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

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Abstract
Summary. 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.