I don't think it's technically an amino acid, at least I can't find anything on that. It's synthesized from tryptophan. One neurotransmitter connection is through tryptophan, both niacin and some neurotransmitters, serotonin's the one that comes to mind, are made from tryptophan, so a lack of niacin might increase the requirement for tryptophan, that might explain some of the neurological symptoms of pellegra.