Anatomical localization of the effects of reproductive state, castration, and social milieu on cells immunoreactive for gonadotropin-releasing hormone-I in male European starlings (Sturnus vulgaris).

Gonadotropin-releasing hormone-I (GnRH-I) cells are localized primarily to the septopreoptic area (POA) and are responsible for regulating gonadotropin release from the anterior pituitary. Some songbird species exhibit dramatic seasonal variation in the number of detectable GnRH-I immunoreactive cells, with higher numbers being observed during the breeding season. Here we investigated the anatomical distribution of GnRH-I-immunoreactive cells in male starlings that varied in response to manipulations of reproductive state, social context, and gonadal condition. We housed photostimulated, intact and castrated male starlings with a female or alone. Additionally, a fifth treatment group consisted of photorefractory males (i.e., in a nonreproductive state) housed alone. All photostimulated males had significantly greater numbers of GnRH-I cells compared with photorefractory male starlings. There was a significant main effect of castration and social context. Castrated males had significantly greater numbers of GnRH-I cells compared with intact males, and males housed in male-female dyads also had greater numbers of GnRH-I cells. Furthermore, the significant main effects of castration and social context were the result of an increase in GnRH-I cell numbers specifically in the rostral and intermediate regions of the POA. These findings indicate that social context and hormonal milieu have profound effects on GnRH-I immunoreactivity in addition to the previously described effects of reproductive state. These data provide novel insight into the environmental regulation of the hypothalamopituitary axis and suggest that gonadal hormones and female presence independently regulate GnRH-I cells in specific regions of the POA in male starlings.