Effects of gonadectomy and hormone replacement on a spontaneous novel object recognition task in adult male rats

Abstract

Recent studies in adult male rats have shown that gonadal hormones influence performance on certain working memory and other types of cognitive tasks that are sensitive to lesions of the medial and/or orbital prefrontal cortices. This study asked whether gonadal hormone modulation of prefrontal cortical function in males also extends to the perirhinal division of the rat prefrontal cortex. Specifically, sham-operated control, gonadectomized, and gonadectomized rats supplemented with testosterone propionate or estradiol were tested on a spontaneous novel object recognition task, a paradigm where performance has been shown to be impaired by perirhinal cortical lesions. Using analyses of variance, regression analyses and post-hoc testing to evaluate group differences, it was found that during both the sample and test trials of the task all four groups spent similar absolute and proportional amounts of time ambulating, rearing, stationary, and exploring the two objects present. All groups also explored each of the two identical objects present during sample trials equally. However, during the test trials, only the control and gonadectomized rats given testosterone showed the expected increase in exploration of the novel objects presented, whereas the gonadectomized and gonadectomized, estradiol-supplemental groups continued to explore the novel and familiar objects equally. That regression analyses also identified significant correlations between low bulbospongiosus muscle weight and impaired novel vs. familiar object discrimination further indicates that gonadectomy in adult male rats adversely affects spontaneous novel object recognition in an androgen-sensitive, estrogen-insensitive manner.

Figure 1

Scatterplots showing bulbospongiosus muscle (BSM) weights in grams (g) of individual animals (dots). Horizontal lines show the average weight for each group. As expected, BSM weights in gonadectomized (GDX) rats and in GDX rats given estradiol (GDX-E) were significantly lower (*) than weights in sham-operated (CTRL) and GDX rats given testosterone propionate (GDX-TP).

Figure 2

Bar graph showing the mean amount of time (s) animals explored each of four sample objects present in the testing field. Control (CTRL), gonadectomized (GDX), and GDX rats given testosterone propionate (GDX-TP) or estradiol (GDX-E) all spent significantly (*) more time with large/complex objects (A), significantly to near-significantly (#, p<0.07) less time with small/smooth objects (D), and intermediate amounts of time with large/smooth (B) and small/complex (C) objects.

Figure 3

Stacked bar graphs showing the mean amounts of time spent ambulating (black), stationary (dark grey) and rearing (light grey) during the first and final 180 seconds of a six-minute open field session for each group. For control (CTRL), gonadectomized (GDX), and GDX rats given testosterone propionate (GDX-TP) or estradiol (GDX-E), significantly different amounts of time were devoted to each principal behavior (*). In addition, during the second half of the session all groups spent significantly (#) more time stationary and less time ambulating than initially. However, there were no significant main effects of hormone treatment or significant interactions between hormone treatment and behavior.

Figure 4

A) Stacked bar graphs showing the mean amounts of time spent ambulating (black), stationary (dark grey), rearing (light grey) and exploring objects (white) during the first and final 90 seconds of a three-minute object recognition task habitation session for each group. For control (CTRL), gonadectomized (GDX), and GDX rats given testosterone propionate (GDX-TP) or estradiol (GDX-E) significantly different amounts of time were devoted to each principal behavior (*). During the second half of the session all groups also spent significantly (#) more time stationary and less time ambulating than initially. No significant main effects of hormone treatment or significant interactions between hormone treatment and object exploration were found. B) Bar graphs showing the Discrimination Index (DI) of object exploration during the first and second halves of the object recognition task habitation trial. For all groups DI values were consistently near zero, indicating that all groups interacted with the two identical objects present for similar amounts of time.

Figure 5

Behavioral data collected during the sample (A, C, E) and test (B, D, F) trials of the spontaneous novel object recognition task with the 1.5 hour delay. Stacked bar graphs showing the mean amounts of time spent ambulating (black), stationary (dark grey), rearing (light grey) and exploring objects (white) during the first and final 90 seconds of a three-minute sample trial (A) and test trial (B) show that the control (CTRL), gonadectomized (GDX), and GDX rats given testosterone propionate (GDX-TP) or estradiol (GDX-E) spent significantly (*) different amounts of time on each principal behavior and that during the second half of the session all groups spent significantly (#) more time stationary and less time ambulating than initially. During the test phase only (B) significant main effects of hormone treatment were observed that were driven by the ambulatory and stationary behaviors of the GDX-TP group (¹). Bar graphs showing the Discrimination Index (DI) of object exploration during the first and second halves of the sample (C) and test (D) trials show that all groups explored both sample objects equally (C) and that the CTRL and GDX-TP groups spent significantly (*) more time with the novel object during the first half of the test trial (D) while the GDX and GDX-E groups did not (D). Regression plots graphing individual DI values as a function of bulbospongiosus muscle (BSM) weight in grams (g) show no correlation between the two for the sample trials (E), but a significant (*) correlation between low BSM weight and low DI values/poor object discrimination during the test trial (F).

Figure 6

Behavioral data collected during the sample (A, C, E) and test (B, D, F) trials of the spontaneous novel object recognition task with the 4 hour delay. Stacked bar graphs showing the mean amounts of time spent ambulating (black), stationary (dark grey), rearing (light grey) and exploring objects (white) during the first and final 90 seconds of a three-minute sample trial (A) and test trial (B) show that the control (CTRL), gonadectomized (GDX), and GDX rats given testosterone propionate (GDX-TP) or estradiol (GDX-E) spent significantly (*) different amounts of time on each principal behavior and that during the second half of the session all animal groups spent significantly (#) more time stationary and less time ambulating than initially. Bar graphs showing the Discrimination Index (DI) of object exploration during the first and second halves of the sample (C) and test (D) object recognition trials show that all animal groups explored both sample objects equally (C) and that the CTRL and GDX-TP groups spent more time with the novel object during the test trial (D) while the GDX and GDX-E groups spent similar amounts of time with the two objects (D; #, GDX compared to CTRL p=.072). Regression plots graphing individual DI values as a function of bulbospongiosus muscle (BSM) weight in grams (g) show no correlation between the two for the sample trial (E) and a weak relationship between low BSM weight and impaired object discrimination in the test trial (F).