To compare the distributions of gold particles in sections labeled with either the GluR4 or GluR2/3 antiserum, the gold particles were categorized into two groups based on their localization with respect to the midpoint of the synaptic cleft set at 7.5 nm from the outer leaflet of the postsynaptic membrane. Compared with particles signaling GluR2/3, particles signaling GluR4 were significantly enriched presynaptic to the cleft midline. Chi square test,p = 0.0072 (no. of synapses studied = 25). Note that because of the distance between epitope and gold particles (Fig. 7), some particles derived from a postsynaptic epitope will end up presynaptic to the cleft midline. The distribution of gold particles signaling GluR2/3 is thus consistent with an exclusively postsynaptic localization of this receptor subunit.