Dietary control of maternal pheromone in the lactating rat

Increases in Bdnf DNA Methylation in the Prefrontal Cortex Following Aversive Caregiving Are Reflected in Blood Tissue

Child maltreatment not only leads to epigenetic changes, but also increases the risk of related behavioral deficits and mental disorders. These issues presumably are most closely associated with epigenetic changes in the brain, but epigenetic changes in peripheral tissues like blood are often examined instead, due to their accessibility. As such, the reliability of using the peripheral epigenome as a proxy for that of the brain is imperative. Previously, our lab has found aberrant methylation at the Brain-derived neurotrophic factor (Bdnf) gene in the prefrontal cortex of rats following aversive caregiving. The current study examined whether aversive caregiving alters Bdnf DNA methylation in the blood compared to the prefrontal cortex. It was revealed that DNA methylation associated with adversity increased in both tissues, but this methylation was not correlated between tissues. These findings indicate that group trends in Bdnf methylation between blood and the brain are comparable, but variation exists among individual subjects.

Adverse caregiving in infancy blunts neural processing of the mother

The roots of psychopathology frequently take shape during infancy in the context of parent-infant interactions and adversity. Yet, neurobiological mechanisms linking these processes during infancy remain elusive. Here, using responses to attachment figures among infants who experienced adversity as a benchmark, we assessed rat pup cortical local field potentials (LFPs) and behaviors exposed to adversity in response to maternal rough and nurturing handling by examining its impact on pup separation-reunion with the mother. We show that during adversity, pup cortical LFP dynamic range decreased during nurturing maternal behaviors, but was minimally impacted by rough handling. During reunion, adversity-experiencing pups showed aberrant interactions with mother and blunted cortical LFP. Blocking pup stress hormone during either adversity or reunion restored typical behavior, LFP power, and cross-frequency coupling. This translational approach suggests adversity-rearing produces a stress-induced aberrant neurobehavioral processing of the mother, which can be used as an early biomarker of later-life pathology.

Diet-based assortative mating through sexual imprinting

Speciation is facilitated by "magic traits," where divergent natural selection on such traits also results in assortative mating. In animal populations, diet has the potential to act as a magic trait if populations diverge in consumed food that incidentally affects mating and therefore sexual isolation. While diet-based assortative mating has been observed in the laboratory and in natural populations, the mechanisms causing positive diet-based assortment remain largely unknown. Here, we experimentally created divergent diets in a sexually imprinting species of mouse, Peromyscus gossypinus (the cotton mouse), to test the hypothesis that sexual imprinting on diet could be a mechanism that generates rapid and significant sexual isolation. We provided breeding pairs with novel garlic- or orange-flavored water and assessed whether their offspring, exposed to these flavors in utero and in the nest before weaning, later preferred mates that consumed the same flavored water as their parents. While males showed no preference, females preferred males of their parental diet, which is predicted to yield moderate sexual isolation. Thus, our experiment demonstrates the potential for sexual imprinting on dietary cues learned in utero and/or postnatally to facilitate reproductive isolation and potentially speciation.

OPEN RESEARCH BADGES

This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/dryad.n1qq6v3.

Female pups receive more maltreatment from stressed dams

The effects of exposure to developmental stress often diverge for males and females. Using the scarcity-adversity model of low nesting resources outside the home cage, our lab has discovered sex differences in both behavioral and epigenetic consequences of repeated exposure to caregiver maltreatment. For the measures we have performed to date, we have found more consequences for females. The reasons underlying this sex disparity are unknown. In the current experiment, we aimed to discern the quality of maternal care received by male and female pups in our model. As we have previously found more behavioral and epigenetic consequences in females, we hypothesized that females receive more adverse care compared to their male littermates. Our hypothesis was supported; in our maltreatment condition, we found that female pups received more adverse care than males. This sex difference in adverse care was not present in our two control conditions (cross-foster and normal maternal care). These data lend support to the notion that one reason females in our model incur more behavioral and epigenetic consequences is a result of greater mistreatment by the dam.

Pharmacological manipulation of DNA methylation normalizes maternal behavior, DNA methylation, and gene expression in dams with a history of maltreatment

The quality of parental care received during development profoundly influences an individual's phenotype, including that of maternal behavior. We previously found that female rats with a history of maltreatment during infancy mistreat their own offspring. One proposed mechanism through which early-life experiences influence behavior is via epigenetic modifications. Indeed, our lab has identified a number of brain epigenetic alterations in female rats with a history of maltreatment. Here we sought to investigate the role of DNA methylation in aberrant maternal behavior. We administered zebularine, a drug known to alter DNA methylation, to dams exposed during infancy to the scarcity-adversity model of low nesting resources, and then characterized the quality of their care towards their offspring. First, we replicate that dams with a history of maltreatment mistreat their own offspring. Second, we show that maltreated-dams treated with zebularine exhibit lower levels of adverse care toward their offspring. Third, we show that administration of zebularine in control dams (history of nurturing care) enhances levels of adverse care. Lastly, we show altered methylation and gene expression in maltreated dams normalized by zebularine. These findings lend support to the hypothesis that epigenetic alterations resulting from maltreatment causally relate to behavioral outcomes.

Developing a neurobehavioral animal model of poverty: Drawing cross-species connections between environments of scarcity-adversity, parenting quality, and infant outcome

Children reared in impoverished environments are at risk for enduring psychological and physical health problems. Mechanisms by which poverty affects development, however, remain unclear. To explore one potential mechanism of poverty's impact on social-emotional and cognitive development, an experimental examination of a rodent model of scarcity-adversity was conducted and compared to results from a longitudinal study of human infants and families followed from birth (N = 1,292) who faced high levels of poverty-related scarcity-adversity. Cross-species results supported the hypothesis that altered caregiving is one pathway by which poverty adversely impacts development. Rodent mothers assigned to the scarcity-adversity condition exhibited decreased sensitive parenting and increased negative parenting relative to mothers assigned to the control condition. Furthermore, scarcity-adversity reared pups exhibited decreased developmental competence as indicated by disrupted nipple attachment, distress vocalization when in physical contact with an anesthetized mother, and reduced preference for maternal odor with corresponding changes in brain activation. Human results indicated that scarcity-adversity was inversely correlated with sensitive parenting and positively correlated with negative parenting, and that parenting fully mediated the association of poverty-related risk with infant indicators of developmental competence. Findings are discussed from the perspective of the usefulness of bidirectional-translational research to inform interventions for at-risk families.

Pharmacological Manipulation of DNA Methylation in Adult Female Rats Normalizes Behavioral Consequences of Early-Life Maltreatment

Exposure to adversity early in development alters brain and behavioral trajectories. Data continue to accumulate that epigenetic mechanisms are a mediating factor between early-life adversity and adult behavioral phenotypes. Previous work from our laboratory has shown that female Long-Evans rats exposed to maltreatment during infancy display both aberrant forced swim behavior and patterns of brain DNA methylation in adulthood. Therefore, we examined the possibility of rescuing the aberrant forced swim behavior in maltreated-adult females by administering an epigenome-modifying drug (zebularine) at a dose previously shown to normalize DNA methylation. We found that zebularine normalized behavior in the forced swim test in maltreated females such that they performed at the levels of controls (females that had been exposed to only nurturing care during infancy). These data help link DNA methylation to an adult phenotype in our maltreatment model, and more broadly provide additional evidence that non-targeted epigenetic manipulations can change behavior associated with early-life adversity.

Chronic early life stress induced by limited bedding and nesting (LBN) material in rodents: critical considerations of methodology, outcomes and translational potential

The immediate and long-term effects of exposure to early life stress (ELS) have been documented in humans and animal models. Even relatively brief periods of stress during the first 10 days of life in rodents can impact later behavioral regulation and the vulnerability to develop adult pathologies, in particular an impairment of cognitive functions and neurogenesis, but also modified social, emotional, and conditioned fear responses. The development of preclinical models of ELS exposure allows the examination of mechanisms and testing of therapeutic approaches that are not possible in humans. Here, we describe limited bedding and nesting (LBN) procedures, with models that produce altered maternal behavior ranging from fragmentation of care to maltreatment of infants. The purpose of this paper is to discuss important issues related to the implementation of this chronic ELS procedure and to describe some of the most prominent endpoints and consequences, focusing on areas of convergence between laboratories. Effects on the hypothalamic-pituitary adrenal (HPA) axis, gut axis and metabolism are presented in addition to changes in cognitive and emotional functions. Interestingly, recent data have suggested a strong sex difference in some of the reported consequences of the LBN paradigm, with females being more resilient in general than males. As both the chronic and intermittent variants of the LBN procedure have profound consequences on the offspring with minimal external intervention from the investigator, this model is advantageous ecologically and has a large translational potential. In addition to the direct effect of ELS on neurodevelopmental outcomes, exposure to adverse early environments can also have intergenerational impacts on mental health and function in subsequent generation offspring. Thus, advancing our understanding of the effect of ELS on brain and behavioral development is of critical concern for the health and wellbeing of both the current population, and for generations to come.

Development of Odor Hedonics: Experience-Dependent Ontogeny of Circuits Supporting Maternal and Predator Odor Responses in Rats

A major component of perception is hedonic valence: perceiving stimuli as pleasant or unpleasant. Here, we used early olfactory experiences that shape odor preferences and aversions to explore developmental plasticity in circuits mediating odor hedonics. We used 2-deoxyglucose autoradiographic mapping of neural activity to identify circuits differentially activated by biologically relevant preferred and avoided odors across rat development. We then further probed this system by increasing or decreasing hedonic value. Using both region of interest and functional connectivity analyses, we identified regions within primary olfactory, amygdala/hippocampal, and prefrontal cortical networks that were activated differentially by maternal and male odors. Although some activated regions remained stable across development (postnatal days 7-23), there was a developmental emergence of others that resulted in an age-dependent elaboration of hedonic-response-specific circuitry despite stable behavioral responses (approach/avoidance) to the odors across age. Hedonic responses to these biologically important odors were modified through diet suppression of the maternal odor and co-rearing with a male. This allowed assessment of hedonic circuits in isolation of the specific odor quality and/or intensity. Early experience significantly modified odor-evoked circuitry in an age-dependent manner. For example, co-rearing with a male, which induced pup attraction to male odor, reduced activity in amygdala regions normally activated by the unfamiliar avoided male odor, making this region more consistent with maternal odor. Understanding the development of odor hedonics, particularly within the context of altered early life experience, provides insight into the development of sensory processes, food preferences, and the formation of social affiliations, among other behaviors.

SIGNIFICANCE STATEMENT

Odor hedonic valence controls approach-avoidance behaviors, but also modulates ongoing behaviors ranging from food preferences and social affiliation with the caregiver to avoidance of predator odors. Experiences can shape hedonic valence. This study explored brain circuitry involved in odor hedonic encoding throughout development using maternal and predator odors and assessed the effects of early life experience on odor hedonic encoding by increasing/decreasing the hedonic value of these odors. Understanding the role of changing brain circuitry during development and its impact on behavioral function is critical for understanding sensory processing across development. These data converge with exciting literature on the brain's hedonic network and highlight the significant role of early life experience in shaping the neural networks of highly biologically relevant stimuli.

Developmental neurobiology of the rat attachment system and its modulation by stress

Stress is a powerful modulator of brain structure and function. While stress is beneficial for survival, inappropriate stress dramatically increases the risk of physical and mental health problems, particularly when experienced during early developmental periods. Here we focus on the neurobiology of the infant rat's odor learning system that enables neonates to learn and approach the maternal odor and describe the unique role of the stress hormone corticosterone in modulating this odor approach learning across development. During the first nine postnatal days, this odor approach learning of infant rats is supported by a wide range of sensory stimuli and ensures attachment to the mother's odor, even when interactions with her are occasionally associated with pain. With maturation and the emergence of a stress- or pain-induced corticosterone response, this odor approach learning terminates and a more adult-like amygdala-dependent fear/avoidance learning emerges. Strikingly, the odor approach and attenuated fear learning of older pups can be re-established by the presence of the mother, due to her ability to suppress her pups' corticosterone release and amygdala activity. This suggests that developmental changes in stress responsiveness and the stimuli that produce a stress response might be critically involved in optimally adapting the pup's attachment system to its respective ecological niche.

Early life trauma and attachment: immediate and enduring effects on neurobehavioral and stress axis development

Over half a century of converging clinical and animal research indicates that early life experiences induce enduring neuroplasticity of the HPA-axis and the developing brain. This experience-induced neuroplasticity is due to alterations in the frequency and intensity of stimulation of pups' sensory systems (i.e., olfactory, somatosensory, gustatory) embedded in mother-infant interactions. This stimulation provides "hidden regulators" of pups' behavioral, physiological, and neural responses that have both immediate and enduring consequences, including those involving the stress response. While variation in stimulation can produce individual differences and adaptive behaviors, pathological early life experiences can induce maladaptive behaviors, initiate a pathway to pathology, and increase risk for later-life psychopathologies, such as mood and affective disorders, suggesting that infant-attachment relationships program later-life neurobehavioral function. Recent evidence suggests that the effects of maternal presence or absence during this sensory stimulation provide a major modulatory role in neural and endocrine system responses, which have minimal impact on pups' immediate neurobehavior but a robust impact on neurobehavioral development. This concept is reviewed here using two complementary rodent models of infant trauma within attachment: infant paired-odor-shock conditioning (mimicking maternal odor attachment learning) and rearing with an abusive mother that converge in producing a similar behavioral phenotype in later-life including depressive-like behavior as well as disrupted HPA-axis and amygdala function. The importance of maternal social presence on pups' immediate and enduring brain and behavior suggests unique processing of sensory stimuli in early life that could provide insight into the development of novel strategies for prevention and therapeutic interventions for trauma experienced with the abusive caregiver.

Diet manipulation affects social behavior of catfish : Importance of body odor

Diet manipulation, an habituation test, and chemical analysis of urinary free amino acids were used to demonstrate that bullhead catfish (Ictalurus nebulosus) naturally detect the body odors of conspecifics and respond to them in a predictable fashion. These signals are used in dominance and territorial relationships and lead to increased aggression toward chemical "strangers." The results support the general notion that nonspecific metabolites, as well as specific pheromones, are important in chemical mediation of social behavior.

Citronellal ingestion decreases the appeal of male mouse urinary pheromone for female mice

To determine whether ingestion of citronellal decreases the attractive power of the male mouse urinary odor, female mice were used in preference tests. A series of tests revealed that the female mice preferred voided urine odors from aged mice over those from younger adult mice. However, exogenous citronellal directly inhibited the advantage of the aged males with regard to attraction.

The development and neurobiology of infant attachment and fear

Survival of altricial infants depends on attachment to the caregiver - a process that requires infants to identify, learn, remember, and approach their attachment figure. Here we review the neurobiology of attachment in infant rats where learning about the caregiver is supported by a specialized attachment neural circuitry to promote the infant-caregiver relationship. Specifically, the attachment circuit relies on infants acquiring learned preferences to the maternal odor, and this behavior is supported by the hyperfunctioning locus coeruleus and generous amounts of norepinephrine to produce experience-induced changes in the olfactory bulb and anterior piriform cortex. Infants also possess a reduced ability to acquire learned aversions or fear, and this behavior is facilitated through attenuated amygdala plasticity to block fear learning. Presumably, this attachment circuitry constrains the infant animal to express only learned preferences regardless of the quality of care received. As pups mature, and begin to travel in and out of the nest, the specialized attachment learning becomes contextually confined to when pups are with the mother. Thus, when outside the nest, these older pups show learning more typical of adult learning, presumably to prepare for independent life outside the nest. The quality of attachment can alter this circuitry, with early life stress prematurely terminating the pups' access to the attachment system through premature functional activation of the amygdala. Overall, the attachment circuit appears to have a dual function: to keep pups close to the caregiver but also to shape pups' behavior to match the environment and define long-term emotion and cognition.

Alteration of mouse urinary odor by ingestion of the xenobiotic monoterpene citronellal

Body odors provide a rich source of sensory information for other animals. There is considerable evidence to suggest that short-term fluctuations in body odor can be caused by diet; however, few, if any, previous studies have demonstrated that specific compounds can directly mask or alter mouse urinary odor when ingested and thus alter another animal's behavior. To investigate whether the ingestion of citronellal, a monoterpene aldehyde that produces an intense aroma detected by both humans and mice, can alter mouse urinary odor, mice (C57BL6J) were trained in a Y maze to discriminate between the urinary odors of male donor mice that had ingested either citronellal in aqueous solution or a control solution. Trained mice could discriminate between urinary odors from the citronellal ingestion and control groups. A series of generalization tests revealed that citronellal ingestion directly altered mouse urinary odor. Moreover, trained mice that had successfully discriminated between urinary odors from donor mice of different ages failed to detect age-related changes in urine from male mice that had ingested 50 ppm of citronellal. This study is the first to show that ingestion of a xenobiotic can alter mouse urinary odor and confuse the behavioral responses of trained mice to age-related scents.

Defining age limits of the sensitive period for attachment learning in rat pups

Enhanced odor preference learning and attenuated fear learning characterizes rat pups' attachment learning Sensitive Period for learning the maternal odor. This period terminates at 10 days old (PN10) with increasing endogenous levels of the stress hormone, corticosterone. Increasing Sensitive Period pups' corticosterone prematurely terminates the Sensitive Period, while decreasing corticosterone in older pups delays Sensitive Period termination. Here we extend these findings and define the age range corticosterone alters learning and question whether corticosterone permanently terminates the Sensitive Period. Pups were odor-0.5 mA shock conditioned with either corticosterone increased (PN5-6; 4 mg/kg vs. saline) or decreased (PN15-16; naturally by maternal presence or corticosterone synthesis blocker, Metyrapone). Finally, PN7-8 pups were conditioned with corticosterone and reconditioned without corticosterone to assess whether the Sensitive Period was permanently terminated. Results indicate developmental limits for corticosterone regulation of pup learning are PN6 through PN15. Furthermore, inducing precocious corticosterone induced fear learning was not permanent, since reconditioning without corticosterone enabled odor preference learning. Results suggest pups are protected from learning aversions to maternal odor until approaching weaning.

Developing a neurobehavioral animal model of infant attachment to an abusive caregiver

BACKGROUND

Both abused and well cared for infants show attachment to their caregivers, although the quality of that attachment differs. Moreover, the infant's attachment to the abusive caregiver is associated with compromised mental health, especially under stress. In an attempt to better understand how abuse by the caregiver can compromise mental health, we explore the neural basis of attachment in both typical and abusive environments using infant rats, which form attachments to the mother through learning her odor. Here, we hypothesize that the neural circuitry for infant attachment differs based on the quality of the attachment, which can be uncovered during stressful situations.

METHODS

We used infant rats to compare infant attachment social behaviors and supporting neurobiology using natural maternal odor, as well as two odor-learning attachment paradigms: odor-stroke (mimics typical attachment) and odor-.5 mA shock conditioning (mimics abusive attachment). Next, to uncover differences in behavior and brain, these pups were injected with systemic corticosterone. Finally, pups were reared with an abusive mother to determine ecological relevance.

RESULTS

Our results suggest that the natural and learned attachment odors indistinguishably control social behavior in infancy (approach to the odor and interactions with the mother). However, with corticosterone injection, pups with an abusive attachment show disrupted infant social behavior with the mother and engagement of the amygdala.

CONCLUSIONS

This animal model of attachment accommodates both abusive and typical attachment and suggests that pups' social behavior and underlying neural circuitry may provide clues to understanding attachment in children with various conditions of care.

Transitions in sensitive period attachment learning in infancy: the role of corticosterone

Survival of altricial infants, including humans and rats, depends on attachment to the caregiver - a process that requires infants to recognize, learn, and remember their attachment figure. The demands of a dynamic environment combined with a maturing organism require frequent neurobehavioral reorganization. This restructuring of behavior and its supporting neural circuitry can be viewed through the unique lens of attachment learning in rats in which preference learning is enhanced and aversion learning is attenuated. Behavioral restructuring is well adapted to securing the crucial infant-caregiver relationship regardless of the quality of care. With maturation and the end of the infant-caregiver attachment learning period, the complex interplay of neural structures, hormones, and social behavior coordinates the developing rat's eventual transition to life outside of the nest. Nevertheless, early-life environmental and physiological stressors can alter the resilient nature of this system, particularly with respect to the amygdala, and these changes may provide important clues to understanding the lasting effects of early stress.

Genetically-based olfactory signatures persist despite dietary variation

Individual mice have a unique odor, or odortype, that facilitates individual recognition. Odortypes, like other phenotypes, can be influenced by genetic and environmental variation. The genetic influence derives in part from genes of the major histocompatibility complex (MHC). A major environmental influence is diet, which could obscure the genetic contribution to odortype. Because odortype stability is a prerequisite for individual recognition under normal behavioral conditions, we investigated whether MHC-determined urinary odortypes of inbred mice can be identified in the face of large diet-induced variation. Mice trained to discriminate urines from panels of mice that differed both in diet and MHC type found the diet odor more salient in generalization trials. Nevertheless, when mice were trained to discriminate mice with only MHC differences (but on the same diet), they recognized the MHC difference when tested with urines from mice on a different diet. This indicates that MHC odor profiles remain despite large dietary variation. Chemical analyses of urinary volatile organic compounds (VOCs) extracted by solid phase microextraction (SPME) and analyzed by gas chromatography/mass spectrometry (GC/MS) are consistent with this inference. Although diet influenced VOC variation more than MHC, with algorithmic training (supervised classification) MHC types could be accurately discriminated across different diets. Thus, although there are clear diet effects on urinary volatile profiles, they do not obscure MHC effects.

Effect of dietary cues on kin discrimination of juvenile Atlantic salmon (Salmo salar) and brook trout (Salvelinus fontinalis)

Several salmonid species can discriminate kin from non-kin using recognition cues. These are known to involve both genetic and environmental components. This study examined the effect of diet, an environmental cue, on kin discrimination in juvenile Atlantic salmon ( Salmo salar L., 1758) and brook trout ( Salvelinus fontinalis (Mitchill, 1814)). Juveniles were reared in kin and non-kin groups. Three different diets were fed to three kin groups and three non-kin groups. Juveniles were then tested for their ability to discriminate kin. We found that juveniles discriminated individuals between kin and non-kin when they shared a common diet with kin, but did not do so when kin were fed a different diet. Moreover, we predicted that diet and genotype together would provide stronger cues than sharing either dietary cues or genetic cues. When the juveniles were given a choice between kin fed the same diet and kin fed a different diet, they did not show a consistent preference for the former. However, in the absence of kinship, juveniles preferred cues from a common diet, i.e., they preferred cue water conditioned by non-kin sharing a common diet than cue water from non-kin fed a different diet. Our results show that dietary cues affect kin discrimination in juvenile Atlantic salmon and brook trout and provide the first empirical evidence for the influence of diet on kin discrimination in salmonids. The functional explanation of these results is that the preferences in kin discrimination are likely context dependent.

Ontogenetic development of the mammalian circadian system

This review summarizes the current knowledge about the ontogenetic development of the circadian system in mammals. The developmental changes of overt rhythms are discussed, although the main focus of the review is the underlying neuronal and molecular mechanisms. In addition, the review describes ontogenetic development, not only as a process of morpho-functional maturation. The need of repeated adaptations and readaptations due to changing developmental stage and environmental conditions is also considered. The review analyzes mainly rodent data, obtained from the literature and from the author's own studies. Results from other species, including humans, are presented to demonstrate common features and species-dependent differences. The review first describes the development of the suprachiasmatic nuclei as the central pacemaker system and shows that intrinsic circadian rhythms are already generated in the mammalian fetus. As in adult organisms, the period length is different from 24 h and needs continuous correction by environmental periodicities, or zeitgebers. The investigation of the ontogenetic development of the mechanisms of entrainment reveals that, at prenatal and early postnatal stages, non-photic cues deriving from the mother are effective. Light-dark entrainment develops later. At a certain age, both photic and non-photic zeitgebers may act in parallel, even though the respective time information is 12 h out of phase. That leads to a temporary internal desynchronization. Because rhythmic information needs to be transferred to effector organs, the corresponding neural and humoral signalling pathways are also briefly described. Finally, to be able to transform a rhythmic signal into an overt rhythm, the corresponding effector organs must be functionally mature. As many of these organs are able to generate their own intrinsic rhythms, another aspect of the review is dedicated to the development of peripheral oscillators and mechanisms of their entrainment. The latter includes control by the central pacemaker as well as by distinct environmental signals. Ecological aspects of the described developmental changes in the circadian system and some practical consequences are also briefly discussed.

Postnatal development of dendritic spines on olfactory bulb granule cells in rats

Postnatal morphological changes in granule cell dendritic spines and filopodia (collectively referred to as "spines/filopodia") were examined in the rat main olfactory bulb to characterize the development of the neural circuitry for olfaction. Granule cells were labeled with a membrane dye and confocal laser scanning microscope images of labeled spines/filopodia were acquired in the following three dendritic domains: apical dendrites in the external plexiform layer, those in the granule cell layer, and basal dendrites. In all three domains the proportion of typical spines slightly increased during development, with a concomitant decrease in the proportion of "stubby" spines lacking a neck; the proportion of filopodia remained unchanged, accounting for 20-40% of all protrusions. The mean diameter and length of the spine/filopodium population were nearly constant throughout development. On the other hand, the developmental pattern of the spine/filopodium density varied markedly, depending on the domain of the dendrites. In the external plexiform layer, the density did not change remarkably during development. The density in apical dendrites in the granule cell layer increased during the initial 2 postnatal weeks, then gradually decreased. The spine/filopodium density in basal dendrites, however, continued to increase until 4 weeks of age, and then began to decrease. These results suggest that a substantial amount of input-specific synaptic remodeling occurs in granule cells during development, which proceeds from superficial dendritic domains to deeper ones, occurring most prominently in the basal dendrites.

Unique Characteristics of Neonatal Classical Conditioning: The Role of the Amygdala and Locus Coeruleus

The central nervous system of altricial infants is specialized for optimizing attachments to their caregiver. During the first postnatal days, infant rats show a sensitive period for learning and are particularly susceptible to learning an attraction to their mother's odor. Classical conditioning appears to underlie this learning that is expressed behaviorally as an increased ability to acquire odor preferences and a decreased ability to acquire odor aversions. Specifically, in neonatal rats, pairing an odor with moderately painful shock (0.5mA) or milk produces a subsequent relative preference for that odor. The neural circuitry supporting the increased ability to acquire odor preferences appears to be the heightened functioning of the noradrenergic pontine nucleus locus coeruleus. Indeed, norepinephrine from the locus coeruleus appears to be both necessary and sufficient for learning during the sensitive period. On the other hand, the decreased ability to acquire odor aversions seems to be due to the lack of participation of the amygdala in at least some aversive learning situations. The site of plasticity in the pup's brain appears to be limited to the olfactory bulb. This neonatal sensitive period for learning ends around postnatal day 9-10, at which time pups make the transition from crawling to walking and classical conditioning becomes "adultlike." The neonatal behavioral and neural induced changes are retained into adulthood where it modifies sexual behavior.

The mechanisms of kin discrimination and the evolution of kin recognition in vertebrates: a critical re-evaluation

I re-examine the four most widely proposed mechanisms of kin discrimination among vertebrates and conclude that the current categorization of kin discrimination mechanisms has been counterproductive because it has a hindered a clear understanding of the basic mechanisms by which animals discriminate kin. I suggest that there likely is only one authentic mechanism of kin discrimination and that this mechanism is learning, particularly associative learning and habituation. Observed differences in the way animals discriminate between kin and non-kin are due only to the cues (e.g., individually-distinctive, family-distinctive, or self) that are used, and not to different mechanisms per se. I also consider whether kin discrimination is mediated by specially evolved kin recognition systems, defined as neural mechanisms that allow animals to directly classify conspecifics as either kin or non-kin. A preliminary analysis of vertebrate recognition systems suggests that specialized neural, endocrine, and developmental mechanisms specifically for recognizing kin have not evolved. Rather, kin discrimination results from an extension of other, non-specialized sensory and cognitive abilities of animals, and may be derived from other forms of social recognition, such as individual, group, or species recognition.

Parent-offspring discrimination in the prairie vole and the effects of odors and diet

The objectives of this study on the prairie vole (Microtus ochrogaster) were to determine whether (i) parents are able to distinguish their own young from alien young, (ii) conspecific odors influence parent-offspring discrimination, and (iii) diet affects the cues used in parent-offspring discrimination. Parent-offspring discrimination was inferred from differential behaviors directed by adults towards their own offspring and alien offspring. In experiment 1, parent-offspring dyads showed high frequencies of cohesive behaviors and low frequencies of agonistic behaviors, while dyads of adults and alien offspring showed significantly more agonistic and fewer cohesive behaviors. In experiment 2, dyads of parents with their own offspring were tested in an arena containing their own soiled shavings, soiled shavings from another family, or clean shavings. Dyads engaged in significantly fewer investigatory behaviors and more cohesive behaviors when tested with their own familiar odors than when tested with unfamiliar odors or odors from clean shavings. In experiment 3, we tested dyads of unrelated adults and young that had been fed either the same or different dietary supplements. There were no significant differences in the frequencies of investigatory, cohesive, or agonistic behaviors in dyads fed the same diet and those fed a different diet.

Hypothalamic-pituitary-adrenal responses to brief social separation

Involuntary separation from close social companions is widely held to lead to pathophysiological outcomes. Presumably, the relationship with, or category of, the separated individual determines the nature of the physiological response. Here, experiments examining the consequences of brief involuntary separation on the activity of the stress-responsive hypothalamic-pituitary-adrenal (HPA) system are reviewed. Only those studies designed specifically to assess the effect of the absence of the social partner are considered. Evidence for HPA activation in response to social separation has been obtained in a number of species; yet, many studies find no effect of separation of affiliative partners on HPA activity. The occurrence of an HPA response to separation does not appear to be related to the phylogenetic position or cognitive capacity of the species studied, nor is it a universal response to mother-infant separation. Rather, it is suggested that the pattern of results can be largely understood in the context of attachment. Separation of partners exhibiting signs of emotional attachment leads to an immediate and persistent HPA response, whereas separation of partners that are affiliative, but do not exhibit attachment, has little or no effect on HPA activity.

A learned odor evokes an enhanced Fos-like glomerular response in the olfactory bulb of young rats

Young rats exposed to peppermint odor and reinforcing tactile stimulation from postnatal days (PND) 1-18 increase their preference for that odor relative to controls. This early olfactory memory is accompanied by an 80% increase in the density of glomerular-layer cells displaying Fos-like immunoreactivity in response to the learned odor on PND 19. The difference is observed in midlateral portions of the olfactory bulb that align with foci of 2-deoxyglucose (2-DG) uptake in adjacent sections. Trained and control animals are not different in the Fos-like response of juxtaglomerular cells within ventrolateral 2-DG foci. Ratios of midlateral/ventrolateral response differ significantly between trained and control animals and include differences among cells of three staining intensities. These ratios are correlated with ratios of 2-DG uptake (midlateral/ventrolateral foci), which also differ significantly between trained and control rats. Juxtaglomerular cells associated with 2-DG foci also express Egr-1-like immunoreactivity. However, the midlateral Egr-1 response does not differ between trained and control rats. These results show that early memories can be associated with an increased Fos-like response in a primary sensory area of the CNS. They also suggest that only specific regions within the olfactory bulb are modified following the learning of a given odor in early life.

What is the role of the immune system in determining individually distinct body odours?

Genetically inbred mice and rats which are identical except for the genes of the major histocompatibility complex (MHC) produce unique urinary odours which can be discriminated by other animals. Congenic strains differing in both the Class I and Class II regions of the MHC produce distinct urinary odours. These urine odours can be used for mate selection and parental recognition, and it has been suggested that they provide a unique genetic mechanism for kin recognition. However, the non-MHC genes and the X and Y chromosomes also modulate the urinary odours of rodents, and rearing rats in a bacteria-free environment inhibits the production of unique MHC-related odours. We have found that dietary differences produce a greater effect on individual odours than differences at one MHC locus. These results suggest that the MHC, commensal bacteria, and dietary products interact to produce urinary odours which can be used for individual recognition in rodents. The problem is: what is the role of the immune system in determining individually distinct body odours? A model that suggests possible answers to this question is proposed.

Neurobiology of associative learning in the neonate: early olfactory learning

Mammalian neonates have been simultaneously described as having particularly poor memory, as evidenced by infantile amnesia, and as being particularly excellent learners with unusually plastic nervous systems that are easily influenced by experience. An understanding of the neurobiological constraints and mechanisms of early learning may contribute to a unified explanation of these two disparate views. Toward that end, we review here our work on the neurobiology of learning and memory in neonates. Specifically, we have examined the neurobiology of early learning using an olfactory classical conditioning paradigm. Olfactory classical conditioning in neonates at the behavioral level conforms well with the requirements and outcomes of classical conditioning described in adults. Furthermore, specific neural correlates of this behavioral conditioning have been described including anatomical and physiological changes, neural pathways, and modulatory systems. In this Review, we outline the behavioral paradigm, the identified neural correlates, and apparent mechanisms of this learning. Finally, we compare the neurobiology of early learning with that reported for mature animals, with specific reference to the role of US-CS convergence, memory modulation, consolidation, and distributed memory.

The locus coeruleus, norepinephrine, and memory in newborns

Use of learned odor cues by newborn rats is critical for pup survival. Rat pups acquire approach responses to maternal odors through an associative conditioning mechanism. This learned behavioral response is accompanied by a modification of olfactory bulb neural response patterns to the learned odor. Both the behavioral and neural response changes involved and require norepinephrine release in the olfactory bulb. The source of this norepinephrine is the locus coeruleus. It is proposed that the unique response properties of the locus coeruleus during the early postnatal period in the rat may facilitate acquisition of these critical early memories.

The effects of olfactory experience on nest odor preferences and pup retrieval in rats

Previous studies have shown that maternal rats are attracted to their own bedding and that of other dams but not to that of virgins. The present studies were designed to test two hypotheses: First, that the addition of a novel botanical odor to the bedding of maternal animals would result in subsequent preferences for such an odor. Second, such a preference should enable dams to discriminate between pups odorized with familiar and unfamiliar odors as measured by differential retrieval of the pups. Both hypotheses were confirmed. In addition, we found that preferences established during a first pregnancy and lactation affected the retrieval of pups during a second lactational period, but could be overturned by new olfactory experience during that second lactational period.

Neuroethology of olfactory preference development

Young mammals come to approach the odor of their mother, a response that facilitates their survival during early life. Young rats induce a cascade of events in their mother to induce the emission of her odor. The pups increase circulating prolactin levels, which increases food intake and the emission of large quantities of cecotrophe containing the maternal odor. This odor is synthesized by the action of cecal microorganisms and changes with maternal diet. The diet-dependence of the odor requires the pups to acquire their attraction to the odor postnatally. The acquisition of this preference occurs when an odor is paired with the tactile stimulation that pups receive during maternal care. The action of the tactile stimulation appears to be mediated by noradrenaline. The development of this type of olfactory attraction is accompanied by changes in the regions of the olfactory bulb that are responsive to the attractive odor. Metabolic, anatomical, and neurophysiological changes in response to the attractive odor emerge in such regions of the bulb after early olfactory preference training.

Individual olfactory signatures as major determinants of early maternal discrimination in sheep

Responses of recently parturient ewes to their familiar versus separated (i.e., unfamiliar) twin lambs were observed in an attempt to elucidate further the characteristics of the phenotypic traits (signatures) mediating maternal recognition and bonding. Ewes responded more positively to their familiar lamb than to its twin that had been isolated at birth. Nonetheless, those same mothers also discriminated between their separated twin and unfamiliar alien lambs. Alien lambs elicited similar rejection behavior regardless of whether they had been housed with their own mother or in isolation prior to testing. Lambs appear to have individually distinct (olfactory) signatures; however, the signatures of twin siblings may be sufficiently similar to enable the mother to detect a resemblance. No evidence suggests that acquired maternal labels play a role in either the acceptance of the ewe's own lamb(s) or rejection of aliens.

Modified behavioral and olfactory bulb responses to maternal odors in preweanling rats

Rat pups acquire an attraction for maternal odors, which can vary with maternal diet. In the two experiments reported here, maternal diet was modified and both pup behavioral responses and pup olfactory bulb neural responses [( 14C]2-DG uptake) to maternal odors were examined. In experiment 1, pups were reared from birth to postnatal day 19 with either a dam fed normal rat chow or a dam fed a sucrose-based diet which suppressed her normal maternal odor. In experiment 2, pups were raised from birth to postnatal day 19 with either a dam fed the sucrose-based diet adulterated with peppermint, or the non-scented sucrose-based diet. Pups selectively expressed both a behavioral attraction and an enhanced olfactory bulb neural response to odors that they experienced in the nest.

Role of the main olfactory system in recognition between individual spiny mice

Recognition between spiny mice requires a period of exposure to learn the olfactory cues of individual conspecifics that can serve as recognition signatures in subsequent discrimination. Animals received nasal irrigations of zinc sulfate to disrupt sensory input to the main olfactory system (MOS) either prior to the 9-day exposure period (Experiment 1) or immediately after the exposure period (Experiment 2). Animals rendered anosmic by zinc sulfate did not show evidence of recognition as defined by huddling preferences. In contrast, animals who received irrigations of saline were able to preferentially huddle with their cagemate who they had been housed with during the exposure period. The results suggest that the MOS is necessary in mediating behavioral discrimination between conspecifics.

Sensitive period for neural and behavioral response development to learned odors

Olfactory preference training early in life produces both a behavioral preference and an enhanced uptake of 2-deoxy-[14C]glucose (2-DG) in specific areas of the olfactory bulb glomerular layer. We now describe a sensitive period during the first week after birth for the development both of the enhanced neural response and the behavioral preference.

Single-unit analysis of postnatal olfactory learning: modified olfactory bulb output response patterns to learned attractive odors

Neonatal rats learn to approach odors associated with stimulation normally provided by their mother. The present report describes changes in olfactory bulb single-unit activity following olfactory learning in young rats. Rat pups were exposed from postnatal day 1 to 18 to either (1) peppermint-scented air while receiving tactile stimulation (Pepp-Stroked), (2) peppermint-scented air with no tactile stimulation (Pepp-Only), (3) clean air and tactile stimulation (Stroked-Only), or (4) clean air and no tactile stimulation (Naive). On day 19, single-unit activity was recorded from mitral/tufted cells in urethane-anesthetized, freely breathing pups in response to either peppermint or a novel orange odor. Mitral/tufted cell response patterns to peppermint were significantly altered in Pepp-Stroked animals compared to control pups. Peppermint exposure alone, not associated with tactile stimulation (Pepp-Only), did not affect subsequent single-cell response patterns to that odor. In addition, the modification of response patterns was specific to peppermint and was not associated with a change in respiration rate. Furthermore, Pepp-Stroked pups had a relative behavioral preference for peppermint on day 19 compared to control pups. These results demonstrate that postnatal olfactory learning selectively modifies the subsequent response patterns of olfactory bulb output cells to the attractive odor. Furthermore, these results indicate that the initial coding of an odor's attractive value occurs within the olfactory bulb.

Localized changes in olfactory bulb morphology associated with early olfactory learning

Young rats exposed to an odor while receiving reinforcing stimulation come to approach that odor upon subsequent presentation. In addition, such pups have increased 14C-2-deoxyglucose (2DG) uptake within focal areas of the glomerular layer in response to that odor, compared to control animals experiencing the odor for the first time. In this study, the morphology of the glomerular areas underlying these 2DG foci was examined to determine whether early olfactory learning imposed local structural changes that could produce the enhanced 2DG uptake. Alternate sections either were processed with a silver and a Nissl stain to label both cell bodies and their processes or were histochemically treated for the mitochondrial enzymes cytochrome oxidase (CO) or succinic dehydrogenase (SDH) to define the glomerular core of the bulb; 2DG autoradiographs were aligned with adjacent stained sections, and regions underlying the high 2DG uptake foci were examined. In odor-familiar animals, large glomerular clusters that protruded into the external plexiform layer or the olfactory nerve layer were associated with the focal areas of increased 2DG uptake. Morphometric analysis of these regions revealed that the glomerular layer underlying the foci of high 2DG uptake was 30% wider in odor-familiar animals than comparable areas in odor-unfamiliar animals; the cross-sectional areas of individual glomeruli were 21% larger in odor-familiar animals. The foci of enhanced 2DG uptake therefore appear to be associated with groups of enlarged glomeruli. These data demonstrate that early olfactory learning influences the morphology of the olfactory bulb.