Fear response of rat pups to a non-aversive social stimulus: Evidence for the involvement of memory processes.

Learning processes in rats during early development are importantly mediated by the mother, which represents the primary source of environmental information. This study aimed to determine whether aversive early experiences can induce the expression of pups' fear responses toward a non-aversive stimulus as a consequence of a memory process. First, we determined pups' fear responses toward an anesthetized female after being exposed to this stimulus or an empty cage together with their mothers from Postnatal Day (PNDs) 1 to 4. Second, we evaluated if the administration of the protein synthesis inhibitor cycloheximide (CHX; 0.2 mg/kg, subcutaneously (sc).) disrupted the reconsolidation processes and abolished the fear response on PND 9. Only female pups previously exposed to the female intruder expressed fear responses toward an anesthetized female on PND 8. CHX administration to female pups immediately after exposure to an anesthetized female on PND 8 suppressed fear responses on PND 9, indicating that the fear expression was the result of a memory process, probably mediated by the mother. These findings demonstrated that early experiences can shape responses to social stimuli in a sex-dependent manner and emphasize the critical role of the mother in influencing fear learning in a social context.

Increase in sexual motivation throughout adolescence in the cycling female rat.

Sexual behavior in the female rat is a highly motivated behavior first displayed during adolescence, a developmental period when neural circuits underlying motivation are not mature. This study characterizes the natural development of sexual motivation and behavior of female rats. We compared the incentive value of the male for mid-adolescent (PNDs:39-43), late adolescent (PNDs:49-53), and adult (PNDs:90-115) cycling females, using a male-female preference task and an ultrasonic vocalization emission test following exposure to a male or female stimulus animal. Furthermore, display of sexual and social behaviors during an interaction with a male or a nonreceptive female was assessed. Mid-adolescent rats exhibited a reduced preference for the male than adults and performed less attempts to access the male. Unlike late adolescent and adult females, mid-adolescent rats did not increase their ultrasonic vocalization emission after interacting with a male relative to a female. Although most of the sexual behavior did not differ between groups, mid-adolescent females showed lower lordosis magnitude and higher levels of play and social investigation during a sexual interaction, giving rise to a unique behavioral profile. Present results indicate that the sexual behavior repertoire is fully displayed by mid-adolescence, but sexual motivation is low and increases into late adolescence.

Sexual behaviour of the female rat during late adolescence: effect of chronic cocaine treatment.

Sexual behaviour is highly motivated and female rats begin to express it during adolescence. The circuitries implicated in the control of motivated behaviours continue to mature during adolescence and seem more sensitive to the effects of psychostimulants such as cocaine. However, a putative differential effect of this drug on the sexual behaviour of females according to age has not yet been studied. Therefore, we compared the motivational value of a male and the expression of sexual behaviour of late-adolescent and adult female rats after chronic treatment with a vehicle or 15.0 mg/kg cocaine. The strong incentive value of a male rat, in a male versus female preference test, for adolescent and adult female rats, was not affected by cocaine. During sexual interaction, adolescents were as sexually receptive as were adults; however, they expressed more runaways and social investigation. Cocaine treatment did not modify the expression of sexual behaviour in either group, but increased social investigation in adolescent rats. These results indicate that late-adolescent pro-oestrus females are highly sexually motivated and might express behaviours typical of this life period during sexual interaction. Moreover, although chronic cocaine treatment seemed to affect more adolescents, it did not alter the sexual motivation or behaviour of females.

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.

Maternal anxiety from pregnancy to 2 years postpartum: transactional patterns of maternal early adversity and child temperament.

The aims of this study were to examine the anxiety trajectories of women from pregnancy to 2 years postpartum and to assess the influence of their early life experiences and the temperament of the child on these trajectories. We evaluated state anxiety (State-Trait Anxiety Inventory) at pregnancy and 3, 6, 12, 18, and 24 months postpartum and determined its course as a function of self-reported early adverse experiences (Childhood Trauma Questionnaire) and the temperament of the child at 18 months (Early Child Behavior Questionnaire). Based on growth curve modeling, we found that anxiety followed a general U-shape pattern from gestation to 2 years postpartum, which was modified by early life experience of women. Greater early adversity was associated with higher gestational anxiety, followed by a marked decrease once the baby was born, and subsequent increase during the later postpartum period. The temperament of the child also modulated anxiety trajectories. Thus, mothers of children high in negative affectivity and who also experienced greater early adversity had elevated and flat anxiety trajectories, while child extraversion was associated with increasing anxiety courses approaching 2 years postpartum. These results show that maternal anxiety dynamically changes through the postpartum period with a course that is affected by previous and current experiences.

Social aversive stimuli presented to the mother produce the precocious expression of fear in rat pups.

During the stress hypo-responsive period, rat pups do not display fear responses toward adult males, yet they exhibit distress behavior in isolation. Since the mother modulates her offspring's affective development, we hypothesized that by altering the mother's behavior, a prolonged stressful situation would modify the ontogeny of the fear responses and distress behaviors in pups. Therefore, we repeatedly exposed the mother-litter dyad to different socially stressful stimuli and subsequently evaluated in 8-day-old pups their fear responses toward an anesthetized male, as well as their distress behavior in isolation. Our results show that repeated exposure to unfamiliar males and females, which altered maternal behavior by eliciting aggression in the mother, was associated with the precocious fear responses in pups, though without altering their distress behavior in isolation. We propose that the mother, as the principal mediator of environmental influences, provokes the precocious expression of fear in pups through alterations in her maternal behavior.

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.

Cocaine treatment before pregnancy differentially affects the anxiety and brain glucose metabolism of lactating rats if performed during adulthood or adolescence.

Cocaine exposure disrupts the maternal behavior of lactating rats, yet it is less known whether it alters the affective changes that accompany motherhood. As the long-term action of cocaine on anxiety varies according to the developmental stage of the individuals, this study aimed to compare the effect of a chronic treatment with cocaine to adult and adolescent non-pregnant females on their anxiety-like behavior and basal brain metabolic activity during lactation. Thus, adult and adolescent virgin rats were exposed to cocaine (0.0 or 15.0 mg/kg ip) during 10 days and were mated four days later. Anxiety behavior was evaluated on postpartum days 3-4 in the elevated plus maze test, and the basal brain glucose metabolism was determined on postpartum days 7-9 by means of [18F] fluorodeoxyglucose positron emission tomography. Cocaine treatment during adulthood increased the anxiety-like behavior of lactating females whereas its administration during adolescence decreased it. Also, the basal glucose metabolism of the medial prefrontal cortex differed between lactating females treated with cocaine during adulthood and adolescence. These differential effects of cocaine, according to the age at which the drug was administered, support the idea that the adolescent and adult brains have a distinct susceptibility to this drug, which leads to divergent long-term changes in the neural circuits that regulate anxiety during lactation.

Raising overlapping litters: Differential activation of rat maternal neural circuitry after interacting with newborn or juvenile pups.

The maternal behaviour of a rat dynamically changes during the postpartum period, adjusting to the characteristics and physiological needs of the pups. This adaptation has been attributed to functional modifications in the maternal circuitry. Maternal behaviour can also flexibly adapt according to different litter compositions. Thus, mothers with two overlapping litters can concurrently take care of neonate and juvenile pups, mostly directing their attention to the newborns. We hypothesised that the maternal circuitry of these mothers would show a differential activation pattern after interacting with pups depending on the developmental stage of their offspring. Thus, we evaluated the activation of several areas of the maternal circuitry in mothers of overlapping litters, using c-Fos immunoreactivity as a marker of neuronal activation, after interacting with newborns or juveniles. The results showed that mothers with overlapping litters display different behavioural responses towards their newborn and their juvenile pups. Interestingly, these behavioural displays co-occurred with specific patterns of activation of the maternal neural circuitry. Thus, a similar expression of c-Fos was observed in some key brain areas of mothers that interacted with newborns or juveniles, such as the medial preoptic area and the nucleus accumbens, whereas a differential activation was quantified in the ventral region of the bed nucleus of the stria terminalis, the infralimbic and prelimbic subregions of the medial prefrontal cortex and the basolateral and medial nuclei of the amygdala. We posit that the specific profile of activation of the neural circuitry controlling maternal behaviour in mothers with overlapping litters enables dams to respond adequately to the newborn and the juvenile pups.

Vaporized Cannabis differentially modulates sexual behavior of female rats according to the dose.

Studies exploring the effect of compounds that modulate the endocannabinoid system on sexual behavior have yielded contradictory results. However, the effect of smoked Cannabis in women has been consistently associated with an increase in sexual drive. Therefore, it can be speculated that vaporized Cannabis will augment sexually motivated components of the sexual behavior of female rats. To test this hypothesis, we compared the sexual behavior of late-proestrous female rats in a bilevel chamber after vaporizing 0, 200 or 400 mg of Cannabis flowers (containing 18% of delta-9-THC and undetectable levels of cannabidiol) during 10 min. We found that both doses of Cannabis increased the duration of the lordosis response, whereas the highest dose also reduced the lordosis quotient of females. The lowest dose of Cannabis augmented the display of hops and darts without altering the expression of sexual solicitations of females, while the highest one did not affect the expression of hops and darts but reduced sexual solicitations. These effects were not accompanied by alterations of females' ambulatory behavior. The increment of the duration of lordosis response produced by both doses of Cannabis could be associated to a general effect of this drug in sensory processing, as can be an enhancement of females' sensory reactivity to male's stimulation. However, the reduction in the display of solicitations and lordosis in response to mounting observed in females exposed to the highest dose when compared to control and 200 mg of Cannabis groups indicates a reduction of sexual receptivity and motivation. This differential effect of vaporized Cannabis according to the dose employed, suggests that it modulates sexual behavior in a complex way, impacting neural circuits that control different aspects of this social behavior.

The reproductive stage and experience of sexually receptive mothers alter their preference for pups or males.

Female rats show postpartum estrus, a unique stage in their reproductive cycle in which they are able to display maternal and sexual responses at the same time. To assess the relative value of pups or males for sexually receptive mothers with different hormonal profiles and reproductive experiences, we employed a 3-point star maze with 3 choice compartments containing: pups, a sexually active male, or no stimulus (neutral). Cycling maternal and nonmaternal females in late proestrus, independently of their previous reproductive experience, strongly preferred the male to the pups, although most postpartum estrous dams did not exhibit preference for the male. The majority of the postpartum primiparous females did not prefer the litter's chamber either, but a previous reproductive experience strongly determined their preference for the pups. These results suggest that the hormonal changes of the proestrus, in contrast to those of the postpartum estrus, promote a strong preference for the male that is not diminished by the maternal condition. Conversely, the endocrine changes of the postpartum facilitate the effect of previous reproductive experience in strengthening the incentive value of the pups.

The Expression of Hypoxia-Induced Gene 1 (Higd1a) in the Central Nervous System of Male and Female Rats Differs According to Age.

HIGD1A (hypoxia-induced gene domain protein-1a), a mitochondrial inner membrane protein present in various cell types, has been mainly associated with anti-apoptotic processes in response to stressors. Our previous findings have shown that Higd1a mRNA is widely expressed across the central nervous system (CNS), exhibiting an increasing expression in the spinal cord from postnatal day 1 (P1) to 15 (P15) and changes in the distribution pattern from P1 to P90. During the first weeks of postnatal life, the great plasticity of the CNS is accompanied by cell death/survival decisions. So we first describe HIGD1A expression throughout the brain during early postnatal life in female and male pups. Secondly, based on the fact that in some areas this process is influenced by the sex of individuals, we explore HIGD1A expression in the sexual dimorphic nucleus (SDN) of the medial preoptic area, a region that is several folds larger in male than in female rats, partly due to sex differences in the process of apoptosis during this period. Immunohistochemical analysis revealed that HIGD1A is widely but unevenly expressed throughout the brain. Quantitative Western blot analysis of the parietal cortex, diencephalon, and spinal cord from both sexes at P1, P5, P8, and P15 showed that the expression of this protein is predominantly high and changes with age but not sex. Similarly, in the sexual dimorphic nucleus, the expression of HIGD1A varied according to age, but we were not able to detect significant differences in its expression according to sex. Altogether, these results suggest that HIGD1A protein is expressed in several areas of the central nervous system following a pattern that quantitatively changes with age but does not seem to change according to sex.

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.

Dopaminergic activity mediates pups' over male preference of postpartum estrous rats.

Pups have greater incentive value than males for rats during the postpartum estrus (PPE); a period when females are both maternally and sexually motivated. Mesolimbic dopaminergic system has been proposed as a general motivational circuit; however in the literature it has been more related to the control of the motivational aspects of maternal than sexual motivation of females. Therefore, we aimed to assess the effect of antagonizing dopaminergic neurotransmission of PPE females on their preference for pups over a male. To achieve this objective we tested PPE rats in a Y-maze with three-choice chambers (one containing eight pups, the other a male and the last one no stimulus) after the systemic administration of the dopaminergic antagonist haloperidol (0.0; 0.025 or 0.05 mg/kg). Furthermore, to determine if this dopaminergic antagonist differentially affects maternal and sexual motivations when pups and male are not competing, we evaluated the effect of haloperidol in the preference of females for pups vs. a non-receptive female and for a male vs. a non-receptive female. In the preference test for pups vs. male, both doses of haloperidol decreased the time that females spent in pups' chamber while increased the time that they spent in male's chamber, resulting in a lack of preference between both incentives. Besides, haloperidol reduced the effort -attempts to get access to the stimuli- made by the females to obtain the pups. Conversely, 0.05 mg/kg of haloperidol did not affect the preference for both incentives when they were confronted to a non-receptive female. Together, these results indicate that the dopaminergic activity mediates pups' preference over male during the PPE and point toward a more relevant role of this system in females' behavioral output when incentives are competing.

Ovarian steroids counteract serotonergic drugs actions in an animal model of obsessive-compulsive disorder.

Recently, we reported the existence of differences according to the reproductive stage of female rats in a pharmacologically induced animal model of obsessive-compulsive disorder. Therefore, the aim of the present study was to examine the role of endogenous and exogenous ovarian steroids in the induction of perseverative responses in a T-maze by the 5-HT1A agonist 8-OH-DPAT (1.0 and 2.0 mg/kg, SC) and in the preventive action of the selective serotonin reuptake inhibitor, fluoxetine (10.0 mg/kg, three times, SC). The results showed that the perseverant action of 8-OH-DPAT as well as the prevention of such perseverance by fluoxetine were reduced during the estrous-metestrous phases of the female reproductive cycle, and by the exogenous ovarian steroids administration to ovariectomized animals. Data are discussed from the standpoint of the action of ovarian steroids on the serotonergic system and on the putative influence of these hormones on the physiopathology and treatment of this disorder in women.

Characterization of hypoxia induced gene 1: expression during rat central nervous system maturation and evidence of antisense RNA expression.

Although recent studies have provided a detailed understanding of cellular interactions occurring during the development of the CNS, little is known about the molecular signals which during the peri and postnatal periods ensure its maturation and functionality. Using the mammalian spinal cord as a model, we have designed experiments to examine the main changes in gene expression occurring during this critical transition. In this paper we describe the cloning and characterization of the rat hypoxia induced gene-1 (Hig-1), its expression pattern during spinal cord maturation and in situ localization of its mRNA. We show an increase in Hig-1 expression between P1 and P15 in the spinal cord and a differential spatial pattern. In the P1 spinal cord we observed preferential expression in regions of dorsal laminae II and III and laminae IX ventrally; while in P8, the distribution was more widespread and overall expression was increased. Hig-1 is also widely expressed in the brain. Results of in situ hybridization experiments, as well as particular features concerning ESTs, led us to propose the expression of an antisense mRNA. Primer-specific RTPCR demonstrates the presence of this aHig-1 transcript whose structure has not yet been characterized. The high homology between putative rHig-1 protein and human- and murine-predicted sequences, as well as its characteristic expression in the Central Nervous System, are indicative of a specific role which could be related to apoptosis signaling during postnatal maturation.

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.

Compulsive-like behaviour according to the sex and the reproductive stage of female rats.

The aim of the present study was to explore putative differences in the responses assessed in an animal model of obsessive-compulsive disorder (OCD) according to the sex and the reproductive cycle of female rats. The model consists of the induction of perseveration (repetitive choices of the same arm in a T-maze) by 8-OH-DPAT (1.0mg/kg). Males and females (pooled in all stages of their oestrous cycle) persevered after 8-OH-DPAT administration and no differences were observed between groups. During the oestrous cycle, this 5-HT(1A) agonist induced perseveration in metoestrus, dioestrus and prooestrus and reduced levels of this behaviour in oestrus. 8-OH-DPAT provoked perseveration in mid-gestation, an effect that was reduced in late-gestation and blocked during lactation. Reproductive cycle changes in the induced perseveration are discussed from the standpoint of the ovarian steroids' action on the serotoninergic system and on the bases of the variations in stress responsiveness along the reproductive cycle of the female. Present results validate the use of females in this model of OCD and could be relevant for studying the role of reproductive hormones in the pathophysiology of this disorder.

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.

Maternal and affective behaviors of lactating rats reared in overlapping litters.

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