Novel approaches to targeting neuropeptide systems

Enkephalin release from VIP interneurons in the hippocampal CA2/3a region mediates heterosynaptic plasticity and social memory

The hippocampus contains a diverse array of inhibitory interneurons that gate information flow through local cortico-hippocampal circuits to regulate memory storage. Although most studies of interneurons have focused on their role in fast synaptic inhibition mediated by GABA release, different classes of interneurons express unique sets of neuropeptides, many of which have been shown to exert powerful effects on neuronal function and memory when applied pharmacologically. However, relatively little is known about whether and how release of endogenous neuropeptides from inhibitory cells contributes to their behavioral role in regulating memory formation. Here we report that vasoactive intestinal peptide (VIP)-expressing interneurons participate in social memory storage by enhancing information transfer from hippocampal CA3 pyramidal neurons to CA2 pyramidal neurons. Notably, this action depends on release of the neuropeptide enkephalin from VIP neurons, causing long-term depression of feedforward inhibition onto CA2 pyramidal cells. Moreover, VIP neuron activity in the CA2 region is increased selectively during exploration of a novel conspecific. Our findings, thus, enhance our appreciation of how GABAergic neurons can regulate synaptic plasticity and mnemonic behavior by demonstrating that such actions can be mediated by release of a specific neuropeptide, rather than through classic fast inhibitory transmission.

Pharmacological Characterization of µ-Opioid Receptor Agonists with Biased G Protein or β-Arrestin Signaling, and Computational Study of Conformational Changes during Receptor Activation

In recent years, G protein vs. β-arrestin biased agonism at opioid receptors has been proposed as an opportunity to produce antinociception with reduced adverse effects. However, at present this approach is highly debated, a reason why more information about biased ligands is required. While the practical relevance of bias in the case of µ-opioid receptors (MOP) still needs to be validated, it remains important to understand the basis of this bias of MOP (and other GPCRs). Recently, we reported two cyclopeptides with high affinity for MOP, the G protein biased Dmt-c[d-Lys-Phe-pCF3-Phe-Asp]NH2 (F-81), and the β-arrestin 2 biased Dmt-c[d-Lys-Phe-Asp]NH2 (C-33), as determined by calcium mobilization assay and bioluminescence resonance energy transfer-based assay. The biased character of F-81 and C-33 has been further analyzed in the [35S]GTPγS binding assay in human MOP-expressing cells, and the PathHunter enzyme complementation assay, used to measure β-arrestin 2 recruitment. To investigate the structural features of peptide-MOP complexes, we performed conformational analysis by NMR spectroscopy, molecular docking, and molecular dynamics simulation. These studies predicted that the two ligands form alternative complexes with MOP, engaging specific ligand-receptor contacts. This would induce different displays of the cytosolic side of the seven-helices bundle, in particular by stabilizing different angulations of helix 6, that could favor intracellular coupling to either G protein or β-arrestin.

Biased Agonism as an Emerging Strategy in the Search for Better Opioid Analgesics

Morphine and related drugs that act through activating opioid receptors are the most effective analgesics for the relief of severe pain. They have been used for decades, despite the range of unwanted side effects that they produce, as no alternative has been found so far. The major goal of opioid research is to understand the mechanism of action of opioid receptor agonists and to improve the therapeutic utility of opioid drugs. In the search for safer and more potent analgesics, analogs with mixed opioid receptor profile gained a lot of interest. However, recently the concept of biased agonism, that highlights the fact that some ligands are able to differentially activate receptor downstream pathways, became a new approach in the design of novel drug candidates for clinical application. In this review, we summarize current knowledge on the development of opioid ligands of peptide and nonpeptide structure, showing how much opioid pharmacology evolved in recent years.

A cholecystokinin B receptor antagonist and cocaine interaction, phase I study

AIMS

RPR 102681, a cholecystokinin-B antagonist, increased dopamine (DA) release and reduced cocaine self-administration in animals. This pilot study sought to assess the safety and pharmacokinetics (PK) of co-administration of RPR 102681 and cocaine, and to confirm the DA release mechanism of RPR 102681.

METHODS

Sixteen cocaine-dependent participants were randomized to either placebo or RPR102681 at 3 ascending doses; cocaine was co-administered at steady state of RPR 102681. [¹¹ C]raclopride positron emission tomography scans were conducted at baseline and at each RPR102681 dose.

RESULTS

RPR 102681 was well tolerated, and safe to co-administer with cocaine. RPR 102681 did not alter the PK of either cocaine or its metabolite benzoylecgonine and showed no intrinsic abuse liability. There was a trend toward reduction of cocaine craving scores. In contrast to animal studies, RPR 102681 significantly increased the binding potential of [¹¹ C]raclopride in the ventral striatum (t test, P < .001) and caudate nucleus (t test, P < .0001) in a small subset of patients, suggesting that it may reduce intrasynaptic striatal DA.

CONCLUSION

Overall, this pilot study suggests that RPR 102681 would be unlikely candidate, as an agonist medication for the treatment for cocaine addiction but worth investigating further for possible role in reducing craving.

Antinociceptive potency of a fluorinated cyclopeptide Dmt-c[D-Lys-Phe-p-CF3-Phe-Asp]NH2

Opioid peptides and opiate drugs such as morphine, mediate their analgesic effects, but also undesired side effects, mostly through activation of the mu opioid receptor. However, delta- and kappa-opioid receptors can also contribute to the analgesic effects of opioids. Recent findings showed that simultaneous activation of multiple opioid receptors may result in additional analgesia with fewer side effects. Here, we evaluated the pharmacological profile of our formerly developed mixed mu/kappa-opioid receptor ligands, Dmt-c[D-Lys-Phe-Phe-Asp]NH₂ (C-36) and Dmt-c[D-Lys-Phe-p-CF₃-Phe-Asp]NH₂ (F-81). The ability of these peptides to cross the blood–brain barrier was tested in the parallel artificial membrane permeability (PAMPA) assay. On the basis of the hot-plate test in mice after central and peripheral administration, analog F-81 was selected for the anti-nociceptive and anti-inflammatory activity assessment after peripheral administration.

Neuroimmune Interaction in the Regulation of Peripheral Opioid-Mediated Analgesia in Inflammation

Peripheral immune cell-mediated analgesia in inflammation is an important endogenous mechanism of pain control. Opioid receptors localized on peripheral sensory nerve terminals are activated by endogenous opioid peptides released from immune cells to produce significant analgesia. Following transendothelial migration of opioid-containing leukocytes into peripheral sites of inflammation, opioid peptides are released into a harsh milieu associated with an increase in temperature, low pH, and high proteolytic activity. Together, this microenvironment has been suggested to increase the activity of opioid peptide metabolism. Therefore, the proximity of immune cells and nerve fibers may be essential to produce adequate analgesic effects. Close associations between opioid-containing immune cells and peripheral nerve terminals have been observed. However, it is not yet determined whether these immune cells actually form synaptic-like contacts with peripheral sensory terminals and/or whether they secrete opioids in a paracrine manner. This review will provide novel insight into the peripheral mechanisms of immune-derived analgesia in inflammation, in particular, the importance of direct interactions between immune cells and the peripheral nervous system.

Contribution of Delta-Opioid Receptors to Pathophysiological Events Explored by Endogenous Enkephalins

Very few discoveries in the neurosciences have triggered clinical speculation and experimentation regarding the etiology of psychiatric illness to the same extent as that following identification of the opiate receptor(s) and subsequent isolation of endogenous morphine-like peptides. There is overwhelming evidence in animals and in human that opioids are involved in behaviorally relevant issues such as the modulation of pain, the response to stress, motivation, addiction, sexuality, food intake, etc., but our knowledge on the possible relation between opioids and mental illness is still very limited.These responses could be explored eitheir by using higlhy selective delta agonist or by emphasizing the effects of phasically secreted endogenous opioid peptides, enkephalin. Both approaches were investigated in particular through protection of enkephalin degradation by dual enkephalinase ihibitors DENKIs such as RB101, PL37 or PL265.

Influence of polar side chains modifications on the dual enkephalinase inhibitory activity and conformation of human opiorphin, a pain perception related peptide

The dual inhibitory action of the pain related peptide opiorphin (H-Gln-Arg-Phe-Ser-Arg-OH) against neutral endopeptidase (NEP) and aminopeptidase N (AP-N) was further investigated by a SAR study involving minor modifications on the polar side chains of Arg residues and glycosylation with monosaccharides at Ser. None of them exerted dual or individual inhibitory potency superior than opiorphin. However, the correlations deduced offer further proof for the key role of these residues upon the binding and bioactive conformational stabilization of opiorphin. NMR conformational studies on the glycopeptides suggest that they are still very flexible compounds that may attain their respective bioactive conformations.

Long-lasting oral analgesic effects of N-protected aminophosphinic dual ENKephalinase inhibitors (DENKIs) in peripherally controlled pain

The peripheral endogenous opioid system is critically involved in neuropathic and inflammatory pain generation as suggested by the modulation of opioid receptors expression and enkephalins (ENKs) release observed in these painful conditions. Accordingly, an innovative approach in the treatment of these nocifensive events is to increase and maintain high local concentrations of extracellular pain-evoked ENKs, by preventing their physiological enzymatic inactivation by two Zn metallopeptidases, the neutral endopeptidase (NEP, neprilysin, EC 3.4.24.11) and the neutral aminopeptidase (APN, EC 3.4.11.2). With this aim, new orally active dual ENKephalinase inhibitors (DENKIs) were designed as soluble prodrugs by introducing a N-terminal cleavable carbamate in the previously described aminophosphinic inhibitors. This induces long-lasting antinociceptive responses after oral administration, in various rodent models of inflammatory and neuropathic pain. These responses are mediated through stimulation of peripheral opioid receptors by DENKIs-protected ENKs as demonstrated by naloxone methiodide reversion. In all tested models, the most efficient prodrug 2a (PL265) was active, at least during 150–180 min, after single oral administration of 25–50 mg/kg in mice and of 100–200 mg/kg in rats. In models of neuropathic pain, both hyperalgesia and allodynia were markedly reduced. Interestingly, combination of inactive doses of 2a (PL265) and of the anti-epileptic drug gabapentin had synergistic effect on neuropathic pain. Pharmacokinetic studies of 2a (PL265) in rats show that the active drug is the only generated metabolite produced. These encouraging results have made 2a (PL265) a suitable candidate for clinical development.

Involvement of neuronal nitric oxide synthase in desensitisation of µ-opioid receptors in the rat locus coeruleus

Nitric oxide (NO) has been recently shown to enhance µ-opioid receptor (MOR) desensitisation in locus coeruleus (LC) neurons. The aim of this study was to evaluate by single-unit extracellular recordings in rat brain slices whether the neuronal NO synthase is involved in MOR desensitisation in LC neurons. As expected, a high concentration of the opioid agonist Met(5)-enkephalin (ME; 10 µM, 10 min) strongly desensitised the inhibition induced by a test application of ME (0.8 µM, 1 min), whereas lower ME concentrations (1 and 3 µM) only weakly desensitised it. The neuronal NO synthase inhibitors 7-nitroindazole (10-100 µM), S-methyl-L-thiocitrulline (0.01-10 µM) and N(ω)-propyl-L-arginine (1-10 µM) attenuated ME (10 µM)-induced opioid desensitisation, although the endothelial NO synthase inhibitor N(5)-(1-iminoethyl)-L-ornithine (3-30 µM) failed to change it. The NO donor sodium nitroprusside (1 mM), but not its inactive analog potassium ferricyanide (1 mM), enhanced the ME (3 µM)-induced desensitisation and prevented the effect of S-methyl-L-thiocitrulline (10 µM). Sodium nitroprusside (1 mM) failed to change the desensitisation of α2-adrenoceptors by noradrenaline (100 µM, 10 min). These results suggest the contribution of NO and a neuronal type of NO synthase in homologous MOR desensitisation in rat LC neurons.

Central antinociception induced by ketamine is mediated by endogenous opioids and μ- and δ-opioid receptors

It is generally believed that NMDA receptor antagonism accounts for most of the anesthetic and analgesic effects of ketamine, however, it interacts at multiple sites in the central nervous system, including NMDA and non-NMDA glutamate receptors, nicotinic and muscarinic cholinergic receptors, and adrenergic and opioid receptors. Interestingly, it was shown that at supraspinal sites, ketamine interacts with the μ-opioid system and causes supraspinal antinociception. In this study, we investigated the involvement of endogenous opioids in ketamine-induced central antinociception. The nociceptive threshold for thermal stimulation was measured in Swiss mice using the tail-flick test. The drugs were administered via the intracerebroventricular route. Our results demonstrated that the opioid receptor antagonist naloxone, the μ-opioid receptor antagonist clocinnamox and the δ-opioid receptor antagonist naltrindole, but not the κ-opioid receptor antagonist nor-binaltorphimine, antagonized ketamine-induced central antinociception in a dose-dependent manner. Additionally, the administration of the aminopeptidase inhibitor bestatin significantly enhanced low-dose ketamine-induced central antinociception. These data provide evidence for the involvement of endogenous opioids and μ- and δ-opioid receptors in ketamine-induced central antinociception. In contrast, κ-opioid receptors not appear to be involved in this effect.

Cholecystokinin: a multi-functional molecular switch of neuronal circuits

Cholecystokinin (CCK), a peptide originally discovered in the gastrointestinal tract, is one of the most abundant and widely distributed neuropeptides in the brain. In spite of its abundance, recent data indicate that CCK modulates intrinsic neuronal excitability and synaptic transmission in a surprisingly cell-type specific manner, acting as a key molecular switch to regulate the functional output of neuronal circuits. The central importance of CCK in neuronal networks is also reflected in its involvement in a variety of neuropsychiatric and neurological disorders including panic attacks and epilepsy.

Identification of an endothelin-converting enzyme-2-specific fluorigenic substrate and development of an in vitro and ex vivo enzymatic assay

Endothelin-converting enzyme-2 (ECE-2) is a membrane-bound zinc-dependent metalloprotease that shares a high degree of sequence homology with ECE-1, but displays an acidic pH optimum characteristic of maturing enzymes acting late in the secretory pathway. Although ECE-2, like ECE-1, can cleave the big endothelin intermediate to produce the vasoconstrictive endothelin peptide, its true physiological function remains to be elucidated, a task that is hampered by the lack of specific tools to study and discriminate ECE-2 from ECE-1, i.e. specific substrates and/or specific inhibitors. To fill this gap, we searched for novel ECE-specific peptide substrates. To this end, peptides derived from the big endothelin intermediate were tested using ECE-1 and ECE-2, leading to the identification of an ECE-1-specific substrate. Moreover, screening of our proprietary fluorigenic peptide Fluofast® libraries using ECE-1 and ECE-2 allowed the identification of Ac-SKG-Pya-F-W-Nop-GGK-NH(2) (PL405), as a specific and high affinity ECE-2 substrate. Indeed, ECE-2 cleaved PL405 at the Pya-F amide bond with a specificity constant (k(cat)/K(m)) of 8.1 ± 0.9 × 10(3) M(-1) s(-1). Using this novel substrate, we also characterized the first potent (K(i) = 7.7 ± 0.3 nM) and relatively selective ECE-2 inhibitor and developed a quantitative fluorigenic ECE-2 assay. The assay was used to study the ex vivo ECE-2 activity in wild type and ECE-2 knock-out tissues and was found to truly reflect ECE-2 expression patterns. The PL405 assay is thus the first tool to study ECE-2 inhibition using high throughput screening or for ex vivo ECE-2 quantification.

Electroosmotic sampling. Application to determination of ectopeptidase activity in organotypic hippocampal slice cultures

We hypothesize that peptide-containing solutions pulled through tissue should reveal the presence and activity of peptidases in the tissue. Using the natural zeta-potential in the organotypic hippocampal slice culture (OHSC), physiological fluids can be pulled through the tissue with an electric field. The hydrolysis of the peptides present in the fluid drawn through the tissue can be determined using capillary HPLC with electrochemical detection of the biuret complexes of the peptides following a postcolumn reaction. We have characterized this new sampling method by measuring the flow rate, examining the use of internal standards, and examining cell death caused by sampling. The sampling flow rate ranges from 60 to 150 nL/min with a 150 microm (ID) sampling capillary with an electric field (at the tip of the capillary) from 30 to 60 V/cm. Cell death can be negligible with controlled sampling conditions. Using this sampling approach, we have electroosmotically pulled Leu-enkephalin through OHSCs to identify ectopeptidase activity in the CA3 region. These studies show that a bestatin-sensitive aminopeptidase may be critical for the hydrolysis of exogenous Leu-enkephalin, a neuropeptide present in the CA3 region of OHSCs.

Enkephalins, dynorphins, and beta-endorphin in the rat dorsal horn: an immunofluorescence colocalization study

To characterize neuronal pathways that release opioid peptides in the rat dorsal horn, multiple-label immunohistochemistry, confocal microscopy, and computerized co-localization measures were used to characterize opioid-containing terminals and cells. An antibody that selectively recognized beta-endorphin labeled fibers and neurons in the ventral horn as well as fibers in the lateral funiculus and lamina X, but practically no fibers in the dorsal horn. An anti-enkephalin antibody, which recognized Leu-, Met-, and Phe-Arg-Met-enkephalin, labeled the dorsolateral funiculus and numerous puncta in laminae I-III and V of the dorsal horn. An antibody against Phe-Arg-Met-enkephalin, which did not recognize Leu- and Met-enkephalin, labeled the same puncta. Antibodies against dynorphin and prodynorphin labeled puncta and fibers in laminae I, II, and V, as well as some fibers in the rest of the dorsal horn. Dynorphin and prodynorphin immunoreactivities colocalized in some puncta and fibers, but the prodynorphin antibody additionally labeled cell bodies. There was no co-localization of dynorphin (or prodynorphin) with enkephalin (or Phe-Arg-Met-enkephalin). Enkephalin immunoreactivity did not colocalize with the C-fiber markers calcitonin gene-related peptide (CGRP), substance P, and isolectin B4. In contrast, there was some colocalization of dynorphin and prodynorphin with CGRP and substance P, but not with isolectin B4. Both enkephalin and dynorphin partly colocalized with vesicular glutamate transporter 2, a marker of glutamatergic terminals. The prodynorphin-positive neurons in the dorsal horn were distinct from neurons expressing mu-opioid receptors, neurokinin 1 receptors, and protein kinase C-gamma. These results show that enkephalins and dynorphins are present in different populations of dorsal horn neurons. In addition, dynorphin is present in some C-fibers.

Peripheral non-viral MIDGE vector-driven delivery of beta-endorphin in inflammatory pain

Background

Leukocytes infiltrating inflamed tissue produce and release opioid peptides such as β-endorphin, which activate opioid receptors on peripheral terminals of sensory nerves resulting in analgesia. Gene therapy is an attractive strategy to enhance continuous production of endogenous opioids. However, classical viral and plasmid vectors for gene delivery are hampered by immunogenicity, recombination, oncogene activation, anti-bacterial antibody production or changes in physiological gene expression. Non-viral, non-plasmid minimalistic, immunologically defined gene expression (MIDGE) vectors may overcome these problems as they carry only elements needed for gene transfer. Here, we investigated the effects of a nuclear localization sequence (NLS)-coupled MIDGE encoding the β-endorphin precursor proopiomelanocortin (POMC) on complete Freund's adjuvant-induced inflammatory pain in rats.

Results

POMC-MIDGE-NLS injected into inflamed paws appeared to be taken up by leukocytes resulting in higher concentrations of β-endorphin in these cells. POMC-MIDGE-NLS treatment reversed enhanced mechanical sensitivity compared with control MIDGE-NLS. However, both effects were moderate, not always statistically significant or directly correlated with each other. Also, the anti-hyperalgesic actions could not be increased by enhancing β-endorphin secretion or by modifying POMC-MIDGE-NLS to code for multiple copies of β-endorphin.

Conclusion

Although MIDGE vectors circumvent side-effects associated with classical viral and plasmid vectors, the current POMC-MIDGE-NLS did not result in reliable analgesic effectiveness in our pain model. This was possibly associated with insufficient and variable efficacy in transfection and/or β-endorphin production. Our data point at the importance of the reproducibility of gene therapy strategies for the control of chronic pain.

Effects and underlying mechanisms of human opiorphin on colonic motility and nociception in mice

In the present study, we investigated the effects of human opiorphin on colonic motility and nociception in mice. In in vitro bioassay, opiorphin (10(-6) to 10(-4)M) caused colonic contraction in a concentration-dependent manner, which was completely blocked by naloxone and partially attenuated by beta-funaltrexamine and naltrindole. Moreover, opiorphin (10(-4)M) significantly enhanced the contractile response induced by Met-enkephalin. The data suggested that the effect of opiorphin on colonic contraction may be due to the protection of enkephalins. In in vivo bioassay, intracerebroventricular (i.c.v.) administration of opiorphin (1.25-10 microg/kg) dose- and time-dependently induced potent analgesic effect (ED(50)=3.22 microg/kg). This effect was fully blocked by naloxone and significantly inhibited by co-injection (i.c.v.) with beta-funaltrexamine or naltrindole, but not by nor-binaltorphimine, indicating the involvement of both mu- and delta-opioid receptors in the analgesic response evoked by opiorphin. In addition, i.c.v. administration of 5 microg/kg opiorphin produced the comparative effect as 10 microg/kg morphine on the analgesia, suggesting that opiorphin displayed more potent analgesic effect than that induced by morphine.

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.

Profiling of behavioral changes and hippocampal gene expression in mice chronically treated with the SSRI paroxetine

INTRODUCTION

Monoamine-based antidepressants inhibit neurotransmitter reuptake within short time. However, it commonly takes several weeks until clinical symptoms start to resolve--indicating the involvement of effects distant from reuptake inhibition.

OBJECTIVE

To unravel other mechanisms involved in drug action, a "reverse" pharmacological approach was applied to determine antidepressant-induced alterations of hippocampal gene expression.

MATERIALS AND METHODS

The behavioral response to long-term paroxetine administration of male DBA/2Ola mice was assessed by the forced swim test (FST), the modified hole board (mHB), and the dark/light box. Hippocampi of test-naive mice were dissected, and changes in gene expression by paroxetine treatment were investigated by means of microarray technology.

RESULTS AND DISCUSSION

Robust effects of paroxetine on passive stress-coping behavior in the FST were observed. Furthermore, anxiolytic properties of long-term antidepressant treatment could be identified in DBA mice in both, the mHB and dark/light box. Analysis of microarray results revealed a list of 60 genes differentially regulated by chronic paroxetine treatment. Preproenkephalin 1 and inhibin beta-A showed the highest level of transcriptional change. Furthermore, a number of candidates involved in neuroplasticity/neurogenesis emerged (e.g., Bdnf, Gfap, Vim, Sox11, Egr1, Stat3). Seven selected candidates were confirmed by in situ hybridization. Additional immunofluorescence colocalization studies of GFAP and vimentin showed more positive cells to be detected in long-term paroxetine-treated DBA mice.

CONCLUSION

Candidate genes identified in the current study using a mouse strain validated for its responsiveness to long-term paroxetine treatment add, in our opinion, to unraveling the mechanism of action of paroxetine as a representative for SSRIs.

Decreased motivational properties of morphine in mouse models of cancerous- or inflammatory-chronic pain: implication of supraspinal neuropeptide FF(2) receptors

Our main purpose was to evaluate the influence of cancer pain on the rewarding properties of morphine. Opioids are very addictive when used by healthy persons, conversely the occurrence of an opioid addiction seems very low when patients suffering from cancer are treated with morphine. We investigated the reinforcing properties of morphine in the place preference paradigm on a new model of mice suffering from a cancer pain induced by syngenic melanoma cells injected in the hind paw. These data were compared with mice suffering either from a short-term- or a chronic-inflammatory pain induced respectively by injection of carrageenan or complete Freund's adjuvant. Remarkably, mice suffering from cancer pain or chronic inflammatory pain did not develop any preference for the environment associated with the injection of morphine. In mice injected with melanoma cells, the specific binding of [(125)I]EYWSLAAPQRF-NH(2), an agonist of neuropeptide FF(2) receptors, was increased in several brain areas involved in the rewarding properties of opiates, including the shell of the nucleus accumbens, the major islands of Calleja, the ventral endopiriform nucleus and the amygdaloid area. Our study is the first to reveal a modification of morphine rewarding properties under cancer pain in rodents. We postulate that anti-opioid neuropeptides might contribute to the suppression of morphine rewarding effects in this murine model of cancer pain.

Inhibition of osteosarcoma-induced thermal hyperalgesia in mice by the orally active dual enkephalinase inhibitor PL37. Potentiation by gabapentin

We have previously shown that stimulation of peripheral opioid receptors by exogenous opiates counteracts the thermal hyperalgesia elicited by a tibial osteosarcoma due to intraosteal inoculation of NCTC 2472 cells to mice. Aiming to study whether pheripheral endogenous enkephalins could also counteract this painful symptom, we assayed in this model the effects of PL37, an orally active dual inhibitor of enkephalin inactivating enzymes. Oral administration of PL37 (25 mg/kg) completely supressed osteosarcoma-induced thermal hyperalgesia through the activation of micro-opioid receptors, since the administration of cyprodime (1 mg/kg) inhibited its antihyperalgesic effect. Neither naltrindole (0.1 mg/kg) nor nor-binaltorphimine (10 mg/kg) modified this PL37-induced antihyperalgesic effect. Moreover, the inhibition of the antihyperalgesic effect induced by PL37 after the administration of naloxone-methiodide (2 mg/kg), a non selective opioid antagonist that does not cross the blood-brain barrier, demonstrates the involvement of peripheral opioid receptors. In contrast, centrally mediated effects may be detected when assaying a higher dose of PL37 (50 mg/kg). Besides, the administration of gabapentin (6.25-25 mg/kg, i.p.) dose-dependently inhibited osteosarcoma-induced thermal hyperalgesia. Interestingly, the combined administration of subeffective doses of PL37 and gabapentin completely prevented this type of thermal hyperalgesia. An isobolographic analysis of this interaction demonstrated a synergistic interaction between both drugs.

Low doses of cyclic AMP-phosphodiesterase inhibitors rapidly evoke opioid receptor-mediated thermal hyperalgesia in naïve mice which is converted to prominent analgesia by cotreatment with ultra-low-dose naltrexone

Systemic (s.c.) injection in naïve mice of cyclic AMP-phosphodiesterase (cAMP-PDE) inhibitors, e.g. 3-isobutyl-1-methylxanthine [(IBMX) or caffeine, 10 mg/kg] or the more specific cAMP-PDE inhibitor, rolipram (1 mug/kg), rapidly evokes thermal hyperalgesia (lasting >5 h). These effects appear to be mediated by enhanced excitatory opioid receptor signaling, as occurs during withdrawal in opioid-dependent mice. Cotreatment of these mice with ultra-low-dose naltrexone (NTX, 0.1 ng/kg-1 pg/kg, s.c.) results in prominent opioid analgesia (lasting >4 h) even when the dose of rolipram is reduced to 1 pg/kg. Cotreatment of these cAMP-PDE inhibitors in naïve mice with an ultra-low-dose (0.1 ng/kg) of the kappa-opioid receptor antagonist, nor-binaltorphimine (nor-BNI) or the mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA) also results in opioid analgesia. These excitatory effects of cAMP-PDE inhibitors in naïve mice may be mediated by enhanced release of small amounts of endogenous bimodally-acting (excitatory/inhibitory) opioid agonists by neurons in nociceptive networks. Ultra-low-dose NTX, nor-BNI or beta-FNA selectively antagonizes high-efficacy excitatory (hyperalgesic) Gs-coupled opioid receptor-mediated signaling in naïve mice and results in rapid conversion to inhibitory (analgesic) Gi/Go-coupled opioid receptor-mediated signaling which normally requires activation by much higher doses of opioid agonists. Cotreatment with a low subanalgesic dose of kelatorphan, an inhibitor of multiple endogenous opioid peptide-degrading enzymes, stabilizes endogenous opioid agonists released by cAMP-PDE inhibitors, resulting in conversion of the hyperalgesia to analgesia without requiring selective blockade of excitatory opioid receptor signaling. The present study provides a novel pharmacologic paradigm that may facilitate development of valuable non-narcotic clinical analgesics utilizing cotreatment with ultra-low-dose rolipram plus ultra-low-dose NTX or related agents.

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.

Use of preproenkephalin knockout mice and selective inhibitors of enkephalinases to investigate the role of enkephalins in various behaviours

RATIONALE

The most simple and efficient method to study the physiological role of enkephalins is to increase the lifetime of these endogenous opioid peptides by inhibiting their inactivating enzymes. Enkephalins are degraded by the concomitant action of two metallopeptidases: neutral endopeptidase (NEP, EC3.4.21.11) and aminopeptidase N (APN, EC3.4.11.2), both enzymes releasing inactive metabolites.

OBJECTIVES

Potent dual inhibitors have been developed, such as RB101. However, NEP and APN have a broad specificity and can cleave various peptides in vitro. Therefore, it was essential to investigate the specific involvement of enkephalins in the various pharmacological responses induced by dual inhibitors.

MATERIALS AND METHODS

We compared the pharmacological responses induced by RB101 in wild-type and preproenkephalin-deficient mice (Penk1-/-) using several behavioural assays.

RESULTS

In all the tests used (hot plate test, force swim test, castor-oil-induced diarrhoea), RB101 induced strong effects in wild-type animals, whereas slight effects were observed in Penk1-/- animals. These residual effects are blocked by pre-administration of the opioid antagonist naloxone, supporting the involvement of the opioid receptors in the responses observed.

CONCLUSIONS

The pharmacological effects induced by dual inhibitors acting on both NEP and APN are mainly due to the protection of the endogenous enkephalins at supraspinal and peripheral levels. It could be speculated that the residual effects observed in Penk1-/- mice after RB101 administration could be due to the direct action of other opioid peptides or through an indirect effect involving the protection of other peptide substrates of NEP or APN, as substance P or angiotensin.

Molecular basis of branched peptides resistance to enzyme proteolysis

We found that synthetic peptides in the form of dendrimers become resistant to proteolysis. To determine the molecular basis of this resistance, different bioactive peptides were synthesized in monomeric, two-branched and tetra-branched form and incubated with human plasma and serum. Proteolytic resistance of branched multimeric sequences was compared to that of the same peptides synthesized as multimeric linear molecules. Unmodified peptides and cleaved sequences were detected by high pressure liquid chromatography and mass spectrometry. An increase in peptide copies did not increase peptide resistance in linear multimeric sequences, whereas multimericity progressively enhanced proteolytic stability of branched multimeric peptides. A structure-based hypothesis of branched peptide resistance to proteolysis by metallopeptidases is presented.

Stage-specific modulation of neprilysin and aminopeptidase N in the limbic system during kindling progression

Aminopeptidase N (APN) and neprilysin (NEP) inactivate neuropeptides released into the brain extracellular fluid. We previously showed that the expression of pyroglutamyl peptidase II (PPII), the TRH degrading ecto-enzyme, is regulated in rat brain by amygdaline kindling, a paradigm that activates neuronal pathways in the limbic system increasing the expression of several neuropeptides including TRH and opioids. To understand the specificity of this phenomenon, we studied APN and NEP expression in brains of partially or fully kindled rats (stage II and V), sacrificed 6 h after last stimulus, compared with sham-operated animals. In situ hybridization analysis of NEP mRNA levels showed decreased expression at stage II in CA1, CA2, olfactory tubercle and medial mammillary nucleus, and increased at stage V in CA1 and CA2 cells. These changes were specific for the ipsilateral side. APN mRNA levels, semi-quantified by RT-PCR, were decreased at stage II and increased at stage V, in frontal cortex-olfactory tubercle, and hippocampus. NEP and APN enzymatic activities, determined by fluorometric assays, followed similar variations to their respective mRNA levels. The coordinated changes (in some regions) of NEP and APN expression were opposite to those previously observed for PPII mRNA and activity levels in limbic regions. These results demonstrate that expression of ectopeptidases can be regulated when peptide neurons are activated and, that regulation is enzyme-, region-, and stage-specific.

Protection of endogenous enkephalin catabolism as natural approach to novel analgesic and antidepressant drugs

The most efficient drugs to alleviate severe pain are opioid compounds. However, their chronic use could be associated with serious drawbacks, such as tolerance, respiratory depression and constipation. Therefore, there is a need for compounds able to efficiently alleviate inflammatory and neurogenic pain following chronic treatment. The discovery that the endogenous opioid peptides, enkephalins, are inactivated by two metallopeptidases, neutral endopeptidase and aminopeptidase N, which can be blocked by synthetic dual inhibitors, represents a promising way to develop 'physiological' analgesics devoid of morphine side effects. These dual inhibitors also have antidepressant-like properties through enkephalin-related activation of delta-opioid receptors. This is expected to reduce the emotional component of pain in humans. This article reviews the promising data obtained for future development of a new class of analgesic that could be of major interest in a number of severe and chronic pain syndromes.

Differential modulation of sodium- and chloride-dependent opioid peptide transport system by small nonopioid peptides and free amino acids

We recently identified a novel opioid peptide transport system in the retinal pigment epithelium that transports opioid peptides by a Na+/Cl--dependent process. Here we describe a similar transport system expressed in SK-N-SH cells (a human neuronal cell line) and show for the first time that the activity of the transport system is modulated differentially by lysine and small nonopioid peptides. The transport process in SK-N-SH cells, monitored with deltorphin II as the substrate, is Na+/Cl--dependent and interacts with several opioid peptides, consisting of 5 to 13 amino acids. The activity of this transport system is markedly stimulated by specific dipeptides and tripeptides, with significant stimulation observable at low micromolar concentrations. The ion dependence, Na+/Cl--activation kinetics, and opioid peptide selectivity of the transport system, however, remain unchanged. The stimulation by the modulatory peptides is associated with an increase in maximal velocity with no change in substrate affinity of the system. Amino acids have no or little effect on the transport system, with the exception of lysine. This cationic amino acid inhibits the transport system, with significant inhibition occurring at physiologic concentrations of the amino acid. The inhibitory effect is primarily associated with a decrease in the maximal velocity of the transport system with little change in substrate affinity. Methyl and ethyl esters of lysine retain the inhibitory potency, but most other structural analogs have no effect. The differential modulation of the transport system by lysine and specific small peptides has important implications in the biology and pharmacology of opioid peptides.

Opioids

Opioids are the most effective and widely used drugs in the treatment of severe pain. They act through G protein-coupled receptors. Four families of endogenous ligands (opioid peptides) are known. The standard exogenous opioid analgesic is morphine. Opioid agonists can activate central and peripheral opioid receptors. Three classes of opioid receptors (mu, delta, kappa) have been identified. Multiple pathways ofopioid receptor signaling (e.g., G(i/o) coupling, cAMP inhibition, Ca++ channel inhibition) have been described. The differential regulation of effectors, preclinical pharmacology, clinical applications, and side effects will be reviewed in this chapter.

Endogenous Signaling Complexity in Neuropeptides- Leucine- and Methionine-Enkephalin

1. In addition to his many fine contributions in furthering our understanding of the neurochemical action of ecosanoids, catchelomines, steroids, anandamines, cannabinoids, endorphins, and the many modifications made to these neural factors, twenty years ago Julius Axelrod published a noteworthy paper concerning the nature of neuropeptides and their potential for multiple neurophysiological effects (Redgate et al., 1986). 2. In that report, Axelrod and coworkers described the neurological actions of the then recently discovered leucine- and methionine-enkephalins, and their biological functions which were novel, atypical, and in possession of neurological effects that were significantly "much more than additive." 3. In this short communication I would like to expand on this observation concerning the "additive effects" contained within the amino acid sequence of the atypical neurotransmitter peptides leucine- and methonine-enkephalin.

CGS 35601, a Triple Inhibitor of Angiotensin Converting Enzyme, Neutral Endopeptidase and Endothelin Converting Enzyme

CGS 35601 (L-tryptophan, N-[[1-[[(2S)-2-mercapto-4-methyl-1-oxopentyl]amino]-cyclopentyl]carbonyl]) is one of a few single molecules capable of inhibiting the activities of angiotensin-converting enzyme (ACE), neutral endopeptidase (NEP) and endothelin converting enzyme (ECE) simultaneously, with IC(50) values of 22, 2, and 55 nM, respectively. Through the inhibition of ACE and ECE, it blocks the conversion of angiotensin I (AI) and big endothelin-1 (big ET-1) into the two most potent peptidic vasoconstrictors, angiotensin II (AII) and ET-1, respectively. By inhibiting NEP, CGS 35601 also prevents the degradation of peptidic vasodilators such as bradykinin (BK), natriuretic peptides (NPs) and adrenomedullin (ADM) and, hence, modulates the secondary release of other vasoactive mediators such as nitric oxide (NO) and prostaglandins. In chronic (30 days) experiments, CGS 35601 is well tolerated with a very good safety profile in healthy normotensive, hypertensive and type 2 diabetic rats. The antihypertensive efficacy of CGS 35601 was demonstrated in chronically instrumented, unrestrained and conscious rat models of hypertension (SHR and DSS) and type 2 diabetes (ZDF-fatty). It lowered blood pressure effectively as well as modulated plasma concentrations of a number of circulating vasoactive peptidic mediators that are keys to the regulation of the vascular tone. These data suggest that CGS 35601, a triple vasopeptidase inhibitor (VPI), may represent a novel class of antihypertensive drugs and may have the potential to reduce morbidity and mortality from cardiovascular disorders, diabetes and subsequent renal complications. Similar in vivo ACE, NEP, and ECE inhibitory activities were also observed with the orally active prodrug, CGS 37808 (L-tryptophan, N-[[1-[[(2S)-2-(acetylthio)-4-methyl-1-oxopentyl]amino]cyclopentyl]-carbonyl]-, methyl ester.

Endogenous opioids upregulate brain-derived neurotrophic factor mRNA through delta- and micro-opioid receptors independent of antidepressant-like effects

Systemic administration of delta-opioid receptor (DOR) agonists decreases immobility in the forced swim test (FST) and increases brain-derived neurotrophic factor (BDNF) mRNA expression in rats, indicating that DOR agonists may have antidepressant-like effects. The aim of this study was to investigate the effects of central administration of endogenous opioid peptides on behavior in the FST and on brain BDNF mRNA expression in rats. Effects of endogenous opioids were compared with those produced by intracerebroventricular administration of a selective non-peptidic DOR agonist (+)BW373U86. Antidepressant-like effects were measured by decreased immobility in the FST. BDNF mRNA expression was determined by in situ hybridization. Centrally administered (+)BW373U86 decreased immobility and increased BDNF mRNA expression in the frontal cortex through a DOR-mediated mechanism, because these effects were blocked by the DOR antagonist naltrindole, but not by the micro-opioid receptor (MOR) antagonist naltrexone (NTX) or the kappa-opioid receptor antagonist nor-binaltorphimine. Of all the endogenous opioids tested, only leu- and met-enkephalin produced behavioral effects like those of (+)BW373U86 in the FST. Unlike (+)BW373U86, the enkephalins upregulated BDNF mRNA expression in the hippocampus through DOR- and MOR-mediated mechanisms. beta-Endorphin, endomorphin-1 and endomorphin-2 significantly increased BDNF mRNA levels in the frontal cortex, hippocampus and amygdala without reducing immobility; and most of these effects were reversed by NTX. This study is the first to provide evidence that endogenous opioids can upregulate BDNF mRNA expression through the DOR and MOR, and that leu- and met-enkephalin have similar pharmacological profiles to synthetic DOR agonists in producing antidepressant-like effects.

Facilitation of fear extinction by midbrain periaqueductal gray infusions of RB101(S), an inhibitor of enkephalin-degrading enzymes

Mu-opioid receptors in the ventrolateral quadrant midbrain periaqueductal gray (vIPAG) contribute to extinction of conditioned fear. The present experiment studied whether fear extinction could be facilitated by infusions of a peptidase inhibitor that reduces catabolism of vIPAG enkephalins. Rats were trained to fear an auditory conditioned stimulus. Fear was then extinguished. Extinction training was preceded by infusions of vehicle or RB101(S), an inhibitor of enkephalin catabolising enzymes. RB101(S) dose dependently facilitated extinction as indexed by performance during extinction and on a drug-free test. This facilitation was not observed when RB101(S) was infused outside the vIPAG. These results confirm that vIPAG endogenous opioids contribute to fear extinction and show that extinction can be facilitated by manipulations that increase vIPAG opioid neuromodulation.

Physiological control of emotion-related behaviors by endogenous enkephalins involves essentially the delta opioid receptors

The endogenous pentapeptide enkephalins bind to the mu and delta opioid receptors, with a slightly higher affinity for the latter. It remains a controversy regarding the respective physiological role of mu and delta opioid receptors in the control of emotion and motivation. One of the difficulties to investigate this problem is the low tonic extracellular release of enkephalins in various brain structures. To overcome this problem the synaptic levels of these pentapeptides were enhanced by inhibition of enzymes involved in their catabolism with the selective inhibitor H3N-CH(CH2-CH2-S-CH3)-CH2-S-S-CH2-CH(CH2phi)-CONH-CH(CH2phi)-COOCH2phi (RB101). This compound was shown to increase the extracellular levels and lifetime of endogenous enkephalins. Similar responses were obtained in wild-type and mu opioid receptor knockout mice following RB 101 administration in behavioral tests measuring locomotor activity, anxiety (elevated O-maze), and motivation (forced swim test and conditioned suppression of motility). In contrast, RB 101 led to antinociceptive responses only in wild-type animals using hot plate and tail immersion tests. These results clearly demonstrate the critical role of delta opioid receptors activated by the endogenous opioid peptides, in the physiological control of emotion- and motivation-related behaviors. In contrast, antinociceptive modulation, at least with respect to thermal nociceptive stimuli, involves enkephalin-activated mu opioid receptors. These findings could open new perspectives in the treatment of mood disorders using either inhibitors of enkephalin catabolism or delta opioid agonists.

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.

Partial Protection against Botulinum B Neurotoxin-Induced Blocking of Exocytosis by a Potent Inhibitor of Its Metallopeptidase Activity

Clostridium botulinum neurotoxins (BoNTs) cause botulism, which is characterized by a flaccid paralysis, through inhibition of acetylcholine release by peripheral cholinergic nerve terminals. This is due to the zinc metallopeptidase activity of the neurotoxin, cleaving one component (synaptobrevin for BoNT/B) of the exocytosis machinery. Yet, there are no specific agents able to control the peptidase-related effects of BoNT/B. We recently developed the first compounds to inhibit this enzymatic activity in the nanomolar range. Here we report that two of our best inhibitors prevent the BoNT/B-induced cleavage of native synaptobrevin on synaptic vesicles, and partially inhibit the suppression of [3H]noradrenaline release from synaptosomes that is caused by BoNT/B. These results were obtained at micromolar concentrations, consistent with the measured inhibitory potency of these inhibitors on the native toxin. These compounds provide a new way to possibly prevent and/or to control the neurotoxin effects of botulinum.

Regional brain cholecystokinin changes as a function of friendly and aggressive social interactions in rats

Cholecystokinin (CCK) is the most abundant neuropeptide in the mammalian brain, and has been implicated in the regulation of a diversity of emotions and motivations including negative affect and stress responses. In this experiment, we assayed levels of CCK (CCK4/5 and CCK8) from tissue homogenates in intruder animals 6 h after resident-intruder inter-male aggression. Intruder animals that demonstrated submissive behavior (freezing and 22-kHz ultrasonic vocalizations) had higher levels of CCK in the tegmentum and posterior cortex as compared to non-submissive (i.e., "Friendly") intruder animals. Ultrasonic vocalizations (22-kHz) were positively correlated with CCK levels in the tegmentum, posterior cortex and pituitary. These data suggest that CCK may play a role in the generation of negative affective states indexed by 22-kHz ultrasonic calls in certain regions of the brain.

Purification of a specific inhibitor of pyroglutamyl aminopeptidase II from the marine annelide Hermodice carunculata. in vivo effects in rodent brain

An inhibitor of the metallo-ectoenzyme, pyroglutamyl aminopeptidase II (PPII), a thyrotropin releasing hormone-specific peptidase, was identified by screening extracts from marine species of the Cuban coast-line belonging to the phylla Chordata, Echinodermata, Annelida, Mollusca, Cnidaria, Porifera, Chlorophyta and Magnoliophyta. Isolation of the inhibitor (HcPI), from the marine annelide Hermodice carunculata, was achieved by trichloroacetic acid treatment of the aqueous extract, followed by ion-exchange chromatography on DEAE Sephacel, gel filtration on Sephadex G-25 and reverse phase-HPLC. HcPI had a small apparent molecular weight (below 1000 Da) and was not a peptide. It inhibited rat PPII (a membrane preparation with 8.5mg protein/ml) with an apparent K(i) of 51 nM. HcPI did not inhibit serine (trypsin, chymotrypsin, elastase and dipeptidyl aminopeptidase IV), cysteine (papain, bromelain and pyroglutamyl aminopeptidase I), aspartic (pepsin and recombinant human immunodeficiency virus 1 protease (HIV1-PR)) nor other metallo proteinases (collagenase, gelatinase, angiotensin converting enzyme, aminopeptidase N and carboxypeptidase A). HcPI was non-toxic and active in vivo. Intraperitoneal injection of HcPI reduced mouse pituitary and brain PPII activity. Potency of the effect was higher in hypophysis and hypothalamus than in other brain regions. Intrathecal administration to male rats reduced PPII activity in the spinal cord. In conclusion we have identified a specific inhibitor of PPII that is the first M1 family zinc metallo-peptidase inhibitor isolated from marine invertebrates. It may be useful for elucidating the in vivo role of PPII in the pituitary and central nervous system.

In vivo properties of thiol inhibitors of the three vasopeptidases NEP, ACE and ECE are improved by introduction of a 7-azatryptophan in P2′ position

Three zinc metallopeptidases are implicated in the regulation of fluid homeostasis and vascular tone and represent interesting targets for the treatment of chronic heart failure. We have previously reported the synthesis of a triple inhibitor able to simultaneously inhibit neprilysin (NEP, EC 3.4.24.11), angiotensin-converting enzyme (ACE, EC 3.4.15.1) and endothelin-converting enzyme (ECE-1, EC 3.4.24.71) with nanomolar potency towards NEP and ACE and a lesser affinity for ECE. Here, we report the optimization and biological activities of analogs derived from lead compound 1 (2S)-2-[(2R)-2-((1S)-5-bromo-indan-1-yl)-3-mercapto-propionylamino]-3- (1H-indol-3-yl)-propionic acid by a structural approach. Among several inhibitors, compound 21, (2S)-2-[(2R)-2-((1S)-5-bromo-indan-1-yl)-3-mercapto-propionylamino]-3-(1H-pyrrolo[2,3-b]pyridin-3-yl)-propionic acid was selected by taking into account its good molecular adaptation with the recently published structures of the three vasopeptidases. This optimization procedure led to an improved pharmacologic activity when compared with 1.