Enkephalins and endorphins

Descending antinociceptive pathway from the rostral ventrolateral medulla: a correlative anatomical and physiological study

Microinjections of the excitatory amino acid L-glutamate were made into the rostral ventrolateral medulla (RVLM) of anesthetised cats, to map the sites at which selective stimulation of cell bodies elicited a significant antinociceptive response (> or = 15% inhibition of the increase in L7 ventral root activity reflexly evoked by stimulation of C-fiber afferents). Antinociceptive sites were largely confined to the RVLM subregion ventromedial to the retrofacial nucleus, extending from the caudal pole of the facial nucleus to the level approximately 2.5 mm more caudal. Increases in arterial pressure were also elicited from some sites in the RVLM, but these were mainly lateral to the antinociceptive sites. In a second series of experiments, rhodamine labeled microspheres or cholera toxin B-gold (CTB-gold) were injected into the dorsal horn of the L7 segment. In three of these experiments in which the injection sites were restricted to the dorsal horn, retrogradely labeled cells in the caudal pons and medulla were virtually all within either the nucleus raphe magnus or the RVLM. Furthermore, the labeled cells in the RVLM were virtually confined to a discrete group located just ventromedial to the retrofacial nucleus, i.e. within the antinociceptive region as mapped by glutamate microinjection. The results of the present study indicate that antinociceptive effects are elicited by stimulation of a subregion in the RVLM, which is located medial to the pressor region. Further, the antinociceptive effects may be mediated, at least in part, by cells projecting directly to the dorsal horn in the spinal cord.

Rostrocaudal differences in morphology and neurotransmitter content of cells in the subretrofacial vasomotor nucleus

The rostral ventrolateral medulla (RVLM) contains sympathoexcitatory neurons that exert a powerful control over the sympathetic outflow to the cardiovascular system. In the cat there is a concentration of such neurons (but not neurons subserving other functions) within a narrow longitudinal column in the RVLM termed the subretrofacial (SRF) nucleus. Furthermore, it has been suggested that there are subgroups of cells, located at different rostrocaudal levels of the SRF nucleus, that preferentially or exclusively control different vascular beds (e.g. in the kidney and hindlimb). The aim of this study was to map quantitatively the rostrocaudal distribution within the nucleus of different cell types, defined according to morphological and/or chemical criteria, and to correlate this with the regional vasomotor effects (in hindlimb and kidney) evoked by stimulation of SRF cells at the corresponding rostrocaudal levels. SRF cells were highly heterogeneous with respect to both their morphology and chemical properties. They varied greatly in size (equivalent diameter ranging from 10-40 microns) as well as in shape and orientation. An immunohistochemical examination using the avidin-biotin procedure revealed that many SRF cells (estimated 57% of all SRF cells) were immunoreactive for tyrosine hydroxylase (TH, a marker of catecholamine cells). In addition, there were SRF cells immunoreactive for neuropeptide Y (NPY, 11% of total), enkephalin (ENK, 16% of total), and serotonin (5HT, 10% of total), but not for substance P, galanin or somatostatin. Different cell types, defined according to their morphology and/or chemical properties, were unevenly distributed throughout the nucleus. In the most caudal part of the SRF nucleus, virtually all cells were TH-positive, and the large majority (estimated 80%) were NPY-positive, suggesting that many cells at this level contained both TH and NPY. In contrast, in the most rostral part of the SRF nucleus, only 30% of cells were TH-positive, and no NPY-positive cells were observed. Both 5HT- and ENK-positive cells were found throughout the rostrocaudal extent of the nucleus, but predominantly within its rostral part. Furthermore, TH-positive cells in the rostral SRF nucleus were on average significantly larger (mean equivalent diameter 18-43% greater) than TH/NPY-positive cells in the caudal part of the nucleus, but smaller than 5HT- or ENK-positive cells at the same level. Overall, rostral cells (regardless of their chemical type) were larger than caudal cells within the SRF nucleus (mean equivalent diameter 13-28% greater).(ABSTRACT TRUNCATED AT 400 WORDS)

The binding of opioid peptides to the Mcg light chain dimer: flexible keys and adjustable locks

Enkephalins and beta-casomorphins (opioid peptides) were found to bind in a variety of conformations to a human light chain (Bence-Jones) dimer from a patient (Mcg) with amyloidosis. The peptides were diffused into crystals of the protein and their positions, relative occupancies and modes of binding were determined at 2.7 A resolution by difference Fourier analyses. Collectively, the opioid peptides occupied practically all of the available space in the concave, internal parts of the binding region, as well as flat or convex external surfaces around the rim of the binding cavity. Suitable ligands ranged in size from four to seven residues. As many as five residues could be accommodated inside the binding region, and there was space for at least four residues on the external surfaces. External binding was influenced by solvent effects and local packing interactions among adjacent protein molecules in the crystal lattice. In the enkephalin series the presence of amino-terminal tyrosine was necessary, but not sufficient for binding. [Met]-enkephalin, a pentapeptide, showed two different modes of binding in overlapping subsites. In one subsite, preferred over the second in a ratio of 1.3:1.0, the side chain of amino-terminal tyrosine penetrated through the floor of the main cavity to lodge in the deep binding pocket about 20 A from the entrance. The remainder of the peptide spanned the length of the main cavity in an extended conformation. In the second subsite the amino end was restricted to the main cavity and the peptide backbone turned abruptly upward at residue 3 to interact with external surfaces. An (Arg-6, Phe-7) heptapeptide extension of [Met]-enkephalin entered the deep pocket and assumed an extended conformation in the main cavity like the pentapeptide. Its last two residues flattened against the external surfaces. [Leu]-enkephalin and its analogues displayed a combination of internal and external binding like [Met]-enkephalin in its secondary subsite. Enkephalin analogues with D-amino acids in position 2 generally adopted conformations which were more convoluted than those in the L-isomers. Moreover, external interactions tended to be more prominent in the D-derivatives. The beta-casomorphin-7 heptapeptide penetrated into the deep pocket and traversed the main cavity in as extended a conformation as the presence of two proline residues would permit. On removal of the ligand there was an unexpected hysteresis effect involving permanent structural alterations in the walls of the binding region. beta-casomorphins-4 and -5 were bound in the main cavity with the carboxyl ends protruding from the entrance.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for a central but not adrenal, opioid mediation in hypertension induced by brief isolation in the rat

Naloxone was found to provoke a hypotensive effect related to the dose on high blood pressure (BP) induced by short-term isolation in young rats. Another opiate antagonist, nalorphine, also reduced the arterial pressure of socially deprived rats. In contrast, naltrexone methylbromide that selectively blocked peripheral opiate receptors did not alter the elevated BP. To investigate whether adrenomedullary opioids were somehow implicated in the development of isolation-induced hypertension, bilaterally adrenalectomized rats were kept under social deprivation for 7 consecutive days. The data obtained indicated that high systolic BP developed in the same manner as in intact rats run in parallel. In conclusion, central opioids appear to be involved in BP elevation due to the stress generated by brief social deprivation in young rats.

Reduced mechanical activity of perfused rat heart following morphine or enkephalin peptides administration

In the isolated and perfused rat heart, the addition of morphine, methionine-enkephalin or leucine-enkephalin to the coronary perfusate, significantly reduces the mechanical activity by negatively affecting both the heart rate and the developed tension. These effects are dose dependent and maximally evident with leucine-enkephalin. Furthermore all the opioids strongly reduce the activity of isoproterenol-stimulated hearts. The suggestion is made that opioid peptides directly influence the cardiac mechanical activity possibly by interacting with membrane-receptor systems.

Enkephalin degrading enzymes are present in the electric organ of Torpedo californica

Two proteolytic activities that degrade [Leu5]enkephalin were found in Torpedo californica electric organ. One is a soluble aminopeptidase that degrades enkephalin at the Tyr1-Gly2 peptide bond, and the second is an endopeptidase that degrades enkephalin at the Gly3-Phe4 peptide bond. The aminopeptidase is inhibited by low concentrations of puromycin and bestatin. More than 60% of the endopeptidase is associated with the particulate fraction and is almost completely inhibited by low concentrations of captopril (SQ 14225) or SQ 20881 (potent inhibitors of angiotensin converting enzyme). Thiorphan and phosphoramidon (potent enkephalinase inhibitors) are much less effective. The pattern of cleavage and inhibition of the particulate endopeptidase thus resembles that of angiotensin converting enzyme.

The stinging response of the honeybee: effects of morphine, naloxone and some opioid peptides

Changes in responsiveness for the stinging reaction of honeybees fixed in a holder after receiving 3 electrical shocks delivered with 1 min interval, was registered and used as measurement for the effect of 2 microliter of different solutions injected. Every shock consisted of a train of pulses of 1 msec each, delivered for 2 sec at a frequency of 100 Hz. Injection of morphine-HCl (50 to 200 n-moles/bee) produced a dose dependent reduction of the honeybee stinging response to the electrical shocks. The morphine dose that produced a 50% inhibition of the response (D50) was 148 n-moles/bee (927 micrograms/g), i.e., a value far greater than that reported for vertebrates in behavioral test of analgesia. Naloxone 1.1 micrograms/g produces a significant reduction of morphine D50 effect and at 4-5 micrograms/g, a full disinhibition. Thus, whereas the D50 of morphine for honeybees is far greater than that for vertebrates, the doses of naloxone that antagonize morphine are similar for bees and vertebrates. Possible explanations of this difference are mentioned. Injections of met-enkephalin, leu-enkephalin, kyotorphin and (D-Ala2) methionine-enkephalinamide, given in doses of 200 n-moles/bee, an amount greater than that of the morphine D50, exhibited no effect on the stinging response.

Is schizophrenia an immunologic receptor disorder?

A model is proposed for an autoimmune etiology for schizophrenia. We propose that schizophrenia is a syndrome (not a disease). We suggest that autoantibodies (and/or cell-mediated immunity) directed against autologous neurotransmitter receptors are responsible for the ebb and flow of psychotic symptomatology. The hypothesis is predicated on autoimmune models in other known receptor diseases as well as on the newly emerging recognition that general immune dysfunction exists in certain schizophreniform psychoses.

Protection of enkephalins from enzymatic degradation utilizing selective metal-chelating inhibitors

Metal ion-chelating agents inhibited enkephalin degradation by a rat striatal membrane-associated endopeptidase termed 'enkephalinase'. The combination of a hydrophobic dipeptidyl moiety and a transition metal-chelating moiety in the same molecule resulted in very efficient and selective inhibitors of enkephalinase. The mercaptoacetyl dipeptides (2-mercaptoacetyl-Leu-Phe and 2-mercaptoacetyl-Phe-Leu) and the N-phosphorylated dipeptides (phosphoryl-Leu-Phe and phosphoramidon) inhibited enkephalinase with IC50 values of 15, 70, 0.3 and 1 nM respectively, but were much less potent against the aminopeptidase and angiotensin converting enzyme, two other metalloenzymes implicated in the degradation of the enkephalins in brain. The inhibition of enkephalinase, using phosphoryl-Leu-Phe as a selective inhibitor, resulted in a 4 fold increase in the amount of enkephalin recovered following K+ depolarization of rat striatal slices.

Immunocytochemical identification of alpha-endorphin-like material in neurones of the brain and corpus cardiacum of the blowfly, Calliphora vomitoria (Diptera)

A group of the 24-26 paraldehyde fuchsin-positive median neurosecretory cells (MNC) in the pars intercerebralis of the brain of the blowfly, Calliphora vomitoria, has shown immunoreactivity towards three different antibodies to alpha-endorphin, a peptide that corresponds to the amino acid sequence present between residues 61 and 76 of the precursor molecule, beta-lipotropin (beta-LPH). The immunoreactive material could be followed in axons within the median bundle, the tract through which neurosecretory material from the MNC is passed down to the corpus cardiacum (CC). The alpha-endorphin-immunoreactive material was observed leaving the CC in the cardiac-recurrent nerve, dorsal to the proventriculus, in the direction of the abdomen. The cells that contain the alpha-endorphin-like material are different from those of the MNC that contain insulin-, pancreatic polypeptide-, and gastrin/CCK-like peptides. This finding demonstrates the considerable complexity and peptidergic nature of the MNC and constitutes further evidence that morphinomimetic-like peptides are present in the nervous system of invertebrates.

Characterization and location of Met5-enkephalin-arg6-phe7 stored in various rat brain regions

A specific antiserum against met5-enkephalin-arg6-phe7 was raised and used to study the distribution and characterization of met5-enkephalin-arg6-phe7-like immunoreactive material in rat brains by radioimmunoassay and immunohistochemical procedures. The antiserum appears to be directed to the COOH-terminus of the peptide, as it fails to cross-react with met5-enkephalin, met5-enkephalin-arg6, met5-enkephalin-arg6-arg7, met6-enkephalin-lys6, and leu-enkephalin. However, it cross-reacts with phe-met-arg-phe by about 10% and with phe-met-arg-phe-NH2 to an insignificant degree. The highest content of met5-enkephalin-arg6-phe7 was found in the striatum, which contains a dense network of immunoreactive varicose fibers and terminals, as well as immunoreactive cell bodies. The met5-enkephalin-arg6-phe7 in striatum can be released in a Ca2+-dependent manner by a depolarizing concentration of KCl, raising the possibility of a neuroregulatory role for met5-enkephalin-arg6-phe7. Characterization of the immunoreactive material by gel filtration and high pressure liquid chromatography revealed the presence of multiple forms of immunoreactive material in some brain regions.

Mechanism of the cardiovascular response to systemic intravenous administration of leucine-enkephalin in the conscious dog

Leucine-enkephalin (Leu5-ENK) (35 micrograms/kg) increased heart rate and mean systemic arterial blood pressure following intravenous injection into chronically-instrumented, conscious dogs. Repeated injections at five-minute intervals were not associated with a diminished response. Naloxone (1 mg/kg) pre-treatment inhibited both heart rate and blood pressure increases. Prazosin (1 mg/kg) attenuated the increase in blood pressure but did not influence the heart rate response. Propranolol (1 mg/kg) attenuated the heart rate response but not the pressor response. Clonidine (30 micrograms/kg) attenuated the positive chronotropic effect of Leu5-ENK. Atropine (1 mg/kg) plus propranolol (1 mg/kg) blocked the heart rate response but the pressor effect was still present. The attenuation of the heart rate response by propranolol and the pressor response by prazosin suggests an adrenergic component to the enkephalin response; the reduction in the heart rate response by clonidine and atropine-propranolol indicates a role for cholinergic mechanisms in the chronotropic response. Hexamethonium (10 mg/kg) blocked the heart rate response and markedly inhibited the pressor response. Vagal interruption attenuated both heart rate and blood pressure responses. It is concluded that intravenous Leu5-ENK stimulates afferent pathways located in fibers which are contained in the vagosympathetic trunk to reflexly increase heart rate and blood pressure.

Postnatal change in receptivity for methionine5-enkephalin in rat duodenum: transition from neurogenic to myogenic receptivity

The study concerned the postnatal ontogenesis of the response to methionine5-enkephalin (ENK) in the duodenum of developing rats. Sustained relaxation was produced by ENK in the isolated duodenum of 8 day old rats. The response was blocked by tetrodotoxin or naloxone. The response increased by day 18, decreased thereafter and was extinguished after 40 days. On the other hand, a tetrodotoxin resistant response to ENK, transient relaxation, appeared at day 20 and was augmented thereafter. The development of neurogenic receptivity for ENK preceded the appearance of myogenic receptivity in the rat duodenum.

Leucine-enkephalin: reversal of intrinsic cardiovascular stimulation by pentobarbital

In the conscious, chronically instrumented dog, leucine-enkephalin ([Leu5]ENK), 35 micrograms/kg injected intravenously, increased heart rate, respiratory rate, systolic and diastolic systemic arterial pressures, mean pulmonary arterial pressure, and cardiac output. After pentobarbital-induced anesthesia, the same dose of [Leu5]ENK decreased heart rate, systemic and pulmonary arterial pressures, and cardiac output. Responses both before and after pentobarbital were blocked by naloxone. These results indicate that barbiturate anesthesia can reverse the cardiovascular stimulatory activity of intravenously administered [Leu5]ENK.

Synthesis and biological properties of enzyme-resistant analogues of substance P

Six analogues of substance P were synthesized with the aim of developing a metabolically stable peptide that would retain the biological activity of substance P. A recently isolated and characterized substance-P-degrading enzyme from human brain with a high specificity for substance P described in the preceding paper in this journal was used as a model for the enzymatic inactivation of substance P. The synthetic analogues were designed to protect the peptide bonds on the carboxyl side of residues 6, 7 and 8 of substance P, which represent the sites of cleavage by substance-P-degrading enzyme. To test for increased enzymatic resistance, the analogues were incubated with the enzyme, the digests were separated on a high-performance liquid chromatography reverse-phase column and the peptide fragments were collected and identified by amino acid analysis. Of the analogues described, an heptapeptide analogue of residues 5-11, less than Glu-Gln-Phe-MePhe-MeGly-Leu-MetNH2, showed almost complete resistance both towards substance-P-degrading enzyme and to degradation on exposure to rat hypothalamic slices. This analogue was about a third as potent as substance P in competing for binding to receptor sites for this peptide in rat brain membranes and a tenth as potent in eliciting contractions of the guinea pig ileum. The peptides were synthesized using the solid-phase technique with polydimethylacrylamide as a solid support and the coupling was achieved with pre-formed symmetrical anhydrides in dimethylacetamide. Fluorenylmethyloxycarbonyl was used as an alpha-amino protecting group in conjunction with t-butyloxycarbonyl as an epsilon-amino protecting group. Ammoniolytic cleavage from the resin was followed by stepwise elution from an SP-Sephadex column, deprotection with trifluoroacetic acid and chromatography on a Bio-Rex 70 ion-exchanger. The peptides were finally purified on a semi-preparative reverse-phase column.

Cardiopulmonary pharmacology of enkephalins in the conscious dog

Various enkephalins (ENK) have been reported to alter heart rate, systemic blood pressure, and respiratory function but with variable results. We previously found that pentobarbital anesthesia reversed the increases in heart rate and systemic arterial blood pressure produced by Leu5-ENK in the dog. We have now studied the cardiopulmonary responses to systemically injected enkephalins in the awake, chronically instrumented dog. In these unanesthetized dogs, Leu5-ENK, Met5-ENK, and an ENK dimer increased heart rate, systemic arterial pressure, and respiratory rate in a dose-dependent fashion. D-Phe4, Met5-ENK, an analog with behavioral activity but without analgesic or in vitro opiate effects was inactive. Leu5-ENK was studied in detail; typically, heart rate and systemic pressure increased within 20 sec and returned to baseline in less than 180 sec. Intra-arterial injections produced a smaller increase in heart rate than did intravenous injections, but similar increases in systemic arterial blood pressure were recorded. These cardiovascular responses suggest that systemically injected enkephalins suppress baroreceptor reflexes and may have a role in cardiopulmonary regulation.

Premenstrual syndrome

The premenstrual syndrome (PMS) is a major clinical entity afflicting a large segment of the female population. Available information are descriptive in nature and the etiology of this syndrome remains unclear. In this review, both biochemical and psychosocial elements of the syndrome have been explored in an effort to redefine the pathophysiology of this seemingly multifactorial psychoneuroendocrine dysfunction. We propose that luteal phase sensitivity to and subsequent withdrawal from the central effects of the neuropeptides beta-endorphin and alpha-melanocyte-stimulating hormone result in a cascade of neuroendocrine changes within the brain-hypothalamus-pituitary complex. Modulation of neurotransmitter function by these peptides may produce alterations in mood and behavior as well as enhance pituitary release of prolactin and vasopressin. Variable gonadal steroid modulation of these responses from subject to subject likely accounts for the heterogeneous clinical manifestations of the PMS.

Synthesis, properties, and reactions of alpha- and beta-D-glucopyranosyl esters of some tripeptides

The 2,3,4,6-tetra-O-benzyl-1-O-(N-benzyloxycarbonyltripeptidyl)-D-glucopyranoses ), 8, and 13 were synthesised from 2,3,4,6-tetra-O-benzyl-alpha-D-glucopyranose and the active esters of the appropriate N-protected tripeptides (Gly-Cly-Gly-, L-Phe-Gly-Gly-, and Gly-Gly-L-Phe-) in the presence of imidazole; the anomeric mixtures were resolved and the alpha and beta anomers characterised. The beta anomer of 13, containing the L and D enantiomers (ratio approximately 3:1) of Gly-Gly-Phe- as the aglycon, could be resolved by column chromatography into the pure isomeric forms. Catalytic hydrogenolysis of the beta anomers, in the presence and absence of a strong acid, yielded the free 1-esters 2 beta, 9 beta, and 14 beta, which were characterised as the monoxalate or trifluoroacetate salts and as free bases. Similarly, the alpha anomers afforded 2 alpha, 9 alpha, and 14 alpha, whereas omission of the strong acid led to accompanying 1 leads to 2 acyl migration, to give the 2-O-acyl derivatives. All of the compounds prepared were converted into the N-acetyl and/or peracetylated derivatives. The 1-esters 2 beta and 9 beta, both in the charged and uncharged form, and the trifluoroacetate salt of 14 beta, are susceptible to cleavage by beta-D-glucosidase; the enzyme had no effect on the uncharged form of 14 beta. This difference between 14 beta and its salt is discussed in conformational terms.

Control of guinea pig intestinal electrolyte secretion by a delta-opiate receptor

The effects of opioids on transepithelial potential difference and short-circuit current across guinea pig ileum stripped of one muscle layer were measured in vitro in Ussing chambers. Opioid peptides such as [DAla2, DLeu5]enkephalin and [DAla2, DMet5]enkephalin, which are primarily agonists at delta-opiate receptors, were able to reduce transepithelial potential difference and short-circuit current at concentrations as low as 1 nM. The narcotic drug etorphine was also very potent in reducing short-circuit current, but fentanyl and morphine, which are primarily agonists at mu-opiate receptors, were almost completely ineffective. Ketocyclazocine was relatively ineffective, and beta-endorphin had intermediate potency. All opioid effects could be reversed by the opiate antagonist naloxone. Somatostatin also reduced short-circuit current, but its effect was not reduced by naloxone. Chloride flux measurements indicated that the effect of etorphine on short-circuit current is associated with an enhancement of active Cl- absorption. The relative effects of opioids in this system suggest that their actions are being mediated by a specific delta-opiate receptor. In contrast, opioid effects on guinea pig intestinal smooth muscle seem to be primarily mediated by a mu-opiate receptor.

Electron-microscopic localization of enkephalin-like imnmunoreactivity in axon terminals of human sympathetic ganglia

The peroxidase-antiperoxidase method was applied to demonstrate enkephalin-like immunoreactivity in the human sympathetic ganglia. A network of immunoreactive fibres was found in light microscopy between the nonreactive neurons. Electron microscopy showed immunostaining to be restricted to axonal profiles containing both large and small vesicles. The strongest reaction was associated with large vesicles.