SOME FACTORS INFLUENCING THYROTROPIN RELEASE IN THE RABBIT

Abstract

The influence of a number of biologically active compounds, many of them naturally occurring, on thyrotropin secretion in the rabbit Has been investigated. The technique used involved microinjection of these agents into discrete locations within the hypothalamus and pituitary and observation of the effect of such microinjections on the thyroidal release of I¹³¹. Of the thyroxine analogs tested, d-thyroxine and tetraiodothyroacetic acid were found to have no inhibitory influence on thyrotropin secretion in the large doses employed. It is concluded that their effect on the feedback control of thyroid hormone secretion, if present at all, is slight. Both L-thyroxine and l-triiodothyronine were capable of inhibiting thyrotropin release. Acetylcholine injected directly into the pituitary or I.V. inhibited thyrotropin release as did the injection of purified substance P into the region of the mamillary body and ventromedian hypothalamic nucleus. Gamma amino butyric acid and serotonin were without effect in any location. Of three catechol hormones naturally occurring in the hypothalamus only adrenaline was able to reflexly inhibit the rate of thyroid hormone release significantly. Systemic reserpinization resulted in delay of thyroid secretion but did not affect the response to hypothalamic microinjection of adrenaline. No evidence was found to support the concept of a hypothalamic thyroxine “receptor” and the hazards of relatively large injection volumes, .02 ml., are depicted with radioautographic studies.