Peptide YY (PYY)3-36 modulates thyrotropin secretion in rats

Peptide YY (PYY)3-36 is a gut-derived hormone, with a proposed role in central mediation of postprandial satiety signals, as well as in long-term energy balance. In addition, recently, the ability of the hormone to regulate gonadotropin secretion, acting at pituitary and at hypothalamus has been reported. Here, we examined PYY3-36 effects on thyrotropin (TSH) secretion, both in vitro and in vivo. PYY3-36-incubated rat pituitary glands showed a dose-dependent decrease in TSH release, with 44 and 62% reduction at 10(-8) and 10(-6) M (P < 0.05 and P < 0.001 respectively), and no alteration in TSH response to thyrotropin-releasing hormone. In vivo, PYY3-36 i.p. single injection in the doses of 3 or 30 cg/kg body weight, administered to rats fed ad libitum, was not able to change serum TSH after 15 or 30 min. However, in fasted rats, PYY3-36 at both doses elicited a significant rise (approximately twofold increase, P < 0.05) in serum TSH observed 15 min after the hormone injection. PYY3-36 treatment did not modify significantly serum T4, T3, or leptin. Therefore, in the present paper, we have demonstrated that the gut hormone PYY3-36 acts directly on the pituitary gland to inhibit TSH release, and in the fasting situation, in vivo, when serum PYY3-36 is reduced, the activity of thyroid axis is reduced as well. In such a situation, systemically injected PYY3-36 was able to acutely activate the thyrotrope axis, suggesting a new role for PYY3-36 as a regulator of the hypothalamic-pituitary-thyroid axis.