The thyrotropin-releasing hormone (TRH) hypothesis of homeostatic regulation: implications for TRH-based therapeutics

Thyrotropin-releasing hormone increases behavioral arousal through modulation of hypocretin/orexin neurons

Thyrotropin-releasing hormone (TRH) has previously been shown to promote wakefulness and to induce arousal from hibernation. Expression of TRH-R1 (TRH receptor 1) is enriched in the tuberal and lateral hypothalamic area (LHA), brain regions in which the hypocretin/orexin (Hcrt) cells are located. Because the Hcrt system is implicated in sleep/wake control, we hypothesized that TRH provides modulatory input to the Hcrt cells. In vitro electrophysiological studies showed that bath application of TRH caused concentration-dependent membrane depolarization, decreased input resistance, and increased firing rate of identified Hcrt neurons. In the presence of tetrodotoxin, TRH induced inward currents that were associated with a decrease in frequency, but not amplitude, of miniature postsynaptic currents (PSCs). Ion substitution experiments suggested that the TRH-induced inward current was mediated in part by Ca(2+) influx. Although TRH did not significantly alter either the frequency or amplitude of spontaneous excitatory PSCs, TRH (100 nm) increased the frequency of spontaneous inhibitory PSCs by twofold without affecting the amplitude of these events, indicating increased presynaptic GABA release onto Hcrt neurons. In contrast, TRH significantly reduced the frequency, but not amplitude, of miniature excitatory PSCs without affecting miniature inhibitory PSC frequency or amplitude, indicating that TRH also reduces the probability of glutamate release onto Hcrt neurons. When injected into the LHA, TRH increased locomotor activity in wild-type mice but not in orexin/ataxin-3 mice in which the Hcrt neurons degenerate postnatally. Together, these results are consistent with the hypothesis that TRH modulates behavioral arousal, in part, through the Hcrt system.

Attenuation of kindled seizures by intranasal delivery of neuropeptide-loaded nanoparticles

Thyrotropin-releasing hormone (TRH; Protirelin), an endogenous neuropeptide, is known to have anticonvulsant effects in animal seizure models and certain intractable epileptic patients. Its duration of action, however, is limited by rapid tissue metabolism and the blood-brain barrier. Direct nose-to-brain delivery of neuropeptides in sustained-release biodegradable nanoparticles (NPs) is a promising mode of therapy for enhancing CNS neuropeptide bioavailability. To provide proof of principle for this delivery approach, we used the kindling model of temporal lobe epilepsy to show that 1) TRH-loaded copolymer microdisks implanted in a seizure focus can attenuate kindling development in terms of behavioral stage, afterdischarge duration (ADD), and clonus duration; 2) intranasal administration of an unprotected TRH analog can acutely suppress fully kindled seizures in a concentration-dependent manner in terms of ADD and seizure stage; and 3) intranasal administration of polylactide nanoparticles (PLA-NPs) containing TRH (TRH-NPs) can impede kindling development in terms of behavioral stage, ADD, and clonus duration. Additionally, we used intranasal delivery of fluorescent dye-loaded PLA-NPs in rats and application of dye-loaded or dye-attached NPs to cortical neurons in culture to demonstrate NP uptake and distribution over time in vivo and in vitro respectively. Also, a nanoparticle immunostaining method was developed as a procedure for directly visualizing the tissue level and distribution of neuropeptide-loaded nanoparticles. Collectively, the data provide proof of concept for intranasal delivery of TRH-NPs as a viable means to 1) suppress seizures and perhaps epileptogenesis and 2) become the lead compound for intranasal anticonvulsant nanoparticle therapeutics.

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

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

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.

The thyrotropin-releasing hormone (TRH)-immune system homeostatic hypothesis

Decades of research have established that the biological functions of thyrotropin-releasing hormone (TRH) extend far beyond its role as a regulator of the hypothalamic-pituitary-thyroid axis. Gary et al. [Gary, K.A., Sevarino, K.A., Yarbrough, G.G., Prange, A.J. Jr., Winokur, A. (2003). The thyrotropin-releasing hormone (TRH) hypothesis of homeostatic regulation: implications for TRH-based therapeutics. J Pharmacol Exp Ther 305(2):410-416.] and Yarbrough et al. [Yarbrough, G.G., Kamath, J., Winokur, A., Prange, A.J. Jr. (2007). Thyrotropin-releasing hormone (TRH) in the neuroaxis: therapeutic effects reflect physiological functions and molecular actions. Med Hypotheses 69(6):1249-1256.] provided a functional framework, predicated on its global homeostatic influences, to conceptualize the numerous interactions of TRH with the central nervous system (CNS) and endocrine system. Herein, we profer a similar analysis to interactions of TRH with the immune system. Autocrine/paracrine cellular signaling motifs of TRH and TRH receptors are expressed in several tissues and organs of the immune system. Consistent with this functional distribution, in vitro and in vivo evidence suggests a critical role for TRH during the developmental stages of the immune system as well as its numerous interactions with the fully developed immune system. Considerable evidence supports a pivotal role for TRH in the pathophysiology of the inflammatory process with specific relevance to the "cytokine-induced sickness behavior" paradigm. These findings, combined with a number of documented clinical actions of TRH strongly support a potential utility of TRH-based therapeutics in select inflammatory disorders. Similar to its global role in behavioral and energy homeostasis a homeostatic role for TRH in its interactions with the immune system is consonant with the large body of available data. Recent advances in the field of immunology provide a significant opportunity for investigation of the TRH-immune system homeostatic hypothesis. Moreover, this hypothesis may provide a foundation for the development of TRH-based therapeutics for certain medical and psychiatric disorders involving immune dysfunction.

Promising avenues of therapeutics for bipolar illness

Basic scientific advances in understanding the neuropsychobioloqy of bipolar disorder have given us a multitude of opportunities to explore and exploit new avenues of therapeutics. Pharmacotherapeutic approaches include: neuropeptides (agonists such as thyrotropin-releasing hormone and antagonists such as corticotropin-releasing hormone), neurotrophic factors (especially brain-derived neurotrophic factor), and glutamatergic mechanisms (such as riluzole, ketamine, and antagonists of the NR-2B subunit of the glutamate receptor). Physiological interventions that would offer alternatives to electroconvulsive therapy include: repeated transcranial magnetic stimulation, especially at more intense stimulation parameters; magnetic stimulation therapy (seizures induced more focally by magnetic rather than electrical stimulation with resulting reduced meaning loss); vagal nerve stimulation, and deep brain stimulation. However, these, as well as the panoply of existing treatments, require further intensive investigation to place each of them in the proper therapeutic seguence and combination for the individual patient, based on development of better clinical and biological predictors of response. Large clinical trial networks and development of systematic research in clinical practice settings, such as that featured by the National Cancer institute for cancer chemotherapy, would greatly accelerate the progress in incorporating new, as well as existing, agents into the best treatment strategies. The bipolar disorders, which are increasingly recognized as complex, highly comorbid conditions with a high morbidity and mortality, of which the majority start in childhood and adolescence, are not likely to respond completely to any single new treatment agent, and new public health initiatives and research strategies are needed as much as any new single treatment advance.

Chronic ethanol intake modifies pyrrolidon carboxypeptidase activity in mouse frontal cortex synaptosomes under resting and K+ -stimulated conditions: role of calcium

Pyrrolidon carboxypeptidase (Pcp) is an omega peptidase that removes pyroglutamyl N-terminal residues of peptides such as thyrotrophin-releasing hormone (TRH), which is one of the neuropeptides that has been localized into many areas of the brain and acts as an endogenous neuromodulator of several parameters related to ethanol (EtOH) consumption. In this study, we analysed the effects of chronic EtOH intake on Pcp activity on mouse frontal cortex synaptosomes and their corresponding supernatant under basal and K+ -stimulated conditions, in presence and absence of calcium (Ca2+) to know the regulation of Pcp on TRH. In basal conditions, chronic EtOH intake significantly decreased synaptosomes Pcp activity but only in absence of Ca2+. However, supernatant Pcp activity is also decreased in presence and absence of calcium. Under K+-stimulated conditions, chronic EtOH intake decreased synaptosomes Pcp activity but only in absence of Ca2+, whereas supernatant Pcp activity was significantly decreased only in presence of Ca2+. The general inhibitory effect of chronic EtOH intake on Pcp activity suggests an inhibition of TRH metabolism and an enhancement of TRH neurotransmitter/neuromodulator functions, which could be related to putative processes of tolerance to EtOH in which TRH has been involved. Our data may also indicate that active peptides and their degrading peptidases are released together to the synaptic cleft to regulate the neurotransmitter/neuromodulator functions of these peptides, through a Ca2+ -dependent mechanism.

The thyrotropin-releasing hormone secretory system in the hypothalamus of the Siberian hamster in long and short photoperiods

Thyrotropin-releasing hormone (TRH) is not only essential for the regulation of the pituitary-thyroid axis, but also exerts complementary effects on energy metabolism within the brain. We hypothesised that increased activity of the TRH secretory system may contribute to seasonal adaptations in the Siberian hamster whereby food intake is decreased in winter, and catabolism of fat stores is increased to support thermogenesis. We determined the distribution of TRH producing neurones and TRH-R1 receptor expressing cells in the hypothalamus, and investigated whether photoperiod regulated this system. TRH-immunoreactive (ir) cell somata and preproTRH mRNA expression were found to be widely distributed throughout the medial hypothalamus, with particular clusters in the paraventricular nucleus, the medial preoptic area and periventricular nucleus, and in the dorsomedial hypothalamus extending into the lateral hypothalamic area. A partial sequence encoding TRH-R1 was cloned from hamster hypothalamic cDNA and used to generate a riboprobe for in situ hybridisation studies. TRH-R1 mRNA expressing cells were abundant throughout the hypothalamus, corresponding to the widespread presence of TRH-ir fibres. Photoperiod did not affect the expression of preproTRH mRNA in any region, and the only significant change in TRH-R1 expression was in the dorsomedial posterior arcuate region. This wide distribution of TRH-producing and receptive cells in the hypothalamus is consistent with its hypothesised neuromodulatory roles in the short-term homeostatic control of appetite, thermoregulation and energy expenditure, but the lack of photoperiodic change in TRH mRNA expression does not support the hypothesis that changes in this system underlie long-term seasonal changes in body weight.

Analysis of the anxiolytic-like effect of TRH and the response of amygdalar TRHergic neurons in anxiety

Thyrotropin-releasing hormone (TRH) was first described for its neuroendocrine role in controlling the hypothalamus-pituitary-thyroid axis (HPT). Anatomical and pharmacological data evidence its participation as a neuromodulator in the central nervous system. Administration of TRH induces various behavioural effects including arousal, locomotion, analepsy, and in certain paradigms, it reduces fear behaviours. In this work we studied the possible involvement of TRHergic neurons in anxiety tests. We first tested whether an ICV injection of TRH had behavioural effects on anxiety in the defensive burying test (DBT). Corticosterone serum levels were quantified to evaluate the stress response and, the activity of the HPT axis to distinguish the endocrine response of TRH injection. Compared to a saline injection, TRH reduced cumulative burying, and decreased serum corticosterone levels, supporting anxiolytic-like effects of TRH administration. The response of TRH neurons was evaluated in brain regions involved in the stress circuitry of animals submitted to the DBT and to the elevated plus maze (EPM), tests that allow to correlate biochemical parameters with anxiety-like behaviour. In the DBT, the response of Wistar rats was compared with that of the stress-hypersensitive Wistar Kyoto (WKY) strain. Behavioural parameters were analysed in recorded videos. Animals were sacrificed 30 or 60min after test completion. In various limbic areas, the relative mRNA levels of TRH, its receptors TRH-R1 and -R2, and its inactivating ectoenzyme pyroglutamyl peptidase II (PPII), were determined by RT-PCR, TRH tissue content by radioimmunoassay (RIA). The extent of the stress response was evaluated by measuring the expression profile of CRH, CRH-R1 and GR mRNA in the paraventricular nucleus (PVN) of the hypothalamus and in amygdala, corticosterone levels in serum. As these tests demand increased physical activity, the response of the HPT axis was also evaluated. Both tasks increased the levels of serum corticosterone. WKY rats showed higher anxiety-like behaviour in the DBT than Wistar, as well as increased PVN mRNA levels of CRH and GR. TRH mRNA levels increased in the PVN and TSH values remained unchanged in both strains although TRH content decreased in the medial basal hypothalamus of Wistar rats only. TRH content was measured in several limbic regions but only amygdala showed specific task-related changes after DBT exposure of both strains: increased TRH content. Expression of TRH mRNA decreased in the amygdala of Wistar, suggesting inhibition of TRHergic neuronal activity in this region. The participation of amygdalar TRH neurons in anxiety was confirmed in the EPM where TRH expression and release correlated with the number of entries, and the % of time spent in open arms, supporting an anxiolytic role of these TRH-neurons. These results contribute to the understanding of the involvement of TRH during emotionally charged situations and shed light on the participation of particular circuits in related behaviours.

Intranasal delivery of a thyrotropin-releasing hormone analog attenuates seizures in the amygdala-kindled rat

PURPOSE

Thyrotropin-releasing hormone (TRH) is known to have anticonvulsant effects in several animal seizure models and is efficacious in treating patients with certain intractable epilepsies. However, the duration of TRH's action is limited due to low bioavailability and difficulty penetrating the blood-brain barrier (BBB). Since direct nose to brain delivery of therapeutic compounds may provide a means for overcoming these barriers, we utilized the kindling model of temporal lobe epilepsy to determine if intranasal administration of a TRH analog, 3-methyl-histidine TRH (3Me-H TRH), could significantly inhibit various seizure parameters.

METHODS

Kindling was accomplished using a 1s train of 60 Hz biphasic square wave (200 microA peak to peak) administered daily to the basolateral amygdala until the animal was fully kindled. Afterdischarge duration (ADD) was assessed via electroencephalographs (EEGs) recorded bilaterally from bipolar electrodes in the basolateral amygdala and behavioral seizure severity (stage I-V) was simultaneously recorded digitally. Kindled subjects received 3Me-H TRH (10(-9), 10(-8), 10(-7) M) intranasally 60 and 30 min prior to amygdala stimulation. The ADD and seizure stage was compared to control kindled animals receiving physiological saline intranasally.

RESULTS

Intranasal application of 3Me-H TRH resulted in a concentration-dependent reduction in total seizure ADD. Additionally, the analog had significant concentration-dependent effects on behavioral stages I through IV (partial) and stage V (generalized) seizures. However, 3Me-H TRH significantly reduced clonus duration only at the highest concentration.

DISCUSSION

The results indicate that intranasal delivery of TRH/analogs may be a viable means to suppress temporal lobe seizures and perhaps other seizure disorders.

Discovery of a dual action first-in-class peptide that mimics and enhances CNS-mediated actions of thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH) displays multiple CNS-mediated actions that have long been recognized to have therapeutic potential in treating a wide range of neurological disorders. Investigations of CNS functions and clinical use of TRH are hindered, however, due to its rapid degradation by TRH-degrading ectoenzyme (TRH-DE). We now report the discovery of a set of first-in-class compounds that display unique ability to both potently inhibit TRH-DE and bind to central TRH receptors with unparalleled affinity. This dual pharmacological activity within one molecular entity was found through selective manipulation of peptide stereochemistry. Notably, the lead compound of this set, L-pyroglutamyl-L-asparaginyl-L-prolyl-D-tyrosyl-D-tryptophan amide (Glp-Asn-Pro-D-Tyr-D-TrpNH(2)), is effective in vivo at producing and potentiating central actions of TRH without evoking release of thyroid-stimulating hormone (TSH). Specifically, this peptide displayed high plasma stability and combined potent inhibition of TRH-DE (K(i) 151 nM) with high affinity binding to central TRH receptors (K(i) 6.8 nM). Moreover, intraperitoneal injection of this peptide mimicked and augmented the effects of TRH on behavioural activity in rat. Analogous to TRH, it also antagonized pentobarbital-induced narcosis when administered intravenously. This discovery provides new opportunities for probing the role of TRH actions in the CNS and a basis for development of novel TRH-based neurotherapeutics.

An analog of thyrotropin-releasing hormone (TRH) is neuroprotective against glutamate-induced toxicity in fetal rat hippocampal neurons in vitro

TRH has been found to be efficacious in treating certain neurodegenerative disorders such as epilepsy, Alzheimer's disease, neurotrauma and depression, however, its mechanism of action is poorly understood. Since glutamate (Glu) toxicity has been implicated in these disorders, we utilized primary enriched cultures of rat fetal (E 17) hippocampal neurons to test the hypothesis that an analog of TRH, 3-Methyl-Histidine TRH (3Me-H TRH), given concurrently with Glu would protect such neurons against cell damage and cell death. Cell viability was assessed via Trypan Blue exclusion cell counts, and neuronal damage was determined by assaying lactic acid dehydrogenase (LDH) released in the conditioned media. Fetal hippocampal neurons were cultured in neurobasal media for 7 days. On day 7, neurons (10(6)/well) were treated with: control media, 10 microM 3Me-H TRH, 500 microM Glu or 500 microM Glu with either 10, 1, 0.1, 0.01 or 0.001 microM 3Me-H TRH. Both media and neurons were harvested 16 h after treatment. Prolonged exposure to 10 microM 3Me-H TRH was not toxic to the cells, whereas neurons exposed to 500 microM Glu resulted in maximal cell death. Notably, 10, 1 and 0.1 microM 3Me-H TRH, when co-treated with 500 microM Glu, protected fetal neurons against cell death in a concentration-dependent manner. These results provide support for an important neuroprotective effect of TRH/analogs against glutamate toxicity in primary hippocampal neuronal culture and implicate a potentially beneficial role of TRH/analogs in neurodegenerative diseases.

Evidence for interplay between thyrotropin-releasing hormone (TRH) and its structural analogue pGlu-Glu-Pro-NH2 ([Glu2]TRH) in the brain: an in vivo microdialysis study

Local perfusion of pGlu-Glu-Pro-NH2, an endogenous peptide structurally related to thyrotropine-releasing hormone (TRH), via in vivo microdialysis into the rat hippocampus did not change the basal level of extracellular acetylcholine. However, co-perfusion of pGlu-Glu-Pro-NH2 with TRH in equimolar concentrations yielded a significant attenuation of TRH-induced acetylcholine release. The results have supported the study's hypothesis that pGlu-Glu-Pro-NH2 opposes the cholinergic effect of TRH in the mammalian central nervous system. The enantiomer pGlu-d-Glu-Pro-NH2 affected neither basal extracellular nor TRH-induced increase of acetylcholine levels.

The 'drive for activity' and "restlessness" in anorexia nervosa: potential pathways

This paper discusses the hypothesis that a 'drive for activity" in the presence of physiological and endocrine changes consistent with starvation is a characteristic symptom of acute anorexia nervosa (AN). This 'drive for movement', along with alertness and lack of fatigue, so unlike the motor slowing and loss of energy observed in simple starvation has been recognized in AN throughout history, but has received little attention in the past fifty years. Clinical reports and experimental evidence suggest that 'restlessness' and a 'drive for activity' vary in intensity, they appears to be starvation-dependent and to wane with food intake. Central nervous system (CNS) systems known to be involved in mediating activity and arousal levels that are altered by the negative energy expenditure in AN are reviewed. Among these, the corticotropin-releasing hormone (CRH) system, the melanocyte stimulating hormone/agouti-related protein (MSH/AGRP) system and the norepinephrine/epinephrine (NE/EPI) and dopamine (DA) system may contribute to the 'drive for activity' and alertness in AN. AN appears to represent a disorder of gene/environment interaction. Future research will reveal whether in individuals predisposed to AN, the 'drive for activity' reflects the reactivation of mechanisms important in food scarcity, controlled by one or more evolutionary conserved genes including those regulating foraging behavior. Recognition of the 'drive for activity' as a diagnostic symptom of AN and its assessment prior to re-nutrition would permit clarification of its role in the etiology of AN.

Homology modeling and site-directed mutagenesis of pyroglutamyl peptidase II. Insights into omega-versus aminopeptidase specificity in the M1 family

Pyroglutamyl peptidase II (PPII), a highly specific membrane-bound omegapeptidase, removes N-terminal pyroglutamyl from thyrotropin-releasing hormone (<Glu-His-Pro-NH(2)), inactivating the peptide in the extracellular space. PPII and enzymes with distinct specificities such as neutral aminopeptidase (APN), belong to the M1 metallopeptidase family. M1 aminopeptidases recognize the N-terminal amino group of substrates or inhibitors through hydrogen-bonding to two conserved residues (Gln-213 and exopeptidase motif Glu-355 in human APN), whereas interactions involved in recognition of pyroglutamyl residue by PPII are unknown. In rat PPII, the conserved exopeptidase residue is Glu-408, whereas the other one is Ser-269. Given that variations in M1 peptidase specificity are likely due to changes in the catalytic region, we constructed three-dimensional models for the catalytic domains of PPII and APN. The models showed a salt bridge interaction between PPII-Glu-408 and PPII-Lys-463, whereas the equivalent APN-Glu-355 did not participate in a salt bridge. Docking of thyrotropin-releasing hormone in PPII model suggested that the pyroglutamyl residue interacted with PPII-Ser-269. According to our models, PPII-S269Q and -K463N mutations should leave Glu-408 in a physicochemical context similar to that found in M1 aminopeptidases; alternatively, PPII-S269E replacement might be sufficient to transform PPII into an aminopeptidase. These hypotheses were supported by site-directed mutagenesis; the mutants lost omegapeptidase but displayed alanyl-aminopeptidase activity. In conclusion, recognition of a substrate without an N-terminal charge requires neutralization of the aminopeptidase anionic binding site; furthermore, shortening of side chain at PPII-269 position is required for adjustment to the pyroglutamyl residue.

Treatments in depression

Major depression is believed to be a multifactorial disorder involving predisposing temperament and personality traits, exposure to traumatic and stressful life events, and biological susceptibility. Depression, both unipolar and bipolar, is a "phasic" disease. Stressful life events are known to trigger depressive episodes, while their influence seems to decrease over the course of the illness. This suggests that depression is associated with progressive stress response abnormalities, possibly linked to impairments of structural plasticity and cellular resilience. It therefore appears crucial to adequately treat depression in the early stages of the illness, in order to prevent morphological and functional abnormalities. While evidence suggests that a severely depressed patient needs antidepressant drug therapy and that a non-severely depressed patient may benefit from other approaches (ie, "nonbiological"), little research has been done on the effectiveness of different treatments for depression. The assertion that the clinical efficacy of antidepressants is comparable between the classes and within the classes of those medications may be true from a statistical viewpoint, but is of limited value in practice. The antidepressant drugs may produce differences in therapeutic response and tolerability. Among the possible predictors of outcome in depression treatment, those derived from clinical assessment, neuroendocrine investigations, polysomnographic sleep parameters, genetic variables, and brain imaging techniques have been extensively studied. This article also reviews therapeutic strategies used when initial treatment fails, and describes briefly new concepts in antidepressant therapies such as the regulation of disturbances in circadian rhythms. The treatment of depressive illness does not stop with treatment of acute episodes, and has to be envisaged as a continuous therapeutic intervention, of which we are still not able to determine the optimal duration of treatment and the moment that it should be ceased.

Structure-activity studies with high-affinity inhibitors of pyroglutamyl-peptidase II

Inhibitors of PPII (pyroglutamyl-peptidase II) (EC 3.4.19.6) have potential applications as investigative and therapeutic agents. The rational design of inhibitors is hindered, however, by the lack of an experimental structure for PPII. Previous studies have demonstrated that replacement of histidine in TRH (thyrotropin-releasing hormone) with asparagine produces a competitive PPII inhibitor (Ki 17.5 microM). To gain further insight into which functional groups are significant for inhibitory activity, we investigated the effects on inhibition of structural modifications to Glp-Asn-ProNH2 (pyroglutamyl-asparaginyl-prolineamide). Synthesis and kinetic analysis of a diverse series of carboxamide and C-terminally extended Glp-Asn-ProNH2 analogues were undertaken. Extensive quantitative structure-activity relationships were generated, which indicated that key functionalities in the basic molecular structure of the inhibitors combine in a unique way to cause PPII inhibition. Data from kinetic and molecular modelling studies suggest that hydrogen bonding between the asparagine side chain and PPII may provide a basis for the inhibitory properties of the asparagine-containing peptides. Prolineamide appeared to be important for interaction with the S2' subsite, but some modifications were tolerated. Extension of Glp-Asn-ProNH2 with hydrophobic amino acids at the C-terminus led to a novel set of PPII inhibitors active in vitro at nanomolar concentrations. Such inhibitors were shown to enhance recovery of TRH released from rat brain slices. Glp-Asn-Pro-Tyr-Trp-Trp-7-amido-4-methylcoumarin displayed a Ki of 1 nM, making it the most potent competitive PPII inhibitor described to date. PPII inhibitors with this level of potency should find application in exploring the biological functions of TRH and PPII, and potentially provide a basis for development of novel therapeutics.

Thyrotropin-releasing hormone (protirelin) inhibits potassium-stimulated glutamate and aspartate release from hippocampal slices in vitro

Excess excitatory amino acid release is involved in pathways associated with seizures and neurodegeneration. Thyrotropin-releasing hormone (TRH; protirelin), a brain-derived tripeptide, has shown efficacy in the treatment of such disorders, yet its mechanism of neuroprotection is poorly understood. Using superfused hippocampal slices, we tested the hypothesis that TRH could inhibit evoked glutamate/aspartate release in vitro. Rat hippocampal slices were first equilibrated in oxygenated Krebs buffer (KRB) (120 min) then superfused for 10 min with KRB (control), or KRB containing 0.1, 1, or 10 microM TRH respectively, prior to and during 5 min depolarization with high potassium KRB (50 mM [K(+)] +/- TRH). Fractions (1 min) were collected during the 5 min stimulation and for an additional 10 min thereafter and analyzed for glutamate and aspartate by HPLC. TRH had no effect on baseline glutamate/aspartate release, while all three TRH doses significantly (P < 0.05) inhibited peak 50 mM [K(+)]-stimulated glutamate/aspartate release, and glutamate remained below control (P < 0.05) at 15 min post stimulation. A 5 min pulse of TRH (10 microM) had no affect on basal glutamate/aspartate release, whereas the TRH pre-pulsed slices failed to release glutamate/aspartate by [K(+)]-stimulation given 15 min later. These results are the first to show a potent and prolonged inhibitory effect of TRH on evoked glutamate/aspartate release in vitro. These initial studies suggest that exogenous and/or endogenous TRH may function, in part, to modulate excess glutamate release in specific CNS loci. Additional studies are in progress to fully understand the mechanism of this potent effect of TRH and its implication in various CNS disorders.

1-(Phenyl)isoquinoline carboxamides: a novel class of subtype selective inhibitors of thyrotropin-releasing hormone (TRH) receptors

We report the synthesis of and binding to the two subtypes of mouse thyrotropin-releasing hormone (TRH) receptors, TRH-R1 and TRH-R2, of several 1-(phenyl)isoquinoline carboxamide analogues. These analogues showed a degree of selectivity for binding at TRH-R2. These are the first ligands reported that show selective binding to these receptors.

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.