Estrogen and Aging Affect the Subcellular Distribution of Estrogen Receptor-α in the Hippocampus of Female Rats

Abstract

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.

Aging and substitutive hormonal therapy influence in regional and subcellular distribution of ERα in female rat brain

“…In the diencephalon, the change in ERα distribution in aging is not observed, but the immunostaining remains in the nuclei through the experiment in most cases, supporting some previous works (McEwen and Alves 1999;Ishunina et al 2000;Adams et al 2002). As it occurs in the telencephalon, ovariectomy makes the immunostaining decrease in an age-related way.…”

“…Furthermore, it was reported that not only is there an age regulation of the intensity of immunostaining, but also an evident influence of aging in the subcellular distribution of this receptor (Ishunina et al 2000;Milner et al 2001;Kalesnykas et al 2005). In ovariectomized animals, the effect of the sexual hormones in the ERα level and its localization within the cell, as is described by many researchers has also been widely demonstrated (Adams et al 2002;Wilson et al 2002;Stirone et al 2003).…”

“…As well as this, this alternative ER-X might be mostly expressed in some exceptional situations, such as ovariectomy, which would justify the high levels of ERα detected in the OVX group by immunohistochemistry. Nevertheless, whatever technique was used, as a direct consequence of ovariectomy, there is a decrease in ERα, intensifying the agingassociated loss of protein (Adams et al 2002;Mehra et al 2005), while 17β-estradiol treatment is capable of restoring the effect of ovariectomy at later stages of the experiment.…”

“…One way involves canonical ER activation, which is constitutively expressed in many brain regions and is able to initiate gene transcription after specifically binding to estradiol (Green and Simpkins 2000;Marin et al 2005). The second way that has been growing interest and refers to the rapid non-genomic (or alternative) signaling pathways, involves extranuclear ER in response to physiological concentration of estrogens to elicit neuroprotection (Singer et al 1999;Adams et al 2002;Mendez et al 2003;Simoncini et al 2003). Recent studies point to a coupling between both mechanisms (for review see Vasudevan and Pfaff 2008).…”

“…As we discussed above, ERα has a distribution that varies among the different cerebral areas studied, as well as with different situations. Because of the potential importance of this receptor in the mediating effects of changing estradiol levels with aging, many studies achieved the amount of global immunostaining and distribution of ERα in several brain areas, such as the hippocampus (Adams et al 2002;Mehra et al 2005) or hypothalamus (Funabashi et al 2000). Furthermore, it was reported that not only is there an age regulation of the intensity of immunostaining, but also an evident influence of aging in the subcellular distribution of this receptor (Ishunina et al 2000;Milner et al 2001;Kalesnykas et al 2005).…”