Corticocortical and thalamocortical projections to layer I of the frontal neocortex in rats

Layer I of the neocortex is a dense synaptic zone consisting of horizontal corticocortical and widespread layer VII projections, in addition to thalamic inputs. In order to determine the origin and extent of corticocortical and thalamocortical projections to layer I of the frontal/premotor area M2 of the rat neocortex, we have used fluorescent anatomical tracing methods to determine the precise sources of cortical and thalamic input to the rostral and caudal aspects of layer I of M2. Retrograde tracer diamidino yellow (DY), applied directly to the pial surface on rostral or caudal areas of rat M2 (RM2 and CM2, respectively) labeled cells ipsilaterally throughout layers II/III, V, and VII of the adjacent primary motor area and the parietal areas (SI and SII). In addition, retrograde transport labeled contralateral CM2 or RM2 in layers II/III and V at sites homotopic to either CM2 or RM2 application sites. Contralateral layer VII was retrogradely labeled by the application to layer I of CM2, but not by the RM2 application. Retrograde DY transport from layer I of RM2 or CM2 of was seen in the ventral medial (VM), ventral lateral (VL), and posterior (Po) thalamic nuclei. However layer I transport from CM2 additionally labeled the thalamic central medial (CM) nucleus, while the RM2 labeled the mediodorsal (MD) thalamic nucleus. Upon determination that thalamic nuclei VM and VL were of primary interest in this study, due to their dense retrograde labeling, injections of anterograde tracer rhodamine dextranamine (RDA) into VM or VL were performed in order to study the projection patterns of these nuclei to layer I of the frontal cortex. RDA injections into VM labeled fibers extending through layer I of both RM2 and CM2 and throughout the cingulate cortex. Injections of RDA into VL consistently labeled dense fibers in layer I of both CM2 and RM2, although labeling was sharply decreased anterior to CM2. This study adds to a growing body of evidence that projections to layer I from all sources of cortical input make a significant contribution to integration throughout the neocortex.