Effects of orexin A on thyrotropin-releasing hormone and thyrotropin secretion in rats

Effects of orexin A on secretion of thyrotropin-releasing hormone (TRH) and thyrotropin (TSH) in rats were studied. Orexin A (50 mu g/kg) was injected iv, and the rats were serially decapitated. The effects of orexin A on TRH release from the rat hypothalamus in vitro and on TSH release from the ant erior pituitary in vitro were also investigated. TRH and thyroid hormone we re measured by individual radioimmunoassays, TSH was determined by the enzy me-immunoassay method. The hypothalamic TRH contents increased significantl y after orexin A injection, whereas its plasma concentrations tended to dec rease, but not: significantly, The plasma TSH levels decreased significantl y in a dose-related manner with a nadir at 15 min after injection. The plas ma thyroid hormone levels showed no changes. TRH release from the rat hypot halamus in vitro was inhibited significantly in a dose-related manner with the addition of orexin A. TSH release from the anterior pituitary in vitro was not affected with the addition of orexin A. The findings suggest that o rexin A acts on the hypothalamus to inhibit: TRH release.