Receptor-binding Affinities and Biological Activities of Analogs of Thyrotropin-releasing Hormone in Prolactin-producing Pituitary Cells in Culture

Computational Studies on the Mechanisms of Nonenzymatic Intramolecular Cyclization of the Glutamine Residues Located at N-Termini Catalyzed by Inorganic Phosphate Species

Glutamine (Gln) residues located at N-termini undergo spontaneous intramolecular cyclization, causing the formation of pyroglutamic acid (pGlu) residues. pGlu residues have been detected at the N-termini in various peptides and proteins. The formation of pGlu residues during the fermentation and purification processes of antibody drugs is one of the concerns in the design and formulation of these drugs and has been reported to proceed rapidly in a phosphate buffer. In this study, we have examined the phosphate-catalyzed mechanisms of the pGlu residue formation from N-terminal Gln residues via quantum chemical calculations using B3LYP density functional methods. Single-point energies were calculated using the second-order Møller-Plesset perturbation theory. We performed the calculations for the model compound in which an uncharged N-terminal Gln residue is capped with a methyl amino group on the C-terminal. The activation energy of the formation of pGlu residues was calculated as 83.8 kJ mol^-1, which was lower than that of the typical nonenzymatic reaction of amino acid residues. In addition, the computational results indicate that the flexibility of the main and side chains in N-terminal Gln residues was necessary for the formation of pGlu residues to proceed. In the obtained pathway, inorganic phosphate species act as the catalyst by mediating the proton transfer.

Conformationally restricted TRH analogues: constraining the pyroglutamate region

A modified synthetic route has been developed so that the steric size of constraints added to the pyroglutamate region of TRH (pGluHisProNH(2)) can be varied. Both an analogue with a smaller ethylene bridge and a larger, more flexible propane bridge in this region have been synthesized. These analogues were synthesized in order to probe why the initial incorporation of an ethane bridge into this region of the molecule had led to an analogue with a binding constant and potency three times lower than that of an directly analogous unconstrained analogue. The data for both analogues indicated that the fall off in activity caused by the ethane bridge in the initial analogue was not caused by the size of the bridge.

Antidepressant effects of thyrotropin-releasing hormone analogues using a rodent model of depression

The antidepressant potential of two naturally occurring analogues of thyrotropin-releasing hormone (TRH), pGLU-GLU-PRO-NH2 (EEP) and pGLU-PHE-PRO-NH2 (EFP), were examined using a rodent model of antidepressant efficacy. The Porsolt Swim Test was used to assay the antidepressant properties of these two peptides. Both analogues of TRH produced significant antidepressant effects, with EEP producing the stronger response. No effect of EEP upon triiodothyronine (T3) was observed at the dosage used. EFP, which has previously been demonstrated to crossreact with the TRH receptor, significantly increased serum T3. Since an effect upon T3 was only observed in the weaker of the two compounds, these data suggest that the behavioral effect of EEP was not secondary to stimulation of thyroid hormone. Additionally, the differential behavioral response to the two compounds suggests a degree of sequence specificity in the ability of TRH-like tripeptides to produce an antidepressant effect.

Thyrotropin releasing hormone analogs: a building block approach to the construction of tetracyclic peptidomimetics

A building block based approach was used to synthesize a pair of tetracyclic peptidomimetics that constrain all but one of the rotational degrees of freedom of the hypothalamic tripeptide hormone thyroliberin. One of the analogs bound to the thyroliberin endocrine receptor (TRH-R) with an affinity greater than that of an analog without constraints. The tetracyclic peptidomimetics were found to be partial agonists for the TRH-R receptor.

A refined model of the thyrotropin-releasing hormone (TRH) receptor binding pocket. Experimental analysis and energy minimization of the complex between TRH and TRH receptor

Seven transmembrane (TM) spanning, G protein-coupled receptors (GPCRs) appear to bind large glycoprotein hormones predominantly within their extracellular domains, small nonpeptidic ligands within the TM helical bundle, and peptide ligands within the extracellular domains and TM bundle. The tripeptide thyrotropin-releasing hormone (TRH, pyroGlu-His-ProNH2) may bind entirely within the TM bundle of the TRH receptor (TRH-R). We have previously demonstrated direct binding contacts between the pyroGlu of TRH and two residues in TM helix 3 (TM-3) of TRH-R and proposed a model of the binding pocket of TRH-R [Perlman, J. H., Laakkonen, L., Osman, R., & Gershengorn, M. C. (1994) J. Biol. Chem. 269, 23383-23386]. Here, we provide evidence for two additional direct interactions between TRH and TRH-R. One interaction is between the aromatic ring of Tyr 282 of TM-6 and His of TRH. This is based on a large increase in the half-maximally effective concentration (EC50) of TRH for stimulation of inositol phosphate formation by Y282A TRH-R and a loss of selectivity of this mutant receptor for TRH analogs substituted at His. We provide evidence for another interaction between Arg 306 of TM-7 and the terminal carboxamide of TRH. Using four direct interactions as anchors, a refined model of the TRH-R binding pocket was constructed using geometry optimization through energy minimization. A novel method for modeling GPCRs based on Monte Carlo and stochastic dynamics simulations is presented in the accompanying paper [Laakkonen, L. J., Guarnieri, F., Perlman, J. H., Gershengorn, M. C., & Osman, R. (1996) Biochemistry 35, 7651-7663].

Specific binding of [3H]pGlu-3-Me-His-Pro-NH2 ([3H]MeTRH) to hypothalamic membranes of juvenile rainbow trout, Oncorhynchus mykiss

We investigated the existence and nature of specific [3H]pGlu-3-Me-His-Pro-NH2 ([3H]MeTRH) binding sites in juvenile rainbow trout (Oncorhynchus mykiss) hypothalamus. Washed hypothalamic membranes were incubated with [3H]MeTRH in the absence (B0) or presence of pGlu-His-Pro-NH2 (TRH) or MeTRH under various experimental paradigms; incubations were terminated by filtration and bound radioactivity was determined by liquid scintillation spectroscopy. Specific binding (Bsp) was tissue dependent, associable, dissociable, and thermolabile. Estimated rates of association (k+1) and dissociation (k-1) were 1.64 x 10(7) M-1 min-1 and 1.98 x 10(-2) min-1, respectively, providing a kinetically derived dissociation rate constant (Kd) of 1.21 x 10(-9) M. [3H]MeTRH binding was displaceable; LIGAND-analysis of three independent homologous displacement experiments consistently indicated a single class of binding sites with an average Kd = 6.91 (+/- 4.32) x 10(-9) M and average maximum binding capacity (Bmax) of 8.84 (+/- 2.72) x 10(-15) mol/mg protein. Native TRH also displaced the radiolabel in a dose dependent manner; LIGAND-estimates for Kd and Bmax were 1.52 (+/- 0.12) x 10(-9) M and 3.79 (+/- 0.99) x 10(-15) mol/mg protein (n = 3 experiments), respectively. Our data indicate that presence of a single class of specific high-affinity TRH-binding sites in the rainbow trout hypothalamus; these findings suggest a role for TRH in regulating the release of hypophysiotrophic factors in the teleost hypothalamus.

Properties of monoclonal antibodies to thyroliberin (TRH) induced by different immunogens: comparison with pituitary TRH receptor

Thyroliberin E-H-P-NH2 (TRH) is a small neuropeptide pGlu-His-Pro-NH2 widely distributed in neural sites. The aim of this work was to obtain an antibody molecule with the nearest properties to that of TRH-receptor in GH3 cells. Different TRH-protein conjugates were prepared and utilized to induce monoclonal antibodies in mice. Several monoclonal antibodies were obtained using E-H-P-NH2 (TRH) coupled either to the histidyl residue (immunogen I) or to the prolyl residue (immunogen II). Antibodies generated using immunogen I and immunogen II were characterized in a radioimmunoassay system and an enzyme immunoassay system respectively. Their selectivities regarding a series of TRH related peptides were compared to those of rabbit polyclonal antibodies using three differently labelled TRH (tritiated-TRH, mono-iodinated-TRH and TRH-OH-acetyl-cholinesterase) as tracers and to prolactin secreting cells TRH receptors using 3H-TRH. Whatever the immunogen, the stereospecificity of monoclonal antibodies tested were found more different from TRH receptor characteristics than rabbit polyclonal antibodies.

Analogs of thyrotropin-releasing hormone (TRH): receptor affinities in brains, spinal cords, and pituitaries of different species

[3H](3-Me-His2) thyrotropin-releasing hormone ([3H]MeTRH) bound to TRH receptors in rodent, rabbit and dog brain and spinal cord (SC), and in rat, sheep, bovine and dog anterior pituitary (PIT) glands, with high affinity (dissociation constants, KdS = 5-9 nM; n = 3-4) but to different densities of these sites (Bmax range 6-145 fmol/mg protein) (rabbit SC greater than sheep PIT much greater than G.pig brain greater than dog brain greater than rat brain greater than bovine and dog PIT). Various TRH analogs competitively inhibited [3H]MeTRH binding in these tissues with a similar rank order of potency: MeTRH greater than TRH greater than CG3703 greater than or equal to RX77368 greater than or equal to MK-771 greater than TRH Glycinamide greater than Glu1-TRH much greater than CG3509 greater than or equal to NVal2-TRH much much greater than TRH free acid much much greater than and cyclo-His-Pro, indicating a pharmacological similarity of CNS and pituitary TRH receptors. While most TRH analogs displaced [3H]MeTRH binding with a similar potency in the different species, TRH exhibited a 2-fold lower affinity in the rat and G.pig brain than in other tissues of other species. Similarly, CG3703 was 2.4-4.5 times more active in the rabbit brain than in the rodent and dog brain, and also more potent in the rabbit brain as compared to the sheep PIT.(ABSTRACT TRUNCATED AT 250 WORDS)

Norvaline2-TRH: binding to TRH receptors in rat brain homogenates

Norvaline2-thyrotropin-releasing hormone ([Nva2]TRH) has been described as a thyrotropin-releasing hormone (TRH) analog with no thyrotropin (TSH)-releasing capacity but enhanced analeptic activity compared with TRH, as shown by the reversal of haloperidol-induced catalepsy. We have evaluated the receptor-binding properties of [Nva2]TRH in homogenates of rat anterior pituitary, hypothalamus, brainstem and cortex tissue, using [3H]TRH and [3H][3-Me-His2]TRH as radioligands. Apparent Ki values at high affinity TRH-binding sites, labelled predominantly by [3H][3-Me-His2]TRH, ranged from 17.0 to 36.9 microM in all tested regions. Additionally, [Nva2]TRH was shown to compete with [3H]TRH at low affinity TRH-binding sites with similar affinities. It is concluded that the loss of TSH-releasing activity of [Nva2]TRH appears to be due to a drastic reduction in binding affinity to the high affinity TRH receptor subtype. Its analeptic activity, however, may be mediated by low affinity TRH binding sites which are predominantly labelled by [3H]TRH or by yet unidentified mechanisms.

Quantitative autoradiography of TRH receptors in discrete brain regions of different mammalian species

The results clearly show marked heterogeneity and ubiquity of the CNS distribution of TRH receptors across several mammalian species including man. The use of high resolution autoradiography coupled with image analysis has permitted the visualization and quantification of TRH receptor density in even very small regions and nuclei of the CNS. This technique will undoubtedly help elucidate the other areas of TRH receptor localization that have thus far escaped detection in mammals and that are yet to be studied in lower vertebrates. Although an attempt has been made to correlate the presence of the peptide, its receptors, and its possible physiological functions, only further detailed physiological/behavioral investigations will ultimately unravel and support the diverse neurotransmitter and trophic roles of TRH in CNS and endocrine function.

Naloxone-like influence of TRH and ACTH-(4-7) on hypothalamic blood flow autoregulation in the rat

The influence of intracerebroventricularly (ICV) administered thyrotropin-releasing hormone pGlu-His-Pro-NH2 (TRH), pGlu-His-Phe-NH2 (TRH analog, (TRHa)), Met-Glu-His-Phe(ACTH-(4-7)) and His-Phe-Arg-Trp-Gly (ACTH-(6-10)) on autoregulation of cerebral blood flow was studied in anesthetized, ventilated rats. Autoregulatory capacity of the cerebrovascular bed was tested by hypothalamic blood flow (HBF) and total cerebral blood volume (CBV) determinations during consecutive stepwise lowering of the systemic mean arterial pressure to 80, 60 and 40 mmHg, by hemorrhage. None of the peptides caused a change in resting HBF or CBV upon ICV administration (5 micrograms/kg). However, the same dose of TRH, TRHa and ACTH-(4-7) resulted in impairment of autoregulation. ACTH-(6-10) was not effective. Thus, the disturbed autoregulation may be due to the presence of the dipeptide Glu-His which is common to TRH, TRHa and ACTH-(4-7).

Ontogeny of receptors for thyrotropin-releasing hormone in the rat brain

Developmental changes in the thyrotropin-releasing hormone (TRH) receptor concentrations of hypothalamic and certain extrahypothalamic structures in the rat were evaluated from 2 days after birth until sexual maturity. The maturational pattern of TRH receptors in hypothalamus, striatum and amygdala follow that of the pituitary and all show the same kinetic properties. The developmental pattern also coincides with that of other components of the hypothalamic-pituitary-thyroid axis. In all tissues, the concentration of receptors increased in early life to reach a maximum between 10 and 30 days of age and then decreased gradually to adult values. Development of TRH receptors reflected changes in the number of receptor sites but not in binding affinity, which remained constant and suggests that the TRH receptor is identical in all 4 tissues studied. These results indicate the parallel development of pituitary and central nervous system TRH receptors.

Use of GH4C1 cell variants to demonstrate a non-spare receptor model for thyrotropin-releasing hormone action

Thyrotropin-releasing hormone (TRH) stimulates maximally both the release of previously synthesized prolactin and the de novo synthesis of prolactin by GH4C1 rat pituitary cells at concentrations less than those necessary to fully occupy the TRH receptor at equilibrium. We have examined the dependency of maximal TRH-enhanced prolactin release and synthesis on receptor number using GH4C1 cell variants with different numbers of TRH receptors. GH4C1 cell variants with increased and decreased numbers of TRH receptors were selected by using a morphological response known as stretching which renders the cells more adherent to the tissue culture substrate. We found that maximal TRH-enhancement of prolactin release or synthesis increased proportionally to the number of TRH receptors per cell, indicating that spare receptors do not exist for TRH on these GH4C1 cells. We also found that occupancy of the TRH receptor by the analogue, N3im-methyl-TRH (MeTRH), in contrast to TRH, closely paralleled stimulated prolactin release in a manner consistent with Clark's receptor-occupancy model. We conclude that differences between apparent Kd and ED50 for TRH do not necessarily result from spare receptors in GH4C1 cells.

Limbic, hypothalamic, cortical and spinal regions are enriched in receptors for thyrotropin-releasing hormone: evidence from [3H]ultrofilm autoradiography and correlation with central effects of the tripeptide in rat brain

Light microscopic autoradiographic localization of specific recognition sites for thyrotropin-releasing hormone (TRH) was determined on thin sections of rat brain using the potent analogue [3H](3-Me-His2)-TRH ([3H]MeTRH). Microdensitometric analysis of the relative optical densities of TRH receptor labelling revealed the following brain regional enrichment: lateral and cortical amygdaloid nuclei greater than ventral dentate gyrus greater than n. accumbens greater than medial septum greater than piriform cortex greater than paraventricular thalamic and hypothalamic nuclei greater than preoptic area greater than diagonal band of Broca greater than lateral septum greater than I-IV layers of frontoparietal cortex greater than dorsal hippocampus greater than olfactory tubercle greater than caudate putamen; globus pallidus. In the spinal cord the apparent relative enrichment of TRH receptors was: substantia gelatinosa = central canal gray greater than ventral gray greater than dorsal gray (layers III-VII) greater than white matter. This heterogeneous distribution of TRH binding sites correlated well with our previous data obtained from membrane binding studies. Furthermore, the specific anatomical localization of receptors for TRH in many nuclei was consistent with those loci involved in the mediation of many physiological and behavioural actions of the peptide in rodent brain.

The interaction of trh and dopaminergic mechanisms in the regulation of stimulated prolactin release in man

The manner by which dopaminergic and TRH mechanisms interact to control PRL release is not known. Whilst dopamine receptor antagonists and TRH both release PRL, it is not known if the PRL released by these two mechanisms reflects similar aspects of physiological control, or if PRL responses to these mechanisms of release can be dissociated. We addressed this question by studying the PRL responses to maximal stimulatory dose of TRH and domperidone (a DA receptor antagonist), which were administered sequentially, simultaneously or separately on different occasions. Six normal volunteers undertook three sets of studies: (1) standard PRL stimulation tests to 400 micrograms TRH, 5 mg domperidone or simultaneous TRH/domperidone administration, (2) domperidone bolus-infusion study in which either 5 mg domperidone or 400 micrograms TRH was administered i.v. at 120 min during a 240 min infusion of domperidone (50 micrograms/min) which was preceded by a 5 mg i.v. bolus of the drug, and (3) TRH bolus-infusion study in which domperidone or TRH was administered i.v. at 120 min during a 240 min infusion of TRH (0.4 micrograms/min) which was preceded by a 400 micrograms i.v. bolus of the drug. In Study 1, simultaneous TRH/domperidone administration induced an incremental rise in PRL (5195 +/- 940 mIU/l) which was significantly greater (P less than 0.0005) than with either domperidone (3730 +/- 825 mIU/l) or TRH (1335 +/- 300 mIU/l) alone. In study 2, TRH administration at 120 min resulted in a significant rise (P less than 0.01) in PRL (delta PRL 960 +/- 232 mIU/l) whilst the second dose of domperidone did not, thus suggesting that the initial bolus and subsequent infusion had resulted in complete DA receptor blockade. In Study 3, domperidone administered at 120 min induced a marked rise in PRL (delta PRL 3609 +/- 963 mIU/l). In contrast, the corresponding TRH stimulus resulted in a small rise (delta PRL 142 +/- 32 mIU/l) suggesting that the PRL release induced by the initial bolus and subsequent infusion had been near maximal. Thus, TRH is able to induce significant PRL release in the presence of maximal DA receptor blockade, and domperidone, in the presence of maximal TRH stimulation, is also capable of inducing significant PRL release. These observations together with the ability of TRH/domperidone to induce a greater PRL response than either agent alone, suggest that each stimulus has a specific releasing action on a fraction of intracellular PRL which is not accessible to the other.(ABSTRACT TRUNCATED AT 400 WORDS)

TRH-induced membrane hyperpolarization in rat clonal anterior pituitary cells

Thyrotropin-releasing hormone (TRH) induces biphasic membrane potential changes, a transient hyperpolarization followed by a prolonged enhancement of the generation of action potentials in the clonal GH3 pituitary cell. The nature of the TRH-induced hyperpolarization was studied in Cl--free solutions. Among various test substances, only TRH and its analogue, which stimulates the release of prolactin from the GH3 cells, were capable of inducing the transient membrane hyperpolarization. The Ca2+ ionophore A23187 also caused a transient hyperpolarization accompanied by an increase in the membrane conductance, although it failed to mimic the late facilitation of spike generation. The reversal potential of the TRH-induced hyperpolarization was identical with that induced by A23187. Reduction of the K+ concentration of the bathing medium caused a similar shift of both these reversal potentials toward a more hyperpolarized level. Injection of the Ca2+-chelator EGTA into the cell suppressed both TRH and Ca2+ ionophore-induced hyperpolarizations. These results suggest that TRH mobilizes the cellular-bound Ca, which in turn activates Ca2+-mediated K+ channels, thus causing the transient membrane hyperpolarization. The relationship between the membrane hyperpolarization and the TRH-stimulated hormone release is discussed.

TRH receptors in fish

The brains of goldfish and other teleosts contain high-affinity binding sites for [3H][3-methyl-His2]thyrotropin-releasing hormone [( 3H]MeTRH) which closely resemble TRH receptors in mammalian brain and pituitary gland in apparent dissociation constant (KD = 3-4 nM), in pharmacology for eight TRH analogs, and in exhibiting marked regional differences in the density of binding sites, with highest binding in the cerebrum and lowest in the cerebellum. Fish brain differs from mammalian brain in containing a prominent additional class of much lower affinity [3H]MeTRH binding sites (KD about 15 microM), of unknown nature, which also exhibit regional differences. Fish pituitary glands contain high-affinity [3H]MeTRH binding sites, but insufficient tissue has prevented full characterization. Little or no saturable binding of [3H]MeTRH was detected in several goldfish peripheral tissues.

Sulfhydryl groups in receptor binding of thyrotropin-releasing hormone to rat amygdala

Binding of [3H]-[3-Me-His2]thyrotropin-releasing hormone [( 3H]MeTRH) to TRH receptors in rat amygdala was decreased by sulfhydryl reagents in a time-, temperature-, and concentration-dependent manner. A pronounced reduction in receptor density, with little or no change in binding affinity, was apparent following disulfide bond reduction by dithiothreitol (DTT), alkylation of thiol groups by N-ethylmaleimide (NEM), and their oxidation by 5,5'-dithiobis (2-nitrobenzoic acid). Heavy metals (Cd2+, Hg2+), which complex with reactive -SH residues, also potently inhibited binding. The pharmacological specificity of residual [3H]MeTRH binding in chemically modified amygdala membranes was the same as that in control preparations. Sequential exposure to thiol reagents, in the presence or absence of cations, revealed possible additive effects. Pretreatment of membranes with TRH (10(-8) - 10(-6) M), and its continued presence during modification, afforded protection against DTT and NEM. These results indicate the possible importance of thiol groups in the maintenance of TRH receptor conformation.

Biochemical similarity of rat pituitary and CNS TRH receptors

Chemical properties of receptor binding sites for thyrotropin-releasing hormone (TRH) in rat pituitary, retina, amygdala and hypothalamus were compared by examining the influence of sulfhydryl reagents on specific binding of [3H](3-Me-His2)-TRH ([3H]MeTRH). Dithiothreitol-induced reduction of disulfide bonds, alkylation of thiol residues by N-ethylmaleimide and their oxidation by 5,5-dithiobis(2-nitrobenzoic acid) (DTNB), all produced marked reduction of [3H]MeTRH binding, which could be prevented in part by preincubation with exogenous TRH. In all tissues, concentration-dependent loss of binding activity was observed following exposure to micromolar heavy metals and mono- and divalent cations, with apparent additive effects between cations and DTT and NEM. Most changes appeared to reflect only a decrease in receptor density (Bmax). The similar sensitivity of all tissues to these compounds complements existing evidence for a close resemblance of TRH receptors in the CNS and pituitary.

Receptors for thyrotropin-releasing hormone (TRH) in rabbit spinal cord

Specific binding of 3H-labeled [3-Me-His2]TRH ([3H]MeTRH) to membranes of rabbit spinal cord (thoracic) involved a homogeneous population of high-affinity sites (Kd = 2.7 +/- 0.17 (n = 5) nM, Bmax = 204 +/- 12(5) fmol/mg protein). TRH analogs competed for the binding in the following rank order of potency: MeTRH greater than TRH greater than TRH-Gly-NH2 greater than Ser-His-Pro-NH2 greater than Thr-His-Pro-NH2 greater than pGlu-His-Pro-NH-C2H5 greater than TRH-free acid. Competition experiments with rat amygdala, run in parallel with rabbit spinal cord, revealed a closely similar pattern of activity. These properties help identify binding sites in the rabbit spinal cord as physiological receptors for TRH. The binding sites resemble receptors previously demonstrated in pituitary and CNS tissues of other species. The densities of [3H]MeTRH binding sites in different segments were generally similar, although density in the thoracic segment appeared to be somewhat higher. In all segments, binding seemed to be enriched in the dorsal gray matter. Dorsal roots and their associated ganglia, however, displayed little or no binding.

Effects of TRH on cell proliferation of rat pituitary cells (GH3)

Chronic treatment (more than 3 d) of GH3 cells, cloned rat pituitary cells producing prolactin, with 100 nM TRH resulted in a 41% reduction in the rate of cell growth in a medium containing 0.5% fetal bovine serum. These effects of TRH appeared both in the medium containing a higher concentration of serum and in that containing six growth factors, i.e. insulin, transferrin, parathyroid hormone, fibroblast growth factor, triiodothyronine, and multiplication-stimulating activity (MSA) instead of serum. TRH stimulated prolactin production by GH3 cells in a dose-dependent manner both in the serum-supplemented and serum-free media. On the other hand, TRH, at 1 nM, elicited a 130% stimulation in the cellular growth, whereas, at concentrations of more than 10 nM, it inhibited the growth significantly. In the defined culture system, it was demonstrated that TRH stimulated prolactin production in the presence or absence of six growth factors, whereas its inhibitory effects on cellular growth appeared only in the presence of MSA regardless of the presence or absence of the other five factors. Furthermore, it was shown that a dose-dependent stimulatory effect of MSA on the growth of GH3 cells was suppressed by TRH. TRH exhibited only a stimulatory effect on cellular growth in the medium containing the five factors other than MSA. In conclusion, TRH could inhibit cell growth of GH3 in the presence of MSA in the defined medium or MSA-like factor(s) in the serum-supplemented medium.

TRH receptor binding in retina and pituitary: major species variation

Retinas and anterior pituitary glands of nine readily available mammalian and one avian species have been examined for their TRH-sensitive binding of [3H]-[3-Me-His2]TRH. Among mammals, major species variations in TRH receptor binding have been detected in both tissues, amounting to about 100-fold in the retina and 20-fold in the pituitary. In the retina, TRH receptor binding was very high in the rat, quite high in sheep and guinea pig, intermediate in rabbit, pig, and mouse, and low but detectable in chicken, cat, calf and dog. In the pituitary gland, binding was very high in sheep, quite high in rabbit, rat, pig, calf and guinea pig, intermediate in chicken, and fairly low in mouse, cat, and dog. A number of possible interfering variables, including affinity differences, albino vs. non-albino strains, barbiturate anesthesia, time after death, sex and estrous cycle, age, and history of light exposure, were considered and/or tested directly, with generally negative results.

Characteristics of thyrotropin releasing hormone (TRH) receptors in rat brain

Characteristics of TRH-receptors were studied in the rat central nervous system (CNS). Ion species, pH and temperature importantly influenced TRH-receptor binding. Subcellular distribution of TRH-receptor binding revealed that synaptic membranes had the greatest percentage of total sites. Scatchard analysis suggested that the rat CNS had two distinct TRH binding sites with apparent dissociation constants (Kd) of 5 X 10(09) M and 13 X 10(-8) M. Receptor activity is sensitive to trypsin and phospholipase A digestion, suggesting that protein and phospholipid moieties are essential for the binding of [3H]TRH. Thiol reagents reduced the binding activity of the receptor, suggesting that an intrachain disulfide bond may form an important constituent of the binding site for TRH. The TRH-receptor in the rat brain was successfully solubilized with non-ionic detergent Triton X-100. On gel chromatography with Sepharose 6B column, the solubilized TRH-receptor molecule eluted at the fraction corresponding to an apparent molecular weight of 300,000 daltons, with Stokes' radius of 5.8 nm. The regional distribution of TRH-receptor binding was examined to clarify the site of TRH action. The highest level of binding was in the hypothalamus, cerebral cortex and hippocampus, indicating that TRH affects the CNS function mainly through the limbic system, cerebral cortex and hypothalamus. Moreover, tricyclic anti-depressants and Li+ decreased the binding of [3H]TRH. These findings suggest that endogenous TRH and TRH receptor may play the role of a neurotransmission modulator in the brain to control emotional and mental functions.

In vivo autoregulation of rat adenohypophyseal thyrotropin-releasing hormone receptor

The possible mechanism of attenuation of thyrotropin response to exogenous thyrotropin-releasing hormone in vivo after repeated administrations of the releasing hormone has been studied. To this end, the effect of prolonged hormone treatment on the binding of hormone to its receptor in the anterior pituitary gland has been evaluated. The data show that prolonged hormonal treatment resulted in a reduction in the number (B max) but no the binding affinity (KD) of the receptor. The effect was reversible and depended on the duration of treatment. This phenomenon of down regulation or the decrease in the receptor number was found not to be due to either the metabolism of releasing hormone or its ability to activate pituitary-thyroid-axis.

Species differences in the brain regional distribution of receptor binding for thyrotropin-releasing hormone

A survey of the regional distribution of binding of 1 nM [3H](3-Me-His2)thyrotropin-releasing hormone ([3H]MeTRH) to TRH receptors in the brains of eight mammalian species revealed major species differences in both the absolute and relative values of TRH receptor binding in different brain regions. Several brain regions exhibited binding equal to or exceeding that in the anterior pituitary gland of the same species, including the amygdala in the guinea pig and rat, the hypothalamus in the guinea pig, the nucleus accumbens in the rabbit, and all these and other regions in the cat and dog, for which pituitary binding was exceptionally low. Species could be divided into two groups according to which brain region appeared highest in binding: rabbits, sheep, and cattle had highest binding in the nucleus accumbens/septal area, whereas guinea pigs, rats, dogs, cats, and pigs had highest binding in the amygdala/temporal cortex area. The nucleus accumbens consistently exceeded the caudate-putamen in receptor binding. For most brain regions, rabbits, rodents, and sheep tended to be higher than carnivores, cattle, or pigs. Further regions that exhibited appreciable binding in most species included the olfactory bulb and tubercle, hippocampus, and various cortical and brain stem areas. In fact, essentially all brain regions appeared to have detectable levels of TRH receptors in at least some species, but no rat peripheral tissues have yet shown detectable receptor binding. The species differences appeared to reflect largely if not entirely differences in receptor density, although this was not tested in every species.

Properties of [3H](3-Me-His2)TRH binding to apparent TRH receptors in the sheep central nervous system

[3H](3-methyl-His2)thyrotropin releasing hormone ([3H]MeTRH) binds to sites in the sheep central nervous system (CNS) whose properties closely resemble both those of CNS binding sites for [3H]TRH and those of pituitary binding sites for [3H]MeTRH. Detailed studies for binding of [3H]MeTRH in the sheep nucleus accumbens and retina have yielded equilibrium dissociation constants of about 4 nM and densities of binding sites of about 3 and 2 pmol/g wet weight, respectively. The binding affinity of [3H]MeTRH was 8- to 10-fold higher than that of [3H]TRH, resulting in much lower non-specific binding with the new ligand. The association reaction had a rate constant of about 2-3 x 10(7) M-1 min-1, while the biphasic dissociation reaction had rate constants of 8-9 x 10(-2) min-1 for the fast phase and 1-2 x 10(-2) min-1 for the slow phase. The regional distribution of binding in the sheep CNS was similar to that observed previously with [3H]TRH. Highest binding outside the pituitary was in the nucleus accumbens area and retina, with another peak in the amygdala-temporal cortex area. Binding was widely distributed, so that no CNS region appeared totally devoid of binding. Nineteen TRH analogs, ranging in potency over 6 orders of magnitude, showed nearly identical abilities to complete for binding of [3H]MeTRH in the CNS areas and in the sheep anterior pituitary gland in side-by-side experiments. These findings argue strongly for identification of [3H]MeTRH binding sites in the CNS as TRH receptors.