Direct hyperpolarizing action of NPY via Y1 receptor. Representative traces reflecting the actions of the NPY agonist and antagonist in the presence of TTX (0.5 μm) in the bath. A, NPY (1 μm) consistently hyperpolarizes hypocretin neurons. B, The Y1R agonist [Pro³⁴]-NPY (1 μm) mimics the hyperpolarizing actions of NPY. C, In the presence of 1 μm BIBP3226 (Y1 receptor antagonist), NPY does not hyperpolarize hypocretin neurons. D, Bar graph showing the alteration in the membrane potential after application of the NPY receptor agonists and antagonist: NPY (^*p < 0.01), Y1R agonist [Pro³⁴]-NPY (Pro; ^*p < 0.05), Y2R agonist NPY13-36 (13-36), Y5R agonist [d-Trp³²]-NPY (D-Trp).