Control of NMDA receptor-mediated activity by GABAergic mechanisms in mature and developing rat neocortex

The effects of diazepam and low concentrations of bicuculline methiodide (BMI) on the expression of long-latency N-methyl-D-aspartate (NMDA) receptor-mediated activity was studied in supragranular layers of juvenile (14-18 days old) and adult rat (greater than or equal to 28 days) primary somatosensory cortex. In juvenile slices, orthodromic stimulation of layer VI/white matter evoked a long-lasting oscillatory field potential response which could be blocked by the NMDA receptor antagonist D(-)-2-amino-5-phosphonovaleric acid (D-APV) and by diazepam. Similar D-APV-sensitive responses could be observed in adult slices when the GABAergic system was slightly suppressed by adding low doses of BMI to the bathing solution. Our findings indicate that a small decrease in the efficacy of the inhibitory system, whether caused by developmental events or by processes that modulate inhibitory electrogenesis, can lead to NMDA receptor-mediated synchronized afterdischarges, which might play an important role in the functional maturation of the neocortex and in susceptibility to epileptogenesis.