Topographic organization of the striatal and thalamic connections of rat medial agranular cortex.

The rostral and caudal portions of rat medial agranular cortex (AGm) play different functional roles. To refine the anatomical framework for understanding these differences, axonal tracers were used to map the topography of the connections of AGm with the striatum and thalamus. The striatal projections follow mediolateral and rostrocaudal gradients that correspond to the locations of the neurons of origin within AGm. Projections from rostral AGm are widespread and dense rostrally, then coalesce into a circumscribed dorsocentral region at the level of the pre-commissural septal nuclei. Projections from mid and caudal AGm are less widespread and less dense, and are focused more caudally. Striatal projections from the adjacent anterior cingulate and lateral agranular areas overlap those of AGm but are concentrated more medially and laterally, respectively. Thalamic connections of AGm are organized so that more caudal portions of AGm have connections with progressively more lateral and caudal regions of the thalamus, and the full extent of AGm is connected with the ventrolateral (VL) nucleus. Rostral AGm is interconnected with the lateral portion of the mediodorsal nucleus (MD1), VL, and the central lateral (CL), paracentral (PC), central medial, rhomboid and ventromedial nuclei. Caudal AGm has robust connections with VL, the posterior, lateral posterior and lateral dorsal nuclei, but little or none with MD1, CL/PC and VM. These differences in the subcortical connections of rostral and caudal AGm parallel their known differences in corticocortical connections, and represent another basis for experimental explorations of the functional roles of these cortical territories.