AXONAL ARBORIZATION OF CORTICOSTRIATAL AND CORTICOTHALAMIC FIBERS ARISING FROM PRELIMBIC CORTEX IN THE RAT

This study aimed at elucidating the branching pattern of striatal and thalamic projections arising from prelimbic (Cg3) cortex in the rat. S mall pools (5-15 cells) of neurons were microiontophoretically injecte d with biotin-dextran or biocytin and their labeled axons were individ ually reconstructed from serial horizontal sections immunostained for calbindin-D28k to delineate striatal patch/matrix compartments. Recons truction of >40 axons shows that all Cg3 corticofugal fibers, includin g corticothalamic axons from layer VI, course through the patch networ k in the rostromedial sector of the striatum. Corticostriatal projecti ons arise from two types of layer V cells: (i) long-range corticofugal neurons, whose main axons reach the brainstem and/or spinal cord, and (ii) neurons arborizing into both striatum and claustrum, either ipsi -. contra- or bilaterally. The axons of these two types of neurons arb orize profusely in striatal patches and only sparsely in the matrix. L ayer VI neurons do not arborize in the striatum but target principally the thalamus. The same corticothalamic axon can innervate the anterio r, rostral intralaminar and mediodorsal thalamic nuclei. These finding s support the concept that no corticofugal fiber system exists that is solely devoted to the striatum. They also shed new light on how neura l information from prelimbic cortex is conveyed to various subcortical limbic structures in the rat.