Low doses of 17β-estradiol rapidly improve learning and increase hippocampal dendritic spines

Abstract

While a great deal of research has been performed on the long-term genomic actions of estrogens, their rapid effects and implications for learning and memory are less well characterized. The often conflicting results of estrogenic effects on learning and memory may be due to complex and little understood interactions between genomic and rapid effects. Here, we investigated the effects of low, physiologically relevant, doses of 17β-estradiol on three different learning paradigms that assess social and non-social aspects of recognition memory and spatial memory, during a transcription independent period of memory maintenance. Ovariectomized female CD1 mice were subcutaneously administered vehicle, 1.5 μg/kg, 2 μg/kg, or 3 μg/kg of 17β-estradiol 15 minutes before social recognition, object recognition, or object placement learning. These paradigms were designed to allow the testing of learning effects within 40 min of hormone administration. In addition, using a different set of ovariectomized mice, we examined the rapid effects of 1.5 μg/kg, 2 μg/kg, or 3 μg/kg of 17β-estradiol on CA1 hippocampal dendritic spines. All 17β-estradiol doses tested impacted learning, memory, and CA1 hippocampal spines. 17β-Estradiol improved both social and object recognition, and may facilitate object placement learning and memory. In addition, 17β-estradiol increased dendritic spine density in the stratum radiatum subregion of the CA1 hippocampus, but did not affect dendritic spines in the lacunosum-moleculare, within 40 min of administration. These results demonstrate that the rapid actions of 17β-estradiol have important implications for general learning and memory processes that are not specific for a particular type of learning paradigm. These effects may be mediated by the rapid formation of new dendritic spines in the hippocampus.

Figure 1

Rapid effects of 17β-estradiol on learning and memory paradigms. Bar graphs illustrate investigation percent (gray bars average habituation, black bars test investigation percent). Line graphs illustrate total investigation durations (H indicates habituation). (a) 3.0 μg/kg of 17β-estradiol improved social recognition above vehicle controls, and all groups administered 17β-estradiol successfully demonstrated social recognition. (b) Total mouse investigation durations were not affected by 17β-estradiol treatment. (c) 1.5 and 3.0 μg/kg of 17β-estradiol improved object recognition above vehicle controls, and all groups administered 17β-estradiol successfully demonstrated novel object discrimination. (d) Total object investigation durations were unaffected by 17β-estradiol treatment. (e) All groups administered 17β-estradiol successfully demonstrated novel object location discrimination, in the object placement paradigm. (f) 17β-Estradiol treatment did not affect total object investigation durations. Asterisks above black bars indicate a significant difference between habituation and test within treatment group. Mean±SEM ^*p<0.05, ^**p<0.01, ^***p<0.001.

Figure 2

Images of Golgi-Cox-stained hippocampal CA1 neurons. Pyr, pyramidal cell layer; rad, stratum radiatum; l-m, lacunosum-moleculare. (a) Scale bar 200 μm. (b) Scale bar 100 μm. (c–f) Images of secondary dendrites from CA1 pyramidal neurons, from the stratum radiatum and lacunosum-moleculare of female mice treated with vehicle, 2.0 or 3.0 μg/kg of 17β-estradiol. Scale bars 5 μm.

Figure 3

Rapid effects of 17β-estradiol on dendritic spines in the stratum radiatum of female mice. (a, b), Treatment with 1.5, 2.0, or 3.0 μg/kg of 17β-estradiol increased dendritic spine density compared with vehicle. (c, d), Treatment with 17β-estradiol did not affect spine length. Asterisks indicate a significant difference between vehicle and 17β-estradiol treatment groups. Mean+SEM. ^***p<0.001.

Figure 4

Rapid effects of 17β-estradiol on dendritic spines in the lacunosum-moleculare of female mice. Treatment with 17β-estradiol did not significantly affect dendritic spine density (a, b) or dendritic spine length (c, d). Mean+SEM.