Influence of Testosterone Deprivation and Replacement on Cognition and Oxidative Stress in Middle-Aged Male Rats

Purpose: Data from aged men suggests a negative correlation between testosterone levels and cognitive function, including the development of mild cognitive impairment and Alzheimer’s disease. The purpose of this study was to 1) determine whether long-term testosterone deprivation (LTTD) impairs cognition and increases oxidative stress in the middle-aged male rat brain and 2) determine whether testosterone (T) replacement after LTTD can reverse these effects. Methods: Twelve-month old male Fischer 344 rats (13 per group) were left intact or castrated for 2 weeks and replaced with subcutaneous implants containing T (short-term T deprivation; STTD). Additional groups were castrated for 10 weeks before being treated with T (long-term T deprivation; LTTD+T) or cholesterol (LTTD). Rats underwent cognitive testing with the Morris water maze (MWM). A 4-day acquisition phase was used for rats to learn the location of a hidden platform. A retention day was used to determine whether rats remembered the platform location after it was removed. A 2-day reversal trial in which the platform was moved to a new location was used to examine mental flexibility. These tests require both hippocampal and cortical areas of the brain. Following MWM rats were euthanized and brains were collected for immunoblotting for markers of cell death (Spectrin) and oxidative stress responses (NFkB, COX2, NOX2) in the hippocampus and cerebral cortex. Plasma advanced oxidative protein products (AOPP) were used as a peripheral marker of oxidative stress. Total testosterone was measured by ELISA. Results: Castration reduced total testosterone to 40% of intact levels whereas testosterone implants increased levels back to those of intact males. Overall, intact rats performed significantly worse on the MWM than STTD and LTTD with or without T replacement. We saw no significant changes in blood AOPP among treatment groups. Similarly, there were no significant differences in the expression of oxidative stress regulated genes or Spectrin cleavage in the hippocampus. Cortical measurements are on-going. Conclusions: These data suggest that castration with or without T replacement improves cognitive function in middle-aged rats, but does not significantly alter oxidative stress in the brain or periphery. These data support the safety profile of testosterone replacement to physiological levels and do not recapitulate correlative data observed in men.