17β-Estradiol Acutely Potentiates Glutamatergic Synaptic Transmission in the Hippocampus through Distinct Mechanisms in Males and Females

Abstract

Estradiol (E2) acutely potentiates glutamatergic synaptic transmission in the hippocampus of both male and female rats. Here, we investigated whether E2-induced synaptic potentiation occurs via presynaptic and/or postsynaptic mechanisms and which estrogen receptors (ERs) mediate E2's effects in each sex. Whole-cell voltage-clamp recordings of mEPSCs in CA1 pyramidal neurons showed that E2 increases both mEPSC frequency and amplitude within minutes, but often in different cells. This indicated that both presynaptic and postsynaptic mechanisms are involved, but that they occur largely at different synapses. Two-photon (2p) glutamate uncaging at individual dendritic spines showed that E2 increases the amplitude of uncaging-evoked EPSCs (2pEPSCs) and calcium transients (2pCaTs) at a subset of spines on a dendrite, demonstrating synapse specificity of E2's postsynaptic effects. All of these results were essentially the same in males and females. However, additional experiments using ER-selective agonists indicated sex differences in the mechanisms underlying E2-induced potentiation. In males, an ERβ agonist mimicked the postsynaptic effects of E2 to increase mEPSC, 2pEPSC, and 2pCaT amplitude, whereas in females, these effects were mimicked by an agonist of G protein-coupled ER-1. The presynaptic effect of E2, increased mEPSC frequency, was mimicked by an ERα agonist in males, whereas in females, an ERβ agonist increased mEPSC frequency. Thus, E2 acutely potentiates glutamatergic synapses similarly in both sexes, but distinct ER subtypes mediate the presynaptic and postsynaptic aspects of potentiation in each sex. This indicates a latent sex difference in which different molecular mechanisms converge to the same functional endpoint in males versus females.

Significance statement: Some sex differences in the brain may be latent differences, in which the same functional endpoint is achieved through distinct underlying mechanisms in males versus females. Here we report a latent sex difference in molecular regulation of excitatory synapses in the hippocampus. The steroid 17β-estradiol is known to acutely potentiate glutamatergic synaptic transmission in both sexes. We find that this occurs through a combination of increased presynaptic glutamate release probability and increased postsynaptic sensitivity to glutamate in both sexes, but that distinct estrogen receptor subtypes underlie each aspect of potentiation in each sex. These results indicate that therapeutics targeting a specific estrogen receptor subtype or its downstream signaling would likely affect synaptic transmission differently in the hippocampus of each sex.

Keywords: dendritic spines; estrogen receptor; sex difference; synapse.

Figure 1.

E2 acutely potentiates mEPSC frequency and mEPSC amplitude in both sexes. A, B, Sample experiment showing (A) mEPSC recording during baseline and after E2 and (B) the time course of the E2-induced increase in instantaneous mEPSC frequency and mEPSC amplitude in the same cell. C, Plots showing mean mEPSC frequency during baseline and after E2 for all cells in both females and males. Overall, E2 increased mEPSC frequency in both sexes. **p < 0.001 (paired t test). Connected symbols represent data from an individual cell. Colored symbols represent the subset of cells in which within-cell t tests showed a significant effect of E2. White symbols represent cells with no significant effect of E2 (also in D–G). D, Plots showing mean mEPSC amplitude during baseline and after E2 for the same cells as in C. Overall, E2 increased mEPSC amplitude in both sexes. **p < 0.01 (paired t test). *p < 0.05 (paired t test). E, F, Plotting mean (±SEM) mEPSC frequency (E) or amplitude (F) after E2 versus during baseline for each cell shows that E2 potentiated mEPSC frequency and/or amplitude in cells that began with a wide range of baseline values. G, Plotting the normalized change in mEPSC frequency versus amplitude for each cell shows that E2 rarely increased both mEPSC frequency and amplitude in the same cells (black); more often, E2 increased mEPSC frequency only (blue), mEPSC amplitude only (orange), or had no effect on mEPSCs (white). H, The proportion of cells in each category of mEPSC response to E2 is similar in females and males.

Figure 2.

2p-glutamate uncaging at individual spines shows that the postsynaptic effects of E2 are synapse-specific. A, Image of a CA1 pyramidal cell during recording showing the dendrite targeted for 2p-glutamate uncaging (box). B, Higher-magnification view of the dendrite showing three spines that were targeted for uncaging (*). C, Currents from 20 individual sweeps during uncaging at the spines in B. Flashes from the uncaging laser (red line) yielded 2pEPSCs consistently throughout the recording; individual sweeps (black) are averaged (red) below. D, E, Sample experiment showing (D) two spines targeted for uncaging with averaged 2pEPSCs evoked at those spines during baseline and after E2 and (E) the time course of the E2-induced increase in 2pEPSC amplitude at spine 1 with no change in 2pEPSC amplitude at spine 2. F, Plotting mean (±SEM) 2pEPSC amplitude after E2 versus baseline for all spines tested shows that E2 potentiated 2pEPSC amplitude in spines that began with a wide range of baseline values. Red symbols represent the subset of spines in which within-spine t tests indicated a significant effect of E2. White symbols represent spines with no significant effect of E2. G, Plotting distributions of the normalized change in 2pEPSC amplitude after E2 for each sex shows no sex difference. H, Schematic illustrating analysis of how the distance between spines on a dendrite influenced the similarity of their response to E2. For each pair of spines, the ratio of normalized responses to E2 was calculated and compared with the linear distance between the spines. I, Plotting normalized response ratio for each spine pair versus linear distance between spines in a pair shows that spines that are closer together respond to E2 more similarly than spines that are further apart (r = −0.53, p = 0.0001).

Figure 3.

2p-glutamate-evoked calcium transients in dendritic spines are potentiated by E2. A–C, At one spine per cell, a line scan (dashed blue line) was taken over the spine and its parent dendrite as shown in A. 2p-glutamate uncaging at that spine evoked a transient increase in OGB fluorescence in the spine head (B) with no change in Alexa-594 fluorescence (C). D, The change in OGB over Alexa-594 fluorescence (ΔF/F) for the spine shown in A–C plotted as a 2pCaT before and after E2. Individual trials (black) were averaged (green). E, Plotting mean (±SEM) 2pCaT amplitude after E2 versus baseline for each spine shows that E2 potentiated 2pCaTs in spines that began with a wide range of baseline values. Colored symbols represent the subset of spines in which within-spine t tests indicated a significant effect of E2. White symbols represent spines with no significant effect of E2. F, Plotting the normalized change in 2pCaT amplitude versus 2pEPSC amplitude within each spine shows that 2pCaTs were potentiated in a subset of spines from which the 2pEPSC was also potentiated by E2.

Figure 4.

The ERβ agonist WAY200070 acutely potentiates mEPSC frequency in females and mEPSC amplitude in males. A, B, Sample mEPSC recordings during baseline, after WAY, and after E2 in a female (A) and a male (B) cell. C, Time course of mEPSC frequency changes for the same female cell as in A showing that WAY acutely increased mEPSC frequency and that E2 after WAY had no further effect. D, Time course of mEPSC frequency changes for the same male cell as in B showing that WAY had no effect on mEPSC frequency but that E2 applied after WAY increased mEPSC frequency, confirming that mEPSC frequency in this cell was responsive to E2. E, Summary of mEPSC frequency analysis in female and male experiments with WAY. Colored symbols represent points in the experiment in which within-cell t tests indicated a significant difference from the preceding condition (also in H). F, Time course of mEPSC amplitude changes for the same female cell as in A showing that WAY had no effect on mEPSC amplitude but that E2 applied after WAY increased mEPSC amplitude, confirming that mEPSC amplitude in this cell was responsive to E2. G, Time course of mEPSC amplitude changes for the same male cell as in B showing that WAY acutely increased mEPSC amplitude and that E2 after WAY had no further effect. H, Summary of mEPSC amplitude analysis in female and male experiments with WAY.

Figure 5.

The ERα agonist PPT has no effect on mEPSCs in females but acutely potentiates mEPSC frequency in males. A, B, Sample mEPSC recordings during baseline, after PPT, and after E2 in a female (A) and a male (B) cell. C, Time course of mEPSC frequency changes for the same female cell as in A showing that PPT had no effect on mEPSC frequency but that E2 applied after PPT increased mEPSC frequency, confirming that mEPSC frequency in this cell was responsive to E2. D, Time course of mEPSC frequency changes for the same male cell as in B showing that PPT acutely increased mEPSC frequency and that E2 after WAY had no further effect. E, Summary of mEPSC frequency analysis in female and male experiments with PPT. Colored symbols represent points in the experiment in which within-cell t tests indicated a significant difference from the preceding condition (also in H). F, Time course of mEPSC amplitude changes for the same female cell as in A showing that PPT had no effect on mEPSC amplitude but that E2 applied after PPT increased mEPSC amplitude, confirming that mEPSC amplitude in this cell was responsive to E2. G, Time course of mEPSC amplitude changes for the same male cell as in B showing that PPT had no effect on mEPSC amplitude but that E2 applied after PPT increased mEPSC amplitude, confirming that mEPSC amplitude in this cell was responsive to E2. H, Summary of mEPSC amplitude analysis in female and male experiments with PPT.

Figure 6.

The GPER1 agonist G1 acutely potentiates mEPSC amplitude in females but has no effect on mEPSCs in males. A, B, Sample mEPSC recordings during baseline, after G1, and after E2 in a female (A) and a male (B) cell. C, Time course of mEPSC frequency changes for the same female cell as in A showing that G1 had no effect on mEPSC frequency but that E2 applied after G1 increased mEPSC frequency, confirming that mEPSC frequency in this cell was responsive to E2. D, Time course of mEPSC frequency changes for the same male cell as in B showing that G1 had no effect on mEPSC frequency but that E2 applied after G1 increased mEPSC frequency, confirming that mEPSC frequency in this cell was responsive to E2. E, Summary of mEPSC frequency analysis in female and male cells with G1. Colored symbols represent points in the experiment in which within-cell t tests indicated a significant difference from the preceding condition (also in H). F, Time course of mEPSC amplitude changes for the same female cell as in A showing that G1 acutely increased mEPSC amplitude and that E2 after G1 had no further effect. G, Time course of mEPSC amplitude changes in the same male cell as in B. Like all other male cells recorded with G1, this cell showed no effect of G1 on mEPSC amplitude. E2 also did not affect mEPSC amplitude in this cell, although E2 applied after G1 did increase mEPSC amplitude in other male cells tested for G1 responsiveness (see H). There were no male cells tested with G1 that showed E2-induced increases in both mEPSC frequency and amplitude. H, Summary of mEPSC amplitude analysis in female and male cells with G1.

Figure 7.

2p-glutamate uncaging at individual spines confirms postsynaptic effects of the GPER1 agonist G1 in females and the ERβ agonist WAY200070 in males. A, Sample recording of 2pEPSCs from spines on a female (top) and a male (bottom) cell during baseline, after WAY, and after E2 and a summary of 2pEPSC results with WAY in females and males. Colored symbols represent points in the experiment in which within-spine t tests indicated a significant difference from the preceding condition (also in B–F). WAY increased 2pEPSC amplitude in a subset of spines from males and occluded a further effect of E2, whereas WAY had no effect on 2pEPSCs in females, even those that responded to E2 after WAY. B, Representative 2pCaTs from the same spines shown in A during baseline, after WAY, and after E2 and a summary of 2pCaT results with WAY in females and males. 2pCaT analysis showed the same pattern as 2pEPSC analysis in that WAY increased 2pCaT amplitude only in males but had no effects in females, even in spines that responded to E2 after WAY. C, Sample 2pEPSCs from spines on a female (top) and a male (bottom) cell during baseline, after PPT, and after E2, and a summary of 2pEPSC results with PPT in females and males. PPT had no effects on 2pEPSC amplitude in either sex, even in spines that responded to E2 after PPT. D, Sample 2pCaTs from the same spines shown in C during baseline, after PPT, and after E2 and a summary of 2pCaT experiments with PPT in females and males showing the same pattern as with 2pEPSCs. E, Sample 2pEPSCs from spines on a female (top) and a male (bottom) cell during baseline, after G1, and after E2, and a summary of 2pEPSC results with G1 in females and males. G1 increased 2pEPSC amplitude in a subset of spines from females and occluded any further effect of E2, whereas G1 had no effect on 2pEPSC amplitude in males, even those that responded to E2 after G1. F, Sample 2pCaTs from the same spines shown in E during baseline, after G1, and after E2, and a summary of 2pCaT results with G1 in females and males. 2pCaT results mirrored 2pEPSC results in that G1 increased 2pCaT amplitude in a subset of spines from females, but had no effect in males, even in spines that responded to E2 after G1. In all sample recordings, red arrow indicates the time of the uncaging.

Figure 8.

A latent sex difference in the mechanisms of E2-induced synaptic potentiation in the hippocampus. Our results support a model in which E2-induced synaptic potentiation is due to both an increase in presynaptic glutamate release probability and postsynaptic sensitivity to glutamate in each sex. In both sexes, the presynaptic and postsynaptic effects of E2 occur largely at separate groups of synapses. Despite these commonalities, however, a distinct combination of ER subtypes mediates E2's effects in each sex. In females, the presynaptic increase in glutamate release probability is mediated by ERβ and the postsynaptic increase in glutamate sensitivity is mediated by GPER1. In males, the presynaptic increase in glutamate release probability is mediated by ERα and the postsynaptic increase in glutamate sensitivity is mediated by ERβ.