Distribution and release of immunoreactive thyroid-stimulating hormone in the rat hypothalamus: effects of thyroidectomy, hypophysectomy and treatment with thyroid hormones.

Immunoreactive thyroid-stimulating hormone (IR-TSH) has been detected in the hypothalamus and is released in vitro by a calcium-dependent mechanism when the tissue is depolarized. Recently, immunocytochemical studies have revealed that IR-TSH is present in thyrotropes in the pars tuberalis. Therefore, because these thyrotropes are associated with the median eminence, the area with the highest concentration of IR-TSH, it is of interest to determine if 'hypothalamic' IR-TSH is from neural or pituitary cells. We addressed this issue by studying the effects of hypophysectomy, thyroidectomy, or chronic administration of triiodothyronine (T3) or thyroxine (T4) on the distribution and in vitro release of IR-TSH in the hypothalamus. We reasoned that, if hypothalamic IR-TSH is dependent on the thyrotropes of the pars tuberalis, then changes in hypothalamic IR-TSH concentration and release should be parallel to those measured in pituitary extracts. IR-TSH was measured in tissue extracted in ice-cold 2% NaCl, with a final pH of 4.5. For the in vitro studies, tissues were incubated for 20-min periods in Krebs-Ringer bicarbonate buffer at 37 degrees C. In untreated rats, the concentration of IR-TSH is greater in the ventral than the dorsal portion of the hypothalamus (39.3 +/- 8.2 vs. 4.0 +/- 1.5 ng/mg wet wt.). Upon finer dissection of the hypothalamus into median eminence and anterior, middle, and posterior portions of the remainder, IR-TSH was only detectable in the middle hypothalamus (5.3 +/- 1.5 ng/mg), and the median eminence (149 +/- 41 ng/mg).(ABSTRACT TRUNCATED AT 250 WORDS)