Comparison of the effects of estradiol and progesterone on serotonergic function

Abstract

Background: Ovarian hormones may contribute to the vulnerability to depression, as well as to the response to antidepressants (ADs). Previously, we reported that acute systemic treatment with estradiol or progesterone blocked the ability of the selective serotonin reuptake inhibitor, fluvoxamine, to inhibit serotonin transporter function in ovariectomized rats. In this study, behavioral consequences, as well as receptor mechanisms underlying these hormonal effects, were investigated.

Methods: Using the forced swimming test, the acute effect of estradiol and/or progesterone on fluvoxamine's AD-like effects was investigated. Using in vivo chronoamperometry, the effect of local application of estradiol or progesterone into the hippocampus of ovariectomized rats on serotonin (5-HT) clearance, as well as on the ability of fluvoxamine to slow 5-HT clearance, were investigated.

Results: The decreased immobility and increased swimming caused by fluvoxamine in the forced swimming test was blocked in rats treated with estradiol and/or progesterone. Local application of estradiol, but not progesterone, slowed 5-HT clearance and both hormones blocked the ability of fluvoxamine to slow 5-HT clearance. Use of hormone receptor agonists and antagonists, revealed that the effects of estradiol are mediated by activation of membrane, as well as nuclear estrogen receptors (ER). The AD-like effect of estradiol involved ER beta and G-protein coupled receptor 30, whereas its blockade of fluvoxamine's effects was ER alpha-mediated. The effects of progesterone occurred solely by activation of intracellular progesterone receptors.

Conclusions: Targeting of ER beta or G-protein coupled receptor 30 might reveal a strategy to permit beneficial effects of estrogen without its deleterious effect on selective serotonin reuptake inhibitor efficacy.

Figure 1

Timeline for administration of hormones (EB and/or P) and/or fluvoxamine treatment in the FST.

Figure 2

Effect of hormone treatments on fluvoxamine-induced behaviors in the FST. OVX rats were treated subcutaneously (sc) with estradiol benzoate (EB, 25μg, in 100μl peanut oil, about 75h prior to the experiment) and/or progesterone (P, 0.5mg, in 100μl peanut oil, 24h prior to the experiment) or vehicle (peanut oil, 24 or 75 h prior to the experiment). Fluvoxamine (10mg/kg) or saline were given 3 times within a 24h period, at 23.5, 5 and 1 hour before the test session. Mean counts of climbing, swimming and immobility were sampled every 5 sec during the 5 min test period. Bars and brackets represent the mean value ± SEM, n= 6/group. Two way ANOVA revealed a significant main effect for hormone treatment (F(3,51) = 4.583, p< 0.01). *p<0.02, Student-Newman-Keuls post hoc comparison of fluvoxamine’s effect with the corresponding control value for each behavior in each treatment group.

Figure 3

Representative 5-HT electrochemical signals illustrating the effect of locally applied 17-β estradiol into the CA3 region of the hippocampus of an OVX rat. The signal was generated by local application of 5-HT (5.2 pmoles). Estradiol 20 pmoles was pressure ejected 10min before the next application of 5-HT. Shown here is the clearance time, T₈₀ parameter: the time it takes for the peak signal amplitude to be reduced by 80%. For clarity, only oxidation signals are shown.

Figure 4

Time course of the effect of local application of E₂ on the 5-HT clearance time parameter, T₈₀. The time course was examined from 1 to 120 min after local application of E₂ (20 pmoles) or PBS as the control vehicle. For these experiments, the same amount of 5-HT was pressure-ejected in a given rat at all-time points after zero. Bars and brackets represent the increase from baseline level in T₈₀ values (in seconds) after the application of E₂, ± SEM (n=4-5). Two way ANOVA revealed a significant effect of hormone (F(1,47) = 30.076, p<0.001). *p<0.05, Student-Newman-Keuls post hoc tests comparing each E₂ value treatment with the corresponding control value at each time point.

Figure 5

Effect of local application of E₂ on 5-HT clearance and on the ability of fluvoxamine to slow clearance after ER antagonism by ICI 182,780. Pretreatment with ICI 182,780 (60 pmoles) was carried out 15min before local application of E₂ (20 pmoles) and the effect on the 5-HT clearance parameter T₈₀, as well as the effect on the ability of fluvoxamine [fluvoxamine is always used at 4x the amount of 5-HT applied] to increase T₈₀ was measured. ICI 182,780, by itself, had no effect on basal 5-HT clearance or on the ability of fluvoxamine to slow clearance (data not shown). The clearance value prior to any PBS, fluvoxamine or hormone treatment was set at 100%. The control group was given PBS. Bar and brackets represent the T₈₀ value as a percentage of the pre-treatment value ± SEM (n=6-10). Two way ANOVA was carried out for each time point. There was a significant main effect for fluvoxamine, both at 10min (F(1,46) = 48.336, p<0.001) and 40-60min (F(1,45) = 14.356, p<0.001) and for hormones, both E₂ and ICI 182,780 + E₂, at 10min (F(2,46) = 14.401, p<0.001) and 40-60min (F(2,45)=3.866, p < 0.05). A significant interaction between hormone X fluvoxamine was detected both at 10min (F(2,46) = 7.146, p <0.002) and 40-60min (F(2,45) = 5.617, p = 0.01). Student-Newman-Keuls post hoc analysis was carried out. *p<0.01, comparing the post-fluvoxamine value with the pre-fluvoxamine values in each treatment group. ^#p<0.001, comparing pre-fluvoxamine values in E₂ or ICI 182,780 + E₂ groups with the pre-fluvoxamine value in the PBS group. ^¥p<0.01, comparing the fluvoxamine value in the ICI 182,780+E₂ group with its effect in the other two groups.

Figure 6

Effects of ER subtype-selective agonists on the 5-HT clearance time parameter, T₈₀. E₂ (20pmoles), PPT (ERα agonist, 60pmoles), DPN (ERβ agonist, 90pmoles), THC (agonist at ERα and also antagonist at the ERβ, 60pmoles) or G1 (GPR30 agonist, 0.2nmoles), were locally applied into the CA3 region of hippocampus and their effects were measured at an early time point (1-10min) and later time point (40-60min) post drug administration. Bar and brackets represent the T₈₀ value as a percentage of the pre-treatment value ± SEM (n=5-12). *p<0.05, Kruskal-Wallis one way analysis of variance on ranks, followed by Dunn’s test comparing percent change in T₈₀ values for the drugs with the percent change in T₈₀ value in the corresponding control (PBS) group.

Figure 7

Effect of ER subtype agonists on the fluvoxamine-induced increase in the 5-HT clearance time parameter, T₈₀. E₂ (20pmoles), PPT (ERα agonist, 60pmoles), DPN (ERβ agonist, 90pmoles), THC (agonist at ERα and also antagonist at the ERβ, 60pmoles) or G1 (GPR30 agonist, 0.2nmoles), were locally applied into the CA3 region of hippocampus and their effects on the ability of fluvoxamine [used at 4x the amount of 5-HTapplied] to increase theT₈₀ value were measured at an early time point (1-10min) and later time point (40-60min) after drug administration. Bar and brackets represent the percent change in T₈₀ value after fluvoxamine ± SEM (n=5-12). *p<0.05, Kruskal-Wallis one way analysis of variance on ranks, followed by Dunn’s test comparing percent change in the T₈₀ value post-fluvoxamine after E₂ or the hormone agonists with fluvoxamine’s percent change in the T₈₀ value of the corresponding control (PBS) group.

Figure 8

Time course for local application of progesterone alone to inhibit the ability of fluvoxamine to slow 5-HT clearance (left panel); in combination with the progesterone receptor antagonist, RU486 (middle panel); or progesterone-BSA conjugate’s inability to alter the effect of fluvoxamine (right panel). RU486 (1.2 nmoles) was given 15 min before the application of P (0.4 nmoles). The ability of fluvoxamine to increase the T₈₀ value was examined at time 0 (before application of P, P+RU486 or P-BSA) and after their application. Bars and brackets represent the increase in the T₈₀ value, in seconds, after fluvoxamine ± SEM (n=6-8). The effect of fluvoxamine was statistically different at different times post-progesterone administration, as shown by one way ANOVA (F(5,45)=14.180, p<0.001). *p<0.002, Student-Newman-Keuls post hoc analysis comparing the fluvoxamine-induced increase in the T₈₀ value after progesterone at each time point with the value before progesterone administration. RU486 prevented the inhibitory effect of progesterone and P-BSA did not mimic the inhibitory effect of progesterone.