Estradiol-induced conditioned place preference may require actions at estrogen receptors in the nucleus accumbens

Ovarian Hormones Regulate Nicotine Consumption and Accumbens Glutamatergic Plasticity in Female Rats

Women report greater cigarette cravings during the menstrual cycle phase with higher circulating levels of 17β-estradiol (E2), which is metabolized to estrone (E1). Both E2 and E1 bind to estrogen receptors (ERs), which have been highly studied in the breast, uterus, and ovary. Recent studies have found that ERs are also located on GABAergic medium spiny neurons (MSNs) within the nucleus accumbens core (NAcore). Glutamatergic plasticity in NAcore MSNs is altered following nicotine use; however, it is unknown whether estrogens impact this neurobiological consequence. To test the effect of estrogen on nicotine use, we ovariectomized (OVX) female rats that then underwent nicotine self-administration acquisition and compared them to ovary-intact (sham) rats. The OVX animals then received either sesame oil (vehicle), E2, or E1+E2 supplementation for 4 or 20 d before nicotine sessions. While both ovary-intact and OVX females readily discriminated levers, OVX females consumed less nicotine than sham females. Further, neither E2 nor E1+E2 increased nicotine consumption back to sham levels following OVX, regardless of the duration of the treatment. OVX also rendered NAcore MSNs in a potentiated state following nicotine self-administration, which was reversed by 4 d of systemic E2 treatment. Finally, we found that E2 and E1+E2 increased ERα mRNA in the NAcore, but nicotine suppressed this regardless of hormone treatment. Together, these results show that estrogens regulate nicotine neurobiology, but additional factors may be required to restore nicotine consumption to ovary-intact levels.

The effects of early life stress on motivated behaviors: A role for gonadal hormones

Motivated behaviors are controlled by the mesocorticolimbic dopamine (DA) system, consisting of projections from the ventral tegmental area (VTA) to the nucleus accumbens (NAc) and prefrontal cortex (PFC), with input from structures including the medial preoptic area (mPOA). Sex differences are present in this circuit, and gonadal hormones (e.g., estradiol and testosterone) are important for regulating DA transmission. Early life stress (ELS) also regulates the mesocorticolimbic DA system. ELS modifies motivated behaviors and the underlying DA circuitry, increasing risk for disorders such as substance use disorder, major depression, and schizophrenia. ELS has been shown to change gonadal hormone signaling in both sexes. Thus, one way that ELS could impact mesocorticolimbic DA is by altering the efficacy of gonadal hormones. This review provides evidence for this idea by integrating the gonadal hormone, motivation, and ELS literature to argue that ELS alters gonadal hormone signaling to impact motivated behavior. We also discuss the importance of these effects in the context of understanding risk and treatments for psychiatric disorders in men and women.

TTC9A deficiency induces estradiol-mediated changes in hippocampus and amygdala neuroplasticity-related gene expressions in female mice

The involvement of tetratricopeptide repeat domain 9A (TTC9A) deficiency in anxiety-like responses and behavioral despair through estradiol action on the serotonergic system has been reported. Emerging evidence suggests that estradiol is a potent modulator of neuroplasticity. As estradiol and neuroplasticity changes are both implicated in mood regulation, and estradiol activity is negatively regulated by TTC9A, we hypothesized that the behavioral changes induced by Ttc9a^-/- is also mediated by neuroplasticity-related mechanisms. To understand the effects of TTC9A and estradiol modulation on neuroplasticity functions, we performed a behavioral analysis of tail suspension immobility and neuroplasticity-related gene expression study of brain samples collected in a previous study involving ovariectomized (OVX) Ttc9a^-/- mice with estradiol or vehicle treatment. We observed that OVX-Ttc9a^-/- mice had significantly reduced the tail suspension immobility compared to OVX-Ttc9a^-/- estradiol-treated mice. Interestingly, there was an upregulation in gene expression of tropomyosin receptor kinase B (Trkb) in the ventral hippocampus, as well as brain-derived neurotrophic factor (Bdnf) and postsynaptic density protein-95 (Psd-95) in the amygdala of OVX-Ttc9a^-/- mice compared to those treated with estradiol. These findings indicate that estradiol plays an inhibitory role in neuroplasticity in Ttc9a^-/- mice. These observations were not found in the wildtype mice, as the presence of TTC9A suppressed the effects of estradiol. Our data suggest the behavioral alterations in Ttc9a^-/- mice were mediated by estradiol regulation involving neuroplasticity-related mechanisms in both the hippocampus and amygdala regions.

Tamoxifen Blocks the Development of Motivational Features of an Addiction-Like Phenotype in Female Rats

Women become addicted sooner after initiating cocaine use as compared to men. Preclinical studies reveal a similar vulnerability in females, with findings from ovariectomized rats suggesting that estradiol mediates the enhanced vulnerability. However, since ovariectomy depletes not only estradiol, but all ovarian hormones, its role in a physiological context is not clear. Thus, the goal of this study was to determine the role of estradiol in the development of an addiction-like phenotype in ovary-intact females treated chronically with the selective estrogen receptor (ER) modulator tamoxifen. We hypothesized that tamoxifen, by antagonizing ERs, would block the development of an addiction-like phenotype as defined by an enhanced motivation for cocaine (assessed under a progressive-ratio schedule), and a heightened vulnerability to relapse (assessed under an extinction/cue-induced reinstatement procedure). Effects were examined following extended access cocaine self-administration (24-h/day; 4-discrete trials/h; 1.5 mg/kg/infusion) and 14-days of abstinence, conditions optimized for inducing an addiction-like phenotype. As predicted, motivation for cocaine was increased following extended-access self-administration and protracted abstinence in the vehicle (sesame oil) and no-injection control groups, but not in the tamoxifen group indicating that ER signaling is critical for the development of this feature of an addiction-like phenotype. Surprisingly, the increase in motivation for cocaine following abstinence was also attenuated in the vehicle group as compared to no-injection controls suggesting that oil/injections also affected its development. Contrary to our hypothesis, tamoxifen did not decrease vulnerability to relapse as this group responded at similar levels during initial extinction sessions and cue-induced reinstatement testing as compared to controls. Tamoxifen did, however, impair extinction learning as this group took longer to extinguish as compared to controls. Taken together, these findings indicate that estradiol is critical for the extinction of drug-associated cues and the development of motivational features of addiction.

Sex differences in nicotine preference

Smoking is the major cause of preventable deaths worldwide, and although there is a decline in overall smoking prevalence in developed countries, the decline in women is less pronounced than in men. Women become dependent faster and experience greater difficulties in quitting. Similar trends have been observed in animal models of nicotine/tobacco addiction. Individual differences in vulnerability to drug abuse are also observed in nicotine/tobacco addiction and point to the importance of sex differences. This Review, summarizes findings from three experimental approaches used to depict nicotine preference in animal models, intravenous and oral nicotine self-administration and nicotine-induced conditioned place preference. Nicotine preference is considered to be reflected in the animal's motivation to administer the drug (intravenously or orally) or to prefer an environment paired with the presence of the drug (conditioned place preference). These approaches all point to the importance of sex and age of the subjects; the preference of females and adolescents appear to be more pronounced than that of males and adults, respectively. A closer look at these factors will help us understand the mechanisms that underlie nicotine addiction and develop strategies to cope. Ignoring sex differences and reaching conclusions based only on studies using male subjects has resulted in erroneous generalizations in the past. Sex differences in nicotine preference have been clearly documented, and awareness on this aspect of nicotine dependence will significantly impact our success in translational research. © 2016 Wiley Periodicals, Inc.

Behavioral and molecular effects of prenatal continuous light exposure in the adult rat

Disruption of the maternal environment during pregnancy leads to behavioral changes and diseases in the adult offspring. To explore the influence of prenatal continuous light exposure (PCLE) on the adult offspring, we exposed pregnant Wistar rats to constant light during late gestation. Adult PCLE offspring showed an anxiety-like behavior and impairment of short-term memory in different tests. Measurements in the whole brain homogenates from newborn and adult offspring indicated decreased melatonin and serotonin levels and increased reactive oxygen species level in PCLE offspring. Further, we determined melatonin-, serotonin-, oxidative stress-, apoptosis-, and circadian system-related genes expression in different brain areas of adult offspring. The serotonin reuptaker Slc6a4 displayed a decreased expression in the prefrontal cortex of PCLE group. The circadian rhythm-related gene Rora was upregulated in the amygdala of PCLE offspring. Our results point to adverse behavioral effects of PCLE on adult offspring, involving serotonin and melatonin signaling dysregulation, increased chronic oxidative stress, and altered gene expression.

The effect of the anabolic steroid, nandrolone, in conditioned place preference and D1 dopamine receptor expression in adolescent and adult mice

Adolescents and adults engage in anabolic-androgenic steroid (AAS) misuse seeking their anabolic effects, even though later on, many could develop neuropsychological dependence. Previously, we have shown that nandrolone induces conditioned place preference (CPP) in adult male mice. However, whether nandrolone induces CPP during adolescence remains unknown. In this study, the CPP test was used to determine the rewarding properties of nandrolone (7.5 mg/kg) in adolescent mice. In addition, since D1 dopamine receptors (D1DR) are critical for reward-related processes, the effect of nandrolone on the expression of D1DR in the nucleus accumbens (NAc) was investigated by Western blot analysis. Similar to our previous results, nandrolone induced CPP in adults. However, in adolescents, nandrolone failed to produce place preference. At the molecular level, nandrolone decreased D1DR expression in the NAc only in adult mice. Our data suggest that nandrolone may not be rewarding in adolescents at least during short-term use. The lack of nandrolone rewarding effects in adolescents may be due, in part to differences in D1DR expression during development.

Sex differences in cocaine conditioned place preference in C57BL/6J mice

Men and women respond differently to the subjective effects of cocaine and cocaine-associated cues, which has implications for the development and maintenance of cocaine addiction. Preclinical studies performed in rats, modeling various aspects of cocaine addiction, have largely validated these results, indicating that female rats may be more sensitive to the rewarding properties of cocaine. The molecular mechanisms leading to sex differences in cocaine reward have largely not been determined, although sex hormones are thought to play a role. The mouse is commonly used as a model organism to study the molecular and genetic factors that influence a variety of psychiatric disorders. In particular, the inbred C57BL/6 mouse strain is often used for behavioral studies related to substance abuse. To begin to understand the hormonal, molecular and genetic mechanisms that might affect cocaine reward, we directly compared male and female C57BL/6J mice in cocaine conditioned place preference (CPP), a test that examines the rewarding and cue-associated properties of drugs of abuse. We conditioned mice at three doses of cocaine and examined preference and extinction of preference. We found that the acquisition of cocaine CPP did not differ between male and female mice. However, extinction of cocaine CPP was delayed in male mice compared with females at the lowest dose of cocaine. We conclude that sex differences in cocaine CPP can be observed in C57BL/6J mice at very low doses of cocaine.

Rapid oestrogenic regulation of social and nonsocial learning

Much research on oestrogens has focused on their long-term action, exerting behavioural effects within hours to days through gene transcription. Oestrogens also affect behaviour on a much shorter time scale. These rapid effects are assumed to occur through cell signalling and can elicit a behavioural effect as early as 15 min after treatment. These effects on behaviour have primarily been explored through the action of oestradiol at three well-known oestrogen receptors (ERs): ERα, ERβ and the more recently described G protein-coupled ER1 (GPER1). The rapid effects of oestradiol and ER agonists have been tested on both social and nonsocial learning paradigms. Social learning refers to a paradigm in which an animal acquires information and modifies its behaviour based on observation of another animal, commonly studied using the social transmission of food preferences paradigm. When administered shortly before testing, oestradiol rapidly improves social learning on this task, although no ER agonist has definitive, comparable improving effects. Some evidence points to GPER1, whereas ERα impairs, and ERβ activation has no effect on social learning. Conversely, ERα and GPER1 play a larger role than ERβ in the rapid improving effect of oestrogens on nonsocial learning, including social and object recognition. In addition, when administered immediately post-acquisition, oestrogens also rapidly improve memory consolidation in a variety of learning paradigms: object recognition, object placement, inhibitory avoidance and the Morris water maze, indicating that oestradiol affects the consolidation of multiple types of memory. Evidence suggests that these improvements are the result of oestrogens acting in the dorsal hippocampus where selective activation of all three ERs shows rapid improving effects on spatial learning comparable to oestradiol. However, the hippocampus is not necessary for rapid oestradiol improvements on social recognition. Although acute treatment with oestradiol enhances learning and memory on various social and nonsocial learning paradigms, the specific ERs play different roles in each type of learning. Future research should aim to further determine the roles of ERs with respect to the enhancing effects of oestradiol on learning and memory, and also determine where in the brain oestradiol acts to affect social and nonsocial learning.

Effect of cocaine sensitization prior to pregnancy on maternal care and aggression in the rat

RATIONALE

Although many studies have investigated the effects of gestational and lactational cocaine use on maternal behavior, few studies have examined the effects of prior adult cocaine use on maternal behavior.

METHODS

In the current study, intraperitoneal cocaine or saline was administered to adult female rats for 10 days, treatments were stopped for 4 days, and the females were then mated. Litter size and weight were recorded on days 2 and 16 of lactation, and maternal care, maternal aggression, and light/dark box activity were assessed on days 2, 9, and 16 of lactation.

RESULTS

The cocaine-treated females exhibited behavioral sensitization on treatment day 10. Mothers previously treated with cocaine retrieved pups more quickly, spent more time caring for the pups (increased pup retrieval, pup grooming, and crouching over the pups by the mother), and were more aggressive towards a male intruder compared to saline-treated control animals on day 2 of lactation. These effects were specific to maternal behavior, as other behaviors were mostly unaffected by prior cocaine treatment. Data from light/dark box tests indicate that cocaine did not affect activity in this test. There were few significant behavioral effects of prior cocaine treatment on lactation days 9 and 16.

CONCLUSIONS

Prior cocaine use may have lasting effects on offspring care and maternal aggression, specifically during early lactation. The present data support the hypothesis that maternal behavior is increased by prior cocaine sensitization, possibly due to cross-sensitization between cocaine and the natural reward of maternal behavior.

Tamoxifen produces conditioned taste avoidance in male rats: an analysis of microstructural licking patterns and taste reactivity

Estrogen receptor activation has been shown to reduce body weight and produce a conditioned reduction in food intake in male rats that is putatively mediated by estradiol's suggested aversive effects. Evidence has shown that the selective estrogen receptor modulator tamoxifen used in the prevention and treatment of breast cancer may also produce changes in food intake and body weight, which are known to impact cancer development and survival. The purpose of the present study was to examine whether tamoxifen produces a conditioned reduction in intake similar to estradiol by producing a conditioned aversion. A one bottle lickometer test was used to examine conditioned changes in sucrose drinking, while the taste reactivity test was used to measure rejection reactions, which serve to index aversion in rats. A backward conditioning procedure that consisted of 3 conditioning days and one vehicle test day was used to examine conditioned changes in 0.3 M sucrose intake and taste reactivity. Our results show that tamoxifen produced a conditioned reduction in sucrose drinking in a one bottle fluid intake test that was similar to the effects produced by estradiol (positive control); however, no active rejection reactions were produced by either tamoxifen (1 and 10 mg/kg) or estradiol. The present results suggest that tamoxifen, at the doses used in the present study, acts as an estrogen receptor agonist to regulate food intake and that the conditioned reduction in intake produced by tamoxifen and estradiol reflects conditioned taste avoidance rather than conditioned taste aversion.

Progesterone can enhance consolidation and/or performance in spatial, object and working memory tasks in Long-Evans rats

Progesterone has a ubiquitous role in reproduction and fitness and may influence cognitive performance. We examined the effects of administration of progesterone (a regimen that facilitates sexual behaviour) on consolidation of complex information in Long-Evans rats, Rattus norvegicus, that may be relevant for social engagement. We also examined the effects of subcutaneous progesterone administration (4 mg/kg versus oil vehicle placebo) on memory of ovariectomized rats during various cognitive tasks. Ovariectomized rats that received progesterone, versus the vehicle, immediately post-training were better able to find a hidden platform in the water maze. In a recognition task, rats that received progesterone spent more time in the novel arm of the Y-maze task than rats that received the vehicle. Ovariectomized rats that received progesterone immediately after training spent significantly more time exploring a novel object (compared to a familiar object) than did vehicle-administered rats. When socially relevant stimuli (i.e. objects with the scent of familiar or novel conspecifics) were used in the social cognition task, ovariectomized rats that received progesterone spent more time exploring the object with the novel conspecifics' scent than did vehicle-administered rats. Pairing of progesterone, but not the vehicle, conditioned a place preference to the originally nonpreferred side of the conditioning chamber. We found no significant differences in motor activity measures in these tasks due to progesterone treatment. These results suggest that progesterone's effects to improve cognitive processes with nonsocial and socially relevant stimuli, as well as have reinforcing effects, may underlie some of its salient effects on reproduction-related behaviours.

A single injection of 17beta-estradiol at the time of pup presentation promotes the onset of maternal behavior in pregnancy-terminated rats

Female rats that are hysterectomized and ovariectomized on day 15 of pregnancy (15HO) and presented with pups 48 h later show maternal behavior after 2 or 3 days of pup exposure. In contrast, if 15HO females are administered (sc) 20 microg/kg of estradiol benzoate (EB) on day 15 of pregnancy after HO, they show near immediate maternal behavior when pups are presented 48 h later. EB has typically been administered on day 15 because of the underlying assumption that EB exerts genomic effects which require a long duration before being expressed in changes in neuronal phenotype. In light of the more recent evidence that estradiol can generate rapid changes in cellular function, we examined whether injection of a water-soluble form of 17beta-estradiol (E(2)) can facilitate maternal behavior in pregnancy-terminated females when it is administered at the time of pup presentation rather than at the time of HO. Female rats treated with 100 microg/kg of E(2) showed a robust facilitation of maternal behavior, requiring a median of 1 day of pup exposure before showing maternal behavior, compared with 3 days in vehicle-treated rats.

Tamoxifen disrupts consolidation and retrieval of morphine-associated contextual memory in male mice: interaction with estradiol

RATIONALE

Tamoxifen (TMX), a selective estrogen receptor modulator, can affect cognitive functions of the brain. The conditioned place preference (CPP) paradigm involves memory for the association between contextual cues and the rewarding properties produced by a drug.

OBJECTIVES

The effects of TMX alone and in combination with estradiol (E2) on reward-related memory of morphine were investigated in adult male mice.

MATERIALS AND METHODS

Using an unbiased CPP paradigm, the ability of morphine sulfate (0.5-10 mg/kg, s.c.) to produce CPP was studied. Afterwards, the effects of TMX (1-10 mg/kg, s.c.) on the acquisition, consolidation, and expression of morphine-induced CPP were assessed. We have also evaluated the possible effects of s.c. E2 (10-200 mug/kg) and its co-administration with TMX (10 mg/kg, s.c.) on the consolidation and retrieval of morphine-associated contextual memory.

RESULTS

(1) Morphine (0.5-10 mg/kg) significantly induced CPP in a dose-dependent manner. (2) TMX (10 mg/kg) significantly reduced the time spent by mice in the morphine compartment when given immediately after each conditioning session (consolidation) or 30 min before testing for place preference in the absence of morphine (expression), whereas it had no effect when administered 30 min before each training session (acquisition). (3) Post-training or pre-testing administration of E2 increased morphine-induced CPP in a dose-dependent manner. (4) In addition, concomitant administration of E2 with TMX appears to prevent the impairing effect produced by TMX.

CONCLUSIONS

TMX appears to disrupt consolidation and retrieval of morphine-associated contextual memory and this impairing effect might be prevented by E2 treatment.

The anabolic steroids testosterone propionate and nandrolone, but not 17alpha-methyltestosterone, induce conditioned place preference in adult mice

Anabolic androgenic steroids (AAS) are often misused by adolescents and athletes. Their effects vary according to chemical structure and metabolism, route of administration, and AAS regimen. In this study, adult C57Bl/6 male mice were systemically exposed to testosterone propionate (TP), nandrolone or 17alpha-methyltestosterone (17alpha-meT), type I, type II and type III AAS, respectively, in order to determine the hedonic or aversive properties of each drug. For this purpose, the conditioned place preference (CPP) test was employed at three different AAS doses (0.075, 0.75 and 7.5 mg/kg). Other behavioral domains monitored were light-dark transitions (side changes) and general activity. TP shifted place preference at all doses tested, and nandrolone shifted place preference at 0.75 and 7.5 mg/kg, but not at 0.075 mg/kg, the lower dose tested. Conversely, mice receiving 17alpha-meT did not show alteration in the preference score. The lower dose of nandrolone did modify exploratory-based anxiety showing a decrease in light-dark transitions if compared to vehicle-treated animals, while mice treated with TP or 17alpha-meT were not affected. Our data suggest that when studying hedonic and rewarding properties of synthetic androgens, distinction has to be made based on type of AAS and metabolism.

Estrogen pretreatment modulates morphine-induced conditioned place preference in ovariectomized mice

Estrogen is known to modulate the neurotransmission in the brain. The main aim of this study was to investigate the effects of estrogen on the rewarding properties of morphine using conditioned place preference (CPP) paradigm in adult female mice. The possible rewarding effect of estrogen was also examined in ovariectomized mice. Following a 6-day conditioning procedure, sham operated animals showed a significant preference towards the side previously paired with a range of morphine doses (2, 5 and 10--but not 20--mg/kg, SC). However, ovariectomized mice showed decreased CPP compared to gonadally intact mice with a right shift in their morphine dose-response curve. These effects were reversed by chronic daily administration of estradiol benzoate (EB; 20 microg/kg, SC). Furthermore, in ovariectomized mice, EB per se was able to induce CPP. In conclusion, our findings indicate that estradiol has a facilitating effect on morphine reward while its deficiency increases the threshold dose of morphine to induce CPP.

Medial preoptic area interactions with dopamine neural systems in the control of the onset and maintenance of maternal behavior in rats

The medial preoptic area (MPOA) and dopamine (DA) neural systems interact to regulate maternal behavior in rats. Two DA systems are involved: the mesolimbic DA system and the incerto-hypothalamic DA system. The hormonally primed MPOA regulates the appetitive aspects of maternal behavior by activating mesolimbic DA input to the shell region of the nucleus accumbens (NAs). DA action on MPOA via the incerto-hypothalamic system may interact with steroid and peptide hormone effects so that MPOA output to the mesolimbic DA system is facilitated. Neural oxytocin facilitates the onset of maternal behavior by actions at critical nodes in this circuitry. DA-D1 receptor agonist action on either the MPOA or NAs can substitute for the effects of estradiol in stimulating the onset of maternal behavior, suggesting an overlap in underlying cellular mechanisms between estradiol and DA. Maternal memory involves the neural plasticity effects of mesolimbic DA activity. Finally, early life stressors may affect the development of MPOA-DA interactions and maternal behavior.

Anabolic-androgenic steroid dependence? Insights from animals and humans

Anabolic-androgenic steroids (AAS) are drugs of abuse. They are taken in large quantities by athletes and others to increase performance, with negative health consequences. As a result, in 1991 testosterone and related AAS were declared controlled substances. However, the relative abuse and dependence liability of AAS have not been fully characterized. In humans, it is difficult to separate the direct psychoactive effects of AAS from reinforcement due to their systemic anabolic effects. However, using conditioned place preference and self-administration, studies in animals have demonstrated that AAS are reinforcing in a context where athletic performance is irrelevant. Furthermore, AAS share brain sites of action and neurotransmitter systems in common with other drugs of abuse. In particular, recent evidence links AAS with opioids. In humans, AAS abuse is associated with prescription opioid use. In animals, AAS overdose produces symptoms resembling opioid overdose, and AAS modify the activity of the endogenous opioid system.

Rapid and estrogen receptor beta mediated actions in the hippocampus mediate some functional effects of estrogen

The steroid hormone, estradiol (E(2)), has numerous targets in the central nervous system, including the hippocampus, which plays a key role in cognition and affective behavior. This review focuses on our evidence from studies in rodents that E(2) has diverse mechanisms in the hippocampus for its functional effects. E(2) has rapid, membrane-mediated effects in the hippocampus to enhance cognitive performance. Administration of E(2) to the hippocampus of rats for 10 min following training enhances performance in a hippocampus-mediated task. Increased cell firing in the hippocampus occurs within this short-time frame. Furthermore, administration of free E(2) or an E(2) conjugate, E(2):bovine serum albumin (BSA), to the hippocampus produces similar performance-enhancing effects in this task, suggesting that E(2) has membrane actions in the hippocampus for these effects. Further evidence that E(2) has rapid, membrane-mediated effects is that co-administration of E(2) and inhibitors of mitogen-activated protein kinase (MAPK), rather than intracellular E(2) receptors (ERs) or protein synthesis, attenuate the enhancing effects of E(2) in this task. Despite these data that demonstrate E(2) can have rapid and/or membrane-mediated effects in the hippocampus, there is clear evidence to suggest that intracellular ERs, particularly the beta (rather than alpha) isoform of ERs, may be important targets for E(2)'s functional effects for hippocampal processes. Administration of ligands that are specific for ERbeta, but not ERalpha, have enhancing effects on hippocampal processes similar to that of E(2) (which has similar affinity for ERalpha and ERbeta). These effects are attenuated when ERbeta expression is knocked down in transgenic models or with central administration of antisense oligonucleotides. Thus, there may be a convergence of E(2)'s actions through rapid, membrane-mediated effects and intracellular ERs in the hippocampus for these functional effects.

Progesterone enhances performance of aged mice in cortical or hippocampal tasks

Ovarian steroids alter cognitive performance of young individuals. Whether progesterone enhances learning and memory in tasks involving the prefrontal cortex and/or hippocampus in aged mice was investigated. Aged mice received progesterone (10 mg/kg, s.c.) or vehicle and were tested for cortical and/or hippocampal learning and memory. Progesterone increased spontaneous alterations in the T-maze and time spent exploring novel objects in the object recognition task. Progesterone increased the time mice spent in the quadrant of the water maze where the hidden platform had been during training, increased latencies to crossover to the shock-associated side of the inhibitory avoidance chamber, and increased freezing in the contextual fear conditioning task. Progesterone did not enhance performance in tasks mediated by the amygdala (cued conditioning), striatum (conditioned place preference), or cerebellum (rotarod) in these aged mice. Thus, progesterone improved learning and memory in tasks mediated by the prefrontal cortex and/or hippocampus of aged mice.

Antisense oligodeoxynucleotides for estrogen receptor-beta and alpha attenuate estradiol's modulation of affective and sexual behavior, respectively

Estradiol (E(2)) modulates affective and socio-sexual behavior of female rodents. E(2)'s functional effects may involve actions through alpha and beta isoforms of estrogen receptor (ERs). The importance of E(2)'s actions at these isoforms for anxiety (open field, elevated plus maze), depression (forced swim test), and sexual behavior (lordosis) was investigated using an antisense oligonucleotide (AS-ODN) strategy. If ERbeta is required for anti-anxiety and antidepressant-like effects, and ERalpha is required for sexual receptivity, of E(2), then intracerebroventricular administration of AS-ODNs against these ERs should attenuate these effects and reduce immunoreactivity of ERs in brain regions that mediate these behaviors, such as the hippocampus and ventral medial hypothalamus (VMH). Ovariectomized rats were primed with 17beta-E(2) (10 microg) 48 h before testing (hour 0). At hours 0, 24, and 47.5, rats were infused with saline vehicle, scrambled control AS-ODNs, or AS-ODNs targeted against ERalpha and/or ERbeta, and were tested at hour 48. Rats infused with ERbeta AS-ODNs, alone, or with ERalpha AS-ODNs had significantly decreased open field central entries, decreased plus maze open arm time and entries, increased time spent immobile, and decreased time spent swimming in the forced swim test, and decreased ERbeta immunoreactivity in the brain than did rats administered ERalpha AS-ODNs, vehicle, or scrambled AS-ODNs. Rats that were administered ERalpha AS-ODNs, alone, or with ERbeta AS-ODNs had significantly decreased lordosis and decreased ERalpha immunoreactivity in the brain compared to rats administered ERbeta AS-ODNs, vehicle, or scrambled AS-ODNs. Thus, ERbeta and ERalpha may be required for E(2)'s modulation of affective and sexual behavior, respectively.

Evidence for the involvement of ERbeta and RGS9-2 in 17-beta estradiol enhancement of amphetamine-induced place preference behavior

Estrogen enhances dopamine-mediated behaviors, which make women and female rats more sensitive to the effects of the psychostimulant drugs, cocaine and amphetamine. How cocaine and amphetamine elicit more robust behavioral responses in females remains unclear, but studies have shown that the Regulator of G-protein Signaling 9-2 (RGS9-2) protein is an important modulator of the behavioral responses to these drugs. Previously, we reported that 17-beta estradiol reduced RGS9-2 mRNA expression in the shell of the nucleus accumbens, but not the core. The present studies were designed to further evaluate the involvement of RGS9-2 in estradiol enhancement of amphetamine-induced place preference behavior and to examine which estrogen receptor subtype mediates the effect of estradiol. Female Sprague-Dawley rats were ovariectomized and treated for 14 days with an inert vehicle or 17-beta estradiol (by Silastic implant or injection [80 microg/kg]). 17-beta-Estradiol-treated female rats had enhanced amphetamine-induced conditioned place preference behavior compared to vehicle-treated, ovariectomized female rats. In situ hybridization histochemistry and Western blotting identified an inverse relationship between RGS9-2 protein expression in the nucleus accumbens shell and the hormonal enhancement of amphetamine-induced place preference behavior. A similar relationship was not found between place preference behavior and RGS9-2 expression in the accumbens core. Moreover, treatment of ovariectomized female rats with the selective estrogen receptor-beta agonist, diarylpropionitrile (1 mg/kg), for 2 weeks also facilitated amphetamine-induced place preference behavior and selectively reduced nucleus accumbens shell RGS9-2 protein expression. These data provide insight into a potential mechanism by which estrogen and/or sex modulate mesoaccumbal dopamine receptor signaling and possibly, addictive behaviors.

Androgens' effects to enhance learning may be mediated in part through actions at estrogen receptor-beta in the hippocampus

Testosterone (T) may enhance cognitive performance. However, its mechanisms are not well understood. First, we hypothesized that if T's effects are mediated in part through actions of its 5alpha-reduced metabolites, dihydrotestosterone (DHT) and/or 3alpha-androstanediol (3alpha-diol) in the hippocampus, then T, DHT, and 3alpha-diol-administration directly to the hippocampus should enhance learning and memory in the inhibitory avoidance task. In order to test this hypothesis, gonadectomized (GDX) male rats were administered T, DHT, or 3alpha-diol via intrahippocampal inserts immediately following training in the inhibitory avoidance task. We found that T tended to increase, and DHT and 3alpha-diol significantly increased, performance in the inhibitory avoidance task compared to vehicle-administered GDX rats. Second, we hypothesized that, if androgens' effects are due in part to actions of 3alpha-diol in the hippocampus, then systemic or intrahippocampal administration of 3alpha-diol should significantly enhance cognitive performance of GDX male rats. Third, we hypothesized that, if androgen metabolites can have actions at estrogen receptors (ERs) in the hippocampus, then administration of ER antisense oligonucleotides (AS-ODNs) directly to the hippocampus of GDX, 3alpha-diol replaced, rats would decrease learning in the inhibitory avoidance task. We found that intrahippocampal administration of AS-ODNs for ERbeta, but not ERalpha, significantly decreased learning and memory of 3alpha-diol replaced rats. Together, these findings suggest that T's effects to enhance learning and memory may take place, in part, through actions of its metabolite, 3alpha-diol, at ERbeta in the dorsal hippocampus.