CGP71683 hydrochloride is a competitive neuropeptide Y5 receptor antagonist with a K_i of 1.3 nM, and shows no obvious activity at Y1 receptor (K_i, >4000 nM) and Y2 receptor (K_i, 200 nM) in cell membranes. IC50 & Target: Ki: 1.3 nM (Y5 receptor), 200 nM (Y2 receptor), >4000 nM (Y1 receptor)^[1] In Vitro: CGP71683 hydrochloride is a competitive neuropeptide Y5 receptor antagonist with a K_i of 1.3 nM, and shows no obvious activity at Y1 receptor (K_i, >4000 nM) and Y2 receptor (K_i, 200 nM) in cell membranes^[1]. In Vivo: CGP71683 (15 nmol/rat, icv, twice daily) shows anorexigenic effect, reducing food intake and bady weight of fed rats. CGP71683 causes 3-times higher serum total T4 and 37% increase in free T4 in the fasted rats than in the fasted controls rats^[2].