We confirm with our model that the balance between glutamatergic and GABAergic neurons plays an important role in the observed network activity. This might explain why it is challenging to obtain synchronized oscillations3241. During early cortical development, GABA is predominantly excitatory45, followed by a switch to inhibitory properties during further maturation4647. This supports our observations that both glutamatergic and GABAergic inhibitors reduce the amount of active neurons, bursting frequency and synchronicity at three weeks post final plating while at later time points (5–7 weeks) only inhibition of excitatory glutamatergic activity seems to affect network activity, while protein expression levels do not change over time. Notably, a trend towards increased synchronicity is observed after addition of GABAergic inhibitor picrotoxin at later time points suggesting that inhibitory neurons play an important role in controlling network activity (reviewed in ref. 48). As shown in primary rodent cortical neurons before, burst amplitude was significantly increased at later time points by picrotoxin as well49. This is another important difference to the study conducted by Kirwan and colleagues30, who were unable to show an effect of GABAergic inhibition on calcium oscillations.
