Region-specific expression of thyrotrophin-releasing hormone-degrading ectoenzyme in the rat central nervous system and pituitary gland

Evolution of thyrotropin-releasing factor extracellular communication units

Thyroid hormones (THs) are ancient signaling molecules that contribute to the regulation of metabolism, energy homeostasis and growth. In vertebrates, the hypothalamus-pituitary-thyroid (HPT) axis links the corresponding organs through hormonal signals, including thyrotropin releasing factor (TRF), and thyroid stimulating hormone (TSH) that ultimately activates the synthesis and secretion of THs from the thyroid gland. Although this axis is conserved among most vertebrates, the identity of the hypothalamic TRF that positively regulates TSH synthesis and secretion varies. We review the evolution of the hypothalamic factors that induce TSH secretion, including thyrotropin-releasing hormone (TRH), corticotrophin-releasing hormone (CRH), urotensin-1-3, and sauvagine, and non-mammalian glucagon-like peptide in metazoans. Each of these peptides is part of an extracellular communication unit likely composed of at least 3 elements: the peptide, G-protein coupled receptor and bioavailability regulator, set up on the central neuroendocrine articulation. The bioavailability regulators include a TRH-specific ecto-peptidase, pyroglutamyl peptidase II, and a CRH-binding protein, that together with peptide secretion/transport rate and transduction coupling and efficiency at receptor level shape TRF signal intensity and duration. These vertebrate TRF communication units were coopted from bilaterian ancestors. The bona fide elements appeared early in chordates, and are either used alternatively, in parallel, or sequentially, in different vertebrate classes to control centrally the activity of the HPT axis. Available data also suggest coincidence between apparition of ligand and bioavailability regulator.

The Thyrotropin-Releasing Hormone-Degrading Ectoenzyme, a Therapeutic Target?

Thyrotropin releasing hormone (TRH: Glp-His-Pro-NH2) is a peptide mainly produced by brain neurons. In mammals, hypophysiotropic TRH neurons of the paraventricular nucleus of the hypothalamus integrate metabolic information and drive the secretion of thyrotropin from the anterior pituitary, and thus the activity of the thyroid axis. Other hypothalamic or extrahypothalamic TRH neurons have less understood functions although pharmacological studies have shown that TRH has multiple central effects, such as promoting arousal, anorexia and anxiolysis, as well as controlling gastric, cardiac and respiratory autonomic functions. Two G-protein-coupled TRH receptors (TRH-R1 and TRH-R2) transduce TRH effects in some mammals although humans lack TRH-R2. TRH effects are of short duration, in part because the peptide is hydrolyzed in blood and extracellular space by a M1 family metallopeptidase, the TRH-degrading ectoenzyme (TRH-DE), also called pyroglutamyl peptidase II. TRH-DE is enriched in various brain regions but is also expressed in peripheral tissues including the anterior pituitary and the liver, which secretes a soluble form into blood. Among the M1 metallopeptidases, TRH-DE is the only member with a very narrow specificity; its best characterized biological substrate is TRH, making it a target for the specific manipulation of TRH activity. Two other substrates of TRH-DE, Glp-Phe-Pro-NH2 and Glp-Tyr-Pro-NH2, are also present in many tissues. Analogs of TRH resistant to hydrolysis by TRH-DE have prolonged central efficiency. Structure-activity studies allowed the identification of residues critical for activity and specificity. Research with specific inhibitors has confirmed that TRH-DE controls TRH actions. TRH-DE expression by β2-tanycytes of the median eminence of the hypothalamus allows the control of TRH flux into the hypothalamus-pituitary portal vessels and may regulate serum thyrotropin secretion. In this review we describe the critical evidences that suggest that modification of TRH-DE activity in tanycytes, and/or in other brain regions, may generate beneficial consequences in some central and metabolic disorders and identify potential drawbacks and missing information needed to test these hypotheses.

Tanycytes and the Control of Thyrotropin-Releasing Hormone Flux Into Portal Capillaries

Central and peripheral mechanisms that modulate energy intake, partition and expenditure determine energy homeostasis. Thyroid hormones (TH) regulate energy expenditure through the control of basal metabolic rate and thermogenesis; they also modulate food intake. TH concentrations are regulated by the hypothalamus-pituitary-thyroid (HPT) axis, and by transport and metabolism in blood and target tissues. In mammals, hypophysiotropic thyrotropin-releasing hormone (TRH) neurons of the paraventricular nucleus of the hypothalamus integrate energy-related information. They project to the external zone of the median eminence (ME), a brain circumventricular organ rich in neuron terminal varicosities and buttons, tanycytes, other glial cells and capillaries. These capillary vessels form a portal system that links the base of the hypothalamus with the anterior pituitary. Tanycytes of the medio-basal hypothalamus express a repertoire of proteins involved in transport, sensing, and metabolism of TH; among them is type 2 deiodinase, a source of 3,3',5-triiodo-L-thyronine necessary for negative feedback on TRH neurons. Tanycytes subtypes are distinguished by position and phenotype. The end-feet of β2-tanycytes intermingle with TRH varicosities and terminals in the external layer of the ME and terminate close to the ME capillaries. Besides type 2 deiodinase, β2-tanycytes express the TRH-degrading ectoenzyme (TRH-DE); this enzyme likely controls the amount of TRH entering portal vessels. TRH-DE is rapidly upregulated by TH, contributing to TH negative feedback on HPT axis. Alterations in energy balance also regulate the expression and activity of TRH-DE in the ME, making β2-tanycytes a hub for energy-related regulation of HPT axis activity. β2-tanycytes also express TRH-R1, which mediates positive effects of TRH on TRH-DE activity and the size of β2-tanycyte end-feet contacts with the basal lamina adjacent to ME capillaries. These end-feet associations with ME capillaries, and TRH-DE activity, appear to coordinately control HPT axis activity. Thus, down-stream of neuronal control of TRH release by action potentials arrival in the external layer of the median eminence, imbricated intercellular processes may coordinate the flux of TRH into the portal capillaries. In conclusion, β2-tanycytes appear as a critical cellular element for the somatic and post-secretory control of TRH flux into portal vessels, and HPT axis regulation in mammals.

TRH regulates action potential shape in cerebral cortex pyramidal neurons

Thyrotropin releasing hormone (TRH) is a neuropeptide with a wide neural distribution and a variety of functions. It modulates neuronal electrophysiological properties, including resting membrane potential, as well as excitatory postsynaptic potential and spike frequencies. We explored, with whole-cell patch clamp, TRH effect on action potential shape in pyramidal neurons of the sensorimotor cortex. TRH reduced spike and after hyperpolarization amplitudes, and increased spike half-width. The effect varied with dose, time and cortical layer. In layer V, 0.5µM of TRH induced a small increase in spike half-width, while 1 and 5µM induced a strong but transient change in spike half-width, and amplitude; after hyperpolarization amplitude was modified at 5µM of TRH. Cortical layers III and VI neurons responded intensely to 0.5µM TRH; layer II neurons response was small. The effect of 1µM TRH on action potential shape in layer V neurons was blocked by G-protein inhibition. Inhibition of the activity of the TRH-degrading enzyme pyroglutamyl peptidase II (PPII) reproduced the effect of TRH, with enhanced spike half-width. Many cortical PPII mRNA+ cells were VGLUT1 mRNA+, and some GAD mRNA+. These data show that TRH regulates action potential shape in pyramidal cortical neurons, and are consistent with the hypothesis that PPII controls its action in this region.

GIRK-like and TRPC-like conductances mediate thyrotropin-releasing hormone-induced increases in excitability in thalamic paraventricular nucleus neurons

The thalamic paraventricular nucleus (PVT), reported to participate in arousal and motivated behaviors, contains abundant receptors for thyrotropin-releasing hormone (TRH), a neuropeptide also known to modulate arousal and mood. To test the hypothesis that TRH could influence the excitability of PVT neurons, whole cell patch-clamp recordings obtained in rat brain slice preparations were evaluated during bath applied TRH. In the majority of neurons tested, TRH induced reversible TTX-resistant membrane depolarization. Under voltage-clamp, TRH induced a concentration-dependent G protein- mediated inward current. The mean net TRH-induced current exhibited a decrease in membrane conductance. Further analyses identified two concurrent conductances contributing to the TRH-induced response. One conductance featured a Na(+)-independent and K(+)-dependent net current that displayed rectification and was suppressed by micromolar concentrations of Ba(2+) and two GIRK antagonists, tertiapin Q and SCH 23390. The second conductance featured a Na(+)-dependent net inward current with an I-V relationship that exhibited double rectification with a negative slope conductance below -40 mV. This conductance was suppressed by nonselective TRPC channel blockers 2-APB, flufenamic acid and ML204, enhanced by La(3+) in a subpopulation of cells, and unchanged by the TRPV1 antagonist capsazepine or a Na(+)/Ca(2+) exchanger blocker KB-R7943. TRH also enhanced hyperpolarization-activated low threshold spikes, a feature that was sensitive to pretreatment with either 2-APB or ML204. Collectively, the data imply that TRH enhances excitability in PVT neurons via concurrently decreasing a G-protein-gated inwardly rectifying K(+) conductance and activating a cationic conductance with characteristics reminiscent of TRPC-like channels, possibly involving TRPC4/C5 subunits.

Thyroxine-induced expression of pyroglutamyl peptidase II and inhibition of TSH release precedes suppression of TRH mRNA and requires type 2 deiodinase

Suppression of TSH release from the hypothyroid thyrotrophs is one of the most rapid effects of 3,3',5'-triiodothyronine (T(3)) or thyroxine (T(4)). It is initiated within an hour, precedes the decrease in TSHβ mRNA inhibition and is blocked by inhibitors of mRNA or protein synthesis. TSH elevation in primary hypothyroidism requires both the loss of feedback inhibition by thyroid hormone in the thyrotrophs and the positive effects of TRH. Another event in this feedback regulation may be the thyroid hormone-mediated induction of the TRH-inactivating pyroglutamyl peptidase II (PPII) in the hypothalamic tanycytes. This study compared the chronology of the acute effects of T(3) or T(4) on TSH suppression, TRH mRNA in the hypothalamic paraventricular nucleus (PVN), and the induction of tanycyte PPII. In wild-type mice, T(3) or T(4) caused a 50% decrease in serum TSH in hypothyroid mice by 5  h. There was no change in TRH mRNA in PVN over this interval, but there was a significant increase in PPII mRNA in the tanycytes. In mice with genetic inactivation of the type 2 iodothyronine deiodinase, T(3) decreased serum TSH and increased PPII mRNA levels, while T(4)-treatment was ineffective. We conclude that the rapid suppression of TSH in the hypothyroid mouse by T(3) occurs prior to a decrease in TRH mRNA though TRH inactivation may be occurring in the median eminence through the rapid induction of tanycyte PPII. The effect of T(4), but not T(3), requires the type 2 iodothyronine deiodinase.

Thyrotropin-releasing hormone d,l polylactide nanoparticles (TRH-NPs) protect against glutamate toxicity in vitro and kindling development in vivo

Thyrotropin-releasing hormone (TRH) is reported to have anticonvulsant effects in animal seizure models and certain intractable epileptic patients. However, its duration of action is limited by rapid tissue metabolism and the blood brain barrier. Direct nose-brain delivery of neuropeptides in sustained-release biodegradable nanoparticles (NPs) is a promising mode of therapy for enhancing CNS bioavailability. Bioactivity/neuroprotection of d,l polylactide nanoparticles containing TRH was assessed against glutamate toxicity in cultured rat fetal hippocampal neurons. Subsequently, we utilized the kindling model of temporal lobe epilepsy to determine if intranasal administration of nanoparticles containing TRH (TRH-NPs) could inhibit kindling development. Animals received daily treatments of either blank (control) or TRH-NPs for 7 days before initiation of kindling. On day 8 and each day thereafter until either fully kindled or until day 20, the animals received daily treatments before receiving a kindling stimulus 3 h later. Afterdischarge duration (ADD) was assessed via electroencephalographs recorded from electrodes in the basolateral amygdalae and behavioral seizure stereotypy was simultaneously recorded digitally. Intranasal application of TRH-NPs resulted in a significant reduction in seizure ADD as kindling progressed, while the number of stimulations required to reach stage V seizures and to become permanently kindled was significantly greater in TRH-NP-treated subjects. Additionally, delay to clonus was significantly prolonged while clonus duration was reduced indicating a less severe seizure in TRH-NP-treated subjects. Our results provide proof of principle that intranasal delivery of sustained-release TRH-NPs may be neuroprotective and can be utilized to suppress seizures and perhaps epileptogenesis.

Tanycyte pyroglutamyl peptidase II contributes to regulation of the hypothalamic-pituitary-thyroid axis through glial-axonal associations in the median eminence

Pyroglutamyl peptidase II (PPII), a highly specific membrane-bound metallopeptidase that inactivates TRH in the extracellular space, is tightly regulated by thyroid hormone in cells of the anterior pituitary. Whether PPII has any role in the region where axons containing hypophysiotropic TRH terminate, the median eminence, is unknown. For this purpose, we analyzed the cellular localization and regulation of PPII mRNA in the mediobasal hypothalamus in adult, male rats. PPII mRNA was localized in cells lining the floor and infralateral walls of the third ventricle and coexpressed with vimentin, establishing these cells as tanycytes. PPII mRNA extended in a linear fashion from the tanycyte cell bodies in the base of the third ventricle to its cytoplasmic and end-feet processes in the external zone of the median eminence in close apposition to pro-TRH-containing axon terminals. Compared with vehicle-treated, euthyroid controls, animals made thyrotoxic by the i.p. administration of 10 microg L-T(4) daily for 1-3 d, showed dramatically increased accumulation of silver grains in the mediobasal hypothalamus and an approximately 80% increase in enzymatic activity. PPII inhibition in mediobasal hypothalamic explants increased TRH secretion, whereas i.p. injection of a specific PPII inhibitor increased cold stress- and TRH-induced TSH levels in plasma. We propose that an increase in circulating thyroid hormone up-regulates PPII activity in tanycytes and enhances degradation of extracellular TRH in the median eminence through glial-axonal associations, contributing to the feedback regulation of thyroid hormone on anterior pituitary TSH secretion.

NMDA receptor up-regulates pyroglutamyl peptidase II activity in the rat hippocampus

Ecto-peptidases hydrolyze peptides in the extracellular fluid of the brain. This process is critical for defining the strength of peptidergic communication. A few studies suggest that brain ecto-peptidase activities are regulated by brain function but the extracellular messengers involved are generally unknown. Pyroglutamyl peptidase II (PPII) is specific for thyrotropin releasing hormone (TRH), a tripeptide with multiple homeostatic functions in brain. The purpose of this study was to identify regulators of brain PPII activity. Electrical stimulation (multiple tetani) did not change PPII activity in cortical or hippocampal slices. However, in hippocampal slices, blockade of calcium channels with high magnesium, or of L-type calcium channels (LTCC) or NMDA receptors, decreased PPII activity, while blockade of AMPA or GABA(A) receptors did not. Blockade of NMDA receptors did not change PPII mRNA levels but decreased PPII levels. The activity of another ecto-peptidase, aminopeptidase N, was also down regulated by a magnesium blockade, not regulated by NMDA receptor blockade and increased by LTCC blockade. The data show a differential regulation of the activity of ecto-peptidases by that of Ca(2+) channel and that synaptic activity, through the NMDA receptor, specifically regulates that of pyroglutamyl peptidase II.

Anterior pituitary pyroglutamyl peptidase II activity controls TRH-induced prolactin release

Ecto-peptidases modulate the action of peptides in the extracellular space. The relationship between peptide receptor and ecto-peptidase localization, and the physiological role of peptidases is poorly understood. Current evidence suggests that pyroglutamyl peptidase II (PPII) inactivates neuronally released thyrotropin-releasing hormone (TRH). The impact of PPII localization in the anterior pituitary on the endocrine activities of TRH is unknown. We have studied whether PPII influences TRH signaling in anterior pituitary cells in primary culture. In situ hybridization (ISH) experiments showed that PPII mRNA was expressed only in 5-6% of cells. ISH for PPII mRNA combined with immunocytochemistry for prolactin, beta-thyrotropin, or growth hormone, showed that 66% of PPII mRNA expressing cells are lactotrophs, 34% somatotrophs while none are thyrotrophs. PPII activity was reduced using a specific phosphorothioate antisense oligodeoxynucleotide or inhibitors. Compared with mock or scrambled oligodeoxynucleotide-treated controls, knock-down of PPII expression by antisense targeting increased TRH-induced release of prolactin, but not of thyrotropin. Similar data were obtained with either a transition-state or a tight binding inhibitor. These results demonstrate that PPII expression in lactotrophs coincides with its ability to control prolactin release. It may play a specialized role in TRH signaling in the anterior pituitary. Anterior pituitary ecto-peptidases may fulfill unique functions associated with their restricted cell-specific expression.

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.

In situ hybridization localization of TRH precursor and TRH receptor mRNAs in the brain and pituitary of Xenopus laevis

We examined the distribution of the mRNAs encoding proTRH and the three TRH receptor subtypes (xTRHR1, xTRHR2, and xTRHR3) in the Xenopus laevis CNS and pituitary. A positive correlation was generally observed between the expression patterns of proTRH and xTRHR mRNAs. xTRHRs were widely expressed in the telencephalon and diencephalon, where two or even three xTRHR mRNAs were often simultaneously observed within the same brain structures. In the pituitary, xTRHR2 was selectively expressed in the distal lobe, and xTRHR3 was found exclusively in the intermediate lobe of white background-adapted animals, indicating that, in amphibians, the effect of TRH on alpha-melanotropin (alpha-MSH) secretion from melanotrope cells is mediated through the novel receptor subtype xTRHR3.

Distribution of the mRNAs encoding the thyrotropin-releasing hormone (TRH) precursor and three TRH receptors in the brain and pituitary of Xenopus laevis: effect of background color adaptation on TRH and TRH receptor gene expression

In amphibians, thyrotropin-releasing hormone (TRH) is a potent stimulator of alpha-melanotropin (alpha-MSH) secretion, so TRH plays a major role in the neuroendocrine regulation of skin-color adaptation. We have recently cloned a third type of TRH receptor in Xenopus laevis (xTRHR3) that has not yet been characterized in any other vertebrate species. In the present study, we have examined the distribution of the mRNAs encoding proTRH and the three receptor subtypes (xTRHR1, xTRHR2, and xTRHR3) in the frog CNS and pituitary, and we have investigated the effect of background color adaptation on the expression of these mRNAs. A good correlation was generally observed between the expression patterns of proTRH and xTRHR mRNAs. xTRHRs, including the novel receptor subtype xTRHR3, were widely expressed in the telencephalon and diencephalon, where two or even three xTRHR mRNAs were often simultaneously observed within the same brain structures. In the pituitary, xTRHR2 was expressed selectively in the distal lobe, and xTRHR3 was found exclusively in the intermediate lobe. Adaptation of frog skin to background illumination had no effect on the expression of proTRH and xTRHRs in the brain. In contrast, adaptation of the animals to a white background provoked an 18-fold increase in xTRHR3 mRNA concentration in the intermediate lobe of the pituitary. These data demonstrate that, in amphibians, the effect of TRH on alpha-MSH secretion is mediated through the novel receptor subtype xTRHR3.

Marginal Involvement of Pyroglutamyl Aminopeptidase I in Metabolism of Thyrotropin-Releasing Hormone in Rat Brain

On thyrotropin-releasing hormone (TRH) metabolism, pyroglutamyl aminopeptidase II (PAP-II), a zinc-dependent ectoenzyme primarily located in the central nervous system, is believed to play a predominant role. Recently we cloned pyroglutamyl aminopeptidase I (PAP-I) which is known for specifically removing a L-pyroglutamate (L-pGlu) residue from the amino terminus of proteins and peptides including TRH. To investigate possible contribution of PAP-I toward TRH metabolism, we conducted biochemical and immunohistochemical characterization using recombinant rat, mouse and human PAP-Is and an antibody raised against rat PAP-I. The Km values toward TRH by the recombinant PAP-Is were about 0.05 mM, being similar value to the reported value of recombinant PAP-II. The L-pGlu-cleaving activities toward TRH in rat brain homogenate were inhibited by a PAP-II specific inhibitor 1,10-phenanthroline, but not inhibited by the antibody against rat PAP-I. Immunohistochemical study in rats revealed heterogeneous distribution of PAP-I in the pituitary, the target tissue of TRH, but the distribution was cytosolic. Taken together, these results suggested that PAP-I might not be dominantly involved in the degradation of TRH in rats. Additionally, we found that PAP-I was localized in the renal proximal tubules. Further investigations are needed for elucidating the function of PAP-I in these restricted sites.

Connective tissue growth factor: a novel marker of layer VII neurons in the rat cerebral cortex

Connective tissue growth factor (CTGF) belongs to a family of secreted, extracellular matrix-associated proteins that are involved in the regulation of cellular functions such as adhesion, migration, mitogenesis, differentiation and survival. Recent studies have also suggested the up-regulation of CTGF in response to trauma, scar formation and excitotoxicity in the CNS. To further elucidate the localization and regulation of this molecule in the rat brain we performed in situ hybridization experiments and found a very strong and selective expression of CTGF messenger ribonucleic acid (mRNA) on the band of layer VII neurons throughout the adult cerebral cortex. Similarly strong neuronal expression was also present in the dorsal endopiriform nucleus, extending rostrally from the ventrocaudal cortical layer VII, and in the deep layers of the olfactory glomeruli and the accessory olfactory nucleus. Double in situ hybridization confirmed selective CTGF mRNA expression on a subpopulation (approximately 35%) of microtubule-associated protein 2 mRNA-positive neurons in the cortical layer VII and the dorsal endopiriform nucleus. The nucleus of lateral olfactory tract showed moderate signal intensity; other parts of the forebrain, mesencephalon and brain stem only revealed a very weak level of CTGF mRNA expression. Non-neuronal expression was rare, considerably weaker than on cortical layer VII neurons, and normally associated with blood vessels. Developmental analysis of CTGF mRNA expression in embryonic and postnatal mouse also showed a moderately late onset at embryonic day 16-18, and confirmed the presence of CTGF mRNA in cortical layer VII in a second rodent species. Interestingly, injury experiments using direct cerebral trauma or injection of excitotoxic kainic acid into rat brain failed to up-regulate CTGF mRNA after injury and during the ensuing period of neuronal cell death, gliosis and neural scar tissue formation. Altogether, the current data suggest a constitutive role of CTGF, particularly in the adult cerebral cortex. In view of the strong ascending projections of subplate neurons into cortical layer 1, this molecule may be involved in the modulation of synaptic input to apical dendrites of pyramidal neurons.

Lithium modulates expression of TRH receptors and TRH-related peptides in rat brain

Lithium is an established mood stabilizer and neuroprotective agent frequently used in the treatment of bipolar disorder and as an adjuvant in drug-resistant unipolar depression. The mechanisms underlying both the therapeutic efficacy of lithium and the exacerbation of symptoms following rapid withdrawal are not understood. From previous studies showing antidepressant and neuroprotective activities of thyrotropin releasing hormone (TRH) and TRH-related neuropeptides we hypothesized that lithium may have substantial effects on the expression and secretion of these peptides and/or their receptors in various rat brain regions involved in the regulation of mood. Chronic lithium effect on TRH receptor binding studies: The effect of 1 and 2 weeks of dietary lithium on [(3)H]3-Me-His-TRH binding to plasma membranes of nucleus accumbens, amygdala and pituitary of young adult male Wistar and the endogenously 'depressed' Wistar Kyoto (WKY) rats was measured by the method of Burt and Taylor [Burt, D.R., Taylor, R.L., Endocrinology 106 (1980) 1416-1423]. Acute, chronic and withdrawal effect of lithium on TRH and TRH-like peptide levels in young, adult male Sprague-Dawley rats: Rats were divided into four lithium treatment groups. Control animals received a standard laboratory rodent chow. The acute group received a single i.p. injection of 1.5 milli-equivalents of LiCl 2 h prior to killing. The chronic and withdrawal groups received standard rodent chow containing 1.7 g/kg LiCl for 2 weeks. Withdrawal rats were returned to standard chow 48 h prior to killing while the chronic animals continued on the LiCl diet. TRH, TRH-Gly (pGlu-His-Pro-Gly, a TRH precursor), EEP (pGlu-Glu-Pro-NH(2), a TRH-like peptide with antidepressant activity) and Ps4 (a prepro-TRH-derived TRH-enhancing decapeptide) immunoreactivity (IR) were measured in 13 brain regions. The remaining samples were pooled and fractionated by high-pressure liquid chromatography followed by EEP radioimmunoassay. Chronic lithium treatment increased [(3)H]3Me-TRH binding in the nucleus accumbens and amygdala about two-fold in both Wistar and WKY rats but no change was observed in pituitary binding. The most widespread changes in TRH and TRH-related peptide levels were observed in the withdrawal group compared to the controls. The direction of change for the total IR was consistent for all TRH-IR and TRH-related peptide-IR within a given tissue. For example, withdrawal increased all peptide levels in the pyriform cortex and striatum but decreased these levels in the anterior cingulate and lateral cerebellum. Both acute injection and chronic treatment with LiCl decreased TRH and TRH-related peptide levels in the entorhinal cortex. Acute injection and withdrawal both increased EEP-IR in striatum by more than two-fold. The acute effects are most likely due to changes in the release of these peptides since 2 h is not sufficient time for alterations in peptide biosynthesis. Chronic treatment increased levels of pGlu-Phe-Pro-NH(2) levels in hippocampus, pGlu-Leu-Pro-NH(2), and peak '2' in septum by more than four-fold. The present results are consistent with a component role for TRH and related peptides in the mood-altering effects of lithium administration and withdrawal frequently observed during treatment for depression and bipolar disorder.

Distribution of thyrotropin-releasing hormone immunoreactivity in the brain of the dogfish Scyliorhinus canicula

To improve knowledge of the peptidergic systems of elasmobranch brains, the distribution of thyrotropin-releasing hormone-immunoreactive (TRHir) neurons and fibers was studied in the brain of the small-spotted dogfish (Scyliorhinus canicula L.). In the olfactory bulbs, small granule neurons richly innervated the olfactory glomeruli. In the telencephalic hemispheres, small TRHir neurons were observed in the superficial dorsal pallium, whereas TRHir fibers were widely distributed in pallial and subpallial regions. In the preoptic region, TRHir neurons formed a caudal ventrolateral group in the preoptic nucleus. In the hypothalamus, the most conspicuous TRHir populations were associated with the lateral hypothalamic recess, but small TRHir populations were found in the posterior tubercle and ventral wall of the posterior recess. The preoptic region and hypothalamus exhibited rich innervation by TRHir fibers. TRHir fibers were observed coursing to the neurohypophysis and the neuroepithelium of the saccus vasculosus, but not to the neurohemal region of the median eminence. Some stellate-like TRHir cells were observed in a few cell cords of the neurointermediate lobe of the hypophysis. The thalamus, pretectum, and midbrain lacked TRHir neurons. Further TRHir neuronal populations were observed in the central gray and superior raphe nucleus of the isthmus, and a few TRHir cells were located in the nucleus of the trigeminal descending tract at the level of the rostral spinal cord. In the brainstem, the central gray, interpeduncular nucleus, secondary visceral region of the isthmus, rhombencephalic raphe, inferior olive, vagal lobe, and Cajal's commissural nucleus were all richly TRHir-innervated. Comparison of the distribution of TRHir neurons observed in the dogfish brain with that observed in teleosts and tetrapods reveals strong resemblance but also interesting differences, indicating the presence of both a conserved basic vertebrate pattern and a number of derived characters.

Cocaine regulates TRH-related peptides in rat brain

Cocaine administration has previously been reported to alter the levels of prepro-TRH mRNA and TRH (pGlu-His-Pro-NH(2)) in the limbic system of rats (J. Neurochem. 60 (1993) 1151). We have now demonstrated that a previously unrecognized family of TRH-like peptides is involved in the actions of cocaine. We treated young adult male Sprague-Dawley rats (five per group, 250g body weight at sacrifice) for 2 weeks with either twice daily injections of saline (control group), twice daily injections of 15mg/kg cocaine until sacrifice (chronic group), single injection of 15mg/kg cocaine 2h prior to sacrifice (acute group) or chronic cocaine injections replaced by saline injections 72h prior to sacrifice (withdrawal group (WD)). Twelve different brain regions were dissected and immunoreactivity for TRH (TRH-IR), EEP (pGlu-Glu-Pro-NH(2); EEP-IR) and related peptides were measured by radioimmunoassay (RIA). High pressure liquid chromatography (HPLC) revealed that in many brain regions EEP-IR and TRH-IR consisted of a mixture of TRH, and other TRH-like peptides including EEP, pGlu-Val-Pro-NH(2) (Val(2)-TRH), pGlu-Tyr-Pro-NH(2) (Tyr(2)-TRH), pGlu-Leu-Pro-NH(2) (Leu(2)-TRH), and pGlu-Phe-Pro-NH(2) (Phe(2)-TRH). Following i.p. injection, these TRH-like peptides readily crossed the blood-brain barrier but cleared very slowly from brain tissues. Acute cocaine produced a 4.1-fold increase in Val(2)-TRH level in medulla while Val(2)-TRH and Tyr(2)-TRH, increased 6.2- and 2.9-fold, respectively in pyriform cortex PYR. TRH and Leu(2)-TRH, decreased 47 and 93%, respectively in the nucleus accumbens (AM) while other EEP-IR peaks decreased 50-100% consistent with the significant decrease in total EEP-IR in the AMs following acute cocaine treatment. Because 2h is too short a time to alter levels of neuropeptides via changes in the rate of biosynthesis, the acute cocaine-induced elevation or reduction in TRH and related peptides is most likely due to suppression or stimulation, respectively, of the corresponding peptide secretion rate. Because TRH and TRH-like peptides have antidepressant, analeptic and euphorigenic properties, we conclude that these endogenous substances are potential mediators of both the cocaine "high" and withdrawal symptoms.

Chronic ingestion of ethanol or glucose solutions affects hypothalamic and limbic TRH metabolism in dams and their pups

The effect of chronic ethanol consumption during pregnancy and lactation on thyrotropin releasing hormone (TRH) metabolism was investigated in the hypothalamus and limbic areas of female rats and their weaned pups. Pregnant female rats received ethanol or isocaloric glucose solution during pregnancy either alone, or also during the 3 weeks of lactation. Thyrotropin (TSH) and corticosterone levels were measured in serum; TRH and TRH-gly concentrations were determined in hypothalamus, hippocampus, n.accumbens, frontal cortex and amygdala of dams and pups at 21 days after parturition. Ethanol or glucose consumption during pregnancy and lactation produced a decrease in TSH levels compared with control animals fed at libitum; water replacement during lactation normalized TSH levels only in glucose-fed dams. Pups from ethanol or pair-fed dams showed low weight and increased TSH levels compared with normal rats. Variations in TRH metabolism were detected in limbic areas. Chronic ethanol caused a decrease in the levels of TRH in the hippocampus and frontal cortex of dams. In contrast, glucose chronic ingestion increased TRH content specifically in n.accumbens and amygdala of dams. Most of the variations in TRH content of limbic areas of pups were not specific for glucose or ethanol treatment and correlated with the deleterious effect of the mother's thyroid condition, although some differences were observed depending on pup's gender. These results support the involvement of TRHergic neurons in the limbic system of the female rat exposed to alcohol or glucose during pregnancy and lactation.

Distribution of thyrotropin-releasing hormone (TRH) immunoreactivity in the brain of the zebrafish (Danio rerio)

The distribution of thyrotropin-releasing hormone (TRH) in the brain of the adult zebrafish was studied with immunohistochemical techniques. In the telencephalon, abundant TRH-immunoreactive (TRHir) neurons were observed in the central, ventral, and supra- and postcommissural regions of the ventral telencephalic area. In the diencephalon, TRHir neurons were observed in the anterior parvocellular preoptic nucleus, the suprachiasmatic nucleus, the lateral hypothalamic nucleus, the rostral parts of the anterior tuberal nucleus and torus lateralis, and the posterior tuberal nucleus. Some TRHir neurons were also observed in the central posterior thalamic nucleus and in the habenula. The mesencephalon contained TRHir cells in the rostrodorsal tegmentum, the Edinger-Westphal nucleus, the torus semicircularis, and the nucleus of the lateral lemniscus. Further TRHir neurons were observed in the interpeduncular nucleus. In the rhombencephalon, TRHir cells were observed in the nucleus isthmi and the locus coeruleus, rostrally, and in the vagal lobe and vagal motor nucleus, caudally. In the forebrain, TRHir fibers were abundant in several regions, including the medial and caudodorsal parts of the dorsal telencephalic area, the ventral and commissural parts of the ventral telencephalic area, the preoptic area, the posterior tubercle, the anterior tuberal nucleus, and the posterior hypothalamic lobe. The dorsal thalamus exhibited moderate TRHir innervation. In the mesencephalon, the optic tectum received a rich TRHir innervation between the periventricular gray zone and the stratum griseum centrale. A conspicuous TRHir longitudinal tract traversed the tegmentum and extended to the rhombencephalon. The medial and lateral mesencephalic reticular areas and the interpeduncular nucleus were richly innervated by TRHir fibers. In the rhombencephalon, the secondary gustatory nucleus received abundant TRHir fibers. TRHir fibers moderately innervated the ventrolateral and ventromedial reticular area and richly innervated the vagal lobe and Cajal's commissural nucleus. Some TRHir fibers coursed in the lateral funiculus of the spinal cord. Some TRHir amacrine cells were observed in the retina. The wide distribution of TRHir neurons and fibers observed in the zebrafish brain suggests that TRH plays different roles. These results in the adult zebrafish reveal a number of differences with respect to the TRHir systems reported in other adult teleosts but were similar to those found during late developmental stages of trout (Díaz et al., 2001).

Antidepressant effects of thyrotropin-releasing hormone analogues using a rodent model of depression

The antidepressant potential of two naturally occurring analogues of thyrotropin-releasing hormone (TRH), pGLU-GLU-PRO-NH2 (EEP) and pGLU-PHE-PRO-NH2 (EFP), were examined using a rodent model of antidepressant efficacy. The Porsolt Swim Test was used to assay the antidepressant properties of these two peptides. Both analogues of TRH produced significant antidepressant effects, with EEP producing the stronger response. No effect of EEP upon triiodothyronine (T3) was observed at the dosage used. EFP, which has previously been demonstrated to crossreact with the TRH receptor, significantly increased serum T3. Since an effect upon T3 was only observed in the weaker of the two compounds, these data suggest that the behavioral effect of EEP was not secondary to stimulation of thyroid hormone. Additionally, the differential behavioral response to the two compounds suggests a degree of sequence specificity in the ability of TRH-like tripeptides to produce an antidepressant effect.

Mutational analysis of the thyrotropin-releasing hormone-degrading ectoenzyme. similarities and differences with other members of the M1 family of aminopeptidases and thermolysin

Thyrotropin-releasing hormone-degrading ectoenzyme (TRH-DE) is a TRH-specific peptidase which catalyzes the inactivation of the peptidergic signal substance TRH. As indicated by sequence alignment, TRH-DE and the other members of the M1 family of aminopeptidases have a distinct set of conserved amino acid residues in common. By replacing amino acid residues that are putatively involved in catalysis, we could demonstrate that the enzymatic activities of the mutants E408D, E442D, E464Q, E464D, Y528F, H507R, and H507F are dramatically decreased, essentially due to the changes of V(max). The mutant enzymes E408Q and E442Q are inactive, whereas the specific enzymatic activity of the mutants R488Q, R488A, and Y554F are similar to that of the wild-type enzyme. These data strongly suggest that E408, E442, Y528, and H507 are involved in the catalytic process of TRH-DE while E464 presumably represents the third zinc-coordinating residue and may be equivalent to E166 in thermolysin. In contrast, amino acid residues R488 and Y554 seem not to be involved in the catalytic mechanism of TRH-DE.

Regulation of TRH and TRH-related peptides in rat brain by thyroid and steroid hormones

To investigate the possibility that TRH (pGlu-His-Pro-NH(2)) and EEP (pGlu-Glu-Pro-NH(2)) contribute to the behavioral and mood changes attending hypothyroidism, hyperthyroidism and hypogonadism, we have treated young, adult, male Sprague-Dawley rats (5/group, 250 g bw at time of sacrifice) for one week with either daily ip injections of saline, 5 microg T(4), 3 mg PTU or castration. Immunoreactivity for TRH (TRH-IR), TRH-Gly (pGlu-His-Pro-Gly, a TRH precursor), EEP and Ps4 (prepro-TRH-derived TRH-enhancing peptide) was measured in 8 brain regions by RIA. Castration reduced the Ps4-IR levels in hippocampus by 80%. High pressure liquid chromatography revealed that in many brain regions EEP-IR and TRH-IR consisted of a mixture of TRH and other TRH-like peptides including EEP, Val(2)-TRH, Tyr(2)-TRH, Leu(2)-TRH and Phe(2)-TRH. Transition from the hyperthyroid to the hypothyroid state increased the Val(2)-TRH and Tyr(2)-TRH levels in the accumbens by 10-fold and 15-fold, respectively, and the corresponding ratios for the pyriform cortex increased 9-fold and 12-fold, respectively. Hypothyroidism and castration reduced the levels of TRH and the majority of other TRH-like peptides in the entorhinal cortex. This is the first report that thyroid and steroid hormones alter the levels of TRH, prepro-TRH-derived peptides, and a newly discovered array of TRH-like neuropeptides in limbic brain regions.

Regulation of adenohypophyseal pyroglutamyl aminopeptidase II activity by thyrotropin-releasing hormone and phorbol esters

Thyrotropin-releasing hormone (TRH) is inactivated by a narrow specificity ectopeptidase, pyroglutamyl aminopeptidase II (PPII), in the proximity of target cells. In adenohypophysis, PPII is present on lactotrophs. Its activity is regulated by thyroid hormones and 17beta-estradiol. Studies with female rat adenohypophyseal cell cultures treated with 3,3',5'-triiodo-L-thyronine (T3) showed that hypothalamic/paracrine factors, including TRH, can also regulate PPII activity. Some of the transduction pathways involve protein kinase C (PKC) and cyclic adenosine monophosphate (cAMP). The purpose of this study was to determine whether T3 levels or gender of animals used to propagate the culture determine the effects of TRH or PKC. PPII activity was lower in cultures from male rats. In cultures from both sexes, T3 induced the activity. The percentages of decrease due to TRH or PKC were independent of T3 or gender; the percentage of decrease due to cAMP may also be independent of gender. These results suggest that T3 and hypothalamic/paracrine factors may independently control PPII activity in adenohypophysis, in either male or female animals.

Analysis of the thyrotropin-releasing hormone-degrading ectoenzyme by site-directed mutagenesis of cysteine residues. Cys68 is involved in disulfide-linked dimerization

Thyrotropin-releasing hormone-degrading ectoenzyme is a member of the M1 family of Zn-dependent aminopeptidases and catalyzes the degradation of thyrotropin-releasing hormone (TRH; Glp-His-Pro-NH2). Cloning of the cDNA of this enzyme and biochemical studies revealed that the large extracellular domain of the enzyme with the catalytically active site contains nine cysteine residues that are highly conserved among species. To investigate the functional role of these cysteines in TRH-DE we used a site-directed mutagenesis approach and replaced individually each cysteine by a serine residue. The results revealed that the proteolytically truncated and enzymatically fully active enzyme consists of two identical subunits that are associated noncovalently by protein-protein interactions but not via interchain S-S bridges. The eight cysteines contained within this region are all important for the structure of the individual subunit and the enzymatic activity, which is dramatically reduced in all mutant enzymes. This is even true for the four cysteines that are clustered within the C-terminal domain remote from the Zn-binding consensus sequence HEICH. In contrast, Cys68, which resides within the stalk region seven residues from the end of the hydrophobic membrane-spanning domain, can be replaced by serine without a significant change in the enzymatic activity. Interestingly, this residue is involved in the formation of an interchain disulfide bridge. Covalent dimerization of the subunits, however, does not seem to be essential for efficient biosynthesis, enzymatic activity and trafficking to the cell surface.

Human TRH-degrading ectoenzyme cDNA cloning, functional expression, genomic structure and chromosomal assignment

Thyrotropin-Releasing Hormone (TRH) is an important extracellular signal substance that acts as a stimulator of hormone secretion from adenohypophyseal target cells and fulfills many criteria for the function of a neuromodulator/neurotransmitter within the central and peripheral nervous systems. The inactivation of TRH-signals is catalysed by a highly specific ectoenzyme. Here, we characterize the human TRH-degrading ectoenzyme (TRH-DE) by primary sequence, functional expression, genomic structure and chromosomal assignment. By screening a cDNA-library constructed from human lung, 5.7 kb of cDNA were identified. The longest open reading frame predicts a type II integral membrane protein of 117 kDa. The extracellular domain contains the HEXXH + E motif that is characteristic of a certain family of Zn-dependent aminopeptidases. Within this family, the sequences of human and rat TRH-DE reveal an unusual high degree of conservation (96% identical residues). Specific enzymatic activity was observed after transfecting COS-7 cells with human TRH-DE cDNA yielding a Km for TRH hydrolysis of 29.7 microM. Northern blot analysis demonstrated a restricted tissue distribution with highest transcript levels in the brain. Using fluorescent in situ hybridization with the cDNA and a genomic lambda clone, respectively, we localized the TRH-DE gene to the long arm of human chromosome 12. Five independent P1 artificial chromosome clones were required to span the complete cDNA sequence and revealed that it is distributed on 19 exons. Interspecies Southern analysis suggests that the gene is present as a single copy in human, monkey, rat, mouse, dog, bovine, rabbit and chicken DNA. All of these data further the notion that the TRH-DE is not an ordinary enzyme but a specific neuropeptidase that has been highly conserved among species.

Pyroglutamyl peptidase: an overview of the three known enzymatic forms

Pyroglutamyl peptidase can be classified as an omega peptidase which hydrolytically removes the amino terminal pyroglutamate (pGlu) residue from specific pyroglutamyl substrates. To date, three distinct forms of this enzyme have been identified in mammalian tissues. Type I is typically a cytosolic, cysteine peptidase displaying a broad pyroglutamyl substrate specificity and low molecular mass. Type II has been shown to be a membrane anchored metalloenzyme of high molecular mass with a narrow substrate specificity restricted to the hypothalamic releasing factor, thyrotropin-releasing hormone (TRH, pGlu-His-Pro-NH2). A third pyroglutamyl peptidase activity has also been observed in mammalian serum which displays biochemical characteristics remarkably similar to those of tissue Type II, namely a high molecular mass, sensitivity to metal chelating agents, and a narrow substrate specificity also restricted to TRH. This serum activity has subsequently been designated 'thyroliberinase'. This review surveys the biochemical, enzymatic, and structural properties of this interesting and unique class of peptidases. It also addresses the putative physiological roles which have been ascribed to these enzymes. Pyroglutamyl peptidase activities isolated and characterized from bacterial sources are also reviewed and compared with their mammalian counterparts.

The thyrotropin-releasing hormone-degrading ectoenzyme: the third element of the thyrotropin-releasing hormone-signaling system

The peptidergic signal substance thyrotropin-releasing hormone (TRH) is inactivated by the TRH-degrading ectoenzyme (TRH-DE), a peptidase that exhibits an extraordinary high degree of substrate specificity and other unusual characteristics. There is no other ectopeptidase known capable of degrading this tripeptideamide, and vice versa, TRH is the only known substrate of this unique enzyme. Thus, studies on this enzyme may reveal new aspects on the function of the TRH signaling system. After succeeding in purifying this enzyme to homogeneity and cloning the cDNA encoding rat TRH-DE, molecular tools became available to study the expression of this enzyme by Northern blot analysis and in situ hybridization histochemistry. The stringent and tissue-specific regulation of the adenohypophyseal TRH-DE by estradiol and thyroid hormones strongly suggests that this enzyme may act as a regulatory element modulating pituitary hormone secretion. In brain, the expression of TRH-DE is not influenced by peripheral hormones but the distinct distribution pattern, and the high activities support the concept that in this tissue TRH-DE may act as a terminator of TRH signals.