Insufficient developmental excitatory neuronal activity fails to foster establishment of normal levels of inhibitory neuronal activity

GABA treatment reduced total signals in unstimulated networks and networks stimulated for 5 days, but instead increased signaling in networks stimulated for 1 day. This increase was prevented by co-treatment with (2R)-amino-5-phosphonopentanoate and 6-cyano-7-nitroquinoxaline-2,3-dione, confirming that GABA invoked excitatory activity in networks stimulated for 1 day. Glutamate increased signals in networks subjected to all stimulation regimens; the GABA receptor antagonist bicuculline prevented this increase only in networks stimulated for 1 day. These latter findings are consistent with the induction of so-called “mixed” synapses (which release a combination of excitatory and inhibitory neurotransmitters) in networks stimulated for 1 day, and support the hypothesis that a critical level of excitatory activity fosters the developmental transition of GABAergic neurons from excitatory to inhibitory.