maternal behavior. The MPOA MCH system may mediate the maternal behavior decline by suppressing the maternal approach motivation and/or increasing maternal withdrawal via its inhibitory action onto the mesolimbic dopamine D1/D2 receptors and its stimulating action on serotonin 5-HT2C receptors in the ventral tegmental area. Research into the MCH maternal effects will enhance our understanding of the neurochemical mechanisms underlying the maternal behavior decline.

Roadmap for maternal behavior research in domestic dogs: lessons from decades of laboratory rodent work

Maternal behavior research in laboratory rats has revealed important behavioral and neurobiological mechanisms governing the onset, maintenance and decline of maternal behavior. However, the extent to which these mechanisms are evolutionarily conserved across species is less clear. This manuscript proposes that examining these mechanisms in dogs may be a viable approach to test their generality and help bridge the gap between rodent and human research, as domestic dogs show greater individual differences and exhibit more human-like maternal characteristics than rodents. These aspects represent advantages over rodent models, which in turn allow systems biological approaches not available in rodents. Additionally, domestic dogs share similar social environments with humans, suffer from the same mental disorders as humans, and can be treated with the same medications. This paper begins with a summary of key findings and theoretical developments from decades of rat maternal behavior research, followed by a literature review of the extant maternal behavior research on dogs and related methodology, highlighting the unique behavioral characteristics of dog maternal behavior and similarities and differences from rat maternal behavior. Finally, several knowledge gaps in dog maternal behavior research, as well as the future research in this area is discussed. It concludes that research on dog maternal behavior will not only advance our understanding of the universality of the neurobiological and behavioral mechanisms in maternal behavior, but also improve our understanding of risk factors associated with postpartum mental disorders.

Parental brain through time: The origin and development of the neural circuit of mammalian parenting

This review consolidates current knowledge on mammalian parental care, focusing on its neural mechanisms, evolutionary origins, and derivatives. Neurobiological studies have identified specific neurons in the medial preoptic area as crucial for parental care. Unexpectedly, these neurons are characterized by the expression of molecules signaling satiety, such as calcitonin receptor and BRS3, and overlap with neurons involved in the reproductive behaviors of males but not females. A synthesis of comparative ecology and paleontology suggests an evolutionary scenario for mammalian parental care, possibly stemming from male-biased guarding of offspring in basal vertebrates. The terrestrial transition of tetrapods led to prolonged egg retention in females and the emergence of amniotes, skewing care toward females. The nocturnal adaptation of Mesozoic mammalian ancestors reinforced maternal care for lactation and thermal regulation via endothermy, potentially introducing metabolic gate control in parenting neurons. The established maternal care may have served as the precursor for paternal and cooperative care in mammals and also fostered the development of group living, which may have further contributed to the emergence of empathy and altruism. These evolution-informed working hypotheses require empirical validation, yet they offer promising avenues to investigate the neural underpinnings of mammalian social behaviors.

Imprinted genes and the manipulation of parenting in mammals

Genomic imprinting refers to the parent-of-origin expression of genes, which originates from epigenetic events in the mammalian germ line. The evolution of imprinting may reflect a conflict over resource allocation early in life, with silencing of paternal genes in offspring soliciting increased maternal provision and silencing of maternal genes limiting demands on the mother. Parental caregiving has been identified as an area of potential conflict, with several imprinted genes serendipitously found to directly influence the quality of maternal care. Recent systems biology approaches, based on single-cell RNA sequencing data, support a more deliberate relationship, which is reinforced by the finding that imprinted genes expressed in the offspring influence the quality of maternal caregiving. These bidirectional, reiterative relationships between parents and their offspring are critical both for short-term survival and for lifelong wellbeing, with clear implications for human health.

Oxytocin antagonist does not disrupt rabbit maternal behavior despite binding to brain oxytocin receptors

We explored a possible role of oxytocin (OXT) for the onset and maintenance of rabbit maternal behavior by: (a) confirming that a selective oxytocin receptor antagonist (OTA) widely used in rodents selectively binds to OXT receptors (OXTR) in the rabbit brain and (b) determining the effect of daily intracerebroventricular (icv) injections of OTA to primiparous and multiparous does from gestation day 29 to lactation day 3. OTA efficiently displaced the high affinity, selective oxytocin receptor (OXTR) radioligand, 125 I-labeled ornithine vasotocin analog (125 I-OVTA), but was much less effective at displacing the selective V1a vasopressin receptor radioligand, 125 I-labeled linear vasopressin, thus showing high affinity and selectivity of OTA for rabbit OXTR as in rodents. Further, ICV OTA injections did not modify nest-building, latency to enter the nest box, time spent nursing or the amount of milk produced, relative to vehicle-injected does. The percentage of mothers suckling the litter was also similar between both groups, regardless of parity. Together, our results do not support a role of OXT for the initiation or maintenance of rabbit maternal behavior. Future studies are warranted to determine if OXT participates in fine-tuning additional aspects of the maternal ethogram, for example, circadian periodicity of nursing and nest defense.

Dysfunctions of brain oxytocin signaling: Implications for poor mothering

Good mothering has profound impact on both the mother's and the young's well-being. Consequently, experiencing inadequate maternal care - or even neglect - in the first stages of life is a major risk factor for the development of psychiatric disorders, and even for poor parenting towards the future offspring. Thus, understanding the neurobiological basis of maternal neglect becomes crucial. Along with other neurotransmitters and neuropeptides, oxytocin (OXT) has long been known as one of the main modulators of maternal behavior. In rodents, disruptions of central OXT transmission have been associated with poor maternal responses, like impaired onset of nursing behaviors, and reduced care and defense of the pups. Importantly, such behavioral and molecular deficits can be transmitted through generations, creating a vicious circle of low-quality maternal behavior. Similarly, evidence from human studies shows that OXT signaling is defective in conditions of inadequate mothering and child neglect. On those premises, this review aims at providing a comprehensive overview of animal and human studies linking perturbed OXT transmission to poor maternal behavior. Considering the important fallouts of inadequate maternal responses, we believe that unraveling the alterations in OXT transmission might provide useful insights for a better understanding of maternal neglect and, ultimately, for future intervention approaches.

Species differences in the effect of oxytocin on maternal behavior: A model incorporating the potential for allomaternal contributions

Oxytocin has historically been linked to processes involved with maternal behavior. However, the relative importance of oxytocin for maternal behavior widely varies among mammalian species, from indispensable to apparently nonessential. This review proposes a new model in which the relative importance of oxytocin for mothering across species is explained by an evolutionary pressure which we term "allomaternal potential", or the degree to which other conspecifics are capable and likely to assist with caregiving. It is notable that in animals where allomaternal potential is high (i.e., many quality helpers are available), oxytocin is decoupled from mothering. However, in animals where allomaternal potential is low (i.e., conspecifics refuse to, or do not provide, quality help), oxytocin is crucial for mothering. We posit that this relationship is a form of kin selection, whereby oxytocin is a signal that leads mothers to preferentially dispense resources to their own young when quality helpers are unlikely.

Brain orchestration of pregnancy and maternal behavior in mice: A longitudinal morphometric study

Reproduction induces changes within the brain to prepare for gestation and motherhood. However, the dynamic of these central changes and their relationships with the development of maternal behavior remain poorly understood. Here, we describe a longitudinal morphometric neuroimaging study in female mice between pre-gestation and weaning, using new magnetic resonance imaging (MRI) resources comprising a high-resolution brain template, its associated tissue priors (60-µm isotropic resolution) and a corresponding mouse brain atlas (1320 regions of interest). Using these tools, we observed transient hypertrophies not only within key regions controlling gestation and maternal behavior (medial preoptic area, bed nucleus of the stria terminalis), but also in the amygdala, caudate nucleus and hippocampus. Additionally, unlike females exhibiting lower levels of maternal care, highly maternal females developed transient hypertrophies in somatosensory, entorhinal and retrosplenial cortices among other regions. Therefore, coordinated and transient brain modifications associated with maternal performance occurred during gestation and lactation.

Comparison of the distribution of oxytocin and vasopressin 1a receptors in rodents reveals conserved and derived patterns of nonapeptide evolution

Oxytocin (OT) and vasopressin (VP) are known modulators of social behaviour across rodents. Research has revealed the location of action of these nonapeptides through localization of their associated receptors, which include the oxytocin receptor (OTR) and the vasopressin 1a receptor (V1aR). As research into these complex systems has progressed, studies investigating how these systems modulate behaviour have remained relatively narrow in scope (ie, focused on how a single brain region shapes behaviour in only a handful of species). However, the brain regions that regulate social behaviour are part of interconnected neural networks for which coordinated activity enables behavioural variation. Thus, to better understand how nonapeptide systems have evolved under different selective pressures among rodent species, we conducted a meta-analysis using a multivariate comparative method to examine the patterns of OTR and V1aR density expression in this taxon. Several brain regions were highly correlated based on their OTR and V1aR binding patterns across species, supporting the notion that the distribution of these receptors is highly conserved in rodents. However, our results also revealed that specific patterns of V1aR density differed from OTR density, and within-genus variance for V1aR was low compared to between-genus variance, suggesting that these systems have responded and evolved quite differently to selective pressures over evolutionary time. We propose that, in addition to examining single brain regions of interest, taking a broad comparative approach when studying the OT and VP systems is important for understanding how the systemic action of nonapeptides modulate social behaviour across species.

Maternal care activates the ventral tegmental area but not dopaminergic cells in the rat

The ventral tegmental area (VTA), together with the preoptic area, is part of a neural circuit necessary for the expression of maternal behaviour (MB); destruction of either area disrupts MB in postpartum rats. Central to the proposal of VTA activation are dopaminergic cells, for which the cell bodies lie in the VTA and project to forebrain structures. This mesolimbic system is a motivational circuit involved in rewarding behaviours such as sex and MB. Despite their recognised importance, surprisingly, unlike the preoptic area, there are no anatomical descriptions of the pattern of VTA activation or of the dopaminergic cell activation, specifically in relation to MB in the rat. In the present study, we explore the possible activation (as indicated by Fos protein via immunohistochemistry) of the anterior and medial portions of the VTA and in the dopaminergic cells in these regions, as well as in the medial preoptic area, in lactating rats, at postpartum day 7 (after a 12-hour mother/pups separation), and in dioestrous females. After 12 hours, mothers were perfused at that moment or after a 90 minutes of interaction, or not, with their pups. We found a strong significant Fos induction in both the preoptic area and in the anterior portion of VTA in dams that interacted with their pups. The number of dopaminergic cells that coexpressed Fos did not differ across groups. Additionally, we determined Fos and GABA colocalisation in the anterior part of the VTA and found dense GABAergic processes, possibly varicosities, in the area of increased Fos expression. The results of the present study support a proposed GABAergic pathway from medial preoptic area to VTA cells, critical for the expression of MB. Future experiments are warranted to explore the neurochemical identity of the Fos and no-Fos expressing cells that are recipients of GABAergic processes in the VTA, aiming to better understand the neural circuitry of the VTA in relation to MB.

Experience-dependent mechanisms in the regulation of parental care

Maternal behavior is a defining characteristic of mammals, which is regulated by a core, conserved neural circuit. However, mothering behavior is not always a default response to infant conspecifics. For example, initial fearful, fragmented or aggressive responses toward infants in laboratory rats and mice can give way to highly motivated and organized caregiving behaviors following appropriate hormone exposure or repeated experience with infants. Therefore hormonal and/or experiential factors must be involved in determining the extent to which infants access central approach and avoidance neural systems. In this review we describe evidence supporting the idea that infant conspecifics are capable of activating distinct neural pathways to elicit avoidant, aggressive and parental responses from adult rodents. Additionally, we discuss the hypothesis that alterations in transcriptional regulation within the medial preoptic area of the hypothalamus may be a key mechanism of neural plasticity involved in programming the differential sensitivity of these neural pathways.

Functional circuit architecture underlying parental behaviour

Parenting is essential for the survival and wellbeing of mammalian offspring but we lack a circuit-level understanding of how distinct components of this behaviour are orchestrated. Here we investigate how Galanin-expressing neurons in the medial preoptic area (MPOA^Gal) coordinate motor, motivational, hormonal and social aspects of parenting. These neurons integrate inputs from a large number of brain areas, whose activation depends on the animal’s sex and reproductive state. Subsets of MPOA^Gal neurons form discrete pools defined by their projection sites. While the MPOA^Gal population is active during all episodes of parental behaviour, individual pools are tuned to characteristic aspects of parenting. Optogenetic manipulation of MPOA^Gal projections mirrors this specificity, affecting discrete parenting components. This functional organization, reminiscent of the control of motor sequences by pools of spinal cord neurons, provides a new model for how discrete elements of a social behaviour are generated at the circuit level.

Behavioral mechanisms underlying the maternal disruptive effect of serotonin 5-HT2A receptor activation in Sprague-Dawley rats

Recent evidence indicates that acute activation of 5-HT_2A receptors causes a disruption of maternal behavior in rats. However, the behavioral mechanisms underlying such a disruption are not known. We addressed this issue using two behavioral approaches targeting the maternal motivational and emotional processing systems. First, we used the pup-separation technique to increase maternal motivation to see whether pup separation is capable of reducing the maternal disruptive effect of TCB-2 (a high-affinity 5-HT_2A agonist) treatment. On postpartum days 4 and 6, different groups of Sprague-Dawley dams were treated with the TCB-2 (5.0 mg/kg, sc) or vehicle and their maternal behaviors were tested after either a 4-h pup-separation or no-pup-separation condition. Although acute TCB-2 injection disrupted maternal behavior, this disruption was not attenuated by pup separation, even after we optimized the timing of separation to maximize its increase on maternal motivation. Acute TCB-2 also impaired the retrieval of food pellets, suggesting a general effect on motivated behaviors. Next, we used a pup preference test and found that dams treated with TCB-2 exhibited an even stronger preference to pups over a male conspecific than vehicle-treated dams, indicating an enhanced motivational and emotional processing of the rewarding property of pups. These findings suggest that TCB-2 has a disruptive effect on rat maternal behavior, and this disruption is not likely due to the drug's effect on mothers' motivational and emotional processing of the incentive salience of pups, although this motivational suppression account cannot be completely ruled out. Future work could explore other possible behavioral mechanisms, such as the drug's effect on executive function.

Oxytocin and Parental Behaviors

The oxytocin/vasopressin ancestor molecule has been regulating reproductive and social behaviors for more than 500 million years. In all mammals, oxytocin is the hormone indispensable for milk-ejection during nursing (maternal milk provision to offspring), a process that is crucial for successful mammalian parental care. In laboratory mice, a remarkable transcriptional activation occurs during parental behavior within the anterior commissural nucleus (AC), the largest magnocellular oxytocin cell population within the medial preoptic area (although the transcriptional activation was limited to non-oxytocinergic neurons in the AC). Furthermore, there are numerous recent reports on oxytocin's involvement in positive social behaviors in animals and humans. Given all those, the essential involvement of oxytocin in maternal/parental behaviors may seem obvious, but basic researchers are still struggling to pin down the exact role oxytocin plays in the regulation of parental behaviors. A major aim of this review is to more clearly define this role. The best conclusion at this moment is that OT can facilitate the onset of parental behavior, or parental behavior under stressful conditions.In this chapter, we will first review the basics of rodent parental behavior. Next, the neuroanatomy of oxytocin systems with respect to parental behavior in laboratory mice will be introduced. Then, the research history on the functional relationship between oxytocin and parental behavior, along with advancements in various techniques, will be reviewed. Finally, some technical considerations in conducting behavioral experiments on parental behavior in rodents will be addressed, with the aim of shedding light on certain pitfalls that should be avoided, so that the progress of research in this field will be facilitated. In this age of populism, researchers should strive to do even more scholarly works with further attention to methodological details.

Behavioral, pharmacological and neuroanatomical analysis of serotonin 2C receptor agonism on maternal behavior in rats

As a highly motivated social behavior, maternal behavior in rats has been routinely used to study psychoactive drugs for clinical, neuroscience and pharmacological purposes. Recent evidence indicates that acute activation of serotonin 2C (5-HT_2C) receptors causes a disruption of rat maternal behavior. The present study was designed to elucidate the behavioral, pharmacological mechanisms and neuroanatomical basis of this 5-HT_2C effect. First, we replicated the finding that acute MK212 injection (2.0mg/kg, a highly selective 5-HT_2C agonist) disrupts maternal behavior, especially on pup retrieval. Interestingly, this disruption was significantly attenuated by 4-h pup separation (a procedure putatively increased maternal motivation). MK212 also suppressed food retrieval, indicating that it has a general effect on motivated behaviors. Second, we showed that MK212 disrupts maternal behavior by specifically activating 5-HT_2C receptor, as pretreatment with a 5-HT_2C receptor antagonist SB242084 (0.6 and 1.0mg/kg) alleviated MK212-induced disruption on pup retrieval. Third, we microinjected MK212 into various brain regions implicated in the regulation of maternal behavior: nucleus accumbens shell (25, 75, 250ng/0.5μl/side), medial prefrontal cortex (25 and 250ng, 1, 2 and 5μg/0.5μl/side), and medial preoptic area (MPOA, 75ng, 1 and 5μg/0.5μl/side). Pup retrieval and other maternal responses were not affected by any of these manipulations. Finally, we used c-Fos immunohistochemistry to identify the central mechanisms of the acute and repeated MK212 effects on maternal behavior. Acute MK212 (2.0mg/kg) disrupted pup retrieval and concurrently decreased c-Fos expression in the ventral part of lateral septal nucleus (LSv), MPOA, dentate gyrus (DG) and dorsal raphe (DR), but increased it in the central amygdala (CeA). Five days of repeated MK212 (2.0mg/kg) treatment produced a persistent disruption of pup retrieval and only decreased c-Fos expression in the DR. These findings not only confirm a role of 5-HT_2C receptor in rat maternal behavior, but also suggest that the coordinated 5-HT_2C activity in various limbic (e.g., LSv, DG, CeA), hypothalamic regions (e.g., MPOA) and brainstem areas (e.g. DR), is likely involved in the mediation of important psychological processes (e.g. motor function, motivation) necessary for the normal expression of maternal behavior.

Thinking and doing: the effects of dopamine and oxytocin genes and executive function on mothering behaviours

Animal and human studies suggest that initial expression of maternal behaviour depends on oxytocin and dopamine systems. However, the mechanism by which these systems affect parenting behaviours and the timing of these effects are not well understood. This article explores the role of mothers' executive function in mediating the relation between oxytocin and dopamine gene variants and maternal responsiveness at 48 months post-partum. Participants (n = 157) were mothers recruited in the Maternal Adversity, Vulnerability and Neurodevelopment Study, which assesses longitudinally two cohorts of mothers and children in Canada. We examined single nucleotide polymorphisms (SNPs) related to the dopamine and oxytocin systems (DRD1 rs686, DRD1 rs265976, OXTR rs237885 and OXTR rs2254298), assessed mothers' decision-making at 48 months using the Cambridge Neurological Automated Testing Battery (CANTAB) and evaluated maternal responsiveness from videotaped interactions during the Etch-A-Sketch co-operation task. Mediation analyses showed that OXTR rs2254298 A-carriers had an indirect effect on positive parenting which was mediated by mothers' performance on decision-making task (estimate = 0.115, P < 0.005), while OXTR rs2254298 A-carriers had both direct and indirect effects on physically controlling parenting, also mediated through enhanced performance on decision-making (estimate = -0.059, P < 0.005). Dopamine SNPs were not associated with any measure of executive function or parenting (all P > 0.05). While oxytocin has previously been associated with only the early onset of maternal behaviour, we show that an OXTR polymorphism is involved in maternal behaviour at 48 months post-partum through mothers' executive function. This research highlights the importance of the oxytocin system to maternal parenting beyond infancy.

Antagonism of V1b receptors promotes maternal motivation to retrieve pups in the MPOA and impairs pup-directed behavior during maternal defense in the mpBNST of lactating rats

Recent studies using V1b receptor (V1bR) knockout mice or central pharmacological manipulations in lactating rats highlighted the influence of this receptor for maternal behavior. However, its role in specific brain sites known to be important for maternal behavior has not been investigated to date. In the present study, we reveal that V1bR mRNA (qPCR) and protein levels (Western blot) within either the medial preoptic area (MPOA) or the medial-posterior part of the bed nucleus of the stria terminalis (mpBNST) did not differ between virgin and lactating rats. Furthermore, we characterized the effects of V1bR blockade via bilateral injections of the receptor subtype-specific antagonist SSR149415 within the MPOA or the mpBNST on maternal behavior (maternal care under non-stress and stress conditions, maternal motivation to retrieve pups in a novel environment, maternal aggression) and anxiety-related behavior in lactating rats. Blocking V1bR within the MPOA increased pup retrieval, whereas within the mpBNST it decreased pup-directed behavior, specifically licking/grooming the pups, during the maternal defense test. In addition, immediately after termination of the maternal defense test, V1bR antagonism in both brain regions reduced nursing, particularly arched back nursing. Anxiety-related behavior was not affected by V1bR antagonism in either brain region. In conclusion our data indicate that V1bR antagonism significantly modulates different aspects of maternal behavior in a brain region-dependent manner.

Vasopressin V1a, but not V1b, receptors within the PVN of lactating rats mediate maternal care and anxiety-related behaviour

The brain neuropeptide arginine-vasopressin (AVP) mediates a wide range of social behaviours via its V1a (V1aR) but also its V1b receptor (V1bR). With respect to maternal behaviour, V1bR are still less investigated, whereas V1aR have been shown repeatedly to trigger maternal behaviour, depending on the brain region. Here, we aimed to study the role of both V1aR and V1bR within the hypothalamic paraventricular nucleus (PVN), a major source of AVP, in maternal care (lactation day (LD) 1), maternal motivation in the pup retrieval test (LD 3) and anxiety-related behaviour on the elevated plus maze (EPM; LD 5) by acute local infusion of receptor subtype-specific antagonists for V1aR (d(CH2)5Tyr(Me)(2)AVP) or V1bR (SSR149415). Furthermore, we compared V1bR expression in the PVN of virgin versus lactating rats (LD 4). Our results demonstrate that within the PVN neither V1bR mRNA (qPCR) nor protein (Western Blot) content differed between virgin and lactating rats. Regarding behaviour, acute antagonism of V1aR, but not of V1bR, decreased the occurrence of nursing as well as anxiety-related behaviour as reflected by higher percentage of time spent on and of entries into the open arms of the EPM. Maternal motivation was not affected by any treatment. In summary, we demonstrate subtype-specific involvement of V1 receptors within the PVN in mediating various maternal behaviours. The lack of effects after V1bR blockade reveals that AVP acts mainly via V1aR in the PVN, at least in lactating rats, to mediate maternal care and anxiety.

Impairments in the initiation of maternal behavior in oxytocin receptor knockout mice

Oxytocin (Oxt) acting through its single receptor subtype, the Oxtr, is important for the coordination of physiology and behavior associated with parturition and maternal care. Knockout mouse models have been helpful in exploring the contributions of Oxt to maternal behavior, including total body Oxt knockout (Oxt −/−) mice, forebrain conditional Oxtr knockout (Oxtr FB/FB) mice, and total body Oxtr knockout (Oxtr −/−) mice. Since Oxtr −/− mice are unable to lactate, maternal behavior has only been examined in virgin females, or in dams within a few hours of parturition, and there have been no studies that have examined their anxiety-like and depression-like behavior following parturition. To improve our understanding of how the absence of Oxt signaling affects maternal behavior, mood and anxiety, we designed a study using Oxtr −/− mice that separated nursing behavior from other aspects of maternal care, such as licking and grooming by thelectomizing (i.e. removing the nipples) of Oxtr +/+ mice and sham-thelectomizing Oxtr −/− mice, and pairing both genotypes with a wet nurse. We then measured pup abandonment, maternal behavior, and postpartum anxiety-like and depression-like behaviors. We hypothesized that genetic disruption of the Oxtr would impact maternal care, mood and anxiety. Specifically, we predicted that Oxtr −/− dams would have impaired maternal care and increased anxiety-like and depression-like behaviors in the postpartum period. We found that Oxtr −/− dams had significantly higher levels of pup abandonment compared to controls, which is consistent with previous work in Oxtr FB/FB mice. Interestingly, Oxtr −/− dams that initiated maternal care did not differ from wildtype controls in measures of maternal behavior. We also did not find any evidence of altered anxiety-like or depressive-like behavior in the postpartum period of Oxtr −/− dams. Thus, our data suggest that Oxt lowers the threshold for the initiation of maternal behavior.

Flexibility and adaptation of the neural substrate that supports maternal behavior in mammals

Maternal behavior is species-specific and expressed under different physiological conditions, and contexts. It is the result of neural processes that support different forms (e.g. postpartum, cycling sensitized and spontaneous maternal behavior) and modalities of mother-offspring interaction (e.g. maternal interaction with altricial/precocious young; selective/non-selective bond). To understand how the brain adapts to and regulates maternal behavior in different species, and physiological and social conditions we propose new neural models to explain different forms of maternal expression (e.g. sensitized and spontaneous maternal behavior) and the behavioral changes that occur across the postpartum period. We emphasize the changing role of the medial preoptic area in the neural circuitry that supports maternal behavior and the cortical regulation and adjustment of ongoing behavioral performance. Finally, we discuss how our accumulated knowledge about the psychobiology of mothering in animal models supports the validity of animal studies to guide our understanding of human mothering and to improve human welfare and health.

Both oxytocin and vasopressin are mediators of maternal care and aggression in rodents: from central release to sites of action

In the mammalian peripartum period, the activity of both the brain oxytocin and vasopressin system is elevated as part of the physiological adaptations occurring in the mother. This is reflected by increased expression and intracerebral release of oxytocin and vasopressin, as well as increased neuropeptide receptor expression and binding. In this review we discuss the functional role of the brain oxytocin and vasopressin system in the context of maternal behavior, specifically maternal care and maternal aggression in rodents. In order to enable the identification of significant and peptide-specific contributions to the display of maternal behavior, various complementary animal models of maternal care and/or maternal aggression were studied, including rats selectively bred for differences in anxiety-related behavior (HAB and LAB dams), monitoring of local neuropeptide release during ongoing maternal behavior, and local pharmacological or genetic manipulations of the neuropeptide systems. The medial preoptic area was identified as a major site for oxytocin- and vasopressin-mediated maternal care. Furthermore, both oxytocin and vasopressin release and receptor activation in the central amygdala and the bed nucleus of the stria terminalis play an important role for maternal aggression. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Oxytocin-messages via the cerebrospinal fluid: behavioral effects; a review

The cerebrospinal fluid (CSF) usually is considered as a protective 'nutrient and waste control' system for the brain. Recent findings suggest, however, that the composition of CSF is actively controlled and may play an influential role in the changes in brain activity, underlying different behavioral states. In the present review, we present an overview of available data concerning the release of oxytocin into the CSF, the location of the oxytocin-receptive brain areas and the behavioral effects of intracerebroventricular oxytocin. About 80% of the oxytocin-receptive areas are located close to the ventricular or subarachnoid CSF, including the hypothalamic 'Behavior Control Column' (L.W.Swanson, 2003). As a conclusion we suggest that 'CSF-oxytocin' contributes considerably to the non-synaptic communication processes involved in hypothalamic-, brainstem- and olfactory brain areas and behavioral states and that the flowing CSF is used as a 'broadcasting system' to send coordinated messages to a wide variety of nearby and distant brain areas.

Inactivation or inhibition of neuronal activity in the medial prefrontal cortex largely reduces pup retrieval and grouping in maternal rats

Previous research suggests that the maternal medial prefrontal cortex (mPFC) may play a role in maternal care and that cocaine sensitization before pregnancy can affect neuronal activity within this region. The present work was carried out to test whether the mPFC does actually play a role in the expression of maternal behaviors in the rats and to understand what specific behaviors this cortical area may modulate. In the first experiment, tetrodotoxin (TTX) was used to chemically inactivate the mPFC during tests for maternal behavior latencies. Lactating rats were tested on postpartum days 7-9. The results of this first experiment indicate that there is a large effect of TTX-induced inactivation on retrieval behavior latencies. TTX nearly abolished the expression of maternal retrieval of pups without significantly impairing locomotor activity. In the second experiment, GABA-mediated inhibition was used to test maternal behavior latencies and durations of maternal and other behaviors in postpartum dams. In agreement with experiment 1, it was observed that dams capable of retrieving are rendered incapable by inhibition in the mPFC. GABA-mediated inhibition in the mPFC largely reduced retrieval without altering other indices of maternal care and non-specific behavior such as ambulation time, self-grooming, and inactivity. Moreover, in both experiments, dams were able to establish contact with pups within seconds. The overall results indicate that the mPFC may play an active role in modulating maternal care, particularly retrieval behavior. External factors that affect the function of the frontal cortical site may result in significant impairments in maternal goal-directed behavior as reported in our earlier work.

The changing role of the medial preoptic area in the regulation of maternal behavior across the postpartum period: facilitation followed by inhibition

Maternal behavior in rats undergoes considerable plasticity in parallel to the developmental stage of the pups, resulting in distinct patterns of maternal behavior and care at different postpartum time points. The medial preoptic area (mPOA) of the hypothalamus is one critical neural substrate underlying the onset and early expression of maternal behavior in rats but little is known about its specific functional role in the evolving expression of maternal behavior across the postpartum period. The present study uses a reversible local neural inactivation method to examine the role of the mPOA in the regulation of maternal behavior throughout the postpartum period, particularly extending into the late postpartum, a little examined period. This approach avoids the compensatory plasticity in CNS that occurs after permanent lesions, and allows the repeated testing of same individuals. Early (PPD7-8) and late (PPD13-14) postpartum maternal behavior was evaluated in female rats following infusions of bupivacaine or vehicle into the mPOA or into control areas. As expected, mPOA inactivation severely but transiently disrupted early postpartum maternal behavior whereas infusion of vehicle or inactivation of adjacent control sites did not. Later in the postpartum period, however, transient mPOA inactivation facilitated the expression of maternal behaviors, highly contrasting the behavioral expression levels characteristic of late postpartum. Results strongly demonstrate that the mPOA is differentially engaged throughout postpartum in orchestrating appropriate maternal responses with the developmental stage of the pups.

Medial preoptic area interactions with dopamine neural systems in the control of the onset and maintenance of maternal behavior in rats

The medial preoptic area (MPOA) and dopamine (DA) neural systems interact to regulate maternal behavior in rats. Two DA systems are involved: the mesolimbic DA system and the incerto-hypothalamic DA system. The hormonally primed MPOA regulates the appetitive aspects of maternal behavior by activating mesolimbic DA input to the shell region of the nucleus accumbens (NAs). DA action on MPOA via the incerto-hypothalamic system may interact with steroid and peptide hormone effects so that MPOA output to the mesolimbic DA system is facilitated. Neural oxytocin facilitates the onset of maternal behavior by actions at critical nodes in this circuitry. DA-D1 receptor agonist action on either the MPOA or NAs can substitute for the effects of estradiol in stimulating the onset of maternal behavior, suggesting an overlap in underlying cellular mechanisms between estradiol and DA. Maternal memory involves the neural plasticity effects of mesolimbic DA activity. Finally, early life stressors may affect the development of MPOA-DA interactions and maternal behavior.

Impact of gestational cocaine treatment or prenatal cocaine exposure on early postpartum oxytocin mRNA levels and receptor binding in the rat

Prior research reported decreased oxytocin levels in specific brain regions correlated with disruptions in maternal care following gestational cocaine treatment in rats. Similarly, prenatal exposure to cocaine impaired subsequent maternal behavior in adulthood, but behavioral alterations were not associated with decreases in oxytocin levels in the same brain regions as were found in their cocaine-treated rat dams. To determine if other aspects of the oxytocin system are disrupted by cocaine treatment or prenatal exposure to cocaine during critical time points associated with maternal care, oxytocin mRNA transcription and receptor binding were examined on postpartum day two in relevant brain regions following gestational treatment with, or prenatal exposure to, either cocaine or saline. We hypothesized that oxytocin mRNA levels and receptor binding would be differentially affected by cocaine in the early postpartum period of dams and their offspring. Our findings indicate that gestational cocaine treatment resulted in significant increases in oxytocin mRNA levels in only the paraventricular nucleus of cocaine-treated dams, with almost significant increases in both generations in the supraoptic nucleus, but no significant effects of cocaine on receptor binding in either generation of dams. These findings indicate that in addition to oxytocin levels, cocaine treatment or prenatal exposure primarily affects oxytocin mRNA synthesis, with little effect on receptor binding in specific brain regions associated with maternal behavior in the early postpartum period of the rat.

Comparison of medial preoptic, amygdala, and nucleus accumbens lesions on parental behavior in California mice (Peromyscus californicus)

We have previously shown that medial preoptic area (MPOA) lesions disrupt parental behavior in both male and female California mice (P. californicus). In the present study, we compare the effects of lesions in the MPOA, with those in the basolateral amygdala (BA) and nucleus accumbens (NA) on male and female parental behaviors in the biparental California mouse. A male or multiparous female from each male-female pair was given an electrolytic or sham lesion in the MPOA, BA, or NA and tested for parental responsiveness. Since female P. californicus show postpartum estrus, they were likely pregnant during parental testing. MPOA lesions produced deficits in both male and female parental behaviors, and BA lesions disrupted male, and to a lesser extent, female parental behavior. NA lesions produced mild effects on pup-retrieval in males and no effect on parental behavior in females. However, NA lesions incompletely destroyed the NA shell, the region most relevant for maternal behavior in rats, and should be investigated further. These results support a role for the MPOA and BA in both male and female parental behaviors.

Hypothalamic neural circuits regulating maternal responsiveness toward infants

A theoretical neural model is developed, along with supportive evidence, to explain how the medial preoptic area (MPOA) of the hypothalamus can regulate maternal responsiveness toward infant-related stimuli. It is proposed that efferents from a hormone-primed MPOA (a) depress a central aversion system (composed of neural circuits between the amygdala, medial hypothalamus, and midbrain) so that novel infant stimuli do not activate defensive or avoidance behavior and (b) excite the mesolimbic dopamine system so that active, voluntary maternal responses are promoted. The effects of oxytocin and maternal experience are included in the model, and the specificity of MPOA effects are discussed. The model may be relevant to the mechanisms through which other hypothalamic nuclei regulate other basic motivational states. In addition, aspects of the model may define a core neural circuitry for maternal behavior in mammals.

Cocaine treatment alters oxytocin receptor binding but not mRNA production in postpartum rat dams

Gestational cocaine treatment in rat dams results in decreased oxytocin (OT) levels, up-regulated oxytocin receptor (OTR) binding density and decreased receptor affinity in the whole amygdala, all concomitant with a significant increase in maternal aggression on postpartum day six. Rat dams with no gestational drug treatment that received an infusion of an OT antagonist directly into the central nucleus of the amygdala (CeA) exhibited similarly high levels of maternal aggression towards intruders. Additionally, studies indicate that decreased OT release from the hypothalamic division of the paraventricular nucleus (PVN) is coincident with heightened maternal aggression in rats. Thus, it appears that cocaine-induced alterations in OT system dynamics (levels, receptors, production, and/or release) may mediate heightened maternal aggression following cocaine treatment, but the exact mechanisms through which cocaine impacts the OT system have not yet been determined. Based on previous studies, we hypothesized that two likely mechanisms of cocaine's action would be, increased OTR binding specifically in the CeA, and decreased OT mRNA production in the PVN. Autoradiography and in situ hybridization assays were performed on targeted nuclei in brain regions of rat dams on postpartum day six, following gestational treatment twice daily with cocaine (15 mg/kg) or normal saline (1 ml/kg). We now report cocaine-induced reductions in OTR binding density in the ventromedial hypothalamus (VMH) and bed nucleus of the stria terminalis (BNST), but not the CeA. There was no significant change in OT mRNA production in the PVN following cocaine treatment.

Functional magnetic resonance imaging shows oxytocin activates brain regions associated with mother-pup bonding during suckling

Oxytocin is released in the maternal brain during breastfeeding and may help strengthen the mother-infant relationship. Here, we used functional magnetic resonance imaging to determine whether oxytocin modulates brain activity in postpartum day 4-8 dams receiving suckling stimulation. During imaging sessions, dams were exposed to pup suckling before and after administration of an oxytocin receptor antagonist. Another group of dams received oxytocin alone. Changes in brain activation in response to suckling closely matched that elicited by oxytocin administration. The overlapping brain areas included the olfactory system, nucleus accumbens, insular cortex, prefrontal cortex, ventral tegmental area, cortical amygdala, and several cortical and hypothalamic nuclei. Blockade of oxytocin receptors largely attenuated activation in these regions. The data suggest that oxytocin may strengthen mother-infant bond formation partly by acting through brain areas involved in regulating olfactory discrimination, emotions, and reward.

Maternal behavior deficits in nulliparous oxytocin knockout mice

The first observations of postpartum oxytocin knockout (OTKO) mice found no maternal behavior deficits. However, it is unclear how detailed those observations were. In this study, we compared maternal behavior exhibited by OTKO and wild-type (WT) nullipara toward six 2-4-day-old foster pups during test sessions conducted on 3 successive days. Each day, subjects were placed in a clean cage 30 min prior to introduction of pups which were deposited in a clump adjacent to the middle of a long wall of each test cage. Behavior was measured for 3.5 h after which pups and test subjects were returned to their home cages. On test days 1 and 3, a significantly smaller proportion of OTKO females retrieved pups to a corner of their cage. Also, significantly fewer pups were retrieved to corners by OTKO females. In contrast to most WTs, most OTKO females mothered pups in the center of the cage where they were initially deposited. Pup-licking frequencies were significantly lower in OTKO females. Their self-grooming frequencies also trended toward being lower. Latencies to retrieve and lick pups, latencies to and frequencies of still crouching over pups and proportion of time in nest did not differ between groups. Our findings suggest that OT stimulates a significant proportion of pup-licking in nulliparous mice, a situation similar to lactating rat mothers. Our results also indicate that OT may play a role in the motivation to retrieve pups to a more secure location.

Biological Aspects of Social Bonding and the Roots of Human Violence

The brain systems that motivate humans to form emotional bonds with others probably first evolved to mobilize the high-quality maternal care necessary for reproductive success in placental mammals. In these species, the helplessness of infants at birth and their dependence upon nutrition secreted from their mothers' bodies (milk) and parental body heat to stay warm required the evolution of a new motivational system in the brain to stimulate avid and sustained mothering behavior. Other types of social bonds that emerged subsequently in placental mammals, in particular monogamous bonds between breeding pairs, appear to have evolved from motivational brain systems that stimulate maternal behavior. This chapter focuses on aspects of the evolution and neurobiology of maternal and pair bonding and associated behavioral changes that may provide insights into the origins of human violence. The roles of the neuropeptides oxytocin and vasopressin as well as the neurotransmitter dopamine will be emphasized. Maternal and pair bonding are accompanied by increased aggressiveness toward perceived threats to the object of attachment as well as diminished fear and anxiety in stressful situations. The sustained closeness with mother required for the survival of infant mammals opened a new evolutionary niche in which aspects of the mother's care became increasingly important in regulating development in offspring. The quantity and quality of maternal care received during infancy determines adult social competence, ability to cope with stress, aggressiveness, and even preference for addictive substances. Indeed, the development of neurochemical systems within the brain that regulate mothering, aggression, and other types of social behavior, such as the oxytocin and vasopressin systems, are strongly affected by parental nurturing received during infancy. Evidence will be reviewed that the neural circuitry and neurochemistry implicated in studies of lower mammals also facilitate primate/human interpersonal bonding. It is hypothesized that neural bonding systems may also be important for the development in individuals of loyalty to the social group and its culture. Neglect and abuse during early life may cause bonding systems to develop abnormally and compromise capacity for rewarding interpersonal relationships and commitment to societal and cultural values later in life. Other means of stimulating reward pathways in the brain, such as drugs, sex, aggression, and intimidating others, could become relatively more attractive and less constrained by concern about violating trusting relationships. The ability to modify behavior based on negative experiences may be impaired. Unmet needs for social bonding and acceptance early in life might increase the emotional allure of groups (gangs, sects) with violent and authoritarian values and leadership. Social neurobiology has the potential to provide new strategies for treating and preventing violence and associated social dysfunction.

Mapping of brain networks involved in consolidation of lamb recognition memory

In sheep, ewes at parturition are responsive to any newborn lamb, but within less than 1 h, mothers learn to recognize the odor of their lamb and restrict maternal care to their own offspring (maternal selectivity). In a first experiment, we investigated the long-term retention of maternal selectivity after various mother-young contact and separation durations. After 4 h of contact, 36 h of separation leads to a total loss of selectivity. Increasing contact duration to 7 days prior to this separation maintains selectivity. These data suggest that lamb memory after going through an initial labile state after parturition, is consolidated over time into a more stable long-term memory. Fos immunohistochemistry reveals that reintroduction of the lamb after 4 h of mother-young contact and 3 h of separation activates different maternal brain regions than reintroduction of the lamb after 7 days of mother-young contact and 3 h of separation. While the piriform cortex shows an enhanced activation at both times, a selective enhancement of activation is observed in the frontal medial and orbitofrontal cortices only after 7 days of mother-young contact. These data suggest that as consolidation occurs, the neurobiological networks sustaining lamb memory involve different structures.

Identification of neural pathways involved in genital reflexes in the female: a combined anterograde and retrograde tracing study

The medial preoptic area (MPOA) is important for reproductive behavior in females. However, the descending pathways mediating these responses to the spinal motor output are unknown. The MPOA does not directly innervate the spinal cord. Therefore, pathways mediating MPOA-induced changes in sexual behavior must relay in the brain. The nucleus paragigantocellularis (nPGi) projects heavily to spinal circuits involved in female sexual reflexes and is involved in the tonic inhibition of genital reflexes. However, the periaqueductal gray (PAG) is also important for female sexual behavior. The present study examined the hypothesis that the MPOA output relays through PAG and the nPGi before descending to the spinal cord. We used anterograde and retrograde tracing techniques to examine the descending pathways and relay sites from the MPOA to the spinal cord and the nPGi in the female rat. Injection of biotinylated dextran amine into the MPOA produced dense labeling in specific regions of the PAG and Barrington's nucleus; anterogradely labeled fibers terminated close to neurons retrogradely labeled from the spinal cord in the PAG, Barrington's nucleus, nPGi, lateral hypothalamus and paraventricular nucleus (PVN). Anterogradely labeled fibers and varicosities were also found close to neurons retrogradely labeled from the nPGi in the PAG, lateral hypothalamus and PVN. These results suggest that the major MPOA output relays in the PAG and nPGi before descending to innervate spinal circuits regulating female genital reflexes and that the MPOA plays a multifaceted role in female reproductive behavior through its modulation of PAG output systems.

Oxytocin antagonism alters rat dams' oral grooming and upright posturing over pups

Studies involving intracerebral administration of antiserum or antagonists have demonstrated that central oxytocin (OT) plays a prominent role in initiating but not maintaining postpartum maternal behavior in rats. There has been little investigation, however, of OT's influence on the levels of maternal behavior exhibited during the maintenance phase. We measured rat dam behavior during the 105-min observation periods preceding and beginning 2 h after intracerebroventricular infusion of the selective OT antagonist (OTA) (1 microg), or normal saline (NS) vehicle (5 microl) on postpartum days 2-3 and 6-7. Compared to NS, OTA significantly decreased pup licking as a proportion of dams' total oral grooming, increased self-grooming, decreased the frequency of elevated upright posture over pups and increased the frequency of lying prone on pups. Quiescent, kyphotic nursing was also significantly lower in OTA-treated dams. Other components of maternal behavior were not significantly affected by OTA or NS treatment. These findings suggest that central OT may shift the focus of the dams' oral grooming from self to pups and may also facilitate elevation of dams' upright posture over pups. Acute stress responses, maternal behavior and central OT receptor binding in adult rats have been linked to the amount of maternal licking and arched back, upright nursing received during infancy. OT activity in dams' brains may influence these developmental outcomes in their offspring by selectively regulating their pup licking and crouching posture.

Medial preoptic lesions disrupt parental behavior in both male and female California mice (Peromyscus californicus)

California mice (Peromyscus californicus) are monogamous and naturally biparental, making them an ideal species in which to study the neural basis of paternal behavior. A male or female from each male-female pair was given an electrolytic or sham lesion in the medial preoptic area (MPOA), an area known to be critical for the expression of maternal behavior in rats, and retested for parental responsiveness. MPOA-lesioned males and females showed significantly longer latencies to show parental behavior and spent significantly less time near pups, sniffing pups, and licking pups than sham-lesioned mice. However, MPOA lesions did not reduce time spent hovering over pups. The results suggest that the neural mechanisms mediating paternal behavior are similar to those mediating maternal behavior in this species.

Maternal behavior, milk ejection, and plasma hormones in Hatano high- and low-avoidance rats

Hatano high- and low-avoidance (in a two-way active avoidance task) animals (HAA and LAA, respectively) were successfully selected from a Sprague-Dawley rat population. Pup growth of both strains was dependent on the maternal strain in a cross-fostering study. To determine whether there are strain-specific differences between HAA and LAA in maternal care, both strains of dams were subjected to a test battery as measured by nest building, home cage behavior, pup retrieval, and milk ejection tests. In addition, changes in plasma concentrations of lactotropic and corticotropic hormones such as prolactin, ACTH, and corticosterone were examined during lactation. The test battery indicated that the dams of both strains built good nests and spent an identical amount of time with their offspring. However, LAA dams showed a prolonged latency time for pup retrieval and often left pups outside the nest until the end of the test period. LAA dams also showed a decreased amount of milk ejection, whereas no strain differences were observed in milk ejection after oxytocin treatment. During lactation, a lesser increase in plasma concentrations of prolactin and a greater increase in ACTH were found in LAA dams. There were no differences between the two strains in plasma concentrations of corticosterone. These results clearly demonstrated decreases in maternal behavior and milk ejection in LAA as compared to HAA dams. The present results also suggest that maternal motivation and mechanisms responsible for maternal hormones related to suckling are involved in the degree of pup growth.

The neuroendocrine basis of social recognition

All social relationships are dependent on an organism's ability to remember conspecifics. Social memory may be a unique form of memory, critical for reproduction, territorial defense, and the establishment of dominance hierarchies in a natural context. In the laboratory, social memory can be assessed reliably by measuring the reduction in investigation of a familiar partner relative to novel conspecifics. The neurohypophyseal neuropeptides oxytocin and vasopressin have been shown to influence a number of forms of social behavior, including affiliation, aggression, and reproduction. This article reviews vasopressin and oxytocin effects on social cognition, particularly the acquisition and retention of social recognition in rats and mice. Studies in rats have demonstrated that vasopressin in specific neural pathways, such as the lateral septum, is necessary for social recognition. As vasopressin facilitates recall when given after an initial encounter, the peptide appears important for the consolidation not the acquisition of a social memory. Although oxytocin has complex effects on social memory in rats, mice with a null mutation of the oxytocin gene are completely socially amnestic without other cognitive deficits evident. As oxytocin given centrally before but not after the initial encounter restores social recognition in these mutant mice, the neuropeptide appears critical for the acquisition rather than the consolidation phase of memory. Oxytocin's effects on social memory are mediated via a discrete cell population in the medial amygdala. These findings support the hypothesis that vasopressin and oxytocin are essential for social memory, although they appear to influence different cognitive processes and may modulate different neural systems. (c) Elsevier Science.

Oxytocin receptor mRNA expression in rat brain: implications for behavioral integration and reproductive success

The nonapeptide, oxytocin (OT), has been implicated in a wide range of physiological, behavioral and pharmacological effects related to learning and memory, parturition and lactation, maternal and sexual behavior, and the formation of social attachments. Specific G-protein linked membrane bound OT receptors mediate OTs effects. The unavailability of highly selective pharmacological ligands that discriminate the OT receptor from the highly homologous vasopressin receptors (V1a, V1b and V2 subtypes) has made it difficult to confirm specific effects of oxytocin, particularly in brain regions where OT and multiple AVP receptor subtypes may be coexpressed. Here, data on the oxytocin receptor (OTR) messenger ribonucleic acid (mRNA) localization in brain are presented in the context of a model that proposes a reproductive state-dependent role for steroid-hormone restructuring of neural circuits, and a role for oxytocin in the integration of neural transmission in pathways subserving: (1) steroid-sensitive reproductive behaviors; (2) learning; and (3) reinforcement. It is hypothesized that social attachments emerge as a consequence of a conditioned association between OT-related activity in these pathways and the eliciting stimulus.

Exogenous oxytocin attenuates suckling-induced prolactin release but not maternal or infant behavior in lactating rats

The influence of exogenous oxytocin (OT) on maternal and infant behavior over 60 min of suckling, which followed 6 h of isolation, was investigated on Day 12 of lactation in rats. Mothers administered 1 IU of OT or saline through an indwelling atrial catheter and their litters indicated a similar nursing and suckling pattern, which was estimated by the crouching time of the mothers and the number of stretch reactions performed by the litters during a suckling period. To assess the alteration of the suckling intensity by OT administration, the plasma prolactin (PRL) level was determined by an Nb2 lymphoma cell bioassay. In the control group, the plasma PRL level increased and reached a peak at 45 min after the onset of suckling in 60% of the animals. The suckling-induced PRL release was completely inhibited and/or markedly delayed by OT administration. The difference in body weight of the litters before and after a suckling period was estimated as an index of the amount of milk suckled by the litters. There was no difference in the amount of milk between the control and OT-treated groups during a 60-min suckling period. However, it was significantly greater in the OT-treated group during the first 20 min of the suckling period. These results indicate that a dose of OT is a factor in the attenuation of the intensity of suckling done by the pups, whereas the nursing and suckling behavior is not influenced by OT administration.

Dose-dependent effects of multiple acute cocaine injections on maternal behavior and aggression in Sprague-Dawley rats

Rat dams, which had no prior drug treatment, were either nontreated controls or were injected subcutaneously 4 times during a 10-day period with a single dose of 30, 15 or 7.5 mg/kg of cocaine hydrochloride HCl, or normal saline. Injections were given immediately postpartum following delivery of their final pup (PPD 1), and again on postpartum day 3 (PPD 3), postpartum day 6 (PPD 6) and postpartum day 10 (PPD 10). Dams were observed 30 min following injections for maternal behavior (MB) towards 8 surrogate male pups on PPD 1 and PPD 3 and for aggression towards a male or female intruder in the presence of their litter on PPD 6 and PPD 10. Compared to saline and untreated controls, cocaine-treated dams exhibited more disruptions in MB on both PPD 1 and PPD 3 and were less aggressive towards an intruder, regardless of intruder sex, on PPD 6 and PPD 10. In most cases MB was altered in a dose-dependent manner with the higher doses of cocaine resulting in a greater disruption of behavior.

Gene targeting approaches to neuroendocrinology: oxytocin, maternal behavior, and affiliation

Transgenic technology affords exciting new opportunities in the field of behavioral neuroendocrinology. We have extended our research into the behavioral function of oxytocin in maternal and social behavior using two transgenic approaches: (i) targeted deletion of the oxytocin gene in mice and (ii) augmented oxytocin receptor expression in the brain. Mice genetically deficient in oxytocin can mate, give birth, and display normal maternal behavior; however, milk ejection and certain aspects of social behavior are affected. Comparative studies of oxytocin receptors have led to the observation that species differences in social organization are associated with differences in receptor distribution. Specifically, monogamous prairie voles and nonmonogamous, asocial montane voles exhibit different patterns of OT receptor expression in the brain. Transgenic mice have been created with a reporter gene driven by the prairie vole oxytocin receptor gene promoter. Analysis of the expression pattern suggests that it should be possible to manipulate receptor expression in the vole brain in order to examine the effects of receptor distribution on behavior.