Organization of the rostral thalamus in the rat: evidence for connections to layer I of visual cortex.

Abstract

The present study demonstrates the organization of a thalamocortical projecting system which terminates within layer I of the visual cortex in the hooded rat. Horseradish peroxidase (HRP) injections restricted to layer I resulted in retrograde labeling of large and medium-sized multipolar and fusiform neurons that are located within the ventromedial (VM) nucleus and a dorsomedial subunit of the ventral anterolateral nucleus (VAL). Retrograde cellular labeling also occurs within the anteromedial nucleus (AM) following these injections. After restriction of HRP injections to layer I, peroxidase labeling was not found within neurons of the classically defined intralaminar system, i.e., central medial, paracentral, and central lateral nuclei, or within the rostral continuations of the intralaminar system. Since the VM, dorsomedial VAL, and AM nuclei are directly adjacent to portions of the internal medullary lamina, we refer to this amalgam of rostral thalamic nuclei that project to layer I as the "paralaminar" system. We also provide cytoarchitectonic criteria that can be used to distinguish three separate subdivisions within the VAL complex, including that portion of the VAL which is part of the "paralaminar" system. In contrast, when control injections of WGA-HRP are placed within either the cellular supragranular or infragranular layers of the visual cortex, no appreciable number of neurons are labeled within the VM, VAL, or AM.