GONADAL HORMONE LEVELS AND SPATIAL-LEARNING PERFORMANCE IN THE MORRISWATER MAZE IN MALE AND FEMALE MEADOW VOLES, MICROTUS-PENNSYLVANICUS

The present study examined the relationships between spatial learning and circulating levels of plasma estradiol and testosterone in adult m ale and female meadow vole, Microtus pennsylvanicus. Meadow voles are induced ovulators and most females that are housed with females or in isolation are in constant diestrus, whereas most females that are hous ed with males are in constant behavioral estrus. In this study sexuall y mature, adult male and female meadow voles housed with either female s (constant diestrus) or males (constant behavioral estrus) were requi red to learn the spatial position of a hidden, submerged platform in t he Morris water maze. Individual voles were tested using two blocks of four trials twice a day for 3 days for a total of six blocks. Task re tention was examined with a probe trial 1 day after the last acquisiti on trial. Females were divided into two groups based on the median lev el of plasma estradiol [High Estradiol (15.79 +/- 1.20 pg/ml) and Low Estradiol (6.22 +/- 2.79 pg/ml) Females]. Males were similarly divided on the basis of median plasma testosterone levels [High (2.53 +/- 0.9 6 ng/ml) and Low Testosterone (0.45 +/- 0.08 ng/ml) Males]. High Estra diol females exhibited significantly longer latencies to reach the hid den platform, indicating poorer acquisition, than did either males (P = 0.025) or Low Estradiol females (for Blocks, 2, 3, 4, and 6, P = 0.0 37). Male superiority in spatial learning performance was evident only when High Estradiol females were compared to males. There were no sig nificant performance differences between High and Low Testosterone mal es. There were also no group differences in retention, with all voles displaying significant retention of the spatial task. There was, howev er, a significant correlation between plasma estradiol levels in femal es and retention, with higher estradiol levels being associated with p oorer retention. These results suggest that levels of estradiol in adu lt female meadow voles are significantly related to spatial learning, with low levels of estradiol being associated with better spatial lear ning. There was no evidence that levels of testosterone were related t o spatial performance in adult male meadow voles. The results suggest that estradiol may have activational effects on spatial learning in th e adult meadow vole and that sex differences in spatial learning are e vident only when High Estradiol females are compared to adult males. ( C) 1995 Academic Press, Inc.