Thyrotropin Secretagogues Reduce Rat Pituitary Neuromedin B, a Local Thyrotropin Release Inhibitor

Abstract

Neuromedin B (NB), a bombesin-like peptide, highly concentrated in rat pituitary gland, has been shown to act as an autocrine/paracrine inhibitor of thyrotropin (TSH) release. Here it is shown that a single injection of thyrotropin-releasing hormone (TRH, 1.5 μg/animal, Ip), the most important stimulator of thyrotropin secretion, induced approximately 35%–45% decrease in pituitary NB content in rats, as well as an important decrease in NB mRNA at 15 and 30 min ( P < 0.05). Acute cold exposure, which induced higher serum TSH with a peak at 30 min, was associated with progressive decrease in pituitary NB, starting at 15 min although only reaching statistical significance after 2 hr ( P < 0.05). Although not involved in the early peak, the decrease in NB may be contributing to maintenance of higher serum TSH in cold-exposed animals compared with those at room temperature. Fed rats, 2 hr after being subcutaneously injected with mouse recombinant leptin (8 μg /100 g body wt), showed a ×2 increase in serum TSH and 38% reduction in pituitary NB ( P < 0.05). In conclusion, TRH and leptin rapidly decreased pituitary NB and it is first proposed that the reduction of the inhibitory tonus of NB on TSH release will ultimately contribute to the amplification of TSH secretion elicited by TSH secretagogues.