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 la yer 1 of the developing rodent somatosensory cortex. Incertocortical axons contact the distal apical dendrites of postmigratory cortical pyramidal cel ls. Although recent work has shown that these GABAergic incertocortical fib ers 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-cul tures in which a region of embryonic diencephalon containing the ZI is main tained adjacent to a region of embryonic somatosensory cortex. Diencephalic explants from transgenic mice expressing enhanced green fluorescent protei n (EGFP) enabled direct visualization of diencephalocortical connections. I sochronic cocultures exhibited diencephalocortical fiber ingrowth immunorea ctive for both GABA and the presynaptic vesicle-associated protein synaptop hysin that was restricted to neocortical layer 1. This pattern of lamina-sp ecific diencephalocortical ingrowth occurred irrespective of placement of t he afferent explant, and persisted in the absence of action potential activ ity and GABA, receptor activation. Heterochronic co-cultures containing old er cortex demonstrated that the cortical explants remain permissive for lam ina-specific ingrowth through the first postnatal week. Organotypic slice c ocultures provide a system in which to study the mechanisms underlying the layer 1-specific ingrowth of extrinsic GABAergic inputs to the perinatal ne ocortex. (C) 2000 Wiley-Liss, Inc.