Neonatal Exposure to the Glutamate Receptor Antagonist MK-801: Effects on Locomotor Activity and Pre-pulse Inhibition Before and After Sexual Maturity in Rats

Neonatal lesions of the ventral hippocampus in rats lead to post- but not pre-pubertal behavioral changes suggesting adolescent onset of dopaminergic hypersensitivity and providing an animal model of schizophrenia. Neonatal exposure to glutamate receptor antagonists produces accelerated apoptosis leading to neuronal loss in central nervous system structures including the hippocampus. This suggested that neonatal MK-801 might lead to behavioural changes like those reported following ventral hippocampal lesions. Thus, rats received MK-801 (0, 0.5, 1.0 mg/kg ip) on postnatal day 3 (P3) and were tested pre- (P35) and post-pubertally (P56). MK-801 produced an increase in TUNEL staining in the hippocampus and other forebrain structures, confirming the induction of apoptosis. Results showed little difference in locomotor activity between neonatal saline- and MK-801-treated groups during habituation or following saline injection but increased activity was seen in the 0.5 mg/kg MK-801 group following amphetamine (1.5 mg/kg i.p.) at P35 but not P56. In tests of pre-pulse inhibition (PPI), neonatal saline and MK-801 groups showed stable startle amplitudes, minimal responding to the pre-pulse stimuli alone, an increase in PPI with increases in pre-pulse intensity, and reduced PPI with apomorphine (0.1 mg/kg s.c.). At P56, neonatal MK-801 groups tested following vehicle showed less sensitivity to changes in pre-pulse intensity. It was concluded that neonatal MK-801 increases apoptotic cell loss in the hippocampus but does not produce behavioural effects like those seen after neonatal ventral hippocampal lesions. However, neonatal MK-801 did lead to increases in locomotor activity in juveniles but not adults and reduced sensitivity to pre-pulse intensity in PPI tests in adulthood.