Release of pro-thyrotropin-releasing hormone connecting peptides PS4 and PS5 from perifused rat hypothalamic slices

Potent hyperglycemic and hyperinsulinemic effects of thyrotropin-releasing hormone microinjected into the rostroventrolateral medulla and abnormal responses in type 2 diabetic rats

We identified ventrolateral medullary nuclei in which thyrotropin-releasing hormone (TRH) regulates glucose metabolism by modulating autonomic activity. Immunolabeling revealed dense prepro-TRH-containing fibers innervating the rostroventrolateral medulla (RVLM) and nucleus ambiguus (Amb), which contain, respectively, pre-sympathetic motor neurons and vagal motor neurons. In anesthetized Wistar rats, microinjection of the stable TRH analog RX77368 (38-150 pmol) into the RVLM dose-dependently and site-specifically induced hyperglycemia and hyperinsulinemia. At 150 pmol, blood glucose reached a peak of 180+/-18 mg% and insulin increased 4-fold. The strongest hyperglycemic effect was induced when RX77368 was microinjected into C1 area containing adrenalin cells. Spinal cord transection at cervical-7 abolished the hyperglycemia induced by RVLM RX77368, but not the hyperinsulinemic effect. Bilateral vagotomy prevented the rise in insulin, resulting in a prolonged hyperglycemic response. The hyperglycemic and hyperinsulinemic effects of the TRH analog in the RVLM was peptide specific, since angiotensin II or a substance P analog at the same dose had weak or no effects. Microinjection of RX77368 into the Amb stimulated insulin secretion without influencing glucose levels. In conscious type 2 diabetic Goto-Kakizaki (GK) rats, intracisternal injection of RX77368 induced a remarkably amplified hyperglycemic effect with suppressed insulin response compared to Wistar rats. RX77368 microinjected into the RVLM of anesthetized GK rats induced a significantly potentiated hyperglycemic response and an impaired insulin response, compared to Wistar rats. These results indicate that the RVLM is a site at which TRH induces sympathetically-mediated hyperglycemia and vagally-mediated hyperinsulinemia, whereas the Amb is mainly a vagal activating site for TRH. Hyperinsulinemia induced by TRH in the RVLM is not secondary to the hyperglycemic response. The potentiated hyperglycemic and suppressed hyperinsulinemic responses in diabetic GK rats indicate that an unbalanced "sympathetic-over-vagal" activation by TRH in brainstem RVLM contributes to the pathophysiology of impaired glucose homeostasis in type 2 diabetes.

Inhibitory effect of prepro-thyrotrophin-releasing hormone (178-199) on adrenocorticotrophic hormone secretion by human corticotroph tumours

Prepro-thyrotrophin-releasing hormone (TRH) (178-199), a 22-amino acid cleavage product of the TRH prohormone, has been postulated to act as an adrenocorticotrophin hormone (ACTH)-release inhibitor. Indeed, although in vitro evidence indicates that this peptide may inhibit basal and stimulated ACTH secretion in rodent anterior pituitary primary cultures and cell lines, not all studies concur and no study has as yet evaluated the effect of this peptide in Cushing's disease. The present study aimed to test the effect of preproTRH(178-199) in human tumoural corticotrophs. Twenty-four human ACTH-secreting pituitary tumours (13 macroadenomas, 11 microadenomas) were collected during surgery and incubated with 10 or 100 nm preproTRH(178-199). ACTH secretion was assessed after 4 and 24 h of incubation by immunometric assay and expressed relative to levels observed in control, unchallenged wells (= 100%). Parallel experiments were performed in rat anterior pituitary primary cultures. A clear inhibition of ACTH secretion at 4 and 24 h was observed in 12 specimens (for 10 nm ppTRH: 70 +/- 4% control at 4 h and 83 +/- 5% control at 24 h; for 100 nm ppTRH: 70 +/- 4% control at 4 h and 85 +/- 5% control at 24 h), whereas a mild and short-lasting stimulatory effect was observed in three tumours and no changes in ACTH secretion in the remaining nine tumoural specimens. The inhibitory effect of preproTRH(178-199) was more evident in macroadenomas and significantly correlated with sensitivity to dexamethasone inhibition. Significant inhibition of ACTH secretion by preproTRH(178-199) in rat pituitary cultures was observed after 24 h of incubation. The present study conducted in a large series of human corticotroph tumours shows that preproTRH(178-199) inhibits tumoural ACTH secretion in a sizable proportion of specimens, in close relation to the size of the tumour and its sensitivity to glucocorticoid negative feedback. This appears a promising avenue of research and further studies are warranted to explore the full scope of preproTRH(178-199) as a regulator of ACTH secretion.

Mice lacking the thyrotropin-releasing hormone gene: what do they tell us?

Thyrotropin-releasing hormone (TRH) is localized in the brain hypothalamus and stimulates the secretion and synthesis of pituitary thyrotropin (TSH). Although TRH deficiency caused by artificial hypothalamic destructions has been reported to result in significant decreases in TSH secretion in rodents, clinical observations from the patients with possible TRH deficiency did not entirely agree with these animal results. Because of its ubiquitous distribution throughout the brain and in the peripheral tissues, TRH has been suggested to possess a wide variety of functions in these regions. However, the neurobehavioral and peripheral actions of TRH still remains to be established. It has been, therefore, anticipated that detailed analysis of TRH-knockout mice might provide insight into the physiological significance of endogenous TRH. The present review focuses on the phenotypic findings of mice deficient in TRH.

Behavioral, hormonal and histological stress markers of anxiety-separation in postnatal rats are reduced by prepro-thyrotropin-releasing hormone 178-199

We investigated in the present study whether systemic injections of prepro-thyrotropin-releasing-hormone 178-199 (PPTRH 178-199) in postnatal 3-days old rat pups can provide ameliorative effects in a model of anxiety-separation disorder. The pups were individually separated from their mother and placed in a novel environment. PPTRH 178-199-treated animals started exploring the novel environment in a significantly shorter time and elicited significantly less distress vocalizations than control animals. PPTRH 178-199-treated animals also had markedly lower serum adrenocorticotropic hormone and corticosterone compared to control animals. Furthermore, we observed a significant increase in PPTRH 178-199 immunoreactive cell bodies in the hypothalamus of PPTRH 178-199-treated animals compared to controls, suggesting that the peptide crossed the blood-brain barrier. PPTRH 178-199 treatment can help to reduce behavioral and hormonal disturbances associated with anxiety-separation situations.

Neuroregulation of ProTRH biosynthesis and processing

This review presents an overview of the current knowledge on proTRH biosynthesis, its processing, its tissue distribution, and the role of known processing enzymes in proTRH maturation. The neuroendocrine regulation of TRH biosynthesis, the biological actions of its products, and the signal transduction and catabolic pathways used by those products are also reviewed. The widespread expression of proTRH, PC1, and PC2 rnRNAs in hypophysiotropic and extrahypophysiotropic areas of the brain, with their overlapping distribution in many areas, indicates the striking versatility provided by tissue-specific processing in generating quantitative and qualitative differences in nonTRH peptide products as well as TRH. Evidence is presented suggesting that differential processing for proTRH at the intracellular level is physiologically relevant. It is clear that control over the diverse range of proTRH-derived peptides within a specific cell is accomplished most from the regulation at the posttranslational level rather than the translational or transcriptional levels. Several examples supporting this hypothesis are presented in this review. A better understanding of proTRH-derived peptides role represents an exciting new frontier in proTRH research. These connecting sequences in between TRH molecules to form the precursor protein may function as structural or targeting elements that guide the folding and sorting of proTRH and its larger intermediates so that subsequent processing and secretion are properly regulated. The particular anatomical distribution of the proTRH end products, as well as regulation of their levels by neuroendocrine or pharmacological manipulations, supports a unique potential biologic role for these peptides.

Effect of the removal of extracellular Ca2+ on the response of cytosolic concentrations of Ca2+ to ouabain in carotid body glomus cells of adult rabbits

Effect of the removal of extracellular Ca2+ on the response of cytosolic concentrations of Ca2+ ([Ca2+]i) to ouabain, an Na+/K+ exchanger antagonist, was examined in clusters of cultured carotid body glomus cells of adult rabbits using fura-2AM and microfluorometry. Application of ouabain (10 mM) induced a sustained increase in [Ca2+]i (mean+/-S.E.M.; 38+/-5% increase, n=16) in 55% of tested cells (n=29). The ouabain-induced [Ca2+]i increase was abolished by the removal of extracellular Na+. D600 (50 microM), an L-type voltage-gated Ca2+ channel antagonist, inhibited the [Ca2+]i increase by 57+/-7% (n=4). Removal of extracellular Ca2+ eliminated the [Ca2+]i increase, but subsequent washing out of ouabain in Ca2+-free solution produced a rise in [Ca2+]i (62+/-8% increase, n=6, P<0.05), referred to as a [Ca2+]i rise after Ca2+-free/ouabain. The magnitude of the [Ca2+]i rise was larger than that of ouabain-induced [Ca2+]i increase. D600 (5 microM) inhibited the [Ca2+]i rise after Ca2+-free/ouabain by 83+/-10% (n=4). These results suggest that ouabain-induced [Ca2+]i increase was due to Ca2+ entry involving L-type Ca2+ channels which could be activated by cytosolic Na+ accumulation. Ca2+ removal might modify the [Ca2+]i response, resulting in the occurrence of a rise in [Ca2+]i after Ca2+-free/ouabain which mostly involved L-type Ca2+ channels.

A novel endogenous corticotropin release inhibiting factor

ACTH is the major regulator of the body's adaptive response to stress and the physiological stimulus for glucocorticoid secretion. A hypothalamic corticotropin release inhibiting factor (CRIF) that inhibits ACTH synthesis and secretion has long been postulated but was not characterized until recently. We have recently identified a 22 amino acid peptide, prepro-thyrotropin releasing hormone (TRH) 178-199 that inhibits basal and stimulated ACTH synthesis and secretion in vitro and stress-induced ACTH secretion in vivo. Prepro-TRH 178-199 is abundant in several brain regions, including the external zone of the median eminence, where its concentration changes in response to stress. We propose that this peptide is a physiological regulator of ACTH production: an endogenous CRIF. Because prepro-TRH 178-199 is encoded within the same precursor as TRH, its expression is likely to be negatively regulated by thyroid hormones leading to changes in endogenous glucocorticoid levels. Streptococcal cell wall (SCW)-induced inflammation, a model of rheumatoid arthritis (RA), was alleviated after long-term thyroxine treatment. Inversely, a hypothyroid milieu led to decreased basal hypothalamic-pituitary-adrenal activity, but increased expression of IL-1 beta and MIP-1 alpha, specific markers for RA in humans. These results suggest that this putative CRIF may be an important component in the development of RA and that regulation of prepro TRH may be highly relevant to the development of other autoimmune diseases that are also exacerbated by low endogenous glucocorticoid levels.

Inhibition of stress-induced neuroendocrine and behavioral responses in the rat by prepro-thyrotropin-releasing hormone 178-199

A corticotropin release-inhibiting factor (CRIF) in brain has been postulated for several decades, based on increased plasma levels of ACTH and corticosterone after hypothalamic-pituitary disconnection. Recent in vitro studies indicate that prepro-TRH178-199 may function as an endogenous CRIF, prompting us to examine stress-related neuroendocrine and behavioral responses after in vivo administration to the adult male rat. Animals that were administered prepro-TRH178-199 intravenously 5 min before restraint stress exhibited a significant attenuation of stress-induced elevations of ACTH, corticosterone, and prolactin, as compared with controls infused with vehicle, whereas thyroid-stimulating hormone (TSH) secretion was not changed. In behavioral studies of stress responsiveness, either the vehicle or prepro-TRH178-199 was administered intracerebroventricularly (ICV) 5 min before testing. In the open field, prepro-TRH178-199 significantly increased grooming, locomotor activity, rearing, and sniffing behaviors. In the light/dark box, it significantly increased the time animals spent in the light compartment and increased the number of crossings between the light/dark compartments. In the plus maze, the peptide significantly increased the amount of time animals spent in the open arms. The same dose of peptide, administered ICV, had no effect on peripheral hormone release in response to restraint stress. Overall, these results support a role for prepro-TRH178-199 in the inhibition of the neuroendocrine responses to stress at the level of the pituitary and indicate that it has central modulatory influences over stress-related behaviors.

Prepro-TRH-(160-169) potentiates gastric acid secretion stimulated by TRH microinjected into the dorsal motor nucleus of the vagus

Prepro-TRH-(160-169) (Ps4), one of the predicted connecting peptides of pro-TRH, potentiates TRH-induced TSH release in vivo and in vitro. The influence of Ps4 on TRH in medullary nuclei-induced vagal stimulation of gastric acid secretion was studied in urethane-anesthetized rats with gastric cannula. Ps4 injected into the dorsal motor nucleus of the vagus (DMN) (200 ng) did not influence basal gastric secretion. Ps4 (100, 150 and 200 ng) co-injected with TRH (50 ng) into the DMN potentiated the acid response to TRH by 14%, 76% and 182% respectively while prepro-TRH-(178-199) (Ps5, 438 ng) had no effect. Ps4 (200 ng) co-injected with TRH into the nucleus ambiguus did not modify TRH (50 ng)-induced acid secretion. These results indicate that Ps4 potentiates TRH in the DMN-evoked gastric acid secretion through an action that is peptide and site specific.

Analysis of structural requirements for TRH-potentiating peptide receptor binding by analogue design

Previous studies established that the [125I-Tyr0]Ps4 derivative of TRH-potentiating peptide (Ps4), Ser-Phe-Pro-Trp-Met-Glu-Ser-Asp-Val-Thr, displays high affinity and selectivity for an orphan membrane receptor in rat anterior pituitary. To identify the sites in Ps4 that determine receptor binding affinity, we have synthesized and screened a panel of 15 single-point substituted analogues for their ability to displace bound [125I-Tyr0]Ps4. The affinity of [Tyr0]Ps4 for rat anterior pituitary membranes [inhibitory constant (Ki), approximately 5 nM] was drastically reduced by the substitution of either Tyr0 with Gly or Asp8 with Asn (Ki approximately 95 and approximately 51 nM, respectively). Deamination of [Tyr0]Ps4 also sharply reduced affinity (Ki approximately 1100 nM). In contrast, Ser1-->Ala, Pro3-->Ala and Thr10-->Val substitutions led to analogues showing a tenfold increase in binding affinity relative to the parent peptide. The change of Phe2-->Leu, Trp4-->Ala, Glu6-->Gln, Val9-->Thr and carboxamidation of Thr10 had no effect on binding affinity. The data suggest that substitutions of the amino-terminal group, Asp8 and Tyr0, have a marked effect on the ability of [Tyr0]Ps4 to compete with [125I-Tyr0]Ps4 for binding to TRH-potentiating peptide pituitary receptor.

Modulation of the biological activity of thyrotropin-releasing hormone by alternate processing of pro-TRH

Thyrotropin-releasing hormone prohormone contains multiple copies of TRH linked together by connecting sequences. Like other plurifunctional prohormone proteins, pro-TRH undergoes differential proteolytic processing in various tissues to generate, beside authentic TRH, several other novel peptides corresponding to C-terminally extended forms of TRH and connecting fragments. The pro-TRH connecting peptides are, together with TRH, predominant storage forms of TRH-precursor related peptides in the rat hypothalamus. Connecting peptides are co-localized with TRH in the median eminence nerve endings and co-released through a mechanism involving voltage-operated Ca2+ channels. The connecting peptide Ps4 is involved in potentiation of the action of TRH on thyrotropin hormone release by pituitary in vitro and in vivo through interactions with a specific pituitary cell receptor coupled to dihydropyridine and omega-connotoxin sensitive Ca2+ channels of the L-type. It also causes dose-dependent increases in the steady state levels of mRNAs of TSH and prolactin through stimulation of the respective gene promoter activities. These findings indicate that Ps4 and TRH, two peptides which originate from a single multifunctional biosynthetic precursor, can function on the same target tissues in a coordinate manner to promote hormonal secretion. This suggests that differential processing of the TRH prohormone may have the potential to modulate the biological activities of TRH.

Neuroendocrine regulation of thyrotropin-releasing hormone (TRH) in the tuberoinfundibular system

[...] It is now required to list each part needed for mucous excretion. They are two ducts in the brain substance, then a thin portion of membrane shaped as the infundibulum, then the gland that receives the tip of this infundibulum and the ducts that drive the mucus (pituita) from this gland to the palate and nares. [...] and I said that one (duct) [...] from the middle of the common cavity (third ventricle) descends [...] into the brain substance, and the end of this duct is [...] the sinus of the gland where the brain mucus is collected [...].

Isolation and amino acid sequence of the TRH-potentiating peptide from bovine hypothalamus

A neuropeptide termed TRH-potentiating peptide, which potentiates TRH-evoked thyrotropin secretion by antehypophysis in vitro, was isolated from an acetonic powder of bovine hypothalamus. The peptide was purified to homogeneity by a 3-step protocol involving molecular sieve filtration, ion-exchange chromatography and reverse phase high performance liquid chromatography. The complete amino acid sequence of the decapeptide was determined as Ser-Phe-Pro-Trp-Met-Glu-Ser-Asp-Val-Thr by automated Edman degradation with a solid-phase sequencer. Bovine TRH-potentiating peptide is structurally identical to Ps4, a decapeptide which was deduced from the cDNA encoding the rat TRH precursor. This study provides for the first time a direct chemical evidence for the existence of non-TRH peptides originating from posttranslational processing of the TRH precursor in vivo.

Omega-conotoxin- and nifedipine-insensitive voltage-operated calcium channels mediate K(+)-induced release of pro-thyrotropin-releasing hormone-connecting peptides Ps4 and Ps5 from perifused rat hypothalamic slices

The rat thyrotropin-releasing hormone (TRH) precursor (prepro-TRH) contains five copies of the TRH progenitor sequence linked together by intervening sequences. Recently, we have shown that the connecting peptides prepro-TRH-(160-169) (Ps4) and prepro-TRH-(178-199) (Ps5) are released from rat hypothalamic neurones in response to elevated potassium concentrations, in a calcium-dependent manner. In the present study, the role of voltage-operated calcium channels in potassium-induced release of Ps4 and Ps5 was investigated, using a perifusion system for rat hypothalamic slices. The release of Ps4 and Ps5 stimulated by potassium (70 mM) was blocked by the inorganic ions Co2+ (2.6 mM) and Ni2+ (5 mM). In contrast, the stimulatory effect of KCl was insensitive to Cd2+ (100 microM). The dihydropyridine antagonist nifedipine (10 microM) had no effect on K(+)-evoked release of Ps4 and Ps5. Furthermore, the response to KCl was not affected by nifedipine (10 microM) in combination with diltiazem (1 microM), a benzothiazepine which increases the affinity of dihydropyridine antagonists for their receptor. The dihydropyridine agonist BAY K 8644, at concentrations as high as 1 mM, did not stimulate the basal secretion of Ps4 and Ps5. In addition, BAY K 8644 had no potentiating effect on K(+)-induced release of Ps4 and Ps5. The marine cone snail toxin omega-conotoxin, a blocker of both L- and N-type calcium channels had no effect on the release of Ps4 and Ps5 stimulated by potassium. Similarly, the omega-conopeptide SNX-111, a selective blocker of N-type calcium channels, did not inhibit the stimulatory effect of potassium. The release of Ps4 and Ps5 evoked by high K+ was insensitive to the non-selective calcium channel blocker verapamil (20 microM). Amiloride (1 microM), a putative blocker of T-type calcium channels, did not affect KCl-induced secretion of the two connecting peptides. Taken together, these results indicate that two connecting peptides derived from the pro-TRH, Ps4 and Ps5, are released by K(+)-induced depolarization through activation of voltage-sensitive calcium channels. The calcium channels appear to have a pharmacological profile different from that of L- and N-type channels. Although, their insensitivity to low Cd2+ concentrations and sensitivity to Ni2+ ions would support the involvement of T-type calcium channels, the lack of effect of amiloride suggests that they belong to a yet undefined class of calcium channels.