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Multifunctional opioid receptor agonism and antagonism by a novel macrocyclic tetrapeptide prevents reinstatement of morphine‐seeking behaviour

The macrocyclic tetrapeptide natural product CJ‐15,208 (cyclo[Phe‐d‐Pro‐Phe‐Trp]) is a multifunctional μ‐opioid receptor and κ‐opioid receptor agonist and κ‐opioid receptor antagonist that produces antinociception and prevents stress‐induced reinstatement of extinguished cocaine‐conditioned place preference (CPP). We hypothesized that an analogue of CJ‐15,208, cyclo[Pro‐Sar‐Phe‐d‐Phe], would demonstrate multifunctional μ‐opioid receptor and κ‐opioid receptor ligand activity, producing potent antinociception with fewer liabilities than selective μ‐opioid receptor agonists, while preventing both drug‐ and stress‐induced reinstatement of morphine‐induced CPP.