How does codeine affect our brain's neurotransmitters?

Slows: Narcotics typically slow neurotransmission, especially the ones signalling pain.

Similar to endorphin: The chemical structure of Codeine is similar to that of the neurotransmitter, endorphin, a natural pain-killer produced by the body. As such, Codeine molecules can fit readily into endorphin receptor sites, and stimulate those neurons to accomplish the same end result - suppressing the perception of pain.