Cloning and Characterization of a cDNA Encoding a Novel Subtype of Rat Thyrotropin-releasing Hormone Receptor

Abstract

The thyrotropin-releasing hormone (TRH) 1 is a tripeptide (pyroglutamic acid-histidine-proline-amide) synthesized from a precursor polypeptide whose sequence contains 5 copies of the TRH sequence (5-7).Originally isolated from the hypothalamus, TRH is present in the central nervous system (thalamus, cerebral cortex, and spinal cord,) as well as in the periphery (pancreas, gastrointestinal tracts, and placenta). In the hypothalamus, TRH is synthesized by peptidergic neurons of supraoptic and paraventricular nuclei. It is then axonally transported to be stored in the median eminence. When secreted in the bloodstream, it reaches the pituitary where it stimulates the production of thyroid stimulating hormone which in turn stimulates the production of thyroxin (T 4 ) in the thyroid gland (8).In addition to this pivotal role in controlling the synthesis and secretion of thyroid stimulating hormone and other hormones from the anterior pituitary, TRH has been implicated as a neurotransmitter (9). TRH abundantly exists in the central nervous system and exogenous administration of TRH elicits a variety of behavioral changes (see Ref. 10 for a review).The distribution of TRH containing cells, fibers, or receptors suggests a potential role of TRH in the perception of noxious stimuli. TRH is present in the periaqueductal gray, the nuclei raphe magnus and pallidus, and the dorsal horn of the spinal cord. TRH-binding sites have been described in the brain, the pituitary, in both the dorsal and ventral horns of the spinal cord as well as in peripheral tissues. When injected centrally, TRH induces a short lasting supraspinal antinociception. The analgesia induced by intracerebroventricular TRH injection is powerful since it is twice as great, on a molar basis, as that of morphine (11). This TRH-induced antinociception is detected in models of chemically and mechanically, but not thermally induced pain. On the other hand, intrathecal TRH injections do not affect basal antinociceptive thresholds (11). However, it is known that TRH enhances spinal reflexes (both in vivo and in vitro) and modulates pain transmission (4,12,13). Although the mode of action of TRH at the level of the spinal cord is unclear, there is evidence suggesting that the TRH-induced facilitation of spinal transmission involves the activation of the N-methyl-D-asparatate receptor (14).TRH actions are mediated by the stimulation of specific cell surface receptors. Studies of pituitary TRH receptors have suggested that the TRHR1 receptor triggers the phospholipase C-protein kinase C transduction pathway (3,15). A cDNA sequence encoding a G-protein-coupled TRHR was originally isolated from mouse pituitary using an expression cloning strategy (15). Subsequently, several groups have described the cloning of rat TRH receptor cDNAs expressed in a pituitary tumor cell line (GH 3 ) or in the pituitary gland (1, 3). In addition, two isoforms of the rat TRHR have been shown to be generated from a single gene by alternative splicing (2). These isoforms are 387 and 412 amin...

First-in-class thyrotropin-releasing hormone (TRH)-based compound binds to a pharmacologically distinct TRH receptor subtype in human brain and is effective in neurodegenerative models

“…Two G Protein-Coupled Receptor (GPCR) subtypes, designated TRHR1 and TRHR2, have been identified to date in mediating TRH actions in non-human mammalian species (Cao et al, 1998;Itadani et al, 1998;Sun et al, 2003). Recent knock-out studies in mouse indicated that TRHR2 receptors account for no more that 5% of total binding in brain and that the CNS effects of TRH and TRH analogs were lost in mice in which TRHR1 was deleted (Thirunarayanan et al, 2013).…”

Homo- and Hetero-oligomerization of Thyrotropin-releasing Hormone (TRH) Receptor Subtypes

“…The two TRH receptor subtypes are ϳ50% homologous, and they exhibit similar binding affinities for TRH (12)(13)(14) and activate the same signaling pathways, although TRHR2 exhibits higher basal signaling activity (14 -16). TRHR1 and 2 are expressed in distinct compartments of the brain and spinal cord (12,14,17,18), although areas have been defined where both receptor subtypes are found (12,14). This raises the question regarding the mechanism of selectivity and receptor regulation used when two individual receptors, or a putative TRHR hetero-oligomeric unit, are activated by one ligand to produce differential cellular responses.…”

“…TRH is also known to have extrapituitary actions in the brain, spinal cord, cardiovascular, and gastrointestinal systems (10,11). The cloning of a second receptor for TRH (TRHR2) from rat brain and spinal cord provided a possible explanation for certain neurotransmitter actions of TRH, in particular the nociceptive and spinal cord regenerative actions (12)(13)(14). The two TRH receptor subtypes are ϳ50% homologous, and they exhibit similar binding affinities for TRH (12)(13)(14) and activate the same signaling pathways, although TRHR2 exhibits higher basal signaling activity (14 -16).…”

“…The cloning of a second receptor for TRH (TRHR2) from rat brain and spinal cord provided a possible explanation for certain neurotransmitter actions of TRH, in particular the nociceptive and spinal cord regenerative actions (12)(13)(14). The two TRH receptor subtypes are ϳ50% homologous, and they exhibit similar binding affinities for TRH (12)(13)(14) and activate the same signaling pathways, although TRHR2 exhibits higher basal signaling activity (14 -16). TRHR1 and 2 are expressed in distinct compartments of the brain and spinal cord (12,14,17,18), although areas have been defined where both receptor subtypes are found (12,14).…”

The human neuroendocrine thyrotropin-releasing hormone receptor promoter is activated by the haematopoietic transcription factor c-Myb

“…cDNAs encoding the highly conserved TRHR have been cloned from mouse, rat and hamster cell lines [8][9][10], from human brain [11] and from avian and bovine tissues [12,13]. This receptor is now also designated TRHR1, to be distinguished from TRHR2, a recently discovered subtype of the rat TRH receptor, 51 % identical to TRHR1 but with a distinct mRNA tissue distribution [14,15].…”