Review
Novel approaches to targeting neuropeptide systems

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Abstract

Generation and/or interruption of cell signalling by neuropeptides has been shown to be essentially, although not exclusively, mediated by one or several membrane-bound enzymes, giving rise to the concept of selective versus dual enzyme inhibitors. Because most of these enzymes are zinc metallopeptidases, novel inhibitors are now being designed based on the structure of these proteins. The physiological role of neuropeptides and their relationships with other peptide systems can be investigated by comparing results obtained using peptidase inhibitors and selective receptor antagonists with those obtained using mice in which genes encoding the various components of a peptide system have been deleted. The potential use of peptidase inhibitors, compared with exogenous agonists, as therapeutic agents (particularly as analgesics or antidepressants) and their use in the investigation of the neurobiology of drug abuse will be discussed with particular focus on enkephalins and cholecystokinin 8 (CCK-8).

Section snippets

Membrane-bound zinc metallopeptidases and the interruption of neuropeptide effects

Peptide messengers are secreted, in active or inactive forms, by specialized cells via a Ca2+-dependent mechanism (Fig. 1). Most frequently, the peptide is released in its active form, which, in general, stimulates receptors located nearby in the CNS [e.g. two opioid peptides Met- and Leu-enkephalin both interact with G-protein-coupled mu and delta opioid receptors close to the site of peptide release3 (Fig. 1a) and the sulfated fragment of cholecystokinin, CCK-8, binds to CCK1 and CCK2

Peptidase-dependent diversification of physiological responses

Among the neuropeptides, CCK-8 was shown to be efficiently cleaved in vivo by APA (Ref. 22), leading to formation of CCK-7, and by the ubiquitously distributed tripeptidyl peptidase II (TPPII, EC 3.4.14.10), which is mainly enriched in the cytosol but is also present as an internal and/or external component of the cell membrane23 (Fig. 1). In contrast to various neuropeptides, including enkephalins, the degradation of CCK-8 by rat brain membranes is much slower, which suggests that peptidases

Dual inhibitors of NEP and aminopeptidase N

Compounds that elicit analgesic responses that are intermediate between NSAIDs and opioid analgesics are needed for efficient treatment of post-operative pain, severe osteoarticular pain, neuropathic pain, chronic inflammatory pain, and pain in children and the elderly.

Owing to the complementary roles of NEP and aminopeptidase N (APN, EC 3.4.11.2) in enkephalin inactivation, selective inhibition of only one of these peptidases provides a non-significant or weak antinociceptive effect (Fig. 3a),

Delta opioid receptors as possible targets for antidepressants

It has been known for a long time that opium and morphine possess euphorogenic and disinhibitory properties that are sometimes still used in psychiatry. Accordingly, enkephalins protected from their metabolizing enzymes have been shown to be active in all tests used to screen antidepressant drugs (e.g. forced swim, conditioned suppression of motility and learned helplessness). The effects of dual inhibitors such as RB101 or selective delta opioid receptor agonists were reversed by naltrindole

Mechanisms of opioid-induced physical and psychic dependence

Numerous studies have suggested that drug abuse could be a result of pre-existent or induced modifications of the endogenous opioid system. Accordingly, in animals and humans, opioid receptor antagonists were shown to reduce the craving of various drugs such as cocaine, ethanol and THC (Refs 12., 49., 50.). The use of dual NEP–APN inhibitors to either potentiate the various effects of enkephalins and their eventual association with agonists or antagonists of counteracting systems [e.g. CCK-8 (

Indirect modulation of rewarding effects of opiates by endogenous cannabinoids and substance P

The existence of dopamine-independent drug-craving mechanisms have also been postulated and recently reviewed55. Thus, deletion of genes encoding either the cannabinoid CB1 receptor56., 57. or the NK1 receptor58., 59. were recently shown to produce a large decrease or a complete lack of morphine-induced psychic dependence, measured by using self-administration and place-preference paradigms, respectively. Interestingly, mice with deletion of NK1 receptors showed a reduced agressivity towards an

Physiological antagonism between two neuropeptide systems (enkephalins and CCK)

Opponent processes have been proposed to play a crucial role in behavioural homeostasis60. Enkephalins and CCK-8 were shown to coexist within numerous brain regions and CCK and opioid receptors are colocalized in neurones of some structures61. The combined use of a dual NEP–APN inhibitor with a CCK2 receptor antagonist (Fig. 4) allowed the mechanism of this opponent process to be investigated.

Thus, the analgesic effects of RB101 in rodents is potentiated (two–eightfold) by co-administration of

Defects in endogenous opioids–CCK balance: anxiety and depressive syndrome

Another benefit of the opioid–CCK system might be gained in the field of depression. Some studies have reported that patients with severe depression have an increase in the concentration of CCK in the cerebrospinal fluid67. Chronic and unpredictable mild stress (CMS) leads to a reduction in the motility of mice and provides an animal model of anhedonia, which is often used to evaluate antidepressants. This test has been used to measure, by microdialysis in the nucleus accumbens, the

Concluding remarks

The need for potent and selective inhibitors of enzymes that metabolize neuropeptides is necessary to establish the exact physiological role of a given peptide in a given tissue. This approach is complementary to that which deletes the various targets for a peptide messenger. Pharmacological experiments achieved by using these two approaches is expected to direct the choice of a therapeutic route (stimulation or blockade of specific receptors). Indeed, modulation of the concentrations of

Acknowledgements

I gratefully acknowledge C. Dupuis for expert manuscript drafting. This work was supported by institutional grant from the European Community (BMH4 CT98 2267).

References (71)

  • T.J. Phillips

    Alcohol preference and sensitivity are markedly reduced in mice lacking dopamine Δ2 receptors

    Nat. Neurosci.

    (1998)
  • R. Spanagel et al.

    The dopamine hypothesis of reward: Past and current status

    Trends Neurosci.

    (1999)
  • C. Ledent

    Unresponsiveness to cannabinoids and reduced addictive effects of opiates in CB1 receptor knockout mice

    Science

    (1999)
  • C. Becker

    2-opioid receptor mediation of morphine-induced CCK release in the frontal cortex of the freely moving rat

    Synapse

    (1999)
  • F. Ruiz

    Similar decrease in spontaneous morphine abstinence by methadone and RB 101, an inhibitor of enkephalin catabolism

    Br. J. Pharmacol.

    (1996)
  • A. Sebret

    Rat hippocampal neurons are critically involved in physiological improvement of memory processes induced by cholecystokinin-B receptor stimulation

    J. Neurosci.

    (1999)
  • M.S. Kramer

    Distinct mechanism for antidepressant activity by blockade of central substance P receptors

    Science

    (1998)
  • B.P. Roques

    Neutral endopeptidase 24.11: structure, inhibition, and experimental and clinical pharmacology

    Pharmacol. Rev.

    (1993)
  • B.L. Kieffer

    Recent advances in molecular recognition and signal transduction of active peptides: receptors for opioid peptides

    Cell Mol. Neurobiol.

    (1995)
  • F. Noble

    International union of pharmacology. XXI. Structure, distribution, and functions of cholecystokinin receptors

    Pharmacol. Rev.

    (1999)
  • A. Réaux

    Aminopeptidase A inhibitors as potential central antihypertensive agents

    Proc. Natl. Acad. Sci. U. S. A.

    (1999)
  • N.D. Rawlings et al.

    Evolutionary families of metallopeptidases

    Methods Enzymol.

    (1995)
  • R. Peyron

    Haemodynamic brain response to acute pain in humans: sensory and attentional networks

    Brain

    (1999)
  • A. Saria

    Opioid-related changes in nociceptive threshold and in tissue levels of enkephalins after target disruption of the gene for neutral endopeptidase (EC 3.4.24.11) in mice

    Neurosci. Lett.

    (1997)
  • M.A.F.M. Gerrits

    Endogenous opioids implicated in the dynamics of experimental drug addiction. An in vivo autoradiographic analysis

    Neuroscience

    (1999)
  • G.Y. Ng

    Genotypic differences in mesolimbic enkephalin gene expression in DBA/2J and C57BL/6J inbred mice

    Eur. J. Pharmacol.

    (1996)
  • L.S. Brady

    Region-specific up-regulation of opioid receptor binding in enkephalin knockout mice

    Mol. Brain Res.

    (1999)
  • H.W.D. Matthes

    Loss of morphine-induced analgesia, reward effect and withdrawal symptoms in mice lacking the μ-opioid-receptor gene

    Nature

    (1996)
  • I. Sora

    Opiate receptor knockout mice define μ receptor roles in endogenous nociceptive responses and morphine-induced analgesia

    Proc. Natl. Acad. Sci. U. S. A.

    (1997)
  • Noble, F. and Roques, B.P. (1997) New research approach to the treatment of pain: the development of complete...
  • K. Karlsson et al.

    Purification of substance P endopeptidase (SPE) activity in human spinal cord and subsequent comparative studies with SPE in cerebrospinal fluid and with chymotrypsin

    J. Mol. Recognit.

    (1998)
  • B. Lu

    Neutral endopeptidase modulation of septik shock

    J. Exp. Med.

    (1995)
  • S. Sturiale

    Neutral endopeptidase (EC 3.4.24.11) terminates colitis by degrading substance P

    Proc. Natl. Acad. Sci. U. S. A.

    (1999)
  • C. Rose

    Characterization and inhibition of a cholecystokinin-inactivating serine peptidase

    Nature

    (1996)
  • S. Pavlasevic

    Brain cholecystokinin tetrapeptide levels are increased in a rat model of anxiety

    NeuroReport

    (1993)
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