Analgesic doses of the enkephalin degrading enzyme inhibitor RB 120 do not have discriminative stimulus properties

The inhibition of enkephalin catabolism by dual enkephalinase inhibitor: A novel possible therapeutic approach for opioid use disorders

Despite the increasing impact of opioid use disorders on society, there is a disturbing lack of effective medications for their clinical management. An interesting innovative strategy to treat these disorders consists in the protection of endogenous opioid peptides to activate opioid receptors, avoiding the classical opioid-like side effects. Dual enkephalinase inhibitors (DENKIs) physiologically activate the endogenous opioid system by inhibiting the enzymes responsible for the breakdown of enkephalins, protecting endogenous enkephalins and increasing their half-lives and physiological actions. The activation of opioid receptors by the increased enkephalin levels, and their well-demonstrated safety, suggests that DENKIs could represent a novel analgesic therapy and a possible effective treatment for acute opioid withdrawal, as well as a promising alternative to opioid substitution therapy minimizing side effects. This new pharmacological class of compounds could bring effective and safe medications avoiding the major limitations of exogenous opioids, representing a novel approach to overcome the problem of opioid use disorders. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

The role of enkephalinergic systems in substance use disorders

Enkephalin, an endogenous opioid peptide, is highly expressed in the reward pathway and may modulate neurotransmission to regulate reward-related behaviors, such as drug-taking and drug-seeking behaviors. Drugs of abuse also directly increase enkephalin in this pathway, yet it is unknown whether or not changes in the enkephalinergic system after drug administration mediate any specific behaviors. The use of animal models of substance use disorders (SUDs) concurrently with pharmacological, genetic, and molecular tools has allowed researchers to directly investigate the role of enkephalin in promoting these behaviors. In this review, we explore neurochemical mechanisms by which enkephalin levels and enkephalin-mediated signaling are altered by drug administration and interrogate the contribution of enkephalin systems to SUDs. Studies manipulating the receptors that enkephalin targets (e.g., mu and delta opioid receptors mainly) implicate the endogenous opioid peptide in drug-induced neuroadaptations and reward-related behaviors; however, further studies will need to confirm the role of enkephalin directly. Overall, these findings suggest that the enkephalinergic system is involved in multiple aspects of SUDs, such as the primary reinforcing properties of drugs, conditioned reinforcing effects, and sensitization. The idea of dopaminergic-opioidergic interactions in these behaviors remains relatively novel and warrants further research. Continuing work to elucidate the role of enkephalin in mediating neurotransmission in reward circuitry driving behaviors related to SUDs remains crucial.

Cerebral decreases in opioid receptor binding in patients with central neuropathic pain measured by [11 C]diprenorphine binding and PET

Central neuropathic pain (CNP) is pain resulting from damage to the central nervous system. Up till now, it has not been possible to identify a common lesion or pharmacological deficit in these patients. This preliminary study in a group of patients with CNP with predominantly post-stroke pain, demonstrates that there is significantly less opioid receptor binding in a number of cortical and sub-cortical structures that are mostly, but not exclusively, within the medial pain system in patients compared to age-matched pain-free controls. The reductions in opioid receptor binding within the medial system were observed mainly in the dorsolateral (Brodman area 10) and anterior cingulate (Brodman area 24 with some extension into area 23) and insula cortices and the thalamus. There were also reductions in the lateral pain system within the inferior parietal cortex (Brodman area 40). These changes in binding could not be accounted for by the cerebral lesions shown by CT or MRI, which were outside the areas of reduced binding and the human pain system. To our knowledge this is the first systematic demonstration of a reduction in opioid receptor-binding capacity in neurones within the human nociceptive system in patients with CNP. This may be a key common factor resulting in undamped nociceptor activity within some of the structures that are predominantly within the medial nociceptive system. If confirmed, these findings may explain why certain patients with CNP require high doses of synthetic opiates to achieve optimum analgesia. The findings also raise the possibility of new pharmacological approaches to treatment.

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.

Depressant effect on a C-fibre reflex in the rat, of RB101, a dual inhibitor of enkephalin-degrading enzymes

The effect of N-[(R,S)-2-benzyl-3[(S)-(2-amino-4-methylthio)butyldithiol]-1-oxopropyl]-L-phenylalanine benzyl ester (RB101), a dual inhibitor of the enkephalin-degrading enzymes, neutral endopeptidase and aminopeptidase N, was assessed in anaesthetised rats on the C-fibre reflex elicited by electrical stimulation within the sural nerve territory and recorded from the ipsilateral biceps femoris muscle. The temporal evolution of the pharmacological response was monitored by the repeated application of a constant stimulus intensity, namely three times threshold (3 T). In addition, recruitment curves were built by varying the stimulus intensity from 0 to 7 T. RB101 (7.5, 15 and 30 mg kg(-1), i.v.) induced a dose-dependent, naloxone-reversible depression of the reflex, which lasted around 60 min with the highest dose. The ED(50) was calculated as 16.9 mg kg(-1). Analyses of the recruitment curves revealed: (1) a significant increase of threshold; (2) a significant depression of the reflex in the ascending part of the curve; and (3) a lack of major depressive effects on the responses elicited by the strongest stimuli (corresponding to the plateau of the curve). The increase in the nociceptive threshold by enkephalin-degrading enzyme inhibitors, confirms previous data obtained from behavioural tests. In addition, the present study revealed an efficacy of these compounds over a wide range of stimulus intensities, albeit excluding the highest.

Central discriminative effects of morphine in rats: training via intracerebroventricular administration

There have been studies of the discriminative effects of intracerebroventricularly (ICV)-administered morphine (MOR) in rats trained to discriminate MOR systemically, but the converse has not been done. In this study, rats were trained to discriminate between ICV (1-10 microg/3 microl, 1 h) or subcutaneous (SC) (3.0 mg/kg, 30 min) injections of MOR vs. saline/vehicle in a discrete-trial avoidance/escape procedure. On generalization testing, subjects in both the ICV- and SC-trained groups responded on the MOR-appropriate lever at ICV MOR doses < or =1-3 microg, and at SC MOR doses 2 to 3 orders of magnitude higher (vs. ICV). Naltrexone (SC) blocked the stimulus effects of MOR (ICV) equipotently in both training groups. In ICV-trained subjects, levorphanol (SC), the mu-opioid selective peptide [D-Ala2, NMePhe4, Gly-ol]-enkephalin (DAMGO) (ICV), and the enkephalinase inhibitor N-[L-(1-carboxy-2-phenyl)ethyl]-L-phenylalanyl-beta-alanine (SCH 32615) (ICV) produced complete MOR-appropriate responding, whereas the dextrorotary enantiomer of levorphanol dextrorphan (SC; < or = 3.0 mg/kg) and the delta-opioid selective peptide [D-Pen2, D-Pen5]-enkephalin (DPDPE) (ICV, < or = 0.03 mg) did not. SC-trained subjects did not generalize to SCH 32615, which suggests qualitative differences in the discriminative stimulus effects of novel drugs as a function of the route of administration of the training drug. These data demonstrate that it is feasible to train rats to discriminate an opioid administered by the ICV route, and to perform extended tests of generalization to novel drugs (SC or ICV) in rats so trained.

Facilitation of enkephalins catabolism inhibitor-induced antinociception by drugs classically used in pain management

The aim of this study was to investigate the facilitatory effects of subanalgesic or low doses of different drugs (acetylsalicylic acid, ibuprofen and morphine) on the antinociceptive responses induced by the endogenous opioid peptides, enkephalins, protected from their catabolism by the dual enkephalin-degrading enzymes inhibitor RB101. According to the analgesic profile of the three studied compounds different antinociceptive assays were used: the hot plate and formalin tests in mice, and the tail flick and paw pressure tests on inflamed paws in rats and polyarthritic rats. Facilitatory effects of subanalgesic doses of acetylsalicylic acid and ibuprofen on RB101-induced antinociceptive responses were observed in the early and late phases of the formalin test, respectively. In the hot plate, tail flick and paw pressure tests, the dose-dependent analgesic effects of RB101 were strongly potentiated by subanalgesic doses of morphine (0.5 mg/kg), while in these tests, acetylsalicylic acid and ibuprofen were unable to modify the RB101-induced antinociceptive responses. The synergism in antinociceptive effects observed with the combination of RB101 and morphine supported by isobolographic analysis, may have interesting clinical implications, considering both the lack of opiate drawbacks observed with RB101 and the high potentiation of its antinociceptive effects with very low doses of morphine.

Novel approaches to targeting neuropeptide systems

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).