Medroxyprogesterone acetate impairs memory and alters the GABAergic system in aged surgically menopausal rats

Abstract

In women, medroxyprogesterone acetate (MPA) is the most commonly used progestin component of hormone therapy (HT). In vitro, MPA negatively impacts markers of neuronal health and exacerbates experimentally-induced neurotoxicity. There is in vitro evidence that these factors are driven by GABAergic and neurotrophic systems. Whether these effects translate to a negative impact on brain function has not been tested in vivo, clinically or preclinically. Here we evaluate the mnemonic and neurobiological effects of MPA in the surgically menopausal rat. Aged ovariectomized (OVX) rats were given subcutaneous vehicle, natural progesterone, low-dose MPA or high-dose MPA. Multiple cognitive domains were analyzed via the water radial-arm maze (WRAM) and Morris maze (MM). Cognitive brain regions were assayed for changes in the GABAergic system by evaluating GAD protein, the synthesizing enzyme for GABA, and neurotrophins. On the WRAM, both progestin types impaired learning. Further, high-dose MPA impaired delayed memory retention on the WRAM, and exacerbated overnight forgetting on the MM. While neurotrophins were not affected by progesterone or MPA treatment, both progestin types altered GAD levels. MPA significantly and progesterone marginally decreased GAD levels in the hippocampus, and both MPA and progesterone significantly increased GAD levels in the entorhinal cortex. These findings suggest that MPA, the most commonly used progestin in HT, is detrimental to learning and two types of memory, and modulates the GABAergic system in cognitive brain regions, in aged surgically menopausal rats. These findings, combined with in vitro evidence that MPA is detrimental to neuronal health, indicates that MPA has negative effects for brain health and function.

Figure 1

Mean error scores for WMC (±SE) on the water radial-arm maze for Day 12 and delay Day 13. Figure 1a shows the working memory load effect where Sham, OVX+PROG, and OVX+High MPA animals exhibited more errors than OVX animals. Figure 2b shows the trial by treatment interaction between OVX and OVX+High MPA where OVX+High MPA animals increased in errors across trials during the delay day of testing while OVX animals did not.

Figure 2

2a) Mean distance scores in centimeters (+SE) on Morris maze days 1–5. 2b) Mean latency scores in seconds (+SE) on Morris maze days 1–5. There were no group differences on distance swum or latency to locate the platform on the Morris Maze. 2c) Mean distance scores in centimeters (+SE) on Morris maze representing overnight forgetting. The figure depicts the collapsed total mean distance score from trial 6 of the previous day to trial 1 of the next day. The OVX+High MPA group significantly increased distance swum across the overnight interval while the OVX group did not; thus, OVX animals remembered the platform location from one day to the next, while OVX+High MPA animals did not. 2d) Mean % distance scores (+SE) spent in the quadrants of the Morris maze during the probe trial. There was no preference, as represented by % distance swum, for the previously platformed quadrant (NW) over the opposite quadrant (SE) by any group. 2e) Mean % distance scores (+SE) spent in the annuli of the Morris maze during the probe trial. All groups exhibited a preference for the outer annulus over the inner and middle annuli confirming animals were not using a motoric strategy of circling the platformed annulus (middle annulus) to solve the task. 2f) A swim path of an animal during regular testing. 2g) A swim path of an animal during the probe trial. 2h) Mean escape scores (+SE) on Morris maze days 1–5. To confirm that animals did learn the platform location, the number of trials within a day that each animal successfully found the platform (and thus escaped) within the 60 second time limit was quantified. Every group showed a significant increase in number of escapes across days. ** p < .0001

Figure 3

Mean latency scores in seconds (+SE) on the visible platform maze days 1–2. There was no Treatment effect for this task, nor were there interactions with Treatment. There was a significant Day × Trial interaction with latencies to locate the platform decreasing across days and trials. These data confirm visual and motor competence by all subjects for platform search.

Figure 4

4a) Mean %OVX by gel (+SE) of GAD protein expression and representative bands, as determined by Western blot luminescence, in the hippocampus. Combined, the OVX+Low MPA and OVX+High MPA groups had significantly and OVX+PROG group had marginally decreased levels of GAD in the Hippocampus compared to the OVX group. 4b) Mean %OVX by gel (+SE) of GAD luminescence and representative bands, in the entorhinal cortex. The combined OVX+MPA group and the OVX+PROG group had significantly increased levels of GAD in the Entorhinal Cortex compared to the OVX group. * p < .05; # p < .10.