It is hard to know exactly why the measurements of the internal control, untransfected neuron appear to vary between the experimental groups. While an effort was made to try and evoke synaptic responses with a consistent stimulus voltage setting, the effectiveness of the stimulus for evoking an NMDA-EPSC depends on, among other things, the condition of the stimulating electrode (e.g. resistance, stray capacitance, bubbles etc). It was rarely practical to record all mutant conditions in the same experiment, so variation in the stimulating electrode condition over the course of this series of experiments could potentially lead to some apparent differences between untransfected neurons with respect to their mean NMDA-EPSC peak amplitude and charge transfer. What this does serve to illustrate though is how important it was for us to use untransfected neurons as an internal control.
The results presented are convincing and I only have a few minor comments.