*2.1. Materials*

The dipeptides WY, tyrosine-tryptophan (YW), tryptophan-methionine (WM), methionine-tryptophan (MW), tryptophan-valine (WV), tryptophan-leucine (WL), and tryptophan-phenylalanine (WF) (purity > 98%) were purchased from NARD Chemicals, Ltd. (Amagasaki, Japan). Tryptophan, tyrosine, (−)-scopolamine hydrobromide trihydrate, and R(+)-SCH-23390 hydrochloride were purchased from Sigma Aldrich Co. (St. Louis, MO, USA).
