Disruption of morphine-conditioned place preference by a δ2-opioid receptor antagonist: study of μ-opioid and δ-opioid receptor expression at the synapse

The addictive properties of morphine limit its clinical use. Learned associations that develop between the abused opiate and the environment in which it is consumed are engendered through Pavlovian conditioning processes. Disruption of the learned associations between the opiate and environmental cues may be a therapeutic approach to prevent morphine dependence. Although a role for the delta-opioid receptor in the regulation of the rewarding properties of morphine has already been shown, in this study we further characterized the role of the delta-opioid receptor in morphine-induced conditioned responses by examining the effect of a selective delta 2-opioid receptor antagonist (naltriben), using a conditioned place preference paradigm in rats. Additionally, we used a subcellular fractionation technique to analyze the synaptic localization of mu-opioid and delta-opioid receptors in the hippocampus, in order to examine the molecular mechanisms that may underlie this morphine-induced conditioned behavior. Our data show that the administration of 1 mg/kg naltriben (but not 0.1 mg/kg) prior to morphine was able to block morphine-induced conditioned place preference. Interestingly, this naltriben-induced disruption of morphine conditioned place preference was associated with a significant increase in the expression of the delta-opioid receptor dimer at the postsynaptic density. In addition, we also observed that morphine conditioned place preference was associated with an increase in the expression of the mu-opoid receptor in the total homogenate. Overall, these results suggest that modulation of the delta-opioid receptor expression and its synaptic localization may constitute a viable therapeutic approach to disrupt morphine-induced conditioned responses.