Acute effect of thyroxine on pituitary neuromedin B content of hypothyroid rats and its correlation with TSH secretion

RESUMO

Neuromedin B (NB) is a bombesin-like peptide that has been recently characterized as a physiological paracrine/autocrine inhibitor of thyrotropin (TSH) secretion. We report here the time course of the effect of thyroxine (T4) administration to hypothyroid rats on the anterior pituitary content of NB. Dutch-Miranda male rats weighing 250-300 g received 0.03 percent methimazole in the drinking water for 3 weeks. T4(0.8µg/100 g body weigh, sc) was given 1/2, 1,2 or 6 h before sacrifice. One group recived saline rather than T4 (hypothyroid control). The groups contained 6 to 8 animals each. NB, extracted from tissue by boiling in acetic acid, was measured by radioimmunoassay, using a highly specific anti-serum. Pituitary NB content was significantly increased 4-fold, as ealry 1/2 h after T4 injection, while serum TSH level was similar to that of the hypophyroid control group. The peak response to T4 was at 4 h after injection, when NB content was increased 8-fold (hypothyroid 45 ñ 8; 1/2h, 223 ñ 15; 1h, 203 ñ 48; 3h, 383 ñ 31; 6h, 224 ñ 30 fmol/mg protein) and serum TSH decreased to the level of normal rats (0.93-1.5 ng/ml) generally observed in our laboratory (jypothyroid 31 ñ 3; 1/2h, 26 ñ 3; 1h, 31 ñ 2; 3h, 1.3 ñ 0.1; 6h, 3.7 ñ 0.6 ng/ml). These data suggest that NB synthesis is rapidly induced by thyroxine and this might represent a new regulatory pathy involved in the acute inhibitory effect of thyroid hormones on TSH secretion