Endogenous opioid peptides: a guide to structures and terminology

Carboxypeptidase activity in synaptic vesicles isolated from striatum and cortex of calf brain

A carboxypeptidase activity has been found in synaptic vesicles (secretory granules) isolated from the cortex and striatum of calf brain which removes amino acids from the carboxy terminus of enkephalin-containing (EC) peptides. The formed enkephalin molecules are not further degraded by this enzyme activity. The preparations were found to be free of cytoplasmic and lysosomal constituents as determined by marker enzyme activities. The vesicle preparations of both cortex and striatum showed differences in the degradation velocities of the various EC peptides depending on size and charge of the amino acid present at the carboxy terminus. The pH optimum of the release of Met-enkephalin from Met-enkephalin-Arg6 has been shown to be between pH 5 and 6. The enzyme activity is inhibited by thiol-blocking agents such as p-hydroxymercuribenzoate and copper ions, but only slightly by metal-chelating agents.

Met-enkephalin and morphine but not dynorphin inhibit noxious stimuli-induced release of substance P from rabbit dorsal horn in situ

The effects of locally applied opioids on the release of immunoreactive Substance P (iSP), induced by mechanical stimuli, from the dorsal horn of the rabbit in situ, were investigated. Morphine and met-enkephalin (met-enk), but not dynorphin A (1-17) (DYN), in a concentration of 10 microM, significantly inhibited the evoked release. These inhibitory effects of morphine and met-enkephalin were antagonized by the local application of naloxone (10 microM) to the dorsal horn. These results suggest that the inhibition of the release of Substance P induced by noxious mechanical stimuli may be mediated by mu and delta, but not by kappa opioid receptors.

Increased dynorphin immunoreactivity in spinal cord after traumatic injury

Opiate antagonists, at high doses, have been shown to improve physiological variables and outcome after experimental spinal injury. Dynorphin appears to be unique amongst opioids in producing hindlimb paralysis after intrathecal injection. Taken together, these findings suggest a possible pathophysiological role for endogenous opioids, particularly dynorphin, in spinal injury. In the present studies we examined the relationship between changes in dynorphin immunoreactivity (Dyn-ir) in rat spinal cord after traumatic injury and the subsequent motor dysfunction. Trauma was associated with significantly increased Dyn-ir at the injury site, but not distant from the lesion. Dyn-ir was found elevated as early as 2 h and as late as 2 weeks after trauma, and was significantly correlated with the degree of injury. These data are consistent with the hypothesis that dynorphin systems may be involved in the secondary injury that follows spinal trauma.

Endogenous opioid immunoreactivity in rat spinal cord following traumatic injury

It has been postulated that endogenous opioids play a pathophysiological role in spinal cord injury, based on the therapeutic effects of the opiate receptor antagonist naloxone in certain experimental models. The high doses of naloxone required to exert a therapeutic action suggest that naloxone's effects may be mediated by non-mu opiate receptors, such as the kappa receptor. This notion is supported by recent pharmacological studies demonstrating that an opiate antagonist more active at kappa sites is effective and far more potent than naloxone in improving outcome after spinal cord injury. Moreover, dynorphin--postulated to be the endogenous ligand for the kappa receptor--is unique among opioids in producing hindlimb paralysis following intrathecal administration in the rat. In the present studies we have examined changes in endogenous opioid immunoreactivity following traumatic spinal cord injury in the rat. Dynorphin A was found to increase progressively with graded injury; changes were restricted to the injury segment and adjacent areas and were time dependent. Dynorphin A-(1-8) showed no marked changes. Methionine and leucine enkephalin were either unaltered or reduced at the injury site; changes were not well localized and were not clearly related to the injury variables. These findings provide further support for a potential pathophysiological role of prodynorphin-derived peptides in spinal cord injury.

Peripheral opioid receptors influencing heart rate in rats: evidence for endogenous tolerance

Opioid peptides injected into the circulation of rats evoke a vagally mediated bradycardia. The intravenous ED50 of morphine for producing a greater than or equal to 10% fall in heart rate was determined in urethane-anesthetized rats. Hypophysectomy, or adrenalectomy plus treatment with dexamethasone (0.5 microgram/h, s.c., 1 day), procedures that remove endogenous sources of opioid peptides, increased the sensitivity of the animal to morphine bradycardia by 6-10-fold. Conversely, stressing the animals by exposure to cold (4-6 degrees C for two days) elevated the ED50 for morphine sulfate and for beta h-endorphin by about 5-fold. Dexamethasone infusions prevented the cold-induced desensitization to morphine. Intravenous administration of rat corticotropin-releasing factor (CRF) also desensitized the animals to morphine. CRF alone produced a fall in blood pressure and heart rate. The bradycardia was prevented by pretreatment with naloxone. These results indicate that the sensitivity of vagal opioid chemoreceptors is influenced by endogenous sources of opioid peptides. This phenomenon can be called 'endogenous tolerance'.

Induction of human cutaneous mast cell degranulation by opiates and endogenous opioid peptides: evidence for opiate and nonopiate receptor participation

In order to examine the capacity of pharmacologically useful opiates to stimulate human mast cell secretion, subjects were skin tested with morphine, codeine, or meperidine hydrochloride. All three agents acted equipotently in eliciting positive immediate skin reactions from all subjects tested. Each agent demonstrated 10 mm of net whealing at 5 to 10 micrograms base (16.7 to 40.4 nmol) injected intradermally. The ability to elicit immediate skin test reactions with endogenous opioid peptides was examined with the use of dynorphin, [D-Ala, 2-D-Leu5] enkephalin, beta-endorphin, and morphiceptin . All four compounds induced wheal-and-flare reactions with the order of potency: dynorphin, greater than beta-endorphin, and greater than [D-Ala, 2-D-Leu5] enkephalin approximately equal to morphiceptin at dose ranges of 0.3 to 8.45 nmol. The inhibition of reactivity by hydroxyzine and the demonstration of mast cell degranulation by electron microscopy suggest that the immediate skin responses to opioid stimulation occur as a consequence of mast cell degranulation. Experiments with the opioid receptor antagonist, naloxone, suggest that both opioid and nonopioid receptors may be involved. These results imply that endogenous opioid peptides possibly may play a role in mast cell function and/or degradulation .

Effect of enkephalin on the micturition cycle of the cat

The effect of the synthetic opiate [D-Ala2, Me-Phe4]-leu-enkephalin ( DAMLE ) on the micturition cycle of the cat was studied. In vivo assays were performed with young male cats under two different conditions: 1) decerebrated cats (D-cats), with an intercollicular transection of the brainstem, and 2) spinal cats (S-cats), with a spinal transection between C5-C6. In vitro studies were carried out on bladder strips taken from adult male cats. The D-cats showed two types of voiding patterns: the first type (I) was characterized by a smooth wave of pressure and an incomplete emptying of the bladder; the second type (II) began like the type I, but ended with a series of small contractions accompanied by small jets of liquid, resulting in the complete emptying of the bladder. DAMLE inhibited vesical contractions and completely inhibited voiding in D-cats at doses equal or superior to 250 micrograms/kg i.v.; no effect was noted with lower doses. Vesical contractions were hardly affected in S-cats, even at high doses (greater than 350 micrograms/kg i.v.). DAMLE did not affect electrically induced contractions of isolated bladder strips. Naloxone not only antagonized the inhibitory effects of DAMLE , but also induced per se a contraction of the bladder. These results indicate that enkephalinergic neurons are involved in the central neural circuitry of the micturition cycle in the cat, with an inhibitory effect at the level of either the ascending spinal pathways or the pontine Barrington 's center.

Contrasting interactions of the locus coeruleus as compared to the ventral noradrenergic bundle with CNS and pituitary pools of vasopressin, dynorphin and related opioid peptides in the rat

The present study examines the influences of selective destruction of the locus coeruleus (LC) or of the ventral noradrenergic bundle (VB) upon discrete CNS and pituitary pools of vasopressin, dynorphin and related opioid peptides in the rat. The selectivity of the lesions was indicated by the fact that destruction of the LC strongly depressed levels of noradrenaline in the cortex in contrast to the hypothalamus, whereas destruction of the VB decreased noradrenaline in hypothalamus but not cortex. Rats sustaining VB lesions displayed a parallel depletion in neurointermediate, but not anterior, lobe levels of immunoreactive-(ir-dynorphin (DYN), ir-DYN, ir-alpha-neo-endorphin (ir-alpha-NE) and ir-vasopressin (ir-VP) whereas those of ir-Met-enkephalin (ir-ME) were unaffected. In the hypothalamus, the content of ir-DYN and ir-VP tended to rise and that of ir-DYN and ir-alpha-NE was significantly elevated, whereas that of ir-ME was not altered. LC destruction failed, in contrast, to modify levels of ir-VP, ir-DYN, ir-DYN, ir-alpha-NE or ir-ME in any of the above structures. It was found to, however, result in a depression in levels of ir-DYN and ir-alpha-NE, but not of ir-ME or ir-VP, in both the hippocampus and striatum whereas VB lesions were, in this respect, ineffective. Further, in the spinal cord, LC lesions resulted in a significant elevation in levels of ir-DYN and ir-alpha-NE in comparison to those of ir-DYN, ir-VP and ir-ME. Neither type of lesion significantly altered the content of any opioid peptide examined in thalamus, cortex, septum or midbrain. These data indicate that: the LC as compared to the VB interact differently with discrete pools of ir-DYN, ir-DYN, ir-alpha-NE and ir-VP in brain, pituitary and spinal cord; it is the VB rather than the LC which modulates the activity of magnocellular neurones projecting to the neural lobe of the pituitary; ir-DYN, ir-DYN and ir-alpha-NE are, in all tissues, regulated independently of ir-ME; levels of ir-DYN, ir-DYN and ir-alpha-NE are co-regulated with those of ir-VP in the hypothalamus-neural lobe axis but not in extrahypothalamic brain tissues nor the spinal cord; and DYN, DYN and alpha-NE might, in certain cases, be modulated differentially of one another, possibly reflecting alterations in precursor processing.

Comparison of the effectiveness of different opioid peptides in suppressing heroin withdrawal

The effectiveness of beta-endorphin, dynorphin-(1-13), dynorphin-(1-10) amide, alpha-neoendorphin and [D-Ala2,D-Leu5]enkephalin in suppressing withdrawal in heroin addicts was compared in this study. Groups of six patients were stabilized overnight in the hospital and were treated with either saline or peptide when withdrawal symptoms began to appear the following morning. Withdrawal was scored before and after treatment by the patient himself and an independent observer. Peptides were administered in a bolus dose of 60 micrograms/kg body weight. The patient, the observer and the physician who administered the injection were all blind to the nature of the compound given. All treatments, including those with saline, produced an overall reduction of withdrawal score. However, by statistical analysis, only treatments with beta-endorphin, [D-Ala2,D-Leu5]enkephalin and dynorphin-(1-13) were effective in producing a significant decrease of withdrawal symptoms. The length of relief brought about by the different peptides varied from less than an hour to a maximum of 5 h in one case. The average period of relief brought about by beta-endorphin, dynorphin-(1-13) and [D-Ala2,D-Leu5]enkephalin was 44, 46 and 60 min, respectively. Of the five peptides administered [D-Ala2,D-Leu5]enkephalin produced the largest number of side-effects.

[Leu5]enkephalin containing peptides derived from a common precursor: evaluation of opioid activity in in vitro bioassays

Opioid peptides containing the sequence of [Leu5]enkephalin were studied in two isolated organ preparations sensitive to opiates, the guinea pig ileum (GPI) and the mouse was deferens (MVD). All peptides tested were able to inhibit the electrically stimulated contraction in both tissues by interacting with specific receptors sensitive to the antagonist naloxone. Some of these peptides, mainly the shorter sequences, showed considerable potency differences in the two systems, suggesting that at least two different types of receptors are involved. Dynorphin-(1-17) displayed the highest agonistic potency in both preparations. In its case, there were no differences in IC50s nor in the shapes of the dose response curves in the two systems, suggesting a common receptor type; however, the reversal of its inhibitory effect following washout of the peptide was much more complete in the MVD than in GPI. Dynorphin B exhibited a higher potency in the GPI. Extension to dynorphin B-29 peptide did not induce changes in the agonistic activity in either system. An increase in one amino acid residue, dynorphin-(1-9) to -(1-10) or beta-neo-endorphin to alpha-neo-endorphin, resulted in a large potency increase in GPI and an opposite effect in MVD. While it has been reported that dynorphin interacts with the kappa opiate receptor in both mouse vas deferens and guinea pig ileum, our results suggest that the observable differences in the kinetics of the interaction in these systems could be due to the presence of different receptor types in these tissues.

Selected opioids, ethanol and intake of ethanol

Rats were given an opportunity to drink tap water or a sweetened ethanol solution once a day. Across initial days of opportunity, rats increased their intake of the ethanol solution. Prior to some days' sessions with presented fluids, rats received either an injection of placebo (the carrier of drugs) or doses of ethylketocyclozocine, diprenorphine, or ethanol. Diprenorphine increased rats' intake of the ethanol solution compared to placebo. The other agents did not reliably modify intakes. These findings support a conclusion that selected activity in opioid systems of brain increase the propensity to drink alcoholic beverages.

Morphine and naloxone modulate intake of ethanol

Rats, deprived of food and water for 18 hr, were given an opportunity to drink water and sweetened ethanol solution for 1 hr prior to being fed and watered for 5 hr, daily. One group received water and sucrose solution without ethanol and other groups received water and sucrose solution with 3, 6, 12 or 24% ethanol. Prior to some days' opportunities to drink, rats were injected with morphine (2.5 mg/kg), naloxone (10 mg/kg), or saline. Morphine increased intake of solutions containing ethanol as compared to intake under placebo. Naloxone reduced intakes of both fluids. Since morphine only increased sucrose solution intake when it contained ethanol, it was concluded that increments in opioid activity increase rats' avidity for ethanol.

The pharmacology of molluscan neurons

It is commonly accepted that the basic physiological properties of the neurons as well as the nature of transmitter substances have remained relatively unchanged through evolution, while brain size and neuron number have greatly increased. Among invertebrates the molluscs, due to the large size of their neurons and lesser complexity of the neural networks controlling specific behavior, have proved to be especially useful for studying elementary properties of single neurons, network organization as well as various forms of learning and memory. The study of putative neurotransmitters has indicated that molluscs use the same low molecular-weight substances and peptides or their metabolites and cyclic nucleotides as transmitters and second messengers as the other species of various phyla. At the same time the receptors of neurotransmitters were found to have certain characteristic properties in the molluscs. The large molluscan neurons have permitted the isolation of individual identifiable nerve cells, and the subsequent analysis of quantities of the transmitters and their metabolic enzymes. These studies have demonstrated that single neurons frequently can contain more than one putative neurotransmitter. It can be expected that this model will contribute to an understanding of the role of multiple transmitters within a single neuron assuring the plasticity of the nervous system. The cellular mechanisms of plasticity have been demonstrated first in molluscan nervous systems. It was proved in identified Aplysia neurons that the same transmitter (ACh) can be released from an interneuron onto two or more follower neurons and can excite one and inhibit another or evoke a biphasic response on a third type of cell. The biphasic response of the molluscan neurons to neurotransmitters was the first demonstration of the plastic synaptic changes. The discovery of individual neurons with their groups of follower cells acting as chemical units has provided an insight into the organization of various behavioral acts. Study of the gastropod molluscs has also shown that the giant serotonergic cells can act as peripheral modulator neurons, as well as interneurons, and in this way they can affect their target organs at more than one level. The molluscan studies have provided more information on transmitter receptors as it was shown that molluscan neurons have at least six different 5HT receptors, three Ach receptors which can be separated pharmacologically. This type of study has led to the discovery of numerous new antagonists and poisons.(ABSTRACT TRUNCATED AT 400 WORDS)

Nerve elements containing Met-enkephalin-Arg6-Gly7-Leu8-like immunoreactivity in canine pancreas - a histochemical study

Canine pancreas was examined by Sternberger's peroxidase-antiperoxidase method using an antiserum against Met-enkephalin-Arg6-Gly7-Leu8 (Met-Enk-Arg-Gly-Leu), an opioid octapeptide included in preproenkephalin A. Immunoreactive Met-Enk-Arg-Gly-Leu-containing nerve terminals with beaded appearance were demonstrated to be juxtaposed to small blood vessels of the exocrine and endocrine pancreas. Eighty-five out of 174 ganglion cell somas in the pancreas were immunopositive. The specific immunostaining was absorbed by preincubation of the antiserum (dilution, 1:2000) with Met-Enk-Arg-Gly-Leu (50 micrograms/ml), but not with Leu-enkephalin and Met-enkephalin (100 micrograms/ml).

Relative contents and concomitant release of prodynorphin/neoendorphin-derived peptides in rat hippocampus

The contents and molecular forms of five different prodynorphin-derived opioid peptides were compared in extracts of rat hippocampus by radioimmunoassay after C18-HPLC resolution. Dynorphin (Dyn) A(1-17) immunoreactivity (ir) and Dyn B-ir were heterogeneous in form; Dyn A(1-8)-ir, alpha-neoendorphin (alpha neo)-ir and beta-neoendorphin (beta neo)-ir each eluted as single homogeneous peaks of immunoreactivity. The fraction of immunoreactivity having the same retention as the appropriate synthetic standard was used to estimate the actual hippocampal content of each peptide. Comparison of these values showed that the concentrations of Dyn B, alpha neo, and Dyn A(1-8) were nearly equal, whereas both Dyn A(1-17) and beta neo were 1/5th to 1/10th the value of the other three. Calcium-dependent K+-stimulated release of these prodynorphin-derived opioids from hippocampal slices was detected. The stimulated rates of release were highest for Dyn B-ir followed by alpha neo-ir, then beta neo-ir and Dyn A(1-8)-ir with Dyn A(1-17)-ir lowest. The relative rates of stimulated release were in agreement with the relative proportions of peptide present within the tissue. This evidence of the presence and release of these opioid peptides considerably strengthens the hypothesis that this family of endogenous opioids plays a neurotransmitter role in the hippocampus.

Further evidence for an opioid receptor complex

We recently presented evidence that distinct morphine and enkephalin receptors coexist in an opioid receptor complex. These studies used membranes prepared from whole rat brain. In this paper the receptor complex is demonstrated to occur in membranes prepared from rat striatum, cortex, and pooled nonstriatal-noncortical regions of the brain. The observation that morphine masks enkephalin receptors is confirmed using 3H-methionine enkephalin to label the enkephalin receptor. These data further support the hypothesis that populations of morphine and enkephalin receptors coexist in an opioid receptor complex.

Peptide E and other proenkephalin-derived peptides are potent kappa opiate receptor agonists

Various proenkephalin-derived peptides such as peptide E and the bovine adrenal medulla peptides BAM-12P and BAM-22P are potent competitors on mu and kappa binding sites in guinea pig brain sections. Moreover, they are all potent agonists in the rabbit vas deferens, a specific kappa opiate receptor bioassay. As described before, dynorphin and some of its fragments are also potent kappa agonists. Our results suggest that not only prodynorphin-derived peptides could act as endogenous kappa ligands but also some proenkephalin-derived peptides such as peptide E.

Endogenous opiates: 1982

This article is the fifth installment in an annual series of reviews of successive year's research dealing with the endogenous opiate peptides. Due to the continuing massive increase in the number of studies in this field, it has become impossible to continue comprehensive reviews of all aspects of this work. As a result we have decided that beginning this year the coverage will be abbreviated to emphasize non-analgesic and behavioral work. The specific areas discussed include stress, tolerance and dependence, consummatory responses, alcohol consumption, schizophrenia and emotional disorders, learning and memory, cardiovascular responses, respiratory effects, thermoregulatory effects, neurological deficits and other disorders, activity, and other, miscellaneous behaviors. As in previous years, we have attempted a relatively comprehensive review of the subjects covered only for the previous year and have not made an attempt to evaluate their contributions relative to those of past years.

Multiple endorphins in neuronal hybrid cell lines

Neuroblastoma x glioma hybrid cells (NG 108CC15) and tumors derived thereof were examined for dynorphin- and alpha-neoendorphin-like material. The techniques employed for analyses of opiate-like material were the isolated mouse vas deferens bioassay and gel chromatography and high pressure liquid chromatography in combination with radioimmunoassays. Dynorphin- and alpha-neoendorphin-like material was detected in both the hybrid cells and the corresponding tumors. Immunoreactive dynorphin and alpha-neoendorphin was also in NCB 20 hybrid cells and in tumors thereof assayed. In all samples investigated, the amounts of alpha-neoendorphin-like material was higher than that of dynorphin-like material. The results revealed considerable variability in the amount of dynorphin- and alpha-neoendorphin-activity between particular samples, suggesting the need for studies into the responsible mechanisms.

The role of amino-terminal sequence of beta-endorphin and dynorphin in the determination of opiate receptor type selectivity

Previous studies have shown that morphiceptin is a highly selective mu-receptor agonist. Recently we have obtained a more potent and stable analog, Tyr-Pro-NMePhe-D-Pro-NH2 (PL017). This peptide retains mu-receptor selectivity. beta-Endorphin is known to be a potent but non-selective opioid peptide for mu-, delta- and benzomorphan binding sites. Dynorphin is a putative kappa-agonist with significant affinity to mu-, delta- and benzomorphan binding sites in rat brain membranes. To understand the structural requirement for receptor type selectivity the enkephalin sequence of beta-endorphin and dynorphin was replaced by that of morphiceptin analog. Replacing the Met-enkephalin sequence of beta h-endorphin by PL017 yields a peptide highly selective for mu-binding sites. Substituting the Leu-enkephalin sequence of dynorphin-17 produces a peptide [PL017-dynorphin(6-17)] that retains high affinities for mu- and kappa-binding sites and has very low affinities for delta- and benzomorphan binding sites. These results suggest that a morphiceptin sequence at the amino-terminus of large opioid peptides can dictate mu-receptor selectivity. An enkephalin sequence at the amino-terminus of large opioid peptides seems to be required to retain high affinity for delta- and benzomorphan binding sites. The high affinity of PL017-dynorphin(6-17) for kappa-binding sites but not for benzomorphan binding sites suggests that benzomorphan sites of rat brain are not kappa-sites.