Opioid activities of fragments of beta-endorphin and of its leucine65-analogue. Comparison of the binding properties of methionine- and leucine-enkephalin

Synthesis and bioactivity studies of 1-adamantanamine derivatives of peptides

Small enkephalin-related peptides containing a 1-adamantanamine moiety coupled through an amide linkage at the C-terminus were synthesized. Several of the compounds showed high mu opioid activity and mu receptor selectivity. The new adamantanamine derivatives were also examined for antiviral activity against HIV-1 in a cell culture system. Some of them inhibited syncytia formation even when the antigen assay gave evidence for viral replication.

Tyrosine-iodination converts the delta-opioid peptide antagonist TIPP to an agonist

The binding properties and pharmacological activities of H-Tyr(3'-I)-Tic-Phe-Phe-OH ([Tyr(3'-I)1]TIPP) were studied. Similar to the delta-opioid receptor antagonist H-Tyr-Tic-Phe-Phe-OH (TIPP), [Tyr(3'-I)1]TIPP is a selective and potent ligand at delta-opioid receptors. The displacement curve of [3H]diprenorphine binding by [Tyr(3'-I)1]TIPP was shifted to the right in the presence of Na+ and 5'-guanylylimidodiphosphate, suggesting that it acted as a delta-opioid receptor agonist. [Tyr(3'-I)1]TIPP also behaved as a full agonist in the mouse vas deferens assay and its effect was both naloxone- and TIPP-reversible. These data show that monoiodination at the 3'-position of the N-terminal tyrosine aromatic ring of TIPP converted it from a potent and selective antagonist to a full agonist at delta-opioid receptors.

Structure-activity relationships of dermorphin analogues containing N-substituted amino acids in the 2-position of the peptide sequence

A series of dermorphin analogues containing an N-alkylated amino-acid residue Xaa in the 2-position of the peptide sequence was synthesized (Xaa = N-methylalanine, proline, pipecolic acid, N-methylphenylalanine, 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid [Tic]). These peptides have the potential of assuming a cis Tyr1-Xaa2 peptide bond. Their in vitro opioid activity profiles were determined in mu- and delta-receptor-representative binding assays and bioassays. Aside from [D-Pro2]dermorphin, all analogues showed high affinity for mu- and/or delta-opioid receptors. Whereas most compounds were found to be full mu-agonists in the guinea pig ileum (GPI) assay, [Tic2]dermorphin (compound 7) was a partial mu-agonist. Replacement of Gly4 in 7 with Phe resulted in an analogue (8) with weak mu-antagonist activity. Furthermore, analogues 7 and 8 both were potent delta-antagonists (Ke = 3-40 nM) against the delta-agonists Leu-enkephalin, DPDPE and deltorphin I in the mouse vas deferens (MVD) assay. Compound 3, containing L-Pro in the 2-position, turned out to be one of the most mu-receptor-selective linear dermorphin analogues reported to date. Low-temperature HPLC experiments using micropellicular octadecyl silica as stationary phase revealed conformational heterogeneity of the dermorphin analogues which was ascribed to cis-trans isomerization around the Tyr1-Xaa2- and Tyr5-Pro6 peptide bonds. In the case of analogue 7 four separate peaks corresponding to the four possible isomers were apparent at -5 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Highly potent side chain-main chain cyclized dermorphin-deltorphin analogues: an integrated approach including synthesis, bioassays, NMR spectroscopy and molecular modelling

Our continuing efforts to study structure-activity relationships of peptide opioids have resulted in the synthesis of a series of cyclic opioids related to dermorphins and deltorphins. The biological activities of the compounds have been determined and the conformational analyses carried out using 1H-NMR spectroscopy and molecular modelling. The three compounds in the series Tyr-c[D-Orn-Phe-Ala], Tyr-c[D-Lys-Phe-Ala], and Tyr-c[A2bu-Phe-Ala-Leu] are cyclized via a lactam bridge from the side-chain of the residue at the second position with the carboxyl terminus of each compound. The molecules incorporate 12-, 13- and 14-membered rings, respectively. They include a phenylalanine at the third position which is a distinguishing characteristic of dermorphins and deltorphins. The guinea pig ileum and mouse vas deferens assays show that the compounds are highly active at both mu- and delta-opioid receptors. The compounds are all highly effective antinociceptive agents as measured by the intrathecal rat hot plate test. Conformational analyses of the molecules indicate that they can adopt topochemical arrays required for bioactivity at both mu- and delta-receptors which explains their high activity in both guinea pig ileum and mouse vas deferens in vitro assays. The results support our models for mu- and delta-receptor activity for constrained peptide opioids.

Single diastereomeric desaminotyrosylalanyl tetra- and heptapeptides with opioid antagonistic activity

The N-terminal dipeptide Tyr-d-Ala of a mu-selective agonist, dermorphin tetrapeptide (DT, H-Tyr-D-Ala-Phe-Gly-NH2) and delta-selective agonist deltorphin C (DEL-C, H-Tyr-D-Ala-Phe-Asp-Val-Val- Gly-NH2) was changed into an aminodiacyl moiety. The relevant synthetic step is a nucleophilic substitution of bromine from a chiral 2-bromopropanamide by the amino group of tyrosine, with overall retention of configuration. The resulting pseudo tetra- and heptapeptides I-VI were characterized for mu- and delta-opioid receptor binding properties using [3H]DAGO and [3H]DPDPE, respectively, and in a bioassay using guinea pig ileum (GPI) and mouse vas deferens (MVD). As a result of chemical alteration of N-terminal depeptide moiety, all synthesized analogs showed considerable reduction in opioid receptor affinity compared to mu- and delta-prototypes (500-fold on the mu-site, analog I, and 125-fold on the delta-site, analog IV). Interestingly, analogs I and IV showed moderate antagonist activity, respectively, on GPI and MVD, with pA2 values of 6.05 and 6.82. Analog IV did not exhibit the delta-antagonist potency and delta-selectivity of TIPP peptides.

Acetylated glucopyranosyl esters of enkephalins

Acetylated D-glucopyranosyl esters of enkephalins were prepared by two different fragment condensation procedures involving direct participation of imidazole in the ester linkage formation. By both methods anomeric mixtures of D-glucosyl esters were obtained and resolved by column chromatography. Depending on coupling conditions, racemization of either the C-terminal or the penultimate amino acid residue of the enkephalin molecule occurred. The glucoconjugates with inverted stereochemistry were quantitated and separated from the main product by reversed-phase high-performance liquid chromatography. The opioid agonist potencies of the synthesized glucopyranosyl esters of enkephalins on electrically stimulated guinea pig ileum and mouse vas deferens preparations were determined in comparison with [Leu5]enkephalin.

Intranigral stimulation of oral movements by [Pro9] substance P, a neurokinin-1 receptor agonist, is enhanced in chronically neuroleptic-treated rats

Bilateral intranigral infusions of three different peptide agonists were made in rats exposed to fluphenazine decanoate, 30 mg/kg/month (FLU) or vehicle (CON) for seven months. Oral movements were monitored repeatedly during the neuroleptic pretreatment period, as well as before the intranigral infusion and during a 90-min period postinfusion. The FLU group had an increased frequency of vacuous chewing movements (VCM) during the pretreatment period in comparison to controls. Intranigral infusion of the neurokinin-1 (NK1) receptor agonist, [Pro9]Substance P (2.5 nmol on each side), 5-7 weeks after the last FLU injection, caused a significant increase of VCM in both pretreatment groups, lasting 7 min after the infusion. The VCM response to [Pro9]Substance P in the FLU group was significantly higher than in the CON group. A NK2 agonist [Lys5, MeLeu9, Nle10]Neurokinin A(4-10) (2.5 nmol) failed to produce significant changes in oral activity. A Leu-enkephalin analogue [D-Ala2,D-Leu5]enkephalin (3.8 nmol) induced a massive biting behavior in both FLU and CON rats. Using VCM as a behavioral assay, an increased nigral sensitivity to a NK1 agonist is demonstrated in rats chronically exposed to neuroleptics. No corresponding alterations could be ascribed for the NK2 receptor agonist or the Leu-enkephalin analogue.

Relationship between primary structure and activity in exorphins and endogenous opioid peptides

We have found a correlation between the certain characteristics of primary structure and biologic activity in exorphins and endogenous opioid peptide family. The characteristics of primary structure are the content of certain segment pairs as well as the density of their arrangement in a peptide. These segment pairs represent basic elements of the regulatory peptide primary structure pattern, which was found recently [Dokl. Akad. Nauk USSR 289 (1986) 721-724; Int. J. Peptide Prot. Res. 38 (1991) 505-510].

Direct electrophilic fluorination of tyrosine in dermorphin analogues and its effect on biological activity, receptor affinity and selectivity

In a preliminary communication we reported [(Tetrahedron Lett. 31, 619 (1990)] that acetyl hypofluorite can be used efficiently to introduce fluorine regiospecifically (ortho to OH) into the phenolic ring of tyrosine-containing peptides. This procedure has been applied to the fluorination of a number of mu-selective opioid peptides derived from dermorphin. While the procedure can be used even when the side chains of Arg, Lys, and Tyr are left unprotected, the sulfoxide of a Met(O)-containing analogue was oxidized to sulfone faster than fluorination of the phenolic ring. This method can also be used when the peptide is attached to Merrifield resin. Thus, Tyr(3-F)-D-Ala-Phe-Gly-NH2 and Tyr(3-F)-D-Arg-Phe-Lys-NH2 (F-DALDA) have been prepared, purified, and characterized. Affinities of these fluorinated peptides for both mu- and delta- opioid receptors are reduced (two- to nine-fold) relative to their nonfluorinated analogues, but their selectivity for mu-opioid receptors is not significantly altered. Similarly, the in vitro biological potencies (GPI and MVD assays) of the fluorinated analogues are reduced (two- to seven-fold) relative to their nonfluorinated parent peptides. Thus, F-DALDA, which has high affinity (Ki mu = 15.2 nM) and selectivity (Ki delta/Ki mu = 5390) for mu-opioid receptors, has potential use in biochemical studies which utilize 19F or 18F- labeled compounds.

Spectroscopic analysis of [Trp3]-beta-casomorphin analogs. Comparative structure conformation-activity studies

A series of [3-tryptophan]-beta-casomorphin-5([Trp3]-beta-CM-5) analogs were investigated by circular dichroism (CD) and fluorescence spectroscopy to explore their structure-conformation properties in solution. In addition, the comparative opioid activities of these compounds were evaluated using the in vitro guinea pig ileum (GPI) and mouse vas deferens (MVD) assays. Specifically, the pentapeptide sequence of [Trp3]-beta-CM-5, H-Tyr-Pro-Trp-Pro-Gly-OH (I) was modified at Pro-2 and Pro-4 by D-Pro substitutions to provide two diastereometric analogs, [Trp3-D-Pro-4]-beta-CM-5 (II) and [D-Pro2,4,Trp3]-beta-CM-5 (III). In the GPI and MVD assays, beta-CM-5 effected IC50 values of 1.3 microM and 8.9 microM, respectively, which confirmed its known mu/delta-selectivity on these two peripheral opioid receptor subtypes. The potencies of compounds I, II, and III were 0.2, 2.0, and less than 0.005 relative to beta-CM-5 on the GPI assay. Compounds I and II exhibited pronounced mu/delta-selectivities (greater than 18.9- and 12.4-fold respectively), whereas compound III was essentially inactive in both the GPI and MVD assays. CD studies of beta-CM-5 and its [Trp3]-beta-CM-5 analogs showed striking differences in their near-UV and far-UV spectra in aqueous or organic solvents. In the far UV CD spectra, weak (20%) alpha-helicity (maximum at 193 nm and minima at 208 and 222 nm) for beta-CM-5 was obtained in trifluoroethanol (TFE); however, none of the [Trp3]-beta-CM-5 analogs showed such CD bands. Of potential relevance to gamma-turn or C7 secondary structure was the observation of a strong negative band at 245 nm for compounds II and III which was not solvent-dependent in H2O or TFE, whereas compound I showed this CD band exclusively in TFE. In the near-UV CD at 275 nm (Trp electronic transition), the relative order of intensities of this band were determined for the [Trp3]-beta-CM-5 compounds to be II greater than I greater than III, which was identical to their relative biological potencies in both the GPI and MVD assays. Fluorescence energy transfer (FET) experiments of compounds I-III provided the intramolecular distances (r) between their Tyr (donor) to Trp (acceptor) side-chains, by the Förster method, and were as follows: [Trp3]-beta-CM-5, r = 10.6 A; [Trp3, D-Pro4]-beta-CM-5, r = 9.6 A; and [D-Pro2,4,Trp3]-beta-CM-5, r = 11.0 A.(ABSTRACT TRUNCATED AT 400 WORDS)

Intranigral infusion of enkephalins elicits dyskinetic biting in rats

Leu- and Metenkephalin (Lenk and Menk) and their more stable analogues D-Ala-Leu- and D-Ala-Metenkephalin (DALenk and DAMenk) as well as D-Ala-D-Leu- and D-Ala-D-Metenkephalin (DADLenk and DADMenk) were infused bilaterally into substantia nigra in awake rats and oral movements were recorded for 90 min. DADLenk and DADMenk elicited dose-dependent biting dyskinesias with a chewing rate of about 90 jaw movements/min. DALenk produced a similar but weaker effect, whereas DAMenk, Lenk and Menk were ineffective in the doses given. These findings suggest a possible enkephalinergic mechanism underlying neuroleptic-induced tardive dyskinesias.

Synthesis and biological activity of [Leu5]enkephalin derivatives containing D-glucose

The synthesis of some [Leu5]enkephalin derivatives is described in which D-glucose has been linked to the opioid pentapeptide through the ester bond involving the carboxyl function at the C-terminal with C-1 or C-6 of the D-glucopyranose moiety. Enkephalin derivatives were assayed for opioid activity and found to be full agonists in bioassays based on inhibition of electrically evoked contractions of the guinea pig ileum (GPI) and of the mouse vas deferens (MVD). The obtained results suggest that the opioid activity of the tested glucoconjugates depend upon the ester bond position in the molecule. Whereas 1-O conjugate 5 was somewhat more potent than [Leu5]enkephalin in the GPI assay, the 6-O conjugates, with the exception of 1-O-benzyl derivative 11, were considerably less potent. All enkephalin derivatives were delta-receptor selective; in particular, the acetylated analog 8 was three times more delta-receptor selective than [Leu5]enkephalin.

Synthesis and biological activity of some glucose-enkephalin conjugates

Two O-glucopeptides, H-Tyr(beta-D-Glc)-Gly-Gly-Phe-OH(10) and H-Tyr(beta-D-Glc)-Gly-Gly-Phe-Leu-OH (11), having the amino acid sequence of enkephalin, were synthesized to determine the influence of the carbohydrate molecule on the biological activity and conformation of these opioid peptides. The synthesis were carried out in a stepwise and/or direct manner by fusing the activated O-glucosylpseudourea intermediate with suitably protected amino acid or peptide derivatives, followed by hydrogenolytic removal of protecting groups. The pure compounds were tested for opiate-like activity by using the guinea pig ileum (GPI) and mouse vas deferens (MVD) preparations.

Side chain to side chain cyclization of an enkephalin analog results in loss of opioid receptor selectivity

The cyclic enkephalin analog H-Tyr-D-Lys-Gly-Phe-Glu-NH2 (I) and the structurally related open chain analogs H-Tyr-D-Nle-Gly-Phe-Gln-NH2 (II) and H-Tyr-D-Lys(For)-Gly-Phe-Abu-NH2 (III) were tested in mu and delta opioid receptor-representative binding assays and bioassays. Whereas both linear analogs showed a pronounced preference for mu receptors over delta receptors, the conformationally restricted cyclic peptide I was found to be unselective. This finding represents the first reported example of a peptide cyclization resulting in a loss of receptor selectivity. From this and earlier studies, it was concluded that the receptor selectivity of cyclized peptide analogs relative to that of their linear correlates may depend on the size and relative rigidity of their ring structures.

The significance of mu- and delta-receptors in rat pancreatic islets for the opioid-mediated insulin release

The binding and the insulinotropic effects of enkephalin analogs and of morphine were investigated in rat pancreatic islets. Binding of [3H]Met-enkephalin was saturable, specific and reversible; the rank order for inhibition competition of [3H]Met-enkephalin binding by various compounds was Met-enkephalin = D-Ala2-MePhe4, Met(0)ol enkephalin) greater than Leu-enkephalin greater than morphine with half-maximal inhibitory constants (IC50) of approx. 0.3, 0.3, 100 and greater than 100 nM, respectively. Both the natural enkephalins exerted their insulinotropic effect only at stimulatory glucose concentrations. They had a dual action; whereas insulin secretion was increased at low enkephalin concentration, this effect was reversed at higher concentrations. However, the various enkephalins exerted this effect at different concentrations; only the EC50 values (half-maximal effective concentrations) of their insulinotropic effect were in the same range as the IC50 values of inhibition of [3H]met-enkephalin binding. Cysteamine pretreatment of rats (depletion of somatostatin containing D-cells and decrease in somatostatin secretion) did not change the Met-enkephalin effect on insulin secretion. In contrast to Met-enkephalin, binding of [3H]morphine to islets was not saturable, and morphine had no effect on insulin secretion unless at unphysiologically high concentrations. The data, therefore, indicate that: mu-receptors (affinity for morphine) do not play a role in rat pancreatic islets; delta-receptors (binding site for Met-enkephalin when mu-receptors are not present) mediate the insulinotropic effect of low Met-enkephalin concentrations; and the insulinotropic action of Met-enkephalin is not mediated indirectly via the paracrine effect of an inhibition of somatostatin secretion.

The relative potency of enkephalins and beta-endorphin in guinea-pig ileum, mouse vas deferens and rat vas deferens after the administration of peptidase inhibitors

Previous studies have shown that three distinct enzymes, amastatin-sensitive aminopeptidase, captopril-sensitive peptidyl dipeptidase A, and phosphoramidon-sensitive endopeptidase-24.11, played a critical role in the inactivation of enkephalins in isolated preparations. In the present study, therefore, the rank order of the potency of three endogenous opioid peptides, [Met5]-enkephalin, [Leu5]-enkephalin, and beta-endorphin, in three isolated preparations, guinea-pig ileum, mouse vas deferens, and rat vas deferens, was estimated in the presence of the mixture of three peptidase inhibitors, amastatin, captopril, and phosphoramidon. [Met5]-Enkephalin was approximately three-fold more potent than [Leu5]-enkephalin and four-fold more potent than beta-endorphin in guinea-pig ileum in which three opioid peptides were indicated to act on mu-receptors. Additionally, [Met5]-enkephalin was slightly but significantly more potent than [Leu5]-enkephalin and approximately twenty-fold more potent than beta-endorphin at delta-receptor sites in mouse vas deferens. Moreover, [Met5]-enkephalin was approximately three-fold more potent than [Leu5]-enkephalin, but sixty-fold less potent than beta-endorphin in rat vas deferens in which the opioid-receptor type interacting with enkephalins could not be determined. In conclusion, the well-known rank order of the potency of three endogenous opioid peptides was shown to be altered in both guinea-pig ileum and mouse vas deferens but not in rat vas deferens by the pretreatment of the preparations with the mixture of three peptidase inhibitors.

In vitro activity profiles of cyclic and linear enkephalin pseudopeptide analogs

The peptide bond in the 4-5 position of the cyclic and linear enkephalin analogs H-Tyr-cyclo[-D-Lys-Gly-Phe-L(or D)-Leu-] and H-Tyr-D-Ala-Gly-Phe-L(or D)-Leu-OH was replaced by a thiomethylene ether linkage. Each of the configurational isomers of the cyclic pseudopeptide H-Tyr-cyclo[-D-Lys-Gly-Phe psi [CH2S]L(or D)-Leu-] showed high potency in both the guinea pig ileum and the mouse vas deferens assay and, therefore, had no preference for either mu- or delta-opioid receptors, in contrast to the cyclic parent peptides H-Tyr-cyclo[-D-Lys-Gly-Phe-L(or D)-Leu-] which are mu-receptor selective. The loss of selectivity observed with the cyclic pseudopeptides may be due to the greater flexibility of their 18-membered ring structures as a consequence of the peptide bond substitution. The linear pseudopeptide analogs were both less potent and less delta-receptor selective than their parent compounds. These results indicate that thiomethylene ether peptide bond replacements can have a pronounced effect on the activity profile of peptide hormones and neurotransmitters.

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.

A novel cyclic opioid peptide analog showing high preference for mu-receptors

The side-chain to side-chain cyclized opioid peptide analogs H-Tyr-D-Orn-Phe-Asp-NH2 (I) and H-Tyr-D-Lys-Phe-Glu-NH2 (II) were synthesized and tested in the guinea pig ileum and mouse vas deferens assays and in binding assays based on displacement of mu- and delta-opioid receptor-selective radioligands from rat brain membranes. The more rigid cyclic analog I containing a 13-membered ring structure showed very high preference for mu-receptors over delta-receptors, whereas the more flexible cyclic peptide II (15-membered ring) was non-selective. These results indicate that variation in the degree of conformational restriction of opioid peptides can produce drastic shifts in their receptor selectivity profile. Because of its high mu-receptor selectivity and rigidity cyclic analog I will be useful for determining the conformational requirements of mu-opioid receptors.

Phencyclidine-induced inhibition of striatal acetylcholine release: comparisons with mu, kappa, and sigma opiate agonists

The effects of phencyclidine (PCP) on ACh release were compared to those of morphine, ethylketocyclazocine (EKC), and N-allylnormetazocine (SKF10047) in a superfused striatal slice preparation. The (+)-isomer of the prototypic sigma opiate agonist, SKF10047, and the prototypic kappa opiate agonist, EKC, had essentially the same pharmacological profile as did PCP. That is, they each inhibited ACh release in a concentration dependent manner (with EKC being the most potent) and this effect was antagonized by 0.1 microM naloxone. Since morphine was without effect on ACh release, it is unlikely that these drugs inhibit ACh release by acting at mu receptors. In addition, we observed that the inhibitory effect of PCP, (+) SKF10047, and EKC on ACh release was reversed by 0.1 microM haloperidol. Given that PCP has been shown to stimulate basal DA release in this preparation, it is possible that PCP, EKC and (+) SKF10047 inhibit ACh release indirectly by stimulating DA release. The naloxone-induced blockade of the effect of PCP and these benzomorphans is discussed in relation to the effects of naloxone on other systems known to influence ACh release.

Effect of intrahypothalamic injection of [D-Ala2,D-Leu5]enkephalin on feeding and temperature in the rat

The effectiveness of the delta-opioid agonist [D-Ala2,D-Leu5]enkephalin (DADLE) in eliciting alterations in feeding and core temperature in rats was compared to morphine. When injected into the ventromedial hypothalamus this peptide caused a dose-related increase in feeding which was rapid in onset and of short duration, and a short-lived increase in temperature. Neither of these effects was blocked by the intrahypothalamic injection of naloxone. The alpha-adrenoreceptor antagonist phentolamine significantly reduced DADLE-stimulated feeding, although it did not counteract the hyperthermia. Since the delta-opioid agonist DADLE is more potent than the mu-agonist, morphine, and produces its effect more rapidly, the delta-opiate receptor may have an influence on the regulation of feeding.

Increase in potencies of opioid peptides after peptidase inhibition

Various agents that have been reported to reduce the enzymatic degradation of the enkephalins have been tested for their ability to potentiate the activity of [Met5]enkephalin in three in vitro assay tissues. The greatest effect was obtained with the combination of bestatin (10 microM or 30 microM), captopril (10 microM), thiorphan (0.3 microM) and L-Leucyl-L-leucine (2 mM) which increased the potency of [Met5]enkephalin 18-fold in the guinea-pig myenteric plexus, 13-fold in the mouse vas deferens and 200-fold in the rat vas deferens. The increased potency is attributed to inhibition of the peptidases since the mixture of inhibitors did not change the activity of either normorphine or the metabolically stable synthetic opioid peptides. The potencies of the hexa-, hepta- and octapeptide C-terminus extensions of [Met5]enkephalin and [Leu5]enkephalin were increased by the peptidase inhibitors in all three preparations; the greatest effects were found in the rat vas deferens. No significant changes in the potencies of fragments of beta-endorphin longer than beta-endorphin-(1-19) were obtained. It may now be possible to inhibit enzymatic degradation of opioid peptides sufficiently to measure their release from neurones activated by electrical field stimulation.

Tricyclic antidepressant desipramine induces stereospecific opiate binding and lipid modifications in rat glioma C6 cells

Incubation for 48 hours of C6 glioma cell cultures with 10(-4)M tricyclic antidepressant desipramine gave rise to a quantitative increase of total lipids and to qualitative modifications of glycosphinegolipids involving detection by thin-layer chromatography of spots migrating according to cerebroside and sulfatide and presence of an abnormal ganglioside pattern. These lipid modifications were associated with the appearance of stereospecific binding of opiates (dihydromorphine) with a dissociation constant of 30-60 nM. These results favor an important role of lipids in opioid receptor function.

A comparison of the ability of opioid peptides and opiates to affect active avoidance conditioning in rats

Enkephalins reduce acquisition of an active avoidance response when administered intraperitoneally shortly before training. The present study examined whether microgram or delta opiate receptors are involved in this enkephalin effect. This was done by comparing the efficacy of micro- and delta-receptor agonists; by attempting to block the enkephalin effect with micro- and delta-receptor antagonists; and by comparing the characteristics of the effects of Met-enkephalin and Leu-enkephalin. In addition, the efficacy of kappa-agonists in reducing acquisition was assessed. It was found that micro-agonists are inactive in this assay; several delta- and kappa-agonists are active. However, not all of the data are consistent with the adequacy of this receptor classification. The micro-receptor antagonist naloxone did not readily block the effect of Met- or Leu-enkephalin but neither did the micro/delta-antagonist, diprenorphine. An additional complexity is the emergence of differences in behavioral activity of Met- snd Leu-enkephalin.

Morphiceptin (NH4-tyr-pro-phe-pro-COHN2): a potent and specific agonist for morphine (mu) receptors

The synthetic peptide NH2-Tyr-Pro-Phe-Pro-CONH2 (morphiceptin), which is the amide of a fragment of the milk protein beta-casein, has morphinelike activities and is highly specific for morphine (mu) receptors but not for enkephalin (delta) receptors. It is as active as morphine in the guinea pig ileum but much less active in the mouse and rat vas deferens. The discovery of this specific morphine receptor ligand substantiates the hypothesis of multiple opiate receptors. The ligand, which may be of physiological significance since a very similar, or identical, activity can be detected in enzymatic digests of beta-casein, may prove useful for further investigation of the functions of opiate receptor subtypes.

Opiate and peptide interaction: effect of enkephalins on morphine analgesia

Interactions between the weakly analgesic enkephalins and morphine on morphine-induced analgesia were studied. Met-enkephalin exhibited morphine analgesia whereas Leu-enkephalin potentiated it. Both Met- and Leu-enkephalin, when tested alone, were not analgesic. The strongly analgesic FK33824 (Sandoz) compound, like Leu-enkephalin, also potentiated morphine analgesia. Tolerance developed to morphine analgesia but not to Met-enkephalin inhibition of morphine analgesia.

Analgesic action of intrathecal and intracerebral beta-endorphin in rats: comparison with morphine

beta-Endorphin and morphine were given into the nucleus reticularis gigantocellularis (NRGC), spinal subarachnoid space and third ventricle of rats, and the antinociceptive effects were assessed by the tail-pinch method. The NRGC was 4-5 times as sensitive to beta-endorphin as was the spinal subarachnoid space and third ventricle, while the NRGC was 13-19 times as sensitive to morphine as were the other regions. Thus, it would appear that beta-endorphin has more sites of action for the production of analgesia than does morphine.

Effects of intravenously administered Leu- or Met-enkephalin on arterial blood pressure

The purpose of these studies was to determine if two endogenous opioids, leucine (Leu) and methionine (Met) -enkephalin, alter blood pressure and, if so, by what mechanisms. Studies from our laboratory show that intravenous administration of Leu-enkephalin in doses of 0.032-320 microgram/kg induced a biphasic response in pentobarbital-anesthetized cats. A transient rise in mean arterial pressure was followed by a more prolonged decline. Administration of Met-enkephalin caused only a decline in mean arterial pressure. Neither agent significantly altered heart rate, venous pressure or the EKG. Having determined that both enkephalins altered blood pressure and observed that the responses were qualitatively different, selected pharmacological antagonists were employed to see if the alterations in blood pressure could be blocked. Naloxone blocked the hypertensive responses and antagonized the hypotensive effects seen with the administration of Leu-enkephalin. Naloxone also shifted the dose-effect curve of Met-enkephalin to the right. Diphenhydramine attenuated both the hypertensive and hypotensive responses of Leu-enkephalin. However, diphenhydramine pretreatment did not alter the decline in blood pressure seen with the higher doses of Met-enkephalin. Propranolol exerted some antagonistic activity in association with the rise in blood pressure seen with Leu-enkephalin, but propranolol did not alter the drop in pressure observed with the administration of either enkephalin. These results show that intravenous administration of the enkephalins can alter blood pressure and these effects are not alike for each enkephalin. Additionally, the enkephalins are not blocked in the same fashion by antagonists, giving support to the hypothesis that the two enkephalins interact with different receptors.

Effects of changes in the structure of enkephalins and of narcotic analgesic drugs on their interactions with mu- and delta-receptors

1 The activity pattern of analogues of the enkephalins was determined in four parallel assays, the inhibition of the electrically evoked contraction of the guinea-pig ileum and mouse vas deferens at 36 degrees C and the inhibition of [(3)H]-naltrexone and [(3)H]-leucine-enkephalin binding at 0 to 4 degrees C in homogenates of guinea-pig brain.2 The activity pattern was best characterized by the ratio of the potency in the guinea-pig ileum to that in the mouse vas deferens (G.p.i./M.v.d.) and the ratio of the potency in inhibiting [(3)H]-naltrexone binding to that in inhibiting [(3)H]-leucine-enkephalin binding (Nal/Leu).3 The enkephalins had low G.p.i./M.v.d. (0.02 to 0.09) and low Nal/Leu (0.05 to 0.18) ratios whereas the corresponding values for morphine were 7.0 and 7.5.4 Analogues obtained by substituting D-Ala for Gly(2) and D-Met or D-Leu for L-Met(5) or L-Leu(5) showed only minor changes in G.p.i./M.v.d. (0.01 to 0.11) and in Nal/Leu (0.06 to 0.13) ratios.5 Analogues in which resistance to enzymatic degradation was brought about by amidation of the C-terminal carboxylic group or methylation of the amino group of tyrosine or both modifications, had G.p.i./M.v.d. ratios of 1.2 to 5.5 and Nal/Leu ratios of 0.5 to 21. High values (2.1 and 3.4) were found for the potent antinociceptive analogue of Sandoz, Tyr-D-Ala-Gly-NCH(3)Phe-Met(O)-ol.6 In the mouse vas deferens, some of the analogues with high G.p.i./M.v.d. and Nal/Leu ratios were tested for antagonism by naloxone and found to require less than the high concentration needed for the natural enkephalins. C57/BL mice, which have a lowered sensitivity to morphine but a normal response to peptides with low G.p.i./M.v.d. and Nal/Leu ratios, had a lowered sensitivity to analogues with high ratios.7 In the alkaloid-like series of narcotic analgesic drugs, ketobemidone, levorphanol, methadone, etorphine and the antagonist Mr 2266 had lower Nal/Leu ratios (1.0 to 2.8) than morphine, normorphine, naloxone and naltrexone (8 to 12).8 It would appear that compounds with low G.p.i./M.v.d. and Nal/Leu ratios interact mainly with delta-receptors in the brain and peripheral nervous system while compounds with high ratios interact mainly with mu-receptors. For antinociceptive action mu-receptors may be more important than delta-receptors.