Evidence for "short loop" feedback regulation of prolactin and growth hormone secretion in the pigeon

When pigeons were injected with an extract of chicken hypothalamus, the increase in weight of the crop sac showed that prolactin secretion was stimulated. Crop sac weight was no higher after 16 days of injection than it was after 8 days. Pituitary gland content of prolactin and growth hormone was elevated at 8 days, but showed a relative decrease after a further 8 days of injections. Prolactin and growth hormone releasor activity in the hypothalamus was investigated in vitro. A decline during treatment with hypothalamic extract was seen, and by 8 days no releasor activity could be detected. Injection with ovine prolactin also reduced prolactin releasor activity in the hypothalamus. The results suggest that the secretion of prolactin and possibly also of growth hormone is controlled by a "short loop" negative feedback mechanism which acts on hypothalamic hypophysiotrophic hormones.