Endogenous Opioid-Like Peptides in Headache. An Overview

Endogenous Opioid-Like Peptides in Headache. An Overview. Aron D. Mosnaim, Ph.D.,1,2 Seymour Diamond, M.D.,1,4 Marion E. Wolf, M.D.,2,3 Javier Puente, Ph.D.,1 and Frederick G. Freitag, D.O.4 1Department of Pharmacology and Molecular Biology, University of Health Sciencec/The Chicago Medical School, North Chicago, Illinois 60064, 2The North Chicago Veterans Administration Medical Center, 3Department of Psychiatry, Loyola University and 4Diamond Headache Clinic, Chicago, Illinois. Reprint requests to: Dr. Aron D. Mosnaim, Professor of Pharmacology and Molecular Biology, The Chicago Medical School, 3333 Green Bay Road, North Chicago, Illinois 60064. Accepted for Publication: March 20, 1989. (Headache 29:368-372, 1989) Considerable research has been focused in identifying biochemical changes in biological tissues and fluids which may be specifically associated with the occurrence of headaches, in particular migraine (MI) and cluster headaches (CH).1-4 Whereas most of this work has been directed to elucidate the possible role of vasoactive substances in the etiology of these conditions,1-4 more recently some interest has shifted towards a better understanding of the mechanisms involved in the abnormal modulation of the pain response observed in these patients.5-23 The reports that morphine and related exogenous opiates exert their analgesic effects by interacting with specific postsynaptic receptors24-26 were soon followed by the isolation from pig brain of the first endogenous opiates,27 the pentapeptides methionine-enkephalin and leucine-enkephalin (MET and LEU, respectively), leading to the present knowledge of at least twenty different naturally occurring neuropeptides with analgesic effects.28 Using recombinant DNA techniques, it has been shown that the majority of these substances arise from the processing of one of three polyprotein precursors e.g., proopiomelanocortin, proenkephalin and prodynorphin; each one of them being in turn, the product of one of three distinct genes expressed with a different, and in some cases, overlapping tissue distribution.29 The opiate neuropeptides have been classified in three groups: endorphins, enkephalins and dynorphins.29 Since these peptides arise from the precursors by proteolysis, the production of the individual opioid peptides can be regulated by both genetic factors and by tissue-specific enzymatic processing.29 Furthermore, the peptide levels and biological activity in a given tissue or fluid will reflect, in part, other factors such as kinetics of degradation and elimination, as well as storage and binding protection.30 It should be noted that analgesia, like other endogenous opiate actions, appears to be mediated on a cellular level by different classes of opiate receptors. Based on drug actions and affinity studies, five such receptors have been already identified: mu, kappa, sigma, delta and epsilon (agonist: morphine, ketocyclazocine, SKF 10.047, enkephalins and ß-endorphin, respectively). The wide distribution of the opioid peptides and their receptors in the central nervous system and peripheral tissues suggests that these substances have a variety of biological roles in addition to pain modulation.31 In fact, and similarly to what has occurred in other promising new research areas, the opioid peptides have been hypothesized to play a number of physiological roles, to produce a host of pharmacological actions, and to be involved in some of the mechanisms responsible for the pathophysiology associated with several disease conditions.32-36 Initial studies showing abnormal cerebrospinal fluid (CSF) levels of "morphine-like factors" and plasma ß-endorphin-like immunoreactivity in migraine (MI) patients (between and during MI attacks, respectively), and in subjects suffering from chronic, daily headaches,5-6,10-12 have led several authors to theorize that the hyperalgesia associated with a MI headache is due, at least in part, to a failure of the "opiate system" for these individuals to properly modulate their "normal" response to pain.5,7,10,15 As could be expected from the possibilities offered by this hypothesis (i.e., an insight into the basic mechanisms of action of analgesic drugs with all the potential applications in the rational development of new and improved therapeutic modalities for pain in general and headache pain in particular), research in this area has grown dramatically.5-23 The lack of an adequate animal model to study the basic mechanisms responsible for the occurrence of the different kinds of headaches and their associated pathophysiology is a formidable obstacle in relating the opioid system activity to the pain accompanying these conditions. In searching for an appropriate paradigm for these studies, scientists have examined plasma, CSF and platelet MET and ß-endorphin levels in patients and controls, and attempted to correlate observed changes with the pain associated with the different phases of the headaches (Tables 1 and 2). Results from preliminary studies analyzing this biological material for opioiddegrading enzyme activity show changes in the kinetic parameters (Km, Vmax and half-life) of plasma aminopeptidase (responsible for breaking the tyr-gly MET bond) in some MI and chronic CH patients,30-37 as well as increases in circulating enkephalinase activity (an enzyme responsible for cleaving the gly-phe MET bond) in chronic headache suffers.38,39 This work has opened a new perspective to the study of headache pain which has already produced some encouraging results (Table 1 and 2); interpretation of which should, however, be made with great caution. It Peptide and Source MET, Plasma MET, Plasma MET, Plasma MET, Plasma MET, Plasma MET, Platelet MET, Platelet MET, CSF Morphine-like Factors, CSF ß-EP, Plasma ß-EP, Plasma ß-EP, Plasma ß-EP, Plasma ß-EP, Plasma Table 1 Endogenous Opioid-Like Peptides in Headache Migraine Daily Comments During Between Chronic Attacks Attacks + Elevated RIA (1) + Elevated Elevated RIA (1) + Elevated Elevated RIA (1) + Elevated Elevated & RIA (2) Normal + Elevated Elevated RIA (1) Normal Normal RIA (1) + Elevated Elevated RIA (1) + Reduced Normal RIA(1) Normal Reduced Reduced RIA (1) Elevated Normal Normal + Reduced Normal Normal Normal (Common) Normal (Common) Reduced (Class.) Normal (Common) ß-EP, Plasma Normal Reduced Mosnaim, et al., 1982 Mosnaim, et al., 1984 Mosnaim, et al., 1985 Mosnaim, et al., 1986 RIA (2) RIA (1) RIA (1) RIA (1) Ferrari, et al., 1987 Mosnaim, et al, 1986 Ferrari, et al., 1987 Anselmi, et al., 1980 Sicuteri, et al., 1978, 1982, Anselmi, et al., 1980 Anselmi, et al., 1980; Sicuteri, 1982; Della Bella, et al., 1982 Appenzeller, 1981; Back, 1985 Baldi, et al., 1982 Facchinetti, et al., 1981 Fettes, et al., 1983, 1985 RIA (1) Genazzani, et al., 1984 RIA (1) + Reduced Reduced References Results compare values obtained for the same individual (2), or for groups of patients (1) during the different stages of headache. RIA, MET, and ß-EP refers to radioimmunoassay, methionine-enkephalin and ß-Endorphin, respectively. Peptide and Source MET, Plasma MET, Plasma MET, Plasma MET, Platelet MET, Platelet MET, Platelet MET, CSF MET, CSF B-EP, CSF During Acute Episode Elevated Precluster (Elevated), During (Variable, Normal), Postcluster (Reduced) Table 2 Endogenous Opioid-Like Peptides in Headache Cluster Tension Comments Within Outside During Series Series Attack Elevated RIA (1) Low Normal Low Normal RIA(1,2) & Reduced & Reduced Pre, During & Postcluster; (Normal) Normal Reduced Normal Hardebo, et al., 1985(3) Diamond, et al., 1987 Mosnaim, et al, 1987 Elevated RIA (1) RIA (1) Ferrari, et al., 1987 Mosnaim, et al., 1987 Low Normal RIA(1) RIA (2) RIA (1) RIA (1) RIA(1) Ferrari, et al., 1987 Mosnaim, et al., 1987 Hardebo, et al., 1985 (3) Hardebo and Ekman, 1987 Hardebo, et al., 1985(3) Normal Reduced Normal Normal References Results compare values obtained for the same individual (2), or for groups of patients (1) during the different stages of headache. RIA, MET and ß-EP refers to radioimmunoassay, methionine enkephalin and ß-Endorphin, respectively. (3) Presented at the Annual Meeting of the American Association for the Study of Headache. should be noted that there is insufficient evidence at the present time to indicate a direct link between any specific, or even general, alterations in the endogenous opioid peptide system, as reflected in the plasma or CSF levels of the neuropeptides studied, and the triggering, sustaining, amelioration or relative severity of headache pain. Changes observed in peptide levels may be interpreted as reflecting one or more of a variety of internal and/or external stimuli, including stress, anxiety, physical exercise and drugs, which may or may not be involved in the etiology of, or even associated with the occurrence of, headache pain.2,3.40 Most published reports tend to support the general findings that some varieties of headaches are accompanied by change in circulating enkephalins and/or endorphin levels.5,6,8-17,19-23 It should be emphasized that there are important differences in the origin, as well as in the mechanisms regulating the biosythesis, release and degradation of the members of the three different opioid peptide families;28,29,31 this could explain conflicting results on the direction and extent of the changes in circulating MET and b-EP levels in some headache studies (Table 1 and 2). Conflicting results may also reflect the differences in the design of the clinical protocols, including diagnostic criteria, as well as the specificity, reproducibility, and equivalence of the analytical procedure used.16 Whereas early results were expressed as levels of "morphine-like" substances and b-endorphin-like immunoreactivity,1,5,10 more recent work reports the levels of individual peptides, including in many instances the intraassay and interassay variation coefficient.16,23 Unfortunately, some of the reported studies have been open trials including small numbers of patients and, in some cases, they lack appropriate controls to account for possible influences of such factors as gender, age, race, and other disease conditions. This makes it somewhat difficult at this time to draw firm conclusions from this work and imposes on the reader a need for caution when comparing results from different laboratories (Tables 1 and 2). One important factor to be considered in evaluating this research is the 'particular phase' of the headache episode at which samples were taken, since neuropeptide levels, particularly MET, may respond rapidly and dramatically to the fast cycling nature of a CH acute attack.22,23 This is also true when comparing MI patients during the acute episode and when pain-free (two to three weeks later).16-20 A potentially important, but often neglected factor in the present studies5-23 has been the medication status of the patients. It is known that a variety of chemicals, some endogenous, some drugs, affect the availability of MET and/or b-EP; these substances may act for example by changing the rate of peptide metabolism or release e.g., the enkephalins are costored with catecholamines in the adrenal chromaffin granules from where they are coreleased. 28-32-34 Despite the limitations imposed by the factors discussed above, which are in many aspects similar to those encountered in carrying out patient research in other complex clinical areas, progress has been steady and encouraging.5-23 Work in our laboratory has dealt mainly with MET, a substance often studied for its anti nociceptive properties.41 The study of MET has a number of important advantages over the study of other neuropeptides with analgesic effects. Circulating MET, a small molecule (pentapeptide, H-Tyr-Gly-Gly-Phe-Me-OH) derived mainly from the adrenal gland,22 can be readily synthesized and is commercially available with a high degree of radio-chemical purity (>95%). This factor, its stability in ice-cold heparinized plasma (3,000xg blood supernatant),30 and the availability of techniques allowing for the rapid and accurate separation of MET metabolic products e.g., column and thin-layer chromatography, makes it relatively easy to study its kinetics of degradation.30,43,44 In vitro studies have shown that MET is metabolized by human plasma to yield mostly (>95%) tyrosine and the remaining tetrapeptide. In young male, healthy drug-free volunteers this enzymatic reaction, which is essentially completed within 90 min., has a half-life, Km and Vmax (x ± S.D.) of 12.8±2.5 min., 0.70±0.01 mM and 17.90 ± 1.05 micromoles/L/min., respectively.30 There are however, some important qualitative and quantitative differences both in the product(s) and in the kinetic characteristics of MET degradation by plasma samples from different animal species examined.30,45,46 This is also true when studying MET degradation by peripheral as well as central nervous system tissue preparations, emphasizing interspecies differences and the diversity of enzymes which may be involved in MET metabolism e.g., [amino-peptidases, dipeptidylcarboxypeptidase (also called endopeptidase 24.11 and enkephalinase A), dipeptidylaminopeptidase (also called enkephalinase b), and angiotensin-converting enzyme]30,45-54 This shows again that a great deal of caution should be exercised when extrapolating results from one species to another, as well as when comparing different tissues or tissue preparations from the same species. Determination of MET levels in plasma, CSF and other biological fluids and tissues is currently carried out using either high-pressure liquid chromatographic techniques or a readily available commercial RIA kit, which allows for the accurate measurement of MET concentration (intraassay and interassay variation coefficient of approximately 10%) in as little as 1 ml of plasma and CSF sample. EDTA (disodium salt) containing plasma samples (0.5-1.0% final conc.) can be stored (-20 C, up to 3 years) with little MET loss.16,23,46 Besides of its documented involvement in the molecular mechanisms of pain, research on MET presents some other important advantages when compared to studies of other neuropeptides with analgesic properties. There appears to be some specificity between changes in MET plasma levels and the different stages of certain headaches; in contrast the concentration of this peptide remains similar to controls in chronic alcoholics20 and in patients suffering from a number of neuropsychiatric conditions.55,56 Results from a preliminary longitudinal study (blood samples were drawn between 10 a.m. and 2 p.m., once or twice a month during a period of 6 to 9 months), indicate a significantly smaller range of plasma MET in individual controls (n =3; male, white, drug-free volunteers) than that observed in a similar protocol involving race and sex-matched, age comparable chronic CH patients (n =4). Furthermore most individual patient values fell at the lower end of, or below the range of controls. (For details, see reference 23). Whereas the acute phase of common or classic MI attacks is associated with substantial increases in plasma MET levels, which return to baseline "control similar" values slowly over a period of days or even weeks (pain-free period),15,17,20 the different phases of an acute CH attack (pre, during and post CH episode) are characterized in many cases by rapid and important changes in circulating MET levels (For details see references 22,23). These findings may eventually prove useful in designing a protocol to monitor the different phases of cluster and/or MI headache, and perhaps serve as possible indicators of the severity of the existing or underlying headache pain. One could also suggest that plasma MET levels could serve as a biological marker for MI (state) and chronic CH patients (trait). Although most of the research in the field of headache pain and neuropeptides has dealt with MET and b-EP there still are many unanswered questions as to their possible role in the etiology of, or the pathophysiology associated with these conditions.1-23 When discussing the basic and clinical research opportunities present in this field, one should also keep in mind those offered by the study of the many other endogenous neuropeptides with analgesic activity (for reviews see 28,29,31,34,41,54). Acknowledgements. Discussions with Dr. S. Ehrenpreis were most helpful in the preparation of this manuscript. This work was supported, in part, by the National Headache Foundation. Dr. Puente is with the University of Chile. We wish to thank Melodee Minikel for editing and typing this manuscript. REFERENCES 1. Critchley M, Friedman AP, Gorini S, Sicuteri F, ed. Advances in Neurology: 33. Headache: Physiopathological and Clinical Concepts. Raven Press, New York, 1982. 1-417. 2. 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