Brain Research 925 (2002) 28–41 www.elsevier.com / locate / bres Research report The cellular origin of corticofugal projections to the superior olivary complex in the rat * John R. Doucet , Liana Rose, David K. Ryugo Center for Hearing Sciences, Departments of Otolaryngology-Head and Neck Surgery and Neuroscience, Johns Hopkins University School of Medicine, 720 Rutland Avenue, Baltimore, MD 21205, USA Accepted 5 October 2001 Abstract 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.  2002 Elsevier Science B.V. All rights reserved. Theme: Sensory systems Topic: Auditory systems: central anatomy Keywords: Auditory; Hearing; Corticobulbar projection; Granule cell domain; Cochlear nucleus 1. Introduction timing and intensity differences between the two ears, stimulus attributes that are crucial in localizing the position The auditory cortex resides at the summit of the auditory of a sound in space [13,14]. They also provide efferent pathway. It interprets ascending information and gives rise feedback to the cochlea [11,34,35,38,42] and the cochlear to pathways that descend to subcortical auditory nuclei nucleus [4,8,26,30,31,39,46], and have been implicated in [9,10,28,40]. These corticofugal pathways suggest that the pathway mediating the acoustic startle reflex [17,37]. auditory cortex not only processes ascending neural in- As a first step towards understanding the role of cor- formation but also actively influences it. One target of ticofugal inputs within this diverse array of functions, our these descending pathways is the superior olivary complex goal in this study was to define the cortical areas and cells (SOC [9,20]). The SOC is a collection of distinct nuclei that project to the SOC in the rat. located rostral to the facial nucleus along the floor of the We addressed these questions by injecting the retrograde pons. The neurons in the SOC form circuits that compute tracer Fast Blue (FB) or Diamidino Yellow (DiY) into the SOC and analyzing the distribution of retrogradely labeled cells over broad regions of temporal cortex. FB and DiY *Corresponding author. Tel.: 11-410-955-0026; fax: 11-410-614- are fluorescent tracers that are commonly employed in 4748. E-mail address: jdoucet@bme.jhu.edu (J.R. Doucet). tract-tracing studies because of their sensitivity and their 0006-8993 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0006-8993(01)03248-6