Estrogen replacement increases both the number of dendritic spines and the density of axospinous synapses in the hippocampal CA1 region in young rats, yet this is attenuated in aged rats. The estrogen receptor-␣ (ER-␣) is localized within select spines of CA1 pyramidal cells in young animals and thus may be involved locally in this process. The present study investigated the effects of estrogen on the ultrastructural distribution of ER-␣ in the CA1 of young (3-4 months) and aged (22-23 months) Sprague Dawley rats using postembedding immunogold electron microscopy. Within dendritic spines, most ER-␣ immunoreactivity (IR) was seen in plasmalemmal and cytoplasmic regions of spine heads, with a smaller proportion within 60 nm of the postsynaptic density. In presynaptic terminals, ER-␣-IR was clustered and often associated with synaptic vesicles. Significant effects of both aging and estrogen were observed. Quantitative analysis revealed that nonsynaptic pools of ER-␣-IR within the presynaptic and postsynaptic compartments were decreased (35 and 27%, respectively) in the young estrogen-replaced animals compared with those that received vehicle. Such localized regulation of ER-␣ in response to circulating estrogen levels might directly affect synaptic signaling in CA1 pyramidal cells. No estrogen treatment-related differences were observed in the aged animals. However, 50% fewer spines contained ER-␣ in the aged compared with young hippocampus. These data suggest that the decreased responsiveness of hippocampal synapses to estrogen in aged animals may result from age-related decrements in ER-␣ levels and its subcellular localization vis-à -vis the synapse. Such a role for spinous ER-␣ has important implications for age-related attenuation of estrogen-induced hippocampal plasticity.

Estrogen regulates hippocampal dendritic spine density and synapse number in an N-methyl-D-aspartate (NMDA) receptor-dependent manner, and these effects may be of particular importance in the context of age-related changes in endocrine status. We investigated estrogen's effects on axospinous synapse density and the synaptic distribution of the NMDA receptor subunit, NR1, within the context of aging. Although estrogen induced an increase in axospinous synapse density in young animals, it did not alter the synaptic representation of NR1, in that the amount of NR1 per synapse was equivalent across groups. Estrogen replacement in aged female rats failed to increase axospinous synapse density; however, estrogen up-regulated synaptic NR1 compared with aged animals with no estrogen. Therefore, the young and aged hippocampi react differently to estrogen replacement, with the aged animals unable to mount a plasticity response generating additional synapses, yet responsive to estrogen with respect to additional NMDA receptor content per synapse. These findings have important implications for estrogen replacement therapy in the context of aging.A t the turn of the century, the life expectancy of American women was roughly equivalent to the average age of the onset of menopause (1). Presently, there is a 30-year discrepancy between these two demographic indices, with a life expectancy of Ϸ80 years and the average onset of menopause remaining in the early fifties (1). As such, it is critical that we understand the interaction of reproductive senescence with the aging of other systems, particularly the nervous system. The regulation of the reproductive axis by estrogens has been characterized in great detail (2). However, estrogens also impact synaptic communication in brain regions involved in cognitive processing, such as the hippocampus (3), and these effects may be of particular importance in the context of aging when both circulating estrogen levels change and hippocampal-dependent functions decline. With respect to its nonreproductive functions, estrogen improves verbal memory and the capacity for learning new associations in both naturally and surgically menopausal women (4). In rats, although findings are somewhat controversial (5-7), learning was enhanced during the proestrus phase (i.e., high estrogen) of the estrous cycle compared with the estrus phrase (i.e., low estrogen) (8). Another study found improved learning and memory on the Morris water maze task in ovariectomized animals with intrahippocampal estrogen injection compared with saline injection (9).Our current understanding of estrogen effects on synaptic plasticity in the hippocampus is based almost exclusively on data from young animals. For example, dendritic spine density in CA1 pyramidal cells is sensitive to naturally occurring estrogen fluctuations in young animals (10), as well as experimentally induced estrogen depletion and replacement (11)(12)(13)(14). Recent evidence suggests that estrogens mediate these morphological changes through N-methyl-D-...

Estrogen interacts with N-methyl-D-aspartate (NMDA) receptors to regulate multiple aspects of morphological and functional plasticity. In hippocampus, estrogen increases both dendritic spine density and synapse number, and NMDA antagonists block these effects. Thus, estrogen-mediated hippocampal plasticity may be of particular importance in the context of age-related changes in endocrine status and cognitive performance. NR1 levels per synapse are increased in CA1 by estrogen in aged rats but not young rats, although no information is available on estrogen-induced synaptic alterations in other NMDA receptor subunits that might impact function. Therefore, the present study was designed to investigate the effect of estrogen on the synaptic and subsynaptic distributions of the NMDA receptor subunits, NR2A and NR2B in CA1 pyramidal cells, within the context of aging. Our results demonstrated that the overall synaptic levels of NR2A and NR2B are similar in young and aged female rats, regardless of estrogen treatment. However, in the aged CA1, estrogen restores NR2B levels back to young levels in the lateral portions of the active synaptic zone. Thus, estrogen may impact the mobility of NMDA receptors across the synapse and, in the process, restore a more youthful synaptic profile. These findings have important implications for the mechanism of estrogen-induced alterations in NMDA receptor-mediated processes, particularly in the context of aging.

In recent years, several mouse models of amyotrophic lateral sclerosis (ALS) have been developed. One, caused by a G86R mutation in the superoxide dismutase‐1 (SOD‐1) gene associated with familial ALS, has been subjected to extensive quantitative analyses in the spinal cord. However, the human form of ALS includes pathology elsewhere in the nervous system. In the present study, analyses were extended to three motor nuclei in the brainstem. Mutant mice and control littermates were evaluated daily, and mutants, along with their littermate controls, were killed when they were severely affected. Brains were removed after perfusion and processed for Nissl staining, the samples were randomized, and the investigators were blinded to their genetic status. Stereologic methods were used to estimate the number of neurons, mean neuronal volumes, and nuclear volume in three brainstem motor nuclei known to be differetially involved in the human form of the disease, the oculomotor, facial, and hypoglossal nuclei. In the facial nucleus, neuron number consistently declined (48%), an effect that was correlated with disease severity. The nuclear volume of the facial nucleus was smaller in the SOD‐1 mutant mice (45.7% difference from control mice) and correlated significantly with neuron number. The oculomotor and hypoglossal nuclei showed less extreme involvement (<10% neuronal loss overall), with a trend toward fewer neurons in the hypoglossal nucleus of animals with severe facial nucleus involvement. In the oculomotor nucleus, neuronal loss was seen only once in five mice, associated with very severe disease. There was no significant change in the volume of individual neurons in any of these three nuclei in any transgenic mouse. These results suggest that different brainstem motor nuclei are differentially affected in this SOD‐1 mutant model of ALS. The relatively moderate and late involvement of the hypoglossal nucleus indicates that, although the general patterns of neuronal pathology match closely those seen in ALS patients, some differences exist in this transgenic model compared with the progression of the disease in humans. However, these patterns of cellular vulnerability may provide clues for understanding the differential susceptibility of neural structures in ALS and other neurodegenerative diseases. J. Comp. Neurol. 416:112–125, 2000. © 2000 Wiley‐Liss, Inc.

Background. Sudden death is the leading cause of mortality in medically refractory cases of epilepsy. Younger persons with epilepsy (PWE), particularly those <40 years, have higher all-cause mortality than those without. However, data are conflicting about mortality and burden of cardiovascular disease (CVD) in middle-aged PWE. Objective: Determine all-cause and sudden death-specific mortality and burden of CVD in PWE in a middle-aged population. Methods. Using UK Biobank, we identified 7,786 (1.6%) participants with a diagnosis of epilepsy; 566 individuals with prior history of stroke were excluded. The 7,220 PWE comprised the study cohort with the remaining 494,676 without epilepsy as the comparator group. PWE were identified based on clinical diagnostic code (validated) or self-reported diagnosis at assessment centre interview. Prevalence of CVD was determined using validated diagnostic codes. Cox proportional hazards regression were used to assess all-cause mortality and sudden death risk, in PWE vs those without epilepsy. Results. Hypertension, coronary artery disease, heart failure, valvular heart disease, and congenital heart disease were all more prevalent in PWE. Arrhythmias including atrial fibrillation/flutter (12.2% vs 6.9%; p<0.01), bradyarrhythmias (7.7% vs 3.5%; p<0.01), conduction defects (6.1% vs 2.6%; p<0.01), and ventricular arrhythmias (2.3% vs 1.0%; p<0.01), as well as cardiac implantable electric devices (4.6% vs 2.0%; p<0.01) were all more common in PWE compared to comparator group. PWE had higher all-cause mortality (HR 3.9 [95% CI, 3.01-3.39]), higher sudden death-specific mortality (HR 6.65 [95% CI, 4.53-9.77]) both adjusted for age, sex and comorbidities; and were almost 2 years younger at death [68.1 vs 69.8; p<0.001]. Conclusions. PWE have markedly higher burden of CVD including arrhythmias and heart failure. Middle-aged PWE have increased all-cause and sudden death specific mortality and higher burden of CVD. While efforts have focused on SUDEP in the young, further work is required to elucidate mechanisms underlying all-cause mortality and sudden death risk in PWE of middle age, to identify prognostic biomarkers and develop preventative therapies in PWE.