Olfactory mechanisms in the control of maternal aggression, appetite, and fearfulness: Effects of lesions to olfactory receptors, mediodorsal thalamic nucleus, and insular prefrontal cortex

Hypothalamic connectivities and self-evaluated aggression in young adults

Introduction

The hypothalamus plays a pivotal role in supporting motivated behavior, including aggression. Previous work suggested differential roles of the medial hypothalamus (MH) and lateral hypothalamus (LH) in aggressive behaviors, but little is known about how their resting-state functional connectivity (rsFC) may relate to aggression in humans.

Methods

We employed the data from the Human Connectome Project (HCP) and examined the rsFC's of LH and MH in 745 young adults (393 women). We also explored sex differences in the rsFC's. We processed the imaging data with published routines and evaluated the results of voxel-wise regression on aggression score, as obtained from Achenbach Adult Self Report, with a corrected threshold.

Results

The analysis revealed significant rsFC between the LH and clusters in the middle temporal and occipital gyri across all subjects and in the thalamus for men, both in negative correlation with aggression score. Slope test confirmed sex differences in the correlation between LH-thalamus rsFC and aggression score. No significant rsFC was observed for MH.

Conclusions

These findings suggest a role of LH rsFCs and sex differences in LH-thalamus rsFC in the manifestation of aggression in humans. The findings highlight the need for further research into sex-specific neural pathways in aggression and other related behavioral traits of importance to mental health.

Enhanced maternal behaviors in a mouse model of congenital blindness

In mammals, mothering is one of the most important prosocial female behavior to promote survival, proper sensorimotor, and emotional development of the offspring. Different intrinsic and extrinsic factors can initiate and maintain these behaviors, such as hormonal, cerebral, and sensory changes. Infant cues also stimulate multisensory systems and orchestrate complex maternal responsiveness. To understand the maternal behavior driven by complex sensory interactions, it is necessary to comprehend the individual sensory systems by taking out other senses. An excellent model for investigating sensory regulation of maternal behavior is a murine model of congenital blindness, the ZRDBA mice, where both an anophthalmic and sighted mice are generated from the same litter. Therefore, this study aims to assess whether visual inputs are essential to driving maternal behaviors in mice. Maternal behaviors were assessed using three behavioral tests, including the pup retrieval test, the home cage maternal behavior test, and the maternal aggression test. Our results show that blind mothers (1) took less time to retrieve their offspring inside the nest, (2) spent more time nursing and licking their offspring in the second- and third-week postpartum, and (3) exhibited faster aggressive behaviors when exposed to an intruder male, compared to the sighted counterparts. This study provides evidence that congenitally blind mothers show more motivation to retrieve the pups, care, and protection towards their pups than sighted ones, likely due to a phenomenon of sensory compensation.

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.

Mother-Young Bonding: Neurobiological Aspects and Maternal Biochemical Signaling in Altricial Domesticated Mammals

Mother-young bonding is a type of early learning where the female and their newborn recognize each other through a series of neurobiological mechanisms and neurotransmitters that establish a behavioral preference for filial individuals. This process is essential to promote their welfare by providing maternal care, particularly in altricial species, animals that require extended parental care due to their limited neurodevelopment at birth. Olfactory, auditory, tactile, and visual stimuli trigger the neural integration of multimodal sensory and conditioned affective associations in mammals. This review aims to discuss the neurobiological aspects of bonding processes in altricial mammals, with a focus on the brain structures and neurotransmitters involved and how these influence the signaling during the first days of the life of newborns.

Incentive value of newborn pups relative to juveniles for mother rats raising overlapping litters

In rats, successful mating during the postpartum estrus results in the temporal overlapping of successive litters within the maternal nest. Mothers with two overlapping-litters (OLM) simultaneously take care of neonate and juvenile pups; however, they mostly direct their attention to the neonates. We hypothesized that these differences reflect an adaptation to the specific characteristics and needs of the two litters and not a lack of interest in the juveniles. To test this hypothesis, we assessed the relative incentive value of newborns and juveniles for OLM in a preference test and compared it with that exhibited by mothers in early (EPM) and late (LPM) postpartum, which were raising only newborns or only juveniles, respectively. Results showed that OLM spent similar time in the newborns and juveniles compartments and did not prefer the newborns as did the EPM, however, similarly to them, OLM made more attempts to get access to the newborns than the juveniles. On the other hand, OLM and LPM did not exhibit a clear preference between the stimuli. These results indicate that both neonates and juveniles have incentive value for OLM, although these mothers invest more effort in the newborns. These results point out to a unique behavioral profile of OLM, which shows similarities with EPM and LPM on different behavioral measures. They also support the idea that motivational processes underlying maternal behavior are complex and dynamic, adapting the response of the mother to pups' needs and the context.

The role of social relationships in the link between olfactory dysfunction and mortality

Recent work suggests that olfactory dysfunction is a strong predictor of five-year mortality in older adults. Based on past work showing: 1) that olfactory dysfunction impairs social functioning and 2) that social ties are linked with mortality, the current work explored whether impairments in social life mediated the relationship between olfactory dysfunction and mortality. Additionally, based on work showing gender differences in the social consequences of olfactory dysfunction, gender was assessed as a potential moderator of this association. Social network size mediated the olfactory-mortality link for females. To probe what feature of social networks was driving this effect, we investigated two subcomponents of social life: emotional closeness (e.g., perceived social support, loneliness) and physical closeness (e.g., physical contact, in-person socializing with others). Physical closeness significantly mediated the olfactory-mortality link for females, even after controlling for social network size. Emotional closeness did not mediate this link. Possible mechanisms underlying this relationship are discussed.

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.

Affective changes during the postpartum period: Influences of genetic and experiential factors

This article is part of a Special Issue "Parental Care". The postpartum period involves some truly transformational changes in females' socioemotional behaviors. For most female laboratory rodents and women, these changes include an improvement in their affective state, which has positive consequences for their ability to sensitively care for their offspring. There is heterogeneity among females in the likelihood of this positive affective change, though, and some women experience elevated anxiety or depression (or in rodents anxiety- or depression-related behaviors) after giving birth. We aim to contribute to the understanding of this heterogeneity in maternal affectivity by reviewing selected components of the scientific literatures on laboratory rodents and humans examining how mothers' physical contact with her infants, genetics, history of anxiety and depression and early-life and recent-life experiences contribute to individual differences in postpartum affective states. These studies together indicate that multiple biological and environmental factors beyond female maternal state shape affective responses during the postpartum period, and probably do so in an interactive manner. Furthermore, the similar capacity of some of these factors to modulate anxiety and depression in human and rodent mothers suggests cross-species conservation of mechanisms regulating postpartum affectivity.

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

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

Sensory, hormonal, and neural basis of maternal aggression in rodents

We review existing knowledge of the neural, hormonal, and sensory basis of maternal aggression in the female rat. Although females may express different kinds of aggression, such as defense or dominance, the most frequent and conspicuous form of aggressive behavior among females is the one associated with motherhood. Maternal aggression occurs in various vertebrate and invertebrate species; however, our emphasis will be on maternal aggression in rats because most of the physiological investigations have been performed in this species. Firstly, we address those factors that predispose the female to attack, such as the endocrine profile, the maternal state, and the stimulation provided by the pups, as well as those that trigger the aggressive response, as the intruder's characteristics and the context. As the postpartum aggression is a fundamental component of the maternal repertoire, we emphasize its association with maternal motivation and the reduction of fear and anxiety in dams. Finally, we outline the neurocircuitry involved in the control of maternal aggression, stressing the role of the ventro-orbital region of prefrontal cortex and the serotoninergic system.

Injections of muscimol into the paraventricular thalamic nucleus, but not mediodorsal thalamic nuclei, induce feeding in rats

The paraventricular thalamic nucleus (PVT) is a component of the midline thalamic group that is interconnected with several brain regions known to play important roles in the control of food intake, including the lateral hypothalamus and nucleus accumbens shell, suggesting that the PVT itself may be involved in mediating feeding behavior. In the current study, we examined whether inhibition of cells in the PVT with the GABA(A) agonist muscimol could alter food intake in non-deprived rats. To control for possible spread of the drug, we also observed food intake after injections of muscimol into the overlying ventricle or laterally adjacent mediodorsal thalamic nuclei (MD). We found that muscimol injections into the central PVT dose-dependently increased food intake. In contrast, intra-MD injections of muscimol resulted in a potent dose-dependent suppression of food intake, while those into the overlying ventricle had no effect. These results support the proposal that the PVT is a component of the neural circuitry controlling feeding behavior.

Social instigation and aggression in postpartum female rats: role of 5-Ht1A and 5-Ht1B receptors in the dorsal raphé nucleus and prefrontal cortex

RATIONALE

5-HT(1A) and 5-HT(1B) receptor agonists effectively reduce aggressive behavior in males that has been escalated by social instigation. Important sites of action for these drugs are the receptors in dorsal raphé nuclei (DRN) and the ventral-orbital prefrontal cortex (VO PFC). DRN and VO PFC areas are particularly relevant in the inhibitory control of escalated aggressive and impulsive behavior.

OBJECTIVES

The objectives of this study are to assess the anti-aggressive effects of 5-HT(1A) (8-OH-DPAT) and 5-HT(1B) (CP-93,129) receptor agonists microinjected into DRN and VO PFC, respectively, and to study the aggressive behavior in postpartum female Wistar rats using the social instigation protocol to increase aggression.

METHODS AND RESULTS

8-OH-DPAT (0.56 μg) in the DRN increased aggressive behavior in postpartum female rats. By contrast, CP-93,129 (1.0 μg) microinjected into VO PFC decreased the number of attack bites and lateral threats. 5-HT(1A) and 5-HT(1B) receptor agonists differed in their effects on non-aggressive activities, the former decreasing rearing and grooming and the latter increasing these acts. When 8-OH-DPAT was microinjected into DRN and CP-93,129 was microinjected into VO PFC in female rats at the same time, maternal aggression decreased. Specific participation of 5-HT(1B) receptors was verified by reversal of the anti-aggressive effects using the selective antagonist SB-224,289 (1.0 μg).

CONCLUSIONS

The decrease in maternal aggressive behavior after microinjections of 5-HT(1B) receptor agonists into the VO PFC and DRN of female postpartum rats that were instigated socially supports the hypothesis that activation of these receptors modulates high levels of aggression in a behaviorally specific manner, due to activation of 5-HT(1B) receptors at the soma and terminals.

Oxytocin modulates unconditioned fear response in lactating dams: an fMRI study

Oxytocinergic neurotransmission during lactation contributes to reduction of anxiety levels and fear. However, our knowledge of where oxytocin acts in the brain to achieve this effect, particularly to an unconditioned fear stimulus, is incomplete. We used blood oxygenation level dependent (BOLD) fMRI to test whether central administration of oxytocin 45-60 min before fMRI scanning alters maternal brain activation in response to a predator scent (TMT, trimethylthiazoline). Comparison behavioral experiments that examined maternal responses to this unconditioned fear-inducing odor were carried out in a separate cohort of lactating rats given similar treatments. Behavioral experiments confirmed the effectiveness of oxytocin at reducing freezing behavior as compared to vehicle controls. Our fMRI findings indicate that oxytocin modulated both positive and negative BOLD responses across several olfactory and forebrain nuclei. Significantly greater percent increases in BOLD signal in response to TMT were observed in the anterior cingulate, bed nucleus of stria terminalis and perirhinal area of oxytocin pretreated rats. These animals also showed significantly larger percent decreases in BOLD in mammillary bodies, secondary motor cortex, gustatory cortex, prelimbic prefrontal cortex, orbital cortex, and the anterior olfactory nucleus. The observed pattern of brain activity suggests that oxytocin enhances neural processing in emotion and cognition driven brain areas such as the cingulate cortex, while dramatically reducing activity in areas also controlling autonomic, visceromotor and skeletomotor responses. The present data contribute to the growing literature suggesting the oxytocin modulate the integration of emotional and cognitive information through myriad brain regions to facilitate decreases in anxiety (even to an unconditioned stimulus) while potentially promoting pair-bonding, social memory and parental care.

Blood oxygen level-dependent signal responses in corticolimbic 'emotions' circuitry of lactating rats facing intruder threat to pups

Lactating rats must continuously maintain a critical balance between caring for pups and aggressively responding to nest threats. We tested the neural response of lactating females to the presentation of their own pups and novel intruder males using blood oxygen level-dependent functional magnetic resonance imaging at 7 T. Dams were presented with a single sequence of a control stimulus, pups or a male intruder in one imaging session (n = 7-9). To further determine the selectivity of neural processing, dams were imaged for their response to a male intruder in both the absence and presence of their pups (n = 6). Several maternal cortical and limbic brain regions were significantly activated by intruder presentation but not by pups or a control stimulus. These included the nucleus accumbens, periaqueductal gray, anterior cingulate, anterior thalamus, basal nucleus of the amygdala, temporal cortex, prelimbic/orbital area and insula. The nucleus accumbens, periaqueductal gray, temporal cortex and mediodorsal thalamus still showed greater neural activity when compared with intruder presentation in the absence of pups. The present results suggest that the high emotional state generated by a threat to pups involves robust activation of classical limbic regions controlling emotional responses. This pattern of blood oxygen level-dependent activity may precede behavioral states upon which lactating rats initiate attacks against a potential threat to offspring.

Central vasopressin V1a receptors modulate neural processing in mothers facing intruder threat to pups

Vasopressin V1a receptors in the rat brain have been studied for their role in modulating aggression and anxiety. In the current study blood-oxygen-level-dependent (BOLD) functional MRI was used to test whether V1a receptors modulate neural processing in the maternal brain when dams are exposed to a male intruder. Primiparous females were given an intracerebroventricular (ICV) injection of vehicle or V1a receptor antagonist ([beta-Mercapto-beta,beta-cyclopentamethylenepropionyl(1), O-me-Tyr(2),Arg(8)]-Vasopressin, [corrected] 125 ng/10 microL) 90-120 min before imaging. During fMRI, awake dams were presented with a male intruder threat to pups using a specialized chamber that contained separate compartments for pups and a male intruder. Our results indicate that the number of activated voxels was reduced in the cortical amygdala with V1a receptor blockade, while an increase was observed in the anterior olfactory nucleus and other areas. Dams treated with V1a antagonist showed significantly greater BOLD responses in the anterior olfactory nucleus, infralimbic prefrontal cortex, gustatory cortex, somatosensory cortex, and substantia innominata when presented with a novel male intruder. BOLD responses were reduced in the cortical amygdala and ventromedial hypothalamus. The V1a receptor sensitive areas play roles in the processing of smell, taste and touch and emotional reactivity. Thus one interpretation of the present fMRI data is that vasopressin, acting through V1a receptors, may modulate sensory processing and perhaps coordinate this effect with changes in visceromotor activity during the initial stages of maternal aggressive motivation and/or anxiogenic responses.

Mother-infant bonding and the evolution of mammalian social relationships

A wide variety of maternal, social and sexual bonding strategies have been described across mammalian species, including humans. Many of the neural and hormonal mechanisms that underpin the formation and maintenance of these bonds demonstrate a considerable degree of evolutionary conservation across a representative range of these species. However, there is also a considerable degree of diversity in both the way these mechanisms are activated and in the behavioural responses that result. In the majority of small-brained mammals (including rodents), the formation of a maternal or partner preference bond requires individual recognition by olfactory cues, activation of neural mechanisms concerned with social reward by these cues and gender-specific hormonal priming for behavioural output. With the evolutionary increase of neocortex seen in monkeys and apes, there has been a corresponding increase in the complexity of social relationships and bonding strategies together with a significant redundancy in hormonal priming for motivated behaviour. Olfactory recognition and olfactory inputs to areas of the brain concerned with social reward are downregulated and recognition is based on integration of multimodal sensory cues requiring an expanded neocortex, particularly the association cortex. This emancipation from olfactory and hormonal determinants of bonding has been succeeded by the increased importance of social learning that is necessitated by living in a complex social world and, especially in humans, a world that is dominated by cultural inheritance.

Brain oxytocin correlates with maternal aggression: link to anxiety

The oxytocinergic system is critically involved in the regulation of maternal behavior, which includes maternal aggression. Because aggression has been linked to anxiety, we investigated the maternal aggression and the role of brain oxytocin in lactating Wistar rats selectively bred for high anxiety-related behavior (HAB) or low anxiety-related behavior (LAB) during the 10 min maternal defense test. HAB dams displayed more maternal aggression against a virgin intruder compared with LAB dams, resulting in more defensive behavior and higher anxiety of HAB-defeated virgins. The different levels of aggression were accompanied by opposite oxytocin release patterns within the paraventricular nucleus (PVN; HAB, increase; LAB, decrease). Furthermore, oxytocin release was higher within the central nucleus of the amygdala (CeA) of HAB dams compared with LABs. A direct correlation between the offensive behavior displayed during the maternal defense test and local oxytocin release was found in both the PVN and CeA. Using retrodialysis, blockade of endogenous oxytocin action by infusion of an oxytocin receptor antagonist (des-Gly-NH2,d(CH2)5[Tyr(Me)2,Thr4]OVT) into the PVN or CeA reduced maternal aggression of HAB dams, whereas infusion of synthetic oxytocin into the PVN tended to increase aggression toward the intruder in LAB dams. There were no significant differences in oxytocin receptor mRNA expression or oxytocin receptor binding between lactating HAB and LAB dams. Therefore, differences in intracerebral release patterns of oxytocin, rather than differences at the level of oxytocin receptors, are critical for the regulation of maternal aggressive behavior.

Reduced anxiety in postpartum rats requires recent physical interactions with pups, but is independent of suckling and peripheral sources of hormones

Changes in emotional behavior occur across the reproductive cycle in female rodents, with reduced anxiety found during the postpartum period, but relatively little is known about factors contributing to this decreased anxiety. Using increased duration of time spent in the open arms of an elevated plus-maze as an indicator of reduced anxiety, it was found in a series of experiments that (1) anxiety is significantly reduced in Long-Evans females during the first week of lactation, but not thereafter, (2) relatively recent contact with pups before testing (within 4 h) is necessary for their reduced anxiety, (3) dams that receive only distal sensory cues from pups for the 4 h prior to testing do not show reduced anxiety, (4) the absence of nipples, and therefore a lack of suckling by pups, has no effect on dams' anxiety, (5) cesarean delivery of pups 2 days prior to expected parturition did not alter later anxiety in dams, (6) hypophysectomy during mid-pregnancy or ovariectomy within 24 h after parturition also did not prevent reduced anxiety in dams, and (7) differences in anxiety between lactating and virgin females are greatest 4-8 min after being placed in the plus-maze. Therefore, exposure to their own peripheral hormones through mid-pregnancy is sufficient to prime female rats to show reduced anxiety, but only if they later have recent physical interaction with pups. Furthermore, because suckling and the peripheral hormones released during suckling appear to be unnecessary, decreased anxiety in maternal rats may instead be regulated by the transient intracerebral release of neuropeptides or neurotransmitters while dams receive other types of tactile inputs from their infants.

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

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

Pups presence eliminates the stress hyporesponsiveness of early lactating females to a psychological stress representing a threat to the pups

Blunted neuroendocrine responses to stress are reported in lactating females after exposure to various stressors. However, many of the stimuli used in these studies have little ethological relevance for maternal protection of the litter in a threatening environment. The question that arises is whether the relevance of the stressor to the infant is critical in the 'gating' of the neuroendocrine response. We hypothesized that the presence of pups with their mothers at the time of exposure to an intruder or a predator odour is an effective way to increase the emotional salience of the psychological stressor, thus eliminating the stress hyporesponsiveness in lactating females. We first compared neuroendocrine responses [corticotropin-releasing factor (CRF) mRNA in the paraventricular nucleus of the hypothalamus (PVN) and central nucleus of the amygdala (CeA), plasma adrenocorticotropic hormone (ACTH) and corticosterone] between early (EL, PPD3-5), late (LL, PPD 15) lactating and virgin (V) females to a male intruder in the home cage. We next investigated the effect of pups' presence at the time of stressor exposure on the magnitude of the hormonal response to a male intruder in the home cage or to a predator odour (fox urine) in a novel environment. In the male intruder paradigm, levels of CRF mRNA expression in the PVN and CeA were lower in LL compared to EL or V females and plasma ACTH and corticosterone secretion was not as elevated in LL compared to EL females. Aggression towards the intruder was high in EL females in the presence of their pups and a positive correlation was found with the integrated ACTH response. Aggression rapidly declined after pup separation (2.5 h or 48 h) or in LL nursing females. In EL females, the presence of the pups with their mothers (EL + pups) at the time of stress significantly increased plasma ACTH and corticosterone responses to either male intruder or predator odour compared to EL females without their pups for 2.5 h or 48 h (EL - pups). Plasma ACTH response to fox urine in EL + pups females was comparable to that of virgin females, suggesting that increasing the salience of emotionally relevant stimuli by keeping the pups present in the cage could eliminate the hyporesponsiveness detected for EL females without their pups. These studies indicate the critical role of the pups in modulating the maternal response to stressors that represent a threat for the litter. We hypothesize that the amygdala, because of its ability to process olfactory stimuli and stimuli with affective properties, might play an essential role in 'gating' the neuroendocrine response to stress during lactation.

Sensory, hormonal, and neural control of maternal aggression in laboratory rodents

Parental animals of many rodent species display fierce and persistent aggression toward unfamiliar conspecifics that appears to protect their often altricial and defenseless young. We herein review studies of the sensory, hormonal, neuroanatomical, and neurochemical mechanisms underlying maternal aggression in laboratory rodents. The relationship between maternal aggression and fearfulness or anxiety is also discussed.

Role of maternal behavior on aggression, fear and anxiety

Concomitant to the expression of maternal behavior, the lactating female develops anxiolysis in the elevated plus maze test, aggression towards intruders and reduced fear in response to a sudden auditory stimulus. This study aims to determine if these behavioral changes are associated with maternal behavior independently of the endocrine status that characterizes gestation, parturition and lactation. To assess this purpose, the behavior of lactating females was compared to that exhibited by maternal and nonmaternal ovariectomized rats untreated with steroid hormones. In contrast with lactating dams, sensitized animals (rats that displayed maternal behavior after a continuous contact with young pups) did not display reduced anxiety in the plus maze test. However, the sensitized females showed behaviors characteristic of lactating rats, such as some components of maternal aggression and reduced fear, though much less intensely than dams. These results suggest that aggression and reduced fear, but not anxiolysis, partially depend on the development of maternal behavior.

Maternal defence as an emotional stressor in female rats: correlation of neuroendocrine and behavioural parameters and involvement of brain oxytocin

In order to study neuroendocrine and behavioural stress responses in female rats post partum we aimed to establish a relevant emotional stressor -- the maternal defence test based on maternal aggression of a lactating resident towards a virgin or lactating intruder approaching the cage. Exposure to maternal defence significantly elevated corticotropin (ACTH) and corticosterone responses of the residents and of virgin or lactating intruders, with an attenuated response in lactating residents and lactating intruders. Exposure to maternal defence increased plasma oxytocin in virgin intruders only. The aggressive behaviour displayed by the residents was directly correlated with the amount of defensive behaviour of the intruder and independent of the intruder's reproductive state. However, the amount of maternal and explorative behaviours displayed by the lactating residents was significantly higher when exposed to a lactating, compared to a virgin, intruder. ACTH responses in lactating residents exposed to virgin intruders were significantly correlated to the amount of offensive (direct correlation) and maternal (inverse correlation) behaviours they displayed. Plasma prolactin concentrations, elevated in lactating compared to virgin rats under basal conditions, were found to be reduced in the lactating residents and intruders in response to exposure to the maternal defence test, whereas it was unchanged in virgin intruders. To test for the involvement of brain oxytocin in neuroendocrine and behavioural responses of the lactating residents an oxytocin receptor antagonist (0.1 microg/5 microL) was infused icv 10 min prior to testing. This treatment increased basal, but not stress-induced, ACTH, corticosterone and oxytocin secretion. Whereas parameters of aggressive behaviour were unchanged, the antagonist reduced signs of maternal behaviour during maternal defence. In summary, the maternal defence test has been characterized as a relevant emotional stressor for female rats which is useful for studying neuroendocrine and emotional responses in females, in particular in the context of reproductive adaptations.

Food availability affects behavior but not circulating gonadal hormones in maternal Belding's ground squirrels

We tested predictions of hypotheses suggesting that the steroid hormones, testosterone (T), progesterone (P), and estradiol (E2), contribute to the energetic regulation of behaviors associated with rearing young in free-living female Belding's ground squirrels (Spermophilus beldingi). We provisioned some female S. beldingi with food rich in fat and calories, and used unprovisioned females as controls. We observed the behavior of females throughout the reproductive cycle, and regularly collected blood samples to measure plasma hormone concentrations. Circulating concentrations of T, P, and E2 were similar in provisioned and unprovisioned females, as were temporal patterns of variation in these hormones. Peaks in rates of nest maintenance and aggressive behavior occurred during gestation and were associated with elevated concentrations of circulating T, P, and E2, raising the possibility that one or more of these hormones mediates behaviors that help females establish maternal nest sites and territories after mating. Temporal patterns of variation in behavior were similar among provisioned and unprovisioned females; however, rates of resting, vigilance, and aggression were higher among provisioned females, whereas unprovisioned females devoted significantly more time to feeding and locomotion. Thus, our data suggest that in maternal S. beldingi, gonadal steroids play a role in mediating behavior associated with raising offspring, but do not facilitate changes in rates of behavior associated with increased energy availability.

Learning of olfactory cues is not necessary for early lamb recognition by the mother

Ewes identify their young through the use of different sensory modalities. Olfactory recognition, which mediates selective acceptance at the udder, is established at 4 h postpartum (pp). Visual and auditory cues are involved in recognition at a distance, which is evident at 12 h pp. This study investigates whether anosmic ewes are able (a) to develop visual and auditory recognition and (b) to restore selective acceptance of their lamb at the udder. Visual and auditory recognition was assessed in anosmic and intact ewes at 12 h and 24 h pp by a test of two choices: their own and an alien lamb. Selectivity at allowing suckling was tested by presenting successively an alien and the familiar lamb at 4 h, 3 days, and 1 month pp. In the two-choice recognition test, at both 12 h and 24 h pp, anosmic as well as intact ewes showed a preference for their familiar lamb. Although anosmic ewes showed no difference in their acceptance of alien and familiar lambs for suckling at 4 h and 3 days pp, they nursed the alien lamb less at 1 month pp and showed more rejection behaviors toward it. Thus, visual, auditory, or both those types of recognition can be rapidly established, independent of olfactory recognition. Moreover, differential behavior of anosmic ewes toward their own versus an alien lamb at the udder at 1 month suggests that vision and audition may compensate to some extent for the loss of olfaction.

Deafferentation of the olfactory bulbs of male rats reduces erection to remote cues from females

If male rats are paired with a receptive, soliciting female rat in a setting that prevents physical contact, but permits auditory, visual, and olfactory contact, some will have erections of the penis--noncontact erections (NCE). Surgical deafferentation of the olfactory bulbs from all the known chemosensory systems of the nasal septum renders rats anosmic, decreases sexual performance in copulation tests with females, and substantially reduces the frequency of NCE. Thus, NCE appear to be primarily regulated though the olfactory perception of volatile chemosensory cues from receptive females.

Intracerebroventricular infusions of morphine, and blockade with naloxone, modify the olfactory preferences for pup odors in lactating rats

Systemic morphine exposure disrupts both maternal behavior (MB) and postpartum aggression, possibly through alterations of olfactory preferences [29]. In the current studies, adult female rats were timed mated and implanted with a unilateral cannula in the lateral ventricle. On day 5 or 6 of lactation, the females were infused with either morphine (2.0 micrograms) or saline (5 microliters) (Experiment 1); or, they were infused with 2.0 micrograms morphine and saline or morphine plus 0.5 micrograms naloxone (Experiment 2). One hour later, they were exposed to bedding soiled by pups, or to clean bedding. The amount of time spent investigating the two bedding types was compared. Morphine produced an aversion to the odor of pups, relative to the saline condition, wherein the females expressed a preference for the odor of pups. Naloxone reversed the effect of the morphine, restoring the preference for pup odors in the females. Thus, morphine disruption of MB may be due to central action on olfactory sensory mechanisms.

Peripheral olfactory deafferentation of the primary olfactory system in rats using ZnSO4 nasal spray with special reference to maternal behavior

A modified method of applying ZnSO4 to the olfactory mucosa is described. Treated rats experienced severe nasal congestion that cleared within 24 h; more persistent morbidity did not occur. Nonpregnant females observed with male intruders 24 h following ZnSO4 showed no alterations in behavior other than a reduction in anogenital sniffing, indicating that they were not hypoactive or irritable. In other experiments, lactating females were observed in a hole-board apparatus; 2 days posttreatment anosmia was confirmed in 80% of bilaterally ZnSO4-treated females by the absence of preference for pup odors. After bilateral but not unilateral ZnSO4 treatment, initially activity scores and nose pokes were equivalent in all groups, but later they both were lower than in controls, probably due to a more rapid habituation to the novel apparatus. We conclude that intranasal ZnSO4 by small-volume spray is a useful experimental tool.

Hormone-dependent aggression in male and female rats: experiential, hormonal, and neural foundations

Hormone-dependent aggression in both male and female rats includes the distinctive behavioral characteristics of piloerection and lateral attack. In males the aggression is dependent on testicular testosterone and is commonly known as intermale aggression. In females, the aggression is most commonly observed as maternal aggression and is dependent on hormones whose identity is only beginning to emerge. The present review examines the experiential events which activate hormone-dependent aggression, the relation of the aggression to gonadal hormones, and the neural structures that participate in its modulation. In males and females, the aggression is activated by cohabitation with a conspecific of the opposite sex, by competitive experience, and by repeated exposure to unfamiliar conspecifics. In the female, the presence of pups also activates aggression. In both males and females, hormones are necessary for the full manifestation of the aggression. The essential hormone appears to be testosterone in males and a combination of testosterone and estradiol in females. The information available suggests the neural control systems for hormone-dependent aggression may be similar in males and females. It is argued that hormone-dependent aggression is behaviorally and biologically homologous in male and female rats.

Peripheral anosmia attenuates female-enhanced aggression in male rats

It is well established that male rats with prior access to sexually active females show enhanced offensive aggression toward unfamiliar male intruders. The present study assessed the importance of the sense of smell for this facilitatory effect. It was found in 2 independent experiments that anosmia, induced peripherally by surgically removing the olfactory epithelium and cutting the olfactory nerves, reduced baseline levels of offensive aggression and significantly attenuated the female-enhanced aggression effect. It was also found that sexual performance of anosmic rats was context-dependent, in that it was more impaired in the homecage environment than in standard observation cages. In contrast to sham-operated males, the experimental animals showed no preference for estrous over anestrous females in a mate choice test. Anosmic males did not appear more fearful than controls, as assessed in a hyponeophagia test, but they showed less exploratory behavior (rearing and head-dipping) in the hole-board test, and less rearing activity in automated activity boxes.

Sex-related differences in dendritic branching of cells in the prefrontal cortex of rats

Abstract Male and female Sprague-Dawley rats were given either gonadectomies or sham surgery on the day of birth. In adulthood they were sacrificed and prepared for Golgi-Cox staining. Using a camera lucida technique, layer II/III pyramidal neurons were drawn in the anterior cingulate cortex areas 1 and 3 and the agranular insular cortex and the number of dendritic branches on the apical and basilar dendrites were then summarized. The results showed that the dendritic arbor of prefrontal cortical cells varied as a function of sex and neonatal gonadectomy. In cingulate 3, castrated males had a smaller arbor on the left relative to normal males, whereas, in cingulate 1 the reverse was found. In addition, females had less apical arbor than males in the cingulate areas, whereas, in the agranular insular cortex, females had greater apical arbor than the males. Furthermore, in this region, gonadectomized groups had less arbor than their respective control groups. Finally, in the agranular insular cortex, the dendritic arbor was greater in the left hemisphere.

Vomeronasal organ removal and female mouse aggression: the role of experience

Removal of the vomeronasal organ (VNX) from female mice had little effect upon fecundity, nest building and pup retrieval. Surprisingly, VNX significantly affected aggression by lactating females. Normally, lactating females will attack and fight intruders placed into the home cage, especially if the intruder is an unfamiliar male. In Experiment 1, we determined that VNX prior to sexual experience totally eliminated this type of aggression. Intact females were highly aggressive, but much more so to unfamiliar than to familiar males. Copulation prior to surgery did not ameliorate the effects of VNX; again, none of the VNX females were aggressive (Experiment 2). In Experiment 3, attacks and fights by previously aggressive primiparous females also were eliminated or significantly suppressed following VNX. Even when the testing occurred later during the postpartum period and for extended periods of time, this same suppression of aggression held following VNX. We conclude that aggression by lactating female mice is under strict mediation by chemosignals detected by the vomeronasal organ.