Z-Pro-D-Leu is a benzyloxycarbonyl-protected dipeptide related to the C-terminal end of oxytocin, and it has a small pharmacological literature of its own dating to the late 1970s. Walter and colleagues reported in PNAS that it inhibited the development of tolerance to, and physical dependence on, morphine in mice, which prompted a series of follow-up studies of how such a short peptide could produce that effect. Later work examined the mechanism rather than the phenomenon. The compound's effects on morphine tolerance and withdrawal were reported to be mediated through the limbic system, and separate studies described an interaction with brain dopaminergic neurotransmission: drugs affecting brain dopamine interfered with its action on morphine withdrawal, and its influence on catecholamine utilisation was mapped to specific mouse brain nuclei. The D-leucine configuration is part of the design rather than incidental, and the compound is generally discussed alongside oxytocin itself in this literature. This is an older body of work, largely from the 1978-1984 period and carried out in rodents. It is summarised here so the reagent can be placed in its published context; it is not a claim about activity in any other setting. | 