Stereoselective effect of morphine on antinociception and endogenous opioid peptide levels in plasma but not cerebrospinal fluid of dogs

A review of the kappa opioid receptor system in opioid use

The kappa opioid receptor (KOR) system is implicated in dysphoria and as an "anti-reward system" during withdrawal from opioids. However, no clear consensus has been made in the field, as mixed findings have been reported regarding the relationship between the KOR system and opioid use. This review summarizes the studies to date on the KOR system and opioids. A systematic scoping review was reported following PRISMA guidelines and conducted based on the published protocol. Comprehensive searches of several databases were done in the following databases: MEDLINE, Embase, PsycINFO, Web of Science, Scopus, and Cochrane. We included preclinical and clinical studies that tested the administration of KOR agonists/antagonists or dynorphin and/or measured dynorphin levels or KOR expression during opioid intoxication or withdrawal from opioids. One hundred studies were included in the final analysis. Preclinical administration of KOR agonists decreased drug-seeking/taking behaviors and opioid withdrawal symptoms. KOR antagonists showed mixed findings, depending on the agent and/or type of withdrawal symptom. Administration of dynorphins attenuated opioid withdrawal symptoms both in preclinical and clinical studies. In the limited number of available studies, dynorphin levels were found to increase in cerebrospinal fluid (CSF) and peripheral blood lymphocytes (PBL) of opioid use disorder subjects (OUD). In animals, dynorphin levels and/or KOR expression showed mixed findings during opioid use. The KOR/dynorphin system appears to have a multifaceted and complex nature rather than simply functioning as an anti-reward system. Future research in well-controlled study settings is necessary to better understand the clinical role of the KOR system in opioid use.

Characterisation of tramadol, morphine and tapentadol in an acute pain model in Beagle dogs

OBJECTIVE

To evaluate the analgesic potential of the centrally acting analgesics tramadol, morphine and the novel analgesic tapentadol in a pre-clinical research model of acute nociceptive pain, the tail-flick model in dogs.

STUDY DESIGN

Prospective part-randomized pre-clinical research trial.

ANIMALS

Fifteen male Beagle dogs (HsdCpb:DOBE), aged 12-15 months.

METHODS

On different occasions separated by at least 1 week, dogs received intravenous (IV) administrations of tramadol (6.81, 10.0 mg kg(-1) ), tapentadol (2.15, 4.64, 6.81 mg kg(-1) ) or morphine (0.464, 0.681, 1.0 mg kg(-1) ) with subsequent measurement of tail withdrawal latencies from a thermal stimulus (for each treatment n = 5). Blood samples were collected immediately after the pharmacodynamic measurements of tramadol to determine pharmacokinetics and the active metabolite O-demethyltramadol (M1).

RESULTS

Tapentadol and morphine induced dose-dependent antinociception with ED50-values of 4.3 mg kg(-1) and 0.71 mg kg(-1) , respectively. In contrast, tramadol did not induce antinociception at any dose tested. Measurements of the serum levels of tramadol and the M1 metabolite revealed only marginal amounts of the M1 metabolite, which explains the absence of the antinociceptive effect of tramadol in this experimental pain model in dogs.

CONCLUSIONS AND CLINICAL RELEVANCE

Different breeds of dogs might not or only poorly respond to treatment with tramadol due to low metabolism of the drug. Tapentadol and morphine which act directly on μ-opioid receptors without the need for metabolic activation are demonstrated to induce potent antinociception in the experimental model used and should also provide a reliable pain management in the clinical situation. The non-opioid mechanisms of tramadol do not provide antinociception in this experimental setting. This contrasts to many clinical situations described in the literature, where tramadol appears to provide useful analgesia in dogs for post-operative pain relief and in more chronically pain states.

Volume transmission of beta-endorphin via the cerebrospinal fluid; a review

There is increasing evidence that non-synaptic communication by volume transmission in the flowing CSF plays an important role in neural mechanisms, especially for extending the duration of behavioral effects. In the present review, we explore the mechanisms involved in the behavioral and physiological effects of β-endorphin (β-END), especially those involving the cerebrospinal fluid (CSF), as a message transport system to reach distant brain areas. The major source of β-END are the pro-opio-melano-cortin (POMC) neurons, located in the arcuate hypothalamic nucleus (ARH), bordering the 3^rd ventricle. In addition, numerous varicose β-END-immunoreactive fibers are situated close to the ventricular surfaces. In the present paper we surveyed the evidence that volume transmission via the CSF can be considered as an option for messages to reach remote brain areas. Some of the points discussed in the present review are: release mechanisms of β-END, independence of peripheral versus central levels, central β-END migration over considerable distances, behavioral effects of β-END depend on location of ventricular administration, and abundance of mu and delta opioid receptors in the periventricular regions of the brain.

Non-suicidal self-injurious behavior, endogenous opioids and monoamine neurotransmitters

BACKGROUND

Self-inflicted injury, including cutting or burning, is the most frequent reason for psychiatric visits to medical emergency departments. This behavior, particularly when there is no apparent suicidal intent, is poorly understood from both biological and clinical perspectives.

OBJECTIVE

To examine the role of endogenous opioids and monoamine neurotransmitters in non-suicidal self-injury (NSSI).

METHODS

We compared cerebrospinal fluid (CSF) levels of endogenous opioids, 5 hydroxyindolacetic acid (5-HIAA) and homovanillic acid (HVA) in individuals with a history of repetitive non-suicidal self-injury with a diagnostically-matched group of individuals who had never engaged in non-suicidal self-injury. History of suicidal behavior, demographic background and psychopathology was assessed. All patients were diagnosed with a Cluster B personality disorder (i.e. borderline, antisocial, narcissistic or histrionic) (N=29) and had a history of at least one suicide attempt. Fourteen participants had a history of repeated non-suicidal self-injurious behavior (NSSI) in adulthood and 15 did not (no NSSI).

RESULTS

The NSSI group had significantly lower levels of CSF beta-endorphin and met-enkephalin when compared with the non-NSSI group. CSF dynorphin, HVA and 5-HIAA levels did not differ. Severity of depression, hopelessness and overall psychopathology was greater in the NSSI group.

CONCLUSION

beta-endorphin and met-enkephalin, opioids acting upon receptors involved in mediating stress-induced and physical pain analgesia respectively, are implicated in NSSI. Serotonergic and dopaminergic dysfunctions do not appear to be related to NSSI. Based on our findings, we propose a model of non-suicidal self-injury. Our results suggest that drugs acting on the opioid system warrant exploration as pharmacological treatments for NSSI.

Metabolism of beta-endorphin in plasma studied by liquid chromatography-electrospray ionization mass spectrometry

Degradation of synthetic human beta-endorphin by a human plasma proteinase was studied with high-performance liquid chromatography in combination with mass spectrometry. The peptide was metabolized at a rate of 25 pmol/min to the major fragments beta-endorphin (1-19) and (20-31), the latter reported as a potent inhibitor of morphine- and beta-endorphin-induced analgesia in mice. The proteinase responsible for this process was classified as a metal-dependent serine proteinase and was effectively inactivated by phenylmethylsulfonyl fluoride and ethylenediaminetetraacetic acid. Identification of the products formed during the enzymatic reaction was performed by liquid chromatography on-line with electrospray mass spectrometry, using a reversed-phase or a novel size-exclusion column capable of separating molecules between 0.1-7 kilodaltons. Peptide sequences were verified by tandem mass spectrometry experiments. The conversion of beta-endorphin may have physiological implications in the mechanism of pain. The obtained data suggest that several precautions should be considered during recovery and measurement of beta-endorphin in plasma by immunological techniques. The applied strategy may also be useful for studying metabolism of various peptidergic compounds with potential pharmacological significance.

Morphine metabolites

Morphine is a potent opioid analgesic widely used for the treatment of acute pain and for long-term treatment of severe pain. Morphine is a member of the morphinan-framed alkaloids, which are present in the poppy plant. The drug is soluble in water, but its solubility in lipids is poor. In man, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) are the major metabolites of morphine. The metabolism of morphine occurs not only in the liver, but may also take place in the brain and the kidneys. The glucuronides are mainly eliminated via bile and urine. Glucuronides as a rule are considered as highly polar metabolites unable to cross the blood-brain barrier. Although morphine glucuronidation has been demonstrated in human brain tissue, the capacity is very low compared to that of the liver, indicating that the M3G and M6G concentrations observed in the cerebrospinal fluid (CSF) after systemic administration reflect hepatic metabolism of morphine and that the morphine glucuronides, despite their high polarity, can penetrate into the brain. Like morphine, M6G has been shown to be relatively more selective for mu-receptors than for delta- and kappa-receptors while M3G does not appear to compete for opioid receptor binding. The analgesic properties of M6G were recognised in the early 1970s and more recent work suggests that M6G might significantly contribute to the opioid analgesia after administration of morphine. The analgesic potency of M6G after intracerebroventricular (ICV) or intrathecal (IT) administration in rats is from 45-800 timer greater than that of morphine, depending on the animal species and the experimental antinociceptive test used. Furthermore, the development of a sensitive high-performance liquid chromatography (HPLC) assay for the quantitative determination of morphine, M6G and M3G has revealed that M6G and M3G were present in abundance after chronic oral morphine administration and that the area under the plasma concentration-time curve exceeded that of morphine. M3G has been found to antagonise morphine and M6G induced analgesia and ventilatory depression in the rat, which has led to the hypothesis that M3G may influence the development of morphine tolerance. M3G exhibits no analgesic effect after ICV or IT administration. Some studies do, however, indicate that M3G may cause non-opioid mediated hyperalgesia/allodynia and convulsions after IT administration in rats. These observations led to the hypothesis that M3G might be responsible for side-effects, hyperalgesia/allodynia and myoclonus seen after high-dose morphine treatment.

Differential biotransformation of dynorphin A (1-17) and dynorphin A (1-13) peptides in human blood, ex vivo

The biotransformation in human blood in vitro of three dynorphin A (Dyn A) peptides was studied by matrix assisted laser desorption mass spectrometry to determine whether the natural peptide, Dyn A(1-17), is biotransformed differently from Dyn A (1-13), the natural sequence shortened form used in numerous neurobiological and pharmacological studies. In addition to studies of Dyn A(1-17), a natural product from prodynorphin and Dyn A(1-13), a natural sequence truncation of Dyn A(1-17), Dyn A(1-10)amide, a synthetic analogue of Dyn A(1-17) presumed to be protected from rapid biotransformation was also studied Synthetic Dyn A peptides were incubated in freshly drawn blood for various periods of time prior to mass spectrometric analysis. Several peptide products were identified from each precursor; the time profiles of appearance and disappearance of the major products were followed. Substantial differences in products and especially in the rate of biotransformation were observed between the processing of Dyn A(1-17) and the two shorter Dyn A peptides, Dyn A(1-13) and Dyn A(1-10)amide. Significant amounts of the natural Dyn A(1-17) survived 4 h of incubation (half-life 3 h). Dyn A (2-17), a major processed product of Dyn A(1-17) in blood, continued to accumulate during the 4-h incubation period. By contrast, both Dyn A(1-13) and Dyn A(1-10) amide were biotransformed very rapidly with half-lives of < 1 min and 10 min, respectively. Most of the products from these two peptide precursors were also further processed rapidly, with the exception of Dyn A(4-12) and Dyn A(4-10)amide, which were detected for over 2 h. Dyn A(1-6) was found as a minor biotransformation product from all three precursor peptides. These findings suggest that an important function of the four C-terminal amino acid residues of the natural form, Dyn A(1-17) [compared to Dyn A(1-13)], is to stabilize or protect the peptide from biotransformation by enzymes, by preserving a natural hairpin structure possibly near the carboxyl-terminus.

Morphine: new aspects in the study of an ancient compound

Morphine is the most widely used compound among narcotic analgesics and remains the gold standard when the effects of other analgetic drugs are compared. Apart from its presence in the poppy plant Papaver somniferum, morphine has been shown to be present in milk, cerebrospinal fluid and also in nervous tissue extracts. Recent evidence suggests that biosynthetic pathways for morphine exist in animal and even human tissues such as liver, blood and brain. The most characteristic effect of morphine is the modulation of pain perception resulting in an increase in the threshold of noxious stimuli. Antinociception induced by morphine is mediated via opioid receptors and therefore can be inhibited by opioid antagonists, e.g., naloxone. Nevertheless, consideration of morphine as endogenous ligand for opioid receptors seems to be speculative. Recently, the primary receptor for morphine-type drugs called the mu-opioid receptor has been cloned from rat brain. There is accumulating evidence that morphine actions are, at least partly, due to one of its major metabolite morphine-6-glucuronide in man. It is concluded that further investigations are necessary to elucidate the mechanisms, whereby multiple actions of morphine are expressed in the nervous system.