Dorsal hippocampal progesterone infusions enhance object recognition in young female mice

Abstract

The effects of progesterone on memory are not nearly as well studied as the effects of estrogens. Although progesterone can reportedly enhance spatial and/or object recognition in female rodents when given immediately after training, previous studies have injected progesterone systemically, and therefore, the brain regions mediating this enhancement are not clear. As such, this study was designed to determine the role of the dorsal hippocampus in mediating the beneficial effect of progesterone on object recognition. Young ovariectomized C57BL/6 mice were trained in a hippocampal-dependent object recognition task utilizing two identical objects, and then immediately or 2 h afterwards, received bilateral dorsal hippocampal infusions of vehicle or 0.01, 0.1, or 1.0 microg/microl water-soluble progesterone. Forty-eight hours later, object recognition memory was tested using a previously explored object and a novel object. Relative to the vehicle group, memory for the familiar object was enhanced in all groups receiving immediate infusions of progesterone. Progesterone infusion delayed 2 h after training did not affect object recognition. These data suggest that the dorsal hippocampus may play a critical role in progesterone-induced enhancement of object recognition.

Figure 1

Cannula placements for each group in both experiments. Each point represents the tip of the guide cannulae; infusion cannulae extended 0.8 mm beyond the tip of the guide cannulae. All injection sites were within the dorsal hippocampus. del = delayed. Figure adapted from Paxinos & Franklin (2003).

Figure 2

Time spent with novel object during retention testing. Each bar represents the group mean ± the standard error of the mean (SEM). The dotted line indicates chance performance (15 s). Asterisks indicate a significant (p < 0.05) difference from the vehicle group. All mice receiving progesterone spent significantly more time exploring the novel object than mice receiving vehicle, thereby demonstrating that progesterone enhanced memory for the familiar object 48 hours after infusion.

Figure 3

Time spent with the novel object during the retention test after delayed infusions. Each bar represents the group mean ± the standard error of the mean (SEM). The dotted line indicates chance performance (15 s). The delayed vehicle and delayed 0.1 μg progesterone groups did not differ in the time spent exploring the novel object.