The demonstration of the multiplicity of opiate receptor types has led to the understanding that, depending on their site of action, opioid peptides as well as opiate alkaloids may bind to more than one opiate receptor subtype. In addition to the two main mu opiate receptor subtypes, mu1 and mu2, our laboratory has demonstrated a third mu opiate receptor (mu3) that is selective for opiate alkaloids but insensitive to opioid peptides. Recently, the mu3 opiate receptor subtype has been cloned from human immune, vascular and neural tissues. This mu3 story complements many biochemical reports; demonstrating morphine is an endogenous signaling molecule, functioning in the capacity of a neurotransmitter and hormone. Adding additional evidence to this hypothesis are the findings of morphine precursors in mammalian and invertebrate tissues. The reports published in this issue of MSM complement this story while advancing the hypothesis by placing opiate alkaloid signaling in limbic structures. The pharmacological characteristics of exogenous morphine find a role for explaining morphine action in an "emotional" and belief setting.