An analysis of specific stimuli causing the release of prolactin and growth hormone at milking in the goat

Experiments to investigate the relative importance of the tactile, conditioned and possible metabolic components of the milking stimulus on the release of prolactin and growth hormone (GH) from the anterior pituitary of the goat are described. A comparison of the hormonal responses to milking the auto-transplanted mammary gland (i.e. denervated gland) with that obtained by milking the intact mammary gland of the same goat showed that the concentration of prolactin in the plasma increased only after milking the intact gland, whereas in two out of four goats an increase in plasma GH was detected several minutes after milking the transplanted gland. In the intact animal significantly more prolactin (P less than 0-01) was released in response to milking both teats for 6 min as compared with that released by milking only one teat for the same time. No significant difference (P less than 0-1) was found for GH. Similar quantities of prolactin and GH were released by goats milked when conscious or under anaesthesia. A comparison of the hormonal responses to teat stimulation in the same anaesthetized goats with and without the removal of milk from the mammary gland showed a significant reduction (P less than 0-001) in the quantity of prolactin and GH released in the absence of milk removal. The significance of these results is discussed.