Why estrogens matter for behavior and brain health

Abstract

The National Institutes of Health (NIH) has required the inclusion of women in clinical studies since 1993, which has enhanced our understanding of how biological sex affects certain medical conditions and allowed the development of sex-specific treatment protocols. However, NIH's policy did not previously apply to basic research, and the NIH recently introduced a new policy requiring all new grant applications to explicitly address sex as a biological variable. The policy itself is grounded in the results of numerous investigations in animals and humans illustrating the existence of sex differences in the brain and behavior, and the importance of sex hormones, particularly estrogens, in regulating physiology and behavior. Here, we review findings from our laboratories, and others, demonstrating how estrogens influence brain and behavior in adult females. Research from subjects throughout the adult lifespan on topics ranging from social behavior, learning and memory, to disease risk will be discussed to frame an understanding of why estrogens matter to behavioral neuroscience.

Figure 1

Schematic models illustrating mechanisms through which 17β-estradiol may modulate hippocampal memory. (A) In the classical mechanism, 17β-estradiol diffuses through the membrane to bind intracellular ERα and ERβ in the cytoplasm or nucleus. The 17β-estradiol-ER complex then binds to an estrogen response element (ERE) on the DNA with co-regulator proteins (Co) and histone acetyltransferases (HAT) to stimulate gene transcription. (B) Non-classical mechanisms reported thus far support a model in which 17β-estradiol activates neurotransmitter (NT) and/or growth factor (GF) receptors at the cell membrane or activates membrane ERs to trigger cell signaling cascades. Cell-signaling activation may lead to local protein synthesis, post-translational protein modifications in the cytoplasm or nucleus, and activation of transcription factors and epigenetic processes that increase gene transcription. Any of these 17β-estradiol-induced alterations could enhance hippocampal memory consolidation, although gene transcription is likely necessary for long-term memory storage.

Figure 2

(A) Treatment (20–30 min) of hippocampal sections with 50 nM 17b-Estradiol (E2) increased dendritic spine density in the stratum radiatum (shown) and stratum oriens (not shown) of the CA1 subregion of the dorsal hippocampus and shown in the biocytin filled pyramidal neurons (scale bar, 100mm). In parallel, infusion of 50 nM 17-b estradiol (E2) in the dorsal hippocampus of ovariectomized female mice 15 min prior to learning (habituation) and testing rapidly enhanced performance in the Object Placement (OP), Object Recognition (OR), and Social Recognition (SR) task within 40 min of treatments. In this specific paradigm vehicle control mice (only control for OP task is shown here) do not show learning while E2 treated mice did, as indicated by a significant increase from habituation (black bars) to test (white bars) in the percent of time spent investigating a novel or displaced stimulus vs a familiar stimulus. Within the same timeframe, patch clamping recordings from CA1 pyramidal neurons of learning-naïve mice showed a significant inhibitory effect of the same 50nM dose of E2 on the frequency of miniature Excitatory Postsynaptic Current (mEPSC) and on the amplitude of AMPA-induced membrane depolarization. This effects is consistent with the notion that E2 rapidly induces the formation of silent/immature synapses. * P<0.05, *** P<0.001 in comparison to vehicle control; # P<0.05, ## P<0.01, ### P<0.001 in comparison to habituation phase. Modified from Phan et al., PNAS 112: 16018–16023. (B) Dorsal hippocampal mTOR activation is necessary for E₂ to enhance object recognition memory consolidation. Levels of phospho-p42 ERK, but not phsopho-p44 ERK, are increased in ovariectomized mice 5 min after bilateral dorsal hippocampal infusion of 5 μg/hemisphere E₂ (*p < 0.05 relative to vehicle). This effect was blocked by the ERK inhibitor U0126 (0.5 μg/hemisphere), the PI3K inhibitor LY298002 (0.005 μg/hemisphere), or the mTOR inhibitor rapamycin (0.25 μg/hemisphere). Each inhibitor also prevented E₂ from enhancing object recognition memory; only mice infused with E₂ alone spent more time than chance (15 s) with the novel object (*p < 0.05). Error bars=mean ± standard error of the mean (SEM). Phosphorylated ERK normalized to total ERK. Insets are representative Western blots of phosphorylated and total ERK. Adapted with permission from Fortress et al., 2013.

Figure 3

(A) Chronic estrone decreased the number of BrdU-immunoreactive (ir) cells whereas chronic 17β-estradiol increased the total number of BrdU-ir cells. Asterisks (*) indicate significant difference between groups (controls vs 17β-estradiol p=0.046; controls vs estrone p=0.005; 17β-estradiol vs estrone p=0.0002). Photomicrographs indicate a BrdU(red)-ir cell colablled with NeuN(green), indicating that the newly synthesized cell expressed a mature neuronal marker. NeuN-neuronal nuclei; BrdU- bromodeoxyuridine. Modified and reprinted with permission from McClure et al., 2013, Hormones and Behavior, 63: 144–157. (B) Effect of estrogen treatment on infarct volume is dependent on “reproductive” age: Estrogen treatment to ovariectomized adult females decreased MCAo-induced infarct volume in the cortex and striatum, while the same dose of estrogen treatment increased stroke-induced infarction in middle aged females. CTX: cortex, STR: striatum, *:p<0.05 (modified from Selvamani and Sohrabji, 2010, Neurobiology of Aging, 31, 1618–1628.

Figure 4

Total number of working memory (WM) errors committed by 3 groups of postmenopausal women (mean age 55.6 yrs) on the SPWM test of working memory. WM errors were significantly more frequent among women not receiving hormone therapy (Non-HT, n = 35), compared with women who were receiving estrogens only (E-Only, n = 38) or women receiving estrogens plus a progestin (E + P, n = 23) at the time of testing. Difference between the 2 treated groups (E-Only, E + P) was not significant. Bars represent standard errors. (Data redrawn from Duff & Hampson, 2000, Horm Behav, 38, 262–276). Horizontal line shows mean level of performance on the same task in a group of young women not using oral contraceptives (n = 39, mean age 21.6 yrs) (first 2 trials only; for details see Hampson & Morley, 2013, Psychoneuroendocrinology, 38, 2897–2904).

Figure 5

The density of doublecortin (DCX)/NeuN expression in the dentate gyrus is increased in depressed patients. A) The density of DCX/NeuN expression was increased in depressed patients compared to depressed patients with psychosis (p=.02) and with a strong trend in controls (p=0.08). B) The density of DCX/NeuN expression was significantly greater in depressed women compared to all other groups. C) Representative DCX-expression is shown from depressed patients, depressed patients with psychosis and controls. Data shown is mean + SEM (standard error of the mean). Reprinted with permission from Epp JR, Beasley CL, Galea LAM. (2013). Neuropsychopharmacology, 38:2297–2306.