Table 1.

Subcellular distribution of GFP–NRX1β4(+) in cortex and hippocampus of transgenic mice

Tissue	% SIG particles
	Axonal		Dendritic		Axodendritic
	Intracellular	Membranous	Intracellular	Membranous	Contacts
P14 cortex	11 ± 2	51 ± 4	19 ± 3	11 ± 2	8 ± 2
P14 hippocampal formation	6 ± 2	55 ± 6	10 ± 3	17 ± 3	12 ± 3

Sections containing the cerebral cortex and hippocampal formation were subjected to EM analysis after immunolabeling for GFP, the extracellular tag of transgenically expressed NRX1β4(+), using SIG as the label. SIG particles were identified as residing in dendrites versus axons and at the plasma membrane or intracellularly. A significant pool of SIG particles resided exactly at the point of contact between a dendrite and an axon (classified as contacts). For the cortical tissue, the number of SIG particles encountered was 501, all of which were identified within 11 micrographs sampled randomly from the infragranular layers. For the hippocampus, 156 SIG particles were encountered from 12 micrographs, taken from the molecular layer of the dentate gyrus. In cortex, all subcellular domains were identifiable. However, in the hippocampus, 19% of the SIG particles resided within subcellular profiles that were unidentifiable as a result of immaturity. The values shown represent percentages ± SEM in each subcellular domain. ANOVA revealed statistically significant differences in the group means (p < 0.00001), indicating highly specific and heterogeneous distribution of NRX1β4(+) expression across the subcellular domains. Student's t test revealed that this heterogeneity reflects significant enrichment of NRX1β4(+) in axonal membranes (p < 0.00001). Specificity of labeling in dendritic membrane and the axodendritic junctional cleft were revealed by comparing SIG particle distribution between tissue of transgenic and wild-type mice (Student's t test, p < 0.005).