Sex differences in open-field behavior in the rat: the inductive and activational role of gonadal hormones

Preoptic and hypothalamic regulation of multi-tiered, chronologically arranged female rat sexual behavior

As in many mammalian behaviors, sexual behavior exhibits structure. Each modular components of the structure, that are linked together over time, occur in probabilistic manner. Endocrine milieu, in particular sex hormones, define the probability to synchronize the behavior with the production of gametes. Developmental experience and environmental cues affect the hormonal milieu of the brain. This is especially true in female mammals, in which ova mature with certain intervals along with ovarian secretion of sex hormones. Estrogens secreted by mature ovarian follicles support both affiliative and executive components of female sexual behavior. In the absence of the ovarian steroids, females avoid males when possible, or antagonize and reject males when put together. Female sexual behavior is intimately linked with the estrous cycle in many species such that females are only receptive for a brief period at the estrus stage surrounding ovulation. Thus, in the rat, females strongly influence the outcome of mating encounter with a male. Affiliative or solicitatory behavior shown by females in estrus leads to the female adapting the lordosis posture, which is characterized by hindleg postural rigidity and lordotic dorsiflexion of the spine, in response to touch-pressure somatosensory stimuli on the skin of the flanks, rump-tail base, perineum region given by male partner. The posture facilitates intromission and consequently fertilization. Although dependence on estrogens is the most important feature of female rat sexual behavior, cervical probing combined with palpation of the hindquarter skin acts as a supranormal stimulus to elicit lordosis. Thus, lordosis behavior is a hub of multi-tiered, chronologically arranged set of behaviors and estrogen appear to alter excitability of neural network for lordosis.

EFFECTS OF LITTER SIZE AND CAGING ON PHYSICAL AND MENTAL DEVELOPMENT IN RATS

As a multidisciplinary field, laboratory animal science promotes or accelerates the emergence of innovative ideas and products. As research has increased, so has the demand for laboratory animals with reliable, standardized characteristics. Thus, the breeding, reproduction, and welfare of laboratory animals are now animals reliable and more. The aim of this study to investigate whether different litter sizes of mothers and different husbandry methods affect the physical and mental development of pups. 30 adults female Wistar Hanover albino rats weighing 200-250 g were used for the study. The weight of the pups was measured once a week from birth until the end of the study, and their physical development was observed. After the pups were weaned, they were randomly divided into cages by sex. The 45 male and 45 female pups were housed in groups of three, five, and seven per cage. When the pups were 12 weeks old, open field test, elevated plus-maze test and Morris water maze behavioral tests were performed every other day, and then plasma corticosterone levels were measured. When the male and female pups in the groups were 14 weeks old, six females were taken from each housing group and mated, and the conception rates and maternal behavior of the pups were observed. During lactation, physical developmental parameters and the body weight of the rats were affected by litter size. Among the post-weaning housing groups, cage density was found to affect weight gain and body weight between groups. It was found that only the sex factor caused significant differences in the behavior of the animals. Females housed with seven rats per cage had higher corticosteroid levels than other females. As a result, it was observed that cages with seven female rats were more physically and psychologically affected than those with three and five rats.

Prepubertal ovariectomy confers resilience to stress-induced anxiety in adult female mice

The increased vulnerability to stress-induced neuropsychiatric disorders in women, including anxiety disorders, does not emerge until pubertal onset, suggesting a role for ovarian hormones in organizing sex-specific vulnerability to anxiety. Parvalbumin (PV) interneurons in the prefrontal cortex are a potential target for these ovarian hormones. PV+ interneurons undergo maturation during the adolescent period and have been shown to be sensitive to stress and to mediate stress-induced anxiety in female mice. To test the idea that ovarian hormones at puberty are necessary for the acquisition of sensitivity to stress, hypothetically driving the response of PV+ interneurons to stress, we performed ovariectomy or sham surgery before pubertal onset in female mice. These mice then were exposed to four weeks of unpredictable chronic mild stress in adulthood. We then assessed anxiety-like behavior and PV/FosB colocalization in the medial PFC. Additionally, we assessed stress-induced anxiety-like behavior in female mice following ovariectomy in adulthood to determine if puberty is a sensitive period for ovarian hormones in mediating vulnerability to stress. We found that prepubertal ovariectomy protects against the development of anxiety-like behavior in adulthood, an effect not found following ovariectomy in adulthood. This effect may be independent of ovarian hormones on prefrontal PV+ interneurons response to stress.

Evaluating the Cognitive Impacts of Drospirenone, a Spironolactone-Derived Progestin, Independently and in Combination With Ethinyl Estradiol in Ovariectomized Adult Rats

Oral contraceptives and hormone therapies require a progestogen component to prevent ovulation, curtail uterine hyperplasia, and reduce gynecological cancer risk. Diverse classes of synthetic progestogens, called progestins, are used as natural progesterone alternatives due to progesterone’s low oral bioavailability. Progesterone and several synthetic analogs can negatively impact cognition and reverse some neuroprotective estrogen effects. Here, we investigate drospirenone, a spironolactone-derived progestin, which has unique pharmacological properties compared to other clinically-available progestins and natural progesterone, for its impact on spatial memory, anxiety-like behavior, and brain regions crucial to these cognitive tasks. Experiment 1 assessed three drospirenone doses in young adult, ovariectomized rats, and found that a moderate drospirenone dose benefited spatial memory. Experiment 2 investigated this moderate drospirenone dose with and without concomitant ethinyl estradiol (EE) treatment, the most common synthetic estrogen in oral contraceptives. Results demonstrate that the addition of EE to drospirenone administration reversed the beneficial working memory effects of drospirenone. The hippocampus, entorhinal cortex, and perirhinal cortex were then probed for proteins known to elicit estrogen- and progestin- mediated effects on learning and memory, including glutamate decarboxylase (GAD)65, GAD67, and insulin-like growth factor receptor protein expression, using western blot. EE increased GAD expression in the perirhinal cortex. Taken together, results underscore the necessity to consider the distinct cognitive and neural impacts of clinically-available synthetic estrogen and progesterone analogs, and why they produce unique cognitive profiles when administered together compared to those observed when each hormone is administered separately.

Role of Organic Cation Transporter 3 and Plasma Membrane Monoamine Transporter in the Rewarding Properties and Locomotor Sensitizing Effects of Amphetamine in Male andFemale Mice

A lack of effective treatment and sex-based disparities in psychostimulant addiction and overdose warrant further investigation into mechanisms underlying the abuse-related effects of amphetamine-like stimulants. Uptake-2 transporters such as organic cation transporter 3 (OCT3) and plasma membrane monoamine transporter (PMAT), lesser studied potential targets for the actions of stimulant drugs, are known to play a role in monoaminergic neurotransmission. Our goal was to examine the roles of OCT3 and PMAT in mediating amphetamine (1 mg/kg)-induced conditioned place preference (CPP) and sensitization to its locomotor stimulant effects, in males and females, using pharmacological, decynium-22 (D22; 0.1 mg/kg, a blocker of OCT3 and PMAT) and genetic (constitutive OCT3 and PMAT knockout (−/−) mice) approaches. Our results show that OCT3 is necessary for the development of CPP to amphetamine in males, whereas in females, PMAT is necessary for the ability of D22 to prevent the development of CPP to amphetamine. Both OCT3 and PMAT appear to be important for development of sensitization to the locomotor stimulant effect of amphetamine in females, and PMAT in males. Taken together, these findings support an important, sex-dependent role of OCT3 and PMAT in the rewarding and locomotor stimulant effects of amphetamine.

Effect of Estrous Cycle on Behavior of Females in Rodent Tests of Anxiety

Anxiety disorders are more prevalent in women than in men. In women the menstrual cycle introduces another variable; indeed, some conditions e.g., premenstrual syndrome, are menstrual cycle specific. Animal models of fear and anxiety, which form the basis for research into drug treatments, have been developed almost exclusively, using males. There remains a paucity of work using females and the available literature presents a confusing picture. One confound is the estrous cycle in females, which some authors consider, but many do not. Importantly, there are no accepted standardized criteria for defining cycle phase, which is important given the rapidly changing hormonal profile during the 4-day cycle of rodents. Moreover, since many behavioral tests that involve a learning component or that consider extinction of a previously acquired association require several days to complete; the outcome may depend on the phase of the cycle on the days of training as well as on test days. In this article we consider responsiveness of females compared to males in a number of commonly used behavioral tests of anxiety and fear that were developed in male rodents. We conclude that females perform in a qualitatively similar manner to males in most tests although there may be sex and strain differences in sensitivity. Tests based on unconditioned threatening stimuli are significantly influenced by estrous cycle phase with animals displaying increased responsiveness in the late diestrus phase of the cycle (similar to the premenstrual phase in women). Tests that utilize conditioned fear paradigms, which involve a learning component appear to be less impacted by the estrous cycle although sex and cycle-related differences in responding can still be detected. Ethologically-relevant tests appear to have more translational value in females. However, even when sex differences in behavior are not detected, the same outward behavioral response may be mediated by different brain mechanisms. In order to progress basic research in the field of female psychiatry and psychopharmacology, there is a pressing need to validate and standardize experimental protocols for using female animal models of anxiety-related states.

Sex-dependent development of enhanced sympathoexcitation in sedentary versus physically active rats

KEY POINTS

The rostral ventrolateral medulla (RVLM) may contribute to sex-based differences in cardiovascular disease (CVD) based on overactivation of the sympathetic nervous system observed in sedentary male rats; however, the added influence of the reproductive cycle in females is currently unknown. To our knowledge this is the first study to demonstrate greater increases in sympathetic nerve activity in response to direct activation of the RVLM in female versus male rats prior to the onset of the reproductive cycle, which persisted after the onset of the reproductive cycle. Lower resting blood pressures in females also suggest peripheral adaptations contribute to sex-based differences in CVD. Sedentary versus physically active conditions appear to promote higher resting sympathetic outflow independent of age and sex. Our results demonstrate the importance of examining sedentary conditions in the context of sex differences and the reproductive cycle in contributing to sympathetic overactivity associated with cardiovascular disease.

ABSTRACT

Female reproductive hormones are considered cardioprotective based on higher risks of cardiovascular disease (CVD) in post- versus pre-menopausal women. Similarly, based on epidemiological studies, a sedentary lifestyle is also a major risk factor for CVD. The mechanisms by which sedentary conditions contribute to CVD, and their influences in the presence and absence of female reproductive hormones are unknown. We hypothesized that sexually immature male and female rats would have similar centrally mediated regulation of blood pressure, but upon sexual maturation, female rats would have lower resting blood pressure and centrally-mediated sympathoexcitation compared to age-matched males. We also predicted resting sympathetic activity would increase upon exposure to sedentary versus active conditions (voluntary wheel running) in males but not in females. We recorded splanchnic sympathetic nerve activity (SSNA) and blood pressure in 4-, 8- and 16-week-old male and female rats under Inactin anaesthesia before and during microinjections of glutamate (1-100 mM) into the rostral ventrolateral medulla (RVLM). Four-week-old female rats had lower resting blood pressure and greater sympathoexcitation following activation of the RVLM, as did 8- and 16-week-old female rats, independent of age or activity condition. Sedentary animals had higher baseline SSNA compared to active animals, independent of sex or age. Our results reveal a complex influence of the interactions between the female reproductive cycle and sedentary conditions. They also demonstrate the importance of examining sedentary conditions in the context of sex- and female reproductive cycle-dependent incidences of cardiovascular disease.

Hippocampal neurogenesis mediates sex-specific effects of social isolation and exercise on fear extinction in adolescence

Impaired extinction of conditioned fear is associated with anxiety disorders. Common lifestyle factors, like isolation stress and exercise, may alter the ability to extinguish fear. However, the effect of and interplay between these factors on adolescent fear extinction, and the relevant underlying neural mechanisms are unknown. Here we examined the effects of periadolescent social isolation and physical activity on adolescent fear extinction in rats and explored neurogenesis as a potential mechanism. Isolation stress impaired extinction recall in male adolescents, an effect prevented by exercise. Extinction recall in female adolescents was unaffected by isolation stress. However, exercise disrupted extinction recall in isolated females. Extinction recall in isolated females was positively correlated to the number of immature neurons in the ventral hippocampus, suggesting that exercise affected extinction recall via neurogenesis in females. Pharmacologically suppressing cellular proliferation in isolated adolescents using temozolomide blocked the effect of exercise on extinction recall in both sexes. Together, these findings highlight sex-specific outcomes of isolation stress and exercise on adolescent brain and behavior, and highlights neurogenesis as a potential mechanism underlying lifestyle effects on adolescent fear extinction.

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Periadolescent isolation stress disrupted extinction recall in male adolescents.

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Running prevented isolation-induced extinction recall deficit in male adolescents.

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Exercise impaired extinction recall in isolated female adolescents.

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Exercise increased hippocampal neurogenesis, except in isolated males.

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Suppression of neurogenesis blocked exercise effects in isolated adolescents.

Organization of exploratory behavior under dark conditions in female and male rats

Sexually dimorphic performance has been observed across humans and rodents in many spatial tasks. In general, these spatial tasks do not dissociate the use of environmental and self-movement cues. Previous work has demonstrated a role for self-movement cue processing in organizing open field behavior; however, these studies have not directly compared female and male movement characteristics. The current study examined the organization of open field behavior under dark conditions in female and male rats. Significant differences between female and male rats were observed in the location of stopping behavior relative to a cue and the topography exhibited during lateral movements. In contrast, no sex differences were observed on measures used to detect self-movement cue processing deficits. These results provide evidence that female and male rats are similar in their use of self-movement cues to organize open field behavior; however, other factors may be contributing to differences in performance.

Sex differences in the elevated plus-maze test and large open field test in adult Wistar rats

There is a growing need for a better understanding of sex differences in animal models of psychiatric disorders. The elevated plus-maze (EPM) test and large open field (LOF) test are widely used to study anxiety-like behavior in rodents. Our studies explored sex differences in anxiety and activity parameters in the LOF and EPM and determined whether these parameters correlate within and between tests. Drug naïve adult male and female Wistar rats (n = 47/sex) were used for the studies, and the rats were tested for 5 min in the EPM and 10 min in the LOF. The females spent more time on the open arms of the EPM and made more open arms entries than the males. The females also spent more time in the center zone of the LOF and made more center zone entries. The females traveled a greater distance in the LOF and EPM. There was a moderate positive correlation between time on the open arms of the EPM and time in the center zone of the LOF. There was also a moderate positive correlation between open arms entries in the EPM and center zone entries in the LOF. A hierarchical cluster analysis revealed one cluster with LOF parameters, one cluster with EPM parameters, and one cluster with parameters related to the avoidance of open spaces. In conclusion, these findings indicate that female rats display less anxiety-like behavior in the EPM and LOF. Furthermore, there are sex differences for almost all behavioral parameters in these anxiety tests.

Adolescent stress during, but not after, pubertal onset impairs indices of prepulse inhibition in adult rats

Exposure to stress during adolescence is a risk factor for developing several psychiatric disorders, many of which involve prefrontal cortex (PFC) dysfunction. The human PFC and analogous rodent medial prefrontal cortex (mPFC) continue to mature functionally and anatomically during adolescence, and some of these maturational events coincide with pubertal onset. As developing brain regions are more susceptible to the negative effects of stress, this may make puberty especially vulnerable. To test this, we exposed male and female rats to isolation and restraint stress during the onset of puberty or during the post-pubertal period of adolescence. In young adulthood, both stressed groups and an unstressed control group underwent testing on a battery of tasks to assess emotional and cognitive behaviors, and the volume of the mPFC was quantified postmortem. Factor analysis revealed only subjects stressed peri-pubertally showed a long-term deficiency compared to controls in prepulse inhibition. Additionally, both sexes showed volumetric mPFC decreases following adolescent stress, and these losses were most pronounced in females. Our findings suggest that pubertal onset may be a vulnerable window wherein adolescents are most susceptible to the negative consequences of stress exposure. Furthermore, it highlights the importance of accounting for pubertal status when studying adolescents.

Motor learning rapidly increases synaptogenesis and astrocytic structural plasticity in the rat cerebellum

Motor-skill learning is associated with cerebellar synaptogenesis and astrocytic hypertrophy, but most of these assessments of cerebellar ultrastructure have been completed after one month of training. After one month of training, the motor-skills necessary to complete these tasks have been acquired for weeks. This experiment aimed to characterize cerebellar ultrastructure during the acquisition phase of motor-skill learning, at a point when performance is still improving. Male and female rats trained for four days on the acrobatic motor learning task, which involved traversing challenging obstacles such as narrow beams and ladders. Concurrently, rats in the motor control condition walked a flat alleyway requiring no skilled movements. After training was complete, all rats were euthanized, and tissue was prepared for electron microscopy. Unbiased stereology techniques were used to assess synaptic and astrocytic plasticity. Results indicated that during the initial days of training, female rats made fewer errors and had shorter latencies on the acrobatic course compared to male rats. However, there were no sex differences in cerebellar ultrastructure. Male and female rats that completed four days of acrobatic training displayed an increase in the density of parallel fiber-Purkinje cell synapses per Purkinje cell and an increase in astrocytic volume, relative to rats in the motor control group.

Rapid Jetlag Resetting of Behavioral, Physiological, and Molecular Rhythms in Proestrous Female Mice

A gradual adaptation to a shifted light-dark (LD) cycle is a key element of the circadian clock system and believed to be controlled by the central circadian pacemaker, the suprachiasmatic nucleus (SCN). Endocrine factors have a strong influence on the regulation of the circadian clock network and alter acute photic responses of the SCN clock. In females, endocrine function depends on the stage of the ovarian cycle. So far, however, little is known about the effect of the estrous cycle on behavioral and molecular responses to shifts in the LD rhythm. Based on this, we investigated whether estrous state affects the kinetics of phase shift during jetlag in behavior, physiology, and molecular clock rhythms in the SCN and in peripheral tissues. Female mice exposed to an advanced LD phase at proestrous or metestrous showed different phase-shift kinetics, with proestrous females displaying accelerated adaptation in behavior and physiology. Constant darkness release experiments suggest that these fast phase shifts do not reflect resetting of the SCN pacemaker. Explant experiments on SCN, adrenal gland, and uterus confirmed this finding with proestrous females showing significantly faster clock phase shifts in peripheral tissues compared with the SCN. Together, these findings provide strong evidence for an accelerated adaptation of proestrous compared with metestrous females to new LD conditions that is accompanied by rapid behavioral, physiological, and molecular rhythm resetting. Not only do these findings open up a new avenue to understand the effect of estrous cycle on the clock network under changing environmental conditions but also imply a greater susceptibility in proestrous females.

Sex and age differences in locomotor and anxiety-like behaviors in rats: From adolescence to adulthood

Risk-taking behaviors are a primary contributor to elevated adolescent injury and mortality. Locomotor and anxiety-like behaviors in rodents have been used to examine risk-taking. Here, we examined risk-taking behavior (i.e., changes in locomotor and anxiety-like behaviors) from early to late adolescence and adulthood in male and female rats in the open-field (OF) apparatus and the light-dark (LD) test. We also examined whether these behaviors are affected by an early adolescent immune stressor, lipopolysaccharide (LPS). Long-Evans male and female rats were injected with LPS (200 μg/kg) or vehicle control in early adolescence (postnatal day [PND] 30 and 32). Anxiety-like behavior and locomotor activity were measured in early (PND 38-40), late adolescence (PND 50), and adulthood (PND 88 and 98) in the OF and in early adolescence (PND 42) and adulthood (PND 90) in the LD test. Early and late adolescent rats displayed significantly greater locomotor and anxiety-like behaviors than adult rats in the OF and LD test. Sex differences were also found, with adolescent and adult females displaying greater locomotor and anxiety-like behaviors than male rats in the OF and LD tests. LPS administered two times in early adolescence did not have a significant impact on either locomotor or anxiety-like behaviors suggesting minimal impact of the immune stressor.

Interactions of the estrous cycle, novelty, and light on female and male rat open field locomotor and anxiety-related behaviors

Animal behavior can be modulated by multiple interacting factors. In rodents such as rats, these factors include, among others, the female estrous cycle, exposure to a novel environment, and light. Here, we used the open field test to disassociate differences in behavior resulting from each of these factors by testing the hypothesis that locomotor and anxiety-related behaviors differ between estrous cycle phases in female rats and that novelty and light exposure concurrently influence these behaviors in both female and male rats. Adult female rats were tested twice under red or white light in estrus and diestrus estrous cycle phases. Adult male rats were also tested twice under either red or white light. In females, an interaction between novelty and estrous cycle phase influenced locomotor and anxiety-related behaviors. In males, novelty influenced locomotor and anxiety-related behaviors differentially under red and white light. Light exposure increased anxiety-related behaviors in both males and females, but reduced locomotor behavior only in females. These findings reveal the complexities of behavioral testing and highlight the importance of factors such as the estrous cycle, novelty, and light exposure.

Lack of M4 muscarinic receptors in the striatum, thalamus and intergeniculate leaflet alters the biological rhythm of locomotor activity in mice

The deletion of M₄ muscarinic receptors (MRs) changes biological rhythm parameters in females. Here, we searched for the mechanisms responsible for these changes. We performed biological rhythm analysis in two experiments: in experiment 1, the mice [C57Bl/6NTac (WT) and M₄ MR -/- mice (KO)] were first exposed to a standard LD regime (12/12-h light/dark cycle) for 8 days and then subsequently exposed to constant darkness (for 24 h/day, DD regime) for another 16 days. In experiment 2, the mice (after the standard LD regime) were exposed to the DD regime and to one light pulse (zeitgeber time 14) on day 9. We also detected M₁ MRs in brain areas implicated in locomotor biological rhythm regulation. In experiment 1, the biological rhythm activity curves differed: the period (τ, duration of diurnal cycle) was shorter in the DD regime. Moreover, the day mean, mesor (midline value), night mean and their difference were higher in KO animals. The time in which the maximal slope occurred was lower in the DD regime than in the LD regime in both WT and KO but was lower in KO than in WT mice. In experiment 2, there were no differences in biological rhythm parameters between WT and KO mice. The densities of M₁ MRs in the majority of areas implicated in locomotor biological rhythm were low. A significant amount of M₁ MR was found in the striatum. These results suggest that although core clock output is changed by M₄ MR deletion, the structures involved in biological rhythm regulation in WT and KO animals are likely the same, and the most important areas are the striatum, thalamus and intergeniculate leaflet.

Prepulse Inhibition of the Startle Reflex as a Predictor of Vulnerability to Develop Locomotor Sensitization to Cocaine

Prepulse inhibition (PPI) of the startle reflex is a measure of sensory-motor synchronization. A deficit in PPI has been observed in psychiatric patients, especially those with schizophrenia and vulnerable subjects, since the neural bases of this disorder are also involved in the regulation of PPI. Recently, we have reported that baseline PPI levels in mice can predict their sensitivity to the conditioned reinforcing effects of cocaine in the conditioned place preference (CPP) paradigm. Mice with a low PPI presented a lower sensitivity to the conditioned rewarding effects of cocaine; however, once they acquired conditioned preference with a higher dose of the drug, a more persistent associative effect of cocaine with respect to environmental cues was evident in these animals when compared with High-PPI mice. Therefore, we proposed that the PPI paradigm can determine subjects with a higher vulnerability to the effects of cocaine. Developing locomotor sensitization after pre-exposure to cocaine is considered an indicator of transitioning from recreational use to a compulsive consumption of the drug. Thus, the aim of the present study was to evaluate whether subjects with a low PPI display a higher locomotor sensitization induced by cocaine. First, male and female OF1 mice were classified as High- or Low-PPI according to their baseline PPI levels. Subsequently, the motor effects induced by an acute dose of cocaine (Experiments 1 and 2) and the development of locomotor sensitization induced by pre-exposure to this drug (Experiments 3 and 4) were recorded using two apparatuses (Ethovision and actimeter). Low-PPI mice presented low sensitivity to the motor effects of an acute dose of cocaine, but a high increase of activity after repeated administration of the drug, thus suggesting a great developed behavioral sensitization. Differences after pretreatment with cocaine vs. saline were more pronounced among Low-PPI subjects than among High-PPI animals. These results endorse our hypothesis that the PPI paradigm can detect subjects who are more likely to display behaviors induced by cocaine and which can increase the risk of developing a cocaine use disorder. Herein, we further discuss whether a PPI deficit can be considered an endophenotype for cocaine use disorder.

Early exposure to general anesthesia impairs social and emotional development in rats

Several animal and emerging human studies suggest an association between an early exposure to general anesthesia (GA) and long-lasting problems with complex social and emotional behaviors such as inattentiveness, impulsivity, anxiogenic tendencies, as well as difficulties engaging in proper social intercourse, with significant increase in attention deficit and hyperactivity-type behaviors. To further investigate these behaviors, and to examine the potential of presently available rodent behavioral models to guide future assessments of long-term socio-emotional impairments in humans, we examined the long-term effects of GA on anxiety/fear and social behaviors. We exposed male and female Sprague-Dawley infant rats at the peak of their synaptogenesis to either GA containing midazolam (9 mg/kg, i.p.), 70% nitrous oxide (N2O) and 0.75% isoflurane (Iso) administered in 29-30% oxygen (experimental), or air (with 30% oxygen) plus the vehicle, 0.1% dimethyl sulfoxide (Sham) for 6 h. Behavioral experiments were conducted at adolescence (the open-field test) and young adulthood (the open-field test, the elevated plus-maze and the social novelty test). We report that an early exposure to GA during critical stages of brain development results in long-lasting increase in risk-taking tendencies and significant changes in the anxiety-related behaviors when tested in young adult rats. In addition, we noted novelty-seeking tendencies/less guarded behavior with changes in social discrimination. We conclude that early exposure to anesthesia may have lasting influences on emotional and social development. Importantly, our results show that currently used rodent behavioral models could be a good correlate to assess long-term socio-emotional GA-induced impairments observed in humans.

A predator-based psychosocial stress animal model of PTSD in females: Influence of estrous phase and ovarian hormones

Traumatized women are more likely than traumatized men to develop post-traumatic stress disorder (PTSD). Still, the inclusion of females in animal models of PTSD has largely been avoided, likely due to the variable hormone profile of female rodents. Because a valid animal model of PTSD that incorporates females is still needed, we examined the influence of estrous stage and ovarian hormones on the female rat response to a predator-based psychosocial stress model of PTSD. Female Sprague-Dawley rats were exposed to psychosocial stress or control conditions for 31 days. Stressed rats were given two cat exposures and daily social instability; control rats were handled daily. Beginning on Day 32, rats underwent physiological or behavioral testing. In Experiment 1, vaginal smears were collected on days of the first and second cat exposures and each day of behavioral testing to determine estrous stage. In Experiments 2 and 3, ovariectomized or sham control rats were exposed to stress or control conditions. Then, they were given behavioral testing (Exp 2), or their hearts were isolated and subjected to ischemia/reperfusion on a Langendorff isolated heart system (Exp 3). Chronic stress increased anxiety-like behavior, irrespective of estrous stage or ovariectomy condition. Ovariectomized females displayed greater startle responses and anxiety-like behavior than sham rats. Stress had no impact on myocardial sensitivity to ischemic injury; however, ovariectomized females exhibited greater ischemia-induced infarction than sham rats. These findings suggest that ovarian hormones may prevent anxiety-like behavior and be cardioprotective in non-stressed controls, but they do not interact with chronic stress to influence the development of PTSD-like sequelae in female rats.

Hysterectomy Uniquely Impacts Spatial Memory in a Rat Model: A Role for the Nonpregnant Uterus in Cognitive Processes

Approximately one-third of women experience hysterectomy, or the surgical removal of the uterus, by 60 years of age, with most surgeries occurring prior to the onset of natural menopause. The ovaries are retained in about half of these surgeries, whereas for the other half hysterectomy occurs concurrently with oophorectomy. The dogma is that the nonpregnant uterus is dormant. There have been no preclinical assessments of surgical variations in menopause, including hysterectomy, with and without ovarian conservation, on potential endocrine and cognitive changes. We present a novel rat model of hysterectomy alongside sham, ovariectomy (Ovx), and Ovx-hysterectomy groups to assess effects of surgical menopause variations. Rats without ovaries learned the working memory domain of a complex cognitive task faster than did those with ovaries. Moreover, uterus removal alone had a unique detrimental impact on the ability to handle a high-demand working memory load. The addition of Ovx, that is, Ovx-hysterectomy, prevented this hysterectomy-induced memory deficit. Performance did not differ amongst groups in reference memory-only tasks, suggesting that the working memory domain is particularly sensitive to variations in surgical menopause. Following uterus removal, ovarian histology and estrous cycle monitoring demonstrated that ovaries continued to function, and serum assays indicated altered ovarian hormone and gonadotropin profiles by 2 months after surgery. These results underscore the critical need to further study the contribution of the uterus to the female phenotype, including effects of hysterectomy with and without ovarian conservation, on the trajectory of brain and endocrine aging to decipher the impact of common variations in gynecological surgery in women. Moreover, findings demonstrate that the nonpregnant uterus is not dormant, and indicate that there is an ovarian-uterus-brain system that becomes interrupted when the reproductive tract has been disrupted, leading to alterations in brain functioning.

M(4) muscarinic receptors and locomotor activity regulation

M(4) muscarinic receptors (M(4) MR) represent a subfamily of G-protein coupled receptors serving a substantial role in spontaneous locomotor activity regulation, cognition and modulation of cholinergic system. With increasing body of literature discussing the role of M(4) MR some controversies arose. Thus, we try here to summarize the current evidence regarding the M(4) MR, with the special focus on their role in Locomotor activity control. We review the molecular function of M(4) MR in specific brain areas implicated in locomotor regulation, and shortly in other CNS processes that could be connected to locomotor activity. We also focus on brain areas implicated in locomotor activity biorhythm changes like suprachiasmatic nucleus, subparaventricular zone posterior hypothalamic area, striatum and thalamus. Gender-related aspects and differences in locomotor activity in males and females are discussed further.

The deletion of M4 muscarinic receptors increases motor activity in females in the dark phase

OBJECTIVES

M4 muscarinic receptors (MR) presumably play a role in motor coordination. Previous studies have shown different results depending on genetic background and number of backcrosses. However, no attention has been given to biorhythms.

MATERIAL AND METHODS

We therefore analyzed biorhythms under a light/dark cycle obtained telemetrically in intact animals (activity, body temperature) in M4 KO mice growth on the C57Bl6 background using ChronosFit software. Studying pure effects of gene knockout in daily rhythms is especially important knowledge for pharmacological/behavioral studies in which drugs are usually tested in the morning.

RESULTS

We show that M4 KO mice motor activity does not differ substantially from wild-type mice during light period while in the dark phase (mice active part of the day), the M4 KO mice reveal biorhythm changes in many parameters. Moreover, these differences are sex-dependent and are evident in females only. Mesor, night-day difference, and night value were doubled or tripled when comparing female KO versus male KO. Our in vitro autoradiography demonstrates that M4 MR proportion represents 24% in the motor cortex (MOCx), 30% in the somatosensory cortex, 50% in the striatum, 69% in the thalamus, and 48% in the intergeniculate leaflet (IGL). The M4 MR densities were negligible in the subparaventricular zone, the posterior hypothalamic area, and in the suprachiasmatic nuclei.

CONCLUSIONS

We conclude that cholinergic signaling at M4 MR in brain structures such as striatum, MOCx, and probably with the important participation of IGL significantly control motor activity biorhythm. Animal activity differs in the light and dark phases, which should be taken into consideration when interpreting the results.

Evaluating mice lacking serum carboxylesterase as a behavioral model for nerve agent intoxication

Mice and other rodents are typically utilized for chemical warfare nerve agent research. Rodents have large amounts of carboxylesterase in their blood, while humans do not. Carboxylesterase nonspecifically binds to and detoxifies nerve agent. The presence of this natural bioscavenger makes mice and other rodents poor models for studies identifying therapeutics to treat humans exposed to nerve agents. To obviate this problem, a serum carboxylesterase knockout (Es1 KO) mouse was created. In this study, Es1 KO and wild type (WT) mice were assessed for differences in gene expression, nerve agent (soman; GD) median lethal dose (MLD) values, and behavior prior to and following nerve agent exposure. No expression differences were detected between Es1 KO and WT mice in more than 34 000 mouse genes tested. There was a significant difference between Es1 KO and WT mice in MLD values, as the MLD for GD-exposed WT mice was significantly higher than the MLD for GD-exposed Es1 KO mice. Behavioral assessments of Es1 KO and WT mice included an open field test, a zero maze, a Barnes maze, and a sucrose preference test (SPT). While sex differences were observed in various measures of these tests, overall, Es1 KO mice behaved similarly to WT mice. The two genotypes also showed virtually identical neuropathological changes following GD exposure. Es1 KO mice appear to have an enhanced susceptibility to GD toxicity while retaining all other behavioral and physiological responses to this nerve agent, making the Es1 KO mouse a more human-like model for nerve agent research.

Animal tests for anxiety-like and depression-like behavior in rats

An animal model of human behavior represents a complex of cognitive and/or emotional processess, which are translated from animals to humans. A behavioral test is developed primarily and specifically to verify and support a theory of cognition or emotion; it can also be used to verify a theory of a psychopathology, but it is not developed for a particular type of psychopathology. The paper reviews tests commonly used in novel drug discovery research. Focus is especially on tests which can evaluate anxiety-like (openfield test, novelty suppressed feeding, elevated plus maze, light/dark box, stressinduced hyperthermia) and depression-like behaviors (forced swim test, tail suspension test, sucrose preference test) as they represent an important methodological tool in pre-clinical as well as in behavioral toxicology studies.

Late pregnancy is vulnerable period for exposure to BPA

Bisphenol A (BPA) is among the better-known endocrine disruptors. BPA is used in various food-contacting materials and is easily eluted into food; as a result, we are exposed to BPA on a daily basis. In adults, BPA is metabolized and eliminated rapidly from the body. However, numerous reports suggest that fetuses and young children are susceptible to BPA. One of the concerning adverse effects of BPA is disruption of behavior, especially anxiety-like behavior. In order to study the mechanism of influences on offspring, it is important to clarify the most vulnerable gestation period. We hypothesized that offspring in late pregnancy would be more susceptible to BPA, because late pregnancy is a critical time for functional brain development. In this study, C57BL/6 mouse fetuses were exposed prenatally by oral dosing of pregnant dams, once daily from gestational day 5.5 to 12.5 (early pregnancy) or 11.5 to 18.5 (late pregnancy), with BPA (0 or 10 mg/kg body weight). Following birth and weaning, the resulting pups were tested using an elevated plus maze at postnatal week 10. The behavior of the offspring was altered by prenatal BPA exposure during late pregnancy but not during early pregnancy. These results indicated that offspring are more vulnerable to exposure to BPA in late pregnancy.

Acupuncture does not ameliorate metabolic disturbances in the P450 aromatase inhibitor-induced rat model of polycystic ovary syndrome

NEW FINDINGS

What is the central question of this study? The effectiveness of low-frequency electroacupuncture in the treatment of metabolic disorders associated with polycystic ovary syndrome (PCOS), an endocrine-metabolic disorder characterized by an imbalance in sex steroid production, is controversial. What is the main finding and its importance? In a rat model of PCOS induced by the inhibition of P450 aromatase, low-frequency electroacupuncture increased low-density lipoprotein-cholesterol but did not improve the insulin resistance or the adipose tissue dysfunction, suggesting that a balance of sex steroids is needed to restore the metabolic function in this rat model of PCOS. Low-frequency electroacupuncture restores sex steroid synthesis and sympathetic activity in women with polycystic ovary syndrome, which may ameliorate its metabolic disturbances, probably by modulating sympathetic nerve activity or sex steroid synthesis. We investigated whether low-frequency electroacupuncture regulates the metabolic function to the same extent as treatment with estradiol or β-adrenergic blocking in a rat model of polycystic ovary syndrome induced by a P450 aromatase inhibitor (letrozole). Letrozole (200 μg day^-1 ) or placebo pellets were implanted in prepubertal Wistar rats. Six weeks thereafter, rats were treated for 5-6 weeks with the following: low-frequency electroacupuncture (5 days per week); a β-adrenergic blocker (propranolol hydrochloride, 0.1 mg kg^-1 , 5 days per week); or 17β-estradiol (2.0 μg) every fourth day. Body weight development, body composition, locomotor activity, insulin sensitivity, tissue-specific glucose uptake, lipid profile, adipocyte size, serum concentrations of adiponectin and insulin, and gene expression in inguinal fat were measured. All treatments increased circulating levels of low-density lipoprotein-cholesterol. Estradiol treatment restored locomotor activity and increased insulin sensitivity but did not modify the glucose uptake in muscle and fat. An upregulation of genes related to insulin sensitivity and downregulation of genes related to adipogenesis were observed in subcutaneous adipose tissue from rats exposed to letrozole. Only estradiol treatment normalized the expression of these genes. In conclusion, low-frequency electroacupuncture increased low-density lipoprotein-cholesterol without affecting insulin sensitivity or adipose tissue function, which could suggest effects on hepatic lipid regulation, probably mediated by the action of estradiol or the β-adrenergic pathway.

Sex-dependent differences in voluntary physical activity

Numbers of overweight and obese individuals are increasing in the United States and globally, and, correspondingly, the associated health care costs are rising dramatically. More than one-third of children are currently considered obese with a predisposition to type 2 diabetes, and it is likely that their metabolic conditions will worsen with age. Physical inactivity has also risen to be the leading cause of many chronic, noncommunicable diseases (NCD). Children are more physically inactive now than they were in past decades, which may be due to intrinsic and extrinsic factors. In rodents, the amount of time engaged in spontaneous activity within the home cage is a strong predictor of later adiposity and weight gain. Thus, it is important to understand primary motivators stimulating physical activity (PA). There are normal sex differences in PA levels in rodents and humans. The perinatal environment can induce sex-dependent differences in PA disturbances. This Review considers the current evidence for sex differences in PA in rodents and humans. The rodent studies showing that early exposure to environmental chemicals can shape later adult PA responses are discussed. Next, whether there are different motivators stimulating exercise in male vs. female humans are examined. Finally, the brain regions, genes, and pathways that modulate PA in rodents, and possibly by translation in humans, are described. A better understanding of why each sex remains physically active through the life span could open new avenues for preventing and treating obesity in children and adults. © 2016 Wiley Periodicals, Inc.

New method to induce mild traumatic brain injury in rodents produces differential outcomes in female and male Sprague Dawley rats

BACKGROUND

Mild traumatic brain injuries (mTBI) are an increasing health concern due to persistent behavioral and neurological effects. To better understand these effects, researchers frequently rely on animal injury models. Existing models, however, may not adequately reproduce the mechanism of injury as it occurs in humans.

NEW METHOD

Our new model for inducing mTBI in rodents entails acceleration of the animal toward a stationary impact zone to produce rapid rotational movement of the head. The aim of the present experiment was to characterize the effects of this injury in female and male rats on behavior, cognition, and neural plasticity.

RESULTS

mTBI produced the most widespread effects in females: they were more active during recovery within minutes of mTBI and more active in the center of the open field 4days after mTBI. Spatial learning deficits in the water maze were mild but persistent and accompanied by reduced numbers of immature neurons in the hippocampus along with reductions in sera levels of the neurotrophin, BDNF. By contrast, male mTBI rats mainly exhibited mild spatial learning deficits, with no other observed effects.

COMPARISON WITH EXISTING METHODS

Our model induced effects on behavior and biology in rats that aligned with existing models. However, new patterns were observed, particularly when comparing females and males.

CONCLUSIONS

Taken together, these findings confirm the validity of this model and point to key differences between females and males in symptom severity and type. Additionally, our model adds a novel injury mechanism that complements existing rodent models.

Sexually Dimorphic Risk Mitigation Strategies in Rats

The scientific understanding of fear and anxiety-in both normal and pathological forms-is presently limited by a predominance of studies that use male animals and Pavlovian fear conditioning-centered paradigms that restrict and assess specific behaviors (e.g., freezing) over brief sampling periods and overlook the broader contributions of the spatiotemporal context to an animal's behavioral responses to threats. Here, we use a risky "closed economy" system, in which the need to acquire food and water and the need to avoid threats are simultaneously integrated into the lives of rats, to examine sex differences in mitigating threat risk while foraging. Rats lived for an extended period (∼2 months) in enlarged chambers that consisted of a safe, bedded nest and a risky foraging area where footshocks could be delivered unpredictably. We observed that male and female rats used different strategies for responding to the threat of footshock. Whereas male rats increased the size of meals consumed to reduce the overall time spent foraging, female rats sacrificed their metabolic needs in order to avoid shocks. Ovarian hormone fluctuations were shown to exert slight but reliable rhythmic effects on risky decision-making in gonadally intact female rats. However, this did not produce significant differences in approach-avoidance trade-offs between ovariectomized and intact female groups, suggesting that male-female sex differences are not due to the activational effects of gonadal hormones but, rather, are likely to be organized during early development.

Preconception Alcohol Increases Offspring Vulnerability to Stress

The effect of preconception drinking by the mother on the life-long health outcomes of her children is not known, and therefore, in this study using an animal model, we determined the impact of preconception alcohol drinking of the mother on offspring stress response during adulthood. In our preconception alcohol exposure model, adult female rats were fed with 6.7% alcohol in their diet for 4 weeks, went without alcohol for 3 weeks and were bred to generate male and female offspring. Preconception alcohol-exposed offsprings' birth weight, body growth, stress response, anxiety-like behaviors, and changes in stress regulatory gene and protein hormone levels were evaluated. In addition, roles of epigenetic mechanisms in preconception alcohol effects were determined. Alcohol feeding three weeks prior to conception significantly affected pregnancy outcomes of female rats, with respect to delivery period and birth weight of offspring, without affecting maternal care behaviors. Preconception alcohol negatively affected offspring adult health, producing an increased stress hormone response to an immune challenge. In addition, preconception alcohol was associated with changes in expression and methylation profiles of stress regulatory genes in various brain areas. These changes in stress regulatory genes were normalized following treatment with a DNA methylation blocker during the postnatal period. These data highlight the novel possibility that preconception alcohol affects the inheritance of stress-related diseases possibly by epigenetic mechanisms.

The standardized functional observational battery: Its intrinsic value remains in the instrument of measure: The rat

The International Conference on Harmonisation's (ICH) Tripartite Guideline on Safety Pharmacology Studies for Human Pharmaceuticals has adopted the requirement that each new test substance must be tested for effects on the central nervous system prior to "first dose in man". This assessment is required to measure, at a minimum, the effects of the substance on general motor activity, behavioral changes, coordination, sensory/motor reflex responses, and body temperatures. To achieve this goal, ICH S7A recommends a neurobehavioral assessment (usually a functional observational battery (FOB) or modified Irwin test), which is generally undertaken in the rat. There seems to be a growing lack of consensus on the value of the FOB to determine CNS safety. This review highlights the importance of the time, effort and cost of training technicians to familiarize with their instrument of measure, so that each observer is better able to identify and document very subtle changes in behavior that will serve to increase the reliability and validity of these assays with respect to CNS safety assessments.

Sex-dependent programming effects of prenatal glucocorticoid treatment on the developing serotonin system and stress-related behaviors in adulthood

Prenatal stress and overexposure to glucocorticoids (GC) during development may be associated with an increased susceptibility to a number of diseases in adulthood including neuropsychiatric disorders, such as depression and anxiety. In animal models, prenatal overexposure to GC results in hyper-responsiveness to stress in adulthood, and females appear to be more susceptible than males. Here, we tested the hypothesis that overexposure to GC during fetal development has sex-specific programming effects on the brain, resulting in altered behaviors in adulthood. We examined the effects of dexamethasone (DEX; a synthetic GC) during prenatal life on stress-related behaviors in adulthood and on the tryptophan hydroxylase-2 (TpH2) gene expression in the adult dorsal raphe nucleus (DRN). TpH2 is the rate-limiting enzyme for serotonin (5-HT) synthesis and has been implicated in the etiology of human affective disorders. Timed-pregnant rats were treated with DEX from gestational days 18-22. Male and female offspring were sacrificed on the day of birth (postnatal day 0; P0), P7, and in adulthood (P80-84) and brains were examined for changes in TpH2 mRNA expression. Adult animals were also tested for anxiety- and depressive- like behaviors. In adulthood, prenatal DEX increased anxiety- and depressive- like behaviors selectively in females, as measured by decreased time spent in the center of the open field and increased time spent immobile in the forced swim test, respectively. Prenatal DEX increased TpH2 mRNA selectively in the female caudal DRN at P7, whereas it decreased TpH2 mRNA selectively in the female caudal DRN in adulthood. In animals challenged with restraint stress in adulthood, TpH2 mRNA was significantly lower in rostral DRN of prenatal DEX-treated females compared to vehicle-treated females. These data demonstrated that prenatal overexposure to GC alters the development of TpH2 gene expression and these alterations correlated with lasting behavioral changes found in adult female offspring.

Prenatal and lactational exposure to low-doses of bisphenol A alters adult mice behavior

Bisphenol A (BPA) is an endocrine-disrupting chemical, widely used in dentistry and various industries. We previously reported that BPA affected murine neocortical development by accelerating neuronal differentiation/migration, resulting in abnormal neocortical architecture as well as aberrant thalamocortical connections in the brains of adult mice. The aim of this study was to investigate whether prenatal and lactational BPA exposure affected behavior in adult mice. Pregnant mice were injected subcutaneously with 20μg/kg of BPA daily from embryonic day 0 (E0) until postnatal day 21 (P21). Control animals received a vehicle alone. Behavioral tests (n=15-20) were conducted at postnatal 3weeks (P3W) and P10-15W. After an open-field test, an elevated plus maze and Morris water maze tests were performed. The total distance in the elevated plus maze test at P3W and in the open-field test at P10W was significantly decreased in the BPA-exposed group, compared with the control group. Significant sex differences were observed in the time spent in the central area in the open-field test at P3W and in the total distance in the elevated plus maze test at P11W. These results indicated that prenatal and lactational BPA exposure disturbed the murine behavior in the postnatal development period and the adult mice.

Inhibition of 5α-reductase activity in late pregnancy decreases gestational length and fecundity and impairs object memory and central progestogen milieu of juvenile rat offspring

Psychological, physical and/or immune stressors during pregnancy are associated with negative birth outcomes, such as preterm birth and developmental abnormalities. In rodents, prenatal stressors can alter the expression of 5α-reductase enzymes in the brain and may influence cognitive function and anxiety-type behaviour in the offspring. Progesterone plays a critical role in maintaining gestation. In the present study, it was hypothesised that 5α-reduced progesterone metabolites influence birth outcomes and/or the cognitive and neuroendocrine function of the offspring. 5α-Reduced steroids were manipulated in pregnant Long-Evans rats via the administration of vehicle, the 5α-reduced, neuroactive metabolite of progesterone, 5α-pregnan-3α-ol-20-one (3α,5α-THP, allopregnanolone; 10 mg/kg/ml, s.c.), or the 5α-reductase inhibitor, finasteride (50 mg/kg/ml, s.c.), daily from gestational days 17-21. Compared to vehicle or 3α,5α-THP treatment, finasteride, significantly reduced the length of gestation and the number of pups per litter found in the dams' nests after parturition. The behaviour of the offspring in hippocampus-dependent tasks (i.e. object recognition, open field) was examined on post-natal days 28-30. Compared to vehicle-exposed controls, prenatal 3α,5α-THP treatment significantly increased motor behaviour in females compared to males, decreased progesterone content in the medial prefrontal cortex (mPFC) and diencephalon, increased 3α,5α-THP and 17β-oestradiol content in the hippocampus, mPFC and diencephalon, and significantly increased serum corticosterone concentrations in males and females. Prenatal finasteride treatment significantly reduced object recognition, decreased hippocampal 3α,5α-THP content, increased progesterone concentration in the mPFC and diencephalon, and increased serum corticosterone concentration in female (but not male) juvenile offspring, compared to vehicle-exposed controls. Thus, inhibiting the formation of 5α-reduced steroids during late gestation in rats reduces gestational length, the number of viable pups per litter, and impairs cognitive and neuroendocrine function in the juvenile offspring.

Progesterone turnover to its 5α-reduced metabolites in the ventral tegmental area of the midbrain is essential for initiating social and affective behavior and progesterone metabolism in female rats

BACKGROUND

Among women and female rodents, progesterone (P) influences social affiliation and affect. These effects may be partly due to formation of its 5α-reduced, 3α- hydroxylated metabolite, 5α-pregnan-3α-ol-20-one (3α,5α- THP).

AIM

To elucidate whether actions of 3α,5α-THP in the midbrain ventral tegmental area (VTA) are both necessary and sufficient to enhance non-sexual and sexual social behaviors, affect, and central 3α,5α-THP metabolism.

MATERIALS AND METHODS

P and 3α,5α-THP formation were unperturbed or blocked in VTA via infusions of vehicle, PK11195 (400 ng), and/or indomethacin (10 μg). Rats then received subsequent infusions of vehicle or 3α,5α-THP (100 ng) and were assessed in a battery of tasks that included open field (exploration), elevated plus maze (anxiety behavior), social interaction (social affiliation), and paced mating (sexual behavior) or were not tested. Metabolic turnover of P to its 5α-reduced metabolites was assessed in plasma, midbrain, hippocampus, frontal cortex, diencephalon, and remaining subcortical tissues (control interbrain).

RESULTS

Infusions of any combination of inhibitors significantly reduced social and affective behavior in all tasks compared to vehicle, concomitant with reduced turnover of P to its 5α-reduced metabolites, in midbrain only. Subsequent infusions of 3α,5α-THP significantly reinstated/enhanced anti- anxiety behavior, lordosis, and P turnover to its 5α-reduced metabolites in midbrain, as well as hippocampus, cortex, and diencephalon (but not plasma or interbrain).

CONCLUSIONS

These data are the first to provide direct evidence that actions of 3α,5α-THP in the VTA are both necessary and sufficient for social and affective behavior, as well as initiation of central 5α-reduction.

Sex differences in the burying behavior test in middle-aged rats: effects of diazepam

The full behavioral profile displayed during the burying behavior test was studied in middle aged (11-14 months) males, females with irregular estrous cycles, and females in persistent diestrus, with and without diazepam (0.5-2.0mg/kg). Ambulation and motor coordination were also tested to discern behavioral changes from general motor alterations. Without diazepam treatment, middle-aged males showed longer burying behavior latencies, more prod explorations and less freezing than both groups of females. Untreated middle aged males also showed less cumulative burying and more immobility compared to females with irregular cycles. None of the parameters showed any difference between the female groups. Diazepam (0.5 and 1.0mg/kg) increased burying behavior latency in females, but had no effect on any parameter in middle aged males. However, a higher dose (2.0mg/kg) of diazepam increased immobility, freezing and the number of prod shocks and decreased prod explorations and groomings, but impaired motor coordination in males. In contrast with young males and females, diazepam at any dose reduced cumulative burying. Data are discussed on the bases of (1) sex and age differences in burying behavior and on (2) the anxiolytic-like action of diazepam and its side effects.

Neurotoxicity of endocrine disruptors: possible involvement in brain development and neurodegeneration

Environmental chemicals that act as endocrine disruptors do not appear to pose a risk to human reproduction; however, their effects on the central nervous systems are less well understood. Animal studies suggested that maternal exposure to endocrine-disrupting chemicals (EDC) produced changes in rearing behavior, locomotion, anxiety, and learning/memory in offspring, as well as neuronal abnormalities. Some investigations suggested that EDC exert effects on central monoaminergic neurons, especially dopaminergic neurons. Our data demonstrated that EDC attenuate the development of dopaminergic neurons, which might be involved in developmental disorders. Perinatal exposure to EDC might affect neuronal plasticity in the hippocampus, thereby potentially modulating neuronal development, leading to impaired cognitive and memory functions. Endocrine disruptors also attenuate gender differences in brain development. For example, the locus ceruleus is larger in female rats than in males, but treatments with bisphenol-A (BPA) enlarge this region in males. Some reports indicated that EDC induce hypothyroidism, which might be evidenced as abnormal brain development. Endocrine disruptors might also affect mature neurons, resulting in neurodegenerative disorders such as Parkinson's disease. The current review focused on alterations in the brain induced by EDC, specifically on the possible involvement of EDC in brain development and neurodegeneration.

Effects of perinatal cocaine exposure on open field behavior and the response to corticotropin releasing hormone (CRH) in rat offspring

Previous reports indicate that prenatal cocaine exposure alters specific behaviors and hypothalamic-pituitary-adrenal axis (HPA) function in the offspring. In most previous studies, cocaine was given via subcutaneous injections. However intravenous administration more closely mimics human cocaine abuse during pregnancy. Therefore, we investigated the effects of prenatal cocaine exposure via intravenous injection to the mothers on open field behavior and HPA axis function of the offspring. We hypothesized that prenatal cocaine exposure decreases immobility in a novel environment, and enhances the HPA response to stress. Dams received cocaine (COC) or vehicle (control, CON) intravenously from gestation day 8 to postnatal day (PD) 5. Behaviors were recorded in the open field on PD 28 (weanlings). As expected, perinatally cocaine-exposed offspring spent less time immobile and had a longer latency to entering the center zone. No other behavioral activities were different between the groups. On PD 43-50, adolescent male and female offspring received either corticotropin releasing hormone (CRH) or saline intravenously. Plasma adrenocorticotropic hormone (ACTH) and corticosterone (CORT) levels were determined before, and up to 60 min after injection. COC-exposed offspring of both sexes had higher basal CORT levels. Prenatal cocaine enhanced the CORT response to CRH/saline injections up to 60 min in males but not in females. These novel results show that perinatal administration of cocaine in a manner that most closely mimics human cocaine use has long-term effects on the offspring's behavioral response to stress and on HPA axis functions.

The ontogeny of exploratory behavior in male and female adolescent rats (Rattus norvegicus)

During adolescence, rats gain independence from their mothers and disperse from the natal burrow, with males typically dispersing further than females. We predicted that, if dispersal patterns are associated with responsiveness to novelty, exploratory behavior in novel environments would increase across adolescence, and males would explore more than females. Alternatively, females might explore more than males, if females are more motivated than males to learn about the immediate environment or if females have poorer spatial abilities than males. Twenty-five male and 21 female rats were exposed to two novel environments (open field and elevated plus-maze) during early, mid-, or late adolescence. Total locomotion and amount of exploration directed towards aversive areas increased across adolescence, even when body weight was included as a covariate. Female adolescents locomoted more and spent more time exploring aversive areas than males. Developmental changes in neural function potentially underlie age and sex differences in exploratory behavior. © 2009 Wiley Periodicals, Inc. Dev Psychobiol 51: 513–520, 2009.

Adolescent female rats are more resistant than males to the effects of early stress on prefrontal cortex and impulsive behavior

We tested the hypothesis that adolescent Sprague-Dawley females may be more resistant than males to display impulsive behavior and lower prefrontal cortex thickness after mother-infant separation (MS). Starting at postnatal day 2 (P2), the MS group was separated 6 hr/day and the early handled (EH) group 15 min/day for 10 days, and another group was standard facility reared (SFR). Subjects were examined for novel open-field activity (P28), light-dark apparatus (P29), familiar open-field (P30) and frontal cortical thickness. This protocol resulted in impulsive behavior in MS rats relative to EH and SFR, but this effect was less pronounced in females than males. MS affected the two sexes differently in terms of decreased prefrontal cortex dorsoventral thickness, with this effect being significant in males but not females. Neuroanatomical and behavioral documentation that adolescent females are more resistant than males to ADHD-like effects of maternal separation have not been previously reported.