Long-term effects of neonatal exposure to MK-801 on recognition memory and excitatory-inhibitory balance in rat hippocampus

Blockade of the N-methyl-d-aspartate receptors (NMDARs) during the neonatal period has been reported to induce long-term behavioral and neurochemical alterations that are relevant to schizophrenia. In this study, we examined the effects of such treatment on recognition memory and hippocampal excitatory and inhibitory (E/I) balance in both adolescence and adulthood. After exposure to the NMDAR antagonist, MK-801, at postnatal days (PND) 5-14, male Sprague-Dawley rats were tested for object and object-in-context recognition memory during adolescence (PND 35) and adulthood (PND 63). The parvalbumin-positive (PV+) γ-aminobutyric acid (GABA)-ergic interneurons and presynaptic markers for excitatory and inhibitory neurons, vesicular glutamate transporter-1 (VGLUT1) and vesicular GABA transporter (VGAT) were examined in the hippocampus to reflect the E/I balance. We found that rats receiving MK-801 treatment showed deficits of recognition memory, reduction in PV+ cell counts and upregulation of the VGLUT1/VGAT ratio in both adolescence and adulthood. Notably, the changes of the VGLUT1/VGAT ratio at the two time points exhibited distinct mechanisms. These results parallel findings of hippocampal abnormalities in schizophrenia and lend support to the usefulness of neonatal NMDAR blockade as a potential neurodevelopmental model for the disease.