Thyrotropin or thyroid-stimulating hormone (TSH) secretion in the
chicken is controlled by several hypothalamic hormones. It is stimulated
by thyrotropin-releasing hormone (TRH) and corticotropin-releasing
hormone (CRH), whereas somatostatin (SRIH) exerts an inhibitory effect.
In order to determine the mechanism by which these hypothalamic hormones
modulate chicken TSH release, we examined the cellular localization
of TRH receptors (TRH-R), CRH receptors type 1 (CRH-R1) and somatostatin
subtype 2 receptors (SSTR2) in the chicken pars distalis by in situ
hybridization (ISH), combined with immunological staining of thyrotropes.
We show that thyrotropes express TRH-Rs and SSTR2s, allowing a direct
action of TRH and SRIH at the level of the thyrotropes. CRH-R1 expression
is virtually confined to corticotropes, suggesting that CRH-induced
adrenocorticotropin release is the result of a direct stimulation
of corticotropes, whereas CRH-stimulated TSH release is not directly
mediated by the known chicken CRH-R1. Possibly CRH-induced TSH secretion
is mediated by a yet unknown type of CRH-R in the chicken. Alternatively,
a pro-opiomelanocortin (POMC)-derived peptide, secreted by the corticotropes
following CRH stimulation, could act as an activator of TSH secretion
in a paracrine way.
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