Vertical bias in dendritic trees of non-pyramidal neocortical neurons expressing GAD67-GFP in Vitro

The neocortical neuropil has a strong vertical (orthogonal to pia) orientat ion, constraining the intracortical flow of information and farming the bas is for the functional parcellation of the cortex into semi-independent vert ical columns or 'modules'. Apical dendrites of excitatory pyramidal neurons are a major component of this Vertical neuropil, but the extent to which i nhibitory, GABAergic neurons conform to this structural and functional desi gn is less well documented. We used a gene gun to transfect organotypic sli ce cultures of meese and rat neocortex with the enhanced green fluorescent protein (eGFP) gene driven by the promoter for glutamic acid decarboxytase 67 (GAD67), an enzyme expressed exclusively in GABAergic cells. Many GAD67- GFP expressing cells were highly fluorescent; and their dendritic morpholog ies and axonal patterns, revealed in minute detail, were characteristic of GABAergic neurons. We traced 150 GFP-expressing neurons from confocal image stacks, and estimated the degree of vertical bias in their dendritic trees using a novel computational metric. Over 70% of the neurons in our sample had dendritic trees with a highly significant vertical bias. We conclude th at GABAergic neurons make an important contribution to the vertical neocort ical neuropil, and are likely to integrate synaptic inputs from axons termi nating within their own module.