**3.3 Expression analysis**

TRH is present not only in the hypothalamus but also in many other brain loci as a neuromodulator / neurotransmitter. Besides, expression of *TRH* gene is found in gastrointestinal tract, pancreas, reproductive tissues, heart, spleen, adrenals and thymus. Its receptor, *TRHR* is also found in gastrointestinal tract, pancreas, testis and adrenals. Paracrine effects of TRH on gastrointestinal tract and pancreas have been suggested. Furthermore, *TSHR* is widely expressed in many organs and tissues such as pituitary, heart, skeletal muscle and adipose. Species-specific expression of HPT genes in certain tissues might exist. For example, *TRHR* was detected in medaka spleen (Mekuchi et al., 2010) but undetectable in bovine spleen (Takata et al., 1998), and *TSHB* has been reported in the liver and reproductive organs of red drum (Cohn et al., 2010) but undetectable in the same organs of ducks (Hsieh et al., 2007).

Tissue distribution of HPT genes in pigs have been investigated by using real-time quantitative RT-PCR with RNA samples from 15 different organs/tissues including brain, hypot4halamus, pituitary, thyroid, lung, kidney, gastrointestinal tract, muscle and fat tissue (Jiang 2011). Highest expression level of *TRH* was found in the brain, and considerable level of *TRH* also expressed in testis, spleen, fat tissue, small intestine and pancreas in addition to hypothalamus. Porcine *TRHR* mRNA was detected in almost all the investigated organs/tissues except the spleen, with high expression levels in the brain, hypothalamus, pituitary, testis and fat tissue. The two subunit genes of hormone TSH showed different tissue distribution patterns. While *CGA* was detectable in almost all the tissues, *TSHB* was mainly existed in the pituitary and undetectable or in extremely low level in the other tissues except in the fat tissue and stomach. Expression of porcine *TSHR* existed in all the fifteen organs/tissues, with highest expression level in the thyroid, then the testis and spleen.
