Regulation of body temperature by thyrotropin-releasing hormone in neonatal chicks

To understand thermal regulation of neonatal chicks, the contribution of thyrotropin-releasing hormone (TRH), a key regulator of the hypothalamus–pituitary–thyroid axis, was investigated. Intracerebroventricular (i.c.v.) injection of TRH (5 and 20 μg) increased body temperature, but did not change plasma T₃ and T₄ concentrations. Intraperitoneal (i.p.) injection of triiodothyronine (T₃) and thyroxine (T₄) did not influence body temperature.

Thereafter, the relationships between TRH and the hypothalamus–pituitary–adrenal axis and sympathetic nervous system were further investigated. Central TRH stimulated both corticosterone and epinephrine release. The i.c.v. injection of a corticotropin-releasing factor receptor antagonist attenuated the change in body temperature and corticosterone concentration caused by TRH, but did not influence plasma T₃ and T₄ concentrations. The i.p. injection of epinephrine did not induce hyperthermia. Therefore, the thermoregulatory response to TRH may differ in neonatal stages being dependent upon the stimulation of the hypothalamus–pituitary–adrenal axis rather than the hypothalamus–pituitary–thyroid axis.