Pituitary TRH receptors

Chronobiology in the endocrine system

Biological signaling occurs in a complex web with participation and interaction of the central nervous system, the autonomous nervous system, the endocrine glands, peripheral endocrine tissues including the intestinal tract and adipose tissue, and the immune system. All of these show an intricate time structure with rhythms and pulsatile variations in multiple frequencies. Circadian (about 24-hour) and circannual (about 1-year) rhythms are kept in step with the cyclic environmental surrounding by the timing and length of the daily light span. Rhythmicity of many endocrine variables is essential for their efficacy and, even in some instances, for the qualitative nature of their effects. Indeed, the continuous administration of certain hormones and their synthetic analogues may show substantially different effects than expected. In the design of drug-delivery systems and treatment schedules involving directly or indirectly the endocrine system, consideration of the human time organization is essential. A large amount of information on the endocrine time structure has accumulated, some of which is discussed in this review.

An analog of thyrotropin-releasing hormone (TRH) is neuroprotective against glutamate-induced toxicity in fetal rat hippocampal neurons in vitro

TRH has been found to be efficacious in treating certain neurodegenerative disorders such as epilepsy, Alzheimer's disease, neurotrauma and depression, however, its mechanism of action is poorly understood. Since glutamate (Glu) toxicity has been implicated in these disorders, we utilized primary enriched cultures of rat fetal (E 17) hippocampal neurons to test the hypothesis that an analog of TRH, 3-Methyl-Histidine TRH (3Me-H TRH), given concurrently with Glu would protect such neurons against cell damage and cell death. Cell viability was assessed via Trypan Blue exclusion cell counts, and neuronal damage was determined by assaying lactic acid dehydrogenase (LDH) released in the conditioned media. Fetal hippocampal neurons were cultured in neurobasal media for 7 days. On day 7, neurons (10(6)/well) were treated with: control media, 10 microM 3Me-H TRH, 500 microM Glu or 500 microM Glu with either 10, 1, 0.1, 0.01 or 0.001 microM 3Me-H TRH. Both media and neurons were harvested 16 h after treatment. Prolonged exposure to 10 microM 3Me-H TRH was not toxic to the cells, whereas neurons exposed to 500 microM Glu resulted in maximal cell death. Notably, 10, 1 and 0.1 microM 3Me-H TRH, when co-treated with 500 microM Glu, protected fetal neurons against cell death in a concentration-dependent manner. These results provide support for an important neuroprotective effect of TRH/analogs against glutamate toxicity in primary hippocampal neuronal culture and implicate a potentially beneficial role of TRH/analogs in neurodegenerative diseases.

Thyrotropin-releasing hormone (protirelin) inhibits potassium-stimulated glutamate and aspartate release from hippocampal slices in vitro

Excess excitatory amino acid release is involved in pathways associated with seizures and neurodegeneration. Thyrotropin-releasing hormone (TRH; protirelin), a brain-derived tripeptide, has shown efficacy in the treatment of such disorders, yet its mechanism of neuroprotection is poorly understood. Using superfused hippocampal slices, we tested the hypothesis that TRH could inhibit evoked glutamate/aspartate release in vitro. Rat hippocampal slices were first equilibrated in oxygenated Krebs buffer (KRB) (120 min) then superfused for 10 min with KRB (control), or KRB containing 0.1, 1, or 10 microM TRH respectively, prior to and during 5 min depolarization with high potassium KRB (50 mM [K(+)] +/- TRH). Fractions (1 min) were collected during the 5 min stimulation and for an additional 10 min thereafter and analyzed for glutamate and aspartate by HPLC. TRH had no effect on baseline glutamate/aspartate release, while all three TRH doses significantly (P < 0.05) inhibited peak 50 mM [K(+)]-stimulated glutamate/aspartate release, and glutamate remained below control (P < 0.05) at 15 min post stimulation. A 5 min pulse of TRH (10 microM) had no affect on basal glutamate/aspartate release, whereas the TRH pre-pulsed slices failed to release glutamate/aspartate by [K(+)]-stimulation given 15 min later. These results are the first to show a potent and prolonged inhibitory effect of TRH on evoked glutamate/aspartate release in vitro. These initial studies suggest that exogenous and/or endogenous TRH may function, in part, to modulate excess glutamate release in specific CNS loci. Additional studies are in progress to fully understand the mechanism of this potent effect of TRH and its implication in various CNS disorders.

Receptors for hypothalamic releasing hormones TRH and GnRH: oligomerization and interactions with intracellular proteins

Studies of TRH and GnRH receptors have revealed much information about the roles of G-proteins and beta-arrestins, as well as receptor residues important for signaling, desensitization and internalization. However, the proteins involved are only just beginning to be identified and characterized. Additional complexity now exists with the observation that these receptors form oligomers in live cells. Indeed, hetero-oligomerization of TRH receptor subtypes 1 and 2 potentially alters interactions with intracellular regulatory proteins. Knowledge of proteins that interact with TRH or GnRH receptors will increase our understanding of receptor function and provide potential drug targets for a range of receptor-associated conditions.

Thyrotropin-releasing hormone increases phospholipase D activity through stimulation of protein kinase C in GH3 cells

Activation of phospholipase D was investigated after treatment of GH3 cells with thyrotropin-releasing hormone. Thyrotropin-releasing hormone treatment resulted in both time- and dose-dependent increases of phospholipase D activity, translocation of protein kinase C-alpha and -beta I isozymes from cytosol to membrane within 30 min, and approx 43-fold increase of phosphatidylinositol-specific phospholipase C activity. Intracellular calcium concentration was rapidly increased and diacyglycerol level remained high up to 3 h after the treatment. Pretreatment of the cells with U73122, a potent inhibitor of phosphatidylinositol-specific phospholipase C, inhibited thyrotropin-releasing hormone-induced phospholipase D activation. Protein kinase C activity was down-regulated by pretreatment of the GH3 cells with either protein kinase C inhibitors (RO320432, GF109203X) or preincubation of the cells with phorbol myristrate acetate (500 nM) for 24 h. This treatment largely abolished the thyrotropin-releasing hormone-induced activation of phospholipase D, thus further confirming the involvement of protein kinase C in the activation. These results suggest that thyrotropin-releasing hormone-induced phospholipase D activation may be due to phosphatidylinositol-specific phospholipase C, and activation of protein kinase C isozymes is responsible for this stimulation.

Central nervous system effects of thyrotropin-releasing hormone and its analogues: opportunities and perspectives for drug discovery and development

Besides its well-known endocrine role in the thyroid system, thyrotropin-releasing hormone (L-pyroglutamyl-L-histidyl-L-prolinamide) has been long recognized as a modulatory neuropeptide. After a brief overview of the extrahypothalamic and receptor distribution, and of the neurophysiological, neuropharmacological and neurochemical effects of this tripeptide, this review discusses efforts devoted to enhance therapeutically beneficial central nervous system effects via structural modifications of the endogenous peptide. An enormous array of maladies affecting the brain and the spinal cord has been a potential target for therapeutic interventions involving agents derived from thyrotropin-releasing hormone as a molecular lead. Successful development of several centrally active analogues and recent accounts of efforts aimed at improving metabolic stability, selectivity and bioavailability are highlighted.

Effect of the disruption of body fluid balance on pyroglutamyl aminopeptidase (Type-1) in rat brain structures

The activity of soluble and membrane-bound pyroglutamyl aminopeptidase Type-1 (PAP I) was evaluated in the hypothalamus, hippocampus, thalamus, brain cortex, and pituitary gland of rats after applying certain hydromineral challenges. Compared to euhydrated rats, decreased enzyme activity was found in the hypophysis of rats deprived of water for 48 h, or rats drinking ad libitum hypertonic sodium chloride solution (2%) for 6 days or distilled water for 6 days and then submitted to acute water overload. PAP I cleaves the pGlu-amino acid bond of neuropeptides such as thyroliberin, luliberin, neurotensin, and bombesin. The decay of particulate PAP I activity may cause an increase of these pyroglutamate peptides in the whole pituitary. Although the deleterious or pro-homeostatic influence of this decay remains to be elucidated, the present data provide evidence for the involvement of this enzyme activity at this anatomical location in the water-electrolyte imbalance.

Dimerization and phosphorylation of thyrotropin-releasing hormone receptors are modulated by agonist stimulation

Dimerization and phosphorylation of thyrotropin-releasing hormone (TRH) receptors was characterized using HEK293 and pituitary GHFT cells expressing epitope-tagged receptors. TRH receptors tagged with FLAG and hemagglutinin epitopes were co-precipitated only if they were co-expressed, and 10-30% of receptors were isolated as hemagglutinin/FLAG-receptor dimers under basal conditions. The abundance of receptor dimers was increased when cells had been stimulated by TRH, indicating that TRH either stabilizes pre-existing dimers or increases dimer formation. TRH increased receptor dimerization and phosphorylation within 1 min in a dose-dependent manner. TRH increased phosphorylation of both receptor monomers and dimers, documented by incorporation of (32)P and an upshift in receptor mobility reversed by phosphatase treatment. The ability of TRH to increase receptor phosphorylation and dimerization did not depend on signal transduction, because it was not inhibited by the phospholipase C inhibitor. Receptor phosphorylation required an agonist but was not blocked by the casein kinase II inhibitor apigenin, the protein kinase C inhibitor GF109203X, or expression of a dominant negative form of G protein-coupled receptor kinase 2. TRH receptors lacking most of the cytoplasmic carboxyl terminus formed dimers constitutively but failed to undergo agonist-induced dimerization and phosphorylation. TRH also increased phosphorylation and dimerization of TRH receptors expressed in GHFT pre-lactotroph cells.

Prostate-thyroid axis: prostatic TRH is one of the stimulators of thyroid hormone

Ventral prostatectomy decreased serum thyroid hormones and histology of the thyroid gland indicate that hypothyroid condition. Co-culture of thyroid gland and ventral prostate stimulates thyroid hormone secretion. In the present study we report prostatic thyrotropin releasing hormone (TRH) is the stimulating factor of thyroid hormone secretion. Mature rat (90 days old) ventral prostate, anterior pituitary and thyroid glands were co-cultured in vitro with or without TRH antibody to assess the direct influence ofprostatic TRH on thyroid hormone secretion. Total thyroxine (T4) and triiodothyronine (T3) were increased significantly in the culture media of ventral prostate, anterior pituitary and thyroid gland when compared with thyroid gland plus anterior pituitary culture media. However, media T4 and T3 concentration decreased significantly in thyroid gland alone; also in thyroid gland plus ventral prostate, thyroid gland plus anterior pituitary and thyroid gland plus anterior pituitary plus ventral prostate were co-cultured with TRH antibody (Ab) in a dose dependent manner. The results suggest that ventral prostatic TRH is one ofthe stimulating factors of thyroid hormone secretion under these in vitro conditions.

Fertilization promoting peptide — A possible regulator of sperm function in vivo

Fertilization promoting peptide (FPP), a tripeptide related to thyrotrophin releasing hormone (TRH), is found in seminal plasma. Recent evidence obtained in vitro suggests that FPP may play an important role in regulating sperm fertility in vivo. Specifically, FPP initially stimulates nonfertilizing (uncapacitated) spermatozoa to "switch on" and become fertile more quickly, but then arrests capacitation so that spermatozoa do not undergo spontaneous acrosome loss and therefore do not lose fertilizing potential. These responses are mimicked, and indeed augmented, by adenosine, known to regulate the adenylyl cyclase (AC)/cAMP signal transduction pathway. Both FPP and adenosine have been shown to stimulate cAMP production in uncapacitated cells but inhibit it in capacitated cells, with FPP receptors somehow interacting with adenosine receptors and G proteins to achieve regulation of AC. These events affect the tyrosine phosphorylation state of various proteins, some being important in the initial "switching on," others possibly being involved in the acrosome reaction itself. Calcitonin and angiotensin II, also found in seminal plasma, have similar effects in vitro on uncapacitated spermatozoa and can augment responses to FPP, suggesting that all four molecules may be involved in regulating availability of cAMP. It is plausible that these molecules have similar effects in vivo, affecting fertility by stimulating and then maintaining fertilizing potential. Either reductions in the availability of FPP, adenosine, calcitonin, and angiotensin II or defects in their receptors could contribute to male infertility. These exciting results may provide new approaches for diagnostic tests and treatments of certain categories of male infertility.

TRH-R2 exhibits similar binding and acute signaling but distinct regulation and anatomic distribution compared with TRH-R1

TRH (thyroliberin) is a tripeptide (pGlu-His-ProNH2) that signals via G protein-coupled receptors. Until recently, only a single receptor for TRH was known (TRH-R1), but two groups identified a second receptor, TRH-R2. We independently discovered TRH-R2. Using an extensive set of TRH analogs, we found no differences in TRH-R1 and TRH-R2 binding or in acute stimulation of signaling. TRH-R2 was more rapidly internalized upon binding TRH and exhibited a greater level of TRH-induced down-regulation than TRH-R1. During prolonged exposure to TRH, cells expressing TRH-R2 exhibited a lower level of gene induction than cells expressing TRH-R1. TRH-R2 receptor mRNA was present in very discrete nuclei and regions of rat brain. A major mRNA transcript for TRH-R2 was seen in the cerebral cortex, pons, thalamus, hypothalamus, and midbrain with faint bands found in the striatum and pituitary. The extensive distribution of TRH-R2 in the brain suggests that it mediates many of the known functions of TRH that are not transduced by TRH-R1. The variations in agonist-induced internalization and down-regulation/desensitization, and anatomic distribution of TRH-R2 compared with TRH-R1, suggest important functional differences between the two receptors.

Neuroregulation of ProTRH biosynthesis and processing

This review presents an overview of the current knowledge on proTRH biosynthesis, its processing, its tissue distribution, and the role of known processing enzymes in proTRH maturation. The neuroendocrine regulation of TRH biosynthesis, the biological actions of its products, and the signal transduction and catabolic pathways used by those products are also reviewed. The widespread expression of proTRH, PC1, and PC2 rnRNAs in hypophysiotropic and extrahypophysiotropic areas of the brain, with their overlapping distribution in many areas, indicates the striking versatility provided by tissue-specific processing in generating quantitative and qualitative differences in nonTRH peptide products as well as TRH. Evidence is presented suggesting that differential processing for proTRH at the intracellular level is physiologically relevant. It is clear that control over the diverse range of proTRH-derived peptides within a specific cell is accomplished most from the regulation at the posttranslational level rather than the translational or transcriptional levels. Several examples supporting this hypothesis are presented in this review. A better understanding of proTRH-derived peptides role represents an exciting new frontier in proTRH research. These connecting sequences in between TRH molecules to form the precursor protein may function as structural or targeting elements that guide the folding and sorting of proTRH and its larger intermediates so that subsequent processing and secretion are properly regulated. The particular anatomical distribution of the proTRH end products, as well as regulation of their levels by neuroendocrine or pharmacological manipulations, supports a unique potential biologic role for these peptides.

Transcription of the human thyrotropin-releasing hormone receptor gene-analysis of basal promoter elements and glucocorticoid response elements

The gene for the human thyrotropin-releasing hormone receptor (TRHR) spans 35 kb and contains three exons and two introns (Matre et al. (1999) J. Neurochem. 72, 1-11). Despite a reported transcription start site (TSS) mapped to position -885 upstream of the translation initiation codon (Iwasaki et al. (1996) J. Biol. Chem. 271, 22183-8), we found cell type specific promoter activity directed by a fragment downstream of this site (-770 to +1). To elucidate the basis for this unexpected activity, we analyzed basal promoter elements in this region of the gene. One divergent TATA box, TTTAAA in position -759, was found by mutational analysis to be critical for promoter activity, providing a likely explanation for the basal activity observed. This proximal region apparently contains several promoter elements, including Pit-1 binding sequences within the first intron of the TRHR gene as previously reported. Here we describe the analysis of two putative glucocorticoid response elements (GREs) that we identified in this region, one (distal) half site overlapping the proposed TSS at -885 and one (proximal) full site within the first intron at position -624. Accordingly, stimulation of rat pituitary GH3 and GH4C1 cells with dexamethasone strongly enhanced transcription activity of a reporter construct containing the distal GRE half site and the proximal GRE site. Both sites bound the glucocorticoid receptor (GR) in a specific manner. Deletion of the distal GRE half site abolished the dexamethasone induction of CAT transcription, as did mutations in the proximal site. We therefore conclude that both sites are necessary for regulation of the TRHR gene transcription by glucocorticoids.

Diversity of thyrotropin-releasing hormone receptors in the pituitary and discrete brain regions of rats

In order to analyze the receptor properties of central nervous system (CNS)-stimulant thyrotropin-releasing hormone (L-pyroglutamyl-L-histidyl-L-prolinamide, TRH), we evaluated the binding of TRH and its analog taltirelin hydrate ((-)-N-[(S)-hexahydro-1-methyl-2,6-dioxo-4-pyrimidinylcarbonyl]-L- histidyl-L-prolinamide tetrahydrate; taltirelin, TA-0910) in rat anterior pituitary and several brain regions. There was a specific binding of [3H]methyl TRH (MeTRH) in the anterior pituitary, hypothalamus, brain stem, cerebral cortex and cerebellum with Kd values of 1.0-1.6 nM. The inhibition of [3H]MeTRH binding by TRH and taltirelin was monophasic in the anterior pituitary, hypothalamus and brain stem with Ki values of 6.3-8.0 nM and 145.5-170.4 nM for TRH and taltirelin, respectively. In contrast, the biphasic inhibition was revealed in the cerebral cortex and cerebellum. The Ki values for TRH and taltirelin were 4.1-4.3 nM and 67.8-73.4 nM for the high affinity binding site and 3.6-4.2 microM and 82.3-197.5 microM for the low affinity binding site, respectively. Addition of 100 microM GTP or its analog 5'-guanylylimidodiphosphate (Gpp[NH]p) affected neither the biphasic inhibition by TRH nor that by taltirelin. Thus the results suggest the presence of distinct high and low affinity TRH receptors in the CNS in contrast to the pituitary.

Receptors for thyrotropin-releasing hormone on rat lactotropes and thyrotropes

Primary cultures of rat pituitary cells were stained with an antibody to the native thyrotropin-releasing hormone (TRH) receptor and with a bioactive, fluorescent analogue of TRH, Rhod-TRH. Rhod-TRH specifically stained 86% of lactotropes and 21% of nonlactotropes from primary pituitary cell cultures. Lactotropes and thyrotropes accounted for 90% of cells that stained with Rhod-TRH, but there were occasional lactotropes and thyrotropes that did not show detectable staining with antireceptor antibodies or with Rhod-TRH. The intensity of staining was generally higher in the GH3 line of tumor cells than in normal pituicytes, and 100% of the tumor cells stained with Rhod-TRH. To determine whether the TRH receptor undergoes ligand-directed endocytosis in normal cells, TRH receptor immunocytochemistry was performed before and after TRH binding. TRH receptors were localized on the surface of cells prior to TRH exposure, and Rhod-TRH fluorescence was confined to the plasma membrane when TRH binding was performed at 0 degrees C, where endocytosis is blocked. When cells were incubated with TRH at 37 degrees C, receptors were found in intracellular vesicles in both lactotropes and thyrotropes, and Rhod-TRH was rapidly internalized into endosomes at elevated temperatures. Internalization of Rhod-TRH was inhibited by hypertonic sucrose, indicating that it occurs through clathrin-coated pits. These findings show that some of the heterogeneity in the secretory and calcium responses of pituicytes to TRH occurs at the level of the TRH receptor.

Modulation of mammalian sperm function by fertilization promoting peptide (FPP)

Fertilization promoting peptide (FPP; pGlu-Glu-ProNH2) is produced by the prostate gland and secreted into seminal plasma. When added to uncapacitated mouse and human sperm suspensions, it stimulates capacitation as demonstrated by both cytological changes and increased fertilizing ability in vitro. When added to capacitated suspensions, FPP inhibits spontaneous acrosome loss but cells retain high fertility in vitro. Adenosine elicits similar responses to FPP in both uncapacitated and capacitated cells and FPP + adenosine has a greater effect on uncapacitated cells than either used individually. We have proposed that these two molecules modulate the same pathway (adenylate cyclase/cAMP) but act via different receptors. The structure of FPP is crucial for bioactivity: loss of the terminal amide group abolishes activity and substitution of the central glutamic acid can markedly alter activity. Most recently we have found that stimulation of TCP-11, the product of the mouse t-complex gene Tcp-11, elicits responses indistinguishable from those obtained with FPP and we have hypothesized that the protein TCP-11 is the receptor for FPP. The existence of a human homologue for Tcp-11 suggests that the gene product, in conjunction with FPP, could play an important role in human fertility.

Signal transduction, desensitization, and recovery of responses to thyrotropin-releasing hormone after inhibition of receptor internalization

Three independent methods were used to block internalization of the TRH receptor: cells were infected with vaccinia virus encoding a dominant negative dynamin, incubated in hypertonic sucrose, or stably transfected with a receptor lacking the C-terminal tail. Internalization was blocked in all three paradigms as judged by microscopy using a fluorescently labeled TRH agonist and biochemically. The initial inositol trisphosphate (IP3) and Ca2+ responses to TRH were normal when internalization was inhibited. The IP3 increase was sustained rather than transient, however, in cells expressing the truncated TRH receptor, implying that the C-terminal tail of the receptor may be important for uncoupling from phospholipase C. After withdrawal of TRH, cells were refractory to TRH until both ligand dissociation and resensitization of the receptor had occurred. When surface-bound TRH was removed by a mild acid wash, which did not impair receptor function, neither wild-type nor truncated receptors were able to generate full IP3 responses for about 10 min. The rate of recovery was not altered by blocking internalization. Recovery of intracellular Ca2+ responses also depended on the rate of Ca2+ pool refilling. In summary, in the continued presence of TRH, phospholipase C activity declines quickly due to receptor uncoupling; this desensitization does not take place for the truncated receptor. After TRH is withdrawn, cells are refractory to TRH. Before cells can respond, TRH must dissociate and a resensitization step, which takes place on the plasma membrane and does not require the C-terminal tail of the receptor, must occur.

A G protein beta gamma dimer-mediated pathway contributes to mitogen-activated protein kinase activation by thyrotropin-releasing hormone receptors in transfected COS-7 cells

Activation of mitogen-activated protein kinase (MAPK) is induced by adding thyrotropin-releasing hormone (TRH) to COS-7 cells cotransfected with TRH receptors and an epitope-tagged MAPK. Long term treatment of the cells with pertussis toxin has no effect on TRH-induced MAPK activation. Incubation of the cells with the protein kinase C (PKC) inhibitor GF109203X causes an almost complete inhibition of MAPK activation by the PKC activator phorbol-12-myristate-13-acetate. In contrast, only approximately 50% of the TRH-induced MAPK activity is inhibited by GF109203X, indicating that activation of MAPK by TRH is only partially dependent on PKC. The inhibitory effect of GF109203X is additive with that of p21(N17ras), a dominant negative mutant of p21(ras) that exerts little effect on PKC-dependent MAPK activation by phorbol-12-myristate-13-acetate. The TRH-induced activation of MAPK also is inhibited partially by overexpression of transducin alpha subunits (alpha t), an agent known to sequester free G protein beta gamma dimers. However, the inhibitory potency of alpha t on TRH-induced activation is about half of that obtained in cells transfected with m2 muscarinic receptors, which activate MAPK exclusively through beta gamma dimers. The effect of alpha t is also additive with that of GF109203X but not with that of p21(N17ras). MAPK activation is not induced by the constitutively active form of G alpha q due to an inhibitory effect of its expression at a step downstream of that at which PKC-dependent and -independent routes to MAPK converge. Our results demonstrate that TRH receptors activate MAPK by a pathway only partially dependent on PKC activity. Furthermore, they indicate that beta gamma dimers of a pertussis and cholera toxin-insensitive G protein are involved in the PKC-independent fraction of the dual signaling route to MAPK initiated in the TRH receptor.

Region-specific expression of thyrotrophin-releasing hormone-degrading ectoenzyme in the rat central nervous system and pituitary gland

Thyrotrophin-releasing hormone (TRH), a hypothalamic neuropeptide hormone and a putative neuromodulator/ neurotransmitter in the central nervous system is inactivated by the TRH-degrading ectoenzyme (TRH-DE), a TRH-specific metallopeptidase localized on the surface of neuronal brain cells in culture and on lactotrophic cells of the pituitary. After succeeding in cloning the cDNA of TRH-DE we now report on the cellular distribution pattern of this enzyme in rat brain, spinal cord and pituitary gland using in situ hybridization histochemistry. In the pituitary, TRH-DE mRNA was found both in the anterior and the neural lobe but not in the intermediate lobe. After treatment with triiodothyronine (T3) a dramatic increase in the mRNA levels of the TRH-DE and a decrease in the intensity of the TRH receptor could be observed in the anterior lobe of the pituitary. In brain, TRH-DE transcripts were predominantly found in neo- and allocortical regions with strongest signals in the olfactory bulb, the piriform cortex, the cerebral cortex, the granular layer of the cerebellar cortex and the pyramidal cells of the Ammon's horn. In the diencephalon, the highest TRH-DE mRNA levels were observed in the medial habenulae followed by several hypothalamic subregions. In the mesencephalon and brainstem, moderate signals were present in the superior colliculi, substantia nigra, dorsal raphe and in the periolivar region. In the spinal cord, TRH-DE mRNA positive neurons were present in all layers. The very distinct distribution of TRH-DE in the brain and the hormonal regulation of the adenohypophyseal enzyme support the concept that this peptidase serves very specialized functions.

Regulation of the adenohypophyseal thyrotropin-releasing hormone-degrading ectoenzyme by estradiol

TRH is inactivated by the TRH-degrading ectoenzyme, a TRH-specific metallopeptidase. At the pituitary level, this enzyme is stringently regulated by thyroid hormones. We describe here gender-related differences and the effect of estradiol (E2) on the expression of this enzyme in the anterior pituitary. Compared with male rats, only about one third of the enzymatic activities and the messenger RNA levels were found in the anterior pituitary of female rats, whereas the TRH receptor transcript levels were found inversely related. When male rats received a single injection of 0.5 microg E2/100 g BW, the enzymatic activity decreased to 65% of control values within 14 h, preceded by a decrease of the transcript levels to 25% of control within 6 h. Basal values were reached again 24-48 h after the injection. E2 had no effect on the expression of the enzyme in the brain. In vivo and with GH3 cells in vitro, E2 effectively counteracted the increase in enzymatic activity induced by T3, whereas neither testosterone nor progesterone, aldosterone, or dexamethasone showed any significant effects. Because the expression of the adenohypophyseal TRH-degrading ectoenzyme is tightly regulated by both T3 and E2 with adequate dynamics, we conclude that this peptidase serves integrative functions for the control of TRH-stimulated hormone secretion.

Effect of cell type on the subcellular localization of the thyrotropin-releasing hormone receptor

The localization of an epitope-tagged receptor for thyrotropin-releasing hormone (TRH) expressed in different cell contexts was studied with immunofluorescence microscopy. In pituitary lactotrophs, which normally express TRH receptors, and in AtT20 pituitary corticotrophs, TRH receptor immunoreactivity was primarily confined to the plasma membrane. In HEK 293 and COS7 cells, TRH receptors were predominantly intracellular. In transiently transfected COS7 cells, the TRH receptor colocalized with endoplasmic reticulum and Golgi markers. The pattern of TRH receptor immunofluorescence was the same over a wide range of receptor expression in transiently transfected COS7 cells, and all cell lines bound similar amounts of 3H- and rhodamine-labeled TRH analogs, suggesting that cell-specific differences in TRH receptor localization were not simply the result of overexpression. In all cell contexts, TRH receptors on the plasma membrane underwent extensive ligand-driven endocytosis. Inhibitors of glycosylation did not alter the subcellular distribution of receptors. In HEK 293 cells expressing the transfected TRH receptor, protein synthesis inhibitors caused translocation of intracellular receptors to the cell surface, as shown by a marked increase in cell surface immunofluorescence and [3H][N3-methyl-His2]TRH binding. These results demonstrate that the subcellular localization of the TRH receptor depends on the cell context in which it is expressed and that intracellular receptors are capable of translocation to the plasma membrane.

Cellular uptake of lead is activated by depletion of intracellular calcium stores

The mechanisms of cellular lead uptake were characterized using a fluorescence method in cells loaded with indo-1. Pb2+ bound to intracellular indo-1 with much higher affinity than Ca2+ and quenched fluorescence at all wavelengths. Pb2+ uptake into pituitary GH3 cells, glial C6 cells, and a subclone of HEK293 cells was assessed by fluorescence quench at a Ca2+-insensitive emission wavelength. Pb2+ uptake was concentration- and time-dependent. Pb2+ uptake in all three cell types occurred at a much faster rate when intracellular Ca2+ stores were depleted by two different methods: addition of drugs that inhibit the endoplasmic reticulum Ca2+ pump (thapsigargin, cyclopiazonic acid, and tert-butylhydroquinone), and prolonged incubation of cells in Ca2+-free media. Application of receptor agonists, which deplete intracellular Ca2+ stores via inositol trisphosphate-sensitive channels, did not activate Pb2+ uptake. Agonists were just as effective as thapsigargin in stimulating uptake of Ca2+ but less so in stimulating uptake of Mn2+. Basal and stimulated Pb2+ uptake were partially reduced by 1 mM extracellular Ca2+ and strongly inhibited by 10 mM Ca2+. Pb2+ entry in GH3 cells was inhibited by two drugs that block capacitative Ca2+ entry, La3+ and SK&F 96365. Depolarization of electrically excitable GH3 cells increased the initial rate of Pb2+ uptake 1.6-fold, whereas thapsigargin increased uptake 12-fold. In conclusion, Pb2+ crosses the plasma membrane of GH3, C6, and HEK293 cells via channels that are activated by profound depletion of intracellular Ca2+ stores.

Organization of the receptor-mediated phosphoinositide cycle: relationship between receptor occupancy and accession of phosphatidylinositol

We have previously reported the existence of separate hormone-responsive and -unresponsive pools of inositol phospholipids in WRK-1 cells. In order to further explore this concept, we have performed experiments to examine the relationship between the plasma membrane receptor and the pool of phosphatidylinositol (Ptdlns) that is metabolized in response to hormonal stimulation. The results support the following conclusions. 1) The amount of Ptdlns metabolized in WRK-1 cells in response to vasopressin is proportional to the number of receptors occupied; neither prolonged activation with nor readdition of submaximal concentration of vasopressin induced the same degree of Ptdlns metabolism as maximal concentration of vasopressin. 2) Dissociation of cytoskeletal structures by incubation with cytochalasin D did not alter the amount of Ptdlns accessed during hormonal stimulation. 3) Accession of Ptdlns from internal membranes does not depend on internalization and recycling of the receptor; cells incubated in potassium-free medium failed to internalize receptor-ligand complexes, yet they accessed the same amount of Ptdlns in response to vasopressin as did control cells. 4) Golgi-mediated phosphatidylinositol transport is not involved in hormone-stimulated phosphoinositide turnover, since brefeldin A, which interferes with Golgi-mediated transport processes, had no effect on the amount of Ptdlns accessed during vasopressin stimulation. 5) Phosphoinositide breakdown and compensatory resynthesis is not a closed process; newly synthesized Ptdlns is not preferentially localized to a hormone-responsive pool but is generally redistributed between responsive and unresponsive pools.

Drugs that influence tubulin polymerization modulate thyrotropin-releasing hormone receptor number in AtT-20 cells

In order to examine the role of cytoskeleton in modulating the cell surface receptors, AtT-20 cells (stably expressing thyrotropin-releasing hormone receptors) were incubated with drugs that are known to modify the tubulin-microtubule system. The binding of [3H]methyl thyrotropin-releasing hormone ([3H]mTRH) to intact cells increased as a function of time, and was linear from 1.25 x 10(6) to 6.25 x 10(6) cells/ml. Cells incubated with colchicine, vinblastine, and taxol for 16 hr were harvested and the cell concentration was determined using a haemocytometer. Because the drugs inhibited the cell proliferation at 100 nM, it was decided to examine the effect of 100 nM of each of the three drugs on the ability of [3H]mTRH to bind cell surface receptors. Cells were incubated with the drugs for 16 hr at 37 degrees. After the incubation, cells (5 x 10(6) cells/ml) from each group were assayed for [3H]mTRH binding. Colchicine, vinblastine, and taxol stimulated [3H]mTRH binding by up to 27, 27, and 21%, respectively, without altering the Ka of the ligand to the receptor. These results suggest that perturbation of cytosolic microtubules leads to a reorganization of the spatial location of hormone receptors.

A possible mechanism of action for fertilization promoting peptide, a TRH-related tripeptide that promotes capacitation and fertilizing ability in mammalian spermatozoa

Fertilization promoting peptide (FPP), a tripeptide structurally related to thyrotrophin releasing hormone (TRH), has been shown to stimulate capacitation and fertilizing ability in both mouse and human spermatozoa, but the mechanisms of action involved in these responses are currently unknown. In the present study utilizing epididymal mouse spermatozoa, we have compared the ability of FPP, TRH, and pyroglutamylphenylalanineprolineamide (an uncharged structurally related tripeptide found in seminal plasma) to stimulate capacitation. At 50 nM, the mean concentration of FPP found in human seminal plasma, only FPP produced a significant response. This suggests that if a receptor is involved, it is one distinct from the TRH receptor. A significant response to FPP required the presence of extracellular Ca2+, with 90 microns Ca2+ being sufficient to support a stimulation of capacitation. The addition of FPP to suspensions at later stages of capacitation indicated that the nature of the response changed, such that addition of FPP to capacitated suspensions inhibited spontaneous acrosome reactions; however, FPP-treated cells were still able to undergo acrosomal exocytosis in response to progesterone, a physiological agonist of acrosomal exocytosis. Because earlier studies had identified a similar capacitation-related change in response to adenosine, being stimulatory early in capacitation and inhibitory later in capacitation, we investigated the possibility that FPP and adenosine might be acting via the same pathway. The combination of FPP plus adenosine, whether used at low, non-stimulatory concentrations or high, maximally-stimulatory concentrations, was more effective in promoting capacitation than either compound used individually. As observed with FPP, addition of adenosine to capacitated cells inhibited spontaneous acrosome loss but did not inhibit exocytosis in response to progesterone. This suggests that the two molecules are affecting a common pathway. Since adenosine, acting via specific cell surface receptors, can stimulate fertilizing ability and adenylate cyclase activity in uncapacitated cells and then inhibit enzyme activity in capacitated cells, we propose that FPP may act by modulating the adenylate cyclase/cyclic AMP signal transduction pathway. In vivo, FPP, which would contact spermatozoa at ejaculation and probably remain bound to cells for some time, could stimulate capacitation as the spermatozoa ascend the female tract; adenosine, present in seminal plasma and the female tract, could either augment FPP's action or replace it if FPP is lost from the cell surface. We therefore suggest that FPP and adenosine, by modulating adenylate cyclase activity to promote capacitation but inhibit spontaneous acrosomal exocytosis, may provide an endogenous mechanism that helps to optimize the fertilizing potential of the few sperm cells that reach the site of fertilization in vivo.

Vinblastine and nocodazole inhibit basal and thyrotropin-releasing hormone-stimulated prolactin secretion in GH(3) cells

To investigate the efficacy of vinblastine as a possible therapeutic agent in prolactinomas, we have examined the effects of vinblastine on GH(3) cell function. The effects of vinblastine were compared to another anti-microtubule drug, nocodazole. At 24 h, prolactin (PRL) secretion was 737±63 ng/ml in control cells. In cells treated with 0.1, 1 and 10μM: nocodazole for 24 h, PRL secretion was reduced to 200±30 ng/ml. After a 24 h incubation with the drugs, cells were washed with drug-free medium and challenged with 100NM: TRH for 10 min. TRH-stimulated PRL secretion was 35±7 ng/ml in control cells, 14±0.5 ng/ml in vinblastine-treated cells and 8.8±0.1 ng/ml in nocodazole-treated cells. [(3)H]TRH binding to GH(3) cell membrane was inhibited by about 15% by vinblastine and nocodazole. In vinblastine and nocodazole treated cells, polymerized tubulin levels decreased by 46 and 55%, respectively. These observations that vinblastine suppresses PRL secretion by GH(3) cells suggest that this drug might be useful as a therapeutic agent for prolactinomas.

Inactivation of thyrotropin-releasing hormone (TRH) by the hormonally regulated TRH-degrading ectoenzyme A potential regulator of TRH signals?

The peptidergic signal substance TRH is inactivated by the TRH-degrading enzyme, a peptidase that exhibits a high degree of substrate specificity and other unusual characteristics. The tissue-specific regulation of the adenohypophyseal enzyme by estradiol and thyroid hormones suggests that it may serve an integrative function in modulating the response of adenohypophyseal target cells to TRH and thus pituitary hormone secretion. The high enzymatic activity of neuronal cells indicates that centrally this peptidase also might act as a terminator of neural TRH signals.

Analogs of thyrotropin-releasing hormone in potentiating the spinal monosynaptic reflex in vitro

The efficacy of thyrotropin-releasing hormone (TRH) and its analogs to potentiate the spinal monosynaptic reflex was studied in isolated cords. The analogs examined were L-pyro-2-aminoadipyl-histidyl-thizolidine-4-carboxyamide (MK-771); pyroglutamyl-histidyl-prolineamide (TRH); pyroglutamyl-L-histidyl-3,3'-dimethyl-prolineamide (RX77368); (3-methyl-His2)TRH(methyl-TRH); gamma-buturolactone-gamma-carbonyl-histidyl-prolineamide citrate (DN-1417); pyroglutamyl-histidyl-proline (TRH-free acid); and histidyl-proline-diketopiperazine (cyclo(His-Pro)). The TRH analogs potentiated the monosynaptic reflex in a dose-dependent manner and the maximal potentiation occurred at about 1 microM. TRH-free acid potentiated the monosynaptic reflex but the maximal potentiation occurred at 100 times the TRH concentration. Cyclo(His-Pro) was totally ineffective. The concentration required to potentiate the monosynaptic reflex by 50% of the maximal response (EC50) was taken as an index for comparing various analogs in relation to TRH. The EC50 values of the analogs did not differ significantly from each other. However, the ratio of the mean value of an analog to that of TRH was of the following order: MK-771 (N- and C-terminally altered) > or = TRH > or = DN-1417 (N-terminal) > or = methyl-TRH > or = RX77368 (C-terminal) >>> TRH-free acid. Cyclo(His-Pro) was ineffective.

Cloning of a cDNA encoding an ectoenzyme that degrades thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH) is an important extracellular signal substance that acts as a hypothalamic-releasing factor, which stimulates the release of adenohypophyseal hormones and functions as a neurotransmitter/neuromodulator in the central and peripheral nervous system. The inactivation of TRH after its release is catalyzed by an ectoenzyme localized preferentially on neuronal cells in the brain and on lactotrophic pituitary cells. This enzyme exhibits a very high degree of substrate specificity as well as other unusual properties. The activity of the adenohypophyseal enzyme is stringently controlled by estradiol and thyroid hormones, indicating that this enzyme itself may serve regulatory functions. Fragments of the enzyme isolated from rat or pig brain were generated by enzymatic digestion or cyanogen bromide cleavage, purified by reverse-phase HPLC, and sequenced. PCR amplification and screening of cDNA libraries from rat brain and pituitary led to the identification and isolation of a cDNA that encodes a protein of 1025 amino acids. The analysis of the deduced amino acid sequence was consistent with the identification of the enzyme as a glycosylated, membrane-anchored Zn metallopeptidase. Furthermore, Northern blot analysis demonstrated that the mRNA levels paralleled the tissue distribution of the enzyme and that in pituitary tissue the transcript levels rapidly increased when the animals were treated with triiodothyronine. Finally, transient transfection of COS-7 cells with this cDNA led to the expression of an active ectopeptidase that displayed the characteristics of the TRH-degrading ectoenzyme.

Dual modulation of K+ currents and cytosolic Ca2+ by the peptide TRH and its derivatives in guinea-pig septal neurones

1. We describe a dual effect of the peptide TRH (thyrotrophin-releasing hormone) and its derivatives at concentrations between 0.1 and 1 microM on the K+ currents and cytosolic Ca2+ concentration in enzymatically dispersed septal neurones. 2. In response to membrane depolarization, septal neurones recorded under whole-cell patch clamp can generate two major K+ currents: (i) a fast and transient K+ current (I(t)), that after a maximum at 2-5 ms inactivates completely at all membrane potentials in less than 50 ms; and (ii) a slowly activating current (I(s)), which reaches a maximum in 15-20 ms and does not exhibit appreciable inactivation during short-lasting voltage pulses. 3. In about 70% of the neurones tested (n = 48) TRH induced a reversible, and often transient, increase of I(t), I(s) or both K+ conductaNces. In approximately 10% of the cells the peptide had an opposite effect and caused a more protracted and partially reversible attenuation of the amplitude of I(t) and I(s). 4. The dual action of TRH on the K+ currents was mimicked by its derivatives but the effects varied depending on their structural relationship with the precursor neuropeptide. The physiological metabolite cyclo-His-Pro and the synthetic analogue methyl-TRH, in which the carboxyl terminus of the molecule is conserved, increased the K+ currents, whereas depression of the K+ conductances was predominantly observed in the presence of TRH-OH, in which the amino end of TRH is maintained intact. 5. In fura-2-loaded unclamped cells, TRH induced either release of Ca2+ from internal stores, Ca2+ entry, or both. With TRH-OH we never observed mobilization of internal Ca2+ but this peptide evoked a large Ca2+ influx. 6. The results demonstrate that the physiological metabolites of brain TRH (cyclo-His-Pro and TRH-OH) have biological activity. TRH and its derivatives exert two types of regulatory actions on the voltage-gated K+ channels and cytosolic Ca2+ concentration in central neurones, which can be explained assuming that TRH and TRH-derived products interact with different subtypes of brain receptors recognizing preferentially either the amino or the carboxyl termini of the TRH molecule.

Activation of CCK-B receptors elevates cytosolic Ca2+ levels in a pituitary cell line

Cytosolic Ca2+ levels ([Ca2+]i) in GH3 cells, a rat anterior pituitary tumor cell line, were monitored with fura-2 by fluorescence measurements. Cholecystokinin octapeptide (CCK-8) produced a transient elevation of [Ca2+]i. The elevation of [Ca2+]i by CCK-8 was inhibited by L-365,260, but not by devazepide. It was still observed when extracellular Ca2+ was eliminated, indicating that CCK-8 mobilizes Ca2+ from intracellular storage sites after interaction with CCK-B receptors. Cholecystokinin octapeptide increased the turnover of phosphatidylinositol, but it did not affect cyclic AMP levels. A possible involvement of phosphatidylinositol breakdown and calcium mobilization in the transduction system of CCK-B receptors in GH3 cells is suggested.

Cloning and expression of the thyrotropin-releasing hormone receptor from GH3 rat anterior pituitary cells

Functional thyrotropin-releasing hormone (TRH) receptors have been expressed in Xenopus laevis oocytes following the microinjection of total and poly(A)+ RNA from GH3 rat anterior pituitary tumour cells. Under voltage-clamp conditions, application of the peptide induced a biphasic Ca(2+)-dependent chloride current. The amplitude of the initial, fast, component of the response was dependent on the concentration of the hormone and on the amount of mRNA injected. Size fractionation of poly(A)+ RNA on a continuous sucrose gradient and Northern blot analysis indicated that the receptor was encoded by an mRNA of approx. 3.5 kb. A 3.28 kbp cDNA encoding the TRH receptor has been cloned and sequenced. Full functionality of the predicted 412-amino-acid receptor protein was demonstrated by functional expression of cell surface receptors in Xenopus oocytes after both cytoplasmic injection of sense RNA transcribed in vitro from this cDNA and nuclear injection of the cDNA under the control of the Herpes simplex virus thymidine kinase promoter. The predicted protein contains seven putative membrane-spanning domains and shows significant sequence identify with some G-protein-coupled receptors. RNA blot analysis indicates that the mRNA for the TRH receptor is exclusively expressed in the pituitary gland. Expression studies performed with clones in which the 3' region of the mRNA has been successively shortened indicate that the 3' terminal region is not an important determinant for efficient functional expression in oocytes.

Ontogeny of thyrotropin-releasing hormone biosynthesis and release in hypothalamic neurons

Thyrotropin-releasing hormone (TRH) is expressed at early postmitotic stages of hypothalamic neuron development, in the mouse and rat, as revealed by the presence of the mature peptide, of pro-TRH mRNAs, and of large precursor forms. This indicates a coordinate expression of several genes encoding, respectively, pro-TRH, its processing enzymes, and the cell machinery for intracellular transport, sorting, and release of TRH. During development, an acceleration of pro-TRH processing is revealed by an increased proportion of the mature peptide. This is correlated with changes in the respective distribution of pro-TRH and TRH along neurites and the ontogenesis of neurosecretory granules.

Synthesis and characterization of a high-affinity photoactivatable analogue of thyrotropin-releasing hormone

An analogue of thyrotropin-releasing hormone (TRH, pGlu-His-ProNH2), i.e. pGlu-His-ProNH-(CH2)6-(4-azidosalicylamide) (TRH-ASA), has been synthesized and, in a radioiodinated form (TRH-IASA), characterized and used as a photoaffinity reagent to label the TRH receptor on rat pituitary GH4C1 cells. TRH-IASA bound to GH4C1 cells with high affinity (Kd = 8 nM), comparable with that of TRH binding. The binding of TRH-IASA was competitive with binding of TRH, two TRH analogues and a TRH receptor antagonist, chlordiazepoxide. TRH-IASA did not bind to or label GH12C1 cells, which lack functional TRH receptors. Labelling of GH4C1 cells with TRH-IASA followed by SDS/PAGE and autoradiography of membrane proteins demonstrated labelling of a single polypeptide which ran as a diffuse band between 71 and 91 kDa, centred at 76 kDa. No change in this labelling pattern was observed as a function of the length of time (between 5 min and 2 h) that GH4C1 cells were incubated with 3 nM-TRH-IASA. Using either a very short (5 s) photolysis interval or low TRH-IASA concentrations, only the 76 kDa band was labelled. Minor bands appeared only after extended photolysis and use of high TRH-IASA concentrations. We conclude that the TRH receptor from rat pituitary GH4C1 cells is a single peptide with an apparent molecular mass of 76 kDa. Details of the chemical synthesis of TRH-ASA are given in Supplementary Publication SUP 50167 (5 pages), which has been deposited at the British Library Document Supply Centre, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1992) 281, 5.

The TRH-like peptide pGlu-Glu-ProNH2 is present in the porcine pituitary but not in reproductive tissues

The peptide pGlu-Glu-ProNH2, which differs from thyrotrophin-releasing hormone (TRH) by only one amino acid, was initially detected and characterised in the rabbit prostate complex and more recently in human semen and rat pituitary. A previous study reported that TRH and a homologous peptide were present in a range of porcine tissues and it was of interest to further characterise these peptides. In this study, high levels of TRH-immunoreactivity have been demonstrated in the porcine pituitary, the majority of which was authentic TRH; although 9% was found to be chromatographically identical to pGlu-Glu-ProNH2. In contrast, TRH-immunoreactivity was not detected in follicular fluid, ovary or prostate. The unexpected finding that pGlu-Glu-ProNH2 is present in the porcine pituitary but absent from regions of the reproductive tract may be of biological significance.

Expression cloning of a cDNA encoding the mouse pituitary thyrotropin-releasing hormone receptor

Thyrotropin-releasing hormone (TRH) is an important extracellular regulatory molecule that functions as a releasing factor in the anterior pituitary gland and as a neurotransmitter/neuromodulator in the central and peripheral nervous systems. Binding sites for TRH are present in these tissues, but the TRH receptor (TRH-R) has not been purified from any source. Using Xenopus laevis oocytes in an expression cloning strategy, we have isolated a cDNA clone that encodes the mouse pituitary TRH-R. This conclusion is based on the following evidence. Injection of sense RNA transcribed in vitro from this cDNA into Xenopus oocytes leads to expression of cell-surface receptors that bind TRH and the competitive antagonist chlordiazepoxide with appropriate affinities and that elicit electrophysiological responses to TRH with the appropriate concentration dependency. Antisense RNA inhibits the TRH response in Xenopus oocytes injected with RNA isolated from normal rat anterior pituitary glands. Finally, transfection of COS-1 cells with this cDNA leads to expression of receptors that bind TRH and chlordiazepoxide with appropriate affinities and that transduce TRH stimulation of inositol phosphate formation. The 3.8-kilobase mouse TRH-R cDNA encodes a protein of 393 amino acids that shows similarities to other guanine nucleotide-binding regulatory protein-coupled receptors.

Thyrotropin-releasing hormone receptor occupancy determines the fraction of the responsive pool of inositol lipids hydrolysed in rat pituitary tumour cells

We report that there are distinct thyrotropin-releasing hormone (TRH)-responsive and -unresponsive pools of inositol (Ins) lipids in rat pituitary tumour (GH3) cells, and present evidence that the size of the responsive pool is determined by the number of activated TRH-receptor complexes. By use of an experimental protocol in which cycling of [3H]Ins is inhibited and resynthesis occurs with unlabelled Ins only, we were able to measure specifically the effects of TRH on the hydrolysis of the Ins lipids present before stimulation. A maximally effective dose of TRH (1 microM) caused a time-dependent decrease in 3H-labelled Ins lipids that attained a steady-state value of 42 +/- 1% of the initial level between 1.5 and 2 h. After 2 h, even though there was no further decrease in 3H-labelled Ins lipids, and no increase in [3H]Ins or [3H]Ins phosphates, turnover of Ins lipids, as assessed as incorporation of [32P]Pi into PtdIns, continued at a rate similar to that in cells incubated without LiCl or unlabelled Ins. These data indicate that Ins lipid turnover was not desensitized during prolonged TRH stimulation. Depletion of lipid 3H radioactivity by TRH occurred at higher TRH doses on addition of the competitive antagonist chlordiazepoxide. Addition of 1 microM-TRH after 3 h of stimulation by a sub-maximal (0.3 nM) TRH dose caused a further decrease in 3H radioactivity to the minimum level (40% of initial value). We propose that the TRH-responsive pool of Ins lipids in GH3 cells is composed of the complement of Ins lipids that are within functional proximity of activated TRH-receptor complexes.

Gonadotropin-releasing hormone and thyrotropin-releasing hormone regulation of g protein function in the rat anterior pituitary lobe

Abstract In order to evaluate the role of guanine nucleotide-dependent transducer proteins (G proteins) in hormone-mediated signal transduction in the anterior pituitary lobe, we examined the effect of gonadotropin-releasing hormone (GnRH) and thyrotropin-releasing hormone (TRH) on two parameters of G protein function, namely [(35) S]GTP(gamma)S binding and low K(m)GTPase activity. Plasma membranes were prepared from anterior pituitary lobes of adult male rats using conventional procedures. GTP binding was determined by incubating 2 to 5 mug membrane protein with approximately 100,000 cpm [(35) S]GTP(gamma)S in a buffer containing 20 mM Tris- HCl, 1 mM EDTA, 1 mM dithiothreitol, and 100 mM NaCI at a pH of 7.4 for 10 or 15 min at 37 degrees C GnRH agonist and TRH stimulated high affinity [(35) S]GTP(gamma)S binding in a concentration-dependent manner. GTP binding was maximally stimulated by GnRH agonist (1 muM) and TRH (0.1 muM) by up to 27% and 34%, respectively. A time-course study revealed that 1 muM GnRH agonist stimulated GTP binding by 30% at 15 min; 0.1 muM TRH stimulated GTP binding by 23% at 1 min, 18% at 5 min and 25% at 10 min. A stable GTP analog, 5'-guanylylimidodiphosphate, inhibited GnRH- as well as TRH-stimulated GTP binding. GnRH antagonist did not affect GTP binding. However, in the presence of the antagonist, stimulation of GTP binding by the GnRH agonist was completely blocked. The low K(m)GTPase activity (EC 3.6.1.-), another parameter of G protein function, was assayed in 2 to 5 mug membrane protein using [gamma-(32) P]GTP at 37 degrees C in an ATP-regenerating buffer containing 1 muM unlabeled GTP. GnRH agonist (0.1 muM) and TRH (1 muM) maximally stimulated this GTPase activity by up to 50% and 40%, respectively. GnRH agonist (1 muM) stimulated the GTPase activity by 30% at 10 min and 48% at 30 min. TRH (1 muM) stimulated the GTPase activity at all time points monitored; stimulation was 46% at 5 min, 49% at 20 min, and 41% at 30 min. Interestingly, the GnRH antagonist stimulated GTPase activity by about 20%, but inhibited GnRH agonist-stimulated GTPase activity in a concentration-dependent manner. These results indicate that the binding of GnRH and TRH to their receptors results in interaction of the receptor with a G protein and activation of the G protein cycle.

A novel substance P binding site in rat brain regions modulates TRH receptor binding

Binding sites for thyrotropin-releasing hormone (TRH) were labelled with [3H](2-Me-His3)TRH ([3H]MeTRH) on membranes from rat brain regions at 0 degrees C for 5 h. Amygdaloid membranes bound [3H]MeTRH with high-affinity (Kd = 3.1 +/- 0.5 nM (n = 4)). Five TRH analogs competed for this binding with the same rank order and with affinities that matched the pharmacological specificity of pituitary TRH receptors. Substance P (SP) and its C-terminal fragments reduced amygdaloid TRH receptor binding in a concentration dependent manner (IC50 for SP = 65 microM). The rank order of potency of SP analogs at inhibiting TRH receptor binding was: SP greater than nonapeptide (3-11) greater than hexapeptide (6-11) greater than heptapeptide (5-11) greater than pentapeptide (7-11). However, other tachykinins were inactive in this system. SP was a potent inhibitor of [3H]MeTRH binding in hippocampus greater than spinal cord greater than retina greater than n. accumbens greater than hypothalamus greater than amygdaloid greater than olfactory bulb greater than or equal to pituitary greater than pons/medulla in parallel assays. In amygdaloid membranes SP (50 microM) reduced the apparent maximum receptor density by 39% (p less than 0.01) without altering the binding affinity, and 100 microM SP induced a biphasic dissociation of [3H]MeTRH with kinetics faster than those induced by both TRH (10 microM) and serotonin (100 microM). In contrast, other neuropeptides such as neurotensin, proctolin, angiotensin II, bombesin and luteinizing hormone releasing hormone did not significantly inhibit [3H]MeTRH binding to amygdaloid membranes.(ABSTRACT TRUNCATED AT 250 WORDS)