The long term objectives of this research are to better understand the interactions of the opioid peptides, specifically dynorphin, with opioid receptors and to study at a molecular level how interactions of antagonists with opioid receptors differ from those of agonists. Another objective is ultimately to develop specific and potent antagonists for kappa receptors based on dynorphin, which could then be used as pharmacological tools in further studies. Such knowledge of how receptors function at a molecular level and the availability of specific antagonists for different opioid receptor types will hopefully lead to the development of non-addictive analgesics, possibly based on the endogenous opioid peptides, and to improved drugs for the treatment of drug addiction.
The specific aim of this project is to investigate modifications to the opioid peptide dynorphin which might impart antagonist activity to this peptide. Structural changes which will be made in the N-terminal heptapeptide portion of dynorphine A- (1-13) amide are alkylation of the amine terminal, replacement of Phe4, and alkylation of the side chains of the basic residues in positions 6 and 7. Conformational constraints will then be incorporated into these dynorphine analogues and dynorphine A-(1-13) amide in order to investigate and compare the conformational preferences for kappa antagonist and agonist activity. The constraints were chosen to differentiate between extended and alpha helical conformations, both of which have been proposed as important conformations for dynorphin. The constraints include cyclizations between residues separated by two residues (i.e., cyclization between the 2 and 5, 3 and 6, or 5 and 8 positions) and cyclic dipeptide replacements (trans-4- aminocyclohexanecarboxylic acid or cyclic dipeptides in the 2-3 or 5-6 positions). The conformations of the constrained peptide fragments and larger peptides containing them will be examined using various experimental techniques (NMR, X-ray crystallography, FT- IR and energy transfer) and computer-assisted molecular modeling with the AMBER program. The analogs will be synthesized by solid phase peptide synthetic methods. The peptides will then be tested for opioid activity, both agonism and antagonism, in isolated smooth muscle preparations, the guinea pig ileum, mouse vas deferens, rabbit vas dererens and hamster vas deferens. Receptor affinity will be measured by radioligand binding to guinea pig cerebellum and rat brain.
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