Chronic inhibition of enkephalinase induces changes in the antinociceptive and locomotor effects of the enkephalinase inhibitor acetorphan in rats

Mu and delta opioid receptors play opposite nociceptive and behavioural roles on nerve-injured mice

BACKGROUND AND PURPOSE

Mu and delta opioid receptors(MOP, DOP) contribution to the manifestations of pathological pain is not understood. We used genetic approaches to investigate the opioid mechanisms modulating neuropathic pain and its comorbid manifestations.

EXPERIMENTAL APPROACH

We generated conditional knockout mice with MOP or DOP deletion in sensoryNav1.8-positive neurons (Nav1.8), in GABAergic forebrain neurons (DLX5/6) orconstitutively (CMV). Mutant mice and wild-type littermates were subjected topartial sciatic nerve ligation (PSNL) or sham surgery and their nociception wascompared. Anxiety-, depressivelike behaviour and cognitive performance were also measured. Opioid receptor mRNA expression, microgliosis and astrocytosis were assessed in the dorsalroot ganglia (DRG) and/or the spinal cord (SC).

KEY RESULTS

Constitutive CMV-MOP knockouts after PSNL displayed reduced mechanical allodynia and enhanced heat hyperalgesia. This phenotype was accompanied by increased DOP expression in DRG and SC, and reduced microgliosis and astrocytosis in deep dorsal horn laminae. Conditional MOP knockouts and control mice developed similar hypersensitivity after PSNL, except for anenhanced heat hyperalgesia by DLX5/6-MOP male mice. Neuropathic pain-induced anxiety was aggravated in CMV-MOP and DLX5/6-MOP knockouts. Nerve-injured CMV-DOP mice showed increased mechanical allodynia, whereas Nav1.8-DOP and DLX5/8-DOP mice had partial nociceptive enhancement. CMV-DOP and DLX5/6-DOP mutants showed increased depressive-like behaviour after PSNL.

CONCLUSIONS AND IMPLICATIONS

MOP activity after nerve injury increased anxiety-like responses involving forebrain GABAergic neurons and enhanced mechanical pain sensitivity along with repression of DOP expression and spinal cord gliosis. In contrast, DOP shows a protective function limiting nociceptive and affective manifestations of neuropathic pain.

A comprehensive review of the pharmacodynamics, pharmacokinetics, and clinical effects of the neutral endopeptidase inhibitor racecadotril

Racecadotril, via its active metabolite thiorphan, is an inhibitor of the enzyme neutral endopeptidase (NEP, EC 3.4.24.11), thereby increasing exposure to NEP substrates including enkephalins and atrial natriuretic peptide (ANP). Upon oral administration racecadotril is rapidly and effectively converted into the active metabolite thiorphan, which does not cross the blood–brain-barrier. Racecadotril has mainly been tested in animal models and patients of three therapeutic areas. As an analgesic the effects of racecadotril across animal models were inconsistent. In cardiovascular diseases such as hypertension or congestive heart failure results from animal studies were promising, probably related to increased exposure to ANP, but clinical results have not shown substantial therapeutic benefit over existing treatment options in cardiovascular disease. In contrast, racecadotril was consistently effective in animal models and patients with various forms of acute diarrhea by inhibiting pathologic (but not basal) secretion from the gut without changing gastro-intestinal transit time or motility. This included studies in both adults and children. In direct comparative studies with loperamide in adults and children, racecadotril was at least as effective but exhibited fewer adverse events in most studies, particularly less rebound constipation. Several guidelines recommend the use of racecadotril as addition to oral rehydration treatment in children with acute diarrhea.

Effects of Loperamide on Mechanical Allodynia Induced by Herpes Simplex Virus Type-1 in Mice

In the present study, we investigated whether the peripherally acting micro-opioid receptor agonist loperamide would inhibit allodynia in the non-inflamed dermatome of mice with herpetic pain. Subcutaneous (s.c.) injection of loperamide (1 and 3 mg/kg) inhibited allodynia. Local (intraplantar) injection of loperamide (1 and 5 microg/site) also produced an anti-allodynic effect. The peripheral opioid receptor antagonist naloxone methiodide (0.1 mg/kg, s.c.) and the micro-opioid receptor-selective antagonist beta-funaltrexamine (40 nmol/site, intraplantar and 20 mg /kg, s.c.) antagonized the anti-allodynic effects of systemic and local loperamide. Local injection of loperamide into the contralateral hind paw was without effect, suggesting that the effect is mediated through local action, not systemic action. Acute and subacute tolerance did not develop to the anti-allodynic effect of loperamide. In addition, there were no cross-tolerance between local opioids (morphine and loperamide) and systemic morphine. These results suggest that stimulation of peripheral micro-opioid receptors suppresses herpetic allodynia without tolerance development. The non-narcotic micro-opioid receptor agonist loperamide may relieve acute herpetic pain in patients with herpes zoster.

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

Neutral endopeptidase EC 3.4.24.11 (NEP) is localized in peptidergic neurons and various colocalized peptides or other humoral mediators may serve as substrates. Target disruption of the NEP gene was reported to enhance the lethal response to endotoxin shock in mice. We examined thermonociceptive thresholds and enkephalin (ENK) tissue levels in transgenic NEP (-/-) and control wild type NEP (+/+) mice. Hot plate (52 degrees C) latency was 13.1 +/- 1.4 s in NEP (+/+) mice (n = 16) while latency increased significantly (P = 0.031) to 17.7 +/- 1.6 s in NEP (-/-) mice. Naloxone (10 mg/kg) had no effect on hot plate latency in NEP (+/+) mice (12.5 s, n = 8), but significantly decreased the latency in NEP (-/-) mice compared to untreated NEP (-/-) deficient mice (10.5 s, n = 8). Morphine (3 or 10 mg/kg) analgesic response was similar in knockout mice and wild type mice. Methionine-ENK (MET-ENK) and leucine-ENK (LEU-ENK) levels were determined in extracts from cortex, brain stem, hypothalamus, striatum, spinal cord, trigeminal ganglion and heart in treated and untreated mice. ENK-levels varied in a regionally-dependent manner and were significantly decreased in hypothalamus and spinal cord. We conclude that deletion of the NEP gene results in an opioid-related increase in thermonociceptive threshold. Regional differences in opioid metabolism indicate that NEP evokes tissue-specific patterns of ENK-regulation. NEP selectively controls opioid biosynthesis in hypothalamus and spinal cord presumably by feedback regulation.

Nociceptin stimulates locomotion and exploratory behaviour in mice

The recently characterized heptadecapeptide nociceptin, the endogenous agonist of the orphan opioid receptor-like 1 (ORL1 receptor), has been tested for its effects on locomotion and exploratory behaviour in mice. I.c.v. administration of as little as 10 ng of nociceptin/animal stimulated locomotor activity. This effect was dose-dependent, increasing in intensity up to 100 ng and in duration for doses in the range of 1000-10000 ng. The stimulation of horizontal locomotion elicited by 100 ng nociceptin was accompanied by a stimulation of the vertical component of locomotion. These effects were not reversed by high doses (1.5 and 4.5 mg/kg s.c.) of the opioid receptor antagonist naloxone. Increasing doses of the dopamine D2 receptor antagonist haloperidol (0.1-0.5 mg/kg i.p.) as well as of the dopamine D1 receptor antagonist SCH 23390 [R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine hydrochloride] (0.0075-0.03 mg/kg s.c.) reversed this effect, suggesting that nociceptin exerts its motor-stimulant actions by increasing central dopaminergic transmission. Nociceptin was also found to increase the number of head dips in the hole-board test, indicating that the peptide stimulates exploratory behaviour.

No tolerance to peripheral morphine analgesia in presence of opioid expression in inflamed synovia

Pain treatment with centrally acting opiates is limited by tolerance. Tolerance is a decreasing effect of a drug with prolonged administration of that drug or of a related (e.g., endogenous) compound acting at the same receptor. This is often associated with a downregulation of receptors. In peripheral inflamed tissue, both locally expressed opioid peptides and morphine can produce powerful analgesia mediated by similar populations of opioid receptors. We hypothesized that the chronic presence of endogenous opioids in inflamed joints might convey downregulation of peripheral opioid receptors and tolerance to the analgesic effects of intraarticular morphine. We assessed these effects after arthroscopic surgery in patients with and without histologically verified synovial cellular infiltration, and we examined synovial opioid peptides and opioid receptors by immunocytochemistry and autoradiography, respectively. We found that, despite an abundance of opioid-containing cells in pronounced synovitis, morphine is at least as effective as in patients without such cellular infiltrations, and there is no major downregulation of peripheral opioid receptors. Thus, opioids expressed in inflamed tissue do not produce tolerance to peripheral morphine analgesia. Tolerance may be less pronounced for peripherally than for centrally acting opioids, which provides a promising perspective for the treatment of chronic pain in arthritis and other inflammatory conditions.

Locomotor and analgesic effects of morphine and acetorphan in rats chronically treated with morphine or thiorphan

A continuous 8-day s.c. administration of morphine (450 microgram/kg/h) sensitized rats to the morphine-induced stimulation of locomotion (morphine test dose = 3 mg/kg, s.c.) but not to the acetorphan (5 mg/kg, i.v.)-induced stimulation of locomotion. On the other hand, a continuous 10-day intracerebroventricular infusion of the enkephalinase inhibitor, thiorphan (25 micrograms/rat/h), known to desensitize the acetorphan-induced stimulation of locomotion, also desensitized the morphine (3 mg/kg, s.c.)-induced stimulation of locomotion. The continuous 10-day, s.c. administration of morphine desensitized to the morphine (3 mg/kg, s.c.)-but not acetorphan (5 mg/kg, i.v.)-induced analgesia, as measured by the latency to jump from a hot plate (55 degrees C). On the other hand, the continuous 10-day intracerebroventricular infusion of thiorphan did not desensitize to morphine (3 mg/kg, s.c.)-induced analgesia. Thus, the chronic actions of morphine and thiorphan, according to the tested function, did not result in cross-sensitization (locomotion) or cross-tolerance (nociception). These differences could depend on the involvement of different opioid receptors (mu vs. delta) and/or on different functional organizations.

Inhibitors of enkephalin-degrading enzymes as potential therapeutic agents

A limited number of enzymes such as membrane metalloendopeptidase (enkephalinase) and angiotensin converting enzyme appear to be involved in deactivation and modulation of circulatory regulatory peptides. Peptides such as the enkephalins are also involved in a large number of physiological processes. This multiplicity of physiological roles has made it difficult to establish the therapeutic role of enkephalin-degrading enzyme inhibitors. Other factors such as difficulty in quantification and thus measurement of processes involved in pain and mental illness have also hindered the process of establishing any therapeutic role of enkephalin-degrading enzyme inhibitors in these conditions. However, they have proved to be useful pharmacological 'tools'. The most likely therapeutic role at present appears to be in the treatment of cardiovascular disorders. As a 'profile' of pharmacological actions of enkephalin-degrading enzymes emerges, it is becoming apparent that bioavailability rather than a high degree of specificity or inhibitory potency may be the most important factor. This may be used to an advantage in future developments by the use of less specific or combined inhibitors in the form of prodrugs, designed to be active at specific sites such as the central nervous system.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.