Increased analgesic activities of a fluorinated and a dimeric analogue of [D-Ala-2]-methionine enkephalinamide

Preparation of bivalent agonists for targeting the mu opioid and cannabinoid receptors

In order to obtain novel pharmacological tools and to investigate a multitargeting analgesic strategy, the CB₁ and CB₂ cannabinoid receptor agonist JWH-018 was conjugated with the opiate analgesic oxycodone or with an enkephalin related tetrapeptide. The opioid and cannabinoid pharmacophores were coupled via spacers of different length and chemical structure. In vitro radioligand binding experiments confirmed that the resulting bivalent compounds bound both to the opioid and to the cannabinoid receptors with moderate to high affinity. The highest affinity bivalent derivatives 11 and 19 exhibited agonist properties in [³⁵S]GTPγS binding assays. These compounds activated MOR and CB (11 mainly CB₂, whereas 19 mainly CB₁) receptor-mediated signaling, as it was revealed by experiments using receptor specific antagonists. In rats both 11 and 19 exhibited antiallodynic effect similar to the parent drugs in 20 μg dose at spinal level. These results support the strategy of multitargeting G-protein coupled receptors to develop lead compounds with antinociceptive properties.

Peptides at the blood brain barrier: Knowing me knowing you

When the Davis Lab was first asked to contribute to this special edition of Peptides to celebrate the career and influence of Abba Kastin on peptide research, it felt like a daunting task. It is difficult to really understand and appreciate the influence that Abba has had, not only on a generation of peptide researchers, but also on the field of blood brain barrier (BBB) research, unless you lived it as we did. When we look back at our careers and those of our former students, one can truly see that several of Abba's papers played an influential role in the development of our personal research programs.

Development of a potent bombesin receptor antagonist with prolonged in vivo inhibitory activity on bombesin-stimulated amylase and protein release in the rat

Of the various types of potent bombesin(Bn)/gastrin releasing peptide receptor antagonists that have been discovered, the desMet14-methyl ester peptides are devoid of residual agonist activity and are among the most potent in terms of in vitro receptor blockade and also in terms of their prolonged inhibition of bombesin-stimulated amylase and protein release in the rat. We have now examined the in vitro and in vivo properties of a new series of methyl ester analogues, [D-Phe6]Bn(6-13)OMe, [D-Phe6,D-Ala11]Bn(6-13)OMe, N alpha-propionyl-[D-Ala24]GRP(20-26)OMe, and [D-pentafluoro-Phe6,D-Ala11]Bn(6-13)OMe, which have an additional D-amino acid substituent and some highly lipophilic moieties at the N-terminus. All analogues were able to potently antagonize the ability of Bn to stimulate amylase release from rat acinar cells, with IC50 values of 2.4, 2.5, 0.6, and 1.3 nM, respectively. The four peptides were found to have binding affinities for these cells comparable to Bn itself, with K(i)s of 10.3, 2.8, 5.5, and 3.6 nM, respectively, but all had little or no affinity for neuromedin B receptors on murine C6 cells. Single bolus IV injections of these peptides were found to potently inhibit amylase and protein release caused by IV infusion of bombesin into the rat. Generally the peptides containing the D-Ala substituent were longer acting than [D-Phe6]Bn(6-13)OMe, so that [D-Phe6,D-Ala11]Bn(6-13)OMe and N alpha-propionyl-[D-Ala24]GRP(20-26)OMe displayed significant inhibitory effects for up to 1.5 h after administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioid receptors and their pharmacological profiles

Opioid receptors can be divided into three major classes, which are called mu, delta and kappa-receptors. The molecular basis of the receptors is discussed and a hypothesis of the binding of bivalent ligands to the receptor is presented. Furthermore the mechanism of action, the distribution and the probable function of these classes is overviewed. Increasing evidence is accumulating that the classical binding model cannot explain completely the interaction of opioids with their receptors. In addition to the mu-receptors, high affinity mu 1 binding sites have been demonstrated. Similarly, the delta receptors may be divided in delta 1 and delta 2. The significance of these subclasses is not yet fully understood. The high affinity mu 1-binding sites, however, represent probably an activated receptor complex, e.g. the complex between the receptor and the guanine-nucleotide-binding protein.

Binding to opioid receptors of enkephalin derivatives taking alpha-helical conformation and its dimer

The opioid receptor binding of [Leu]enkephalin derivatives with extended address segment to the C-terminal was studied. The extension peptide is designed to take an amphiphilic helical structure in order to evaluate effects of helical conformation and membrane affinity of enkephalin moiety of the derivatives on receptor binding. In the delta-receptor-selective binding assay, Tyr-Gly-Gly-Phe-Leu-Lys-Aib-Leu-Aib-OH (1) showed the same affinity as enkephalinamide, whereas in the mu-receptor-selective binding assay, a 7-fold reduction in affinity was observed. On the other hand, Tyr-Gly-Gly-Phe-Leu-(Lys-Aib-Leu-Aib)2-OH (2) showed 51- and 96-fold decreases in affinities for delta- and mu-receptors, respectively, compared with enkephalinamide. The low receptor affinity of derivative 2 is considered due to alpha-helical conformation, which might not be compatible with topological requirements of delta- and mu-receptors. A dimer, Tyr-Gly-Gly-Leu-Phe-(Lys-Aib-Leu-Aib)2-Lys(X)-Aib-OCH3 (X = Tyr-Gly-Gly-Phe-Leu-, (4], showed 2.5- and 3.0-fold increases in affinities respectively for delta- and mu-receptors compared with the monovalent derivative 2, possibly due to cross-linking of neighboring receptors. The Hill plot of the binding of the dimer to bovine brain membranes was composed of two phases, although such a heterogeneity of receptors was not observed in the presence of naloxone or in the binding to NG108-15 cell membranes. These findings indicate the presence of the bivalent-ligand-induced interactions between delta- and mu-receptors in bovine brain membranes.

Effect of covalent dimer conjugates of angiotensin II on receptor affinity and activity in vitro

Angiotensin II [1-8 or 2-8] analogues and [4-8] fragments were dimerized through the amino- or carboxy-terminal groups in order to try to increase their potency as reported for other hormones. The binding affinity to the angiotensin II receptor subtypes A (A IIA) and B (A IIB) was tested and compared to the potency in rabbit aortic ring. The [2-8] dimers coupled through the N-terminus show no significant change in potency in aortic ring. The [4-8] fragments coupled through the N-terminus are inactive in the ring. They have however a significantly increased affinity for the A IIA receptor, the specific function of which has not yet been reported. When angiotensin II analogues or fragments are coupled through the C-terminus, there was a significant drop in affinity and potency, confirming the importance of the free carboxyl group in position 8 for binding and activity. It is concluded that binding to the A IIB receptor correlates well with the effectiveness in aortic ring. However, in contrast to the beneficial effect reported for a large number of other hormones, dimerization of angiotensin II or its fragments is not accompanied by an increased biological activity in aortic ring.

Potentiation of pentazocine analgesia by low-dose naloxone

The analgesia produced by combinations of low-dose naloxone with pentazocine or morphine was studied in 105 patients with moderately severe postoperative pain after standardized surgery for removal of impacted third molars. Pain intensity was quantified using a visual-analogue scale. To eliminate the release of endogenous opioids produced by the placebo component of open drug administration, all injections were made by a preprogrammed infusion pump. The analgesia produced by pentazocine, an agonist-antagonist opiate-analgesic acting predominantly at the kappa opiate receptor, was potentiated by low-dose naloxone, whereas the analgesia produced by morphine, a mu-agonist, was attenuated by low-dose naloxone. To evaluate whether similar potentiation would be present in an animal model, and specifically, in the absence of diazepam, which patients receive, we performed an analogous experiment in rats in which nociceptive threshold was determined using the Randall-Selitto paw-withdrawal test. The results were completely analogous to the clinical results: pentazocine analgesia was potentiated by low-dose naloxone, whereas morphine analgesia was attenuated by low-dose naloxone. These data demonstrate a novel interaction between opiates, and suggest a rationale for opiate combinations to produce potent analgesia with fewer autonomic side effects and less abuse potential than presently available analgesics.

[D-Ala2, (F5) Phe4]-dynorphin 1-13-NH2 (DAFPHEDYN): a potent analog of dynorphin 1-13

Intracerebroventricular administration of the dynorphin analog, [D-Ala2,(F5)Phe4]-dynorphin 1-13-NH2 (DAFPHEDYN) in rats produced diuresis and profound analgesia. Both effects were antagonized by central administration of naltrexone or naloxone. Intravenous administration of 10, 25, and 50 mg/kg of DAFPHEDYN failed to induce diuresis. The increased potency of DAFPHEDYN was apparent from the failure of an equal dose of the parent compound (dynorphin 1-13) to produce diuresis and the failure of [D-Ala2]-dynorphin 1-13-NH2 to produce analgesia. Radioligand binding studies indicated the DAFPHEDYN retains the same degree of kappa selectivity as the parent compound (dynorphin 1-13) though a drop in affinity occurred. DAFPHEDYN may be of significant interest because it retains the essential pharmacology of the parent compound and exhibits marked in vivo potency.

The degradation of dynorphin A in brain tissue in vivo and in vitro

The demonstration of analgesia following in vivo administration of dynorphin A (Dyn A) has been difficult. In contrast, a number of electrophysiological and behavioral effects reported with in vivo injection of Dyn A can be produced by des-tyrosine dynorphin A (Dyn A 2-17). This suggested the extremely rapid amino terminal degradation of dynorphin A. To test this hypothesis, we examined the degradation of dynorphin A following in vivo injection into the periaqueductal gray (PAG) as well as in vitro using rat brain membranes under receptor binding conditions. In vivo, we observed the rapid amino terminal cleavage of tyrosine to yield the relatively more stable destyrosine dynorphin A. This same cleavage after tyrosine was observed in vitro. Inhibition of this aminopeptidase activity in vitro was observed by the addition of dynorphin A 2-17 or dynorphin A 7-17 but not after the addition of dynorphin A 1-13, dynorphin A 1-8, dynorphin B or alpha-neo-endorphin suggesting a specific enzyme may be responsible. The detection of the behaviorally active des-tyrosine dynorphin A following in vivo injection of dynorphin A suggests that this peptide may play an important physiological role.

Two beta-adrenergic pharmacophores on the same molecule. A set of agonist-antagonist combinations

A series of compounds containing combinations of one or two pharmacophores of the agonist type (isoproterenol) or the antagonist type (propranolol or alprenolol) on the same molecule were prepared. The pharmacophores were connected by a derivative of polyethylene glycol with an average length of six atoms (carbon and oxygen). Furthermore, compounds containing two alprenolol residues, separated by chains of average lengths of 70 or 145 atoms, were synthesized. The abilities of these compounds to interact with beta-adrenoceptors of rat heart and lung tissues were examined by measuring the following parameters: competitive binding with [3H]dihydroalprenolol, activation of adenylate cyclase, and inhibition of isoproterenol-stimulated adenylate cyclase. The affinity of the compound with two isoproterenol pharmacophores for receptor was about the same as that with one isoproterenol pharmacophore and between 30 and 200 times weaker than that of (+/-)isoproterenol. Both mono- and bis-pharmacophore compounds partially stimulated catecholamine sensitive adenylate cyclase and at high concentrations inhibited the stimulation produced by (-)isoproterenol. The affinity of the compound with antagonist (propranolol) and agonist (isoproterenol) pharmacophores on the same molecule was intermediate between that of propranolol and isoproterenol. The compound was only able to inhibit adenylate cyclase activity. Compounds containing two antagonist (alprenolol) pharmacophores bound to receptors with affinities from an order of magnitude lower to about equal to that of the compound containing one pharmacophore. When membranes were preincubated with compounds containing two antagonist pharmacophores and then washed extensively, there were persistent effects of all of these compounds on the binding constants of [3H]dihydroalprenolol. All of these compounds were only able to inhibit adenylate cyclase activity and none exhibited any subtype selectivity at beta-adrenoceptors. The results suggest that, in the beta-adrenergic system, compounds with agonist and antagonist substituents on the same molecule exhibit properties of the substituent with the higher affinity for beta-adrenoceptor, and no agonist activity is evident when two antagonist pharmacophores are linked on the same molecule. All of the above results may be explained without recourse to cross-linking of beta-adrenoceptors with two pharmacophores, a phenomenon cited in similar studies of receptors for opiates and gonadotropin-releasing hormone.

A novel side-chain-linked antiparallel cyclic dimer of enkephalin

The dimeric cyclic enkephalin analog, (H-Tyr-D-Lys-Gly-Phe-Glu-NH2)2, was isolated as a second major component from the crude product obtained in a solid-phase synthesis of the corresponding cyclic monomer, H-Tyr-D-Lys-Gly-Phe-Glu-NH2. In comparison with [Leu5]enkephalin the cyclic dimer is about equipotent in assays representative for mu-opioid receptor interactions and 1/10 as potent at the delta-receptor. The fact that the enkephalin dimer shows a receptor selectivity pattern distinct from that of the cyclic monomer and of the corresponding linear analog suggests that cyclodimerization via side-chain linkages might be generally useful as a means to produce shifts in the activity profiles of peptide hormones and neurotransmitters.

Increased affinity and selectivity of enkephalin tripeptide (Tyr-D-Ala-Gly) dimers

The binding of alkylendiamide dimers of the three N-terminal residues of [D-Ala2,D-Leu5]enkephalin (DADL) to rat brain and Ng108-15 neuroblastoma-glioma cell membranes was compared with that of DADL, Tyr-D-Ala-Gly-NMe-Phe-Gly-ol (DAGO) and morphiceptin. Tritiated DADL and DAGO were used as labeled ligands for delta- and mu-receptors, respectively. Dimerization of the tripeptides resulted in dramatic increases in both mu and delta binding. The binding to mu-receptors showed two peaks at an alkyl chain length of n = 2 and approximately n = 16. In contrast, delta binding (NG108-15 cells) increased steadily with increasing chain length. The dimers with n less than 18 were mu-preferential, and the one with n = 2 showed the most dramatic increase in mu selectivity with a 400 fold higher affinity to mu- than to delta-receptors. For long-chain alkyl spacers the compounds became delta selective.

Increased affinity of dimeric enkephalins is not dependent on receptor density

The equilibrium binding and dissociation kinetics of the enkephalin dimer bis-(D-Ala2-D-Leu5-enkephalin)-ethylenediamide (designated DPE2) to neuroblastoma glioma NG108-15 cells were investigated and compared with the monomers D-Ala2-D-Leu5-enkephalin (DADL) and D-Ala2-Leu5-enkephalinamide (DALEA). Binding was studied after exposure of the membrane to increasing concentrations of the irreversible delta receptor selective ligand FIT in order to decrease the density of binding sites on the cell membrane. The increased affinity of DPE2 did not revert to that of the monomer DADL by this reduction of binding sites. Similarly, the dissociation of DPE2 did not approach that of the monomer DALEA in the presence of 1 microM DALEA. These data strongly suggest that crosslinking does not occur, and fail to confirm the hypothesis that dimers with short spanning chain length aid the clustering of receptors. We postulate: 1) If the dimer binds to a bivalent binding site, the monovalent binding state of our bivalent ligand may not exist to an appreciable extent, and 2) the bivalent ligand cannot bind when the binding site is irreversibly blocked by a monovalent ligand.

Receptor binding and biological activity of bivalent enkephalins

Two series of dimeric enkephalin analogues were assayed for opioid activity in two isolated smooth muscle preparations: the guinea pig ileum (GPI) and the mouse vas deferens (MVD). Dimers have the general structure: X-(CH2)n-X, where X is H-Tyr-D-Ala-Gly-Phe-Leu-NH-(n = 0, 2, 4, 6, 8, 10, 12), for the first series of dimeric pentapeptide enkephalins (DPEn), and H-Tyr-D-Ala-Gly-Phe-NH-(n = 2, 4, 6, 8, 12), for the series of dimeric tetrapeptide enkephalins (DTEn). Comparison of biological activities with binding affinities revealed that: (1) the DPE series with n = 2-8 showed increased potency in the MVD assay relative to monomeric [D-Ala2, Leu5]enkephalinamide (DALEA); (2) there was an associated increase affinity for the delta receptor of rat brain or neuroblastoma-glioma hybrid cells. (however, the relative potencies were higher in the MVD assay then predicted on the basis of binding affinities); (3) the DTE series also showed an increase in delta receptor affinities and MVD potencies relative to DALEA, for n = 2-12; (4) for the DTE series, the increase in MVD activities was less than that expected on the basis of delta binding affinity; (5) for both the DPE and DTE series, activities in the GPI assay and mu-receptor affinities were highly correlated: as the length of the methylene bridge increased from 2 to 12, there was a progressive loss of activity in both assays, with a similar pattern for DPE and DTE. Two selected dimers and their corresponding monomers were also assayed for antinociceptive activity in vivo: results were consistent with GPI and mu-binding but not with MVD and delta-binding. Two alkylamide analogs of penta- and tetrapeptide monomers, representing the monomer with the attached spacer of the most active dimers, were also assayed in biological and binding assays. Comparison of these compounds with the corresponding dimers suggest that the changes in activities and selectivities induced by dimerization are not a spurious effect of the presence of an akylamide derivative of the carboxy terminal of enkephalin but rather may represent a specific effect due to the bivalent nature of the ligands.

Enkephalin analogs: synthesis and properties of analogs with lipophilic or extended carboxyl-terminus. Quantitative structure-activity relationship of analogs modified in residue position 5

Four analogs of enkephalin (EK) have been synthesized by the solid-phase method and their biological activities have also been investigated. All four analogs were less active than Met-enkephalin (Met-EK) as shown by relative potencies in the guinea pig ileum (GPI) assay: Met-EK, 100; [Phe5]-EK-NH2, 59; [Trp5]-EK-NH2, 11; Met-EK-Cys(Cam)-OH, 37; and N,N'-bis(Met-EK)-cystine, 34. Two of the analogs were more potent than Met-EK as shown by relative potencies in the mouse tail-flick assay for analgesia: Met-EK, 100; [Phe5]-EK-NH2, 1340; [Trp5]-EK-NH2, 1640. Quantitative structure-activity relationship calculations were carried out for GPI potencies of analogs substituted in position 5. The calculation indicated that, in this position, the bulkiness had the main influence.

Differential penetration of DSIP peptides into rat brain

Delta sleep-inducing peptide (DSIP) or five closely related peptides were injected peripherally and the levels of DSIP-like immunoreactivity measured in the brains of 100 g and 500 g rats decapitated a minute later after washout with 0.9% NaCl. Higher concentrations of immunoreactive material were found in brain tissue with injection of des Trp1-DSIP, D-Ala4-DSIP, and, after correction for cross-reactivity, with D-Ala4-DSIP-NH2 than with the other peptides. Both peptides substituted in the fourth position with D-alanine resulted in higher concentrations in 500 g rats than in 100 g rats, indicating a possible effect of age. The 5-10 fold greater brain to blood ratios of radioactivity after 125I-N-Tyr-DSIP than after 125-I radioiodinated serum albumin (RISA) provided additional evidence against non-specific leakage or contamination of the brains with substantial amounts of residual blood. The results suggest that penetration of the blood-brain barrier (BBB) by small amounts of peptides, at least DSIP peptides, may be somewhat selective.

Opioid agonist and antagonist bivalent ligands as receptor probes

Bivalent ligands are molecules which contain two pharmacophores linked by a connecting chain (spanner). The present report describes the use of oxymorphamine (Oxy) and naltrexamine (Nal) as the opioid agonist and antagonist pharmacophores separated by a variable length spanner composed of succinyl-bis-oligoglycine. The agonist series, [CH2CO(Gly)nOxy]2, and antagonist series, [CH2CO(Gly)nNal]2, were synthesized (n = 0-4) and tested on the electrically stimulated GPI. All of the antagonist bivalent ligands (Nal) antagonized the effects of morphine, with the greatest potency enhancement (60 x) residing with the succinyl (n = 0) congener. A dramatically different SAR profile was observed in the agonist (Oxy) series where the greatest potency enhancement (17 x) occurs when n = 2. By contrast with the antagonist series the agonist bivalent ligand with n = 0 is equipotent to its monovalent agonist analogue. The significance of these results with respect to the possibility of discrete opioid agonist and antagonist recognition sites are discussed.

Double-enkephalins--synthesis, activity on guinea-pig ileum, and analgesic effect

We have synthesized enkephalin analogues in which C-terminal methionine or leucine residues are replaced by a second active fragment of the enkephalin analogue. Synthesis of two compounds is described: in one, two fragments of a D-Ala2-enkephalin analogue are connected by a -NH-NH-bridge, and in the other, three methylene groups are incorporated between the amino groups. The first compound is a very potent inhibitor of electrically induced contractions of guinea-pig ileum and produces a strong analgesia when administered intraperitoneally in mice. The second compound is less active on the ileum and fails to produce analgesia after systemic injection. The double-enkephalins may interact with mu-receptors.

Induction of analgesia by central administration of ORG 2766, an analog of ACTH4--9

Dose-dependent analgesia was produced by microinjection of ORG 2766 into the periaqueductal gray (PAG). This analgesia was found to be potent and long-lasting and occurred at doses which were equimolar to those necessary for morphine analgesia. The same doses failed to produce analgesia by the cerebroventricular route, suggesting that the PAG was the site of action of this effect. Naloxone failed to reduce the analgesia and morphine tolerant did not diminish the effect significantly. Additionally, ORG 2766 at concentrations up to 10 micrometer failed to inhibit binding of [3H]naloxone to brain opiate receptors in vitro. These results suggest a non-opiate mechanism of action and are discussed in terms of a proposed alpha-MSH or ACTH receptor.

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.

Differential effects of neuropeptides on short-term memory in primates

In a within-subject design, six rhesus monkeys (3 males and 3 females) received a 100 micrograms/kg injection of one of seven neuropeptides or a diluent control solution and were then tested for activity level, learning (discrimination reversal), short-term memory (delayed response), and for responsiveness to noxious stimuli. One daily injection was made with a different peptide for 10 consecutive days, including pre- and post tests on th first and last days with the diluent control. DSIP and D-Phe4-Met enkephalin seemed to produce some interference with short-term memory, while alpha-MSH showed some facilitation of it, as indicated by interactions of the peptides with the delay periods of 0, 15, or 30 sec. Sex differences were found in the learning task and the responsiveness to a noxious stimulus, suggesting the possibility of interactions between the peptides and endogenous hormones.

LH-RH antagonists: further analogs with ring-substituted aromatic residues

Although the systematic substitution of benzene and other aromatic ring systems with various atoms and groups has been a standard approach in conventional pharmaceutical research, it has only recently received the attention it deserves in peptide research. The observation that D-p-Cl-Phe inserted in position 2 of certain LH-RH (luteinizing hormone-releasing hormone) analogs results in large improvements in antagonist activity led us to examine the effect of this and other substituents on position 1 and 2 D-phenylalanyl analog side chains. Analogs containing two D-p-Cl-phenylalanines were found to be particularly powerful competitive inhibitors when assayed in cycling rat for blockade of ovulation. Analogs with non-aromatic amino acids in the first position exhibited much lower activities.

Behavioral effects of dynorphin 1-13 in the mouse and rat: initial observations

Dynorphin is a recently identified, pharmacologically potent endogenous opioid peptide. Heretofore it has not been characterized for its behavioral effects. The effects of centrally infused dynorphin upon a variety of behaviors were therefore examined in mice and rats. The present findings point to a specific profile of behavioral activity. The peptide was active in facilitating feeding and grooming, but was inactive in modifying pain sensitivity and rearing behavior. Naloxone was generally ineffective in reversing behavioral effects. Dynorphin thus appears to have some opiate-like effects upon exogenous administration but may be rapidly broken down into a behaviorally potent non-opiate peptide fragment.

MIF-I suppresses deprivation-induced fluid consumption in rats

Rats were injected IP with a 0.1 mg/kg dose of MIF-I, naloxone, dynorphin, [D-Phe4]-Met-enkephalin, [D-Ala2, F5Phe4]-Met-enkephalin-NH2, or the diluent vehicle, placed in their home cages for ten minutes, and then given ad lib access to either 20% sucrose, 10% sucrose, water, 0.01% quinine, or 0.02% quinine in a repeated measures design with solutions counter-balanced over five days. Fluid consumption was measured very hour for 4 hours. A mixed analysis of variance yielded significant results for all main effects and the peptides by fluid and hours by fluid interactions. For the 4-hr test period, naloxone and [D-Phe4]-Met-enkephalin produced reliable increased in consumption while MIF-I produced a reliable decrease. Differences were obtained only with sucrose solutions, and the results clearly suggest that peptides modulate fluid consumption at positive levels of incentive motivation. To reconcile the findings of increased consumption after naloxone with the many studies suggesting a decrease in such paradigms, 0.1, 1.0, and 10.0 mg/kg of naloxone and MIF-I were administered as before but to independent groups of rats and intake was measured every 30 min. These results replicate and extend the above findings by showing that during the first 30-min period, both naloxone and MIF-I suppressed intake in a dose-dependent fashion, with MIF-I being more effective at each dose. The 0.1 mg/kg naloxone group, however, increased consumption over time and achieved a total consumption greater than control animals but comparable to that observed in the first study. It appears that at very low doses naloxone increases consumption over time, but at more commonly tested higher doses it has a suppressant effect. The results support the concept that in many situations MIF-I can produce the same effects as naloxone.

Tyrosine-modified analogs of methionine-enkephalin and their effects on the mouse vas deferens

Eighteen analogs of Met-enkephalin were synthesized in order to examine features of the N-terminal tyrosine (Tyr) residue responsible for activity on the mouse vas deferens. The most critical part of the tyrosine side-chain was its phenolic hydroxyl group which, in terms of biological activity, was highly sensitive to small changes and to the inclusion of fluorine or methyl groups in the aromatic ring. In contrast, the free amino group was not as sensitive to alterations. Single amino acid extensions had only modest effects on activity; however, beta and D-amino acid extensions virtually destroyed activity. Although the Tyr residue might be considered a promising part of the opiate peptides for the development of competitive antagonists, none of our inactive analogs were able to antagonize enkephalin and were, therefore, without binding affinity towards opiate receptors in the vas deferens.

Endogenous opiates: through 1978

The discovery of receptors in the brain for opiates and the structure of the endogenous peptides for these receptors has led to an explosion of interest in this field. The present review is the first of an annual series. It summarizes many of the highlights of research with opiate peptides published with a date of 1978 or earlier.

Possible non-narcotic component to action of opiate peptides on tonic immobility

Chickens were tested in a tonic immobility paradigm after a single intraperitoneal injection of either 0.0, 0.1, 1.0, 10.0; 100.0, or 1000.0 microgram/kg of the potent opiate analog, (D-Ala2, F5Phe4)-Met-enkephalin-NH2. An inverted-U relationship was obtained, with 100 microgram/kg being the most effective in prolonging immobility. This dose was used in subsequent studies involving pretreatment with naloxone or diluent followed by treatment with diluent, (D-Ala2, F5Phe4)-Met-enkephalin-NH2 (a strong opiate), or (D-Phe4)-Met-enkephalin (a weak opiate). The results indicated that although naloxone had mixed effects in attenuating the duration of tonic immobility, even the analog with negligible opiate activity reliably potentiated the response. Therefore, a component of the effect of opiate peptides on tonic immobility could be due to a non-narcotic action.

Analgesia after peripheral administration of enkephalin and endorphin analogues

Several analogues of Met-enkephalin and beta-endorphin were tested for their analgesic properties after systemic injection. The latencies of mice to flick their tails away from a source of heat revealed that analogues of the opiate peptides can cause analgesia when injected by this route. In particular, compounds specifically designed to be more lipophilic or to possess additional binding sites were shown to be potent analgesic after peripheral administration.

Effects of an enkephalin analog on complex learning in the rhesus monkey

Facilitation of the learning of a discrimination reversal task for a reward of food was found in rhesus monkeys after subcutaneous administration of a potent pentafluorinated enkephalin analog. (D-Ala2)-F5-Phe4-enkephalin-NH2. General activity, short-term memory, startle, and analgesia, however, were not significantly affected. In a within-subject design, each of 6 monkeys (3 males and 3 females) received each of 5 doses of the enkephalin analog (0.1, 1, 10, 100, and 1,000 microgram/kg). One daily injection was made for 7 consecutive days, including pre- and posttests on the first and last days with the diluent control. The enkephalin doses, with the exception of the 0.1 microgram/kg level, produced significantly faster learning than the diluent. Some sex differences were suggested by the data, but these effects are difficult to interpret. The results suggest that relatively small amounts of this analog given systematically can exert a reliable effect on a complex behavior such as reversal learning at doses devoid of opiate effects, due perhaps to enhanced cognitive flexibility rather than improvement in short-term memory or association formation.

Partially modified retro-inverso-enkephalinamides: topochemical long-acting analogs in vitro and in vivo

The synthesis of four enkephalinamide analogs is described in which the peptide bond between residues 4 and 5 is reversed with or without simultaneous reversal of the carboxyl-terminal amide bond. These so-called partially modified retro-inverso-isomers are new, potent, topochemical analogs of the enkephalins. Tests, both in vitro and in vivo, have shown that these analogs are considerably longer acting than any previously studied enkephalins. Thus, partial reversal of the peptide bonds of the backbone can result in peptides with enhanced activity compared to a parent compound, provide that the structural complementarity of both the side chains and end groups are conserved.

Metabolism of potent enkephalin analogs (FK 33-824, D-Ala2, pentafluorophenylalanine-4-enkephalinamide and a dimer of D-Ala2-enkephalin) and D-amino acid substituted derivatives of human beta-endorphin

New analogs of the opiate peptides containing novel substitutions were compared in terms of their metabolic stability in the presence of an ultrafiltrate of mouse brain. The enkephalin analog FK 33-824 was more stable at short incubation periods (30 min) than Met-enkephalin but less stable than D-Ala2-enkephalinamide at longer periods (180 min) as shown by the complete release of N-terminal Tyr. In contrast, D-Ala2-enkephalinamide substituted in position 4 with pentafluorophenylalanine was completely stable at the time periods tested. A dimer of D-Ala2-enkephalin was relatively stable at short periods but subject to a 30% hydrolysis (vs. 100% for FK 33-824) in terms of Tyr release at longer periods of incubation. A doubly substituted human beta-endorphin (D-Leu17, D-Lys29-beta-endorphin) showed greater stability than the native peptide based on release of internal residues after incubation with the ultrafiltrate of brain. The presence of D-Leu17 blocked release of intermediate sized endorphins but did not affect liberation of Tyr. The additional presence of D-Thr in position 6 of a triply substituted beta-endorphin (D-Thr6, D-Leu17, D-Lys29-beta-endorphin) did not affect liberation of Tyr, indicating that formation of gamma-endorphin (cleavage of Leu17-Phe) and of enkephalin (cleavage of Met5-Thr) need not occur before the action of brain peptidases. The results demonstrate the feasibility of altering the resistance of analogs of enkephalin and endorphin to degradation by brain enzymes.

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.

Enkephalin and other peptides reduce passiveness

Enkephalin and other brain peptides previously have been shown to be active in the dopa potentiation test which may be considered an animal model of mental depression. A recently described model of passive immobility during swimming, also sensitive to tricyclic antidepressants, was therefore used to study a large number of naturally occurring peptides and some of their analogues. It was found that several enkephalins with no opiate activity after peripheral injection reduced the immobility and thus increased the activity of swimming rats. alpha-MSH, but not its 4--10 core or a 4--9 analogue, also caused significantly more swimming than did the diluent control. As we have previously found in several animal and clinical studies, a smaller dose of MIF-I was more effective than larger doses. The results confirm our concept of the CNS actions of brain peptides and support the suggestion that some of them, like the enkephalins, might be useful after peripheral administration in mental depression or other CNS disorders.