title: Central regulation of hypothalamic-pituitary-thyroid axis under physiological and pathophysiological conditions.
abstract: TRH is a tripeptide amide that functions as a neurotransmitter but also serves as a neurohormone that has a critical role in the central regulation of the hypothalamic-pituitary-thyroid axis. Hypophysiotropic TRH neurons involved in this neuroendocrine process are located in the hypothalamic paraventricular nucleus and secrete TRH into the pericapillary space of the external zone of the median eminence for conveyance to anterior pituitary thyrotrophs. Under basal conditions, the activity of hypophysiotropic TRH neurons is regulated by the negative feedback effects of thyroid hormone to ensure stable, circulating, thyroid hormone concentrations, a mechanism that involves complex interactions between hypophysiotropic TRH neurons and the vascular system, cerebrospinal fluid, and specialized glial cells called tanycytes. Hypophysiotropic TRH neurons also integrate other humoral and neuronal inputs that can alter the setpoint for negative feedback regulation by thyroid hormone. This mechanism facilitates adaptation of the organism to changing environmental conditions, including the shortage of food and a cold environment. The thyroid axis is also affected by other adverse conditions such as infection, but the central mechanisms mediating suppression of hypophysiotropic TRH may be pathophysiological. In this review, we discuss current knowledge about the mechanisms that contribute to the regulation of hypophysiotropic TRH neurons under physiological and pathophysiological conditions.
authors.affiliation: Department of Endocrine Neurobiology (C.F.), Institute of Experimental Medicine, Hungarian Academy of Sciences, 1083 Budapest, Hungary; Department of Medicine, Division of Endocrinology, Diabetes, and Metabolism (C.F., R.M.L.), Tupper Research Institute, Tufts Medical Center, Boston, Massachusetts 02111; and Department of Neuroscience (R.M.L.), Tufts University School of Medicine, Boston, Massachusetts 02111.
authors.forename: Csaba
authors.initials: C
authors.lastname: Fekete
authors.forename: Ronald M
authors.initials: RM
authors.lastname: Lechan
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology*; D006801:Humans; D007031:Hypothalamus / Q000502:physiology*; D010902:Pituitary Gland / Q000502:physiology*; D013961:Thyroid Gland / Q000502:physiology*; D013973:Thyrotropin-Releasing Hormone / Q000502:physiology*
chemical_list: D013973:Thyrotropin-Releasing Hormone