Psychopharmacological effects of MK-801 in infant and preweanling rat pups.

-Neonatal (3-4-day-old) and preweanling (17-18-day-old) Sprague-Dawley rat pups were tested following SC administration of saline, 0.01, 0.05, 0.1, 0.5, or 1.0 mg/kg MK-801. In neonatal rat pups, reductions in a number of behaviors (forward locomotion, mouthing) were seen at the higher (0.5 and 1 mg/kg) doses. In contrast, evidence of behavioral stimulation in forward locomotion at 30 min postinjection was seen at a lower dose (0.1 mg/kg). In preweanling rat pups, marked sedative effects of MK-801 were seen at higher doses (decreases in forward locomotion, headlift and sniff), with signs of behavioral stimulation (increases in forward locomotion and mouthing) evident at low doses. Thus, as in adults, low doses of MK-801 may be behaviorally stimulatory and higher doses inhibitory to both neonatal and preweanling pups, although the stimulatory effects appear to be somewhat less pronounced in these young animals than has been previously reported in adulthood.