Ontogeny of sex differences in defensive burying behavior in rats: effect of social isolation

Anxiety-Like Behaviors in Mice Unmasked: Revealing Sex Differences in Anxiety Using a Novel Light-Heat Conflict Test

Anxiety and chronic pain afflict hundreds of millions worldwide. Anxiety and pain are more prevalent in females compared to males. Unfortunately, robust sex differences in human anxiety are not recapitulated in rodent tests, and results from rodent pain studies frequently fail to translate clinically. Therefore, there is a need to develop tests that reflect the differential salience of anxiety or pain-related stimuli between the sexes. Accordingly, here we introduce the Thermal Increments Dark-Light (TIDAL) conflict test. The TIDAL test places an anxiety-relevant stimulus (dark vs. illuminated chamber) in conflict with a heat-related stimulus (incrementally heated vs. isothermic chamber); mice freely explore both apparatus chambers. Here, we aim to determine whether the TIDAL conflict test reveals in mice underappreciated sex differences in anxiety and/or heat sensitivity. We establish in four distinct experiments that females on the TIDAL conflict test persist substantially longer on the dark-heated plate, suggesting that female mice exhibit elevated anxiety-like behavior. Mice more strongly prefer the heated-dark plate on the TIDAL conflict test compared to control thermal place preference with both chambers illuminated. We also reveal that an anxiety-relieving drug, paroxetine, reduces mouse preference for the heating dark plate, supporting the validity of the TIDAL test. Therefore, our new TIDAL conflict test reliably unmasks the relative salience of anxiety (vs. heat sensitivity): mice that are female exhibit robust anxiety-like behaviors not consistently observed in classical tests. Future studies should incorporate TIDAL and other conflict tests to better understand rodent behavior and to identify mechanisms underlying anxiety and pain.

Social housing promotes cognitive function and reduces anxiety and depressive-like behaviours in rats

The aim of the study was to assess the impact of social isolation of rats in the post-weaning period using behavioural tests aimed at assessing cognitive function, anxiety, and depressive-like behaviours. The monitoring was performed in male Wistar rats which were housed after weaning either individually (n = 8) or in pairs (n = 8) for 33 days. In the open field, rats kept in isolation reared less often (P < 0.05) than pair-housed rats. In the elevated plus-maze test, pair-housed rats entered the open arm more frequently (P = 0.002) and stayed in the closed arm less often (P = 0.019) compared to rats housed in isolation. In the forced swim test, climbing was seen more frequently (P = 0.016) in pair-housed rats whereas immobility was more common (P = 0.006) in rats housed individually. In the novel object recognition test, the pair-housed rats preferred (P = 0.014) the novel object whereas there was no difference (P = 0.107) in time spent by exploring familiar and novel objects in rats housed in isolation. Furthermore, juvenile rats housed for 33 days in isolation showed higher (P = 0.003) body weight gain during the monitored period than rats housed for the same period in pairs. Our findings are important not only in terms of assessing the impact of rat housing on their mental and physical development but also in terms of the accurate interpretation of the results of other experiments where the rat is used as a model organism.

The role of the nucleus reuniens in regulating contextual conditioning with the predator odor TMT in female rats

RATIONALE

Experiencing intrusive distressing memories of a traumatic event(s) is a prominent symptom profile for post-traumatic stress disorder (PTSD). Understanding the neurobiological mechanisms associated with this symptom profile can be invaluable for effective treatment for PTSD.

OBJECTIVES

Here, we investigated the functional role of the nucleus reuniens (RE), a midline thalamic in modulating stressor-related memory.

METHODS

Female Long Evans rats were implanted with a cannula aimed at the RE. The RE was pharmacologically inactivated via muscimol (0.5 mM) prior to exposure to the predator odor stressor trimethylthiazoline (TMT; synthetically derived fox feces component) or water (controls) in a distinct context with bedding material (experiment 1) or no bedding (experiment 2). To measure context reactivity, the index of the contextual memory, 2 weeks following exposure to TMT, rats were re-exposed to the TMT-paired context (in the absence of TMT).

RESULTS

In experiment 1, during context re-exposure (with bedding), inactivation of the RE had no effect on context reactivity. In experiment 2, during context re-exposure (no bedding), rats previously exposed to TMT showed decreased immobility compared to controls, indicating reactivity to the context and likely related to theincreased exploration of the environment. Rats in the TMT group that received RE inactivation showed increased immobility relative to rats that received aCSF, suggesting that muscimol pre-treatment blunted context reactivity.

CONCLUSION

In conclusion, recruitment of the RE in stressor-related contextual memory appears to be dependent on the contextual environment and whether the animal is able to engage in different stress coping strategies.

Increased alcohol self-administration following exposure to the predator odor TMT in active coping female rats

Post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) are highly comorbid. Additionally, individual differences in response to stress suggest resilient and susceptible populations. The current study exposed male and female Long Evans rats to the synthetically produced predator odor 2,5-dihydro-2,4,5-trimethylthiazoline (TMT) to examine individual differences in stress-reactive behaviors (digging and immobility) and whether these differences were related to subsequent alcohol drinking. Male and female Long Evans rats were trained on operant alcohol self-administration. After 9 sessions, rats underwent exposure to TMT or water (Control) in a distinct context. 6 days after TMT exposure, rats underwent re-exposure to the TMT-paired context (without TMT), and a series of behavioral assessments (acoustic startle, zero maze, light/dark box), after which rats resumed alcohol self-administration. TMT subgroups were created using a ratio of digging to immobility behavior during TMT exposure and rats with a ratio score < 1.0 or> 1.0 were grouped into TMT-1 (low digging/high immobility) or TMT-2 (high digging/low immobility), respectively. All male rats exposed to TMT met criteria for TMT-1, while female rats were divided into the two subgroups. In females, high digging/low immobility behavior during TMT exposure (TMT-2) was related to increased alcohol self-administration, but this was not observed in males or females that engaged in low digging/high immobility (TMT-1). These data show that individual differences in stress-reactivity can lead to lasting behavioral changes which may lead to a better understanding of increases in alcohol drinking following stress in females.

Conserved Serotonergic Background of Experience-Dependent Behavioral Responsiveness in Zebrafish (Danio rerio)

Forming effective responses to threatening stimuli requires the adequate and coordinated emergence of stress-related internal states. Such ability depends on early-life experiences and, in connection, the adequate formation of neuromodulatory systems, particularly serotonergic signaling. Here, we assess the serotonergic background of experience-dependent behavioral responsiveness using male and female zebrafish (Danio rerio). For the first time, we have characterized a period during behavioral metamorphosis in which zebrafish are highly reactive to their environment. Absence of social stimuli during this phase established by isolated rearing fundamentally altered the behavioral phenotype of postmetamorphic zebrafish in a challenge-specific manner, partially due to reduced responsiveness and an inability to develop stress-associated arousal state. In line with this, isolation differentially affected whole-brain serotonergic signaling in resting and stress-induced conditions, an effect that was localized in the dorsal pallium and was negatively associated with responsiveness. Administration of the serotonin receptor 1A partial agonist buspirone prevented the isolation-induced serotonin response to novelty in the level of the whole brain and the forebrain as well, without affecting catecholamine levels, and rescued stress-induced arousal along with challenge-induced behaviors, which together indicates functional connection between these changes. In summary, there is a consistent negative association between behavioral responsiveness and serotonergic signaling in zebrafish, which is well recognizable through the modifying effects of developmental perturbation and pharmacological manipulations as well. Our results imply a conserved serotonergic mechanism that context-dependently modulates environmental reactivity and is highly sensitive to experiences acquired during a specific early-life time window, a phenomenon that was previously only suggested in mammals.SIGNIFICANCE STATEMENT The ability to respond to challenges is a fundamental factor in survival. We show that zebrafish that lack appropriate social stimuli in a sensitive developmental period show exacerbated alertness in nonstressful conditions while failing to react adequately to stressors. This shift is reflected inversely by central serotonergic signaling, a system that is implicated in numerous mental disorders in humans. Serotonergic changes in brain regions modulating responsivity and behavioral impairment were both prevented by the pharmacological blockade of serotonergic function. These results imply a serotonergic mechanism in zebrafish that transmits early-life experiences to the later phenotype by shaping stress-dependent behavioral reactivity, a phenomenon that was previously only suggested in mammals. Zebrafish provide new insights into early-life-dependent neuromodulation of behavioral stress-responses.

Sensorimotor developmental factors influencing the performance of laboratory rodents on learning and memory

Behavioral studies in animal models have advanced our knowledge of brain function and the neural mechanisms of human diseases. Commonly used laboratory rodents, such as mice and rats, provide a useful tool for studying the behaviors and mechanisms associated with learning and memory processes which are cooperatively regulated by multiple underlying factors, including sensory and motor performance and emotional/defense innate components. Each of these factors shows unique ontogeny and governs the sustainment of behavioral performance in learning tasks, and thus, understanding the integrative processes of behavioral development are crucial in the accurate interpretation of the functional meaning of learning and memory behaviors expressed in commonly employed behavioral test paradigms. In this review, we will summarize the major findings in the developmental processes of rodent behavior on the basis of the emergence of fundamental components for sustaining learning and memory behaviors. Briefly, most sensory modalities (except for vision) and motor abilities are functional at the juvenile stage, in which several defensive components, including active and passive defensive strategies and risk assessment behavior, emerge. Sex differences are detectable from the juvenile stage through adulthood and are considerable factors that influence behavioral tests. The test paradigms addressed in this review include associative learning (with an emphasis on fear conditioning), spatial learning, and recognition. This basic background information will aid in accurately performing behavioral studies in laboratory rodents and will therefore contribute to reducing inappropriate interpretations of behavioral data and further advance research on learning and memory in rodent models.

Exposure to early adversity: Points of cross-species translation that can lead to improved understanding of depression

The relationship between developmental exposure to adversity and affective disorders is reviewed. Adversity discussed herein includes physical and sexual abuse, neglect, or loss of a caregiver in humans. While these stressors can occur at any point during development, the unique temporal relationship to specific depressive symptoms was the focus of discussion. Further influences of stress exposure during sensitive periods can vary by gender and duration of abuse as well. Data from animal studies are presented to provide greater translational and causal understanding of how sensitive periods, different types of psychosocial stressors, and sex interact to produce depressive-like behaviors. Findings from maternal separation, isolation rearing, chronic variable stress, and peer-peer rearing paradigms clarify interpretation about how various depressive behaviors are influenced by age of exposure. Depressive behaviors are broken down into the following categories: mood and affect, anhedonia, energy, working memory, sleep-wake, appetite changes, suicide, and general malaise. Cross-species evidence from humans, nonhuman primates, rats, and mice within each of these categories is discussed. In conclusion, sensitive periods for affective-related behaviors (anxiety, mood, and controllability) occur earlier in life, while other aspects of depression are associated with adversity later during adolescence.

Defensive burying test in postweaning rats: use of a small round chamber

The defensive burying test is an experimental model that is used to explore anxiety-like behavior in adult rats. Because the expression of anxiety-like behavior may differ between infant and adult rats, we tested the impact of chambers with different sizes and shapes on defensive burying in 28-day-old Wistar rats. The first two chambers had base areas of 560 cm, but one was rectangular and the other round. The base areas of the other two chambers were 282 cm, also with one rectangular and one round. We examined the effects of vehicle and 1 mg/kg diazepam on defensive burying in the various chambers. Locomotor activity was also measured to identify or exclude any sedative effects. Independent of the treatments used, the infant rats showed a shorter burying latency in the three modified chambers and a longer cumulative burying time compared with the original apparatus. The effects of diazepam (i.e. increased latency and decreased burying time) were only significant in the small round chamber, without significant effects on general motor activity. These results suggest that a small round chamber that is used to test burying behavior is sensitive to the anxiolytic actions of diazepam when the experimental subjects are very young rats.

Rearing conditions differently affect the motor performance and cerebellar morphology of prenatally stressed juvenile rats

The cerebellum is one of the most vulnerable parts of the brain to environmental changes. In this study, the effect of diverse environmental rearing conditions on the motor performances of prenatally stressed juvenile rats and its reflection to the cerebellar morphology were investigated. Prenatally stressed Wistar rats were grouped according to different rearing conditions (Enriched=EC, Standard=SC and Isolated=IC) after weaning. Six weeks later, male and female offspring from different litters were tested behaviorally. In rotarod and string suspension tests, females gained better scores than males. Significant gender and housing effects were observed especially on the motor functions requiring fine skills with the best performance by enriched females, but the worst by enriched males. The susceptibility of cerebellar macro- and micro-neurons to environmental conditions was compared using stereological methods. In female groups, no differences were observed in the volume proportions of cerebellar layers, soma sizes and the numerical densities of granule or Purkinje cells. However, a significant interaction between housing and gender was observed in the granule to Purkinje cell ratio of males, due to the increased numerical densities of the granule cells in enriched males. These data imply that proper functioning of the cerebellum relies on its well organized and evolutionarily conserved structure and circuitry. Although early life stress leads to long term behavioral and neurobiological consequences in the offspring, diverse rearing conditions can alter the motor skills of animals and synaptic connectivity between Purkinje and granular cells in a gender dependent manner.

Development × environment interactions control tph2 mRNA expression

Adverse early life experience is thought to increase an individual's susceptibility to mental health disorders, including anxiety and affective disorders, later in life. Our previous studies have shown that post-weaning social isolation of female rats during a critical period of development sensitizes an anxiety-related serotonergic dorsal raphe nucleus (DR) system in adulthood. Therefore, we investigated how post-weaning social isolation, in combination with a challenge with the anxiogenic drug, N-methyl-beta-carboline-3-carboxamide (FG-7142; a partial inverse agonist at the benzodiazepine allosteric site on the GABAA receptor), affects home cage behavior and serotonergic gene expression in the DR of female rats using in situ hybridization histochemistry. Juvenile female rats were reared in isolation or groups of three for a 3-week period from weaning (postnatal day (PD) 21 to mid-adolescence (PD42)), after which all rats were group-reared for an additional 16 days until adulthood. Among vehicle-treated rats, isolation-reared rats had decreased rodent tryptophan hydroxylase 2 (tph2) mRNA expression in ventral and ventrolateral subdivisions of the DR, a pattern observed previously in a rat model of panic disorder. Isolation-reared rats, but not group-reared rats, responded to FG-7142 with increased duration of vigilance and arousal behaviors. In addition, FG-7142 decreased tph2 expression, measured 4h following treatment, in multiple subregions of the DR of group-reared rats but had no effect in isolation-reared rats. No treatment effects were observed on 5-HT1A receptor or serotonin transporter gene expression. These data suggest that adolescent social isolation alters tph2 expression in specific subregions of the DR and alters the effects of stress-related stimuli on behavior and serotonergic systems.

Post-weaning social isolation of female rats, anxiety-related behavior, and serotonergic systems

Our previous studies have shown that post-weaning social isolation of male rats leads to sensitization of serotonergic systems and increases in anxiety-like behavior in adulthood. Although studies in humans suggest that females have an increased sensitivity to stress and risk for the development of neuropsychiatric illnesses, most studies involving laboratory rats have focused on males while females have been insufficiently studied. The objective of this study was to investigate the effects of post-weaning social isolation on subsequent responses of an anxiety-related dorsal raphe nucleus (DR)-basolateral amygdala system to pharmacological challenge with the anxiogenic drug, N-methyl-beta-carboline-3-carboxamide (FG-7142; a partial inverse agonist at the benzodiazepine allosteric site on the γ-aminobutyric acid (GABA)(A) receptor). Juvenile female rats were reared in isolation or in groups of three for a 3-week period from weaning to mid-adolescence, after which all rats were group-reared for an additional 2 weeks. We then used dual immunohistochemical staining for c-Fos and tryptophan hydroxylase in the DR or single immunohistochemical staining for c-Fos in the basolateral amygdala. Isolation-reared rats, but not group-reared rats, injected with FG-7142 had increased c-Fos expression within the basolateral amygdala and in serotonergic neurons in the dorsal, ventrolateral, caudal and interfascicular parts of the DR relative to appropriate vehicle-injected control groups. These data suggest that post-weaning social isolation of female rats sensitizes a DR-basolateral amygdala system to stress-related stimuli, which may lead to an increased sensitivity to stress- and anxiety-related responses in adulthood.

Early adolescence as a critical window during which social stress distinctly alters behavior and brain norepinephrine activity

Many neural programs that shape behavior become established during adolescence. Adverse events at this age can have enduring consequences for both adolescent and adult mental health. Here we show that repeated social stress at different stages of adolescent development differentially affects rat behavior and neuronal activity. Early-adolescent (PND 28, EA), mid-adolescent (PND 42, MA), and adult (PND 63) rats were subjected to resident-intruder social stress (7 days) and behavior was examined 24-72 h later. In EA rats selectively, resident-intruder stress increased proactive responses in the defensive burying and forced swim tests. In adult rats, resident-intruder stress decreased burying behavior regardless of whether the animal was stressed as an adult or during early adolescence. As the locus coeruleus (LC)-norepinephrine system has been implicated in proactive defense behaviors, LC neuronal activity was quantified in separate cohorts. Stressed EA rats had elevated spontaneous LC discharge rates and diminished responses to sensory stimuli compared with controls. Microinjection of a CRF antagonist into the LC selectively inhibited neurons of stressed EA rats, suggesting that EA social stress induces tonic CRF release onto LC neurons, shifting the mode of discharge to an activated state that promotes active defensive behaviors. In all adult groups, resident-intruder stress resulted in an increased phasic response to sensory stimuli with no change in spontaneous rates. MA was a transition period during which social stress did not affect behavior or LC activity. The results suggest that social stress interacts with the brain norepinephrine system to regulate defensive strategies in an age-dependent manner.

Adolescent development, hypothalamic-pituitary-adrenal function, and programming of adult learning and memory

Chronic exposure to stress is known to affect learning and memory in adults through the release of glucocorticoid hormones by the hypothalamic-pituitary-adrenal (HPA) axis. In adults, glucocorticoids alter synaptic structure and function in brain regions that express high levels of glucocorticoid receptors and that mediate goal-directed behaviour and learning and memory. In contrast to relatively transient effects of stress on cognitive function in adulthood, exposure to high levels of glucocorticoids in early life can produce enduring changes through substantial remodeling of the developing nervous system. Adolescence is another time of significant brain development and maturation of the HPA axis, thereby providing another opportunity for glucocorticoids to exert programming effects on neurocircuitry involved in learning and memory. These topics are reviewed, as is the emerging research evidence in rodent models highlighting that adolescence may be a period of increased vulnerability compared to adulthood in which exposure to high levels of glucocorticoids results in enduring changes in adult cognitive function.

Post-weaning social isolation increases activity in a novel environment but decreases defensive burying and subchronic MK-801 enhances the activity but not the burying effect in rats

Subchronic treatment with a non-competitive glutamate NMDA-receptor antagonist [e.g., MK-801 or phencyclidine] or social isolation (SI) from weaning (age 21 days) to adulthood (age 56 days) produce deficits similar to some of the positive and negative symptoms of schizophrenia. Few studies have evaluated the effects of these treatments on emotional behavior. We hypothesized that subchronic MK-801, post-weaning SI or the two in combination would alter activity in a novel environment, anxiety-like behaviors in the elevated plus-maze, coping responses in the defensive burying paradigm and social behavior. In experiment 1, SI rats (n=17) showed increased locomotor activity when exposed to a novel environment, no change in plus-maze behavior and decreased defensive burying when compared to group housed rats (n=16). Subchronic MK-801 enhanced the increase in activity but not the decrease in burying in SI rats. Experiment 2 evaluated the effects on social behavior of post-weaning SI. The locomotor and burying results of experiment 1 were replicated and SI rats (n=9) were found to decrease orientation towards a novel conspecific social target when compared to group housed rats (n=8). The behavioral abnormalities of SI rats may be a manifestation of GABAergic dysfunction that has recently become evident in schizophrenia.

Ontogenetic interaction between social relationships and defensive burying behavior in the rat

The present experiments clarify sexual and social relationship factors related to the development of defensive burying behavior in rats. Rats were raised in isolation, or in a variety of pairs differing in sex, age or familiarity during the juvenile and post-juvenile period. In Experiment 1, decreased burying behavior was found in both male and female rats during the juvenile stage when they were reared in isolation, or with an adult female, or for males reared with a same-age female. In Experiment 2, female rats isolated during the juvenile stage who were reared after the juvenile stage with a same-sex, non-isolated rat, showed as much burying behavior as rats reared with a littermate; this was not found for male rats. When both male and female rats isolated during the juvenile stage were reared with each other after isolation, they maintained reduced burying behavior in adulthood. These sex differences in the effect of different social groupings are likely due to the differences in social relationships during the juvenile and after puberty, when social dominance relationships emerge in male rats. In Experiment 3, the effects of social dominance relationships on burying behavior were investigated in male rats. Subordination increased the freezing tendency as a passive defense, while social tension accompanied with rearing with an adult male produced decreased burying behavior as a proactive defense. These findings suggest that affiliative relationships involving playful contacts activate and maintain burying behavior, but familiarity is not a significant factor, while dominance relationships modulate the patterns of burying behavior.

Age-dependent change in exploratory behavior of male rats following exposure to threat stimulus: Effect of juvenile experience

The ontogeny of exploratory behavior depending on the intensity of threat in a modified open-field was investigated in male rats aged 40, 65, and 130 days, by comparing with less threatening condition with no shock and more threatening condition where they were exposed to mild electric shock. The number of crossings in a dim peripheral alley was counted as the level of activity. The total duration of stay in the central area was measured as the level of exploration. The number of entries and stretch-attend postures into a bright center square were measured as active exploratory behavior and the risk assessment behavior, respectively. When exposed to mild shock prior to the test, 40-day-old rats decreased these exploratory behaviors, while 65- and 130-day-old rats increased active exploratory behavior (Experiment 1). A lower level of exploratory behavior following a mild shock was found in 65 and 130-day-old rats isolated during the juvenile stage, but not in rats isolated after puberty (Experiment 2). These findings suggest that the direction of changes in exploratory behavior of male rats following an increase in potential danger showed ontogenetic transition, which is mediated by social experiences as juveniles, but not as adults. This transition may be associated with the emergence of active exploratory behavior during the juvenile stage, which is activated by social interaction.