Sex differences and the influence of the Y chromosome on learning in the C57BL/6 mouse strain

This thesis sought to identify the physiological factor(s) influencing known sex differences in non-reproductive behaviors in rodents. Sexually dimorphic hormone levels have long been asserted to control sex differences in activity, spatial and complex maze learning, and shock conditioning. However, evidence of genetic influences on some sexually dimorphic morphological structures and influences of the Y chromosome on behavior led to an alternative hypothesis. It was proposed that the presence of the Y chromosome, being sexually dimorphic and only present in males, could also lead to sex differences in behavior. ^ To explore this hypothesis three inbred mouse strains were behaviorally tested. The C57BL/6J strain contained normal XX females and XY males, allowing an assessment of sex differences. The C57BL/6J-YPOS strain contained sex-reversed XY-females thereby assessing the influence of the Y chromosome in the absence of testosterone. The C57BL/6J-YPOS+Tg strain contained an XX-female and XY-female (genetically identical to the YPOS strain) and an XX-male with the Sry transgene, and an XY-male with the Sry transgene. The YPOS+Tg strain allowed an assessment of both the independent and interactive effects of testosterone and the Y chromosome. ^ XY-females from the YPOS strain demonstrated improved spatial ability in the Morris water maze, but no difference in open field, Lashley III maze or active avoidance learning, tasks in which we detected a sex difference. These data suggested a specific effect of the Y chromosome on spatial performance. However, data from the YPOS+Tg strain were less clear. Testosterone influenced open field activity level. The Y chromosome influenced Morris maze retention. However we detected no influence of the Y chromosome on Morris maze learning, as was expected from Y POS data, and no influence of testosterone or the Y chromosome on active avoidance learning. These data suggested that the Sry transgene was nonlinearly influencing the behavior of offspring. Spatial Morris maze performance of the YPOS+Tg strain suggested that both testosterone and the Y chromosome were independently capable of influencing this behavior. These data support both an influence of testosterone and the Y chromosome on the known sex differences in non-reproductive behaviors in mice.