Pre-weaning paternal deprivation impairs social recognition and alters hippocampal neurogenesis and spine density in adult mandarin voles

Unraveling sex differences in maternal and paternal care impacts on social behaviors and neurobiological responses to early-life adversity

Early-life stress (ELS) affects the development of prosocial behaviors and social-cognitive function, often leading to structural brain changes and increased psychosocial disorders. Recent studies suggest that mother- and father-child relationships independently influence social development in a sex-specific manner, but the effects of impaired father-child relationships are often overlooked. This review examines preclinical rodent studies to explore how parental neglect impacts neuroplasticity and social behaviors in offspring. We highlight that disruptions in maternal interactions may affect male pups more in uniparental rodents, while impaired paternal interactions in biparental rodents tend to impact female pups more. Due to limited research, the separate effects of maternal and paternal neglect on brain development and social behaviors in biparental species remain unclear. Addressing these gaps could clarify the sex-specific mechanisms underlying social and neurobiological deficits following parental neglect.

Shaping the development of complex social behavior

Early life experiences can have an enduring impact on the brain and behavior, with implications for stress reactivity, cognition, and social behavior. In particular, the neural systems that contribute to the expression of social behavior are altered by early life social environments. However, paradigms that have been used to alter the social environment during development have typically focused on exposure to stress, adversity, and deprivation of species-typical social stimulation. Here, we explore whether complex social environments can shape the development of complex social behavior. We describe lab-based paradigms for studying early life social complexity in rodents that are generally focused on enriching the social and sensory experiences of the neonatal and juvenile periods of development. The impact of these experiences on social behavior and neuroplasticity is highlighted. Finally, we discuss the degree to which our current approaches for studying social behavior outcomes give insight into "complex" social behavior and how social complexity can be better integrated into lab-based methodologies.

Behavioral evidence of the functional interaction between the main and accessory olfactory system suggests a large olfactory system with a high plastic capability

Olfaction is fundamental in many species of mammals. In rodents, the integrity of this system is required for the expression of parental and sexual behavior, mate recognition, identification of predators, and finding food. Different anatomical and physiological evidence initially indicated the existence of two anatomically distinct chemosensory systems: The main olfactory system (MOS) and the accessory olfactory system (AOS). It was originally conceived that the MOS detected volatile odorants related to food, giving the animal information about the environment. The AOS, on the other hand, detected non-volatile sexually relevant olfactory cues that influence reproductive behaviors and neuroendocrine functions such as intermale aggression, sexual preference, maternal aggression, pregnancy block (Bruce effect), puberty acceleration (Vandenbergh effect), induction of estrous (Whitten effect) and sexual behavior. Over the last decade, several lines of evidence have demonstrated that although these systems could be anatomically separated, there are neuronal areas in which they are interconnected. Moreover, it is now clear that both the MOS and the AOS process both volatile and no-volatile odorants, indicating that they are also functionally interconnected. In the first part of the review, we will describe the behavioral evidence. In the second part, we will summarize data from our laboratory and other research groups demonstrating that sexual behavior in male and female rodents induces the formation of new neurons that reach the main and accessory olfactory bulbs from the subventricular zone. Three factors are essential for the neurons to reach the AOS and the MOS: The stimulation frequency, the stimulus's temporal presentation, and the release of opioids induced by sexual behavior. We propose that the AOS and the MOS are part of a large olfactory system with a high plastic capability, which favors the adaptation of species to different environmental signals.

Cigarette smoke-induced pulmonary impairment is associated with social recognition memory impairments and alterations in microglial profiles within the suprachiasmatic nucleus of the hypothalamus

Chronic obstructive pulmonary disease (COPD) is a major, incurable respiratory condition that is primarily caused by cigarette smoking (CS). Neurocognitive disorders including cognitive dysfunction, anxiety and depression are highly prevalent in people with COPD. It is understood that increased lung inflammation and oxidative stress from CS exposure may 'spill over' into the systemic circulation to promote the onset of these extra-pulmonary comorbidities, and thus impacts the quality of life of people with COPD. The precise role of the 'spill-over' of inflammation and oxidative stress in the onset of COPD-related neurocognitive disorders are unclear. The present study investigated the impact of chronic CS exposure on anxiety-like behaviors and social recognition memory, with a particular focus on the role of the 'spill-over' of inflammation and oxidative stress from the lungs. Adult male BALB/c mice were exposed to either room air (sham) or CS (9 cigarettes per day, 5 days a week) for 24 weeks and were either daily co-administered with the NOX2 inhibitor, apocynin (5 mg/kg, in 0.01 % DMSO diluted in saline, i.p.) or vehicle (0.01 % DMSO in saline) one hour before the initial CS exposure of the day. After 23 weeks, mice underwent behavioral testing and physiological diurnal rhythms were assessed by monitoring diurnal regulation profiles. Lungs were collected and assessed for hallmark features of COPD. Consistent with its anti-inflammatory and oxidative stress properties, apocynin treatment partially lessened lung inflammation and lung function decline in CS mice. CS-exposed mice displayed marked anxiety-like behavior and impairments in social recognition memory compared to sham mice, which was prevented by apocynin treatment. Apocynin was unable to restore the decreased Bmal1-positive cells, key in cells in diurnal regulation, in the suprachiasmatic nucleus of the hypothalamus to that of sham levels. CS-exposed mice treated with apocynin was associated with a restoration of microglial area per cell and basal serum corticosterone. This data suggests that we were able to model the CS-induced social recognition memory impairments seen in humans with COPD. The preventative effects of apocynin on memory impairments may be via a microglial dependent mechanism.

Losing a parent in early-life impairs flock size discrimination and lowers oxytocin receptor abundance in a medial amygdala homologue of adult zebra finches

Experiencing inadequate parental care during early-life diminishes adult social competencies. For example, low parental care impairs adult socio-cognitive abilities (e.g., recognizing familiar conspecifics) and affiliation (e.g., close social proximity); outcomes attributed to diminished medial amygdala nonapeptide functioning in rodents. Whether parental care has effects beyond familiarity, and if siblings have similar effects to parents, is unclear. Here, zebra finches were used to explore if parent and/or sibling number shape adult recognition and preference of small versus large flocks and nonapeptide (oxytocin, vasotocin) receptor expression in an avian homologue of the mammalian medial amygdala. Chicks were raised by single mothers or fathers in small broods or paired parents in small or large broods matched to single parents for chicks per nest or per parent, respectively. Pair-raised birds had preferred flock sizes as adults, but birds raised by single parents had equal preference for either size. Oxytocin receptor expression was lower in birds raised by single parents versus paired parents, but vasotocin receptor levels were unaffected. Such results highlight parents as formative antecedents of their offspring's social competencies related to group size preference and their nonapeptide mechanisms, outcomes that influence an animal's ability to live in social groups.

Sex-specific effects of neonatal paternal deprivation on microglial cell density in adult California mouse (Peromyscus californicus) dentate gyrus

Adverse early-life experiences are risk factors for psychiatric disease development, resulting in stress-related neuronal modeling and neurobehavioral changes. Stressful experiences modulate the immune system, contributing to neuronal damage in higher cortical regions, like the hippocampus. Moreover, early-life stressors dysregulate the function of microglia, the resident immune cells of the brain, in the developing hippocampus. Paternal deprivation, an early-life stressor in many biparental species, facilitates sex-dependent inhibitions in hippocampal plasticity, but parental contributors to these sex-specific outcomes are unknown. Also, neurobiological mechanisms contributing to impairments in hippocampal neuroplasticity are less known. Thus, our goals were to 1) determine whether parental behavior is altered in maternal females following removal of the paternal male, 2) assess the effects of paternal deprivation on dentate gyrus (DG) volume and microglia proliferation, and 3) determine if early-life experimental handling mitigates sex-specific reductions in DG cell survival. California mice were born to multiparous breeders and reared by both parents (biparental care) or by their mother alone (i.e., father removed on postnatal day 1; paternal deprivation). One cohort of offspring underwent offspring retrieval tests for eight days beginning on postnatal day 2. On PND 68, these offspring (and a second cohort of mice without behavioral testing) were euthanized and brains visualized for bromodeoxyuridine (BrdU) and neuron-specific class III beta-tubulin (TuJ-1) or ionized calcium binding adaptor molecule 1 (Iba1). While mate absence did not impair maternal retrieval, paternal deprivation reduced DG volume, but Iba1+ cell density was only higher in paternally-deprived females. Neither sex or paternal deprivation significantly altered the number of BrdU+ or Tuj1+ cells in the DG - an absence of a reduction in cell survival may be related to daily handing during early offspring retrieval tests. Together, these data suggest that paternal deprivation impairs hippocampal plasticity; however, sex and early environment may influence the magnitude of these outcomes.

Enduring effects of early-life adversity on reward processes: A systematic review and meta-analysis of animal studies

Two-thirds of individuals experience adversity during childhood such as neglect, abuse or highly-stressful events. Early-life adversity (ELA) increases the life-long risk of developing mood and substance use disorders. Reward-related deficits has emerged as a key endophenotype of such psychiatric disorders. Animal models are invaluable for studying how ELA leads to reward deficits. However, the existing literature is heterogenous with difficult to reconcile findings. To create an overview, we conducted a systematic review containing multiple meta-analyses regarding the effects of ELA on reward processes overall and on specific aspects of reward processing in animal models. A comprehensive search identified 120 studies. Most studies omitted key details resulting in unclear risk of bias. Overall meta-analysis showed that ELA significantly reduced reward behaviors (SMD: -0.42 [-0.60; -0.24]). The magnitude of ELA effects significantly increased with longer exposure. When reward domains were analyzed separately, ELA only significantly dampened reward responsiveness (SMD: -0.525[-0.786; -0.264]) and social reward processing (SMD: -0.374 [-0.663; -0.084]), suggesting that ELA might lead to deficits in specific reward domains.

Amphetamine exposure alters behaviors, and neuronal and neurochemical activation in the brain of female prairie voles

Previous studies have shown that 3-day d-amphetamine (AMPH) treatment effectively induced conditioned place preferences (CPP) and impaired pair bonding behaviors in prairie voles (Microtus ochrogaster). Using this established animal model and treatment regimen, we examined the effects of the demonstrated threshold rewarding dose of AMPH on various behaviors and their potential underlying neurochemical systems in the brain of female prairie voles. Our data show that 3-day AMPH injections (0.2 mg/kg/day) impaired social recognition and decreased depressive-like behavior in females without affecting their locomotion and anxiety-like behaviors. AMPH treatment also decreased neuronal activation indicated by the labeling of the early growth response protein 1 (Egr-1) as well as the number of neurons double-labeled for Egr-1 and corticotrophin-releasing hormone (CRH) in the dentate gyrus (DG) of the hippocampus and paraventricular nucleus of the hypothalamus (PVN) in the brain. Further, AMPH treatment decreased the number of neurons double-labeled for Egr-1 and tyrosine hydroxylase (TH) but did not affect oxytocinergic neurons in the PVN or cell proliferation and neurogenesis markers in the DG. These data not only demonstrate potential roles of the brain CRH and dopamine systems in mediating disrupted social recognition and depressive-like behaviors by AMPH in female prairie voles, but also further confirm the utility of the prairie vole model for studying interactions between psychostimulants and social behaviors.

Under or Absent Reporting of Light Stimuli in Testing of Anxiety-Like Behaviors in Rodents: The Need for Standardization

Behavioral neuroscience tests such as the Light/Dark Test, the Open Field Test, the Elevated Plus Maze Test, and the Three Chamber Social Interaction Test have become both essential and widely used behavioral tests for transgenic and pre-clinical models for drug screening and testing. However, as fast as the field has evolved and the contemporaneous involvement of technology, little assessment of the literature has been done to ensure that these behavioral neuroscience tests that are crucial to pre-clinical testing have well-controlled ethological motivation by the use of lighting (i.e., Lux). In the present review paper, N = 420 manuscripts were examined from 2015 to 2019 as a sample set (i.e., n = ~20–22 publications per year) and it was found that only a meager n = 50 publications (i.e., 11.9% of the publications sampled) met the criteria for proper anxiogenic and anxiolytic Lux reported. These findings illustrate a serious concern that behavioral neuroscience papers are not being vetted properly at the journal review level and are being released into the literature and public domain making it difficult to assess the quality of the science being reported. This creates a real need for standardizing the use of Lux in all publications on behavioral neuroscience techniques within the field to ensure that contributions are meaningful, avoid unnecessary duplication, and ultimately would serve to create a more efficient process within the pre-clinical screening/testing for drugs that serve as anxiolytic compounds that would prove more useful than what prior decades of work have produced. It is suggested that improving the standardization of the use and reporting of Lux in behavioral neuroscience tests and the standardization of peer-review processes overseeing the proper documentation of these methodological approaches in manuscripts could serve to advance pre-clinical testing for effective anxiolytic drugs. This report serves to highlight this concern and proposes strategies to proactively remedy them as the field moves forward for decades to come.

Effects of early life adversities upon memory processes and cognition in rodent models

Exposure to stressors in early postnatal life induces long-lasting modifications in brainfunction.Thisplasticity,an essential characteristic of the brain that enables adaptation to the environment, may also induce impairments in some psychophysiological functions, including learning and memory. Early life stress (ELS) has long-term effects on thehypothalamic-pituitary-adrenal axisresponse to stressors, and has been reported to lead toneuroinflammation,altered levelsof neurotrophic factors, modifications inneurogenesis andsynaptic plasticity,with changes in neurotransmitter systems and network functioning. In this review, we focus on early postnatal stress in animal models and their effects on learning and memory.Many studies have reported ELS-induced impairments in different types of memories, including spatial memory, fear memory, recognition (both for objects and social) memory, working memory and reversal learning. Studies are not always in agreement, however, no effects, or sometimes facilitation, being reported, depending on the nature and intensity of the early intervention, as well as the age when the outcome was evaluated and the sex of the animals. When considering processes occurring after consolidation, related with memory maintenance or modification, there are a very reduced number of reports. Future studies addressing the mechanisms underlying memory changes for ELS should shed some light on the understanding of the different effects induced by stressors of different types and intensities on cognitive functions.

Sex-dependent effects of pair bond interruption on anxiety- and depression-like behaviors in adult mandarin voles

Stable and positive social bonds are pretty vital to the development of animals. Instability and disruptions of social bonds, such as maternal separation and social isolation, always produce disastrous influence on physiology, neuroendocrine and behaviors. Pair bond is one of the most important social bonds in adulthood. But the different effects of pair bond interruption between males and females are rarely studied. In the present study, the monogamous mandarin voles (Microtus mandarinus) were used to confirm the time window of pair bond formation. After that, voles were separated from their partner for 1 or 2 weeks. Then anxiety- and depression-like behaviors were investigated by using open field test, light-dark box test, tail suspension test and forced swimming test, respectively. The results showed that: (1) cohabitation for 5 days is sufficient and necessary for mandarin voles to form pair bond; (2) loss of partner is always crucial for the effects of pair bond interruption, while social isolation works in certain behavioral tests.; (3) pair bond interruption for 2 weeks significantly increased the level of anxiety and depression in adult males, but not female mandarin voles. Overall, this research suggested that loss of partner plays a key role in pair bond interruption in male mandarin voles.

Altered social recognition memory and hypothalamic neuropeptide expression in adolescent male and female rats following prenatal alcohol exposure and/or early-life adversity

Prenatal alcohol exposure (PAE) and early-life adversity (ELA) both negatively impact social neurobehavioral development, including social recognition memory. Importantly, while individuals with PAE are more likely to experience ELA, relatively few studies have assessed the interaction of these two early insults on adolescent social behavior development. Here, we combine animal models of PAE and ELA to investigate both their unique and interactive effects on social neurobehavioral function in early and late adolescent male and female rats. Behavioral testing was followed by assessment of hypothalamic expression of oxytocin (OT) and vasopressin (AVP), key neuropeptides in the regulation of social behavior. Our results indicate that PAE and ELA have unique sex- and age-specific effects on social recognition memory and OT/AVP expression, with more pronounced neurobehavioral changes observed in males than in females in both early and late adolescence. Specifically, ELA impaired social recognition in early adolescent females regardless of prenatal treatment, while males showed deficits in both early and late adolescence in response to unique and interactive effects of PAE and ELA. Neurobiological data suggest that these perinatal insults differentially impact the OT and AVP systems in a sexually dimorphic manner, such that the OT system appears to be particularly sensitive to PAE in males while the AVP system appears to be more vulnerable to ELA in females. Taken together, our data provide novel insight into how the early postnatal environment may mediate outcomes of PAE as well as the power of animal models to interrogate the relationship between these pre- and postnatal insults.

Paternal Deprivation and Female Biparental Family Rearing Induce Dendritic and Synaptic Changes in Octodon degus: I. Medial Prefrontal Cortex

In most mammalian species parent-offspring interactions during early life periods primarily comprise social contacts with the mother, whereas the role of males in parental care is one of the most overlooked and understudied topics. The present study addressed the hypothesis that the complete deprivation of paternal care delays or permanently retards synaptic connectivity in the brain, particularly in the medial prefrontal cortex (mPFC) of the offspring in a sex-specific manner. Another aim of this study was to address the question whether and in which way replacing the father with a female caregiver (in our experiments the “aunt”) can “buffer” the detrimental effects of paternal deprivation on neuronal development. The comparison of: (a) single mother rearing; (b) biparental rearing by father and mother; and (c) biparental rearing by two female caregivers revealed that: (i) paternal care represents a critical environmental factor for synaptic and dendritic development of pyramidal neurons in the vmPFC of their offspring; (ii) a second female caregiver (“aunt”) does not “buffer” the neuronal consequences of paternal deprivation; and that (iii) neuronal development in the vmPFC is differentially affected in male and female offspring in response to different family constellations.

Preweaning Paternal Deprivation Impacts Parental Responses to Pups and Alters the Serum Oxytocin and Corticosterone Levels and Oxytocin Receptor, Vasopressin 1A Receptor, Oestrogen Receptor, Dopamine Type I Receptor, Dopamine Type II Receptor Levels in Relevant Brain Regions in Adult Mandarin Voles

Although maternal separation and neonatal paternal deprivation (PD) have been found to exert a profound and persistent effects on the physiological and behavioural development of offspring, whether preweaning PD (PPD; from PND 10 to 21) affects maternal and parental responses to pups and the underlying neuroendocrine mechanism are under-investigated. Using monogamous mandarin voles (Microtus mandarinus), the present study found that PPD increased the latency to approach a pup-containing ball, decreased the total durations of sniffing and contacting a pup-containing ball and walking and increased the total duration of inactivity in both sexes. Moreover, PPD decreased serum oxytocin levels and increased corticosterone levels, but only in females. Furthermore, in both males and females, PPD decreased the expression of oxytocin receptor mRNA and protein in the medial preoptic area (MPOA), nucleus accumbens (NAcc) and medial prefrontal cortex (mPFC), but increased it in the medial amygdala (MeA) and decreased the expression of oestrogen receptor mRNA and protein in the MPOA. PPD increased the expression of dopamine type I receptor in the NAcc, but decreased it in the mPFC. PPD decreased dopamine type II receptor (D2R) in the NAcc both in males and females, but increased D2R in the mPFC in females and decreased D2R protein expression in males. Moreover, PPD decreased vasopressin 1A receptor (V1AR) in the MPOA, MeA and mPFC, but only in males. Our results suggest that the reduction of parental responses to pups induced by PPD may be associated with the sex-specific alteration of several neuroendocrine parameters in relevant brain regions.

Sex-dependent effects of paternal deprivation and chronic variable stress on novel object recognition in adult California mice (Peromyscus californicus)

Early-life stress exposure can confer vulnerability for development of psychiatric illnesses and impaired cognition in adulthood. It is well-known that early-life stress can dysregulate the hypothalamic-pituitary-adrenal (HPA) axis in a sex-dependent manner. Specifically, uniparental rodent models of prolonged disrupted mother-offspring relationships (e.g., maternal separation) have demonstrated greater alterations in stress responsivity in adult males, compared to females. Also, chronic early-life stressors (e.g., limited bedding model) impair cognitive function in males more than females. However, the sex-dependent effects of early-life stress and later-life chronic HPA axis activation on cognition have not been well-characterized. Here, we utilized the biparental California mouse (Peromyscus californicus) to model the early-life adversity of paternal deprivation (PD). Fathers either remained in the nest (biparental care) or were permanently removed (PD) on postnatal day (PND) 1. Adult offspring were exposed to daily handling (control) or chronic variable stress (CVS; three stressors for seven days). Twenty-four hours after the final stressor, the novel object recognition (NOR) task commenced, followed by serum collection for corticosterone (CORT) analysis. Independent of sex or rearing, CVS increased CORT. Exploration during acquisition for the NOR task was increased as a result of CVS and PD. During NOR testing, non-stressed females exhibited greater difference scores (i.e., increased recognition memory), compared to non-stressed males. However, the addition of CVS diminished difference scores in females - an effect not observed in CVS-exposed males. Overall, these data suggest that neonatal paternal experience, sex, and chronic stress contribute to exploratory behavior, cognition, and stress hormone concentrations in a biparental species.

The serotonin system in the hippocampus CA3 involves in effects of CSDS on social recognition in adult female mandarin voles (Microtus mandarinus)

Chronic social defeat stress (CSDS) exacerbated the development of stress-related psychiatric disorders, and the social recognition dysfunction is the core feature of many psychiatric disorders. However, the effects of CSDS on female social recognition and the underlying neural mechanisms remain unclear. Using highly aggressive adult female mandarin voles (Microtus mandarinus) as animal model, the aim of this work is to investigate the effects of CSDS on social recognition in adult female rodents and the neurobiological mechanisms underlying these effects. Our results indicate the CSDS disrupted the normal social recognition in adult female voles. Meanwhile, defeated voles exhibited increased neural activity in the DG, CA1 and CA3 of the hippocampus. Furthermore, CSDS reduced levels of serotonin (5-HT) and serotonin 1A receptors (5-HT_1AR) in the CA3. We also discovered that microinjection of 8-OH-DPAT into the CA3 effectively reversed the social recognition deficits induced by CSDS, and an infusion of WAY-100635 into the CA3 of control female voles impaired social recognition. Moreover, targeted activation of the 5-HT neuron projection from the DRN to CA3 by long-term administration of CNO significantly prevented the CSDS induced social recognition deficits. Taken together, our study demonstrated that CSDS induced social recognition deficits in adult female voles, and these effects were mediated by the action of 5-HT on the 5-HT_1AR in the hippocampus CA3. The projection from the DRN to CA3 may be involved in social recognition deficits induced by CSDS.

The neural mechanisms and consequences of paternal caregiving

In recent decades, human sociocultural changes have increased the numbers of fathers that are involved in direct caregiving in Western societies. This trend has led to a resurgence of interest in understanding the mechanisms and effects of paternal care. Across the animal kingdom, paternal caregiving has been found to be a highly malleable phenomenon, presenting with great variability among and within species. The emergence of paternal behaviour in a male animal has been shown to be accompanied by substantial neural plasticity and to be shaped by previous and current caregiving experiences, maternal and infant stimuli and ecological conditions. Recent research has allowed us to gain a better understanding of the neural basis of mammalian paternal care, the genomic and circuit-level mechanisms underlying paternal behaviour and the ways in which the subcortical structures that support maternal caregiving have evolved into a global network of parental care. In addition, the behavioural, neural and molecular consequences of paternal caregiving for offspring are becoming increasingly apparent. Future cross-species research on the effects of absence of the father and the transmission of paternal influences across generations may allow research on the neuroscience of fatherhood to impact society at large in a number of important ways.