I would not classify LSD (or psilicobyn) as an addictive substance, but for some usage does bring pleasure, and pleasure itself is a result of activation of the reward centers of the brain.
My comment still stands, pleasure itself (if anything) is the addictive thing here, and to some (including myself many, many times in my past) a good hard trip is extremely pleasurable. For others it might be ice cream, or sex, or skydiving.. regardless all of these examples do activate the rewards system of the brain, which is why they are pleasurable to begin with.
Interaction between LSD and dopamine D2/3 binding si... [Synapse. 2005] - PubMed - NCBITogether, these results are consistent with a direct interaction between LSD and a portion of dopamine D2/3 receptors in pig brain, possibly contributing to the psychopharmacology of LSD.
Lysergic acid diethylamide (LSD) is a partial agoni... [Life Sci. 1998] - PubMed - NCBIThese results suggest that LSD not only interacts with dopaminergic receptors but also has a unique capacity for modulating dopaminergic transmission.
I am not an expert on addiction here, but that is just the first couple of hits from a quick google scholar search. I think that once the drug wears off it is no longer bound to the seratonin receptors so they again function normally, and any releases of seratonin while under the influence are not a drastic departure from normal to begin with.