Acute and chronic estradiol treatments reduce memory deficits induced by transient global ischemia in female rats

Transient global ischemia induces selective, delayed neuronal death in the hippocampal CA1 and delayed cognitive deficits. Estrogen treatment ameliorates hippocampal injury associated with global ischemia. Although much is known about the impact of estrogen on neuronal survival, relatively little is known about its impact on functional outcome assessed behaviorally. We investigated whether long-term estradiol (21-day pellets implanted 14 days prior to ischemia) or acute estradiol (50 μg infused into the lateral ventricles immediately after ischemia) attenuates ischemia-induced cell loss and improves visual and spatial working memory in ovariectomized female rats. Global ischemia significantly impaired visual and spatial memory, assessed by object recognition and object placement tests at 6-9 days. Global ischemia did not affect locomotion, exploration, or anxiety-related behaviors, assessed by an open-field test at 6 days. Long-term estradiol prevented the ischemia-induced deficit in visual working memory, maintaining normal performance in tests with retention intervals of up to 1 h. Long-term estradiol also prevented ischemia-induced deficits in spatial memory tests with short (1 and 7 min), but not longer (15 min), retention intervals. Acute estradiol significantly improved visual memory assessed with short retention intervals, but did not prevent deficits in spatial memory. Acute estradiol significantly increased the number of surviving CA1 neurons, assessed either at 7 days after ischemia or after the completion of behavioral testing 9 days after ischemia. In contrast, chronic estradiol did not reduce CA1 cell death 9 days after ischemia. Thus, long-term estradiol at near physiological levels and acute estradiol administered after ischemic insult improve functional recovery after global ischemia. These findings have important implications for intervention in the neurological sequellae associated with global ischemia.     

Premarin improves memory, prevents scopolamine-induced amnesia and increases number of basal forebrain choline acetyltransferase positive cells in middle-aged surgically menopausal rats

Conjugated equine estrogen (CEE) is the most commonly prescribed estrogen therapy, and is the estrogen used in the Women's Health Initiative study. While in-vitro studies suggest that CEE is neuroprotective, no study has evaluated CEE's effects on a cognitive battery and brain immunohistochemistry in an animal model. The current experiment tested whether CEE impacted: I) spatial learning, reference memory, working memory and long-term retention, as well as ability to handle mnemonic delay and interference challenges; and, II) the cholinergic system, via pharmacological challenge during memory testing and ChAT-immunoreactive cell counts in the basal forebrain. Middle-aged ovariectomized (Ovx) rats received chronic cyclic injections of either Oil (vehicle), CEE-Low (10 μg), CEE-Medium (20 μg) or CEE-High (30 μg) treatment. Relative to the Oil group, all three CEE groups showed less overnight forgetting on the spatial reference memory task, and the CEE-High group had enhanced platform localization during the probe trial. All CEE groups exhibited enhanced learning on the spatial working memory task, and CEE dose-dependently protected against scopolamine-induced amnesia with every rat receiving the highest CEE dose maintaining zero errors after scopolamine challenge. CEE also increased number of ChAT-immunoreactive neurons in the vertical diagonal band of the basal forebrain. Neither the ability to remember after a delay nor interference, nor long-term retention, was influenced by the CEE regimen used in this study. These findings are similar to those reported previously for 17 β-estradiol, and suggest that CEE can provide cognitive benefits on spatial learning, reference and working memory, possibly through cholinergic mechanisms.     

Chronic 17beta-estradiol pretreatment and ischemia-induced hippocampal degeneration and memory impairments: a 6-month survival study

Exogenous administration of estrogen has been shown to significantly reduce ischemia-induced neuronal degeneration. However, the long-term impact of such treatment on neuronal protection and functional recovery remain largely unknown. The present study assessed the effects of a 15-day pretreatment with 17beta-estradiol on memory deficits and neuronal damage up to 6 months following a 10-min global ischemia in rats. Four groups of ovariectomized female rats [sham-operated and ischemic rats receiving a 15-day pretreatment of either the vehicle or 17beta-estradiol (100 microg/kg)] were tested. The 8-arm radial maze and object recognition tests served to evaluate the impact of 17beta-estradiol treatment on ischemia-induced spatial and recognition memory impairments, respectively. Testing in the radial maze was initiated at two distinct time intervals following reperfusion (7 and 120 days) to evaluate changes in memory functions over time. Our findings revealed long-lasting neuroprotective effects of 17beta-estradiol treatment on hippocampal CA1 pyramidal cells in ovariectomized ischemic rats (43.5% greater neuronal survival than observed in vehicle-treated ischemic animals). Importantly, this neuronal protection translated into significant improvements of recognition and spatial memory functions in estradiol-treated ischemic rats.     

Effects of long-term treatment with 17 β-estradiol and medroxyprogesterone acetate on water maze performance in middle aged female rats

Although previous research has indicated that hormone replacement therapy benefits memory in menopausal women, several recent studies have shown either detrimental or no effects of treatment. These inconsistencies emphasize the need to evaluate the role of ovarian hormones in protecting against age-related cognitive decline in an animal model. The present study investigated the effects of long-term hormone treatment during aging on the Morris water maze. Female Long Evans hooded rats were ovariectomized at middle age (12-13 months) and were immediately placed in one of five groups: no replacement, chronic 17 β-estradiol only, chronic 17 β-estradiol and progesterone, chronic 17 β-estradiol and medroxyprogesterone acetate (MPA), or cyclic 17 β-estradiol only. 17 β-estradiol was administered in the drinking water in either a chronic or cyclic (3 out of 4 days) fashion. Progesterone and MPA were administered via subcutaneous pellets. Following 6 months of hormone treatment, animals were tested on the Morris water maze. Animals performed four trials a day for 4 days and after the final day of testing a subset of animals completed a probe trial. Across 4 days of testing, rats receiving 17 β-estradiol in combination with MPA performed significantly worse than all other groups receiving hormone replacement. In addition on the last day of testing, chronic 17 β-estradiol administration was more beneficial than cyclic administration and no replacement. Thus compared to other hormone-treated groups, long-term 17 β-estradiol treatment in combination with MPA results in impaired performance on the spatial Morris water maze.     

Sex differences in the long-term effect of preweanling isolation stress on memory retention

Social experiences during development can powerfully modulate later neuroendocrine and behavioral system. In the present study, male and female rat pups experienced daily bouts of social isolation for 6 h per day or control conditions during the third postnatal week. Performance on a 12-arm radial maze with 8 arms consistently baited with food reward was examined in adulthood. During the social isolation, both male and female pups exhibited a significant increase in plasma corticosterone levels. When tested on the radial arm maze as adults, the performance of female rats that had experienced social isolation during development was not affected; however, male rats in the isolation condition initially exhibited impairments in working memory but not reference memory. Despite achieving comparable asymptotic levels of performance on the maze, male rats that experienced social isolation during the third week demonstrated disruption in working memory retention when radial arm maze trials were interrupted after the fourth arm choice. Thus, while male rats that experience social isolation during the third week of life eventually perform comparably to controls on the standard radial arm maze task, their ability to retain information over a delay remains impaired. These findings highlight an important sex difference in the long-term effects of stress during this period of late preweanling development.     

Acute estrogen treatment facilitates recognition memory consolidation and alters monoamine levels in memory-related brain areas

Acute effects of estrogens on mnemonic processes were examined at the behavioral and neurochemical levels. 17β-estradiol and 17α-estradiol influences on memory consolidation were assessed using object placement (OP) and object recognition (OR) tasks. Subjects received treatment immediately after a sample trial (exploring two novel objects), and memory of objects (OR memory) or location of objects (OP memory) was tested 4 h later. Both isomers of estradiol enhanced memory. For spatial memory, 15 and 20 µg/kg of 17β-estradiol facilitated OP, while lower and higher doses were ineffective. 17α-estradiol had a similar pattern, but a lower dose was effective. When treatment was delayed until 45 min after a sample trial, memory was not enhanced. For non-spatial memory, OR was facilitated at 5 µg/kg of 17β-estradiol and at 1 and 2 µg/kg of 17α-estradiol and, similar to OP, lower and higher doses were ineffective. These data demonstrate that beneficial effects of estrogens are dose, time and task dependent, and the dose-response pattern is an inverted U. Because monoamines are known to have contributions to memory, brains were removed 30 min after treatment for measurements of dopamine (DA), norepinephrine (NE), serotonin (5-HT), and metabolites. Estrogen elevated 5HT, NE metabolite MHPG, turnover ratio of NE to MHPG, and DA metabolite DOPAC levels in the prefrontal cortex, while NE and MHPG were decreased in the hippocampus. Thus, acute estrogens exert rapid effects on memory consolidation and neural function, which suggests that its mnemonic effects may involve activation of membrane associated estrogen receptors and subsequent signaling cascades, and that monoamines may contribute to this process.     

Human chorionic gonadotropin (a luteinizing hormone homologue) decreases spatial memory and increases brain amyloid-beta levels in female rats

Numerous studies have suggested that estradiol (E) improves spatial memory as female rats with E perform better than those without E. However there is an inverse relationship between E and luteinizing hormone (LH) levels and LH could play a role. We examined whether treatment with the LH homologue human chorionic gonadotropin (hCG), would impair spatial memory of adult E-treated female rats. In the object location memory task, ovariectomized (ovxed) rats treated with E and either a single high dose (400 IU/kg) or a lower repeated dose of hCG (75 IU/kg hourly for 8 h) showed spatial memory disruption compared to ovxed rats treated with estradiol alone. Impairment was attributed to memory disruption as performance improved with shortened delay between task exposure and testing. Tests on another spatial memory task, the Barnes maze, confirmed that hCG (400 IU/kg) can impair memory: although E + veh treated animals made significantly fewer hole errors across time, E + hCG-treated did not. In humans, high LH levels have been correlated with Alzheimer's disease (AD). Because brain amyloid-beta (Aβ) species have been implicated as a toxic factor thought to cause memory loss in AD, we analyzed whether hCG-treated animals had increased Aβ levels. Levels of Aβ from whole brains or hippocampi were assessed by Western blot. hCG treatment to E-implanted females significantly increased soluble Aβ40 and Aβ42 levels. These results indicate that high levels of LH/hCG can impair spatial memory, and an increase in brain Aβ species may account for the memory impairment in hCG-treated rats.     

Spatial memory deficits in maternal iron deficiency paradigms are associated with altered glucocorticoid levels

"The goal of this study was to examine the effect of maternal iron deficiency on the developing hippocampus in order to define a developmental window for this effect, and to see whether iron deficiency causes changes in glucocorticoid levels. The study was carried out using pre-natal, post-natal, and pre + post-natal iron deficiency paradigm. Iron deficient pregnant dams and their pups displayed elevated corticosterone which, in turn, differentially affected glucocorticoid receptor (GR) expression in the CA1 and the dentate gyrus. Brain Derived Neurotrophic Factor (BDNF) was reduced in the hippocampi of pups following elevated corticosterone levels. Reduced neurogenesis at P7 was seen in pups born to iron deficient mothers, and these pups had reduced numbers of hippocampal pyramidal and granule cells as adults. Hippocampal subdivision volumes also were altered. The structural and molecular defects in the pups were correlated with radial arm maze performance; reference memory function was especially affected. Pups from dams that were iron deficient throughout pregnancy and lactation displayed the complete spectrum of defects, while pups from dams that were iron deficient only during pregnancy or during lactation displayed subsets of defects. These findings show that maternal iron deficiency is associated with altered levels of corticosterone and GR expression, and with spatial memory deficits in their pups."     

Impairment of spatial learning by estradiol treatment in female mice is attenuated by estradiol exposure during development

High doses of estradiol (E(2)) can impair spatial learning in the Morris water maze, in ovariectomized mice, but the same dose has no effect on adult castrated males. Here, we test the hypothesis that this sex difference is caused by neonatal actions of E(2). In Experiment 1, C57BL/6J pups were given daily estradiol benzoate (EB) or oil injections from the day of birth until postnatal Day 3. Adults were gonadectomized and received EB (s.c.) or oil 28 h before the first day of training, and 4 h before each of four daily training sessions on the Morris water maze. Females given oil as neonates, and EB prior to training displayed the poorest performance. Females that received EB as neonates and EB prior to training were insensitive to the deleterious effects of adult EB and performed better than males given the same hormone treatments. We conducted a second experiment using aromatase enzyme knockout (ArKO) mice. Adult male and female ArKO and wild-type (WT) littermates were gonadectomized and received either injections of oil or EB prior to and during water maze training (as described above). Hormone treatment failed to affect performance, yet, female but not male ArKO mice showed impaired learning compared to WT littermates. Thus, exposure to estradiol during neonatal development can counteract the deleterious effects of EB on adult spatial learning.     

Assessment of estradiol influence on spatial tasks and hippocampal CA1 spines: evidence that the duration of hormone deprivation after ovariectomy compromises 17beta-estradiol effectiveness in altering CA1 spines

Two pulses of 17β-estradiol (10 µg) are commonly used to increase hippocampal CA1 apical dendritic spine density and alter spatial performance in ovariectomized (OVX) female rats, but rarely are the measures combined. The goal of this study was to use this two-pulse injection protocol repeatedly with intervening wash-out periods in the same rats to: 1) measure spatial ability using different tasks that require hippocampal function and 2) determine whether ovarian hormone depletion for an extended 10-week period reduces 17β-estradiol's effectiveness in elevating CA1 apical dendritic spine density. Results showed that two injections of 10 µg 17β-estradiol (72 and 48 h prior to testing and timed to maximize CA1 apical spine density at behavioral assessment) corresponded to improved spatial memory performance on object placement. In contrast, two injections of 5 µg 17β-estradiol facilitated spatial learning on the water maze compared to rats given two injections of 10 µg 17β-estradiol or the sesame oil vehicle. Neither 17β-estradiol dose altered Y-maze performance. As expected, the intermittent two-pulse injection protocol increased CA1 apical spine density, but 10 weeks of OVX without estradiol treatment decreased the effectiveness of 10 µg 17β-estradiol to increase CA1 apical spine density. Moreover, two pulses of 5 µg 17β-estradiol injected intermittently failed to alter CA1 apical spine density and decreased basal spine density. These results demonstrate that extended time without ovarian hormones reduces 17β-estradiol's effectiveness to increase CA1 apical spine density. Collectively, these findings highlight the complex interactions among estradiol, CA1 spine density/morphology, and task requirements, all of which contribute to behavioral outcomes.     

The effects of chronic estradiol treatment on working memory deficits induced by combined infusion of beta-amyloid (1-42) and ibotenic acid

Estrogen limits in vitro neuron death induced by application of beta-amyloid, the cytotoxic peptide linked to Alzheimer's disease. However, the ability of estrogen to protect neurons and preserve cognitive function in vivo following exposure to beta-amyloid has not been demonstrated. Our objective was to evaluate the potential of estrogen to reduce spatial working memory deficits in female rats induced by administration of a neurotoxic form of beta-amyloid in combination with the excitotoxin, ibotenic acid. The interaction of beta-amyloid with excitotoxic factors may underlie cognitive deficits associated with Alzheimer's disease. Therefore, to create an experimental model typical of early Alzheimer's disease a low dose of ibotenic acid was administered with beta-amyloid into the dorsal hippocampus. Ovariectomized rats were implanted subcutaneously with Silastic capsules that produce physiological levels of 17beta-estradiol 10 days before bilateral intrahippocampal injections of aggregated beta-amyloid (1-42) and ibotenic acid. Capsules remained in situ throughout behavioral testing. When tested 3-10 weeks after neurotoxin treatment, females without estrogen capsules exhibited delay-dependent impairments in working memory performance on a water maze and a radial arm maze. Females treated with estrogen and combined neurotoxins displayed working memory performance comparable to unlesioned females on both tasks. Neurotoxin treatment increased immunoreactivity for glial fibrillary acidic protein but this measure was unaffected by estradiol treatment indicating that estrogen did not limit glial proliferation. Results indicate that estrogen prevented deficits in spatial working memory induced by neurotoxin treatments intended to mimic the pathology of early Alzheimer's disease.     

Chronic estradiol replacement impairs performance on an operant delayed spatial alternation task in young, middle-aged, and old rats

The current study examined effects of chronic estradiol replacement on a prefrontally-mediated working memory task at different ages in a rodent model. Ovariectomized young, middle-aged, and old Long–Evans rats were given 5% or 10% 17β-estradiol in cholesterol vehicle via Silastic implants and tested on an operant delayed spatial alternation task (DSA). The two estradiol exposed groups did not perform as well as the vehicle control group did. Deficits were present at all but the longest delay, where all groups including the vehicle control group performed poorly. Surprisingly, there was not a significant effect of age or an age by estradiol interaction, despite the fact that old rats had longer latencies to respond after both correct and incorrect lever presses. These data confirm our earlier finding that chronic estradiol treatment has an impairing effect on working memory as measured on DSA task. However, contrary to expectations, young, middle-aged and old rats were similarly impaired by chronic estradiol treatment; there were no indications of differential effects at different periods of the lifespan. Also contrary to expectations, there were no indications of a decline in DSA performance with advancing age. Overall, the results demonstrate that chronic estradiol exposure causes deficits in the DSA performance of ovariectomized female rats, not only in young adulthood, but also at older ages analogous to those at which hormone replacement therapy is commonly prescribed in humans.     

Acute pretreatment with estradiol protects against CA1 cell loss and spatial learning impairments resulting from transient global ischemia

Estradiol can act to protect against hippocampal damage resulting from transient global ischemia, but little is known about the functional consequences of such neuroprotection. The present study examines whether acute estradiol administered prior to the induction of transient global ischemia protects against hippocampal cell death and deficits in performance on a spatial learning task. Ovariectomized female rats were primed with estradiol benzoate or oil vehicle 48 and 24 h prior to experiencing one of three durations of 4-vessel occlusion (0, 5, or 10 min). Performance on the cued and hidden platform versions of the Morris water maze was assessed 1 week following ischemia. On the cued platform task, neither hormone treatment nor ischemia significantly influenced acquisition. When tested on the hidden platform task, however, oil-treated rats exhibited impairments in spatial learning after either 5 or 10 min of ischemia while estradiol-treated rats showed no impairments after 5 min of ischemia and only mild impairments after 10 min of ischemia. Immediately following behavioral testing, rats were perfused and survival of CA1 pyramidal cells was assessed. Ischemia was associated with the loss of CA1 pyramidal cells but rats that received estradiol prior to ischemia showed less severe damage. Furthermore, the extent of cell loss was correlated with degree of spatial bias expressed on a probe trial following hidden platform training. These findings indicate that acute exposure to estradiol prior to ischemia is both neuroprotective and functionally protective.     

Increased daily handling of ovariectomized rats enhances performance on a radial-maze task and obscures effects of estradiol replacement

Estrogen impacts performance on tasks of learning and memory, although there are inconsistencies in the direction and magnitude of the reported effects. Contributory factors to the inconsistencies may be methodological differences associated with different regimens of treatment. The goal of the present experiment was to assess the effect of increased handling, such as that commonly associated with pharmacological or other experimental manipulations, on the ability of estrogen to influence working memory performance. Young adult rats were ovariectomized and implanted with capsules containing either cholesterol or 25% estradiol diluted in cholesterol. Half of each hormone treatment group received standard handling, which consisted of handling required to carry out experimental procedures and half received increased handling, which consisted of standard handling as well as 2 min of additional daily handling by the experimenter. Animals were trained daily on a working memory task on an eight-arm radial maze for 24 days of acquisition and for eight additional daily trials in which delays of either 1 min or 3 h were imposed between the fourth and fifth arm choices. Animals that received increased handling exhibited significantly enhanced performance during acquisition and delay trials compared to those that received standard handling. Estradiol significantly enhanced performance during delay trials in animals that received standard handling but had no effect in animals that received increased handling. These results suggest that the amount of handling that animals receive as part of experimental procedures may obscure the memory enhancing effects of estradiol replacement on certain tasks of cognition.     

Spatial memory, navigation and dance behaviour in Apis mellifera

Navigation and dance communication in Apis mellifera have been extensively studied on the level of sensory processing, but the structure and content of the spatial memory underlying such phenomena have yet to be addressed. Here we survey new findings indicating that the memory used by bees to navigate within the range of their orientation flights is much more complex than hitherto thought. It appears to allow them to decide between at least two goals in the field, and to steer towards them over considerable distances. Two models concerning the structure of bees’ spatial memory are developed from new empirical evidence. The first one relies on the integration of at least two flight vectors, while the second assumes the existence of a ‘functional’ map based on the information available on-site. These findings also raise questions about the process of encoding and decoding information in the context of the waggle dance. We review published data and recent evidence indicating that memories of topographical features might also be involved in dance communication, and point out what needs to be addressed to elucidate the corresponding memory demands. The flight paths of recruited bees can now be traced by means of radar techniques, and thus tools are available to tackle these questions.     

Estrogen treatment effects on cognition, memory and mood in male-to-female transsexuals

Gonadal hormones, particularly estrogens, have been suggested to influence memory and cognitive tasks that show sex differences. Previously, we reported that male-to-female (M-F) transsexuals undergoing estrogen treatment for sex re-assignment scored higher on verbal Paired Associate Learning (PAL) than a transsexual control group awaiting estrogen treatment. The present study used a more robust design to examine further associations between estrogen and cognition. We assessed additional aspects of memory, including visual, spatial, object and location memory, other cognitive abilities that show reliable sex differences, including verbal and visual-spatial abilities, and mood variables that could mediate associations between estrogen and cognition. In addition to comparing groups of individuals on and off estrogen, we used two repeated measures designs (AB and BA). The AB group was tested prior to hormone treatment and then again after treatment had begun; the BA group was tested while on estrogen treatment and then again when hormones had been withdrawn prior to surgery. Few changes in memory or cognition were observed, and changes that were observed were not consistent across study designs. The lack of significant effects did not relate to mood changes or to the sexual orientation of participants. These findings suggest that estrogen treatment associated with sex change for M-F transsexuals has little or no influence on sex-typed aspects of cognition or memory.     

Oxytocinergic activity is linked to lower blood pressure and vascular resistance during stress in postmenopausal women on estrogen replacement

Estrogen administration results in increased release of the oxytocin (OT) prohormone reflected by increases in oxytocin intermediate peptide (OT Int) in both animal models and humans, and sequential treatment of ovariectomized rats with estrogen/progesterone then progesterone withdrawal leads to increased hypothalamic OT mRNA. Blood pressure (BP) reductions have been related to increased exogenous and endogenous OT in rats and to higher endogenous OT activity in premenopausal women, but not previously in postmenopausal women. Thus, we used plasma obtained at rest and during a speech stressor from 54 postmenopausal women who participated in a 6-month randomized trial of oral conjugated estrogens vs. placebo to examine effects of estrogen replacement therapy (ERT) on plasma OT and OT Int levels and their relationships to changes in BP during the trial. ERT alone and with progesterone (but not placebo) led to significant increases in plasma levels of OT Int, but no change in plasma OT levels. Women showing greater increases in OT Int during treatment showed greater decreases in BP and total vascular resistance during a series of behavioral stressors compared to women with moderate or no increases in OT Int, even after controlling for effects related to treatment condition or to changes in plasma estradiol. The findings suggest that enhanced oxytocinergic activity may contribute to BP decreases associated with ERT in more responsive postmenopausal women.     

Spatial memory and foraging competition in captive western lowland gorillas (Gorilla gorilla gorilla)

Spatial memory and foraging competition were investigated in three mother/offspring pairs of western lowland gorillas,Gorilla gorilla gorilla, using a naturalistic foraging task at the Toronto Zoo. Sixteen permanent food sites were placed throughout the animals’ enclosures. All of the sites were baited and a pair of animals was free to visit the sites and collect the food. Five of the subjects collected the food with accuracy better than chance. Most of the subjects visited the sites using a pattern, and for half the subjects this was one of adjacency. The high accuracy of five of the subjects and the lack of a consistent adjacency pattern suggest that the animals did in fact use spatial memory. Furthermore, the gorillas tended to avoid visiting food sites that had been previously depleted by their partner. They also appeared to split their search of the enclosures, each visiting only a proportion of the food sites. This indicated that the animals were competing and altering their foraging behaviour based on the behaviour of their partner. Therefore, accuracy was recalculated to take this into account. When the sites depleted by either animal in a pair during a given trial were worked into the accuracy calculations for individual animals, three of the animals still maintained accuracy above chance. This suggests that the animals were not only able to remember which sites they had depleted, but those sites depleted by their foraging partner as well.     

Effects of long-term estrogen replacement on social investigation and social memory in ovariectomized C57BL/6 mice

Estrogen has been shown to play a role in modulating social recognition memory. However, the literature regarding the influence of estrogen on social memory is sparse and only covers two experimental manipulations: acute injections and receptor knockout. Long-term effects of estrogen replacement on social investigation and social recognition are unknown. Furthermore, existing social recognition protocols focus on memory of very short durations (<2 h). In the present study, we examined long-term effects of estrogen replacement on both short- (<30 min) and long-term (24 h) social recognition in ovariectomized female C57BL/6 mice by implanting 60-day time-release pellets containing physiological doses of estradiol (0, 0.18, or 0.72 mg of 17beta-estradiol). After 55 days of treatment, evidence of social recognition memory, measured by 24-h habituation, was found only in mice receiving the 0.72-mg pellet. This result is remarkable as previous reports indicate that individually-housed untreated rats and mice do not show habituation beyond 2 h. Our study further revealed that estrogen also increased frequencies of baseline social investigation without affecting general activity levels and decreased delayed post-swim-stress serum corticosterone concentration. Thus, these results suggest that long-term estrogen replacement increased the interest in social interaction as well as decreased stress responses. It is likely that the 24-h habituation observed in the estrogen replacement group is mediated jointly by the non-mnemonic effects of estrogen on the behavior displayed during the stage of memory encoding as well as mnemonic effects during the stage of memory consolidation.