Shifts in preferred learning strategy across the estrous cycle in female rats

The current status of the effects of ovarian steroids on learning and memory remains somewhat unclear, despite a large undertaking to evaluate these effects. What is emerging from this literature is that estrogen, and perhaps progesterone, influences learning and memory, but does so in a task-dependent manner. Previously, we have shown that ovariectomized rats given acute treatments of estrogen acquire allocentric or "place" tasks more easily than do rats deprived of estrogen, but acquire egocentric or "response" learning tasks more slowly than do those deprived of hormone, suggesting that estrogen treatment may bias the strategy a rat is able to use to solve tasks. To determine if natural fluctuations in ovarian hormones influence cognitive strategy, we tested whether strategy use fluctuated across the estrous cycle in reproductively intact female rats. We found that in two tasks in which rats freely choose the strategy used to solve the task, rats were more likely to use place strategies at proestrous, that is, when ovarian steroids are high. Conversely, estrous rats were biased toward response strategies. The data suggest that natural fluctuations in ovarian steroids may bias the neural system used and thus the cognitive strategies chosen during learning and memory.     

Hippocampal and striatal dependent navigation: Sex differences are limited to acquisition

Estrogen has been demonstrated to enhance the use of hippocampal-based place learning while reducing the use of striatal-based motor-response strategy (Korol, D.L., Malin, E.L., Borden, K.A., Busby, R.A., & Couper-Leo, J. (2004). Shifts in preferred learning strategy across the estrous cycle in female rats. Horm. Behav. 45, 330–338). Previous research has focused on task acquisition and the switch from a place to motor-response navigation with training. The current paradigm allowed an examination of the interplay between these two systems by having well-trained animals switch strategies “on demand.” Female and male Sprague–Dawley rats were taught a motor-response task on a plus maze. The rats were then introduced to a place task and taught to switch, by cue, from the motor-response to place strategy. Finally, the rats were trained to continuously alternate between place and motor-responses strategies. The maze configuration allowed for an analysis of cooperative choices (both strategies result in the same goal arm), competitive choices (both strategies result in different goal arms), and single strategy choices (can only use the motor-response strategy). The results indicate that sex and estrogen-related effects on navigation strategy are limited to the initial stages of learning a task. The role of sex and estrogen is diminished once the task is well learned, and presumably, the relative involvement of the hippocampal and striatal systems is established.     

Conjugated estrogens in the endometrium during the estrous cycle in pigs

Estrogen metabolism results in the formation of inactive estrogen sulphates and glucuronides. Despite the lack of receptor binding, circulating conjugated estrogens might serve as a reservoir for the active form through the involvement of specific cleaving enzymes. In order to elucidate the potential role that estrogen conjugates play in the regulation of the estrous cycle, we determined the concentration of progesterone, estrogen and estrogen conjugates in serum and endometrial homogenates of cycling gilts. In addition, we determined the mRNA expression changes of enzymes (UDP glucuronosyltransferase (UGT), β-glucuronidase (GUSB), sulphotransferases (SULT) and steroid sulphatase (STS)) and transporters (multidrug resistance-associated protein (MRP), organic anion-transporting polypeptide (OATPs)) involved in the estrogen metabolism in the endometrium across the estrous cycle. GUSB displayed highest expression at estrous (day 0), decreasing expression during metestrus (day 3 and 6), minimal expression on day 10 and 12, and increasing expression towards proestrus (day 18), suggesting either a stimulation by estrogens or a negative impact of progesterone. The mRNA expression of the influx-transporter OATP1A2 significantly increased from day 0 to 6 and decreased again by day 10, while the efflux-transporters (MRP1, MRP2, and MDR1) displayed minimal expression at day 3 and 6. The mRNA expression of the UDP-glucuronsyltransferases followed a similar pattern, with minimal expression found at day 6. The analyses of the concentration of local and circulating steroid hormones points towards an interaction of the analyzed transporters and enzymes with steroid hormones, thereby possibly regulating the reservoir of active steroids contributing to the endometrial function.     

Evidence for the influence of testosterone in the performance of spatial navigation in a virtual water maze in women but not in men

Testosterone (T) may be associated with enhanced spatial navigation in a number of rodent species, although the nature of the relation is equivocal. Similarly, numerous studies in humans generally have found that T is associated with enhanced spatial ability on a variety of paper and pencil tasks that may relate to navigational ability. However, relatively few studies have reported effects of T on navigational ability in humans. We investigated the relationship between endogenous T and performance on a virtual water maze (vWM) and mental rotations test (MROT). ELISA for T was performed on salivary samples that were obtained from participants before and after completion of both spatial tasks. Results indicated that women with low T required more time to locate the hidden platform in the vWM than either group of men or women with high T. Significant negative correlations were found for the entire sample between vWM performance and T, and between vWM latency to escape and MROT. Similar significant correlations were found in women but not men. Thus, our data support the position that T improves performance in the vWM in a linear fashion, most strongly in women. However, further work is needed to confirm this hypothesis in humans.     

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.     

Effects of sex and estrous cycle on the brain and plasma arginine metabolic profile in rats

l-arginine is a versatile amino acid with a number of bioactive metabolites. Increasing evidence implicates altered arginine metabolism in the aging and neurodegenerative processes. The present study, for the first time, determined the effects of sex and estrous cycle on the brain and blood (plasma) arginine metabolic profile in naïve rats. Female rats displayed significantly lower levels of l-arginine in the frontal cortex and three sub-regions of the hippocampus when compared to male rats. Moreover, female rats had significantly higher levels of l-arginine and γ-aminobutyric acid, but lower levels of l-ornithine, agmatine and putrescine, in plasma relative to male rats. The observed sex difference in brain l-arginine appeared to be independent of the enzymes involved in its metabolism, de novo synthesis and blood-to-brain transport (cationic acid transporter 1 protein expression at least), as well as circulating l-arginine. While the estrous cycle did not affect l-arginine and its metabolites in the brain, there were estrous cycle phase-dependent changes in plasma l-arginine. These findings demonstrate the sex difference in brain l-arginine in the estrous cycle-independent manner. Since peripheral blood has been increasingly used to identify biomarkers of brain pathology, the influences of sex and estrous cycle on blood arginine metabolic profile need attention when experimental research involves female rodents.

     

The impact of flavonoids on spatial memory in rodents: from behaviour to underlying hippocampal mechanisms

Emerging evidence suggests that a group of dietary-derived phytochemicals known as flavonoids are able to induce improvements in memory, learning and cognition. Flavonoids have been shown to modulate critical neuronal signalling pathways involved in processes of memory, and therefore are likely to affect synaptic plasticity and long-term potentiation mechanisms, widely considered to provide a basis for memory. Animal dietary supplementation studies have further shown that flavonoid-rich foods are able to reverse age-related spatial memory and spatial learning impairments. A more accurate understanding of how a particular spatial memory task works and of which aspects of memory and learning can be assessed in each case, are necessary for a correct interpretation of data relating to diet-cognition experiments. Further understanding of how specific behavioural tasks relate to the functioning of hippocampal circuitry during learning processes might be also elucidative of the specific observed memory improvements. The overall goal of this review is to give an overview of how the hippocampal circuitry operates as a memory system during behavioural tasks, which we believe will provide a new insight into the underlying mechanisms of the action of flavonoids on cognition.     

The anorectic effect of fenfluramine is influenced by sex and stage of the estrous cycle in rats

The controls of food intake differ in male and female rats. Daily food intake is typically greater in male rats, relative to female rats, and a decrease in food intake, coincident with the estrous stage of the ovarian reproductive cycle, is well documented in female rats. This estrous-related decrease in food intake has been attributed to a transient increase in the female rat's sensitivity to satiety signals generated during feeding bouts. Here, we investigated whether sex or stage of the estrous cycle modulate the satiety signal generated by fenfluramine, a potent serotonin (5-HT) releasing agent. To examine this hypothesis, food intake was monitored in male, diestrous female, and estrous female rats after intraperitoneal injections of 0, 0.25, and 1.0 mg/kg D-fenfluramine. The lower dose of fenfluramine decreased food intake only in diestrous and estrous females, suggesting that the minimally effective anorectic dose of fenfluramine is lower in female rats, relative to male rats. Although the larger dose of fenfluramine decreased food intake in both sexes, the duration of anorexia was greater in diestrous and estrous female rats, relative to male rats. Moreover, the magnitude of the anorectic effect of the larger dose of fenfluramine was greatest in estrous rats, intermediate in diestrous rats, and least in male rats. Thus our findings indicate that the anorectic effect of fenfluramine is modulated by gonadal hormone status.     

Oxytocin secretion induced by osmotic stimulation in rats during the estrous cycle and after ovariectomy and hormone replacement therapy

Hyperosmolality is a potent stimulus for the secretion of oxytocin. Oxytocinergic neurons are modulated by estrogen and oxytocin secretion in rats varies according to the phase of the estrous cycle, with higher activity during proestrus. We investigated the oxytocin secretion induced by an osmotic stimulus (0.5 M NaCl) in female rats. Plasma oxytocin and the oxytocin contents in the neurohypophysis and the paraventricular and supraoptic nuclei were determined during the morning (8-9 h) and afternoon (17-18 h) of the estrous cycle and after ovariectomy followed or not by hormone replacement. Plasma oxytocin peaked in control animals during proestrus. Oxytocin content decreased in the paraventricular and supraoptic nuclei during proestrus and estrus compared to diestrus and increased in the neurohypophysis during proestrus morning. No significant difference was observed in the oxytocin content of the neurohypophysis, nuclei or plasma between ovariectomized animals and ovariectomized animals treated with estrogen or estrogen plus progesterone. Therefore, any ovarian factor other than estrogen or progesterone seems to play a direct or indirect role in the increase in oxytocin secretion. The osmotic stimulus caused an increase in plasma oxytocin throughout the estrous cycle. A reduction in oxytocin content during diestrus and an increase during proestrus were observed in the paraventricular nuclei. In ovariectomized animals, the treatment with estrogen potentiated the response of oxytocin to the osmotic stimulus, with the response being even stronger in the case of estrogen plus progesterone. In conclusion, the ovarian steroids estrogen plus progesterone could modulate the osmoreceptor mechanisms related to oxytocin secretion.     

Induction of pseudopregnancy using artificial VCS: importance of lordosis intensity and prestimulus estrous cycle length

In cycling female rats, vaginocervical stimulation (VCS) received naturally during mating or by artificial mechanical stimulation induces neuroendocrine and behavioral responses that are critical for reproduction, including bi-circadian prolactin surges which result in pregnancy or an 8-14-day diestrous period called pseudopregnancy (PSP). Following mating, the incidence of PSP is higher when females receive high (10) as opposed to low (3-5) numbers of intromissions. Therefore, a threshold level of VCS must be exceeded before hypothalamic changes required for PSP can occur. This study characterized the threshold curve for PSP induction for artificial VCS (VCS-a). Proestrous females were given 1, 2, 3, 4, or 8 VCS-a applied with a glass rod using 200 g of force for 2 s, with an 8-min interval between stimulations. The lordosis response (LR) to the stimulus was measured on a scale of increasing intensity from 0 to 3, and the occurrence of PSP was measured by daily vaginal lavage. In contrast to previous findings, VCS-a induced robust lordosis responses without concurrent flank and perineal stimulation. The frequency of PSP induction did not increase in females as a function of amounts of VCS-a. However, the occurrence of PSP was strongly tied to the maximum lordosis response (LR(max)) observed. PSP was observed only among multiply stimulated females that showed the highest LR(max) (3.0) to at least one of the stimulations. Multiply stimulated females that showed a LR(max) < 3.0 or females that received only one VCS-a never became PSP. PSP and a stronger LR(max) were more likely to occur in females that had 5-day compared to 4-day prestimulus estrous cycle lengths. We conclude that central mechanisms important for VCS-induced PSP and lordosis may be potentiated by estradiol's actions in estrogen-concentrating forebrain areas.     

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.     

View-based navigation in Hymenoptera: multiple strategies of landmark guidance in the approach to a feeder

In order to analyse how landmarks guide the last stages of an insect's approach to a goal, we recorded many flights of individual wasps and honeybees as they flew to an inconspicuous feeder on the ground that was marked by one or by two nearby landmarks. An individual tends to approach the feeder from a constant direction, flying close to the ground. Its body is oriented in roughly the same horizontal direction during the approach so that the feeder and landmarks are viewed over a narrow range of directions. Consequently, when the insect arrives at the feeder, the landmarks take up a standard position on the retina. Three navigational strategies govern the final approach. The insect first aims at a landmark, treating it as a beacon. Secondly, bees learn the appearance of a landmark with frontal retina and they associate with this stored view a motor trajectory which brings them from the landmark sufficiently close to the goal that it can be reached by image matching. Insects then move so as to put the landmark in its standard retinal position. Image matching is shown to be accomplished by a control system which has as set points the standard retinal position of the landmark and some parameter related to its retinal size. Accepted: 1 March 1997     

Influence of sex steroid hormones on spatial memory in a songbird

In mammals, sex steroid hormones influence spatial learning and memory abilities but there are few data regarding such effects in birds. We investigated whether non-invasive sex steroid hormone treatment would affect spatial memory task performance of great tits (Parus major). For five consecutive days, birds were fed wax moth larvae injected with either 80 μg testosterone or 80 μg estradiol carried in peanut oil immediately prior to behavioral testing. During the 5 days prior to and the 5 days following hormone treatment, birds were fed vehicle-injected larvae. Both hormone manipulations resulted in an elevation of circulating hormone levels within 5 min of larva ingestion. This elevation was sustained for at least 30 min but had no short-term (<1 day) effect on spatial memory performance. However, performance tended to increase during the first 5 days of vehicle treatment and during both sex steroid treatments whereas it decreased during the 5 days of vehicle treatment following either hormone treatment. These results suggest that both hormones led to some improvement in spatial memory that declined once treatment ended. The great tit hippocampus was found to express androgen and estrogen receptors which would provide a direct site of sex steroid action.     

Molecular mechanism of cell proliferation in rodent uterus during the estrous cycle

The rodent uterus is a widely studied target tissue for sexual steroid hormone action. The aim of the present study was to assess the molecular mechanism that participates in the initiation of cell proliferation of the rat uterine epithelial cells during the estrus (E)–metestrus (M) transition. Cell proliferation, ERα, c-fos, cyclin D1 and D3, cdk4, and cdk6 proteins were assessed in these animals by immunohistochemistry. Estradiol (E₂) and progesterone (P₄) plasma levels were assessed by RIA. The results indicate that the glandular epithelium starts to proliferate at 21:00 h on estrus day, and initiates at least 3 h before the luminal epithelium does. Fos expression was markedly increased during the afternoon of estrus day, and its increase was in parallel to ERα expression. Interestingly, both, cyclin D1 and D3 were abundantly expressed in the luminal and glandular epithelia, and nuclear immunolabelling of cyclin D1 and D3 precedes BrdU incorporation in the cell. cdk4 and cdk6 were localized in the nuclei in both epithelia throughout the studied time course. In addition, cdk4 was more abundant throughout estrus and metestrus days than cdk6. The overall results indicate that ERα, Fos and cyclins D1 and D3, cdk4 and cdk6 are expressed in both glandular and luminal epithelia of the rat uterus during the E–M transition. In conclusion, there is a good correlation between sequential expression of these proteins and cell cycle progression in the rat uterine epithelial cells during the estrous cycle. However, the differences observed in the cellular localization, time course of expression and the cellular types that express both cyclins between physiological and pharmacological conditions, demonstrated different mechanisms of regulation and should be due to the complex hormonal milieu during the estrous cycle.     

Homing and home-site fidelity in rock-dwelling cichlids (Pisces: Teleostei) of Lake Malawi,Africa

Synopsis Existence of home site fidelity and homing ability was established for Pseudotropheus aurora. Territorial males stayed up to 1 1/2 years within the same territories. Females showed long-term fidelity to certain places. Transplantation experiments revealed that P. aurora males home over distances of up to at least 2500 m but would not cross deep water. The territorial males of five other sympartic species returned from 1000 m to their territories. The mechanisms and functions of homing in cichlids are discussed.     

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.     

QRFP in female rats: effects on high fat food intake and hypothalamic gene expression across the estrous cycle

Pyroglutamylated arginine-phenylalanineamide peptide (QRFP) is a neuropeptide involved in feeding behavior. Central administration of QRFP selectively increases the intake of a high fat diet in male rats. QRFP administration also stimulates the hypothalamic-pituitary-gonadal axis via gonadotrophin-releasing hormone in male and female rats. Prepro-QRFP mRNA is expressed in localized regions of the mediobasal hypothalamus which are abundant in neurotransmitters, neuropeptides and receptor systems important for food intake regulation and reproductive behaviors. The current experiments were conducted to investigate the effects of centrally administered QRFP-26 on the intake of a high fat diet (HFD, 60% kcal from fat) in female rats and to investigate alterations in hypothalamic prepro-QRFP and its receptors, GPR130a and GPR103b, mRNA levels over the estrous cycle. In Experiment 1, female rats were administered QRFP-26 (intracerebroventricular; 0.3 nmol, 0.5 nmol, 1.0 nmol) in rats consuming either a HFD or a low fat diet. All doses of QRFP-26 selectively increased the intake of the HFD in female rats. These data suggest that QRFP-26 regulates the intake of energy dense foods in female rats, which is similar to previous findings in male rats. In Experiment 2, hypothalamic levels of prepro-QRFP mRNA and its receptors were assessed during diestrus, proestrus, or estrus. The level of prepro-QRFP mRNA in the ventromedial/arcuate nucleus (VMH/ARC) of the hypothalamus was increased during proestrus, which suggests that endogenous estrogen levels regulate QRFP expression in the VMH/ARC. These data suggest that QRFP may play a role in coordinating feeding behaviors with reproductive function when energy demand is increased.     

Uterine epithelial and eosinophil estrogen receptors in rats during the estrous cycle

Summary In the uterus of the adult female rats, the luminal epithelial cells and the eosinophil leukocytes are rich in cytoplasmic estrogen receptors. During the estrous cycle, the epithelial estrogen receptor concentration reaches its peak level, in proestrus, drops precipitously in estrus, and hits the trough, at metestrus. Repopulation of the cytoplasm with estrogen binding sites occurs during diestrus. This pattern of cyclic change is indicative of a rapid turnover of estrogen receptors in the epithelial cells and its regulation by endogenous estrogens. The concentration of estrogen receptors in the cytoplasm of the eosinophils does not appear to fluctuate during the cycle. But the intrauterine, distribution of these leukocytes is clearly cyclic in pattern, ostensibly influenced by estrogens. While progesterone binding activity is consistently demonstrated in tandem with estrogen receptors in the cytoplasm of the epithelial cells, it has not been observed in the eosinophil leukocytes. These findings support the claim that there are two estrogen receptor systems in the rat uterus, one mediating the intracellular events of the genomic response to estrogens, and the other being concerned with non-genomic responses.Abbreviations BSA Bovine serum albumin - FITC fluorescein isothiocyanate - TMRITC tetramethylrhodamine isothiocyanate - CMO O-carboxymethyl oxime