Identification of thyrotropin-releasing hormone receptor messenger RNA in the rat central nervous system and eye

Hypothalamic neurohormones and immune responses

The aim of this review is to provide a comprehensive examination of the current literature describing the neural-immune interactions, with emphasis on the most recent findings of the effects of neurohormones on immune system. Particularly, the role of hypothalamic hormones such as Thyrotropin-releasing hormone (TRH), Corticotropin-releasing hormone (CRH) and Gonadotropin-releasing hormone (GnRH). In the past few years, interest has been raised in extrapituitary actions of these neurohormones due to their receptors have been found in many non-pituitary tissues. Also, the receptors are present in immune cells, suggesting an autocrine or paracrine role within the immune system. In general, these neurohormones have been reported to exert immunomodulatory effects on cell proliferation, immune mediators release and cell function. The implications of these findings in understanding the network of hypothalamic neuropeptides and immune system are discussed.

Release of retinal growth hormone in the chick embryo: local regulation?

The neural retina is an extrapituitary site of growth hormone (GH) production and an autocrine or paracrine site of retinal GH action. Retinal GH is released from retinal tissue and may be secreted into the vitreous. Ontogenetic changes in the abundance of retinal GH during embryogenesis indicate that the amount of GH released may be regulated. The presence of pituitary GH secretagogues (GH-releasing hormone, GHRH; thyrotropin-releasing hormone, TRH; and ghrelin) and pituitary GH inhibitors (somatostatin, SRIF and insulin-like growth factor, IGF-1) within the neural retina may indicate the involvement of these factors in retinal GH release. This possibility is supported by the finding that GHRH is colocalized with GH in chick retinal ganglion cells (RGCs) and in immortalized cells (QNRD) derived from quail neuroretinal cells and by the induction of GH mRNA in incubated QNRD cells. In summary, these results provide evidence for the autocrine or paracrine regulation of retinal GH release in the ganglion cells of the embryonic chick retina.

Tat-binding protein-1 (TBP-1), an ATPase of 19S regulatory particles of the 26S proteasome, enhances androgen receptor function in cooperation with TBP-1-interacting protein/Hop2

The 26S proteasome, which degrades ubiquitinated proteins, appears to contribute to the cyclical loading of androgen receptor (AR) to androgen response elements of target gene promoters; however, the mechanism whereby the 26S proteasome modulates AR recruitment remains unknown. Using yeast two-hybrid screening, we previously identified Tat-binding protein-1 (TBP-1), an adenosine triphosphatase of 19S regulatory particles of the 26S proteasome, as a transcriptional coactivator of thyroid hormone receptor. Independently, TBP-1-interacting protein (TBPIP) was also identified as a coactivator of several nuclear receptors, including AR. Here, we investigated whether TBP-1 could interact with and modulate transcriptional activation by AR cooperatively with TBPIP. TBP-1 mRNA was ubiquitously expressed in human tissues, including the testis and prostate, as well as in LNCaP cells. TBP-1 directly bound TBPIP through the amino-terminal domain possessing the leucine zipper structure. AR is physically associated with TBP-1 and TBPIP in vitro and in LNCaP cells. TBP-1 similarly and additively augmented AR-mediated transcription upon coexpression with TBPIP, and the ATPase domain, as well as leucine zipper structure in TBP-1, was essential for transcriptional enhancement. Overexpression of TBP-1 did not alter AR protein and mRNA levels. In the chromatin immunoprecipitation assay, TBP-1 was transiently recruited to the proximal androgen response element of the prostate-specific antigen gene promoter in a ligand-dependent manner in LNCaP cells. These findings suggest that a component of 19S regulatory particles directly binds AR and might participate in AR-mediated transcriptional activation in cooperation with TBPIP.

Thyrotropin-releasing hormone (TRH) in the cerebellum

Thyrotropin-releasing hormone (TRH) was originally isolated from the hypothalamus. Besides controlling the secretion of TSH from the anterior pituitary, this tripeptide is widely distributed in the central nervous system and regarded as a neurotransmitter or modulator of neuronal activities in extrahypothalamic regions, including the cerebellum. TRH has an important role in the regulation of energy homeostasis, feeding behavior, thermogenesis, and autonomic regulation. TRH controls energy homeostasis mainly through its hypophysiotropic actions to regulate circulating thyroid hormone levels. Recent investigations have revealed that TRH production is regulated directly at the transcriptional level by leptin, one of the adipocytokines that plays a critical role in feeding and energy expenditure. The improvement of ataxic gait is one of the important pharmacological properties of TRH. In the cerebellum, cyclic GMP has been shown to be involved in the effects of TRH. TRH knockout mice show characteristic phenotypes of tertiary hypothyroidism, but no morphological changes in their cerebellum. Further analysis of TRH-deficient mice revealed that the expression of PFTAIRE protein kinase1 (PFTK1), a cdc2-related kinase, in the cerebellum was induced by TRH through the NO-cGMP pathway. The antiataxic effect of TRH and TRH analogs has been investigated in rolling mouse Nagoya (RMN) or 3-acetylpyridine treated rats, which are regarded as a model of human cerebellar degenerative disease. TRH and TRH analogs are promising clinical therapeutic agents for inducing arousal effects, amelioration of mental depression, and improvement of cerebellar ataxia.

High levels of thyrotropin-releasing hormone receptors activate programmed cell death in human pancreatic precursors

OBJECTIVES

Thyrotropin-releasing hormone (TRH) is expressed in rodent and human adult pancreata and in mouse pancreas during embryonic development. However, expression of TRH receptors (TRHRs) in the pancreas is controversial. We sought to provide evidence that the TRH/TRHR system might play a role in fetal development.

METHODS

We used quantitative reverse transcription-polymerase chain reaction to measure TRH and TRHR messenger RNA (mRNA). To study the effects of TRHR expression in a pancreatic progenitor population, we expressed TRHRs in human islet-derived precursor cells (hIPCs) by infection with adenoviral vector AdCMVmTRHR. Thyrotropin-releasing hormone receptor signaling was measured as inositol phosphate production and intracellular calcium transients. Thyrotropin-releasing hormone receptor expression was measured by [H]methyl-TRH binding. Apoptosis was monitored by release of cytochrome c from mitochondria.

RESULTS

We show that TRH mRNA is expressed in human fetal and adult pancreata, and that TRHR mRNA is expressed in fetal human pancreas but not in adult human pancreas. Thyrotropin-releasing hormone receptors expressed in hIPCs were shown to signal normally. Most importantly, TRH treatment for several days stimulated apoptosis in hIPCs expressing approximately 400,000 TRHRs per cell.

CONCLUSIONS

These findings suggest a possible role for TRH/TRHR signaling in pancreatic precursors to promote programmed cell death, a normal constituent of morphogenesis during embryonic development in humans.

Isolation and characterization of a transcriptional cofactor and its novel isoform that bind the deoxyribonucleic acid-binding domain of peroxisome proliferator-activated receptor-gamma

Using the DNA-binding domain (DBD) and hinge region of human peroxisome proliferator-activated receptor (PPAR)-gamma as bait in yeast two-hybrid screen, we isolated partial cDNA identical with that of the C terminal of KIAA1769. KIAA1769 encodes a 2080-amino acid protein (molecular mass, 231 kDa) that was recently identified to interact with PPARalpha and termed PPARalpha-interacting cofactor 285 (here referred to as PPARgamma-DBD-interacting protein 1 (PDIP1)-alpha). PDIP1 mRNA was expressed in 3T3-L1 adipocytes and THP-1 macrophages. We also identified the expression of the N terminal extended form of PDIP1alpha (referred to as PDIP1beta) consisting of 2649 amino acids (295 kDa) in human cultured cell lines by RT-PCR, and 5' rapid amplification of cDNA ends. Ribonuclease protection assay revealed that PDIP1beta mRNA was expressed more abundantly than PDIP1alpha mRNA. The C-terminal region of PDIP1 directly binds DBD of PPARgamma, and multiple LXXLL motifs in PDIP1 were not required for the interaction. PDIP1alpha and -beta similarly enhanced PPARgamma-mediated transactivation in transfection assays and short interfering RNA targeting PDIP1 mRNA significantly reduced transactivation by PPARgamma. No potent intrinsic activation domain was identified in either PDIP1 isoforms in mammalian one-hybrid assays, and mutation of all LXXLL motifs did not affect enhancement of PPARgamma-mediated transactivation. PDIP1alpha and -beta similarly augmented transactivation by PPARalpha, PPARdelta, thyroid hormone receptor (TR)-alpha1, TRbeta1, and retinoid X receptor-alpha. PDIP1alpha also enhanced estrogen receptoralpha- and androgen receptor-mediated transactivation, whereas PDIP1beta did not. PDIP1alpha showed receptor-specific synergism with activation function-2-interacting coactivators in PPARgamma- and TRbeta1-mediated transactivation. Together, PDIP1 might function as a transcriptional cofactor for a broad range of nuclear receptors, possibly in collaboration with specific activation function-2 interacting coactivators.

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

Thyrotropin-releasing hormone (TRH) receptor (TRHR) is a G-protein-coupled receptor playing a crucial role in the anterior pituitary where it controls the synthesis and secretion of thyroid-stimulating hormone and prolactin. Its widespread presence not only in the central nervous system, but also in peripheral tissues, including thymus, indicates other important, but unknown, functions. One hypothesis is that the neuropeptide TRH could play a role in the immune system. We report here that the human TRHR promoter contains 11 putative response elements for the haematopoietic transcription factor c-Myb and is highly Myb-responsive in transfection assays. Analysis of Myb binding to putative response elements revealed one preferred binding site in intron 1 of the receptor gene. Transfection studies of promoter deletions confirmed that this high-affinity element is necessary for efficient Myb-dependent transactivation of reporter plasmids in CV-1 cells. The Myb-dependent activation of the TRHR promoter was strongly suppressed by expression of a dominant negative Myb-Engrailed fusion. In line with these observations, reverse transcriptase PCR analysis of rat tissues showed that the TRHR gene is expressed both in thymocytes and bone marrow. Furthermore, specific, high-affinity TRH agonist binding to cell-surface receptors was demonstrated in thymocytes and a haematopoietic cell line. Our findings imply a novel functional link between the neuroendocrine and the immune systems at the level of promoter regulation.

Distribution of thyrotropin-releasing hormone immunoreactivity in the brain of the dogfish Scyliorhinus canicula

To improve knowledge of the peptidergic systems of elasmobranch brains, the distribution of thyrotropin-releasing hormone-immunoreactive (TRHir) neurons and fibers was studied in the brain of the small-spotted dogfish (Scyliorhinus canicula L.). In the olfactory bulbs, small granule neurons richly innervated the olfactory glomeruli. In the telencephalic hemispheres, small TRHir neurons were observed in the superficial dorsal pallium, whereas TRHir fibers were widely distributed in pallial and subpallial regions. In the preoptic region, TRHir neurons formed a caudal ventrolateral group in the preoptic nucleus. In the hypothalamus, the most conspicuous TRHir populations were associated with the lateral hypothalamic recess, but small TRHir populations were found in the posterior tubercle and ventral wall of the posterior recess. The preoptic region and hypothalamus exhibited rich innervation by TRHir fibers. TRHir fibers were observed coursing to the neurohypophysis and the neuroepithelium of the saccus vasculosus, but not to the neurohemal region of the median eminence. Some stellate-like TRHir cells were observed in a few cell cords of the neurointermediate lobe of the hypophysis. The thalamus, pretectum, and midbrain lacked TRHir neurons. Further TRHir neuronal populations were observed in the central gray and superior raphe nucleus of the isthmus, and a few TRHir cells were located in the nucleus of the trigeminal descending tract at the level of the rostral spinal cord. In the brainstem, the central gray, interpeduncular nucleus, secondary visceral region of the isthmus, rhombencephalic raphe, inferior olive, vagal lobe, and Cajal's commissural nucleus were all richly TRHir-innervated. Comparison of the distribution of TRHir neurons observed in the dogfish brain with that observed in teleosts and tetrapods reveals strong resemblance but also interesting differences, indicating the presence of both a conserved basic vertebrate pattern and a number of derived characters.

Distribution of thyrotropin-releasing hormone (TRH) immunoreactivity in the brain of the zebrafish (Danio rerio)

The distribution of thyrotropin-releasing hormone (TRH) in the brain of the adult zebrafish was studied with immunohistochemical techniques. In the telencephalon, abundant TRH-immunoreactive (TRHir) neurons were observed in the central, ventral, and supra- and postcommissural regions of the ventral telencephalic area. In the diencephalon, TRHir neurons were observed in the anterior parvocellular preoptic nucleus, the suprachiasmatic nucleus, the lateral hypothalamic nucleus, the rostral parts of the anterior tuberal nucleus and torus lateralis, and the posterior tuberal nucleus. Some TRHir neurons were also observed in the central posterior thalamic nucleus and in the habenula. The mesencephalon contained TRHir cells in the rostrodorsal tegmentum, the Edinger-Westphal nucleus, the torus semicircularis, and the nucleus of the lateral lemniscus. Further TRHir neurons were observed in the interpeduncular nucleus. In the rhombencephalon, TRHir cells were observed in the nucleus isthmi and the locus coeruleus, rostrally, and in the vagal lobe and vagal motor nucleus, caudally. In the forebrain, TRHir fibers were abundant in several regions, including the medial and caudodorsal parts of the dorsal telencephalic area, the ventral and commissural parts of the ventral telencephalic area, the preoptic area, the posterior tubercle, the anterior tuberal nucleus, and the posterior hypothalamic lobe. The dorsal thalamus exhibited moderate TRHir innervation. In the mesencephalon, the optic tectum received a rich TRHir innervation between the periventricular gray zone and the stratum griseum centrale. A conspicuous TRHir longitudinal tract traversed the tegmentum and extended to the rhombencephalon. The medial and lateral mesencephalic reticular areas and the interpeduncular nucleus were richly innervated by TRHir fibers. In the rhombencephalon, the secondary gustatory nucleus received abundant TRHir fibers. TRHir fibers moderately innervated the ventrolateral and ventromedial reticular area and richly innervated the vagal lobe and Cajal's commissural nucleus. Some TRHir fibers coursed in the lateral funiculus of the spinal cord. Some TRHir amacrine cells were observed in the retina. The wide distribution of TRHir neurons and fibers observed in the zebrafish brain suggests that TRH plays different roles. These results in the adult zebrafish reveal a number of differences with respect to the TRHir systems reported in other adult teleosts but were similar to those found during late developmental stages of trout (Díaz et al., 2001).

Presence of thyrotropin-releasing-hormone-immunoreactive (TRHir) amacrine cells in the retina of anuran and urodele amphibians

The presence of thyrotropin-releasing-hormone-immunoreactive (TRH-ir) amacrine cells in the retina of amphibians is reported for the first time. The anuran and urodele retinas studied exhibit major differences in the distribution of TRH-ir cells. In the two urodele species investigated, most TRH-ir amacrine cells were located in the ganglion cell layer (GCL). These pear-shaped cells originate a dense TRH-ir dendritic plexus in strata 4-5 of the inner plexiform layer (IPL). A small number of TRH-ir amacrine cells were observed in the inner nuclear layer (INL). Most of these INL TRH-ir cells were multipolar neurons with radiating dendrites that originate a loose plexus in the IPL stratum 1. In the three anuran species investigated, most TRH-ir amacrine cells were located in the INL. Distribution of TRH-ir processes in the IPL of anurans was not so clearly layered as in urodeles, dendrites being observed throughout strata 1-5. In the toad retina THR-ir material was also observed in the outer plexiform layer, which suggests that toads may have some TRH-ir interplexiform neurons. In the frog and toad, TRH-ir fibers were also observed in the optic nerve, although their origin could not be ascertained. The number of TRH-ir amacrine cells per whole retina was higher in anurans than in urodeles, though urodeles have higher cell densities. The marked differences in distribution of TRH-ir amacrine cells observed between anurans and urodeles, and among the three anuran species, suggest different functions of TRH in retinal processing, perhaps related to the different specializations of the visual systems of these species.

Presence and development of thyrotropin-releasing hormone-immunoreactive amacrine cells in the retina of a teleost, the brown trout (Salmo trutta fario)

The presence of thyrotropin-releasing hormone-immunoreactive (TRHir) amacrine cells is described for the first time in the retina of a teleost. These amacrine cells were mostly located in the inner nuclear layer, with occasional perikarya in the ganglion cell layer. Their processes formed a conspicuous plexus at the level of the ganglion cell perikarya. The TRHir amacrine cells appeared in posthatching stages, with the total number in retinas of juveniles approximately four times the number of cells in adults. Two types of TRHir cells, large and small, can be distinguished in developing stages, small cells outnumbering large cells. The TRHir cells of adults appears mainly to correspond to large, multistratified amacrine cells of developing stages. The possibility of transient expression of TRH in small amacrine cells during development is discussed.

Expression of thyrotropin-releasing hormone (TRH) receptor subtype 1 in mouse pancreatic islets and HIT-T15, an insulin-secreting clonal beta cell line

Thyrotropin-releasing hormone (TRH), originally isolated as a hypothalamic hormone, has been reported to be present and released from the pancreatic beta cells, affecting pancreatic functions. However, it still remains unclear whether TRH receptor is expressed in the pancreas. In the present study, we characterized TRH receptors (TRHR) in mouse pancreatic islets and HIT-T15 cells, a hamster clonal beta cell line. RT-PCR study showed significant expression of TRHR subtype 1 (TRHR1) mRNA in both mouse pancreatic islets and HIT-T15 (HIT) cells. In contrast, there was no expression of TRHR2 mRNA, a novel subtype of TRHR which is expressed predominantly in the central nervous system. Sequencing analysis demonstrated that TRHR1 of the islets was identical to that in the pituitary, and cloned hamster TRHR1 shared 93.3 % homology with that of the mouse at the nucleic acid level. Northern blot analysis of TRHR 1 mRNA in HIT-T15 cells showed a single strong hybridization signal approximately 3.7 kb in length. Furthermore, Scatchard plot analysis in HIT-T15 cells revealed that the Kd value for MeTRH was 0.63 nM. Significant elevation of intracellular calcium concentration was observed in response to as little as 10 nM TRH , and this was not affected by removal of extracellular calcium. This is the first description indicating the presence of functional TRH receptor subtype 1 in the pancreatic beta cells, and our observations suggested the regulation of pancreatic function by TRH through autocrine or paracrine mechanisms.

Regulation of the mouse preprothyrotropin-releasing hormone gene by retinoic acid receptor

Retinoic acid (RA) has been reported to inhibit the secretion and synthesis of the pituitary TSH in vivo and in vitro. However, little is known about the influence of RA on the expression of the prepro-TRH gene. We therefore investigated whether the promoter activity of the mouse TRH gene is directly regulated by RA using a transient transfection assay into CV-1 cells. In the absence of cotransfected RA receptor (RAR), all-trans-RA did not affect the promoter activity. In contrast, the cotransfected RARalpha significantly stimulated promoter activity in the absence of ligand, and all-trans-RA reversed basal promoter activation. The cotransfected thyroid hormone receptor-beta (TRbeta), but not 9-cis-RA receptor (RXR), had an additive effect on the RAR-dependent stimulation. TR and RAR can similarly interact with the corepressor proteins, and the cotransfected nuclear receptor corepressor (N-CoR) has been demonstrated to augment the transcriptional stimulation of the TRH gene by unliganded TR. As observed with TR, the coexpression of a N-CoR variant significantly enhanced the ligand-independent stimulation by RAR. A mutant RAR (RAR403) lacking the C-terminal activation function-2 (AF-2) activation domain that was essential for ligand-induced corepressor release constitutively stimulated the promoter activity. The constitutive stimulation by RAR403 was augmented by the cotransfected N-CoR variant. A deletion analysis of the 5'-flanking region of the TRH gene revealed that the minimal promoter region for the regulation by RAR was -83 to +53, with a consensus half-site motif for the thyroid hormone response element at -57. In contrast to the strong binding of TR to the thyroid hormone response element half-site in gel retardation assays, no binding of RAR homodimer, RAR/ RXR heterodimer, or RAR/TR heterodimer was observed to the minimal promoter region. These results collectively suggest that RAR without heterodimerization with RXR and TR regulates transcription of the mouse TRH gene in cooperation with the corepressor, and that the DNA binding of RAR appeared to be unnecessary for regulation of the TRH gene promoter.

DNA binding and interaction with the nuclear receptor corepressor of thyroid hormone receptor are required for ligand-independent stimulation of the mouse preprothyrotropin-releasing hormone gene

A negative thyroid hormone response element (TRE) in the mouse preprothyrotropin-releasing hormone (TRH) gene was previously mapped within the proximal promoter element between -83 and +53 that contained a TRE half-site motif at -57 (-57TGACCT-51). In transfection experiments, the promoter activity is stimulated by unliganded thyroid hormone receptor (TR) and T3 reverses the basal promoter stimulation. In this study, we determined whether the direct binding of TR to the TRE half-site in the mouse TRH gene is required for the ligand-independent stimulation using a transient transfection assay into CV-1 cells and electrophoretic mobility shift assays (EMSA). In addition, the role of a corepressor protein for the ligand-independent stimulation was examined using a putative splicing variant of the nuclear receptor corepressor (N-CoRI). Point mutations introduced into the TRE half-site at -57 eliminated the binding of TR and the stimulatory effect of unliganded TR. Two mutant TRs lacking DNA-binding activity and two CoR box mutant TRs showed no stimulation in the wild-type TRH promoter. The cotransfected N-CoRI potentiated the ligand-independent stimulation by the wild-type TR, but did not compensate for the impaired function of the CoR box mutant TR. In EMSA, TR strongly bound as homodimers and weakly as heterodimers with retinoid X receptor (RXR) to the element containing the TRE half-site at -57. Binding of TR to the TRE half-site was essential to form homo- and heterodimers, and the RXR binding site appeared to be located downstream of the TRE half-site. In vitro translated N-CoRI preferentially bound TR homodimers over TR/RXR heterodimers. These results collectively suggest that the DNA-bound TR/corepressor complex might be directly involved in the ligand-independent stimulation of the mouse TRH gene promoter.

Structural and functional organization of the gene encoding the human thyrotropin-releasing hormone receptor

The thyrotropin-releasing hormone (TRH) receptor (TRHR) is widely distributed throughout the central and peripheral nervous systems. In addition to its role in controlling the synthesis and secretion of thyroid-stimulating hormone and prolactin from the anterior pituitary, TRH is believed to act as a neurotransmitter as well as a neuromodulator. We have isolated genomic lambda and P1-derived artificial chromosome clones encoding the human TRHR. The gene was found to be 35 kb with three exons and two introns. A 541-bp intron 1 (-629 to -89 relative to the translation start site) is conserved between human and mouse. A large intron 2 of 31 kb disrupts the open reading frame (starting in position +790) in the sequence encoding the supposed junction between the third intracellular loop and the putative sixth transmembrane domain. A similar intron was found in chimpanzee and sheep but not in rat and mouse. Promoter analysis of upstream regions demonstrated cell type-specific reporter activation, and sequencing of 2.5 kb of the promoter revealed putative cis-acting regulatory elements for several transcription factors that may contribute to the regulation of the TRHR gene expression. Functional analysis of potential response elements for the anterior pituitary-specific transcription factor Pit-1 revealed cell type-specific binding that was competed out with a Pit-1 response element from the GH gene promoter.

Cloning and characterization of a cDNA encoding a novel subtype of rat thyrotropin-releasing hormone receptor

A cDNA encoding a thyrotropin-releasing hormone (TRH) receptor expressed in the pituitary was previously cloned (De La Pena, P., Delgado, L. M., Del Camino, D., and Barros, F. (1992) Biochem. J. 284, 891-899; De La Pena, P., Delgado, L. M., Del Camino, D., and Barros, F. (1992) J. Biol. Chem. 267, 25703-25708; Duthie, S. M., Taylor, P. L., Anderson, J., Cook, J., and Eidne, K. A. (1993) Mol. Cell Endocrinol. 95, R11-R15). We now describe the isolation of a rat cDNA encoding a novel subtype of TRH receptor (termed TRHR2) displaying an overall homology of 50% to the pituitary TRH receptor. Introduction of TRHR2 cDNA in HEK-293 cells resulted in expression of high affinity TRH binding with a different pharmacological profile than the pituitary TRH receptor. De novo expressed receptors were functional and resulted in stimulation of calcium transient as assessed by fluorometric imaging plate reader analysis. The message for TRHR2 was exclusive to central nervous system tissues as judged by Northern blot analysis. Studies of the expression of TRHR-2 message by in situ hybridization revealed a pattern of expression remarkably distinct (present in spinothalamic tract, spinal cord dorsal horn) from that of the pituitary TRH receptor (present in hypothalamus, and ventral horn of the spinal cord, anterior pituitary). Therefore, we have identified a novel, pharmacologically distinct receptor for thyrotropin-releasing hormone that appears to be more restricted to the central nervous system particularly to the sensory neurons of spinothalamic tract and spinal cord dorsal horn, which may account for the sensory antinociceptive actions of TRH.

Postnatal ontogeny of the thyrotropin-releasing hormone receptor messenger ribonucleic acids in the rat forebrain

Postnatal developmental change of the thyrotropin-releasing hormone receptors (TRHR) in the rat forebrain was investigated using TRH binding assays and Northern blot analyses from postnatal day 8 to the age of 2 years. TRH binding assays, with [3H]MeHisTRH as the radioactive ligand, demonstrated that the binding capacity in the forebrain was lowest at postnatal day 8 and increased to a maximum level at postnatal day 20. The TRH binding significantly decreased to adult levels between days 20 and 35, and no significant change was observed thereafter. Northern blot analysis, with a 32P-labeled TRHR cRNA probe, revealed that expression of the TRHR gene in the forebrain was not detectable on day 8 after birth, whereas apparent gene expression could be detected in the anterior pituitary. In contrast to the binding capacities, TRHR mRNA levels were very low until postnatal day 20, and increased significantly between days 20 and 35. No significant alteration in mRNA levels was observed after day 35. These results indicated that: (1) TRH binding capacities in the forebrain increased to a maximum levels between the second and third postnatal week and thereafter decreased to adult level, (2) the levels of TRHR mRNA and the TRH-binding capacities did not correlate in that period, suggesting that the TRHR number in the immature forebrain might be regulated by a posttranscriptional mechanism, and (3) expression of the TRHR gene in the forebrain and pituitary seemed to be regulated differentially during development.

Identification of thyrotropin-releasing hormone receptor mRNA in the Leydig cells of the mouse testis by in situ hybridization

Thyrotropin-releasing hormone receptor (TRH-R) mRNA was detected in cryostat sections of the mouse testis using biotinylated oligonucleotides complementary to the cDNA encoding the mouse pituitary TRH-R by in situ hybridization. Hybridization signals were detected exclusively in the Leydig cells. The intensity of the signal was probe-concentration dependent. This result suggests that testicular TRH may serve as an autocrine regulator of reproductive function and development via TRH-R in a fashion that is similar or identical to that in the pituitary.

Identification of thyrotropin-releasing hormone receptor in the rat testis

We have recently documented the expression of preprothyrotropin-releasing hormone (TRH) gene in murine, human and rat testis. Moreover, we have localized TRH to rat Leydig cells immunohistochemically, and found that both prepro TRH mRNA and TRH levels are developmentally regulated in the rat testis. To investigate the potential roles of TRH in testicular function, characterization of TRH receptors (TRH-R) in this tissue was undertaken. Recently, a cDNA encoding murine TRH-R has been isolated, making possible cloning of a rat TRH-R cDNA from the anterior pituitary gland. This cDNA was used for detection of TRH-R gene expression in the rat testis by Northern blot analysis and reverse transcription-polymerase chain reaction (RT-PCR). TRH receptor assays were also performed with (3H)MeHisTRH as the radioactive ligand. In Northern blot analysis, a single and specific hybridization band, approximately 3.8 kb in size, was identified in whole testis RNA, identical in size with that found in the anterior pituitary gland. The concentration of TRH-R mRNA in the testis was approximately 10% of that in the pituitary. TRH-R mRNA was also detected by RT-PCR in Metrizamide gradient-purified Leydig cells. TRH receptor binding assays revealed the presence of specific, high affinity binding sites with a Kd of 1.6 x 10(-8) M in the testis. Such TRH binding was inhibited by chlordiazepoxide, a specific antagonist of TRH receptor binding. We conclude that TRH may exert local, probably autocrine, actions in the testis via a transmembrane receptor very similar or identical to that in pituitary.

A truncated isoform of the thyrotropin-releasing hormone receptor is expressed in the rat central nervous system as well as in the pituitary gland

Using the reverse transcription-polymerase chain reaction (RT-PCR), a cDNA encoding the entire rat thyrotropin-releasing hormone receptor (TRH-R) was isolated from normal rat pituitary gland mRNA. In addition, a novel truncated isoform of TRH-R which lacks 52 base pairs (bp) in the carboxyl (C-) terminal tail was isolated. This truncation, probably generated by alternate splicing, causes a frame-shift and results in a truncated TRH-R 25 amino acids shorter and with a different C-terminal amino acid sequence than the longer type receptor. This truncated TRH-R mRNA, along with the longer receptor form, was found to be expressed throughout the rat pituitary gland and brain.