Hypothalamo-pituitary portal blood concentrations of β-endorphin during suckling in the ewe

Matched hypothalamo-pituitary portal and jugular blood samples were collected over about 6 h from 7 lactating Corriedale ewes penned with their lambs, and a careful record was kept of ewe/lamb behaviour. Hypothalamo-pituitary portal blood concentrations of β-endorphin were measured by radioimmunoassay and the secretion rates were calculated; these were related to peripheral plasma prolactin and LH concentrations, and the sucking bouts of the lambs. Basal LH concentrations remained < 1 ng/ml with 0-2 pulses of 1.5-3.5 ng/ml amplitude per 6-h collection period. Prolactin secretion was episodic with individual baselines varying from 24 to 286 ng/ml, and peak concentrations of 50-631 ng/ml. Portal β-endorphin was secreted in an episodic pattern with individual baseline secretion rates varying from 0.125 to 0.495 ng/min, and peak secretion rates of 0.768 to 3.216 ng/min. A close correlation was seen between sucking bouts and the secretion of portal β-endorphin and peripheral prolactin; 86% of sucking bouts resulted in a significant release of β-endorphin, and 46% of sucking bouts resulted in a significant release of prolactin. These results show that hypothalamic β-endorphin is released in response to the sucking stimulus. This provides support for the hypothesis that, during lactation, β-endorphin acts within the hypothalamus to reduce GnRH release and hence depress pituitary gonadotrophin secretion.