Circadian rhythms of TRH-like peptide levels in rat brain

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

Introduction

Methods

Results

Conclusion

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

Background

The 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.

Results

Significant 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).

Conclusions

The 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.

Aging and the hypothalamic-pituitary-thyroid axis

The world's population is increasingly aging, this noted particularly in the Western world where there are ever greater numbers of centenarians and those over 85 years. Given the immense importance of the thyroid gland for optimal health and the fact that morphological and functional changes in the hypothalamic-pituitary-thyroid (HPT) axis take place as a natural adaptation to the aging process, a clear distinction must be made in older individuals between these and the onset of disease. However, this is problematic since, frequently, subtle differences exist between them, making diagnosis a challenging task, especially as concerns subclinical disease. The newly emerging interdisciplinary field of geroscience offers the prospect of being used as a platform to investigate the effect of disrupted HPT function on functional capacity and cognitive ability among the aged, as well as the risk or onset of age-associated diseases, thus enhancing healthspan and lifespan. Because optimal functioning of the thyroid gland is a prerequisite for longevity as well as for mental and physical wellbeing, this review summarizes the recent scientific data regarding HPT and aging while discussing alternative and personalized treatment approaches to maintaining a healthy thyroid as a means to ensuring a long, active, and healthy life.

The forgotten effects of thyrotropin-releasing hormone: Metabolic functions and medical applications

Thyrotropin-releasing hormone (TRH) causes a variety of thyroidal and non-thyroidal effects, the best known being the feedback regulation of thyroid hormone levels. This was employed in the TRH stimulation test, which is currently little used. The role of TRH as a cancer biomarker is minor, but exaggerated responses to TSH and prolactin levels in breast cancer led to the hypothesis of a potential role for TRH in the pathogenesis of this disease. TRH is a rapidly degraded peptide with multiple targets, limiting its suitability as a biomarker and drug candidate. Although some studies reported efficacy in neural diseases (depression, spinal cord injury, amyotrophic lateral sclerosis, etc.), therapeutic use of TRH is presently restricted to spinocerebellar degenerative disease. Regulation of TRH production in the hypothalamus, patterns of expression of TRH and its receptor in the body, its role in energy metabolism and in prolactin secretion are addressed in this review.

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 162 mg/kg ip ketamine and then infused intranasally with 20 μl of sterile saline containing either 0 or 5 mg/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.

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.

Comparing label-free quantitative peptidomics approaches to characterize diurnal variation of peptides in the rat suprachiasmatic nucleus

Mammalian circadian rhythm is maintained by the suprachiasmatic nucleus (SCN) via an intricate set of neuropeptides and other signaling molecules. In this work, peptidomic analyses from two times of day were examined to characterize variation in SCN peptides using three different label-free quantitation approaches: spectral count, spectra index and SIEVE. Of the 448 identified peptides, 207 peptides were analyzed by two label-free methods, spectral count and spectral index. There were 24 peptides with significant (adjusted p-value < 0.01) differential peptide abundances between daytime and nighttime, including multiple peptides derived from secretogranin II, cocaine and amphetamine regulated transcript, and proprotein convertase subtilisin/kexin type 1 inhibitor. Interestingly, more peptides were analyzable and had significantly different abundances between the two time points using the spectral count and spectral index methods than with a prior analysis using the SIEVE method with the same data. The results of this study reveal the importance of using the appropriate data analysis approaches for label-free relative quantitation of peptides. The detection of significant changes in so rich a set of neuropeptides reflects the dynamic nature of the SCN and the number of influences such as feeding behavior on circadian rhythm. Using spectral count and spectral index, peptide level changes are correlated to time of day, suggesting their key role in circadian function.

The output signal of Purkinje cells of the cerebellum and circadian rhythmicity

Measurement of clock gene expression has recently provided evidence that the cerebellum, like the master clock in the SCN, contains a circadian oscillator. The cerebellar oscillator is involved in anticipation of mealtime and possibly resides in Purkinje cells. However, the rhythmic gene expression is likely transduced into a circadian cerebellar output signal to exert an effective control of neuronal brain circuits that are responsible for feeding behavior. Using electrophysiological recordings from acute and organotypic cerebellar slices, we tested the hypothesis whether Purkinje cells transmit a circadian modulated signal to their targets in the brain. Extracellular recordings from brain slices revealed the typical discharge pattern previously described in vivo in single cell recordings showing basically a tonic or a trimodal-like firing pattern. However, in acute sagittal cerebellar slices the average spike rate of randomly selected Purkinje cells did not exhibit significant circadian variations, irrespective of their specific firing pattern. Also, frequency and amplitude of spontaneous inhibitory postsynaptic currents and the amplitude of GABA- and glutamate-evoked currents did not vary with circadian time. Long-term recordings using multielectrode arrays (MEA) allowed to monitor neuronal activity at multiple sites in organotypic cerebellar slices for several days to weeks. With this recording technique we observed oscillations of the firing rate of cerebellar neurons, presumably of Purkinje cells, with a period of about 24 hours which were stable for periods up to three days. The daily renewal of culture medium could induce circadian oscillations of the firing rate of Purkinje cells, a feature that is compatible with the behavior of slave oscillators. However, from the present results it appears that the circadian expression of cerebellar clock genes exerts only a weak influence on the electrical output of cerebellar neurons.

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-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 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 α(1)-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-NH(2)) 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-NH(2), 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 α(1)-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 1mg/kg rat leptin and peptide and protein levels were measured in brain and peripheral tissues at 0, 0.5, 1 and 2h 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 0.5h consistent with leptin-induced release of these peptides: STR (7 downward arrow). Significant changes in TRH and TRH-like peptide levels for other brain regions were: CBL (5 downward arrow), ENT (5 downward arrow), HC (4 downward arrow), AY (4 downward arrow), FCX (3 downward arrow), and ACNG (1 downward arrow). 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-3beta 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-3beta (GSK-3beta), 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-3beta in the modulation of depression by TRH and TRH-like peptides and the therapeutic potential of GSK-3beta inhibitors for neuropsychiatric and metabolic diseases, young adult male Sprague-Dawley (SD) rats were (a) injected ip with 1.8mg/kg of GSK-3beta inhibitor VIII (GSKI) and sacrificed 0, 2, 4, 6, and 8h later or (b) injected with 0, 0.018, 0.18 or 1.8mg/kg GSKI and bled 4h 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 4h. In contrast, a nearly complete loss of TRH and TRH-like peptides from testis within 2h and pancreas by 4h 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-beta 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-3beta.

TRH-receptor-type-2-deficient mice are euthyroid and exhibit increased depression and reduced anxiety phenotypes

Thyrotropin-releasing hormone (TRH) is a neuropeptide that initiates its effects in mice by interacting with two G-protein-coupled receptors, TRH receptor type 1 (TRH-R1) and TRH receptor type 2 (TRH-R2). Two previous reports described the effects of deleting TRH-R1 in mice. TRH-R1 knockout mice exhibit hypothyroidism, hyperglycemia, and increased depression and anxiety-like behavior. Here we report the generation of TRH-R2 knockout mice. The phenotype of these mice was characterized using gross and histological analyses along with blood hematological assays and chemistries. Standard metabolic tests to assess glucose and insulin tolerance were performed. Behavioral testing included elevated plus maze, open field, tail suspension, forced swim, and novelty-induced hypophagia tests. TRH-R2 knockout mice are euthyroid with normal basal and TRH-stimulated serum levels of thyroid-stimulating hormone (thyrotropin), are normoglycemic, and exhibit normal development and growth. Female, but not male, TRH-R2 knockout mice exhibit moderately increased depression-like and reduced anxiety-like phenotypes. Because the behavioral changes in TRH-R1 knockout mice may have been caused secondarily by their hypothyroidism whereas TRH-R2 knockout mice are euthyroid, these data provide the first evidence for the involvement of the TRH/TRH-R system, specifically extrahypothalamic TRH/TRH-R2, in regulating mood and affect.

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.

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.