Preliminary Studies on Animals Suggest That Opioids Can Exert Analgesic

Preliminary studies on animals suggest that opioids can exert analgesic effects when administered directly into peripheral tissues (e.g. ,injected into an inflamed joint) because:

A) opioids decrease the production of inflammatory substances in peripheral tissues.
B) opioids increase vasodilation in peripheral tissues.
C) opioids cause vasoconstriction in peripheral tissues.
D) opioids may bind to receptors located on the distal (peripheral) ends of primary afferent (sensory) neurons.
E) none of the above;there is no evidence that opioids can produce analgesic effects outside to the central nervous system.