The source of corticocollicular and corticobulbar projections in area Te1 of the rat.

Cortical area Te1 in the rat commonly is associated with primary auditory cortex. It is the source of direct projections to the inferior colliculus (IC), superior olivary complex (SOC), and the cochlear nucleus (CN). A question that arises is whether these descending pathways derive from a common source or separate populations of cortical neurons. We addressed this question in seven rats by injecting either Diamidino yellow (DiY) or Fast blue (FB) into the IC and injecting the other tracer into the CN (n=4) or SOC (n=3). All injections were made on the left side of the brain. In a sample of sections through area Te1 in both hemispheres, we counted single- and double-labeled cells. We estimate that IC-projecting cells outnumber those projecting to the CN or SOC by at least a factor of ten. The source of corticofugal pathways to the left IC was heavily biased towards the same side of the brain (ipsi/contra ratio 8 +/- 2.5), whereas it was more equally distributed between the two hemispheres for the left CN and SOC (ipsi/contra ratios ranged from 0.7-2.3). Finally, we observed that only 10-20% of those cells filled with a tracer injection in the CN or SOC also contained the tracer injected into the IC. In a previous study, we observed a similarly small percentage of double labeled cells when FB and DiY were injected into the CN and SOC, respectively. Combined with the distinct laminar distribution of IC-, SOC-, and CN-projecting neurons within layer V, the results suggest that these three pathways largely derive from different populations of cortical neurons.