Morphine-like Factor Research Articles

Algesiogenic and analgesic activities of synthetic substance P.

The objective of our study was to determine whether the pure synthetic substance P(SP) is algesiogenic or analgesic when administered centrally or peripherally. The relationships between SP-induced analgesia and the content of morphine-like factor (MLF) in the brain were also studied. Intracarotid arterial administration of SP (20-200 microgram) produced no pseudoaffective responses to pain in six out of nine rats, but in the remaining three, there was an exhibition of these responses. Chlorpheniramine pretreatment antagonized these responses. On cantharidin blister base experiments in humans, SP (10(-3) g/ml) produced slight pain and an itchy sensation. SP given intracerebroventricularly produced an analgesia in mice in a dose of 5 ng/mouse, as determined by the acetic acid-induced writhing and hot plate methods. These SP-induced analgesia were antagonized by naloxone pretreatment. SP did not alter the content of MLF in the mouse whole brain. However, SP5-11 not only produced an analgesia but also increased the content of MLF. These results suggest that SP has a slight algesiogenic activity which might be mediated by histamine and a slight analgesic activity which might be mediated by MLF.

ALGESIOGENIC AND ANALGESIC ACTIVITIES OF SYNTHETIC SUBSTANCE P

AbstractThe objective of our study was to determine whether the pure synthetic substance P(SP) is algesiogenic or analgesic when administered centrally or peripherally. The relationships between SP-induced analgesia and the content of morphine-like factor (MLF) in the brain were also studied. Intracarotid arterial administration of SP (20-200 μg) produced no pseudo-affective responses to pain in six out of nine rats, but in the remaining three, there was an exhibition of these responses. Chlorpheniramine pretreatment antagonized these responses. On cantharidin blister base experiments in humans, SP (10−3 g/ml) produced slight pain and an itchy sensation. SP given intracerebroventricularly produced an analgesia in mice in a dose of 5 ng/mouse, as determined by the acetic acid-induced writhing and hot plate methods. These SP-induced analgesia were antagonized by naloxone pretreatment. SP did not alter the content of MLF in the mouse whole brain. However, SP5-11 not only produced an analgesia but also increased the content of MLF. These results suggest that SP has a slight algesiogenic activity which might be mediated by histamine and a slight analgesic activity which might be mediated by MLF.

SII-4 - Mechanism of the change of morphine-like factor (MLF) in the brain by dental pulp stimulation.

SII-4 - Mechanism of the change of morphine-like factor (MLF) in the brain by dental pulp stimulation.

Biology of Opioid Peptides

Opioid peptides are endogenous or synthetic peptides, with a spectrum of pharmacological activity similar to that of morphine and other narcotic agonist drugs. The designation opioid peptide can be made if (a) it produces morphine-like, naloxone reversible effects in several in vitro bioassay sys­ tems such as the guinea pig ileum (1), the cat nictitating membrane (2), the mouse vas deferens (3), and the neuroblastoma X glioma tissue culture preparation (4), and (b) if the peptide competes at low concentrations with opiate ligands for binding at opiate receptor binding sites (5-7). An addi­ tional, although not essential, criterion might include the production of naloxone reversible analgesia. These criteria exclude substance P which does have analgesic activity (8, 9), fragments of adrenocorticotropic hor­ mone which have very weak opiate receptor binding activity (10), and the morphine-like factor identified on the basis of morphine antibody recog­ nition (11). At present only three endogenous opiate peptides have been identified that have any claim to a physiological function. These are leucine5-enkepha­ lin (12), methionine5-enkephalin (12), and ,a-endorphin (13-16). Other frag­ ments of ,B-lipotropin such as LPH 61-69, LPH 61-76, LPH 61-77, and LPH 61-87 have opioid peptide activity (17-19) but current evidence sug­ gests that these may be artifacts of extraction or degradation products of ,B-endorphin (20). Other unidentified opioid peptides undoubtedly exist (21-24), but in the absence of structural and chemical identification it is impossible to assess their significance or biological function. The term endorphin as a generic name for opioid peptides has little to recommend

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

A partially purified morphine-like peptide ‘enkephalin’ (PPE) extracted from bovine brain elicted 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.

Antagonism of General Anesthesia by Naloxone in the Rat

The effect of naloxone, a narcotic antagonist, on the response of animals to painful stimuli during anesthesia was studied. Rats were anesthetized with cyclopropane, halothane, or enflurane in groups of 12. Following induction, inspired anesthetic concentration was gradually reduced to a point at which 35-60 per cent of animals responded to tail clamping. Thereafter the anesthetic concentration was held constant for 30 minutes. Rats in each group then received saline solution or naloxone, 10mg/kg, given intravenously. The response to tail clamping was retested 5 minutes later. In additional experiments EEG's were recorded from rats anesthesized with one of these anesthetics. After a stable light plane of anesthesia had been attained, each animal was given naloxone, 10 mg/kg, iv, and the EEG recorded for an additional 5 minutes. In the tail-clamping experiments, naloxone approximately doubled the number of rats responding during cyclopropane, halothane, or enflurane anesthesia. The EEG patterns of several animals anesthetized with either cyclopropane or halothane changed to patterns consistent with lighter planes of anesthesia after naloxone administration. That naloxone alters the depth of inhalational anesthesia suggests that anesthetics may release an endogenous morphine-like factor (MLF) in the central nervous system.

MORPHINE–NALOXONE INTERACTION IN THE CENTRAL CHOLINERGIC SYSTEM: THE INFLUENCE OF SUBCORTICAL LESIONING AND ELECTRICAL STIMULATION

1 The opiate antagonist naloxone, injected or topically applied to the cerebral cortex, had no significant effect on the spontaneous output of cortical acetylcholine (ACh) in rats. 2 Morphine (2.5 mg/kg) administered intravenously inhibited the release of cortical ACh. A subsequent injection of naloxone rapidly reversed morphine-induced inhibition, and produced a sustained increase in the release of ACh. Topical application of naloxone solutions, after morphine, produced a slow and weak reversal of its inhibitory action. 3 Destruction of the medial thalamus abolished both the inhibitory effects of morphine on the cortical ACh release, and its antagonism by naloxone administered after the agonist. 4 Injection of naloxone in a low dose (0.1 mg/kg) increased the release of cortical ACh provoked by electrical stimulation of either the medial thalamus or the reticular formation in normal rats. In the morphine-dependent rat, naloxone also facilitated the evoked release and its action was greater than in control animals. The facilitatory effect of naloxone on the cortical release evoked by stimulation of the medial thalamus was greater than its effect on the release evoked by stimulation of the reticular formation in both normal and morphine-dependent rats. 5 Naltrexone, a narcotic antagonist, also facilitated the electrically stimulated release of cortical ACh. 6 It is suggested that (a) morphine and naloxone act at a subcortical site, probably the medial thalamus, to modify the cortical ACh release and that (b) naloxone may facilitate the electrically-induced release of ACh in the CNS by antagonizing the effect of the endogenous morphine-like factor, enkephalin.

Inactivity of enkephaline on human serum esterase

While opioid agonist accelerate the hydrolytic action of human serum esterase, and opioid antagonists competitively antagonize this acceleration, the endogenous morphine-like factor methionine-enkephaline neither accelerates the enzyme nor competes with an opioid accelerator. It is proposed that the pentapeptide enkephaline is only the prosthetic group of the true endogenous morphine-like factor and too short a peptide chain to have a stable enough secondary structure for opioid action in the presence of dissolved protein.

Elevated levels of enkephalin in morphine-dependent rats.

NUMEROUS theories propose mechanisms regulating opiate dependence. Hughes1,2 elegantly identified a morphine-like factor “enkephalin” which mimics effects of morphine on nerve, plexus in smooth muscle in a fashion that can be antagonised by opiate antagonists, while a compound with the same properties has been identified in brain extracts by competition for opiate receptor binding3–6. Enkephalin activity has a regional and subcellular distribution similar to that of the opiate receptor7,8 and was first shown to be a mixture of two peptides H–Tyr–Gly–Gly–Phe–Met–OH and H–Tyr–Gly–Gly–Phe–Leu–OH by Hughes et al.9 with a preponderance of the methionine peptide in pig brain9. Independently, we identified the same two peptides in bovine brain but with four times more of the leucine than of the methionine peptide10,11. Brain enkephalin differs from substances in pituitary gland and hypothalamic extracts which mimic morphine12,13.

Effects of microiontophoretically applied methionine-enkephalin on single neurones in rat brain.

THE identification of a specific opiate receptor in mammalian brain1, and also of an endogenous morphine-like factor, enkephalin, which possesses potent opiate agonist activity peripherally2, raises the question of the possible physiological significance of this substance in the central nervous system. There is evidence from a number of studies suggesting that the brainstem is an important site for antinociceptive action3–5. In previous studies8, morphine applied iontophoretically to single neurones in the rat brainstem, has been found to cause either an increase or a decrease in firing rate, but only the depressant response showed stereospecificity and could be blocked by the specific opiate antagonist, naloxone7,8. Similar findings on cortical neurones have been reported9. We have therefore investigated the effects of iontophoretically applied methionine-enkephalin (met-enkephalin), the more potent peripherally of the two pentapeptides, on the activity of single neurones in the rat brain stem and compared these effects with those of morphine, and also of another narcotic analgesic, etorphine, which is considerably more potent. Since naloxone has been shown to block peripheral effects of enkephalin10, we have examined its effects on the neuronal responses to met-enkephalin, morphine and etorphine. Our findings support the concept of a physiological role for endogenous enkephalin in the central nervous system.

A morphine-like factor ‘enkephalin’ in rat brain: subcellular localization

A morphine-like factor ‘enkephalin’ in rat brain: subcellular localization

The regional distribution of a morphine-like factor enkephalin in monkey brain

The regional distribution of a morphine-like factor enkephalin in monkey brain

Morphine-like ligand for opiate receptors in human CSF

Human CSF was fractionated chromatographically and tested for affinity to opiate receptors in synaptic plasma membrane preparations from rat brain. A unique fraction of an apparent molecular weight of 1000 to 1200 dalton, probably identical with that previously found by us in brain extracts, showed affinity. Since the active factor behaved similar to morphinomimetic analgesics in its interaction with the receptors and not like the narcotic antagonist naltrexone, it is termed MLF (morphine-like factor). The level of MLF varied between different individuals and was apparently lower in patients with trigeminal neuralgia than in other patients.

An endogenous morphine-like factor in mammalian brain

An endogenous morphine-like substance (MLF) found in rat and calf brains has a regional distribution correlating with that of opiate receptors, with the highest levels in the caudate and negligible amounts in the cerebellum. In binding assays MLF behaves like an opiate agonist. Sodium ion and enzyme and reagent treatment of membranes decrease its potency and manganese ion enhances it. MLF is localized in synaptosomal fractions, stored in an osmotically labile compartment, and can be degraded by carboxypeptidase A and leucine amino peptidase, implying a peptide structure. Its molecular weight is about 1000 as determined by gel chromatography.