Synthesis and central nervous system actions of thyrotropin-releasing hormone analogues containing a dihydroorotic acid moiety

A series of thyrotropin-releasing hormone (TRH) analogues in which the pyroglutamic acid residue was replaced by (S)-4,5-dihydroorotic acid (Dio-OH) and the related derivatives were prepared. Their central nervous system actions based on spontaneous locomotor activity, antagonistic effect on reserpine-induced hypothermia, and antagonistic effect on pentobarbital anesthesia were evaluated and the structure-activity relationships are discussed. Of these, (1-methyl-(S)-4,5-dihydroorotyl)-L-histidyl-L-prolinamide (14b) showed the most potent activities, which were 30-90 times greater than those of TRH. Moreover, the thyrotropin-releasing activity of 14b was about 50 times weaker than that of TRH, and compound 14b (TA-0910) was selected as a potent candidate.     

Centrally acting and metabolically stable thyrotropin-releasing hormone analogues by replacement of histidine with substituted pyridinium

Metabolically stable and centrally acting thyrotropin-releasing hormone (TRH) analogues were designed by replacing the central histidine with substituted pyridinium moieties. Their analeptic and acetylcholine-releasing actions were evaluated to assess their potency as central nervous system (CNS) agents. A strong experimental connection between these two CNS-mediated actions of the TRH analogues was obtained in subject animals. The analogue 3-(aminocarbonyl)-1-(3-[2-(aminocarbonyl)pyrrolidin-1-yl]-3-oxo-2-[[(5-oxopyrrolidin-2-yl)carbonyl]amino]propyl)pyridinium (1a) showed the highest (TRH-equivalent) potency and longest, dose-dependent duration of action from a series of homologous compounds in antagonizing pentobarbital-induced narcosis when administered intravenously in its CNS-permeable prodrug form (2a) obtained via reduction of the pyridinium moiety to the nonionic dihydropyridine. The maximum change in hippocampal acetylcholine concentration upon perfusion of the pyridinium-containing tripeptides into the hippocampus of rats was also achieved with 1a. No binding to the endocrine TRH receptor was measured for the TRH analogues reported here; therefore, our design afforded a novel lead for centrally acting TRH analogues. We have also demonstrated the benefits of the prodrug approach on the pharmacokinetics and brain uptake/retention of pyridinium-containing TRH analogues (measured by in vivo microdialysis sampling) upon systemic administration.     

Stimulus properties of thyrotropin-releasing hormone

Male Sprague-Dawley rats were trained in a two-lever operant discrimination task using 20 mg/kg thyrotropin-releasing hormone (TRH) and saline as cues. Following completion of 40 daily training sessions, 22 of 25 subjects demonstrated a high level of discriminative responding based on the TRH and saline cues. An evaluation of the time course of TRH indicated that the stimulus properties peak between 5 and 15 min and dissipate substantially by 55–65 min. During additional testing, rats showed dose-dependent generalization between the training treatments (20 mg/kg TRH and saline) and novel doses of TRH (1, 5, 10, and 40 mg/kg). However, animals failed to show generalization between the training drug (20 mg/kg TRH) and d-amphetamine sulfate (0.8, 1.6, or 2.4 mg/kg); likewise, animals trained to discriminate d-amphetamine (0.8 or 1.6 mg/kg) from saline failed to show generalization between d-amphetamine and TRH (10, 20, or 30 mg/kg). Microgram quantities of TRH (2.5–25 g) administered into either the lateral or third ventricle elicited dose-dependent generalization to the training drug (TRH 20 mg/kg, i.p.), suggesting a CNS mechanism of action for this effect of TRH.     

Synthesis and central nervous system actions of thyrotropin-releasing hormone analogs containing a 1-substituted 2-oxoimadazolidine moiety*

A series of thyrotropin-releasing hormone (TRH) analogs in which the pyroglutamic acid residue was replaced by (S)-2-oxoimidazolidine-4-carboxylic acid (Oic-OH) and the related derivatives was prepared, and the central nervous system (CNS) actions were examined. Of these, 1-benzyl-Oic-His-Pro-NH₂ (2c) showed the most potent activities, which were 1.5-8 times greater than those of TRH. Moreover, the thyrotropin (TSH)-releasing activity of 2c was about 1/16 times weaker than that of TRH.     

Effects of hypothermia on thyrotropin-releasing hormone content in the rat brain

The thyrotropin-releasing hormone (TRH) content in the brain was determined in normothermic and hypothermic rats subjected to immobilization stress. TRH contents in the hypothalamus, midbrain and cerebral cortex significantly decreased during mild hypothermia (body temperature about 34°C), but not during profound hypothermia (about 24°C). The decreases in the TRH content during mild hypothermia were readily reversed by rewarding the animal. These results indicate that cerebral TRH is involved in the response to a mild body temperature drop when the animal is exposed to a cold environment.     

Interaction between thyrotropin-releasing hormone and the mesolimbic dopamine system

The involvement of the mesolimbic dopamine (DA) system in the excitatory behavioral effects of thyrotropin-releasing hormone (TRH) has been a controversial topic. In this study TRH was injected into the nucleus accumbens, lateral ventricles or ventral tegmental area and changes in spontaneous motor activity and metabolism of DA in the nucleus accumbens and striatum measured. Injection of TRH into all three areas of the brain produced an increase in photocell counts of locomotor activity and, in the nucleus accumbens, a significant decrease in photocell counts of rearing was measured. Injection of TRH into the nucleus accumbens caused a marked increase in metabolism of DA in both the nucleus accumbens and striatum. A smaller increase in metabolism of DA was also observed after injection of TRH into the lateral ventricles, but no significant change was found after intra-ventral tegmental administration of TRH. These data indicate that while TRH probably acts in the nucleus accumbens to enhance the metabolism of DA, and presumably release of DA, the excitatory behavioral effect of TRH is only partially mediated by this dopaminergic mechanism.     

Carboxyl tail cysteine mutants of the thyrotropin-releasing hormone receptor type 1 exhibit constitutive signaling: role of palmitoylation

We studied the role of carboxyl tail cysteine residues and their palmitoylation in constitutive signaling by the thyrotropin-releasing hormone (TRH) receptor type 1 (TRH-R1) in transfected mammalian cells and in Xenopus laevis oocytes. To study palmitoylation, we inserted a factor Xa cleavage site within the third extracellular loop of TRH-R1, added a carboxyl-terminal C9 immunotag and expressed the mutant receptor in Chinese hamster ovary cells. We identified TRH-R1-specific palmitoylation in the transmembrane helix-7/carboxyl-tail receptor fragment mainly at Cys-335 and Cys-337. In contrast to a mutant truncated at Cys-335 that was reported previously to be constitutively active, a receptor truncated at Lys-338 (K338Stop), which preserves Cys-335 and Cys-337, and C337Stop and N336Stop, which preserve Cys-335, did not exhibit increased constitutive signaling. TRH-R1 mutants substituted singly by Gly or Ser at Cys-335 or Cys-337 did not exhibit constitutive signaling. By contrast, substitution of both cysteines (C335G/C337G or C335S/C337S) yielded TRH-R1 mutants that exhibited marked constitutive signaling in mammalian cells. In the oocyte, constitutive signaling by C335G/C337G resulted in homologous (of C335G/C337G) and heterologous (of M1 muscarinic receptor) desensitization. Because both Cys-335 and Cys-337 have to be substituted or deleted for constitutive signaling, we propose that a single palmitoylation site in the proximal carboxyl tail is sufficient to constrain TRH-R1 in an inactive conformation.     

Ethoxyformylation of histidine residues in equine growth hormone

Reactivity of histidine residues in equine growth hormone to ethoxyformic anhydride was studied. The existence of two kinetically different sets was demonstrated: one of them including only the slow reacting histidine 169 (k = 0.164min^-1) and the other containing fast reacting histidines 19 and 21 (k = 0.892 min^-1). A correlation between the decrease in the capacity to compete with ¹²⁵I-labeled hormone for rat liver binding sites and the degree of ethoxyformylation of the fast group was found. Circular dichroism studies indicated no significant conformational changes in the protein with all three residues modified. These results fully agree with those obtained for bovine growth hormone which is further evidence supporting the vinculation of histidines 19 and/or 21 with the binding site of these hormones to their specific receptors.     

Solubilization and characterization of pituitary thyrotropin-releasing hormone receptors

Thyrotropin-releasing hormone (TRH) receptors were solubilized from a rat pituitary tumor cell line, GH4C1, with digitonin. Convenient assays were developed based on the ability of hydroxylapatite and polyethyleneimine-soaked glass fiber filters to adsorb the solubilized [3H]methyl-TRH-receptor complex but not free [3H]methyl-TRH. The kinetics of [3H]methyl-TRH binding to solubilized receptors were extremely temperature dependent. Binding reached equilibrium at 10-20 nM [3H]methyl-TRH in 30 min at 23 degrees and 6 hr at 0 degree. The half-times for dissociation were less than 5 min at 23 degrees and 7.6 hr at 0 degree. Equilibrium binding experiments yielded linear Scatchard plots at 0 degree with Kd = 3 nM, whereas the Kd was greater than 20 nM at 23 degrees. A series of TRH congeners displaced [3H]methyl-TRH with the rank order reported for membrane receptors, N3-methyl-HisTRH greater than or equal to TRH greater than pGlu-N3-methyl-HisProNH(CH2)6NH2 greater than or equal to pGluHisProNH(CH2)6NH2 greater than pGluHisTyrNH2 much greater than TRH free acid. The antagonist chlordiazepoxide exhibited an IC50 of 10 microM. [3H]methyl-TRH binding to solubilized receptors displayed a broad pH optimum, from 6.5 to 7.5. The solubilized receptor could be obtained from cultured GH4C1 cells and in much larger quantities from GH4C1 tumors. Tumors from 12 rats yielded greater than 700 pmol of specific soluble TRH binding activity (1 g of protein). The solubilized receptor could be purified 10-20-fold by chromatography on wheat germ agglutinin columns and could be concentrated by adsorption on either DEAE-Sephadex or hydroxylapatite. The procedures outlined allow the solubilization of pituitary TRH receptors from a rich and abundant source, the rapid and reproducible assay of [3H]methyl-TRH binding, and substantial enrichment of receptor activity. These findings should be valuable for the purification and identification of the TRH receptor protein.     

Aspects of chronic oral treatment with thyrotropin-releasing hormone: the hypothalamic-pituitary-thyroid axis in rats. A study with a pharmacological dose of thyrotropin-releasing hormone.

The effects of 16 days of oral treatment with thyrotropin-releasing hormone (TRH, 1 mg/24 h) on serum levels of thyrotropin (TSH), thyroxine (T4) and triiodothyronine (T3) and the kinetics of TRH in the blood were studied in normal rats. A second group of animals served as controls. TRH was dissolved by sonification (10 mg/l) and was stable in tap water. TRH was measured by a radioimmunoassay procedure (normal range: 20-80 pmol/l, antiserum K2B9 1:120,000 final dilution). An increase in basal TSH (7,200 +/- 440 ng/l, mean +/- SD) was found after 2 days of treatment (11,420 +/- 810 ng/l), but a significant increase was observed after 5 days of treatment (12,530 +/- 640 ng/l, p less than 0.001). T4 serum concentrations remained in the normal range during the entire period of study, whereas T3 serum concentrations (0.76 +/- 0.1 micrograms/l) were increased to 1.22 +/- 0.2 micrograms/l on day 5 (p less than 0.001). A subsequent decline of TSH, T4 and T3 up to the end of the study was observed. TRHmax concentrations were registered on day 5 (790 +/- 24 pmol/l). The mean value of TRHmax was 723 +/- 34 pmol/l. To improve the stability of TRH in tap water, 1-ml samples of drinking water with dissolved TRH were measured. The mean TRH concentration in drinking water was 73 +/- 1.5% (SD). No significant correlations were found between the area under the curve of TSH (184,340 ng.l-1.24 h) and that of TRH (14,954 pmol.l-1.24 h).(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibitory effects of thyrotropin-releasing hormone on neuronal activity in the nucleus accumbens

The effects of thyrotropin-releasing hormone (TRH) were examined on neuronal activity in the nucleus accumbens, receiving an input from the parafascicular nucleus of the thalamus or the hippocampus, in chloral hydrate-anesthetized rats, using a microiontophoretic technique. The spikes produced by stimulation of the parafascicular nucleus were predominantly and dose-dependently inhibited during iontophoretic application of TRH. When the effects of TRH and dopamine were tested on the same neurons of the nucleus accumbens, inhibition of the generation of spikes by both drugs was observed in most neurons. In contrast, spikes elicited by stimulation of the hippocampus in most neurons of the nucleus accumbens were not affected by TRH or dopamine. Both TRH- and dopamine-induced inhibition of the spikes induced by stimulation of the parafascicular nucleus was antagonized by simultaneous application of haloperidol. In animals treated with reserpine, inhibition of the generation of spikes upon stimulation of the parafascicular nucleus did not occur in any neurons in the nucleus accumbens during application of TRH, whereas the dopamine-induced inhibition was still observed. These results suggest that inhibitory effects of TRH on the neurons of the nucleus accumbens receiving an input from the parafascicular nucleus are mediated by dopamine released from the dopaminergic nerve terminals located in the nucleus accumbens.     

Enhancement by intracerebroventricular thyrotropin-releasing hormone of indomethacin-induced gastric lesions in the rat

Effects of the intracerebroventricular thyrotropin-releasing hormone (TRH) on gastric mucosa were studied in rats. TRH (3 and 10 micrograms rat-1 i.c.v.) produced slight gastric lesions and also aggravated indomethacin-, aspirin- or 5-hydroxytryptamine (5-HT)-induced gastric lesions, while restraint and cold stress-induced lesions were not influenced by TRH. Bethanechol used at a dose sufficient to produce acid secretion did not influence the gastric mucosa in intact or indomethacin-treated rats. Enhancement of indomethacin-induced gastric lesions by TRH was not inhibited to any significant degree by atropine 0.1 mg kg-1 s.c., which prevented TRH-induced gastric acid secretion, but tended to be inhibited by phentolamine, 2.5 mg kg-1 i.p. It is concluded that the enhancement by TRH of indomethacin-induced gastric lesions is due to a combination of the central and peripheral actions of the ulcerogenic agents.     

促甲状腺素释放激素改善失血性休克心功能的作用机理

TRH iv后显著升高失血性休克家兔MAP,±dP/dt_(max),Vpm,V_(max)和血浆E。利血平预处理后,TRH的上述作用消失。β受体阻滞剂普萘洛尔预处理后,TRH增强±dP/dt_(max),Vpm和V_(max)的作用消失,并在iv TRH后20和30 min取消了TRH的升压作用。α受体阻滞剂酚苄明预处理后,TRH仍能升高MAP,±dP/dt_(max),Vpm和V_(max)。离体大鼠左心房实验证明TRH(0.1mmol/L)可显著增强异丙肾上腺素和多巴胺的正性肌力作用。     

Inhibition by thyrotropin-releasing hormone of epileptic seizures in spontaneously epileptic rats

The effects of thyrotropin-releasing hormone (TRH) were investigated on absence-like seizures, which are characterized by the sudden appearance of 5-7 Hz spike-wave-like complexes in the cortical and hippocampal EEG, and on tonic convulsions of spontaneously epileptic rats (SER; zi/zi, tm/tm), a double mutant obtained by mating zitter homozygote (zi/zi) with tremor heterozygote rats (tm/+). TRH (5 and 10 mg/kg i.v.) inhibited the appearance of both absence-like seizures and tonic convulsions of SER without inducing obvious changes in the background EEG. The inhibitory effects were seen 5-20 min after injection of 10 mg/kg TRH and were antagonized by pretreatment with haloperidol (0.5 and 1.0/kg i.p.), although haloperidol alone did not affect the seizures. These results suggest that TRH has an antiepileptic effect in the genetically defined animal model, SER, and that the effect is mediated by the central dopaminergic system.