ed murine macrophage phagocytosis (Tomassini et al., 2003). Activity at all three opioid receptors, μ, κ, and δ, does clearly contribute to regulation of the immune system, but their effects depend on their anatomical location, differential signaling, and interactions with various other systems of the body. It is therefore necessary to look at how opioids interact with different parts of the nervous and immune systems.
             Opioids exert their actions on the immune system through binding to receptors in the central nervous system and/or on cells in the immune system. The immunomodulatory effects of opioids exerted through their interaction with CNS opioid receptors have been demonstrated by studies showing that ICV but not systemic administration of N-methylnaltrexone, a nonspecific opioid receptor antagonist, attenuates the effects of opioids on the immune system (Nelson & Lysle, 2001). It was further demonstrated that morphine administered within the periaqueductal gray matter specifically may have immunosuppressive effects (We
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