Organization of the corticostriatal projection from rat medial agranular cortex to far dorsolateral striatum.

The rat medial agranular cortex (AGm) projects bilaterally to the striatum, mainly to the dorsocentral striatum (DCS) and in a narrow band in the far dorsolateral striatum (FDLS). For the projection from AGm to DCS, axonal and synaptic densities are known to be two times greater in the ipsilateral versus contralateral DCS, producing a contralateral to ipsilateral ratio of 0.5. In the present study we wished to determine if this was also the case for the projection from AGm to FDLS. Injections of biotinylated dextran amine were made into AGm in normal rats, and unbiased sampling was used to quantify the density of axons and axonal varicosities present in the FDLS. Unlike the contra/ipsi ratio of 0.5 found in DCS, the FDLS exhibited a contra/ipsi ratio of 1.0 for both axonal and synaptic densities. While overall axonal densities were roughly equivalent in ipsilateral and contralateral FDLS, axonal density in the ipsilateral FDLS was uniform, whereas density in the contralateral FDLS increased from the medial edge of the band to its boundary at the external capsule. These differences in the bilateral projections of AGm to separate striatal regions raise questions regarding the populations of corticostriatal neurons in AGm that contribute to these projections.