The mediobasal hypothalamus regulates pituitary function through the actions of hypophyseal neurons. Prolactin, for example, is regulated by release of dopamine from hypothalamic neurons that project to the median eminence (ME). The ME transports dopamine via portal blood to the anterior pituitary, where it inhibits release of prolactin. Prolactin itself is able to mediate its own regulation by signalling back on hypothalamic cells through the phosphorylation of Signal Transducer and Activator of Transcription (STAT)5. Our previous data has shown prolactin induced phospho-STAT5 in the nuclei of many cells of the mouse mediobasal hypothalamus. These experiments also show both nuclear and granular phospho-STAT5expression in the ME. The aim of this current study was  to examine the time course of prolactin-induced phospho-STAT5 in the hypothalamus compared to the ME of the male mouse.

Male C57BL/6 mice were treated with bromocriptine over 24 h in order to block endogenous prolactin release. Intraperitoneal injections of 2mg/kg ovine prolactin were administered over 2, 5, 20 or 60 min. Animals were euthanised with pentobarbital and perfused with 4% paraformaldehyde. Brains were excised and cryostat sections prepared. Phospho-STAT5 staining was visualised through immunohistochemical techniques. Phospho-STAT5 in the arcuate nucleus and ME was counted, where nuclear, or analysed using densometric measures where apparent in a punctate form. The results show a temporal change in the nuclear staining observed in the arcuate nucleus, with immediate responses at 2 min (p<0.005), increasing by four-fold at 60 min. In the ME, the response visible at 2 min was three-fold the saline response (p<0.05), but remained at this level across the time course. This data demonstrates the responses of the ME are temporally distinct to that observed in the arcuate nucleus suggesting a possible functional separation