Extrinsic GABAergic innervation of developing neocortical layer 1 in organotypic slice co-cultures

Afferents from the zona incerta (ZI) of the ventral thalamus contribute to the dense, transient gamma-aminobutyric acid (GABA)ergic fiber plexus in layer 1 of the developing rodent somatosensory cortex. Incertocortical axons contact the distal apical dendrites of postmigratory cortical pyramidal cells. Although recent work has shown that these GABAergic incertocortical fibers are likely to provide widespread fast synaptic excitation of pyramidal cells in layers 2-6 during peak periods of cortical synaptogenesis, little is known about the mechanisms by which these axons project to the neocortex and are confined to layer 1. Here we characterize organotypic slice co-cultures in which a region of embryonic diencephalon containing the ZI is maintained adjacent to a region of embryonic somatosensory cortex. Diencephalic explants from transgenic mice expressing enhanced green fluorescent protein (EGFP) enabled direct visualization of diencephalocortical connections. Isochronic co-cultures exhibited diencephalocortical fiber ingrowth immunoreactive for both GABA and the presynaptic vesicle-associated protein synaptophysin that was restricted to neocortical layer 1. This pattern of lamina-specific diencephalocortical ingrowth occurred irrespective of placement of the afferent explant, and persisted in the absence of action potential activity and GABA(A) receptor activation. Heterochronic co-cultures containing older cortex demonstrated that the cortical explants remain permissive for lamina-specific ingrowth through the first postnatal week. Organotypic slice cocultures provide a system in which to study the mechanisms underlying the layer 1-specific ingrowth of extrinsic GABAergic inputs to the perinatal neocortex.