The medial preoptic area is necessary for motivated choice of pup- over cocaine-associated environments by early postpartum rats

The Integration of the Maternal Care with Sleep During the Postpartum Period

Our entire life occurs in a constant alternation between wakefulness and sleep. The impossibility of living without sleep implies that any behavior must adapt to the need for sleep, and maternal behavior does not escape from this determination. Additionally, maternal behavior in mammals is a highly motivated behavior, essential for the survival of the offspring. Thus, the mother has to adapt her physiology of sleep to the constant demands of the pups, where each species will have different strategies to merge these two physiological needs. However, all studied female mammals will experience sleep disturbances at some point of the postpartum period.

What can challenging reproductive contexts tell us about the rat's maternal behavior?

Maternal behavior in mammals encompasses a complex repertoire of activities that ensure the survival of the offspring and shape their neural and behavioral development. The laboratory rat has been employed as a classic model for investigating maternal behavior, and recently with the use of advanced techniques, the knowledge of its neural basis has been expanded significantly. However, the standard laboratory testing conditions in which rats take care of a single litter impose constraints on the study of maternal flexibility. Interestingly, the reproductive characteristics of this species, including the existence of a fertile postpartum estrus, allow us to study maternal behavior in more complex and ethologically relevant contexts, even in laboratory settings. Here we review how maternal and sexual motivations interact during the postpartum estrus, shaping the behavioral response of females according to the presence of the pups and males. Next, we describe how impregnation during the postpartum estrus creates a new reproductive context in which mothers simultaneously care for two successive litters, adapting their responses to different behavioral and physiological demands of pups. These findings illustrate the behavioral adaptability of maternal rats to pups' needs and the presence of other reinforcers, as well as its dependence on the context. In our view, future perspectives in the field, by incorporating the use of cutting-edge techniques, should analyze maternal flexibility and its neural substrates in models that incorporate complex and challenging contexts. This approach would allow a more comprehensive understanding of brain circuits involved in the adaptive and flexible nature of parenting.

The Protective Effect of Social Reward on Opioid and Psychostimulant Reward and Relapse: Behavior, Pharmacology, and Brain Regions

Until recently, most modern neuroscience research on addiction using animal models did not incorporate manipulations of social factors. Social factors play a critical role in human addiction: social isolation and exclusion can promote drug use and relapse, while social connections and inclusion tend to be protective. Here, we discuss the state of the literature on social factors in animal models of opioid and psychostimulant preference, self-administration, and relapse. We first summarize results from rodent studies on behavioral, pharmacological, and circuit mechanisms of the protective effect of traditional experimenter-controlled social interaction procedures on opioid and psychostimulant conditioned place preference, self-administration, and relapse. Next, we summarize behavioral and brain-mechanism results from studies using newer operant social-interaction procedures that inhibit opioid and psychostimulant self-administration and relapse. We conclude by discussing how the reviewed studies point to future directions for the addiction field and other neuroscience and psychiatric fields, and their implications for mechanistic understanding of addiction and development of new treatments.SIGNIFICANCE STATEMENT In this review, we propose that incorporating social factors into modern neuroscience research on addiction could improve mechanistic accounts of addiction and help close gaps in translating discovery to treatment. We first summarize rodent studies on behavioral, pharmacological, and circuit mechanisms of the protective effect of both traditional experimenter-controlled and newer operant social-interaction procedures. We then discuss potential future directions and clinical implications.

Experience-Regulated Neuronal Signaling in Maternal Behavior

Maternal behavior is shaped and challenged by the changing developmental needs of offspring and a broad range of environmental factors, with evidence indicating that the maternal brain exhibits a high degree of plasticity. This plasticity is displayed within cellular and molecular systems, including both intra- and intercellular signaling processes as well as transcriptional profiles. This experience-associated plasticity may have significant overlap with the mechanisms controlling memory processes, in particular those that are activity-dependent. While a significant body of work has identified various molecules and intracellular processes regulating maternal care, the role of activity- and experience-dependent processes remains unclear. We discuss recent progress in studying activity-dependent changes occurring at the synapse, in the nucleus, and during the transport between these two structures in relation to maternal behavior. Several pre- and postsynaptic molecules as well as transcription factors have been found to be critical in these processes. This role reflects the principal importance of the molecular and cellular mechanisms of memory formation to maternal and other behavioral adaptations.

The Prolactin Family of Hormones as Regulators of Maternal Mood and Behavior

Transition into motherhood involves profound physiological and behavioral adaptations that ensure the healthy development of offspring while maintaining maternal health. Dynamic fluctuations in key hormones during pregnancy and lactation induce these maternal adaptations by acting on neural circuits in the brain. Amongst these hormonal changes, lactogenic hormones (e.g., prolactin and its pregnancy-specific homolog, placental lactogen) are important regulators of these processes, and their receptors are located in key brain regions controlling emotional behaviors and maternal responses. With pregnancy and lactation also being associated with a marked elevation in the risk of developing mood disorders, it is important to understand how hormones are normally regulating mood and behavior during this time. It seems likely that pathological changes in mood could result from aberrant expression of these hormone-induced behavioral responses. Maternal mental health problems during pregnancy and the postpartum period represent a major barrier in developing healthy mother-infant interactions which are crucial for the child's development. In this review, we will examine the role lactogenic hormones play in driving a range of specific maternal behaviors, including motivation, protectiveness, and mother-pup interactions. Understanding how these hormones collectively act in a mother's brain to promote nurturing behaviors toward offspring will ultimately assist in treatment development and contribute to safeguarding a successful pregnancy.

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.

The neurochemistry of social reward during development: What have we learned from rodent models?

Social rewards are fundamental to survival and overall health. Several studies suggest that adequate social stimuli during early life are critical for developing appropriate socioemotional and cognitive skills, whereas adverse social experiences negatively affect the proper development of brain and behavior, by increasing the susceptibility to develop neuropsychiatric conditions. Therefore, a better understanding of the neural mechanisms underlying social interactions, and their rewarding components in particular, is an important challenge of current neuroscience research. In this context, preclinical research has a crucial role: Animal models allow to investigate the neurobiological aspects of social reward in order to shed light on possible neurochemical alterations causing aberrant social reward processing in neuropsychiatric diseases, and they allow to test the validity and safety of innovative therapeutic strategies. Here, we discuss preclinical research that has investigated the rewarding properties of two forms of social interaction that occur in different phases of the lifespan of mammals, that is, mother-infant interaction and social interactions with peers, by focusing on the main neurotransmitter systems mediating their rewarding components. Together, the research performed so far helped to elucidate the mechanisms of social reward and its psychobiological components throughout development, thus increasing our understanding of the neurobiological substrates sustaining social functioning in health conditions and social dysfunction in major psychiatric disorders.

Infralimbic Cortex Biases Preference Decision Making for Offspring over Competing Cocaine-Associated Stimuli in New Mother Rats

In the context of drug abuse, converging evidence suggests that cocaine use in new mothers is significantly reduced by the competing motivation related to child rearing. Given the demonstrated importance of the medial prefrontal cortex (mPFC) in decision-making processes, we investigated the contribution of distinct regions of the mPFC [anterior cingulate (Cg1), prelimbic (PrL), infralimbic (IL)] to decision making in new mother rats performing a concurrent pup/cocaine choice conditioned place preference (CPP) task. When given a choice, inactivation of IL cortex significantly biased decision making of mother rats toward cocaine-associated cues, highly contrasting the distribution of preferences by control groups. In contrast, inactivation of PrL cortex had the opposite effect, significantly increasing offspring bias in the decision making, such that none of the mothers chose the cocaine-associated alternative. Cg1 inactivation was without effect. Functional inactivation of these same mPFC subregions had no effect in a non-conflict CPP task in which context-induced cocaine or pup seeking were examined separately. Notably, inactivation of the IL cortex also interfered with maternal behavior. Taken together, we have identified a specific role of the IL cortex in the prioritization of offspring over drug competing alternatives, thus promoting resistance to drug use in new mothers.

Adaptations in reward-related behaviors and mesolimbic dopamine function during motherhood and the postpartum period

Initiation and maintenance of maternal behavior is driven by a complex interaction between the physiology of parturition and offspring stimulation, causing functional changes in maternal brain and behavior. Maternal behaviors are among the most robust and rewarding motivated behaviors. Mesolimbic dopamine (DA) system alterations during pregnancy and the postpartum enable enhanced reward-related responses to offspring stimuli. Here, we review behavioral evidence demonstrating postpartum rodents exhibit a bias towards pups and pup-related stimuli in reward-related tasks. Next, we provide an overview of normative adaptations in the mesolimbic DA system induced by parturition and the postpartum, which likely mediate shifts in offspring valence. We also discuss a causal link between dopaminergic dysfunction and disrupted maternal behaviors, which are recapitulated in postpartum depression (PPD) and relevant rodent models. In sum, mesolimbic DA system activation drives infant-seeking behavior and strengthens the mother-infant bond, potentially representing a therapeutic target for reward-related deficits in PPD.

Social modulation of drug use and drug addiction

This review aims to demonstrate how social science and behavioral neurosciences have highlighted the influence of social interactions on drug use in animal models. In neurosciences, the effect of global social context that are distal from drug use has been widely studied. For human and other social animals such as monkeys and rodents, positive social interactions are rewarding, can overcome drug reward and, in all, protect from drug use. In contrast, as other types of stress, negative social experiences facilitate the development and maintenance of drug abuse. However, interest recently emerged in the effect of so-called "proximal" social factors, that is, social interactions during drug-taking. These recent studies have characterized the role of the drug considered, the sharing of drug experience and the familiarity of the peer which interaction are made with. We also examine the few studies regarding the sensorial mediator of social behaviors and critically review the neural mediation of social factors on drug use. However, despite considerable characterization of the factors modulating distal influences, the mechanisms for proximal influences on drug use remain largely unknown. This article is part of the Special Issue entitled 'The neuropharmacology of social behavior: from bench to bedside'.

Thalamic integration of social stimuli regulating parental behavior and the oxytocin system

Critically important components of the maternal neural circuit in the preoptic area robustly activated by suckling were recently identified. In turn, suckling also contributes to hormonal adaptations to motherhood, which includes oxytocin release and consequent milk ejection. Other reproductive or social stimuli can also trigger the release of oxytocin centrally, influencing parental or social behaviors. However, the neuronal pathways that transfer suckling and other somatosensory stimuli to the preoptic area and oxytocin neurons have been poorly characterized. Recently, a relay center of suckling was determined and characterized in the posterior intralaminar complex of the thalamus (PIL). Its neurons containing tuberoinfundibular peptide 39 project to both the preoptic area and oxytocin neurons in the hypothalamus. The present review argues that the PIL is a major relay nucleus conveying somatosensory information supporting maternal behavior and oxytocin release in mothers, and may be involved more generally in social cue evoked oxytocin release, too.

The role of ΔfosB in the medial preoptic area: Differential effects of mating and cocaine history

The transcription factor deltaFosB (ΔFosB) is induced in the nucleus accumbens (NAc) by repeated exposure to drugs of abuse and natural rewards. Less is known about its role in other brain areas. Here, we compared the effects of mating versus cocaine history on induction of ΔFosB in the medial preoptic area (MPOA), an integral site for reproductive behavior, and in the NAc. ΔFosB immunoreactivity (ir) was increased in the MPOA of previously naïve and experienced male rats that mated the day before euthanasia, compared to unmated controls and experienced males with recent mating abstinence. Western immunoblots confirmed that the 35-37-kDa isoform of ΔFosB was increased more in recently mated males. Conversely, previous plus recent cocaine did not increase ΔFosB-ir in the MPOA, despite an increase in the NAc. Next, a viral vector expressing ΔFosB, its dominant negative antagonist ΔJunD, or green fluorescent protein (GFP) control, were microinjected bilaterally into the MPOA. ΔFosB overexpression impaired copulation and promoted female-directed aggression, compared to ΔJunD and control males. These data suggest that ΔFosB in the mPOA is expressed in an experience-dependent manner and affects systems that coordinate mating and aggression. (PsycINFO Database Record

Presence of pups suppresses hunger-induced feeding in virgin adult mice of both sexes

Despite recent progress on neural pathways underlying individual behaviors, how an animal balances and prioritizes behavioral outputs remains poorly understood. While studying the relationship between hunger-induced feeding and pup-induced maternal behaviors in virgin female mice, we made the unexpected discovery that presence of pups strongly delayed and decreased food consumption. Strikingly, presence of pups also suppressed feeding induced by optogenetic activation of Agrp neurons. Such a suppressive effect inversely correlated with the extents of maternal behaviors, but did not rely on the display of these behaviors, and was also present in virgin males. Furthermore, chemogenetic activation of Vglut2+ neurons in the medial preoptic area (mPOA), a region critical for maternal behaviors and motivation, was sufficient to suppress hunger-induced feeding. However, muscimol inhibition of the mPOA, while disrupting maternal behaviors, did not prevent pup suppression of feeding, indicating that neural pathways in other brain regions may also mediate such an effect. Together, these results provide novel insights into neural coordination of pup care and feeding in mice and organizations of animal behaviors in general.

Previous and recent maternal experiences modulate pups' incentive value relative to a male without affecting maternal behavior in postpartum estrous rats

This study extends the behavioral analysis of the postpartum estrus (PPE) which represents a unique period in the female rat's lifetime when maternal and sexual motivations co-exist. The aim of this study was to explore how previous and recent maternal experiences influence the maternal responses to pups when confronted with a male in a preference test or when they are presented independently in the home cage. To achieve this objective, we firstly compared the maternal behavior in the home cage and the preference for pups or a male in a Y-maze of primiparous and multiparous females approximately twelve hours after delivery. No differences were observed in the active and passive components of the maternal behavior of primiparous and multiparous rats; however second-time mothers made more efforts to gain access to the pups and tended to spend more time with them in the Y-maze than maternally inexperienced dams. In a second experiment, we assessed the influence of recent maternal experience with pups on PPE females' behavior by comparing pups vs. male preference and maternal behavior of females that had experienced continuous or limited (approximately two hours) interaction with their litters after parturition was completed. PPE rats subjected to reduced interaction with their pups preferred the male, while females continuously exposed to pups chose them over the male. This change in females' preference was not accompanied by significant alterations of maternal performance in the home cage, although anogenital licking tended to decrease in females with limited mother-litter interaction. Together, the results of these experiments indicate that previous and recent maternal experiences influence the motivational responses of PPE females, and that these effects are more evident when both motivations compete.

Neuroanatomical and neurochemical basis of parenting: Dynamic coordination of motivational, affective and cognitive processes

This article is part of a Special Issue "Parental Care". Becoming a parent is arguably the most profound transforming experience in life. It is also inherently very emotionally and physically demanding, such that the reciprocal interaction with the young changes the brain and behavior of the parents. In this review, we examine the neurobiological mechanisms of parenting primarily discussing recent research findings in rodents and primates, especially humans. We argue that it is essential to consider parenting within a conceptual framework that recognizes the dynamics of the reciprocal mother-young relationship, including both the complexity and neuroplasticity of its underlying mechanisms. Converging research suggests that the concerted activity of a distributed network of subcortical and cortical brain structures regulates different key aspects of parenting, including the sensory analysis of infant stimuli as well as motivational, affective and cognitive processes. The interplay among these processes depends on the action of various neurotransmitters and hormones that modulate the timely and coordinated execution of caregiving responses of the maternal circuitry exquisitely attuned to the young's affect, needs and developmental stage. We conclude with a summary and a set of questions that may guide future research.

Early postpartum pup preference is altered by gestational cocaine treatment: associations with infant cues and oxytocin expression in the MPOA

Cross-fostering studies suggest cocaine-induced deficits in maternal behavior could be associated with altered behavior of offspring following prenatal cocaine-exposure. Neonatal vocalizations are an important offspring cue facilitating early interactions between dam and rodent pup offspring and have been shown to be altered following prenatal cocaine-exposure. It is unclear how variations in acoustic parameters of USVs impact maternal behavior and the mechanism(s) underlying these processes. The present study examined differences in cocaine-exposed and control rodent dam maternal preference of cocaine-exposed or untreated pups in a dual choice apparatus. Relationship of preference-like behavior with pup USVs and dam oxytocin expression was explored. Gestational cocaine-exposure interfered with preference-like behavior of dams on postpartum day 1 with cocaine-exposure associated with decreased time spent on the cocaine-exposed pup side compared to the control pup side, and decreases in preference-like behavior associated in part with decreased number of USVs being emitted by cocaine-exposed pups. On postpartum day 5, decreased oxytocin expression in the medial preoptic area was associated with altered preference-like behavior in cocaine-exposed dams, including frequency and latency to touch/sniff pups. Results indicate cocaine's effects on the mother-infant relationship is likely synergistic, in that cocaine influences mother and offspring both independently and concertedly and that variations within pup vocalizations and the oxytocin system may be potential mechanism(s) underlying this synergistic relationship during the postpartum period.

Behavioural changes in mothers and maternally sensitised female mice

The maternal motivation and depression-like behaviour of primiparous mother and maternally sensitised virgin female mice were investigated. During a 1-h test period, dams and sensitised female mice spent significantly more time in pup-associated than in control cages when they could freely choose between them, while virgin control and ovariectomised females had no such preference. In the forced swim test, the time spent in active (swimming and struggling) and passive (floating) behaviours was measured for 6 min. Mother mice spent more time engaged in active behaviours than virgin and sensitised female mice, while the latter two groups did not differ from each other in the forced swim test. The results suggest that maternal motivation is increased in postpartum mothers and maternally sensitised female mice. We also provide the first demonstration that postpartum mother mice display anti-depression-like behaviours in the forced swim test, while maternally sensitised females do not show such emotional changes.

Maternal and affective behaviors of lactating rats reared in overlapping litters

Postpartum mating in rats gives rise to complex family units consisting of the mother and two overlapping litters. As a consequence, newborn pups of the second litter, since the moment they are born, acquire experience not only from interaction with the mother and age-matched littermates but also from interaction with older siblings. Newborn pups reared in overlapping litters (OLs) receive a different pattern of maternal stimulation compared to those reared in single litters (SL: one litter of same aged pups), as the mothers reduce some maternal behavior components and juvenile pups from the first litter develop maternal behavior. Since there is strong evidence showing that variations in maternal behavior are transmitted throughout generations, we hypothesized that the altered pattern of maternal stimulation received by OL reared females would modify their behavior during motherhood. To test this hypothesis maternal behavior, maternal aggression and experimental anxiety of dams reared under OL and SL conditions during the first postpartum week were compared. No differences were found between the groups in their maternal behavior and aggression. This result may be explained by the maternal behavior of the juveniles that could compensate for the deficits in the caregiving behaviors received by OL litters. However, a subtle temporal reorganization of the licking behavior was found in OL reared mothers, together with an increased anxiety-related behavior in the plus maze test. These results suggest dissociation in the effects provoked by early environmental alterations on different behavioral systems, and more importantly, that independently of their early family composition, both groups can cope effectively with the changing demands of the pups.

The effects of pregnancy, lactation, and primiparity on object-in-place memory of female rats

Maternal physiology and behavior change dramatically over the course of pregnancy to nurture the fetus and prepare for motherhood. Further, the experience of motherhood itself continues to influence brain functioning well after birth, shaping behavior to promote the survival of offspring. To meet these goals, cognitive abilities, such as spatial memory and navigation, may be enhanced to facilitate foraging behavior. Existing studies on pregnant and maternal rats demonstrate enhanced cognitive function in specific spatial domains. We adopted a novel object-in-place task to assess the ability of female rats to integrate information about specific objects in specific locations, a critical element of foraging behavior. Using a longitudinal design to study changes in spatial memory across pregnancy and motherhood, an advantage in the object-in-place memory of primiparous female rats compared to nulliparous females emerged during lactation not during pregnancy, and was maintained after weaning at 42 days postpartum. This enhancement was not dependent on the non-mnemonic variables of anxiety or neophobia. Parity did not affect the type of learning strategy used by females to locate a cued escape platform on a dual-solution water maze task. Results indicate that the enhancement of object-in-place memory, a cognitive function that facilitates foraging, emerged after pregnancy during the postpartum period of lactation and persisted for several weeks after weaning of offspring.

Storing maternal memories: hypothesizing an interaction of experience and estrogen on sensory cortical plasticity to learn infant cues

Much of the literature on maternal behavior has focused on the role of infant experience and hormones in a canonical subcortical circuit for maternal motivation and maternal memory. Although early studies demonstrated that the cerebral cortex also plays a significant role in maternal behaviors, little has been done to explore what that role may be. Recent work though has provided evidence that the cortex, particularly sensory cortices, contains correlates of sensory memories of infant cues, consistent with classical studies of experience-dependent sensory cortical plasticity in non-maternal paradigms. By reviewing the literature from both the maternal behavior and sensory cortical plasticity fields, focusing on the auditory modality, we hypothesize that maternal hormones (predominantly estrogen) may act to prime auditory cortical neurons for a longer-lasting neural trace of infant vocal cues, thereby facilitating recognition and discrimination. This couldthen more efficiently activate the subcortical circuit to elicit and sustain maternal behavior.

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.

Modulation of nociception by social factors in rodents: contribution of the opioid system

RATIONALE

The opioid system is involved in the regulation of several behavioral and physiological responses, controlling pain, reward, and addictive behaviors. Opioid administration, depending on drugs and doses, usually affects sociability reducing interactions between conspecifics, whereas some affiliative behaviors such as sexual activity, social grooming, and play behavior increase the endogenous opioid activity.

OBJECTIVES

The possible interaction between endogenous opioids released during socio/sexual behavior and their analgesic effect on pain response is reviewed in the rodent literature.

RESULTS

Direct evidence for socially mediated opioid changes resulting in increase in nociceptive threshold derives from studies exploring the effects of defeat experiences, social isolation, maternal, sexual behavior, and social reunion among kin or familiar animals in laboratory rodents. Indirect evidence for endogenous activation of the opioid system, possibly affecting pain sensitivity, derives from studies investigating the relevance of natural social reward using the conditioned place preference protocols or analyzing ultrasonic vocalizations associated to positive affective contexts. Finally, genetic and epigenetic factors that affect the opioid system during development are reported to be involved in modulating the response to social stimuli as well as nociception.

CONCLUSIONS

All studies highlight the relevance of affiliative contact behavior between conspecifics that is responsible for the activation of the endogenous mu-opioid system, inducing nociceptive threshold increase.

Functional mapping of the neural circuitry of rat maternal motivation: effects of site-specific transient neural inactivation

The present review focuses on recent studies from our laboratory examining the neural circuitry subserving rat maternal motivation across postpartum. We employed a site-specific neural inactivation method by infusion of bupivacaine to map the maternal motivation circuitry using two complementary behavioural approaches: unconditioned maternal responsiveness and choice of pup- over cocaine-conditioned incentives in a concurrent pup/cocaine choice conditioned place preference task. Our findings revealed that, during the early postpartum period, distinct brain structures, including the medial preoptic area, ventral tegmental area and medial prefrontal cortex infralimbic and anterior cingulate subregions, contribute a pup-specific bias to the motivational circuitry. As the postpartum period progresses and the pups grow older, it is further revealed that maternal responsiveness becomes progressively less dependent on the medial preoptic area and medial prefrontal cortex infralimbic activity, and more distributed in the maternal circuitry, such that additional network components, including the medial prefrontal cortex prelimbic subregion, are recruited with maternal experience, and contribute to the expression of late postpartum maternal behaviour. Collectively, our findings provide strong evidence that the remarkable ability of postpartum females to successfully care for their developing infants is subserved by a distributed neural network that carries out efficient and dynamic processing of complex, constantly changing incoming environmental and pup-related stimuli, ultimately allowing the progression of appropriate expression and waning of maternal responsiveness across the postpartum period.

A bold view of the lactating brain: functional magnetic resonance imaging studies of suckling in awake dams

Functional magnetic resonance imaging (fMRI) has been used to investigate the responsiveness of the maternal rat brain to pup-suckling under various experimental paradigms. Our research employing the lactating rat model has explored the cortical sensory processing of pup stimuli and the effect of suckling on the brain's reward system. Suckling was observed to increase blood-oxygen-level-dependent (BOLD) signal intensity in the midbrain, striatum and prefrontal cortex, which are areas that receive prominent dopaminergic inputs. The BOLD activation of the reward system occurs in parallel with the activation of extensive cortical sensory areas. The observed regions include the olfactory cortex, auditory cortex and gustatory cortex, and could correspond to cortical representations of pup odours, vocalisations and taste that are active during lactation. Activation patterns within reward regions are consistent with past research on maternal motivation and we explore the possibility that exposure to drugs of abuse might be disruptive of maternal neural responses to pups, particularly in the prefrontal cortex. Our ongoing fMRI studies support and extend past research on the maternal rat brain and its functional neurocircuitry.

Characteristic neurobiological patterns differentiate paternal responsiveness in two Peromyscus species

Rodent paternal models provide unique opportunities to investigate the emergence of affiliative social behavior in mammals. Using biparental and uniparental Peromyscus species (californicus and maniculatus, respectively) we assessed paternal responsiveness by exposing males to biological offspring, unrelated conspecific pups, or familiar brothers following a 24-hour separation. The putative paternal circuit we investigated included brain areas involved in fear/anxiety [cingulate cortex (Cg), medial amygdala (MeA), paraventricular nucleus of the hypothalamus (PVN), and lateral septum (LS)], parental motivation [medial preoptic area (MPOA)], learning/behavioral plasticity (hippocampus), olfaction [pyriform cortex (PC)], and social rewards (nucleus accumbens). Paternal experience in californicus males reduced fos immunoreactivity (ir) in several fear/anxiety areas; additionally, all californicus groups exhibited decreased fos-ir in the PC. Enhanced arginine vasopressin (AVP) and oxytocin (OT)-ir cell bodies and fibers, as well as increased neuronal restructuring in the hippocampus, were also observed in californicus mice. Multidimensional scaling analyses revealed distinct brain activation profiles differentiating californicus biological fathers, pup-exposed virgins, and pup-naïve virgins. Specifically, associations among MPOA fos, CA1 fos, dentate gyrus GFAP, CA2 nestin-, and PVN OT-ir characterized biological fathers; LS fos-, Cg fos-, and AVP-ir characterized pup-exposed virgins, and PC-, PVN-, and MeA fos-ir characterized pup-naïve virgins. Thus, whereas fear/anxiety areas characterized pup-naïve males, neurobiological factors involved in more diverse functions such as learning, motivation, and nurturing responses characterized fatherhood in biparental californicus mice. Less distinct paternal-dependent activation patterns were observed in uniparental maniculatus mice. These data suggest that dual neurobiological circuits, leading to the inhibition of social-dependent anxiety as well as the activation of affiliative responses, characterize the transition from nonpaternal to paternal status in californicus mice.

Cocaine alters dendritic spine density in cortical and subcortical brain regions of the postpartum and virgin female rat

Cocaine use during pregnancy induces profound neural and behavioral deficits in both mother and offspring. The present study was designed to compare the effects of cocaine exposure on spine density of postpartum and virgin female rat brains. Timed, pregnant, primiparous rats were injected with either cocaine (30 mg/kg) or saline, once daily, from gestational day 8 to 20. Twenty-four hours after giving birth, dam brains were processed for Golgi-impregnation. Virgin females were also injected with the same dose of cocaine or saline for 12 days and sacrificed 24 h after the last injection for comparison. Pregnant rats had significantly greater spine density in the medial amygdala (MeA) and medial preoptic area (MPOA) and lower spine density in CA1 than virgin females independent of cocaine treatment. Cocaine significantly increased dendritic spine density on the apical branch of pyramidal cells in the prefrontal cortex (PFC, 15%), both apical (13%) and basal (14.8%) branches of CA1 and cells in the MeA (28%) of pregnant rats. In the MPOA, cocaine administration resulted in a decrease in dendritic spine density (14%) in pregnant rats. In virgin females, cocaine had fewer effects but did increase dendritic spine density on both branches of CA1 neurons and in the MeA. The present study is the first to demonstrate that spine density differs between pregnant and virgin females and that pregnancy makes the brain more vulnerable to cocaine, which has important clinical implications.