Fear In Female Rats Research Articles

Conspecific interactions predict social transmission of fear in female rats

Social transmission of fear occurs in a subset of individuals, where an Observer displays a fear response to a previously neutral stimulus after witnessing or interacting with a conspecific Demonstrator during memory retrieval. The conditions under which fear can be acquired socially in rats have received attention in recent years, and suggest that social factors modulate social transmission of information. We previously found that one such factor, social rank, impacts fear conditioning by proxy in male rats. Here, we aimed to investigate whether social roles as determined by nape contacts in females, might also have an influence on social transmission of fear. In-line with previous findings in males, we found that social interactions in the home cage can provide insight into the social relationship between female rats and that these relationships predict the degree of fear acquired by-proxy. These results suggest that play behavior affects the social transfer/transmission of information in female rats.

Estrous cycle contributes to state-dependent contextual fear in female rats

The increased susceptibility of women to stress and trauma-related disorders compared to men suggests a role for ovarian hormones in modulating fear and anxiety. In both humans and rodents, estrogen and progesterone have been shown to influence fear learning during acquisition, expression, and extinction. Recently, we showed that allopregnanolone (ALLO), a progesterone (PROG) metabolite and GABAA receptor potentiator, confers state-dependent contextual fear when infused into the bed nucleus of the stria terminalis of male rats. In order to determine whether estrous cycle-related fluctuations in circulating PROG confer state-dependent contextual fear in female rats, animals received Pavlovian fear conditioning during an estrous cycle phase when PROG was either low (late diestrus) or high (late proestrus). After conditioning, animals were tested for contextual fear in either the same or different estrous cycle phase. Subjects conditioned in diestrus and tested in proestrus showed lower levels of contextual fear compared to subjects conditioned and tested in the same estrous cycle phase (either diestrus or proestrus), suggesting a state-dependent effect of estrous cycle phase on fear learning. This state dependence was asymmetric, however, as animals trained in proestrus and tested in diestrus exhibited high levels of contextual fear. In ovariectomized (OVX) females treated acutely with either PROG or vehicle, state dependence was not observed. These results suggest that the hormonal state in diestrus may play a role in conferring state dependence to conditioned fear in naturally cycling female rats but not in an OVX model.

Toxoplasma gondii Infection Causes an Atypical Abundance of Oxytocin and Its Receptor in the Female Rat Brain.

Infection with the protozoan Toxoplasma gondii causes loss of innate fear of cat odors in both male and female rats. This behavioral change is presumed to reflect a parasitic manipulation that increases transmission of the parasite from its intermediate to definitive host. The host behavioral change in male rats is dependent on gonadal steroids. In contrast, the loss of fear in female rats is not accompanied by greater gonadal steroids and cannot be rescued by gonadectomy. This disparity suggests that proximate mechanisms of the post infection host behavioral change in rats are sexually dimorphic. Here, we report that female rats infected with Toxoplasma gondii exhibit greater abundance of messenger RNA for oxytocin and oxytocin receptors in the paraventricular nucleus of the hypothalamus and posterodorsal medial amygdala, respectively. Brain oxytocin is critical for sex-typical social and sexual behaviors in female rodents. The change in oxytocin and its receptor could potentially alter activity in the social salience circuits, leading to a reduction in defensive behaviors and an increase in approach to ambivalent environmental cues. Our results argue that sexually dimorphic neural substrates underpin sexually monomorphic host behavioral change in this host–parasite association.

Sex Differences in Foraging Rats to Naturalistic Aerial Predator Stimuli

SummaryRodents in the wild are under nearly constant threat of aerial predation and thus have evolved adaptive innate defensive behaviors, such as freezing or fleeing, in response to a perceived looming threat. Here we employed an ethologically relevant paradigm to study innate fear of aerial predators in male and female rats during a goal-oriented task. Rats foraging for food in a large arena encountered either a 2D or 3D looming stimulus, to which they instinctively fled back to a safe nest. When facing a direct aerial threat, female rats exhibited a greater fear response than males and this divergence maintained when exposed to the environment on subsequent days with no predator interaction, suggesting stronger contextual fear in female rats. These results may have relevance toward exploring neurobiological mechanisms associated with higher diagnosis rates of fear and anxiety-related disorders in women as compared with men.

Anterior cingulate cortex and dorsal hippocampal glutamate receptors mediate generalized fear in female rats

Exhibiting fear to non-threatening cues or contexts—generalized fear—is a shared characteristic of several anxiety disorders, which afflict women more than men. Female rats generalize contextual fear at a faster rate than males and this is due, in part, to actions of estradiol in the dorsal CA1 hippocampus (dCA1). To understand the mechanisms underlying estradiol’s effects on generalization, we infused estradiol into the anterior cingulate cortex (ACC) or ventral CA1 hippocampus (vCA1) of ovariectomized (OVX) female rats. Estradiol acts within the ACC, but not the vCA1, to promote generalized fear. We next examined if AMPA or NMDA receptor antagonists (NBQX, APV) infused into the dCA1 or the ACC of female rats could block generalized fear induced by systemic injections of estradiol. Immediate pre-testing infusions of NBQX or APV into either region eliminated estradiol-induced generalization. Specific blockade of GluN2B receptors with infusions of Ro 25-6981 into the dCA1 or ACC also eliminated generalized fear. Our results suggest that in addition to the dCA1, the ACC is an important locus for the effects of estradiol on fear generalization. Moreover, within these regions, AMPA and NMDA-GluN2B receptors are necessary for estradiol-induced generalization of fear responses, suggesting a critical involvement of glutamatergic transmission. Furthermore, we identified a novel role for GluN2B in mediating the effects of estradiol on generalized fear in female rats. These data potentially implicate GluN2B receptors in more general forms of memory retrieval inaccuracies, and form the foundation for exploration of glutamate receptor pharmacology for treatments of anxiety disorders involving generalization.

Sex Differences in Fear of Aerial Threats in Foraging Rats: An Ethological Approach

Rodents in the wild are under nearly constant threat of aerial predation, and thus haveevolved adaptive innate defensive behaviors, such as freezing or fleeing, in response to aperceived looming threat. Here we investigated sex differences using an ethologically relevant paradigm to study innate-fear of aerial predators in rats during a goal-oriented task. Rats foraging for food in a large arena encountered either a 2D or 3D looming stimulus, to which they instinctively fled back to a safe nest. When facing a direct predatory threat, female rats exhibited a greater fear response than males and this divergence maintained when exposed to the environment on subsequent days with no predator interaction, suggesting stronger contextual fear in female rats. These results closely align with human conditions, where a higher diagnosis rate of fear- and anxiety-related disorders is found in females than in males.

5-HT1A receptors of the prelimbic cortex mediate the hormonal impact on learned fear expression in high-anxious female rats

Hormones highly influence female behaviors. However, research on this topic has not usually considered the variable hormonal status. The prelimbic cortex (PrL) is commonly engaged in fear learning. Connections from and to this region are known to be critical in regulating anxiety, in which serotonin (5-HT) plays a fundamental role, particularly through changes in 5-HT1A receptors functioning. Also, hormone fluctuations can greatly influence anxiety in humans and anxiety-related behavior in rodents, and this influence involves the functioning of 5-HT brain systems. The present investigation sought to determine whether fluctuations in ovarian hormones relative to the estrous cycle would influence the expression of learned fear in female rats previously selected as low- (LA) or high-anxious (HA). Furthermore, we investigate the role of the 5-HT system of the PrL, particularly the 5-HT1A receptors, as a possible modulator of estrous cycle influence on the expression of learned fear through intra-PrL microinjections of 5-HT itself or the full 5-HT1A agonist 8-OH-DPAT (8-hydroxy-2-(di-n-propylamine)tetralin). Behavioral changes were assessed using the fear-potentiated startle (FPS) procedure. The results showed that fear intensity is associated with hormonal decay, being more accentuated during the estrus phase. This increase in fear levels was found to be negatively correlated with the expression of potentiated startle. In rats prone to anxiety and tested during the proestrus and estrus phases, 5-HT mechanisms of the PrL seem to play a regulatory role in the expression of learned fear. These results were not replicated in the LA rats. Similar but less intense results were found regarding the early and late diestrus. Our data indicate that future studies on this subject need to take into account the dissociation between low- and high-responsive females to understand how hormones affect emotional behavior.

Social transmission of Pavlovian fear: fear-conditioning by-proxy in related female rats

Pairing a previously neutral conditioned stimulus (CS; e.g., a tone) to an aversive unconditioned stimulus (US; e.g., a foot-shock) leads to associative learning such that the tone alone will elicit a conditioned response (e.g., freezing). Individuals can also acquire fear from a social context, such as through observing the fear expression of a conspecific. In the current study, we examined the influence of kinship/familiarity on social transmission of fear in female rats. Rats were housed in triads with either sisters or non-related females. One rat from each cage was fear conditioned to a tone CS+ shock US. On day two, the conditioned rat was returned to the chamber accompanied by one of her cage mates. Both rats were allowed to behave freely, while the tone was played in the absence of the foot-shock. The previously untrained rat is referred to as the fear-conditioned by-proxy (FCbP) animal, as she would freeze based on observations of her cage-mate’s response rather than due to direct personal experience with the foot-shock. The third rat served as a cage-mate control. The third day, long-term memory tests to the CS were performed. Consistent with our previous application of this paradigm in male rats (Bruchey et al. in Behav Brain Res 214(1):80–84, 2010), our results revealed that social interactions between the fear conditioned and FCbP rats on day two contribute to freezing displayed by the FCbP rats on day three. In this experiment, prosocial behavior occurring at the termination of the cue on day two was significantly greater between sisters than their non-sister counterparts, and this behavior resulted in increased freezing on day three. Our results suggest that familiarity and/or kinship influences the social transmission of fear in female rats.

Estrogen disrupts the inhibition of fear in female rats, possibly through the antagonistic effects of estrogen receptor alpha (ERalpha) and ERbeta.

The ambiguous role of estrogen in emotional learning may result from opposing actions of estrogen receptor alpha (ERalpha) and ERbeta. Using a fear-conditioning paradigm called the AX+, BX- discrimination, in which cue A comes to elicit fear and cue B becomes a safety signal, we examined the effect of 17beta-estradiol (E) and selective ERalpha and ERbeta agonists on excitatory and inhibitory fear learning. Gonadectomized (GDX) male and female rats implanted with E or selective ERalpha or ERbeta agonists were trained on the AX+, BX- discrimination and tested periodically to A, B, and AB. GDX sham-implanted male and female rats and GDX E-implanted males, but not GDX E-implanted females, exhibited less fear to AB than to A, suggesting that estrogen interferes with generalization of safety signals in female rats. ERalpha and ERbeta agonists disrupted discrimination learning in both sexes. ERalpha-implanted groups had higher fear responses to all cues than did ERbeta-implanted groups, suggesting that these two receptors have opposing effects in aversive discrimination learning. In contrast, neither E nor ERalpha and ERbeta agonists affected single-cue fear conditioning in either sex. These data suggest that E does not enhance fear in emotional learning but acts to disrupt the inhibition of fear in females only.

Early life stress impairs fear conditioning in adult male and female rats

We demonstrated that neonatal isolation (1-h pup isolation; postnatal days 2–9) impairs context-induced fear conditioning in adult male rats and tends to enhance this effect and foot shock sensitivity in females. In this study, we examine the effects of brief (i.e., handling; 15 min) and prolonged (3 h) maternal separations (postnatal days 1–21) on fear conditioning and foot shock sensitivity in adult male and female rats. Identical training and test conditions from our prior study were employed so comparisons of the three early life stressors could be made. Context- and cue-elicited freezing and ultrasonic vocalizations (USVs; 22 kHz) were measured after 10 tone-shock training trials in Experiment 1. In Experiment 2, foot shock responses (flinch, jump, sonic vocalizations) to escalating shock levels were assessed. Brief maternal separation impaired context- and cue-conditioned fear in rats of both sexes as assessed by USVs. Prolonged maternal separation only impaired context fear in female rats. There were no effects on foot shock sensitivity. Results of this and other studies suggest that early life stress impairs fear conditioning in adult rats whereas stress experienced in adulthood has the opposite effect. These opposing effects may reflect developmental differences on stress-induced alterations on hippocampal regulation of the hypothalamic–pituitary–adrenal axis.

Sex-selective effects of neonatal isolation on fear conditioning and foot shock sensitivity

Our previous work demonstrates enduring effects of the early life stress of neonatal isolation (ISO). ISO facilitates appetitive response learning in adult female, but not male rats, and enhances corticosterone levels and stress responsivity in infant and juvenile rats of both sexes. Corticosterone acts at brain areas such as hippocampus that are rich in glucocorticoid receptors, differentiate postnatally, are sexually dimorphic, and involved in learning. Thus, ISO is hypothesized to alter aversive learning in adult rats in a sex-specific manner. This study tests this hypothesis using context and cue fear conditioning. Pups were isolated for 1 h a day on postnatal (PN) days 2–9 or were non-handled and were then tested in adulthood (PN70-90). In Experiment 1, context- and cue-elicited freezing and ultrasonic vocalizations (USVs; 22 kHz range) were measured. Experiments 2–4, respectively, examined three unconditioned foot shock responses (flinch, jump, vocalization), unconditioned fear (time in center of an open, novel arena), and appetitive (stroking-induced; 50 kHz range) USVs. ISO had a sex-selective effect on context-induced USVs that may reflect changes in foot shock sensitivity. ISO increases foot shock sensitivity and tends to enhance context-induced fear in female rats, whereas ISO tends to impair context-induced fear in male rats. Overall, male rats show greater conditioned fear, and female rats show greater unconditioned fear as well as enhanced responses to the aversive and appetitive stimuli. The sex-specific effect of ISO on context fear may reflect neuronal reorganization in stress responsive areas and/or sex differences in some unconditioned responses.