A peptide-like substance from pituitary that acts like morphine. 2. Purification and properties

Postoperative analgesia

Postoperative analgesia both by drugs and regional techniques is reviewed. In the United Kingdom in the last 25 years or more there has been little advance on either front. Some marginal improvement in regard to drugs might be brought about by better education of both doctors and nurses and better patient contact. Extradural analgesia and intercostal block do not offer a complete solution, though a judicious increase in the use certainly of the former might be beneficial. The problem awaits a radical new approach.

Detection of antagonist activity for narcotic analgesics in mouse hot-plate test

On the assumption that by the use of the hot-plate procedure the antagonist properties of narcotic analgesics could be detected, the effect of morphine, pentazocine, nalorphine and naloxone were investigated. The latency of paw-licking and jumping-off were determined and compared. The agonist, morphine, at doses of 0.025, 0.05 and 0.1 mmole/kg injected IP significantly increased paw-lick and jump-off latency above that seen in saline controls. The mixed agonist-antagonist, pentazocine, at doses of 0.048, 0.096 and 0.192 mmole/kg and nalorphine, an antagonist with some agonist activity, at doses of 0.032, 0.064 and 0.128 mmole/kg significantly increased the latency of paw-licking, but did not significantly change the jump-off latency. At a dose of 0.016 mmole/kg naloxone treated mice jumped from the hot-plate significantly sooner than controls but no effects of naloxone on paw-licking latency were observed. These results suggest that agonist properties are involved in the paw-lick response and that antagonistic properties determine jumping-off behavior.

The effects of opiate receptor agonists and antagonists on the stress-induced secretion of corticosterone in mice

1 Intraperitoneal administration of normorphine, morphine or naloxone or exposure to ether vapour for 1 min, elevated plasma corticosteroid concentrations in mice. 2 Injection of saline or exposure to ether vapour rendered mice less sensitive to a subsequent exposure to ether vapour 15 min later. 3 Treatment with normorphine (50 mg/kg) potentiated the corticosteroid response to ether stress whilst pentazocine (20 mg/kg), naltrexone (10 mg/kg), morphine (24 mg/kg), levorphanol (20 mg/kg) and naloxone (50 mg/kg) prevented the stress-induced elevation of plasma corticosteroids. 4 Both naloxone and morphine inhibited the potentiation by normorphine of the response to ether, the dose of naloxone required being higher than that for inhibition of normorphine analgesia. 5 It is concluded that endogenous opioid peptides may be involved in the control of the response to ether stress in mice.

Activity of VIP, somatostatin and other peptides in the mouse vas deferens assay

Non-opiate peptides such as vasoactive intestinal peptide (VIP) and somatostatin were tested for their effects on electrically induced contractions of the vas deferens. VIP(ED50 = 2.7 X 10(-8) M) and to a lesser extent somatostatin (ED50 = 5.2 X 10(-8) M) were found to be in the same general range of activity as enkephalin and the endorphins in this system. Human pancreatic polypeptide (HPP) exerted a biphasic effect, inhibiting the contractions at high concentrations but enhancing them at lower concentrations. A number of other natural occurring brain peptides were ineffective at concentrations of 1 X 10(-6) M. Several somatostatin analogues were tested and their activity on the vas deferens was found to more closely parallel their potency to inhibit the release of gastric acid than of growth hormone. In contrast to the brain opiates, however, the inhibitory effects of VIP, somatostatin and its analogues, and HPP were not reversed by the opiate antagonist naloxone. The results suggest that the vas deferens can be readily used for evaluation of analogues of VIP, somatostatin, and other peptides.

Isolation of the C-fragment and C'-fragment of lipotropin from pig pituitary and C-fragment from brain

Three novel peptides derived from lipotropin, the C-Fragment (residues 61-91), C'-Fragment (61-87) and N-Fragment (1-38), were isolated from pig pituitary, and the C-Fragment was shown to be present in brain. The experimental procedures developed for their isolation are described. The formation of each of the fragments involves enzymic cleavage of lipotropin at consecutive basic residues, with specificity identical with that involved in the activation of known prohormones. In brain assays C-Fragment exhibits a range of biological activities related to its ability to act as an inhibitory neurotransmitter.

Enhancement of a nociceptive reaction by opioid antagonists in mice

1. The opioid antagonists, naloxone, GPA 2163, levallorphan and Mr-2266 reduced the latency of the jumping reaction of mice in the hot plate test. The (+)-isomers of levallorphan and Mr-2266 which are devoid of antagonistic activity did not increase this latency. 2. In the same nociceptive reaction test, the enhancing effect of naloxone progressed in a dose-range similar to that required for the antagonism by naloxone of the depressive action of morphine. 3. The facilitatory effect of naloxone was not blocked by the previous administration of morphine or etorphine but it was prevented by pretreatment with a high dose of buprenorphine. 4. The antagonism by naloxone of morphine and of buprenorphine did not follow the same pattern. 5. The factors which are or may be involved in the efficacy of naloxone in enhancing nociceptive reactions are discussed. 6. The enhancing effect of naloxone may be due to an antagonism of endogenous ligands for the opiate receptor. If so, these ligands would be involved in reaction to but not in perception of nociceptive stimuli which need not be harmful ones.

Effects of Met-enkephalin on body temperature of normal and morphine-tolerant rats

The endogenous opioid met-enkephalin intraventricularly adminstered to the rat at the dose of 100 microgram raised rectal temperature, whereas 400 microgram of the pentapeptide caused a diphasic effect, i.e., hypothermia followed by hyperthermia. Met-enkephalin was ineffective when administered i.p. The effects on temperature were substantially similar to those elicited, for both routes of administration, by morphine, which may either raise or lower rat temperature depending on the dose. More naloxone was required to antagonize thermic effects of met-enkephalin than morphine. Finally, there was a lack of effects on temperature for met-enkephalin centrally administered to morphine-tolerant animals, thus providing further evidence, in vivo, of cross tolerance between opiates and naturally occurring ligands of opiate receptors.

Isolation and thin-layer chromatographic identification of several peptides with NH2-terminal tryptophan and their histochemical demonstration in the ACTH cells of the rat hypophysis

Methods for the isolation and thin-layer chromatographic identification of amino-terminal tryptophyl-peptides presumably responsible for histochemical tryptophyl-peptide reactions in the ACTH cells of the rat hypophysis are described. In the hypophyseal extract several tryptophylpeptide bands--depending on the homogenization solution--were demonstrated on thin-layer chromatograms. Tryptophyl-peptides were demonstrated from their fluorescence induced 1) with glyoxylic acid (glyoxylic acid introduced into the homogenization solution), 2) by exposure of the chromatographic plates to combined formaldehyde and chloral vapour or 3) by exposure to combined formaldehyde and acetyl chloride vapour. A positive PAS reaction was demonstrated in some tryptophyl-peptide bands. Thus, some tryptophylpeptides seem to contribute to the observed PAS positivity of the ACTH cells.

Endorphins: naloxone fails to alter experimental pain or mood in humans

In 30 human subjects, experimental pain was produced by either ischemia or cold-water immersion. In a double-blind procedure, intravenous doses of up to 10 milligrams of naloxone hydrochloride in saline were indistinguishable from similarly administered saline alone. There were no effects on subjective pain ratings, finger plethysmograph recordings, or responses to mood-state questionnaires. These laboratory procedures do not activate any functionally significant pain-attenuating or mood-altering effect of endorphins.

An effect of beta-melanocyte stimulating hormone (beta-MSH) on alpha-motoneurones of cat spinal cord

Actions of beta-MSH and of melatonin on the recovery cycle of single spinal neurones were studied in the decerebrate-spinal cat. beta-MSH facilitated the rate of post-excitation recovery of alpha-motoneurones and some internuncial neurones, and melatonin inhibited the rate of post-excitation recovery. These observations provide additional evidence that beta-MSH functions in the nervous system as a modulator, and may help explain actions of beta-MSH in modifying acquisition of conditioned avoidance responses as well as its interaction with drugs such as morphine.

The distribution of methionine-enkephalin and leucine-enkephalin in the brain and peripheral tissues

1 A method is described for the rapid extraction of opioid peptides from the brain and other tissues. The method is based on acid extraction of tissues followed by adsorption of the extract onto Amberlite XAD-2 resin. Elution with methanol separates the enkephalins and alpha-endorphin from beta-endorphin.2 Over 90% of the opioid peptide activity isolated from brain and gut of several species by our method was due to methionine- and leucine-enkephalin. In contrast, the major opioid peptide activity recovered from the pituitary was due to peptides of much greater mol. wt. than the enkephalins.3 An opioid peptide with properties unlike those of the known endorphins or enkephalins was present in brain extracts. This peptide, termed epsilon-endorphin, has an apparent mol. wt. of 700 to 1200; it constituted between 5 to 10% of the total opioid activity in our extracts.4 A differential assay of methionine- and leucine-enkephalin was made either by destroying methionine-enkephalin activity with cyanogen bromide or by separating the peptides by thin layer chromatography.5 The ratio of methionine-enkephalin to leucine-enkephalin varied greatly in different brain regions. The highest proportions of leucine-enkephalin were found in the cerebral cortex and hippocampus.6 Formaldehyde perfusion and fixation of the brain in vivo had no significant effect on the brain content of enkephalin, indicating that proteolytic breakdown is not a major problem in the extraction of these peptides.7 It is suggested that the enkephalins may have a neurotransmitter role in both brain and peripheral tissues and that methionine- and leucine-enkephalin may subserve separate neuronal functions.

A morphine-like factor in mammalian brain: analgesic activity in rats

A partially purified morphine-like peptide 'enkephalin' (PPE) extract from bovine brain elicited pronounced apparent analgesia after injection into the periaqueductal gray matter of rat brain. This analgesia was reversed by the opiate antagonist naloxone in a dose-dependent fashion. Analgesia was more rapid in onset and much shorter in duration after PPE than after morphine administration. Analgesia was elicited only by those ion exchange column fractions of PPE that competed potently for opiate receptor binding. No analgesia could be detected when PPE or morphine injections were administered at a site 2 mm lateral to the periaqueductal gray matter. The potencies of synthetic methionine- and leucine-enkephalin in eliciting analgesia were less than 1% of those of partially purified enkephalin extracts when doses of equivalent ability to compete for opiate receptor binding were compared.

Structural requirements for opioid activity of analogues of the enkephalins

Structure-activity relations of a series of analogues of the two endo­genous morphine-like peptides, leucine-enkephalin and methionine-enkephalin are examined on the basis of ( a ) effects on the mouse vas deferens and the guinea pig ileum and ( b ) affinities for the rat brain opiate receptor. In the mouse vas deferens, metabolism of the peptides by proteolysis is not a major influence on activity. In contrast, however, brain opiate re­ceptor preparations contain an abundance of proteolytic enzymes, the effects of which can be minimized by conducting opiate receptor binding assays at 0 °C and in the presence of bacitracin. The potentiation of biological activity and opiate receptor binding affinity by replacing the Gly 2 residue in the natural enkephalins by d-Ala, is discussed both in terms of increased stability of the Tyr-d-Ala bond to aminopeptidases and of the stabilization of the peptide conform­ation as present in the receptor-peptide complex. The substitution of the Leu 5 - or Met 5 -residue by the corresponding d-amino acid contributes little to proteolytic stability, which emphasizes that the predominating site at which metabolism occurs is the Tyr 1 -Gly 2 bond. Of the analogues described, [d-Ala 2 , d-Leu 5 ]-enkephalin is the most active peptide in the three assay systems, the mouse vas deferens, the guinea pig ileum and the rat brain opiate receptor preparations. Substitutions by the respective d-amino acids d-Tyr and d-Phe at positions 1 and 4 reduce both the potency and binding affinity and emphasize the importance of stereochemical acceptability at these positions. The promotion of receptor binding by d-amino acids is examined, particularly with respect to implied peptide conformations. The experi­mental data have been analysed for the relative influence of metabolic and conformational factors.

Some effects of naloxone on behavior in the mouse

The effect of naloxone on the latency of mice to enter a dark box was determined and the study of Jacob et al. (1974) on the effect of naloxone on hotplate behavior was replicated. Mice took significantly longer to enter a dark box after receiving 10 mg/kg of naloxone than did the saline controls. With the hot-plate technique, at all doses (0.1, 0.3, 1, 3, and 10 mg/kg) except the lowest (0.03 mg/kg), naloxone-treated mice jumped from the hot plate significantly sooner than did the saline controls. No effect of naloxone on paw-lick latency was found. All the data confirm the findings of Jacob et al. (1974). The hotplate results suggest that prolonged exposure to a noxious stimulus may be necessary to activate the endorphin system.

Endogenous opiates-a progress report

The "opiate receptor" has become a well-accepted entity, stimulating the search for an endogenous agonist. This has successfully revealed several different endogenous peptide opiates whose physiological role is under intense investigation, contributing to our understanding of pain and of opiate tolerance and dependence. A review of the current literature is presented.

Distribution of endorphins (peptides with morphine-like pharmacological activity) in pituitary

Endorphin was found in the pituitary gland of rat, sheep, pig, cow and man. In bovine pituitary, highest concentrations have been found in that part of the posterior lobe containing the pars intermedia, although endorphin was also present in the pars distalis. Analysis by gel filtration and adsorption chromatography suggested that the major endorphins from rat, pig and cow pituitary were similar, with apparent molecular weights in the range 3000-3500 daltons.

Evaluation of the periaqueductal central gray (PAG) as a morphine-specific locus of action and examination of morphine-induced and stimulation-produced analgesia at coincident PAG loci

Experiments were carried out in rats to (1) elaborate upon the sepcificity of drug action in the periaqueductal gray matter (PAG), and (2) to evaluate the posible congruence of PAG sites of morphine-induced and stimulation-produced analgesis (SPA) applied at virtually identical PAG loci. It was demonstrated that the effect of morphine intracerebrally (i,c.) administered into the PAG was not duplicated by other centrally acting agents (chlorpromazine, chlordiazepoxide, pentobarbital or naloxone) administered i.c. at the same PAG site. This selective action of morphine in the PAG was further demonstrated not to be test-bound since morphine significantly altered responding in all four of the analgesiometric tests employed. Thus, multiple i.c. injections of drugs at the same PAG locus were useful in demonstrating site specificity of drug action where behavioral and electroencephalographic methods alone had previously provided ambiguous information. Morphine-induced analgesia and SPA, evaluated at virtually coincident PAG sites, revealed only a general congruence of efficacious loci. The most effective PAG loci for morphine-induced analgesia were not the same as those for SPA; analgesia effected by one analgesia-producing manipulation did not reliably predict that analgesia would also be produced by the other analgesia-producing manipulation at the PAG sites examined. In general, the more efficacious analgesia-producing PAG loci were localized in the ventral-ventrolateral PAG.

Effects of naloxone on experimentally induced ischemic pain and on mood in human subjects

The hypothesis that painful stimuli activate the endogenous opioid (endorphin) system in humans was tested by examining the effect of the opiate antagonist naloxone on experimentally induced ischemic pain and on subjective mood ratings. Intravenous injections of saline or naloxone hydrochloride (2 and 10 mg) were administered under double-blind conditions to 12 subjects. Naloxone did not affect the pain ratings. However, a significant dose-related effect of naloxone on tension-anxiety was found, suggesting that the endorphins. like exogenously administered opiates, may have antianxiety properties.

Effect of opiate receptor blockade on pain sensitivity in the rat

Blockade of opiate receptors by naloxone (2 mg/kg) was found to produce a significant increase in pain sensitivity as measured by the tail-flick test. This finding supports the view that endogenous opiate systems may play a role in the modulation of pain sensitivity. Naloxone, however, was found to have no effect on pain responsiveness as measured by tail-pinch. These findings, together with additional reports, suggest that endogenous opiate systems may exert differential actions on different sensory modalities.