Elsevier

Neuroscience

Volume 394, 1 December 2018, Pages 303-315
Neuroscience

Research Article
Estrogen Alters the Synaptic Distribution of Phospho-GluN2B in the Dorsolateral Prefrontal Cortex While Promoting Working Memory in Aged Rhesus Monkeys

https://doi.org/10.1016/j.neuroscience.2018.09.021Get rights and content

Highlights

  • The majority of monkey dlPFC dendritic spines contain pGluN2B regardless of age or estradiol treatment.

  • In young monkeys, estradiol reduces the percentage of cytoplasmic pGluN2B.

  • In aged monkeys, estradiol increases cytoplasmic pGluN2B and reduces synaptic pGluN2B.

  • In aged monkeys, lower synaptic and higher cytoplasmic pGluN2B distribution correlates with better working memory scores.

Abstract

Age- and menopause-related deficits in working memory can be partially restored with estradiol replacement in women and female nonhuman primates. Working memory is a cognitive function reliant on persistent firing of dorsolateral prefrontal cortex (dlPFC) neurons that requires the activation of GluN2B-containing glutamate NMDA receptors. We tested the hypothesis that the distribution of phospho-Tyr1472-GluN2B (pGluN2B), a predominant form of GluN2B seen at the synapse, is sensitive to aging or estradiol treatment and coupled to working memory performance. First, ovariectomized young and aged rhesus monkeys (Macaca mulatta) received long-term cyclic vehicle (V) or estradiol (E) treatment and were tested on the delayed response (DR) test of working memory. Then, serial section electron microscopic immunocytochemistry was performed to quantitatively assess the subcellular distribution of pGluN2B. While the densities of pGluN2B immunogold particles in dlPFC dendritic spines were not different across age or treatment groups, the percentage of gold particles located within the synaptic compartment was significantly lower in aged-E monkeys compared to young-E and aged-V monkeys. On the other hand, the percentage of pGluN2B gold particles in the spine cytoplasm was decreased with E treatment in young, but increased with E in aged monkeys. In aged monkeys, DR average accuracy inversely correlated with the percentage of synaptic pGluN2B, while it positively correlated with the percentage of cytoplasmic pGluN2B. Together, E replacement may promote cognitive health in aged monkeys, in part, by decreasing the relative representation of synaptic pGluN2B and potentially protecting the dlPFC from calcium toxicity.

Introduction

Working memory is a type of short-term memory that involves transient storage, organization, and processing of information, which together guide reasoning and goal-directed behavior (Goldman-Rakic, 1995, Arnsten et al., 2012). This executive function is highly vulnerable to age- and menopause-related decline in humans and nonhuman primates and relies on the integrity of the dorsolateral prefrontal cortex (dlPFC), Area 46 (Bartus et al., 1978, Rapp and Amaral, 1989, Oscar-Berman et al., 1992, Roberts et al., 1997, Voytko and Tinkler, 2004, Hara et al., 2015). We previously showed that long-term cyclic administration of 17β-estradiol in aged ovariectomized monkeys restores working memory deficits while concurrently increasing the density of plastic, thin dendritic spines on dlPFC neurons (Rapp et al., 2003, Hao et al., 2007).

In rhesus monkeys, working memory function can be assessed using the well-characterized delayed response (DR) test (Bartus et al., 1978, Rapp and Amaral, 1989, Hara et al., 2012c, Hara et al., 2012b, Hara et al., 2012a). During this task, “delay cells” in the dlPFC exhibit spatially tuned recurrent firing while spatial information is held in working memory (Funahashi et al., 1989). Computational modeling has suggested that the slower kinetics of GluN2B-containing NMDA receptors are well-suited to support such prolonged network firing (Wang, 1999, Compte et al., 2000). Studies in vivo have since verified that GluN2B is critical to working memory by demonstrating that local application of a selective GluN2B subunit antagonist produces a significant reduction in “delay cell” firing (Wang et al., 2013).

Estrogen has wide-ranging effects on the brain, such as enhancing cognitive function, increasing spinogenesis, and inducing rapid signaling events that alter synaptic transmission and plasticity (Gould et al., 1990, Woolley and McEwen, 1992, Hara et al., 2015). It is well-documented in the rodent hippocampus literature that estrogen mediates spinogenesis and changes in synaptic plasticity through NMDA receptors (Woolley and McEwen, 1994, Woolley et al., 1997). However, little is known about the effects of estrogen on NMDA receptors in the primate PFC.

The current study tested the hypothesis that the subcellular distribution of GluN2B is altered with aging and estradiol treatment in monkeys and that the distribution pattern is coupled with working memory. We chose an antibody against GluN2B phosophorylated at the tyrosine 1472 residue (pGluN2B) because this is the form predominantly seen at the synapse, and levels of pGluN2B directly correlate with NMDA currents (Alvestad et al., 2003, Zhang et al., 2008, Wang et al., 2013). We discovered that in aged ovariectomized monkeys, E treatment reduces the synaptic representation of pGluN2B while increasing cytoplasmic pGluN2B. In addition, working memory scores inversely correlated with the percentage of synaptic pGluN2B and positively correlated with the percentage of cytoplasmic pGluN2B in aged monkeys. These results suggest that hormone replacement therapy may promote cognitive health in aged monkeys, in part, by decreasing the relative representation of synaptic pGluN2B and potentially protecting the dlPFC from calcium toxicity.

Section snippets

Monkeys

Thirteen young adult (mean ± SEM, 10.01 ± 0.55 years old) and thirteen aged (24.70 ± 0.66 years old) female rhesus monkeys (Macaca mulatta) were included in this study. This cohort is a subset of a larger cohort used in various other studies evaluating the effects of aging and hormone replacement on cognitive and neurobiological measures (Rapp et al., 2003, Hao et al., 2006, Hao et al., 2007, Wang et al., 2010, Hara et al., 2014, Crimins et al., 2017, Hara et al., 2016). Therefore, behavioral

Behavioral characterization

The 24 monkeys included in this current study were part of a larger cohort and their detailed behavioral data have been presented in previous studies (Rapp et al., 2003, Hao et al., 2007, Wang et al., 2010, Crimins et al., 2017). No significant effects of age or treatment were observed with the number of trials to reach task acquisition criterion of 90% correct or better at delay intervals of 0 and 1 s (p > 0.05; Fig. 2A), consistent with literature reporting that aged and menopausal monkeys

Discussion

This study revealed that the effects of estrogen treatment on synaptic pGluN2B distribution in the monkey dlPFC are diametrically opposed depending on age. E treatment reduced synaptic pGluN2B and increased cytoplasmic pGluN2B in aged monkeys, while the direction of change was the opposite in young monkeys. In addition, working memory scores inversely correlated with the percentage of synaptic pGluN2B and positively correlated with the percentage of cytoplasmic pGluN2B in aged monkeys (Fig. 5),

Conclusions

Here we provide the first evidence for the linkage between PFC-reliant cognitive function and the distribution of a synaptic protein—one especially critical to glutamatergic signaling—offering novel insight into both the synaptic mechanisms of age-related cognitive decline, and the protective role of estrogen. A principal finding was that aged monkeys treated with estradiol expressed a lower percentage of synaptic pGluN2B, but a higher percentage of cytoplasmic pGluN2B, compared to those

Acknowledgments

We thank Mary Roberts, Sania Fong, Deborah Kent, Heather McKay, Tweithy Oung, and Anne Canfield at the California National Primate Research Center for their expert technical assistance involving the rhesus monkeys, and Dr. Donald Canfield for assistance in veterinary work.

Author contributions

Yuko Hara designed research, performed research, analyzed data, and wrote the paper. Johanna L. Crimins performed research and wrote the paper. Rishi Puri performed research. Athena C.J. Wang performed research. Sarah E. Motley performed research. Frank Yuk performed research. Tiffany M. Ramos performed research. William G.M. Janssen performed research. Peter R. Rapp designed research and wrote the paper. John H. Morrison designed research and wrote the paper.

Funding

This work was supported by National Institute on Aging grants P01 AG16765 to J.H.M., and in part by the Intramural Research Program of the National Institute on Aging.

Declaration of interest

The authors declare no competing financial interests.

Glossary

17-beta estradiol
the 17-beta isomer of estradiol, an estrogen steroid hormone that is the most abundant form of estrogenic steroid in premenopausal primates
Delayed response test
a test of working memory wherein the subject is asked to guide behavior according to internal representations of trial-specific, cued information retained over a delay period, rather than on the basis of external sensory stimuli no longer available in the environment
Mushroom spine
a morphological subclass of dendritic

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