A molecular function that directly (via physical interaction or direct modification) activates, inhibits or otherwise modulates the activity of a neurotransmitter receptor. Modulation of activity includes changes in desensitization rate, ligand affinity, ion selectivity and pore-opening/closing. [ http://www.ncbi.nlm.nih.gov/pubmed/12740117 http://www.ncbi.nlm.nih.gov/pubmed/25172949 ]