The ability of 17␤-estradiol (E 2 ) to enhance hippocampal object recognition and spatial memory depends on rapid activation of extracellular signal-regulated kinase (ERK) in the dorsal hippocampus (DH). Although this activation can be mediated by the intracellular estrogen receptors ER␣ and ER␤, little is known about the role that the membrane estrogen receptor GPER plays in regulating ERK or E 2 -mediated memory formation. In this study, post-training DH infusion of the GPER agonist G-1 enhanced object recognition and spatial memory in ovariectomized female mice, whereas the GPER antagonist G-15 impaired memory, suggesting that GPER activation, like E 2 , promotes hippocampal memory formation. However, unlike E 2 , G-1 did not increase ERK phosphorylation, but instead significantly increased phosphorylation of c-Jun N-terminal kinase (JNK) in the DH. Moreover, DH infusion of the JNK inhibitor SP600125 prevented G-1 from enhancing object recognition and spatial memory, but the ERK inhibitor U0126 did not. These data suggest that GPER enhances memory via different cell-signaling mechanisms than E 2 . This conclusion was supported by data showing that the ability of E 2 to facilitate memory and activate ERK signaling was not blocked by G-15 or SP600125, which demonstrates that the memory-enhancing effects of E 2 are not dependent on JNK or GPER activation in the DH. Together, these data indicate that GPER regulates memory independently from ER␣ and ER␤ by activating JNK signaling, rather than ERK signaling. Thus, the findings suggest that GPER in the DH may not function as an estrogen receptor to regulate object recognition and spatial memory.

Ample evidence has demonstrated that sex steroid hormones, such as the potent estrogen 17b-estradiol (E 2 ), affect hippocampal morphology, plasticity, and memory in male and female rodents. Yet relatively few investigators who work with male subjects consider the effects of these hormones on learning and memory. This review describes the effects of E 2 on hippocampal spinogenesis, neurogenesis, physiology, and memory, with particular attention paid to the effects of E 2 in male rodents. The estrogen receptors, cell-signaling pathways, and epigenetic processes necessary for E 2 to enhance memory in female rodents are also discussed in detail. Finally, practical considerations for working with female rodents are described for those investigators thinking of adding females to their experimental designs.Hormones have long been known to play key roles in regulating learning and memory. Many hormones, including epinephrine, glucocorticoids, and insulin influence learning and memory via inverted U-shaped dose-response relationships in which memory is enhanced by moderate, but not low or high, hormone levels (Roozendaal 2000;Korol and Gold 2007;McNay and Recknagel 2011). Research from the past two decades has demonstrated that sex steroid hormones, particularly the potent estrogen 17b-estradiol (E 2 ), also regulate learning and memory in male and female rodents via an inverted U-shaped dose-response function that is influenced by estrogen receptor expression (Packard and Teather 1997a;Packard 1998;Foster 2012). Yet E 2 has not gained widespread acceptance as a hormonal modulator of memory in both females and males, perhaps because the majority of this research has been conducted in female rodents. Moreover, the common view of E 2 as a "female" hormone may contribute to the misperception that E 2 is not relevant for cognitive function in males. However, considerable evidence supports a vital role for E 2 in mediating neural function and behavior in male rodents. Therefore, it is important for investigators working with males to understand the ways in which E 2 and other sex steroid hormones (e.g., androgens, progestins, and other estrogens) may influence their brain regions and behaviors of interest.Another reason for investigators to be cognizant of sex steroid hormone-induced regulation of learning and memory is that males and females may respond differently to various treatments and environmental factors. A classic example is that of acute stress, which enhances classical conditioning and increases apical CA1 dendritic spine density in male rats, but impairs classical conditioning and decreases CA1 spine density in female rats (Wood and Shors 1998;Shors et al. 2001). Therefore, investigators hoping to comply with National Institutes of Health policies that encourage the inclusion of females in biomedical research must be aware that adding females to a study is not as simple as adding another group. In some ways, females are fundamentally different from males, the most obvious of which is the presence of reproductive...

The ovarian hormones 17β-estradiol (E2) and progesterone (P4) are potent modulators of hippocampal memory formation. Both hormones have been demonstrated to enhance hippocampal memory by regulating the cellular and molecular mechanisms thought to underlie memory formation. Behavioral neuroendocrinologists have increasingly used the object recognition and object placement (object location) tasks to investigate the role of E2 and P4 in regulating hippocampal memory formation in rodents. These one-trial learning tasks are ideal for studying acute effects of hormone treatments on different phases of memory because they can be administered during acquisition (pre-training), consolidation (post-training), or retrieval (pre-testing). This review synthesizes the rodent literature testing the effects of E2 and P4 on object recognition (OR) and object placement (OP), and the molecular mechanisms in the hippocampus supporting memory formation in these tasks. Some general trends emerge from the data. Among gonadally intact females, object memory tends to be best when E2 and P4 levels are elevated during the estrous cycle, pregnancy, and in middle age. In ovariectomized females, E2 given before or immediately after testing generally enhances OR and OP in young and middle-aged rats and mice, although effects are mixed in aged rodents. Effects of E2 treatment on OR 7and OP memory consolidation can be mediated by both classical estrogen receptors (ERα and ERβ), and depend on glutamate receptors (NMDA, mGluR1) and activation of numerous cell signaling cascades (e.g., ERK, PI3K/Akt, mTOR) and epigenetic processes (e.g., histone H3 acetylation, DNA methylation). Acute P4 treatment given immediately after training also enhances OR and OP in young and middle-aged ovariectomized females by activating similar cell signaling pathways as E2 (e.g., ERK, mTOR). The few studies that have administered both hormones in combination suggest that treatment can enhance OR and OP, but that effects are highly dependent on factors such as dose and timing of administration. In addition to providing more detail on these general conclusions, this review will discuss directions for future avenues of research into the hormonal regulation of object memory.

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Histone acetylation is essential for hippocampal memory formation in young adult rodents. Although dysfunctional histone acetylation has been associated with age-related memory decline in male rodents, little is known about whether histone acetylation is altered by aging in female rodents. In young female mice, the ability of 17b-estradiol (E 2 ) to enhance object recognition memory consolidation requires histone H3 acetylation in the dorsal hippocampus. However, the extent to which histone acetylation is regulated by E 2 in middle-aged females is unknown. The mnemonic benefits of E 2 in aging females appear to be greatest in middle age, and so pinpointing the molecular mechanisms through which E 2 enhances memory at this age could lead to the development of safer and more effective treatments for maintaining memory function without the side effects of current therapies. Here, we show that dorsal hippocampal infusion of E 2 rapidly enhanced object recognition and spatial memory, and increased histone H3 acetylation in the dorsal hippocampus, while also significantly reducing levels of histone deacetylase (HDAC2 and HDAC3) proteins. E 2 specifically increased histone H3 acetylation at Bdnf promoters pII and pIV in the dorsal hippocampus of both young and middle-aged mice, despite age-related decreases in pI and pIV acetylation. Furthermore, levels of mature BDNF and pro-BDNF proteins in the dorsal hippocampus were increased by E 2 in middle-aged females. Together, these data suggest that the middle-aged female dorsal hippocampus remains epigenetically responsive to E 2 , and that E 2 may enhance memory in middle-aged females via epigenetic regulation of Bdnf.