In vitro activity: Ondansetron decreases the intensity of withdrawal signs such as increased defecation, jumping and wet-dog shakes, elevated the nociceptive threshold values which are decreased by precipitated withdrawal, but produces no change in urination, rectal temperature or salivation. Ondansetron and granisetron significantly enhances gastric emptying of glass beads and improves cisplatin-induced slowing of gastric emptying in rats. Ondansetron exhibits a biphasic dose-response profile in mice, with antidepressant-like effects peaking at 0.1 mg/kg, in the forced swim and tail suspension tests. Ondansetron pretreatment augments the antidepressant effects of fluoxetine and venlafaxine but does not influence the effects of desipramine or 8-hydroxy-2-(di-n-propylamino) tetralin. Ondansetron (10 mg/kg) reverses hyperactivity in the open field, and decreases the percentage entry and time spent in open arms in the elevated plus maze. Ondansetron, a selective and potent 5HT3 receptor antagonist, is shown to be effective at blocking the amphetamine-induced disruption of LI at a dose of 0.01 mg/kg, but not at 0.1 mg/kg. Ondansetron is able to attenuate increases in dopamine activity, produces pharmacologically with amphetamine without affecting baseline dopamine activity. Ondansetron facilitates performance in young adult and aged animals, and inhibits an impairment in habituation induced by scopolamine, electrolesions or ibotenic acid lesions of the nucleus basalis magnocellularis. Ondansetron and arecoline antagonizes a scopolamine-induced impairment.

Cell Assay: 5-HT evoked transient inward currents (EC50 = 3.4 microM; Hill coefficient = 1.8) that were blocked by the 5-HT3 receptor antagonist ondansetron (IC50 = 103 pM). The 5-HT3A receptor antagonist ondansetron (0.3 nM) reversibly inhibited the 5-HT (30 microM) signal by 70% and at 3 nM it abolished the response.