Estrogen abolishes latent inhibition in ovariectomized female rats

ZFP804A mutant mice display sex-dependent schizophrenia-like behaviors

Genome-wide association studies uncovered the association of ZNF804A (Zinc-finger protein 804A) with schizophrenia (SZ). In vitro data have indicated that ZNF804A might exert its biological roles by regulating spine and neurite morphogenesis. However, no in vivo data are available for the role of ZNF804A in psychiatric disorders in general, SZ in particular. We generated ZFP804A mutant mice, and they showed deficits in contextual fear and spatial memory. We also observed the sensorimotor gating impairment, as revealed by the prepulse inhibition test, but only in female ZFP804A mutant mice from the age of 6 months. Notably, the PPI difference between the female mutant and control mice was no longer existed with the administration of Clozapine or after the ovariectomy. Hippocampal long-term potentiation was normal in both genders of the mutant mice. Long-term depression was absent in male mutants, but facilitated in the female mutants. Protein levels of hippocampal serotonin-6 receptor and GABAB1 receptor were increased, while those of cortical dopamine 2 receptor were decreased in the female mutants with no obvious changes in the male mutants. Moreover, the spine density was reduced in the cerebral cortex and hippocampus of the mutant mice. Knockdown of ZFP804A impaired the neurite morphogenesis of cortical and hippocampal neurons, while its overexpression enhanced neurite morphogenesis only in the cortical neurons in vitro. Our data collectively support the idea that ZFP804A/ZNF804A plays important roles in the cognitive functions and sensorimotor gating, and its dysfunction may contribute to SZ, particularly in the female patients.

Age, sex and pre-exposure effects on acquisition and generalization of conditioned taste aversion in rats

The main aim of the present study was to assess the effect of sex and aging in two pre-exposure learning effects, latent inhibition (LI) and perceptual learning (PL), with a conditioned taste aversion paradigm. Young adult (90 days) and aged (more than 18 months) males and females received 8 pre-exposure trials either with stimulus AX (LI conditions) or BX (PL conditions). Then, all animals received a conditioning trial with AX and two test trials, one with AX and other with BX. The level of generalization between AX and BX was assessed by means of the absolute level of consumption of BX and by the difference in consumption between both stimuli. The results showed an attenuation of latent inhibition as well a stronger generalization of conditioned taste aversion in females when generalization is inferred from the BX consumption. A facilitation of conditioning for the aged animals was also found regardless of the pre-exposed stimulus. Pre-exposures to BX resulted in little generalization, but pre-exposures to AX resulted in a very similar consumption of both compounds, indicating a strong generalization between them. Overall, the study provided novel evidence about the effect of sex and aging on taste aversion, raising at the same time some relevant questions about perceptual learning and how such pre-exposure effect has been typically assessed.

Sex Differences in Cue Competition Effects With a Conditioned Taste Aversion Preparation

This study aimed to test whether male and female rats might show differences in cue competition effects in a conditioned taste aversion (CTA) model. Experiment 1 tested for sex differences in overshadowing. After conditioning of a flavored compound AB or only one simple flavor A (being A and B a solution of sugar 10% and salt 1%, counterbalanced), consumption of the A solution at test was larger in the former than in the latter case only in males. Thus, the usual effect of overshadowing was observed in males but not in females. Experiment 2 examined sex differences in blocking with the same stimuli used in Experiment 1. After conditioning of AB, the consumption of B was larger for the animals that previously received a single conditioning trial with A than for those that received unpaired presentations of A and the illness. As observed in Experiment 1, the typical blocking effect appeared only in males but not in females. The present findings thus support the hypothesis that sex dimorphism might be expressed in classical conditioning, or at least, in cue competition effects such as overshadowing and blocking with a taste aversion model.

Aromatized testosterone attenuates contextual generalization of fear in male rats

Generalization is a common symptom of many anxiety disorders, and females are 60% more likely to suffer from an anxiety disorder than males. We have previously demonstrated that female rats display significantly accelerated rates of contextual fear generalization compared to male rats; a process driven, in part, by activation of ERβ. The current study was designed to determine the impact of estrogens on contextual fear generalization in male rats. For experiment 1, adult male rats were gonadectomized (GDX) and implanted with a capsule containing testosterone proprionate, estradiol, dihydrotestosterone proprionate (DHT), or an empty capsule. Treatment with testosterone or estradiol maintained memory precision when rats were tested in a different (neutral) context 1day after training. However, male rats treated with DHT or empty capsules displayed significant levels of fear generalization, exhibiting high levels of fear in the neutral context. In Experiment 2, we used acute injections of gonadal hormones at a time known to elicit fear generalization in female rats (e.g. 24h before testing). Injection treatment followed the same pattern of results seen in Experiment 1. Finally, animals given daily injections of the aromatase inhibitor, Fadrozole, displayed significant fear generalization. These data suggest that testosterone attenuates fear generalization likely through the aromatization testosterone into estradiol as animals treated with the non-aromatizable androgen, DHT, or animals treated with Fadrozole, displayed significant generalized fear. Overall, these results demonstrate a sex-dependent effect of estradiol on the generalization of contextual fear.

Perspectives on fear generalization and its implications for emotional disorders

Although generalization to conditioned stimuli is not a new phenomenon, renewed interest in understanding its biological underpinning has stemmed from its association with a number of anxiety disorders. Generalization as it relates to fear processing is a temporally dynamic process in which animals, including humans, display fear in response to similar yet distinct cues or contexts as the time between training and testing increases. This Review surveys the literature on contextual fear generalization and its relation to several views of memory, including systems consolidation, forgetting, and transformation hypothesis, which differentially implicate roles of the hippocampus and neocortex in memory consolidation and retrieval. We discuss recent evidence on the neurobiological mechanisms contributing to the increase in fear generalization over time and how generalized responding may be modulated by acquisition, consolidation, and retrieval mechanisms. Whereas clinical perspectives of generalization emphasize a lack of fear inhibition to CS^- cues or fear toward intermediate CS cues, the time-dependent nature of generalization and its relation to traditional views on memory consolidation and retrieval are often overlooked. Understanding the time-dependent increase in fear generalization has important implications not only for understanding how generalization contributes to anxiety disorders but also for understanding basic long-term memory function. © 2016 Wiley Periodicals, Inc.

Hippocampal cytosolic estrogen receptors regulate fear generalization in females

Generalization of fear responses is a symptom of many anxiety disorders and we have previously demonstrated that female rats generalize fear to a neutral context at a faster rate compared to males. This effect is due in part, to activation of ER and modulation of memory retrieval mechanisms resulting in fear generalization. Given that the effects of estradiol on fear generalization required approximately 24h, our data suggested possible genomic actions on fear generalization. To determine whether these actions were due to cytosolic versus membrane bound receptors, female rats were given infusions of ICI 182,780, a cytosolic estrogen receptor antagonist, into the lateral ventricle or dorsal hippocampus simultaneously with estradiol treatment or with an ER agonist (DPN). Infusions of ICI into the lateral ventricle or the dorsal hippocampus blocked fear generalization induced by peripheral or central treatment with estradiol or DPN, suggesting that estradiol acts through cytosolic ERβ receptors. In further support of these findings, intracerebroventricular or intra-hippocampal infusions of bovine serum conjugated estradiol (E2-BSA), activating membrane-bound estrogen receptors only, did not induce fear generalization. Moreover, rats receiving intra-hippocampal infusions of the ERK/MAPK inhibitor, U0126, continued to display estradiol-induced generalization, again suggesting that membrane-bound estrogen receptors do not contribute to fear generalization. Overall, these data suggest that estradiol-induced enhancements in fear generalization are mediated through activation of cytosolic/nuclear ER within the dorsal hippocampus. This region seems to be an important locus for the effects of estradiol on fear generalization although additional neuroanatomical regions have yet to be identified.

Estrogen receptors in the central nervous system and their implication for dopamine-dependent cognition in females

This article is part of a Special Issue "Estradiol and cognition". Over the past 30 years, research has demonstrated that estrogens not only are important for female reproduction, but also play a role in a diverse array of cognitive functions. Originally, estrogens were thought to have only one receptor, localized exclusively to the cytoplasm and nucleus of cells. However, it is now known that there are at least three estrogen receptors (ERs): ERα, ERβ and G-protein coupled ER1 (GPER1). In addition to being localized to nuclei, ERα and ERβ are localized to the cell membrane, and GPER1 is also observed at the cell membrane. The mechanism through which ERs are associated with the membrane remains unclear, but palmitoylation of receptors and associations between ERs and caveolin are implicated in membrane association. ERα and ERβ are mostly observed in the nucleus using light microscopy unless they are particularly abundant. However, electron microscopy has revealed that ERs are also found at the membrane in complimentary distributions in multiple brain regions, many of which are innervated by dopamine inputs and were previously thought to contain few ERs. In particular, membrane-associated ERs are observed in the prefrontal cortex, dorsal striatum, nucleus accumbens, and hippocampus, all of which are involved in learning and memory. These findings provide a mechanism for the rapid effects of estrogens in these regions. The effects of estrogens on dopamine-dependent cognition likely result from binding at both nuclear and membrane-associated ERs, so elucidating the localization of membrane-associated ERs helps provide a more complete understanding of the cognitive effects of these hormones.

Activation of ERβ modulates fear generalization through an effect on memory retrieval

Women are 60% more likely to suffer from an anxiety disorder than men. One hypothesis for this difference may be that females exhibit increased rates of fear generalization. Females generalize fear to a neutral context faster than males, a process driven, in part, by estrogens. In the current study, ovariectomized adult female Long-Evans rats were given acute injections of estradiol benzoate (15μg/0.1mL sesame oil) or sesame oil during a passive avoidance procedure to determine if estrogens increase fear generalization through an effect on fear memory acquisition/consolidation or through fear memory retrieval. Animals injected 1h prior to training generalized to the neutral context 24h later but not 7days after training. Generalization was also seen when injections occurred 24h before testing, but not when tested at immediate (1h) or intermediate (6h) time points. In Experiment 3, animals were injected with estrogen receptor (ER) agonists, PPT or DPN, to determine which ER subtype(s) increased fear generalization. Only the ERβ agonist, DPN, increased fear generalization when testing occurred 24h after injection. Our results indicate that estradiol increases fear generalization through an effect on fear memory retrieval mechanisms by activation of ERβ.

Sex differences and chronic stress effects on the neural circuitry underlying fear conditioning and extinction

There are sex differences in the rates of many stress-sensitive psychological disorders such as posttraumatic stress disorder (PTSD). As medial prefrontal cortex and amygdala are implicated in many of these disorders, understanding differential stress effects in these regions may shed light on the mechanisms underlying sex-dependent expression of disorders like depression and anxiety. Prefrontal cortex and amygdala are key regions in the neural circuitry underlying fear conditioning and extinction, which thus has emerged as a useful model of stress influences on the neural circuitry underlying regulation of emotional behavior. This review outlines the current literature on sex differences and stress effects on dendritic morphology within medial prefrontal cortex and basolateral amygdala. Such structural differences and/or alterations can have important effects on fear conditioning and extinction, behaviors that are mediated by the basolateral amygdala and prefrontal cortex, respectively. Given the importance of extinction-based exposure therapy as a treatment for anxiety disorders such as PTSD, understanding the neural mechanisms by which stress differentially influences fear learning and extinction in males and females is an important goal for developing sex-appropriate interventions for stress-related disorders.

Deficits in latent inhibition induced by estradiol replacement are ameliorated by haloperidol treatment

There are sex differences in the symptomatology of schizophrenia, and in the response to antipsychotic treatments. One hallmark symptom of schizophrenia is a deficit in selective attention. Selective attention can be measured using a latent inhibition (LI) paradigm in humans; LI can be measured in rodents, and is used as an animal model of the selective attention deficits observed in schizophrenia. In the current experiments LI was used to clarify whether selective attention differs between male rats and ovariectomized (OVX) female rats receiving different estradiol (E2) replacement regimens. An additional aim was to determine whether haloperidol’s (HAL) facilitation of LI is enhanced by E2. Males and OVX female rats were trained in a conditioned emotional response LI paradigm. Females received no E2 replacement, a chronic low dose of E2 via silastic capsule, or a high phasic dose of E2 via silastic capsule accompanied by E2 (10 µg/kg subcutaneous (SC)) injections every 4th day. Actual plasma levels of E2 were determined using an enzyme linked immunosorbent assay. Rats were also administered a vehicle treatment, a 0.05 mg/kg, or a 0.1 mg/kg IP injection of HAL. Males and OVX females that did not receive E2 replacement both exhibited LI, but LI was not observed in the low and high E2 replacement groups. HAL restored LI at a lower dose in the females receiving high E2 replacement compared to females receiving low E2 replacement, indicating that E2 replacement facilitates HAL in restoring LI.

Estradiol and striatal dopamine receptor antagonism influence memory system bias in the female rat

Estradiol (E2) has been shown to influence learning and memory systems used by female rats to find a reward. Rats with high levels of E2 tend to use allocentric, or place, memory while rats with low levels of E2 use egocentric, or response, memory. It has been shown that systemic dopamine receptor antagonism interacts with E2 to affect which memory system is used. Here, dopamine antagonists were administered directly into either the dorsal striatum or nucleus accumbens to determine where in the brain this interaction takes place. Seventy-four young adult, female, Sprague-Dawley rats were trained and tested in a modified plus-maze. All rats were ovariectomized, received a subcutaneous low E2 implant, and were implanted with bilateral cannulae into either the dorsal striatum or the nucleus accumbens. Additionally, high E2 rats received daily injections of E2 in a sesame oil solution while low E2 rats received daily injections of vehicle. After reaching criterion levels of performance in a plus-maze task, rats were administered microinjections of either a dopamine D1 receptor (SCH 23390; 0.1 μg/ml and 0.01 μg/ml) or D2 receptor (raclopride; 2 μg/ml and 0.5 μg/ml) antagonist or a vehicle control (saline) in a counterbalanced manner. High E2 rats exhibited a trend towards a place memory bias while low E2 rats showed a response memory bias. Dorsal striatal administration of a D1, but not D2, dopamine receptor antagonist caused a switch in the memory system used by both high and low E rats. There was no significant effect of dopamine receptor antagonism in the nucleus accumbens group. Thus, E2 determined which memory system controlled behavior in a plus-maze task. Moreover, this effect was modulated by dopamine D1R antagonism in the dorsal but not ventral striatum suggesting that memory systems are, in part, mediated by E2 and dopamine in this region.

Estrogen receptors are found in glia and at extranuclear neuronal sites in the dorsal striatum of female rats: evidence for cholinergic but not dopaminergic colocalization

Estrogens rapidly affect dopamine (DA) neurotransmission in the dorsal striatum (dSTR) and DA-related diseases, such as Parkinson's disease and schizophrenia. How estrogens influence DA function remains unclear, in part, because the ultrastructural localization of estrogen receptors (ER) in the dSTR is not known. Light microscopic studies of the dSTR have suggested the presence of ER. This experiment used electron microscopy to determine whether these ER are at extranuclear sites in the dSTR, providing evidence for a mechanism through which estrogen could rapidly affect DA transmission. The dSTR was labeled with antibodies for ERα, ERβ, and G protein-coupled ER 1 (GPER-1) to confirm whether these ER were present in this brain area. After this, the dSTR was dual labeled with antibodies for ERα or GPER-1 and tyrosine hydroxylase or vesicular acetylcholine transporter to determine whether ER are localized to dopaminergic and/or cholinergic processes, respectively. Ultrastructural analysis revealed immunoreactivity (IR) for ERα, ERβ, and GPER-1 exclusively at extranuclear sites throughout the dSTR. ERα-, ERβ-, and GPER-1-IR are mostly frequently observed in axons and glial profiles but are also localized to other neuronal profiles. Dual labeling revealed that ERα- and GPER-1-IR is not associated with DA axons and terminals but is sometimes associated with cholinergic neurons. Because these receptors are exclusively extranuclear in the dSTR, binding at these receptors likely affects neurotransmission via nongenomic mechanisms.

Sex steroids and schizophrenia

The peak in incidence for schizophrenia is during late adolescence for both sexes, but within this time frame the peak is both earlier and steeper for males. Additionally, women have a second peak in incidence following menopause. Two meta-analyses have reported that men have an overall ∼40% greater chance of developing schizophrenia than do women (Aleman et al., 2003; McGrath et al., 2004). These and other findings have led to the suggestion that ovarian hormones may be protective against schizophrenia. Less explored is the potential role of testosterone in schizophrenia, although disruptions in steroid levels have also been reported in men with the illness. The relationship between increased gonadal hormone release per se and peri-adolescent vulnerability for psychiatric illness is difficult to tease apart from other potentially contributory factors in clinical studies, as adolescence is a turbulent period characterized by many social and biological changes. Despite the obvious opportunity provided by animal research, surprisingly little basic science effort has been devoted to this important issue. On the other hand, the animal work offers an understanding of the many ways in which gonadal steroids exert a powerful impact on the brain, both shaping its development and modifying its function during adulthood. Recently, investigators using preclinical models have described a greater male vulnerability to neurodevelopmental insults that are associated with schizophrenia; such studies may provide clinically relevant insights into the role of gonadal steroids in psychiatric illness.

Progestogens influence cognitive processes in aging

Steroid hormones may alter mnemonic processes. The majority of investigations have focused on the effects of 17β-estradiol (E(2)) to mediate learning. However, progesterone (P(4)), which varies across endogenous hormonal milieu with E(2), may also have effects on cognitive processes. P(4) may have effects in the hippocampus, prefrontal cortex (PFC) and/or striatum to enhance cognitive performance. Cognitive performance/learning has been assessed using tasks that are mediated by the hippocampus (water maze), PFC (object recognition) and striatum (conditioning). Our findings suggest that progestogens can have pervasive effects to enhance cognitive performance and learning in tasks mediated by the hippocampus, PFC and striatum and that these effects may be in part independent of actions at intracellular progestin receptors. Progestogens may therefore influence cognitive processes.

Latent inhibition is affected by phase of estrous cycle in female rats

Estrogen has been shown to have a strong modulatory influence on several types of cognition in both women and female rodents. Latent inhibition is a task in which pre-exposure to a neutral stimulus, such as a tone, later impedes the association of that stimulus with a particular consequence, such as a shock. Previous work from our lab demonstrates that high levels of estradiol (E2) administered to ovariectomized (OVX) female rats abolishes latent inhibition when compared to female rats with low levels of E2 or male rats. To determine if this E2-induced impairment also occurs with the natural variations of ovarian hormones during the estrous cycle, this behavior was investigated in cycling female rats. In addition, pre-pubertal male and female rats were also tested in this paradigm to determine if the previously described sex differences are activational or organizational in nature. In a latent inhibition paradigm using a tone and a shock, adult rats were conditioned during different points of the estrous cycle. Rats conditioned during proestrus, a period of high E2 levels, exhibited attenuated latent inhibition when compared to rats conditioned during estrus or metestrus, periods associated with low levels of E2. Moreover, this effect is not seen until puberty indicating it is dependent on the surge of hormones at puberty. This study confirms recent findings that high E2 interferes with latent inhibition and is the first to show this is based in the activational actions of hormones.

Sex-dependent antipsychotic capacity of 17β-estradiol in the latent inhibition model: a typical antipsychotic drug in both sexes, atypical antipsychotic drug in males

The estrogen hypothesis of schizophrenia suggests that estrogen is a natural neuroprotector in women and that exogenous estrogen may have antipsychotic potential, but results of clinical studies have been inconsistent. We have recently shown using the latent inhibition (LI) model of schizophrenia that 17β-estradiol exerts antipsychotic activity in ovariectomized (OVX) rats. The present study sought to extend the characterization of the antipsychotic action of 17β-estradiol (10, 50 and 150 μg/kg) by testing its capacity to reverse amphetamine- and MK-801-induced LI aberrations in gonadally intact female and male rats. No-drug controls of both sexes showed LI, ie, reduced efficacy of a previously non-reinforced stimulus to gain behavioral control when paired with reinforcement, if conditioned with two but not five tone-shock pairings. In both sexes, amphetamine (1 mg/kg) and MK-801 (50 μg/kg) produced disruption (under weak conditioning) and persistence (under strong conditioning) of LI, modeling positive and negative/cognitive symptoms, respectively. 17β-estradiol at 50 and 150 μg/kg potentiated LI under strong conditioning and reversed amphetamine-induced LI disruption in both males and females, mimicking the action of typical and atypical antipsychotic drugs (APDs) in the LI model. 17β-estradiol also reversed MK-induced persistent LI, an effect mimicking atypical APDs and NMDA receptor enhancers, but this effect was observed in males and OVX females but not in intact females. These findings indicate that in the LI model, 17β-estradiol exerts a clear-cut antipsychotic activity in both sexes and, remarkably, is more efficacious in males and OVX females where it also exerts activity considered predictive of anti-negative/cognitive symptoms.

Chronic stress, cyclic 17β-estradiol, and daily handling influences on fear conditioning in the female rat

Chronic stress and estrogens alter many forebrain regions in female rats that affect cognition. In order to investigate how chronic stress and estrogens influence fear learning and memory, we ovariectomized (OVX) female Sprague-Dawley rats and repeatedly injected them (s.c.) with 17β-estradiol (E, 10 μg/250 g or sesame oil vehicle, VEH). Concurrently, rats were restrained for 6 h/d/21 d (STR) or left undisturbed (CON). Rats were then fear conditioned with 4 tone-footshock pairings and then after 1 h and 24 h delays, given 15 tone extinction trials. Regardless of E treatment, chronic stress (VEH, E) facilitated freezing to tone during acquisition and extinction following a 1h delay, but not during extinction after a 24 h delay. E did not influence freezing to tone during any phase of fear conditioning for either the control or chronically stressed rats, but did influence contextual conditioning that may have been carried predominately by the STR group. In the second experiment, we investigated "handling" influences on fear conditioning acquisition, given the disparate findings from the current study and previous work (Baran, Armstrong, Niren, & Conrad, 2010; Baran, Armstrong, Niren, Hanna, & Conrad, 2009). Female rats remained gonadally-intact since E did not influence tone fear conditioning. Indeed, brief daily handling (1-3 m/d/21 d) facilitated acquisition of fear conditioning in chronically stressed female rats, and either had no effect or slightly attenuated fear conditioning in controls. Thus, chronic stress impacts amygdala-mediated fear learning in both OVX- and gonadally-intact females as found previously in males, with handling significantly influencing these outcomes.

Contrasting effects of increased and decreased dopamine transmission on latent inhibition in ovariectomized rats and their modulation by 17beta-estradiol: an animal model of menopausal psychosis?

Women with schizophrenia have later onset and better response to antipsychotic drugs (APDs) than men during reproductive years, but the menopausal period is associated with increased symptom severity and reduced treatment response. Estrogen replacement therapy has been suggested as beneficial but clinical data are inconsistent. Latent inhibition (LI), the capacity to ignore irrelevant stimuli, is a measure of selective attention that is disrupted in acute schizophrenia patients and in rats and humans treated with the psychosis-inducing drug amphetamine and can be reversed by typical and atypical APDs. Here we used amphetamine (1 mg/kg)-induced disrupted LI in ovariectomized rats to model low levels of estrogen along with hyperfunction of the dopaminergic system that may be occurring in menopausal psychosis, and tested the efficacy of APDs and estrogen in reversing disrupted LI. 17beta-Estradiol (50, 150 microg/kg), clozapine (atypical APD; 5, 10 mg/kg), and haloperidol (typical APD; 0.1, 0.3 mg/kg) effectively reversed amphetamine-induced LI disruption in sham rats, but were much less effective in ovariectomized rats; 17beta-estradiol and clozapine were effective only at high doses (150 microg/kg and 10 mg/kg, respectively), whereas haloperidol failed at both doses. Haloperidol and clozapine regained efficacy if coadministered with 17beta-estradiol (50 microg/kg, an ineffective dose). Reduced sensitivity to dopamine (DA) blockade coupled with spared/potentiated sensitivity to DA stimulation after ovariectomy may provide a novel model recapitulating the combination of increased vulnerability to psychosis with reduced response to APD treatment in female patients during menopause. In addition, our data show that 17beta-estradiol exerts antipsychotic activity.

Prefrontal cortex lesions and sex differences in fear extinction and perseveration

Electrolytic lesions of the medial prefrontal cortex (PFCX) were examined using fear conditioning to assess the recall of fear extinction and performance in the Y-maze, open field, and object location/recognition in male and female Sprague-Dawley rats. Rats were conditioned to seven tone/footshocks, followed by extinction after 1-h and 24-h delays, revealing PFCX effects and sex differences during all phases of fear conditioning. In male rats, PFCX impaired 24-h recall of fear extinction to tone, which required the 1-h delay extinction and was not attributed to nonassociative factors. In contrast, sham and PFCX females increased freezing to tone following a 24-h delay, whether or not 1-h delay tone extinction was presented. Moreover, PFCX females failed to extinguish to tone, contrasting to the robust extinction to tone that was observed for sham females, PFCX, and sham males. Also, sex differences were found during acquisition, with sham females acquiring fear conditioning slower than PFCX females. By the last tone-shock presentation, sham and PFCX females showed a slight but significant reduction in freezing to tone relative to those of sham and PFCX males. Of the other behavioral measures, PFCX females maintained exploration of a novel object during object recognition when sham females habituated. PFCX did not influence other behaviors in the remaining tasks. These findings show important sex differences in PFC function, with the PFC influencing the recall of fear extinction in males and contributing to the acquisition and maintenance of fear extinction memory in females, perhaps through altering perseveration.

Disruption of latent inhibition induced by ovariectomy can be reversed by estradiol and clozapine as well as by co-administration of haloperidol with estradiol but not by haloperidol alone

INTRODUCTION

Epidemiological and clinical life cycle studies have indicated that the more favorable illness course and the better response to antipsychotic drugs (APDs) in women with schizophrenia correlate with high levels of estrogen, whereas increased vulnerability to exacerbation and relapse and reduced sensitivity to treatment are associated with low estrogen levels. Accordingly, the estrogen hypothesis of schizophrenia proposes that estrogen has a neuroprotective effect in women vulnerable to schizophrenia.

MATERIALS AND METHODS

Latent inhibition (LI), the capacity to ignore stimuli that received nonreinforced preexposure prior to conditioning, is disrupted in acute schizophrenia patients and in rats and humans treated with the psychosis inducing drug amphetamine. Disruption of LI is reversible by typical and atypical APDs. The present study tested whether low levels of estrogen induced by ovariectomy (OVX) would lead to disruption of LI in female rats and whether such disruption would be normalized by estrogen replacement treatment and/or APDs.

RESULTS

Results showed that OVX led to LI disruption, which was reversed by 17beta-estradiol (150 microg/kg) and the atypical APD clozapine (5 mg/kg), but not by the typical APD haloperidol (0.1, 0.2, 0.3 mg/kg). Haloperidol regained efficacy when administered with 17beta-estradiol (50 microg/kg).

DISCUSSION

These results provide the first demonstration in rats that low levels of hormones can induce a pro-psychotic state that is resistant to at least typical antipsychotic treatment. This constellation may mimic states seen in schizophrenic women during periods associated with low levels of hormones such as the menopause.

Sex differences in corticolimbic dopamine and serotonin systems in the rat and the effect of postnatal handling

Stress-related psychopathology is particularly prevalent in women, although the neurobiological reason(s) for this are unclear. Dopamine (DA) and serotonin (5-HT) systems however, are known to play important adaptive roles in stress and emotion regulation. The aims of the present study included examination of sex differences in stress-related behaviour and neuroendocrine function as well as post mortem neurochemistry, with the main hypothesis that corticolimbic DA and 5-HT systems would show greater functional activity in males than females. Long-Evans rats of both sexes were employed. Additional factors incorporated included differential postnatal experience (handled vs. nonhandled) and adult mild stress experience (acute vs. repeated (5) restraint). Regional neurochemistry measures were conducted separately for left and right hemispheres. Behaviourally, females showed more exploratory behaviour than males in the elevated plus maze and an openfield/holeboard apparatus. Females also exhibited significantly higher levels of adrenocorticotrophic hormone and corticosterone at all time points in response to restraint stress than males across treatment conditions, although both sexes showed similar habituation in stress-induced ACTH activation with repeated mild stress. Neurochemically, females had significantly higher levels of DA (in ventromedial prefrontal cortex (vmPFC), insular cortex and n. accumbens) and 5-HT (in vmPFC, amygdala, dorsal hippocampus and insula) than males. In contrast, males had higher levels of the DA metabolite DOPAC or DOPAC/DA ratios than females in all five regions and higher levels of the 5-HT metabolite 5-HIAA or 5-HIAA/5-HT ratios in vmPFC, amygdala and insula, suggesting greater neurotransmitter utilization in males. Moreover, handling treatment induced a significant male-specific upregulation of 5-HT metabolism in all regions except n. accumbens. Given the adaptive role of 5-HT and DAergic neurotransmission in stress and emotion regulation, the intrinsic sex differences we report in the functional status of these systems across conditions, may be highly relevant to the differential vulnerability to disorders of stress and emotion regulation.

Chronic stress and sex differences on the recall of fear conditioning and extinction

Chronic stress effects and sex differences were examined on conditioned fear extinction. Male and female Sprague-Dawley rats were chronically stressed by restraint (6 h/d/21 d), conditioned to tone and footshock, followed by extinction after 1 h and 24 h delays. Chronic stress impaired the recall of fear extinction in males, as evidenced by high freezing to tone after the 24 h delay despite exposure to the previous 1 h delay extinction trials, and this effect was not due to ceiling effects from overtraining during conditioning. In contrast, chronic stress attenuated the recall of fear conditioning acquisition in females, regardless of exposure to the 1 h extinction exposure. Since freezing to tone was reinstated following unsignalled footshocks, the deficit in the stressed rats reflected impaired recall rather than impaired consolidation. Sex differences in fear conditioning and extinction were observed in nonstressed controls as well, with control females resisting extinction to tone. Analysis of contextual freezing showed that all groups (control, stress, male, female) increased freezing immediately after the first tone extinction trial, demonstrating contextual discrimination. These findings show that chronic stress and sex interact to influence fear conditioning, with chronic stress impairing the recall of delayed fear extinction in males to implicate the medial prefrontal cortex, disrupting the recall of the fear conditioning acquisition in females to implicate the amygdala, and nonstressed controls exhibiting sex differences in fear conditioning and extinction, which may involve the amygdala and/or corticosterone levels.

Estradiol or diarylpropionitrile administration to wild type, but not estrogen receptor beta knockout, mice enhances performance in the object recognition and object placement tasks

Cognitive processes mediated by the hippocampus and cortex are influenced by estradiol (E(2)); however, the mechanisms by which E(2) has these effects are not entirely clear. As such, studies were conducted to begin to address the role of actions at the beta form of the intracellular estrogen receptor (ERbeta) for E(2)'s cognitive effects in adult female mice. We investigated whether E(2) improved performance of wild type (WT) and ERbeta knockout (betaERKO) mice in tasks considered to be mediated by the cortex and hippocampus, the object recognition and object placement tasks. WT and betaERKO mice were ovariectomized (ovx) and E(2) (0.1 mg/kg), an ERbeta selective ER modulator (SERM), diarylpropionitrile (DPN; 0.1 mg/kg), or oil vehicle was administered to mice following training in these tasks. We hypothesized that if E(2) has mnemonic effects, in part, due to its actions at ERbeta, then WT mice administered E(2) or DPN would have improved performance compared to vehicle WT controls, which would not be different from betaERKO mice administered vehicle, E(2) or DPN. Alternatively, activation of ERalpha (with E(2), which is a ligand for both ERalpha and ERbeta) may produce opposing effects on cognition and/or the activation of ERalpha and ERbeta vs. either receptor isoform alone may produce a different pattern of effects. Results obtained supported the hypothesis that ERbeta activation is important for mnemonic effects. Ovx WT, but not betaERKO, mice administered E(2) or DPN had a greater percentage of time exploring a novel object in the object recognition task and a displaced object in the object placement task. Thus, actions at ERbeta may be important for E(2) or SERMs to enhance cognitive performance of female mice in the object recognition and placement tasks.