(a) Activity levels were monitored in photo-beam arenas for 30 min and at time 0, mice were injected with Saline or MK-801 at 0.15 or 0.25 mg/kg. (a, b) WT mice exhibited MK-801 induced hyperactivity at the 0.25-mg/kg dose only and environmental enrichment had no effect on the response to the antagonist. (c, d) KO mice exhibited hyperactivity in response to MK-801 treatment at both doses and environmental enrichment further increased this hyperactivity. Data expressed as mean±SEM; n=5–7 per group; *p<0.05, **p<0.01, ***p<0.001. Saline vs 0.25 mg/kg dose, #p<0.05, ###p<0.001; Saline vs 0.15 mg/kg, arrow denotes continuous significant data points.