Pre-conditioned place preference treatment of chloral hydrate interrupts the rewarding effect of morphine

The medical use of morphine as a pain killer is hindered by its side effects including dependence and further addiction. As the prototypical μ receptor agonist, morphine's rewarding effect can be measured by conditioned place preference (CPP) paradigms in animals. Chloral hydrate is a clinical sedative. Using a morphine CPP paradigm that mainly contains somatosensory cues, we found that pre-CPP treatment in rats using chloral hydrate for 6 consecutive days could disrupt the establishment of CPP in a U shape. Chloral hydrate had no effect on the body weight of rats. Our results indicate that prior treatment with chloral hydrate can interrupt the rewarding effect of morphine.