TRH-like Peptides Research Articles

Preclinical In-Vivo Safety of a Novel Thyrotropin-Releasing Hormone-Loaded Biodegradable Nanoparticles After Intranasal Administration in Rats and Primates.

Thyrotropin-releasing hormone (TRH) and TRH-like peptides carry a therapeutic potential for neurological conditions. Nanoparticles (NP) made of the biodegradable polymer, Poly(Sebacic Anhydride) (PSA), have been developed to carry TRH, intended for intranasal administration to patients. There is limited information on the safety of biodegradable polymers when given intranasally, and therefore, we have performed two preclinical safety and toxicity studies in cynomolgus monkeys and rats using TRH-PSA nanoparticles. The rats and monkeys were dosed intranasally for 42days or 28days, respectively, and several animals were followed for additional 14days. Animals received either placebo, vehicle (PSA), or different concentrations of TRH-PSA. No systemic adverse effects were seen. Changes in T3 or T4 concentrations were observed in some TRH-PSA-treated animals, which did not have clinical or microscopic correlates. No effect was seen on TSH or prolactin concentrations. In the monkey study, microscopic changes in the nasal turbinates were observed, which were attributed to incidental mechanical trauma caused during administration. Taken together, the TRH-loaded PSA NPs have proven to be safe, with no local or systemic adverse effects attributed to the drug loaded nanoparticles. These findings provide additional support to the growing evidence of the safety of peptide-loaded NPs for intranasal delivery and pave the way for future clinical trials in humans.

A resveratrol derivative modulates TRH and TRH-like peptide expression throughout the brain and peripheral tissues of male rats.

IntroductionResveratrol and related polyphenols have therapeutic effects ranging from treatment of depression, Alzheimer's and Parkinson's disease, obesity, diabetes, neurodegeneration and ageing. TRH and TRH‐like peptides, with the structure pGlu‐X‐Pro‐NH2, where ‘X can be any amino acid reside, have reproductive, caloric‐restriction‐like, anti‐ageing, pancreatic‐β cell‐enhancing, cardiovascular and neuroprotective effects. We hypothesize that TRH and TRH‐like peptides are mediators of the therapeutic actions of the resveratrol derivative pterostilbene (PT).MethodsSixteen young adult male Sprague–Dawley rats were divided into four groups. Control group remained on ad libitum chow and water for 10 days. Acute group received ad libitum chow and water for 9 days and then 0.9 g PT/250 g rat chow for 24 h. Chronic animals received PT in chow for 10 days. Withdrawal rats received PT chow for 8 days and then normal chow for 2 days. TRH and TRH‐like peptide levels were measured in medulla oblongata (MED), frontal cortex (FCX), hypothalamus (HY), amygdala (AY), hippocampus (HC), piriform cortex (PIR), nucleus accumbens (NA), entorhinal cortex (ENT), striatum (STR), cerebellum (CBL), anterior cingulate (ACNG), posterior cingulate (PCNG), prostate (PR), liver (L), testis (T), heart (H), pancreas (PAN), adrenals (AD) and epididymis (EP).ResultsSignificant changes in the levels of TRH and TRH‐like peptides occurred throughout the brain and peripheral tissues in response to PT treatment.ConclusionThe high responsiveness of PIR, CBL, HY, STR, PCNG, MED, FCX, NA, ACNG and AY in brain and EP and PR is consistent with TRH and TRH‐like peptides participating in the therapeutic effects of PT.

TRH and TRH-like peptide levels covary with caloric restriction and oral metformin in rat heart and testis

BackgroundObesity, type 2 diabetes, and major depression are common comorbidities in aging populations. Metformin (MF) mimics effects of caloric restriction (CR) which increases health and life spans. CR suppresses central TRH activity resulting in bradycardia and suppression of energy expenditure. TRH and TRH-like peptides with sequence pGlu-X-Pro-NH2, (X-TRH) where ‘X’ can be any amino acid residue, have anorectic, antidepressant, anxiolytic, analeptic, anti-epileptic, neuroprotective and anti-aging effects. The present studies compare the effects of CR and oral MF on expression of TRH and TRH-like peptides. CR experiment: Sixteen male Sprague-Dawley rats divided into 4 groups: ad libitum fed (CR-CON), 24 h fasted (CR-AC), fed (ADF1) or fasted (ADF2) 24 h after 10 days of alternate day fasting. MF experiment: Sixteen rats divided into 4 groups: water (MF-CON), 6 mg MF/ml water 24 h (MF-AC), 10 days MF (CHR) or 8 days MF then 48 h water (WD). ResultsComparison of chronic MF and ADF2 treatment effects on TRH and TRH-like peptide levels in testis revealed a consistent pattern for peptide responses observed in the two experiments with Glu-TRH > Val-TRH > TRH > Peak 2 > Trp-TRH > Phe-TRH > Tyr-TRH. TRH levels increased in heart during CR and MF treatment consistent with cardiac responses to congestive heart failure and infarction. ConclusionsBecause TRH reverses aging-induced testicular degeneration and mediates the pathophysiology of congestive heart failure these tripeptides may participate in the therapeutic effects of MF in ameliorating the cardiometabolic risk factors and fertility impairments associated with type 2 diabetes.

Rifaximin modulates TRH and TRH-like peptide expression throughout the brain and peripheral tissues of male rats

BackgroundThe TRH/TRH-R1 receptor signaling pathway within the neurons of the dorsal vagal complex is an important mediator of the brain-gut axis. Mental health and protection from a variety of neuropathologies, such as autism, Attention Deficit Hyperactivity Disorder, Alzheimer’s and Parkinson’s disease, major depression, migraine and epilepsy are influenced by the gut microbiome and is mediated by the vagus nerve. The antibiotic rifaximin (RF) does not cross the gut-blood barrier. It changes the composition of the gut microbiome resulting in therapeutic benefits for traveler’s diarrhea, hepatic encephalopathy, and prostatitis. TRH and TRH-like peptides, with the structure pGlu-X-Pro-NH2, where “X” can be any amino acid residue, have reproduction-enhancing, caloric-restriction-like, anti-aging, pancreatic-β cell-, cardiovascular-, and neuroprotective effects. TRH and TRH-like peptides occur not only throughout the CNS but also in peripheral tissues. To elucidate the involvement of TRH-like peptides in brain-gut-reproductive system interactions 16 male Sprague–Dawley rats, 203 ± 6 g, were divided into 4 groups (n = 4/group): the control (CON) group remained on ad libitum Purina rodent chow and water for 10 days until decapitation, acute (AC) group receiving 150 mg RF/kg powdered rodent chow for 24 h providing 150 mg RF/kg body weight for 200 g rats, chronic (CHR) animals receiving RF for 10 days; withdrawal (WD) rats receiving RF for 8 days and then normal chow for 2 days.ResultsSignificant changes in the levels of TRH and TRH-like peptides occurred throughout the brain and peripheral tissues in response to RF. The number of significant changes in TRH and TRH-like peptide levels in brain resulting from RF treatment, in descending order were: medulla (16), piriform cortex (8), nucleus accumbens (7), frontal cortex (5), striatum (3), amygdala (3), entorhinal cortex (3), anterior (2), and posterior cingulate (2), hippocampus (1), hypothalamus (0) and cerebellum (0). The corresponding ranking for peripheral tissues were: prostate (6), adrenals (4), pancreas (3), liver (2), testis (1), heart (0).ConclusionsThe sensitivity of TRH and TRH-like peptide expression to RF treatment, particularly in the medulla oblongata and prostate, is consistent with the participation of these peptides in the therapeutic effects of RF.

Trh and Trh-Like Peptide Levels Covary with Caloric Restriction and Oral Metformin in Rat Heart and Testis

Trh and Trh-Like Peptide Levels Covary with Caloric Restriction and Oral Metformin in Rat Heart and Testis

Evolutionarily conserved TRH neuropeptide pathway regulates growth in Caenorhabditis elegans

In vertebrates thyrotropin-releasing hormone (TRH) is a highly conserved neuropeptide that exerts the hormonal control of thyroid-stimulating hormone (TSH) levels as well as neuromodulatory functions. However, a functional equivalent in protostomian animals remains unknown, although TRH receptors are conserved in proto- and deuterostomians. Here we identify a TRH-like neuropeptide precursor in Caenorhabditis elegans that belongs to a bilaterian family of TRH precursors. Using CRISPR/Cas9 and RNAi reverse genetics, we show that TRH-like neuropeptides, through the activation of their receptor TRHR-1, promote growth in Celegans TRH-like peptides from pharyngeal motor neurons are required for normal body size, and knockdown of their receptor in pharyngeal muscle cells reduces growth. Mutants deficient for TRH signaling have no defects in pharyngeal pumping or isthmus peristalsis rates, but their growth defect depends on the bacterial diet. In addition to the decrease in growth, trh-1 mutants have a reduced number of offspring. Our study suggests that TRH is an evolutionarily ancient neuropeptide, having its origin before the divergence of protostomes and deuterostomes, and may ancestrally have been involved in the control of postembryonic growth and reproduction.

TRH and TRH-Like Peptide Levels Co-Vary with Reproductive and Metabolic Rhythms.

Photoperiod-synchronized rhythms in non-CSN tissues persist in total darkness. Clock genes involved in maintaining regular biorhythms within the suprachiasmatic nucleus (SCN) of the hypothalamus are expressed in extra-CNS tissues and continue periodic expression in vitro. Understanding the details of how the SCN clock is coupled with peripheral clocks is only incompletely understood and may involve a multiplicity of feedback systems. The present study is an extension of our previous work showing that brain levels of TRH (pGlu-His-Pro-NH2) and TRH-like peptides (X-TRH: pGlu-X-Pro-NH2, where "X" can be any amino acid residue) fluctuate throughout the day-night cycle. Male rats were maintained in a stable environment, lights on 6-18 h. TRH and TRH-like peptides in liver, pancreas, testis, prostate, epididymis, and heart were measured at 3, 10, 16, and 22 h. The greatest change in peptide level was a 12-fold increase for TRH in prostate at 16 h relative to the corresponding value at 3 h. The TRH, Tyr-TRH and Phe-TRH levels in liver declined steadily to about 40% of the 3-h values by 22 h. Changes, in the order of decreasing number of significant increases (↑) and/or decreases (↓), were: testis (5↑, 1↓), liver (3↓), epididymis (2↑), prostate (1↑, 1↓) and heart (1↑). Peptide levels in liver and testis correlated with serum leptin and serum corticosterone, respectively, which are potent releasers of these peptides. Testosterone and glucose were also highly correlated. These tripeptides may participate in the regulation of metabolic and reproductive functions, which change during the day-night cycle.

Ketamine modulates TRH and TRH-like peptide turnover in brain and peripheral tissues of male rats

Major depression is the largest single healthcare burden with treatments of slow onset and often limited efficacy. Ketamine, a NMDA antagonist used extensively as a pediatric and veterinary anesthetic, has recently been shown to be a rapid acting antidepressant, making it a potential lifesaver for suicidal patients. Side effects and risk of abuse limit the chronic use of ketamine. More complete understanding of the neurobiochemical mechanisms of ketamine should lead to safer alternatives. Some of the physiological and pharmacological actions of ketamine are consistent with increased synthesis and release of TRH (pGlu-His-Pro-NH2), and TRH-like peptides (pGlu-X-Pro-NH2) where “X” can be any amino acid residue. Moreover, TRH-like peptides are themselves potential therapeutic agents for the treatment of major depression, anxiety, bipolar disorder, epilepsy, Alzheimer's and Parkinson's diseases. For these reasons, male Sprague–Dawley rats were anesthetized with 162mg/kg ip ketamine and then infused intranasally with 20μl of sterile saline containing either 0 or 5mg/ml Glu-TRH. One, 2 or 4h later, the brain levels of TRH and TRH-like peptides were measured in various brain regions and peripheral tissues. At 1h in brain following ketamine only, the levels of TRH and TRH-like peptides were significantly increased in 52 instances (due to increased biosynthesis and/or decreased release) or decreased in five instances. These changes, listed by brain region in order of decreasing number of significant increases (↑) and/or decreases (↓), were: hypothalamus (9↑); piriform cortex (8↑); entorhinal cortex (7↑); nucleus accumbens (7↑); posterior cingulate (5↑); striatum (4↑); frontal cortex (2↑,3↓); amygdala (3↑); medulla oblongata (1↑,2↓); cerebellum (2↑); hippocampus (2↑); anterior cingulate (2↑). The corresponding changes in peripheral tissues were: adrenals (8↑); epididymis (4↑); testis (1↑,3↓); pancreas (1↑); prostate (1↑). We conclude that TRH and TRH-like peptides may be downstream mediators of the rapid antidepressant actions of ketamine.

Discovery of a low affinity thyrotropin-releasing hormone (TRH)-like peptide that exhibits potent inhibition of scopolamine-induced memory impairment in mice.

TRH-like peptides were synthesized in which the critical N-terminus residue L-pGlu was replaced with various heteroaromatic rings, and the central residue histidine with 1-alkyl-L-histidines. All synthesized TRH-like peptides were evaluated in vitro as agonists in HEK mTRH-R1 and HEK mTRH-R2 cell lines, an expressing receptor binding assay (IC50), and cell signaling assay (EC50). The analeptic potential of the synthesized peptides was evaluated in vivo by using the antagonism of a pentobarbital-induced sleeping time. The peptides 6a, 6c and 6e were found to activate TRH-R2 with potencies (EC50) of 0.002 μM, 0.28 μM and 0.049 μM, respectively. In contrast, for signaling activation of TRH-R1, the same peptides required higher concentration of 0.414 μM, 50 μM and 19.1 μM, respectively in the FLIPR assay. The results showed that these peptides were 207, 178 and 389-fold selective towards TRH-R2 receptor subtype. In the antagonism of a pentobarbital-induced sleeping time assay, peptide 6c showed a 58.5% reduction in sleeping time. The peptide 6c exhibited high stability in rat blood plasma, a superior effect on the scopolamine-induced cognition impairment mice model, safe effects on the cardiovascular system, and general behavior using a functional observation battery (FOB).

Increased TRH and TRH-like peptide release in rat brain and peripheral tissues during proestrus/estrus

Women are at greater risk for major depression, PTSD, and other anxiety disorders. ERβ-selective agonists for the treatment of these disorders are the focus of pharmacologic development and clinical testing. Estradiol and its metabolites contribute to the neuroprotective effects of this steroid class, particularly in men, due to local conversion of testosterone to estiradiol in key brain regions which are predisposed to neurodegenerative diseases. We have used young adult female Sprague–Dawley rats to assess the role of TRH and TRH-like peptides, with the general structure pGlu-X-Pro-NH2 where “X” can be any amino acid residue, as mediators of the neurobiochemical effects of estradiol. The neuroprotective TRH and TRH-like peptides are coreleased with excitotoxic glutamate by glutamatergic neurons which contribute importantly to the regulation of the estrus cycle. The levels of TRH and TRH-like peptides during proestrus and/or estrus in the 12 brain regions analyzed were significantly decreased (due to accelerated release) 106 times but increased only 25 times when compared to the corresponding levels during diestrus days 1 and 2. These changes, listed by brain region in the order of decreasing number of significant decreases (↓) and/or increases (↑), were: striatum (20↓,1↑), medulla oblongata (16↓,2↑), amygdala (14↓,1↑), cerebellum (13↓,1↑), hypothalamus (12↓,1↑), entorhinal cortex (6↓,6↑), posterior cingulate (10↓,1↑), frontal cortex (3↓,5↑), nucleus accumbens (5↓,3↑), hippocampus (5↓,2↑), anterior cingulate (2↓,1↑), and piriform cortex (1↑). In peripheral tissues the corresponding changes were: ovaries (23↓), uterus (16↓,1↑), adrenals (11↓,3↑), and pancreas (1↓,6↑). We conclude that these peptides may be downstream mediators of some of the therapeutic effects of estrogen.

Rapid modulation of TRH and TRH-like peptide release in rat brain and peripheral tissues by ghrelin and 3-TRP-ghrelin

Ghrelin is not only a modulator of feeding and energy expenditure but also regulates reproductive functions, CNS development and mood. Obesity and major depression are growing public health concerns which may derive, in part, from dysregulation of ghrelin feedback at brain regions regulating feeding and mood. We and others have previously reported that thyrotropin-releasing hormone (TRH, pGlu-His-Pro-NH2) and TRH-like peptides (pGlu-X-Pro-NH2, where “X” can be any amino acid residue) have neuroprotective, antidepressant, anti-epileptic, analeptic, anti-ataxic, and anorectic properties. For this reason male Sprague-Dawley rats were injected ip with 0.1mg/kg rat ghrelin or 0.9mg/kg 3-Trp-rat ghrelin. Twelve brain regions: cerebellum, medulla oblongata, anterior cingulate, posterior cingulate, frontal cortex, nucleus accumbens, hypothalamus, entorhinal cortex, hippocampus, striatum, amygdala, piriform cortex and 5 peripheral tissues (adrenals, testes, epididymis, pancreas and prostate) were analyzed. Rapid and profound decreases in TRH and TRH-like peptide levels (increased release) occurred throughout brain and peripheral tissues following ip ghrelin. Because ghrelin is rapidly deacylated in vivo we also studied 3-Trp-ghrelin which cannot be deacylated. Significant increases in TRH and TRH-like peptide levels following 3-Trp-ghrelin, relative to those after ghrelin were observed in all brain regions except posterior cingulate and all peripheral tissues except prostate and testis. The rapid stimulation of TRH and TRH-like peptide release by ghrelin in contrast with the inhibition of such release by 3-Trp-TRH is consistent with TRH and TRH-like peptides modulating the downstream effects of both ghrelin and unacylated ghrelin.

Rapid modulation of TRH and TRH-like peptide release in rat brain and peripheral tissues by prazosin

Hyperresponsiveness to norepinephrine contributes to post-traumatic stress disorder (PTSD). Prazosin, a brain-active blocker of α1-adrenoceptors, originally used for the treatment of hypertension, has been reported to alleviate trauma nightmares, sleep disturbance and improve global clinical status in war veterans with PTSD. Thyrotropin-releasing hormone (TRH, pGlu-His-Pro-NH2) may play a role in the pathophysiology and treatment of neuropsychiatric disorders such as major depression, and PTSD (an anxiety disorder). To investigate whether TRH or TRH-like peptides (pGlu-X-Pro-NH2, where “X” can be any amino acid residue) participate in the therapeutic effects of prazosin, male rats were injected with prazosin and these peptides then measured in brain and endocrine tissues. Prazosin stimulated TRH and TRH-like peptide release in those tissues with high α1-adrenoceptor levels suggesting that these peptides may play a role in the therapeutic effects of prazosin.

TRH-Like Peptides

TRH-like peptides are characterized by substitution of basic amino acid histidine (related to authentic TRH) with neutral or acidic amino acid, like glutamic acid, phenylalanine, glutamine, tyrosine, leucin, valin, aspartic acid and asparagine. The presence of extrahypothalamic TRH-like peptides was reported in peripheral tissues including gastrointestinal tract, placenta, neural tissues, male reproductive system and certain endocrine tissues. Work deals with the biological function of TRH-like peptides in different parts of organisms where various mechanisms may serve for realisation of biological function of TRH-like peptides as negative feedback to the pituitary exerted by the TRH-like peptides, the role of pEEPam such as fertilization-promoting peptide, the mechanism influencing the proliferative ability of prostatic tissues, the neuroprotective and antidepressant function of TRH-like peptides in brain and the regulation of thyroid status by TRH-like peptides.

Rapid modulation of TRH and TRH-like peptide release in rat brain and peripheral tissues by leptin

Leptin is not only a feedback modulator of feeding and energy expenditure but also regulates reproductive functions, CNS development and mood. Obesity and major depression are growing public health concerns which may derive, in part, from disregulation of leptin feedback at the level of the hypothalamic feeding centers and mood regulators within the limbic system. Identifying downstream mediators of leptin action may provide therapeutic opportunities. We and others have previously reported that thyrotropin-releasing hormone (TRH, pGlu-His-Pro-NH 2) and TRH-like peptides (pGlu-X-Pro-NH 2, where “X” can be any amino acid residue) have neuroprotective, antidepressant, anti-epileptic, analeptic, anti-ataxic, and anorectic properties. For this reason, young, adult male Sprague–Dawley rats were injected ip with 1 mg/kg rat leptin and peptide and protein levels were measured in brain and peripheral tissues at 0, 0.5, 1 and 2 h later. Eleven brain regions: pyriform cortex (PYR), entorhinal cortex (ENT), cerebellum (CBL), nucleus accumbens (NA), frontal cortex (FCX), amygdala (AY), posterior cingulate (PCNG), striatum (STR), hippocampus (HC), medulla oblongata (MED) and anterior cingulate (ACNG) and five peripheral tissues (adrenals, testes, epididymis, pancreas and prostate) were analyzed. TRH and six TRH-like peptide levels in STR fell by 0.5 h consistent with leptin-induced release of these peptides: STR (7↓). Significant changes in TRH and TRH-like peptide levels for other brain regions were: CBL (5↓), ENT (5↓), HC (4↓), AY (4↓), FCX (3↓), and ACNG (1↓). The rapid modulation of TRH and TRH-like peptide release combined with their similarity in behavioral, neuroendocrine, immunomodulatory, metabolic and steroidogenic effects to that of leptin is consistent with these peptides participating in downstream signaling.

Rapid modulation of TRH and TRH-like peptide release in rat brain, pancreas, and testis by a GSK-3β inhibitor

Antidepressants have been shown to be neuroprotective and able to reverse damage to glia and neurons. Thyrotropin-releasing hormone (TRH) is an endogenous antidepressant-like neuropeptide that reduces the expression of glycogen synthase kinase-3β (GSK-3β), an enzyme that hyperphosphorylates tau and is implicated in bipolar disorder, diabetes and Alzheimer's disease. In order to understand the potential role of GSK-3β in the modulation of depression by TRH and TRH-like peptides and the therapeutic potential of GSK-3β inhibitors for neuropsychiatric and metabolic diseases, young adult male Sprague–Dawley (SD) rats were (a) injected ip with 1.8 mg/kg of GSK-3β inhibitor VIII (GSKI) and sacrificed 0, 2, 4, 6, and 8 h later or (b) injected with 0, 0.018, 0.18 or 1.8 mg/kg GSKI and bled 4 h later. Levels of TRH and TRH-like peptides were measured in various brain regions involved in mood regulation, pancreas and reproductive tissues. Large, 3–15-fold, increases of TRH and TRH-like peptide levels in cerebellum, for example, as well as other brain regions were noted at 2 and 4 h. In contrast, a nearly complete loss of TRH and TRH-like peptides from testis within 2 h and pancreas by 4 h following GSKI injection was observed. We have previously reported similar acute effects of corticosterone in brain and peripheral tissues. Incubation of a decapsulated rat testis with either GSKI or corticosterone accelerated release of TRH, and TRH-like peptides. Glucocorticoids, via inhibition of GSK3-β activity, may thus be involved in the inhibition of TRH and TRH-like peptide release in brain, thereby contributing to the depressogenic effect of this class of steroids. Corticosterone-induced acceleration of release of these peptides from testis may contribute to the decline in reproductive function and redirection of energy needed during life-threatening emergencies. These contrasting effects of glucocorticoid on peptide release appear to be mediated by GSK-3β.

TRH and TRH-like peptide expression in rat following episodic or continuous corticosterone

Sustained abnormalities of glucocorticoid levels have been associated with neuropsychiatric illnesses such as major depression, posttraumatic stress disorder (PTSD), panic disorder, and obsessive compulsive disorder. The pathophysiological effects of glucocorticoids may depend not only on the amount of glucocorticoid exposure but also on its temporal pattern, since it is well established that hormone receptors are down-regulated by continuously elevated cognate hormones. We have previously reported that TRH (pGlu-His-Pro-NH2) and TRH-like peptides (pGlu-X-Pro-NH2) have endogenous antidepressant-like properties and mediate or modulate the acute effects of a single i.p. injection of high dose corticosterone (CORT) in rats. For these reasons, two accepted methods for inducing chronic hyperglucocorticoidemia have been compared for their effects on brain and peripheral tissue levels of TRH and TRH-like peptides in male, 250 g, Sprague-Dawley rats: (1) the dosing effect of CORT hemisuccinate in drinking water, and (2) s.c. slow-release pellets. Overall, there were 93% more significant changes in TRH and TRH-like peptide levels in brain and 111% more in peripheral tissues of those rats ingesting various doses of CORT in drinking water compared to those with 1-3 s.c. pellets. We conclude that providing rats with CORT in drinking water is a convenient model for the pathophysiological effects of hyperglucocorticoidemia in rodents.

Escitalopram Regulates Expression of TRH and TRH-Like Peptides in Rat Brain and Peripheral Tissues

Background: Escitalopram (eCIT) is a highly selective serotonin reuptake inhibitor (SSRI) that can be an effective treatment for a number of neuropsychiatric disorders including major depression. We, and others, have previously reported that thyrotropin-releasing hormone (TRH, pGlu-His-Pro-NH₂) and TRH-like peptides with the general structure pGlu-X-Pro-NH₂, where ‘X’ can be any amino acid residue, have neuroprotective, antidepressant, analeptic, arousal, and anti-epileptic effects that could mediate the neuropsychiatric and therapeutic effects of a variety of neurotropic agents. The present work explores the possible mediation of the therapeutic effects of eCIT by TRH and TRH-like peptides. Methods: In order to extend our understanding of the range of neurotransmitter systems that are modulated by and, in turn, influence the expression of TRH and TRH-like peptides, 16 male Sprague-Dawley rats were injected i.p. with eCIT (24 mg/kg BW) and the brain levels of TRH and TRH-like peptides in various brain regions involved in mood regulation and peripheral tissues with serotonergic innervation were measured 0, 2, 4, and 6 h later by combined HPLC and RIA. Results and Conclusion: Remarkable 3- to 25-fold increases in TRH and TRH-like peptide levels were observed 2 h after i.p. eCIT in the epididymis. This reproductive tissue has the highest level of serotonin found in most mammals. The acute (2 h) effect of eCIT in brain regions involved in mood regulation, particularly the nucleus accumbens and medulla oblongata, cerebellum, and striatum was to increase the levels of TRH-like peptides, most consistently Phe-TRH. An important exception was a decrease in the level of TRH in the nucleus accumbens. These responses, in general, were the opposite of those we have previously observed after acute restraint stress in this same rat strain. We conclude that some of the therapeutic effects of inhibition of serotonin reuptake are mediated by altered release of TRH and TRH-like peptides.

Lipopolysaccharide modulation of thyrotropin-releasing hormone (TRH) and TRH-like peptide levels in rat brain and endocrine organs

Lipopolysaccharide (LPS) is a proinflammatory and depressogenic agent whereas thyrotropin-releasing hormone (TRH; pGlu-His-Pro-NH2) is an endogenous antidepressant and neuroprotective peptide. LPS and TRH also have opposing effects on K+ channel conductivity. We hypothesized that LPS can modulate the expression and release of not only TRH but also TRH-like peptides with the general structure pGlu-X-Pro-NH2, where "X" can be any amino acid residue. The response might be "homeostatic," that is, LPS might increase TRH and TRH-like peptide release, thereby moderating the cell damaging effects of this bacterial cell wall constituent. On the other hand, LPS might impair the synthesis and release of these neuropeptides, thus facilitating the induction of early response genes, cytokines, and other downstream biochemical changes that contribute to the "sickness syndrome." Sprague-Dawley rats (300 g) received a single intraperitoneal injection of 100 microg/kg LPS. Animals were then decapitated 0, 2, 4, 8, and 24 h later. Serum cytokines and corticosterone peaked 2 h after intraperitoneal LPS along with a transient decrease in serum T3. TRH and TRH-like peptides were measured by a combination of high-performance liquid chromatography and radioimmunoassay. TRH declined in the nucleus accumbens and amygdala in a manner consistent with LPS-accelerated release and degradation. Various TRH-like peptide levels increased at 2 h in the anterior cingulate, hippocampus, striatum, entorhinal cortex, posterior cingulate, and cerebellum, indicating decreased release and clearance of these peptides. These brain regions are part of a neuroimmunomodulatory system that coordinates the behavioral, endocrine, and immune responses to the stresses of sickness, injury, and danger. A sustained rise in TRH levels in pancreatic beta-cells accompanied LPS-impaired insulin secretion. TRH and Leu-TRH in prostate and TRH in epididymis remained elevated 2-24 h after intraperitoneal LPS. We conclude that these endogenous neuroprotective and antidepressant-like peptides both mediate and moderate some of the behavioral and toxic effects of LPS.

Circadian rhythms of TRH-like peptide levels in rat brain

This is the first report of diurnal variations in the levels of thyrotropin-releasing hormone-like peptides (pGlu-X-Pro-NH 2, where “X” can be any amino acid residue) in brain regions involved in mood regulation. These peptides have neuroprotective and antidepressant-like properties that may help stabilize chronobiologic systems that are often abnormal in neuropsychiatric disease. We hypothesized that diurnal fluctuations in the levels of these neuropeptides are components of the chronobiologic regulation of autonomic, behavioral and emotional states. Optimal use of these potentially therapeutic agents will benefit from an understanding of their response to, and effect on, normal vegetative, activity and sleep patterns, and the corresponding disordered patterns of mental illness. For these reasons, 16 male, 200 g, Sprague–Dawley rats were maintained for 4 weeks in a stable 12 h lights on, 12 h lights off photoperiod. Levels of TRH and TRH-like peptides were measured at 3.0 h, 10.5 h, 13.5 h and 21.0 h, where the subjective midnight was 0.0 h, by a combination of HPLC and RIA. Highly significant changes in TRH-like peptide levels were observed in the striatum, posterior cingulate, cerebellum, pyriform cortex, nucleus accumbens and medulla oblongata. TRH-like peptide levels, in general, were highly correlated with changes in TRH concentration, within and between brain regions, and may be colocalized in large glutamatergic neurons innervating the rat limbic system. We conclude that TRH-like peptides may be important components of chronobiologic systems involved in maintaining autonomic, behavioral and mood equilibria.

Valproate and copper accelerate TRH-like peptide synthesis in male rat pancreas and reproductive tissues

Treatment with valproate (Valp) facilitates the synthesis of TRH-like peptides (pGlu-X-Pro-NH 2) in rat brain where “X” can be any amino acid residue. Because high levels of TRH-like peptides occur in the pancreas and pGlu-Glu-Pro-NH 2 (Glu-TRH) has been shown to be a fertilization promoting peptide, we hypothesized that these peptides mediate some of the metabolic and reproductive side effects of Valp. Male WKY rats were treated with Valp acutely (AC), chronically (CHR) or chronically followed by a 2 day withdrawal (WD). AC, CHR and WD treatments significantly altered TRH and/or TRH-like peptide levels in pancreas and reproductive tissues. Glu-TRH was the predominant TRH-like peptide in epididymis, consistent with its fertilization promoting activity. Glu-TRH levels in the epididymis increased 3-fold with AC Valp. Phe-TRH, the most abundant TRH-like peptide in the pancreas, increased 4-fold with AC Valp. Phe-TRH inhibits both basal and TRH-stimulated insulin release. Large dense core vesicles (LDCV's) contain a copper-dependent enzyme responsible for the post-translational processing of precursors of TRH and TRH-like peptides. Copper (500 μM) increased the in vitro C-terminal amidation of TRH-like peptides by 8- and 4-fold during 24 °C incubation of homogenates of pancreas and testis, respectively. Valp (7 μM) accelerated 3-fold the processing of TRH and TRH-like peptide precursors in pancreatic LDCV's incubated at 24 °C. We conclude that copper, an essential cofactor for TRH and TRH-like peptide biosynthesis that is chelated by Valp, mediates some of the metabolic and reproductive effects of Valp treatment via acceleration of intravesicular synthesis and altered release of these peptides.