Estrogen receptor-α distribution in male rodents is associated with social organization

It has been hypothesized that site-specific reduction of estrogen receptor-alpha (ER alpha) is associated with the expression of male prosocial behaviors. Specifically, highly social males are predicted to express significantly lower levels of ER alpha than females and less social males in brain regions associated with prosocial behavior including the bed nucleus of the stria terminalis (BST) and the medial amygdala (MeA). This hypothesis was tested by comparing ER alpha immunoreactivity (IR) in three species of microtines, the polygynous montane (Microtus montanus) and meadow (M. pennsylvanicus) voles and the monogamous pine vole (M. pinetorum), and two species of cricetines that differ in the extent of social pair-bond formation, Siberian (Phodopus sungorus) and Djungarian (P. campbelli) hamsters. As predicted, ER alpha-IR was sexually dimorphic in the BST and MeA of the highly social species, with females expressing more ER alpha-IR cells than males. Male and female montane voles did not differ. Male and female meadow voles differed in the ventromedial hypothalamus, with females expressing more ER alpha-IR cells. Male pine voles expressed lower levels of ER alpha-IR in the MeA than male montane and meadow voles and in the BST relative to montane males. Male Djungarian hamsters, which show higher levels of parental care, had fewer ER alpha-IR cells in the BST than male Siberian hamsters. Results indicate that the distribution of ER alpha differs relative to the continuum of species-typical affiliative behavior and supports the hypothesis that ER alpha has a significant role in regulating species-specific social organization.