Widespread projections from subgriseal neurons (Layer VII) to layer I in adult rat cortex

Theories of information processing and plasticity in mammalian cortex often rely on knowledge of intracortical networks studied in rodent cortex. Acco rdingly, the contribution of all cells involved in this circuitry is potent ially significant, including connections from a subset of neurons that pers ist from the developmental subplate, called subgriseal neurons in the prese nt study. Ascending corticocortical connections from subgriseal neurons wer e identified by using in vivo transport of fluorescent retrograde tracers f rom discrete applications confined to cortical layer I (approximately 1 mm( 2)) or from injections placed into superficial cortical layers. Application s restricted to cortical layer I can be identified by a subsequent retrogra de labeling pattern that includes neurons in layers II/III and V but not th ose in layer TV. In contrast, when retrograde tracer is deposited in layers II/III, layer TV cells are also labeled. By using this identification tech nique in juvenile and adult rats, widespread interareal projections to supe rficial layers, including unequivocal connections to cortical layer I, were found to originate from a tangential band of neurons directly below the co nventionally identified gray matter (i.e., subgriseal) and from a smaller n umber of cells in the white matter (WM) proper. Subgriseal and Will neurons were labeled below application and injection sites in somatosensory, audit ory, visual, motor, frontal, and adjacent areas at distances of more than 4 mm. However, the subgriseal-to-superficial pathway was not sensitive to no nfluorescent retrograde tracers including horseradish peroxidase. Because n eurons in the deeper cortical layers can be strongly influenced through inp ut to their apical dendritic extensions in cortical layer I, the widespread connections described in the present study indicate that the ascending sub griseal projections should be considered in models of mature cortical funct ion. J. Comp. Neurol. 407:275-286, 1999. (C) 1999 Wiley-Liss, Inc.