Comparative metabolism and conformation of TRH and its analogues

Protective effects of L-pGlu-(2-propyl)-L-His-L-ProNH2, a newer thyrotropin releasing hormone analog in in vitro and in vivo models of cerebral ischemia

In the present study, the newly synthesized TRH analog (L-pGlu-(2-propyl)-L-His-l-ProNH(2); NP-647) was evaluated for its effects in in vitro (oxygen glucose deprivation (OGD)-, glutamate- and H(2)O(2)-induced injury in PC-12 cells) and in vivo (transient global ischemia) models of cerebral ischemic injury. PC-12 cells were subjected to oxygen and glucose deprivation for 6h. Exposure of NP-647 was given before and during OGD. In glutamate and H(2)O(2) induced injury, exposure of NP-647 was given 1, 6 and 24h prior to exposure of glutamate and H(2)O(2) exposure. NP-647, per se found to be non-toxic in 1-100μM concentrations. NP-647 showed protection against OGD at the 1 and 10μM. The concentration-dependent protection was observed in H(2)O(2)- and glutamate-induced cellular injury. In in vivo studies, NP-647 treatment showed protection of hippocampal (CA1) neuronal damage in transient global ischemia in mice and subsequent improvement in memory retention was observed using passive avoidance retention test. Moreover, administration of NP-647 resulted in decrease in inflammatory cytokines TNF-α and IL-6 as well as lipid peroxidation. These results suggest potential of NP-647 in the treatment of cerebral ischemia and its neuroprotective effect may be attributed to reduction of excitotoxicity, oxidative stress and inflammation.

Antiepileptic potential and behavioral profile of L-pGlu-(2-propyl)-L-His-L-ProNH2, a newer thyrotropin-releasing hormone analog

Thyrotropin-releasing hormone (TRH) and its analogs have a number of neurobiological functions and therapeutic uses in disorders of the central nervous system. In this study, the newly synthesized TRH analogs were evaluated for central nervous system activity in pentobarbital-induced sleeping in mice. The most potent TRH analog (L-pGlu-(2-propyl)-L-His-L-ProNH(2) coded as NP-647) was evaluated for its antiepileptic potential in various seizure models in mice in comparison with TRH. Intravenous pretreatment with NP-647 (10 and 20 micromol/kg body wt) significantly delayed the onset and reduced the frequency of convulsions in the pentylenetetrazole model, but not in the maximum electroshock seizure model. Also, it was found to be protective against picrotoxin- and kainic acid-induced seizures. However, NP-647 did not significantly affect theophylline-induced seizures. Further study of the effect of NP-647 on locomotor activity and a functional observational battery revealed that it did not significantly exhibit any undesirable effects as compared with vehicle and TRH. NP-647 did not significantly affect cerebral blood flow, whereas the native peptide TRH markedly increased cerebral blood flow. Furthermore, NP-647 exerted antiepileptic activity without significantly altering plasma thyroid-stimulating hormone levels and mean arterial blood pressure. This suggests that NP-647 is more selective for central nervous system activity and devoid of hormonal and cerebrovascular system effects. In contrast, TRH exhibited cardiac and endocrine effects as marked by significant elevation in mean arterial blood pressure and plasma thyroid-stimulating hormone levels. This study demonstrates that NP-647 has potential antiepileptic activity devoid of undesirable effects and, thus, can be exploited for the prevention and treatment of epilepsy.

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.

Cerebrovascular effects of the TRH analogues pGlu-3-methyl-His-Pro amide and pGlu-Glu-Pro amide: a comparison with TRH

The goal of the study was to assess whether TRH analogues possess cerebrovascular effects similar to the native peptide. The neuropeptide thyrotropin releasing hormone (TRH) elicits cerebrovasodilation in several species under various conditions. The laser-Doppler method was employed to study the effects of TRH and the analogues pGlu-3-methyl-His-Pro amid (M-TRH) and pGlu-Glu-Pro amide. Intravenous (i.v.) injection of 300 microg kg(-1) of TRH elicited cerebrovasodilation and a 62% increase in blood flow within 1 minute. M-TRH, in a dose of 300 microg kg(-1) i.v., elicited a 80% increase in cerebral blood flow. Even a minute dose of M-TRH (625 ng kg(-1)) caused an increase in cerebral blood flow. No clear difference in effects on the cerebral blood flow was observed between spontaneously and mechanically ventilated animals, pGlu-Glu-Pro amide had no cerebrovascular effect.

Development of a continuous, fluorometric coupled enzyme assay for thyrotropin-releasing hormone-degrading ectoenzyme

Thyrotropin-releasing hormone degrading-ectoenzyme (TRH-DE) (EC 3.4. 19.6), removes the N-terminal pyroglutamyl residue of thyrotropin-releasing hormone (TRH). Discontinuous assays have been used to measure TRH-DE activity; however, a continuous assay is needed to make reliable measurements of initial rates and facilitate kinetic studies. Presented is a continuous, coupled enzyme assay for TRH-DE in which TRH-DE hydrolyzed the substrate, pyroglutamyl-histidyl-prolylamido-4-methyl coumarin (TRHMCA), to give His-ProMCA, which was then cleaved by dipeptidyl peptidase IV (EC 3.4.14.5) to give 7-amino-4-methyl coumarin (MCA). Reaction progress was monitored continuously by measuring the increase in MCA fluorescence. This assay should be especially useful for rapid screening of potential TRH-DE inhibitors. A previously reported discontinuous assay, where nonenzymatic cyclization at 80 degrees C was used to liberate MCA from His-ProMCA, was found to underestimate the amount of product formed. A modified procedure that avoids this is presented. Initial rates and kinetic parameters for TRHMCA hydrolysis by TRH-DE determined using this modified assay correspond with those determined by the continuous assay. Discontinuous and continuous assays gave K(m) values for TRHMCA of 3.4 +/- 0.7 microM (n = 5) and 3.8 +/- 0.5 microM (n = 5), respectively. K(i) values determined by the discontinuous assay for TRH and TRH-OH were 35 +/- 4 microM (n = 3) and 311 +/- 31 microM (n = 5), respectively.

A new biological contribution of cyclo(His-Pro) to the peripheral inhibition of pancreatic secretion

The tripeptide pyro-Glu-His-Pro-NH2[thyrotropin-releasing hormone (TRH)] was isolated from the hypothalamus as a thyrotropin-releasing factor. It has a broad spectrum of central nervous system-mediated actions, including the stimulation of exocrine pancreatic secretion. TRH is also synthesized in the endocrine pancreas and found in the systemic circulation. Enzymatic degradation of TRH in vivo produces other bioactive peptides such as cyclo(His-Pro). Because of the short half-life of TRH and the stability of cyclo(His-Pro) in vivo, we postulated that at least part of the peripheral TRH effects on the exocrine pancreatic secretion may be attributed to cyclo(His-Pro), which has been shown to have other biological activities. This study determines in parallel the peripheral effects of TRH and cyclo(His-Pro) as well as the putative contribution of other TRH-related peptides on exocrine pancreatic secretion in rats. TRH and its metabolite cyclo(His-Pro) dose dependently inhibited 2-deoxy-D-glucose (2-DG)-stimulated pancreatic secretion. TRH and all the related peptides tested had no effect on the basal and cholecystokinin-stimulated amylase release from pancreatic acinar cells in vitro. These data indicate that cyclo(His-Pro) mimics the peripheral inhibitory effect of TRH on 2-DG-stimulated exocrine pancreatic secretion. This effect is not detected on isolated pancreatic acini. Our findings provide a new biological contribution for cyclo(His-Pro) with potential experimental and clinical applications.

Intracisternal TRH and RX 77368 potently activate gastric vagal efferent discharge in rats

The influence of intracisternal (ic) TRH and the stable TRH analog, RX 77368, on gastric vagal efferent discharge (GVED) was investigated in urethane-anesthetized rats. Consecutive IC injections of TRH (3, 30, and 300 ng) at 60 min intervals stimulated dose dependently multi-unit GVED with a peak increase of 90 +/- 21%, 127 +/- 18% and 145 +/- 16% respectively. In two separate studies, IC injection of RX 77368 at 1.5 or 15 ng stimulated multi-unit GVED by 142 +/- 24% and 244 +/- 95% respectively. Saline injection IC had no effect on GVED. RX 77368 (1.5 ng, ic) action was long lasting (84 +/- 13 min) compared with TRH (3 ng: 44 +/- 7 min). Single-unit analysis also showed that 13 of 13 units responded to ic RX 77368 (1.5 ng) by an increase in activity. These data indicate that low doses of TRH injected ic stimulate vagal efferent outflow to the rat stomach and that RX 77368 action is more potent than TRH.

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.

Passive transepithelial absorption of thyrotropin-releasing hormone (TRH) via a paracellular route in cultured intestinal and renal epithelial cell lines

Transport studies using intestinal brush-border membrane vesicles isolated from rats and rabbits have failed to demonstrate proton- or Na(+)-dependent carrier-mediated transport of thyrotropin-releasing hormone (TRH), despite a pharmacologically relevant oral bioavailability. To examine the hypothesis that reported levels of oral bioavailability reflect predominantly a paracellular rather than transcellular route for transepithelial transport of TRH, we have studied TRH transport in cultured epithelial cell types of intestinal (Caco-2 and T84) and renal (MDCK I, MDCK II, and LLC-PK1) origin, whose paracellular pathways span the range of permeability values observed in natural epithelia. Transport of TRH across monolayers of intestinal Caco-2 cells was similar to the flux of mannitol (approximately 1-4% per 4 hr), and unlike other putative substrates for the di-/tripeptide carrier, apical-to-basolateral transport was not increased by the presence of an acidic pH in the apical chamber. TRH transport did not show saturation, being uneffected in the presence of 20 mM cold TRH. In each cell type studied TRH and mannitol transport were similar and positively correlated with the conductance of the cell layers, consistent with a passive mechanism of absorption. This evidence suggests that, providing that a peptide is resistant to luminal hydrolysis, small but pharmacologically significant amounts of peptide absorption may be achieved by passive absorption across a paracellular route.

The effects of thyrotropin-releasing hormone and scopolamine in Alzheimer's disease and normal volunteers

Thyrotropin-releasing hormone (TRH), a neuromodulator and possibly a neurotransmitter in the central nervous system, was shown in a prior study of young normal volunteers to attenuate the memory impairment induced by the anticholinergic drug scopolamine. In the present study, the cognitive, behavioral and physiologic effects of high dose TRH (0.5 mg/kg), both alone and following administration of scopolamine, were examined in 10 Alzheimer's disease (AD) patients (mean age±SD=63.5 years) and 12 older normal volunteers (mean age=64.9±8.8 years). On the day AD subjects received TRH alone, modest but statistically significant improvement from baseline performance was documented on some tests of learning and memory, especially in those with mild dementia severity. In comparing cognitive test performance between the scopolamine alone and scopolamine+TRH conditions, only two test scores were significantly higher in the latter condition. In the group of older volunteers, TRH did not attenuate scopolamine-induced cognitive impairment, contrary to prior findings in a group of younger controls. In fact, older subjects performed worse after receiving scopolamine followed by TRH than after receiving scopolamine alone. In addition, no change from baseline cognitive performance was detected after subjects received TRH alone. These findings raise several questions and speculations on possible age-related changes in the cholinergic system, as well as on the mechanism of the interaction of TRH with the cholinergic system.

Evidence for pyroglutamyl peptidase I and prolyl endopeptidase activities in the rat insulinoma cell line RINm 5F: lack of relationship with TRH metabolism

Thyrotropin-Releasing hormone (TRH)-degrading pyroglutamyl peptidase I (PGP I) and prolylendopeptidase (PE) activities have been demonstrated in rat insulinoma RINm 5F cell line. These two enzymes catalyze the conversion of TRH to Histydyl-Proline-Diketopiperazine and to acid TRH respectively. After cell fractionation, we found all the PGP I and PE activities in the cytosolic fraction. The membrane-bound PGP II activity is not detectable in the RINm 5F cells. Further investigations on these two cytosolic enzymes show that pyroglutamyl- and proline-containing peptides are inhibitors of each TRH-degrading enzyme. Gel filtration chromatography on Sephadex G100 shows that PGP I and PE activity have an apparent molecular mass of about 18 kDa and 57 kDa, respectively. Kinetic analysis with TRH as substrate, gives a Km of 44 microM and 235 microM, and a Vmax of 1.49 and 8.80 pmol/min/micrograms protein for PGP I and PE, respectively. Immunoreactive TRH, His-Pro-Diketopiperazine and acid TRH levels in the cell line extracts are 2.2 +/- 0.9, 22.5 +/- 11.1 and 28.7 +/- 14.6 pg/1O6 cells, respectively. When cells have been incubated for 2 to 72 hours with a P.E. inhibitor (Z-Gly-Pro-CHN2) at 5 x 10(-7) M, both cell PGP I and PE activities are inhibited. No change in the cellular content of immunoreactive TRH, His-Pro-Diketopiperazine and acid TRH have been observed in treated cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Degradation of thyrotropin-releasing hormone and luteinising hormone-releasing hormone by enzymes of brain tissue

In this article, the enzymes of brain and associated tissues that can degrade thyrotropin-releasing hormone (TRH) and luteinising hormone-releasing hormone (LH-RH) are reviewed. As both TRH and LH-RH are considered to act as neurotransmitters or neuromodulators in the CNS, attention is paid to the subcellular location of the enzymes described and how their topographies and substrate specificities fit them to playing roles as inactivating agents for TRH and LH-RH or as regulators of intracellular concentrations of TRH and LH-RH. Consideration is also given to enzymes involved in biotransformation of TRH to secondary metabolites that exhibit biological activity and to enzymes involved in the metabolism of secondary metabolites.