Pregabalin-Tolperisone Combination to Treat Neuropathic Pain: Improved Analgesia and Reduced Side Effects in Rats.

The current treatment of neuropathic pain (NP) is unsatisfactory; therefore, effective novel agents or combination-based analgesic therapies are needed. Herein, oral tolperisone, pregabalin, and duloxetine were tested for their antinociceptive effect against rat partial sciatic nerve ligation (pSNL)-induced tactile allodynia described by a decrease in the paw withdrawal threshold (PWT) measured by a dynamic plantar aesthesiometer. On day 7 after the operation, PWTs were assessed at 60, 120, and 180 min post-treatment. Chronic treatment was continued for 2 weeks, and again, PWTs were measured on day 14 and 21. None of the test compounds produced an acute antiallodynic effect. In contrast, after chronic treatment, tolperisone and pregabalin alleviated allodynia. In other experiments, on day 14, the acute antiallodynic effect of the tolperisone/pregabalin or duloxetine combination was measured. As a novel finding, a single dose of the tolperisone/pregabalin combination could remarkably alleviate allodynia acutely. It also restored the neuropathy-induced elevated CSF glutamate content. Furthermore, the combination is devoid of adverse effects related to motor and gastrointestinal transit functions. Tolperisone and pregabalin target voltage-gated sodium and calcium channels, respectively. The dual blockade effect of the combination might explain its advantageous acute analgesic effect in the present work.

Telmisartan Is a Promising Agent for Managing Neuropathic Pain and Delaying Opioid Analgesic Tolerance in Rats.

Despite the large arsenal of analgesic medications, neuropathic pain (NP) management is not solved yet. Angiotensin II receptor type 1 (AT1) has been identified as a potential target in NP therapy. Here, we investigate the antiallodynic effect of AT1 blockers telmisartan and losartan, and particularly their combination with morphine on rat mononeuropathic pain following acute or chronic oral administration. The impact of telmisartan on morphine analgesic tolerance was also assessed using the rat tail-flick assay. Morphine potency and efficacy in spinal cord samples of treated neuropathic animals were assessed by [35S]GTPγS-binding assay. Finally, the glutamate content of the cerebrospinal fluid (CSF) was measured by capillary electrophoresis. Oral telmisartan or losartan in higher doses showed an acute antiallodynic effect. In the chronic treatment study, the combination of subanalgesic doses of telmisartan and morphine ameliorated allodynia and resulted in a leftward shift in the dose-response curve of morphine in the [35S]GTPγS binding assay and increased CSF glutamate content. Telmisartan delayed morphine analgesic-tolerance development. Our study has identified a promising combination therapy composed of telmisartan and morphine for NP and opioid tolerance. Since telmisartan is an inhibitor of AT1 and activator of PPAR-γ, future studies are needed to analyze the effect of each component.

Shedding Light on the Pharmacological Interactions between µ-Opioid Analgesics and Angiotensin Receptor Modulators: A New Option for Treating Chronic Pain.

The current protocols for neuropathic pain management include µ-opioid receptor (MOR) analgesics alongside other drugs; however, there is debate on the effectiveness of opioids. Nevertheless, dose escalation is required to maintain their analgesia, which, in turn, contributes to a further increase in opioid side effects. Finding novel approaches to effectively control chronic pain, particularly neuropathic pain, is a great challenge clinically. Literature data related to pain transmission reveal that angiotensin and its receptors (the AT1R, AT2R, and MAS receptors) could affect the nociception both in the periphery and CNS. The MOR and angiotensin receptors or drugs interacting with these receptors have been independently investigated in relation to analgesia. However, the interaction between the MOR and angiotensin receptors has not been excessively studied in chronic pain, particularly neuropathy. This review aims to shed light on existing literature information in relation to the analgesic action of AT1R and AT2R or MASR ligands in neuropathic pain conditions. Finally, based on literature data, we can hypothesize that combining MOR agonists with AT1R or AT2R antagonists might improve analgesia.

Pharmacological Evidence on Augmented Antiallodynia Following Systemic Co-Treatment with GlyT-1 and GlyT-2 Inhibitors in Rat Neuropathic Pain Model.

The limited effect of current medications on neuropathic pain (NP) has initiated large efforts to develop effective treatments. Animal studies showed that glycine transporter (GlyT) inhibitors are promising analgesics in NP, though concerns regarding adverse effects were raised. We aimed to study NFPS and Org-25543, GlyT-1 and GlyT-2 inhibitors, respectively and their combination in rat mononeuropathic pain evoked by partial sciatic nerve ligation. Cerebrospinal fluid (CSF) glycine content was also determined by capillary electrophoresis. Subcutaneous (s.c.) 4 mg/kg NFPS or Org-25543 showed analgesia following acute administration (30-60 min). Small doses of each compound failed to produce antiallodynia up to 180 min after the acute administration. However, NFPS (1 mg/kg) produced antiallodynia after four days of treatment. Co-treatment with subanalgesic doses of NFPS (1 mg/kg) and Org-25543 (2 mg/kg) produced analgesia at 60 min and thereafter meanwhile increased significantly the CSF glycine content. This combination alleviated NP without affecting motor function. Test compounds failed to activate G-proteins in spinal cord. To the best of our knowledge for the first time we demonstrated augmented analgesia by combining GlyT-1 and 2 inhibitors. Increased CSF glycine content supports involvement of glycinergic system. Combining selective GlyT inhibitors or developing non-selective GlyT inhibitors might have therapeutic value in NP.

On the Role of Peripheral Sensory and Gut Mu Opioid Receptors: Peripheral Analgesia and Tolerance.

There is growing evidence on the role of peripheral µ-opioid receptors (MORs) in analgesia and analgesic tolerance. Opioid analgesics are the mainstay in the management of moderate to severe pain, and their efficacy in the alleviation of pain is well recognized. Unfortunately, chronic treatment with opioid analgesics induces central analgesic tolerance, thus limiting their clinical usefulness. Numerous molecular mechanisms, including receptor desensitization, G-protein decoupling, ß-arrestin recruitment, and alterations in the expression of peripheral MORs and microbiota have been postulated to contribute to the development of opioid analgesic tolerance. However, these studies are largely focused on central opioid analgesia and tolerance. Accumulated literature supports that peripheral MORs mediate analgesia, but controversial results on the development of peripheral opioid receptors-mediated analgesic tolerance are reported. In this review, we offer evidence on the consequence of the activation of peripheral MORs in analgesia and analgesic tolerance, as well as approaches that enhance analgesic efficacy and decrease the development of tolerance to opioids at the peripheral sites. We have also addressed the advantages and drawbacks of the activation of peripheral MORs on the sensory neurons and gut (leading to dysbiosis) on the development of central and peripheral analgesic tolerance.

Comparisons of In Vivo and In Vitro Opioid Effects of Newly Synthesized 14-Methoxycodeine-6-O-sulfate and Codeine-6-O-sulfate.

The present work represents the in vitro (potency, affinity, efficacy) and in vivo (antinociception, constipation) opioid pharmacology of the novel compound 14-methoxycodeine-6-O-sulfate (14-OMeC6SU), compared to the reference compounds codeine-6-O-sulfate (C6SU), codeine and morphine. Based on in vitro tests (mouse and rat vas deferens, receptor binding and [35S]GTPγS activation assays), 14-OMeC6SU has µ-opioid receptor-mediated activity, displaying higher affinity, potency and efficacy than the parent compounds. In rats, 14-OMeC6SU showed stronger antinociceptive effect in the tail-flick assay than codeine and was equipotent to morphine, whereas C6SU was less efficacious after subcutaneous (s.c.) administration. Following intracerebroventricular injection, 14-OMeC6SU was more potent than morphine. In the Complete Freund's Adjuvant-induced inflammatory hyperalgesia, 14-OMeC6SU and C6SU in s.c. doses up to 6.1 and 13.2 µmol/kg, respectively, showed peripheral antihyperalgesic effect, because co-administered naloxone methiodide, a peripherally acting opioid receptor antagonist antagonized the measured antihyperalgesia. In addition, s.c. C6SU showed less pronounced inhibitory effect on the gastrointestinal transit than 14-OMeC6SU, codeine and morphine. This study provides first evidence that 14-OMeC6SU is more effective than codeine or C6SU in vitro and in vivo. Furthermore, despite C6SU peripheral antihyperalgesic effects with less gastrointestinal side effects the superiority of 14-OMeC6SU was obvious throughout the present study.

Efficacy-Based Perspective to Overcome Reduced Opioid Analgesia of Advanced Painful Diabetic Neuropathy in Rats.

Reduction of the opioid analgesia in diabetic neuropathic pain (DNP) results from µ-opioid receptor (MOR) reserve reduction. Herein, we examined the antinociceptive and antiallodynic actions of a novel opioid agonist 14-O-methymorphine-6-O-sulfate (14-O-MeM6SU), fentanyl and morphine in rats with streptozocin-evoked DNP of 9-12 weeks following their systemic administration. The antinociceptive dose-response curve of morphine but not of 14-O-MeM6SU or fentanyl showed a significant right-shift in diabetic compared to non-diabetic rats. Only 14-O-MeM6SU produced antiallodynic effects in doses matching antinociceptive doses obtained in non-diabetic rats. Co-administered naloxone methiodide (NAL-M), a peripherally acting opioid receptor antagonist failed to alter the antiallodynic effect of test compounds, indicating the contribution of central opioid receptors. Reduction in spinal MOR binding sites and loss in MOR immunoreactivity of nerve terminals in the spinal cord and dorsal root ganglia in diabetic rats were observed. G-protein coupling assay revealed low efficacy character for morphine and high efficacy character for 14-O-MeM6SU or fentanyl at spinal or supraspinal levels (E max values). Furthermore, at the spinal level only 14-O-MeM6SU showed equal efficacy in G-protein activation in tissues of diabetic- and non-diabetic animals. Altogether, the reduction of spinal opioid receptors concomitant with reduced analgesic effect of morphine may be circumvented by using high efficacy opioids, which provide superior analgesia over morphine. In conclusion, the reduction in the analgesic action of opioids in DNP might be a consequence of MOR reduction, particularly in the spinal cord. Therefore, developing opioids of high efficacy might provide analgesia exceeding that of currently available opioids.

Similarity and dissimilarity in antinociceptive effects of dipeptidyl-peptidase 4 inhibitors, Diprotin A and vildagliptin in rat inflammatory pain models following spinal administration.

Dipeptidyl-peptidase 4 (DPP4) enzyme is involved in the degradation of many biologically active peptides including opioids. Its role in pain transmission is poorly elucidated. Recently we reported on the spinal antihyperalgesic effects of DPP4 inhibitors, Ile-Pro-Ile (Diprotin A) and vildagliptin in carrageenan-evoked acute inflammatory pain in rats. The present study investigated the effects of intrathecal (it.) diprotin A and vildagliptin in Complete Freund's Adjuvant- (CFA) and formalin induced pain in rats. The former assay can model the subchronic inflammatory pain condition and the later one reflects both acute tonic and inflammatory pain conditions. The involvement of opioid receptor (OR) subtypes, Y1-, and GLP1 receptors were also investigated. In CFA pain model it. diprotin A or vildagliptin dose-dependently inhibits hyperalgesia in ipsilateral while has no effect in contralateral paws. The peak effect was achieved 30 min following drug administration which was used for further analysis. Both compounds showed naltrexone reversible antihyperalgesia. Co-administration of OR-subtype-selective antagonists with diprotin A and vildagliptin revealed involvement of µ and δ > µ opioid receptors, respectively. Co-administered Y1 but not GLP1 receptor antagonists reversed the antihyperalgesic action of both DPP4 inhibitors. In touch-hypersensitivity both compounds were ineffective. In formalin test only diprotin A showed µ and δ OR-mediated antinociception and only in the 2nd phase. This effect was Y1 or GLP-1 receptor antagonist insensitive. In conclusion, diprotin A and vildagliptin display antinociception of different mechanisms of action in subchronic inflammatory pain. Furthermore, the spinal pain relay points of inflammatory pain of acute or subchronic conditions were more effectively affected by diprotin A than vildagliptin which needs future elucidation.

The Peripheral Versus Central Antinociception of a Novel Opioid Agonist: Acute Inflammatory Pain in Rats.

Opioid analgesics devoid of central side effects are unmet medical need in the treatment of acute pain (e.g. post-operative pain). Recently, we have reported on 14-O-methylmorphine-6-O-sulfate (14-O-MeM6SU), a novel opioid agonist of high efficacy producing peripheral antinociception in subchronic inflammatory pain in certain doses. The present study focused on the antinociceptive effect of 14-O-MeM6SU compared to morphine in formalin test of an early/acute (Phase I) and late/tonic (Phase II) pain phases. Subcutaneous 14-O-MeM6SU (253-1012 nmol/kg) and morphine (3884-31075 nmol/kg) dose dependently reduced the pain behaviors of both phases. Co-administered naloxone methiodide (NAL-M), a peripherally acting opioid antagonist, abolished the antinociceptive effect of 506 nmol/kg 14-O-MeM6SU. On the other hand, the effects of 14-O-MeM6SU (1012 nmol/kg) and morphine (15538 nmol/kg) were only partially affected by NAL-M, indicating the contribution of CNS to antinociception. Locally injected test compounds into formalin treated paws caused antinociception in both phases. Locally effective doses of test compounds were also injected into contralateral paws. Morphine showed effects in both phases, 14-O-MeM6SU in certain doses failed to produce antinociception in either phase. A NAL-M reversible systemic dose of 14-O-MeM6SU and the lowest systemic effective dose of morphine were evaluated for their sedative effects following isoflurane-induced sleeping (righting reflex). In contrast to morphine, 14-O-MeM6SU in certain antinociceptive doses showed no impact on sleeping time. These data highlight that high efficacy opioids of limited CNS penetration in certain doses mitigate somatic and inflammatory pain by targeting MOR at the periphery.

14-O-Methylmorphine: A Novel Selective Mu-Opioid Receptor Agonist with High Efficacy and Affinity.

14-O-methyl (14-O-Me) group in morphine-6-O-sulfate (M6SU) or oxymorphone has been reported to be essential for enhanced affinity, potency and antinociceptive effect of these opioids. Herein we report on the pharmacological properties (potency, affinity and efficacy) of the new compound, 14-O-methylmorphine (14-O-MeM) in in vitro. Additionally, we also investigated the antinociceptive effect of the novel compound, as well as its inhibitory action on gastrointestinal transit in in vivo. The potency and efficacy of test compound were measured by [35S]GTPγS binding, isolated mouse vas deferens (MVD) and rat vas deferens (RVD) assays. The affinity of 14-O-MeM for opioid receptors was assessed by radioligand binding and MVD assays. The antinociceptive and gastrointestinal effects of the novel compound were evaluated in the rat tail-flick test and charcoal meal test, respectively. Morphine, DAMGO, Ile5,6 deltorphin II, deltorphin II and U-69593 were used as reference compounds. 14-O-MeM showed higher efficacy (Emax) and potency (EC50) than morphine in MVD, RVD or [35S]GTPγS binding. In addition, 14-O-MeM compared to morphine showed higher affinity for µ-opioid receptor (MOR). In vivo, in rat tail-flick test 14-O-MeM proved to be stronger antinociceptive agent than morphine after peripheral or central administration. Additionally, both compounds inhibited the gastrointestinal peristalsis. However, when the antinociceptive and antitransit doses for each test compound are compared, 14-O-MeM proved to have slightly more favorable pharmacological profile. Our results affirm that 14-O-MeM, an opioid of high efficacy and affinity for MOR can be considered as a novel analgesic agent of potential clinical value.

New opioid receptor antagonist: Naltrexone-14-O-sulfate synthesis and pharmacology.

Opioid antagonists, naloxone and naltrexone have long been used in clinical practice and research. In addition to their low selectivity, they easily pass through the blood-brain barrier. Quaternization of the amine group in these molecules, (e.g. methylnaltrexone) results in negligible CNS penetration. In addition, zwitterionic compounds have been reported to have limited CNS access. The current study, for the first time gives report on the synthesis and the in vitro [competition binding, G-protein activation, isolated mouse vas deferens (MVD) and mouse colon assay] pharmacology of the zwitterionic compound, naltrexone-14-O-sulfate. Naltrexone, naloxone, and its 14-O-sulfate analogue were used as reference compounds. In competition binding assays, naltrexone-14-O-sulfate showed lower affinity for µ, δ or κ opioid receptor than the parent molecule, naltrexone. However, the µ/κ opioid receptor selectivity ratio significantly improved, indicating better selectivity. Similar tendency was observed for naloxone-14-O-sulfate when compared to naloxone. Naltrexone-14-O-sulfate failed to activate [35S]GTPγS-binding but inhibit the activation evoked by opioid agonists (DAMGO, Ile5,6deltorphin II and U69593), similarly to the reference compounds. Schild plot constructed in MVD revealed that naltrexone-14-O-sulfate acts as a competitive antagonist. In mouse colon, naltrexone-14-O-sulfate antagonized the inhibitory effect of morphine with lower affinity compared to naltrexone and higher affinity when compared to naloxone or naloxone-14-O-sulfate. In vivo (mouse tail-flick test), subcutaneously injected naltrexone-14-O-sulfate antagonized morphine's antinociception in a dose-dependent manner, indicating it's CNS penetration, which was unexpected from such zwitter ionic structure. Future studies are needed to evaluate it's pharmacokinetic profile.

New Morphine Analogs Produce Peripheral Antinociception within a Certain Dose Range of Their Systemic Administration.

Growing data support peripheral opioid antinociceptive effects, particularly in inflammatory pain models. Here, we examined the antinociceptive effects of subcutaneously administered, recently synthesized 14-O-methylmorphine-6-O-sulfate (14-O-MeM6SU) compared with morphine-6-O-sulfate (M6SU) in a rat model of inflammatory pain induced by an injection of complete Freund's adjuvant and in a mouse model of visceral pain evoked by acetic acid. Subcutaneous doses of 14-O-MeM6SU and M6SU up to 126 and 547 nmol/kg, respectively, produced significant and subcutaneous or intraplantar naloxone methiodide (NAL-M)-reversible antinociception in inflamed paws compared with noninflamed paws. Neither of these doses significantly affected thiobutabarbital-induced sleeping time or rat pulmonary parameters. However, the antinociceptive effects of higher doses were only partially reversed by NAL-M, indicating contribution of the central nervous system. In the mouse writhing test, 14-O-MeM6SU was more potent than M6SU after subcutaneous or intracerebroventricular injections. Both displayed high subcutaneous/intracerebroventricular ED50 ratios. The antinociceptive effects of subcutaneous 14-O-MeM6SU and M6SU up to 136 and 3043 nmol/kg, respectively, were fully antagonized by subcutaneous NAL-M. In addition, the test compounds inhibited mouse gastrointestinal transit in antinociceptive doses. Taken together, these findings suggest that systemic administration of the novel compound 14-O-MeM6SU similar to M6SU in specific dose ranges shows peripheral antinociception in rat and mouse inflammatory pain models without central adverse effects. These findings apply to male animals and must be confirmed in female animals. Therefore, titration of systemic doses of opioid compounds with limited access to the brain might offer peripheral antinociception of clinical importance.

A new potent analgesic agent with reduced liability to produce morphine tolerance.

The therapeutic use of opioids is limited by the development of tolerance to the analgesic effect and the cellular and molecular mechanisms underlying this phenomenon are still not completely understood. For this reason the search for new analgesic derivatives, endowed with lower tolerance, is always an active field. The newly synthesized 14-O-Methylmorphine-6-sulfate (14-O-MeM6SU) shows high efficacy in in vitro assays and a strong analgesic action in the rat tail flick test. The aim of present work was to investigate: the analgesic effect of 14-O-MeM6SU in mouse tail-flick test; the tolerance to analgesic effect of 14-O-MeM6SU compared to morphine in mice, the effects of test compounds on glutamatergic neurotransmission by measuring spontaneous excitatory postsynaptic currents (sEPSCs) of layer V pyramidal cells from rat prefrontal cortices; and the effect of acute and chronic 14-O-MeM6SU treatments on opioid receptor gene expression in SH-SY5Y neuroblastoma cells expressing µ-opioid (MOP) and nociceptin/opioid receptor-like 1 (NOP) receptors. 14-O-MeM6SU was 17 times more potent than morphine in analgesia and had long duration of action in analgesic dose equipotent to morphine. Mice were treated subcutaneously (s.c.) either with 200 µmol/kg morphine or with 14-O-MeM6SU (12 µmol/kg) twice daily for three days. The magnitude of tolerance or cross-tolerance indicated by the shift in antinociceptive ED50 measured was greater for morphine compared to 14-O-MeM6SU. Subsequent to behavioral testing, patch-clamp experiments in layer V pyramidal neurons of rat prefrontal cortical slices in the presence of bicuculline were performed. Both 14-O-MeM6SU (0.1 µM) and morphine (1 µM) decreased the frequency of sEPSCs, indicating reduction of glutamate release. The effect of the novel compound was reversed by the opioid receptor antagonist naloxone, indicating an opioid mediated action. In contrast, the amplitude was not affected. Finally, gene expression data showed a dose dependent down-regulation of MOP receptor after 24h and 48 h exposure to 14-O-MeM6SU. Interestingly, no changes were detected for NOP receptor gene expression. The specific lack of this effect could be related to the lower tolerance development to analgesic effect of 14-O-MeM6SU. Furthermore, 14-O-MeM6SU displayed high intrinsic efficacy possibly an important factor in the observed effects. Further, the observed inhibition of glutamatergic signaling might be attributed also to the reduction of opioid tolerance. Based on our results the development of a new clinically important, safe analgesic agent might be possible.

Peripheral antinociceptive efficacy and potency of a novel opioid compound 14-O-MeM6SU in comparison to known peptide and non-peptide opioid agonists in a rat model of inflammatory pain.

This study compared the peripheral analgesic effects of a novel opioid agonist 14-O-methylmorphine-6-O-sulfate (14-O-MeM6SU), to that of non-peptide (morphine, fentanyl) and peptide opioid agonists (Met-enkephalin; met-ENK and ß-endorphin; ß-END) in a model of localized inflammatory pain evoked by intraplantar (i.pl.) Freund's complete adjuvant (FCA). Nociceptive responses to local opioid agonists were measured by pressure paw-withdrawal procedures. In addition, the antinociceptive efficacy and potency of these test compounds in vivo was compared to that in vitro using the rat vas deferens (RVD) bioassay. Intraplantar 14-O-MeM6SU (0.32-2.53 nmol/rat), morphine (14.95-112.15 nmol/rat), fentanyl (0.19-2.36 nmol/rat), met-ENK (0.10-10 nmol/rat) and ß-END (0.77-5.00 nmol/rat) dose dependently increased paw pressure thresholds exclusively in inflamed hindpaws. At higher doses analgesic effects were also seen in noninflamed paws for 14-O-MeM6SU, morphine and fentanyl but not for met-ENK or ß-END. The maximal possible local analgesic effect (%) measured in inflamed paws was 50.6 ± 2.7, 18.23 ± 1.78, 37.44 ± 2.17, 36.00 ± 1.43, and 40.69 ± 0.91 for 14-O-MeM6SU, morphine, fentanyl, met-ENK and ß-END, respectively. Interestingly, i.pl. administered opioid peptides met-ENK and ß-END displayed a peripheral analgesic ceiling effect. This local antinociception was antagonized by co-administered opioid antagonist naloxone-methiodide (NAL-M). Similar to the analgesic testing, the RVD showed the following efficacy order of the test compounds: 14-O-MeM6SU>ß-END>fentanyl>met-ENK≫morphine. Taken together, 14-O-MeM6SU was more potent than morphine, fentanyl and met-ENK and ß-END and displayed superiority in the maximum antinociceptive effects. The superiority of local antinociceptive effects of 14-O-MeM6SU might be due to both pharmacodynamic and pharmacokinetic factors.

Functionalization of the carbonyl group in position 6 of morphinan-6-ones. Development of novel 6-amino and 6-guanidino substituted 14-alkoxymorphinans.

The well-known opioid agonists, oxycodone and oxymorphone, and the opioid antagonists, naloxone and naltrexone, are commonly used clinical agents and research tools in the opioid field. They belong to the class of morphinan-6-ones, and produce their pharmacological effects by interacting with opioid receptors, i.e. mu (MOR), delta (DOR) and kappa (KOR). The search for potent agonists and antagonists has continuously engaged the interest of pharmaceutical research, aiming for the identification of safer therapeutic agents or discovery of opioids with novel therapeutic properties and with lesser unwanted side effects. The chemically highly versatile carbonyl group in position 6 of mophinan-6-ones permits functionalization and modification leading to numerous opioid ligands. We have focused on representative examples of various derivatives and interesting approaches for the development of structurally distinct molecules with substitution at C6 (e.g. 6-methylene, 6-hydroxy, 6-amido, bifunctional ligands), as preclinically and clinically valuable opioids. In this work, the development of 6-amino and 6-guanidino substituted 14-alkoxymorphinans, including the synthesis and pharmacological investigations is presented. The new approach represented by the introduction of amino and guanidino groups into position 6 of the morphinan skeleton of 14-O-methyloxymorphone, led to compounds with high efficacy, MOR affinity and selectivity, which act as potent antinociceptive agents. Altogether, as a consequence of target drug design and synthetic efforts in the field of morphinan-6-ones, we achieve a better understanding of the function of the opioid system, and such efforts may open new avenues for further investigations.

New approach to the neurobiological mechanisms of addiction.

Much progress has been made in the last decade in the understanding the neural substrates of drug addiction, transmitters involved, epigenetic background and their relation to learning and memory but much remains to be elucidated and strong effort is necessary to integrate the rich information at the molecular, cellular systems, and behavioral levels to further clarify the mechanisms and therapy of this complex disease. The aim of this review is to collect and interpret the latest opinions in the development, the underlying mechanisms and therapy of addiction as a disease of central nervous system. The neurocircuitry, the transmitters and the epigenetics of addiction are discussed.

[Behavioural studies during the gestational-lactation period in morphine treated rats]. / Viselkedési vizsgálatok a gesztációs-laktációs idoszakban morfin-kezelt patkányon.

UNLABELLED: Opioids impair the maternal behaviour of experimental animals. The effect of morphine on maternal behaviour in rat dams treated chronically with morphine during the whole pregnancy and lactation has not been yet analysed systematically. OBJECTIVE: The aim of our work was to investigate the behavioural effects of moderate dose morphine administered constantly in the whole perinatal period in rats. METHODS: Nulliparous female rats were treated with 10 mg/kg morphine s.c. once daily, from the day of mating. Maternal behaviour was observed, the effects of acute morphine treatment on the maternal behaviour and whether this effect could be antagonised by naloxone were also investigated. Physical and other behavioural (anxiety-like signals in elevated plus maze, changes in locomotor activity) withdrawal signs precipitated by naloxone were registered. After weaning sensitivity to the rewarding effect of morphine was measured by conditioned place preference and to the aversive effect of naloxone by conditioned place aversion tests. Antinociceptive test on tail-flick apparatus was performed to investigate the changes in morphine antinociceptive effects due to chronic morphine treatment. RESULTS: Maternal behaviour was significantly impaired in morphine-treated dams. This effect of morphine lasted c.a. 2-3 hours a day, it showed dose-dependency and was enhanced in MO-treated group (sensitisation). Only weak physical and no other behavioural (anxiety-like behaviour or hypolocomotion) withdrawal signs were precipitated by naloxone. The positive reinforcing effect of morphine and aversive effect of naloxone were markedly increased on conditioned place paradigm. Significant antinociceptive tolerance was not seen. CONCLUSION: Although human drug abuse can be hardly modelling under experimental circumstances, our constant, relatively moderate dose morphine treatment administered once daily during the whole pregnancy and lactation resulted in several subtle behavioural changes in dams. In perinatally opioid-exposed offspring short- and long-term behavioural disturbances can be detected which is well-known from literature. Besides direct pharmacological effects of morphine impaired maternal responsiveness and pup care could play a role in these disturbances.

Characterization of phenolic compounds and antinociceptive activity of Sempervivum tectorum L. leaf juice.

Sempervivum tectorum L. (houseleek) leaf juice has been known as a traditional herbal remedy. The aim of the present study was the chemical characterization of its phenolic compounds and to develop quantitation methods for its main flavonol glycoside, as well as to evaluate its antinociceptive activity. Lyophilized houseleek leaf juice was studied by HPLC-DAD coupled to electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) to identify flavonol glycosides, hydroxy-benzoic and hydroxy-cinnamic acids. Ten flavonol glycosides and sixteen phenolic acid compounds were identified or tentatively characterized. Structure of the main flavonol compound was identified by nuclear magnetic resonance spectroscopy. Three characteristic kaempferol glycosides were isolated and determined by LC-ESI-MS/MS with external calibration method, using the isolated compounds as standard. The main flavonol glycoside was also determined by HPLC-DAD. Validated HPLC-DAD and LC-ESI-MS/MS methods were developed to quantify kaempferol-3-O-rhamnosyl-glucoside-7-O-rhamnoside and two other kaempferol glycosides. Antinociceptive activity of houseleek leaf juice was investigated by writhing test of mice. Sempervivum extract significantly reduced pain in the mouse writhing test.

The central versus peripheral antinociceptive effects of µ-opioid receptor agonists in the new model of rat visceral pain.

This study describes the antinociceptive effects of µ-opioid agonists, d-Ala(2),N-Me-Phe(4),Gly(5)-ol-enkephalin (DAMGO) and morphine in a model of rat visceral pain in which nociceptive responses were triggered by 2% acetic acid intraperitoneal (i.p.) injections. DAMGO and morphine were administered i.p., to the same site where acetic acid was delivered or intracerebroventricularly (i.c.v.). The antinociceptive actions of i.p. versus i.c.v. administered DAMGO or morphine were evaluated in the late phase of permanent visceral nociceptive responses. Both compounds inhibited the nociceptive responses in a dose-dependent manner and exhibited more potent agonist activity after i.c.v. than i.p. administration. DAMGO and morphine showed comparable ED(50) values after i.p. injections. However, DAMGO was much stronger than morphine after central administration. Co-administration of the peripherally restricted opioid antagonist, naloxone methiodide (NAL-M), significantly attenuated the antinociceptive effects of i.p. DAMGO or morphine. On the other hand, i.c.v. injections of NAL-M partially antagonized the antinociceptive effect of i.p. morphine and failed to affect the antinociceptive action of i.p. DAMGO indicating the partial and pure peripheral antinociceptive effects of morphine and DAMGO, respectively. These results suggest the role of either central or peripheral µ-opioid receptors (MOR) in mediating antinociceptive effects of i.p. µ-opioid agonists in the rat late permanent visceral pain model which closely resembles the clinical situation.

Synthesis and pharmacological activities of 6-glycine substituted 14-phenylpropoxymorphinans, a novel class of opioids with high opioid receptor affinities and antinociceptive potencies.

The synthesis and the effect of a combination of 6-glycine and 14-phenylpropoxy substitutions in N-methyl- and N-cycloproplymethylmorphinans on biological activities are described. Binding studies revealed that all new 14-phenylpropoxymorphinans (11-18) displayed high affinity to opioid receptors. Replacement of the 14-methoxy group with a phenylpropoxy group led to an enhancement in affinity to all three opioid receptor types, with most pronounced increases in δ and κ activities, hence resulting in a loss of µ receptor selectivity. All compounds (11-18) showed potent and long-lasting antinociceptive effects in the tail-flick test in rats after subcutaneous administration. For the N-methyl derivatives 13 and 14, analgesic potencies were in the range of their 14-methoxy analogues 9 and 10, respectively. Even derivatives 15-18 with an N-cyclopropylmethyl substituent acted as potent antinociceptive agents, being several fold more potent than morphine. Subcutaneous administration of compounds 13 and 14 produced significant and prolonged antinociceptive effects mediated through peripheral opioid mechanisms in carrageenan-induced inflammatory hyperalgesia in rats.