Opioid activity of dermenkephalin analogues in the guinea-pig myenteric plexus and the hamster vas deferens

Pharmacologic study of C-terminal fragments of frog skin calcitonin gene-related peptide

The calcitonin gene-related peptide from the skin of the frog Phyllomedusa bicolor (pbCGRP) is a 37-residue neuropeptide that differs from human alpha CGRP (halphaCGRP) at 16 positions. The affinities of the C-terminal fragments of pbCGRP and halphaCGRP were evaluated in SK-N-MC cells: pbCGRP(8-37) (K(i)=0.2nM) and pbCGRP(27-37) (K(i)=95nM) were, respectively, 3 times and 20 times more potent than the human fragments halphaCGRP(8-37) and halphaCGRP(27-37). Their antagonistic potencies were measured in SK-N-MC and Col 29 cells, and the rat vas deferens. pbCGRP(8-37) inhibited the halphaCGRP-stimulated production of cAMP by SK-N-MC and Col 29 cells 3 to 4 times more strongly than halphaCGRP(8-37). Thus pbCGRP(8-37) is the most potent CGRP-1 competitive antagonist of all the natural sequences reported to date. pbCGRP(27-37) was also as potent as [D(31), A(34), F(35)] halphaCGRP(27-37), a prototypic antagonist analog derived from structure-activity relationship studies of halphaCGRP(8-37).

Dmt and opioid peptides: a potent alliance

The introduction of the Dmt (2',6'-dimethyl-L-tyrosine)-Tic pharmacophore into the design of opioid ligands produced an extraordinary family of potent delta-opioid receptor antagonists and heralded a new phase in opioid research. First reviewed extensively in 1998, the incorporation of Dmt into a diverse group of opioid molecules stimulated the opioid field leading to the development of unique analogues with remarkable properties. This overview will document the crucial role played by this residue in the proliferation of opioid peptides with high receptor affinity (K(i) equal to or less than 1 nM) and potent bioactivity. The discussion will include the metamorphosis between delta-opioid receptor antagonists to delta-agonists based solely on subtle structural changes at the C-terminal region of the Dmt-Tic pharmacophore as well as their behavior in vivo. Dmt may be considered promiscuous due to the acquisition of potent mu-agonism by dermorphin and endomorphin derivatives as well as by a unique class of opioidmimetics containing two Dmt residues separated by alkyl or pyrazinone linkers. Structural studies on the Dmt-Tic compounds were enhanced tremendously by x-ray diffraction data for three potent and biologically diverse Dmt-Tic opioidmimetics that led to the development of pharmacophores for both delta-opioid receptor agonists and antagonists. Molecular modeling studies of other unique Dmt opioid analogues illuminated structural differences between delta- and mu-receptor ligand interactions. The future of these compounds as therapeutic applications for various medical syndromes including the control of cancer-associated pain is only a matter of time and perseverance.

Isolation, structure, synthesis, and activity of a new member of the calcitonin gene-related peptide family from frog skin and molecular cloning of its precursor

Calcitonin gene-related peptide has been extracted from the skin exudate of a single living specimen of the frog Phyllomedusa bicolor and purified to homogeneity by a two-step protocol. A total volume of 250 microl of exudate yielded 380 microg of purified peptide. Mass spectrometric analysis and gas phase sequencing of the purified peptide as well as chemical synthesis and cDNA analysis were consistent with the structure SCDTSTCATQRLADFLSRSGGIGSPDFVPTDVSANSF amide and the presence of a disulfide bridge linking Cys(2) and Cys(7). The skin peptide, named skin calcitonin gene-related peptide, differs significantly from all other members of the calcitonin gene-related peptide family of peptides at nine positions but binds with high affinity to calcitonin gene-related peptide receptors in the rat brain and acts as an agonist in the rat vas deferens bioassay with potencies equal to those of human CGRP. Reverse transcriptase-polymerase chain reaction coupled with cDNA cloning and sequencing demonstrated that skin calcitonin gene-related peptide isolated in the skin is identical to that present in the frog's central and enteric nervous systems. These data, which indicate for the first time the existence of calcitonin gene-related peptide in the frog skin, add further support to the brain-skin-gut triangle hypothesis as a useful tool in the identification and/or isolation of mammalian peptides that are present in the brain and other tissues in only minute quantities.

Activity profiles of dalargin and its analogues in mu-, delta- and kappa-opioid receptor selective bioassays

1. To elucidate the structural features ensuring action of [D-Ala2, Leu5]-enkephalyl-Arg (dalargin), a series of dalargin analogues were tested for their effectiveness in depressing electrically-evoked contractions of the guinea-pig myenteric plexus-longitudinal muscle preparations (mu- and kappa-opioid receptors) and the vasa deferentia of the hamster (delta-opioid receptors), mouse (mu-, delta- and kappa-opioid receptors), rat (similar to mu-opioid receptors) and rabbit (kappa-opioid receptors). The naloxone KB values in the myenteric plexus were also obtained. 2. [L-Ala2]-dalargin was 19 times less potent than dalargin, and its pharmacological activity was peptidase-sensitive. The ratio of delta-activity to mu-activity for [L-Ala2]-dalargin was 6.78, and KB was 7.9 nM. This emphasizes the role that D-configuration of Ala2 plays in determining the active folding of dalargin molecule as well as in conferring resistance to peptidases. 3. [Met5]-dalargin was equipotent to dalargin in the myenteric plexus, but was more potent in the vasa deferentia of hamster and mouse (KB=5.5 nM). Leu5 and the interdependence of Leu5 and D-Ala2 are of importance for the selectivity of dalargin for mu-opioid receptors. 4. Dalarginamide was more potent and selective for mu-opioid receptors than dalargin, whilst dalarginethylamide, though equipotent to dalarginamide in the myenteric plexus, was more potent at delta-opioid receptors (KB=5.0 nM). [D-Phe4]-dalarginamide and N-Me-[D-Phe4]-dalarginamide were inactive indicating the contribution of L-configuration of Phe4 to the pharmacological potency of dalargin. 5. N-Me-[L-Phe4]-dalarginamide possessed the highest potency and selectivity for mu-opioid receptors (the ratio of delta-activity to mu-activity was 0.00053; KB=2.6 nM). The CONH2 terminus combined with the N-methylation of L-Phe4 increased the potency and selectivity of dalargin for mu-opioid receptors.

Anti-idiotypic antibodies: a useful alternative for studying the biochemical expression of mu/delta opioid binding sites in mammalian brain

A polyclonal antiserum was produced against opioid binding sites using an anti-idiotypic approach whereby antibodies directed against the opioid agonist DSLET were used as immunogen. The anti-idiotypic antiserum recognized specific brain proteins with molecular masses of 76 +/- 4, 73 +/- 4 and 59 +/- 3 kDa, respectively. The immunolabeling of these proteins was mainly inhibited by mu, delta opioid agonists and a general antagonist, naloxone. The inhibition of immunoprecipitation by opioid agonists and antagonist and the developmental expression of these immunoreactive proteins found to occur during brain ontogeny strongly suggest that these three proteins were mu, delta but not kappa opioid binding sites. The anti-idiotypic antiserum both inhibits 3H-DADLE binding and mimics the inhibitory agonist effects on the stimulated cAMP level of NG 108-15 cells which expressed delta opiate receptors. Numerous mammalian brain opioid binding sites were labeled, due to the fact that the binding site was the epitope recognized by the anti-idiotypic antibodies. From the numerous studies performed with a view to characterizing the specificity of the anti-idiotypic antibodies, it was strongly suggested that the anti-idiotypic antibodies specifically recognize mu/delta opioid binding sites and they can therefore be powerful tools for studying the biochemical expression of these opioid binding sites in mammalian brains.

The action of amlodipine on voltage-operated calcium channels in vascular smooth muscle

1. Amlodipine, a dihydropyridine derivative largely ionized at physiological pH, inhibited calcium channel currents in single vascular smooth muscle cells isolated from rabbit ear artery in a concentration-dependent manner. 2. Amlodipine inhibited the current-voltage relationship for calcium channel currents across the range of test potentials used. However, the effect of amlodipine was more marked on more depolarized test potentials. Amlodipine also shifted the steady-state inactivation curve for calcium channel currents in a hyperpolarized direction. 3. The potency of amlodipine as determined from the steady-state inhibition of calcium channel current induced by the drug was dependent on the holding potential of the cells. Use of a more depolarized holding potential increased the potency of amlodipine. 4. Onset of amlodipine-induced inhibition was relatively rapid at both -60 mV and -40 mV holding potential. The use of a more depolarized holding potential increased the rate of association of amlodipine. No recovery from amlodipine-induced inhibition was seen over a 20 min period following washout of the drug. 5. In addition to voltage-dependence, the action of amlodipine showed use-dependence, in that the effect of amlodipine was more marked when calcium channel currents were evoked frequently. Increasing the frequency of activation of calcium channel currents did not alter the apparent onset rate of amlodipine-induced inhibition, but increased the degree of inhibition achieved by the drug. 6. The electrophysiological properties of amlodipine, particularly its voltage-dependence are probably important determinants of its action in vivo.

The aspartic acid in deltorphin I and dermenkephalin promotes targeting to delta-opioid receptor independently of receptor binding

Recent studies on the highly potent and selective delta-opioid agonists demenkephalin (Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2) and deltorphin I (Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2) suggested that key structural features necessary for specific targetting to the delta-opioid receptor are located within the C-terminal halves of these naturally occurring heptapeptides. To investigate the contribution of aspartic acid 4 residue in deltorphin I and aspartic acid 7 residue in dermenkephalin to the delta-addressing ability of the C-terminal ends, fourteen analogs were synthesized and assessed for their ability to bind to mu and delta-opioid receptors in rat brain membrane homogenates. Results showed that i/ although the tetrapeptide C-terminus of dermenkephalin and deltorphin I differ in amino acid composition, they play a similar role in specifying correct addressing of these peptides to the delta-receptor, ii/ the negatively charged side chain of aspartic acid 4 residue in deltorphin I and aspartic acid 7 residue in dermenkephalin is not involved in binding contact at the delta-receptor site, nor in maintaining a delta-bioactive folding of the peptides, iii/ these side chains are, in contrast, functionally or structurally required to confer high delta-selectivity by preventing mu-site recognition and/or binding.