Dose-dependent effects of post-training estradiol plus progesterone treatment on object memory consolidation and hippocampal extracellular signal-regulated kinase activation in young ovariectomized mice

Abstract

Previous work from our laboratory has shown that the ability of estradiol to enhance object memory consolidation in young ovariectomized mice is dependent on dorsal hippocampal activation of the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) signaling pathway [Fernandez SM, Lewis MC, Pechenino AS, Harburger LL, Orr PT, Gresack JE, Schafe GE, Frick KM (2008) Estradiol-induced enhancement of object memory consolidation involves hippocampal extracellular signal-regulated kinase activation and membrane-bound estrogen receptors. J Neurosci 28:8660-8667]. However, it is unclear if estradiol modulates memory or ERK activation similarly in the presence of progesterone. Therefore, the present study investigated effects of combined estradiol and progesterone treatment on object memory consolidation and dorsal hippocampal ERK activation in young ovariectomized C57BL/6 mice. Object memory was tested in a novel object recognition task. Immediately after training, mice received intraperiotoneal (i.p.) injections of vehicle, 17beta-estradiol (E(2); 0.2 mg/kg), or E(2) plus 5, 10, or 20 mg/kg progesterone (P). Forty-eight hours later, mice receiving E(2) alone or E(2) plus 10 or 20 mg/kg P exhibited significantly enhanced memory for the novel object relative to chance, whereas those receiving vehicle or E(2) plus 5 mg/kg P spent no more time than chance with the novel object. Two weeks later, ERK phosphorylation was measured in the dorsal hippocampus 1 h after i.p. injection of vehicle, E(2), or E(2) plus P. Consistent with our previous work [Fernandez SM, Lewis MC, Pechenino AS, Harburger LL, Orr PT, Gresack JE, Schafe GE, Frick KM (2008) Estradiol-induced enhancement of object memory consolidation involves hippocampal extracellular signal-regulated kinase activation and membrane-bound estrogen receptors. J Neurosci 28:8660-8667], E(2) alone significantly increased phospho-p42 ERK protein levels in the dorsal hippocampus relative to vehicle controls. In contrast, no combination of E(2) and P affected dorsal hippocampal phospho-ERK levels. These data indicate that, unlike E(2) alone, the beneficial effects of combined E(2) plus P treatment on memory are not associated with ERK activation in the dorsal hippocampus 1 h after treatment, and suggest that E(2) alone and combined E(2) plus P may influence ERK activation in different time frames or enhance memory through different mechanisms.

Figure 1

Forty-eight hrs after training, mice treated with 0.2 E₂, 0.2 E₂ + 10 P, and 0.2 E₂ + 20 P demonstrated memory for the familiar object by spending significantly more time with the novel object relative to chance (dashed line at 15 s; * p < 0.05), whereas vehicle-and 0.2 E₂ + 5 P-treated mice did not. Each bar represents the mean (± S.E.M.) time spent with the novel or familiar object 48 hrs after training.

Figure 2

(A) Representative Western blot images illustrating phosphorylated and total p42 and p44 ERK protein levels in each group. (B) Densitometric analyses indicated that phospho-p42 ERK levels in the dorsal hippocampus were significantly increased 1 hr after a single i.p. injection of 0.2 E₂ relative to vehicle controls (* P < 0.05). (C) In contrast, none of the groups treated with 0.2 E₂ + progesterone exhibited increased phospho-p42 ERK levels. Phospho-p44 ERK protein levels were not significantly elevated by 0.2 E₂ alone or any combination of 0.2 E₂ + progesterone. (D and E) No treatment affected total p42 or p44 ERK protein immunoreactivity. Each bar in B–E represents the mean percent change from vehicle controls (± S.E.M.).