The endorphins, novel peptides of brain and hypophysial origin, with opiate-like activity: biochemical and biologic studies

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

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.

Cardioprotection conferred by exercise training is blunted by blockade of the opioid system

OBJECTIVES

To investigate the effect of opioid receptor blockade on the myocardial protection conferred by chronic exercise and to compare exercise training with different strategies of myocardial protection (opioid infusion and brief periods of ischemia-reperfusion) preceding irreversible left anterior descending coronary ligation.

INTRODUCTION

The acute cardioprotective effects of exercise training are at least partly mediated through opioid receptor-dependent mechanisms in ischemia-reperfusion models.

METHODS

Male Wistar rats (n = 76) were randomly assigned to 7 groups: (1) control; (2) exercise training; (3) morphine; (4) intermittent ischemia-reperfusion (three alternating periods of left anterior descending coronary occlusion and reperfusion); (5) exercise training+morphine; (6) naloxone (a non-selective opioid receptor blocker) plus morphine; (7) naloxone before each exercise-training session. Myocardial infarction was established in all groups by left anterior descending coronary ligation. Exercise training was performed on a treadmill for 60 minutes, 5 times/week, for 12 weeks, at 60% peak oxygen (peak VO₂). Infarct size was histologically evaluated.

RESULTS

Exercise training significantly increased exercise capacity and ΔVO2 (VO₂ peak - VO₂ rest) (p < 0.01 vs. sedentary groups). Compared with control, all treatment groups except morphine plus naloxone and exercise training plus naloxone showed a smaller infarcted area (p < 0.05). No additional decrease in infarct size occurred in the exercise training plus morphine group. No difference in myocardial capillary density (p = 0.88) was observed in any group.

CONCLUSIONS

Exercise training, morphine, exercise training plus morphine and ischemia-reperfusion groups had a smaller infarcted area than the control group. The effect of chronic exercise training in decreasing infarct size seems to occur, at least in part, through the opioid receptor stimulus and not by increasing myocardial perfusion.

Biology of hypothalamic neurons and pituitary cells

Results obtained by examining hypothalamic neurons producing precursors to neurohormones, and pituitary cells synthesizing peptide and glycoprotein families of hormones, and recent advances in comparative endocrinology, have been summarized and considered from the following viewpoints: species specificity in the organization and communication of the hypothalamic neurons with different brain areas lying inside the BBB and with CVOs; sensitivity of hypothalamic neurons and pituitary cells to the environmental stimuli; gonadal steroids as modulators of gene expression needed for neuronal differentiation and synaptogenesis; dose(s)-dependent pituitary cell proliferation and differentiation; an inverse relationship between PRL and GH synthesis and release and also between degree of hyperplasia and hypertrophy of PRL cells and retardation of GTH cell differentiation; and responsiveness of neurons producing CRH, and of neurons and pituitary cells synthesizing POMC hormones, to stress and glucocorticosteroids. These data show that growth of the animals may be stimulated, retarded, or inhibited; reproductive properties and behavior may be under hormonal control; and character of responsiveness in reaction to stress, and ability for adaptation and other related functions, may be controlled.

C-terminal amidated peptides: production by the in vitro enzymatic amidation of glycine-extended peptides and the importance of the amide to bioactivity

Peptidylglycine alpha-amidating enzyme (alpha-AE) can be used in an in vitro reaction to convert C-terminal glycine-extended peptides to peptide hormones with a C-terminal amino acid amide. Structure-activity data for 45 bioactive peptides show that the C-terminal amide is required for the full biological activity of most amidated peptide hormones. These data emphasize the role alpha-AE can have in amidated peptide production.

Localization of beta-endorphin-like immunoreactivity in the nervous system of the adult newt, Notophthalmus viridescens

Using indirect immunofluorescence methods, we have localized for the first time in the newt, Notophthalmus viridescens, beta-endorphin (beta-ep)-like immunoreactivity in the neurons of spinal ganglia (SPG), spinal cord (SPC), as well as in the hypothalamic region of the brain. An examination of serially sectioned SPG showed that the beta-ep-positive neurons, cell bodies, and nerve fibers were distributed at all levels of SPG. Peripheral regions of the perikarya of beta-ep-positive SPG neurons exhibited intense staining for beta-ep, the central nuclear region remaining nonreactive. In SPC, brightly staining fibers were seen entering the afferent nociceptive input areas, namely the Lissauer's tracts, substantia gelatinosa, and the dorsal ascending columns. Dot-fiber immunofluorescence pattern was observed throughout the gray matter of SPC representing beta-ep-positive, secondary sensory neurons as well as interneurons. Also, discrete cluster of neurons located deep in the gray matter of SPC stained positively to beta-ep antisera. This study not only demonstrates for the first time the presence of beta-ep like material in the newt, more specifically in SPG and SPC, but also raises the question of a possible link between beta-ep and newt limb regeneration as previous work has shown that SPG support limb regeneration in a denervated-amputated newt forelimb.

Immunohistochemical localization of beta-endorphin-like material in the urodele and anuran amphibian tissues

In the present study, we have localized for the first time beta-endorphin (beta-EP)-like material in the adult and larval urodele and anuran tissues using immunohistochemical techniques. In the adult Notophthalmus viridescens and Ambystoma mexicanum, strong immunoreactivity to beta-EP antisera was observed in the region of the intermediate lobe, the latter fluorescing as a discrete body. The fluorescence was confined to the periphery of the cells, while the nuclei and the immediately surrounding cytoplasmic regions of the cells remained unstained. A few scattered cells in the anterior pituitary gland as well as the tracts in the posterior lobe also exhibited positive staining, although not as strong as the intermediate lobe. In the larval urodele, A. maculatum, beta-EP-like material was localized for the first time in the sensory ganglia and their emerging nerve fibers, in the Leydig cells of the skin, as well as in a few discrete cells scattered among stomach epithelial cells. In addition to the above, immunoreactivity to beta-EP antisera was observed in the cellular intermediate part of the neurointermediate lobe of the pituitary gland in young Xenopus laevis, the neural part of the lobe remaining nonreactive.

Drug effects on active immobility responses: what they tell us about neurotransmitter systems and motor functions

The literature reviewed indicates that active immobility can be promoted by systemic injections of various neurotransmitter systems, as follows: (1) Dopaminergic blockade of both D1 and D2 receptor subtypes. (2) Cholinergic agonism of both muscarinic and nicotinic receptors. (3) Noradrenergic agonism of both alpha-1 and alpha-2 receptors (but these agonists may interfere with haloperidol- and reserpine-induced catalepsy). (4) GABA agonism. (5) Histamine agonism, particularly at the H1 receptor. (6) Opiate agonism, including action of many endogenous opiate peptides, particularly those affecting mu and delta receptors. (7) Agonism by certain other peptides (neurotensin, cholecystokinin). Among the major interactions of neurotransmitter systems that regulate immobility, are the following: (1) Cholinergic-dopaminergic (cholinolytics disrupt catalepsy of dopaminergic blockade and dopaminergic agonists tend to disrupt cholinomimetic catalepsy). (2) Opiate-induced catalepsy is antagonized by the dopamine agonist, apomorphine, but is enhanced by amphetamine. It is also antagonized by certain alpha-2 adrenergic agonists, while it does not seem to be antagonized by anticholinergics. (3) Numerous other interactions have been reported, involving opiates and MSH, serotonin and dopamine mimetics, serotonin and ketamine, GABA and neuroleptics, neurotensin and anticholinergics and histamine. The significance of the multiple neurotransmitter systems is unknown. One possible explanation is that the various neurotransmitter systems participate in mediating the sensory inputs that are involved in triggering immobility and regulate the higher-order limbic and basal ganglia processing reactions that engage a final motor output pathway from the brainstem. The brain is assumed to contain two sets of systems, each with its own, or possibly overlapping, set of neurotransmitter systems, that promote either active immobility or locomotion. The systems reciprocally inhibit each other. Another view, not mutually exclusive, is that output from the locomotor-promoting system provides a negative feedback, via the active immobility pathways, to act as a "brake" on movement, while at the same time maintaining the muscular tonus that is characteristic of active immobility.

Circulatory events following spontaneous muscle exercise in normotensive and hypertensive rats

In previous studies we have shown that spontaneously hypertensive rats (SHR) develop a running behaviour and, secondary to the running behaviour, develop an endorphin-mediated analgesic effect. In the present study the role of the central endorphin system in the cardiovascular responses to spontaneous exercise in normotensive Wistar Kyoto rats (WKY) and SHR was investigated. The experimental design allowed us to record mean arterial pressure (MAP) and heart rate (HR) continuously for more than 1 week without interfering with the daily activities of the animals. They were active in running wheels during the dark period (19.00-07.00 h) and the activity was accompanied by a marked rise in HR. In SHR, a clear depression of blood pressure lasting for about for about 50 min was noted following each running period. The MAP during the post-running depression was 131.4 +/- 1.6 mmHg which was significantly lower than the pre-running control value (145.2 +/- 2.3 mmHg, P less than 0.01). In contrast, MAP in the post-running period in WKY was not significantly different from the pre-running values. In addition, the depression period of SHR had a mean post-running length of 49.7 +/- 3.4 min, which is significantly longer than in the WKYs (37.8 +/- 3.5 min, P less than 0.05). In control rats, naloxone infusion had no effect on blood pressure but a marked bradycardia was observed. In nine running SHR receiving a naloxone infusion, their MAP during the depression period was not different from the control pressure. Our study indicates that endorphin systems are involved in the regulating of blood pressure and HR during muscle exercise in SHR. These systems trigger the transient depression of blood pressure observed immediately after a running period in the SHR.

CSF beta-EP in headache and depression

Cerebrospinal fluid (CSF) levels of beta-endorphin (beta-EP) were measured in 9 migraineurs with interparoxysmal headache (MIH), in 13 patients with major depression in an active phase (5 suffered from MIH), and in 16 age-matched controls. beta-EP was measured by specific RIA after gel-chromatography. While beta-EP levels of depressed patients (58.5 +/- 21.0 fmol/ml, M +/- SD) were similar to those of controls (65.8 +/- 26.6), those of migraineurs (15.0 +/- 11.1) were significantly reduced (p less than 0.01). In depressed patients also suffering from MIH, beta-EP concentrations (22.8 +/- 7.2, p less than 0.05) were half those reported in depressed patients without pain problems. The reduced CSF beta-EP levels in patients whose headache and depression coexist support the notion that this neuropeptide is concerned with chronic pain, independently of the affective state.

beta-Endorphins (beta-EP) in amphibians: higher beta-EP levels during regenerating stages of anuran life cycle and immunocytochemical localization of beta-EP in regeneration blastemata

Plasma beta-endorphin (beta-EP) levels were measured at different stages of metamorphosis in anuran species, namely Rana catesbeiana, R. pipiens, and Xenopus laevis. The mean levels of beta-EP in tadpoles and postmetamorphic juveniles were significantly higher than adult levels in all three species examined. The pattern of high tadpole levels and low adult levels corresponds with the limb regeneration ability of the tadpole and restriction in the adult. We have also reported plasma beta-EP levels in two urodele species, Ambystoma tigrinum and A. jeffersonianum. Localization of beta-EP by immunocytochemistry shows bright fluorescence to beta-EP antiserum in the wound epithelium of the regeneration blastema. The mesoderm and nuclei of all blastema cells remain nonreactive. It is suggested that receptors to beta-EP reside in the epidermis.

Chemical and immunological characterization of salmon endorphins

Five new peptides related to salmon N-acetyl endorphin II (N-Ac-EP II) were isolated from an acid acetone extract of the pituitary. They were identified as EP II and four N-terminal fragments of N-Ac-EP II, namely N-Ac-EP II (1-18), (1-19), (1-20), and (1-29). The sequence comparison with mammalian EPs revealed that N-Ac-EP II (1-18) and (1-19) corresponded to mammalian alpha- and gamma-EP, respectively. The occurrence of peptides similar to mammalian alpha-, beta-, and gamma-EP indicates that further processing of EP takes place in both the teleost and mammalian pituitaries by similar pathways. A radioimmunoassay for salmon EPs has been developed with rabbit antiserum raised against salmon N-Ac-EP II. It was demonstrated that the antiserum showed full cross-reactivity with salmon N-Ac-EP I, N-Ac-EP II (1-20), and N-Ac-EP II (1-18), but none with Met-enkephalin, human beta-EP, and human beta-LPH. Similarly, the salmon EPs did not cross-react with human beta-EP antiserum in the radioimmunoassay.

Beta-adrenoceptor activation mediates stress-induced secretion of beta-endorphin-related peptides from intermediate but not anterior pituitary

Recently, we reported that adrenaline can stimulate the secretion of immunoreactive beta-endorphin in the rat. This response is mediated by beta-adrenoceptors and requires circulating adrenaline concentrations which are known to occur during stress. We therefore studied whether catecholamines are implicated in the stress-induced secretion of immunoreactive beta-endorphin from the pituitary gland. We report here that in rat the beta-adrenoceptor antagonist (-)propranolol reduces or abolishes the rapid increase of immunoreactive beta-endorphin levels during some stress stimuli (footshock, passive avoidance, restraint) but not during others (ether, formalin, laparotomy). The propranolol-sensitive response is largely prevented by extirpation of the neurointermediate lobe of the pituitary gland but is unaffected by dexamethasone, which inhibits peptide secretion from the corticotroph cells of the anterior lobe. These results suggest that catecholamines activate the release of immunoreactive beta-endorphin from the intermediate lobe but not from the anterior lobe of the pituitary gland during certain stress conditions.

Monoclonal antibody to the message sequence Tyr-Gly-Gly-Phe of opioid peptides exhibits the specificity requirements of mammalian opioid receptors

Six myeloma cell hybrids producing antibodies to human beta-endorphin were isolated from a single mouse spleen. The monoclonal antibodies displayed different binding patterns with the antigen. We report the characterization of one antibody which recognizes the tetrapeptide Tyr-Gly-Gly-Phe representing the message sequence found at the NH2 terminus of all naturally occurring mammalian opioid peptides. Competition experiments in radioimmunoassay and immunohistochemistry show that the antibody fails to bind the beta-endorphin precursor beta-lipotrophin, does not discriminate among opioid peptides that share the same message sequence but have different COOH-terminal extensions, and does not react with pharmacologically inactive derivatives of beta-endorphin. The antibody recognition of the message sequence of natural opioid peptides is sensitive to those molecular changes that affect their receptor binding competence.

Different patterns of central and peripheral beta EP, beta LPH and ACTH throughout life

The aim of the present study was to evaluate the relationship between central and peripheral concentrations of proopiocortin-related peptides in different periods of life. One hundred and eighty-nine plasma samples from normal subjects (18-87 years) obtained in basal conditions, and 20 cerebrospinal fluid (CSF) samples obtained by lumbar puncture from healthy volunteers (18-75 years) were studied. beta Lipotropin (beta LPH), beta endorphin (beta EP) and ACTH were measured by specific RIA after silicic acid plasma extraction and gel chromatography (beta LPH and beta EP). No sex differences were found in the patterns of the three peptides either in the plasma or in CSF. In the plasma samples, both beta LPH and beta EP concentrations showed a pattern throughout life which was expressed by a paraboloid function with the lowest values found in young and old subjects and with peaks at 51.3 and 48.2 years, respectively. On the contrary, ACTH values failed to be represented by a significant linear or curvilinear regression and presented only a slight decrease in subjects over 75 years of age. CSF levels of beta LPH were significantly lower in 45-76 year old subjects (18.8 +/- 12.6 fmol/ml, M +/- SD) than in 18-44 year old subjects (34.5 +/- 15.8; p less than 0.05), as were those of beta EP (elderly: 41.2 +/- 19.7; young: 94.2 +/- 36.7; p less than 0.05), which showed a significantly linear inverse correlation with age (r = 0.6062, p less than 0.01). These CSF samples did not show any ACTH variations connected with age.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of opioid peptides in brain mechanisms regulating blood pressure

Beta-endorphin and related opioid peptides are neuropeptides which appear to play a role in cardiovascular regulation which is supported by altered nociceptive responsiveness in hypertensive animals. In spontaneously hypertensive rats the pain threshold for electric stimulation is elevated; these rats show increased response latency time in a hot plate test. The opiate antagonist naloxone reverses these values to that of the normotensive controls. In other forms of experimental hypertension, eg, renal hypertension (one-clip, two-kidney model), no change in pain sensitivity is apparent. Sinoaortic baroreceptor denervation causes a labile hypertension without changes in hot plate response. Administration of beta-endorphin into the nucleus of the solitary tract (NTS) gradually decreases blood pressure and heart rate without affecting respiratory frequency. These cardiovascular effects are blocked by naloxone as well as by an antibody to beta-endorphin. In contrast to the effects of beta-endorphin, microinjection of enkephalins into the NTS increases blood pressure and heart rate. The data suggest the existence of two separate endorphin systems at the level of the NTS, one a depressor and another a pressor system. The depressor influence of beta-endorphin may play a role in the mechanism of action of antihypertensive agents such as methyldopa and clonidine. Our data support a role of endorphins as neuropeptides involved in cardiovascular regulation, exerting a dual influence at the level of the NTS.

Effects of beta-endorphin and its fragments on inhibitory avoidance behavior in rats

The effects on retrieval of a one-trial learning inhibitory avoidance response of beta-endorphin, alpha-endorphin, and gamma-endorphin, given prior to test have been studied in rats. beta-Endorphin (beta-LPH 61-91) in a relatively low dose (1.56 micrograms sc. or 50 ng icv.) facilitated inhibitory avoidance behavior, while a higher dose (10 micrograms sc. or 100 ng icv.) caused bimodal changes (facilitation in 50% of the animals and attenuation in another 40%. Peripheral injection of gamma-endorphin attenuated inhibitory avoidance behaviour in a dose-dependent manner. The C-terminus of beta-endorphin (beta-LPH 78-91) was ineffective. alpha-Endorphin facilitated inhibitory avoidance behavior in a dose dependent manner. Naltrexone pretreatment antagonized the bimodal effect of beta-endorphin: following pretreatment with the opiate antagonist the low latency component disappeared, but the facilitatory effect of the neuropeptide remained the same. It is suggested that beta-endorphin carries more than one bit of behavioral information. Inherent activities either related or unrelated to naltrexone-sensitive opiate as well as biotransformation into alpha- and gamma-endorphin may contribute to the multiple behavioral effects of this neuropeptide.

Cardiovascular effects of beta-endorphin after microinjection into the nucleus tractus solitarii of the anaesthetised rat

The cardiovascular effects of beta-endorphin after administration directly into the nucleus tractus solitari (NTS) of urethane-anaesthetised rats were investigated. Unilateral injection resulted in a U-shaped dose-response relationship with a fall in mean arterial pressure and heart rate occurring at low doses (less than 10 ng). No change in respiratory frequency was observed at any of the doses examined. The hypotensive effects of beta-endorphin were anatomically specific and restricted to the NTS. The depressor response was prevented and bradycardia reduced by naloxone (1 mg/kg s.c. or 10 ng injected into the NTS) and also by beta-endorphin antiserum (1:50 dilution) but not by antiserum to [Met5]enkephalin (1:50 dilution) applied locally into the NTS. The beta-endorphin antiserum caused a rise in blood pressure when administered alone. Conversely microinjection of antiserum to [Met 5]enkephalin resulted in a brief depressor response. Doses of beta-endorphin larger than 10 ng induced a rise in blood pressure accompanied by variable effects on heart rate. Similarly unilateral administration of Des-tyr-endorphin (100 pg) resulted in a blood pressure increase and [D-Ala2,Met5]enkephalin produced a dose-related pressor response and tachycardia. The results indicate that at least two separate endorphin systems are involved in cardiovascular control at the level of NTS, one being depressor in nature (beta-endorphin-like) and the other pressor ([met5]enkephalin-like).

Some perspectives on monoamine-opioid peptide interaction in rat central nervous system

Light microscopic immunocytochemistry was employed to investigate possible sites of interaction between the endogenous opioid peptides and monoamines in the rat central nervous system. The opioid and related peptides examined included beta-endorphin (beta-END), alpha-MSH (alpha-MSH) and leucine-enkephalin (Leu-ENK). The monoamines were examined using antisera generated against tyrosine hydroxylase, dopamine-beta-hydroxylase as well as serotonin. Due to the long-tract nature of the central monoamine projections as well as beta-END/alpha-MSH fiber systems, serial section analyses were performed utilizing parasagittal brain sections. Many areas rich in both the monoamines as well as opioid peptides were investigated. These included several thalamic and hypothalamic nuclei, several limbic structures, mesencephalic periaqueductal gray, brain stem noradrenergic cell groups and their rostral projections, the dopaminergic nigrostriatal system, and the serotonergic raphe nuclei and their projections. The results suggest a more intimate linkage between the monoamines and the opioid peptides than previously realized. Some of the intricacies of monoamine-opioid peptide interaction, in particular those pertaining to their possible role in pain and analgesia, catalepsy, and neuroendocrine effects are also discussed.

Suppressive effect of cholecystokinin and its related peptides on beta-endorphin-induced catalepsy in rats

The effects of the C-terminal octapeptide of cholecystokinin (CCK-8) and its related peptides on the onset and duration of beta-endorphin-induced catalepsy on injection of the peptides into the lateral ventricle were investigated in male rats. The onset of catalepsy was delayed to some extent by nonsulfated CCK-8 and CCK-7 but CCK-8 and caerulein were ineffective. Naltrexone and caerulein significantly shortened the duration of catalepsy, but CCKs were less effective to shorten it. Pentagastrin had no effect on either parameter.

Chemistry and biosynthesis of pro-opiomelanocortin. ACTH, MSH's, endorphins and their related peptides

Studies of lipotropins, melanotropins and endorphins on one hand, and of adrenocorticotropin on the other, has given rise to the concept of a multipotent precursor molecule recently renamed proopiomelanocortin. The preferential sites of cleavage of the precursor to produce its biologically active components are made of pairs of basic amino acid residues as described for the biosynthesis of beta-MSH and pro-insulin. Such structural feature is also found in other pro-hormone molecules. Pulse chase experiments and secretory studies carried out in both anterior and intermediate lobes of rat pituitary glands revealed the transformation of different forms of the precursor into different end-products, the anterior lobe producing preferentially ACTH and beta-LPH while the intermediate produces mainly the alpha-MSH and beta-endorphin. The multiple forms of precursors seem to differ in their carbohydrate content although at least two different gene products are still possible. The presence of similar peptides in the hypothalamus makes it highly probable that neuropeptides are biosynthesized with similar process. Thus the model of beta-LPH precursor, proposed as early as in 1967, is now applicable to the biosynthesis of all other neuropeptides. Major advances in this field are expected in the 1980s.

Opioid activities of fragments of beta-endorphin and of its leucine65-analogue. Comparison of the binding properties of methionine- and leucine-enkephalin

For characterisation in vitro, four parallel assays were used: the guinea-pig ileum and mouse vas deferens as pharmacological models at 36 degrees C and the inhibition of binding of [3H]-naltrexone, [3H]-leucine-enkephalin and [3H]-methione-enkephalin at 0 degrees C. The Leu65-analogue of beta-andorphin and its fragments (61-65, 61-76 and 61-77) have a lower affinity to the [3H]-naltrexone binding site of mu-receptors than the corresponding Met65-peptides wereheas no such difference was found for the [3H]leucine-enkephalin binding sites or delta-receptors. When the binding of [3H]-methionine-enkephalin or [3H]-leucine-enkephalin was inhibited by cold ligands interacting with delta-, mu-, or kappa-receptors, no evidence was obtained for more than one type of delta-binding site.

II - Prostaglandin hyperalgesia: the peripheral analgesic activity of morphine, enkephalins and opioid antagonists

Morphine, enkephalins, nalorphine, naloxone and pentazocine are shown to have a peripheral analgesic effect. In our modification of the Randall-Selitto test these substances were 50--100 times more potent than a standard local anaesthetic, lidocaine. At this peripheral site, naloxone did not antagonize the effect of morphine. Morphine had a marked analgesic effect on the hyperalgesia induced by PGE2 and PGI2, BaCl2, Ca2+ ionophore A23187, isoprenaline but not on that induced by dibutyryl cyclic AMP. It was suggested that the peripheral analgesic effect of morphine is due to an inhibition of adenylate-cyclase activity.

Pharmacological and immunological characterization of the Leu5 analogue of human beta-endorphin

The potencies of beta h-endorphin, Met (O)5-beta h-endorphin, and synthetic Leu5-beta h-endorphin have been compared in three bioassays of opioid activity, and in two radioimmunoassays. In all assays, a peptide isolated from hemodialysates from a psychotic patient behaved like Leu5-beta h-endorphin; it has been distinguished unambiguously from beta h-endorphin and Met(O)5-beta h-endorphin. Leu5-beta h-endorphin was one-fifth as potent as beta h-endorphin in guinea pig ileum myenteric plexus, but was only slightly less active in mouse vas deferens and in guinea pig brain opiate receptor binding assay. The low cross-reactivity of Leu5-beta h-endorphin relative to beta h-endorphin with an antiserum raised to beta-endorphin suggests that the preferred solution conformations of these peptides are different. In all bioassays beta h-endorphin was 2- to 3-fold less potent than beta c-endorphin.

Distribution of the messenger RNA coding for the common precursor of corticotropin and beta-lipotropin within the bovine pituitary

The distribution of the mRNA coding for the common precursor of corticotropin and beta-lipotropin among different parts of the bovine pituitary has been investigated by quantifying the mRNA activity with the use of a cell-free protein-synthesizing system. The results obtained have demonstrated that this mRNA activity is located both in the anterior lobe and in the intermediate lobe, while it is essentially not detectable in the neural lobe nor in the stalk. The structural identity of the translation products of corticotropin/beta-lipotropin mRNA from the anterior and from the intermediate lobe has been indicated by their molecular weight as well as by the electrophoretic patterns of the peptide fragments formed from them upon partial enzymatic proteolysis or upon cyanogen bromide cleavage. The specific activity of corticotropin/beta-lipotropin mRNA in the intermediate lobe is about 20-fold higher than that in the anterior lobe, and the total activity of this mRNA in the former is about 2-fold higher than that in the latter. In the intermediate lobe, the translation product of corticotropin/beta-lipotropin mRNA amounts to almost one-third of the products encoded by total translatable mRNA. These results indicate that corticotropin/beta-lipotropin mRNA represents a major mRNA species in intermediate lobe of the pituitary, thus suggesting that this lobe may perform a highly specialized function in producing a large amount of the common precursor of corticotropin and beta-lipotropin.

Radioimmunoassay of beta-endorphin basal and stimulated levels in extracted rat plasma

A sensitive radioimmunoassay for beta-endorphin is described. Antibodies against human beta-endorphin which exhibit a high avidity for the C-terminal of the peptide were raised in rabbits following the injection of thyroglobulin-coupled human beta-endorphin (betah-E) as immunogen. Methionine-enkephalin, alpha-, gamma-endorphine, as well as ACTH peptides did not cause interference in the radioimmunoassay. beta-Lipotropin, however, showed a 50% cross-reactivity. The sensitivity of the assay is 25 pg/0.5 ml tube volume for beta-endorphin. beta-Endorphin was extracted with a high recovery from the rat plasma using silicic acid and beta-endorphin levels as low as 100 pg/ml could be measured. Basal levels of beta-endorphin-like immunoreactivity in plasma of rats were about 400 pg/ml. beta-Endorphin levels in adrenalectomized rats and in animals chronically treated with the cortisol synthesis blocker metyrapone were found to be markedly increased (about 7-fold). Exposure of the rats to electrically induced footshocks caused a similar increase of immunoreactive beta-endorphin in plasma. A significant increase was also seen after insulin injection.