The cellular origin of corticofugal projections to the superior olivary complex in the rat.

Corticofugal pathways originating in auditory cortex innervate most subcortical auditory nuclei in the ascending pathway [Auditory Neurosci. 1 (1995) 287-308; J. Comp. Neurol. 371 (1996) 15-40]. Our goal is to determine if these projections arise from the same neurons or if different neurons project to each of the separate structures. We also seek to identify the layers and fields of auditory cortex from which these neurons originate. In the present study, we answer these questions with respect to the projections to the superior olivary complex (SOC). Fluorescent retrograde tracers, Fast Blue (FB) or Diamidino Yellow (DiY), were injected into the SOC and the pattern of labeled cells was determined in temporal neocortex. We also injected FB into the granule cell domain (GCD) of the cochlear nucleus. Cortical projections to the GCD derive exclusively from layer V pyramidal cells in primary auditory cortex [Brain Res. 706 (1996) 97-102]. Thus the pattern of labeling produced by injections in the GCD provided a reference for interpreting the labeling after SOC injections. Layer V pyramidal cells project to the SOC, and these neurons were distributed bilaterally in primary and secondary areas of auditory cortex. The projections to the SOC from primary auditory cortex are predominantly uncrossed, whereas those from secondary auditory cortex are nearly equal for the two hemispheres. In animals that received injections of FB in the GCD and DiY in the SOC, cells labeled by each injection had a different laminar distribution and very few cells were double labeled. These data suggest that the cortical pathways ending in the cochlear nucleus and SOC are largely independent. We discuss the implications of these findings with respect to the multifunctional nature of the SOC in brainstem auditory processing.