Zatosetron maleate is a potent and selective 5HT3 receptor antagonist. IC50 & Target: 5HT3 receptor^[1] In Vivo: Acute administration of 0.1 (n=21) and 0.3 (n=5) mg/kg Zatosetron maleate (Zatosetron) in male rats, but not 0.01, 0.05, 1.0 or 10 mg/kg (n=5, 3, 6 and 4, respectively) Zatosetron maleate or saline (n=5), leads to a significant reduction in the number of spontaneously active A10 dopamine cells. The number of spontaneously active A10 dopamine cells is not significantly different from 30 to 60 min post i.p. Zatosetron maleate (0.1 mg/kg) administration, shows a significant decrease by 60 to 90 min (0.65±0.11, P=0.03, n=5), a larger decrease by 90 to120 min (0.53±0.08, P=0.004, n=5) and remains at this significantly decreased level from 2 to 3 h (0.50+0.05, P=0.0004, n=5). Single-unit recordings show that Zatosetron maleate inhibits the activity of A10 dopamine cells following i.v. administration (ED₅₀=0.12 mg/kg, n=8). Chronic administration of 0.1 mg/kg (n=16) Zatosetron maleate, but not 0.01, 1.0 or 10 mg/kg (n=4, 8 and 7, respectively) Zatosetron maleate or saline (n=5), leads to a significant reduction in the number of spontaneously active A10 dopamine cells^[2].