Estradiol selectively regulates metabolic substrates across memory systems in models of menopause.

Estrogen loss at menopause is thought to contribute to specific memory problems commonly encountered by women who are transitioning through or who have experienced menopause. Work in preclinical models suggests that estrogens bidirectionally regulate cognition through direct actions on different neural systems called memory systems, enhancing some types of learning and memory while impairing others. The energy load in the brain during cognitive activity is notoriously high, requiring sufficient provisions of metabolic substrates such as glucose, lactate, or ketones for optimal cognition. Thus, it is possible that estrogens bidirectionally regulate energy substrate availability within each system to produce the improvements and impairments in learning and memory. Indeed, estradiol increases extracellular levels of glucose in the hippocampus, a shift that corresponds to the hormone's beneficial effects on hippocampus-sensitive cognition. In contrast, estradiol decreases levels of lactate and ketones in the striatum, a shift that corresponds to the impairing effects of estradiol on striatum-sensitive cognition. Menopause may thus be associated with both cognitive improvements and impairments depending on estradiol status and on the problem to be solved. We propose that regulation of neural metabolism is one likely mechanism for these bidirectional effects of estradiol on cognition.

Extracellular levels of glucose in the hippocampus and striatum during maze training for food or water reward in male rats.

Glucose potently enhances cognitive functions whether given systemically or directly to the brain. The present experiments examined changes in brain extracellular glucose levels while rats were trained to solve hippocampus-sensitive place or striatum-sensitive response learning tasks for food or water reward. Because there were no task-related differences in glucose responses, the glucose results were pooled across tasks to form combined trained groups. During the first 1-3 min of training for food reward, glucose levels in extracellular fluid (ECF) declined significantly in the hippocampus and striatum; the declines were not seen in untrained, rewarded rats. When trained for water reward, similar decreases were observed in both brain areas, but these findings were less consistent than those seen with food rewards. After the initial declines in ECF glucose levels, glucose increased in most groups, approaching asymptotic levels ∼15-30 min into training. Compared to untrained food controls, training with food reward resulted in significant glucose increases in the hippocampus but not striatum; striatal glucose levels exhibited large increases to food intake in both trained and untrained groups. In rats trained to find water, glucose levels increased significantly above the values seen in untrained rats in both hippocampus and striatum. The decreases in glucose early in training might reflect an increase in brain glucose consumption, perhaps triggering increased brain uptake of glucose from blood, as evident in the increases in glucose later in training. The increased brain uptake of glucose may provide additional neuronal metabolic substrate for metabolism or provide astrocytic substrate for production of glycogen and lactate.

Aging is not equal across memory systems.

The present experiments compared the effects of aging on learning several hippocampus- and striatum-sensitive tasks in young (3-4 month) and old (24-28 month) male Fischer-344 rats. Across three sets of tasks, aging was accompanied not only by deficits on hippocampal tasks but also by maintained or even enhanced abilities on striatal tasks. On two novel object recognition tasks, rats showed impaired performance on a hippocampal object location task but enhanced performance on a striatal object replacement task. On a dual solution task, young rats predominately used hippocampal solutions and old rats used striatal solutions. In addition, on two maze tasks optimally solved using either hippocampus-sensitive place or striatum-sensitive response strategies, relative to young rats, old rats had impaired learning on the place version but equivalent learning on the response version. Because glucose treatments can reverse deficits in learning and memory across many tasks and contexts, levels of available glucose in the brain may have particular importance in cognitive aging observed across tasks and memory systems. During place learning, training-related rises in extracellular glucose levels were attenuated in the hippocampus of old rats compared to young rats. In contrast, glucose levels in the striatum increased comparably in young and old rats trained on either the place or response task. These extracellular brain glucose responses to training paralleled the impairment in hippocampus-sensitive learning and the sparing of striatum-sensitive learning seen as rats age, suggesting a link between age-related changes in learning and metabolic substrate availability in these brain regions.

Intra-striatal estradiol in female rats impairs response learning within two hours of treatment.

Estradiol treatment administered systemically or directly to the dorsolateral striatum across two days impairs performance on a response task in which rats learn to make a specific body turn to locate food on a maze. Estradiol can act through both slow and rapid signaling pathways to regulate learning impairments, however it is impossible to dissociate the slow from the rapid contributions of estradiol following long exposures. To assess the rapid effects of estradiol on striatum-sensitive learning, we trained rats on a response learning task after either relatively short or long treatments of estradiol infused directly into the striatum. Three-month-old female rats were ovariectomized 21 days before training and received guide cannulae implanted bilaterally into the dorsolateral striatum. For short duration treatments, rats were given bilateral infusions (0.5 µl) of 17ß-estradiol-sulfate (0, 5, 50, or 500 nM in aCSF-vehicle) either 2h or 15 min prior to training. For long duration treatments, rats received a series of estradiol infusions (500 nM) at 48, 24, and 2h prior to training. Replicating previous findings (Zurkovsky et al., 2007), intra-striatal estradiol treatments given for two days prior to training impaired response learning. Estradiol-induced impairments in performance were also demonstrated 2h, but not 15 min, after single infusions. Thus, estradiol acts within hours of exposure in the striatum, a structure lacking classical estrogen receptors, to impair response learning.

Epinephrine converts long-term potentiation from transient to durable form in awake rats.

Neuroendocrine responses to an emotional or arousing experience modulate memory for the event. Extensive evidence suggests that epinephrine plays an important role in the regulation of memory formation by emotions and arousal. Some forms of synaptic plasticity are similarly responsive to modulation by stress and arousal. The present experiment examined the effects of epinephrine on induction and maintenance of long-term potentiation (LTP) in awake rats. Rats were prepared with bilaterally implanted electrodes for recording evoked field potentials in dentate granule cells following perforant pathway stimulation. LTP was induced with high-frequency stimulation parameters that resulted in modest early potentiation of the EPSP that decayed within 20 min. Epinephrine enhanced the magnitude of early LTP induction and also extended the durability of LTP from minutes to at least several days. Epinephrine did not alter baseline responses or modulate pre-LTP input-output curves. The enhancement of LTP by epinephrine was dose-dependent, following an inverted-U dose-response curve similar to that seen in memory enhancement experiments, suggesting considerable convergence of epinephrine modulation of memory and LTP. In extending substantially the maintenance of LTP after induction, the present finding offer potential means to study the neurobiology of rapid forgetting seen in aged rodents and other animals and the neurobiology of the impaired forgetting seen in post-traumatic stress disorder.

Estrogen modulates learning in female rats by acting directly at distinct memory systems.

Physiologically high levels of circulating estradiol enhance the use of place learning and impair the use of response learning to find food on a land maze. These two types of learning are impaired by lesions of distinct neuronal structures, i.e. the hippocampus and striatum, respectively. Moreover, it has been shown in male rats that compromising hippocampal function can promote the use of response learning, while compromising striatal function can promote place learning. These findings suggest an ongoing competition between the hippocampus and striatum during cognition, such that intact functioning of one structure somehow obstructs the relative participation of the other. The goal of this study was to determine if estrogen's opposing effects on place and response learning in female rats are due to direct actions, either independent or interacting, at the hippocampus and striatum. We infused 0.5 microM 17beta-estradiol 3-sulfate sodium or vehicle bilaterally into the dorsal hippocampus or dorsolateral striatum of ovariectomized young adult female rats, 48, 24 and 2 h before training. Rats were tested on one of three appetitive tasks in a Y-maze: place learning, response learning, or response learning with reduced visual cues (cue-poor condition). Intrahippocampal estradiol infusions enhanced place learning, reversing a cannula-induced impairment, whereas intrastriatal infusions had no effects on place learning. Estradiol infusions into neither structure significantly affected response learning when extramaze cues were visible. However, in the response task, cue-poor condition, intrastriatal but not intrahippocampal infusions impaired learning. These data demonstrate that estrogen modulates place and response learning at the hippocampus and striatum respectively, most likely through independent actions at these two structures.

Short-term estrogen treatment in ovariectomized rats augments hippocampal acetylcholine release during place learning.

Estrogen modulates learning and memory in ovariectomized and naturally cycling female rats, especially in tasks using spatial learning and navigation. Estrogen also modulates cholinergic function in various forebrain structures. Past studies have shown positive correlations between hippocampal ACh output and performance on hippocampus-dependent tasks. The present study examined whether estradiol replacement would potentiate hippocampal ACh release during place learning. In vivo microdialysis and HPLC were used to measure extracellular ACh levels in the hippocampus of ovariectomized female rats that had received s.c. injections of 17beta-estradiol (10 microg) or sesame oil (vehicle treatment) 48 and 24h prior to training on a place task. Estrogen did not alter baseline levels of extracellular ACh in the hippocampus. During training, hippocampal ACh increased in ovariectomized rats regardless of estrogen status. However, while estradiol did not enhance learning in this experiment, estradiol significantly potentiated the increase in hippocampal ACh release seen during place training. This represents the first demonstration of on-line assessment of ACh output in hippocampus during learning in female rats and suggests that estrogen-dependent modulation of ACh release during training might control activation of different neural systems used during learning.

Glucose enhancement of 24-h memory retrieval in healthy elderly humans.

When administered soon before or after training, glucose facilitates memory in rodents and in several populations of humans, including healthy elderly people. Thus, glucose appears to enhance memory formation in a time- and dose-dependent manner. By assessing the effects of glucose at the time of memory tests, the present experiment examined the role of glucose on memory retrieval in healthy elderly people. On four sessions separated by a week, glucose or saccharin were administered immediately before hearing a narrative prose passage, as in previous experiments, or immediately before being tested for recall of the passage (24 h after training). Subjects recalled significantly more information after glucose ingestion than after saccharin ingestion whether the glucose was given before acquisition or memory tests. In addition, recall was significantly better in the preacquisition glucose condition relative to recall in the retrieval glucose condition. These findings provide evidence that glucose enhances both memory storage and retrieval.

Glucose, memory, and aging.

Circulating glucose concentrations regulate many brain functions, including learning and memory. Much of the evidence for this view comes from experiments assessing stress-related release of epinephrine with subsequent increases in blood glucose concentrations. One application of this work has been to investigate whether age-related memory impairments result from dysfunctions in the neuroendocrine regulation of the brain processes responsible for memory. Like humans, aged rodents exhibit some memory impairments that can be reversed by administration of epinephrine or glucose. In elderly humans, ingestion of glucose enhances some cognitive functions, with effects best documented thus far on tests of verbal contextual and noncontextual information. Glucose also effectively enhances cognition in persons with Alzheimer disease or Down syndrome. Although earlier evidence suggested that glucose does not enhance cognitive function in healthy young adults, more recent findings suggest that glucose is effective in this population, provided the tests are sufficiently difficult. In college students, glucose consumption significantly enhanced memory of material in a paragraph. Glucose also appeared to enhance attentional processes in these students. Neither face and word recognition nor working memory was influenced by treatment with glucose. The neurobiological mechanisms by which glucose acts are under current investigation. Initial evidence suggests that glucose or a metabolite may activate release of the neurotransmitter acetylcholine in rats when they are engaged in learning. Consequently, the issue of nutrition and cognition becomes increasingly important in light of evidence that circulating glucose concentrations have substantial effects on brain and cognitive functions.

Age-related differences in an ecologically based study of route learning.

Spatial learning abilities in younger adults and in healthy elderly adults were examined in 2 tasks. In the first task, participants were tested for their ability to recall relevant route information as well as to recognize and to order temporally landmark information observed along the route. Older participants had relatively greater difficulty retracing the route and temporospatially ordering landmarks but were equally good at recognition of landmarks occurring on the route. In the second task, participants memorized a 2-dimensional representation of a route and subsequently navigated the route from memory. Older participants had greater difficulty memorizing the route and navigating it. Errors of omission, commission, wrong, and forced choice were analyzed. Group differences in the pattern of errors differed by task.

Increased susceptibility to induction of long-term depression and long-term potentiation reversal during aging.

Homosynaptic long-term depression (LTD) and reversal of long-term potentiation (LTP) were examined extracellularly at CA3-CA1 synapses in stratum radiatum of slices from adult (6-9 months) and aged (20-24 months) Fischer 344 rats. Prolonged low-frequency stimulation (LFS) (900 pulses/1 Hz) of the Schaffer collaterals depressed the initial slope of the excitatory postsynaptic potential (EPSP) in aged but not adult rats. LTD at aged synapses was pathway-specific, persistent, and sensitive to the NMDA receptor antagonist DL-2-amino-5-phosphonopentanoic acid (AP5). Adult slices exhibited AP5-sensitive LTD in high [Ca2+] medium, whereas LTD in aged slices was blocked by high [Mg2+], suggesting that differences in Ca2+ regulation may underlie susceptibility to LTD. Despite age-related differences in LTD induction, no age difference in LTP magnitude was revealed. Additionally, LFS delivered 60 min after LTP induction resulted in similar LTP reversal for both age groups. Susceptibility differences to LTP reversal were indicated after multiple short-duration LFS bursts (30 pulses/1 Hz), with each burst separated by 10 min. Aged synapses exhibited significant reversal after a single burst and complete reversal after three LFS episodes. In adult slices, LTP reversal appeared after the fourth burst, and at no time was LTP depressed to initial baseline levels. This study provides the first characterization of homosynaptic LTD/LTP reversal in the aged animal and demonstrates that one form of plasticity, depression attributable to LFS, is increased during aging.

LTP saturation and spatial learning disruption: effects of task variables and saturation levels.

The prediction that "saturation" of LTP/LTE at hippocampal synapses should impair spatial learning was reinvestigated in the light of a more specific consideration of the theory of Hebbian associative networks, which predicts a nonlinear relationship between LTP "saturation" and memory impairment. This nonlinearity may explain the variable results of studies that have addressed the effects of LTP "saturation" on behavior. The extent of LTP "saturation" in fascia dentata produced by the standard chronic LTP stimulation protocol was assessed both electrophysiologically and through the use of an anatomical marker (activation of the immediate-early gene zif268). Both methods point to the conclusion that the standard protocols used to induce LTP do not "saturate" the process at any dorsoventral level, and leave the ventral half of the hippocampus virtually unaffected. LTP-inducing, bilateral perforant path stimulation led to a significant deficit in the reversal of a well-learned spatial response on the Barnes circular platform task as reported previously, yet in the same animals produced no deficit in learning the Morris water task (for which previous results have been conflicting). The behavioral deficit was not a consequence of any after-discharge in the hippocampal EEG. In contrast, administration of maximal electroconvulsive shock led to robust zif268 activation throughout the hippocampus, enhancement of synaptic responses, occlusion of LTP produced by discrete high-frequency stimulation, and spatial learning deficits in the water task. These data provide further support for the involvement of LTP-like synaptic enhancement in spatial learning.

Hippocampal synaptic enhancement and spatial learning in the Morris swim task.

The authors attempted to replicate the study of Castro, Silbert, McNaughton, and Barnes (1989) in which it was concluded that bilateral saturation of hippocampal synaptic enhancement produced a deficit in acquisition of a spatial navigation problem in the Morris swim task. The original protocol was followed as closely as possible, but no effect of long-term enhancement (LTE) saturation on spatial performance in this task was found. This negative result suggests either that the previous finding using the swim task reflected statistical error or that some as yet undetermined variable is of critical importance in this phenomenon. The present negative finding also raises a question concerning the reproducibility of the earlier results of McNaughton, Barnes, Rao, Baldwin, and Rasmussen (1986) in which LTE saturation apparently led to a prolonged deficit on a different spatial task. Although negative results in such experiments do not constitute grounds for rejecting the underlying hypothesis, the present lack of a positive effect renders uncertain, for the time being, one of the lines of experimental support for the theory that LTE at hippocampal synapses reflects a mechanism for the associative, distributed storage of new spatial information.

Unilateral naris closure and vascular development in the rat olfactory bulb.

The blood supply to the brain has been linked closely to nervous system function and metabolism, thereby possibly playing a direct role in brain maturation. Previously, we demonstrated that closure of an external naris early in life results in large changes within the olfactory bulb, including reductions in laminar volume and cell number and a rapid decline in metabolism and protein synthesis. To understand the role of the blood supply in the dramatic changes following naris closure, the present study examines the development of olfactory bulb vasculature in unilaterally odor-deprived and control rats. On post-partum day 1 (P1; the day after birth), littermate rat pups underwent either unilateral naris occlusion or sham surgery. On P5, P10, P15, P20, P30 and P60, animals were perfused with an india ink-gelatin mixture to assess blood vessel amount and complexity. Densitometric analyses were performed to obtain values of blood vessel area ratios (vessel area/tissue area), branch point number and branch point density. Considerable vessel development in all bulbs occurred over the first two to three weeks post-partum. By P20, large reductions in vessel area ratios were observed in all constituent laminae of deprived bulbs. While similar reductions in number of vessel branch points/tissue area were seen, few changes were noted in the number of branch points/vessel area. The effects were primarily confined to early developmental periods: bulb vasculature in animals deprived at older ages (P40) appeared normal. The results indicate that the vasculature responds to alterations in sensory stimulation early in life, therefore potentially playing an important regulative role in neural development.

Unilateral olfactory deprivation: effects on succinate dehydrogenase histochemistry and [3H]leucine incorporation in the olfactory mucosa.

Surgically closing one external naris reduces airflow through one half of the nasal cavity, decreasing the access of odors to the receptor sheet. In rats, unilateral naris occlusion performed near birth results in large reductions in the size of the olfactory bulb, the primary central relay, when examined 30 days later. Previous research has demonstrated that there is a rapid reduction in [3H]2-deoxyglucose (2-DG) and [3H]leucine uptake in the bulb within hours after naris closure. The present study examined whether similar rapid changes could be observed in the sensory periphery. Pups occluded on P1 and examined on P3 with succinate dehydrogenase histochemistry exhibited reduced staining on the closed side of the nasal cavity, suggesting occlusion results in reductions in mucosal metabolism. Larger differences in staining were observed in pups examined at P6. [3H]Leucine incorporation was quite similar on both sides of the nasal septum as late as 30 days post occlusion, suggesting less dramatic changes in protein synthesis. The results suggest that naris closure does indeed have rapid effects on mucosal function, but indicate that the changes are different than those observed in the bulb.

Rapid changes in 2-deoxyglucose uptake and amino acid incorporation following unilateral odor deprivation: a laminar analysis.

Unilateral naris occlusion in neonatal rats results in large alterations in the olfactory bulb, including substantial changes in laminar volume and enhanced cell death. These gross changes are undoubtedly the result of a cascade of more basic cellular regulatory events. The present study assesses the possibility of rapid post-deprivation changes in two such processes: glucose metabolism and protein synthesis. On the day after the day of birth rat pups underwent unilateral naris occlusion or sham surgery. In one study, either 1, 12, 24 or 48 h following surgery [3H]2-deoxyglucose [( 3H]2-DG) was administered to gauge patterns of glucose uptake. In a second study, [3H]leucine was injected to assess patterns of protein synthesis. Autoradiographs were then subjected to quantitative analyses. As early as 1 h following occlusion reduced 2-DG uptake was observed in many bulb regions. By 24 h, leucine incorporation was also uniformly diminished. While 2-DG uptake remained suppressed 48 h after deprivation, levels of amino acid incorporation returned to normal patterns in most laminae, with the exception of the mitral cell layer, where increased uptake was encountered. To evaluate whether the effects were developmental by nature a group of P40-P45 animals treated similarly were also examined. While 24 h of deprivation impaired 2-DG uptake in older animals, no alterations in amino acid incorporation were observed. The results indicate that early odor deprivation has rapid and specific effects on cellular functioning within the developing olfactory bulb.

Prenatal neurogenesis in the telencephalon of the precocial mouse Acomys cahirinus.

[3H]thymidine autoradiography was employed to examine the times of formation of the major neuronal classes in the forebrain of the precocial mouse Acomys cahirinus. Dams received 3 thymidine injections over a 24 h period on either embryonic Day 14, 18, 20, 22, 29 or 36. Age-related changes in the distribution and number of heavily labeled cells were noted. Acomys exhibited later onset and more protracted periods of cell generation than the phylogenetically related, altricial laboratory rat or mouse, indicating considerable differences in patterns of early growth between the species. Understanding the factors responsible for these differences could lend important insights into evolutionary mechanisms involved in the process of speciation.