Identification of two related pentapeptides from the brain with potent opiate agonist activity

Neuronal peptide (enkephalin) receptors in the ear artery of the rabbit

Methionine-enkephalin methylester (MEM) and leucine-enkephalin (LE) inhabit the vasoconstrictor responses of the rabbit ear artery to nerve stimulation by acting on a specific neuronal peptide (enkephalin)-receptor insensitive to opiate agonists. The tetrapeptide: H-Tyr-Gly-Phe-Leu-OCH3 is ineffective. This is the first instance of enkephalins acting in an organ devoid of receptors. In a new test for the analysis of opiate receptors, MEM (ID50=6.9 X 10(-9) M) was a potent inhibitor of transmission. The presence was shown of opiate receptors in the brain which were insensitive to high i.v. or intraventricular doses of enkephalins. It is concluded that enkephalins are not natural ligands to the opiate receptors, but that some of the receptors confuse these structures because of similar characteristics which determine the binding of both opiates and peptides.

Effect of normorphine and enkephalin on spontaneous potentials in the vas deferens

Spontaneous excitatory junction potentials were recorded intracellularly from the smooth muscle of the mouse vas deferens. Exposure of the vas deferens to either normorphine (1 muM) or methionine-enkephalin (0.5 muM) did not alter the frequency or amplitude distribution of the spontaneous potentials. The possible mechanisms by which the narcotic agonists depress the release of noradrenaline on nerve stimulation without inhibiting the spontaneous release of noradrenaline are discussed.

Naltrexone-induced hypothermia in the rat

Naltrexone, in relatively high doses, has been reported to cause a fall in body temperature in human ex-heroin addicts who had been abstinent for at least 6 weeks. The underlying mechanism of this hypothermic effect has been investigated in rats. The first consideration was that the temperature change was a reflection of delayed withdrawal but rats implanted with a morphine pellet 45 days earlier showed no significant change in temperature after a dose of naltrexone that caused marked withdrawal hypothermia in dependent rats implanted 3 days previously. A fall in core temperature was only induced in rats after doses of 80 and 160 mg/kg i.p. of naltrexone. Behavioral thermoregulatory studies revealed that the animals correct the falling body temperature by increased exposure to a radiant heat source indicating that the central thermostats had not been significantly affected by the drug. These data suggest that the major component in the hypothermic effect of naltrexone is activation of efferent heat loss pathways or peripheral heat loss mechanisms. Due to current suggestions that opiate receptors might represent the receptors for an endogenous transmitter the results are discussed in relation to this consideration. When compared to the sites and mechanism of action of opiates on thermoregulation the results with naltrexone lend little support to the hypothesis that the fall in temperature is due to displacement of an endogenous substance from central opiate receptors.

Physical dependence of opiate-like peptides

Methionine-enkephalin and beta-endorphin, endogenous peptides with activities similar to those of opiates, were infused for 70 hours into the periaqueductal gray-fourth ventricular space of the rat brain. When challenged with a naloxone, a specific opiate antagonist, these animals manifested a typical morphine-like withdrawal syndrome. These results show that such peptides can cause physical dependence.

Tolerance and dependence induced by morphine-like pituitary peptides in rats

An extract from porcine pituitaries containing peptides with opiate-like activity (endorphins) was investigated for possible tolerance/dependence liability in rats. Repeated intraventricular administration of endorphins induced a degree of tolerance similar to that induced by normorphine. Rats made tolerant to morphine by repeated pellet implantation proved cross-tolerant to the pituitary extract as well as to synthetic methionine-enkephalin. Naloxone given to rats, repeatedly pretreated with endorphins, precipitated a withdrawal syndrome strongly resembling that induced in rats treated with normorphine. Withdrawal precipitated in morphine-dependent rats was suppressed by intraventricularly applied endorphins. The results suggest that identical sites and mechanisms are involved in the development of tolerance/dependence induced by opiates and pituitary endorphins.

Porcine pituitary peptides with opiate-like activity: partial purification and effects in the rat after intraventricular injection

A peptide material with opiate-like activity in the guinea-pig ileum was extracted from porcine pituitaries using a hot glacial acetic acid extraction method and was partially purified by gel filtration. When injected intraventricularly in rats, these purified peptides induced strong analgesia, catelepsy, respiratory depression and other opiate-like effects, which lasted for several hours.

Tolerance and dependence evoked by an endogenous opiate peptide

Incubation of neuroblastoma X glioma hybrid cells for 12-97 hr with methionine-enkephalin results in an increase in adenylate cyclase activity [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] that is mediated by the opiate receptor. The results show that cells become tolerant to, and dependent upon, enkephalin.

Morphinomimetic activity of synthetic fragments of beta-lipotropin and analogs

In the myenteric plexus-longitudinal muscle bioassay, beta-endorphin, i.e., beta-lipotropin (beta-LPH)-[61-91], has a potency of 450 with confidence limits of 281-966 when Met5-enkephalin is used as a reference standard with a potency of 100. The primary amide and the ethylamide of Met5-enkephalin have potencies statistically overlapping with that of beta-endorphin. The primary amide of alpha-endorphin has twice the potency of the free acid form of alpha-endorphin. An intact NH2-terminal tyrosine is not necessary for full intrinsic activity. The shortest fragment of beta-LPH with morphinomimetic activity is beta-LPH-[61-64].

Preliminary analysis of 1H and 13C spectral and relaxation behavior in methionine-enkephalin

Nuclear magnetic resonance studies on the conformational dynamics of the pentapeptide H-Tyr-Gly-Gly-Phe-Met-OH are reported. This peptide, for which the generic trivial name "methionine-enkephalin" has been suggested, is pharmacologically active as a ligand for the mammalian opiate receptor(s). The studies reported are parallel investigations in two solvents (dimethylsulfoxide and water) of: 1H and 13C high resolution spectral assignments; 1H and 13C spin-lattice relaxation times, temperature dependence of amide proton chemical shifts, and half-times for chemical exchange or amide protons. From these data we conclude that the tyrosine side chain of methionine-enkephalin exhibits restricted motion with respect to the main peptide backbone of the molecule. On the other hand both the phenylalanyl and methionyl side chains are undergoing intramolecular reorientation with relatively high frequency.

Enkephalin: a potential modulator of noradrenaline release in rat brain

In slices of rat occipital cortex preincubated with 3H-noradrenaline, methioine-enkephalin diminished the overflow of tritium evoked by electrical field stimulation or by 20 mM potassium. The effect was not modified by phentolamine, but was antagonized by naloxone. Giver alone, naloxone slightly increased the potassium-evoked overflow. It is concluded that enkephalin reduces transmitter release from cerebrocortical noradrenergic nerve endings. One-function of endogenous opiate receptor ligands may be presynaptic inhibition of central noradrenergic transmission.

Peptide transmitters: a unifying hypothesis for euphoria, respiration, sleep, and the action of lithium

Actions of morphine include analgesia, sleep, euphoria, and depression of respiration. Transmitter or modulator substances in the brain that have actions similar to morphine may control these functions in man. This hypothesis proposes that enkephalin is a controlling neurotransmitter and its binding to opiate receptors determines mood state as well as influencing respiratory and sleep patterns. Lithium may act through modification of the opiate receptor affinity for an endogenous morphine-like substance. The theory predicts blocking action of naloxone in mania and in most drug-induced euphorias. It implies a new chemical pathophysiological basis for the phenomenology of mental illness.

Isolation of a novel endogenous opiate analgesic from human blood

Based upon its ability to inhibit opiate receptor binding, a low-molecular-weight substance (600) has been isolated from human plasma by extraction into butanol and ion exchange, molecular sieve, and thin-layer chromatography. When this substance, termed anodynin, is microinjected into rat periaqueductal gray matter, it causes a profound, long-lasting analgesia which is prevented by prior injection of the opiate antagonist naloxone. Anodynin (opiate receptor binding material) levels in serum from hypophysectomized rats are less than 5% of values obtained in sham-operated controls. Anodynin differs from enkephalin, a morphine-like peptide isolated from brain, in its sensitivity to enzymatic loss of opiate receptor inhibitory potency, thin-layer chromatographic mobility, and behavioral effects. Anodynin might be a hormone that acts on peripheral opiate receptors in the classical manner, but might also, due to its lipophilic nature and small size, penetrate into the brain to produce centrally mediated behavioral effects.

Morphine-like peptides in mammalian brain: isolation, structure elucidation, and interactions with the opiate receptor

A substance that competes for opiate receptor binding has been isolated from calf brain and identified as two pentapeptides, H-Tyr-Gly-Gly-Phe-Met-OH (methionine enkephalin) and H-Tyr-Gly-Gly-Phe-Leu-OH (leucine enkephalin), with about four times more leucine enkephalin than methionine enkephalin. Sodium and manganese effects on opiate receptor interactions show that both peptides are agonists, whereas leucine enkephalin may be a "purer" agonist than methionine enkephalin.

Somatostatin and ACTH are peptides with partial antagonist-like selectivity for opiate receptors

The peptides, Met-enkephalin and somatostatin were analysed for competitive affinity to opiate agonist receptor sites in rat brain. Met-enkephalin, like morphinomimetic compounds, being equal to marked selectivity for agonist sites while somatostatin and ACTH showed much less selectivity, being equal to partial antagonists like nalorphine. These results are compared with those observed for these peptides in vivo. It is suggested that somatostatin and acth act as partial agonist--antagonist on opiate receptors in the cns.

beta-Lipotropin as a prohormone for the morphinomimetic peptides endorphins and enkephalins

The hypophysial homomeric peptide beta-lipotropin (beta-LPH-[1-91]) has no morphinomomimetic activity in a bioassay (myenteric plexus-longitudinal muscle of the guinea pig's ileum) or binding assays with stereospecific opiate-receptors of rat brain synaptosome preparations. Incubating beta-LPH-[1-91] at neutral pH with the supernatant aqueous extracts of rat brain generates (fragments of beta-LPH with) morphinomimetic activity in the same assay systems. These results are related to the recently recognized structural relationships between beta-LPH, the newly isolated peptides met-enkephalin (beta-LPH-[61-65]) and alpha-endorphin (beta-LPH-[61-76]) and also to the biologically active fragments of analogs: beta-LPH-[61-64], beta-LPH-[61-65[-NH2, (Met(O)65)-BETA-LPH-[61-65], beta-LPH-[61-69], and beta-LPH-[61-69]. Enzymatic biogenesis of these morphinomimetic peptides would preclude localizing them as such in cellular or subcellular elements with currently available methodology.