Anti-opioid systems in morphine tolerance and addiction--locus-specific involvement of nociceptin and the NMDA receptor.

Mechanisms for opioid tolerance and addiction are divided into two types of plasticity--cellular level and those occurring through multiple neuronal networks. Receptor desensitization through phosphorylation and endocytosis are currently well discussed using cell lines expressing opioid receptors in relation to acute tolerance mechanisms, while altered gene expression is mainly discussed in relation to the model mechanisms of chronic tolerance and dependence. However, little is known of mechanisms operating through plasticity of neuronal networks. In our approach, we began with the assumption that some non-opioid neurons with anti-opioid activity may cause neuronal plasticity, showing opioid adaptation and dependence. In mice lacking nociceptin/orphanin FQ receptor (NOP), or the NMDA receptor epsilon1 subunit, both of which mediate anti-opioid activities, analgesic tolerance and dependence following chronic morphine treatments were markedly attenuated. Chronic morphine-treatments increased NOP gene or epsilon1 subunit protein expression in the spinal cord or specific brain loci, respectively. Furthermore the rescue of the epsilon1 subunit gene in the specific brain locus of knockout mice recovers the tolerance and dependence. All these results suggest that the enhanced anti-opioid system may contribute to the development of morphine tolerance and dependence, and their contribution could be brain locus specific.